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Chemical Bonding
CHEMICAL BONDING
Updated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde
(Monika Kannadaguli & Ramaiah Kotra)
1. INTRODUCTION
For more than four decades, almost all nonwovens required a chemical binder in order to provide any measure of structural integrity. In addition, the binder was called upon to contribute and convey numerous properties that were necessary for the effective performance of the fabric.
During this extended period, binders were essentially the weak element in developing fully acceptable nonwoven fabrics. The fibers that were available to the nonwoven industry were the same fibers that were available to the textile and other fiber-based industries; hence, the fibers were fully acceptable. Generally, the binder limited the performance of the nonwoven fabric.
The deficiencies cited against nonwovens generally were deficiencies attributable to an inadequate binder. Common complaints are as follows:
The fabric doesn't have enough strength.
The fabric is too stiff.
The fabric has inadequate absorbency.
The fabric shows poor laundering ability.
The fabric has inadequate dry cleaning ability.
The fabric simply doesn't feel like a textile.
Consequently, a great deal of effort has been put into the development and continuous improvement of chemical binders. The steady improvements in nonwovens performance that occurred over a period of many years were, in no small measure, due to improvements in the performance and utility of the binder.
In the very early stages of nonwovens development, different types of natural resins and glues were used to bond nonwovens. While they conveyed some integrity and strength to these webs, they also had many glaring deficiencies. Consequently, synthetic binders were developed to meet the structural and performance requirements of nonwoven fabrics.
Polyvinyl acetate was the first successful synthetic binder used in substantial volume. This material had distinctly superior adhesive properties, strength, and performance compared to the early natural adhesives. This binder is flexible and it can be applied to fiber webs by many ways including print bonding.
The industry was faced with the inevitable compromise in fabric properties of nonwovens bonded with synthetic materials. In order to build strength in the fabric, increasing amounts of resin must be applied, which results in more stiffness. If softness is necessary, it can be achieved, but primarily by sacrificing strength.
A substantial improvement in this trade-off of strength and softness was achieved with the introduction of acrylic-based latex binders in the 1950s and 1960s. By proper selection of co-monomers, it is possible to build improved softness properties with adequate strength. Consequently, these binders became widely used by most of the nonwovens industry, despite the somewhat higher cost.
As polymer technology for manufacturers of synthetic binder systems improved, a greater variety of chemical building blocks became available with much greater flexibility in terms of binder strength, durability, and other properties. The introduction of cross-linkable and self-crosslinking binder polymers turned out an entirely new range of fabric properties. This was particularly noteworthy in durable nonwovens where such durability features as washability and dry cleanability were important.
2. PROPERTIES DESIRED IN A BINDER
The construction of a nonwoven with suitable binders is to achieve improved characteristics such as strength, softness, adhesion, firmness, durability, stiffness, fire retardence, hydrophilicity, hydrophobicity, anti-microbial properties, organic compatibility, reduced surface tension, improved dimensional stability and solvent, wash and acid resistance. The following list illustrates some general considerations required for an ideal binder. The required properties can be varied depending on the end-uses.
· · Strength: The strength of a nonwoven fabric is more closely related to the strength of the applied binder.
Adhesion to Fibers: Even though the mechanism of adhesion is not completely understood, the adhesion strength of the binder-to-fiber bond has to be considered.
Flexibility/handle: The some movements of fibers should be allowed, especially when a soft hand is desired.
Elastic Recovery: To avoid the permanent deformation of fabric, good elastic recovery is required under strain.
Resistance to washing/ Drying cleaning: Some nonwoven products need durability in cleaning processes according to their end-uses.
Resistance to aging: The binder should be stable and not be degraded in the fabric during storage and use.
Good color and color retention: Diverse ranges of colors are required, and the colorfastness and yellowing problems should be considered.
Economical: Minimizing the cost is an ongoing requirement.
Other special requirements: Such as Flame resistance, resistance to chemicals, air, oxygen, light, heat, etc.
3. HOW BINDERS WORK
The process involves three steps:
Binder application to nonwoven web.
Removal moisture or solvent.
Formation of strong bond between binder and nonwoven web.
In general binders contain polymer produced by the reaction of monomers in presence of initiators or catalysts. Surfactants are used to stabilize these polymer particles in water during emulsification (Fig. 1)
Fig.1: Emulsion Polymerization – Schematic. [1]
After application of the binder to the nonwoven web, during moisture removal, film formation takes place. This phenomenon is shown in Fig. 2.
Fig.2: Schematic of film formation [1]
Binder factors influencing nonwoven performance are:
Backbone Structure
Functional Group
Surfactant
Process
4. CLASSIFICATION OF BINDERS
Due to their diversity, binders may be classified into several categories based on polymer (binder) chemical structure, functionality and the type of curing reactions.
4.1. CLASSIFICATION BASED ON CHEMICAL STRUCTURE
There are three main kinds of binders: butadiene copolymers, acrylates, and vinyl copolymers. The chemical compositions influence Tg, hardness and softness, hydrophobicity and hydrophilicity, elasticity, aging, and dry tensile strength of binders. The higher the Tg, the higher will be the dry tensile strength of binders.
Butadiene Copolymers
The structure of the main butadiene copolymer is shown as follows:
The butadiene polymers are cross-linked by polysulphides, and their properties are modified by different copolymers. The butadiene monomers provide elasticity while styrene and acrylonitrile monomers give tensile strength, and oil and solvent resistance, respectively. Their disadvantages are oxidation and discoloration due to residual double bonds in their polymer chains.
Acrylic acid derivatives
Acrylic binders are the most widely used and versatile binders available with various modifications. The properties of acrylic binders differ according to their derivatives and copolymers. The structures of the common acrylic polymers are as follows:
Acrylic Acid Derivatives
They are frequently copolymerized with styrene, acrylonitrile, vinyl chloride or vinyl acetate, depending on the desirable properties. Some of these properties are hardness from styrene, solvent resistance from acrylonitrile, flame retardancy from vinyl chloride, and cost benefits from vinyl acetate.
Vinyl copolymers
There are two main binders for vinyl copolymers: vinyl chloride and vinyl acetate. Since the vinyl binders are stiff, they are plasticized externally or internally. As internal plasticizers, ethylene and acrylate monomers are used, and external plasticizers consist of vinyl chloride. Due to its low Tg, vinyl acetate is not that stiff, and its advantage is low cost. The chlorides cause yellowing problems. The chemical structures are closely related Tg and stiffness of binders.
Vinyl acetate
4.2. CLASSIFICATION BASED ON FUNCTIONALITY
The functionality of binders is in the functional groups attached to polymer chains, which influences wet and solvent properties. To modify binder properties, copolymerization with a small amount of monomers with special functionality is performed. The main functionalities in binders are carboxyl and amide side chains.
Carboxyl functionality
This functionality is related to binders containing acrylic acid or methacrylic acid by copolymerization. The binders are crosslinkable since the functional group, carboxylic acid, provides sites for crosslinking reactions.
Amide functionality
This functionality is related to binders containing acrylamide by copolymerization. The amide functionality provides crosslinking sites, and even the binders are self-crosslinkable.
N-metylol amide (NMA) functionality
This functionality is obtained after acrylamide is reacted with formaldehyde. The binders containing the substituted acrylamide groups have self-crosslinking properties and the possible reaction as follows:
Acrylamide
4.3. CLASSIFICATION BASED ON TYPE OF CURING REACTIONS
The classification of reactivity refers to crosslinkability of binders, which is related to reaction with curing resins, crosslinking agents. The most common curing resin is melamine formaldehyde condensate resin involving reaction of n-methylol groups.
Non-crosslinkable polymers
The polymers do not contain any of the functional groups. They cannot crosslink, even with external curing resins.
Crosslinkable polymers
The polymers contain acid or amide functional groups. They can react with added curing resins, but the degree of crosslinking is limited.
Self-crosslinking polymers
The polymers contain n-methylol functional groups. They can react with themselves, and a high crosslink density can be obtained by adding curing resins.
Recent trends in chemical bonding: Although nonwoven manufacturers are seeking alternative technologies such as thermal bonding, chemical bonding still has its advantages and a promising market. Chemical bonding allows more room for fabric designs and fiber selections. Both disposable and durable products are supplied to roll goods producers and fiber manufacturers. On the environmental front, increasingly strict regulations and guidelines are driving a trend towards alternative products and technologies. Manufacturers and end-product suppliers alike are seeking ultra-low or formaldehyde-free binders. The growing consideration of the environmental impact of chemical binder and additives has become a focus of debate on the national and international level.
Latex binder chemical types
A latex polymer consists of an aqueous medium with extremely fine liquid or solid polymer particles dispersed therein. The latex polymer generally is produced via free radical emulsion polymerization in water, whereby a vinyl monomer is combined with a small amount of other monomer (co-monomer) to create a high molecular weight polymer. The latex dispersion also will carry surfactants, stabilizers, and other additives to convey realistic properties to the latex itself.
When used as a binder, the latex typically is combined with other components to provide the formulated binder ready for application to the fiber web. The formulated binder conveys many characteristics that are not possessed by the straight binder. Consequently, there is a substantial chemistry involved in combining the latex with the other components in order to prepare the formulated binder.
Binders are quite dependent upon the glass transition temperature (Tg) of the monomer unit selected to form the polymer. Differential Scanning Calorimeter (DSC) is used to determine Tg as shown below (fig. 3):
Fig. 3: Typical DSC graph
The lower the (Tg) of the monomer units, the softer is the resulting polymer. A sampling of the most common monomers used in the manufacture of latex polymer for nonwoven binders include the following materials:
Monomer Tg(0C)
Ethylene -125
Butadiene -78
Butyl Acrylate -52
Ethyl Acrylate -22
Vinyl Acetate +30
Vinyl Chloride +80
Methyl Methacrylate +105
Styrene +105
Acrylonitrile +130
The monomers selected for forming the polymeric latex also have considerable influence on the hydrophilic or hydrophobic nature of the binder. This can affect the wet strength of the nonwoven fabric as well as a host of absorbency characteristics.
With the current capabilities of polymerization chemistry, there is considerable versatility for each chemical type. Despite this range of properties, the commonly employed nonwovens binders generally are characterized by a fairly well defined set of properties. These properties can be modified to some degree by incorporation of other agents, but they provide a useful guide in classifying the kind of performance to be expected from each type of binder.
5. TYPES OF BINDERS
The following comparison of latex binder chemical types provides an indication of the relative performance, as well as the advantages and disadvantages of each type of binder. As indicated, the binder properties can be modified considerably by the presence of co-monomers.
i) Acrylic: These binders offer the greatest durability, color stability, and dry/wet performance. Acrylic binders have the widest range of fabric hand properties. They can be formulated to vary from very soft (Tg = - 40°C) to extremely hard (Tg = 105°C). These binders can be used in virtually all nonwovens applications, although they tend to be more costly. These polymers can be made to cross-link, with substantial improvement in durability.
ii) Styrenated Acrylics: These are tough, hydrophobic binders. The resulting textile hand ranges from soft-to-firm (Tg varies from –20°C to +105°C ).These binders can be used in applications where there is a need for some wet strength without crosslinking. The use of this type of latex binder does involve some sacrifice in UV and solvent resistance.
iii) Vinyl Acetate (VAC): The vinyl acetate binders are firm (Tg = +30°C to +40°C); however, they are relatively low cost and find extensive use. They offer good dry strength and toughness, but are somewhat hydrophilic and have a tendency to yellow when subjected to heat.
iv) Vinyl Acrylics: These binders are more hydrophobic than the straight VAC binders. They provide excellent toughness, flexibility, and better color stability. They are the compromise between VAC and acrylic, and can compete on a cost/performance basis. The hand range is limited to intermediate softness (Tg = -10°C) to a firm hand (Tg = +30°C).
v) Ethylene Vinyl Acetate (EVA): These latex binders have a (Tg range of –20°C to +115°C, which is equivalent to soft ranging to an intermediate textile hand. They exhibit high wet strength, coupled with excellent absorbency. In general, they are less costly than acrylics. They do have a tendency to have more of an odor compared to other binders. They are used primarily in wipes, air-laid pulp fabrics and similar applications.
vi) Styrene-Butadiene (S/B, SB, or styrene butadiene rubber): These binders have an excellent combination of flexibility and toughness. They range in hardness from very soft (Tg = -30°C) to very firm (Tg = +80°C). However, the (Tg of an SB binder is not strictly comparable to other classes of nonwoven binders. The styrene-to-butadiene ratio (S/B ratio) is the most common method for describing the relative hand resulting from the use of these binders. When cross-linked, this class of binder is very hydrophobic and durable. They are affected somewhat by heat and light because of their tendency to oxidize.
vii) Polyvinyl Chloride (PVC): The homopolymer of polyvinyl chloride is a very hard, rigid polymer (Tg = +80°C). This polymer must be plasticized to provide flexibility and film-forming properties. Normally, the (PVC) binders used in nonwovens are softened internally by co-polymerizing the vinyl chloride or with softer acrylic monomers. The hand range of most of these polymers is still relatively firm (Tg is greater than the +30°C). Because this type of polymer is a thermoplastic, it performs well in heat and dielectric sealing applications. This can be an advantage in some uses. The chlorine content of the polymer promotes flame retardency. This feature is one of the primary benefits of utilizing this type of binder. However, the chlorine also conveys the tendency to yellow upon heat aging, due to elimination of hydrogen chloride from the polymer.
viii) Ethylene/Vinyl Chloride: Binders in this class have a slightly broader hand range (Tg = 0°C to +30°C) without the external plasticization required of (PVC) binders. The presence of the chlorine again conveys some flame retardancy. These binders exhibit good acid resistance, fair water resistance, and excellent adhesion to synthetic fibers. There is some tendency to yellow upon aging. In essence, this is an internally plasticized (PVC) binder, considering the ethylene monomer to be the softener.
6. FORMULATION
i) Ingredients
The formulation of binding solution is an art since many ingredients are involved and many different possibilities exist for different end-uses. Some of the characteristics, and the types of formulation agents utilized to obtain them include the following.
· · Surfactants : offer improvement in binder adhesion, stability, and ability to be converted into a foam
External cross-linkers: provide cross-links with binder polymer to provide improved performance
Defoamers: utilized to minimize foam in processing
Repellent agents : convey water or oil repellency
Salts: added to impart low flame response properties and to convey antistatic properties
Thickeners: added to control the rheology of the binder liquid
Catalysts: added to facilitate curing and to promote cross-linking
Acids and bases: added to control pH of the latex
Dyes and pigments: provide color to the binder and fabric
Fillers: added to reduce binder tack and to lower cost
Optical brighteners: added to increase whiteness
Sewing aids: added to provide lubrication during fabrication
The purposes of wetting agents, mainly nonionic or anionic surfactants, are to enhance binder penetration through webs, improve the affinity between binder and fibers. The crosslinker, which has multi-functional groups, is generally added to increase crosslink density and to improve durability and resistance to deformation.
ii) Order of Formulation
In terms of adding ingredients into a binding bath, the compatibility of ingredients should be confirmed because the orders are extremely important. The milky white color of most binders impedes a check on the white-color indication of non-compatible ingredients, so most ingredients are first added to the dilution water. After the compatibility is assured, binders are added and then thickeners added to adjust viscosity. For the stability of the binding solution, catalysts are added just before application. Some water may be added to reach a desirable solid level. The summarized order is as follows:
· · Most ingredients
· · Latex binder
· · Thickener
· · Catalyst
· · Some water, and the others, such as dyes and pigments, fillers, clays, optical brighteners, sewing aids, etc.
7. BONDING TECHNOLOGY
Web consolidation or nonwoven bonding processes interlock preferentially arranged fiber or film assemblies by mechanical, chemical, solvent, and/or thermal means. The degree of bonding is a primary factor in determining fabric integrity (strength), porosity flexibility, softness, and density (loft, thickness).
Bonding may be carried out as a separate and distinct operation, but generally is carried out as a sequential operation in tandem with web formation. In some fabric constructions, more than one bonding process may be used to enhance physical or chemical properties.
Mechanical consolidation methods include needlefelting, stitchbonding, and hydroentangling. Chemical consolidation methods involve applying adhesive binders to webs by saturating, spraying, printing, or foaming techniques. Solvent bonding involves softening or partially dissolving fibers with a solvent to provide self-bonding surfaces. Thermal bonding involves the use of heat and often pressure to fuse or weld fibers together at points of intersection or in patterned bond sites.
Important issues to consider when choosing the web consolidation methods are economy; versatility; and product properties, primarily absorbency, strength, softness, loft, and purity. A recurring issue involves environmental requirements of both the process and the product. Many techniques are done for specific properties of unique fabrics; therefore, it is difficult to measure differences in cost. In some instances, two or more bonding techniques compete. The system that is most energy-efficient; environmentally sound or provides the preferred fabric properties generally dominates.
7.1 CHEMICAL BONDING PROCESSES
Chemical or resin bonding is a generic term for interlocking fibers by the application of a chemical binder. The chemical binder most frequently used to consolidate fiber webs today is a water-borne latex. Most latex binders are made from vinyl materials, such as polyvinylacetate, polyvinylchloride, styrene/butadiene resin, butadiene, and polyacrylic, or their combinations.
Latexes are extensively used as nonwoven binders, because they are economical, versatile, easily applied, and effective adhesives. The versatility of a chemical binder system can be indicated by enumerating a few factors that are considered when such a system is formulated.
The chemical composition of the monomer or backbone material determines stiffness/softness properties, strength, water affinity (hydrophilic/hydrophobic balance), elasticity, durability, and aging. The type and nature of functional side groups determines solvent resistance, adhesive characteristics, and cross-linking nature. The type and quantity of surfactant used influences the polymerization process, polymer stability, and the application method.
Chemical binders are applied to webs in amounts ranging from about 5% to as much as 60% by weight. In some instances, when clays or other weighty additives are included, add-on levels can approach or even exceed the weight of the web. Waterborne binders are applied by spray, saturation, print, and foam methods. A general objective of each method is to apply the binder material in a manner sufficient to interlock the fibers and provide fabric properties required of the intended fabric usage.
