In conventional polymer manufacturing, monomers, the initial kernels of the chemical reaction that produces polymers, are suspended in large quantities of water or chemical solvents. According to DeSimone, polymers produced on a large scale such as polystyrene, polyvinyl chloride (PVC), polyethylene co-vinylacetate, polyacrylic acid, and styrene and butadiene rubber, are created using heterogeneous dispersion polymerization. In this process, polymers form in two phases in which the initial monomer or the resulting polymer, or both, are finely dispersed in a solvent and are controlled by adding a surfactant (stabilizer) that conforms particle sizes to within a relatively narrow range. Once the polymers are formed, manufacturers remove the water or evaporate the solvents used to disperse the polymers. Companies then face the challenge of properly disposing of and remediating these harmful by-products--a daunting task as EPA regulations grow more stringent. In 1992, the U.S. plastics industry produced 567 million pounds of toxic waste, according to the EPA. In the same year, companies put about one-quarter of their total waste back into the nation's environment.
Because most organic solvents (typical ones include toluene and methylene chloride) are petroleum-based, many are flammable, volatile, and often cause narcotic effects to humans at high concentrations. Yet pharmaceutical, chemical, and polymer industries must use vast amounts of these solvents to extract, separate, and manufacture chemicals.
According to Paul Anastas, chief of the new chemicals section in the EPA's Office of Pollution Prevention and Toxics in Washington, DC, these solvents have "serious health implications." Companies that release these solvents must comply with various requirements, says Anastas: "They have to be concerned with waste treatment and control, [monitoring] water effluent and air emissions, and minimizing exposure to workers."
Human health stands to benefit from polymerization using supercritical carbon dioxide because the process avoids the use of harmful solvents that are known health hazards, contends DeSimone, whose research is funded through the Environmentally Benign Chemical Synthesis and Processing Program sponsored by the EPA and the National Science Foundation. Manufacturers have traditionally dealt with toxic by-products in two ways: remediation and control. "What we're talking about now is a new option for dealing with waste-avoidance," DeSimone says.
Therein lies this technology's single greatest advantage, says Anastas: "Natural carbon dioxide is absolutely innocuous. The gas is harmful only if it is produced through combustion. "If you're just converting existing carbon dioxide, there is no harm," Anastas explains. After the polymers form, the carbon dioxide is simply depressurized, returning it to a gaseous state, and is then vented off. Anastas quickly offers further reassurances: "The use of carbon dioxide is not going to contribute to the greenhouse effect; only the creation of carbon dioxide [is going to do that]."
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