"Recycling, Plastics". In: Encyclopedia of Polymer Science and

RECYCLING, PLASTICS

Introduction

The unabated growth in the use of plastics in recent years is of considerable con-cern because of the difficulties involved in waste disposal. Increasingly, plasticsare replacing wood, glass, paper, and metal in numerous applications, especiallyin packaging. Items such as plastic cups, food utensils, and convenience containerssuch as “clam shells” (sandwich boxes) contribute substantially to the burgeoningmunicipal solid waste (MSW) stream. As a means of dealing with this waste, therecycling of plastics increased dramatically from about 1990, when the emphasisshifted from landfilling to recycling and energy recovery. To facilitate recycling,ever-revitalized infrastructures are needed for collecting, sorting, cleaning, repro-cessing, and manufacturing new products to be marketed to end-users. This articlefocuses on recycling of thermoplastics in the MSW.

Plastics in MSW: The Americas. In the United States, plastic resin salesand captive use reached 46.2 million tons in 2001, a 4% decrease from 2000, ac-cording to the American Plastics Council (1). Resin production rose to 45.9 milliontons in 2001, up 4.8% from the previous year. The U.S. plastics industry contin-ues to expand into new markets as plastic products come to replace ones madeof wood and metal (Fig. 1). In the United States, some 232 million tons of MSWwere generated in 2000, an increase of 0.9 million tons over 1999 (Fig. 2). Of thisstream, plastics constitute about 10.7 wt%. Plastic containers and packaging dom-inate, followed by materials in goods such as automobiles, appliances, electronics,furniture, and carpeting. Plastic resins used in containers and packaging includepoly(ethylene terephthalate) (PET; in soft drink bottles with polypropylene [PP]caps), high density polyethylene (HDPE; in milk and water bottles), poly(vinyl

657Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

658 RECYCLING, PLASTICS Vol. 7

Consumer and institutional(14%)

All others (14%)

Transportation (4%)

Exports (12%)

Industrial/machinery (1%)

Electrical and electronic (3%)

Building and construction(17%)

Adhesives/inks/coatings(2%)

Furniture and furnishings(4%)

Packaging (29%)

Fig. 1. Major market distribution for plastics (2001). From Ref. 2.

Yard trimmings(12%)

Food scraps(11.2%)

Plastics (10.7%)

Metals (7.8%)

Rubber, leather,and textiles

(6.7%) Glass (5.5%)

Wood (5.5%)

Other (3.2%)

Paper (37.4%)

Fig. 2. Total waste generation in the United States (2000). From Ref. 2.

chloride) (PVC; in cooking-oil bottles), low density polyethylene (LDPE) and linearlow density polyethylene (LLDPE; for bags, sacks, and wraps), and polystyrene(PS; in cups and containers). Table 1 shows the generation, recovery, and discardamounts for various resin types (see also POLYESTERS, THERMOPLASTIC; EthylenePolymers; VINYL CHLORIDE POLYMERS; STYRENE POLYMERS).

Although the portion of plastics recovered by recycling in the United Statesis relatively small—1.34 million tons, or 5.4% of plastics generation in 2000—recovery of certain plastics has been increasing (2). Recently, 40% of discarded PETsoft drink bottles and 31.9% of HDPE milk and water bottles were recovered (2). In

Vol. 7 RECYCLING, PLASTICS 659

Table 1. Total Plastics in MSW (2000) by Resin Typea

Generation, Recovery, Generation, Discards,Resin 103 ton 103 ton % 103 ton

PET 2,490 430 2,060HDPE 4,830 420 4,410PVC 1,390 1,390LDPE/LLDPE 5,740 150 5,590PP 3,350 10 3,340PS 2,280 2,280Other resins 4,630 330 4,300Total plastics in MSW 24,710 1,340 5.4 23,370aSource: Ref. 2.

1960, an estimated 390,000 ton of plastics was discarded; by 1999, that figure hadrisen to 24.2 million tons. As a portion of MSW generation in the United States,plastics were less than 1% in 1960, increasing to 10.5% in 1999 (2). In Canada,the majority of collected plastic waste is represented by PET (54%) and HDPE(37%), followed by PP (6%) and PVC (3%). Post-consumer plastics recycling inCanada has focused on recycling of plastic packaging, employing manual sortingfollowed by reprocessing of resin-specific streams (eg, HDPE). In 1998, Canadiansrecycled almost all collected PET and HDPE waste (3,4). In Latin America, somecountries have recently adopted regulations governing the MSW and have startedto address recycling infrastructure issues. Brazil has adopted national take-backregulations on batteries and has passed laws regarding electronic scrap and end-of-life vehicles. In Mexico, legislation has been prepared regarding the recyclingof lamps and batteries. In 2002, El Salvador, Jamaica, and Argentina consideredregulations on batteries and printer cartridges.

