Environmental Engineering Reference
In-Depth Information
detergent, pesticide, solvents) also end up in the recycling stream and pose
a contamination risk (FDA 2006). The value of cleaning technologies is
assessed on their efficacy in removing contaminants acquired by the bottles
exposed to a cocktail of chemicals. Super-clean technologies available today
are able to deliver “food-grade” PET for bottle applications even where
the PET waste streams are highly contaminated with non-food polyester
containers (Blanchard et al., 2003).
Trace contaminants derived from the first use of the resin survive in the
lower parts per million range in conventional recycling of bottles to flakes
(Bayer, 2002; Franz et al., 2004). Bottle-to-bottle applications demand
more stringent, super-clean recycling processes to reduce the level of
contaminations to nearly the same levels as that in virgin resin. This is
accomplished by a combination of thermal treatment (180-230°C), inert
gas stripping or vacuum degassing, and re-extrusion (280-290°C) into
pellets (Welle, 2011). The contaminants, such as flavor ingredients are
generally localized in a thin layer of approximately 10-15 µm of the
food-contact surface (see Fig. 9.14 ). Some super-clean techniques therefore
rely on base-catalyzed hydrolytic depolymerization of the layer at
approximately 150°C followed by washing, degassing, and reextrusion
(Welle, 2008). The reaction produces ethylene glycol and terephthalic acid
monomers that have to be removed by vacuum degassing. Alternatively
waste PET might be glycolyzed (with ethylene glycol) to achieve partial
depolymerization into oligomers that can supplement the prepolymer (prior
to the melt condensation phase) in the PET manufacturing process (Welle,
2008). As thewaste PET is not reduced tomonomers, this is still considered
a material recovery operation.
 
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