The Mystery of Physical Aging, Part 1: Knowing the Difference

This is part one of a three-part series discussing the physical aging of amorphous polymer products. Click the links below for more: 

• Part Two: Slow Down Aging
• Part Three: Accelerated Aging Tests

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In the late 1970s the use of plastic bottles to contain water was popularized when the FDA approved the use of PET polyester as a suitable material for this purpose. The amorphous variety of this polymer is clear and tough and has appropriate barrier properties when a grade of the correct molecular weight is used and properly processed. Shortly after the introduction of this product, a troubling phenomenon came to light.

When amorphous PET products were stored at elevated temperatures found in warehouses that were not climate controlled (approximately 50°C or 120°F), they lost much of their as-molded impact performance in a matter of weeks. At the same time, tests showed that the material was becoming stronger and stiffer. At that time there were some well-understood mechanisms that could explain the onset of brittle behavior in plastic materials. The most obvious of these is a reduction in molecular weight. During processing, this is typically caused by prolonged exposure to elevated temperatures while the material is in the melt state. 

For some materials, such as PET, the presence of excess moisture in the polymer during processing presents an additional hazard to the integrity of the polymer. Once the part is molded, the application environment can also pose problems along the same lines. Long-term elevated-temperature exposure can consume the stabilizers placed into the materials, permitting oxidation to occur. In some materials, a combination of high application-temperature exposure and high humidity can present the same challenges over the long term that wet material during processing presents in the short term.

Other influences such as exposure to ultraviolet light or chemicals can create additional problems. With molecular-weight reduction, impact performance is the first short-term property to be diminished. However, at some point early in the aging process, the strength of the polymer will also start to decline. It is this property that is typically monitored in documenting the effects of aging.

Another known cause for reduced impact performance is an increase in crystallinity. In polymers that are capable of crystallizing, a higher degree of crystallinity will result in an increase in strength and modulus and a corresponding decrease in toughness. This decrease can be measured using an impact test, or a tensile test will document a decrease in elongation to break. In this mechanism, the strength and stiffness will continue to increase as the degree of crystallinity increases, as long as no competing processes such as polymer degradation intervene.

What Causes Material to Brittle

Unfortunately, none of the above processes accounted for the observed change in behavior. The conditions under which the material was stored were not aggressive enough to cause thermal or hydrolytic degradation. No sunlight was getting into the storage facilities, and no one was introducing any chemicals such as cleaning agents.

While amorphous PET is certainly capable of crystallizing, this process requires exposure to temperatures above the glass transition temperature. This transition occurs at 80°C and would cause a significant loss in strength and stiffness and the great likelihood that the parts would deform. In addition, crystallization would cause clear material to turn cloudy. 

This is something that can be seen in plants that perform reheat stretch-blow molding to convert injection molded PET preforms into bottles. If the preheat temperature goes too high the preforms will begin to develop a haze that is associated with the development of a crystal structure in the material. None of the parts displayed distortion or a cloudy appearance. So why were they so brittle?

At about this time, L.C.E. Struik in the Netherlands was authoring a paper on a process known as physical aging. It is nine pages of absolute brilliance and can be read online. Struik picked up on something that had been known for some time: that amorphous materials, including polymers, are not in a state of equilibrium when they are below their glass-transition temperature (Tg), due to the rapid rate at which they are cooled during normal processing. At a molecular level, this means that the spacing between the polymer chains is greater than it would be if the material was in an equilibrium state.

He also pointed out that the mobility of an amorphous polymer below the Tg is not quite zero, that there is a very small amount of motion that drives the polymer chains to collapse into this excess free volume. As a result, the chains in an amorphous polymer are constantly in the process of getting closer together. As they do, the intermolecular attractions between the chains become stronger, resulting in reduced freedom of movement. At a macro level, this results in a material that becomes stronger and stiffer over time while also losing ductility.

This behavior occurs at a rate that is dependent on temperature. If the temperature declines to a point below what is known as the beta transition of the material, physical aging will cease completely. However, in most amorphous polymers the beta transition is subambient. In polystyrene it is -5°C (23°F), in PVC it is -50°C (-58°F), and in PC it occurs at -100°C (-148°F). In other words, the range of temperatures over which physical aging occurs includes the range of temperatures at which we typically use these materials.

Perhaps more importantly, the rate of physical aging increases with temperature. Therefore, parts molded in an amorphous material will undergo physical aging faster when they are used at an elevated temperature. This explained the mystery of the loss of impact performance in the PET bottles.

As already mentioned, the Tg of amorphous PET polyester is approximately 80°C (176°F). Parts produced in this material and stored at room temperature are significantly farther from the Tg than those stored in those hot warehouses, which were reaching 50°C. Studies showed that the same change in physical properties that took two years to occur in PET stored at room temperature took less than a month at 50°C. And no attempts to account for this behavior by invoking hydrolysis, oxidation or crystallization made any sense.

Oxidation vs. Physical Aging

In the vocabulary that describes the behavior of polymers, aging is essentially synonymous with oxidation. There are important differences between this type of aging and physical aging. First, as we have already discussed, oxidation degrades all mechanical properties. Impact resistance may be the first property to show evidence of decline, but others will follow eventually. Physical aging reduces ductility but will improve load-bearing properties such as strength, modulus and creep resistance.

Oxidation will eventually result in a reduction in molecular weight; it is a chemical process that breaks the polymer chain. Physical aging causes no such disruption of the polymer building block. 

Finally, oxidation is an irreversible reaction; it is a one-way path to the demise of the material. Physical aging is reversible. If the material that has undergone physical aging is raised to a temperature above its glass transition and then rapidly quenched, the effects of aging that have occurred previously are erased regardless of the time frame over which they have occurred.

In part two we will discuss the practical importance of this phenomenon and illustrate some interesting and often misunderstood consequences related to its occurrence.