Braz J Oral Sci. October-December 2007 - Vol. 6 - Number 23
Influence of chemical and mechanical polishing on water sorption of acrylic resins polymerized by water bath and microwave irradiation Morgana Nicoleti Gabrioti1* Lais Regiane da Silva Concilio2* Margarete Cristiane Ribeiro-Dasilva2* Jeremiah Edillor Juson3 Célia Marisa Rizzatti Barbosa4* 1*
Undergraduate student PhD student in Dental Prosthesis 3 Resident – Eastman Dental Center, University of Rochester,,New York, USA 4* Full Professor - Department of Prosthesis and Periodontology *Piracicaba Dental School, State University of Campinas 2*
Received for publication: May 16, 2007 Accepted: November 21, 2007
Abstract Previous studies have investigated the possible influences to different types of polymerization and polishing procedures in some properties of acrylic resins. Water sorption is an important property, which may be induced by these factors. The aim of this work was to evaluate water sorption in acrylic resin processed in microwave energy or water bath, after chemical and mechanical polishing. Forty heatpolymerized acrylic resin (Vipi-Cril, Dental VIPI Ltd, Pirassununga, São Paulo, Brazil) specimens were made according to ADA Specification No.12 for Denture Base Polymers and divided into 4 groups: Groups I and II were processed in microwave energy (500 W for 3 minutes); Groups III and IV, in water bath (73ºC ± 1ºC for 9 hours). Groups I and III were polished chemically; Groups II and IV were polished mecahnically. The specimens were submitted to water sorption test. The water sorption values (mg/cm2) were calculated and submitted to ANOVA and Tukey´s test (5% significance level). The obtained results were: Group I: 0.000953, Group II: 0.001069, Group III: 0.000958 and Group IV: 0.001491. No statistical difference were found among the groups (p>.05). Polymerization by microwave energy or water bath associated with chemical or mechanical polishing did not influence the water sorption of the heat polymerized acrylic resin evaluated. Key-words: resin; microwave energy; water sorption; polishing.
Correspondence to: Célia Marisa Rizzatti Barbosa Departamento de Prótese e Periodontia Faculdade de Odontologia de Piracicaba, UNICAMP Avenida Limeira, 901 CEP: 13414-018 Piracicaba, SP, Brasil Phone: + 019 2106 5373 E-mail:
[email protected]
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Braz J Oral Sci. 6(23):1442-1444 Influence of chemical and mechanical polishing on water sorption of acrylic resins polymerized by water bath and microwave irradiation
Introduction Acrylic resin denture bases may be altered by many factors during laboratorial steps1. Loss of retention and stability of dentures under clinical conditions is usually caused by dimensional changes that occur in the acrylic resin base due to heat from the polymerization process upon resin base water loss or uptake, stress release and base flexural fatigue2-3. Water sorption seen in clinical use following denture processing promotes expansion of the resin, thus compensating the polymerization shrinkage2,4. It is because of the resin ability to absorb water when immersed in a moist environment during a time period1. Water molecules, as a superficial plasticizer agent, promote expansion of polymers in the polymeric chain1. Both resin polymerization and polishing procedures may influence water sorption 4 . Studies confirm that resin polymerization by water bath processing (73ºC during 9 hours) promote higher water sorption amount than heating it by microwave energy4-5. The resin surface polishing and amount of residual monomer are also directly related to water sorption of acrylic resin4,6. It was proved that mechanical polishing can promote distortions on resin surface due to superficial heating4. In addition superficial heating promoted in