A SYNOPSIS ON COATING & LAMINATION IN TEXTILES

A SYNOPSIS ON COATING & LAMINATION IN TEXTILES:

PROCESS & APPLICATIONS

Mayur Basuk & Girish Kherdekar

Wool Research Association – Center of Excellence for Sportech, Thane- 400607,

Maharashtra, India; Correspondence: Phone no: +91 9699821340; +91 22 2586 8109,

Fax: +91 22 2531 8365; Email: [email protected], [email protected]

Abstract

Coating and laminating are increasingly important techniques for adding value to technical textiles. Coating and lamination enhance and extend the range of functional performance properties of textiles and the use of these techniques is growing rapidly as the applications for technical textiles become more diverse. They are two functional processes which are used make a proper finishing to the textile material. Cheaper fabric structures may be coated or laminated to provide higher added value to end-users and higher profit margins to manufacturers. The present paper deals with the fabric preparation, methods used, application areas, tests required and market scenario for coating and lamination.

Key words: Coating, Laminations, methods & applications

1. Introduction

Coated fabrics are engineered composite materials, produced by a combination of a textile fabric and a polymer coating applied to the fabric surface. The polymer coating confers new properties on the fabric, such as impermeability to dust particles, liquids and gases, and it can also improve existing physical properties, such as fabric abrasion. The fabric component generally determines the tear and tensile strength, elongation and dimensional stability, while the polymer mainly controls the chemical properties, abrasion resistance and resistance to penetration by liquids and gases. Many properties, however, are determined by a combination of both these components, and both base fabric and polymer must be carefully selected by a thorough consideration of the properties required in the finished product.

Coated textiles is defined as a material composed of two or more layers, at least one of which is a textile fabric and at least one of which is a substantially continuous polymeric layer. The layers are bonded closely together by means of an added adhesive or by the adhesive properties of one or more of the component layers. In other word it is described as a textile fabric on which there has been formed in situ, on one or both surfaces, a layer or layers of adherent coating material. The coating formulation with different textile grade polymer like PVC, PU, acrylic, PTFE are hugely used to make a textile product with multipurpose way like waterproof protective clothing, tarpaulin, protective clothing, electrical insulation etc.

Coated fabrics can be used as

• Coverings or as a barrier for protection, separation containment.
• For appearance modification for decorative or functional purpose.
• Improving dimensional stability, controlling stretch, preventing edges from frying or curling.
• For control of porosity, e.g. for filtration.
• As a matrix for holding some functional material, chemical, pigment or other agent.
• As a processing aid, for example in ‘in situ’ moldings, vacuum technique or thermo-moldings.
• Combining the specialist properties of polymers with the flexibility, strength, drapability and covering power of a fabric.

The most familiar coated fabric is probably the waterproof jacket worn for protection from the rain. Protective clothing for the police, firemen, postmen and the services also made from coated fabric. Examples of domestic articles made from coated fabrics include shower curtains, mattress ticking, flame retardant (FR) upholstery, wipe-clean table cloths and curtain linings. Aesthetic effects and easy clean features can also be achieved by fabric and polymer combinations, and man-made leathers used for seat covers and apparel are essentially coated fabrics. Coated fabrics in the industrial area include materials for tarpaulins, life jackets, life rafts, aircraft safety chutes, hovercraft skirts, protective coverings, awnings, aircraft fuel tanks and flexible containers. Seat upholstery for automobiles, coaches and trains is likely to be coated to improve abrasion resistance, improve FR properties and reduce dust particle penetration.

On the other hand, lamination process is used to prepare some important textile products with daily uses in our practical life like- blackout curtains and blinds etc^1-7.

The key to success in textile coating and lamination depends upon the application of appropriate technology using modern machinery. Machine productivity is important, but flexibility in terms of production speed and the versatility of coating/lamination methods are important factors to consider, as well as a high level of process monitoring, process control and automation to satisfy demanding technical specifications.

 

 

2. Material & their properties
4. • Preparation of fabrics^(3,4,5)

1. Woven fabric:

The original and still most commonly used is the woven fabric. The woven fabrics have three basic construction .the plain weave, the satin weave, and the twill weave. The plain weave is by far strongest because it has interlacing of the fibers, it is used most often. Twill weaving produces distinct surface appearances and it used for affects. Because it is the weakest of the woven, satin weave is used principally for shifting. For shoe upper and other application where strength is important, woven cotton fabrics are used.

1. Knitted fabric:

Knitted fabrics are used where moderate strength and elongation are required. The knit market, it has recently been replaced by polyester or cotton yarn and filament. Where high elongation is required, nylon is used. Knit are predominantly circular jersey, however, patterned knit are becoming more and more prevalent when a polymeric coating is put on a knit fabric. The strength properties are somewhat less than that of fabric. The main use of knit fabric is in apparel, automotive and furnishing furniture upholstery, shoe liner, boot shanks –any place elongation is required.

1. Non-woven:

Many types of nonwoven fabric are used as a substrate .The wet web process gives a non-woven fabric with paper like properties, low elongation, low strength and poor drape. When these substrates are coated, the papery characteristic show through the coating, and fabric esthetics are not satisfactory.

It is difficult to achieve strength as well as a smooth surface; therefore, a high quality non-woven of this is very expensive. Spun bonded non-woven is available in both polyester and nylon in a range of weight. The strength qualities are very high and elongation is low. Since these material is quite stiff. So, these materials are used only when strength and price are the major considerations.

