Thermal expansion characteristics of light-cured dental resins and resin composites
Introduction
Thermal expansion is a crucial factor that challenges the adhesive bond between restorations and tooth structure. A great difference in the coefficient of linear thermal expansion (CLTE) between tooth and the restorative material leads to different dimensional changes, occurring when there is a temperature change in the restored tooth. Such expansions and contractions develop stresses at the tooth–restorative interface, which may lead to the formation of microleakages at the margins of the restoration [1]. These microleakages can have potentially serious effects. The penetration of acid and microorganisms can result in the patient's experience of sensitivity and ultimately the occurrence of secondary caries. Pulpal damage can result from toxic products liberated by microorganisms. Staining can occur at the margin of the restoration resulting from accumulation of debris [2]. Bullard et al. found a strong correlation between microleakages and the coefficient of linear thermal expansion [3]. Of course the failure of an adhesive bond is complex and it is affected by a number of factors; however, the driving force was found to be the difference in the CLTE between tooth structure and restoration. It is for this reason that ideally restorative materials should have similar coefficients of thermal expansion to enamel and dentine of tooth. For some commercial pit and fissure sealants this coefficient was found to range from 70.9 to 93.7×10−6/°C [4], while for various commercial resin composites from 26 to 35×10−6/°C [2], [4], or from 26 to 83.5×10−6/°C [5], or from 20 to 80×10−6/°C [6]; all for the temperature range 0–60°C. For the enamel and dentin it is 17×10−6/°C and about 11×10−6/°C, respectively [7].
The most widely used resin in dental composites is that based on the copolymer network prepared from a combination of bisphenol A glycol dimethacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA). TEGDMA is usually added to Bis-GMA in order to achieve workable viscosity limits since the latter monomer possesses very high viscosity due to the intermolecular hydrogen bonding [8]. The thermal expansion characteristics of the resin prepared from the mixture (50/50 wt/wt) of these monomers have been extensively studied under dry [9], [10] or water-saturated (wet) conditions [11], [12], [13]. Differences are observed in the values for the coefficient of linear thermal expansion, depending on the temperature range examined. It must be mentioned that although the CLTE is a material constant, its value does not remain constant over wide temperature ranges. Especially in the case of polymers it undergoes a great change, as the polymer goes from the glassy to the rubbery state at the glass transition temperature (Tg).
Besides the above-mentioned dimethacrylate monomers, the use of urethane dimethacrylate (UDMA; product of 2,2,4 (2,4,4)-trimethylhexyl diisocyanate and 2-hydroxyethyl methacrylate) and ethoxylated bisphenol A glycol dimethacrylate (Bis-EMA) in the preparation of some composites [14] has also been reported in the literature. Bis-EMA has a structure resembling that of Bis-GMA but without the two hydrophilic –OH groups, which makes it less viscous as well as less hydrophylic [15]. Recently, a new hydrophobic dimethacrylate was produced from Jeneric/Pentron Inc. (named by the company polycarbonate dimethacrylate, PCDMA), which had been used as a mixture with Bis-EMA for the preparation of resin-composite dental materials [16]. In the literature no data were found with respect to the thermal expansion characteristics of these three dental resins.
The aim of the present work was to determine the coefficient of linear thermal expansion under the same conditions of the light-cured resins made from Bis-GMA, TEGDMA, UDMA, Bis-EMA (4) or PCDMA monomer (Scheme 1) in order to study the effect of the chemical structure of the monomer used in the thermal expansion behavior of the corresponding resin. The CLTE of four commercial light cured restorative resin composites (Z-100 MP, Filtek Z-250, Sculpt-It and Alert) was also determined, the resin matrix of which is based on copolymers of these monomers. In Z-100 MP it is a copolymer of Bis-GMA/TEGDMA, in Filtek Z-250 a copolymer of Bis-GMA/Bis-EMA/UDMA, and in Sculpt-It and Alert a copolymer of Bis-EMA/PCDMA. The study of these composites was aimed to give us some information regarding whether the chemical structure of the resin has any effect on the CLTE of the corresponding resin composite. In the literature there is a controversy about this view. Some researchers observed that there is an inverse linear relationship between the coefficient of linear thermal expansion and the filler volume fraction of the composite [6], [17], [18], [19] while, others did not find any significant correlation between these parameters and suggest that the thermal expansion is also affected by some other potential factors, such as the thermal characteristics of the filler particles, the condition of silanate bonding between filler and matrix and composition of the matrix resin [5].
The coefficient of linear thermal expansion of resins and composites was measured by using a thermomechanical analyzer in dry or in water-saturated (wet) samples and in samples heated at 100°C for 1, 3 or 6 h.
Section snippets
Materials
The dimethacrylate monomers used were Bis-GMA, TEGDMA, UDMA, Bis-EMA (4) and PCDMA and they were used as received without further purification (details on the product Lot number and the manufacturer appear in Table 1). To make the monomers light curing, 0.65 wt% camphoroquinone (CQ) (Polysciences, Lot no. 497117) was used as photosensitizer and 0.61 wt% N,N-dimethylaminoethyl methacrylate (DMAEMA) (Riedel-de-Haen, Lot no. 20770) as the reducing agent. Because the dimethacrylates, except TEGDMA
Results and discussion
In Fig. 2, the TMA expansion curves of the resins prepared by light curing of Bis-GMA, TEGDMA, Bis-EMA (4), UDMA and PCDMA, are presented, showing the linear dimensional changes, ΔL/L0 (mm/mm), versus temperature. Great differences appear between the values of the first and second run, while between those of the second and third run the discrepancies are almost negligible. For the resins prepared from Bis-GMA, Bis-EMA (4), and UDMA, the curve of the first run shows an unexpected transition.
Conclusions
The thermomechanical analysis (TMA) of dental resins prepared by light-curing of Bis-GMA, TEGDMA, UDMA, Bis-EMA(4) and PCDMA monomer showed that the coefficient of linear thermal expansion (CLTE) of the resins depends on their chemical structure. For the temperature intervals 0–60°C it was found to be 120.3×10−6/°C, 110.1×10−6/°C and 118.3×10−6/°C correspondingly for the resins of Bis-GMA, TEGDMA and UDMA; these values are similar and lower than those of the resins of Bis-EMA(4) and PCDMA
Acknowledgements
We would like to thank Ivoclar-Vivadent and Jeneric/Pentron Inc for generously providing the urethane dimethacrylate monomer (UDMA) and polycarbonate dimethacrylate monomer (PCDMA) used in this work.
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