Resin composite blocks via high-pressure high-temperature ...

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 529–534 533Fig. 1 – Weibull plots of flexural strength results; open symbols were used for photo-polymerized (PP) materials andfilled-symbols were used for high-temperature high-pressure (HT/HP) polymerized materials.Fig. 2 – SEM micrographs of specimens fractured during fracture toughness (K IC ) testing: (a) Grandio photo-polymerized (PP);(b) Grandio polymerized under high-temperature high-pressure (HT/HP).materials were higher than those of the commercially availableCAD/CAM material Paradigm.The most significant observation during the SEM characterizationsof fractured K IC specimens from all the materialstested was the presence of fewer and smaller voids in HT/HPpolymerized composites. As an example, Fig. 2a and b showshigh (1500×) magnification micrographs of PP and HT/HP polymerizedGrandio samples, respectively. No attempt had beenmade to quantify the differences identified.4. DiscussionHigh pressure polymerization is an appealing approach in theattempt to improve the properties of polymers/composites.As early as 1930, Conant and Tongberg [10] reported theirresult on the application of high pressure [(0.9–1.2) GPa] inthe polymerization of isoprene. A high pressure apparatusused to study a series of organic reactions and severalpolymerization reactions under pressure was described byFawcett and Gibson. [11]. Later on, high density polyethyleneand poly(methyl methacrylate) (PMMA) [12,13], among severalother polymers, were obtained under high pressure. Studyingthe polymerization of tetraethylene glycol dimethacrylate(TEGDMA) under pressure (1 GPa) and with no catalyst, Kaminskiet al. [14] reported a degree of crosslinking of 12% after96 h, similar to that obtained in the industrial production ofPMMA.The primary effects of pressure on a monomer mixtureare to decrease intermolecular distances and to reduce thefree volume [15]. Furthermore, as the pressure increases, themobility of the monomers is reduced and limited to relativelysmall translations, which suggests that the polymerizationkinetics are slower as the pressure increases. It is likely thatas the pressure increases above the GPa range, monomerslose their mobility necessary for polymerization and mostlikely transform into solids, forming monomer crystals [16].Low degrees of conversion would result in polymers withpoor mechanical properties while pressures in the GPa rangewould make the process unsuitable for large scale industrial