Moghbelli, Ehsan (2014-08). Scratch Behavior of Multiphase Styrenic Copolymers and Effects of Environmental Conditioning. Doctoral Dissertation. Thesis uri icon

abstract

  • Scratch-induced surface deformation is a tribological research area that falls under the abrasive wear category. A variety of factors including high strain rate, large-scale deformation, non-linear material response, heat dissipation, and the evolution of a complex stress field, renders scratch a complex mechanical process. The dependence of polymers on testing rates, temperature, and pressure, along with the surface characteristics of the two materials in contact bring the rate, time, temperature, and pressure dependent behaviors of polymers, and the surface condition of the interacting surfaces also add to the complications of scratch analysis. In order to gain an in-depth understanding of polymer scratch behavior, this dissertation focuses on the scratch response of multiphase systems made up of a plastic matrix and a dispersed rubber phase. The introduction of a rubber phase and the effect it has on scratch behavior is explored through a number of factors including rubber size and type, environmental conditioning through heat processing, moisture exposure, and water immersion. A standardized progressive load scratch test (ASTM D7027/ISO 19252) is used to examine the mechanical response to scratch deformation in ASA and ABS systems with varying rubber particle size. Previous simulation results from finite element methods are used to assess the scratch response of the multiphase systems and comparisons are made to results based on single phase plastics and their respective scratch behavior. The key scratch damage transitions identified and studied are: (1) the onset of scratch groove formation, (2) the onset of periodic cracking, (3) the onset of material removal (plowing), and (4) the onset of scratch visibility. The onset of groove formation is generally related to the secant modulus at the point of compressive yielding. The onsets of crack formation and plowing are more complex to quantitatively evaluate, and are strongly influenced by the material tensile and/or shear strength. For ASA copolymers, enhanced scratch performance is observed in systems with rubber particles the size of 1 micron relative to 100 nm sized rubber particle systems, while ABS copolymers containing 100 nm sized rubber particles are more scratch resistant than ASA copolymer systems with similar rubber particle size and distribution. The fact that these three model systems exhibit similar mechanical properties in uniaxial tension and compression bulk testing does not explain their differences in scratch resistance based on our previous FEM modeling and experimental results for single phase systems. The local stress state generated by the rubber particles and the scratch process at the surface, along with changes in surface coefficient of friction, are used to explain these findings. In order to minimize orientation and residual stress effects from the injection molding process, heat treatments at temperatures above and below Tg were carried out on ASAs with varying rubber content, rubber size, and rubber type. Low temperature annealing (LTA) was seen to reduce rubber orientation while having no impact on bulk mechanical properties, surface characteristics, or scratch resistance. On the other hand, high temperature annealing (HTA) minimized orientation and residual stress and showed no impact on bulk mechanical properties or surface characteristics, while also leading to a significant improvement in scratch resistance for high rubber content systems.

publication date

  • August 2014