Influence of humidity on the scratch behavior of polystyrene–acrylonitrile random copolymers

The effect of exposure to a humid environment on the scratch behavior of a set of model polystyrene–acrylonitrile (SAN) random copolymers was investigated over a period of 10 days. Linear increasing load scratch tests were performed according to ASTM D7027/ISO 19252. The critical loads for the onset of key scratch deformation mechanisms like periodic micro-cracking, plowing, and scratch visibility were used as metrics for evaluating scratch resistance. The scratching coefficient of friction was evaluated, as well. It was found that, in general, the scratch resistance decreases during the first few days of moisture exposure, but then experiences a degree of recovery upon saturation. It is proposed that the initially absorbed moisture causes plasticization, making the surface weaker until saturation where the water molecules gather together on the surface to impart a degree of lubrication and consequently improve the scratch resistance. Implications of moisture absorption on the scratch behavior of polymers will be discussed.