chemical polishing can also affect properties of acrylic resins, modifying the polymeric molecular structure7- 8. The aim of this work was to evaluate water sorption in a heat-polymerized Brazilian resin, processed by microwave energy or water bath, after chemical or mechanical polishing. Material and Methods Forty heat-polymerized acrylic resin specimens were made according to the ADA Specification No 12 for Denture Base Polymers9, and divided into 4 groups with 10 samples each. The groups were classified according to processing and polishing treatments: Group I: microwave irradiation and chemical polishing; Group II: microwave irradiation and mechanical polishing; Group III: warm water bath and chemical polishing; Group IV: warm water bath and mechanical polishing (Table 1). The specimens of Groups I and II were invested in poly-vinyl chloride flasks (PVC) (Onda Cril; Dental VIPI Ltd, Pirassununga, São Paulo, Brazil),
and the specimens of Groups III and IV were invested in metallic flasks (DCL n.5.5; Bethil Comp Ltd, Sao Paulo, Brazil), with type III stone (Herodent-Soli-Rock; Vigodent S/A Industry, Rio de Janeiro, Brazil). After stone setting, each flask was opened and washed with current water and detergent to remove petroleum jelly residues. Sodium alginate coating (VIPI Coating; Dental VIPI Ltd, Pirassununga, São Paulo, Brazil) was applied on the stone surface1 and heat-polymerized acrylic resin (Vipi Cril; Dental VIPI Ltd, Pirassununga, São Paulo, Brazil) was prepared and placed in the flasks according to manufacturer’s instructions. The flasks were placed in a hydraulic press for an hour, under the pressure of 1.25 tons. Polymerization procedures started half an hour after pressing, in order to stabilize tensions. The specimens invested in metallic flasks were processed in water bath (74±1ºC for 9 hours), and those invested in PVC flasks were processed in microwave oven (500 W for 3 minutes) (Panasonic, Brazil, model MN 7806 BH - 1380 W). After resin polymerization, the flasks were cooled on the bench until it reached the room temperature1. Specimens were deflasked and had underwent finishing procedures with tungsten drills in low rotation and aluminum oxide sandpapers of decreasing granulations (180, 220 and 400)1. A cylindrical stainless steel support with the same size of the specimens was used in order to apply the same pressure to all specimens during finishing and mechanical polishing. The thickness of the specimens was controlled using a digital caliper ruler. Chemical polishing was carried through the immersion of the specimens in a proper warm fluid for polishing (Dental VIPI Ltd, Pirassununga, Sao Paulo, Brazil) using a chemical polisher (model Pq-9000, Termotron, Sao Paulo, Brazil) for 10 seconds on 75±1ºC. After that, the specimens were removed, left in room temperature for 15 seconds and washed in current water during 1 minute to eliminate excess of polishing fluid. Mechanical polishing was carried through using bench vise, soft brush, flannel, chalky powder and slurry pumice1. The superficial area (A) of each specimen was calculated (cm2) based on mean values obtained after 3 measurements of diameter. The specimens were placed in a recipient, containing dry and fresh silica gel and were stored in a kilt under 37±1ºC for 23±1 hours. After this period, the recipient
Table 1 - Groups classification according to processing and polishing treatments, and averages of water sorption (mg/cm2) obtained. GROUP
POLYMERIZATION
POLISHING
I
Microwave irradiation500 W / 3 min.
Chemical
MEANS 0.000953 a
II
Microwave irradiation500 W / 3 min.
Mechanical
0.001069 a
III
Water bath73ºC / 9 hours
Chemical
0.000958 a
IV
Water bath73ºC / 9 hours
Mechanical
0.001491 a
* Groups with equal letter did not differ, considering 5% of significance level by ANOVA and T test.
