Effect of microwave irradiation and water storage on the viscoelastic properties of denture base and reline acrylic resins

Abstract

This study evaluated the effect of microwave irradiation and water storage on the viscoelastic properties of two denture base resins (Lucitone 550-L and Vipi Wave-VW) and two reline resins (Kooliner-K and Tokuyama Rebase Fast II-TR II). Eight specimens (40×10×3.3 mm) of each material were evaluated by dynamic mechanical thermal analysis (DMTA) after processing, water storage for 7 days (WS), one (MW1) and 7 cycles of microwave irradiation (MW7). For each specimen, DMTA runs were carried out within different temperature intervals. Values of storage modulus ($E^{\prime }$) and loss tangent (tan $\delta$) at 37 °C were obtained from the first and last runs. From the last run, values of $E^{\prime }$ at the glass transition temperature ($Tg$) and maximum tan $\delta$ were also recorded. Data were analyzed by a 2-way ANOVA followed by Student–Newman–Keuls test ($\alpha =0.05$). Measurements of the inside temperature of each specimen during microwave irradiation (650 W/3 min) were conducted using a fiber optic temperature sensor. Six specimens of each material were evaluated. During microwave irradiation, all specimens reached the boiling temperature of water at approximately 130 s. From the first DMTA run, MW1 and WS significantly increased the $E^{\prime }$ and decreased the tan $\delta$ of K at 37 °C ($P<0.05$). From the last run, the tan $\delta$ of K and TR II was significantly decreased after MW 1 ($P<0.05$). MW 7 decreased the tan $\delta$ of K ($P<0.05$). The $E^{\prime }$ of L and VW was similar and higher than K and TR II ($P<0.05$). At Tg, K produced the lowest $E^{\prime }$ and its maximum tan $\delta$ was increased after MW1 ($P<0.05$). The Tg mean values were arranged as: L>V W>TR II>K ($P<0.05$). Microwave irradiation and WS did not detrimentally affect the viscoelastic properties of the denture base and reline resins evaluated.

Introduction

The use of dental prosthesis can change the oral environment, increasing the total amount of oral microorganisms or part of the oral microflora. Among the disinfection protocols used for eradicating microorganisms from the denture base acrylic resins and controlling cross-contamination, microwave irradiation has proved to be an effective method (Neppelenbroek et al., 2003, Sanita et al., 2009, Silva et al., 2006). In preliminary studies, microwave irradiation at 650 W for 6 min (Neppelenbroek et al., 2003) and 3 min (Sanita et al., 2009) was effective to eliminate pathogenic microorganisms from acrylic resin specimens.

During microwave irradiation, the acrylic resins are immersed in 200 mL of water (Neppelenbroek et al., 2003, Sanita et al., 2009, Mima et al., 2008), which reaches the boiling temperature of water within 1–2 min depending on the power and frequency of the applied microwave (Neppelenbroek et al., 2003). Therefore, it is likely that the glass transition temperature ($T_g$) of the materials may be reached, providing mobility of molecular chains and affecting their physical–mechanical and viscoelastic properties (Urban et al., 2007). It has been observed that the heating generated during microwave irradiation may increase the shrinkage and the water sorption process, thus affecting the dimensional stability of acrylic resins (Urban et al., 2007, Seo et al., 2007, Seo et al., 2008). The plasticizing effect of absorbed water is a well known process (Braden, 1963) and has been implicated in the reduction of mechanical properties of denture base polymers submitted to microwave disinfection (Pavarina et al., 2005). However, to the authors’ knowledge, none of the earlier studies have measured the inside temperature of the acrylic resins during microwave irradiation.

The dynamic mechanical thermal analysis (DMTA) is one of the most sensitive methods to study the behavior of polymers and has been used to characterize the viscoelastic properties of dental materials (Clarke, 1989, Huggett et al., 1990, Phoenix et al., 2004, Saber-Sheikh et al., 1999, Wetton et al., 1991). The DMTA is based on observing the viscoelastic response of a material subjected to a small oscillatory strain, within its elastic limit, at a specific frequency while temperature changes (Wetton et al., 1991). In general, the following factors can be determined: storage modulus ($E^{\prime }$) which determines rigidity and depends on the materials’ ability to store mechanical energy; loss modulus ($E^{″}$) that is associated with the energy absorbed during dynamic deformation and loss tangent ($tan\delta$) that is the ratio between $E^{″}$ and $E^{\prime }$ and is a measure of the fraction of energy lost. In addition, the peak of $tan\delta$ corresponds to the $T_g$ and is the temperature at which polymer chains are able to move rather freely in the polymeric mass, changing from a frozen glassy condition with limit mobility to a totally mobile system (Wetton et al., 1991, Øysaed, 1990). From these properties, the degree of polymerization, presence of plasticizers, effect of post-cure treatments and rigidity can be evaluated.

The aim of this study was to evaluate the effect of microwave irradiation and water storage on the viscoelastic properties of the denture base and reline acrylic resins by DMTA. The temperature change inside resins during microwave irradiation was also assessed. The null hypothesis tested was that the microwave irradiation and water storage would not affect the viscoelastic properties of the materials.