A thin layer of polyester based polyurethane foam can be needed into the non-woven to improve the surface coating properties. The furniture upholstery market was the first to accept this product. Table 1 shows the different type of fabrics used for coating as below:

 

 

 

Table 1: Fabrics for coating:

Fabric	Advantages	Disadvantages
Cotton	§ Excellent coating adhesion § No bonding agent § Low thermal shrinkage	§ Low shrinkage per weight ratio § Absorb moisture § Vulnerable to mildew rotting ,insects
Polyester	§ Strong with HT, low shrinkage § Relative inexpensive § Resistance to mildew, rotting and insects. § Good overall properties as considered price	§ Low moisture absorbency § Limited resilience.
Nylon	§ Strong with HT variant available § Good elasticity § Resistance to mildew rotting and insects § Good thermal absorbency	§ Fabrics may sag due to moisture absorption § Relative inexpensive
Polyethylene Polypropylene	§ Light weight § Inexpensive § Chemically inert resistance to mildew rotting and insects.	§ Adhesive difficult to some substances
Aramid	§ High tensile strength § High melting point § Excellent strength per weight ratio	§ Expensive degraded by UV or sunlight
Glass	§ High temperature resistances § Zero moisture absorbency § Excellent UV stability	§ Heavy § Brittle, poor flexing properties.

Apart from the natural fiber such as cotton and wool, most other material is used in coating. During the coating both fabric substrate and polymer resin will generally subjected to heat treatment.

• Material/Polymers used in Textile Coating^(2,3)

The wide ranges of polymer used, are available for different types of coatings. The functional property of coated fabrics will depend on the chemical nature of the polymer used. Following table 2 indicates main polymer used in textile coating & Table 3 depicts the function of functional properties of different coating composition:

Table 2: Material/Polymers used in Textile Coating

Polymer	Advantages	Disadvantages	Products
PVC	Versatile material Good inherent FR Good oil, solvent resistance Heat and RF weld able for good watertight seams	Cracks when cold Moderate heat and age resistance Plasticizers migration	Tarpaulins covering, large tents, and architecture uses. Seat upholstery Lather cloths Protective clothing Leisure products, banner, bunting,
PVDC Polyvinylidene chloride	Very good FR Low gas permeability Heat weld able, clear, high gloss	Hard and brittle	Blends with acrylics to improve FR in coating
PU Polyurethane	Tough good extensibility God weathering abrasion resistance	Moderate FR Relatively expensive Limited hydrolysis resistance	Waterproof protective clothing Aircraft life jackets
Silicon	Odorless, inert, good resistance to chemical and micro-organism High tear resistance and puncture resistance Low toxicity, water repellant Some breath ability Temp service range from – 60 to 200 oC	Attracts oily soiling Seaming is difficult due to release properties Expensive Difficult to print or apply artwork	Lightweight covering, tarpaulins, Sacks FIBCs, bulk bags Airbags Food, medical application Gaskets, seats Parachutes Oven curtains
PTFE (PFOA release issue)	Excellent resistance to acid,  alkalies, chemicals, Solvent, oil, oxidation and weathering Non sticks properties Temp service upto 260 oC	Very expensive	Architecture application calendar belts Food and medical uses Gaskets, seals
Natural rubber	Excellent stretch and flexibility Working temp up to     70oC Many properties obtained by blending	Moderate sunlight and oxidative resistance Moderate solvent and oil resistances	Carpet backing Tyres Life crafts Conveyer belts Protective clothing Escape churtes
Styrene butadiene rubber	Similar to natural rubber but somewhat better resistances to abrasion flexing and microrg	Similar to natural rubber	Carpet backing
Nitrile rubber	Good oil resistance, which increases with acrylonitrile content. Better resistance to sunlight and heat.	Limited fire resistances	Oil resistance clothing, belts and items handling oily or greasy product
Butyl rubber	Low permeability to gases, better resistances to heat and chemical, oxidation.	Solvent resistance limited, FR resistances, seaming difficult	Air cushion, pneumatic spring, protective clothing for chemical and acid. Light weight jacket, life rafts

Table 3: Function of functional properties of different coating composition ^(2,3)

Functional Properties	Coating chemicals
Stain release	Perfluoro chemical, Polyacrylates, Silicone based products & PVC, Vinylacetate
Soil release
Water repellent
Hot oil repellent/resistance
Waterproof
Deodrant/antibacterial clean room fabric finishes,Chemicals	Poly – hexamethylene biguanide hydrochloride-PHMB,Cyclo dextrin
Chemical odour absorbing	Activated carbon based coatings.
Chemical protective	Based on Aramide, Teflon, PTFE, Carbon, Neoprene Coating
Breathable coating	Hydrophilic, polyacrilamide based products, Polyurethanes
Thermal resistance & insulating	PVC, Teflon, Carbon coating, Silicon rubber
Heat & corrosive resistant coating	Polytetrafluoroethylene, Perfluorooctanoic acid,Teflon
Stiffness/interlining coating	LDPE/HDPE/PVC
Solvent resistance, abrasion resistance, ageing resistance, ozone resistance	Polyurethane coating
Waterproofing, electrical encapsulation, sealants	Butadiene polyurethane resin

 

2.3 Materials for lamination

2.3.1 Membrane Laminated fabric: Membranes are extremely thin films made from polymeric material and engineered in such a way that they have a very high resistance to liquid water penetration, yet allow the passage of water vapour. Films have the advantage over fabrics of 100% cover at light weights; they are generally completely impermeable to liquids and gases and are also excellent barriers to dust and other particles. Film/fabric laminates are used in sail making, hot air balloons and airships etc. Films can be used to produce reflective surfaces with efficiencies much higher than can be produced with a fabric. A typical membrane is only about 10 micrometer thick and, therefore, is laminated to a conventional textile fabric to provide the necessary mechanical strength. They are of two types of membrane, microporous and hydrophilic

