"Environmentally Degradable Plastics". - Wiley Online Library

2 of the 12 “principles of green chemistry” elucidated by Anastas and co-workers ... practicable and (2) products, at the end of their function, should break down into .... waste to avoid contamination of the compost, (2) imperfect separation ...... siderations are perforce relegated to secondary status, and where interim solutions.
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ENVIRONMENTALLY DEGRADABLE PLASTICS Introduction Environmentally degradable plastics are sometimes referred to as “green plastics” where the word “green” is used in an ecological sense. The impact of plastics on the environment is hardly a new theme, but with increased interest in “green chemistry” and the broader “green revolution,” the relationship between plastics and the environment is a topic of current interest. Overviews on environmentally degradable plastics have recently appeared for the nonspecialist (1–4) as well as more technical symposia proceedings (5–13). Several extensive Web sites are maintained (14,15) and a commercial market report has been available (16) (see BIODEGRADABLE POLYMERS, MEDICAL APPLICATIONS). Throughout the 1990s the two major issues relevant to a plastic’s “greenness” were feedstock renewability and alternative late-stage disposal options based on degradability, biodegradability, and compostability. These issues are related to 2 of the 12 “principles of green chemistry” elucidated by Anastas and co-workers (17,18): (1) a feedstock should be renewable wherever technically and economically practicable and (2) products, at the end of their function, should break down into innocuous degradation products (17, p. 30). Political considerations not particularly connected with the environment have also played a role in focusing attention on the production of plastics, including growing concern over dependence on foreign sources of crude oil, the desire to find additional markets for agricultural products, and the increasing costs and other difficulties of waste management (particularly strong in Europe and Japan) (3). Since 1993, the International Standards Organization (ISO), through its Technical Committee 207, has been developing the 14000 series,

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

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“Standardizations in the Field of Environmental Management.” The long-term goal of this effort is to formulate life-cycle assessment (LCA) programs, environmental auditing procedures, product standards, and product-labeling programs that incorporate environmental aspects (14). In particular, LCA would provide the analytical tools and protocols for producing an inventory analysis of the material and energy inputs and outputs for a product system, and an environmental impact assessment that includes the inventory analysis and a product’s complete environmental profile (19). One central concern in LCA is the origin and fate of the carbon contained in or released as carbon dioxide in the production, use, and disposal of the product. ISO 14040 describes the uses and limitations of LCA methodology. LCA of plastic products is still in the early stages of development. While future reports on “green plastics” may have the benefit of well-documented LCAs containing quantitative results obtained with consensus analytical tools, the present report is limited to the two factors originally motivating attention to the environmental impact of plastics—the origin of the plastics feedstocks (renewable vs nonrenewable) and the ultimate environmental fate of discarded plastics (degradable/biodegradable/compostable vs recalcitrant). Greenness and price are two separate issues. Even green plastics will have to have favorable price–performance ratios in order to compete in the marketplace, but complicating the issue of price is the reality that no accepted method for determining total cost exists that would account for such factors as the depletion of natural resources, the costs of guaranteeing the supply of raw materials, or the environmental burden of waste management. Definitions. Standardization of definitions, test methods, and performance criteria are required for degradable, biodegradable, and compostable plastics (as for other plastics), to allow product stewardship, to enable regulatory programs, and to support consumer confidence. In the United States these responsibilities lie with the American Society for Testing and Materials (ASTM). ASTM definitions of technical terms used in the plastics industry are the responsibility of ASTM Committee D20 on Plastics. Its Subcommittee D20.92 on Terminology originated, for example, ASTM document D883-00, “Standard Terminology Relating to Plastics.” Several of the most important ASTM definitions from that standard related to the degradation of plastics are reproduced here: Degradation: A deleterious change in the chemical structure, physical properties, or appearance of a plastic (see DEGRADATION). Degradable plastic: A plastic designed to undergo a significant change in its chemical structure under specific environmental conditions, resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification. Hydrolytically degradable plastic: A degradable plastic in which the degradation results from hydrolysis. Oxidatively degradable plastic: A degradable plastic in which the degradation results from oxidation. Photodegradable plastic: A degradable plastic in which the degradation results from the action of natural daylight.

