| As dental composite properties are continually being improved, enhancements in strength, toughness and durability are highly desired. One approach to enhancing toughness and resistance to wear is through engineering the filler/matrix interface. The ultimate goal was the development of a ductile interface to provide additional plasticity and enhance toughness by the crack-pinning mechanism. To vary the interface between the particulate filler (glass beads average diameter 3.6 μm) and the standard light initiated dental resin, the beads were coated with varying amounts of silane coupling agents which either had reactivity towards the resin matrix (3-methacryloxypropyltrimethoxysilane, MPS), or were inert towards it (3-glycidoxypropyltrimethoxysilane, GPS). A chemically bound layer of a ductile rubber (polybutadiene-co-acrylonitrile, Mw = 5,500) whose thickness could be varied was also applied by novel methods, and tested as a composite toughness enhancer. A possible correlation between interfacial strengths (from microbond shear tests of glass fibers) following various treatments of glass and the mechanical performance of their composites (three point bending modulus, strength and fracture toughness values from mechanical testing) was investigated. Although there was no linear correlation between interfacial adhesion and property enhancement, samples with the highest matrix-filler interfacial strength displayed the best strength and fracture toughness values. Composites with silanated particles were found to show better performance when compared to the unsilanated specimens, especially after soaking in 50:50 ethanol:water (accelerated aging in a simulated oral environment). The layer of rubber at the interface did not improve the toughness of the composites. Ultrahigh molecular weight polyethylene (UHMWPE, Mw = 3–6 × 106), because of its relatively high modulus and high ductility, was also investigated as a toughness-enhancing interface. Amongst various novel techniques developed, a simple two step coating method involving an intermediate step for PE-silane attachment was found to provide the maximum and most stable UHMWPE attachment to glass beads. Also, simple swelling treatments were developed for increasing the reactivity of the chemically inert UHMWPE surface towards the methacrylate resin. The improvements in interfacial adhesion (a maximum of fourfold increase was obtained) were either comparable or better than those reported by comparatively harsher treatments like nitric acid, plasma or UV etching. |
