Surface characterization of precious alloys treated with thione metal primers

Objectives To characterize the effect of two thione metal primers with phosphate groups on the surface morphology and composition of two noble prosthodontic alloys. Methods Cast specimens from Argen 81(Au–Pd) and Argipal (Hi–Pd) alloys which were ground, polished and ultrasonicated in water, were divided in two groups (2 × 3) and treated with single layers of Alloy Primer (AP) and Metal Primer II (MP) primers respectively. The treated alloy surfaces were washed off with acetone and then examined by polarized light microscopy (PLM), reflection FTIR microspectroscopy (FTIRM) and X-ray photoelectron spectroscopy (XPS). Results After AP treatment, PLM revealed a crystalline phase (VBATDT) dispersed in an amorphous phase (MDP plus soluble VBATDT) on both the alloys tested. MP demonstrated a fibrial arrangement with the most dense structure found on the Hi–Pd alloy. FTIRM failed to clearly resolve the presence of single bondSsingle bondH peaks on alloy surfaces. Moreover, single bondNH and single bondPdouble bond; length as m-dashS peaks were identified denoting the presence of original thione tautomers. In both primers, phosphates were detected in a dissociative state (single bondPO32−). FTIR molecular mapping confirmed separation of VBATDT from MDP and MEPS from residual MMA. XPS showed that on alloy surfaces approximately 50% of sulphur was in the sulphide state, the rest being organic sulphur. AP showed higher sulphide percentage than MP on both alloys and higher sulphide percentage on the Au–Pd alloy (p < 0.05). Clinical significance Phase separation of the primer components on alloy surfaces may adversely affect their clinical performance. Sulphide formation on alloy surfaces was confirmed only by XPS under ultra-high vacuum and not by environmental techniques like FTIR; this poses serious questions on the chemical bonding capacity of these primers with the noble alloys tested under environmental conditions.