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Biodegradable plastic: A degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae. Compostable plastic: A plastic that undergoes degradation by biological processes during composting to yield carbon dioxide, water, inorganic compounds, and biomass at a rate consistent with other known compostable materials and leaves no visually distinguishable or toxic residues. A plastic can be degradable but not necessarily biodegradable; it can be biodegradable without being compostable (ie, the rate of degradation is part of the definition of compostable); and it can be compostable without being biodegradable (ie, the degradation may be abiotic). Also notice that the definition of biodegradable plastic refers only to degradation by microorganisms, not macroorganisms. Test Methods. Industry standard test methods for determining the environmental degradability, biodegradability, and compostability are the responsibility, in the United States, of ASTM’s Subcommittee D20.96 on Environmentally Degradable Plastics. Standard ASTM test methods used to assess biodegradability include measurements of carbon dioxide production (eg, Sturm test and soil test), weight loss, extent of fragmentation, tensile properties, molecular weight and molecular weight distribution, enzyme assays, biochemical oxygen demand (BOD), and ecotoxicity (eg, cress seed test and earthworm test). Multiple test procedures are necessary in evaluating a material because some tests are subject to false-positive interpretations. Examples are (1) an observed weight loss may result from the leaching of additives rather than polymer degradation; (2) carbon dioxide production might result from the degradation only of a low molecular weight polymer fraction; and (3) a large loss of material strength might come from a very small change in chemical makeup. To illustrate the application of such test procedures it can be said, for example, that poly(ε-caprolactone), by its disappearance in a soil burial test, is potentially biodegradable in soil, whereas unmodified polyolefins like polyethylene, polypropylene, and polystyrene are recalcitrant. A distinction is necessary between evaluations of a polymer and a plastic made from that polymer because Additives (qv) (Plasticizers (qv), stabilizers, etc), processing, and fabrication may all have effects on environmental behavior. For example, a polymer or blend may be biodegradable and compostable as film, fibers, or foam, whereas injection-molded products may be inherently biodegradable but not environmentally compostable. ASTM D6400. In 1999 ASTM announced its “Standard Specification for Compostable Plastics” (D6400-99). The standard establishes criteria to be met before a product can be labeled compostable. Briefly, a product must, at minimum, satisfy ASTM tests showing conversion to carbon dioxide at 60% for a homopolymer or a statistical random copolymer and 90% for other types of copolymers and blends in 180 days or less, and leave no more than 10% of the original weight on a 2-mm screen. If carbon-14 tests are used a test period of 365 days is allowable. Environmental toxicity issues are also addressed, including limits for heavy metals. The ASTM standard has counterparts in German (DIN 54900), European (EN 13432), and international (ISO 14855) documents. The relatively short time scale of 6 months in ASTM D6400 identifies plastics that compost quickly, completely, and safely, and serves well the purpose

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of diverting waste from landfills and incinerators to composting facilities. Some common and inherently compostable biomaterials, however, including wood and leaves, do not satisfy the D6400 specification. Discussion continues as to whether complete biodegradability is required for compostability. A longer time scale for compostability, with less rapid carbon dioxide evolution, may be suitable for purposes of providing long-term soil enhancers, and it may turn out that ASTM will develop a second standard of compostability aimed at plastics that degrade more slowly. But the potential impact of the accumulation of undegraded plastic residues on long-term agricultural productivity is an important issue that has not yet been resolved either by long-term scientific measurements or by consensus. Products that comply with the ASTM specification, as confirmed by independent testing, can bear a logo developed jointly by the international Biodegradable Products Institute (BPI) and the United States Composting Council (USCC). Certified products are listed on the BPI Web site at www.bpiworld.org. Markets. Green plastics typically can be produced in various forms (pellets, film, and fibers) and are compatible with a variety of processing methods (Extrusion (qv) and molding). Markets for green plastics are, therefore, potentially the same as those for conventional plastics. In order to be successful in those markets, green plastics will have to possess the physical properties required for the intended application and be cost-competitive; that is, regardless of the intended market, success will be determined by the cost–performance ratio in comparison to the conventional plastics targeting the same market. Beyond that, however, markets for green plastics can be distinguished according to how biodegradability and compostability are related to the application. In some applications, biodegradability and compostability serve intrinsic functional purposes and give the product a performance capability that often cannot be matched by alternative products. For example, agricultural mulching films control soil temperature and moisture, reduce leaching of nutrients, and prevent weed growth, increasing productivity for vegetable and fruit crops (eg, tomatoes, peppers, melons, and sweet corn) by 50–350% (2). Compostable agricultural mulching film that can be plowed under after use eliminates the costs of collecting and disposing of nondegradable polyethylene covers. Similar advantages accrue with other compostable agricultural and horticultural products, such as planting pots, baling rope and twine, string, clips and hooks, labels, seed mats, time-released fertilizers (greensticks), and others. Biodegradability and compostability are also central to the function of collection bags for compostable materials (like yard waste and food waste) and of trash bin liners. Approximately 10% of the 1.5 billion pounds of trash bags produced in the United States are used for lawn and leaf bags. Nondegradable bags and liners add to the costs of composting: (1) they have to be separated from the waste to avoid contamination of the compost, (2) imperfect separation degrades compost quality, and (3) nondegradable bags require disposal. Compostable waste bags and trash bin liners avoid these problems. Biodegradability is an intrinsic feature of some other applications, such as the institutional use of laundry bags that dissolve during the washing and then biodegrade after disposal into sewage. In other applications the product’s compostability, rather than serving a functional requirement of the application, provides an alternative waste-disposal method. Composting diverts waste from landfills and incinerators and contributes

