I.C. Schoonover

Crazing of Acrylic Resins

A PPEARANCE of small surface cracks is often observed in dentures and t plastic teeth. These cracks lower the esthetic qualities of the restorations and greatly reduce their strength. Crazing is the result of local stresses set up in the resin. When the stress exceeds that necessary to fracture the resin, crazing occurs. The object of this investigation was to determine the factors which produce crazing in dental acrylic resin and to explain the crazing mechanism. Typical photographs of crazing in a denture and in plastic teeth are shown in Figs. 1 and 2. It has been shown1-4 that crazing in polymethyl methacrylate is caused either by application of external mechanical stresses or by the action of chemical agents on stressed material (solvent crazing). The latter type of crazing may occur in the presence of residual monomer, solvents, or depolymerization products. These substances will produce local strains greater than the material can withstand. Often the simultaneous action of solvents and external stresses produces crazing. It is probable that the crazing of dentures results mainly from solvent action in combination with both internal and external stresses. Examination of dentures in service and of laboratory specimens showed that there was a Wide variation in the amount of crazing occurring in denture bases and in plastic teeth made of the same brands of material. It was also observed that plastic teeth which did not craze on the application of a solvent before being placed on a denture were often susceptible to crazing after being subjected to the denture processing procedure. Because these observations indicated that the craze susceptibility of the resins was dependent upon variations in the processing procedure, the effects of several of these variations were investigated. Included were the effect of the type of investment or mold material used in processing the denture, the effect of water sorption during processing, and the effect of the molecular weight of the resin.