Studies on curing chemistry of aluminum-chromium-phosphates as low temperature curable binders

The curing behavior of metal phosphate binders, Al(H2PO4)(3) and Al3Cr(H2PO4)(9,12), was compared and characterized in order to understand the changes in chemical structure and crystalline phase that occur when they are annealed at temperatures up to 1200degreesC. Based on IR, NMR and XRD measurements, Al(H2PO4)(3) as a starting phase of the aluminum phosphate (AP) binder transformed to AlH3(PO4)(2) at 200degreesC, to a linear Al(PO3)(3) at 300degreesC, and finally to a cyclic Al(PO3)(3) beginning at 500degreesC. The chemical and crystalline behaviors of the Al3Cr(H2PO4)(9) (ACP-319) binder were developed in an almost identical manner to those of AP but with much a slower rate. In particular, the former was amorphous up to 500degreesC with slow crystallization kinetics. Furthermore, the presence of a filler or an increase in the Cr mole ratio enhanced the dehydrolytic condensation chemistry. In particular, the ACP-319 binder containing fillers showed the completion of curing reaction when pre-treated at 200degreesC.