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EFFECTS OF OXIDE POWDERS AND EGLASS FIBERS ON SOME MECHANICAL AND PHYSICAL PROPERTIES OF ACRYLIC RESINS FOR PROVISIONAL FIXED RESTORATIONS

Arnaldo G. Zuccari

Acrylic resins are employed in many different applications in dentistry, one of which is for the fabrication of provisional fixed partial dentures. Some of the problems associated with this use are related to the material's poor mechanical properties, causing mediocre performance of long-span provisional restorations. It has been demonstrated that acrylic resins can be strengthened through the addition of structural components of different size, shape and chemical composition, distributed in the acrylic matrix, thus forming a composite structure.

The purpose of this study was to investigate the changes in mechanical and physical properties of three different acrylic resins for provisional fixed restorations, poly(methylmethacrylate), poly(ethyl methacrylate) and poly(isobutyl methacrylate), when metal oxide powders (alumina, magnesia, zirconia) and Eglass fibers, primed with a metal primer (MP II), were added as structural components.

Following one week of water conditioning, a three-point bending test was conducted (from which modulus of elasticity, transverse strength, toughness and 0.2% offset yield strength were obtained), followed by a Knoop Hardness test. Weight gain and oxide leaching also were assessed. Data obtained were analyzed with two-way analysis of variance (ANOVA) and Student-Newman-Keuls multiple comparison tests. For the combinations that showed the best results, exothermic reaction and dimensional change were surveyed, along with a fractographic analysis.

The addition of particles generally increases the water sorbed by the composite systems.

The amount of oxides leached out from the resin, when it occurred, seemed to increase along with oxide concentration.

It was found that the 2 percent admixtures by volume in a poly(methyl methacrylate) resin matrix had significant beneficial effects on the mechanical properties (p < 0.05). Among these, zirconia exhibited the greatest improvements in modulus of elasticity (MOE), transverse strength, toughness and hardness. Poly(ethyl methacrylate) showed a general decrease in mechanical properties (p < 0.05). Poly(isobutyl methacrylate) showed no general trend for mechanical properties.

It was also demonstrated that hardness, MOE and strength of acrylic resins are reliably linearly related (R2 > 0.8).

During the three-point bending tests, it was noticed that some specimens significantly deformed, suggesting that if the pretest readings of specimen breadth and depth were used to calculate the maximum breaking strength, as usually done, the resultant data might be misleading. Accordingly, breadth and depth of broken specimens were remeasured, and breaking strengths were recalculated. It was found that there were significant discrepancies (up to 20 percent) between sB0 (based on pretest measurements) and sBB (based on post-test measurements). It is therefore suggested to use the post-test measurements.

The heat generated during polymerization was not significantly influenced by the different admixtures. On the other hand, the polymerization shrinkage was influenced, with increases of up to 0.9 percent.

The fractographic analysis showed that autopolymerizing acrylic resins are resinresin composite materials, constituted of prepolymerized beads embedded in a newly formed acrylic matrix. Although the main fracture modality appears to occur through the matrix and at the interface, some transbead fractures were identified, suggesting that the metal particle reinforcement could be studied further by incorporating structural components directly in the beads.