Chemical analysis of textile coatings and membranes

Coated and laminated textiles usually consist of a textile substrate, which will typically be a woven, knitted or nonwoven fabric, combined with a thin, flexible fdm composed of a natural or synthetic polymeric substance. A coated fabric is one in which the textile substrate has a polymer film applied directly to one or both surfaces as a viscous liquid in a solvent or water, the thickness of which is controlled by application via a blade or similar aperture. A transfer coated fabric is an intermediate product in which a thermoplastic film is first prepared on a release paper prior to thermally bonding to the textile substrate. A laminated fabric usually consists of one or more textile substrates that are combined with a pre-prepared polymer film or membrane by adhesives or heat and pressure (Hall, 2000). 

A wide range of polymers which form thin, flexible films are used. They can be split into two categories, polymeric elastomers and rubbers. The polymers include polyurethanes, polyvinyl chloride, polyvinylidene chloride, polyethylene, polytetrafluoroethylene, silicone elastomers, polyacrylates, and chlorinated and chlorosulphonated polyethylenes. 

The first step in the production of complex polymers is to prepare segmented prepolymers by coupling a hydroxyl-terminated polyester or polyether to a polyfunctional isocyanate. This extends the molecular size through the urethane linkages. 

A typical polyester could be poly (diethyleneglycol adipate) with a molecular weight of between 2000 and 3000. A typical polyether is obtained by polymerisation of propylene oxide, usually in the presence of a small proportion of glycerol or sorbitol to provide branched structures. Two of the most commonly used isocyanates are 2,4- and 2,6- tolylene di-isocyanate (TDI). These are chosen because of their reactivity, cheapness and relative low toxicity. The resulting prepolymer has -OH terminal groups when a deficiency of isocyanate is used, but -NCO terminal groups if an excess of isocyanate is employed. 

This is similar to PVC, but made from the monomer 1,1-dichloroethylene (CH, = CC I j. It has a specific gravity of 1.67-1.71 and a degree of polymerisation of over 200. The molecular weight of commercial polymers is about 20 0(X). It is often used in the form of a copolymer with vinyl chloride or ethyl acrylate to improve its properties. Coating formulations are plasticised with highly chlorinated aromatics, as common plasticisers are ineffective. 

---