This work presents the results obtained from the study of sorption/desorption process of some food/oral simulating liquids (FSLs) by the new marketed dental light-cured nanohybrid composite Kalore GC.
The sorption/desorption process followed is recommended by ISO 4049:2009. The samples were immersed in various liquids proposed by ADA as FSLs, such as H 2 O, artificial saliva, EtOH, EtOH/H 2 O solution (75 vol%) or C 7 H 16 , while the mass change for totally 30 days was recorded on defined time intervals. Afterwards the samples were put in dry desiccators at 37 °C for the study of desorption process.
The weight percentage of sorption of the above mentioned FSLs by Kalore GC was determined; also the wt% of the desorbed liquid, the diffusion coefficient of sorption and desorption, the wt% solubility and the % volume increase due to the liquid sorption.
The sorption characteristics of a dental composite depend both on composite structure and liquid. Ethanol/water and ethanol showed the highest effect on the determined characteristics. Then, the water and SAGF ® saliva follows and finally the heptane solvent.
Dental polymer composites known also as resin composites are constructed by two major phases: the organic polymer matrix, which is a mixture of dimethacrylates, such as Bis-GMA, Bis-EMA, UDMA or TEGDMA and the inorganic filler, usually SiO 2 , silicates or ceramics, dispersed within the matrix offering better mechanical properties and endurance. Other ingredients for the efficiency of the composite include organosilanes acting as coupling agents, camphorquinone and tertiary amine for the photo-initiation of polymerization, pigments for color matching, inhibitors or stabilizers for lifetime. Their final properties are affected by the constituents and their percentage. Years of research on dental polymer composites gave products with a variety of applications, quick shaping and invisible restorations.
The sorption properties of various liquids by dental composites have been studied extensively, since the longevity of those restorations depends on their response toward water, food, biological liquids, mouth hygiene, etc. Small molecules of liquids intrude and remain into the polymer network shortly after restoration. A portion of the liquid locates into the microvoids and the free volume among polymer chains without noticeable volume changes; a portion of the liquid however separates the polymer chains leading to an increase of the volume (swelling). Simultaneously, the sorbed liquid extracts molecules of unreacted monomers which may have toxic effects on human cells. The sorption percentages of a liquid should remain under certain levels, so the composite would be safely placed without abruption or swelling occurring. Hydrophilic or hydrophobic liquids act differently; the former form intermolecular forces mainly hydrogen bonds with the functional groups of the polymer matrix ( OH, NH , C O, CO ) and sorbed by the matrix in larger amounts. The intraoral environment is significantly different than that of a pure liquid and the intraoral conditions are far more complex. The FDA has proposed some liquids as food/oral simulating liquids (FSLs) in order to be used as accelerating environments in sorption experiments, trying to indicate the impact of analogous foods on the polymer composites in long term. The kinetics of the sorption and desorption processes are also of great importance indicating the rate of the phenomena; the diffusion coefficient D which express the rate of the process is a valuable parameter .
Recently sorption, solubility, shrinkage and mechanical properties of commercial dental resin composites characterized as “low-shrinkage” were studied before and after four months aging in 75 vol% ethanol/water solution . Also the hygroscopic expansion kinetics of dental resin composites after storage in water for 150 days was studied . Water sorption and solubility was studied recently and in core built-up materials .
The last few years a new dental light-cured nanohybrid composite has appeared in the market known with the name Kalore GC. This composite is of special interest due to its polymeric matrix, which contains a recently prepared dimethacrylate especially for this use, the DX-511 ( Fig. 1 ). This is a large dimethacrylate molecule (MW = 895) with stiff structural core. DX-511 is based on the chemistry of urethane dimethacrylates and is compatible with every other ester or adhesive usually used. Its long rigid core seems to reduce rapidly polymerization shrinkage and stress, whereas the flexible arms increase the activity of the monomer. There is no issue of toxicity because the structure size of the molecule prohibits that. The matrix contains also, along with the DX-511, the well-known dental dimethacrylates Bis-EMA and UDMA .
Water sorption and dimensional changes after immersion in water of Kalore GC have been studied recently and interesting results have accrued since that product showed lower values compared to other composites due to the stiff structure of its matrix .
The aim of this research is the study of the sorption/desorption kinetics of deionised H 2 O, artificial saliva SAGF ® , EtOH/H 2 O (75 vol%) or EtOH commonly used as FSLs, by the Kalore GC. Also the determination of volume increase of Kalore GC after immersion in these FSLs. The null hypothesis stated is that the sorption/desorption process of a FLS by a dental composite is strongly affected by the chemical structure and morphology of the composite and the chemical structure of the liquid.