The common methods of bonding include saturation, foam, spray, print and powder bonding. They are briefly introduced in the following paragraphs:
i) Saturation
Saturation bonding is used in conjunction with processes which require rapid binder addition, such as card-bond systems, and for fabric applications which require strength, stiffness, and maximum fiber encapsulation, such as carrier fabrics. Fiber encapsulation is achieved by totally immersing the web in a binder bath or by flooding the web as it enters the nip point of a set of pressure rolls. Excess binder is removed by vacuum or roll pressure.
Three variations of saturation bonding exist: screen, dip/squeeze, and size-press. Screen saturation is used for medium-weight nonwovens, such as interlinings. Dip/squeeze saturation is used for web structures with strength sufficient to withstand immersion without support, such as spunbonds. Size-press saturation is used in high speed processes, such as wet-laid nonwovens. Drying and curing may be carried out on steam-heated drying cans or in thru-air ovens or perforated-drum dryers. Binder addition levels range from 20% to 60%. Two techniques, single screen saturator and applicator roll technique, are illustrated in fig. 4 & 5. Advantages of this method are simplicity, controllable tensile strength and softness by choice and amount of binders. The disadvantages are the great influence of binders on softness, and the limitation in loftiness.
Fig. 4: Saturation bonding
Fig. 5: Applicator roll method
ii) Foam bonding
Foam bonding is a means to apply binder at low water and high binder-solids concentration levels. The basic concept employed involves using air as well as water as the binder diluent and carrier medium. Foam-bonded nonwovens require less energy in drying, since less water is used. The foam is generated by introducing air into the formulated latex while mechanically agitating the binder solution.
Air/latex dilutions or blow ratios in the order of 5:25 are practiced for various products. With the addition of a stabilizing agent to the binder solution, the foam can resist collapsing during application and curing, and the bonded fabric will exhibit enhanced loft, hand, and resilience. Non-stabilized foams are referred to as froths; froth-bonded fabrics are similar in properties to some saturation-bonded nonwovens. One example of this bonding is illustrated in fig. 6. The advantages include less energy required to dry the web, less binder migration and controllable softness by choices and amount of binders. The disadvantages are difficulties in controlling process and adequate foaming.
Fig 6: Foam bonding process
iii) Spray bonding
In spray bonding, binders are sprayed onto moving webs. Spray bonding is used for fabric applications that which require the maintenance of highloft or bulk, such as fiberfill and air-laid pulp wipes. The binder is atomized by air pressure, hydraulic pressure, or centrifugal force and is applied to the upper surfaces of the web in fine droplet form through a system of nozzles.
Lower-web-surface binder addition is accomplished by reversing web direction on a second conveyor and passing the web under a second spray station. After each spraying, the web is passed through a heating zone to remove water, and the binder is cured (set/cross-linked) in a third heating zone. For uniform binder distribution, spray nozzles are carefully engineered. Typical spray bonding is illustrated in fig. 7 & 8
Fig. 7: Schematic of spray bonding process
Fig. 8: Industrial spray bonding process
iv) Print bonding
Print bonding applies binder only in predetermined areas. It is used for fabric applications that require a part of the area of the fabric to be binder-free, such as wipes and coverstocks. Many lightweight nonwovens are print bonded. Printing patterns are designed to enhance strength, fluid transport, softness, hand, absorbency, and drape. Print bonding is most often carded out with gravure rolls. Binder addition levels are dependent on engraved area and depth as well as binder-solids level. Increased pattern versatility can be achieved with the use of rotary screen rolls. Drying and curing are carried out on heated drums or steam-heated cans.
In print bonding, high viscose binders are applied to limited, patterned areas. A prewet/prebond step is required for enough strength of webs, and typical steps in this bonding are in fig. 9.
Fig. 9: Latex printing process
There are two types of printers: rotary screen and rotogravure printers. Binders are applied through a hollow applicator roll in rotary screen printer, while in rotogravure printer they are applied by an engraved applicator roll as shown in fig. 10 a & b. The main advantage is that outstanding softness of nonwoven fabrics with adequate strength can be achieved.
Fig. 10a: Print bonding
Fig. 10b: Printing equipment
v) Powder bonding
In powder bonding, the adhesive powder of thermoplastic polymers is applied onto webs by heat and pressure. Polyesters and polyolefins with low Tg's and molecular weight can be used as powder binders. A typical bonding line is illustrated in fig. 11 a, b & c. The advantages are the bulky structure of dense nonwovens and the applicability of polyester or polypropylene webs. The disadvantage lies in difficulties of suitable particle sizes and ranges, and their distribution.
Fig. 11a: Powder bonding line
Fig. 11b: Powder adhesive sprinkling
Fig. 11c: Powder bonding with electrostatic assistance
8. APPLICATIONS
Nonwoven products in which binders are utilized:
Wipes and towels
Medical nonwovens
Roofing products
Apparel interlinings
Filter media
Coating substrates
Automotive trim
Carrier fabrics
Bedding products (high loft)
Furniture applications (high loft)
Apparel
Pillows (high loft)
9. SUMMARY
In the latter part of the 1970s and 1980s, thermal bonding technology grew rapidly, providing the industry with a realistic method to produce strong and soft nonwoven fabrics without the use of a chemical binder. This development provided substantial advances in performance and properties of many types of nonwovens. One quality of this new bonding technique was that these nonwovens contained no formaldehyde and no chemical additives to cause consumer concern. Naturally, this has depressed the interest of chemical binders within the industry and has resulted in a decline in binder usage.
Despite this setback, significant improvements and advances have continued to be made by the synthetic polymer industry, to the benefit of the range of nonwoven products that continue to utilize chemical bonding methods. These improvements have involved such developments as formaldehyde-free binders, low-cure temperature binders, complex copolymers with unique characteristics, moldable binders, and others. In the future, new types of binders may be combined with the present choices, for example, by co polymerization. In addition, new ideas such as reactive binders which can be covalently bonded with fibers will be continually investigated.
REFERENCES
1. 1. Michele Mlynar, Rohm & Haas Co, “Chemical Binders”, INTC 2003, Sept 15-18.
2. 2. W.E. Devry, "Latex bonding Chemistry and Processes."
3. 3. Derelich, "Nonwoven Textile Fabrics", Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 16, 3rd Ed, p104-124, 1981.
4. 4. B.M. Lichstein, The Nonwovens Handbook, INDA Association of the Nonwoven Fabric Industry, New York, 1988.
5. 5. J. Lunenschloss and W. Albrecht, Non-woven Bonded Fabrics, John Wiley & Sons Inc., New York, 1985.
6. 6. A.E. Meazey, " Binders used in Bonded Fiber Fabric Production", Nonwovens '71, The textile Trade Press, England, 1971.
7. 7. M.F. Meyer and W. A.; Haili, " Nonwovens and Laminates Made with Polyester Adhesive Powders, "Eastman Kodak Company.
8. 8. J.M. Oelkers and E.J. Sweeney," Latex Binders and Bonding Techniques of Disposables", 1988.
9. 9. Ellen Lees Wuagneux, " And how would you like your nonwovens?" Nonwoven Industry Oct. 64-72 (1997).
10. 10. INDA, Book of Paper, 1997
11. 11. www.nonwovens.com/facts/technology/binders/binders.htm
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SYNTHOBIND
(A pigment printing binder)
PROPERTIES
Chemical Nautre
Based on co-polymer emulsion.
Physical appearance
Milky white liquid.
pH
8 to 9.
Solubility
Easily soluble in hot & cold water.
Stability
Excellent.
Storage
Excellent.
Handling
Non-hazardous
APPLICATIONS
Synthobind is a high quality pigment printing binder and has found wide applications in different processes. It is a very good self cross linking acrylic co- polymer emulsion and gives self emulsifying nature. It provides excellent prints with good fastness properties on cotton & viscose. The print paste should be filtered well before taking for printing.
PACKING
50 Kg HDPE Plastic Carboys.
(This information is given in good faith, without warranty as the conditions of use are beyond our control.)
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SYNTHO L -4000
(A high quality pigment printing binder)
PROPERTIES
Chemical Nautre
Based on co - polymer emulsion
Physical appearance
Off white free flowing paste.
pH & Concentration
7 to 8 Cons. 33% + or - 1% polymer contains
Solubility
Easily soluble in hot & cold water.
Stability
Excellent.
Storage
Excellent. Containers should be kept away from direct sunlight.
Handling
Non - hazardous.
APPLICATIONS
Some of the applications of Syntho L-4000, which is a high quality pigment printing binder are:
It is a very good cross linking acrylic co - polymer emulsion & offers self emulsifying nature
It gives glossy , bright prints with attractive fastness properties on cotton & viscose.
PACKING
50 Kg HDPE Plastic Carboys.
Binder
The quality of a print depends on the quality of the binder. The binder film formed on the fibre must be colourless, clear, even thickness, smooth and neither too hard nor too soft. The binder should be inert so that maximum fastness can be achieved. Polymers based on acrylic esters, vinyl esters and butadiene are suitable. To give regular degree of hardness, monomers whose homopolymers produce hard and brittle films such as styrne, methylmethacrylate, acrylonitrile and vinyl chloride are combined with monomers whose homopolymers produce soft but very tacky film such as butylacrylate and butadiene. In addition to such components, the polymer also contain self cross linking compound such as methylolacrylamide and methylolmethacrylamide or their esters, and nonomers with reactive groups such as acrylamide and methacrylamide for improving stability of the film. Binders of good fastness to dry cleaning generally contain acrylonitrile.
Generally binder is a self-crosslinking acrylic emulsion, which combines soft, durable film properties with excellent thermal and mechanical stability. The binder, on polymerisation, acts as a connecting link by holding the pigment on one hand and fibre on the other.
Good binder satisfy the following special features:
1.Outstanding fastness to washing and rubbing.
2.Outstanding fastness to wet scrubbing.
3.Excellent fastness to light and resistance to ageing.
4.No chocking of the design on screen or printing machine due to its re-emulsification property.
5. High brilliancy of shade and soft handle.
Updated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde
(Monika Kannadaguli & Ramaiah Kotra)
1. INTRODUCTION
For more than four decades, almost all nonwovens required a chemical binder in order to provide any measure of structural integrity. In addition, the binder was called upon to contribute and convey numerous properties that were necessary for the effective performance of the fabric.
During this extended period, binders were essentially the weak element in developing fully acceptable nonwoven fabrics. The fibers that were available to the nonwoven industry were the same fibers that were available to the textile and other fiber-based industries; hence, the fibers were fully acceptable. Generally, the binder limited the performance of the nonwoven fabric.
The deficiencies cited against nonwovens generally were deficiencies attributable to an inadequate binder. Common complaints are as follows:
The fabric doesn't have enough strength.
The fabric is too stiff.
The fabric has inadequate absorbency.
The fabric shows poor laundering ability.
The fabric has inadequate dry cleaning ability.
The fabric simply doesn't feel like a textile.
Consequently, a great deal of effort has been put into the development and continuous improvement of chemical binders. The steady improvements in nonwovens performance that occurred over a period of many years were, in no small measure, due to improvements in the performance and utility of the binder.
In the very early stages of nonwovens development, different types of natural resins and glues were used to bond nonwovens. While they conveyed some integrity and strength to these webs, they also had many glaring deficiencies. Consequently, synthetic binders were developed to meet the structural and performance requirements of nonwoven fabrics.
Polyvinyl acetate was the first successful synthetic binder used in substantial volume. This material had distinctly superior adhesive properties, strength, and performance compared to the early natural adhesives. This binder is flexible and it can be applied to fiber webs by many ways including print bonding.
The industry was faced with the inevitable compromise in fabric properties of nonwovens bonded with synthetic materials. In order to build strength in the fabric, increasing amounts of resin must be applied, which results in more stiffness. If softness is necessary, it can be achieved, but primarily by sacrificing strength.
A substantial improvement in this trade-off of strength and softness was achieved with the introduction of acrylic-based latex binders in the 1950s and 1960s. By proper selection of co-monomers, it is possible to build improved softness properties with adequate strength. Consequently, these binders became widely used by most of the nonwovens industry, despite the somewhat higher cost.
As polymer technology for manufacturers of synthetic binder systems improved, a greater variety of chemical building blocks became available with much greater flexibility in terms of binder strength, durability, and other properties. The introduction of cross-linkable and self-crosslinking binder polymers turned out an entirely new range of fabric properties. This was particularly noteworthy in durable nonwovens where such durability features as washability and dry cleanability were important.
2. PROPERTIES DESIRED IN A BINDER
The construction of a nonwoven with suitable binders is to achieve improved characteristics such as strength, softness, adhesion, firmness, durability, stiffness, fire retardence, hydrophilicity, hydrophobicity, anti-microbial properties, organic compatibility, reduced surface tension, improved dimensional stability and solvent, wash and acid resistance. The following list illustrates some general considerations required for an ideal binder. The required properties can be varied depending on the end-uses.
· · Strength: The strength of a nonwoven fabric is more closely related to the strength of the applied binder.
Adhesion to Fibers: Even though the mechanism of adhesion is not completely understood, the adhesion strength of the binder-to-fiber bond has to be considered.
Flexibility/handle: The some movements of fibers should be allowed, especially when a soft hand is desired.
Elastic Recovery: To avoid the permanent deformation of fabric, good elastic recovery is required under strain.
Resistance to washing/ Drying cleaning: Some nonwoven products need durability in cleaning processes according to their end-uses.
Resistance to aging: The binder should be stable and not be degraded in the fabric during storage and use.
Good color and color retention: Diverse ranges of colors are required, and the colorfastness and yellowing problems should be considered.
Economical: Minimizing the cost is an ongoing requirement.
Other special requirements: Such as Flame resistance, resistance to chemicals, air, oxygen, light, heat, etc.
3. HOW BINDERS WORK
The process involves three steps:
Binder application to nonwoven web.
Removal moisture or solvent.
Formation of strong bond between binder and nonwoven web.
In general binders contain polymer produced by the reaction of monomers in presence of initiators or catalysts. Surfactants are used to stabilize these polymer particles in water during emulsification (Fig. 1)
Fig.1: Emulsion Polymerization – Schematic. [1]
After application of the binder to the nonwoven web, during moisture removal, film formation takes place. This phenomenon is shown in Fig. 2.
Fig.2: Schematic of film formation [1]
Binder factors influencing nonwoven performance are:
Backbone Structure
Functional Group
Surfactant
Process
4. CLASSIFICATION OF BINDERS
Due to their diversity, binders may be classified into several categories based on polymer (binder) chemical structure, functionality and the type of curing reactions.
4.1. CLASSIFICATION BASED ON CHEMICAL STRUCTURE
There are three main kinds of binders: butadiene copolymers, acrylates, and vinyl copolymers. The chemical compositions influence Tg, hardness and softness, hydrophobicity and hydrophilicity, elasticity, aging, and dry tensile strength of binders. The higher the Tg, the higher will be the dry tensile strength of binders.
Butadiene Copolymers
The structure of the main butadiene copolymer is shown as follows:
The butadiene polymers are cross-linked by polysulphides, and their properties are modified by different copolymers. The butadiene monomers provide elasticity while styrene and acrylonitrile monomers give tensile strength, and oil and solvent resistance, respectively. Their disadvantages are oxidation and discoloration due to residual double bonds in their polymer chains.
Acrylic acid derivatives
Acrylic binders are the most widely used and versatile binders available with various modifications. The properties of acrylic binders differ according to their derivatives and copolymers. The structures of the common acrylic polymers are as follows:
Acrylic Acid Derivatives
They are frequently copolymerized with styrene, acrylonitrile, vinyl chloride or vinyl acetate, depending on the desirable properties. Some of these properties are hardness from styrene, solvent resistance from acrylonitrile, flame retardancy from vinyl chloride, and cost benefits from vinyl acetate.
Vinyl copolymers
There are two main binders for vinyl copolymers: vinyl chloride and vinyl acetate. Since the vinyl binders are stiff, they are plasticized externally or internally. As internal plasticizers, ethylene and acrylate monomers are used, and external plasticizers consist of vinyl chloride. Due to its low Tg, vinyl acetate is not that stiff, and its advantage is low cost. The chlorides cause yellowing problems. The chemical structures are closely related Tg and stiffness of binders.
Vinyl acetate
4.2. CLASSIFICATION BASED ON FUNCTIONALITY
The functionality of binders is in the functional groups attached to polymer chains, which influences wet and solvent properties. To modify binder properties, copolymerization with a small amount of monomers with special functionality is performed. The main functionalities in binders are carboxyl and amide side chains.
Carboxyl functionality
This functionality is related to binders containing acrylic acid or methacrylic acid by copolymerization. The binders are crosslinkable since the functional group, carboxylic acid, provides sites for crosslinking reactions.
Amide functionality
This functionality is related to binders containing acrylamide by copolymerization. The amide functionality provides crosslinking sites, and even the binders are self-crosslinkable.
N-metylol amide (NMA) functionality
This functionality is obtained after acrylamide is reacted with formaldehyde. The binders containing the substituted acrylamide groups have self-crosslinking properties and the possible reaction as follows:
Acrylamide
4.3. CLASSIFICATION BASED ON TYPE OF CURING REACTIONS
The classification of reactivity refers to crosslinkability of binders, which is related to reaction with curing resins, crosslinking agents. The most common curing resin is melamine formaldehyde condensate resin involving reaction of n-methylol groups.
Non-crosslinkable polymers
The polymers do not contain any of the functional groups. They cannot crosslink, even with external curing resins.
Crosslinkable polymers
The polymers contain acid or amide functional groups. They can react with added curing resins, but the degree of crosslinking is limited.
Self-crosslinking polymers
The polymers contain n-methylol functional groups. They can react with themselves, and a high crosslink density can be obtained by adding curing resins.
Recent trends in chemical bonding: Although nonwoven manufacturers are seeking alternative technologies such as thermal bonding, chemical bonding still has its advantages and a promising market. Chemical bonding allows more room for fabric designs and fiber selections. Both disposable and durable products are supplied to roll goods producers and fiber manufacturers. On the environmental front, increasingly strict regulations and guidelines are driving a trend towards alternative products and technologies. Manufacturers and end-product suppliers alike are seeking ultra-low or formaldehyde-free binders. The growing consideration of the environmental impact of chemical binder and additives has become a focus of debate on the national and international level.
