A coated fabric combines the benefits of the base fabric with those of the polymer with which it is coated. The resulting coated fabric will have many properties, which cannot be offered by either component individually, and careful consideration is necessary to select both base fabric and coating polymer. The base fabric provides the mechanical strength of the composite material and supports the layer of coating applied to it.

The chemical formulation of the coating, the coating thickness and weight, the number of layers, the form of the technical textile and the nature of any pre-treatment (such as to stabilise the fabric dimensions prior to coating) are of great importance.

A fundamental requirement for any coated or laminated fabric is that the coating shall have strong adhesion and cohesion towards the base fabric and it must not separate throughout the life time of the product.

Coating

It is important that the coating formulation is a homogenous mixture and a variety of propeller stirrers (200 to 1,500 rpm) are used to achieve this. In some instances homogenisers (ie, triple roll-mixing mill) may be required to break up larger aggregates, especially in thick coating layers. Where the coating mixture has to be foamed (in case of high quality synthetic leather, ladies bags material), a dynamic foam generator -- which has twin rotors rotating within stators -- may be used.

The intermeshing pins on the rotors and stators and the supply of air/gas cause the formation of a large number of microbubbles. Consequently, a cellular coating layer is formed, usually of the microporous type, if the foamed coating is cured before the bubbles can collapse. High solids coatings generate heat through friction in mixing and hence the foam-mixing unit is water-cooled.

Properties of thermo-plastic polymers as coating materials

Thermoplastic properties of some plastics are useful in that they allow the material to be used as hot melt adhesives and, in certain cases, to be joined by welding techniques. In general terms, for a given chemical type of thermoplastic, the shorter the molecular length, the lower the melting point, and the longer the molecular length, the higher the melting point. Adhesives are generally shorter chain length molecules and melt at relatively low temperatures, for example polyester fibre melts at about 260°C, but there are polyester based adhesives which melt as low as 100°C.

The main thermoplastics used for textile coating are polyvinyl acetate, acrylics, PVC, polyurethanes and synthetic rubbers.

It is important to realise that coating formulations consists of several additives. Amongst these are UV radiation and heat stabilisers, antioxidants, fillers to improve the mechanical properties, fillers for economy, FR chemicals, reinforcement fibres (turning the plastic into a ‘Composite’), pigments and other compounds necessary to confer further special properties or to assist with processing. It is necessary to make sure that all the ingredients are compatible with each other.

Polyurethanes generally have good adhesion to fabric substrates and have good elongation and excellent flexibility, even at very low temperatures. They have excellent surface abrasion properties and, in fact, are used as lacquers for leather. Acrylics are extremely versatile materials with many variants and price ranges. They have good clarity and good UV resistance. Some low temperature cure acrylic resins are available, which will result also in energy savings. Some of the essential functional properties of coating chemicals are shown below in Table 1.

Table 1. Properties of Coating chemicals

Some of the useful applications of various coating polymers are described below in Table 2.

Table 2. Coating polymers & their applications

Polyvinylchloride (PVC)

PVC is an extremely well researched material that can be compounded to produce very many different properties. It is the most used material in fabric coating, is economical and has a high degree of inherent FR properties. However, it has come under attack from environmental pressure groups and alternatives are being researched. FR properties of PVC can be improved by blending with polyvinylidene chloride (PVDC).

Natural and synthetic rubbers

Rubbers are polymers and are closely related to plastics. They are used in combination with textiles in many end uses, and indeed the fabric coating industry is in some ways an off-shoot of the rubber industry. There are various types to suit different applications, and they are versatile in that they can be blended together and they will accept additives to provide specific properties.

There are a number of specialist rubbers including nitrile rubber, butyl rubber, polychloropene, the best known of which is Neoprene (DuPont) and chlorosulphonated polyethylene, the best known of which is Hypalon (DuPont). SBR rubber latex is an inexpensive and versatile material and has many applications where economy is important, eg, carpets. These rubbers are used widely in fabric coating applications.

