|Year : 2006 | Volume
| Issue : 2 | Page : 63-71
|Thermal cyclic changes on water sorption and solubility of composite restoratives - An in-vitro study
Aduma Meena Reddy, Dhanya Kumar, Vasundhara Shivanna
Department of Conservative Dentistry and Endodontics, College of Dental Sciences, Davangere 577004, India
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| Abstract|| |
Background & Objectives : The aim of the study was to evaluate the effect of thermal cycling on water sorption and solubility of three commercially available composites namely Microfilled (Filtek A110), Hybrid (Z- 100) and Packable (Filtek P-60).
Methodology: Thirty disc specimens of dimension 10 1 mm diameter and I ± 0.1 mm thickness were made using each group of composite and randomly divided into three sub groups. All the specimens were subjected to conditioning in a incubator maintained at 35 ± I °C for 24 hours until a constant mass was achieved and the weight checked using an electronic analytical balance (m,). The subgroups were treated as follows ; Subgroup 1 : Stored in distilled water at 178 hours; Subgroup 2 : stored in distilled water at 35°C for 173 hours and subjected to five hours of thermal cycling with an upper temperature of 60°C. Subgroup 3 : stored in distilled water at 35°C for 173 hours and subjected to five hours of thermal cycling with an upper temperature of 60°C. Mass after treatment were measured anti recorded as m2 and specimens were re-conditioned to constant mass m3. The volume of the specimens was obtained and water sorption and solubility calculated. Data was analysed using factorial ANOVA/ Mann-Whitney test at significance level of 0.05.
Results: Results have shown that highest water sorption was seen for Group B i.e. Z-100 and least for Group C i.e. Filtek P-60. the highest solubility was seen for Group A i.e. Filtek A-1 10 and least for Filtek P-60).
Interpretations and Conclusions: Conclusions from the study can he drawn as the effect of thermal cycling on water sorption and solubility of composite resins was material dependent. A significant increase in water sorption was observed for Filtek-A-110 and Z-100 with thermal cycling whereas solubility was not effected with thermal cycling.
|How to cite this article:|
Reddy AM, Kumar D, Shivanna V. Thermal cyclic changes on water sorption and solubility of composite restoratives - An in-vitro study. J Conserv Dent 2006;9:63-71
|How to cite this URL:|
Reddy AM, Kumar D, Shivanna V. Thermal cyclic changes on water sorption and solubility of composite restoratives - An in-vitro study. J Conserv Dent [serial online] 2006 [cited 2020 Oct 30];9:63-71. Available from: https://www.jcd.org.in/text.asp?2006/9/2/63/42360
| Introduction|| |
The growing demands for esthetics and concern about mercury hygiene, led to the introduction and rapid developments in composite resins. For decades, the dental profession has sought an esthetic material to replace amalgam alloy for stress bearing restoration which will survive the rigors of wet environment, rapid temperature changes, abrasion from food and enamel as well as the disruptive action of slightly basic saliva and regurgitated stomach fluids which are highly acidic.  Resin-based composite materials undergo a series of physical changes as a result of the polymerization reaction and subsequent interaction with oral environment. Following polymerization, the inward movement of water molecules causes mobilization of ions within the resin matrix and an outward movement of unreacted monomers leaches out ions from tillers and activators. They undergo a series of physical changes during polymerization reaction, and due to the subsequent interaction with the wet oral environment there is water absorption which causes two opposing phenomenon:
- Due to diffusion of water, it leaches out the free unreacted monomers and ions from the resin matrix, which leads to loss of weight resulting in solubility.
- Due to absorption of water there is a hygroscopic expansion which may be able to compensate for the effects of polymerization shrinkage and relieve stresses and thereby reduce marginal gaps to a certain extent. 
These phenomenon may allow for some degree of relaxation of polymerization stresses and reduce marginal gaps (thonemann & others, 1997: Carvalho & others 1996). It may also result in staining, breakage in margin contours and decreased mechanical properties. Routine eating, drinking and breathing can produce changes in the intra-oral temperature, an these cyclic temperature changes can be pathogenic in two ways. First mechanical stresses created due to differences in coefficient of thermal expansion can result in bond failure at the tooth restorative interface. Second, the changing gap dimensions are associated with gap volume changes that pump pathogenic oral fluids in and out of the gaps. The physical and mechanical properties of the composite resins may also be affected by thermal cycling, although the effects of thermal cycling on marginal gap, wear and bond strength tests of composites has been widely investigated , studies on its effects on water sorption and solubility are limited. After assessing the reports describing temperature changes of teeth in vivo and an analysis of 130 in vitro studies involving thermal cycling of teeth , a clinically relevant thermal-cycling regimen was suggested by Gale & Darvell(1999).
