Porosity, water sorption and solubility of denture base acrylic resins polymerized conventionally or in microwave

Abstract The proper selection of polymerization cycle is important to prevent overheating of the monomer that could cause degradation, porosity and, consequently, deleterious effects on the denture base properties. Objective This study evaluated the porosity, water sorption and solubility of acrylic resins (Vipi Cril-VC and Vipi Wave-VW) after conventional or microwave polymerization cycles. Material and Methods Specimens (n = 10) were made and cured: 1-WB = 65°C during 90 min + boiling during 90 min (VC cycle - control group); 2-M25 = 10 min at 270 W + 5 min at 0 W + 10 min at 360 W (VW cycle); 3-M3 = 3 min at 550 W; and 4-M5 = 5 min at 650 W. Afterward, they were polished and dried in a dessicator until a constant mass was reached. Specimens were then immersed in distilled water at 37°C and weighed regularly until a constant mass was achieved. For porosity, an additional weight was made with the specimen immediately immersed in distilled water. For water sorption and solubility, the specimens were dried again until equilibrium was reached. Data were submitted to 2 way-ANOVA and Tukey HSD (α=0.05). Results Porosity mean values below 1.52% with no significant difference among groups for both materials were observed. Resins showed water sorption and solubility values without a significant difference. However, there was a significant difference among groups for these both properties (P<0.013). The highest sorption (2.43%) and solubility (0.13%) values were obtained for WB and M3, respectively. Conclusions The conventional acrylic resin could be polymerized in a microwave since both the materials showed similar performance in the evaluated properties. Shorter microwave cycles could be used for both the materials without any detectable increase in volume porosity.


Introduction
Polymethyl methacrylate acrylic resin has been the material of choice for making denture bases since the beginning of the twentieth century. This material has been modified in order to improve physical and mechanical properties and facilitate the laboratory procedure using microwave polymerization, visible light curing, and vacuum pressure at low temperature curing systems 24 .
During the microwave polymerization, there is a rapid and homogeneous internal heating 6 . This alternative polymerization method has advantages such as ease and cleanliness during acrylic resin processing 12 , rapid temperature increase 17 and, thus, a reduction in execution time 6 , minimum color change in the denture base acrylic resin and less risk of artificial teeth fracture during deflasking 16 .
The importance of the proper selection of the microwave curing cycle has been reported in order to prevent overheating of the monomer that could cause degradation 8 , porosity and consequent injury to the properties of the prosthesis 6 . According to Bafile,et al. 2 (1991), resins designed for microwave polymerization could contain triethylene or tetraethylene glycol dimethacrylate in their composition. These dimethacrylates have low vapor pressure, allowing the polymerization to be carried out at elevated temperatures (between 100°C and 150°C) without risk of porosity, which would not occur with methyl methacrylate since it has a high vapor pressure.
The porosity is a non-desirable characteristic to the acrylic resin denture base 24 . Severe porosity can weaken the prosthesis and result in high internal stress, leading to greater vulnerability to distortion and warpage 29 . A porous surface promotes colonization of the material by oral microorganisms such as Candida albicans 14 and facilitates the retention of substances and deposition of calculus, resulting in staining and impaired aesthetic 24,30 . Water absorbed by acrylic resins during the use of prosthesis acts as a plasticizer and can result in volume changes, so the water sorption evaluation also has clinical relevance. Furthermore, residual monomer and other water soluble byproducts are released into the oral cavity and may cause tissue irritation; therefore, it is desired that these materials have low solubility 5 . D e s p i t e t h e a d v a n t a g e s o f m i c r o w a v e polymerization, this method has gained only limited clinical acceptance, and materials specifically formulated for use in a microwave have a higher cost compared to those for conventional polymerization.
Thus, aiming to overcome these disadvantages, the polymerization of a conventional resin was tested in a microwave oven, and this study evaluated the porosity and water sorption and solubility properties of the acrylic resins Vipi Cril (VC) and Vipi Wave (VW) after polymerization using experimental microwave cycles and the cycles recommended by the manufacturer.
The hypotheses evaluated in this study were: 1) there would be differences regarding the porosity results between the materials in the evaluated polymerization cycles; and 2) the water sorption and solubility results would be different between the materials conventionally polymerized or cured in a microwave.

Material
The acrylic resins selected for this study are shown in Figure 1.

Specimen preparation
For the analysis of porosity, specimens (n=10) were made in the dimensions of 50x4x2 ±0.1 mm 10 .
Metal matrixes in such dimensions were molded using laboratory silicone (Zetalabor-Zhermack; Labordental,  in two stages. In the first one, the specimen was daily weighed after a storing period in a desiccator (Vidrolabor, São Paulo, SP, Brazil). In the second one, the specimen was also daily weighed after a storing period in distilled water in an oven (New Instruments, Piracicaba, SP, Brazil) at 37°C. The final record of weighing for dry and wet specimens was carried out at the time that they reached a stable mass, evidenced after stabilization in a milligram scale.

Material
The porosity was analyzed using the gravimetric method based on Archimedes' principle 2,4,18,21 and, therefore, an additional weighing was performed after each stage with the specimen immersed in distilled water.
Thereafter, the porosity was calculated according to the following equations: Vs dry = (m d -m d ')/ρ water [1] Vs wet = (m w -m w ')/ρ water [2] Porosity % = 100 x (Vs dry -Vs wet)/Vs dry [3] where: Vs dry (mL) is the volume of the dry specimen; m d (g) is the mass of the dry specimen recorded in air; m d ' (g) is the mass of the dry specimen recorded with the specimen immediately immersed in water; ρ water (g/mL) is the density of water; Vs wet (mL) is the volume of the wet specimen; m w (g) is the mass of the wet specimen recorded in air; and m w ' (g) is the mass of the wet specimen recorded with the specimen immediately immersed in water.

Water sorption and solubility
Specimens were subjected to a drying process in order to achieve a constant weight. So, they were  kept in a vacuum desiccator and daily weighed on a digital analytical balance until the difference between sequential weight measurements was less than 0.5 mg. After obtaining the constant mass, specimens were stored in distilled water at 37°C and were also daily weighed until stabilization, but always after careful drying with absorbent paper.
To calculate the water sorption and solubility, specimens passed again by the above drying process.
The water sorption and solubility percentages were calculated using the following equations 15 : % Sorption = 100 x (m 2 -m 3 )/m 1 [4] % Solubility = 100 x (m 1 -m 3 )/m 1 [5] where: m 2 is the mass (mg) of the specimen after immersion in water; m 3 is the mass (mg) of the specimen after the second drying; and m 1 is the mass (mg) of the specimen after the first drying.

Statistical analysis
The results of porosity, water sorption and solubility (%) were analyzed using two-way ANOVA ("material" and "polymerization cycle" factors) and followed by

Results
For the number of specimens used to evaluate the porosity, water sorption and solubility properties (n=10) of denture base acrylic resins, this study showed adequate power for both factors "material"

Discussion
The first hypothesis evaluated in this study, in which "there would be differences regarding the porosity results between the materials in the evaluated polymerization cycles", was not accepted, since no significant differences were observed between the acrylic resins conventionally polymerized in the waterbath or processed in a microwave.    26 . This is because the dissolution of the monomer is 0.5 to 1.7x10 -2 times smaller than the water sorption of the resin based on polymethyl methacrylate 25 .
The acrylic resins contain polar carbonyl groups that therefore attract water molecules 26