Characterization of Water Sorption , Solubility and Filler Particles of Light-Cured Composite Resins

In current clinical restorative treatment many types of resin composites are available for the replacement of natural tooth tissues. Composites consist mainly of filler particles and a resin matrix based on different monomers. For purposes of research, clinical applications and communications, composite resins are traditionally classified on the basis of filler particle size, i.e. macrofill, hybrid and microfill. However, a new classification of filler particle sizes could include the nanofillers, nanohybrids, and microhybrid or minifill composites (1-7). The characterization and evaluation of resinbased material properties are assessed by flexural strengths, elasticity modulus, degree of conversion, hardness, wear resistance, polishability and other investigations (2,3,5-10). Water sorption and solubility are important properties of composite resins and influence their strength, abrasion resistance, volume and color Characterization of Water Sorption, Solubility and Filler Particles of Light-Cured Composite Resins


INTRODUCTION
In current clinical restorative treatment many types of resin composites are available for the replacement of natural tooth tissues.Composites consist mainly of filler particles and a resin matrix based on different monomers.For purposes of research, clinical applications and communications, composite resins are traditionally classified on the basis of filler particle size, i.e. macrofill, hybrid and microfill.However, a new classification of filler particle sizes could include the nanofillers, nanohybrids, and microhybrid or minifill composites (1)(2)(3)(4)(5)(6)(7).
As the filler characteristics of composites have been considered a significant factor in their rate of wear, surface roughness, esthetic results, water sorption (WS) and solubility (SO) (11,13,17,18), the objectives of this study were to compare the WS and SO of 3 resin-based filling materials containing different filler contents.This investigation also analyzed filler sizes, shape, type and other characteristics of resins under scanning electron microscopy (SEM).The tested hypothesis was that filler characteristics may influence the WS and SO of resins.

MATERIAL AND METHODS
The compositions of the 3 composite resins tested are presented in   For the WS and SO analyses, 10 disk-shaped specimens of each composite resin were prepared using Teflon molds (0.8 mm in thickness and 6.0 mm in diameter).After filling the mold to excess, the material surface was covered with a polyester strip and glass slide, compressed to avoid porosities, and light-cured from the surface with a halogen light-curing unit (Optilux 501; Demetron/Kerr Corp., Orange, CA, USA).The resin disks were stored in a desiccator (Pyrex, São Paulo, SP, Brazil) at 37°C for 22 h, followed by 2 h at 23°C, until constant mass was achieved (m1).The masses of these completely dried specimens were recorded (Chyo Balance JK 180; Chyo Corp., Tokyo, Japan).Specimens were then stored for 7 days in water at 37°C, and the saturated mass was measured (m2).Finally, the specimens were dried again in the desiccator until obtaining constant mass and their masses were once again determined (m3).The difference in mass between the initial dry and final dry mass represented the amount of SO (m1 -m3/volume of specimen), which was analyzed by one-way ANOVA and Tukey's post-hoc test (α=0.05).The difference in mass between saturated and final dry specimens provided sorption values (m2 -m3/volume of specimen), which were analyzed by one-way ANOVA.
For evaluating the filler particles, 5 disk-shaped specimens of each composite resin were prepared using Teflon molds (3 mm thick and 5.0 mm in diameter) and light-cured for 5 s with the halogen light-curing unit.Each Filtek Supreme or Esthet-X composite resin disk was immersed for 1 week in 2 mL acetone, which was changed daily.For Renamel Microfill, the treatment was the same, but the organic solvent used was chloroform.Thereafter, the specimens were fixed in metallic stubs, sputter-coated with gold (MED 010; Baltec, Balzers, Leichtenstein) and observed with a scanning electron microscope (JSM-5600; JEOL Inc., Peabody, MA, USA).Representative areas showing the filler particles were photographed at ×1,000 and ×2,500 magnifications.

