CHAPTER 8PLASTICS 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. • Recycling of plastics is discussed, with a concentration on postconsumer plastics—those that have served their intended use. • Routine re- processing and use of process scrap is not covered, as it is generally considered part of normal plastic manufacturing rather than categorized as recycling. • Energy recovery, whether through incineration or through thermal depolymerization or pyrolysis, is also not covered. • Biodegradable plastics are covered in detail, with some discussion of biobased plastics that may not be biodegradable.
Municipal Solid Waste • Concerns about municipal solid waste (MSW), deﬁned by the U.S. Environmental Protection Agency as including residential, institutional, ofﬁce, and commercial waste, but not including construction and demolition debris, wastewater treatment sludge, and industrial waste • Stem from three main considerations.: • One is the amount of space occupied by the waste—space that therefore is not available for other uses. • Another is the resources that are “wasted” when materials are sent to disposal. • The third concern is the health and environmental impacts associated with emissions from the waste, either during or after disposal.
Landﬁll remains the primary method for handling MSW, although its prevalence has decreased with time (Fig. 8.3). • The declining number of landﬁlls in the United States, down to 1767 in 2002 from 7924 in 1988, has been compensated by new landﬁlls being much larger, on average, than those that have shut down. • Composting, which was insignificant before the late 1980s, has grown substantially. • Incineration has stayed relatively stable over the last decade.
Plastics in Municipal Solid Waste 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. • As can be seen, the containers and packaging category has the largest share of plastics in MSW.
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? • The two most common options are • determining the amount of material collected for recycling and • determining the amount of material delivered for reuse. Since, typically, 5 to 15 percent of collected material is lost during processing (mostly because it is some type of contaminant such as a paper label, product residue, un- wanted variety of plastic, or other material), recycling rates calculated using these two methods can differ substantially.
Environmental Beneﬁts of Recycling and Use of Biodegradable Plastics • An obvious beneﬁt of recycling and use of biodegradable plastics is that both reduce the requirement for landﬁll or incineration of waste materials. • Items that are recycled are, by deﬁnition, diverted from the waste stream. • Biodegradable plastics can be managed by composting, generally perceived as more environmentally beneﬁcial than landﬁll or incineration. • In fact, advocates of composting often refer to it as natural or biological recycling.
Often, although not always, another beneﬁt of recycling is cost reduction. • For example, use of regrind became routine because of the monetary savings it provided. Similarly, certain plastics industries for years have relied on a combination of off-spec and recycled plastics because of their lower price. • The desire to beneﬁt from consumer preferences for recycled material coupled, in some cases, with legislative pressures have led, on occasion, to the anomalous situation of recycled plastic being worth more per pound than virgin resin, but these situations are usually short lived. • Recent increases in the cost of oil and natural gas, with consequent increases in prices for virgin resins, provide more opportunity for recycled plastics to be economically competitive.
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. • Additional beneﬁts from recycling of plastics result from the fact that use of recycled resin displaces use of virgin materials and thus reduces depletion of natural resources • Recycling processes generally produce fewer environmental efﬂuents than do processes that produce virgin resin, so the use of recycled plastics usually results in a decrease in air and water pollution. • Biobased plastics use renewable materials as a feedstock, so they also can reduce resource depletion.
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. • For example, Fenton calculated the total energy requirement for a low-density polyethylene grocery bag to be 1400 kJ, while a bag with 50 percent recycled content required only 1164 kJ, for a savings of nearly 17 percent. • A DOE report concluded that recycling PET products such as soft drink and ketchup bottles requires only about a third of the energy needed to produce the PET from virgin materials. • Again, recent increases in energy prices make this advantage even more signiﬁcant.
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. • For example, a recent study by the Center for Packaging Technology (Cetea) in Spain reported that PET recycling reduces carbon dioxide emissions by 25 percent and methane emissions by 18 percent. • In the farther-term future, when oil supplies diminish signiﬁcantly, production of plastics from renewable feedstocks will likely be critical
RECYCLING COLLECTION • For plastics recycling to occur (or for recycling of other materials), three basic elements must be in place. • First, there must be a system to collect the targeted materials, to gather them together. • Second, there must be a facility capable of processing the materials into a form that permits them to be used to make a new product. • Third, new products made in whole or part from the recycled materials must be manufactured and sold. • A breakdown in any part of this system eventually stops the whole process. • Because of this, efforts to increase recycling rates must pay attention to markets for the recycled materials as well as to the infrastructure to allow collection and processing of the materials.