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to the sustainability of agriculture. Composting is especially important in Europe and Japan, where land is scarce and high landfill fees are common. In Japan, land is so scarce that over 90% of the solid-waste stream is incinerated, but concern has grown over the observed increasing levels of environmental dioxins. In the United States, many states are developing their composting infrastructures, especially for commercial and institutional sources, in order to increase their overall recycling rates. Plastic packaging is an increasing component of municipal solid waste, and markets for compostable plastics are potentially large in the area of packaging— for industrial, commercial, and household use. Examples include single-use and limited-use disposable materials such as packaging film for food and nonfood items, bottles, jars, loose-fill packaging, and rigid foam packaging. Food-service items constitute another significant end use, including cutlery, plates, cups, trays, and plastic coatings for paper products like plates and cups. Markets include institutions (hospitals, schools, prisons, and the military), the fast-food industry, and the general public. Another end use category is consumer products, including such products as diapers, personal care and hygiene articles, sports and recreation items, and others. Some manufacturers emphasize the physical properties of their products, with little or no emphasis on biodegradability except as a corollary benefit. Green plastics comprise only a very small part (less than 1%) of today’s plastics. They do, however, make up a significant part of some specialty, niche markets: starch-based loose-fill packaging now constitutes 30% of the loose-fill packaging market. The plastics described here are those currently commercially available, and are limited mainly to those available in the United States. Manufacturers are named only for illustrative purposes; the list is not intended to be comprehensive. The plastics materials are described generically, with respect to the major polymer constituent(s); for each generic type there are likely to be many specific formulations. Brief mention is made, at the end, of some materials that have been studied in the laboratory. Biomedical applications are described separately (see BIODEGRADABLE POLYMERS, MEDICAL APPLICATIONS). Many of the polymers mentioned below have separate entries in the encyclopedia. Here the focus is on their specialized use in the production of environmentally degradable plastics.

Petroleum-Based Polymers Activated Polyolefins. Polyolefins degrade by oxidation (eq. 1) (2) but the rate in the environment is extremely slow; polyolefins are environmentally recalcitrant. In “activated polyolefins” the environmental degradation is enhanced. An early example was the photodegradable plastic produced when an olefin is copolymerized with carbon monoxide (CO) (2,20) (see POLYKETONES). A copolymer of ethylene and (∼1%) CO, on exposure to outdoor sunlight for approximately 3 weeks, becomes embrittled and fragments to a friable powder. A major application is the manufacture of plastic six-pack holders for beverage containers, which became important after 1990 when the United States Congress, in response to public concern over litter, passed Public Law 100-556 requiring that plastic ring carriers be made of degradable material. Manufacturers include Dow and DuPont.

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Vol. 6 O

+

+ O2

OH

+

OH

(1) Various conventional ketone monomers can also be used in copolymerizations with olefins to introduce groups that promote oxidation. Transition-metal complexes (iron, nickel, cobalt) introduced after polymerization act as antioxidant stabilizers during processing but, following an induction period (the length of which depends on concentration), the complexes convert to products that are oxidation activators. Through formulation variations, the length of the induction period and the rate of degradation that follows can be controlled. For applications where the expected disposal environment is earth burial, commercially available additive packages can be incorporated (∼3% by weight) into polyethylene or polypropylene during processing to induce accelerated oxidative degradation initiated by natural daylight, heat, and/or mechanical stress. Applications include agricultural mulching film, compost bags, shopping and grocery bags, baling twine, and packaging film. Activated polyolefins do not meet the requirements of compostability specified in ASTM D6400. They have market acceptance where cost is of paramount importance and adherence to the ASTM D6400 specification is not mandated.

Poly(vinyl alcohol), Poly(glycolic acid), and Poly(ε-caprolactone). Poly(vinyl alcohol) (1) (see VINYL ALCOHOL POLYMERS) is the most-produced watersoluble polymer in the world. It is prepared by free-radical polymerization of vinyl acetate followed by alcoholysis. It environmentally degrades (21,22) by oxidation to form carbonyl groups along the chain, and subsequent hydrolysis. Poly(vinyl alcohol) is an exception to the general rule that chains of carbon–carbon single bonds (as in polyolefins) are recalcitrant. Although considered to be a biodegradable synthetic polymer (the oxidation can be both enzymatic and nonenzymatic), the extent of degrading microorganisms appears to be limited. Poly(vinyl alcohol) has applications in hospital laundry bags, water-soluble packaging, agricultural products, and other film products. Poly(vinyl alcohol) is also used in blends (see below). OH CH2

CH n

1

Poly(glycolic acid) (2) (23) is synthesized from condensation polymerization of glycolic acid or catalytic-ring-opening polymerization of the glycolide. (Although glycolic acid is found in nature it is commercially produced mainly through synthesis.) Poly(glycolic acid) is not only biodegradable, it is biocompatible—nontoxic and not rejected by the intended organism. It is therefore used in various biomedical applications (see BIODEGRADABLE POLYMERS, MEDICAL APPLICATIONS). In applications intended for large-scale use, glycolic acid is widely used in combination with either petroleum-based or renewable components (see below). O CH2

C 2

O

n

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Poly(ε-caprolactone) (3) (23) is synthesized by ring-opening polymerization of ε-caprolactone using tin-octanoate as a catalyst (24). It is used, for example, as blown film for compost bags or in materials where it serves as a matrix for the controlled release of pesticides, herbicides, and fertilizers. The low melting point of poly(ε-caprolactone) (60◦ C) (Table 1) limits applications, but it has good mechanical properties and is more hydrophobic than—and compatible with—many biopolymers. It is therefore widely used to modify the properties of other degradable plastics in blends (see below). O CH2