1. Microporous Membrane: The first and probably the best known microporous membrane developed and introduced in 1976 by W. Gore, is known as Gore-Tex. This is a thin film of expanded polytetrafluoroethylene (PTFE) polymer. The latest Gore-Tex membrane is a thin film claimed to contain over 9 billion microscopic pores per square inch, these pores are actually 20,000 times smaller than a water droplet, but 700 times larger than a water vapor molecule. Other manufacturers make similar membranes based on microporous polyvinylidene fluoride (PVDF) cast directly on to the fabric. The hydrophobic nature of the polymer and small pore size requires very high pressure to cause water penetration.
2. Hydrophilic Membranes: Hydrophilic membranes are very thin films of chemically modified polyester or polyurethane containing no holes which, therefore, are sometimes referred to as non-poromeric. Water vapour from perspiration is able to diffuse through the membrane in relatively large quantities. The polyester or polyurethane polymer is modified by incorporating up to 40% by weight of poly (ethylene oxide). The poly (ethylene oxide) constitutes the hydrophilic part of the membrane by forming part of the amorphous regions of the polyurethane polymer system. It has a low energy affinity for water molecules which is essential for rapid diffusion of water vapor. These amorphous regions are described as acting like intermolecular ‘pores’ allowing water vapor molecules to pass through but preventing the penetration of liquid water owing to the solid nature of the membrane. Membranes have to be incorporated into textile products in such a way as to maximize the functional properties without adversely affecting the classical textile properties of handle, drape and visual impression. The membranes are laminated to the textile material with chemical or thermal bonding⁸.

Speciality films are widely used in protective clothing as waterproof and breathable barriers against the weather. They are also used in footwear, and medical applications are being explored.50–55 The market leaders are Gore-Tex which is a PTFE film, Sympatex (Acordis) which is made from polyester, and Porelle (Porvair) which is polyurethane based. There are also several other branded waterproof and breathable films made from polyurethane. These films can be broadly classified into two types: micro porous and ‘solid’ – which is sometimes referred to as ‘monolithic’. The micro porous varieties, such as Gore-Tex and Porelle, have very small pores in their structure which are too small to allow drops of water through, but large enough to allow water vapour molecules through, thus accounting for their breathability. The ‘solid’ film variety, such as Sympatex, breathes by a process of water molecules migrating from hydrophilic site to hydrophilic site within the polymer network. There are advantages and disadvantages claimed for each type; surfactants are believed to cause leaks in microporous films but not solid films, water swells solid films but not microporous films. Generally, solid films have better tear strength and are said to be better barriers to odours and to some microbes.

2.3.2 Polyurethane & Poly Olefin foam

Polyurethane foams are available in various qualities, such as different densities, FR properties, different porosity and other specialist chemical and physical properties. The largest outlet for polyurethane foam is in textile laminates, used for automobile seats covers and for other coverings in the car interior. The usual form is sheet foam with a thickness of anything between 2 and 10 mm or more, laminated to the face fabric on one side, and a lightweight ‘scrim’ fabric on the other side. The polyurethane foam may be either polyester polyurethane or polyether polyurethane. The polyurethane foam is generally more easily laminated by the flame lamination technique. For polyether polyurethane foam chemicals are added to the foam to make this possible. Polyurethane foam develops cracks in hot, humid areas^2,3.

3. Coating Methods:

3.1 Principle of fabric coating^1-5

Coating method can be classified in several ways. Their most fundamental characteristics are the number of layer that can be applied per applicator. Single layer method is applied coating layer at a time and requires several applicator inline to achieve the total number of layer desired in the coating structure. These analyses classify method by number of layer and indicate which give more uniform coating.

Single layer method:

In this type of coating, usually the layer, which is applied on the base fabric, is single coated. This is useful when we required a thin coating to a substrate. It is also possible that by applying single layer, we will get a thick coating. But that time coating solution should be more viscous by nature.

Dip coating: knife coating, air knife coating

Rod coating: blades coating, forward and reverse roll coating

Multi layer coating:

In this type of coating, more than one layer is usually applied on single coated fabric. By using multiplayer coating method we can improve the serviceability, durability even other   coated properties for fabric. Sometimes while applying coating solution on the fabric, then it is difficult to achieve proper property to the fabric because of thin coating so that time it is necessary to apply more than one coating to the fabric to get proper properties by applying thick coating.

Slide coating, curtain coating

 

Basic consideration to select coating method:

None of the basic families of coating technique can be applied for all purpose because coating involve different coating weight, solvent and viscosity achieve though desired quality and functionally of a coating in an economical way .we must select an appropriate coating method ⁽¹⁾

• Number of layer
• Wet layer of thickness
• Viscosity
• Coating accuracy require
• Coating support or web
• Coating speed

3.2 Type of coating^1,2,5,9

The broad classification is based on the form of the coating material as follows:

3.2.1 Lick roll method

In this method, the fabric was passed over the coating roll which was rotated in a trough of the coating liquor as shown in Fig. 1. There were several variations on this theme, which were developed to ensure amore even application of the coating by metering the coating onto the fabric. This was done by two main approaches, the first of which was to use a second roll on the primary coating roll, which picked up a fixed amount. The second was to use a doctor blade on the primary roll, so that again only a fixed amount of liquor was transferred to the fabric. The main disadvantage of these systems was that the amount of coating on the fabric was dependent on the surface tension and viscosity of the coating fluid and also the surface condition of the fabric. To overcome this problem, knife coating was developed.

