To investigate the kinetic process of water diffusion and mass change in new resin–matrix composites during water sorption/desorption cycles.
Five new resin–matrix composites were investigated [Filtek ® Silorane (FS), GC Gradia Direct Anterior (GDA), GC Gradia Direct Posterior (GDP), GC Kalore (GCK), Vertise ® Flow (VF)]. Five disk-shaped specimens, per material (15.0 ± 0.1) mm diameter by (2.0 ± 0.1) mm, were prepared according to ISO 4049. Each disk was immersed separately in de-ionized water for 150 d and then reconditioned for 75 d; all at (37 ± 1) °C. Mass was measured at different time intervals. Perspex disks were used as control. Data analysis was done by repeated measures ANOVA, one-way ANOVA and Tukey’s post hoc test ( p < 0.05).
The water sorption (μg/mm 3 ) after 150 d immersion ranged from 13.51 μg/mm 3 (±0.40) for FS to 71.96 μg/mm 3 (±0.90) for VF. The solubility ranged up to 16.95 μg/mm 3 (±0.79) for VF. A significant mass reduction occurred in VF after the peak value [73.63 μg/mm 3 (±0.31)] of water sorption was reached at 42 d. VF had the highest diffusion-coefficient for sorption: 5.23 × 10 −9 cm 2 /s (±0.38) and desorption: 11.72 × 10 −9 cm 2 /s (±0.16). Percentage sorption differences were significant for all materials ( p < 0.001), except between GCK and GDP. The early correlation between mass change and square root of time was linear.
Each resin–matrix composite varied in sorption/desorption cycles which may affect clinical service. A concurrent solubility process occurred during sorption of the self-adhering composite VF. The silorane composite FS exhibited minimal sorption.
Water plays a key role in the oral environment as the solvent of aqueous solutions and the ingested liquid. Polymerized dental restorative resin composites interact continuously with water following application intra-orally. It is logical to investigate first the direct response of dental resin composites to water.
Previous studies have established that water sorption in dental resin composites is a diffusion-controlled process . Theoretically, this process is regulated by two approaches . One is the free volume approach, through which water molecules can accumulate at microvoids, resin–filler interfaces and morphological defects without any reaction to polar groups. The other is the interaction approach, through which water molecules will form hydrogen bonds with some hydrophilic groups. The diffusion-controlled water sorption in dental resin composites could cause several time-dependent effects, including hygroscopic expansion , hygroscopic stress which could result in micro-cracks or even cracked cusps in the restored tooth , weakened mechanical properties , hydrolytic degradation of bonds particularly at resin–filler interfaces , elution of leachable species , polymer plasticization which leads to reduction in hardness and glass transition temperature , and decreased wear resistance . Water sorption can also induce internal structural damage, such as microvoids and crazing, which will generate expansion stress and accelerate water uptake . Furthermore, water sorption might generate peeling stresses in bonded layers of polymers that may cause serious clinical consequences during the placement of restorations in which adhesives rather than mechanical retentions are used .
In spite of highly cross-linking networks in polymerized composites, a few other components may be eluted into water, such as residual monomers, small chain polymers, polymerization promoters and ions from filler particles. Most of these leachable species are eluted quickly from polymerized resins within a few days . Consequently, the esthetic performance and biocompatibility of the materials may be compromised by the release of those components .
The conventional organic matrix of resin composites is generally based on methacrylate chemistry, especially cross-linking dimethacrylates . The water-related properties of dimethacrylate-based resin composites have been widely studied , while few studies have been carried out on silorane-based resin composites and apparently no previous research has been established on DX-511 monomer (with a long rigid core and high molecular mass, Fig. 1 ) or self-adhering resin composites. Hence, it is worth comparing the water-related properties of newly introduced resin composites based on DX-511/dimethacrylate co-monomers, self-adhering resin composites based on adhesive/methacrylate co-monomers, micro-fine hybrid resin composites based on dimethacrylate and posterior restorative resin composites based on silorane.
The objectives of the current study were to test and compare the water sorption, kinetic process of water diffusion and solubility in a range of resin composites immersed in de-ionized water for 150 d and then reconditioned for 75 d at (37 ± 1) °C. The null hypothesis was that the behaviors of test resin composites immersed in water exhibit no significant differences.
