| 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 (m 1 ). 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 (m 2 ). After 21 days of storage in water, the specimens were once again desiccated until a constant weight was achieved (m 3 ). 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.
Keywords: Composite resins, solubility, water
|How to cite this article:|
Muniz GL, Souza EM, Raposo CC, Santana IL. Influence of heat treatment on the sorption and solubility of direct composite resins. Indian J Dent Res 2013;24:708-12
Composite resins should ideally be stable, yet this usually does not occur. , Several physical changes may happen because of the curing reaction and subsequent interaction with the oral environment. 
|How to cite this URL:|
Muniz GL, Souza EM, Raposo CC, Santana IL. Influence of heat treatment on the sorption and solubility of direct composite resins. Indian J Dent Res [serial online] 2013 [cited 2019 Oct 23];24:708-12. Available from: http://www.ijdr.in/text.asp?2013/24/6/708/127617
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.  The sorption and solubility phenomena may precede several physical and chemical processes that cause deleterious effects in the composite resin structure, which may impair its clinical performance , and mechanical properties. 
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. , 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  because the polymeric chain may present lower density of cross-links. , 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. 
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 indicate that this method enhances the physical and mechanical properties of these materials. ,,
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.  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, Paranα, 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. 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. ,
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 Odontolégicos, 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 (m 1 ). 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 (m 2 ) 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 (m 3 ).
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 mm 3 of each specimen was calculated by multiplying the base area by the thickness (cylinder volume = Πr 2 h).
The initial weight obtained after the first desiccation (m 1 ) was used to calculate the percentage of weight variation at each time interval during the 21 days of water storage.
The properties of water sorption (W sp ) and solubility in water (W sl ) after 21 days of water storage were calculated in micrograms per cubic millimeter (μg/mm 3 ), according to the following equations proposed in the ISO standard 4049:
Data were initially analyzed on the software statistical package for social sciences (SPSS Inc., Chicago, IL, USA) for Windows 17.0. Since they presented normal and homogeneous distribution, the multivariate analysis test (analysis of variance) was applied considering two criteria (type of resin and heat treatment). When the test indicated significant differences, the Tukey's test was used to compare the mean sorption and solubility (P < 0.05).
| Results|| |
Heat treatment significantly influenced the sorption (P = 0.026) [Graph 1] and solubility (P = 0.01), i.e., the heat treatment reduced the sorption and solubility of the resins analyzed.
The type of resin also significantly influenced the sorption (P = 0.01) and solubility (P < 0.01). The Groups G1 and G2 presented lowest water sorption values, followed by G3, G4 and G5, with significant differences between all groups (P < 0.01). With regard to solubility, the Groups G4 and G3 exhibited the lowest values, followed by G2, G1 and G5 (P < 0.01). Except for the G5, all study groups presented negative solubility values [Table 2].
|Table 2: Means±SD of water sorption and solubility values (in μ/mm3) for the five study groups and statistical analysis for each property evaluated* |
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Graph 2 presents the percentage of weight variation of the study groups, according to the period of storage in distilled water. All groups exhibited an increase in weight after 7 days of storage.
| Discussion|| |
According to the ISO standard 4049, in order to be indicated as restorative materials, composite resins should present water sorption smaller than 40 μg/mm 3 and solubility smaller than 7.5 μg/mm 3 after a storage for 7 days.
Both sorption and solubility values obtained in this study were lower than those indicated in the ISO standard [Table 2], even in storage periods of 21 days in water. The specimens exhibited an increase in weight after 7 days of storage [Graph 2], suggesting that the 7 days recommended on the ISO standard are not enough to evaluate the actual water sorption values of resin materials.