Latex binder chemical types
A latex polymer consists of an aqueous medium with extremely fine liquid or solid polymer particles dispersed therein. The latex polymer generally is produced via free radical emulsion polymerization in water, whereby a vinyl monomer is combined with a small amount of other monomer (co-monomer) to create a high molecular weight polymer. The latex dispersion also will carry surfactants, stabilizers, and other additives to convey realistic properties to the latex itself.
When used as a binder, the latex typically is combined with other components to provide the formulated binder ready for application to the fiber web. The formulated binder conveys many characteristics that are not possessed by the straight binder. Consequently, there is a substantial chemistry involved in combining the latex with the other components in order to prepare the formulated binder.
Binders are quite dependent upon the glass transition temperature (Tg) of the monomer unit selected to form the polymer. Differential Scanning Calorimeter (DSC) is used to determine Tg as shown below (fig. 3):
Fig. 3: Typical DSC graph
The lower the (Tg) of the monomer units, the softer is the resulting polymer. A sampling of the most common monomers used in the manufacture of latex polymer for nonwoven binders include the following materials:
Monomer Tg(0C)
Ethylene -125
Butadiene -78
Butyl Acrylate -52
Ethyl Acrylate -22
Vinyl Acetate +30
Vinyl Chloride +80
Methyl Methacrylate +105
Styrene +105
Acrylonitrile +130
The monomers selected for forming the polymeric latex also have considerable influence on the hydrophilic or hydrophobic nature of the binder. This can affect the wet strength of the nonwoven fabric as well as a host of absorbency characteristics.
With the current capabilities of polymerization chemistry, there is considerable versatility for each chemical type. Despite this range of properties, the commonly employed nonwovens binders generally are characterized by a fairly well defined set of properties. These properties can be modified to some degree by incorporation of other agents, but they provide a useful guide in classifying the kind of performance to be expected from each type of binder.
5. TYPES OF BINDERS
The following comparison of latex binder chemical types provides an indication of the relative performance, as well as the advantages and disadvantages of each type of binder. As indicated, the binder properties can be modified considerably by the presence of co-monomers.
i) Acrylic: These binders offer the greatest durability, color stability, and dry/wet performance. Acrylic binders have the widest range of fabric hand properties. They can be formulated to vary from very soft (Tg = - 40°C) to extremely hard (Tg = 105°C). These binders can be used in virtually all nonwovens applications, although they tend to be more costly. These polymers can be made to cross-link, with substantial improvement in durability.
ii) Styrenated Acrylics: These are tough, hydrophobic binders. The resulting textile hand ranges from soft-to-firm (Tg varies from –20°C to +105°C ).These binders can be used in applications where there is a need for some wet strength without crosslinking. The use of this type of latex binder does involve some sacrifice in UV and solvent resistance.
iii) Vinyl Acetate (VAC): The vinyl acetate binders are firm (Tg = +30°C to +40°C); however, they are relatively low cost and find extensive use. They offer good dry strength and toughness, but are somewhat hydrophilic and have a tendency to yellow when subjected to heat.
iv) Vinyl Acrylics: These binders are more hydrophobic than the straight VAC binders. They provide excellent toughness, flexibility, and better color stability. They are the compromise between VAC and acrylic, and can compete on a cost/performance basis. The hand range is limited to intermediate softness (Tg = -10°C) to a firm hand (Tg = +30°C).
v) Ethylene Vinyl Acetate (EVA): These latex binders have a (Tg range of –20°C to +115°C, which is equivalent to soft ranging to an intermediate textile hand. They exhibit high wet strength, coupled with excellent absorbency. In general, they are less costly than acrylics. They do have a tendency to have more of an odor compared to other binders. They are used primarily in wipes, air-laid pulp fabrics and similar applications.
vi) Styrene-Butadiene (S/B, SB, or styrene butadiene rubber): These binders have an excellent combination of flexibility and toughness. They range in hardness from very soft (Tg = -30°C) to very firm (Tg = +80°C). However, the (Tg of an SB binder is not strictly comparable to other classes of nonwoven binders. The styrene-to-butadiene ratio (S/B ratio) is the most common method for describing the relative hand resulting from the use of these binders. When cross-linked, this class of binder is very hydrophobic and durable. They are affected somewhat by heat and light because of their tendency to oxidize.
vii) Polyvinyl Chloride (PVC): The homopolymer of polyvinyl chloride is a very hard, rigid polymer (Tg = +80°C). This polymer must be plasticized to provide flexibility and film-forming properties. Normally, the (PVC) binders used in nonwovens are softened internally by co-polymerizing the vinyl chloride or with softer acrylic monomers. The hand range of most of these polymers is still relatively firm (Tg is greater than the +30°C). Because this type of polymer is a thermoplastic, it performs well in heat and dielectric sealing applications. This can be an advantage in some uses. The chlorine content of the polymer promotes flame retardency. This feature is one of the primary benefits of utilizing this type of binder. However, the chlorine also conveys the tendency to yellow upon heat aging, due to elimination of hydrogen chloride from the polymer.
viii) Ethylene/Vinyl Chloride: Binders in this class have a slightly broader hand range (Tg = 0°C to +30°C) without the external plasticization required of (PVC) binders. The presence of the chlorine again conveys some flame retardancy. These binders exhibit good acid resistance, fair water resistance, and excellent adhesion to synthetic fibers. There is some tendency to yellow upon aging. In essence, this is an internally plasticized (PVC) binder, considering the ethylene monomer to be the softener.
6. FORMULATION
i) Ingredients
The formulation of binding solution is an art since many ingredients are involved and many different possibilities exist for different end-uses. Some of the characteristics, and the types of formulation agents utilized to obtain them include the following.
· · Surfactants : offer improvement in binder adhesion, stability, and ability to be converted into a foam
External cross-linkers: provide cross-links with binder polymer to provide improved performance
Defoamers: utilized to minimize foam in processing
Repellent agents : convey water or oil repellency
Salts: added to impart low flame response properties and to convey antistatic properties
Thickeners: added to control the rheology of the binder liquid
Catalysts: added to facilitate curing and to promote cross-linking
Acids and bases: added to control pH of the latex
Dyes and pigments: provide color to the binder and fabric
Fillers: added to reduce binder tack and to lower cost
Optical brighteners: added to increase whiteness
Sewing aids: added to provide lubrication during fabrication
The purposes of wetting agents, mainly nonionic or anionic surfactants, are to enhance binder penetration through webs, improve the affinity between binder and fibers. The crosslinker, which has multi-functional groups, is generally added to increase crosslink density and to improve durability and resistance to deformation.
ii) Order of Formulation
In terms of adding ingredients into a binding bath, the compatibility of ingredients should be confirmed because the orders are extremely important. The milky white color of most binders impedes a check on the white-color indication of non-compatible ingredients, so most ingredients are first added to the dilution water. After the compatibility is assured, binders are added and then thickeners added to adjust viscosity. For the stability of the binding solution, catalysts are added just before application. Some water may be added to reach a desirable solid level. The summarized order is as follows:
· · Most ingredients
· · Latex binder
· · Thickener
· · Catalyst
· · Some water, and the others, such as dyes and pigments, fillers, clays, optical brighteners, sewing aids, etc.
7. BONDING TECHNOLOGY
Web consolidation or nonwoven bonding processes interlock preferentially arranged fiber or film assemblies by mechanical, chemical, solvent, and/or thermal means. The degree of bonding is a primary factor in determining fabric integrity (strength), porosity flexibility, softness, and density (loft, thickness).
Bonding may be carried out as a separate and distinct operation, but generally is carried out as a sequential operation in tandem with web formation. In some fabric constructions, more than one bonding process may be used to enhance physical or chemical properties.
Mechanical consolidation methods include needlefelting, stitchbonding, and hydroentangling. Chemical consolidation methods involve applying adhesive binders to webs by saturating, spraying, printing, or foaming techniques. Solvent bonding involves softening or partially dissolving fibers with a solvent to provide self-bonding surfaces. Thermal bonding involves the use of heat and often pressure to fuse or weld fibers together at points of intersection or in patterned bond sites.
Important issues to consider when choosing the web consolidation methods are economy; versatility; and product properties, primarily absorbency, strength, softness, loft, and purity. A recurring issue involves environmental requirements of both the process and the product. Many techniques are done for specific properties of unique fabrics; therefore, it is difficult to measure differences in cost. In some instances, two or more bonding techniques compete. The system that is most energy-efficient; environmentally sound or provides the preferred fabric properties generally dominates.
7.1 CHEMICAL BONDING PROCESSES
Chemical or resin bonding is a generic term for interlocking fibers by the application of a chemical binder. The chemical binder most frequently used to consolidate fiber webs today is a water-borne latex. Most latex binders are made from vinyl materials, such as polyvinylacetate, polyvinylchloride, styrene/butadiene resin, butadiene, and polyacrylic, or their combinations.
Latexes are extensively used as nonwoven binders, because they are economical, versatile, easily applied, and effective adhesives. The versatility of a chemical binder system can be indicated by enumerating a few factors that are considered when such a system is formulated.
The chemical composition of the monomer or backbone material determines stiffness/softness properties, strength, water affinity (hydrophilic/hydrophobic balance), elasticity, durability, and aging. The type and nature of functional side groups determines solvent resistance, adhesive characteristics, and cross-linking nature. The type and quantity of surfactant used influences the polymerization process, polymer stability, and the application method.
Chemical binders are applied to webs in amounts ranging from about 5% to as much as 60% by weight. In some instances, when clays or other weighty additives are included, add-on levels can approach or even exceed the weight of the web. Waterborne binders are applied by spray, saturation, print, and foam methods. A general objective of each method is to apply the binder material in a manner sufficient to interlock the fibers and provide fabric properties required of the intended fabric usage.
The common methods of bonding include saturation, foam, spray, print and powder bonding. They are briefly introduced in the following paragraphs:
i) Saturation
Saturation bonding is used in conjunction with processes which require rapid binder addition, such as card-bond systems, and for fabric applications which require strength, stiffness, and maximum fiber encapsulation, such as carrier fabrics. Fiber encapsulation is achieved by totally immersing the web in a binder bath or by flooding the web as it enters the nip point of a set of pressure rolls. Excess binder is removed by vacuum or roll pressure.
Three variations of saturation bonding exist: screen, dip/squeeze, and size-press. Screen saturation is used for medium-weight nonwovens, such as interlinings. Dip/squeeze saturation is used for web structures with strength sufficient to withstand immersion without support, such as spunbonds. Size-press saturation is used in high speed processes, such as wet-laid nonwovens. Drying and curing may be carried out on steam-heated drying cans or in thru-air ovens or perforated-drum dryers. Binder addition levels range from 20% to 60%. Two techniques, single screen saturator and applicator roll technique, are illustrated in fig. 4 & 5. Advantages of this method are simplicity, controllable tensile strength and softness by choice and amount of binders. The disadvantages are the great influence of binders on softness, and the limitation in loftiness.
Fig. 4: Saturation bonding
Fig. 5: Applicator roll method
ii) Foam bonding
Foam bonding is a means to apply binder at low water and high binder-solids concentration levels. The basic concept employed involves using air as well as water as the binder diluent and carrier medium. Foam-bonded nonwovens require less energy in drying, since less water is used. The foam is generated by introducing air into the formulated latex while mechanically agitating the binder solution.
Air/latex dilutions or blow ratios in the order of 5:25 are practiced for various products. With the addition of a stabilizing agent to the binder solution, the foam can resist collapsing during application and curing, and the bonded fabric will exhibit enhanced loft, hand, and resilience. Non-stabilized foams are referred to as froths; froth-bonded fabrics are similar in properties to some saturation-bonded nonwovens. One example of this bonding is illustrated in fig. 6. The advantages include less energy required to dry the web, less binder migration and controllable softness by choices and amount of binders. The disadvantages are difficulties in controlling process and adequate foaming.
Fig 6: Foam bonding process
iii) Spray bonding
In spray bonding, binders are sprayed onto moving webs. Spray bonding is used for fabric applications that which require the maintenance of highloft or bulk, such as fiberfill and air-laid pulp wipes. The binder is atomized by air pressure, hydraulic pressure, or centrifugal force and is applied to the upper surfaces of the web in fine droplet form through a system of nozzles.
Lower-web-surface binder addition is accomplished by reversing web direction on a second conveyor and passing the web under a second spray station. After each spraying, the web is passed through a heating zone to remove water, and the binder is cured (set/cross-linked) in a third heating zone. For uniform binder distribution, spray nozzles are carefully engineered. Typical spray bonding is illustrated in fig. 7 & 8
Fig. 7: Schematic of spray bonding process
Fig. 8: Industrial spray bonding process
iv) Print bonding
Print bonding applies binder only in predetermined areas. It is used for fabric applications that require a part of the area of the fabric to be binder-free, such as wipes and coverstocks. Many lightweight nonwovens are print bonded. Printing patterns are designed to enhance strength, fluid transport, softness, hand, absorbency, and drape. Print bonding is most often carded out with gravure rolls. Binder addition levels are dependent on engraved area and depth as well as binder-solids level. Increased pattern versatility can be achieved with the use of rotary screen rolls. Drying and curing are carried out on heated drums or steam-heated cans.
In print bonding, high viscose binders are applied to limited, patterned areas. A prewet/prebond step is required for enough strength of webs, and typical steps in this bonding are in fig. 9.
Fig. 9: Latex printing process
There are two types of printers: rotary screen and rotogravure printers. Binders are applied through a hollow applicator roll in rotary screen printer, while in rotogravure printer they are applied by an engraved applicator roll as shown in fig. 10 a & b. The main advantage is that outstanding softness of nonwoven fabrics with adequate strength can be achieved.
Fig. 10a: Print bonding
Fig. 10b: Printing equipment
v) Powder bonding
In powder bonding, the adhesive powder of thermoplastic polymers is applied onto webs by heat and pressure. Polyesters and polyolefins with low Tg's and molecular weight can be used as powder binders. A typical bonding line is illustrated in fig. 11 a, b & c. The advantages are the bulky structure of dense nonwovens and the applicability of polyester or polypropylene webs. The disadvantage lies in difficulties of suitable particle sizes and ranges, and their distribution.
Fig. 11a: Powder bonding line
Fig. 11b: Powder adhesive sprinkling
Fig. 11c: Powder bonding with electrostatic assistance
8. APPLICATIONS
Nonwoven products in which binders are utilized:
Wipes and towels
Medical nonwovens
Roofing products
Apparel interlinings
Filter media
Coating substrates
Automotive trim
Carrier fabrics
Bedding products (high loft)
Furniture applications (high loft)
Apparel
Pillows (high loft)
9. SUMMARY
In the latter part of the 1970s and 1980s, thermal bonding technology grew rapidly, providing the industry with a realistic method to produce strong and soft nonwoven fabrics without the use of a chemical binder. This development provided substantial advances in performance and properties of many types of nonwovens. One quality of this new bonding technique was that these nonwovens contained no formaldehyde and no chemical additives to cause consumer concern. Naturally, this has depressed the interest of chemical binders within the industry and has resulted in a decline in binder usage.
Despite this setback, significant improvements and advances have continued to be made by the synthetic polymer industry, to the benefit of the range of nonwoven products that continue to utilize chemical bonding methods. These improvements have involved such developments as formaldehyde-free binders, low-cure temperature binders, complex copolymers with unique characteristics, moldable binders, and others. In the future, new types of binders may be combined with the present choices, for example, by co polymerization. In addition, new ideas such as reactive binders which can be covalently bonded with fibers will be continually investigated.
REFERENCES
1. 1. Michele Mlynar, Rohm & Haas Co, “Chemical Binders”, INTC 2003, Sept 15-18.
2. 2. W.E. Devry, "Latex bonding Chemistry and Processes."
3. 3. Derelich, "Nonwoven Textile Fabrics", Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 16, 3rd Ed, p104-124, 1981.
4. 4. B.M. Lichstein, The Nonwovens Handbook, INDA Association of the Nonwoven Fabric Industry, New York, 1988.
5. 5. J. Lunenschloss and W. Albrecht, Non-woven Bonded Fabrics, John Wiley & Sons Inc., New York, 1985.
6. 6. A.E. Meazey, " Binders used in Bonded Fiber Fabric Production", Nonwovens '71, The textile Trade Press, England, 1971.
7. 7. M.F. Meyer and W. A.; Haili, " Nonwovens and Laminates Made with Polyester Adhesive Powders, "Eastman Kodak Company.
8. 8. J.M. Oelkers and E.J. Sweeney," Latex Binders and Bonding Techniques of Disposables", 1988.
9. 9. Ellen Lees Wuagneux, " And how would you like your nonwovens?" Nonwoven Industry Oct. 64-72 (1997).
10. 10. INDA, Book of Paper, 1997
11. 11. www.nonwovens.com/facts/technology/binders/binders.htm
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SYNTHOBIND
(A pigment printing binder)
PROPERTIES
Chemical Nautre
Based on co-polymer emulsion.
Physical appearance
Milky white liquid.
pH
8 to 9.
Solubility
Easily soluble in hot & cold water.
Stability
Excellent.
Storage
Excellent.
Handling
Non-hazardous
APPLICATIONS
Synthobind is a high quality pigment printing binder and has found wide applications in different processes. It is a very good self cross linking acrylic co- polymer emulsion and gives self emulsifying nature. It provides excellent prints with good fastness properties on cotton & viscose. The print paste should be filtered well before taking for printing.
PACKING
50 Kg HDPE Plastic Carboys.
(This information is given in good faith, without warranty as the conditions of use are beyond our control.)
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SYNTHO L -4000
(A high quality pigment printing binder)
PROPERTIES
Chemical Nautre
Based on co - polymer emulsion
Physical appearance
Off white free flowing paste.
pH & Concentration
7 to 8 Cons. 33% + or - 1% polymer contains
Solubility
Easily soluble in hot & cold water.
Stability
Excellent.
Storage
Excellent. Containers should be kept away from direct sunlight.
Handling
Non - hazardous.
APPLICATIONS
Some of the applications of Syntho L-4000, which is a high quality pigment printing binder are:
It is a very good cross linking acrylic co - polymer emulsion & offers self emulsifying nature
It gives glossy , bright prints with attractive fastness properties on cotton & viscose.
