It is generally known that pipes and fittings in thermoplastic material are widely used in industries where conveyance of highly corrosive liquids and gases requires high-quality construction materials, featuring excellent corrosion resistance. Stainless steel, coated steel, glass and ceramic materials can often be advantageously replaced by thermoplastic materials, ensuring safety, reliability and economic benefits under similar operating conditions.
Chemical Attack on Thermoplastics & Elastomers
Chemicals that attack polymers do so at differing rates and in differing ways. There are two general types of chemical attack on polymer:
- Swelling of the polymer occurs but the polymer returns to its original condition if the chemical is removed. However, if the polymer has a compounding ingredient that is soluble in the chemical, the properties of the polymer may be changed because of the removal of this ingredient and the chemical itself will be contaminated.
- The base resin or polymer molecules are changed by crosslinking, oxidation, substitution reactions or chain scission. In these situations the polymer cannot be restored by the removal of the chemical. Examples of this type of attack on PVC are aqua regia at 20°C and wet chlorine gas.
Factors Affecting Chemical Resistance
A number of factors can affect the rate and type of chemical attack that may occur. These are:
- Concentration: In general, the rate of attack increases with concentration, but in many cases there are threshold levels below which no significant chemical effect will be noted.
- Temperature: As with all processes, rate of attack increases as temperature rises. Again, threshold temperatures may exist.
- Period of Contact: In many cases rates of attack are slow and of significance only with sustained contact.
- Stress: Some polymers under stress can undergo higher rates of attack. In general PVC is considered relatively insensitive to “stress corrosion”.