(77d) Tailor-Made Composites by High Pressure Spray Processes

During the last decade the requirements for industrial products are more and more increasing. This effect is not only apparent for end-market products but also for precursor products manufactured in chemical, pharmaceutical, life science and food industries. For a commercial success it is no longer sufficient to produce particles from pure substances like polymers or pharmaceutical agents. It has become more and more important to design tailor-made particle systems which allow e.g. the controlled release of active agents or offer a durable protection of sensitive ingredients. One of the most promising technologies for customised particle generation is the use of supercritical fluids (SCF). These technologies allow generating powders with properties that are difficult or even impossible to achieve by classical methods like milling, crystallization, spray drying or agglomeration. One of the newest processes to design tailor-made particle systems is the so called PGSS (particles from gas saturated solutions) process which is illustrated in figure 1. The different substances, necessary to form the composite, are stored in separate vessels. The substances can be in liquid, molten or dispersed state. With high pressure pumps the liquids are dosed to a static mixer, where compressed carbon dioxide is added to the composition. In the static mixer, the different components and the gas are homogenised. Under elevated pressures the gas is partly soluble in many organic components. This causes a decrease of the viscosity and the surface tensions of the liquids and facilitates the dispersion of the components. Subsequently the so formed solution is depressurised via a nozzle into a spray tower. Due to the expansion of the gas fine droplets are formed. Caused by the Joule-Thomson phenomenon of the gas, the spray tower is cooled. The process has to be operated in that way, that at least one component solidifies under spray tower conditions and incorporates the other components. The obtained morphologies are illustrated at the right side of figure 1. Particles with dispersed droplets or dispersed solids are formed. In the PGSS technique the powders are handled in an inert carbon dioxide atmosphere. The complete process works at very moderate temperatures. During the mixing of the components the temperature is just slightly above the melting point of the components or in some cases even below the melting point (reduction of the melting temperature by dissolved carbon dioxide). In the spray tower, temperatures of 20°C or even lower (until ?60°C) are normal. Thus, the PGSS-process is suitable for very sensitive substances like in pharmaceuticals and life science products. The use of the compressed gas allows the formation of different morphologies with the PGSS-technique. By adjusting the process parameters like spray pressure, spray temperature and gas to product ratio it is possible to form spheres, sponges, fibres, hollow spheres and foams. Many experiments demonstrate that morphology and size can be tailor-made in a range between approx. 3 µm up to 500 µm, with bulk densities from 90 kg/m³ up to approx 600 kg/m³. Beside the formation of single component particles the process offers the possibility to incorporate soluble and insoluble liquids, dispersed nano-particles, reactive agents and even a combination of all is possible. Figure 2 shows two component composites formed of one solid (light grey regions) and one liquid (dark grey regions). With a liquid content of 64 wt.% open structures are obtained. By reducing the liquid phase to 45 wt.% a complete packaging of the liquid is possible. In this presentation it is shown how high pressure in combination with supercritical fluids offers new possibilities to generate powders with unusual morphologies and properties from pure substances and mixtures. Different experimental results with the lab scaled PGSS plants are illustrated and finally the versatile possibilities of this technique will be illustrated by different applications which are already established in industry.