The temperatures and periods used in some heat treatments may vary and thus cause structural alterations in the composite resin. , In order to enhance the physical and mechanical properties, the temperature of heat treatment should be close than the temperature of glass transition of composite resin in order to effectively homogenize and modify the structure of the polymeric chain, increasing the number of cross-links, making the polymer denser and consequently more resistant.  The temperature and time of heat treatment in this study were standardized by previous thermal analysis (thermogravimetry and differential scanning calorimetry) to determine the temperature of onset of weight loss and the glass transition of composite resins. , The first null hypothesis was rejected, because the heat treatment influenced the sorption (P = 0.026) and solubility (P = 0.01) of both resins analyzed, i.e., the sorption and solubility values of the resins FillMagic and P60 submitted to heat treatment were smaller compared with the same resins submitted only to light curing.
In this study, the type of resin significantly influenced the sorption and solubility and thus the second null hypothesis was also rejected. According to the literature, the different results found in the water sorption and solubility values were probably related to the composition of materials. The hydrophilicity of constituent monomers may influence the water sorption.  Among the commercially available light cured resins, many are combinations of bisphenol A glycidyl dimethacrylate (Bis-GMA), urethane dimethacrylate, triethylene glycol dimethacrylate and more recently the bisphenol A polyethylene glycol diether dimethacrylate (Bis-EMA). The Bis-EMA has been used in more recent composite resins because it is a less hydrophilic monomer, which presents similar molecular structure as the Bis-GMA, yet with two fewer hydroxyl groups.  The resins FillMagic and P60, which presented lower water sorption values than the indirect resin Epricord, present the monomer Bis-EMA in their compositions, which may explain these outcomes.
In addition, the inorganic components may also contribute to the different sorption values. Even though, the water sorption in the fillers is not remarkable,  there may be accommodation of water between the fillers and the matrix, which would impair this union over time.  The water absorbed diffuse into the filler-matrix interface or microvoids of the composite.  The differences observed in the water sorption may be assigned to the nature of fillers and the effectiveness of their silanization. ,, The resin FillMagic is basically composed of glass particles and the resin P60 is composed of silica-zirconia, which may be related to the less effective silanization.  This may explain why the resin P60 presented higher water sorption values compared to the resin FillMagic.
The resin Epricord is cured only by light, in a light curing unit, different from other indirect resins that make use of light and also other resources as heat, vacuum, pressure or nitrogen with a view to optimize the curing. In this study, the resin Epricord presented the highest water sorption and solubility values, even higher than the light cured direct resins (P < 0.01), indicating that indirect resins submitted to only one curing method should not be recommended for indirect restorations, in disagreement with the manufacturer's instructions. These materials should associate other treatments in an attempt to maximize their degrees of conversion and thus their physical properties.
The solubility of composite resins reflects the quantity of unreacted monomers released in the water, as well as fillers and photoinitiators, which are low weight molecules. In fact, any component in the composite resin may be solubilized.  The high quantity of glass particles found in the resin FillMagic may cause greater lixiviation of other inorganic components, especially silicon,  which may explain why this resin presented higher solubility compared to the resin P60.
Negative solubility values were observed for the resins FillMagic and P60, with and without heat treatment, which masked the actual solubility.  This does not indicate that no solubility occurred, but rather that water sorption in these groups was greater than the solubility, because the final weight was greater than the initial weight. It is believed that some water molecules bonded to polymeric chains through hydrogen bridges were maintained firmly bonded to the polar sites along the polymeric chain,  preventing the complete removal of this solvent after desiccation.
Comparison of the sorption and solubility values of the indirect resin Epricord with the direct resins FillMagic and P60 (with and without heat treatment) demonstrates that the idea to use cheaper direct composite resins for indirect restorations may be a viable alternative, however additional laboratory and clinical studies are necessary to allow the utilization of these resins in indirect restorative procedures.
| Conclusion|| |
The heat treatment influenced the water sorption and solubility, i.e., it reduced the sorption and solubility values of the resins analyzed; although, more studies are needed to indicate direct composite resins for indirect restorations.
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Ivone Lima Santana
Departments of Dentistry, Federal University of Maranhão, São Luís MA
Source of Support: This study was partially funded by CAPES Coordination
for the Improvement of Higher Education Personnel,, Conflict of Interest: None
[Table 1], [Table 2]