Plastics in MSW: Europe, Asia, and the Pacific Rim. Consumptionof plastics in Europe is predicted to grow about 4% annually from 24.9 milliontons in 1995 to 36.9 million tons in 2006, with PP and PET showing the greatestincrease (5). In the United Kingdom in 2000, only 8% of collected plastics wasrecycled instead of the targeted 15% (6). A related problem in the United King-dom and elsewhere is the continued growth of the plastic portion in automotiveshredded residue (ASR), which recently stood at about 40% of ASR. For Japan,plastics production and waste discharge data have been compiled by the Plas-tic Waste Management Institute, which reported that resin production in 1999was 14,570,000 ton; total plastic waste discharge the same year was 9,760,000ton (7). The breakdown of general waste and industrial waste in Japan in 1999was 4,860,000 and 4,900,000 ton, respectively. In Australia, total plastic resinconsumption in 2000 was 1,530,783 ton, of which 753,000 ton was landfilled and167,673 ton was recovered (8). Between 1997 and 2000, plastics reprocessing in-creased by 80%, with packaging materials having the highest recovery (42.1%).

For many Asian countries as well as for Russia and other nations, data areeither incomplete, of questionable reliability, or hard to come by. It has been re-ported that in India, the plastics share of the MSW is about 7% by weight (9). For2000, the country’s total production of plastics was estimated at 3 million tons. Al-though mechanical recycling (ie, grinding materials) is widely employed in India,


7654321

PETE HDPE V LDPE PP PS OTHER

Fig. 3. SPI resin identification code. The seven symbols in the SPI Code can be found at thewebsite http://grn.com/grn/library/symbols.htm (and elsewhere). However, the acronymsbelow the images are not part of the website images and have been added separately here.

feedstock recycling (ie, chemical depolymerization) is largely untried. In China,consumption of plastic products was estimated in 2000 to be growing about 20%annually (10). Around 50–60% of plastics waste in China is either uncollected ordisposed of on land, in rivers, and in the sea. Although the remainder is availableto recyclers, the actual rate of recycling has been declining steadily (from 20% to5% in a recent period). This decrease in recycling of domestic materials is offset bythe substantial business devoted to recycling waste imported from abroad, givingrise to allegations of “dumping” by foreigners.

Plastics Coding System and Terminology. In the United States, theEnvironmental Protection Agency (EPA) advocates Three R’s: Reduce the amountof waste to be discarded, Reuse products or repair what is broken, and Recycle asmuch as possible, including buying products with recycled content. In the mid-1980s the Society of Plastics Industry (SPI) introduced a coding system usingnumbered triangles that are now found on containers around the world (Fig. 3).By accelerating the sorting of bottles and containers, the code greatly enhances re-cycling. This system also helps municipalities track plastics in their waste stream.

Terms related to the reprocessing of discarded materials include recycling(a process used to recover plastics waste); recycled (a reprocessed material con-taining some quantity of post-consumer plastic waste, usually more than 25 wt%);reprocessing (activity involving steps such as sorting, grinding, reextrusion, or re-molding into a secondary material for remanufacturing of plastic goods); recycledcontent (the amount of plastics from the post-industrial and/or post-consumerwaste stream that is added to a virgin resin); post-industrial waste (or precon-sumer waste: off-spec parts, plant discards, and the like); and post-consumer waste(materials and products discarded after consumer use).

Challenges for Plastics Recycling

Plastics recycling presents numerous technical, economical, and marketing chal-lenges. One such technical issue is the variability of product composition andcolor, because discarded products are made from a wide array of resins and addi-tives. The contamination of the waste stream is aggravated by labels, glue, print-ing, product residue, and similar elements. Most waste streams include immisci-ble polymers (Table 2). Efforts to recover them via conventional melt processingwithout first sorting them will produce materials with uncontrolled coarse mor-phology, poor interfacial adhesion, and inferior physical properties (11–13). As iswell known, the better the physical properties, the higher the quality of recycled


Table 2. Miscibility Chart for Commonly Used Thermoplasticsa,b

Plastic type PET HDPE PVC LDPE PP PS PA PC

Poly(ethylene terephthalate) M IM IM IM IM IM IM IMHigh density polyethylene M IM IM IM IM IM IMPoly(vinyl chloride) IM IM M IM IM IM IM IMLow density polyethylene IM M IM IM IM IMPolypropylene IM IM IM M IM IM IMPolystyrene IM IM IM M IM IMPolyamide IM IM IM IM IM M IMPolycarbonate M IM IM IM IM IM MaSource: Polymer Technology Center—Northwestern University (Evanston, Ill.).bNote: M = miscible; IM = immiscible.

products. Because all six major thermoplastics in the waste stream (PET, HDPE,PVC, LDPE, PP, and PS) are immiscible, they must be sorted before recovery forreuse can occur via melt processing. Also, reprocessing of mixed plastics with dif-ferent melting points causes degradation, leading in turn to deteriorated physicalproperties. For example, PET and PVC cannot be melt processed together becausePVC burns at the temperature where PET melts (270◦C). If the same mixture isprocessed at 170◦C—suitable for PVC—PET would remain unmelted, thus pre-venting the desired mixing.

The difference in melt flow rates of the various polymers originally selectedfor specific fabrication methods causes generation of heterogeneous mixtures (ie,low melt-flow rate plastics are used for blow molding, while high melt-flow rateplastics are needed for injection molding). Remelting mixed plastics usually low-ers their properties, except for a few polymer blends such as HDPE/LLDPE orHDPE/PP (with 5–7% PP), which exhibit limited compatibility (14,15). To im-prove physical properties and processability of mixed plastic waste, an impactmodifier and a compatibilizer are needed. Both of these can be added as premadeingredients, but only the compatibilizer can be generated in situ during reactiveextrusion. The compatibilizer migrates to the interface, reducing the interfacialtension, improving adhesion, and stabilizing the blend morphology from coales-cence and agglomeration of the dispersed particles. The most common compatibi-lizers are block or graft copolymers (ie, styrene–ethylene–butylene–styrene) andfunctionalized polymers (ie, maleated polyolefins). Although compatibilizers areused sparingly (1–5 wt%), they are expensive.

