Thermal decomposition of human tooth enamel

Summary

Further insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO₃ ²⁻, OH⁻, structurally incorporated H₂O, Cl⁻, and, indirectly, HPO₄ ²⁻ after TE had been heated in N₂ or vacuum in the range 25–1000°C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO₃ ²⁻ components begins at much lower temperature (e.g., 100°C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO₃ ²⁻ in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200° to a maximum at ∼800°C (>10% of the possible A sites filled), where it is responsible for an increase ina lattice parameter. A substantial fraction of the CO₃ ²⁻ in B sites moves to A sites before being evolved, apparently via a CO₂ intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl⁻ was observed at temperatures below 700–800°C. Structural OH⁻ content increases ∼70% to a maximum near 400°C. Structurally incorporated water is lost continuously up to ∼800°C with a sharp loss at 250–300°C. The “sudden”a lattice parameter contraction, ∼0.014Å, occurs at a kinetics-dependent temperature in the 250–300°C range and is accompanied by reordering and the “sharp” loss of ∼1/3 of the structurally incorporated H₂O. The hypothesis that structurally incorporated H₂O is the principal cause of the enlargement of thea lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42Å) is thus allowed by these experimental results.