| dc.description.abstract | The susceptibility of polymers to surface damage like scratch significantly limits their use in applications where both surface structural integrity and aesthetics are important. The present study employs a standardized progressive load scratch test to investigate the fundamental physical and mechanistic origins of scratch-induced deformation in polymers with the purpose of applying the gained knowledge to design polymers with superior scratch resistance. The key aspects of polymer scratch behavior in this study are the scratch cracking resistance and the scratch visibility resistance. Previous experimental and numerical studies have shown that there is a correlation between mechanical properties and scratch deformation mechanisms. In order to obtain a more comprehensive understanding on how the mechanical properties affect the scratch behavior, this research focuses on experimental analysis of a set of well controlled polymer systems to establish structure-property relationships, leading to effective design of polymers with superior scratch-proof properties.
A scratch can be considered a single-pass sliding of a single-asperity across the surface of a polymer under an applied normal load. Understanding the scratch-induced deformation of polymers is complicated due to the viscoelastic nature of polymers. The ASTM scratch test has enabled significant progress in the understanding of the scratch behavior of polymers. This test consists of a linearly increasing normal load, which generates continuous progression of deformation, allowing for the observation of damage formation and evolution. This allows to conduct a straightforward analysis and to develop structure-property relationships. Another challenging aspect of scratch testing is unambiguously evaluating the scratch visibility. Assessing the scratch visibility merely based on human observation is troublesome. Quantitative evaluation of scratch resistance requires the elimination of ambiguity and subjectivity. Employing a reliable testing and analysis methodology that is based upon the principles of material science facilitates the fundamental understanding of polymer scratch behavior.
To successfully design polymers with high scratch resistance, the first step is to fundamentally understand what factors influence the scratch behavior. To do so, a set of model epoxy systems with varying crosslinking density were prepared to investigate how the scratch behavior, specifically, the onset of crack formation, might be influenced. The findings indicate that both the tensile strength and compressive yield stress determine the resistance against scratch damage formation. Moreover, the scratch behavior of a set of injection molded model PC systems with different tensile and compressive constitutive behavior was investigated. The findings suggest that the scratch visibility of the model PC systems is closely linked to the compressive yield stress, which dictate the magnitude of the scratch depth and shoulder height.
Novel material design concepts are required to develop polymers with superior scratch resistance. Polyrotaxane (PR) is a supramolecule with rings threaded onto a backbone linear chain that is capped by bulky end groups. The ring structure, cyclodextrin (CD), can slide along the backbone, allowing for stress redistribution. Due to its dynamic structure, PR has shown to induce significant self-recovery abilities after scratch-type of damage, leading to improvements in the scratch resistance. PR has been extensively investigated in elastomeric coating systems. In this work, the effect of PR on the scratch behavior of more rigid polymers networks like poly(methyl methacrylate) (PMMA) were investigated. Dielectric relaxation spectroscopy, dynamic mechanical analysis, tensile and compressive true stress-strain tests, in conjunction with ASTM scratch test, were conducted to fundamentally understand how PR influences the mechanical and scratch behaviors of PMMA. | en |