Although the additives (qv) in the original product can affect the phys-ical properties and performance of recycled plastics, the extent of their cross-contamination remains unclear. Moreover, multiple remelting cycles cause ther-mal and oxidative degradation in thermoplastics, resulting in changes in molec-ular weight, discoloration, and sometimes cross-linking. Because of such adverseeffects, 100% recycling cannot be achieved for plastic materials.

Successful recycling requires an adequate supply of a reasonably clean feed-stock. Items such as PET soft drink bottles or natural HDPE milk bottles areabundant in the United States, where curbside collection and drop-off centers arecommon, and thus make ideal feedstocks. The collected plastics are sorted, ground,reclaimed as flakes or pellets, and sold to fabricators of new products. In Germany,


where the “Dual” system is operated by Duales System Deutschland GmbH, par-ticipants label their packaging with a green dot. Companies using this system areexempt from “take-back” programs. In 1994, the European Community publisheda directive that resembled the German Packaging Ordinance. Similar legislationknown as Producer Responsibility Obligation was introduced in the United King-dom in 1997. The primary obstacle to increased recycling in all venues, however,remains the imbalance between waste collection and potential end-use; far moreplastic material is collected than can be recovered economically.

Economics

Plastics recycling is both an economic and an environmental activity. Possibleoutcomes of recycling include material recovery (via mechanical or chemical pro-cesses), energy recovery via incineration, or as a carbon source for blast furnaces(16). Of these, material recovery is the most ecologically beneficial. This non-polluting method is applicable to both single and mixed polymeric waste that isgenerated either during product manufacture or from the MSW.

For plastics recyclers (reprocessors), the high costs of both transportationand processing are harsh economic realities. Because of the fuel and equipmentexpenses associated with transporting waste over long distances, most plasticsrecycling is best handled at the local level by municipalities. Sound manage-ment of processing costs suggests that energy recovery by incineration, landfilling,cracking plastics waste back into monomers, and use of biodegradable plastics areamong the most feasible alternatives. The trend of virgin resin suppliers to be-come producers of recycled-content resins, which began in the United States andEurope, will continue in the new millennium. Some resin suppliers see this newmarket as an area for potential growth, especially with refined recycling technolo-gies. For PET and HDPE bottles, for example, the cost of recycling (reprocessing)has been estimated at $100–150/ton, compared with incineration or energy re-covery at $100/ton and landfilling at $30–50/ton (17). Waste-to-energy plants aremore acceptable than incineration, which can produce undesirable toxic ash orfumes. The economics of post-consumer plastics recycling depends on the marginbetween product price and the cost of raw materials as well as the size of therecycling (reprocessing) facility. Crude oil prices also play a role in setting the costof virgin resins.

Since the late 1980s, public awareness of the environmental benefits of re-cycling has increased, and governmental mandates for recycling have kept pace.Such legal requirements have kept the cost of materials from becoming the solefactor in determining the extent of recycling. These developments, however, donot ensure that recycling will be economically efficient. The cost of collection andseparation of plastics remains high in contrast to that for aluminum and paperproducts. The task is further complicated for those plastics in the MSW that arepigmented in bright colors, contaminated, and/or multilayered. Although suchproducts can be recycled using thermal processes such as pyrolysis, gasification,and hydrogenation (see below), mineral- or glass-filled materials and multilay-ered structures can be hard to recycle because of their complexity. The divergenceamong fabrication processes used to make plastic products also contributes to the


difficulty of sorting, cleaning, and reformulating materials cost-effectively. By theearly 2000s in the United States and elsewhere, economical plastics recycling waslimited to products made from HDPE and PET. Extending recycling to other plas-tics will require lowering recycling costs while improving physical properties andperformance relative to those of virgin resins.

Technologies

Technologies for recycling post-consumer plastics were slow to develop until the1990s (18). These technologies are usually divided into four categories: primary(regrind), secondary (mechanical), tertiary (feedstock), and quaternary (inciner-ation). Each of these technologies has a viable role in recycling plastics whileconserving natural resources and avoiding landfills.

In the most straightforward approach, primary recycling of regrind occursin the manufacturing process, where regrind is simply added to virgin resin andthe mixture is then reprocessed. Secondary recycling uses post-consumer sortedor unsorted plastics waste. When reprocessed, these materials are used in lessdemanding applications that do not require virgin resins. Manual sorting of plas-tics waste is still used to separate HDPE and PET. Figure 4 shows the layout ofa manual sorting facility, with four people separating HDPE, PET, PP, and PVC.Automated sorting uses the differences in properties such as density, surface en-ergy, low temperature behavior, and melting temperature (19). Frequently used

Curbsidecollection

Transportto MRF

Sorting (manual or automatic)separate resin types

PET HDPE PVC PP

To recycle operation by resin

Recycleunit

Fabri-cator

Value-addedraw material

Fibersheetfilm

Bottlesengineeringresin

Fig. 4. Layout of a manual bottle-sorting line. Taken from http://www.plasticsrecycling.org/sort.htm.