CH2

CH2

CH2

CH2

C

O n

3

Aliphatic polyesters like poly(glycolic acid) and poly(ε-caprolactone) are biodegradable through the action of nonspecific enzymes including the esterases found abundantly in soil (23,33–36). Copolyesters and Poly(ester amides). Many biodegradable thermoplastic copolyesters (aliphatic and aliphatic–aromatic) and poly(ester amides) are manufactured from conventional petroleum-based monomers (23). (see POLYESTERS, THERMOPLASTIC). The process often involves diols (eg, ethylene glycol, 1,4-butanediol, 1,6-hexanediol) and dicarboxylic acids (eg, adipic, succinic, sebacic). Terephthalic acid is sometimes a component; the aromatic diacid increases chain rigidity. These copolyesters biodegrade in a composting environment by surface growth of bacteria and fungi, and the action of hydrolytic enzymes. Degradation rates in compost depend on composition. They are typically available as resins, films, or fibers; are usually extrusionprocessed; and are characteristically strong, elastic, and waterproof. Applications variously include agricultural mulching film, compost bags, shopping bags that can later be used as compostable garbage bags, transparent film packaging, film coatings for paper and paperboard containers (eg, drinking cups), coatings for starch film, disposable diapers, nonwoven and fiber applications, and others. EastarBio copolyester (4), (Table 1), manufactured by Eastman Chemical, is a random poly(butylene adipate-co-terephthalate) containing 50% terephthalate; film is prepared by solvent casting (Fig. 1). EastarBio copolyester biodegrades to the extent of 80% in 150 days in a compost environment. O O

(CH2)4

O

C

O (CH2)4

C

O n

m

O

O

C

C o

4

Ecoflex, manufactured by BASF, is a random copolyester based on terephthalic acid (40–45%), adipic acid (55%), and 1,4-butanediol, plus chain-extending additives (Table 1). A modified poly(ethylene terephthalate) copolyester is manufactured by DuPont as Biomax (Table 1). A fraction of the ester linkages can be substituted with amide linkages, which modifies interchain hydrogen bonding. Examples are the BAK poly(ester amides) previously manufactured by Bayer.

Table 1. Selected Properties: Typical Values for Film Products or Product Families Trade Manufacturer name

Product

Density, g/cc

T g ,◦ C

Low density Polypropylenea Poly(ε-caprolactone)

Dow

Copolyester

Eastman

Copolyester Copolyester Poly(lactic acid) Poly(hydroxybutyrate-cohydroxyvalerate) Starch/poly(ε-caprolactone)

BASF Dupont Cargill Dow Metabolix

0.921 −35 0.903 −20 TONE P-767 1.145 P-787 1.145 EastarBio GP 1.22 −30 Ultra 1.22 −33 — Ecoflex F BX 701.1 1.25–1.27 Biomax 1.35 — 1.25 52 Natureworks 4042Db 1.25 −25 to 6 Biopol

Novamont

MaterBi

polyethylenea

142

a b

LDPE and PP included for comparison. Biaxially oriented film.

Z-class

1.23



Melting Tensile temperature, strength, Elongation, ◦C MPa %

Tensile modulus, MPa

Reference

— 164 60 60 108 102–115 110–115 200 135 90–178

10.3 35.5 24.8 41.4 22 30 32/36 14–48 110/145 18–43

620 — 800 900 700 >600 580/820 40–500 160/100 10–1000

165.5 1380 435 386 107 113 — 60–2100 3310/3860 100–3600

25 25 26 26 27 27 28 29 30 31



25–45

250–886

100–450

32

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Fig. 1. A compostable garbage bag made from EastarBioTM copolyester. Courtesy of Eastman Chemical Co.

Polymers from Renewable Sources Cellulose. Cellulose (qv), β[1→4]-linked D-glucose (5) (37), is the most abundant biopolymer on earth and accounts for 40% of all organic matter. It is the main component of plant structural fiber that keeps the plant cell wall intact and gives it strength. Oven-dried cotton (qv) contains about 90% cellulose. An average wood (q.v.) has about 50% cellulose, the remaining components being lignin (qv) and hemicellulose. Well over 68 million metric tons of cellulose are produced commercially each year, mainly from wood. H H CH OH H 2 OH O HO O H H H H O HO O OH CH OH H H 2 H n

5

Cellophane is an example of “regenerated cellulose.” In the viscose process (38,39), shredded wood pulp is treated with caustic soda, producing alkali cellulose. After aging to produce the desired extent of cellulose degradation, the cellulose is treated with carbon disulfide to form a thick solution of xanthated cellulose, called viscose. The viscose is “ripened,” and extruded into a bath of sulfuric

