Thermal Expansion of Sheet Molding Compound Materials

The coefficient of linear thermal expansion (CLTE) of composites is an important property that has to be well understood. In this study, using a mathematical expression, graphic relationships were established for predicting the general trend of CLTE variations with temperature as affected by the materials properties. Subsequently, the CLTE of different types of sheet molding compound (SMC) materials was measured at various temperatures in the direction parallel to the cosmetic surface. It was observed that the CLTE variations with temperature go through a maximum. The experimental results were then interpreted with the help of the developed theoretical relationships. More specifically, the effect of low profile additive on the CLTE of SMC was studied, and it was observed that increasing the concentration of the low profile additive, a high CLTE ingredient, reduces the overall CLTE, and at the same time, shifts the position of the maximum to lower temperatures. As to the effect of glass content, it was observed that changing the glass content from 27% to 37% does not affect the position of the maximum in the CLTE-temperature curves noticeably, although this variation reduces the overall CLTE significantly. These observations were all consistent with the theoretical predictions. The information generated in this work enables one to control the CLTE of composites as required by specific applications. An example is the use of SMC in the production of inner panels for deck lids and hoods, in which a controlled CLTE is essential in order to avoid bond line readout.