Table 3. Density of Plastic Materials in theMSWa

Plastic material Density, g/cm3

PET 1.40HDPE 0.95PVC 1.40LDPE 0.91PP 0.90PS 1.05aSource: Polymer Technology Center—NorthwesternUniversity (Evanston, Ill.).

density separators divide materials into light and heavy fractions on the basis ofplastic density (Table 3). Other methods include marking plastic products withone of the seven SPI codes, addition of unique organic markers to the polymerbackbone, and solvent separation (selective dissolution) involving a solvent thatdissolves one plastic followed by the use of another solvent at room temperatureor higher in order to remove another plastic. Mixed plastics can be separated byfroth flotation using surfactants.

As the plastics recycling industry matures, manual sorting will be replacedby automated techniques. One such approach has been developed by the MagneticSeparation System Co. (Nashville, Tenn.), which uses X ray, infrared (IR), andother sensors to separate PET and HDPE by type and by color (20). Their mostrecent automated sorting module, the Alladin, not only sorts plastic resins by typeand color but can also create three outputs from one input (21). For example, clearPET, green PET, and blue PET (or PET, colored HDPE, and natural HDPE) couldbe sorted in one pass with the capacity of 4–6 ton/h. Other sorting methods includethe use of solvents, X-ray fluorescence, near-IR, mid-IR, Fourier-transform Ramanspectroscopy, and electrostatic sorting (22–25). The triboelectric pen (Tribopen), acompact device that senses the triboelectric charges of different plastics, can sortengineering resins such as polyamide and acrylonitrile–butadiene–styrene (ABS)from commodity plastics such as PP and PE (polyethylene) (26).

Another secondary recycling technology is the novel process called solid-stateshear pulverization (S3P), which does not require sorting of discarded plastics (27–29). The S3P technology uses a pulverizer made by Berstorff GmbH (Hannover,Germany), that is based on a modified twin-screw extruder with intensive cooling.This mechanochemical process subjects plastics to simultaneous shear and com-pression, which results in unique changes in chemical structure because of in situcompatibilization and physical properties superior to those made by melt process-ing. Pulverization is accompanied by intimate mixing without melting, resultingin powder of unusual morphology, large surface area, and controlled particle sizeand distribution (Figs. 5–8).

Tertiary recycling involves thermal treatment of plastics waste resulting inchemical intermediates. Several technologies such as pyrolysis, gasification, andhydrogenation are being practiced (30–32). Pyrolysis converts polymers to a liquidin the absence of oxygen; gasification produces carbon monoxide and hydrogen in alimited-oxygen atmosphere, while during hydrogenation some hydrogen is added

Flakesilo

Flakedryer

Plasticbales

Baleconveyor

Balebreaker

Vibratoryconveyor

Bottleconveyor

Granulator

Aspirationsystem

Flakestoragebin

Wash system

Separationsystem

Extruder Screenchanger

Pelletizer

Fig. 5. Layout of a conventional plastics recycling line. Courtesy of Cumberland Engineering, South Attleboro, Mass.

665


Fig. 6. Representative melt-processed pellets from corresponding post-consumer flakes.

RR′A

B

Fig. 7. Illustration of mechanochemsitry in a pulverizer during S3P. Polymers A and Bare compatibilized in situ to create AB block or graft copolymers.


Solid-stateshearpulverization

Fig. 8. Representative S3P powder from corresponding post-consumer flakes.

in the cracking step. Heating plastics such as poly(methyl methacrylate) (PMMA),PS, and acetal homopolymers in the absence of oxygen produces monomers withvery high yields (22). These monomers can be isolated, purified, and reused tomake new polymers. Thermolysis, when carried out in the absence of air, producessynthetic crude oil. Gasification with controlled amounts of oxygen produces Syn-gas, which is used as a fuel. Hydrogenation by heat and pressure in an excess ofhydrogen produces liquid fuel such as gasoline or diesel. Thermolysis is a versa-tile process that can handle contaminated medical waste, auto shredded residue,and other complex mixtures. Depolymerization of plastics by solvolysis producesmonomers or oligomers (30).

Recycling approaches known as advanced recycling technologies (ART) re-claim feedstock value from discarded plastics through depolymerization. ARTcomprises two groups, depending on whether the depolymerization results inmonomer or in hydrocarbon feedstock. The choice of recycling technique is in-fluenced by factors such as the purity of the plastic stream, chemical makeup ofthe plastics, and the nature of additives. The type of plastic also helps to deter-mine which depolymerization process is appropriate. Polyesters, polycarbonates(qv), polyamides, and polyurethanes (qv) undergo reverse polymerization and theycan be converted to their respective monomers for use in making the same virginresins (18,30). Plastics such as PE, PP, and PVC can be converted into petrochem-icals and fuels through pyrolysis. ART processes do not require sorting and areintegrated into existing refinery or petrochemical facilities (31).

