Objective: The objective of this work is to optimize the cationic initiation of SilMix® (3M ESPE) respect to amount of catalyst, resulting exotherm, and propose a possible cure mechanism.
Methods: The general procedure was as follows. Approximately, SilMix (1g) and the acid catalyst were combined in a planetary mixer and mixed for 5min. After which, the material was placed on a microscope slide and tested for hardness after 15min, 30min, 1h, 24h, and 48h using a Gillmore needle (one-pound:pass/fail).
Results: Hexafluorophosphoric acid (HFPA, 5 wt%) was found to polymerize the SilMix to hardness in >15min resulting in a brittle solid. At lesser concentrations such as 4.6, 4, and 2.5wt%, the material polymerized to hardness. The latter concentrations took longer to reach hardness (30 minutes) but the solid was less brittle. Solutions of acetic acid (AA):HFPA (varying from 32:68 to 15:85 by weight; total catalyst 4wt%) differed in results. The AA was mixed into the SilMix first for consistency. At the high AA proportion (32-29%), the resin was springy in consistency but did not polymerize to hardness after 48h. At the lower AA concentrations (14.6wt%), the resin polymerized to hardness after 15min. The maximum temperature from preliminary exotherm data for HFPA (5wt%) was 39.0±6.2°C. Upon inclusion of AA (14.6wt%), the initial data results in a slight increase (44°C), but within HFPA standard deviation.
Conclusion: It was found that the least optimal chemical cure system for SilMix was the HFPA 2.5wt% due to slow cure time with resulting promising exotherms.
This work was supported in part by NIH/NIDCR Grant R21DE018336, NIH/NIDCR T32(DE07294), and Missouri Life Science Research Board Grant (#13234-2007).