Collection of Materials • For postconsumer materials, including plastics, the most difﬁcult part of the recycling process may be getting the material collected in the ﬁrst place. • Industrial scrap is “owned” by the industrial entity that produced it. • If the owners cannot get the scrap recycled, they will either have to dispose of it or pay some other business to do so. • For much consumer scrap, there is little or no monetary incentive for its owner, the individual consumer, to direct it into a recycling system. • Furthermore, industrial scrap tends to be concentrated, with substantial amounts of material in relatively few locations, making it relatively easy to collect. • Postconsumer materials are typically very diffuse, so a more elaborate collection infrastructure is needed to get this material gathered together in quantities that make its processing economically viable.
There are three main approaches to collection: • go out and get the material, • create conditions such that the material will be brought to you, or • use a combined approach. • There is a trade-off between motivation and convenience in getting people to participate in recycling by appropriately diverting the targeted recyclables from the waste stream into the recycle stream. • Highly motivated individuals will participate in recycling even if they have to go to considerable effort to do so.
If systems are set up to be very convenient, less motivation will be required to get people to participate. • Therefore, increasing participation in recycling can be increased by providing greater motivation, by providing greater convenience, or by a combination of the two. • Usually (although not always), systems that go out and get the materials provide greater convenience than those that require individuals to deliver the materials to a collection point.
RECYCLING PROCESSES • Recycling processes for plastics can be classiﬁed in a variety of ways. • One categorization differentiates between primary, secondary, tertiary, and sometimes quaternary recycling • Primary recycling originally was deﬁned as applications producing the same or similar products, • whereas secondary recycling produces products with less demanding speciﬁcations. • EPA’s current deﬁnition considers use of in-plant scrap as primary recycling and use of postconsumer material as secondary recycling, regardless of the end products
Tertiary recycling uses the recycled plastic as a chemical raw material. • Quaternary recycling uses the plastic as a source of energy. This last category is often not considered to be true recycling. • An alternative categorization that is gaining in popularity is mechanical and feedstock recycling. • Mechanical recycling, as the name indicates, uses mechanical processes to convert the plastic to a usable form, thus encompassing the primary and secondary processes outlined above. • Feedstock recycling is essentially equivalent to tertiary recycling, using the recycled plastic as a chemical raw material, generally (but not always) for the production of new plastics. • The term recovery is often used to encompass mechanical and feed- stock recycling plus incineration with energy recovery. This categorization is used in Europe in particular.
Plastic resins differ in terms of which recycling technologies are appropriate. • Thermoplastics are more amenable to mechanical recycling than thermosets, which cannot be melted and reshaped. • Typically, condensation polymers such as PET, nylon, and polyurethane are more amenable to feedstock recycling than addition polymers such as polyoleﬁns, polystyrene, and PVC. • Most addition polymers produce a complex product mixture that is difﬁcult to use economically as a chemical feedstock, while condensation polymers usually produce relatively pure one- or two-component streams.
Separation and Contamination Issues • When plastics are collected for recycling, they are not pure. • They contain product residues, dirt, labels, and other materials and often contain more than one type of plastic resin, resins with different colors, additive packages, and so on. • This contamination is one of the major stumbling blocks to increasing the recycling of plastic materials. • Usefulness of the recovered plastic is greatly enhanced if it can be cleaned and puriﬁed. • Therefore, technologies for cleaning and separating the materials are an important part of most plastics recycling systems.
Separation of Nonplastic Contaminants • Since most household plastics are collected for recycling mixed with other materials, the ﬁrst step in processing is usually to separate the plastics from these other materials • Inclusion of nonplastic contaminants in recycled resins can affect both processing of the material and performance of the products manufactured from these materials. • Presence of remnants of paper labels, for example, can result in black specks in plastic bottles, detracting from their appearance and rendering them unsuitable for some applications. • These paper fragments can also build up in the screens in the extruder during processing, resulting in greater operating pressures (and energy use) and requiring more frequent screen changes..
