A. Terol

PHYSICAL PROPERTIES AND SELF-SETTING MECHANISM OF CALCIUM PHOSPHATE CEMENTS FROM CALCIUM BIS-DIHYDROGENOPHOSPHATE MONOHYDRATE AND CALCIUM OXIDE

An apatitic calcium phosphate cement was developed from calcium bis-dihydro-genophosphate monohydrate (or monocalcium phosphate monohydrate, MCPM) and calcium oxide (CaO). The powder had a Ca/P molar ratio of 1.67, and the liquid was either pure water or 0.25 M–1 M sodium phosphate buffer, pH 7.4. The influence of the powder-to-liquid (P/L) ratio on the setting time and the mechanical strength were studied. The best results were obtained for the 1 M phosphate buffer with a P/L ratio of 1.53; the setting time was 7 min and the compressive strength was 25 MPa after 24 h and 33 MPa after 11 d. The mechanism and kinetics of the setting reaction were investigated by X-ray diffraction, differential scanning calorimetry, 31P magic angle spinning–nuclear magnetic resonance and infrared spectrometry. The setting reaction was found to be biphasic: in the first step, during the mixing time, MCPM reacted with CaO immediately to give calcium hydrogenophosphate dihydrate (or dicalcium phosphate dihydrate, DCPD) which, in the second step, reacted more slowly with the remaining CaO to give hydroxyapatite. The conversion of the starting materials to hydroxyapatite was complete within 24 h when the liquid was water, but was slower and incomplete with the phosphate buffers. Of the starting materials, 30% remained after 3 d.

pH-metric study of the setting reaction of monocalcium phosphate monohydrate/calcium oxide-based cements.

Hydraulic calcium phosphate cements (CPCs) that are used as osseous substitutes, set by an acid–base reaction between an acid calcium phosphate and a basic calcium salt (often a phosphate). In order to gain a better understanding of the setting of the monocalcium phosphate monohydrate–calcium oxide cement that we developed and in the aim to improve its mechanical properties, the setting reaction was studied by pH-metry. The two methods described in the literature were used. In the first, cement samples were prepared then crushed after different storage periods at 37 °C, 100% RH. The powder was then immersed in pure water with stirring and the pH was measured after equilibration. In the second technique, the starting materials were poured into water while stirring and the pH were followed over time. The two methods gave different results. The first procedure provided information concerning the pH of the surrounding liquid following the partial dissolution of the cement components, rather than any information about pH changes during setting. The second method is more appropriate to follow the pH variations during setting. In this second procedure, the effects of different parameters such as crushing time, stirring rate, liquid-to-powder (L/P) ratio and temperature were investigated. These parameters may impact substantially on the shape and position of the pH=f(t) curves. One or three pH jumps were observed during the setting depending on the composition of the liquid phase. The time at which these pH jumps occurred depended on the pH of the liquid phase, the concentration of the buffer, the crushing of starting materials, the L/P ratio and the temperature. Good linear correlations were obtained (i) between the time of the pH jumps and the L/P ratio and the temperature and (ii) between the time of the first pH jump and the compressive strength and the final setting time of the cements prepared with different liquid phases. It may be assumed in view of these correlations that the results obtained in dilute solution may be extrapolated to the conditions of cement sample preparation and that the mechanical properties of the cement are directly related to the phenomena that occur at the first pH jump which corresponds to precipitation of dicalcium phosphate dihydrate.

Effect on composition of dry mechanical grinding of calcium phosphate mixtures

For diverse reasons, calcium phosphates used to prepare hydraulic calcium phosphate cements can be ground mixed. The grinding with a rotating micromill of monocalcium phosphate monohydrate or anhydrous, dicalcium phosphate dihydrate or anhydrous with calcium oxide, calcium hydroxide, calcium carbonate, tetracalcium phosphate, or alpha- or beta-tricalcium phosphate was studied for different calcium to phosphate (Ca/P) ratios, rotating rates, masses of balls, and environmental conditions. During dry grinding by ball milling, anhydrous or hydrated acid calcium phosphates can mechanochemically react with anhydrous or hydrated basic calcium salts to form dicalcium phosphate dihydrate or anhydrous, noncrystalline calcium phosphate, and/or calcium deficient or stoichiometric hydroxyapatite, depending on the Ca/P ratio in the mixture and the time of grinding. The reaction rate is a function of the rotation rate and the mass of the balls. Water is not necessary to initiate the reaction but facilitates it because hydrated salts react faster than the corresponding anhydrous salts. Neither carbon dioxide nor carbonate ions seem to have any influence on the transformation kinetics. The transformations that occur during grinding influence the final mechanical properties of hydraulic calcium phosphate cements prepared from these materials. Thus, if a grinding step of the starting materials is planed, the grinding conditions will have to be particularly well defined to obtain reproducible results.