Quaternary recycling involves waste-to-energy processes. Through combus-tion, plastics can generate energy amounts comparable to the range betweenWyoming coal [22,080 kJ/kg (9600 Btu/lb)] and fuel oil [48,070 kJ/kg (20,900Btu/lb)]. PE, PS, PET, and PVC yield 23,000, 39,000, 16,100, and 20,700 kJ/kg(10,000, 17,000, 7000, and 9000 Btu/lb) respectively (32). Post-consumer plastics(10–30%) mixed with paper (90–70%) are used as processed engineered fuel in


existing energy plants. Increasingly, automotive shredded residue is being co-combusted with the MSW in Europe and North America. Both environmentalistsand citizens afflicted with the “not-in-my-back-yard” (NIMBY) syndrome are con-cerned about toxic fumes and ash by-products from incineration. Improvementsin this process may render it more acceptable; a Japanese incinerator is alreadyfunctioning alongside a public swimming pool in Tokyo.

Restabilization of Plastics for Recycling. To make recyclate suitablefor reprocessing and reuse, it needs to be restabilized because of the partial con-sumption of the original stabilizers during the “first-life” application. During pro-cessing and end-use, plastics undergo an oxidative degradation that results in al-tered molecular weight by chain scission or by cross-linking, and in deteriorationof properties and surface appearance (33). Once restabilized, recycled plastics canbe used again despite many years of service. For example, 10- to 13-year-old pig-mented HDPE crates that are restabilized with antioxidants (Irganox 1010, phos-phite Irganox 168, and light stabilizers Tinuvin 327, including hindered aminesTinuvin 710) make them suitable for the manufacture of new crates (34). RecycledPBT/PC [PBT = poly(butyl terephthalate)] blends and talc-filled PP can be resta-bilized with Recyloblend 660 (35). Other restablized plastics that are successfullyrecycled include PP from battery cases, PS from yogurt cups, PVC from windowframes, and EPDM/PP [EPDM = ethylene–propylene diene monomer] blends fromcar bumpers (34).

Single Waste Streams

Single streams consist of PET and HDPE bottles and are the most successfully re-cycled of all (36). Other types of plastics suffer from either insufficient quantitiescollected or lacking developed markets and favorable economics. In the UnitedStates in 2000, PET and HDPE represented about 50% of plastic bottle recovery,with bottles from PVC, LDPE/LLDPE, PP, and PS accounting for only about 5%(36). One advanced approach converts clear PET bottles into high quality regrindwith low residual contamination (37). Labels and glue are removed in a mixtureof hot water and steam that softens and separates them. After granulation, PETflakes are washed in a mixture of detergents and caustic soda at an elevated tem-perature for a few minutes. Possible heavy contaminants (glass, aluminum, sand,and product residue) are separated in a hydrocyclone. Lower density particles (ie,polyolefins) are separated by the sink/float method. Detergents are removed bywashing followed by de-watering and drying to reach a moisture content rangingbetween 20% and 0.5%. The ability of PET to be converted to fiber has made thefiber market the largest U.S. outlet for recovered PET bottles. Other uses for recy-cled PET include strapping and film/sheet applications. PET can also be recycledby depolymerization to monomers that are used to make virgin PET (38).

HDPE recycling includes granulation, sorting from contaminants (labels,glue, and PP caps), washing, drying, and/or pelletizing. Because of odor retention,recycled HDPE is unsuitable for food packaging but can be used in motor oil bot-tles, pallets, crates, and plastic lumber (39). Unwashed HDPE can be compoundedwith fillers or glass fiber and pigments to be extruded only into low value prod-ucts (14,15,40). LDPE/LLDPE used for packaging are recycled by blending with


Table 4. Physical Properties of 100% Post-Consumer PSa

Property Value Test method

Melt flow rate (g/10 min @ 200◦C, 5-kg load) 5.0–8.0 ASTM D1238Izod impact, J/mb 0.64–0.8 ASTM D256Vicat softening point, ◦C 100 ASTM D638Tensile strength at yield, MPac 30.34 ASTM D638Tensile elongation,% 25–45 ASTM D638aSource: Ref. 41.bTo Convert J/m to ft · lb/in., multiply by 1.875.cTo Convert MPa to psi, multiply by 145.

virgin resins and reused mostly in films (2,3). PP from battery cases and caps,lids, and container packaging is reclaimed by blending with virgin PP. RecycledPP is used in auto parts, appliances, and consumer goods (15). Crystal PS fromCD jewel cases and drinking cups is ground, washed, dried, and reprocessed byinjection molding to be reused as hangers, trays, picture trays, and similar items(41). Physical properties of 100% post-consumer PS are shown in Table 4. Ex-panded PS (EPS) from discarded egg cartons, “clam shells” (sandwich boxes), andloose packaging is recycled by blending with virgin EPS at about a 25:75 ratio andreused in the same applications. PVC from wire and cable waste is recycled by aselective dissolution process called Vinyloop (42,43). Recovered pure PVC is usedfor garden hoses, vinyl wallpaper, and window frames.

