(10e) Coupling Extrusion and Supercritical Co2: a New Process for a Homogeneous Inclusion of Fragile Molecules in a Polymer Matrix

Extrusion is a process already used industrially for the formulation of finely divided solids into polymeric matrices. However, it often involves high temperatures and mechanical constraints, prohibiting the use of fragile molecules, as found in pharmaceuticals. Moreover, porosity is only controllable by the addition of nucleation agents. This could imply the presence of residues in material and the need for an additional stage to eliminate them if they hamper use in medicines. In this work, we have developed a supercritical carbon dioxide (SC CO₂)-assisted extrusion process, which leads to the manufacturing of nano-structured polymeric foams. SC CO₂ modifies the rheological properties of the material in the barrel of the extruder and acts as a blowing agent during the relaxation at the passage through the die. Thus, its high solubilisation in the polymer undergoing extrusion results in extensive expansion at the die outlet. The reduction in viscosity lowers the mechanical constraints and the temperature within the extruder. This allows handling molecules with a limited thermal stability. For experimental purposes, a single-screw extruder (Scamex, France) with a capacity of about 1 kg.h^-1 was modified in order to be able to inject SC CO₂ within the extruded material. Preliminary experiments were carried out using polystyrene (PS) and the effects of two parameters: cooling (by air or in a water bath) and operating temperature were studied.

Pore size appeared to be a positive function of the temperature. For the highest temperature (230°C), the structure of the extruded matter continues to evolve leading to pores of large size until occupying all the central part of the cylinder. This is probably due to the reduction of viscosity of the melted material with the temperature, thus decreasing the constraint being opposed to the growth of pores. This effect is more marked in the centre of the cylinder, which is less quickly cooled. We measured a strong increase in the void ratio (Vf) with the temperature between 175 and 185°C probably caused by this bubble growth which preserves a regular form. Then, Vf remained constant between 185 and 200°C and it thus seemed that it was an increase in the coalescence of bubbles which led to the structure observed at 200°C. By cooling the material in a water bath, the mean pore size was reduced, as was the size distribution. A much more homogeneous material was obtained and Vf was lower (19 % instead of 30 %). It thus seems important to solidify the structure in order to carry out relevant observations of the influence of the operating parameters. In this process used here, the depressurisation of the mixture of both components along the die releases the dissolved CO₂ and creates a controlled micro porosity within material. Polystyrene with void ratio of about 15 to 30 % was thus manufactured. Moreover controlling the temperature allows the control of the porosity by the effects on the phenomena of growth and coalescence. The quantity of dissolved SC CO₂ and the relaxation can be controlled by the adjustment of the operating conditions and, consequently, the expansion, the size and the density of the pores can be fixed to obtain new materials, with a broad range of properties. The SC assisted-extrusion allows the manufacture of solid dispersions in a polymeric matrix, by reducing the operating temperatures and mechanical constraints within the extruded material. This allows the handling of thermolabile molecules, such as pharmaceutical drug substances. Coupling these two technologies also allows the creation of porosity within a solid dispersion, without the use of additives which could require an additional stage for their elimination.. A next stage will consist in studying the influence of other operating parameters such as the CO₂ concentration and the pressure.