STUDY ON STRESS INDUCED CORROSION AND LIFETIME OF DENTAL RESIN

通过断裂韧性（K_<IC>）测量、非线性断裂能分析、裂纹扩展速度(V)和应力强度因子（K_I）测量以及断口形貌观察，从环境应力开裂（ESC）的角度研究热固化义齿基托树脂的开裂机理。虽然K_<IC>取决于水分和应变速率，但估计吸水试样的最适宜值约为1.0 MPa。m^<1/2>时，不发生慢裂纹扩展（SCG）。从无因次荷载-荷载点位移曲线的应用中发现，塑性能量耗散率大于裂纹表面形成所需的净能量。在乙醇溶液和乙酸溶液中，随K_I的增大，未吸水和吸水的基托树脂试样的V值均增大，当K_I接近K_<IC>时，V值收敛在1 μm/s左右。当浓度较高时，即使K_I较小时，V也收敛于相同的值。工业树脂吸水后的V值与义齿基托树脂相似，但在干燥样品中没有收敛，反而增大。这被认为是水阻止了环境溶液的扩散。吸水树脂的断口表面呈现条状，而不吸水树脂的断口在高浓度时呈现光滑的断口。综上所述，在义齿基托树脂中会发生ESC，并考虑以下机制。(1)开裂是由应力和环境溶液共同作用下聚合物链的溶解引起的；(2)在聚合物链与环境溶液直接反应过程中聚合物链的断裂引起的。

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.