Although it is not possible to fully simulate the environment inside the oral cavity under laboratory conditions, including the moisture and stress induced at the interface between the teeth and the restorations, laboratory conditions can, to some extent, simulate the oral cavity environment through the aging procedure. It utilizes the differences in the thermal coefficients of the expansion of the metal and acrylic resin, which induce stress in the adhesive bonding of various functional monomers contained in the utilized primers to the studied metal surfaces. TC was used as a standard procedure for simulating the aging process. On the other hand, when primed with one of the six metal primers, the bond strengths at TC0 were significantly higher than those obtained for the non-primed group ( p 0.05). However, particular types of the sulfur-containing metal primers that result in the most durable bonding between the resin and the metal elements of gold alloys still remain unknown.ДЇor the non-primed specimens at TC0, the bond strengths between the acrylic resin and pure metals were equal to approximately 8вЂ“9 MPa regardless of the metal type. The sulfur-containing monomers contained in commercially available metal primers were effective for improving the bond strength of resin to noble alloys. It has been also reported that the sulfur-containing monomers promoted the adhesion between the resin and pure metals, such as Au, Ag, Cu, and Pd. These monomers contain 6-(4-vinylbenzyl-n-propyl) amino-1,3,5-triazine-2,4-dithiol (VBATDT), thiophosphoric acid derivatives (MEPS), 6-methacryloyloxyhexyl 2-thiouracil-5-carboxylate (MTU-6), or 10-methacryloyloxydecyl-6,6-dithiooctanate (MDDT), and their single-liquid primers can be easily applied onto noble metal surfaces without special equipment or complicated surface treatment procedures. In addition, sulfur-containing monomers for bonding resins to noble metal alloys were synthesized as alternatives to these techniques, and marketed as commercially available metal primers. Therefore, various surface treatment methods have been developed for noble metals, including electrolytic Sn-plating, high-temperature oxidation, the application of a liquid GaвЂ“Sn alloy, and silica coating. However, these functional monomers were found to be inefficient for noble metal alloys. Since the strong and durable bonding of resin luting cements to metal surfaces is extremely important for restorations, their adhesion to base metals and enamel substrates can be effectively improved using carboxylic or phosphate promoting monomers. Metal crowns and fixed partial dentures (often fabricated from noble metals) are widely used as functional restorations for posterior teeth. The bond strengths between the resin and Au and type IV Au alloy at TC2000 were the highest ones when Metal Primer Z was used. The sulfur-containing metal primers, except for Metal Link Primer, were found to be more effective for improving the bond strength between the self-cured acrylic resin and Ag as compared to the other three pure metals ( p < 0.05).

However, at TC2000, the bond strengths between the resin and the five metals significantly decreased with respect to the values obtained at TC0 regardless of the primer ( p < 0.05). All of the sulfur-containing primers significantly improved the resin bond strength as compared to that of the non-primed group at TC0 regardless of the metal type ( p < 0.05). Three-way ANOVA and Tukey compromise post hoc tests were used to analyze the data ( О± = 0.05). The bond strengths were measured after 24 h (TC0) and after 2000 thermal cycles at 4вЂ“60 вЊ (TC2000). Afterwards, a brass ring was placed on the metal surface and filled with self-cured acrylic resin ( n = 10).

Four pure metals (Au, Ag, Cu, and Pd) and type IV Au alloy were either untreated, or treated with one of the five sulfur-containing metal primers (V-Primer, Metaltite, Alloy Primer, Metal Link Primer, and Metal Primer Z). This study investigated the effect of sulfur-containing primers for noble metals on the shear bond strength of self-cured acrylic resin after thermal cycling (TC).

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