31P solid-state MAS-NMR spectroscopy of the compounds that form in phosphate-bonded dental casting investment materials during setting

Objectives
To use 31P solid-state MAS-NMR to determine which compounds form in phosphate-bonded dental casting investment material during setting, when the ambient temperature is altered. To determine whether they differ in material originating at the center of the mix from material that adheres to the mixing bowl wall.
Methods
1H high powered decoupled (HPDC) and 1H cross polarized (CP) 31P solid-state MAS-NMR spectroscopy were used at a resonance frequency of 121.4 MHz to determine molecular structure. Four commercial products were examined. Manufacturer's instructions were followed and special liquid used without dilution. Ambient temperature was between 18 and 37 °C.
Results
Molecular structures change with ambient temperature and product. Amorphous Mg3(PO4)2 or struvite dominate with newberyite, cattiite, amorphous Mg2P2O7 and amorphous MgHPO4 present as minor phases. Exceptionally, amorphous MgHPO4 dominates. Differences in structure were found in material taken from the center of the mix compared with that scraped from the bowl wall, but the incidence may be specific to the product/mixer combination and not a general effect.
Significance
The formation of compounds in phosphate-bonded investment can be affected by ambient temperature. This effect and the use of material adhering to the bowl wall (instead of that from the center of the mix) are possible causes for the unpredictability of setting expansion measurements between laboratories. There is variation between products. When phosphate-bonded investment is required for casting, a consistent ambient temperature must be used and it would be wise to mix sufficient material to avoid scraping the bowl.