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CHAPTER 8 PLASTICS RECYCLING AND BIODEGRADABLE PLASTICS. This chapter begins with an overview of municipal solid waste and the contribution that plastics of various types make to it.
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Landﬁll remains the primary method for handling MSW, although its prevalence has decreased with time (Fig. 8.3).
Plastics account for only about 11.3 percent of materials in the U.S. MSW stream by weight (Fig. 8.4), but this proportion continues to increase.
The proportion by volume is very difﬁcult to determine accurately, but is certainly larger than the percent by weight, as the density of plastics is less than the average of MSW
Much of the plastic in MSW originates in packaging. Figure 8.5 shows the proportions of plastics, paper, metal, glass, and rubber and leather in EPA’s major product categories of durable goods, nondurables, and containers and packaging.
Recycling rates for plastics differ considerably by resin and by product type
A more fundamental problem than data accuracy is the matter of deﬁnition—what should count as recycled?
Environmental Beneﬁts of Recycling and Use of Biodegradable Plastics
Often, although not always, another beneﬁt of recycling is cost reduction.
Biodegradable plastics are still generally more expensive than the synthetic plastics they compete with, although the price differential is decreasing. If these biodegradable plastics are also biobased, increases in price of oil and natural gas may make them more competitive.
A factor that is certain to become increasingly important in the next decade is that the use of recycled plastics often results in signiﬁcant energy savings, compared to the use of virgin resin.
In the near future, efforts to reduce emissions of greenhouse gases may become an important driver for use of plastics in general and for biobased and recycled plastics in particular.
If systems are set up to be very convenient, less motivation will be required to get people to participate.
Plastic resins differ in terms of which recycling technologies are appropriate.
Separation of plastics from nonplastic contaminants typically relies on a variety of fairly conventional processing techniques.
Another problem arises from differences in melting temperatures.
Mixing resins of different colors can also be a problem. Laundry detergent bottle producers were able to fairly easily incorporate unpigmented milk bottle HDPE in a buried inner layer in detergent bottles, but they found it much more difﬁcult to use recycled laundry detergent bottles.
Sorting systems can be divided into macrosorting, designed to operate on whole or nearly whole plastic articles; microsorting, designed to operate on chipped plastics; and molecular sorting, designed to act on dissolved plastics.
This system allows communities to tell consumers in a relatively simple way what materials are desired in the recycling system and what materials should not be included.
Other examples, of course, could also be cited where the small but real risk of unacceptable performance, or of release of some damaging substance coupled with the critical nature of the application, is likely to rule out the use of recycled plastics.
Next came use of recycled plastic in buried inner layers of packaging, such as a recycled PS clamshell used for hamburgers, in which the contact between the food and the recycled plastic was mediated by a layer of virgin plastic that acted as at least a partial barrier.
Systems for processing recycled HDPE have been approved for limited direct food contact applications as well.
Recycled resins, by their very nature, often have a somewhat unknown history.
Similarly, plastics that are designed to be used outdoors generally must be stabilized against photodegradation. Recycled materials are likely to need additional stabilizer to retain adequate performance.
Either with or without oxidation, chain cleavage can also occur. This results in a decrease in molecular weight, with a consequent decrease in many performance properties.
In summary, the general rule is that recycled polymers will have somewhat different properties than virgin polymers.