REDUCTION OF POLYMERIZATION SHRINKAGE IN DENTAL COMPOSITES REINFORCED BY NANOCLAY

Transcription

1 REDUCTION OF POLYMERIZATION SHRINKAGE IN DENTAL COMPOSITES REINFORCED BY NANOCLAY C. D. Mayworm (1), F. L. Bastian (1) Cidade Universitária, Centro de Tecnologia, bl. I sala I-222, Ilha do Fundão, Rio de Janeiro, RJ. zip: camila@metalmat.ufrj.br (1) Laboratório de Compósitos, Universidade Federal do Rio de Janeiro ABSTRACT Present study aims to obtain decrease of polymerization shrinkage of dental composites by inserting layered silicates (montmorillonite). Polymerization shrinkage frequently results in post-operative sensitivity, micro-leakage, and other undesirable effects. Organophilic montmorillonite is structured in layers that interact with the monomers (Bis-GMA and TEG-DMA) used as dental composites matrix. Such layers are separated and exfoliated inside polymer matrix during processing creating free volume that compensates polymerization shrinkage, and, at the same time, montmorillonite layers acquire nanometric dimension (isolated layer dimension). 5% (mass) of montmorillonite powder was inserted in a blend of Bis-GMA and TEG-DMA at mass fraction 1:1. The mixture was kept under agitation during 3h (60 C) and ultrasound during more 3h. Montmorillonite exfoliation was confirmed by x-ray diffraction and its layers were observed by transmission electron microscopy. Polymerization shrinkage was calculated using picnometry and showed a decrease up to 72.3% comparing to commercial dental composites and polymer without filler particles. Key-words: polymerization shrinkage, nanocomposite, nanoclay, montmorillonite

2 INTRODUCTION Composite materials have been extensively used in dental restorations. However, dental composites still present some undesirable effects. Polymerization shrinkage is a frequent problem (1-5). Adhesively bonded composite restoration may result in post-operative sensitivity, marginal micro-leakage, cuspal displacement, micro-cracks in enamel and premature failure of the restoration, due to stresses induced by polymerization shrinkage (4-7). The incorporation of organophilic layered silicates fillers in several types of resin-based composites have been widely used to enhance an assortment of properties. Improvements include higher modulus, increased strength, superior tribological behavior, and decreased polymerization shrinkage. The main reason for these improved properties in nanocomposites is the stronger interfacial interaction between the matrix and layered silicate, compared with usual filler-reinforced systems (8-17). In addition, when silicate is inserted into the resin, the matrix polymer chain is lodged between the silicate layers which can result into three different structures, in the case of this paper the structure of interest is the exfoliated nanocomposite. A complete disorganization and separation of the silicate layers occurs. The layer thickness is around 1 nm, and the lateral dimensions of these layers may vary from 30 nm to several microns or larger, depending on the particular layered silicate, so a nanocomposite is formed (9-10). The separation of the silicate sheets creates free volume that compensates the polymerization shrinkage (16-17). The use of particles in nano-size range as fillers in polymeric composites is increasing. The presence of nanoparticles improves mechanical and tribological properties of the materials, because of their high specific surface area-to volume ratio. The use of nanoparticles can give rise to a material with a new behavior, due to interfacial interactions, resulting in exceptional properties (18-25). The objective of this study is to decrease polymerization shrinkage of dental composites by incorporating esmectite nanoclay (montmorillonite) as reinforcing filler particle. The layered silicate exfoliation was confirmed by x-ray diffraction (XRD) and

3 visualized by transmission electron microscopy (TEM). The polymerization shrinkage was measured by water picnometry. MATERIALS AND METHODS Four experimental composites and one commercial composite were tested (table 1). The experimental composites were formulated from blends of {2,2-bis[4-(2- hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane} (Bis-GMA, Esstech, Essington, USA) and triethylene glycol dimethacrylate (TEGDMA, Aldrich, USA) at mass fractions 1:1 Bis-GMA/TEGDMA. The filler particles used were montmorillonite Cloisite 20A (Southern Clay Products, USA) and Aerosil OX50, a fumed amorphous nano-sized silica (Degussa, Germany). The resins were activated for visible light polymerization by the addition of camphorquinone (CQ, Aldrich, USA) and dimethylaminoethylmethacrylate (DMAEMA, Aldrich, USA). Also a polymerization inhibitor was used in order to avoid premature cure (butyl hydroxy toluene, BHT, Aldrich, USA). The samples were polymerized by an UniXS (Heraeus Kulzer, Germany) during ninety seconds. The emission spectrum was calibrated. Table 1 Composites formulations (w%). Group I Group II Group III Group IV Group V* Bis-GMA 49,45% 24,725% 46,98% 24,725% TEGDMA 49,45% 24,725% 46,98% 24,725% Silica (OX50) - 50% - 47,5% Montmorillonite - - 5% 2,5% CQ 0,5% 0,25% 0,475% 0,25% DMAEMA 0,5% 0,25% 0,475% 0,25% BHT 0,1% 0,05% 0,095% 0,05% * Group V consists on composite TPH3 (Dentsply, USA) principal components: 70% of silica particles (10nm - 3µm) and 30% of Bis-GMA and TEGDMA. Groups I and V were control groups. It was possible to evaluate the results by comparing them with the other groups

