Thermal Expansion Behavior of Nonoriented Polypropylene/Clay Composites

Linear thermal expansion coefficient (LTEC) was measured for compression molding samples of polypropylene (PP)/clay composites with clay loading of 0 to 7？wt%. Composites were prepared by internal batch mixer and specimens were prepared by compression molding. These processing methods are not anticipated to have a preference for orientation; therefore effect of anisotropy was minimal. The LTEC was measured along three different faces of the compression molding sheets, parallel to compression direction S1 and perpendicular to compression directions S2 and S3. The LTEC for neat PP measured by current research, ？mm/mm/°C, was not found to be altered by direction of the measurements. Similar behavior was observed with composites having very moderate clay content, that is, 5？wt%. An interesting finding by current study was that incorporating clay particles into the PP matrix led to better shrinkage or contraction behavior of the samples prepared by compression molding. 1. Introduction Polypropylene (PP) is a versatile polyolefin that finds a strong demand in some of the advanced applications such as automotive industry. What makes PP a good candidate for such advanced applications is the excellent chemical and mechanical properties due to its superior crystallinity nature. PP is a highly crystalline polyolefin with relatively higher melting and softening temperature compared to other commodity vinyl polymers such as polyethylene (PE) and polystyrene (PS). Some of the interior parts of an automotive may be made of PP. Recently, glass fiber reinforced PP was introduced to be used in the under-the-hood parts in automotive. Despite this versatility in applications for the PP, this polymer suffers from dimensional instability due to high linear thermal expansion coefficient (LTEC). Neat PP has different values of LTEC depending on the orientation of crystalline chain domain [1]. Draw ratio and temperature are the main factors that may affect the LTEC of oriented PP. In general, LTEC is higher for the segment that is perpendicular to the draw direction, while for the segment that is parallel to the draw direction, less values of LTEC may be observed [1]. Jawad et al. [2] have postulated that internal shrinkage stresses may play a major role on the magnitude of LTEC for the oriented PP that was subject to annealing processing. Nonannealed PP samples may exhibit a negative value of LTEC in the axial direction (i.e., parallel to draw direction) as a response to the positive LTEC in the transverse direction (i.e., perpendicular to the draw direction) [3]. Anisotropy