Plastic material selection: The four performance killers

By: 
September 18, 2015

This is part of an ongoing series of articles on materials selection authored by Eric Larson, a mechanical engineer with 30 years of experience in plastics design, for PlasticsToday. His most recent book is Plastics Materials Selection: A Practical Guide .

September is often a month when we think about performance. We have third-quarter financial results. In the United States, we have the pennant race in baseball, and also the start of the football season. Fans will spend hours debating the merits of their fantasy football team, as they try to predict performance for the coming months.

For those of us involved in new product development, September is often the month when new projects get kicked off. As product specifications are being developed, performance criteria are also being evaluated. When it comes to products made from thermoplastic materials, the actual in-use performance depends not only on the design of the product, but on the material that is being used. Furthermore, how that material performs depends on the conditions under which it is used. The end-use conditions can usually be grouped into four main areas: temperature, chemicals, radiation and time. Exposure to any of these conditions - or to a combination of them - can wreak havoc on the performance of thermoplastics. I like to call these conditions, The Four Horsemen of the Plastic Apocalypse.

Temperature

All thermoplastics, by definition, soften (and/or melt) at high temperature. The exact temperature at which this occurs will vary depending on the material, but even at lower temperatures, long-term exposure to heat will have a detrimental effect on a thermoplastic material. The primary reason is that this exposure to heat causes a breakdown of the polymer chains, resulting in a lower molecular weight distribution, and a loss of properties. The most common losses are in elasticity and toughness, but other properties are affected, as well. The temperature at which this degradation begins to occur will vary, depending on the chemical family of the polymer, as will the exact chemical mechanism involved (oxidation, depolymerization and so forth). Occasionally, this degradation can be reduced through the use of additives known as heat stabilizers. Polymer degradation will still occur, but at a higher temperature, and at a lower rate.

At the other end of the temperature spectrum, thermoplastics are also affected by extreme cold. Most of this effect is seen in brittleness, in that there is a loss (sometimes a complete loss) of ductility; even low stresses will cause brittle fracture.

Chemicals

Like many materials, thermoplastics are often susceptible to chemical attack. Normally, when we think of chemicals, we think of nasty things such as acids, solvents (paint and lacquer thinner, acetone and toluene, for example), gasoline and other fuels, or detergents and cleaning solutions. But there are also chemicals in all sorts of things that we encounter in our daily life, ranging from suntan lotion to moisturizers to lip balm, even water.

 Most of us think of water as an inert material, but for some materials, such as raw iron, exposure to water causes an immediate chemical

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