PACKING
50 Kg HDPE Plastic Carboys.
Binder
The quality of a print depends on the quality of the binder. The binder film formed on the fibre must be colourless, clear, even thickness, smooth and neither too hard nor too soft. The binder should be inert so that maximum fastness can be achieved. Polymers based on acrylic esters, vinyl esters and butadiene are suitable. To give regular degree of hardness, monomers whose homopolymers produce hard and brittle films such as styrne, methylmethacrylate, acrylonitrile and vinyl chloride are combined with monomers whose homopolymers produce soft but very tacky film such as butylacrylate and butadiene. In addition to such components, the polymer also contain self cross linking compound such as methylolacrylamide and methylolmethacrylamide or their esters, and nonomers with reactive groups such as acrylamide and methacrylamide for improving stability of the film. Binders of good fastness to dry cleaning generally contain acrylonitrile.
Generally binder is a self-crosslinking acrylic emulsion, which combines soft, durable film properties with excellent thermal and mechanical stability. The binder, on polymerisation, acts as a connecting link by holding the pigment on one hand and fibre on the other.
Good binder satisfy the following special features:
1.Outstanding fastness to washing and rubbing.
2.Outstanding fastness to wet scrubbing.
3.Excellent fastness to light and resistance to ageing.
4.No chocking of the design on screen or printing machine due to its re-emulsification property.
5. High brilliancy of shade and soft handle.
Friday, November 23, 2007
Diffusion
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Diffusion
Diffusion means penetration/movement of substance owing the existence of conc. gradient i.e. movement of particles between the two surfaces having different density from higher to lower one.
This is very important because it affects the fastness properties and the color yield.
Diffusion depends on
Dye size and nature of fibers and dyes
Structure of fibers (crystallinity and orientation)
Forces of interaction
Environment pH, solvent, temperature etc
Diffusion coefficient
The behavior of dye movement from higher concentration to lower one is described in terms of Fick’s law, which states that the no. of particles which diffuse through a cross-section in the x direction (S in moles/cm2) in a time t (seconds), the so called flux F (g moles/unit area/unit time) is proportional to the gradient of conc. dc/dx (in moles/cm4)
ds/dt = F = -DA. dc/dx
Where ds/dt = rate of diffusion
A area of cross section (in cm2)
D= diffusion coefficient/ diffusivity (cm2/sec)
The diffusion coefficient, D indicates dye diffusing through unit time through unit cross section area of the fiber under unit concentration gradient. Fick’s law is applicable only in cases in which the concentration gradient dc/dx is independent of time. Thus D is a measure of the diffusion properties of dyes and permeability of fiber.
Methods of measuring diffusion coefficient
Determination of D is important because, firstly to correlate structure of dye and/or fiber
Secondly, to calculate rates of dyeing or rates of desorption which may asses the practical situation
For determination of D, two methods are available
Non steady method: by varying the concentration gradient
Steady method: by maintaining a steady concentration gradient in the substrate throughout the process
Non steady method:
During the dyeing process, the system is in a non steady state since the concentration gradient in the substrate decreases as the dye concentration at the center of the fiber increases and also the dye concentration in the dyebath decreases. Hence Fick’s second law needs to be applied; D d2c/dx2=dc/dt
Steady State method1
In dyeing fiber and films, steady state conditions are only present at the very beginning. Thus measurement of this type are possible only a film substrate.
Theory
The film is interposed between two different concentrated solutions (one of them may be zero) and after allowing a sufficient time for a steady concentration gradient to be set up in the film, the rate at which the dye is transferred from concentrated to dilute is measured. Conditions being so arranged that concentration become that constant. D can be calculated, under this conditions, from equation ds/dt=-DA. dc/dx
[fig: Neale’s apparatus for the diffusing dye through a cellulose sheet]
Procedure
The film is clamped between the flanges of two tubes, dye solution is placed on one side and a blank solution is on the other. Two compartments are stirred to avoid any hydrodynamic complications and the rate at which dye is transferred from the dye solution is measured.
After a sufficient period of time had elapsed for the blank solution to contain a measurable amount of dye, it was replaced by blank solution. This procedure was repeated. As dye diffused through the film, the blank solution was removed at intervals and the quantity of dye estimated colorimetrically so that dye concentration in the compartment was maintained approximately zero throughout the experiment.
Now as dc/dx is constant, ds/dt can be determined directly, so D can be calculated. A is in cm2, dc concentration difference between two sides of membrane, x thickness of membrane.
Diffusion Model
There are two models for diffusion
Pore model
Free Volume Model
Pore Model
The pore model was first proposed for the dyeing of cellulosic fibers in 1935
Details:
This model considers the fiber to be network of interconnecting pores. When filled with water, the latter allow the dye molecules to diffuse and be simultaneously absorbed on the wall of the pore.
The fiber is heterogeneous material and the dye must follow a tortuous/zigzag path in order to avoid the impenetrable crystalline regions. In passing through the amorphous region of the substrate, the large dye molecule must weave its way through a network of chains or along the surfaces of crystalline regions and may even encounter voids. In general the diameter of the pores may be expected to vary from one type of fiber to another. For example direct dye is easily sorped by cotton than viscose rayon due to larger pore size.
For this model, the channels or pores are considered to be zigzag and occupy a fraction of α volume of the substrate.
Defects:
The pore model can’t be accepted in its entirety. The main defect is that structural parameters of the polymer, namely the size, shape and tortuiosity of the pores can’t be defined with any confidence.
If the pores are defined, the polymer structure is treated secondary one, the pores merely supplying surfaces for dye adsorption.
An exert correlation between the dye affinity and the diffusion coefficient can only be accepted if the dye molecule in the pore could be located out of range of the adsorption forces i.e. in the interior of the pore and the solution considered to possess the properties of normal water, which is practically impossible
The model assumes that the dye molecules diffuse without hindrance down the pore. Such a situation is unlikely
It is assumed that the channels (pores) are circular in cross sections, but observed dichroism in oriented materials and as dye molecules are linear, planar and rigid structure, a pore with an elliptical cross section is more realistic.
Fig. change in cross section of a fiber after orienting
Free Volume Model
Theory
Increasing the temperature (above Td)2 will result in an increase in the segmental of the polymer, thereby allowing more ‘holes’ to be made available for the diffusing, according to Williams-Landel-Ferry (WLF) equation:
log DT/DTd = A (T-Td) / (B + T – Td)
DT, DTd is measured diffusion coefficients at temperature at T & Td
A, B semi empirical constant.
Details
The free volume model is that volume in a liquid and solid not occupied by the constituent atoms; in fact it arises from the thermal motion of the actions and hence increases with temperature.
Below Td, the polymer chains may be regarded frozen into position and they only motions they can undergo a thermal vibrations. When Td is reached/ sufficient energy is available for bond rotation in the backbone of the polymer chain. An adequate free volume has been created to provide a large energy to accommodate rotating polymer segment. Once this segment has moved, the space it has vacated allows another segment to move. The onset of the segmental motion occurs over a narrow temperature range which includes Td.
Comparison between pore and free volume model
The controversies between the two models, as which one is more correct, the question is incorrectly formulated as any model is a simplification of reality and is therefore expected to fail in certain cases. Thus it is impossible to improve one is superior to another.
In all dyeing processes on the major fibers used today both models are probably effective simultaneously but in widely varying proportions. This conclusion evolved independently and by different methods and showed that the important factors affecting the diffusion coefficient are:
the degree of swelling
Dye affinity
enthalpy change for the ‘hole’ formation
the temperature dependence given in WLF equation
The diffusion coefficients on the more porous fibers were found to be below and above Td. Therefore both models exists every dyeing process and to measure the magnitude of the diffusion coefficient during dyeing depends on:
chemical structure of polymer
degree of crystallinity
percentages of stable pores in the polymer
segmental mobility
The dimensions of the diffusant molecules or ions.
Thus for most widely used textile fiber, with direct and indirect evidence one can conclude that free volume model is dominant one for polyester dyeing.
1 The steady state methods simply monitor the passage of dye through the material without reference to the internal distribution where non steady states yield a more detailed study of various factors.
2 Td is glass transition temperature measured under dyeing conditions i.e. in water Tg is measured with dry polymers.
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Labels: Theory of Dyeing
Dyeing Mechanism
Affinity
It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.
Substantivity
The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.
Reproducibility of Shades
The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.
Characteristics of highly reproducible dyes are:
Highly soluble
Medium substantivity
Medium reactivity
Good wash off properties
Highly diffusible
Optimization of Dye
The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.
Substrate
Affinity
Circulation speed
Action of chemicals before
Dyestuff
Depth of shade
Optimum quantity/yield
Diffusion ability and regularity
Color fastness
Combination & mixability
Chromphore percentage
Auxiliary Products
Optimum quantity
Compatibility with dyestuff and with each other
Levelness
Control of PH in final exhaustion
Reproducibility
No adverse effect
Temperature and time
Low initial temperature to avoid rapid absorption of dye
Control of critical temperature zone for maximum exhaustion
Sufficient time for penetration and fixing
Machine
Control of batch
Volume of flow
Temperature regulation
The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.
Functional Groups of fiber
Cotton: OH-, at higher pH it is ionizable
Wool: -COOH, -NH, -CONH2. At pH 3-4 ionized positively so acid dye is used to dyeing
Acrylic: -COOH, -SO3H, -O SO3H
Silk: -NH2, -CONH
Viscose: -OH, -COOH
Polyester: -OH, -COOH. No ionization effect, high temperature used for dyeing with dispersing.
Diacetate: -OH, -COOCH3
Triacetate: -COOCH3
Dyeing Medium
Aqueous medium
Water
Solvent
Foam
Vapor phase: cationic, anionic, nonionic
Dyeing Mechanism
The sequence of dyeing falls into four stages
Transfer of dye onto fiber surface
Adsorption
Diffusion into the fiber
Interaction
Transfer of dye onto fiber surface
The transfer of dye onto the fiber surface depends on:
Environment of the dyebath: environment of the bath refers to
Solvent and its type, nature, quantity: solvent may be water and or any other solvents which may be soft/hard, acidic, alkaline, ionic, nonionic etc.
pH
Dyeing assistants like electrolytes, leveling agents, carrier, dispersing agents etc.
temperature of the dyebath which depends on material type (cotton or polyester), type of dye (hot brand or cold brand), method of dyeing (padding or exhaust) Suitable environment ensures easy transference of dye on fiber surface.
Substantivity
Mechanical and physical force
Adsorption
The distribution process is called adsorption, if the substance which is to be distributed is retained by a surface. The assembly of dye molecules at the fiber surface is governed by:
Electropotential forces: All fiber when immersed into water or aqueous solution acquires an electric potential known as ‘zeta potential.’ Cellulosic fiber bears a negative charge while protein fibers at higher pH than its isoelectric point bears are negatively charged and at lower pH than isoelectric point is positively charged.
Temperature: most dyes in solution are either in molecular and partially ionized state or exist in the form of ionic micelles; increase in temperature tends to breakdown micelles into less aggregated units. Increase of temperature promotes vibrational activity accelerates the migration of the surface of the fiber.
Agitation: when a fiber is immersed in the dye a large no of molecules tend to enter the fabric at once, thus creating a layer called ‘Barrier.’ If the dye molecules are to reach the fiber surface then the barrier should be broken which is done by agitation.
Dye adsorption has affect on fastness properties.
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Labels: Theory of Dyeing
Pigment Printing
In pigment printing, insoluble pigments, which have no affinity for the fiber, are fixed on to the textile with binding agents in the pattern required. This description is perhaps oversimplified, but it does obviously set pigments apart from dyes that are absorbed into the fiber and fixed there as a result of reactions specific to the dye.
Historical Development of Pigment Printing
§ Until 1937 natural polymers as binders and thickeners (starch, glue)
§ Around 1937 emulsion thickening
§ Around 1960 use of aqueous self-crosslinking dispersions as binders
§ Around 1970 development of synthetic thickening agents based on acrylic acid
§ After 1980 ecological improvements (e.g., emission)
Why Pigment Printing is Important
§ The pigment can be applied to all fibers potentially and it is the only coloration to glass fiber, fabric and polyester
§ No wet treatment is required, so drying and curing is applicable to all fiber.
§ Extensive color range of highly light fast colors
§ Possible to produce good combination shades on blended fiber in one padding operation
§ Application procedure is simple
§ No change of hue of colorant throughout processing
§ Less expensive
A good quality pigment print is characterized by
§ Brilliance and high color value relative to the pigment concentration in material
§ Minimum stiffening in the handle of the textile
§ Generally acceptable fastness properties.
Components of a pigment printing system
A pigment printing system consists of three essential components:
§ Pigment dispersion: Specific pigments are treated in a grinding mill in the presence of suitable non-ionic surfactants. A particle size of 0.1-3 μm is typical. Generally, the pigment pastes are aqueous based and contain the dispersing agent, humectants (to prevent evaporation and drying out).
§ Binders and cross-linking agents (polymers): The binders used in pigment printing systems are film-forming substances made up of long-chain macro molecules which, when heated with a suitable acid-donating catalyst, form a three-dimensional structure in the pigment.
§ Thickeners and auxiliary agents: These give the required print thickening power (rheology).
Binder
The binder is a film forming substance made up of long‑chain macromolecules which, when applied to textile together with the pigment, produce a three dimensionally linked network.
Binder- CH2-OR + HO-Textile Binder –O- textile + HOR
Where R is H or CH3.
The links are formed during some suitable 'fixing' process, which usually consist of dry heat and a change in pH value, bringing about either self-crosslinking or reaction with suitable crosslinking agents.
The degree of cross linking should be limited, to prevent the macromolecules becoming too rigidly bonded, thus preserving some extensibility. The important criteria, which ensure that the pigment within the crosslinked binder film is fast to wear and cleaning, are elasticity, cohesion and adhesion to the substrate, resistance to hydrolysis, as little thermoplasticity as possible and absence of swelling in the presence of dry cleaning solvents.
Required properties for Binders
§ Should be film forming
§ Should be water swell
§ Should not be too thermoplastic
§ Should have atmospheric stability
§ Should be colorless and clear
§ Should be of even thickness and smooth; neither too hard nor too stiff.
§ Should have good adhesion to substrate without being tacky.
§ Should possess good resistance to chemical and mechanical stress
§ Should be readily removable from equipments
§ Should provide good color yield
§ Should be non toxic
Types of Binders
§ According to the origin
Natural: glue, gelatine etc
Synthetic: acramin binders
§ According to chemical groups
Acrylic binders: These are normally an aqueous dispersed co-polymer of butyl acrylate and styrene, having N-methylol acrylamide groups for cross-linking purposes.
Some of the more important properties of this type of binder are:
§ Good resistance to ageing by light
§ Good heat stability
§ Generally a harsh handle
§ Good solvent resistance
Butadiene co-polymer binders : They are made by emulsion co-polymerisation with acrylonitrille and N-methylolmethacrylamide. Some of the more important properties of this type of binder are:
§ Poor resistance to ageing by light
§ Susceptible to yellowing on heat treatment
§ Generally a soft handle, particularly on synthetic fibers
§ Generally the highest binding action on synthetic fibers
§ good solvent resistance
Trade names of binderTrade name Manufacturer Origin
Acramin Bayer Germany
Tinolite, Microfix, orema Ciba Switzerland
Helizarine BASF Germany
Imperon Hoechst Germany
Thickening Systems
There is a wide range of thickener materials available including alginates, natural vegetable gums, synthetic polymers, or even foams. These materials show sensitivity to factors such as temperature, pH, and salt content.
§ Ionic thickener (alginates): Better color yield
§ Nonionic thickener (cellulose ether): stable to pH variation and electrolyte content.
§ Natural and semi synthetic hydrophilic thickeners: should not used in pigment printing because:
- When entrapped in binder film, are either soluble in water or swell in presence of water even after fixation.
- They contain large no of polar groups like hydroxyl group and produce a hard film and stiff handle.
- Aftertreatment to remove them is not effective since they are enclosed in the binder film.
Emulsion Thickener
Two mutually immiscible liquids (oil and water) are stirred to produce an emulsion with the presence of emulsifier. The nature of the emulsifier and the ratio of the two immiscible liquids determine which liquid will be dispersed (the disperse phase) in the other (the outer, continuous phase)
The emulsifier forms a film between the two liquids, reducing interfacial tension. The emulsion stability depends on
- The degree of dispersion
- Type and quality of emulsifying used
- The substance dissolved or dispersed in the dispersed or dispersion medium
Two types of emulsion thickener
§ Oil in water (o/w): kerosene/white spirit in water
§ Water in oil (w/o): water in kerosene or white spirit
Synthetic thickeners
§ A thickener that is made artificially. Synthetic thickeners are typically designed to offer high viscosity at low concentrations, high yield value, shear thinning, stability, integrity over a wide temperature range, and ease of use.
§ Synthetic thickeners are efficient at only 1-3 % concentration level while approximately 10% of a natural thickener is needed to give the required viscosity in the print paste.
§ Other advantages of synthetic thickeners include rapid make-up since they require no waiting for hydration to occur, sharp print boundaries, and controlled penetration which usually provides greater color value and levelness.
Other Auxiliaries
§ Catalysts
§ Diammonium phosphate: - most widely used acid catalyst
§ used in conc. of 0.5% and 0.8% in screen and roller printing respectively
§ when used in correct proportion produces a pH of 3 in fabric and brings a cross linking reaction
§ Ammonium salts: sulphocyanide, sulfate and chloride are suitable. Ammonium nitrate: not recommended and it turns polyamide fiber yellow
§ Urea
These are agents that are added to improve “runnability” on printing machines. Owing to their low volatility these auxiliaries are used sparingly, maximum amounts of 20 parts/1000 being common; otherwise the fastness properties may be adversely affected.
§ Softening agents
After curing fixation the resultant “handle” of the printed fabric depends on a number of factors:
- monomer composition of the binder
- presence of water-soluble protective colloids (e.g. alginates, etc.)
- extent and type of cross-linking.
By the addition of certain compounds (usually termed “plasticisers”) improves the handle of printed goods.