              

Fig. 1: Other lick roll methods. (a) Lick roll method (b) A metering roll

(c) doctor knife

3.2.2 Direct Coating

Direct method is sometimes called the ‘floating knife’ technique. In this method, the coating fluid is applied directly to the textile fabric and spread in a uniform manner by means of a fixed knife. The thickness of the coating is controlled by the gap between the bottom of the knife and the top of the fabric. The fabric is stretched flat to form an even uniform surface and is transported under a stationary doctor blade. As the fabric moves forward, it is scraped by the knife and the polymer resin or compound is spread evenly over the surface. The way in which this gap is controlled determines the type of machinery used. The following are the main techniques used:

• knife on air
• knife over table
• knife over roller
• knife over rubber blanket

The amount of polymer applied, the ‘add-on’ depends on the concentration of the dispersion or solution – this is the so-called ‘solids content’; other parameters also affect resin add-on.  The add-on is also influenced by the blade profile and blade angle, and by the fabric tension, which determines the intimacy of contact with the fabric.

1. Knife on air

In this method, the knife is positioned after a support table and rests directly on the fabric. In this arrangement, compressive force applied on the coating material is greater, and as such, the coating compound enters the interstices of the fabric. This technique is useful for applying very thin, lightweight, impermeable coatings (as low as 7–8 g/m²) suitable for hot air balloons, anoraks, etc¹. Fig 2 shows Knife on air coating.

Fig 2:  Knife on air coating

1. Knife over roller

In this method, a gap is set between the blade and the surface of the fabric to apply a measured amount of coating. It is also used if the weight of the coating being applied is too heavy for the fabric to support. The position of the blade in relation to the crown of the roller is important, because if it is not directly above it, the situation in effect is similar to the knife on air method. This ‘off crown’ position is in fact useful for coating thick fabrics which have raised surface or pile. Fig 3 shows Knife over roller.

Fig 3: (a) Knife over roller

1. b) Blade on the crown c) Blade slightly off crown position

Fig 4: Knife over table

The main advantage of Knife on air is that any irregularities in the fabric do not affect the running of the machine. However, this is not the case with the knife over table or knife over roll methods for although the coating thickness can be accurately controlled, any fabric faults or joints in the fabric are likely to jam under the blade causing fabric breakage. The problem of metering an accurate amount of coating onto the substrate was finally solved by the use of a flexible rubber blanket, which gives a controlled gap for the coating compound and yet is sufficiently flexible to allow cloth imperfections or sewing to pass underneath the blade without getting trapped and causing break outs. In knife over blanket, the web is supported by a short conveyor, in the form of an endless rubber blanket stretched between two rollers. Because the tension applied on the blanket results in a uniform pressure between the knife and the substrate, the fabric is not subjected to stretching in this arrangement^1,4. Fig 5 shows Knife over Blanket.

Fig 5: Knife over Blanket

Blade profile:

The add-on is also influenced by blade profile and blade angle and by fabric tension. The geometry of the coating knife and the angle of application also have an important role to play in the effectiveness and penetration of the coating. There are three main types of knife profile as shown in fig 6.

1. Pointed blade: the sharper the blade the more of the coating compound is scraped off and consequently the lower the coating weight.
2. Round blade: this gives a relatively higher coating weight than a sharp point.
3. Shoe blade: this gives the highest coating of all the blade profiles; the longer the length of the shoe the higher the coating weight.

Fig 6: Different types of blade

Product from direct coating:

• Waterproof protective clothing fabric
• Automotive car seat fabric
• Tarpaulins
• Lightweight fabric
• Blankets (containing more than 40 layers of direct coating)
• Conveyer belts

3.2.3 Foam Coating:

‘Foam coating’ is related to direct coating which involves preparation of foam using a solution or a water dispersion of the textile chemical to be applied. The foam is direct coated in the same way as described above for compounded polymers. Foam processing was developed in the USA during the late 1970^s as a more environmentally friendly alternative to impregnation or padding with a pad mangle. Instead of fully immersing the fabric in an aqueous bath of the chemical finish and then squeezing out the excess water, the foamed chemical, at the appropriate concentration, is direct coated on to one side of the fabric.

Foam processing is extremely useful for very heavy weight materials such as carpets, which are usually treated with anti-soil finishes such as fluorocarbons.

The fluorocarbon is applied from very ‘dry’ foam, i.e. very low density containing very little water, to the face side of the carpet. If the carpet were completely immersed in water it would be quite impossible to dry off all the water at a commercial speed. Fluorocarbons can be applied to carpets made from all fibers, including polypropylene, in this manner.

‘Stain blocker’ chemicals can be applied at the same time as the fluorocarbon to carpets containing either nylon or wool, to produce excellent anti-soiling properties which are due to the combined action of the two types of finish.

Importance:

• Foam doesn’t sink into the fabric but site on the surface.
• The foam collapse on drying and is not actually visible as a separate layer.
• We can apply it on one side of fabric.
• The actual add on of the chemical finish is 2-3 % on dry weight of good.
• Solid content of foam is very low so it is used to coat the spun yarn.

Advantages of foam coating

• less water to dry off and
• The foam does not sink into the fabric, but sits on the surface. The foam collapses on drying and is not actually visible as a separate layer when dry.

• The actual add-on of the chemical finish is usually of the order of 2–3% or less, on dry weight of goods. This technique can also be used as a method of applying low add-ons of polymers to fabrics.
• Fabrics produced from spun yarns can sometimes be coated in this way, because the ‘solids content’ of the foam is very low and the blade need not scrape the fabric surface as much as in a normal direct  coating on top of a fabric finish may lead to poor polymer adhesion.
• The foam processing method is especially useful for applying finishes to only one side of the fabric the non-coated side.

