Some researchers have suggested that the weakest link of dental composites is thefiller-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 therm al expansion. Experimental composites containing different filler fractions of either silane-treatedor untreated fillers were made. Compressive strength and coefficient of thermalexpansion were determined using routine methods, while the hydrolytic degradation was investigated by measuring changes in concentrations of elements in the storagewater using atomic absorption spectrophotom etry. Scanning electron microscopicinvestigations were made on fractured samples. The diffusion coefficient of a representative resin system was determined gravimetrically. Seven commercial composites were investigated regarding hydrolyticdegradation. The filler compositions of these composite materials were determined by emission spectroscopy or energy-dispersive x-ray analysis before storage in distilledwater. This water was replaced and analyzed m onthly using plasma spectrophotom etryor atomic absorption spectrophotom etry. After completed water storage the samples were fractured and investigated by useof scanning electron microscopy.From the results of these studies the following conclusions were drawn: 1. The compressive strength of composites changes linearly with increased fillerfraction. Contrary to bonded fillers, composites containing unbonded fillers loststrength with increased filler fraction. 2. Water diffuses through the polymer m atrix 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 hydrolyticdegradation of the filler proceeds. 4. The hydrolytic degradation of the filler seemed to be associated w ith micro-crackform ation occurring in the matrix. Of the investigated composites, the micro-filledresin showed the lowest frequency of such crack formations. 5. The coefficient of ther al expansion decreases linearly with increased fillerfraction. Silane treatment did not influence this coefficient. 6. Using a simplified model to predict stresses in a particle filled composite indicatesthat rather high stress levels are induced in the polymer m atrix due to polym erizationshrinkage. This shrinkage induces radial compressive and tangential tensilestresses with respect to the filler surface. Increased filler fraction increases thetangential tensile stresses but reduces the compressive radial stresses. / <p>S. 1-66: sammanfattning, s. 67-168: 6 uppsatser</p> / digitalisering@umu