Surface modification of tricalcium phosphate for improvement of the interfacial compatibility with biodegradable polymers.

The surface of tricalcium phosphate (TCP) filler particles was activated by treatment with dilute aqueous phosphoric acid. ATR-IR spectra indicated the formation of calcium hydrogen phosphate dihydrate at the surface. Oligo(lactone)s were formed by the subsequent reaction of the activated TCP with L-lactide and epsilon -caprolactone, respectively, at 150 degrees C without any additional catalysts. After extraction of the oligo(lactide), the residue of modified TCP-included calcium lactate whereas the water of crystallization of the dihydrate disappeared as shown by ATR-IR spectroscopy. Owing to the insolubility of TCP in common solvents, the analogous reaction between water-soluble disodium hydrogen phosphate dihydrate and L-lactide was used to study the kind of chemical bonds by high-resolution NMR spectroscopy. The 1H and 13C NMR spectra of the reaction product also pointed out the presence of calcium lactate. Additionally, signals were found indicating a covalent attachment of lactic acid units onto the phosphorus. For the preparation of composites, poly(L,DL-lactide) was mixed with TCP and modified TCP, respectively, in a ratio of 75/25 (w/w) and directly injection moulded into tensile test specimens at a barrel temperature of 180 degrees C. Although mechanical properties were not improved, scanning electron microscopy (SEM) indicated a better interfacial phase interaction in the composite with the modified TCP. Chemical bonds between filler and polymer matrix are assumed to be formed by transesterification reactions.