3.2.4 Foamed & Crushed Foamed Coating:

This method generally apply polymer to the woven fabric, knitted fabric, yarn, fabric of a general open construction, which cannot generally be direct coated.

Importance:

• More softer handle and better drape than direct coating.
• Degree of permeability and breath ability is good.
• Versatile in nature so that many different varieties of filler including pigment can be added to coating formulation to produce different novel properties.

A typical foam coating formulation may contain

Acrylic resin; Water; foaming agent and Foam stabilizers (ammonium stearate), thickening agent; Filler, e.g. calcium carbonate; FR chemical (if necessary);  Cross linking agents (if necessary).

Product:

• All polymer, acrylic, water based rubber, polyurethane could also be used.
• Floor covering, wall covering, black out curtain, and curtain lining.
• Filter fabric
• Fire retardant chemical are incorporated into foam.
• Crushing the foam under very high pressure and temperature will result in a clear film with low breathability but very good waterproofness and high durability. With two or three-layered crushed foam coating, one of the layers being pigmented black the other layer is necessary to prevent to dulling of shade of fabric.

3.2.5 Transfer Coating:

This technique is used for knitted fabrics which compare to oven fabric are open and stretchy and can’t be coated by direct coating method. Because they would distort under the tension applied to obtain a flat surface and here resin would sink into knitted fabric and probably penetrate through to the face and fabric stiffing would accure i.e. loss in tear strength.

Principle: Transfer coating is first to spread to polymer on to the release paper to form a film and then laminate this film to fabric. In this way polymer doesn’t come in top contact with the fabric until it is actually in the form of a film. See fig 9

Procedure: Transfer coating is carried out in number of stages in that firstly, the top layer is applied first to the release paper by a doctor blades and is dried. The base layer is applied over this top layer using second doctor blade and straight afterward, the fabric is laid over the base layer and joined to it by nip roller. The paper with coating and fabric on it then passes into a second oven, which dried and cross link the second layer together.

The base layer sticks to the fabric while top layer, which was applied first to the release paper doesn’t stick to it because of its release property. After the assembly emerges from the second layer oven, the freshly produced coated fabric is peeled off the release off on to batching roller. It higher specification are required, it may be necessary to coat on a higher add on of polyurethane and this will achieved by a three layer coating i.e. inserting an intermediate layer in between top and base layer. Decorative and embossed design can be obtained using embossed paper or by further processing, here only face side coating. Base material is flexible, knitted fabric and resin penetration doesn’t accure. Extremely soft and flexible coated fabric is obtained.

Disadvantage:

• More expensive than direct coating, partly because of added cost of relatively expensive paper and more expensive double headed plant
• In some cases, the release paper can be rolled up and reused but release property deteriorates each time.
• The amount of any raising agent used should be kept to minimum because they cause negative effect on coating.

Product:

• Main outlet for transfer coated polyurethane fabric is in unmarked, waterproof protective clothing. Luggages, footwear, gloves, waterproof mattress cover.
• Produce coated fabric with excellent high visibility appearance.
• Transfer coating are used in upholstery and produce an attractive product.
• 
• Fig 7: Transfer coating 3.2.6 Kiss roll coating:Kiss roll coating is one of the best-known methods, which comes under the direct application method, which has the product to be applied on its surface. This kiss roller is either fed directly with coating liquor, whereby it is immersed directly into the auxiliary.

  The kiss roller rotates either in the running direction of the substrate or in the opposite direction. If they rotate in the same direction, it is conceivable that the peripheral speed of the roller may be greater than the speed of the nonwoven or it may be slower.

  In practice it is common to operate the kiss roller alternatively in both directions. In this way, with the same liquor concentration the add on and the penetration depth can be altered substantially. When the kiss roller rotates in the same direction the liquor normally penetrate more deeply into the substrates tan if it rotates in the opposite direction. The add on is dependent on the speed of rotation. The result can be varied with the same solid content of the liquor by altering the viscosity. By altering the angle of wrap with which the nonwoven touches the kiss roller. It is carried out using low viscosity dispersion or solution.

  

• Fig.8  Kiss Roller coating1) Guide roller        2) Doctor blade            3) Kiss roller3.2.7 Rotary screen coating:

  The rotary screen technique can also be used for coating polymer on to fabric with add on between 5 to 500 g/m². The add on is controlled by resin viscosity, the mesh of the cylindrical screen, the speed and pressure of the squeegee bar inside the screen. The fabric moves at the same speed as the rotation of the rotary screen and there is no any frictional contact between them. When a resin is deposited on to the fabric, the resin flows and merges together to form a continuous coating.

  This method is generally restricted to water based resin due to the problem of solvent wash off facility. Hot melt adhesive in the form of aqueous paste are applied this method is well suited to light weight nonwoven fabric and delicate films such as water resistances and breathable films for apparel.

  The rotary screen coating machinery consists of squeegees, which press the resin in the screen through the perforated screen wall on to the substrate. Light squeegee pressure produces a small amount of resin add on. The add on can be increased by applying higher pressure and also by their positioning of the squeegee in relation to the center of the backup roller. Other factor influencing adds on are the screen type, perforation shape and the size and resin solid content, viscosity and rheology.

  In the vertical down system, two sides of the fabric are being coated in a single pass. The vertical up arrangement is used for coating textile and nonwoven and also used to apply PVC to tarpaulins. Horizontal techniques are used for combination application

   

  Advantages:

  • This technique allows some stretchy fabrics to be coated.
  • Require less fabric tension.
  • Production of coating with soft handle and good drape.
  • The material can be dried, rolled up, stored, and reactivated when required.