§ Cross-linking agents
These agents are universally based on either urea-formaldehyde types (e.g. dimethylolurea) or melamine-formaldehyde types. They are incorporated into printing compositions in an attempt to increase various aspects of fastness, particularly rub and scrub fastness with synthetic fibers. A maximum addition of 10-20 pts/1000 is normally encountered: larger amounts can have a quite marked effect on the “handle” of the fabric
Pigment Printing Recipe and Procedure
Typical Recipe:
Pigment: 10-20gm
Binder: 40-50 gm
Thickener: 35-50 gm
Catalyst: 5 gm
Dispersing agent 2 gm
Water x ml
Procedure:
§ Preparation of printing paste using dispersing agents and thickener and catalyst.
§ Application of pigment paste and binding resin together
§ Drying at 140 – 150°C
§ Curing to fix the resin pigment
Affect of curing on PET
Temperature Time Strength loss
205°C 1 min 0%
220°C 1 min 0%
235°C 1 min 2%
245°C 1 min 5%
260°C 1 min 13%
Problems of Pigment Printing
§ Adverse effect due to binder as it changes texture of fabrics.
§ The quality of printing or dyeing depends on the characteristics of binder used to affix the pigment even more than the properties of pigment.
§ Some solvents used in emulsion like kerosene, white spirit cause problem like flammability.
§ The chemical and physical influences on the binder and print paste can interfere during production and processing resulting in sticking especially in roller printing.
§ The gumming up of equipments, odor, air and water pollution
§ Difficulty in obtaining the necessary wet treatment fastness and abrasion resistance with certain products, may not be obtained pigment printing or dyeing.
Pigment Dyeing on Fabric
Typical Recipe
Pigment: 10-20gm/L
Binder: 40-50 gm/L
Thickener: 35-50 gm/L
Catalyst: 5 gm/L
Thickener: 2 gm/L
Dispersing agent: 2 gm/L
Procedure:
§ Binder is weighted and diluted with cold water
§ Pigment and thickener is added with cold water
§ Catalyst solution is added
§ Dispersing agent is added
§ The dyeing liquor is well filtered and stirred; material is padded
§ The material is dried at 70 -100°C in hot flue steam but no use of cylinder dryer.
§ Curing is done at 150°C, 2-3 min
Precautions:
§ No alkalinity: The fiber to be dyed should not be alkaline
§ No OBA: OBA may produce faulty shade
§ No formation of skein: Binder should not be allowed to form skein which ultimately give specky shade
Typical procedure for Garment dyeing
§ First bleach the material then treat with a synthetic mordant cationising agent at pH 7
§ Rinse at 60°C at a rate of 2°C/ min for 20 min
§ Cold rinse
§ Apply pigment at 7O°C (pH 5) for 20 min
§ Add salt, acid and raise temperature when necessary
§ Now use binder 4% for 10 min at 70°C
§ Cold rinse with 1 gm/L soap wash for 10 min at 65°C
§ Cold rinse and dried
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Diffusion
Diffusion means penetration/movement of substance owing the existence of conc. gradient i.e. movement of particles between the two surfaces having different density from higher to lower one.
This is very important because it affects the fastness properties and the color yield.
Diffusion depends on
Dye size and nature of fibers and dyes
Structure of fibers (crystallinity and orientation)
Forces of interaction
Environment pH, solvent, temperature etc
Diffusion coefficient
The behavior of dye movement from higher concentration to lower one is described in terms of Fick’s law, which states that the no. of particles which diffuse through a cross-section in the x direction (S in moles/cm2) in a time t (seconds), the so called flux F (g moles/unit area/unit time) is proportional to the gradient of conc. dc/dx (in moles/cm4)
ds/dt = F = -DA. dc/dx
Where ds/dt = rate of diffusion
A area of cross section (in cm2)
D= diffusion coefficient/ diffusivity (cm2/sec)
The diffusion coefficient, D indicates dye diffusing through unit time through unit cross section area of the fiber under unit concentration gradient. Fick’s law is applicable only in cases in which the concentration gradient dc/dx is independent of time. Thus D is a measure of the diffusion properties of dyes and permeability of fiber.
Methods of measuring diffusion coefficient
Determination of D is important because, firstly to correlate structure of dye and/or fiber
Secondly, to calculate rates of dyeing or rates of desorption which may asses the practical situation
For determination of D, two methods are available
Non steady method: by varying the concentration gradient
Steady method: by maintaining a steady concentration gradient in the substrate throughout the process
Non steady method:
During the dyeing process, the system is in a non steady state since the concentration gradient in the substrate decreases as the dye concentration at the center of the fiber increases and also the dye concentration in the dyebath decreases. Hence Fick’s second law needs to be applied; D d2c/dx2=dc/dt
Steady State method1
In dyeing fiber and films, steady state conditions are only present at the very beginning. Thus measurement of this type are possible only a film substrate.
Theory
The film is interposed between two different concentrated solutions (one of them may be zero) and after allowing a sufficient time for a steady concentration gradient to be set up in the film, the rate at which the dye is transferred from concentrated to dilute is measured. Conditions being so arranged that concentration become that constant. D can be calculated, under this conditions, from equation ds/dt=-DA. dc/dx
[fig: Neale’s apparatus for the diffusing dye through a cellulose sheet]
Procedure
The film is clamped between the flanges of two tubes, dye solution is placed on one side and a blank solution is on the other. Two compartments are stirred to avoid any hydrodynamic complications and the rate at which dye is transferred from the dye solution is measured.
After a sufficient period of time had elapsed for the blank solution to contain a measurable amount of dye, it was replaced by blank solution. This procedure was repeated. As dye diffused through the film, the blank solution was removed at intervals and the quantity of dye estimated colorimetrically so that dye concentration in the compartment was maintained approximately zero throughout the experiment.
Now as dc/dx is constant, ds/dt can be determined directly, so D can be calculated. A is in cm2, dc concentration difference between two sides of membrane, x thickness of membrane.
Diffusion Model
There are two models for diffusion
Pore model
Free Volume Model
Pore Model
The pore model was first proposed for the dyeing of cellulosic fibers in 1935
Details:
This model considers the fiber to be network of interconnecting pores. When filled with water, the latter allow the dye molecules to diffuse and be simultaneously absorbed on the wall of the pore.
The fiber is heterogeneous material and the dye must follow a tortuous/zigzag path in order to avoid the impenetrable crystalline regions. In passing through the amorphous region of the substrate, the large dye molecule must weave its way through a network of chains or along the surfaces of crystalline regions and may even encounter voids. In general the diameter of the pores may be expected to vary from one type of fiber to another. For example direct dye is easily sorped by cotton than viscose rayon due to larger pore size.
For this model, the channels or pores are considered to be zigzag and occupy a fraction of α volume of the substrate.
Defects:
The pore model can’t be accepted in its entirety. The main defect is that structural parameters of the polymer, namely the size, shape and tortuiosity of the pores can’t be defined with any confidence.
If the pores are defined, the polymer structure is treated secondary one, the pores merely supplying surfaces for dye adsorption.
An exert correlation between the dye affinity and the diffusion coefficient can only be accepted if the dye molecule in the pore could be located out of range of the adsorption forces i.e. in the interior of the pore and the solution considered to possess the properties of normal water, which is practically impossible
The model assumes that the dye molecules diffuse without hindrance down the pore. Such a situation is unlikely
It is assumed that the channels (pores) are circular in cross sections, but observed dichroism in oriented materials and as dye molecules are linear, planar and rigid structure, a pore with an elliptical cross section is more realistic.
Fig. change in cross section of a fiber after orienting
Free Volume Model
Theory
Increasing the temperature (above Td)2 will result in an increase in the segmental of the polymer, thereby allowing more ‘holes’ to be made available for the diffusing, according to Williams-Landel-Ferry (WLF) equation:
log DT/DTd = A (T-Td) / (B + T – Td)
DT, DTd is measured diffusion coefficients at temperature at T & Td
A, B semi empirical constant.
Details
The free volume model is that volume in a liquid and solid not occupied by the constituent atoms; in fact it arises from the thermal motion of the actions and hence increases with temperature.
Below Td, the polymer chains may be regarded frozen into position and they only motions they can undergo a thermal vibrations. When Td is reached/ sufficient energy is available for bond rotation in the backbone of the polymer chain. An adequate free volume has been created to provide a large energy to accommodate rotating polymer segment. Once this segment has moved, the space it has vacated allows another segment to move. The onset of the segmental motion occurs over a narrow temperature range which includes Td.
Comparison between pore and free volume model
The controversies between the two models, as which one is more correct, the question is incorrectly formulated as any model is a simplification of reality and is therefore expected to fail in certain cases. Thus it is impossible to improve one is superior to another.
In all dyeing processes on the major fibers used today both models are probably effective simultaneously but in widely varying proportions. This conclusion evolved independently and by different methods and showed that the important factors affecting the diffusion coefficient are:
the degree of swelling
Dye affinity
enthalpy change for the ‘hole’ formation
the temperature dependence given in WLF equation
The diffusion coefficients on the more porous fibers were found to be below and above Td. Therefore both models exists every dyeing process and to measure the magnitude of the diffusion coefficient during dyeing depends on:
chemical structure of polymer
degree of crystallinity
percentages of stable pores in the polymer
segmental mobility
The dimensions of the diffusant molecules or ions.
Thus for most widely used textile fiber, with direct and indirect evidence one can conclude that free volume model is dominant one for polyester dyeing.
1 The steady state methods simply monitor the passage of dye through the material without reference to the internal distribution where non steady states yield a more detailed study of various factors.
2 Td is glass transition temperature measured under dyeing conditions i.e. in water Tg is measured with dry polymers.
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Labels: Theory of Dyeing
Dyeing Mechanism
Affinity
It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.
Substantivity
The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.
Reproducibility of Shades
The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.
Characteristics of highly reproducible dyes are:
Highly soluble
Medium substantivity
Medium reactivity
Good wash off properties
Highly diffusible
Optimization of Dye
The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.
Substrate
Affinity
Circulation speed
Action of chemicals before
Dyestuff
Depth of shade
Optimum quantity/yield
Diffusion ability and regularity
Color fastness
Combination & mixability
Chromphore percentage
Auxiliary Products
Optimum quantity
Compatibility with dyestuff and with each other
Levelness
Control of PH in final exhaustion
Reproducibility
No adverse effect
Temperature and time
Low initial temperature to avoid rapid absorption of dye
Control of critical temperature zone for maximum exhaustion
Sufficient time for penetration and fixing
Machine
Control of batch
Volume of flow
Temperature regulation
The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.
Functional Groups of fiber
Cotton: OH-, at higher pH it is ionizable
Wool: -COOH, -NH, -CONH2. At pH 3-4 ionized positively so acid dye is used to dyeing
Acrylic: -COOH, -SO3H, -O SO3H
Silk: -NH2, -CONH
Viscose: -OH, -COOH
Polyester: -OH, -COOH. No ionization effect, high temperature used for dyeing with dispersing.
Diacetate: -OH, -COOCH3
Triacetate: -COOCH3
Dyeing Medium
Aqueous medium
Water
Solvent
Foam
Vapor phase: cationic, anionic, nonionic
Dyeing Mechanism
The sequence of dyeing falls into four stages
Transfer of dye onto fiber surface
Adsorption
Diffusion into the fiber
Interaction
Transfer of dye onto fiber surface
The transfer of dye onto the fiber surface depends on:
Environment of the dyebath: environment of the bath refers to
Solvent and its type, nature, quantity: solvent may be water and or any other solvents which may be soft/hard, acidic, alkaline, ionic, nonionic etc.
pH
Dyeing assistants like electrolytes, leveling agents, carrier, dispersing agents etc.
temperature of the dyebath which depends on material type (cotton or polyester), type of dye (hot brand or cold brand), method of dyeing (padding or exhaust) Suitable environment ensures easy transference of dye on fiber surface.
Substantivity
Mechanical and physical force
Adsorption
The distribution process is called adsorption, if the substance which is to be distributed is retained by a surface. The assembly of dye molecules at the fiber surface is governed by:
Electropotential forces: All fiber when immersed into water or aqueous solution acquires an electric potential known as ‘zeta potential.’ Cellulosic fiber bears a negative charge while protein fibers at higher pH than its isoelectric point bears are negatively charged and at lower pH than isoelectric point is positively charged.
Temperature: most dyes in solution are either in molecular and partially ionized state or exist in the form of ionic micelles; increase in temperature tends to breakdown micelles into less aggregated units. Increase of temperature promotes vibrational activity accelerates the migration of the surface of the fiber.
Agitation: when a fiber is immersed in the dye a large no of molecules tend to enter the fabric at once, thus creating a layer called ‘Barrier.’ If the dye molecules are to reach the fiber surface then the barrier should be broken which is done by agitation.
Dye adsorption has affect on fastness properties.
Posted by Chariots of knowledge 0 comments
Labels: Theory of Dyeing
Pigment Printing
In pigment printing, insoluble pigments, which have no affinity for the fiber, are fixed on to the textile with binding agents in the pattern required. This description is perhaps oversimplified, but it does obviously set pigments apart from dyes that are absorbed into the fiber and fixed there as a result of reactions specific to the dye.
Historical Development of Pigment Printing
§ Until 1937 natural polymers as binders and thickeners (starch, glue)
§ Around 1937 emulsion thickening
§ Around 1960 use of aqueous self-crosslinking dispersions as binders
§ Around 1970 development of synthetic thickening agents based on acrylic acid
§ After 1980 ecological improvements (e.g., emission)
Why Pigment Printing is Important
§ The pigment can be applied to all fibers potentially and it is the only coloration to glass fiber, fabric and polyester
§ No wet treatment is required, so drying and curing is applicable to all fiber.
§ Extensive color range of highly light fast colors
§ Possible to produce good combination shades on blended fiber in one padding operation
§ Application procedure is simple
§ No change of hue of colorant throughout processing
§ Less expensive
A good quality pigment print is characterized by
§ Brilliance and high color value relative to the pigment concentration in material
§ Minimum stiffening in the handle of the textile
§ Generally acceptable fastness properties.
Components of a pigment printing system
A pigment printing system consists of three essential components:
§ Pigment dispersion: Specific pigments are treated in a grinding mill in the presence of suitable non-ionic surfactants. A particle size of 0.1-3 μm is typical. Generally, the pigment pastes are aqueous based and contain the dispersing agent, humectants (to prevent evaporation and drying out).
§ Binders and cross-linking agents (polymers): The binders used in pigment printing systems are film-forming substances made up of long-chain macro molecules which, when heated with a suitable acid-donating catalyst, form a three-dimensional structure in the pigment.
§ Thickeners and auxiliary agents: These give the required print thickening power (rheology).
Binder
The binder is a film forming substance made up of long‑chain macromolecules which, when applied to textile together with the pigment, produce a three dimensionally linked network.
Binder- CH2-OR + HO-Textile Binder –O- textile + HOR
Where R is H or CH3.
The links are formed during some suitable 'fixing' process, which usually consist of dry heat and a change in pH value, bringing about either self-crosslinking or reaction with suitable crosslinking agents.
The degree of cross linking should be limited, to prevent the macromolecules becoming too rigidly bonded, thus preserving some extensibility. The important criteria, which ensure that the pigment within the crosslinked binder film is fast to wear and cleaning, are elasticity, cohesion and adhesion to the substrate, resistance to hydrolysis, as little thermoplasticity as possible and absence of swelling in the presence of dry cleaning solvents.
Required properties for Binders
§ Should be film forming
§ Should be water swell
§ Should not be too thermoplastic
§ Should have atmospheric stability
§ Should be colorless and clear
§ Should be of even thickness and smooth; neither too hard nor too stiff.
§ Should have good adhesion to substrate without being tacky.
§ Should possess good resistance to chemical and mechanical stress
§ Should be readily removable from equipments
§ Should provide good color yield
§ Should be non toxic
Types of Binders
§ According to the origin
Natural: glue, gelatine etc
Synthetic: acramin binders
§ According to chemical groups
Acrylic binders: These are normally an aqueous dispersed co-polymer of butyl acrylate and styrene, having N-methylol acrylamide groups for cross-linking purposes.
Some of the more important properties of this type of binder are:
§ Good resistance to ageing by light
§ Good heat stability
§ Generally a harsh handle
§ Good solvent resistance
Butadiene co-polymer binders : They are made by emulsion co-polymerisation with acrylonitrille and N-methylolmethacrylamide. Some of the more important properties of this type of binder are:
§ Poor resistance to ageing by light
§ Susceptible to yellowing on heat treatment
§ Generally a soft handle, particularly on synthetic fibers
§ Generally the highest binding action on synthetic fibers
§ good solvent resistance
Trade names of binderTrade name Manufacturer Origin
Acramin Bayer Germany
Tinolite, Microfix, orema Ciba Switzerland
Helizarine BASF Germany
Imperon Hoechst Germany
Thickening Systems
There is a wide range of thickener materials available including alginates, natural vegetable gums, synthetic polymers, or even foams. These materials show sensitivity to factors such as temperature, pH, and salt content.
§ Ionic thickener (alginates): Better color yield
§ Nonionic thickener (cellulose ether): stable to pH variation and electrolyte content.
§ Natural and semi synthetic hydrophilic thickeners: should not used in pigment printing because:
- When entrapped in binder film, are either soluble in water or swell in presence of water even after fixation.
- They contain large no of polar groups like hydroxyl group and produce a hard film and stiff handle.
- Aftertreatment to remove them is not effective since they are enclosed in the binder film.
Emulsion Thickener
Two mutually immiscible liquids (oil and water) are stirred to produce an emulsion with the presence of emulsifier. The nature of the emulsifier and the ratio of the two immiscible liquids determine which liquid will be dispersed (the disperse phase) in the other (the outer, continuous phase)
The emulsifier forms a film between the two liquids, reducing interfacial tension. The emulsion stability depends on
- The degree of dispersion
- Type and quality of emulsifying used
- The substance dissolved or dispersed in the dispersed or dispersion medium
Two types of emulsion thickener
§ Oil in water (o/w): kerosene/white spirit in water
§ Water in oil (w/o): water in kerosene or white spirit
Synthetic thickeners
§ A thickener that is made artificially. Synthetic thickeners are typically designed to offer high viscosity at low concentrations, high yield value, shear thinning, stability, integrity over a wide temperature range, and ease of use.
§ Synthetic thickeners are efficient at only 1-3 % concentration level while approximately 10% of a natural thickener is needed to give the required viscosity in the print paste.
