How Environmental Moisture Influences Scratch Resistance Of Polymers

Polymeric materials are replacing metals and ceramics in the manufacturing of a wide range of products owing to their low density, low cost, and high recyclability. Yet, the poor scratch resistance of polymeric materials still limits their applications where functionality and aesthetic appearance are of extreme importance, such as cellphone screen, automobile interiors, and exteriors, etc.

Hence, there is a constant desire to study and improve the resistance of polymeric materials against scratch damage. The previous study was done by Xiao et al. [Xiao, S., Hossain, M. M., Liu, P., Wang, H., Hu, F., & Sue, H. J. (2017). The scratch behavior of model polyurethane elastomers containing different soft segment types. Materials & Design, 132, 419-429.] has successfully correlated the material properties of polymers with the scratch-induced deformations. It has shown that frictional, constitutive and viscoelastic properties are the key factors determine the scratch resistance of polymers.

However, most of the polymeric products are exposed to humid environments during service and the environmental moisture might have a significant impact on the material properties and the corresponding scratch resistance of polymers. The present study utilizes one set of well-designed cast polyurethane (PU) elastomers as the model polymer systems to investigate the variation of scratch resistance of polymers before and after exposure to environmental moisture.

PU elastomers are generally composed of two chemical constituents, a hard segment, and a soft segment. The material properties of PU elastomers can be quite different with each other just by modifying the type or content of each segment. The model PU systems in this study are synthesized containing the same hard segment, but each of them has a different soft segment: two with low polarity and two with high polarity. Scratch tests are performed on the dry and wet model systems according to the ASTM D7027/ ISO 19252 standard. Various material characterizations are also conducted to track the changes in material properties before and after moisture exposure.

The scratch test results show that environmental moisture notably deteriorates the scratch resistance of polymers with low polarity, but slightly improves the scratch resistance of polymers with high polarity. For polymers with low polarity, water molecules tend to distribute throughout the free volume in the matrix, the distributed water molecules plasticize the polymer and result in degradation in tensile properties as well as an increase in surface friction. The plasticization effect of water molecules contributes to the deterioration in scratch resistance. However, for polymers with high polarity, water molecules interact with polar bonding sites to form water clusters on the surface that lubricates the polymer surface. The lubrication effect of water molecules leads to the improvement in scratch resistance.

The present study indicates that environmental moisture can either deteriorate or improve the scratch resistance of polymers. Therefore, careful assessment is necessary for predicting the scratch resistance of polymers upon exposure to moisture.

These findings are described in the article entitled Effect of moisture exposure on scratch behavior of model polyurethane elastomers, recently published in the journal Polymer. This work was conducted by Shuang Xiao and Prof. Hung-Jue Sue from Texas A&M University, as well as Hailin Wang and Dr. Fengchao Hu from BASF Polyurethane Specialties (China) Co. Ltd., China.