Recycled plastic products – a hierarchy of uses
Materials collected and processed as recyclables rarely come back from manufacturers as the same product. Some uses seem superior to others, a sentiment expressed by the phrase “highest and best use,” which used to refer primarily to energy conservation but now is used more broadly. Referring to resources, some uses directly reduce demand for virgin materials, whereas others essentially create unneeded products that do nothing to reduce the consumption of virgin materials. Many recycled-content products are themselves essentially unrecyclable. Based on these kinds of considerations, at least three different product outcomes can be observed, namely, primary, secondary, and tertiary reprocessing.
As applied to plastic packaging, primary reprocessing produces new packaging; secondary reprocessing produces new items that are usually not practically recyclable themselves because of reduced polymer purity and the lack of collection infrastructure; tertiary reprocessing uses high heat or industrial chemicals to break plastic products into their chemical components, some of which can then, in theory, be made into new products.
Primary reprocessing
This entails remanufacturing the recovered product back into the same product. An example is recovered aluminum cans made into new aluminum cans, or a recovered clear glass bottle made into a new clear glass bottle.
In theory, all six of the six resin types used to make packaging plastics are candidates for primary reprocessing. In reality, however, primary reprocessing is rare.
Two chemical properties make it difficult. One is plastic’s sensitivity to heat and handling. Plastic molecules are long and flexible, and they change structurally when subjected to thermal and mechanical stress during melting and extrusion. The molecules interconnect and stiffen, and the plastic becomes weak and brittle. This type of degradation is called “heat history” in the plastics recycling trade. The deterioration accumulates with each reprocessing and is irreversible. In contrast, glass and aluminum, composed of short, robust molecules, are not as sensitive to heat and handling and therefore can be reprocessed many times.
The second chemical property that makes primary reprocessing difficult is that plastics are very susceptible to contamination. If sorting is imperfect, resins may mix with other kinds of organic debris when melted. Mixing leads to defects and disruptions in the molecular structure which, in turn, leads to degraded properties. In some cases, contamination leads to the total breakdown of the polymer. For example, even trace amounts of polyvinyl chloride (PVC) destroy polyethylene when the two are melted together.
An analogous problem is found with glass, which is highly sensitive to ceramic contamination. With plastics, however, potential contaminants are more plentiful and much more difficult to control. Separating plastics is particularly problematic because there is little variation in physical properties (such as density and solubility) to use in sorting. Also, the six basic types of plastic resin include multiple grades and colors within each resin type, and often several resin types are used to make a single container.
Primary plastics reprocessing is therefore strongly limited by the chemical properties of the material. Reprocessors that make plastic containers out of other plastic containers typically blend virgin resin with the recycled resin to boost the product’s performance. One study reported that it is possible to make containers with recycled contents of up to 50%, if the reclaimed containers used are themselves made of pure virgin resin. At least one blow-molder was also able to produce a 100%-recycled content bottle with the desired properties using a particular blend of post-consumer resins. However, large-scale reprocessors have found that using more than 15% to 25% of post-consumer feedstock reduced the strength of their containers.
Secondary reprocessing
This is the most common type of plastic reprocessing in the USA. It uses recovered plastics to produce new items that are usually not recyclable themselves. Secondary reprocessing reduces the quality of the polymer if it reduces its purity. Accordingly (and largely theoretically, since the industry is very new), feedstock does not have to be as pure as for primary reprocessing. Principal products made by secondary reprocessing include textiles, panels, pallets, and plastic lumber.
Secondary reprocessing sometimes diverts material from landfill and sometimes decreases the use of virgin material. For example, if there is a market for a jacket filled with polyester fiber, and that jacket’s filling is made from post-consumer bottles, then the bottles are diverted from landfill and the virgin resources that otherwise would have been used to make the fiber are conserved.
In plastics recycling, secondary reprocessing differs from primary in the following respects
- It reprocesses materials in such a way as to render them less recyclable or unrecyclable.
- It is less likely to be the highest and best use.
- It does not usually reduce the production of plastic packaging from virgin resources.
A comparison of the material flows for alternative plastic disposal schemes (reuse, primary, secondary reprocessing) is shown below. Primary and secondary schemes take material back into the “production” section for the reprocessing operation. All three schemes are based on the same volume of use indicated by the thickness of the material flow arrows in the “use” section. The amount of material produced and wasted increases going from reuse to primary to secondary reprocessing. An interesting point shown in the figure is that secondary reprocessing (the most common type of plastic reprocessing in the US) does not form a closed loop.
Figure 1: Comparison of Material Flows with Alternative Disposal Schemes
Tertiary reprocessing
In tertiary reprocessing, plastics are broken down into basic chemicals that could be reconstituted into virgin-grade material or used as fuel. Converting the output from tertiary processing back into ethylene for plastic synthesis uses cryogenic (low temperature) separation. The process is very similar to producing ethylene from natural gas.