§ Other advantages of synthetic thickeners include rapid make-up since they require no waiting for hydration to occur, sharp print boundaries, and controlled penetration which usually provides greater color value and levelness.
Other Auxiliaries
§ Catalysts
§ Diammonium phosphate: - most widely used acid catalyst
§ used in conc. of 0.5% and 0.8% in screen and roller printing respectively
§ when used in correct proportion produces a pH of 3 in fabric and brings a cross linking reaction
§ Ammonium salts: sulphocyanide, sulfate and chloride are suitable. Ammonium nitrate: not recommended and it turns polyamide fiber yellow
§ Urea
These are agents that are added to improve “runnability” on printing machines. Owing to their low volatility these auxiliaries are used sparingly, maximum amounts of 20 parts/1000 being common; otherwise the fastness properties may be adversely affected.
§ Softening agents
After curing fixation the resultant “handle” of the printed fabric depends on a number of factors:
- monomer composition of the binder
- presence of water-soluble protective colloids (e.g. alginates, etc.)
- extent and type of cross-linking.
By the addition of certain compounds (usually termed “plasticisers”) improves the handle of printed goods.
§ Cross-linking agents
These agents are universally based on either urea-formaldehyde types (e.g. dimethylolurea) or melamine-formaldehyde types. They are incorporated into printing compositions in an attempt to increase various aspects of fastness, particularly rub and scrub fastness with synthetic fibers. A maximum addition of 10-20 pts/1000 is normally encountered: larger amounts can have a quite marked effect on the “handle” of the fabric
Pigment Printing Recipe and Procedure
Typical Recipe:
Pigment: 10-20gm
Binder: 40-50 gm
Thickener: 35-50 gm
Catalyst: 5 gm
Dispersing agent 2 gm
Water x ml
Procedure:
§ Preparation of printing paste using dispersing agents and thickener and catalyst.
§ Application of pigment paste and binding resin together
§ Drying at 140 – 150°C
§ Curing to fix the resin pigment
Affect of curing on PET
Temperature Time Strength loss
205°C 1 min 0%
220°C 1 min 0%
235°C 1 min 2%
245°C 1 min 5%
260°C 1 min 13%
Problems of Pigment Printing
§ Adverse effect due to binder as it changes texture of fabrics.
§ The quality of printing or dyeing depends on the characteristics of binder used to affix the pigment even more than the properties of pigment.
§ Some solvents used in emulsion like kerosene, white spirit cause problem like flammability.
§ The chemical and physical influences on the binder and print paste can interfere during production and processing resulting in sticking especially in roller printing.
§ The gumming up of equipments, odor, air and water pollution
§ Difficulty in obtaining the necessary wet treatment fastness and abrasion resistance with certain products, may not be obtained pigment printing or dyeing.
Pigment Dyeing on Fabric
Typical Recipe
Pigment: 10-20gm/L
Binder: 40-50 gm/L
Thickener: 35-50 gm/L
Catalyst: 5 gm/L
Thickener: 2 gm/L
Dispersing agent: 2 gm/L
Procedure:
§ Binder is weighted and diluted with cold water
§ Pigment and thickener is added with cold water
§ Catalyst solution is added
§ Dispersing agent is added
§ The dyeing liquor is well filtered and stirred; material is padded
§ The material is dried at 70 -100°C in hot flue steam but no use of cylinder dryer.
§ Curing is done at 150°C, 2-3 min
Precautions:
§ No alkalinity: The fiber to be dyed should not be alkaline
§ No OBA: OBA may produce faulty shade
§ No formation of skein: Binder should not be allowed to form skein which ultimately give specky shade
Typical procedure for Garment dyeing
§ First bleach the material then treat with a synthetic mordant cationising agent at pH 7
§ Rinse at 60°C at a rate of 2°C/ min for 20 min
§ Cold rinse
§ Apply pigment at 7O°C (pH 5) for 20 min
§ Add salt, acid and raise temperature when necessary
§ Now use binder 4% for 10 min at 70°C
§ Cold rinse with 1 gm/L soap wash for 10 min at 65°C
§ Cold rinse and dried
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Tuesday, November 20, 2007
Dyeing Mechanism
Dyeing Mechanism
Affinity
It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.
Substantivity
The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.
Reproducibility of Shades
The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.
Characteristics of highly reproducible dyes are:
Highly soluble
Medium substantivity
Medium reactivity
Good wash off properties
Highly diffusible
Optimization of Dye
The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.
Substrate
Affinity
Circulation speed
Action of chemicals before
Dyestuff
Depth of shade
Optimum quantity/yield
Diffusion ability and regularity
Color fastness
Combination & mixability
Chromphore percentage
Auxiliary Products
Optimum quantity
Compatibility with dyestuff and with each other
Levelness
Control of PH in final exhaustion
Reproducibility
No adverse effect
Temperature and time
Low initial temperature to avoid rapid absorption of dye
Control of critical temperature zone for maximum exhaustion
Sufficient time for penetration and fixing
Machine
Control of batch
Volume of flow
Temperature regulation
The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.
Functional Groups of fiber
Cotton: OH-, at higher pH it is ionizable
Wool: -COOH, -NH, -CONH2. At pH 3-4 ionized positively so acid dye is used to dyeing
Acrylic: -COOH, -SO3H, -O SO3H
Silk: -NH2, -CONH
Viscose: -OH, -COOH
Polyester: -OH, -COOH. No ionization effect, high temperature used for dyeing with dispersing.
Diacetate: -OH, -COOCH3
Triacetate: -COOCH3
Dyeing Medium
Aqueous medium
Water
Solvent
Foam
Vapor phase: cationic, anionic, nonionic
Dyeing Mechanism
The sequence of dyeing falls into four stages
Transfer of dye onto fiber surface
Adsorption
Diffusion into the fiber
Interaction
Transfer of dye onto fiber surface
The transfer of dye onto the fiber surface depends on:
Environment of the dyebath: environment of the bath refers to
Solvent and its type, nature, quantity: solvent may be water and or any other solvents which may be soft/hard, acidic, alkaline, ionic, nonionic etc.
pH
Dyeing assistants like electrolytes, leveling agents, carrier, dispersing agents etc.
temperature of the dyebath which depends on material type (cotton or polyester), type of dye (hot brand or cold brand), method of dyeing (padding or exhaust) Suitable environment ensures easy transference of dye on fiber surface.
Substantivity
Mechanical and physical force
Adsorption
The distribution process is called adsorption, if the substance which is to be distributed is retained by a surface. The assembly of dye molecules at the fiber surface is governed by:
Electropotential forces: All fiber when immersed into water or aqueous solution acquires an electric potential known as ‘zeta potential.’ Cellulosic fiber bears a negative charge while protein fibers at higher pH than its isoelectric point bears are negatively charged and at lower pH than isoelectric point is positively charged.
Temperature: most dyes in solution are either in molecular and partially ionized state or exist in the form of ionic micelles; increase in temperature tends to breakdown micelles into less aggregated units. Increase of temperature promotes vibrational activity accelerates the migration of the surface of the fiber.
Agitation: when a fiber is immersed in the dye a large no of molecules tend to enter the fabric at once, thus creating a layer called ‘Barrier.’ If the dye molecules are to reach the fiber surface then the barrier should be broken which is done by agitation.
Dye adsorption has affect on fastness properties.
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Pigment Printing
In pigment printing, insoluble pigments, which have no affinity for the fiber, are fixed on to the textile with binding agents in the pattern required. This description is perhaps oversimplified, but it does obviously set pigments apart from dyes that are absorbed into the fiber and fixed there as a result of reactions specific to the dye.
Historical Development of Pigment Printing
§ Until 1937 natural polymers as binders and thickeners (starch, glue)
§ Around 1937 emulsion thickening
§ Around 1960 use of aqueous self-crosslinking dispersions as binders
§ Around 1970 development of synthetic thickening agents based on acrylic acid
§ After 1980 ecological improvements (e.g., emission)
Why Pigment Printing is Important
§ The pigment can be applied to all fibers potentially and it is the only coloration to glass fiber, fabric and polyester
§ No wet treatment is required, so drying and curing is applicable to all fiber.
§ Extensive color range of highly light fast colors
§ Possible to produce good combination shades on blended fiber in one padding operation
§ Application procedure is simple
§ No change of hue of colorant throughout processing
§ Less expensive
A good quality pigment print is characterized by
§ Brilliance and high color value relative to the pigment concentration in material
§ Minimum stiffening in the handle of the textile
§ Generally acceptable fastness properties.
Components of a pigment printing system
A pigment printing system consists of three essential components:
§ Pigment dispersion: Specific pigments are treated in a grinding mill in the presence of suitable non-ionic surfactants. A particle size of 0.1-3 μm is typical. Generally, the pigment pastes are aqueous based and contain the dispersing agent, humectants (to prevent evaporation and drying out).
§ Binders and cross-linking agents (polymers): The binders used in pigment printing systems are film-forming substances made up of long-chain macro molecules which, when heated with a suitable acid-donating catalyst, form a three-dimensional structure in the pigment.
§ Thickeners and auxiliary agents: These give the required print thickening power (rheology).
Binder
The binder is a film forming substance made up of long‑chain macromolecules which, when applied to textile together with the pigment, produce a three dimensionally linked network.
Binder- CH2-OR + HO-Textile Binder –O- textile + HOR
Where R is H or CH3.
The links are formed during some suitable 'fixing' process, which usually consist of dry heat and a change in pH value, bringing about either self-crosslinking or reaction with suitable crosslinking agents.
The degree of cross linking should be limited, to prevent the macromolecules becoming too rigidly bonded, thus preserving some extensibility. The important criteria, which ensure that the pigment within the crosslinked binder film is fast to wear and cleaning, are elasticity, cohesion and adhesion to the substrate, resistance to hydrolysis, as little thermoplasticity as possible and absence of swelling in the presence of dry cleaning solvents.
Required properties for Binders
§ Should be film forming
§ Should be water swell
§ Should not be too thermoplastic
§ Should have atmospheric stability
§ Should be colorless and clear
§ Should be of even thickness and smooth; neither too hard nor too stiff.
§ Should have good adhesion to substrate without being tacky.
§ Should possess good resistance to chemical and mechanical stress
§ Should be readily removable from equipments
§ Should provide good color yield
§ Should be non toxic
Types of Binders
§ According to the origin
Natural: glue, gelatine etc
Synthetic: acramin binders
§ According to chemical groups
Acrylic binders: These are normally an aqueous dispersed co-polymer of butyl acrylate and styrene, having N-methylol acrylamide groups for cross-linking purposes.
Some of the more important properties of this type of binder are:
§ Good resistance to ageing by light
§ Good heat stability
§ Generally a harsh handle
§ Good solvent resistance
Butadiene co-polymer binders : They are made by emulsion co-polymerisation with acrylonitrille and N-methylolmethacrylamide. Some of the more important properties of this type of binder are:
§ Poor resistance to ageing by light
§ Susceptible to yellowing on heat treatment
§ Generally a soft handle, particularly on synthetic fibers
§ Generally the highest binding action on synthetic fibers
§ good solvent resistance
Trade names of binderTrade name Manufacturer Origin
Acramin Bayer Germany
Tinolite, Microfix, orema Ciba Switzerland
Helizarine BASF Germany
Imperon Hoechst Germany
Thickening Systems
There is a wide range of thickener materials available including alginates, natural vegetable gums, synthetic polymers, or even foams. These materials show sensitivity to factors such as temperature, pH, and salt content.
§ Ionic thickener (alginates): Better color yield
§ Nonionic thickener (cellulose ether): stable to pH variation and electrolyte content.
§ Natural and semi synthetic hydrophilic thickeners: should not used in pigment printing because:
- When entrapped in binder film, are either soluble in water or swell in presence of water even after fixation.
- They contain large no of polar groups like hydroxyl group and produce a hard film and stiff handle.
- Aftertreatment to remove them is not effective since they are enclosed in the binder film.
Emulsion Thickener
Two mutually immiscible liquids (oil and water) are stirred to produce an emulsion with the presence of emulsifier. The nature of the emulsifier and the ratio of the two immiscible liquids determine which liquid will be dispersed (the disperse phase) in the other (the outer, continuous phase)
The emulsifier forms a film between the two liquids, reducing interfacial tension. The emulsion stability depends on
- The degree of dispersion
- Type and quality of emulsifying used
- The substance dissolved or dispersed in the dispersed or dispersion medium
Two types of emulsion thickener
§ Oil in water (o/w): kerosene/white spirit in water
§ Water in oil (w/o): water in kerosene or white spirit
Synthetic thickeners
§ A thickener that is made artificially. Synthetic thickeners are typically designed to offer high viscosity at low concentrations, high yield value, shear thinning, stability, integrity over a wide temperature range, and ease of use.
§ Synthetic thickeners are efficient at only 1-3 % concentration level while approximately 10% of a natural thickener is needed to give the required viscosity in the print paste.
§ Other advantages of synthetic thickeners include rapid make-up since they require no waiting for hydration to occur, sharp print boundaries, and controlled penetration which usually provides greater color value and levelness.
Other Auxiliaries
§ Catalysts
§ Diammonium phosphate: - most widely used acid catalyst
§ used in conc. of 0.5% and 0.8% in screen and roller printing respectively
§ when used in correct proportion produces a pH of 3 in fabric and brings a cross linking reaction
§ Ammonium salts: sulphocyanide, sulfate and chloride are suitable. Ammonium nitrate: not recommended and it turns polyamide fiber yellow
§ Urea
These are agents that are added to improve “runnability” on printing machines. Owing to their low volatility these auxiliaries are used sparingly, maximum amounts of 20 parts/1000 being common; otherwise the fastness properties may be adversely affected.
§ Softening agents
After curing fixation the resultant “handle” of the printed fabric depends on a number of factors:
- monomer composition of the binder
- presence of water-soluble protective colloids (e.g. alginates, etc.)
- extent and type of cross-linking.
By the addition of certain compounds (usually termed “plasticisers”) improves the handle of printed goods.
§ Cross-linking agents
These agents are universally based on either urea-formaldehyde types (e.g. dimethylolurea) or melamine-formaldehyde types. They are incorporated into printing compositions in an attempt to increase various aspects of fastness, particularly rub and scrub fastness with synthetic fibers. A maximum addition of 10-20 pts/1000 is normally encountered: larger amounts can have a quite marked effect on the “handle” of the fabric
Pigment Printing Recipe and Procedure
Typical Recipe:
Pigment: 10-20gm
Binder: 40-50 gm
Thickener: 35-50 gm
Catalyst: 5 gm
Dispersing agent 2 gm
Water x ml
Procedure:
§ Preparation of printing paste using dispersing agents and thickener and catalyst.
§ Application of pigment paste and binding resin together
§ Drying at 140 – 150°C
§ Curing to fix the resin pigment
Affect of curing on PET
Temperature Time Strength loss
205°C 1 min 0%
220°C 1 min 0%
235°C 1 min 2%
245°C 1 min 5%
260°C 1 min 13%
Problems of Pigment Printing
§ Adverse effect due to binder as it changes texture of fabrics.
§ The quality of printing or dyeing depends on the characteristics of binder used to affix the pigment even more than the properties of pigment.
§ Some solvents used in emulsion like kerosene, white spirit cause problem like flammability.
§ The chemical and physical influences on the binder and print paste can interfere during production and processing resulting in sticking especially in roller printing.
§ The gumming up of equipments, odor, air and water pollution
§ Difficulty in obtaining the necessary wet treatment fastness and abrasion resistance with certain products, may not be obtained pigment printing or dyeing.
Pigment Dyeing on Fabric
Typical Recipe
Pigment: 10-20gm/L
Binder: 40-50 gm/L
Thickener: 35-50 gm/L
Catalyst: 5 gm/L
Thickener: 2 gm/L
Dispersing agent: 2 gm/L
Procedure:
§ Binder is weighted and diluted with cold water
§ Pigment and thickener is added with cold water
§ Catalyst solution is added
§ Dispersing agent is added
§ The dyeing liquor is well filtered and stirred; material is padded
§ The material is dried at 70 -100°C in hot flue steam but no use of cylinder dryer.
§ Curing is done at 150°C, 2-3 min
Precautions:
§ No alkalinity: The fiber to be dyed should not be alkaline
§ No OBA: OBA may produce faulty shade
§ No formation of skein: Binder should not be allowed to form skein which ultimately give specky shade
Typical procedure for Garment dyeing
§ First bleach the material then treat with a synthetic mordant cationising agent at pH 7
§ Rinse at 60°C at a rate of 2°C/ min for 20 min
§ Cold rinse
§ Apply pigment at 7O°C (pH 5) for 20 min
§ Add salt, acid and raise temperature when necessary
§ Now use binder 4% for 10 min at 70°C
§ Cold rinse with 1 gm/L soap wash for 10 min at 65°C
§ Cold rinse and dried
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Labels: Printing
Pigment
Pigments implied general insolubility and complete insolubility in water.
Difference between dye and pigment
The difference between dye and pigment is not a clear one. Most organic pigments are closely related to dyes with respect to their chemical structure and there are dyes which become pigments after application. Vat dye is a dye when used in dyeing but a pigment when used in printing.
Dye
Pigment
Solubility in water
All dye must be soluble during process
Almost insoluble
Affinity
Possess a specific affinity towards fiber
Have no affinity but used as coating
Chemical nature
Organic and few are metallic
Most are metallic or organometallic.
Application
Through water medium
Through adhesive or binder
Uses of Pigment
Pigments are used for coloration of a very broad and diverse number of materials
Surface coating for interior, exterior, automotive and other application
Paints based on olegoresinous liquid and water emulsion
Printing ink for papers (lithographic, rotogravure and flexographic systems (and for other materials such as metal plates, foils, artists and writing material)
Coloration of plastics and rubber
Textile printing
Coloration of manmade fibers by mass pigmentation before fiber formation (dope dyeing) etc.
Required Properties of pigments
They should have covering power which is influenced by particle size
Should be inert, stable and have long life
Should have capability of mixing
Good wet fastness, light fastness and abrasion resistance
Good resistance to acid, base, perspiration, chlorine, peroxide and gas fading
Good solvent resistance (insoluble in water, CCl4, Cl2C=CHCl)
Suitable brilliance, hardness and stability
Suitable characteristics for good dispersion including particle size and distribution, electrical charge (most are negatively charged particle), specific gravity, purity and crystalline structure, conditions of precipitation of the pigments
Should be applicable to all fibers.