  3.2.8 Spray Coating

  Spray coating is also an important technique for coating. Chemicals which are extremely stable are used for spray coating because shear stress during spray is very high.  In theory all type of liquid adhesive can be sprayed hot melt, solvent-based, water-based and high solids versions. In practice, however, hot melts need expensive apparatus to ensure that they do not solidify prematurely or char, solvents present problems of flammability and water-based adhesives may not dry at commercial speeds. In recent years, reactive polyurethane adhesives have been developed which allow high bond strengths with low levels of add-on. Moisture cure polyurethane adhesives do not need a high temperature to initiate cross linking, are available as a jelly and are virtually 100% solids content. Machtex of Holland have specialised in spray lamination equipment over many years and their machines can be used to process materials such as raised velvets or velours which would be damaged by high temperatures and pressure if laminated on a calendar. The problems usually associated with spray applications are uniformity and precision of application, penetration of the fabric substrate, occasional blocking of a spray nozzle, control of the liquid being sprayed usually a solvent and continuous drying of the liquid.

  3.2.9 Calendar Coating

  Calendars are primarily used to produce unsupported films of PVC and rubbers from compounded polymer ‘dough’. This process can also be adapted to apply freshly produced film to fabric. Calendars consist of a number of massive rollers, sometimes five or more in various configurations, which rotate to crush the ‘dough’ and smooth it into films of uniform thickness. The thickness of the film is determined by the gap separation of the rollers, but there is usually a limit to the thinness of films which may be produced by this method. The more rollers, film produced is more accu-rate. Some of the rollers also generate heat, and the material is fabricated into a continuous sheet which can be brought into contact with the fabric to which it adheres.

• Polymer componentFig. 9 : Calendar Coating

  3.2.10 Hot Melt Extrusion Coating

  This method is used for thermoplastic polymers such as polyurethane, polyolefins and PVC, which are applied by feeding granules of the material into the nip between moving heated rollers. There are two general designs in use, the Zimmer machine, which has two melt rollers the Bema machine, which has three. They are smaller version of calenders but differ in that they need a fabric (or paper or a film) as a substrate on which to deposit the film as it is produced. This method can apply resin (in the form of films) to fabric at a faster rate than that can be achieved by transfer coating or direct coating. This process is used to produce light weight coverings or tarpaulins⁹.

  Hot -melt adhesives are thermoplastic and can be melted or softened by heat. When spread on the fabric in the hot state, lamination with another fabric can occur on re-solidification as the coating cools down. A variety of methods can be used with hot melt adhesives. Slot die extrusion allows high viscosity hot melt adhesives to be extruded as a continuous film directly on all types of textiles⁹. Alternatively, roller and calendar hot -melt coating and laminating may be used. Hot melt adhesives are 100 percent solids available in their simplest form as pellets, pillows, granules or bulk filled drums. The adhesive is melted and applied to the substrate via rotogravure, spray or the Porous Coat® technology. The use of hot melt adhesives for laminating textiles is relatively new compared to the other processes discussed in this paper. Recent advancements in application equipment and adhesive formulations have made direct hot melt laminating a viable alternative.

   

   

  Advantages

  • Significant material savings can result by direct application of the hot melt adhesive, thus eliminating the cost associated with cryogenically created powders or formed films and webs.
  • Line speeds are often surpassing those of the “old” technologies.
  • Ovens used to activate dry adhesives are not required in most cases, saving both utilities and floor space.
  • A converter’s adhesive choices are broadened to include dry adhesives (polyester, polyamide and EVA), reactive urethanes, pressure sensitive adhesives and others²².

  Rotogravure

  This process uses an engraved roll to deliver a pattern of adhesive to the substrate. Adhesive is melted with a conventional hot melt applicator or extruder and delivered to a holding pan via a heated transfer hose. The engraved roll is submerged in a pool of molten adhesive which fills the pores or engraved areas of the roll. As the engraved roll turns, the excess adhesive is removed, by a wiping blade, leaving only the adhesive in the engraved areas. A backup roll behind the substrate applies the necessary amount of pressure, ensuring that the substrate and the engraved roll contact correctly for effective transfer of the adhesive to the substrate. A transfer roll can also be used to remove the adhesive from the patterned roll and transfer it to the substrate. Once coated, the second substrate is introduced and the two pass through a lamination station consisting of a hard roll and a rubber coated roll to complete the lamination²².

• 
• Spray SystemsSpray systems combine heated air with molten adhesive to produce continuous filaments through a nozzle (Figure 10).The filaments are applied to the moving substrate. Once the substrate is coated, a second substrate is introduced and the two are permanently joined with the use of a lamination station consisting of two rolls that apply pressure to the laminate.Advantages
  • Modular spray heads allow placement of the adhesive to occur at a variety of distances from the substrate. They also allow the spray system’s coating position within a line to be changed as needed.
  • The non-contact nature of the spray systems can bean advantage when working with temperature sensitive substrates such as polypropylene.
  • Adhesives can be applied at operating temperatures without damaging the substrate.
  • These systems are typically used when low add-on weights (as low as 1 gram per square meter) are desired.

  3.2.11 Process flow diagram of coating

  General process of coating involves application of polymer film followed by drying and then curing. As per the requirement pre- padding or post padding is done. In case of some coating, calendaring is also done. In denim coating biopolishing (enzyme washing) is given to get washing effect.

