High equilibrium uptake of water can soften a denture because the absorbed water can act as a plasticizer for polymethylmethacrylate (PMMA) and reduce the strength of the material. Water sorption also effects other physicochemical and mechanical properties. Soft relining materials are mostly applied to dentures immersed in oral fluids for longer time periods. Objectives: The initial bonding properties of different relining materials bonded to dry samples and samples immersed in water for 3 months as a simulation of the oral environment were investigated. Methods: Six commercially available soft relining materials (n=5/ group) were tested against a heat-activated compression molded PMMA denture base polymer (Paladon 65, Heraus-Kulzer, Germany). The tensile test method described in ASTM specification D 897-78 was used. After testing, the fracture surfaces and sections of the interface perpendicular to the bonding surface were examined with microscopic techniques. Results: ANOVA showed significant differences between different materials (p<0.05). Increase in tensile strength of practical adhesion of silicon based relining materials ranging from 25 to 64 % was observed whereas no absolute change was observed in the acrylic based materials. The statistical analysis of the results did not reveal significant differences (p>0.05) among water saturated and dry PMMA samples. The analysis of the samples with microscopical techniques revealed that the tendency of failure propagation was observed more to the bulk of the relining material and bulk-interface border than the interface alone. Conclusions: The absolute increase in tensile strength of practical adhesion and the analysis of the failure surfaces supports that the existence of water effects the silane-PMMA and/or silane-silicone reaction in the PMMA-silicone interface.

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