Mixed Waste Streams

Polyolefins. One of the most common recycling activities involving mixedwaste streams is the conversion of polyolefins to plastic lumber as a replacementfor wood or concrete. Compositions of mixed plastics vary from 100% unwashedfeedstock to refined, tailor-made mixtures containing mostly polyolefins, foam-ing and reinforcing agents, pigments, and other additives. The amount and typeof polyolefins in these mixtures affect physical properties of the resultant plas-tic lumber. Higher LDPE content in mixed waste results in a product with lowflexural modulus, while a higher PP content leads to a more brittle material, es-pecially at subzero temperatures (44). Plastic lumber containing recycled HDPE,fiberglass, and wood flour exhibits increased stiffness without any significant re-duction in tensile strength. The extrusion processes for making plastic lumberresemble those for fabricating thick-wall products such as park benches, railroadties, and backyard decks. Mechanical recycling of mixed polyolefin waste is lim-ited; products generally have a lower resale value than those from single plastics.Examples include pallets, containers, noise insulation walls, and landscape andgarden items (13,45). Feedstock recycling for mixed plastics is gaining acceptancebecause they permit recovery of hydrocarbon components that are used in themanufacture of new chemicals and plastics.

Engineering Alloys and Blends. Many widely used thermoplasticsare engineering alloys and blends created to meet an increasing demand forgreater toughness, higher end-use temperature, and improved processability. This


combination of properties cannot be achieved by the use of single polymers butis made possible by alloying or blending of different polymers through the use ofimpact modifiers, compatibilizing agents, and other ingredients during blending.Modifiers and compatibilizing agents used in producing these blends, however,are sensitive to repetitive thermal history during melt reprocessing.

For example, impact-modified engineering alloys and blends with enhancedtoughness are often susceptible to thermal and oxidative degradation becauserubber degrades and cross-links, resulting in increased viscosity and processingdifficulties. PC/ABS alloys where PC is the matrix require a higher processing tem-perature than ABS and are more vulnerable to thermal degradation (46). Thesealloys lose mechanical properties and exhibit increased melt viscosity after fiveinjection molding rounds because of the cross-linking and oxidation of the rubberphase. Polyamide-6,6 (PA-66) and poly(2.6-dimethyl–1.4-phenylene oxide) (PPO)with a graft copolymer such as styrene–maleic anhydride as a compatibilizer andan elastomeric modifier such as styrene–butadiene–styrene block copolymer ex-hibit good stiffness at high temperature, and solvent resistance (47). Repetitiveprocessing of these alloys at 280◦C resulted in a significant loss of impact strengthdue to degradation. By contrast, reprocessing of a single polymer, PA-66 contain-ing a thermally stable impact modifier, does not show a similar loss of impactstrength. PC/PMMA alloy with an impact modifier used in auto lighting com-ponents shows a decrease in stiffness after repetitive reprocessing (46). PBT/PC[PBT = poly(butylene terephthalate)] blends exhibit a slight reduction in ten-sile properties after three reprocessing cycles attributed to a polybutadiene-basedmodifier. Engineering alloys and blends including PPO/PS, ABS/PC, and PC/PBTare blended with virgin materials and additives for reprocessing and reuse (with aminimum of 25 wt% of recycled material) (48). Thermoplastic polyurethanes (PUs)are recovered by mechanical recycling and are reprocessed by Injection Molding(qv) or Extrusion (qv).

Vehicle Recycling. Recycling of material from vehicles is limited to met-als and plastics, with other nonmetallic materials going to landfills (49). Aftermetals are separated, the remaining lighter fraction is called ASR. Of about 4million tons of ASR produced annually in the United States, more than 1 milliontons consist of PU foam and more than 750,000 ton consists of thermoplastics(50). Energy recovery from ASR via incineration is achieved by adding it to theMSW; stack emissions and ash composition show no measurable environmentalimpact (49). Argonne National Laboratory (Illinois) has developed a unique frothflotation process to separate high purity plastics such as ABS, HIPS (high im-pact polystyrene), and PP from the ASR. The process combines size reduction,two gravity-based separation steps, and a froth-flotation separation step. Froth-flotation recovers equal-density HIPS and ABS particles. When the particles areplaced in a special aqueous mixture, HIPS particles float and ABS particles sink.Recovered plastics can be used again for computer parts, office equipment andaccessories, and similar products. It has been estimated that this process couldrecover more than 136,000 ton of high purity plastics annually from scrapped au-tos and appliances, saving some 92 billion megajoules (87 trillion British thermalunits) of energy and avoiding disposal costs of $10–40/ton (50). Another Argonneprocess separates flexible PU foam from ASR and cleans it to produce high qualityfoam for reuse.


Auto instrument panels that used to be made from 15 kinds of plastics arenow 100% recyclable as a result of redesigning with mutually compatible plas-tics (51). Thermoplastic olefin elastomer (TOE), forms a skin, and PP foam andmineral-filled plasma-treated PP make up the core. Heat resistant and impactresistance are retained by increasing the amount of TOE and talc in the recyclate.New approaches to vehicle recycling include eco-design, employing design for dis-assembly (DFD), and design for recycling (DFR) (52). DFR involves designing forend-of-life, using both recycled and recyclable materials, reducing the number ofmaterials, marking parts for material identification, and using compatible ma-terials and DFD (53,54). In 1995, 24 different plastics were used in the averagesedan made in the United States, for a total of 143 kg, or 9.3% of the vehicle. PU,PVC, and PP were the most common, followed by PA 66, ABS, and PE. Several ofthese plastics are recycled for various secondary applications, including reuse innew vehicles.

