In part two of “Understanding Corrosion-Resistant Thermoplastics” we covered the yardstick most often used in determining the the allowable weight as well as immersion tests and what physical property changes to measure.  Here in our final installment, part 3, we’ll wrap up this important series with a dialogue on what chemicals do attack thermoplastics and the issue of stress cracking.

The so-called mineral acids normally attack metals. In the same regard, organic solvents, being similar to the thermoplastic materials, have an equivalent effect on thermoplastics. As a general rule, for the thermoplastic materials dis­cussed in this article, the degree of attack with organic solvents increases in order with the fol­lowing common solvents: Alcohols, ketones, esters, aromatic and chlorinated solvents. Linear polyethylene and polypropylene exhibit some­what more resistance to organic solvents than rigid polyvinyl chloride, modified polyvinyl chloride, and ABS. Attack by organic solvents results in a very distinct softening of the speci­mens and some solution occurs in most instances. The higher resistance of polyethylene and poly­propylene to organic solvents indicates the reason why no suitable solvent cements have been developed for these two classes of materials.

Stress cracking can occur with all the ma­terials covered. However, Type I, Grade 1, rigid polyvinyl chloride and polypropylene have rather rare instances of stress cracking. The stress cracking potential, in increasing order, is as follows: Type II, Grade 1, PVC, modified high impact PVC, ABS compounds, and linear polyethylene. Some surface active materials such as soaps, de­tergents, and other wetting agents, will adversely affect linear polyethylene. Highly oxidizing acids such as high concentrations of nitric and chromic acids will accelerate stress cracking activities on the aforementioned materials. Im­mersion tests for stress cracking environments should include a static loading of the test speci­men. Methods have been outlined for metallic materials and could be applied for thermoplastic materials in the same relative manner, taking into account the applicable differences in working stress.

Higher temperature applications in corrosive environments are similar to those for metals with respect to increased corrosive attack. Corrosive attack will increase at temperatures some 5-15°F below the heat distortion point. Substantial increases in penetration, as well as weight gain, will become apparent as temperatures approach the heat distortion point. This is especially apparent with chemicals whose higher rat­ing drops from room temperature to a lower rating at the higher limit shown. The decreased corrosion resistance tends to coincide with the upper physical operating temperatures with respect to working stress.

Welds made with ABS compounds, linear polyethylene, and polypropylene will be more subject to stress cracking if inert gases are not utilized in the welding operation.

One of the prime advantages of thermo­plastics is, of course, their high resistance to chem­icals and to subsequent corrosion. Extensive data on the chemical resistance of thermoplastics have been gathered over a period of years. This information may be considered as a basis for rec­ommendation, but not as a guarantee. Materials should be tested under actual service conditions to determine their suitability for a particular purpose.

Material suppliers will be helpful in supply­ing as much information as possible on any of their products. It is rather difficult to publish precise corrosion-resistance information because of the many different possible combinations that can affect the results — temperature, the ma­terial and its consistency, stress and strain, con­centration of the attacking agent, and time of exposure.

And don’t forget, Kamweld has an excellent assortment of welders in a wide variety price ranges, we encourage you to check our welders out at Kamweld plastics welders online store