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acid and sodium sulphite, generating a coagulated gel. The transluscent gel is washed, purified, and bleached. Softening agents—often either glycerol or ethylene glycol—and other chemicals are then added, and the resulting material is cast into a sheet consisting of an amorphous felt. The sheet has high water-vapor permeability, but good resistance to oils and greases. Coatings of nitrocellulosewax or poly(vinylidene chloride) provide barriers to moisture and gases, solvent sealability, and heat sealability. Cellophane has a water-vapor permeability comparable to high density polyethylene. Newer technology, based on the use of N-methylmorpholine-N-oxide as a cellulose solvent (40), has been developed with the aim of reducing hazardous by-products and producing a more environmentally friendly process. The viscose technology, however, still dominates (see CELLULOSE FIBERS, REGENERATED). Production of cellophane peaked in the 1960s at about 340,000 t a year in the United States, but since then has largely been replaced by synthetic plastics such as polypropylene. Cellophane is still used in food packaging for such items as potato chips, candies, and baked goods; its stiffness allows bags to stand upright on shelves. Nonfood applications include cigarette packages, cigar wrappings, and others; it tears well when notched and has excellent printability. Uncoated cellophane film readily biodegrades beginning with a hydrolysis reaction catalyzed by the extracellular enzyme cellulase. It disintegrates in 10– 14 days and totally biodegrades in 1–2 months. Nitrocellulose–wax coatings also totally biodegrade, in 6 months or so. Coatings of poly(vinylidene chloride) do not biodegrade; they disintegrate to a friable powder. Cellulose is not thermoplastic but can be chemically modified to produce a wide variety of cellulosic plastics—some of which are thermoplastic—and fibers. Before 1950 cellulosics were the most important group of thermoplastics; today, cellulosic fibers still make up about 8% of the fiber market. Some chemically modified celluloses do not have the biodegradability of cellulose. Cellulose acetate, one of the most important cellulosics, is produced by the reaction of cotton fiber with acetic acid and acetic anhydride using sulfuric acid as a catalyst. It is thermoplastic, and films can be obtained by extrusion as well as by casting from acetone solution. The film is clear and strong, and has high oxygen and water-vapor permeability. It “breathes” and does not fog, making it useful as packaging film for fresh produce and baked goods. It is also resistant to oils and greases. Cellulose acetate can also be molded (see CELLULOSE ESTERS). The degradation of cellulose acetate begins with abiotic hydrolysis and oxidation, followed by complete biodegradation, but acetylation decreases the biodegradation rate relative to cellulose. For example, increasing the degree of substitution from an average of 1.7 of the three hydroxyl groups per monomer (5) to 2.5 significantly reduces the rate of biodegradation. Starch. Starch, thermoplastic? (qv) (41,42) is a major agricultural commodity and, by far, the most inexpensive commercial biopolymer; it is the only biopolymer that is competitive with polyethylene in price. Annual world production, over 32 million metric tons, is from corn (maize), potatoes, rice, tapioca (cassava), barley, wheat, and other crops. Approximately 16 million metric tons are produced in the United States each year, mainly by extraction from corn but also from potatoes, wheat, and other sources.

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Well over half the starch produced is partially hydrolyzed to manufacture corn syrup, dextrose (glucose), and other hydrolysis products that are used as sweeteners or as feedstocks for various manufacturing processes in the chemical, pharmaceutical, and brewing industries. Approximately 75% of the remaining starch, either in native form or modified, is used for nonfood industrial purposes in the manufacture of paper and cardboard, paper coatings and sizings, textile and carpet sizing, and adhesives. Food uses include baby foods, bread batter, cake mixes, confectioneries, puddings, pie fillings, glazes, and sauces. Starch serves an energy storage function in plants; when starch is metabolized by plants, energy is released and used for plant growth. The major polymer components of starch are amylose and amylopectin. Amylose (6) is α[1 → 4]-linked D-glucose; its molecular weight ranges from 2 × 105 to 2 × 106 . Amylopectin has a backbone of α[1 → 4]-linked D-glucose with α[1 → 6]-linked D-glucose at branch points; it has molecular weights up to 4 × 108 . The relative amounts of amylose and amylopectin vary with the source plant. Cornstarch is typically 28% amylose and 72% amylopectin, but it can be genetically modified to have as much as 85% amylose or, for all practical purposes, 100% amylopectin (waxy maize starch). The physical properties of starch depend on the amylose/amylopectin ratio. H CH2OH O H H HO OH H

H H CH2OH O O H H HO OH H

H O

n

6

Natural starch occurs in the cell in the form of granules. The starch granules are gelatinized upon heating, whereupon the starch molecules become more disordered; the gelatinization temperature ranges from about 70◦ C in excess water to above 200◦ C at low moisture content. Depending upon treatment conditions, the starch granules and their crystalline structure may be completely destroyed. In time the starch may retrograde; that is, it reassociates into aggregates and precipitates at low concentrations or sets to a gel at higher concentrations, eventually becoming cloudy, releasing water, and shrinking into a rubbery consistency. Under proper conditions of temperature, pressure, water content, and shear, the starch granular structure is totally disrupted (43). The starch exists as a continuous phase and in this state is called thermoplastic starch (TPS). TPS can be extruded, injection-molded, or formed into foams (44–48). The major limitation in developing applications, representing an obstacle to be overcome, is the hydrophilicity of starch, which leads to water sensitivity in physical properties. Strategies for decreasing sensitivity to water include derivitization. For example, acetylation at a degree of substitution greater than 1.5 of the three hydroxyl groups per monomer (6) leads to water-resistant “hydrophobic” starch. Chemical modifications often increase water resistance without

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Fig. 2. Water-resistant foam packaging “peanuts” made by extruding starch acetate and water. Courtesy of R. Shogren, U. S. D. A.

