DISTORTION BEHAVIOR OF HEAT-ACTIVATED ACRYLIC DENTURE-BASE RESIN IN CONVENTIONAL AND LONG, LOW-TEMPERATURE PROCESSING METHODS

There have been many reports on fatal distortion of heat-activated acr ylic denture-base resin which is still widely used in the field of rem ovable prosthodontics. However, these reports have failed to report qu antitatively on polymerization and thermal shrinkage factors. In the p resent study, we attempted to verify that the shrinkage of heat-activa ted acrylic denture-base resin was caused mainly by thermal contractio n after processing. Furthermore, we examined the degree of distortion resulting from long, low-temperature processing, and compared the resu lts with that of the conventional method. The strain gauge and thermoc ouple were embedded in a specimen at the time of resin packing. The me asurement started from the beginning of processing and continued until the specimen was bench-cooled and immediately before and after it was de-flasked, as well as during seven-day immersion in water at 37 degr ees C. The resin expanded when processed by the conventional method. M eanwhile, mild shrinkage, possibly polymerization shrinkage, was obser ved when the resin was processed by the low-temperature method. This s uggested that polymerization shrinkage was compensated for by thermal expansion during processing by the conventional method. Moreover, the shrinkage strains in the period from the completion of processing to i mmediately after de-flasking, in both the conventional and low-tempera ture methods, were identical to the theoretical value of thermal shrin kage which we obtained by multiplying the linear coefficients of therm al expansion by temperature differences. The shrinkage strain in the s pecimen processed by the low-temperature method, measured from the end of processing to immediately after de-flasking, averaged 64% of that in the specimen processed by the conventional method. The results reve aled quantitatively that the shrinkage of heat-activated acrylic dentu re-base resin was mainly thermal shrinkage, and demonstrated the advan tage of the low-temperature method in reducing thermal shrinkage.