Hydrolytic degradation of dental composites and effects of silane-treatment and filler fraction on compressive strength and thermal expansion of composites

Sammanfattning: Some researchers have suggested that the weakest link of dental composites is the filler-matrix bond. However, due to incompleteness of information dealing with this bond and its stability in a humid environment, it was considered desirable to investigate the effect of water on this region, as well as the influence of filler bonding and filler fraction on compressive strength and thermal expansionExperimental composites containing different filler fractions of either silane-treated or untreated fillers were made. Compressive strength and coefficient of thermal expansion were determined using routine methods, while the hydrolytic degradation was investigated by measuring changes in concentrations of elements in the storage water using atomic absorption spectrophotometry. Scanning electron microscopic investigations were made on fractured samples.The diffusion coefficient of a representative resin system was determined gravimetrically. Seven commercial composites were investigated regarding hydrolytic degradation. The filler compositions of these composite materials were determined by emission spectroscopy or energy-dispersive x-ray analysis before storage in distilled water. This water was replaced and analyzed monthly using plasma spectrophotometry or atomic absorption spectrophotometry.After completed water storage the samples were fractured and investigated by use of scanning electron microscopy. From the results of these studies the following conclusions were drawn:1. The compressive strength of composites changes linearly with increased filler fraction. Contrary to bonded fillers, composites containing unbonded fillers lost strength with increased filler fraction.2. Water diffuses through the polymer matrix and attacks the filler particles. This degradation is most pronounced for untreated fillers containing glass modifying elements such as sodium, barium and strontium.3. The resin, used as a matrix, influences the speed with which the hydrolytic degradation of the filler proceeds.4. The hydrolytic degradation of the filler seemed to be associated with micro-crack formation occurring in the matrix. Of the investigated composites, the micro-filled resin showed the lowest frequency of such crack formations.5. The coefficient of thermal expansion decreases linearly with increased filler fraction. Silane treatment did not influence this coefficient.6. Using a simplified model to predict stresses in a particle filled composite indicates that rather high stress levels are induced in the polymer matrix due to polymerization shrinkage. This shrinkage induces radial compressive and tangential tensile stresses with respect to the filler surface. Increased filler fraction increases the tangential tensile stresses but reduces the compressive radial stresses.