The purpose of the study is to investigate the effects of cyclic temperature changes on water sorption and solubility of three commercially available composite resins namely Microfill (Filtek A-I10), Hybrid (Z100) and Packable (Filtek P-60) based on regimen advocated by Gale & Darvell. The water sorption and solubility of the different composites was also compared.
| Methods and Materials|| |
The technical profiles of composites evaluated are shown in [Table 1]. They included a microfilled (Filtek A-I 10), a hybrid(Z-100) and a packable (Filtek P-60). The methodology was based on ISO 4049 procedures (International Standard Organisation, 1992) with modifications for specimen dimensions and thermal testing.
A total of 90 specimens from three different composites mentioned above are prepared in the rectangular Teflon mold with circular recesses or opening (10 mm x 1 mm). Teflon mold was placed on a flat surface like glass slab and the composite material was placed in the circular recesses of this customized Teflon molds and covered with mylar strips. A glass slide was placed over the mylar strip and pressure was applied to extrude excess material. The composites were then light cured according to manufacturer's instruction through the glass slide and mylar strip with a halogen curing light (3M). The intensity of curing light was checked with a radiometer to ensure constant output.
GroupA : Filtek A-110 Microfill : 30 specimens
Group B : Z-100 Hybrid :30 specimens
Group C : Filtek P-60 Packable :30 specimens
Further each group of composite material (i.e. 30 specimens) were randomly subdivided into 3 subgroups of 10 specimens each which were subjected to different temperature changes. After the specimens were prepared and grouped, all the specimens were transferred to an incubator maintained at 35° ± 1°C and stored for 24 hours. After 24 hours they were removed and stored in a dessicator maintained at 23 ± 1°C for one hour, then weighed using an analytical electronic balance and the mass obtained for each of the specimen was recorded as m l . The specimens after recording m l were treated in the following way.
Subgroup I (Control) : Stored in distilled water at 35°C for 178 hours.
Subgroup II (Cycled to 45°C) : Stored in distilled water at 35°C for 173 hours and subjected to five hours (300 cycles) of thermal cycling with an upper temperature of 45 o C.
Subgroup III (Cycled to 60°C) : Stored in distilled water at 35°C for 173 hours followed by subjected to five hours (300 cycles) of thermal cycling with an upper temperature of 60°C.
Once the above mentioned treatment for three groups is done, the specimens were removed and the surface water was blotted away with blotting paper until it was free form visible moisture. The specimens were then waved in the air for about 15 seconds and weighed. This weighed mass was recorded as m 2 . After this weighing, the specimens were reconditioned to a constant mass in incubator using the above mentioned cycle i.e. placed in incubator maintained at 35 ± 1 °C for 24 hours followed by storage in incubator maintained at 23 ± 1 °C for one hour. The mass of the specimens after reconditioning is weighed and this is recorded as m 3 . Finally, volume 'V' of specimens were calculated after measuring the diameter at four equally spaced points on the circumference and the thickness at the centre of the specimens in cubic millimeters with the help of digimatic calipers.
The values for water sorption Wsp in mg/cm 3 were calculated using the following formula
Wsp = m 2 -m 3 / V
M 2 -Mass of the specimen in mg after treatment.
M 3 - Is the reconditioned mass of the specimen in mg
V - Volume of the specimen in cm 3 .
The Value for solubility Wsl in mg/cm' for each specimen were obtained using the following formula
Wsl = m 1 - m 3 / V
m 1 - Is the conditioned mass in mg prior to treatment
m 3 - Is the reconditioned mass of the specimen in mg
V - Volume of the specimen in cm 3 .
For all statistical analysis, a significance level of 0.05 was used. The descriptive data that included mean and standard deviation were determined for each of the groups comparison of water sorption and solubility between various groups and 3 sub-groups was done by Mann-Whitney test (Alternative to unpaired 't' test).
| Results|| |
The mean water sorption and solubility are shown in [Table 2] and [Figure 1] and [Figure 2] and results of statistical analysis arc reflected in [Table 3].
