PVC Coated Fabrics

PVC fabrics are of two types, both being plasticised PVC.

Unsupported PVC
- PVC film or sheeting
Supported PVC
- PVC coated fabrics - fabric may be nylon, polyester, poly vinyl alcohol, more exotic types such as aramid or natural fibre fabric.

In the second case we need to consider the effect of chemicals on both the PVC and on the support fabric.

PVC compounds are intimate mixtures of polymer, stabilizers, lubricants, pigment, fillers plus plasticisers in the case of flexible PVC. Each of the ingredients in a compound will behave differently in the presence of a chemical reagent, so the chemical resistance of a compound must be considered in terms of the components present, however the effect of plasticisers is by far the most important.

Unplasticised or rigid PVC is a very chemical resistant material and is often used in the construction of chemical plant equipment.

Resistance of Plasticised PVC

Plasticisers are incorporated in PVC compounds so as to confer flexibility and softness. Plasticised compounds can cover a very wide range of flexibility and softness and also differ in other respects e.g. tensile strength and resilience, according to the type and/or amount of plasticisers used. For example, the type of plasticiser used will affect the chemical resistance, but in this paper it has been assumed that a commonly used, fairly resistant plasticiser such as dioctyl phthalate (DOP)) would be present.

The amount of plasticiser incorporated will also affect the chemical resistance, an increase in plasticiser content leading to a deterioration in chemical resistance because the plasticiser is less resistant to attack than is the PVC.

The chemical resistance to be expected from plasticised PVC when DOP is used as the plasticiser is summarised in the following paragraphs. (Refer also to the tables of chemical resistance of PVC).

Acid and Alkalis

Dilute acids and alkalis have little effect at room temperature, but at elevated temperatures some hydrolysis and extraction of plasticiser may occur. Concentrated acids and alkalis hydrolyse plasticisers slowly in the cold and more rapidly when heated.

Organic Liquids

The main effect of organic liquids on plasticised PVC is to extract the plasticiser and this results in some hardening, particularly when the PVC is removed from contact with the liquid. The compound may become rigid and less tough.

Most organic solvents will extract plasticiser and give rise to these effects, but with aromatic and chlorinated hydrocarbons, aliphatic and aromatic nitro compounds, ketones, aliphatic and cyclic ethers, this plasticiser extraction is accompanied by a softening of the PVC and the overall effect is difficult to predict.

With certain solvents too, the plasticiser extracted is replaced by the solvents so that the compound remains flexible provided it remains in contact with that liquid. On allowing the solvent to evaporate the material will stiffen and will not soften on re-immersion.

The undesirable effects previously mentioned which can occur with PVC compounds containing a monomeric ester-type plasticiser, such as DOP, can be prevented or reduced by the use instead of a polymeric type of plasticiser which affords a higher resistance to extraction. Such plasticisers include polypropylene adipate (PPA) and polypropylene sebacate (PPS), the former being of value in contact with oils and fuels and the latter showing a high degree of resistance to soap and detergent solutions.

Solids/Powders

As well as liquid chemicals, the following dry chemicals will extract plastisiser from flexible PVC, so are not recomended for direct contact:

activated carbon			pentaerythritol
calcium carbide			masonry cement
agricultural lime			silicone carbide
ferric oxide			terephthalic acid
roasted ground coffee			alumina
kaolin			cement
magnesium oxide			zinc oxide

The fineness of the powder will affect the rate of extraction, as would be expected. Refer also to the table from Hoechst Trevira for the effect of other dry chemicals.

Laquer Coating

PVC coated fabrics often have a topcoat of a lacquer to improve the cleanability and chemical resistance of the fabric.

Acrylic lacquer gives improved cleanability, reduces plasticiser loss and improves weather resistance. It may also give some improved chemical resistance.

Fluorinated lacquers are more expensive and difficult to weld, but give much better resistance to dirt, chemicals and the weather. In some cases fluorinated polymer coatings have been used as a top coat, giving even better resistance.