With worldwide consumption of plastics increasing annually, and with fifty percent of the mixed solid waste stream consisting of these materials, the opportunities for recycling and recovery of polymeric materials are high. In this regard SCF technology has been applied to these areas (102, 402).
One application of SCF that may prove commercially viable to the recycling industry is as a cleaning agent to remove contamination from the polymer prior or during mechanical recycling. For example, supercritical fluids can be used to clean polyethylene automotive fuel tanks of residual gasoline or diesel (56). A gas tank may absorb up to 5% by weight of fuel over the course of its service life. By pressurising the shredded materials and exposing them to scCO 2, the gas acts as a solvent to extract the residual fuel.
Supercritical fluids can also be used during recompounding to remove contaminants (373). Controlled experiments with naphthalene doped plastics found 95% or greater of the naphthalene could be removed by introducing SCF into a 34 mm counter-rotating, intermeshing twin-screw extruder.
Unlike mechanical recycling, which collects and uses waste plastics without any further chemical treatment, chemical recycling uses the waste products to turn them into their monomers (or other useful chemicals) by means of chemical reactions. Common examples of chemical recycling processes include cracking and hydrogenation. Thermal conversion technologies on the other hand generate conversion products and heat.
These are used for both a volume reduction in the waste and for...
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