The most widely used plastic in pipe installation is unplasticised polyvinyl chloride (PVC-U). It is highly suitable for both interior and exterior applications as well as for buried pipelines. It has excellent chemical resistance that, combined with smoothness of bore, eliminates build-up of scale and gives good flow characteristics that remain constant throughout its working life. Being odourless and tasteless, it is suitable for conveying drinking water and many food products: it has good abrasion resistance and weathering qualities and affords good thermal and electrical insulation. PVC-U is light and clean to handle and can be easily jointed.

The excellent chemical resistance of polyvinyl chloride (PVC) makes it particularly suitable for a wide range of applications in which this property is of primary importance. PVC has become accepted as an attractive material for the construction of safe and durable plant equipment. The rigidity of PVC in its unplasticised form is a useful feature in this application, as is its inherent fire resistance. However, one limitation is that because of its thermoplastic nature, PVC will soften and subsequently degrade when exposed to excessive heat, but it can be used with confidence at temperatures up to 60°C.

PVC tanks and pipes are used for strong and conveying corrosive materials eg. Alkalis and acids. PVC pipes are also used to convey other materials of a less corrosive nature through plants where there are corrosive atmospheres, or through corrosive soils in which the pipe lies buried. For example, PVC water pipe and conduit are used to carry water and electrical wiring through plant areas. Other items of plant and laboratory equipment such as ducting, effluent drains, extraction fans, fume cupboards, bench tops and protective clothing are also often made of PVC. The resistance of PVC to corrosive soils has led to its introduction into underground piping applications.

Chemical attack on PVC differs from the attack on metals. With metals and other materials, damage is usually confined to the surface and involved

corrosion and a loss in weight. When attack occurs on PVC, however, it generally involves absorption of the chemical reagent, leading to swelling and softening, and a gain in weight. In extreme cases, loss of weight may occur when the surface os the PVC is dissolved or becomes so swollen or decomposed that it crumbles away.

Resistance of unplasticised PVC (PUVC-U)
The type of behaviour to be expected for PVC-U on the basis of laboratory tests is summarised in the following paragraphs with additional information based on practical experience for the fabricated articles being given in some instances. It should be noted that a PVC is not normally recommended for use above 60°C. The tests quoted were not carried out above this temperature.

Water and dilute solutions
At elevated temperatures PVC-U absorbs water and this causes slight swelling. The effect is reduced if the water contains a solute. This slight swelling does not constitute unacceptable attack, and no failure has been observed due to contact with water or aqueous solutions for PVC-U pipe of the type, which conforms to the relevant British standards. Pipe, which has been service for some years, has not failed at temperatures up to those at which softening, expansion and other independent effects can occur. Allowance must be made for this slight swelling which occurs as a result of the absorption of water.

Acids
PVC-U is highly resistant to strong acids, although some oxidising acids in high concentration attack it. Hydrochloric acid can be used at all concentrations at temperature up to 60°C. Sulphuric acid at less than 90% concentration has no effect at temperatures up to 60°C, but acid of 90-95% concentration should not be carried at temperatures in excess of 50°C. Cold nitric acid is satisfactory at concentrations up to 50%, but hot concentrated nitric acid attacks PVC-U.

Alkalis
PVC-U is satisfactory in the presence of alkalis at all concentrations at temperatures up to 60°C

Halogens
Dry chlorine gas does not attack PVC-U at room temperatures but there is some attack at elevated temperatures or if the gas is moist. Bromine and fluorine, even in low concentrations, will attack PVC-U at room temperature.

Oxidising agents
PVC-U is resistant to all, but the most serve oxidising condition. Hydrogen peroxide at all concentrations has no effect, and even concentrated solution of oxidising salts such as potassium permanganate cause only superficial attack.

Reducing agents
These reagents have practically no effect on PVC-U at temperatures up to 60°C

Organic liquids and vapours
PVC-U is resistant to most oils, fats, alcohols and petrol.

General
Generally PVC is unsuitable for use in contact with aromatic and chlorinated hydrocarbons, ketones, nitro compound, esters and cyclic ethers, which penetrate the PVC and cause marked swelling and softening. Some petrol based fuels containing benzene cause swelling. Their penetrating solvents may be harmful to PVC even when dilute, but as they are diluted further their effects fall off noticeable, and very low concentration, such as are present in effluence, can be safely handled.

The consideration of the suitability of PVC pipe for conveying gas falls under this heading. Aliphatic hydrocarbons can be safely handled, but aromatic hydrocarbons can cause unacceptable swelling of the PVC even by absorption from the vapour phase. There is a limiting concentration of any aromatic constituent of the gas to be conveyed, below which the degree of attack will not render the use of PVC pipe unacceptable. The advisability of using PVC pipe for conveying gas needs careful consideration based on detailed information concerning the constituents, particularly any aromatic constituents, of the gas.

Where natural or manufactured gas, which has been treated for the recovery of aromatic vapours, is considered, there should be a sufficiently wide margin of safety to permit the use of PVC-U pipe.