Chemical characterization of in vivo aged zinc phosphate dental cements

The chemical composition of zinc phosphate dental cements aged in vivo was studied. Twenty-seven samples aged 2 to 43 years were investigated using X-ray diffraction, infrared spectroscopy, 31P nuclear magnetic resonance spectroscopy, thermogravimetric analysis and differential scanning calorimetry. Evidence for the presence of zinc oxide, amorphous zinc phosphate, water of hydration and crystalline zinc phosphate tetrahydrate was found. The latter was identified as hopeite; it was present in 92% of the cements studied. No correlation with time concerning either the chemical structure of the components or their relative amounts was found. Zinc phosphate dental cements show very good chemical stability on long-term use.

Effect of mechanical grinding of MCPM and CaO mixtures on their composition and on the mechanical properties of the resulting self-setting hydraulic calcium phosphate cements

Calcium bis-dihydrogenophosphate monohydrate (or monocalcium phosphate monohydrate, MCPM) is often used as the acid calcium phosphate in hydraulic calcium phosphate cement formulations. But commercial MCPM is not pure; it contains a small amount of orthophosphoric acid and moisture. Consequently, MCPM is difficult to mill and the powder is sticky and presents aggregates. Because granularity influences the mechanical properties of the hardened cements, a possible way to get around this difficulty that has been proposed is to premix it with other materials before grinding. We therefore ground commercial MCPM with CaO. A rapid decrease in the amount of MCPM was observed during mechanical grinding by a solid-solid reaction with calcium oxide. The final products were anhydrous or dihydrate dicalcium phosphate and/or hydroxyapatite or calcium-deficient hydroxyapatite depending on the initial calcium-to-phosphate (Ca/P) ratio. The mechanical properties (compressive strength and setting time) of cements made from MCPM and CaO were affected whatever the Ca/P ratio as a consequence of the change in composition of the starting materials. Storage at different temperatures of MCPM and CaO mixtures manually ground in a mortar for only 2 min and without mechanical grinding did not affect their composition, but a decrease was observed in the compressive strength of cements made from these mixtures.