Physical/Chemical Properties of Pigments
Chemical Structure
Inorganic oxide, salts, organometallic toners, organic insoluble azo pigments, phthalocyanine metal complexes
Physical state
Very important, decreasing particle size increase color value but decreasing hiding power
Particle size
5-7 micron
Density
Sp gravity range from 1.17- 1.37 for most cases
Melting points
Usual range 110 -175°C
Boiling point
Decompose at 195- 345°C. phthalocyanine pigments sublimes at 500°C
Water solubility
Insoluble for all practical purposes.
Other solubility
Inorganic pigments are insoluble in most solvent
Spectra
Very strong and high, though not comparatively sharp peaks
Application Properties of Pigments
Fabric dyed
Any fiber can be dyed by selecting a suitable binder, quality greatly depends on binder used to affix the pigment
Fabrics printed
Any fiber by suitable binder even hard to print polyester blends and glass fibers
Disposable fabrics
Well suited for non woven fabrics
Dischargeability
Some pigments are suitable for discharge printing
Alkali fastness
Poor for organometallic azo toners, good for insoluble azo
Heat resistance
Extremely varied. Some are stable up to 200°C and some up to 300°C. optimum for inorganic pigments
Light fastness
Generally very good. Optimum for inorganic pigments
Wash fastness
Generally good to very good
Useful colors
Diarylide yellows and oranges, Hasna yellow, azoic reds, phthalocyanine blues and greens, carbon black, TiO2 white, violet and browns.
Processes used
Padding for dyeing
Aftertreatment
None required
Classification of pigments
According to origin
Natural/Mineral: Iron ores, clays, chalk etc
Synthetic/chemical: white lead, ZnO, TiO2 and large number of inorganic and organic color
According to Reactivity
Reactive pigment: some pigments on account of the chemical character react with oil, fatty acids and soaps. These are called reactive pigments e.g. ZnO, red lead
Inert pigment: TiO2
According to Chemical Nature
Organic pigment: appx 25% (by wt.) of the world production of organic colorant is accounted for organic pigments. They account for only 4% of total pigment production. Of the total organic pigments production yellow, red and blue tones accounted for 89%.
Most organic pigments exhibit a small solubility, typically in polar solvent. All the organic pigments are soluble in one or more of the four chemical: Chloroform (CHCl3), Methyl alcohol (CH3OH), Dimethyl formamide (DMF) and concentrated H2SO4. Organic pigment consists of:
Azo pigment:
Strong tinctorial strength
Good alkali resistance
Excellent brightness
Cover a wide range with regard to other application properties
Poor alkali resistance of certain organometallic pigments make them unsuitable for printing
Diarylide orange and yellows:
Extremely bright color
Inferior light fastness
Phthalocyanine
Blue, greens are dominant shade especially in plastic coloration
Offer low migration
Good temperature stability
Excellent light fastness
Good heat resistance
Excellent alkali resistance
Good solvent resistance
Used extremely in printing, pad dyeing and dope dyeing
Hasna yellow
Good light fastness
Have migration tendency
Inorganic pigment:
They account for 96% (by wt.) of total production. More than half of their production volume is accounted for a single production, TiO2, the most important white pigment
H2SO4 is a good solvent for many inorganic pigments
They are opaque
Less expensive
More weather resistant
More chemical resistant
Insoluble in most organic solvents
Highest degree of light fastness
Excellent heat resistance
They consist of
Salts: Sulfates, carbonates, silicates and chromates of many metal elements like, Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.
Oxides of Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.
Metal Complexes: Naturally occurring oxides and silicates
Difference between organic and inorganic pigment
Organic
Inorganic
Solubility
Soluble in organic solvent
Soluble in inorganic solvent
Tinctorial strength
Higher
Lower
Brightness
Higher
Lower
Purity
Higher
Lower
Transparency
Opaque
Transparent
Weather resistance
Less
More
Chemical resistance
Less
More
Fastness
Good
Excellent
Cost
Expensive
Cheap
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Sunday, November 18, 2007
Textile Printing...3
Pigment Printing
In pigment printing, insoluble pigments, which have no affinity for the fiber, are fixed on to the textile with binding agents in the pattern required. This description is perhaps oversimplified, but it does obviously set pigments apart from dyes that are absorbed into the fiber and fixed there as a result of reactions specific to the dye.
Historical Development of Pigment Printing
§ Until 1937 natural polymers as binders and thickeners (starch, glue)
§ Around 1937 emulsion thickening
§ Around 1960 use of aqueous self-crosslinking dispersions as binders
§ Around 1970 development of synthetic thickening agents based on acrylic acid
§ After 1980 ecological improvements (e.g., emission)
Why Pigment Printing is Important
§ The pigment can be applied to all fibers potentially and it is the only coloration to glass fiber, fabric and polyester
§ No wet treatment is required, so drying and curing is applicable to all fiber.
§ Extensive color range of highly light fast colors
§ Possible to produce good combination shades on blended fiber in one padding operation
§ Application procedure is simple
§ No change of hue of colorant throughout processing
§ Less expensive
A good quality pigment print is characterized by
§ Brilliance and high color value relative to the pigment concentration in material
§ Minimum stiffening in the handle of the textile
§ Generally acceptable fastness properties.
Components of a pigment printing system
A pigment printing system consists of three essential components:
§ Pigment dispersion: Specific pigments are treated in a grinding mill in the presence of suitable non-ionic surfactants. A particle size of 0.1-3 μm is typical. Generally, the pigment pastes are aqueous based and contain the dispersing agent, humectants (to prevent evaporation and drying out).
§ Binders and cross-linking agents (polymers): The binders used in pigment printing systems are film-forming substances made up of long-chain macro molecules which, when heated with a suitable acid-donating catalyst, form a three-dimensional structure in the pigment.
§ Thickeners and auxiliary agents: These give the required print thickening power (rheology).
Binder
The binder is a film forming substance made up of long‑chain macromolecules which, when applied to textile together with the pigment, produce a three dimensionally linked network.
Binder- CH2-OR + HO-Textile Binder –O- textile + HOR
Where R is H or CH3.
The links are formed during some suitable 'fixing' process, which usually consist of dry heat and a change in pH value, bringing about either self-crosslinking or reaction with suitable crosslinking agents.
The degree of cross linking should be limited, to prevent the macromolecules becoming too rigidly bonded, thus preserving some extensibility. The important criteria, which ensure that the pigment within the crosslinked binder film is fast to wear and cleaning, are elasticity, cohesion and adhesion to the substrate, resistance to hydrolysis, as little thermoplasticity as possible and absence of swelling in the presence of dry cleaning solvents.
Required properties for Binders
§ Should be film forming
§ Should be water swell
§ Should not be too thermoplastic
§ Should have atmospheric stability
§ Should be colorless and clear
§ Should be of even thickness and smooth; neither too hard nor too stiff.
§ Should have good adhesion to substrate without being tacky.
§ Should possess good resistance to chemical and mechanical stress
§ Should be readily removable from equipments
§ Should provide good color yield
§ Should be non toxic
Types of Binders
§ According to the origin
Natural: glue, gelatine etc
Synthetic: acramin binders
§ According to chemical groups
Acrylic binders: These are normally an aqueous dispersed co-polymer of butyl acrylate and styrene, having N-methylol acrylamide groups for cross-linking purposes.
Some of the more important properties of this type of binder are:
§ Good resistance to ageing by light
§ Good heat stability
§ Generally a harsh handle
§ Good solvent resistance
Butadiene co-polymer binders : They are made by emulsion co-polymerisation with acrylonitrille and N-methylolmethacrylamide. Some of the more important properties of this type of binder are:
§ Poor resistance to ageing by light
§ Susceptible to yellowing on heat treatment
§ Generally a soft handle, particularly on synthetic fibers
§ Generally the highest binding action on synthetic fibers
§ good solvent resistance
Trade names of binderTrade name Manufacturer Origin
Acramin Bayer Germany
Tinolite, Microfix, orema Ciba Switzerland
Helizarine BASF Germany
Imperon Hoechst Germany
Thickening Systems
There is a wide range of thickener materials available including alginates, natural vegetable gums, synthetic polymers, or even foams. These materials show sensitivity to factors such as temperature, pH, and salt content.
§ Ionic thickener (alginates): Better color yield
§ Nonionic thickener (cellulose ether): stable to pH variation and electrolyte content.
§ Natural and semi synthetic hydrophilic thickeners: should not used in pigment printing because:
- When entrapped in binder film, are either soluble in water or swell in presence of water even after fixation.
- They contain large no of polar groups like hydroxyl group and produce a hard film and stiff handle.
- Aftertreatment to remove them is not effective since they are enclosed in the binder film.
Emulsion Thickener
Two mutually immiscible liquids (oil and water) are stirred to produce an emulsion with the presence of emulsifier. The nature of the emulsifier and the ratio of the two immiscible liquids determine which liquid will be dispersed (the disperse phase) in the other (the outer, continuous phase)
The emulsifier forms a film between the two liquids, reducing interfacial tension. The emulsion stability depends on
- The degree of dispersion
- Type and quality of emulsifying used
- The substance dissolved or dispersed in the dispersed or dispersion medium
Two types of emulsion thickener
§ Oil in water (o/w): kerosene/white spirit in water
§ Water in oil (w/o): water in kerosene or white spirit
Synthetic thickeners
§ A thickener that is made artificially. Synthetic thickeners are typically designed to offer high viscosity at low concentrations, high yield value, shear thinning, stability, integrity over a wide temperature range, and ease of use.
§ Synthetic thickeners are efficient at only 1-3 % concentration level while approximately 10% of a natural thickener is needed to give the required viscosity in the print paste.
§ Other advantages of synthetic thickeners include rapid make-up since they require no waiting for hydration to occur, sharp print boundaries, and controlled penetration which usually provides greater color value and levelness.
Other Auxiliaries
§ Catalysts
§ Diammonium phosphate: - most widely used acid catalyst
§ used in conc. of 0.5% and 0.8% in screen and roller printing respectively
§ when used in correct proportion produces a pH of 3 in fabric and brings a cross linking reaction
§ Ammonium salts: sulphocyanide, sulfate and chloride are suitable. Ammonium nitrate: not recommended and it turns polyamide fiber yellow
§ Urea
These are agents that are added to improve “runnability” on printing machines. Owing to their low volatility these auxiliaries are used sparingly, maximum amounts of 20 parts/1000 being common; otherwise the fastness properties may be adversely affected.
§ Softening agents
After curing fixation the resultant “handle” of the printed fabric depends on a number of factors:
- monomer composition of the binder
- presence of water-soluble protective colloids (e.g. alginates, etc.)
- extent and type of cross-linking.
By the addition of certain compounds (usually termed “plasticisers”) improves the handle of printed goods.
§ Cross-linking agents
These agents are universally based on either urea-formaldehyde types (e.g. dimethylolurea) or melamine-formaldehyde types. They are incorporated into printing compositions in an attempt to increase various aspects of fastness, particularly rub and scrub fastness with synthetic fibers. A maximum addition of 10-20 pts/1000 is normally encountered: larger amounts can have a quite marked effect on the “handle” of the fabric
Pigment Printing Recipe and Procedure
Typical Recipe:
Pigment: 10-20gm
Binder: 40-50 gm
Thickener: 35-50 gm
Catalyst: 5 gm
Dispersing agent 2 gm
Water x ml
Procedure:
§ Preparation of printing paste using dispersing agents and thickener and catalyst.
§ Application of pigment paste and binding resin together
§ Drying at 140 – 150°C
§ Curing to fix the resin pigment
Affect of curing on PET
Temperature Time Strength loss
205°C 1 min 0%
220°C 1 min 0%
235°C 1 min 2%
245°C 1 min 5%
260°C 1 min 13%
Problems of Pigment Printing
§ Adverse effect due to binder as it changes texture of fabrics.
§ The quality of printing or dyeing depends on the characteristics of binder used to affix the pigment even more than the properties of pigment.
§ Some solvents used in emulsion like kerosene, white spirit cause problem like flammability.
§ The chemical and physical influences on the binder and print paste can interfere during production and processing resulting in sticking especially in roller printing.
§ The gumming up of equipments, odor, air and water pollution
§ Difficulty in obtaining the necessary wet treatment fastness and abrasion resistance with certain products, may not be obtained pigment printing or dyeing.
Pigment Dyeing on Fabric
Typical Recipe
Pigment: 10-20gm/L
Binder: 40-50 gm/L
Thickener: 35-50 gm/L
Catalyst: 5 gm/L
Thickener: 2 gm/L
Dispersing agent: 2 gm/L
Procedure:
§ Binder is weighted and diluted with cold water
§ Pigment and thickener is added with cold water
§ Catalyst solution is added
§ Dispersing agent is added
§ The dyeing liquor is well filtered and stirred; material is padded
§ The material is dried at 70 -100°C in hot flue steam but no use of cylinder dryer.
§ Curing is done at 150°C, 2-3 min
Precautions:
§ No alkalinity: The fiber to be dyed should not be alkaline
§ No OBA: OBA may produce faulty shade
§ No formation of skein: Binder should not be allowed to form skein which ultimately give specky shade
Typical procedure for Garment dyeing
§ First bleach the material then treat with a synthetic mordant cationising agent at pH 7
§ Rinse at 60°C at a rate of 2°C/ min for 20 min
§ Cold rinse
§ Apply pigment at 7O°C (pH 5) for 20 min
§ Add salt, acid and raise temperature when necessary
§ Now use binder 4% for 10 min at 70°C
§ Cold rinse with 1 gm/L soap wash for 10 min at 65°C
§ Cold rinse and dried
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Pigment
Pigments implied general insolubility and complete insolubility in water.
Difference between dye and pigment
The difference between dye and pigment is not a clear one. Most organic pigments are closely related to dyes with respect to their chemical structure and there are dyes which become pigments after application. Vat dye is a dye when used in dyeing but a pigment when used in printing.
Dye
Pigment
Solubility in water
All dye must be soluble during process
Almost insoluble
Affinity
Possess a specific affinity towards fiber
Have no affinity but used as coating
Chemical nature
Organic and few are metallic
Most are metallic or organometallic.
Application
Through water medium
Through adhesive or binder
Uses of Pigment
Pigments are used for coloration of a very broad and diverse number of materials
Surface coating for interior, exterior, automotive and other application
Paints based on olegoresinous liquid and water emulsion
Printing ink for papers (lithographic, rotogravure and flexographic systems (and for other materials such as metal plates, foils, artists and writing material)
Coloration of plastics and rubber
Textile printing
Coloration of manmade fibers by mass pigmentation before fiber formation (dope dyeing) etc.
Required Properties of pigments
They should have covering power which is influenced by particle size
Should be inert, stable and have long life
Should have capability of mixing
Good wet fastness, light fastness and abrasion resistance
Good resistance to acid, base, perspiration, chlorine, peroxide and gas fading
Good solvent resistance (insoluble in water, CCl4, Cl2C=CHCl)
Suitable brilliance, hardness and stability
Suitable characteristics for good dispersion including particle size and distribution, electrical charge (most are negatively charged particle), specific gravity, purity and crystalline structure, conditions of precipitation of the pigments
Should be applicable to all fibers.
Physical/Chemical Properties of Pigments
Chemical Structure
Inorganic oxide, salts, organometallic toners, organic insoluble azo pigments, phthalocyanine metal complexes
Physical state
Very important, decreasing particle size increase color value but decreasing hiding power
Particle size
5-7 micron
Density
Sp gravity range from 1.17- 1.37 for most cases
Melting points
Usual range 110 -175°C
Boiling point
Decompose at 195- 345°C. phthalocyanine pigments sublimes at 500°C
Water solubility
Insoluble for all practical purposes.
Other solubility
Inorganic pigments are insoluble in most solvent
Spectra
Very strong and high, though not comparatively sharp peaks
Application Properties of Pigments
Fabric dyed
Any fiber can be dyed by selecting a suitable binder, quality greatly depends on binder used to affix the pigment
Fabrics printed
Any fiber by suitable binder even hard to print polyester blends and glass fibers
Disposable fabrics
Well suited for non woven fabrics
Dischargeability
Some pigments are suitable for discharge printing
Alkali fastness
Poor for organometallic azo toners, good for insoluble azo
Heat resistance
Extremely varied. Some are stable up to 200°C and some up to 300°C. optimum for inorganic pigments
Light fastness
Generally very good. Optimum for inorganic pigments
Wash fastness
Generally good to very good
Useful colors
Diarylide yellows and oranges, Hasna yellow, azoic reds, phthalocyanine blues and greens, carbon black, TiO2 white, violet and browns.
Processes used
Padding for dyeing
Aftertreatment
None required
Classification of pigments
According to origin
Natural/Mineral: Iron ores, clays, chalk etc
Synthetic/chemical: white lead, ZnO, TiO2 and large number of inorganic and organic color
According to Reactivity
Reactive pigment: some pigments on account of the chemical character react with oil, fatty acids and soaps. These are called reactive pigments e.g. ZnO, red lead
Inert pigment: TiO2
According to Chemical Nature
Organic pigment: appx 25% (by wt.) of the world production of organic colorant is accounted for organic pigments. They account for only 4% of total pigment production. Of the total organic pigments production yellow, red and blue tones accounted for 89%.
Most organic pigments exhibit a small solubility, typically in polar solvent. All the organic pigments are soluble in one or more of the four chemical: Chloroform (CHCl3), Methyl alcohol (CH3OH), Dimethyl formamide (DMF) and concentrated H2SO4. Organic pigment consists of:
Azo pigment:
Strong tinctorial strength
Good alkali resistance
Excellent brightness
Cover a wide range with regard to other application properties
Poor alkali resistance of certain organometallic pigments make them unsuitable for printing
Diarylide orange and yellows:
Extremely bright color
Inferior light fastness
Phthalocyanine
Blue, greens are dominant shade especially in plastic coloration
Offer low migration
Good temperature stability
Excellent light fastness
Good heat resistance
Excellent alkali resistance
Good solvent resistance
Used extremely in printing, pad dyeing and dope dyeing
Hasna yellow
Good light fastness
Have migration tendency
Inorganic pigment:
They account for 96% (by wt.) of total production. More than half of their production volume is accounted for a single production, TiO2, the most important white pigment
H2SO4 is a good solvent for many inorganic pigments
They are opaque
Less expensive
More weather resistant
More chemical resistant
Insoluble in most organic solvents
Highest degree of light fastness
Excellent heat resistance
They consist of
Salts: Sulfates, carbonates, silicates and chromates of many metal elements like, Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.
