I am afraid your HA is not really pure HA but a mixture with DCPD or DCPA. Thermal analysis is not quite effective nor sensitive to detect surface silanization it would be better to try DFR or ATR FTIR. Check de DRX pattern of your material in order to determine how pure is your HA.

The weight loss in the range 200-300°C and the corresponding endothermic event are probably due to the dehydration of residual DCPD into DCP (and maybe also already sole Ca pyrophosphate; the exothermic peak at 300°C is likely to be due to the combustion of organic residue (associated to silanes). The diffuse weight loss and corresponding change in heat capacity might be due to the conversion of DCP into Ca pyrophosphate.

More details about the exact synthesis conditions would help in providing a correct interpretation of the TGA-DTA observations.

Good luck.

J. Lemaitre

References:

- Mortier A, Lemaitre J & Rouxhet P. G. « Formation of β−tricalcium di(orthophosphate) from mixtures of hydroxy–apatite and calcium hydrogen orthophosphate » Thermochim. Acta 113, 133-139, 1987.

- Mortier A, Lemaitre J & Rouxhet P. G. « Temperature-programmed characterization of synthetic calcium-deficient phosphate apatites » Thermochim. Acta 143, 265-282, 1989.

The mentioned information is too brief to provide the proper explanation. The detailed information on the synthesis way of hydroxyapatite (including information on the initial chemicals) is absolutely necessary. Without this information one can only respond that the weight loss in the range room temperature – 300 °C is due to elimination of some volatile components. It might be traces of the initial reagents, such as ammonia (at temperatures below ~ 80 °C), various types of water (adsorbed, incorporated, hydrate) at temperatures within 100 – 300 °C, etc. Even the silane coupling agent might be partly or fully evaporated and/or decomposed within this temperature range. Indeed, the exothermic peak at 300 °C might be due to combustion of the organic residue (associated to silanes); however, the weigh percentage of silanes must be disclosed. The diffuse weight loss and corresponding change in heat capacity at higher temperatures might be due to the conversion of DCP into Ca pyrophosphate, as well as due to the decomposition of non-stoichiometric and/or amorphous compounds into the stoichiometric ones.