• 

Fig .10 Process flow diagram of coating

5.1.6 Film Lamination

Laminate film is generally categorized into these five categories: ● Standard thermal laminating films ● Low-temperature thermal laminating films ● Heat-set (or heat-assisted) laminating films ● Pressure-sensitive films ● Liquid laminate⁹

5.7 Faults in Lamination⁹

There some faults can be usually appears in the laminated products during the process. These faults can be removed or partially decreased by a proper m/c setting, controlling different parameters like; heating, temperature, degree of adhesion limit etc. Few of the lamination faults which generally frequently appears in lamination process have been discussed in the table 4 as below:

 

Table 4: List of some common lamination fault in textile finishing

Faults name	Reasons	Remedies
Bubbles	Improper roller nip pressure	Proper roller surface mapping
Particles	Poor mixing of laminating pest	Excellent grinding of the materials in the laminating pest
Age cracks	In stability of the laminating pest/solution/adhesives	Advanced camera technology for highest contrast
Misalignment of edges	Luck of proper supervisation to the process	Easy integration into production lines
Laminating faults	Quick process, less time	Networking for further evaluation

 

6. Application areas of coated & laminated fabrics

Technical textiles are generally recognized to be one of the most dynamic and promising areas for the future of the textile industry and also as an emerging area of development for functional application of textiles. Technical textiles are defined as textile materials and products manufactured primarily for their technical and performance properties rather than their aesthetic or decorative characteristics. With growing dominance of technical textiles, Techtextil, Messe Frankfurt has classified technical textiles into twelve groups from the application point of view as given below table 5 & table 6 shows the field wise end use application of coated & laminated textiles.

Table 5

Type of application	Application area
Agro tech	Agriculture, horticulture and forestry
Build tech	Building  and construction
Cloth tech	Technical component of shoes and clothing
Geotech	Geotextile and civil engineering
Hometech	Component  of furniture, upholstery and floor covering
Indutech	Filtration, cleaning and other industrial application
Medtech	Hygiene and medical application
Oekotech	Environment Protection
Packtech	Packing
Protect	Personal and properly protection
Sportech	Sports and leisure
Mobitech	Automobiles, shipping, railways and aerospace

 

Table 6 ^{19, 25}

S.no.	Field	End use application
1	Agriculture	Bulk containers, Fencing, Seed/ crop covers, Bags, Shade material, Irrigation system, Pond liners, Hoses
2	Construction	Safety fencing, wind covers, concrete curing, safety vests, hoses, conveyer belting, truck covers, drainage ditches, substrate preparation, architectural structural, Tarpaulins Bulk bags
3	Clothing	shoe uppers & lining, artificial  leather/ bags/ belts, rainwear, garment lining, backing/stiffeners, water/ stain repellent, gloves, hats, combining different material
4	Geo textile	Setting pond liners, Irrigation liners, Landfill Liners & covers, Soil stabilizers, Erosion Barriers
5	Home Furnishings	Upholstery, Trim, Carpet Backing, Drapery Backing, Bedding, Artificial Leather
6	Industrial	Conveyor belts, filtration, barrier material , field covers abrasive backing, mechanical rubber goods
7	Sport/ Leisure	Athletic shoes, artificial leather / bags/ belts, rainwear, Sail cloth, Parachute fabric, Ballooning fabric, artificial turf, Tents, Exercise mats, Exercise equipments, Balls, seating, field covers
8	Transportation	Seating/ trims for automotives, truck, aircraft, bus, Airbag, truck covers, hose/ belts , seating, headlining, tyres, Vehicle interiors- textiles often laminated onto interior components such as door panels.
9	Medical	Gloves, hygiene products, Upholstery, body bags, incontinence material, bandage, implants, barrier material, prosthetic device Anti Bacterial Coatings, Waterproof breathable Hydrophilic membranes
10	Packaging	Bulk containers, house wrap, lumber wrap, gas holding, barrier packaging, liquid bulk storage, hauling, waterproof material.
11	Protective	Gloves, cut/slash resistant material, aprons, clean room, chemical/ haz- mat suits, footwear, space suits
12	Interiors	Upholstery- Stain resistance, UV resistance
13	Technical Apparel and PPE	Waterproof Breathable Membranes, Phase Change Materials Fluorescence
14	Apparel	Fashion, luggage and accessories- textured looks such as high shine or ‘wet’ look, PVC/Faux leather

 

7. Testing of coated fabrics ^{1, 2, 6}

Depending on properties desired in particular product, following tests were carried out. Table 7 shows various test standards for coated & laminated textiles.

Table 7: Test Standards for Coated & Laminated Textiles^{1, 6}

S.No.	Test	Test Standard
1	Cone Test	IS 7941
2	Water Repellency – Spray Rating Test	IS 390, AATCC 22 – 1996
3	Adhesion of the Coating to the Fabric	ASTM D751
4	Coating Mass per Unit Area	BS 3424, IS 7016 Part 1
5	Bursting Strength and Puncture Resistance	ASTM D751 and IS 7016 Part 6
6	Hydrostatic Resistance	ASTM D751, BS 3424, IS 7016 Part 7
7	Brittleness Point of Flexible Polymers and Coated Fabrics	ASTM D2137, BS 3424, IS 7016 Part 14
8	Test Methods for Determination of Volatile Loss	ASTM D1203
9	Degree of Fusion (or Degree of Curing) of the Coating	BS 3424, ASTM D4005
10	Wicking Test	ASTM D751
11	Stiffness of Fabric Test	IS 6490
12	Colour Fastness to Dry and Wet Rubbing	BS 3424, IS 1259
13	Flame Resistance Test	IS 11871, BS 3119
14	Abrasion Resistance Test	ASTM D3389, BS 3424
15	Method for Determination of Air Retention	ASTM D5193
16	Damage Due to Flexing Test	ASTM D2097, BS 3424, IS 7016 Part 4
17	Air Permeability Test	BS 3424
18	Water Vapour Permeability Test	ASTM E96
19	Resistance to Permeation by Hazardous Liquid Chemicals	ASTM F739
20	Test Method for Coated Fabrics Used for Oil Spill Control and Storage	ASTM F715
21	Blocking Test	BS 3424, IS 7016 PART 9

