Original Research Influence of heat treatment on the sorption and solubility of direct composite resins Gabrielle Ribeiro Lima Muniz1, Erick Miranda Souza1, Carolina Carramilo Raposo1, Ivone Lima Santana1 Departments of Dentistry, 1 Federal University of Maranhão, São Luís‑MA, Brazil
Received : 30‑01‑13 Review completed : 02‑08‑13 Accepted : 23‑08‑13
ABSTRACT Context: Heat treatment allows the use of direct composite resins for fabrication of inlays/onlays restorations because it improves some mechanical and physical properties. Aim: The aim of this study is to analyze the influence of heat treatment on the water sorption and solubility of direct composite resins compared with an indirect composite resin. Materials and Methods: A total of 50 cylindrical specimens were fabricated (6 mm diameter × 2 mm high) and divided into five groups (n = 10): G1 (FillMagic without heat treatment-control 1), G2 (heat‑treated FillMagic), G3 (P60 without heat treatment-control 1), G4 (heat‑treated P60) and G5 (indirect resin Epricord-control 2). After fabrication, the specimens were placed in a desiccator containing silica gel and maintained at 37°C for 24 h. This cycle was repeated until a constant weight was achieved (m1). Following, the specimens were stored in individual flasks containing 2 ml of distilled water in an oven at 37°C. The specimens were weighed after intervals of 1, 7 and 21 days of immersion in water (m2). After 21 days of storage in water, the specimens were once again desiccated until a constant weight was achieved (m3). The mean diameter and thickness of specimens were obtained using a digital pachymeter. Statistical Analysis Used: Two‑way analysis of variance and Tukey’s test were used to compare the sorption and solubility (α = 0.05). Results: The type of resin significantly influenced the sorption (P = 0.01) and solubility (P = 0.00). The heat treatment also significantly influenced the sorption (P = 0.026) and solubility (P = 0.01). Conclusion: It was concluded that the heat treatment is an additional curing method that improves strength to the sorption and solubility of composite resins. Key words: Composite resins, solubility, water
Composite resins should ideally be stable, yet this usually does not occur.[1,2] Several physical changes may happen because of the curing reaction and subsequent interaction with the oral environment.[3] When the resin contacts the water, two different mechanisms take place: Water sorption, which causes weight gain and solubility of components as fillers and residual monomers, which causes weight loss.[4] The sorption and solubility phenomena may precede several physical Address for correspondence: Prof. Ivone Lima Santana E‑mail:
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Website: www.ijdr.in PMID: *** DOI: 10.4103/0970-9290.127617
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and chemical processes that cause deleterious effects in the composite resin structure, which may impair its clinical performance[1,5] and mechanical properties.[6] It is known that the degree of conversion of composite resins may directly affect their clinical performance. The greater the quantity of monomers transformed in polymers, the higher will be the degree of conversion of composite resins and the better will be the material properties.[7,8] The degree of conversion may also influence the water sorption and solubility of composites, since the inadequate curing of the material increases the sorption and solubility of composites[9] because the polymeric chain may present lower density of cross‑links.[10,11] Polymeric chains with lower density of cross‑links are more susceptible to the action of solvents and consequently to plastification. The cross‑links usually reduce the permeability of the polymer by reducing the existing free volume.[12] Secondary curing is a method employed to increase the degree of conversion of composite resins. Heat treatment is one example of secondary curing. Laboratory studies Indian Journal of Dental Research, 24(6), 2013
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indicate that this method enhances the physical and mechanical properties of these materials.[13‑15] Considering that direct and indirect composite resins have similar compositions, it might be possible to use simple technical modifications, such as additional thermal treatment, to enhance the mechanical resistance of cheaper direct composite resins up to values similar to those of indirect resins.[16] Therefore, the utilization of direct resins for fabrication of indirect resins would be a viable alternative. Taking into account that the additional curing may increase the degree of conversion of composite resins, which in turn may influence the sorption and solubility of composites, this study evaluated the influence of heat treatment on the sorption and solubility of two direct composite resins after 21 days of storage in distilled water, compared with an indirect composite resin. The following null hypotheses were tested: (1) heat treatment does not influence the water sorption and solubility values of the resins evaluated and (2) there is no difference in the water sorption and solubility values of these resins.