Separation of plastics from nonplastic contaminants typically relies on a variety of fairly conventional processing techniques. • Typically, the plastic is granulated, sent through an air classiﬁer to remove light fractions such as labels, washed with hot water and detergent to remove product residues and dirt and to remove or soften adhesives, and screened to remove small heavy contaminants such as dirt and metal. • Magnetic separation is used to remove ferrous metals, and techniques such as eddy current separators or electrostatic separators are also often used to remove metals. • Many of these techniques were originally developed for mineral processing and have been adapted to plastics recycling.
Separation by Resin Type • Contamination of one resin with another can also result in diminished performance. • One of the most fundamental problems is that most polymers are mutually insoluble. • Thus, a blend of resins is likely to consist, on a microscopic scale, of domains of one resin embedded in a matrix of the other resin. • While this sometimes results in desirable properties, more often it does not. • To further complicate matters, the actual morphology, and thus the performance, will be strongly dependent not only on the composition of the material but also on the processing conditions. • Therefore, for most high-value applications, it is essential to separate plastics by resin type.
Another problem arises from differences in melting temperatures. • When PET is contaminated with polyvinyl chloride (PVC), for example, the PVC decomposes at the PET melt temperatures, resulting in black ﬂecks in the clear PET. • A very small amount of PVC contamination can render useless a large quantity of recovered PET. • On the other hand, at PVC processing temperatures, PET ﬂakes fail to melt, resulting in solid inclusions in the PVC articles that can cause them to fail. • Again, a small amount of PET contamination can render recovered PVC unusable.
More subtle problems can arise, even from very similar resins. • When injection-molded HDPE base cups from soft drink bottles were contaminated by newly developed blow- molded HDPE base cups, the recovered HDPE consisted of a blend of a high-melt-ﬂow resin with a low-melt-ﬂow resin that neither blow molders nor injection molders found usable. • This caused serious difﬁculty for some recyclers. • While this problem disappeared with the discontinuation of base cups, the difﬁculty in separating injection-molded HDPE bottles from blow-molded HDPE bottles is an ongoing concern.
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. • The color tended to show through the thin pigmented layer, especially in lighter-colored bottles. • Motor oil bottlers who used black bottles had no such problem. • As a general rule, the lighter the color of the plastic article, the more difﬁcult it is to incorporate recycled content and, conversely, the lighter the color of the plastic article, the easier it is to ﬁnd a use for it when it is recycled (and hence the higher its value).
Categories of Sorting Systems • Initially, many systems for separating plastics by resin type relied on hand sorting. • Sorting by consumers delivering the appropriate types of plastics to the correct bins is nominally free of cost. • However, costs do arise due to errors, and material is lost when consumers choose not to participate • Automated sorting systems use various technological devices to identify plastics in materials rather than relying on embedded codes. • Capital investment is generally higher than for hand sorting, but operating costs are typically lower. • Accuracy may be as good or better.
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. • Hand sorting is always of the macrosorting variety. • In principle, automated sorting can be of any type but is most often macrosorting or microsorting. • Various systems are now available to quickly and automatically identify plastics by resin type or at least to distinguish between desired and undesired materials. • Many of these rely on differences between resins in absorption or transmission of various wave- lengths of electromagnetic radiation. • Many can also separate plastics by color or color family in addition to sorting by resin type. Most of these systems work well with whole containers but are not effective in separating chipped plastics or multilayer materials.
Resin Identiﬁcation Codes • The ﬁrst control point for separation of plastics by resin can be at the time of collection. • To facilitate identiﬁcation of plastics packaging by resin type, and to satisfy pressure by states for such a system, the Society of the Plastics Industry (SPI) developed a coding system, comprised of a triangle formed by three chasing arrows, with a number inside and a letter code below.
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. • In many cases, recycling programs collect only high-density polyethylene (HDPE) and polyethylene terephthalate (PET) bottles. • They can communicate this to consumers by asking for no. 2 and no. 1 plastic bottles, respectively. • In most states where it has been adopted, the SPI code is required to be present on all bottles of 16 oz up to 5 gal capacity and on other containers of 8 oz up to 5 gal. • In some states, there is no top limitation, so the code is required even on very large containers, including 55-gal drums.
