Comparative static curing versus dynamic curing on tablet coating structures

International audienceCuring is generally required to stabilize film coating from aqueous polymer dispersion. This post-coating drying step is traditionally carried out in static conditions, requiring the transfer of solid dosage forms to an oven. But, curing operation performed directly inside the coating equipment stands for an attractive industrial application. Recently, the use of various advanced physico-chemical characterization techniques i.e., X-ray micro-computed tomography, vibrational spectroscopies (near infrared and Raman) and X-ray microdiffraction, allowed new insights into the film-coating structures of dynamically cured tablets. Dynamic curing end-point was efficiently determined after 4 h. The aim of the present work was to elucidate the influence of curing conditions on film-coating structures. Results demonstrated that 24 h of static curing and 4 h of dynamic curing, both performed at 60 degrees C and ambient relative humidity, led to similar coating layers in terms of drug release properties, porosity, water content, structural rearrangement of polymer chains and crystalline distribution. Furthermore, X-ray microdiffraction measurements pointed out different crystalline coating compositions depending on sample storage time. An aging mechanism might have occur during storage, resulting in the crystallization and the upward migration of cetyl alcohol, coupled to the downward migration of crystalline sodium lauryl sulfate within the coating layer. Interestingly, this new study clearly provided further knowledge into film-coating structures after a curing step and confirmed that curing operation could be performed in dynamic conditions

Comprehensive study of dynamic curing effect on tablet coating structure

International audienceThe dissolution method is still widely used to determine curing end-points to ensure long-term stability of film coatings. Nevertheless, the process of curing has not yet been fully investigated. For the first time, joint techniques were used to elucidate the mechanisms of dynamic curing over time from ethylcellulose (Aquacoat (R))-based coated tablets. X-ray micro-computed tomography (X mu CT), Near Infrared (NIR), and Raman spectroscopies as well as X-ray microdiffraction were employed as non-destructive techniques to perform direct measurements on tablets. All techniques indicated that after a dynamic curing period of 4 h, reproducible drug release can be achieved and no changes in the microstructure of the coating were any longer detected. X mu CT analysis highlighted the reduced internal porosity, while both NIR and Raman measurements showed that spectral information remained unaltered after further curing. X-ray microdiffraction revealed densification of the coating layer with a decrease in the overall coating thickness of about 10 pm as a result of curing. In addition, coating heterogeneity attributed to cetyl alcohol was observed from microscopic images and Raman analysis. This observation was confirmed by X-ray microdiffraction that showed that crystalline cetyl alcohol melted and spread over the coating surface with curing. Prior to curing, X-ray microdiffraction also revealed the existence of two coating zones differing in crystalline cetyl alcohol and sodium lauryl sulfate concentrations which could be explained by migration of these constituents within the coating layer. Therefore, the use of non-destructive techniques allowed new insights into tablet coating structures and provided precise determination of the curing end-point compared to traditional dissolution testing. This thorough study may open up new possibilities for process and formulation control

Synthesis of zirconium oxycarbide (ZrCxOy) powders: Influence of stoichiometry on densification kinetics during spark plasma sintering and on mechanical properties

International audienceDifferent stoichiometries of zirconium oxycarbide (ZrCxOy) powders were synthesised by carboreduction to approach the role of the chemical composition on the densification behaviour during spark plasma sintering and on the mechanical properties. Chemical analyses were performed using complementary methods to determine the actual stoichiometry and homogeneity of powders. Evolution of relative density during SPS treatment and microstructure observations by SEM showed that oxygen and vacancy contents would enhance the densification kinetics by promoting the lattice diffusion of limiting species. A study of mechanical properties of SPS sintered specimens has highlighted their significant dependence on the oxycarbide stoichiometry, especially at high temperature

A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

International audienceZirconium oxycarbide powders with controlled composition ZrC0.94O0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (Ea) from the densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n 6 2) governs the densification process, while a dislocation motion mechanism (nP 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687-774 kJ mol1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification

Spark plasma sintering of zirconium carbide and oxycarbide: Finite element modeling of current density, temperature, and stress distributions

International audienceA combined experimental/numerical approach was developed to determine the distribution of current density, temperature, and stress arising within the sample during spark plasma sintering (SPS) treatment of zirconium carbide (ZrCx) or oxycarbide (ZrCxOy). Stress distribution was calculated by using a numerical thermomechanical model, assuming that a slip without mechanical friction exists at the interfaces between the sample and the graphite elements. Heating up to 1950 °C at 100 °C min-1 and a constant applied pressure of 100 MPa were retained as process conditions. Simulated temperature distributions were found to be in excellent agreement with those measured experimentally. The numerical model confirms that, during the zirconium oxycarbide sintering, the temperature measured by the pyrometer on the die surface largely underestimates the actual temperature of the sample. This real temperature is in fact near the optimized sintering temperature for hot-pressed zirconium oxycarbide specimens. It is also shown that high stress gradients existing within the sample are much higher than the thermal ones. The amplitude of the stress gradients was found to be correlated with those of temperature even if they are also influenced by the macroscopic sample properties (coefficient of thermal expansion and elastic modulus). At high temperature, the radial and angular stresses, which are much higher than the vertical applied stress, provide the more significant contribution to the stress-related driving force for densification during the SPS treatment. The heat lost by radiation toward the wall chambers controlled both the thermal and stress gradients

TEM study of the reaction mechanisms involved in the carbothermal reduction of zirconia

International audienceThe carbothermal reduction of zirconia has been studied by TEM. The starting reactants consist in carbon black and monoclinic zirconia. Two successive steps are involved in the formation of the oxycarbide. The first one leads to the formation of an oxygen rich oxycarbide compound surrounded by an amorphous covering layer that regulates the achievement of species between gas and oxycarbide. The growth is controlled by three solid-gas partial reactions corresponding either to the destabilisation of the two products or to the nucleation and growth of the oxycarbide. The oxycarbide nucleation site is strictly located within the amorphous carbon. During the second step of the process, one of the initial constituents is missing (zirconia) and the previous solid-gas equilibriums are broken. The maturation of the oxycarbide into nearly stoichiometric carbide begins. The surrounding amorphous covering layer crystallises in carbon rich secondary carbide which progressively equilibrates in composition within the inner primary one