Technical Notes

Chemical Resistance of Polypropylenes

Polypropylene is essentially inert and resistant to a great variety of chemicals at temperatures much higher than those tolerated by other thermoplastic materials. It is unaffected by aqueous solutions of inorganic salts and by practically all mineral acids and bases, even when highly concentrated and at temperatures over 140° F. It resists up to 80% sulfuric acid and concentrated HCL up to 212° F. It can withstand saline solutions at temperatures over 212° F. Its resistance to organic solvents below 175° is also good (see Table 5).
Polypropylene is subject to slow attack by oxidizing acids. If it is to be used in the presence of strong oxidizing chemicals, suitable anti oxidants must be added to prevent degradation.

Effect of Inorganic Compounds

Water and other inorganic environments have essentially no effect on polypropylene. Only oxidizing compounds have a significant effect upon polypropylene. They attack the polymer chain and modify the chemical structure, causing the material to lose its good mechanical properties. This a typical oxidizing phenomenon. Temperature plays an important role in this type of reaction.

Sulfuric acid should not exceed 50% concentration up to 140° F. Concentrated nitric acid, oleum, and chlorosulphonic acid are not recommend for use even at room temperature. Gaseous bromine will react with polypropylene, destroy the crystalline structure, and render the material brittle. Only dilute solutions of free chlorine or bromine do not attack polypropylene.

The effects of chlorites and hypo chlorites depend largely upon temperature and concentration. Salts, even in concentrated aqueous solutions and solutions of alkaline nature, have no effect up to 212° F, provided they are not oxidative substances.

Effect of Organic Compounds

Polypropylene is more resistant to organic chemicals than are most other commercially available thermoplastics. At room temperature, polypropylene is insoluble in practically all organic solvents.

The extent to which such solvents are absorbed by polypropylene depends on a number of factors, notably temperature and the polarity of the solvents. Absorption becomes greater as temperature increases and polarity of the solvent decreases. In other words, polypropylene is inert to polar compounds such as ethanol and acetone, while non-polar solvents such as benzene, carbon tetrachloride, and petroleum ether will cause swelling. Correspondingly, mechanical properties deteriorate with prolonged exposure.

Table 5 can be used to view how polypropylene reacted with various substances at various temperatures.