Polymerization shrinkage of resin mixtures used in dental composites

16 POLYMERIZATION SHRINKAGE OF RESIN MIXTURES USED IN DENTAL COMPOSITES Krzysztof Pałka1* , Patrycja Janiczuk1 Joanna Kleczewska2 , Lublin University of Technology, Faculty of Mechanical Engineering, Nadbystrzycka 36, 20-618 Lublin, Poland 2 Arkona Laboratory of Dental Pharmacology, Nasutów 99C, 21-025 Niemce, Poland *e-mail: 1 Abstract Polymerization of dental composites generates shrinkage in the matrix resulting in numerous clinical problems. The influence of the most commonly used resins on the shrinkage process has not been thoroughly examined so far in the literature. The purpose of this work is to determine the effect of the resin mixture compositions on the volumetric polymerization shrinkage. The mixtures used in the tests were prepared in specific weight proportions to determine the influence of particular monomers, such as Bis-GMA, Bis-EMA, UDMA, and TEGDMA. The shrinkage measurements were performed using the self-designed video-imaging device. The studies showed that the shrinkage decreased in opposite to the Bis-GMA concentration in the mixture with the simultaneous decrease in the Bis-EMA content. The shrinkage value decreased in opposite to the molecular weight only for some monomers and compositions. Bis-EMA resin as a flexible monomer achieved the lower shrinkage values than UDMA, and the highest shrinkage values, above 6%, were achieved by compositions with the 14wt% content of Bis-GMA. Regarding the shrinkage, the most optimal composition was 56wt% Bis-GMA with the addition of Bis-EMA (24wt%) and TEGDMA (20wt%). The obtained shrinkage value was 4.73%. Keywords: polymer blends, volumetric shrinkage, dimetacrylates, dental composite [Engineering of Biomaterials 154 (2020) 16-21] doi:10.34821/eng.biomat.154.2020.16-21 Introduction Light-curable dental composites are currently the most commonly used materials for the direct reconstruction of enamel and dentin, accounting for about 70% of all dental restorations [1]. They are made of a polymer matrix reinforced with organic or inorganic, mineral, or mixed particles [2]. From the 1960’s, with the invention of the Bis-GMA monomer by Bowen, they became the main direction of development in aesthetic dentistry, gradually replacing amalgams, cements and restorations made of precious metals [3]. The universality in the use of composites based on lightcurable resins is primarily implied by their beneficial mechanical properties, wear resistance, durability, ease of application and aesthetic [1]. In addition to regaining the functionality of the teeth and guaranteeing their aesthetic appearance, composite restorations perform a very important function which is reducing the risk of health-threatening infections associated with the development of caries [4]. The key factor in maintaining the marginal adaptation and durability of these restorations turns out to be the polymerization shrinkage, resulting from the exchange of van der Waals intermolecular interactions to covalent bonds between monomers during polymerization. As a consequence of this phenomenon, stresses arise in the restoration-tooth interface, worsening adhesion and leading to the formation of a marginal fissure where bacteria can enter causing secondary caries [2]. Stresses exceeding the enamel strength may lead to a fracture within the bond with the material [5,6]. Most composites used in restorative dentistry shrink linearly by 0.6-1.4% [7], however, the value of volumetric contraction may be as high as 7% [8]. The polymerization shrinkage depends on the resin composition as well as the amount of filler particles and radiation intensity [9]. The size of the polymerization shrinkage depends, among others, on the molecular weight and functionality of the monomers. When comparing monomers with the same molecular weight, the polymerization shrinkage increases with functionality. Comparing monomers with the same functionality, the polymerization shrinkage increases opposite to the molecular weight, as a consequence, mixtures of Bis-GMA and other monomers will affect the polymerization shrinkage [10]. The purpose of this work is to determine the effect of resin mixture composition on the volumetric polymerization shrinkage. Materials and Methods In order to assess the relationship between the composition of the resins mixtures used as the matrix of dental composites and the resulting polymerization shrinkage, compositions with a suitably wide range of resin proportions were prepared, and then subjected to shrinkage tests at the designed stand and via the authors’ own method. The four most popular dimethacrylate resins: Bis-GMA (CAS 1565-94-2), Bis-EMA (CAS 41637-38-1), UDMA (CAS 72869-86-4) and TEGDMA (CAS 109-16-0) were mixed in the proportions shown in TABLE 1. The weight ratios for the mixtures and their markings are shown in TABLE 2. TABLE 1. Physical properties of tested dimethacrylates. Monomer Molecular weight (average) Double bond concentration (mol/kg) Viscosity (Pa⋅s) Bis-GMA 510.6 3.90 1200.00 Bis-EMA 540.0 3.70 0.90 UDMA 470.0 4.25 23.10 TEGDMA 286.3 6.99 0.01 Each mixture contained camphorquinone (an initiator; CAS 10373-78-1) and N,N-dimethylaminoethyl methacrylate (CAS 2867-47-2) as a co-initiator for light-cured free-radical polymerization, as well as 20wt% of the TEGDMA, which acts as an active solvent decreasing the viscosity of the mixture and facilitating effective mixing. The amount of TEGDMA monomer was constant for all the tested mixtures. The compositions were prepared by mixing precisely weighed ingredients (± 0.1 mg) using the WAS 220 laboratory balance (Radwag). The mixing process was carried out under low energy lighting to avoid an accidental polymerization of the resins. The shrinkage measurements were carried out on the self-designed video-imaging device which is schematically presented in FIG. 1. The specimen was deposited on the stainless steel pin of the 3 mm diameter. The top surface of the pin was placed in the optical axis of the camera to minimize the geometrical deformations of the acquired image. The pin with the resin droplet was rotated during the measurement in a full angle range with the step of 20°, realized by a stepper motor controlled by the dedicated software. In each angular position, an image (640x480 pixels) was taken until the last position of the sample was reached. The image acquisition was also controlled by the software. Yellow diffused light was used to obtain good exposure conditions (without reflections) and to avoid accidental curing. 17 TABLE 2. Resin compositions used to measure polymerization shrinkage. Each specimen contained 20wt% of TEGDMA. Specimen G E U G40E40 G56E24 G24E56 G40U40 G56U24 G24U56 G27E27U27 G26E40U14 G26E14U40 G14E26U40 G40E26U14 G40E14U26 G14E40U26 Bis-GMA (wt%) (G) 80.0 40.0 56.0 24.0 40.0 56.0 24.0 26.7 26.0 26.0 14.0 40.0 40.0 14.0 Bis-EMA (wt%) (E) 80.0 40.0 24.0 56.0 26.7 40.0 14.0 26.0 26.0 14.0 40.0 UDMA (wt%) (U) 80.0 40.0 24.0 56 (...truncated)