RESULTS
WS and SO mean values are presented in Table 2.The tested materials had statistically similar WS (p>0.05), but differed significantly from each other with regard to water SO, ANOVA revealed significant differences among the composites (p<0.0001).The SO of Filtek Supreme was lower than those of Esthet-X and Renamel Microfill, which presented similar mean SO values.
Figures 1 to 3 show the filler particles of Filtek Supreme, Esthet-X and Renamel Microfill, respectively.Fillers were irregular (Esthet-X and Renamel Microfill) or spherical (Filtek Supreme) in shape, depending on    the manufacturer.Spherical agglomerates of 1 to 4 μm nano-sized particles can be observed in the composition of Filtek Supreme (Fig. 1A).Nanoparticles were discrete nonagglomerated and nonaggregated particles situated among and over nanoclusters or spherical agglomerates (Fig. 1B).Esthet-X presented irregularshaped filler particles, ranging from 0.5 to 3 μm.The average size of the fillers was 0.5 to 1 μm (Fig. 2A and  2B).Distribution of filler particles of Renamel Microfill (Cosmedent Inc.) was not uniform and the composite contained many particles larger than 1 μm (<10 μm), but the predominance of particles were of one micron or less (Fig. 3A and 3B).

DISCUSSION
The amount of water that composite resins can absorb depends on the hydrophilicity of polymeric matrices and filler composition (8,9,18,19).The composites tested in this study showed similar WS mean values after one week of water storage and these values can be considered as lower and adequate for resin-based filler materials.The WS values ranged from 17.1 to 20.3 μm/ mm 3 and were lower than those required by the ISO 4049 standard, which establishes that the maximum WS value is 40 μm/mm 3 .
The 3 composites (Filtek Supreme, Renamel Microfill and Esthet-X) contain BisGMA and other di-and methacrylate monomers (Bis-EMA, TEG-DMA and UDMA), and these different resin matrices did not seem to influence the results of WS analyzes.Most resin-based composites are Bis-GMA-based materials and the chemistry of the monomers present in the matrix is a key factor to the hydrophilic nature of the polymer (9).The high viscosity of the polymer requires the addition of diluent monomers, such as TEG-DMA.Such diluent monomers, coupled with the presence of hydroxyl groups in the Bis-GMA molecule, result in an increase in WS.WS can promote the expansion of the restoration, which is detrimental to its longevity (14).
Da Silva et al. (6) showed a correlation between SO and degree of conversion in nanofilled and hybrid composites.Nevertheless, no correlation was found between degree of conversion and saliva sorption for the same materials.Therefore, it may be assumed that the increase in the degree of conversion reduced SO.Since SO is reflected by amount of leachable unreacted monomers, the high degree of conversion reduced the SO because the amount of unreacted monomers available for leaching out was lower due to the high percentage of reacted aliphatic C=C bonds from the dimethacrylate monomers.However, the nanofilled composite presented a higher SO than the hybrid resin, in contrast to the data of the present study.
The water SO mean values presented by the composite resins tested varied from -4.0 to 5.8 μm/mm 3 ; these values were lower than the maximum value established by the ISO 4049 standard (<7.5 μm/mm 3 ).Filtek Supreme demonstrated a negative value, indicating that, possibly not all of the absorbed water was removed by the drying process or that the any reaction was incomplete, thus, increasing the mass of the composite (4).Renamel Microfill and Esthet-X had similar SO means, which were higher than that of Filtek Supreme.
Besides unreacted monomers, inorganic ions present as fillers within composites can leach into the surrounding environment.In addition, water in contact with silica filler surfaces can break siloxane bonds and the hydrolysis induces debonding of the filler particles, increasing the mass loss of the composite.Yap and Wee (18) showed a correlation between filler load and WS and SO when comparing a highly filled composite (Surefil; Dentsply/Caulk, 66% by volume) and 3 other materials; one microfilled (Silux Plus; 3M/ESPE) and 2 microhybrid resins (Z100; 3M/ESPE and Ariston pHc -Ivoclar, Vivadent).The composites used in this study contain similar filler content by volume (59 to 60% of filler), but the size, shape and type of filler differed among materials.
Dental composites have used strontium glass, barium glass, quartz, borosilicate glass, ceramic, silica and prepolymerized resin as filler particles (1,2,20).Filtek Supreme contains 20 nm nanosilica spherical fillers (non-agglomerated/non-aggregated) and agglomerated zirconia/silica spherical nanoclusters of 1 to 4 μm (Fig. 1).The fillers of Esthet-X and Renamel Microfill have irregular shapes and are composed of glass and silica particles (Figs. 2 and 3).However, the average size of the filler particles of the Esthet-X composite resin is lower than that of Renamel Microfill.
In conclusion, filler particles seem to have little influence upon WS and SO.The tested composite resins had similar WS characteristics, while the SO of the nanofilled composite resin was lower than those of the minifill and microfilled composites.

Table 1 .
Characteristics of composite resins tested.