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Polymer Science and Engineering: The Shifting Research Frontiers
are formed. Technology is being developed to convert polymer powders into three-dimensional prototype parts by using a computer-driven laser-sintering technique. There is much to be learned about the physics of sintering polymer powders.
Emulsion polymerization is widely practiced because it is a convenient process for producing high-molecular-weight polymers using free radical mechanisms at high rates while controlling the exothermic nature of the reaction. In some cases, the resulting aqueous dispersion of the very fine polymer particles (called a latex) can be used directly for convenient fabrication of products. For example, the ubiquitous rubber gloves employed nowadays to protect against the spread of AIDS are made by dipping forms into a rubber latex followed by vulcanization. The popular water-based latex paints are made by emulsion polymerization. The paint film is formed by the evaporation of water, as opposed to organic solvents for oil-based paints. Painters also like cleaning up using water rather than paint thinner or solvents. The tiny emulsion polymer particles fuse into a continuous film driven by surface tension forces as the water evaporates. The rheological characteristics of the polymer must be designed such that fusion can occur without macroscopic flow of the coating. Environmental considerations strongly favor formation of coatings using latexes rather than solutions that "cure" by evaporation of organic solvents.
Nonaqueous dispersion technology has emerged as a means of applying high-performance coatings (e.g., automotive paints) while minimizing solvent emissions. These are sophisticated materials in which fine polymer particles are formed by dispersion polymerization in a nonaqueous environment in which dispersant polymer chains prevent coalescence of the particles through steric stabilization. By this route, high-solids fluids of acceptable viscosity can be rapidly applied to metal substrates to form high-performance coatings with reduced emission of organic solvents.
Inverse emulsion homo-or co-polymerization of water-soluble monomers such as acrylamide or acrylamide with co-monomers, such as cationic quaternary acrylates, represents a new commercially practiced method for the generation of materials that are suitable as flocculants. The materials as generated are extremely effective for purifying water, even at a level of 500 parts per million. When activated in water via modification of the surfactant or stabilizer, they can induce large flocs to be formed, which can be centrifuged from waste systems like sewage. The resulting flocs can be certrifuged, dried, and used as fertilizers. The resulting water is, of course, in a much purer state than before this treatment. Such operations are already in wide commercial use in many of the large cities of Europe and North America and are an example of how polymers can help clean the environment. Producing such polymers is already a multimillion dollar business.