Influence of silane treatment and filler fraction on thermal expansion of composite resins.

The coefficient of thermal expansion of experimental composite materials containing either silane-treated or untreated fillers in a triethylene glycol dimethacrylate (TEGDMA) matrix was investigated. The results show that an inverse linear relationship existed between volume fraction filler and coefficient of thermal expansion. No differences were seen between silane-treated and untreated composites, while it was found that repeated heating (aging) caused the thermal expansion to decrease for all material combinations. Reduction in the coefficient of thermal expansion with increased filler fraction of unbonded filler indicates that the polymerization shrinkage of the matrix induces hoop stresses around the fillers. By use of a simplified theoretical model (Appendix), these stresses could be estimated. These estimates revealed that the induced stresses were remarkably high, and that increased filler fraction increased the tensile stress level surrounding the filler particles. Since these tensile stresses could facilitate crazing and crack growth in the matrix, these estimates may explain why filled resins containing low fractions of microfilled particles seem to possess remarkably good clinical wear resistance when compared with composites containing higher filler concentrations, at least during the first years in service.