The Materials Analyst, Part 6: A lesson in chemical resistance

Evaluating the fitness-for-use of a certain plastic material in different chemical environments can be a difficult task. While a lot of testing has been performed over the years on many different combinations of plastics and chemicals, and this information has been assembled into some good databases, surprises still occur. This is often because other application conditions, such as temperature or applied stress, are not faithfully duplicated in the laboratory testing. In addition, the test specimens are typically standard tensile or flex bars, simple shapes with uniform nominal walls, gates that orient the material in the best possible way, and no weld lines. The real world is not so kind.

Years of experience have taught us that certain material families exhibit "good" chemical resistance while other materials are considered to be "poor" when it comes to this category. It is no coincidence that many of the materials that we consider good in this department are such semicrystalline polymers as nylon, acetal, and polyester. Amorphous polymers such as polycarbonate and polystyrene get a reputation as having poor chemical resistance. But the reality is that all materials have an Achilles heel.

Nylon, for example, is found in many under-the- hood automotive applications where the combined demands of high temperature and automotive fluids present no problem. However, battery components are typically made from polypropylene and the clear stems that permit us to look down inside the battery to check its status have long been made of SAN, an amorphous polymer. The reason is not simply cost. Nylon has a problem in strong acids and the sulfuric acid in the battery would produce significant chemical attack in a short period of time.

Often there are subtle distinctions in chemical resistance even within a polymer family. This turned out to be the root of a problem sent to us one day in the form of some acetal parts that were failing in a food application. The parts were blades in a mixer used to make barbecue sauce. The application had been around for some time and had been working well. The blades were huge; the wall thickness in some areas was more than 1/2 inch. And yet suddenly these parts were dissolving. The pieces that we received were badly pitted and eroded; it was hard to believe that they were the same parts as the good samples that were provided to us. The customer knew that the parts were supposed to be made of acetal copolymer and they suspected that a substitution had been made. Our first task was to identify the material in the failed parts.

The technique we used was differential scanning calorimetry (DSC), since it is perfect for identifying the melting process in a material. If it was acetal copolymer, it would have a distinct, sharp melting point near 165C. If it was another semicrystalline material, we would find a different melting point. And although it seemed unlikely, if by some chance it was an amorphous material, we would find no melting point at all. However, we would discover a glass transition temperature, which would allow us to make the identification.

Figure 1 shows the result of our DSC test on the failed part and also shows a DSC scan of an acetal copolymer. They obviously did not match; the higher melting point was a match with another material, acetal homopolymer.

To those who do not spend a lot of time working with problems of chemical attack, the substitution of acetal homopolymer for copolymer may seem trivial or even may seem to be an improvement. After all, chemical resistance is often associated with crystallinity and the larger area under the melting curve for homopolymer shows that it is more crystalline than copolymer. A comparison of short-term property charts for homopolymer and copolymer shows that homopolymer is stronger, stiffer, and has a higher heat deflection temperature than copolymer. These are all a result of the higher crystallinity in the homopolymer. But as we noted above, all polymers have some vulnerability. In the case of acetals, highly polar materials such as strong acids and strong bases are particularly rough. As it turns out, the chemistry that turns a homopolymer into a copolymer is actually more resistant to these strong polar compounds.

Long-term studies of the two materials in a strong acid or base environment show that the properties of the homopolymer, while they start out higher, fall more rapidly as the polymer is attacked. Even hot water can do damage over time, and it is no coincidence that in plumbing applications, acetal copolymer is the material of choice and attempts to use homopolymer have met with performance problems. In fact, a simple test for telling the difference between acetal homopolymer and copolymer involves placing the material in a strong base and heating it to a high temperature. If the material dissolves in a short period of time, it is a homopolymer; if it survives, it is a copolymer.

So what about our failed mixing blade? Well, it is no secret that barbecue sauce contains a number of ingredients that can be considered acidic. In addition, the blending process is performed at high temperatures. In this case, the combination of conditions was not damaging to the acetal copolymer, but the homopolymer could not survive for very long. What had seemed to the processor like an innocent substitution, or even an enhancement, had instead caused an established application to fail.