In other cases, solution processing is required because of the nature of the product being fabricated rather than the nature of the polymer. Application of certain types of paint or coatings is most conveniently done by making the polymer fluid by dissolving it in a solvent and hardening by solvent removal. Melt fabrication cannot be used to economically perform certain operations like tablet coating (for controlled delivery) in the pharmaceutical industry. Formation of polymer membranes with complex structures consisting of extremely thin skins overlaying a porous support layer generally involves a variety of solution-processing protocols that can hardly be accomplished in any other way. Prepregs, used to make continuous-fiber-reinforced composites, often involve a solution processing step. "Molecular composites" based on rigid rod polymers dispersed in a matrix of a random coil chain polymer have attracted enthusiasm recently, and most techniques for forming them will invariably involve solution technology.
The state of scientific knowledge about the thermodynamics, rheology, diffusion, and morphology development in multicomponent polymer-solvent systems will need to be advanced in order to place these types of processes on a solid technical footing. For example, recent attempts to mathematically model the formation of asymmetric membranes have failed to give some of the insight critically needed simply because the fundamental elements of such models are not currently well-enough understood.
Environmental and health concerns associated with solution processing must be dealt with at several levels. There are opportunities for innovation in both the processes and the materials used. One avenue of interest is systems that use more benign solvents. Ongoing work in the area of supercritical fluid technology appears promising for some applications.
Dispersion processing—the generation of particulate forms of polymers and their conversion into products—is a key technique that is likely to grow in importance, driven by both environmental and materials considerations. The polymer may be in the form of a dry powder, an aqueous dispersion (e.g., a latex), or a nonaqueous dispersion (NAD). A number of examples illustrate the current state of this technology and future directions.
Polytetrafluoroethylene, or Teflon®, is a powder after the polymerization process, and its melting point is so high that melt fabrication is usually not practical. However, by combinations of heat and pressure it is possible to fabricate solid forms in much the same way as used in powder metallurgy. Powder coatings are applied to various substrates by a number of techniques, including fluidized beds and flame heating. Powdered liquid crystalline polymers can be "extruded" below their melting points in much the same way as metal powders