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Fractures of polymer material are one of the most frequent reasons for the repair of removable dental prostheses. Therefore, there is a constant endeavor to strengthen them, and polymer materials with high resistance to fracture are being developed. The aim of this study was to determine the flexural strength of polymer materials and their reinforcements and thus give preference to their clinical use. Specimens with dimensions 18 x 10 x 3 mm were tested after polymerization, immersion in water at a temperature 37 degrees C for 28 days, and thermocycling by using the "short-beam" method to determine the flexural strength. Microscopic examination was performed to determine the quality of bonding between the glass fibers and matrix. Common polymer materials (control group) demonstrated the lowest flexural strength, although, when reinforced with fibers they showed higher flexural strength, matching that of the tested high-impact strength resin. Thermocycled specimens had the highest flexural strength, whereas there was no difference (p > 0.05) between specimens tested after polymerization and immersion in water.

The paper describes different effects on the mechanical properties of fibre reinforced polymers, such as direction and the amount of fibres, impregnation of fibres with the polymer matrix, and adhesion between fibres and the polymer matrix. Direction of the fibres can be unidirectional (vertical to the direction of loading), i.e. bidirectional or multidirectional (under different angles to the direction of loading), or even in random order. The amount of fibres also increases the strength of the composite, although reinforcement with fibres can only be efficient when loading is transferred from the matrix to the fibres. This requires good impregnation of the fibres with the polymer matrix, which depends on the type and viscosity of the polymer matrix (prior to polymerisation), and possibility of good wetting of each fibre. For improvement of adhesion between the polymer matrix and glass fibres bonding/ coupling agents are used, particularly silane. They bond to the surface of the glass by silane bridges and by co-polymerisation initiated by free radicals bond with the polymer matrix, thus improving adhesion between fibres and the polymer matrix. Apart from the mechanical properties, such good bonding is also important for reducing the unfavourable effect of water on the marginal surface of the fibre-base. [ABSTRACT FROM AUTHOR]