Safety Concerns • Even when plastics are sorted by type, the performance of recycled plastics may differ from virgin plastic because of the effects of the use cycle. • These changes may be due to chemical changes within the polymer, sorption of materials into the polymer, or other factors. • If materials are sorbed, there is potential for later release of these substances. • For some critical applications, such possible or actual changes in behavior of recycled plastics pose unacceptable risks. • For example, it is probably safe to conclude that recycled plastics will not be used for implantable medical devices. • It is highly unlikely that recycled plastics will be used for the packaging of sensitive drugs
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. • For less critical applications, such as the use of recycled packaging for food products, the conventional wisdom used to be that recycled plastics should not be used. • This changed dramatically in the 1990s. In the United States, one of the earliest applications of recycled plastic for packaging of food products was recycled PET in egg cartons. • The physical barrier of the egg shell provided a degree of added protection to the food that the FDA agreed was sufﬁcient to allow ordinary recycled PET to be used
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. • Still later, repolymerizedPET was used in direct contact with food (blended with virgin material). • The repolymerization process, with its crystallization steps, provided assurance that any impurities present would be removed • Finally came FDA approval of speciﬁc systems for intensive cleaning of physically reprocessed PET, coupled with the limitation of incoming material to relatively pure streams of soft drink bottles returned for deposits. • Then, production of 100 percent recycled content PET bottles using a process for physically reprocessing bottles collected from curbside was approved
Systems for processing recycled HDPE have been approved for limited direct food contact applications as well. • The concern over use of recycled plastics in food contact falls in two general areas. • First is concern about biological contaminants. In most cases, the processing steps for production of plastic packaging materials provide a sufﬁcient heat history to destroy disease producing organisms. Therefore, this is not a major concern. • A more signiﬁcant concern is the possible presence of hazardous substances in the re- cycled feedstock. • FDA regulations require food packagers to ensure that the materials they use are safe for food contact—that they do not contain substances that might migrate into the food and cause deleterious effects on human health.
Recycled resins, by their very nature, often have a somewhat unknown history. • What if, for example, someone put some insecticide, some gasoline, weed killer, or any of a myriad of toxic substances into a soft drink bottle and later turned that bottle in for recycling? How can we prevent that container from contaminating new plastic packages? What we have seen in this, as in other areas, is movement at the FDA away from absolute prohibitions and toward a more reasonable evaluation of risk. • In particular, the FDA has laid out guidelines for challenging recycling processes with known model contaminants and evaluating the ability of the process to remove those contaminants, thus providing some assurance that unacceptable levels of migration will not occur.
Quality Issues • As discussed, the use history of a recycled plastic can affect its properties and performance. • It is well known that plastics undergo chemical changes during processing and use that ultimately lead to deterioration in properties. • In fact, much of the history of plastics is related to the development of appropriate stabilizing agents to prevent this degradation. • We routinely stabilize plastics against thermo-oxidative degradation that would otherwise occur during processing. • We know that some resins are much more sensitive than others. • Depending on the amount of stabilizer initially present, the history of the resin, and the type of resin, a recycled plastic resin may or may not require additional stabilizer to be successfully utilized.
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. • When regrind began to be a common ingredient in plastics processing in the late 1970s, much effort was devoted to studying the effects of multiple processing cycles on polymer performance. • For many polymers, three major types of chemical reaction occur. • First is oxidation. Reaction of the polymer structure with oxygen results in the incorporation of oxygen-containing groups in the polymer, with concomitant changes in properties and increased potential for further reactions.
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. • Chain cleavage can be followed by cross-linking, the forming of new molecular bonds that increases molecular weight and also changes properties. • In some polymers, one or the other of these reactions predominates. In others, such as polyethylene, the effects of one tend to be balanced by the effects of the other. • Some molecular structures are much more reactive than others. • Polypropylene, for example, is signiﬁcantly more susceptible to photo-oxidation than is polyethylene. • Furthermore, for some materials, it is feasible to upgrade the material during reprocessing (such as in solid-stating of recycled PET), while for others it is not.
In summary, the general rule is that recycled polymers will have somewhat different properties than virgin polymers. • These changes are usually detrimental and range in nature from virtually unnoticeable to major. • Just as not all polymers are equally sensitive, not all properties are equally sensitive. • It is not unusual, for example, for a recycled HDPE to have virtually the same tensile strength as virgin HDPE but at the same time have signiﬁcantlydecreased Izod impact strength.