Moplen chemical resistance

Moplen polypropylene resins, like most polyolefins, are highly resistant to solvents and chemicals. The results of extensive laboratory and actual field installation tests of polypropylene's chemical resistance are reported in this publication, which is updated at intervals.

The chemical resistance data presented here is based on ASTM D543. Unstressed specimens resin, 3 in. long by 0.025 in. (76.2 mm x 0.635 mm) thick, in the shape of dumbbells were used. Results are reported after 1- month immersion. As it is difficult to create actual service conditions in the laboratory, the results of many of the environments should be taken only as an indication of behavior in service.

Moplen polypropylene resins have outstanding resistance to water and other inorganic environments. In most aqueous environments, its weight increase is less than 0.2% when it has been stored for 6 months at ambient temperatures. When the temperature is increased to 60°C (140°F), the weight increase is less than 0.5% for a similar period. According to ASTM D570, its 24-hr water absorption rate is 0.03%. It resists most strong mineral acids and bases, but, like the other polyolefins, it is subject to attack by oxidizing agents.

Moplen polypropylene resins are appreciably affected by chlorosulfonic acid and oleum at room temperature, 98% sulfuric acid, 30% hydrochloric acid, and 30% hydrogen peroxide at 100°C (212°F). They are also affected by 98% sulfuric acid at 60°C (140°F) and fuming nitric acid and liquid bromine at room temperatures. Under strain, failure could occur with strong oxidizing acids at temperatures lower than those mentioned. With few exceptions, however, inorganic chemicals produce little or no effect on Moplen resins over a period of 6 months at temperatures up to 120°C (248°F).

The permeation resistance of Moplen polypropylene resins to organic chemicals depends on the rate and extent to which absorption occurs. This, in turn, affects the suitability of the resin to serve in a particular environment. When the resin is removed from the environment, evaporation will take place and cause it to return almost to its original dimensions. Property changes resulting from the absorption will be reversed if evaporation is complete.

Temperature and polarity of the organic medium are the foremost factors in determining the extent of absorption by polypropylene. Absorption becomes greater as temperatures are increased and polarity of the medium is decreased. Polypropylene copolymers swell more than homopolymers, indicating greater absorption. Such nonpolar liquids as benzene, carbon tetrachloride, and petroleum ether have a higher absorption rate with polypropylene than polar media such as ethanol and acetone.

Some reduction in tensile strength and an increase in flexibility and elongation-to-break in tension can be expected, depending on the nature and amount of the organic medium absorbed.

Moplen polypropylene resins have excellent resistance to environmental stress-cracking. When they are tested according to ASTM D1693 the brittle fractures that occur with certain polyethylenes in contact with polar organic liquids, detergents, and silicone fluids are not observed. Failure of this type with polypropylene is rare. Those environments known to cause such cracking to polypropylene are 98% sulfuric acid, concentrated chromic/sulfuric acid mixtures, and concentrated hydrochloric acid/chlorine mixtures.

The useful life of Moplen polypropylene resins at elevated temperatures is limited by oxidative degradation. The expected life of polypropylene at any given temperature is also determined by the nature of the environment, and by the extraction of some of the antioxidant system. Any environment that tends to extract the antioxidants may lead to more rapid breakdown of the polypropylene, especially at elevated temperatures.