significantly decreasing biodegradability. Plasticizers (qv) like glycerol acetate and diethyl succinate can be used to improve processability. Extruded starch foam is produced by the flash generation of steam as the starch leaves the die (49,50). Foam packaging “peanuts,” as a replacement for polystyrene, can be made by extruding starch acetate or hydroxypropylated starch and water (Fig. 2). A commercial example is EcoFoam, a water-soluble extruded hydroxypropylated starch foam manufactured by National Starch and Chemical Co. Other strategies for overcoming the water sensitivity of starch include blending with other polymers and forming composites by the addition of polymeric or mineral fillers (see below). Poly(lactic acid). Lactic acid, CH3 CHOHCOOH, occurs naturally in animals and in microorganisms. It can be produced commercially by chemical synthesis, but in the United States fermentation is the major route. In bioreactors, microorganisms are fed a carbon source substrate, such as dextrose, with yields of lactic acid greater than 90%. The lactic acid is recovered from the fermentation broth and purified in a multistep process that represents a major part of production costs. Eighty percent of the world’s production of lactic acid is from corn sugar. Poly(lactic acid) (PLA) (7) (Table 1) is the polyester synthesized from lactic acid (51–54). Condensation polymerization produces a low molecular weight prepolymer (M n ∼ 5000) that is treated with chain extenders to yield high molecular weight PLA. Condensation polymerization yields high molecular weight PLA directly when carried out in a high boiling point azeotropic solvent with removal of water by distillation at reduced pressure. Alternatively, the prepolymer is depolymerized in the melt under reduced pressure to produce lactide, the cyclic dimer of lactic acid (a dilactone). Metal-catalyzed ring-opening polymerization of lactide

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produces a high molecular weight polymer (M n = 60,000–150,000) PLAs are also called Polylactide (qv). O O CH C CH3

n

7

PLA from L- or D-monomers is semicrystalline, with a melting transition near 180◦ C and glass temperature around 67◦ C. In contrast, a racemic mixture of L- and D-lactic acid produces amorphous polymers with glass temperatures near room temperature. During the depolymerization of the prepolymer the stereoisomers are separated. The L-lactide is the primary component of the high molecular weight PLA; varying the amount of D-lactide allows a variation in physical properties (Table 1). PLA is thermoplastic and can be processed by extrusion, injection molding, blow molding, and thermoforming. It is unstable in acid and alkaline solution, but is insoluble in water and has good moisture and grease resistance. Its mechanical properties depend on molecular weight and crystallinity. Properties can be varied so as to display, for example, the clarity of polystyrene, the stiffness and tensile strength of poly(ethylene terephthalate), and the twist-retention of cellophane. Other features include favorable flavor and aroma barrier properties, low temperature heat-sealing, and printability. Lactic acid can be copolymerized with glycolic acid or ε-caprolactone to extend the range of properties further. Major current applications of PLA include clear packaging film and thermoformed articles for disposable food service items and other containers (Fig. 3). PLA can also be spun, and other current major applications are as fibers used in bedding products, clothing, carpets, sheets and towels, and wall coverings (Fig. 4). In addition, PLA can be used for agricultural mulching film and other compostable agricultural and horticultural products, compost bags, trash bin liners, sporting items such as golf tees, coatings for paper and cardboard, and other products. An example is NatureWorks manufactured by Cargill Dow (Figs. 3 and 4). PLA degrades by hydrolysis, even in the absence of enzymes, and can be recycled back to the monomer. It is compostable, but the rate of biodegradability in a composting environment depends strongly on conditions of temperature, crystallinity, and size and shape of the article (23,34,55–58). The resistance of PLA to degradation by microorganisms plays a role in food packaging applications. PLA is also biocompatible (see BIODEGRADABLE POLYMERS, MEDICAL APPLICATIONS). As new markets develop for the monomer, lactic acid, through, for example, its conversion to ethyl lactate for use as a biobased solvent, the cost of PLA may be reduced. Microbial Polyhydroxyalkanoates. Some microorganisms produce polyesters as energy and carbon storage materials, metabolizing them when no external carbon source is available (59,60). Microbial polyesters are now produced commercially through large-scale fermentation processes carried out in bioreactors. Microorganisms (eg, Ralstonia eutropha) are fed carbon source substrates, promoting cell growth followed by polymer accumulation, which can result in as

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Fig. 3. Sealed packaging tray made from NatureWorksTM PLA. Courtesy of Cargill Dow Co.

much as 80–95% of the cell’s dry weight. The bacteria are collected and washed, and the cells ruptured. The polymer is extracted with a solvent, precipitated from water as a white powder, and converted to pellets (61). The major commercial microial polyesters are polyhydroxyalkanoates (PHAs) (see POLY(3-HYDROXYALKANOATES)) Poly(3-hydroxybutyrate) (PHB) (8) is brittle, but as the number of carbon atoms in the side chains of PHAs increases, eg, poly(3-hydroxyvalerate) (PHV) (9), the properties tend from polypropylene-like to elastomeric. Copolymers, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), have a range of properties depending on composition (Table 1).

O

CH3 O

CH

CH2 8

C n

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Fig. 4. Bedding products made from NatureWorksTM PLA. Courtesy of Cargill Dow Co.