Materials and methods
Kalore GC is produced by GC Corporation (Tokyo, Japan). Its ingredients are shown in Table 1 . The product (Lot. 1312161, shade A3) was used as received with no further purification, while it should be noticed that all experiments were carried out in absence of light.
|Polymer matrix||18 ( i.e. α = 0.18)|
|Dimethacrylate urethane (UDMA)||6–16|
|Dimethacrylate urethane DX-511||5–10|
|Ethoxylate dimethacrylates (Bis-EMA)||<5|
The experimental work was performed in accordance to the method described in ADA Specification No. 27 (1993) for resin based filling composite materials, which is identical to ISO 4049:2009 (§7.12) . A Teflon mold (1 mm thick and 15 mm in diameter) was placed between two glass slides (3 mm thick), covered with a Mylar sheet (thickness 0.05 mm, Stripmat Polydentia SA) to avoid adhesion, and the composite material filled up the empty space, taking care not to entrap air bubbles. The assembly, held together by spring clips, was put under irradiation (in direct contact) at both sides for 100 s each. Then the disk was removed, the material excess was scratched away and an abrasive paper was used to fix the slight side irregularities. The polymerizations took place using a 3000 XL dental curing device (Serial No. 119213, 3M Company, St. Paul, USA). This source consisted of a 75 W tungsten halogen lamp, which emits radiation between 420 and 500 nm ( λ max = 470 nm, ɛ = 3.8 · 10 4 cm 2 /mol) and lighting power at 785 mW/cm 2 measured by Hilux curing light meter (Benlioglu Dental INC, Serial No. 9080935). Twenty specimens were fabricated in total, separated in five groups of four ( n = 4). All specimens were placed in a desiccator and transferred in a pre-conditioned oven (Memmert Model 200) at 37 ± 0.1 °C. Later, they were removed, weighted at an accuracy of ±0.00001 g of a MettlerToledoAG285 balance and stored back to the oven until a constant mass was obtained.
Following, the discs were immersed in five solvents or solutions: deionised H 2 O, artificial saliva SAGF ® medium , EtOH/H 2 O (75 vol%), EtOH (absolute for analysis, Carlo Erba, Batch: 9M 275279 M) and C 7 H 16 (99+% biotech grade solvent, Aldrich, Lot: S41495-177) and stored at preconditioned water-bath of 37 ± 1 °C. The pH of the SAGF ® saliva was adjusted to 6.8, because this value is closer to that of the human saliva in the mouth. The use of the SAGF ® saliva required special care because the solution was super-saturated in CO 2 with regard to the air. As a result, it tended to lose CO 2 gas, which led to pH increase. Often, the pH of solution was controlled and in case of rise drops of CH 3 COOH solution were added. At fixed time intervals discs were removed, wiped with cotton cloth to remove liquid excess and shaken in air for 10–15 s to get dry, weighted and returned to the liquid. The time intervals were more often during the first days and as the uptake slowed down more extended. The uptake of the liquid was recorded until equilibrium was reached. The total immersion time was 30 d. The samples were then transferred into the desiccator vials in oven (silica gel as drying mean) maintained at 37 ± 0.1 °C and a similar process to that above, repeated during desorption. The desiccator material used was renewed daily.
Before and after the process study, density and dimension measurements were taken. Specimens’ density was determined using the Archimedes buoyancy principle. The mass recordings of the specimens in air and water under certain temperature were carried out using a specially designed Mettler-Toledo AG64 balance (±0.0001 g). Those values produce density values using Eq. (1) , where ρ t the material density required (g/cm 3 ), ρ θ the water density at the certain working temperature (g/cm 3 ), m a or m w the disk mass (g) in air or water. The diameter of the disk samples was measured by a caliper (No. 600 Rabone Chesterman, England ±0.001 cm), while their thickness by a Profi micrometer (±0.01 mm) at five points of the circular surface.
Based on the mass recordings the following parameters were calculated as reported below:
a. The mass increase (MI) of specimens during the sorption process was calculated using the following formulae:
where m 0 the initial mass (g) of the sample and m t its mass during sorption process for t time, V 0 the initial sample volume (mm 3 ). Also the mass increase of composite corresponding to the polymer matrix content was calculated as follows:
Where α is the polymer matrix percentage of composite.
b. The mass of liquid desorbed (LD) during desorption process was calculated by the formulae:
L D ( μ g/mm 3 ) = 10 6 Δ m V ∞ ( s ) = m ∞ ( s ) − m t V ∞ ( s )