Surface roughness is an important property of acrylic resin since denture bases are in contact with buccal tissues, and a rough surface may affect tissues health due to microorganism accumulation¹. Smooth and highly polished denture surface are of utmost importance for patient comfort and denture longevity, and it is desired for reducing microorganism’s retention². It can improve good aesthetical results, oral hygiene and low plaque retention, preventing oral diseases³. Finishing and polishing procedures are essential steps in obtaining a smooth acrylic surface³. Two polishing methods are frequently used in the denture finishing: mechanical and chemical process. Mechanical polishing uses abrasive drills and aluminum oxide sandpapers in decreasing granulations, pumice slurry with felt cone and chalk powder with a soft brush at a bench vise^4-5. As the mechanical technique, chemical polishing is biocompatible, and was developed to eliminate the abrasive polishing sequence, and to save time. In this case, the finished prosthesis is immersed in monomer heated at 100.3ºC⁶.

Nishii⁷ introduced polymerization of acrylic resin by microwave energy in 1968. This technique can be considered practicable because it is a clean and practical method and avoids wasting time in addition. Possible homogeneous heating of gypsum and acrylic resin mass during polymerization is also an advantage because it may reduce acrylic tensions and distortions^7-8. However, temperatures over 90ºC could promote tension and distortions of acrylic resin, increasing the possibility of superficial roughness⁹. Careful surface texture measurement cans facilitate understanding of how the acrylic will stain or wear in vivo¹⁰. Response to oral hygiene measure such as tooth brushing is an important factor in clinical performance of the material. Studies related to methods of polishing acrylic resin and microwave energy polymerization are necessary in order to assure the safety and reliability of their use.

The aim of this study was to evaluate the influence of polymerization methods (microwave energy - MW and water bath - WB) and polishing techniques (chemical - C and mechanical - M) on the surface roughness of one heatpolymerizes acrylic resin.

Material and Methods

Steel disk-shaped patterns in accordance to Specification nº 12 of American Dental Association (A.D.A.)¹¹, were used to prepare forty heat polymerized acrylic resin samples (50±1mm in diameter and 0.5±0.05 mm in thickness). The patterns were included in metallic flasks (DCL^® n.5.5, Bethil Ind. Ltd, Campinas, São Paulo, Brazil) and polyvinyl chloride (PVC) flasks (Onda-Cryl^®, Artigos Odontológicos Clássico Ltd, São Paulo, São Paulo, Brazil), to make the gypsum molds for constructing resin samples (Figure 1). Acrylic resin samples were prepared and divided in 4 groups (10 samples each) according to polymerization methods and polishing procedures (Table 1).

Heat-polymerized acrylic resin developed to microwave and heat water polymerization (Vipi Cril^®, Dental Vipi Artigos Odontológicos Ltd, Pirassununga, São Paulo, Brazil) was prepared according to manufacturer‘s instructions, and included in the gypsum molds^12-13.

Microwave energy (MW) was used in processing half samples, and the other half was processed by water bath (WB), according the following methods: metallic flask, by water bath for 9 hours at 74 ± 1°C (Righetto & Salin Comp, Campinas, São Paulo, Brazil); and PVC flasks, by microwave energy (Panasonic MN 7806 BH, 1380W, Amazon, Brazil), at 500W for 3 minutes. After polymerization cycles, the flasks were allowed to bench cool for 3 hours⁸.

Machine grinding APL 04 (Arotec, São Paulo, Brazil) procedures were conducted employing abrasive papers in successive grits of 320, 400 and 600. During finishing procedures, the samples were held by a steal cylindrical support to standardize pressure and thickness^12-13. Steel diskshaped patterns and digital caliper (Starrett^®, Microtec Ltd, São Paulo, Brazil) were used to hold and to control thickness specimens during all finishing procedures.

Ten WB samples and ten MW samples were mechanically polished in a bench vise with soft brush and pumice slurry, flannel wheel and chalk powder^12-13. In order to standardize pressure applied on sample surface during mechanical polishing, the stainless steel support was used¹⁰ (Figure 2). Ten WB samples and ten MW samples were chemically polished in a chemical polisher PQ 9000 (Termotron^®, Piracicaba, São Paulo, Brazil) with monomer heated at approximately 75± 1 ºC (VIPI^®, Dental Vipi Artigos Odontológicos Ltd, Pirassununga, São Paulo, Brazil). The samples were immersed in the fluid for 10 seconds (Figure 3), subsequently dried for 15 seconds, and washed in current water for 1 minute to remove excess monomer^10.

We consider that is extremely important to consider the roughness values attained during the course of denture preservation. Since surface roughness of chemical polishing are dependent on acrylic resin type and processing method, and it don’t occurs when resin is polished by mechanical methods and processed by microwave energy, we suggest that the association of mechanical polishing and microwave processing could be used clinically. However is considered that additional studies are necessary to confirm the log-term behaviors of surface roughness, polymerization and polishing techniques, and other properties of resins.

In conclusion, based on the results obtained, we can conclude that: 1)The polymerization process (microwave or water bath) didn’t influence surface roughness values of heat-polymerized acrylic resin; 2) The polishing method (mechanical or chemical) influence surface roughness values of heat-polymerized acrylic resin; 3) Mechanical polishing promoted smoother surfaces than chemical polishing.

References