Some mechanistic observation on water-deteriorated dental composite resins

Four different types of dental composite resins were subjected to the water sorption environment in both dyed and undyed distilled water for 1 and 5 weeks at 37 degrees C. Weight changes due to the water sorption, effects of water sorption on mechanical properties, and fractographic observations were conducted. It was found that (i) the water sorption was diffusion-controlled, (ii) both break stress and modulus of elasticity reduced by increasing the amount of absorbed water, and (iii) fractographic observations showed that fracture pattern of the wet/dry/wet lamellar structure was not straight line, rather S-shaped pattern, suggesting that (a) mechanical properties (particularly, the modulus of elasticity) of the wet portion were not same as those in the dry portion, and (b) there could be an internal residual stress developed at/in the interfacial zone between the dry and wet portion. By one-layer removal from the wet/dry/wet lamellar structure composites, the residual stress was calculated from measuring the radius of the curvature. It was found that for all water-sorbed composite materials, tensile residual stresses were developed, which were from 30 to 40% of the break stresses for TPH, Charisma, and Durafil VS, and was 78% of the break stress for Z100 material.