MATERIALS AND METHODS Fabrication of specimens
A total of 50 specimens were fabricated (n = 10), being 20 with the microhybrid composite resin FillMagic enamel A3 (Vigodent S.A. Ind. e Com, Bom Sucesso, RJ, Brazil), 20 with the hybrid resin Filtek P60 A3 (3M ESPE, St‑Paul MN, USA) and 10 with the indirect resin Epricord Enamel E1 (Kurakay, CO., LTD., Tokyo, Japan) [Table 1], using a split round metallic template measuring 6 mm diameter and 2 mm height. The two parts of the template were placed in a metallic ring to provide stability. The template was placed on a glass slab followed by a polyester strip (PREVEN Indústria e Comércio de Produtos Odontológicos Ltda EPP, Guapirama, Paraná, Brazil) and the resin was placed in the template using a titanium instrument. Another polyester strip was placed and a glass slab was pressed on it until the excess resin was completely extruded. Table 1: Characteristics of composite resins employed Material Composition Batch Fill Methacrylic monomers, pyrogenic silica, barium 011/10 magic and aluminum silicate, 80% wt. of barium glass (0.5 µm). BIS‑GMA, TEGDMA, BIS‑EMA and UDMA Filtek™ Aluminum oxide, zirconia/silica, 75.9% wt. N155323 P60 (0.01‑3.5 µm). BIS‑GMA, UDMA and BIS‑EMA Epricord Barium/borosilicate glass and aluminum, 82% 136AA in weight (0.6 µm). Filler composed of UDMA and BIS‑GMA BIS‑GMA=Bisphenol A glycidyl dimethacrylate, TEGDMA=Triethylene glycol dimethacrylate, BIS‑EMA=Bisphenol A polyethylene glycol diether dimethacrylate, UDMA=Urethane dimethacrylate
Indian Journal of Dental Research, 24(6), 2013
The glass slab was removed and the resin was cured for 40 s by irradiating the upper aspect using the optic fiber tip of the appliance light emitting diode laser (Dabi Atlante Equipamentos odontológicos, Ribeirão Preto, SP, Brazil) in close contact with the polyester strip. This appliance has a light intensity of 600 mW/cm². Thus, cylindrical resin blocks were obtained with similar dimensions as the template. Afterward, the specimens were stored in dry and dark flasks. The specimens were divided into five groups with 10 specimens each: G1 (FillMagic without heat treatment-control 1), G2 (heat‑treated FillMagic), G3 (P60 without heat treatment-control 1), G4 (heat‑treated P60) and G5 (indirect resin Epricord-control 2). The groups G1 and G3 (controls 1) were not submitted to any additional treatment, maintaining only the light cured condition. The groups G2 and G4 were submitted to heat treatment using dry heat in an oven at 170°C for 10 min, then removed and placed on a surface at room temperature. The time and the temperature had been previously standardized in thermal analysis studies.[14,15] The resin Epricord (group G5-control 2), indicated for indirect restorations, was initially cured using the same template as described for the other resins. Then, following the manufacturer’s instructions, it was placed in a light curing unit for 240 s (Foto‑Lux, Futura Brasil Equipamentos Odontológicos, São Carlos, São Paulo, Brazil).
Water sorption and solubility tests
The water sorption and solubility tests were conducted according to the ISO standard 4049, except for the dimensions of specimens and period of water storage, which was extended up to 21 days. All specimens were placed in a desiccator containing silica gel and stored at 37°C for 24 h. After this period, they were kept in the desiccator for 1 h at 23°C and then weighed in an analytical scale with 0.0001 g precision (Ohaus Adventurer, Toledo do Brasil Indústria de Balanças Ltda, São Bernardo do Campo, SP, Brazil), until a constant weight was achieved (m1). This dehydration process was repeated until the weight loss was smaller than or equal to 0.2 mg in a 24 h period. Following, the specimens were stored in individual flasks containing 2 ml of distilled water in an oven at 37°C. All specimens were weighed in intervals of 1, 7 and 21 days of water storage. For each weighing, the specimens were removed from the water, weighed, dried with absorbent paper (on both sides and without pressure for 15 s), immediately weighed in an analytical scale (m2) and returned to the distilled water at 37°C. The water in the recipients of all specimens was changed weekly. After 21 days of water storage, the specimens were once again submitted to the desiccation process and weighed daily until a constant weight was achieved (m3). 709
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The mean diameter and thickness of specimens were obtained from two measurements of diameter and five measurements of thickness. The diameter was measured by tracing two lines that crossed in the center of each specimen, forming a right angle. The thickness was measured on the center of the specimen and at four equidistant points. These dimensions were measured using a digital pachymeter Starrett 799 (Starrett Indústria e Comércio Ltda, Itu, SP, Brazil) with 0.01 mm precision. The volume in mm3 of each specimen was calculated by multiplying the base area by the thickness (cylinder volume = πr2h). The initial weight obtained after the first desiccation (m1) was used to calculate the percentage of weight variation at each time interval during the 21 days of water storage.
G5 (P
