Crystallization behavior of polypropylene and its effect on woodfiber composite properties

Abstract

This paper describes an approach where polarizing optical microscopy is used to observe the crystallization process of different polypropylenes in the presence of wood fiber. The crystallization behavior was found to be related to the chemical composition of the polymer systems and the addition of maleic anhydride grafted polypropylene (MAPP) to polypropylene dramatically altered the crystal structure around the fiber, even at low levels (1 to 2 percent). For neat polypropylene, the wood fiber showed poor nucleating ability, and little difference in bulk and surface crystallization was observed. However, when MAPP was added (1 to 10 percent), the nucleation density at the wood fiber surface became so pronounced that a transcrystalline layer was formed around the fiber. Because the wood fibers themselves had not undergone any prior chemical modification, the increase in nucleation ability of the fiber surface must be attributed to specific interactions between the wood fibers and the maleic anhydride moiety of the MAPP. These observations reveal that the MAPP may not only interact with wood fibers, but may also change the microstructure of the matrix near the fibers. It was observed that the nucleation of the wood fiber surface was selective, indicating that the local physical or chemical characteristics of the fibers might have some influence on the polypropylene crystallization. Dynamic mechanical analysis was used to monitor isothermal crystallization of the polypropylene blends, and provided some additional insight into the contribution of crystalline morphology to strength properties.

Citation

Yin, Suzhou; Rials, Timothy G.; Wolcott, Michael P. 1999. Crystallization behavior of polypropylene and its effect on woodfiber composite properties. In: Fifth international conference on woodfiber-plastic composites; 1999 May 26-27; Madison, WI. Madison, WI: Forest Products Society: 139-146.