4 Montmorillonite exfoliation and composites preparation Montmorillonite powder was added into a blend of Bis-GMA, TEGDMA, CQ, DMAEMA and BHT. The mixture was kept under agitation during 3h at 60 C and ultrasound during more 3h so as to guarantee complete exfoliation of montmorillonite. After this process group III was ready. To produce group IV it was necessary to insert silica (OX50) into the mixture described above. Group I was only a blend of Bis-GMA, TEGDMA, CQ, DMAEMA and BHT and group II was made by adding silica (OX50) particles into a blend of Bis-GMA, TEGDMA, CQ, DMAEMA and BHT. All filler particle additions were made carefully in small fractions to avoid agglomerates formation and in agreement with percentages presented in table 1. Filler particles were also bolted in a 500 mesh bolter. X-ray diffraction (XRD) In order to certify clay exfoliation three samples made from the mixture of Bis- GMA, TEGDMA, CQ, DMAEMA, BHT and montmorillonite (group III) were analyzed by x-ray diffraction (Rigaku Mini-Flex diffractometer using monochromatic CuKα radiation). Montmorillonite powder was also analyzed. Polymerization shrinkage The polymerization shrinkage was measured using water picnometry. With this technique it was possible to obtain the densities of the experimental and commercial materials before and after polymerization, and then the dimensions variation was calculated from equation (A). D 1 ρ ρ p (%) n = 100 (A) Where: D (%) - dimension variation in percentage; ρ n - density of the material before polymerization; ρ p - density of the polymerized material

5 Six measurements were made for each material before and after polymerization. Transmission electron microscopy (TEM) The same three samples analyzed by XRD were visualized by TEM with the purpose of characterize montmorillonite layers exfoliated inside polymer matrix and observe their dimensions in nanoscale range. RESULTS AND DISCUSSION X-ray diffraction (XRD) Figure 1 shows montmorillonite powder XRD analysis. The presence of a peak in the graph shows that montmorillonite layers are organized and the distance between them is 24,55Å, calculated from Bragg s law. The graph of figure 2 does not present any peak, showing that montmorillonite layers are completely disordered and polymer chains penetrate between them. Therefore the processing method proposed is efficient to produce exfoliated montmorillonite in the composite Montmorillonite powder Intensity θ Figure 1 X-ray diffraction of montmorillonite powder

6 Intensity Mixture of Bis-GMA, TEGDMA, CQ, DMAEMA, BHT and 5% montmorillonite θ Figure 2 X-ray diffraction of group III. Polymerization shrinkage Table 2 shows the polymerization shrinkage results calculated for the five materials. Groups III and IV, the only groups that have nanoclay in their composition, show the smallest polymerization shrinkage percentage, confirming that montmorillonite exfoliation really contributes to decrease dental polymer matrix contraction due to free volume formation. The comparison of TPH3 (Group V) and Group III demonstrates a decrease of 72.3% of polymerization shrinkage. So, nanoclay insertion is, unquestionably, a real alternative to decrease commercial dental composites polymerization shrinkage. Table 2 Polymerization shrinkage percentage. Materials Polymerization shrinkage (standard deviation) Group I 12.44% (1.38) Group II 4.78% (0.52) Group III 0.90% (0.07) Group IV 0.56% (0.06) Group V 3.26% (0.46) Transmission electron microscopy (TEM)

7 Nanoclay layers can be observed in figure 3. It shows montmorillonite layers completely disordered presenting very thin thickness. Additionally to x-ray diffraction, TEM images confirm montmorillonite exfoliation corroborating the success of processing method. Figure 3 TEM image of a group III sample. CONCLUSIONS The obtained results show that: Montmorillonite particles can be exfoliated into studied dental composites matrix using the proposed technique. Incorporation of exfoliated organophilic montmorillonite layers is an efficient method to decrease polymerization shrinkage of dental restorative composites. ACKNOLEDGEMENTS To CNPq for C. D. Mayworm scholarship and financial support. To Dentsply and Southern Clay Products for materials donation

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