Properties of dental methacrylate resins with different urethane-modified side chains
Date Issued
2012
Date
2012
Author(s)
Jan, Yih-Dean
Abstract
Composite resins are currently the most popular dental restorative materials worldwide. Composite resins provide certain advantages such as good esthetics, easy application, and lower costs. However, there remain some disadvantages to their use, such as polymerization shrinkage, low wear resistance, and marginal discoloration. Composite resins are composed of organic monomers and inorganic fillers. High molecular weight dimethacrylate monomers with low polymerization shrinkage and high strength, such as bisphenol A-glycidyl dimethacrylate (bis-GMA), are most commonly used. The high viscosity of bis-GMA reduces the loading of fillers and also the degree of conversion of the monomers in the absence of other low viscosity diluents. Low molecular weight diluent monomers, such as triethylene glycol dimethacrylate (TEGDMA), are often added to reduce viscosity and increase the reactivity and conversion rate. However, the diluent monomers also increase polymerization shrinkage, leading to polymerization stress, debonding at the restoration-tooth interface, secondary caries, postoperative sensitivity, pulpal irritation, and marginal discoloration. Polymerization shrinkage is the principal cause of failure of clinical dental composite resin fillings. Reducing this shrinkage, thus, represents one of the most important goals in the development of new matrices for composite resins.
Currently, there remains a lack of “non-shrinkage” composite resins worldwide. In this study, we aimed to develop low-shrinkage composite resins for dental application. As expected, the higher the molecular weight and volume the monomer, the less extensive the shrinkage when polymerized. Most commercial dental composite resins are composed of bis-GMA or its derivatives. We increased the molecular weight and volume of the dimethacrylate molecule by conjugating functional side chains to the dimethacrylate structure. Urethane, which is a compound of diisocyanate and 2-hydroxyethyl methacrylate (HEMA), is a material suitable for use as a dimethacrylate side chain. Polyurethane displays certain advantages, such as low shrinkage, high wear resistance, and good biocompatibility. We selected three diisocyanates with different chemical structures as side chain
materials: 1,6-Diisocyanatohezane (HDI),
4,4’-diisocyanatodicyclohexylmethane (H12MDI) and toluene 2,4-diisocyanate (TDI). HDI is a linear structure molecule. H12MDI contains two aliphatic rings (cyclohexane) linked by a methyl group, whereas TDI contains a toluene moiety.
When conjugated to dimethacrylate, these three chemical structures reduced polymerization shrinkage and increased the mechanical strength of the composites. Different structures and numbers of side chains on dimethacrylate provided different results. The molecular weight and viscosity of experimental resins were increased as functional side chain density was increased. The polymerization shrinkage and degree of conversion were decreased when functional side chain density was increased. Polymerization shrinkage in the DM-M-1.5c and DM-T-1.5c groups was significantly less extensive than in the other groups (p<0.05). Although the degree of conversions of these two groups were significantly lower than that of the control group, the surface hardness values were equal to or significantly higher than that of the control group because of increasing functionalities of the side chain-modified groups. There were non-significant differences between these two groups and the control group in cell vitality. The biocompatibility of dental resin is related to the stereo hindrance of resin matrix molecular structures. When the ratio of HDI, H12MDI or TDI functional side chain to dimethacrylate is increased, the stereo hindrance of resin structure is increased, more toxic resin monomers are trapped in the complicated resin structure, and thus the resin matrix reveals less cytotoxicity. The urethane modification of dimethacrylate, therefore, represents an effective means of reducing polymerization shrinkage and increasing surface hardness. The modified dimethacrylate with good biocompatibility might be suitable for dental use in the future.
Subjects
biocompatibility
composite resin
degree of conversion
hardness
polymerization shrinkage
urethane-modified resin
Type
thesis
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