CH3

O

CH2

O

CH CH2

C n

9

The composition of the copolymer is controlled through the ratio of feedstocks used: glucose or sucrose (from sugar cane, sugar beets, or starch hydrolysates) can be used as substrates for butyrate; propionic acid (eg, from the fermentation of wood pulp waste) can be used for valerate. PHBV is thermoplastic and can be processed by injection molding, extrusion, blow molding, film and fiber forming, and lamination techniques. Significantly, PHBV is stable in humid air and its barrier properties are not humiditydependent. Commercial PHBV can be processed into packaging materials such as bottles, jars, and films; food service products such as eating utensils, cups, and plates; toiletry articles such as combs and razor handles; and other products including credit cards, golf tees, and plant pots. An example is Biopol and other PHAs manufactured by Metabolix (Table 1). Just as there is a wide range of microorganisms that can synthesize PHAs, there are a large number of microorganisms that can degrade PHAs. PHA biodegradation (23,62,63) begins with bacterial or fungal surface colonization. An extracellular PHA depolymerase degrades and solubilizes the polymer into

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fragments that are then absorbed by the cell, resulting in complete biodegradation. PHAs biodegrade at a rate that depends on copolymer composition, molecular weight, degree of crystallinity, and the size and shape of the article. PHBV is also recyclable. PHAs are biocompatible as well as biodegradable and PHBV is used in biomedical applications (see BIODEGRADABLE POLYMERS, MEDICAL APPLICATIONS). One of its degradation products, butyric acid, is a mammalian metabolite found in low concentrations in humans.

Blends and Composites Starch-Based. Granular starch was used early on as a particulate filler of conventional polymers (eg, polyethylene and polypropylene) in films (64–66). The starch granules are surface-treated to increase compatibility with the hydrophobic matrix: poly(ethylene-co-acrylic acid) or poly(ethylene-co-vinyl alcohol) plasticizers may be used to reduce brittleness. At starch levels of less than 10%, intended applications were degradable agricultural mulching film and compost bags. The polyethylene encapsulates the starch granules and biodegradation of the starch granules leads to fragmentation of the film. Photocatalytic additives (transition-metal compounds, unsaturated polymers, oils, fats) aim to enhance degradation of the polyethylene. Compost bags, of 23–25% starch, have “curb sturdiness.” Starch–polyethylene formulations of 20–80% starch have also been commercialized. Issues connected with the environmental fate and impact of the polyethylene residues remain, and are the subject of current research. TPS (see above) interacts with hydrophilic and hydrophobic polymers, and can be blended with petroleum-based polymers that are biodegradable. By varying the composition and processing of the blends it is possible to manipulate blend properties to produce plastics that resist property changes at high relative humidity but are nevertheless compostable. Starch–poly(vinyl alcohol) blends, for example, are thermoplastic and can be processed by extrusion (qv), injection molding (qv), blow molding (qv), film blowing, and thermoforming (qv) (67). Their properties depend on composition and processing. In films there is some sensitivity to humidity, and in water they swell but do not dissolve. With one formulation, the mechanical properties at 55% relative humidity are similar to those of polyethylene. The degree of crystallinity of the poly(vinyl alcohol) component varies according to the level of residual acetate groups that remains following the preparation of the poly(vinyl alcohol) by alcoholysis of poly(vinyl acetate). A range of water-soluble blends can thereby be produced by varying the amount of poly(vinyl alcohol), its molecular weight, and its crystallinity. One application is water-soluble laundry bags (68). Starch–poly(vinyl alcohol) films degrade hydrolytically at a rate that depends on composition and crystallinity. In sludge, carbon dioxide measurements indicate greater than 90% degradation in 10 months. Starch–poly(vinyl alcohol) blends can also be extruded as foams. The poly(vinyl alcohol) increases water resistance relative to foam made from starch alone, allowing shaped applications such as foamed cups and plates.

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Fig. 5. Agricultural products (mulch film, planting pots, clips) made from MaterBiTM . Courtesy of Novamont S. P. A.

Starch–poly(ε-caprolactone) blends (69) are also water-resistant and totally biodegradable. Their strength and water resistance depend on composition, and film properties can be varied from those resembling low density to high density polyethylene. Applications include agricultural mulching film (12–15 µm) that biodegrades over a controlled period of time to allow cultivation periods from 1 to 5 months. Other applications are trash bin liners, compostable yard waste bags, plant pots, shopping bags, packaging film, cutlery and food service ware, diaper backings, and consumer items. An example of starch-based blends is MaterBi manufactured by Novamont S.P.A. (Italy) (Fig. 5) (Table 1). Minerals—silicates (clay) or carbonates—increase the stiffness and toughness of starch. Earthshell Packaging manufactured by Earthshell Corp. is a starch–calcium carbonate fiber composite made for a variety of applications including the hinged-lid “clamshell” sandwich containers used in the fast-food industry (Fig. 6). Potato starch and limestone make up about 80% of the composite. They are mixed with water and fiber (eg, cellulose from recycled paper) to form a batter. Thickeners and release agents are added and the batter is placed between two heated mold plates. The water is converted to steam, expanding the batter. The product is finished with protective and water-proofing coatings. Others. Triacylglycerols (triglycerides) (10) (70–72) make up a large part of the storage lipids in animal and plant cells. They are now receiving renewed attention as a candidate feedstock for the production of polymer resins. When liquid at room temperature they are called oils. Commercially important oils are produced from the seeds of soybeans, corn (maize), cotton, sunflowers, flax (linseed), rape, castor beans, tung, palms, peanuts, olives, almonds, coconuts, and canola. Over 7.3 million metric tons of vegetable oils are produced in the United States each year, mainly from soybean, flax, and rapeseed. Soy oil alone accounts for 80% of the seed oils produced in the United States. Soy oil contains about 55% linoleic acid (11), 22% oleic acid (12) and 10% palmitic acid (13).