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Braz J Oral Sci. 6(23):1442-1444 Influence of chemical and mechanical polishing on water sorption of acrylic resins polymerized by water bath and microwave irradiation
was placed on a bench for as hour, until reaching room temperature (23 ± 2ºC). The specimens were weighted using an analytical balance scale with the accuracy of 0.0001g. This conditioning cycle of each specimen was repeated for 24 hours until calculated mass (m1) became inferior to 0.2 mg9. The specimens were stored in distilled water, 37 ± 1ºC for 30 days. After this period, the specimens were removed from water, dried with paper towel and was weighted again using the same analytical balance (m2). Sixty seconds was observed during weighting. The second desiccation process was carried through similar to the first one. The obtained mass was called m3. Water sorption (mg/cm2) was calculated using the following formula: Water sorption (mg/cm3) = (mass after immersion (mg) conditioned mass (mg) / Surface Area (cm2) Data were submitted to statistical analysis using ANOVA and Tukey’s Test (5% significance). Results The following average values of water sorption (mg/cm2) were obtained in the groups: I: 0.000953; II: 0.001069; III: 0.000958 and IV: 0.001491 (Table I). No statistical difference was found among the groups (p>.05). Discussion Water sorption is an important property of acrylic resin since it can compensate contraction, promoting better adaptation of prosthesis bases2. Water absorbed by acrylic resin stays in gaps among the interpolymeric chains that form acrylic resin structure. The magnitude of these interpolymeric gaps determines the amount of water to be absorbed. Better polymerization of acrylic resin increases the crosslinking and reduces water sorption values. In this study, water sorption was evaluated for heat-polymerized acrylic resins processed by 2 different methods. No statistically significant difference (p>0.05) was found between the groups processed by microwave energy (I and II) or water bath (III and IV). This result is consistent with previous studies, which have also found that the quality of the polymerization obtained by microwave energy was similar to that obtained by conventional water bath technique 2,5. The findings of the present study suggest that microwave polymerization had no influence on water sorption. This can be explained by the fact that the microwave heating is dielectric, as the heat is generated in the mass to be polymerized. Polar molecules, like water and acrylic resin monomer, vibrate billions of cycles per second when exposed to microwave electromagnetic field. This vibration promotes intermolecular shocks that generate homogeneous heat within the resin mass, resulting in an adequate polymerization2. Some studies have found that the quality of polymerization obtained in microwave irradiation technique was similar to
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conventional water bath 2; other study 4 showed that resin processed by water bath presented higher water sorption in the first hour of water storage. In this experiment it was observed that the arrangement of polymeric chains in the microwave cured specimens had no effect on the water molecules diffusion, showing similar result of water bath specimens. Chemical polishing promotes heating of the polymerized resin mass and may cause softening of most superficial mass, modifying the molecular structure and, consequently, the property of water sorption1. However, there was no statistical difference among the groups polished by chemical (I and III) and mechanical polishing (II and IV). It was probably because change in molecular structures of resin are only observed when the heating is above 90ºC 1 ; in the present study, monomer fluid temperature used for chemical polishing reached no more than 75ºC. So the monomer temperature was not high enough to promote significant alterations in molecular resin chain and consequently change in the water sorption. Despite of superficial layer heating by the from chemical polishing it altered alters some properties of the resin7-8, probably because there was insufficient it was not sufficient heat to modify the magnitude of the interpolymers gaps and did not influence and consequently to alter the water sorption in the whole of the resin mass. Considering the purpose and the experimental conditions of this work, it was observed that there was no influence of the chemical and mechanical polishing in water sorption of acrylic resins polymerized by water bath or microwave irradiation. References 1. 2.
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Phillips RW. Skinner dental materials. 9th ed. Rio de Janeiro: Guanabara Koogan; 1993. p. 140-60 Rizzatti-Barbosa CM, Del Bel Cury AA, Panzeri H. Influence of water sorption and microwave polymerization on denture adaptability. Rev Fac Odont S Paulo. 1995; 9: 197-206. Pavan S, Arioli Filho JN, Dos Santos PH, Mollo Jr. FA. Effect of microwave treatments on dimensional accuracy of maxillary acrylic resin denture base. Braz Dent J. 2005; 16: 119-23 Rahal JS, Mesquita MF, Henriques GEP, Nóbilo MAA. Influence of chemical and mechanical polishing on water sorption and solubility of denture base acrylic resins. Braz Dent J. 2004; 15: 225-30. Nishii M. Studies on the curing of denture base resins with microwave irradiation with particular reference to heat curing resins. J Osaka Dent Univ. 1968; 2: 23-40. Von Fraunhofer JA, Suchatlampong C. The surface characteristics of denture base polymers. J Dent. 1975; 3: 105-9. Araújo PA, Abreu D, Magalhães AGO. Properties of denture base polymers subjected to chemical polishing method. I – transversal, deflexion, hardness and deformations. Estomat Cult. 1972; 6: 40-4. Gotusso MJ. Chemical and superficial treatment of acrylic resins. Rev Asoc Odontol Argent. 1969; 57, 359-61. American Dental Association. Revised American Dental Association Specification N. 12 for denture base polymers. J Am Dent Assoc. 1975; 90: 451-8.