In theory, tertiary reprocessing permits mixed collection without the extensive sorting and cleaning required by primary and, to a lesser extent, secondary reprocessing. However, since tertiary processes are functionally similar to chemical manufacturing, the environmental impacts, including emissions and energy use, are likely to be high compared to primary or secondary reprocessing. Tertiary is not widely practiced in the US because of the high capital and operating costs of the process.
Tertiary reprocessing of plastics has been done using thermal and chemical methods. Chemical processes, including glycolysis, methanolysis, and hydrolysis, decompose plastic by unzipping the polymer chains. Thermal processes, primarily pyrolysis, use heat and catalysts to break plastic down into gases such as ethane and methane. Current thinking is that thermal processing is the only commercially viable type of tertiary reprocessing, since only PET among the packaging resin types can be processed by chemical methods.
The pyrolysis process requires using a large stream of purified inert gas, typically nitrogen, to prevent the plastic from completely decomposing through combustion into carbon dioxide and water. The process requires substantial energy input, since plastics are poor thermal conductors. When clean, pure polymer feed streams are processed under laboratory conditions, pyrolysis generates up to 10% waste material, including coke and often hazardous inorganic compounds. This result suggests that under production conditions, with grossly mixed and contaminated feedstocks, the residue may be substantially higher. On the other hand, some tertiary reprocessors in Germany have claimed they have reduced residual material to 5% of what came in. This level is commendably low by conventional refining or remanufacturing operation standards. The residues of existing tertiary processes are landfilled.
Marketing recovered plastics
While recycling proportions are high for some container types in the US, so far plastic recovery has had only a minor impact on the total amount wasted. The EPA estimates that in 1993, 22% of all discards were recovered. Recovery rates were more than 30% for paper and 60% for metal. But only 3.5% of discarded plastic was recovered.
Most of the plastic packaging that is recovered and reprocessed comes from PET and HDPE bottles. Other plastic resin types are reprocessed at rates that hover around 1%. In 1995 reprocessed resin consumption totaled 1,525 million pounds, or about 2% of the total plastic resin used.
Figure 2 below shows the dramatic disparity between the growth in production of virgin resins and production of recycled resins. Using APC data, the Environmental Defense Fund found that the virgin market grew more than 6 times faster than the recycled market.
Figure 2: Plastic Packaging Produced and Recycled, 1990-1993
The market for products of secondary processing has been limited both by product performance problems and high material costs. Plastic lumber, for example, is heavy compared to wood, cannot be used to bear loads in structures, is subject to warping, and begins to degrade when exposed to sunlight. Recycled HDPE pellets cost $0.34/lb, almost as much as the $0.38/lb price for virgin HDPE. Recycled PET pellets cost about $0.58/lb compared to $0.76/lb for virgin material. Economic return is reduced by the high price of recycled resin and the practical requirement to use at least 50% virgin resin to achieve desired performance.
Domestic and international destinations of recovered materials
After plastic containers are collected (those economic considerations will be discussed later), they must be sold, reprocessed, and made into new products.
The market in post-consumer resin is dominated by a few large plastic-reprocessing facilities in the US and by Pacific Rim countries. Both can pay high prices for the post-consumer resource, the first because of automation and the second because of low labor costs for sorting. Nationally the amount of post-consumer plastics exported is at least 20% to 30%. The state of Oregon, for example, exports about 35% of its plastic scrap. In California, all recovered PET goes to the Plastic Recycling Corporation of California (PRCC). In 1995 the PRCC sold nearly all of California’s recovered PET overseas.
This market structure creates several impacts worth noting. First, California businesses that use recycled PET resin must purchase it out of state. Therefore, the development of local businesses using recycled plastic resin is inhibited, and this new round of transportation uses more energy and generates more pollution.
In addition, plastics are a major component of an international trade in discarded resources that has become a source of serious problems. Discarded materials that are collected in industrialized countries and shipped to third-world countries as recyclables are sometimes badly contaminated. Occasionally the contamination is hazardous waste. The countries that ship the materials rely on the often-weak regulatory climates, huge reservoirs of cheap labor, and desperate economies of the receiving countries. Greenpeace and other organizations have documented conditions at recycling facilities in countries that import this material and have found conditions to be hazardous and exploitive. In addition, Greenpeace found that exported plastics were very poorly sorted. In a seven-country survey, up to 50% of the discards shipped overseas were contaminated and had to be dumped, often in unlined, unmanaged sites. Little or no documentation has been found regarding the market stability or soundness of the products that these countries produce with plastic scrap. The “cradle to grave” approach to waste management does not apply if the “grave” is in another country.
