Effects of Weathering Cycles on the Mechanical Properties of Thermoplastic Polyurethane

Abstract

In the present work, a method for synthesizing pure thermoplastic polyurethane (TPU) and TPU composite test specimens was developed to show stress-stretch dependency, to observe the effects of weathering on samples, and to show the potential of TPU as a superior coating agent over traditional polyurethane in aerospace applications. The TPU specimens were tensile tested using ASTM standards to extremely high deformations: stretch values of 5 times original length in tensile testing. During testing, TPU specimens exhibited typical behavior of nonlinear viscoelastic materials with extensive energy dissipation during stretch-release cycles. A portion of the tested samples were exposed to 168 hours of UV radiation, moisture, and temperature fluctuation to simulate accelerated weathering and exposure to harsh environments. When comparing weathered TPU samples to non-weathered TPU samples, there was no appreciable difference in the mechanical properties or the amount of energy absorbed during deformation; the stretch-stress curves were nearly identical before and after weathering. When returned to a zero-stress state, considerable residual strain remained in all specimens. It is conjectured that strain-induced crystallization is responsible for the unique shape-memory effect that the TPU specimens experience. Slower loading rates with the same peak stretch values showed higher peak stresses in samples from the same batch, showing that the rate of crystallization is dependent upon the rate at which TPU samples are deformed. The same tests were performed with TPU composites, filled with molybdenum disulfide. While TPU composites have slightly different overall mechanical properties from pure TPU, the exposure to weathering also had minimal effect on mechanical properties of composite specimens. Fracture tests were also performed on pure TPU and composite TPU samples. The effect of UV weathering on fracture toughness of pure TPU and TPU composites is more prevalent, as specimens were hardened and their abilities to absorb energy during crack growth was greatly reduced.