Materials and methods
Preparation of specimens
Five commercial restorative resin composites were investigated ( Table 1 ). Twenty-five ( n = 5) disk-shaped specimens (15.0 ± 0.1) mm diameter by (2.0 ± 0.1) mm thickness, were prepared according to ISO FDIS 4049:1999 and the manufacturer’s instructions. In an ordinary laboratory environment at (23 ± 1) °C and relative humidity (RH) of (50 ± 2)%, the material was packed slightly overfilled into a brass ring mold set on a piece of transparent polyester film on a glass microscope slide. It was then covered with another piece of polyester film while being pressed by another glass slide. Care was taken to minimize trapped air bubbles. The exit window (11 mm curved light guide) of a halogen curing light (Optilux 501, Kerr Corporation, USA) was placed against the glass slide. This section of the specimen was irradiated using a standard curing mode (output wavelength range: 400–505 nm; output irradiance: 580–700 mW/cm 2 ). Five overlapping sections on each side of the specimen were irradiated, respectively, each section for 60 s. The glass slide was removed after the first irradiation to avoid the potential light attenuation. The remaining irradiation was delivered through the polyester film to minimize oxygen in the air. Light was periodically checked for stability before every curing session with a radiometer built into the Optilux 501.
|Material||Code||Lot no./shade||Manufacturer||Organic matrix||Fillers||Filler loading (wt %)|
|Filtek ® Silorane (posterior restorative)||FS||20080514 A3||3M ESPE, St. Paul, MN, USA||Silorane (3,4-epoxycyclohexylethylcyclo-polymethylsiloxane, bis-3,4-epoxycyclohexylethyl-phenylmethylsilane)||Quarz (silane layer), radiopaque yttrium fluoride||76|
|GC Gradia ® Direct Anterior (micro-fine hybrid)||GDA||0901134 A3||GC Dental Products Corp., Tokyo, Japan||UDMA and dimethacrylate co-monomers||Silica, prepolymerized filler||73|
|GC Gradia ® Direct Posterior (micro-fine hybrid)||GDP||0905201 A3||GC Dental Products Corp., Tokyo, Japan||UDMA and dimethacrylate co-monomers||Silica, prepolymerized filler, fluoro-alumino-silicate glass||77|
|GC Kalore (universal composite)||GCK||0903171 A3||GC America Inc., USA||DX-511, UDMA (urethane dimethacrylate) and dimethacrylate co-monomers||Prepolymerized filler (with lanthanoid fluoride), fluoro-alumino-silicate glass, strontium/barium glass, silicon dioxide, lanthanoid fluoride||82|
|Vertise ® Flow (self-adhering flowable composite)||VF||3172311 A2||Kerr Corporation, Orange, CA, USA||GPDM (glycerol phosphate dimethacrylate) and methacrylate co-monomers||Prepolymerized filler, barium glass, nano-sized colloidal silica, nano-sized ytterbium fluoride||70|
The specimens were removed from the mold and any with visual voids were excluded. The edges were rotated and finished against 1000 grit silicon carbide abrasive paper to remove irregularities until visually smooth. Debris was blown away with a dust blower. The diameter and thickness of the finished specimens were measured at four points with an electronic digital caliper (Powerfix, OWIM GmbH & Co., KG, Germany).
The specimens were stored in a lightproof desiccator with anhydrous self-indicating silica gel at (37 ± 1) °C. After 22 h, the specimens were transferred to another desiccator at (23 ± 1) °C for 2 h and then weighed to a precision of ±0.1 mg using a calibrated electronic analytical balance with a precision of 0.01 mg (Ohaus Analytical Plus, Ohaus Corporation, USA). This cycle was repeated until the mass change of each specimen was not more than 0.1 mg in any 24 h period to ensure the completion of polymerization and dehydration. This constant mass ( m 1 ) was the initial mass of the specimen.
Two ICI Perspex disks (polymethylmethacrylate; PMMA, Imperial Chemical Industries, UK) were used as control. One was stored dry in a lightproof desiccator at (37 ± 1) °C and the other one was retained in de-ionized water at (37 ± 1) °C.
Water sorption and solubility
The specimens of each material were randomly immersed in 10 mL de-ionized water at (37 ± 1) °C in individual glass vials for periods of time. They were weighed as a function of different immersion time until water sorption equilibrium was reached. The total immersion time was 150 d. The specimen was carefully taken out by tweezers, dried on filter paper until free from visible moisture, waved in the air for 15 s, weighed 1 min later to ±0.01 mg and returned to the replaced water bath. To avoid variations in the pH of the solution, the de-ionized water was replaced every 7 d. The recorded mass was denoted as m 2 ( t ). The specimen was reconditioned to constant mass ( m 3 ) in the desiccators using the cycle described above. The total desorption process was 75 d. Also, the recorded mass at different time points was denoted as m 3 ( t ). The percentage apparent mass change, which excluded the leached species, was calculated by:
M g ( % ) = m 2 ( t ) − m 1 m 1 × 100
The percentage solubility was given by:
S ( % ) = m 1 − m 3 m 1 × 100
The percentage water uptake at time t was then calculated using Eq. (3) :
M t ( % ) = M g ( % ) + S ( % )
The water uptake and solubility in μg/mm 3 were given by Eqs. (4) and (5) , respectively:
W s = m 2 ( t ) − m 3 V