Lamination

A laminated (or combined) fabric consists of two or more layers, one of which is a textile fabric, bonded closely together by means of an added adhesive, or by the adhesive properties of one or more of the component layers. Conventional laminated technical textiles normally consist of one or more textile substrates that are combined using a pre-prepared polymer film or membrane by using adhesives or by using heat and pressure. Usually, the layer in a laminated fabric consists of a polymeric substance; however, in some metallised fabrics the metal is not deposited by chemical deposition but is laminated using an adhesive or by use of an electric arc.

Film lamination

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. Films can be used to produce reflective surfaces with efficiencies much higher than can be produced with a fabric. However, they are generally easily torn and easily punctured and damaged by flexing, but some of these physical limitations can be overcome by lamination to a fabric, and the two components combined can offer a wide variety of useful properties.

Film production is a large volume industry, which uses many different variants of a relatively small number of polymers, produced mainly for packaging. Depending on the method of manufacture and starting material, films can be very inexpensive, such as polyethylene, or extremely expensive, such as Kapton (DuPont). Film coating is widely used to confer heat sealability, low gas permeability, printability and other specialist properties.

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.

Polyurethane lamination

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 form 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. This tri-laminate is used by car companies in the world, and is in this form to produce a material which is soft to touch and which never creases or ‘Bags’. The polyurethane foam may be either polyester polyurethane or polyether polyurethane. Polyurethane foam/textile laminates are also used in the footwear industry and in speciality items such as waistbands, baby diapers and personal hygiene products.

Polyolefin lamination

Polypropylene and polyethylene foams are available in roll form. Polyolefins have excellent microbial, oil, solvent and chemical resistance and can be fabricated by vacuum forming and moulding techniques to produce smooth, well defined contours in articles such as automotive interior components. These foams produce a soft touch to headliners, dashboards, door casings and sunvisors, and they also contribute to heat, noise and vibration insulation. The soft touch is slightly firmer than polyurethane foam and is considered by some in the automotive interior industry to be more ‘up market’. Polyolefin foams have certain advantages over polyurethane foams, such as no ‘fogging’ in automobiles and better chemical resistance and ageing properties, but adhesive lamination requires some careful consideration owing to the inert nature of polyolefins.

Coating formulations

Formulations of water-based coating polymers

Water-based polymers used for fabric coating include polyvinyl acetates, acrylics, polyurethanes, silicones and rubbers. In many cases with water-based resins, a single polymer may be used for a particular application or layer of a multi-coat coating, but different grades can frequently be mixed to obtain the required properties. For example, a very soft resin may be mixed with a harder one to soften the harder one one or vice-versa.

For direct coating, it is necessary to thicken the polymer to a viscosity appropriate to the fabric construction, resin add-on required, and apparatus to be used, including blade profile. A very broad range of viscosity is used, between say 5000 centipoise and 40000 centipoise or even higher. Apart from thickening agents, other additives may be fillers such as chalk or talc for economy and FR agents if required. In actual mixing of components, it is essential that the thickener, usually diluted with water, is added with careful stirring to the polymer and not the other way around – which will cause lumps to form.

Stirring is always started slowly, and speeded up when a stable mix is obtained. Before any bulk mixing of polymers and compounding ingredients is undertaken, it is vital that small scale mixes are first carried out in the laboratory to assess compatibility, stability and any other problems which may arise. There are many different water-based polymers, which may be mixed together to produce a particular property. In addition, there are whole ranges of textile chemicals, which may also be mixed into the compound, eg, anti-static agents, humectanes (moistrusing agents), softening agents, lubricants, etc. Certain waxes, paraffin dispersions or lubricants can also be used as detackifying agents for soft resins.

Thickening achieved by very gradual addition of ammonia with stirring viscosity approximately 15000 centipoise with Brookfield Viscometer.