 

8. Factors affecting properties of coated or laminated textiles²⁰

The properties of a coated or laminated fabric depend on:

1. a) The type of polymer used – rubber, polyvinylchloride, polyurethane, etc which are specially formulated with additives and compounded into a paste suitable for laminating or coating.
2. b) The nature of the textile substrate – cotton, rayon, nylon, polyester, polyester – cotton blends and glass fibers that may be produced in woven, knitted or nonwoven constructions.
3. c) The coating method employed – spread coating, dip coating, melt coating, lamination.
4. SWOT analysis for Coated & Laminated Textiles^{20, 21}

SWOT Analysis
Strengths Standard quality product. Simple and easy production process. User friendly design. Can be supplied in roll form to the customer. Raw materials and utilities are readily available.	Weaknesses High energy consumption. High production cost. High skilled (trained) workers required. Financial constraints. Storing conditions (inventory). Volatile compound can be produced.
Opportunities New products can be developed. New markets can be created. High consumption trends. Research for new specialist applications. Good profit margins. Overseas access.	Threats Possibility of negative publicity. Environmental protection policy. Injurious to health. Operators’ safety. Social restrictions. Political instability.

 

10. Coating Market Scenario

Coated and laminated fabrics appear in most of the areas of technical textile, especially transportation, industrial, clothing and medical uses. Significant growth is expected in all 12 application areas.

A new report available through research firm Markets and Markets predicts global demand for UV coatings will be worth approximately $7.6 billion by 2019, up from approximately $4.6 billion in 2014 and growing at a CAGR of 10.6%. The study reports value for the market in 2013 was approximately $4.2 billion.

According to the study, Asia-Pacific is the most promising market for coatings and is anticipated to continue into the near future. Asia-Pacific is the largest UV coatings market, which consumed almost half of the total demand in 2013. The demand for UV coatings in the region is estimated to grow at a CAGR of about 9.9% from 2014 to 2019.²²

According to a technical market research report, Paints and Coatings, Global Markets & Advanced Technologies from BCC Research, The market for paints and coatings was valued at $111.2 billion in 2012 and should reach $116.9 billion in 2013. Total market valued is expected to reach $141 billion in 2018 after increasing at a five-year compound annual growth rate of 3.8 %.

The market is broken down into applications of architectural and decorative, industrial, and special purpose coatings. Of these, architectural and decorative coatings accounts for the largest share of the market. Valued at nearly $41.0 billion in 2007, this segment is expected to be worth $44.4 billion by 2012, a CAGR of 1.5%.

Industrial coating has the second largest share of the market. Worth nearly $34 billion in 2007, it is expected to reach nearly $40.0 billion by 2012 at a CAGR of 3.2%. Special purpose coating, valued at nearly $16.0 billion in 2007, will experience growth to $18.7 billion over the next 5 years, a CAGR of 3.0%²³.

 

 

Table 8: Global Sale and Forecast of Various Types of Coatings ($ Billions)

Type of Coating	2005	2006	2007	2012	CAGR % 2007-2012
Architectural & decorative coating	38.65	39.84	41.14	44.36	1.5
Industrial coating	31.89	32.87	33.94	39.75	3.2
Special purpose coating	15.16	15.62	16.13	18.67	3.0
Total	85.70	88.33	91.21	102.78	2.4

 

Coating and Laminating Market Trends: Market Growth is driven by automotive and transportation, protective clothing, awnings/canopies (including signage), and small volume niche markets. Coating and Laminating Market Trends includes following points:

• The industry is global and becoming more so
• Environmental issues will have more impact
• Recycling a major environmental issue
• New techniques and materials will evolve to help resolve problems and create better products
• Alliances and partnerships will be more common
• Increased use of nonwovens
• Greater use of composites²⁴

11. Conclusion

The coating and lamination gives a powerful tool for the advancement of textile technology. It provides the opportunities to produce the special fabrics like water-proof resistant tarpaulins, coverings, large tents and architectural uses, back coating for upholstery including autoseats, Artificial turf,  Food, Medical applications, parachutes, Woven curtains, for heat- sensitive fabrics, automotive fabrics, disposable hospital apparel etc. The recent developments also enhanced the lamination and coating technique into state-of-art process of the future in textile field. Coated Textiles have established themselves as one of the important product in the textile industry and as well as in Textile Global Market. Globalization and the import of coated and laminated fabrics into the UK and USA are likely to continue to reduce prices, and therefore for profitability and indeed for survival, the coating and lamination industry must both develop new product and reduce cost of production and raw material. Coating with modified silica nanosol opens up new opportunities to functionalize and refine textiles. The nano particulate size of the sol particles promote excellent adhesion to the textile fibres, which can be further enhanced by subsequent thermal treatment. Due to very low layer thickness the consumption of coating solution is also very low. Also the preparation of multifunctional coating is possible. So there could be synergistic benefits by keeping in touch with area.

Acknowledgement

The authors of this article are grateful to Director and the management of Wool Research Association for their kind support and guidance.

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8. Sengupta & J. Behera, A Synopsis on Waterproof Breathable textiles & clothing, WTIN Web portal magazine, June 2014
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