This work is concerned with the study of the sorption and desorption process of water, ethanol or ethanol/water solution 50% (v/v) or 75% (v/v) by the dental resins prepared by light curing of Bis-GMA, Bis-EMA, UDMA, TEGDMA or D 3 MA.
A thin resin disc is placed in a bath of time to obtain the sorption curve m t = f ( t ). Then the liquid is desorbed until a constant mass for the disc is reached and the desorption curve is recorded. These experimental curves help in the determination of the sorbed/desorbed liquid amount at equilibrium, the percentage of the extracted mass of unreacted monomer known as “solubility”, and the sorption/desorption diffusion coefficient which expresses correspondingly the rate of the liquid sorption/desorption.
The highest liquid uptake by dental resins was 13.3 wt% ethanol for Bis-GMA-resin, 12.0 wt% ethanol for UDMA-resin, 10.10 wt% ethanol/water solution for TEGDMA-resin, 7.34 wt% ethanol for D 3 MA-resin and 6.61 wt% ethanol for Bis-EMA-resin. The diffusion coefficient for all resins was higher in water than in ethanol/water solution or ethanol. Bis-GMA-resin showed the highest diffusion coefficient (11.01 × 10 −8 cm 2 s −1 ) followed by Bis-EMA-resin (7.43 × 10 −8 cm 2 s −1 ), UDMA-resin (6.88 × 10 −8 cm 2 s −1 ), D 3 MA-resin (6.22 × 10 −8 cm 2 s −1 ) and finally by TEGDMA-resin (1.52 × 10 −8 cm 2 s −1 ).
All studied dental resins, except TEGDMA-resin, absorbed higher amount of pure ethanol than water or ethanol water solution. TEGDMA-resin absorbed higher amount of ethanol/water solution (50/50 or 75/25 (v/v)) than water or ethanol. For all studied dental resins the diffusion coefficient was higher in water than in ethanol/water solution or ethanol.
Typically the commercially most widely used dental resin is prepared from polymerization of bisphenol A glycol dimethacrylate (Bis-GMA), bisphenol A ethoxylated dimethacrylate (Bis-EMA), urethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGDMA) and decanediol dimethacrylate (D 3 MA) ( Scheme 1 ). Dental resins in the oral environment may absorb water and chemicals, such as those found in saliva or food (acids, bases, salts, alcohols, etc.) and may release components to their surroundings. The process of sorption and desorption of a liquid may produce deleterious effect on the structure and function of the resin. These effects may include volumetric changes, such as plasticization, softening and chemical changes such as oxidation and hydrolysis . Thus, long-term survival of dental resins under oral conditions is obviously of great concern. Studies of behavior of dental resins in a wet in vitro environment have been mainly conducted in distilled water and in oral/food simulating liquids such as ethanol/water solutions which are among the best solvents for dental resins .
This work is concerned with the study of the sorption and desorption process of water, ethanol, ethanol/water solution 50 vol-% or ethanol/water solution 75 vol-% by the dental resins prepared by light curing of Bis-GMA, Bis-EMA, UDMA, TEGDMA or D 3 MA. The simplest and the most common technique for studying the sorption and desorption process of a liquid in a resin is the method of the sorption/desorption kinetics. In this technique a thin resin disc is placed in a bath of time to obtain the sorption curve m t = f ( t ). Then the liquid is desorbed, until a constant mass for the disc is reached, and the desorption curve is recorded. These experimental curves help in the determination of the sorbed/desorbed liquid amount at equilibrium, the percentage of the extracted mass of unreacted monomer known as “solubility”, and the sorption/desorption diffusion coefficient which expresses correspondingly the rate of the liquid sorption/desorption. The values of all these parameters were determined and compared.
Materials and methods
The dimethacrylate monomers used were Bis-GMA (Polysciences Europe GmbH, Lot no. 493282), UDMA (Ivoclar-Vivadent, Lot No. B00338), TEGDMA (Aldrich Chem. Co., Lot No. 17529EA-503), D 3 MA (Ivoclar-Vivadent, Batch-NR:F36539, Teile-NR: 300373) and Bis-EMA (Aldrich Chem. Co., Lot No. 03514 HF). They were used as received without further purification. To make the samples light curing, 2 mol% of camphoroquinone (CQ) (Aldrich Chem. Co., Lot no. S12442-053) used as photosensitizer, and 2 mol% of N,N-dimethylaminoethyl methacrylate (DMAEMA) (Riedel-de Haën, Lot no. 20770) used as reducing agent, were added to each sample. Because dimethacrylates, except TEGDMA and D 3 MA, are viscous liquids, the CQ and DMAEMA were first dissolved in dichloromethane, then certain amount of this solution was added to the monomer(s) and the solvent was subsequently evaporated under vacuum.
Sorption and “solubility” tests were determined according to the method described in ADA Specification No. 27 for resin based filling materials, which is identical to ISO 4049-1988. Specimen discs were prepared by filling a Teflon mold (15 mm in diameter and 1 mm in thickness) with the unpolymerized material, taking care to minimize entrapped air. The samples were irradiated for 200 s on each side, using a XL3000 dental photocuring source (3M-ESPE, St. Paul, MN, USA). Due to the large size of our specimen the unit was used without the light guide at the contact with the sample. This source consisted of a 75 W tungsten halogen lamp, which emits radiation between 420 and 500 nm and has the maximum peak at 470 nm, where also absorbs CQ ( λ max = 470 nm, ε = 3.8 × 10 4 cm 2 /mol). These irradiation times were found to be adequate for the polymerisation to complete. Four specimen discs were prepared for each resin.
All the specimens of resins were placed in a desiccator and transferred in a pre-conditioning oven at 37 °C. After 24 h they were removed, stored in the desiccator for 1 h and weighted to an accuracy of ±0.00001 g using a Mettler H54AR balance. This cycle was repeated until a constant mass ( m i ) was obtained. Following, the discs were immersed in water, ethanol, 50 vol-% ethanol/water solution or 75 vol-% ethanol/water solution at 37 °C. At fixed time intervals they were removed, blotted dry to remove excess liquid, weighted and returned to the liquid. The time intervals were more during the first days and as the uptake slowed more extended. The uptake of the liquid was recorded for 30 days. The samples were then transferred to a drying oven maintained at 37 °C and a similar process to that above repeated during desorption.
The percentage weight increase of specimens, WI (%), was calculated using the following formula:
W I ( % ) = 100 × m s − m i m i
where m s represents the weight of the saturated specimen after 30 days of immersion. This is an apparent value for the liquid sorbed, because unreacted monomer is simultaneously extracted resulting to decrease of specimens weight.
For the determination of monomer extracted the samples were transferred to a drying oven maintained at 37 °C and a similar process to that sorption repeated during desorption. The percentage amount of water, WD (%), ethanol, ED (%), or ethanol/water solution, EWD (%), desorbed of specimens, were calculated using the following formula:
W D ( % ) or E D ( % ) or E W D ( % ) = 100 × m s − m d m s
In which m d represents the weight of the specimen after desorption for 30 days. The amount of unreacted monomer which extracted by water, ethanol or ethanol solutions, during the immersion for 30 days, known as “solubility” (SL) of the resin in these solvents was calculated from the formulae:
SL ( % ) = 100 × m i − m d m i
The percentage amount of water, WS (%), ethanol, ES (%), or ethanol solutions, EWS (%), sorbed is then given by the formulae:
W S ( % ) or E S ( % ) or E W S ( % ) = W I ( % ) + S L ( % )