Excessive amounts of foaming agent may lead to poor abrasion resistance and lowering of other physical properties. Ammonium stearate can migrate to the coated surface and produce ‘Blooms’, a discolouration effect which may be unacceptable in some applications. More effective foaming agents are based on disodium N-octadecyl sulphosuffinamate; these are more expensive, but can be used in combination with the less expensive ones to provide a boost. In case of black-out curtain coatings minimum three coatings are required. First coat is carried-out using PU/PVC/acrylic coating along with white pigment & subsequent coatings using black & white pigment.

A typical formulation is as given below:

First coat:

Acrylic/PU/PVC binder
Titanium dioxide
Glycerin
Sodium laury/Ether sulphate
Ammonium Nitrate
Ammonium Stearate
Metamine Resin
Liquor Ammonia
Thickener

Coat the foam paste and dry & cure at 95°C.Then allow second coat using black pigment, dry & cure at 95°C and again apply third white-coat to hide black coat – dry & cure at 95°C.

Formulation of solvent-based coating recipes for polyurethane

The solvents used in polyurethane coatings are ethylacetate, DMF (di-methyl formamide), MEK (methyl ethyl ketone), toluene and iso-butyl alcohol.

Table 3. A typical recipe for waterproof coating using polyurethane solvent-based resins

A typical formulation for a waterproof coating on polyester could be as shown above in Table 3.

Formulation of PVC coating recipes

The main commercial outlet for plastisols in fabric coating is the production of tarpaulins and other associated covering material. Raw PVC is generally supplied in powder form and to convert this to a useful material for coating, it is necessary to prepare a plastisol, which is a complex mixture of PVC, plasticizer and several other specialist chemicals – all required to do a specific job. Great care is needed because the chemicals selected must not have any harmful effects during either mixing or use.

A typical formulations of PVC based coating recipes is shown below:

PVC                            – 40% 
Filler (CaCO3)            – 35%
Primary Plasticizer        – Dioctyl Phthalate
Secondary Plasticizer   – Carboxy Pardtin Wax
Solvent                        – Methy lethy/Ketone/DMT

To produce a pore-like structure or a very flexible coated material, a ‘Blowing agent’, such as an azo- dicarbonamide (decomposition range 190 - 230°C), is mixed into the plastisol. These agents decompose during processing to produce small voids in the coating giving a sponge-type structure and a soft touch to the surface. The factors to consider in choice of blowing agent are decomposition temperature, particle size distribution and volume of gas liberated, because these variables will determine the pore structure and other properties.

Rubber coating formulations

This is a very large and specialised subject area, because there are so many different types of rubber and additives used in many thousands of applications. There are over a dozen different grades of Neoprene (DuPont polychloroprene rubber) alone, for example, and there are very many different ingredients in a recipe, each ingredient being necessary to do a specific job or to provide some specialist property.

The essential constituents of a rubber compound are: the rubber itself, vulcanising agents, fillers, oils, antioxidants and plasticizers. Frequently, inclusion of one particular chemical improves one property at the expense of another, and a balance must be reached. Finally, the mix must be processable on the apparatus available, produce the specified properties and be at the correct price to be commercially viable.

The essential component is the vulcanising agent, or activator, the most common one of which is zinc oxide. In practice, this universal vulcanising agent is usually used together with an additional agent to ‘Tailor’ the properties to meet the requirement.

The fillers are powders and the processing aids, which may also be plasticizers, are needed to disperse the solid particles and to help control the viscosity. Fillers are sometimes referred to as black or white, the black type being invariably carbon black.

Antioxidants are necessary to protect the rubber from atmospheric oxygen and to improve the ageing properties. Here again, careful selection is important in order to prevent discolouration and other adverse effects.

A typical basic formulation would include:

Rubber polymers
Vulcanising agent/sulphur
Zinc oxide
Accelerator (Catalyst)
Fillers
Antioxidant/Stabiliser
Softener/Lubricant

There are numerous other ‘Standard’ formulations depending on the type of rubber and the end use properties required. Some recipes may contain 20 or more different ingredients.

