STUDY ON STRESS INDUCED CORROSION AND LIFETIME OF DENTAL RESIN

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The mechanism of cracking in heat-cured denture base resin was investigated with a view of environmental stress cracking (ESC), by the measurement of fracture toughness (K_<IC>), the analysis of non-linear fracture energy, the measurement of crack growth velocity (V) and the stress intensity factor (K_I), and the observation of fracture surface. Although the K_<IC> depends on the water and the strain rate, the most appropriate value of the specimen with absorbed water was estimated to be about 1.0 MPa. m^<1/2> when the slow crack growth (SCG) never occurred. From the application of the dimensionless load-load point displacement curve, it was found that the plastic energy dissipation rate was greater than the net energy required for the formation of the crack surface. The V of both specimens without and with absorbed water in denture base resins increased with increasing K_I in both ethanol and acetic acid solution, and converged on about 1 μm/s when K_I approached K_<IC>. In case of higher concentration, V converged on the same value even when K_I was small. The V of industrial resin with absorbed water behaved much like denture base resins, but increased without convergence in dried specimen. This was attributed that the water prevented the environmental solutions from diffusing. Furthermore, the fracture surface of any resins with absorbed water showed the striped pattern, but without water the smooth surface appeared at heigh concentration. From these results, it is suggested that the ESC in denture base resin would occur and is considered the follwing mechanisms. (1)The cracking is caused by the dissolution of polymer chain under the presence of both stress and environmental solution, and/or (2)by the rupture of primary polymer chain dur to the direct reaction between chain and environmental solution.