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 reaction. Fortunately, most thermoplastics do not chemically react with water. But there are some thermoplastics, such as nylon, which absorb water. This absorption process, which is fully reversible, causes the material to swell, and also acts as a plasticizer, making the material tougher, more flexible, more ductile, and reducing its strength. 

Water can also act as a solvent for other chemicals. In those situations the exposure to water is not the issue, it is the chemical(s) contained in the water. So whether the water is used for irrigation or potable use (or other), it is important to understand its source-a well, river, stream, lake, dam, the ocean-and the chemicals and minerals it may contain. Even tap water can contain chemicals, as municipal water treatment centers in various parts of the world frequently treat water to remove pathogens. So if you are selecting a material that will be in contact with water, you need to be aware of the chemicals that could be in that water.

How a specific plastic is affected by exposure to a given chemical depends on a number of variables. First and foremost is whether the material reacts with that chemical. It may be completely impervious to that chemical, no matter what. Or it may be unaffected at low temperatures, but affected by exposure at high temperatures. Then there is the relative concentration of the chemical; whether the exposure is constant or intermittent; and the duration of the exposure. Finally, there is the chemical mechanism involved. Is the chemical acting as a plasticizer, and if so, is it a reversible action, or permanent? Is the chemical causing an oxidation reaction, polymer degradation, or simply a discoloration of the surface?  

Radiation

Another end-use condition that affects thermoplastics is radiation. Most people think of the term radiation as it pertains to radioactivity-a material that emits particles and energy as part of nuclear decay. But radiation is a much broader term, and describes the process by which electromagnetic waves travel through space. 

Electromagnetic (EM) waves are a form of energy that is composed of an electrical field and a magnetic field. These waves can have a wavelength as small as 1 picometer (10-12 meters) to as large as 100 megameters (106 meters, or 1,000 kilometers). This range of wavelengths, commonly known as the electromagnetic spectrum, begins with gamma rays (at less than 10 picometers), and includes x-rays, ultraviolet UV light, visible light, infrared, microwaves and radio waves.

The amount of energy carried by these waves decreases as the wavelength increases. Gamma rays carry the most energy, followed by x-rays, then UV light. In physics, EM waves are collectively described as "light" waves, although the term "light" typically is used to describe visible light, which is composed of electromagnetic waves with wavelengths between roughly 390 and 750 nanometers. 

In thermoplastic material selection, we are sometimes concerned with whether a given thermoplastic-and the additives it contains-will block a given frequency of EM waves, or transmit them without loss. For example, in optical applications, we typically want all light in the visible spectrum to be transmitted, without concern for other wavelengths. Or, in the case of sunglasses, we may want to block a certain amount of visible light, or wavelengths in the UV range. Or, in an electronic shielding application, we may want to block transmission of EM waves in a certain band of the radio frequency (RF) spectrum.

However, we also need to account for the effects of any EM waves on the polymer itself. Basically, we are putting energy into the polymer matrix, especially at the lower end of the spectrum (gamma rays through UV). If the polymer is transparent to those waves, the energy passes through. However, if the polymer blocks that transmission, the energy will be absorbed and either converted into heat or cause the polymer chains to break apart.

One of the reasons sunlight is so devastating to materials (all materials, not just thermoplastics) is because it contains not just EM waves in the visible spectrum, but also in the infrared and UV spectra. Long-term, continuous exposure to direct sunlight means the material will absorb a lot of energy, usually with detrimental effects. 

Time

The final end-use condition, and in some ways the most critical, is time. The passage of time, especially in combination with one or more other conditions, will almost always result in a loss of performance for a plastic material. In fact, most of the test data that are used to evaluate environmental effects are created using time as a variable.  

For instance, heat-aging tests are used to evaluate the effect of long-term exposure to elevated temperatures, and can be used to show the change in a given property value, say tensile strength, as a function of time. In a similar manner, weatherability tests are often used to assess the long-term effects of exposure to an outdoor environment. These tests typically address a combination of temperature, chemical and radiation (primarily UV) effects measured over a course of days, weeks, months or years. These tests may include a variety of factors: For instance, an Arizona weathering test typically addresses high heat and high UV in a dry environment, while a Florida weathering test addresses high humidity and high UV in a sub-tropical environment, sometimes with the added effect of salt spray. While these tests are often conducted on an accelerated time scale, the intent is to predict long-term performance over months and years of exposure.  

To evaluate the effects of exposure to one of the conditions described above, one needs to compare property data before and after exposure. Any change in property data will be obvious. The net effect on performance can then be easily predicted.

So, before you blindly pick a plastic material and ride off into the sunset, consider which horse you are riding on.

Eric Larson

Eric R. Larson is a mechanical engineer with over 30 years' experience in plastics design. He has helped develop products ranging from boogie boards, water basketball games and SCUBA diving equipment to disposable lighters, cell phones and handheld medical devices.

Larson is owner of the Art of Mass Production (AMP), an engineering consulting company based in San Diego, CA. AMP provides services to manufacturing companies in the consumer electronics, wireless, and medical device industries.

Larson is also moderator of the blog site, plasticsguy.com, where he writes about plastics technology and its effect on people and the planet. His newest book, Plastics Materials Selection: A Practical Guide, can be purchased through his website.

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