The Use of Microwave Energy in Dental Prosthesis

Acrylic resins are used as denture base materials since 1937 (Craig, 1993) and heat-cured Poly(methyl-methacrylate) (PMMA) (Lay et al., 2004) composed by methyl methacrylate monomers chains is the most popular of them. Virtually, dentures are constructed from conventional polymer/monomer (Lay et al., 2004) processed by heat water-bath system and compression molding technique (Ganzarolli et al., 2002). It is prepared by mixing the monomer and polymer in a specific ratio, resulting in a dough mass that is packed and pressed in a flask for polymerization (Ganzarolli et al., 2007). Addition polymerization of PMMA requires the activation of the initiator (benzoyl peroxide) to provide free radicals. Polymerization takes place as the free radicals open the double bonds of the methyl methacrylate, creating a chain reaction where the monomer attaches to polymer free radicals (Bartoloni et al., 2000). When the polymerization reaction is activated by heat, the monomers form polymeric chains joined by high energy linkings (crossed-links) and this reaction would finish when all monomers have supposal been reacted (Machado et al., 2004). Although water-bath polymerization is extensively used to process PMMA, new resins and processing methods have been proposed in order to obtain better physical properties and to simplify the technique (Souza Jr. et al., 2006). Heating curing, chemical curing by pouring technique, light curing and microwave curing resin have been extensively studied for denture base processing (Harman, 1949; Nishii, 1968; Jerolimov et al., 1989; Urabe et al., 1999). Polymerization of heat-cured PMMA is usually carried out in a temperature-controlled water bath for at least 9 hours. However, the use of microwave energy to polymerize PMMA decreases the time to three minutes only, producing acrylic resin bases with the same quality as those polymerized by water bath technique (Nishii, 1968; Kimura et al, 1983; Hayden, 1986; Jerolimov et al., 1989; Del Bel Cury et al., 1994; Rizzatti-Barbosa et al., 1995; Braun et al., 1988; Ganzarolli et al., 2002; Yunus et al., 2005; Rizzatti-Barbosa et al., 2009). The microwave polymerization technique has advantages such us less equipment is required (Schnieder et al., 2002), the method is fast and clean, the final product has the same quality of physical properties, and it presents better accuracy of fit, resulting in improvement adaptation of denture base (Rizzatti-Barbosa et al., 1995; Rodrigues-Garcia et al., 1996; Compagnoni et al., 2004; Shibayama et al., 2009). The advantages of microwave heating over conventional heating are: (1) the inside and outside of substance are almost equally heated,