³¹P solid-state NMR investigation of compounds in phosphate-bonded investments (PBI's)

Methods: A small but representative range (n=5) of commercial products was investigated in the as-received, green-set and burnt-out conditions. They were mixed according to manufacturer's instructions using undiluted special liquid. Structures were investigated by ³¹P MAS-NMR with high-powered-decoupling and/or cross-polarisation.

Results: As received, a single ³¹P resonance peak was observed at either 0ppm on the NMR spectrum [≡ pure (NH₄H₂PO₄)] or ≈-2ppm (additive displaced orthophosphate). Green-set produced the expected orthophosphate struvite peak (MgNH₄PO₄.6H₂O) at +6ppm, but also a second major orthophosphate peak (+1ppm to +3ppm) and in some products a minor pyrophosphate peak. Burn-out at the recommended temperature created spectra with between two and six peaks, no two being the same. Two PBI's intended for base alloys had a dominant pyrophosphate peak (-5ppm or -8ppm) with a minor presence of orthophosphate (+2ppm or +3ppm). A third had a dominant orthophosphate peak (0ppm) with significant metaphosphate peaks (-13ppm and -19ppm). Two PBI's intended for precious alloys had metphosphate main peaks (-13ppm to -22ppm) with a presence of orthophosphate (-1ppm to +2ppm). Traces of struvite persisted after burn-out. Silicophosphate was not detected.

Conclusions: Chemistries are complex and variable. Struvite and unreacted NH₄H₂PO₄ can account for the green-set orthphosphate peaks (pyrophosphate, possibly nucleation seeds). Proposition - under appropriate conditions, Mg(PO₃)₂ is formed during the thermal decomposition of MgNH₄PO₄.H₂O as an alternative to the assumed product Mg₂P₂O₇, and that Mg₂P₂O₇ can react with MgO to form Mg₃(PO₄). To dismiss silicophosphate formation completely, an additional experiment must be conducted due to the possibility of a very long relaxation time.

(Scottish executive Grant CZG/4/1/10).