Nadine Le Bolay

Characterisation of the physicochemical properties of polymers ground in a vibrated bead mill

Abstract The paper presents a study on the evolution of polymer properties during fine grinding in a vibrated bead mill. Two polymers having different initial properties were tested: poly(vinyl acetate) (PVA) and polystyrene (PS). They were ground with various operating conditions and the evolution of the size, the morphology, the molecular weight, the crystalline structure and the glass transition temperature was characterised. It was shown that the fragmentation rate and the fragment morphology depend on the material nature and on the operating parameters. The energy supplied by the mill is first essentially used for particle fragmentation. Then, when a limit size is reached, chains are cut at their extremities. It is thus possible to grind a polymer without damage if the experimental time is well controlled. The crystalline structure of poly(vinyl acetate) becomes amorphous during the first grinding minutes, and the amorphisation time does not depend on the operating conditions. Finally, no variation of the glass transition temperature was observed in this study.

Fragmentation mechanism of poly(vinyl acetate) particles during size reduction in a vibrated bead mill

The fragmentation mechanism taking place during dry grinding of poly(vinyl acetate) particles in a shaker bead mill has been analysed using various shape factors permitting a quantitative description of the particles morphology. The knowledge of the size evolution was not sufficient to understand the phenomena occurring in the mill. Several experiments were carried out with different operating conditions and it was shown that the predominant phenomenon governing the fragmentation depends mainly on the intensity of the stress applied to the particles. When this is high, the entire particles are broken while only small fragments are removed when the stress is low. This study permits the definition of the sub-populations of particles created after fragmentation, which can be used for modelling the grinding kinetics.

Cogrinding significance for calcium carbonate–calcium phosphate mixed cement. II. Effect on cement properties†

In the present study, we aim to evaluate the contribution of the cogrinding process in controlling calcium carbonate-dicalcium phosphate dihydrate cement properties. We set a method designed to evaluate phase separation, usually occurring during paste extrusion, which is quantitative, reliable, and discriminating and points out the determining role of cogrinding to limit filter-pressing. We show that solid-phase cogrinding leads to synergistic positive effects on cement injectability, mechanical properties, and radio-opacity. It allows maintaining a low (<0.4 kg) and constant load during the extrusion of paste, and the pastes composition remains constant and close to that of the initial paste. Analogous behavior was observed when adding a third component into the solid phase, especially SrCO(3) as a contrasting agent. Moreover, the cements mechanical properties can be enhanced by lowering the L/S ratio because of the lower plastic limit. Finally, unloaded or Sr-loaded cements show uniform and increased optical density because of the enhanced homogeneity of dry component distribution. Interestingly, this study reveals that cogrinding improves and controls essential cement properties and involves processing parameters that could be easily scaled up. This constitutes a decisive advantage for the development of calcium carbonate-calcium phosphate mixed cements and, more generally, of injectable multicomponent bone cements that meet a surgeons requirements.

Control of the Injectability of Calcium Carbonate-Calcium Phosphate Mixed Cements for Bone Reconstruction

The purpose of this study was to improve injectability and cohesiveness of original calcium carbonate-calcium phosphate mixed (CaCO3-CaP) self-setting paste for bone filling and repair. With this aim in view dry co-grinding was implemented on the solid phase (vaterite and dicalcium phosphate dihydrate) of this cement. A protocol designed to quantify paste injectability has been established and pointed out the synergistic positive effects of solid phase co-grinding treatment on injectability, cohesiveness and setting time of the paste. The improvement of these properties are related to close and homogeneous association of reactive powders and to the decrease of specific surface area favoring the powders hydration process enhancing setting reaction rate. In addition, the particle size decrease and morphology modification improved flowability of the paste which results in a low and constant (320 g) force level to extrude the paste.