Electrical/Electronics Products Recycling. Electrical and electronicsproducts such as computers, fans, stereos, TV housings, and vacuum cleanersmake up about 1% of the MSW. End-of-life electronics are a fast-growing part ofMSW. More than a dozen different resins are used in electrical/electronics prod-ucts, including ABS, PMMA, polyamides, PC, PP, PS, flame-retardant (FR) HIPS,PU, PVC, blends of PC/ABS, and PET/PBT (55). The six most common resins are PSand HIPS (29%), ABS (14%), PP (12%), PU (9%), PC (8%), and phenol formalde-hyde (5%). Several separation technologies are used to recover these polymers.MBA Polymers (Richmond, Calif.) has developed an automated process to recoverPP, HIPS, ABS, PPO, FR HIPS, PC, and PVC (Fig. 9). The sorting of plastics fromdurable parts by plastic type is an important aspect of achieving high qualityin recycled materials. Because the parts are made from engineering plastics (qv)with a wide variety of additives, fillers, and pigments, an identification by densityis not reliable. Equipment for near-IR (NIR) and mid-IR (MIR) spectroscopy issometimes used, although with limitations; NIR focuses on determining opacity,separating the mixed stream into clear (PET and PVC), translucent (HDPE andPP), and opaque (all colored materials)materials. Paints and coatings have to beremoved for accurate identification; NIR systems cannot identify plastics filledwith carbon black. Size reduction of parts involves high speed granulators androtary grinders that produce particles of different sizes and liberate metal, labels,and other nonplastic materials. After metal and nonplastics removal and sorting,the engineering plastics are repelletized and formulated for manufacture of newparts and reuse in similar applications. About 7000 kJ/kg (3000 Btu/lb) are usedto recover plastics with the MBA process, and no “raw material” energy is con-sumed. Using recovered plastic instead of additional virgin resin results in energysavings of 39,000 kJ/kg (17,000 Btu/lb) of raw material, or more than 85% of theenergy needed to make virgin resin in the first place.

Carpet Recycling. By the beginning of the century, about 4.7 million tonsof residential carpets were disposed of annually in the United States, with lessthan 5% being reused or recycled (56). Scrap carpet is bulky and difficult to collect.Efforts are underway in the United States to reduce landfilled carpets by 40% by2012. Carpeting is a complex product consisting of 40–60 wt% of yarn from PA-6,PA-66, PET, and PP, 4–6% of primary backing made from woven PP, 38–52 wt%of adhesive made from styrene–butadiene latex with calcium carbonate as a filler,


Bulk mixedplastic feed

GrindingStage 3

Plastic purifying

By-products

Pelletizing andformulating

PP

HIPS

ABS

PPO

FR HIPS

PC

PVC

By-products

By-products

Flake

Fines,fluff

Screening

GrindingStages 1 and 2

Preshreddedfeed

Metal & Nonplastics removal Polishing & blending

Fig. 9. Simple process flow diagram for recycling plastics from durable goods. Courtesyof MBA Polymers, Inc., Richmond, Calif.

and 3–5 wt% of secondary backing made from PP or jute (57). Carpets removed fordisposal are often contaminated with dust and stains. In one method, carpets arechopped into small pieces that are fed into an extruder and then pelletized. Thepellets are blended with virgin PP or LDPE for use in plastic lumber. Monsanto’spatented process recycles PA-66 carpet without separation by using reactive ex-trusion and compatibilization (58). The resultant material is suitable for injectionmolding and extrusion. Yet another process produces interlocking floor tiles fromvinyl-backed carpet with all the fibers of the carpet intact (59). The U.S. marketfor resilient flooring is about 1.5 billion square feet per year, representing a majoroutlet for recycled carpet.

Plastic Film Recycling

In contrast to the relative simplicity of recycling commodity resins, recycling plas-tic film has numerous facets, including the efficient consolidation of materials andthe sorting and removal of contaminants. A significant aspect is the high volume-to-weight ratio of films, which translates into increased transportation cost. Filmsare more difficult to sort because most do not have identification codes. Some filmslike the pallet wrap contain plasticizers that make them tacky, and need to be han-dled separately from other films to prevent sticking. Different additives such asantislip agents and lubricants are used to manufacture films, and thus sorting isnecessary. Agricultural film in the form of bags and shrink wrap can be hard tocollect and to transport because of their bulk. A film used outdoors for a long timemay be partially degraded, limiting its recyclability by melt extrusion. The S3Ptechnology discussed earlier allows recycling of densified agricultural film into


blown film without significant loss of physical properties. In Western Europe, theinfrastructure for collecting and recycling PE film is more advanced than in theUnited States and Japan.

Biodegradable Plastics

Biodegradable plastics, designed to decompose through the action of living mi-croorganisms, are an alternative to conventional plastics when recovery or recy-cling are impractical or not economically feasible. Biodegradable polymers, medi-cal applications (qv) are of two types: naturally occurring and synthesized (64,65).The first group includes polysaccharides (qv) (starch, thermoplastic (qv); cellulose(qv)), protein (gelatin (qv); casein), and lignin (qv). Synthetic resins consists ofpolyalkylene esters, poly(lactic acid) and its copolymers, poly(vinyl alcohol), andothers (see POLYLACTIDE; VINYL ALCOHOL POLYMERS). Some biodegradable poly-mers are commercial products: poly(lactic acid), starch-filled PE, poly(vinyl al-cohol), plastics based on starch esters, and blends of starch esters with aliphaticpolyesters, starch–poly(ε-caprolactone) blend, and others. Often, their cost pre-vents wide use of biodegradable plastics. Starch-based polymers are less expen-sive because starch is available from corn and other crops. Poly(vinyl alcohol) iswidely used because of its solubility in water. Poly(lactic acid) is a low cost ma-terial used in medical implants and drug delivery. The mechanical properties ofpoly(lactic acid) are similar to those of PS, except for a lower glass-transition tem-perature and limited stability to hydrolysis. Like other plastics, biodegradablepolymers can be processed by most conventional plastic processing methods suchas injection molding and extrusion.