Dry Mechanosynthesis of Strontium-Containing Hydroxyapatite from DCPD +CaO +SrO

By grinding mixtures of DCPD + CaO + SrO in a planetary ball mill, mixe d calciumstrontium phosphates or hydroxyapatites were mechanochemically s ynthe ized. Calcium and strontium-to-phosphate molar ratios [(Ca+Sr)/P] 1.50, 1.60 and 1.67 were test ed and the relative amount of Sr and Ca was also varied. The kinetic study of the mech ano emical reaction was carried out for all the mixtures by XRD and DSC. The reaction r ate was slightly lower with SrO than with CaO. XRD patterns of the powders after grinding showed an ap titic profile and after heating at 950°C for 2 h, powders were mixed Ca-Sr-hydroxyapatites or tri(Ca/Sr) phosphate or a mixture of them. Introduction Trace elements are an interesting alternative to growth fac ors in order to favor bone formation. Strontium is an element which plays a considerable part in osseous m ineralization and its use is envisaged in osteoporosis treatment [1]. In this goal, some attempt s were already done for introducing strontium in bioceramics [2] to enhance bone ingrowth. We show ed that dry mechanosynthesis was a possible route to obtain calcium deficient hy droxyapatites (CDHAs) that can be used to prepare biphasic calcium phosphate ceramics [3] with the expected calcium-tophosphate ratio Ca/P ± 0.01. CDAHs were prepared by grinding dicalc ium phosphate dihydrate (DCPD) and calcium oxide mixtures. Given the similar chemical properties of CaO and SrO, we synthesized strontium-containing HAs or DHAs by replacing partly or completely CaO by SrO according to Eq. 1 with 0 ≤ x ≤ 4, 0 ≤ y ≤ 4 and 3 ≤ x + y ≤ 4. 6 CaHPO4·2H2O + x CaO + y SrO Ca(6 + x)Sry(HPO4)(4-x-y) (PO4)6-(4-x-y) + (10 + x + y) H2O (1) Material and methods DCPD (from Fluka) had a median particle size d 50 of 8 μm (d10–d90 = 1.6–27 μm; calculated specific surface area, 3.5 m 2 g, Mastersizer, Malvern Instruments) and was used as received. CaO (from Aldrich) was heated at 900°C for 2 h to remove H 2O and CO2 and stored in a vacuum desiccator. After heating, median particle size was around 7 μm (2–40 μm; calculated specific surface area = 4.3 m 2 g). SrO was prepared by heating Sr(OH) 2·8H2O (from Riedel-de-Haën) at 900°C for 2 h. Its XRD pattern complied with the JCPDS file 6-520 wit h a slight amorphous-like background. Mixtures (15 g) of DCPD, CaO and SrO in variable proportions depending on the desir ed Ca replacement by Sr and (Sr+Ca)/P ratios were ground in a planeta ry ball mill (Retsch Instruments: vial eccentricity on the rotating sun disc 3.65 cm) at a rotation vel ocity of 350 rpm with 5 balls, 2.5 cm in diameter (total mass 133 g and surface area 110 cm2). At dif ferent intervals, powder (50 mg) was taken for analysis. The DCPD content at the different inter vals was determined either by XRD (Philips PW3830X, CGR horizontal goniometer, Cu K α1 = 1.5405 Å, Ni filter), surface area of the DCPD peak at 5.80° θ (plane (0 2 0)) after baseline subtraction or by DSC (DSC6 P erkinElmer), Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 103-106 doi:10.4028/www.scientific.net/KEM.254-256.103

Chemical characterization of in vivo aged zinc polycarboxylate dental cements.

The chemical composition of zinc polycarboxylate dental cements aged in vivo was studied. Thirty samples aged from one to 17 years were investigated using X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry. Evidence for the presence of zinc oxide, amorphous zinc polycarboxylate and water of hydration was found. No correlation with age concerning either the chemical structure of the components or their relative amounts was found. Zinc polycarboxylate dental cements show very good chemical stability on long-term use.

Polymorphisme et analyse du comportement thermique d'une serie de 3-(α-hydroxy imino aryl) 1,2,2-trimethyl cyclopentane carboxylate

The infrared,1H NMR and X-ray diffraction characteristics of a series of 3-(α-hydroxy-imino-aryl)-1,2,2-trimethyl cyclopentane carboxylate are reported. The thermal analysis of this series has been studied by differential scanning calorimetry. The results show a large variety of thermal behaviours with some of the compounds exhibiting polymorphism and the two types of isomers were detected.ZusammenfassungEs werden die Infrarot-, die1H NMR- und Röntgendiffraktionsmerkmale einer Reihe von 3-(-Hydroxyiminoaryl)-1,2-2-trimethylcyclopentancarboxylate dargelegt. Die Thermoanalyse dieser Reihe wurde mittels DSC durchgeführt. Die Ergebnisse zeigen ein weites Feld thermischen Verhaltens, wobei einige der Verbindungen Polymorphismus aufweisen und zwei Typen von Isomeren festgestellt werden konnten.

Chemical and structural changes in zinc polycarboxylate cements after immersion in dilute organic acid solutions

The behaviour of zinc polycarboxylate cements in contact with dilute aqueous solutions of organic acids at concentrations close to those existing in buccal medium, was studied. The organic acids were acetic, citric, tartaric and lactic acids, at 0.01 M and 0.001 M. The elution of zinc and magnesium was 10–1000 times greater in acid than in pure water, and correlated with the concentrations and the dissociation constants, pK1, of the acids tested. In all cases, important water losses were observed. In the 0.01 M acids, the cement structure collapsed to form a viscous, compact and homogeneous layer on the cement surface. In this layer, the polymeric carboxylic chains were regenerated from the zinc and magnesium polycarboxylate cement. Comparison with pure water showed that even the smallest concentration of the weak acids greatly modified the cement behaviour. This could explain the well-known differences in erosion processes between theoretical erosion predicted by standard specification tests and the in vivo situation.