Quantitative characterization of surface deformation in polymer composites using digital image analysis

The stress whitening of polymers and polymer composites during surface deformation (scratching) can represent a severe technological problem in certain applications. For example, scratch resistance is particularly important for poly(propylene) automobile interior components. Unfortunately, the addition of reinforcing agents such as talc or mica for improved dimensional stability and rigidity often results in an increased sensitivity to scratching. The ability to design new materials with reduced visible surface deformation requires more sophisticated information about the deformation mechanisms of polymers and polymer composites near surfaces and their relationship to the scattering of incident light. We have developed a technique to quantify the light scattered from polymer composite surfaces due to surface deformation. We first deform the material in a controlled manner using a scratch testing apparatus. We then analyze the region near the scratch with reflected polarized light in an optical microscope coupled to a digital image analysis system. By measuring the light scattering from the sample as a function of incident light polarization and sample orientation, it is possible to obtain information about the nature and extent of deformation at the sample surface. In this report, we describe our technique and demonstrate how it can be used to quantify the surface deformation of poly(propylene)‐talc composites. By examining a series of materials as a function of talc content, we have been able to obtain information that can be related to specific micromechanisms of deformation near the scratch, such as the orientation of the polymer and the voiding near the talc particles. The technique should assist in the improved development of similar materials for applications in which surface appearance is a primary concern.