Results have shown that effects of thermal cycling on water sorption was material dependent. When mean values for water sorption were analyzed and compared between different materials in various treatment subgroups Filtek P-60 has shown least water sorption when compared to other two groups where the difference was statistically significant with Filtek A-110 and highly significant with Z-100. When these values were compared between Filtek A-I10 & Z-100, its shown that in all the subgroups. Z-100 has shown higher water sorption when compared to Filtek A-110 with a statistically significant difference. When mean values for water sorption were analyzed and compared between different treatment sub groups for each group it was seen that there was an increase in water sorption values at higher temperatures for FiltekA-110 and for Z-100 and the differences were statistically significant. But for Filtek P-60 there was no such difference.
When mean values for solubility were analyzed and compared between different materials it had shown that Filtek P-60 showed least values when compared to other two groups where it showed significant difference with Filtek A- 110 and with Z-100. Solubility of Filtek A-I 10 was significantly higher than Z-100. When mean values for water solubility were analyzed and compared between different treatment sub groups for various materials, it showed that the differences between different treatment subgroups were not statistically significant for any of the materials tested. Sub Group 2 (Cycled at 45° C)
| Discussion|| |
The general reduction of dental caries and patient interest in dental aesthetics, both have resulted in the development of new tooth-coloured restoratives and techniques. Basically composites has four basic components namely, matrix phase which contains dimethacrylate resin, polymerization initiators activated either chemically or visible light, dispersed phase made of inorganic fillers and tints and coupling phase that adheres the matrix to the filler particles (eg. Silanes)
The thermal cycling procedure is done in this study to simulate the intraoral temperature changes due to various activities like eating, drinking and breathing. The thermal cycling regimen advocated by Gale and Darvell (1999) (that is 35°C (28 sec), 15°C (2 seconds), 35°C (28 seconds) and 45°C (2 seconds) was derived froth an in vivo information and it was suggested as the benchmark standard for all laboratory testing of dental restorations.  The extremes in temperature were fixed at 15°C and 45°C, as they were the lowest /highest comfortable temperatures reported from the invivo studies (Gale and Darvell)  Plant and others (1974) determined that fluids were too hot to is above 68°C but subjects could sip them with discomfort between 60°C and 68°C. So, an upper temperature limit of 60°C was considered in the study, as this particular temperature had been shown to increase surface degradation and wear of composites. The total contact time with water was 178 hours for both the control and thermal cycled groups (173 hours storage in water followed by five hours of thermal cycling in water).
Water absorption is the amount of water that a material absorbs over a time per unit of surface area / volume. When a restorative material absorbs water, its properties change, and is therefore its effectiveness as a restorative material is usually diminished. A number of factors will determine the diffusion coefficient for this type of polymer based materials. These include types of resin, filler fraction, filler size, reactivity of the glass, presence of silane and non-silane coupling agents. The increase in the dimension during hygroscopic expansion may be beneficial in relieving some of the internal polymerization shrinkage stresses and thereby increasing the longevity of the adhesive union to the surrounding tooth.  During the early stages, hygroscopic expansion due to water absorption may close any gaps and relieve interface stresses that have been generated by polymerization shrinkage. However, a hygroscopic expansion which approximately the value of polymerization shrinkage is of little concern. But sometimes when the coefficient of expansion, which is in excess of polymerization shrinkage, may cause an outward force against cavity walls, causing cracking and fracture of enamel. 
The effect of cyclic temperatures on water sorption was material dependent. Although thermal cycling generally increased water sorption, the increase was only significant for Filtek A-110 and Z-100. Filtek P60 has shown least water sorption when compared to the other two Groups. Due to the higher number of the hydrophilic dialkyl ether moieties in TEGDMA and TeEGDMA, composites containing TEGDMA/ TeEGDMA as diluent in the resin matrix Filtek A-110 and Z-100 absorbed more water than the other systems. Due to the hydrophobic nature of ethoxylated version of Bis-GMA (Bis-EMA) which is used in Filtek P-60 does not contain unreacted hydroxyl groups on main polymer chain which leads to lesser amount of water sorption(Rayter and Nilsen, 1993). Z100 has shown significantly higher water sorption values when compared to Filtek A-110 As the filler content increases as in case of Z-100 the resin filler interface increases. This interface is a very weak junction especially if its uncoupled and leads to increased accommodation of water at the interface between the fillers and matrix resulting in increased water sorption
When mean values for water sorption were analyzed and compared between different treatment sub groups for each group it has shown that there was an increase in water sorption values at higher temperatures for Filtek A-110 and for Z- 100, but for Filtek P-60 there was no such difference.
