Elsevier

Dental Materials

Volume 23, Issue 8, August 2007, Pages 1030-1041
Dental Materials

Synthesis, characterization, shrinkage and curing kinetics of a new low-shrinkage urethane dimethacrylate monomer for dental applications

https://doi.org/10.1016/j.dental.2007.03.004Get rights and content

Abstract

Objectives

The aim of the study was to synthesize and characterize an isophorone-based urethane dimethacrylate (IP-UDMA) resin-monomer and to investigate its shrinkage and curing kinetics.

Methods

The IP-UDMA monomer was synthesized through the reaction of polyethylene glycol 400 and isophorone diisocyanate followed by reacting with HEMA to terminate it with methacrylate end groups. The reaction was followed using a standard back titration method and FTIR spectroscopy. The final product was purified and characterized using FTIR, 1H NMR, elemental analysis and refractive index measurement. The shrinkage-strain of the specimens photopolymerized at circa 700 mW/cm2 was measured using the bonded-disk technique at 23, 35, and 45 °C. Initial shrinkage-strain-rates were obtained by numerical differentiation of shrinkage-strain data with respect to time. Degree-of-conversion of the specimens was measured using FTIR spectroscopy. The thermal curing kinetics of the monomer were also studied by differential scanning calorimetry (DSC).

Results

The characterization methods confirmed the suggested reaction route and the synthesized monomer. A low shrinkage-strain of about 4% was obtained for the new monomer. The results showed that the shrinkage-strain-rate of the monomer followed the autocatalytic model of Kamal and Sourour [Kamal MR, Sourour S. Kinetic and thermal characterization of thermoset cure. Polym Eng Sci 1973;13(1):59–64], which is used to describe the reaction kinetics of thermoset resins. The model parameters were calculated by linearization of the equation.

Significance

The model prediction was in a good agreement with the experimental data. The properties of the new monomer compare favorably with properties of the commercially available resins.

Introduction

A wide variety of the polyurethane structure blocks, i.e. polyols, isocyanates, chain extenders and other modifying ingredients, has enabled the design of materials for different applications. Urethane di(meth)acrylates (UDMAs) are a class of materials with two (meth)acrylate end groups which enable them to be cured either thermally or through photopolymerization processes [1], [2], [3]. The latter is of more interest because of the ease of application and lower energy consumption. UDMAs have been widely used in coatings [4], [5], [6], [7], [8], [9], adhesives [10], [11], pultrusion processes [12], and dental materials [13], [14], [15], [16], [17], [18], [19]. The matrix phases of dental composites, which are now widely used in restorative dentistry, are mostly di(meth)acrylate monomers. Although some UDMAs have been commercialized for dental applications [16], they still have much promise to overcome the shortcomings of the hitherto available resins. One of the major drawbacks, especially in dental restorative materials (dental composites) is their polymerization shrinkage which may lead to marginal gaps and secondary caries in the restored teeth. Shrinkage is the result of matrix-phase polymerization of composites. Numerous studies have been performed to reduce the polymerization shrinkage of the dental resins which have focused mainly on the synthesis of new monomers with reduced shrinkage [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. But in spite of the extensive studies, polymerization shrinkage is still a major drawback in dental resin-monomers.

In this study a new urethane dimethacrylate (UDMA) was synthesized and characterized with the aim of reducing polymerization shrinkage. The shrinkage kinetic of the monomer, which is very important in understanding the phenomena, was also investigated. The autocatalytic model of Kamal [30], [31], which was adopted and modified by Atai and Watts [32] to describe the photopolymerization shrinkage kinetics of dental resin-monomers, was applied to study the curing behavior of the new monomer. As the synthesized UDMA may be used for other applications in which the resins are thermally cured, the thermal curing kinetics of the resin were also studied. The photopolymerization and thermal curing kinetic parameters of the monomer were determined.

Section snippets

Materials

Isophorone diisocyanate (IPDI), polyethylene glycol (PEG) with the molecular weight of 400 g/mol, 2-hydroxyethylethyl methacrylate (HEMA), camphorquinone (CQ), and benzoyl peroxide were purchased from Merck (Germany). Tetrahydrofuran (THF) and N,N′-dimethyl aminoethyl methacrylate (DMAEMA) were obtained from Fluka (Germany). 2,2-Bis-(2-hydroxy-3-methacryloxypropoxy) phenyl] propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) were kindly supplied by Röhm (Degussa group, Germany).

Results

Table 1 presents results of the elemental analysis of the synthesized IP-UDMA. Table 2 shows the degree of the photopolymerization conversion of IP-UDMA, Bis-GMA and their mixtures with the reactive diluent TEGDMA. The neat IP-UDMA shows much higher DC% than the neat Bis-GMA. Table 3 shows the shrinkage-kinetic parameters for the resins (IP-UDMA and Bis-GMA/TEGDMA 80/20 (wt.%)), containing 0.5 wt.% QC and 0.5 wt.% DMAEMA as photoinitiator system, at different temperatures. Table 4 depicts thermal

Synthesis and characterization of the IP-UDMA

The results of FTIR and 1H NMR spectroscopy (Fig. 3, Fig. 4), and the elemental analysis (Table 1) confirm the chemical structure of the synthesized monomer according to the reaction route of Fig. 1 and the structure illustrated in the Fig. 2. The IP-UDMA was a clear monomer with a refractive index of n25D=1.49103 which is close to the refractive indices of glass fillers (≈1.556) used in dental restorative materials. This provides a good appearance in restorative composites in which an esthetic

Conclusions

A new urethane dimethacrylate was synthesized via the reaction of isophorone diisocyanate (IPDI), polyethylene glycol (PEG 400) and 2-hydroxyethylethyl methacrylate (HEMA). The chemical reaction and the generation of the product were followed and confirmed using FTIR and 1H NMR spectroscopy and elemental analysis. The new monomer showed a lower shrinkage-strain and higher degree-of-conversion in comparison with the commercially available resin-monomer, Bis-GMA, suggesting that it is a feasible

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