Polymeric materials are a wonder of modern chemistry and engineering, however, they are susceptible to environmental degradation which can alter their mechanical properties and make them brittle. Hundreds of millions of tons of thermoplastic materials are produced each year, in many cases for outdoor use, and as such, understanding their degradation behavior is paramount to development of new materials and applications. In this work films of two common thermoplastic materials – polyethylene (PE) and polyethylene terephthalate (PET) – were exposed outdoors and under accelerated laboratory conditions. The mechanical properties of these films change with time, which has implications for their use in mechano-structural applications. Two main mechanisms are explored for the transition to brittle mechanical failure due to environmental degradation, including decreased molecular weight and increased crystallinity. Both are due to chain scission and oxidation, though the latter is found to be more highly correlated to brittle fracture of the polymer films. This is because changes in surface density due to crystallization creates points of stress concentration (i.e. surface defects), which are largely responsible for mechanical failure in standard tensile tests, as illustrated in Figure 1. The interplay and relative importance of each mechanism is an active field of discussion, and here detailed surface, bulk, and depth-dependent mechanical, chemical, and structural characterization of these properties is shown. The degradation behavior is more thoroughly explored by examining a diverse range of accelerated laboratory exposure conditions, e.g., 30°C up to 70°C, no and low-ultraviolet (UV) light intensity to high UV intensity. These results aid development of service life prediction methodologies, in which new materials could be tested for generic climates (e.g., hot, sunny, dry vs. cool, dark, wet) on a compressed time scale.

Fracture behavior of thermoplastic films in outdoor and accelerated laboratory exposure

255th National Meeting (2018)

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