Adhesive Lamination-Aqueous based

Adhesive lamination can be used to laminate two fabrics by applying an aqueous-based pressure-sensitive adhesive by knife-over-roller spreading. Alternatively, the pressure-sensitive adhesive can be spread on a release paper and then transfer coated to the textile material, which can then be combined with a second fabric by bringing these into contact under heat and pressure to remove the water. Typical of the adhesives used are natural and synthetic rubber, styrene-butadiene resins (SBRs), polyvinyl alcohol and acrylic polymers.

Solvent-based adhesives can be used to laminate microporous membranes to textile fabrics to provide a barrier against liquids (for use in hospital theatre gowns and to protect mattresses, for instance). A solvent-based polyurethane that cures in the presence of moisture is sprayed on the fabric and the membrane is nipped against the adhesive surface. Then the two are held together while cross-linking takes place to form the necessary bonding. Engraved roller or screen-printing techniques may also apply solvent-based adhesives.

Typical recipe for lamination adhesive based on PVC is as follows:
PVC (Solvent)                                     – Methe/ ethy / ketone
Plasticizer (both primary & secondary) – CPW based

Hot-Melt Coating/Lamination

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 resolidification 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 textiles. Mixing the hot-melt adhesive with air inside a spray nozzle is used to generate discontinuous coatings. Alternatively, roller and calendar hot-melt coating and laminating may be used.

Hot-melt adhesive, ground down to 60 - 200 [mu]m particle size can be applied from an engraved roller (at 200°C or above) in a predetermined pattern to a heat-stable fabric. Then a second fabric or film is brought into contact while the adhesive is still molten.

Table 4. Hot melt adhesive chemical types

Important information regarding manufacturers and suppliers

List of prominent manufacturers for coating & lamination machine, suppliers of coating chemicals, stabilisers, pigments and plasticiser is easily made available here, for reference of readers.

Manufacturers of coating and laminating plant

Some of the prominent manufacturers for coating & lamination machine are enlisted in Table 5.

Suppliers of coating chemicals

Suppliers for coating chemicals are listed in Table 6.
Suppliers for stabilisers, pigments, plasticiser

Suppliers of stabilisers

1. JV Organics
2. CIBA

Suppliers of pigments

1. Sudarshan, Mumbai.
2. Pidilite, Mumbai.
3. Meghmani, Ahmedabad.
4. Clariant, Mumbai.

Suppliers of plasticizer

1.Dioctyl phthalate; KLJ – Vapi.

Summary

The formulation of a coating contains a wide range of chemicals depending upon the nature of the polymer, the necessary additives for the specific end use, whether the coating has to be foamed prior to application, and the type of coating machinery to be used. Coatings may be coloured, translucent or opaque.

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. Cheaper fabric structures may be coated or laminated to provide higher added value to end-users and higher profit margins to manufacturers.

Acknowledgement

The authors gratefully acknowledge the permission given by the Management of MANTRA for publishing this article.

References

1. ANON: High Performance Textiles, 1993 February, pp 11.
2. M M Hirschler and R R Poletti: Coated Fabrics, 1984, pp 90-94.
3. A K Mukherjee, R K Mohanty et al: Adhesive Binders for Nonwoven Fabrics, International Conferene on Nonwovens; The Textile Institute, 1998, pp 70-86.
4. A E Meazey: Binders Used in Bonded Fibre Fabric Production, Nonwovens, 1971, pp 63-71.
5. Nair G P and Pardhan S P: Colorage, 2006 , 53 (10 ), pp 96.
6. A R Horrocks and S C Anand: Coating of Technical Textiles, Handbook of Technical Textiles, 2000, pp 173-179.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal November 2008 issue.

Dr S R Naik
Man-Made Textiles Research Association (MANTRA),
Surat, Gujarat.

Rakesh S Chauhan
Man-Made Textiles Research Association (MANTRA),
Surat, Gujarat.