AUJ Yap et al (2000)  examined the effects of food stimulating liquids on composite surface characteristics. They said that composites based on Bis-GMA matrix had more water sorption and so more susceptible to the softening effects of food stimulating liquid which could result in clinical wear. Decky J. Indrani et al (1995)  studied the relation of resin structure and water sorption on fracture toughness of dimethacrylate resins. They found that water sorption increased in proportion to the concentration of the hydrophilic dialkyl ether moieties where it was seen in higher number for TEGDMA and TeEGDMA based monomers.  AUJ Yap et al (2002)  investigated the effects of cyclic changes on water sorption and solubility of four commercial composite resins. It was found that as the volume of the filler increases, the amount of water absorbed into the matrix increased. Kalachandra (1989)  studied the effect of filler content on water sorption of composite. It was found that when water sorption of filled composites was compared to predictions of ideal systems based solely on polymer content, filled specimens were found to absorb twice as much water as the unfilled materials.
One of the major problems of composite resins is the incomplete polymerization. The degree of conversion has been shown to be in the range of 60-75%. Incomplete conversion may have unreacted monomers which might be dissolved from the material in wet environment and result in solubility. The elution of unreacted leachable components from composite resins is clinically important, as it has a potential impact on both the biocompatibility and structural durability of dental composites  . The elution or leaching of the molecules occurs generally via diffusion through the resin matrix and it is therefore dependent on the size and chemical composition of the leachable molecules. Smaller molecules have enhanced mobility when compared to larger, bulkier molecules.
Solubility of all composites was not affected by thermal cycling. But significant differences in solubility were, however observed between materials. Between all the material Filtek A-110 and Z-100 had higher solubility when compared to Filtek P-60 with a significant differences which can be due to the fact that composite resins based on BisGMA (A 110, Z 100) appeared to be more susceptible to detrimental effects of ethanol solution compared to the composite based on UDMA and modified BisGMA (P60) and elution/leaching process is dependent on size and chemical composition of leachable molecules(ferracane,1994). So smaller molecules like TEGDMA have enhanced mobility when compared to larger , bulkier molecules like BisGMA, UDMA. When compared Filtek A-110 had higher solubility when compared to Z-100 which can be due to air voids incorporated into polymerized resin materials and this depends on the filler content. Lower the filler content more air voids and more number of inhibition zones with unpolymerized materials which may lead to higher solubility as in the case of Filtek A-110 whose filler content is on 40% by volume.
AUJ Yap et al (2000)  evaluated the surface characteristics of composites with food stimulating liquids and concluded that solubility values were expected to be greater when the resin matrices are based on BisGMA. Tanaka et al (1991)  evaluated the residual monomers (TEGDMA & BisGMA) of a set visible-light-cured composite resins when immersed in water and they have found that lower molecular weight TEGDMA molecules eluted faster and were more in number when compared to other components. Han VTS et al (2004)  investigated the influence of curing lights and modes on the elution of leachable components from dental composites and it was seen that regardless of curing light or mode, more TEGDMA was eluted compared to other monomers. H.Oysaed and I.E. Ruyter (1986) evaluated the water sorption and filler characteristics of composites for use in posterior teeth and found that lower the filler content and the higher the resin content, higher is the solubility.
Because of the diversity in methodologies used, this study highlights the need for further research into the beneficial and the negative effects of the post placement expansion and leaching of the filler elements on the tooth structure and the optimal level for which could be aimed for formulating new materials.
| Conclusion|| |
Within the limitations of this in-vitro study, the following conclusions were drawn:
- The effect of cyclic temperature changes on water sorption was found to be material dependent.
- A highly significant increase in water sorption was observed for Z-100 when compared to other materials, Group A i.e. Filtek A-I 10 and Group C i.e. Filtek P-60.
- A significant increase in water sorption was observed for Group A i.e. Filtek A-I10 when thermocycled at higher temperatures of 450cand 600c whereas, Group B i.e. Z- 100 and Group C i.e. Filtek P-60 were not effected by thermocycling.
- A significant increase in solubility was observed for Group A i.e. Filtek A-I 10 when compared to other materials, Group B i.e. Z-100 and Group C i.e. Filtek P-60.
Thermal cycling did not effect the solubility of any of the composites evaluated.
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Aduma Meena Reddy
Department of Conservative Dentistry and Endodontics, College of Dental Sciences, Davangere 577004
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]
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