Oxides of Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.
Metal Complexes: Naturally occurring oxides and silicates
Difference between organic and inorganic pigment
Organic
Inorganic
Solubility
Soluble in organic solvent
Soluble in inorganic solvent
Tinctorial strength
Higher
Lower
Brightness
Higher
Lower
Purity
Higher
Lower
Transparency
Opaque
Transparent
Weather resistance
Less
More
Chemical resistance
Less
More
Fastness
Good
Excellent
Cost
Expensive
Cheap
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Labels: Printing
Roller Printing
Engraved roller printing is a modern continuous printing technique developed in the late 19th and early 20th centuries.
Until the development of rotary screen printing, it was the only continuous technique.
The high fixed cost of copper rollers, expense of engraving process, and possible distortion of fabric during printing have led to its reduced use, now being less than 5% of the worldwide textile printing market.
The fine design detail possible with this technique has always been its main advantage.
Main Parts and their functions
Color Doctor: It is quite essential and is a thin sharp blade of steel that rests on the engraved roller and serves to scrape off color from its surface, leaving only that which rests in the engraving. On the perfect action of this doctor depends the entire success of printing, and as its sharpness and angle of inclination to the copper roller varies with the styles of work in hand. Any roughness, unevenness or the slightest snip in the edge is sufficient to allow color to escape under it, with the result that the finished parts exhibit serious defects in the form of streak or cloudy patches of color.
Lint Doctor: It does two functions
Remove any loose fiber, filaments, neps which are attached from the printing cloth and get stuck up to the wet surface of the roller.
It cleans other colors of a multicolor design which are already printed on the cloth and being still wet impress themselves on the surface of the roller
Blankets: This is a tightly woven uniform woolen piece of equal thickness and elasticity. It can be removed from the machine, well washed to remove the dried paste, dried and again put on the machine as before. They are resilient and hard wearing.
Back Grey: Simple pieces of unbleached cloth that are run between the blanket and the cloth to be printed. This is used to absorb the color forced at the back of the printed piece which would otherwise spoil the blanket.
Furnishers: Color furnishers are usually wooden roller working partly in the color box and partly above. They supply the color to the design rollers.
Color Box/Tray: they are long, narrow, shallow, trough of word or copper to carry the printing paste. It is capable of being moved backward or forward.
Color unit: Consists of color box, two doctors and design roller.
Engraving of copper rollers
In this method, a heavy copper cylinder (roller) is engraved with the print design by carving the design into the copper. Copper is soft, so once the design is engraved, the roller is electroplated with chrome for durability.
Three different methods of engraving
By hand with a graver which cuts the metal away
By etching, in which the pattern is dissolved out in nitric acid; and
By machine, in which the pattern is simply indented.
Pros and Cons
Advantages:
Suited for large batches
Very high speed
Fine line or sharp line can be easily obtained
Highest production for single color is 18000 yds/hr and 10000/yds for 12 colors
Can be used for printing any style
Can be used for all classes of dye on any fabric for all design
Repeats do not exist as printing is continuous
Disadvantages
Changing time is high
Crush effect is produced
Engraving the printing roller is expensive operation
Defects in Roller Printing
Scratches: Due to some hard particles of sand or grit in the color paste. These marks being very fine; they are noticeable only after fully development of the color at the stage of soaping and washing
Removed by burnishing the rollers
Snappers: These are in the form of ugly lines printed parallel to the selvedges. They are caused by some substances getting under the color doctor which is lifted up at that part and color escapes unscrapped from its either side giving a dirty and smeared printing. Loose thread from the cloth or dried particle of paste or bad mixing of thickener can cause this defect.
Can be removed by restraining the color paste.
Lifts: It occurs at regular interval usually equal to the circumference of the design roller. Some hard particle gets stuck into the engraving which lifts the color doctor and allows the color paste to go ahead unscrapped.
Removed by pulled out of the particle.
Scrimps: Due to creases or folds in the cloth. They are visible in the form of a double edge with white unprinted part left over in between, below a crease or fold in the cloth.
Rollers at the back of the feed and drying arrangements must be right.
Streaks: this defects manifest in the form of two or more fine lines running parallel to the selvedges either straight (due to scratches on the roller) or in a zigzag (cut in the color doctor) manner.
Removed by polishing the roller and resharpening the doctor edge
Scumming: due to proper uncleaning of the design roller surfaces. It spoils the whole cloth due to the printing of one or more colors. It may caused by
Rough surface of the roller
An uneven doctor edge
Doctors not properly adjusted
Removed by correcting the factor responsible for it i.e. by positioning the roller or correcting the uneven edge of the doctor or setting the doctor right.
Uneven printing: Due to
Uneven pressure on the ends of design rollers, show light and deep printing at the selvedges
Uneven lapping of the furnishing roller
Uneven addition of the color at the one end of the color tray
Greater percentage of insoluble substances in the paste
Uneven diameter of the design roller due to constant polishing of one end of the roller
Comparison among roller, flat and rotary screen printing
Feature
Roller
Automatic flat screen
Rotary screen
Minimum run (meters)
10,000
500
1000
Design scope
Excellent
Good
Good to excellent
Size of pattern repeat
Limited by roller diameter of 40 to 46 cm
Large designs possible
Limited by screen diameter of about 64 cm
Heaviness of print
Depends on the depth of engravings, generally less than screen prints
Depends on size of holes in screens, percentage of open area and squeegee pressure
Pressure on fabric
More than in screen printing
Low but depends in squeegee pressure
Production rate
Fast
Slower
Fast
Ease of setting up new designs
Costly and length process
Less costly than rotary screens
Technically more difficult than flat screens
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Labels: Printing
Screen Printing
Screen Printing
Screen printing is arguably the most versatile of all printing processes. It can be used to print on a wide variety of substrates, including paper, paperboard, plastics, glass, metals, fabrics, and many other materials including paper, plastics, glass, metals, nylon and cotton. Some common products from the screen printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards.
Screen Printing Process
Hand screen
Semi automatic flat screen
Rotary Screen
Screens
Made of cotton, silk, nylon cloth, polyester or metal gauze
Screen mesh refers to the number of threads per inch of fabric. The more numerous the threads per inch the finer the screen.
The usual mesh of screen employed for cotton and silk printing is 80 threads per inch.
The finer the screen the sharper are the outlines but more effort is needed to force the printing paste through the screen.
Screen Frames
There are two types of screen frames, metal and wood.
Screen frames for commercial use are usually made of steel, or a lighter metal, with a hollow cross section to provide rigidity with minimum weight.
Screen frames are usually 26" x 55" (measured externally) and 23" x 52" (measured internally) for printing 45" wide cloth
Screen Fabric
There are two types of threads for screen fabric:
Monofilament - single strands weaved into fabric
Primarily used in commercial printing and other applications
Advantage: Monofilament is easier to clean than multifilament
Multifilament - multiple strands wound together like a rope, then weaved into fabric.
Primarily used in textile printing.
Disadvantage: ink tends to build up on screen, more difficult to clean. Monofilament mesh has become the industry standard.
Screen Fabric Types
Silk - multifilament weave
loses toughness with frequent use
reclaiming chemicals containing bleach or chlorinated solvents destroy the silk
Today silk is primarily used for printing art, not commercial use as before
Nylon - multifilament or monofilament
good for stretching
compared to polyester, lacks stability
less rigid than polyester
unsuitable for closely registered colors
Polyester - multifilament or monofilament (calendared monofilament polyester, metallized monofilament polyester)
primary material used in commercial screen printing
Polyester is strong and stable when stretched
Other screen materials - carbonized polyester
glass
wire mesh
stainless steel
Screen Preparation
Photochemical method is most widely used for preparing the screen. This is based on the principle that when a coating of a solution of ammonium dichromate-gelatine or ammonium dichromate-polyvinyl alcohol is dried and exposed to light, Insolubilisation takes place
Other method for screen preparation is lacquer and laser screen.
Photochemical method
Coat the flat screen with light-sensitive polymer, and dry it in the dark.
Position a positive transparency of the pattern on the polymer-coated screen.
Expose the screen to ultraviolet light. Ultraviolet light rays pass through the transparent (non-pattern) areas of the transparency on to the screen and harden the polymer.
Wash the screen in warm water to remove the polymer from the unexposed (pattern) areas of the screen through which the printing paste will pass.
Dry the screen
Preparation of Sensitising solution
Sensitising solution may be prepared as follows:
(1) Chrome-Gelatine Solution
Solution A 200 g Pure gelatine
500 g Boiling water
Total 700 g
Solution B 70 g Ammonium dichromate
150 g Boiling water
80 g Liquor Ammonia
Total 300 g
Solution A and Solution B are mixed in a dark room.
(2) Chrome-Polyvinyl Alcohol Solution
600 g Polyvinyl alcohol (15% solution)
120 ml Ammonium dichromate (33% solution)
240 ml Cold water
1 litre with cold water
Squeegee system
Rubber Squeegee
Double Squeegee
Magnetic rod Squeegee
Rubber Squeegee
These vary in Shore hardness from 55 (soft) to 70 (hard). Softer blades give a heavier print. The edge shape of the rubber blades is chosen to suit requirements.
Round ones [Figure (a)] suit, for example, wool and fleece fabrics, where a heavy print is needed to penetrate the fibrous surface.
Long, tapered edges [Figure (c)] are used when penetration is not important as on flat and woven fabrics.
The stubby edge [Figure (b)] is good for one-stroke printing on interlock. The chisel shape
[Figure (d)] is used to flood the screen with printing paste while the screen is raised in preparation for the print stroke when only one print stroke is to be used.
Double SqueegeeThis system is easier to make than a single squeegee, which must be lifted over the pool of print paste at the end of each stroke.
Magnetic rod Squeegee
Arolling rod (a) or a pair of rods (b) is moved by a driven electromagnet moving under the printing blanket. The diameter of the single rod is small enough to allow print paste to flow over and round it at the end of a pass. The twin rods form a well of paste, the volume of which depends on rod spacing and diameter.
Fundamental characteristics of screen printing
In screen printing process – hydrodynamic pressure is built up in the print paste between the squeegee and the screen surface through which the paste is passed.
The hydrodynamic pressure appears to be inversely proportional to the radius of the pore i.e. Hydrodynamic pressure 1/rn (n<2)
Here the pore radius greatly affects the amount of paste flowing through screen; Hydrodynamic pressure is also proportional to the viscosity of paste.
The percentage of open area of the screen also plays a role. More open screens allow more paste to pass.
The fabric is to be printed forms a three dimensional structure with the screen where the absorbency of the fibers and penetration capacity between yarn also affect the uptake of the paste.
The usual hexagonal openings are larger at the outside of the screen than at the inside, the capillarity and surface tension forces etc. result in a printing with actually more color deposited in the areas between holes than opposite holes.
Flat bed screen printing
An automated version of the older hand operated silk screen printing
The flat-bed screen process is a semi-continuous, start-stop operation.
For each color in the print design, a separate screen must be constructed or engraved
Fabric glued to blanket
Screens rise and fall
Printing done while screen in down position
Rod or blade squeegee system
Up to four strokes possible
Productivity is in the range of 15-25 yards per minute.
The design repeat size is limited to the width and length dimensions of the flat screen.
Currently accounted for apprx. 15-18% of printed fabric production worldwide
Slow process
Factors affecting Print paste passing through the screen
The ‘mesh’ (threads per inch) of the screen fabric
The fraction of open area in the screen fabric, this not only depends on the mesh but also on the yarn diameter and the effect of subsequent treatments, such as calendaring
The hardness and cross section of the squeegee blade; a hard rubber squeegee with a sharp cross section is suitable for outlines, whereas soft, rounded blade applies more paste and is suitable for blotches
The hardness of the printing table, if the top of the table is firm a soft squeegee is probably necessary, whereas with a resilient table surface a harder squeegee is preferable.
The viscosity of printing paste
The number of squeegee strokes; from two to four strokes are usually applied
The speed of the squeegee stroke
Flat bed to Rotary screen
Modification of flat bed screen printing: from semi continuous to continuous, low productivity to high productivity.
Quality of end result.
Amount of color that can be applied. Note that the screen area consisting of holes is smaller in rotary screens than in flat screens.
Evenness of color.
Ability to produce fine lines and half-tones. Half-tones are tone graduations within one colored area.
More compact than flat screen machines for the same number of colors in the pattern
Rotary screen machines are highly productive, allow for the quick changeover of patterns, have few design limitations, and can be used for both continuous and discontinuous patterns
Typical speeds are from 50-120 ypm (45-100 mpm)
High investment cost and the machines are generally not profitable for short yardages of widely varying patterns.
Controls approximately 65% of the printed fabric market worldwide
Rotary screen printing
In basic operation, rotary screen and flat screen-printing machines are very similar. Both use the same type of in-feed device, glue trough, rotating blanket (print table), dryer, and fixation equipment. The process involves initially feeding fabric onto the rubber blanket. As the fabric travels under the rotary screens, the screens turn with the fabric.
Print paste is continuously fed to the interior of the screen through a color bar or pipe. As the screen rotates, the squeegee device pushes print paste through the design areas of the screen onto the fabric. As in flat-bed screen printing, only one color can be printed by each screen. After print application, the process is the same as flat screen printing.
Estimates indicate that this technique controls approximately 65% of the printed fabric market worldwide.
Defects on screen printing
Out of registration – pattern out of fit.
Glue streaks – from the rubber blanket.
Color smear.
Color out – from a lack of print paste.
Creased fabric.
Pinholes in any screen.
Damage to the screen leading to misprints.
Lint on the fabric causes pick-off.
The prints may come out lighter in the middle and deeper towards the selvedges. This occurs when too much cloth is steamed in one batch or when the cloth is very thick.
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Labels: Printing
Textile printing
Textile printing
Textile printing is used to signify the production, by various means of colored patterns on designs upon all sorts of textile fiber.
Textile printing is the most important and versatile of the techniques used to add design, color, and specialty to textile fabrics. In other words, dyes and pigments are applied locally or discontinuously to produce the various designs. In fact, printing is described as ‘localized dyeing.’ The forces which operate between the dye and the fiber (on mechanical retention, hydrogen bonding, chemical reaction, electrostatic attraction etc.) are the same in dyeing and printing.
The term 'colorant' is used here because it covers both dyes and Pigments
STEPS OF PRINTING
Preparation of the fabric
Preparation of the print paste.
Printing the fabric.
Drying the printed fabric.
Fixation of the printed dye or pigment.
Afterwashing.
STYLES OF PRINTING
Percentage of rejection is lower than the discharge method
Direct style
Discharge: white and colored
Resist style
Raised style
Flock style
crimp/ crepon style
Burn out style
Difference between Resist and Discharge printing
Discharge printing
Resist Printing
Always sharp in outline, bright in appearance and give perfect and sparkling whites due to the bleaching effect on the discharging agent
Generally subdued and the colors are less bright; the outlines of the printing motifs are less sharp
Drastic chemical action is required to destroy the color
Little or no chemical action is involved to prevent fixation of color
Costs is higher and ingredients has to be selected carefully to facilitate complete destruction of color
Little or no chemical action is involved to prevent fixation of color
Cost is lower requiring less chemicals and auxiliaries
Cost is lower requiring less chemicals and auxiliaries
Applicable to those colors which can be discharged and has limitations; and also it is generally difficult to get reproducible results in all operations
Effective in all cases as almost colors are capable of being resisted
Relatively low chemical stability and fastness property of printed fabric
Printed fabric has great chemical stability and fastness properties
METHODS OF PRINTING
Block printing
Stencil printing
Roller printing
Screen Printing
Hand screen
Semi automatic flat screen
Rotary screen
Transfer printing
Flat bed
Continuous transfer
Vaccum transfer
Digital Inkjet Printing
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Labels: Printing
Course Outline
TXE 405, Wet Processing IIICourse InstructorMd. Abbas UddinRecommended bookMiles, L. W. C. (1994) Textile Printing, Bradford, England, Society of Dyers and Colorists.Peters, R. H. (1975) Textile Chemistry: The physical chemistry of dyeing, Amsterdam, Elsevier Scientific Publishing Company.Shenai, V. A. (1999) Technology of Textile Processing: Technology of Textile Finishing, Vol X, Mumbai, Sevak publications.Tomasino, C. (1992) Chemistry & Technology of Fabric Preparation & Finishing, North Carolina, College of Textiles: North Carolina State University.Course ContentsLec 1: IntroductionLec 2 & 3: Screen printingLec 4: Roller printingLec 5, 6 & 7: Pigment (structure and properties, printing, dyeing, synthetic thickener, emulsion thickeners and binder)Lec 8: Dyeing mechanismLec 9, 10 and 11: Diffusion, diffusion coefficient, diffusion modelLec 12: Dyeing kinetics, Interaction of dyes and fibersLec 13: Forces in Dyeing systems, Aggregation of dyesLec 14: Mineral and oxidation colorsLec 15: Structure and application of Mordant dyesLec 16: PresentationLec 17: Different types of surface active agents (Synthesis, Effects, Degradability); Chemistry, properties and uses of various Acids, Alkalis, salts, Oxidizing Agents and Reducing Agents in Textile Wet Processing.Lec 18 and 19: Softening agents (different types, applications)Lec 20: rot-proofing, mildew proofing, insect and bactericidal finishesLec 21: soil release finishesLec 22: ReviewMarks Distribution
Attendance: 05%
Class sessional (assignment and quiz): 15%
Presentation: 10%
Midterm: 20%
Final: 50%
Course Policy
Attendance is must. Less than 60% attendance will carry zero marks.
No student will be allowed to enter in the class room after 5 mins of the entrance of the instructor.
There will be announced and unannounced quizzes at any time of the class.
Presentation will be done in a group of 4/5, with a 10 minute length. Topic would be chosen by the students from the course outline.
Final will be comprehensive
Good Luck
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