Biodegradable polymers offer unique advantages for the medical market (su-tures, pins, implants) and for agriculture (mulch, seeding strips). In the UnitedStates, more than 125 million nontoxic biodegradable plastic sutures sutures (qv)are used each year in heart operations. Biodegradable polymers may also re-place some hard-to-recycle contaminated products such as silage wrappings, plantpots, seed trays, and empty fertilizer bags containing chemical residue harmful tosewage treatment systems. Ongoing concern for a “green” environment has helpedto increase production of biodegradable polymers from corn-derived sugar, beets,soybeans, rice bran, and potato peelings.

Life Cycle Assessment. The relatively new discipline called life cycleassessment (LCA) has considerable environmental implications with regard toplastic waste. LCA encompasses every phase from raw materials and resin manu-facturers through transportation, processing, usage, recycling, and disposal (66).The life cycle of a plastic product embraces several steps (Fig. 10). Conversionof crude oil into petrochemicals is followed by a synthesis of virgin resins, whichare then processed into products. When their useful life ends, products are eitherrecycled or landfilled. By monitoring the qualities of a plastic throughout its lifecycle, its performance value can be maintained at the highest possible level. Be-cause only 5% of the consumed resources are transferred into useful products and95% ends up as waste or by-products, most processes are resource-intensive (67).These may be modified in the future through LCA, which holds promise for emis-sion studies and similar environmental aspects during the life cycle of a product.


Crudeoil

PetrochemicalsVirginresins

Products

Parts

End user

Assembly

End-user

Recycle

Landfill

Fig. 10. Life cycle of plastics.

Overall, the reputation of plastics recycling has been a mixed one. On theone hand, many post-consumer products still cannot be recycled economically, andfar too much plastic material is landfilled. On the other, emerging technologiesand new attitudes are helping to brighten that picture. Increased commitmentsworldwide to a greener environment are reflected both in enhanced communityparticipation and in stricter governmental regulations, raising expectations forimproved plastics recycling in the future.

Other Economic Aspects

Extended Producer Responsibility. First introduced in Sweden in theearly 1990s, the concept of Extended Producer Responsibility (EPR) has gained at-tention in Europe and elsewhere. EPR extends the manufacturer’s responsibilityregarding environmental impact of their products during the life cycle, recycling,and disposal. EPR was first mandated in Germany in 1991 as the “Green Dot”system. In the United States, EPR is defined more broadly as the shared respon-sibility of the government, consumers, and industry, rather than of the produceralone. Several American companies have voluntarily implemented EPR concepts:Xerox practices a maximum recovery of office equipment and takes back its prod-ucts; Kodak takes back single-use cameras, and DuPont takes back and recyclesPET film (60).

Eco-Design of Plastic Parts. Eco-design or “green design” of plasticparts involves minimizing the amount of material entering the MSW by reducingpart weight and by extending the life of a part through improved performance.Reuse of a plastic for secondary applications is also practiced, along with closed-loop recycling (returning recycled plastic back into the same application) (61,62).Designers should choose compatible plastics to eliminate disassembly if possible,


because separation of materials is one of the critical elements in the economics ofplastics recycling. Auto makers such as Peugeot (France) have adopted a “mono”material approach for large plastic parts such as bumpers in order to simplifyrecycling. Bumpers, fixtures, and fittings are made from one grade of PP, in con-trast to the PP/EPDM blend usually used. Many manufacturers are still reluctant,however, to use recycled plastics or products with recycled content when design-ing their products because very little information about physical properties andlong-term performance is available (63). Specifications such as size, weight, andwall thickness often constrain the use of recycled plastics. Certain products fromrecycled materials must be made thicker to compensate for the material’s reducedproperties.

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GENERAL REFERENCES

J. Brandrup and co-workers, eds., Recycling and Recovery of Plastics, Hanser Publishers,Munich, 1995.


K. C. Frisch and co-workers, eds., Recycling of Polyurethanes—Advances in Plastics Recy-cling, Technomic Publishing Co., Inc., Lancaster, Pa., 1999.V. Goodship, Introduction to Plastic Recycling, RAPRA Technology, Ltd., Shawburg, Shrop-shire, England, 2001.C. D. Papaspyrides and J. G. Poulakis, in J. C. Salamone, ed., Polymeric Materials Encyclo-pedia, Vol. 10, CRC Press, Boca Raton, Fl., 1996, pp. 7403–7419.

K. KHAIT

Northwestern University

REINFORCEMENT. See Volume 4.

RELEASE AGENTS. See Volume 4.

RHEOLOGICAL TESTING. See VISCOMETRY.

Documents

"Recycling, Plastics". In: Encyclopedia of Polymer Science and