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O CH2

O

C

R O

O CH

O

C

R′

CH3

(CH2)4

CH

CH

CH2

O

C

CH

(CH2)7

C OH

O CH2

CH

11 R′′

10 O CH3

(CH2)7

CH

CH

(CH2)7

C

O CH3

(CH2)14

OH 12

C OH

13

Vegetable and animal oils can be made more chemically reactive (eg, by epoxidation), and then cross-linked to form sturdy thermosets. Reinforced with glass fiber the composites have applications in the areas of agricultural equipment and the automotive industry. Plant fiber reinforcement, eg, from jute, hemp, flax, sisal, keraf, wood, straw, or hay, provides a biodegradable option (73). A triglyceride-based thermoset has been developed from soybean and corn polymers as a sheet-metal alternative for side panels in agricultural equipment, replacing the conventional petroleum-based options (Fig. 7). Initial testing in 1997 involved the University of Delaware’s ACRES (Affordable Composites from Renewable Resources) Program and the United Soybean Board. The vegetable-oil composite is strong and weighs 25% less than steel—and less than the petroleumbased products typically used to make panels. It also accepts paint. In 1999, a soybased polymer was tested in the rear panel of John Deere combines, and beginning

Fig. 6. Earthshell ® Packaging disposable food service ware. Courtesy of Earthshell Corp.

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Fig. 7. Prototype John Deere side panel for a hay baler (8 × 3 , 25 lb.). A foam core is enclosed on both sides with a thermoset composite made from soybean oil resin reinforced with glass fiber. Courtesy of University of Delaware Center for Composite Materials.

with the 2002 model year, John Deere Harvester Works’ line of combines has included panels made from the material HarvestForm. Polyurethanes (qv) are now being made from polyols derived from soybean oil, eg, SoyOyl manufactured by Urethane Soy Systems Co. (USSC). Applications include carpet backing and padding, furniture and automobile seating foam, foam insulation, and others.

Laboratory Studies Many other compositions and processes have been studied in the laboratory; some are in the development stage on their way to commercial production. A few illustrative examples are given here.

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Fig. 8. Starch foam cup and tray coated with water-resistant PHBV. An adhesive layer of shellac, a natural resin, prevents delamination of PHBV from the starch. Courtesy of R. Shogren, U. S. D. A.

Nodax, under development by Proctor & Gamble, is made from renewable sources. It consists of a family of copolymers of 3-hydroxybutyrate and one or more 3-hydroxyalkanoates having a longer side chain, where the side chain has anywhere from 3 to 20 carbon atoms. The structures thereby resemble linear low density polyethylene, with the 3-hydroxybutyrate units providing a basis for crystallinity and the other units introducing structural irregularity and reduced crystallinity. Nodax can be converted into sheets, molded articles, foams, films, fibers, and nonwoven fabrics, and is both biodegradable and compostable. Modifying the hydrophilicity of starch has been the goal of many strategies, so as to improve the resistance of starch to water. Starch has been chemically modified by reaction with caprolactone, producing a thermoplastic that needs no plasticizer (74). Coatings can also be used, as when starch films are coated with an waterproofing layer of a nitrocellulose–wax blend, as is done with cellophane. Or, starch sheeting can be laminated with a water-resistant layer of poly(ε-caprolactone) by coextrusion (75). Starch-based foams can provide compostable packaging for the fast-food industry. The extruded starch foam can be coated with a outer layer of acetylated starch to provide water resistance, or a foamed starch-acetylated starch blend can be so coated (76,77). Starch has also been coated with the microbial polyester PHBV to provide resistance to hot and cold water; a natural resin, shellac, prevents delamination (78) (Fig. 8). Starch can be blended with biodegradable polyesters from renewable sources, such as PLA or PHBV, or with biodegradable petroleum-based copolyesters or poly(ester amides). In these cases, the aim is to combine low cost starch with higher cost polymers having greater hydrophobicity and better physical properties. For example, biodegradable and compostable plastic eating utensils can be made using 55% cornstarch and 45% PLA (Fig. 9).

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Fig. 9. Plastic eating utensils. The utensils on the left are made of 55% cornstarch and 45% poly(lactic acid). They are biodegradable and compostable. For comparison, nondegradable polystyrene utensils are shown on the right. Courtesy of R. Shogren, U. S. D. A.

Starch is also compatible with other biopolymers, including polysaccharides such as pectin (79) and agarose (80), and proteins such as casein (81) and gelatin (82). Chitin (qv), a polysaccharide produced from shellfish waste, and chitosan (deacetylated chitin) are not thermoplastic, but films and fibers prepared by evaporation of solvent have good mechanical properties and low oxygen permeability (83). They mainly have pharmaceutical and biomedical applications, such as contact lenses and wound-healing treatments. Pullulan, a polysaccharide, is prepared commercially through yeast fermentation. It can be processed into a bioplastic that is hard, strong, tough, and elastic. Pullulan has good oxygen barrier properties and has been developed for food-packaging applications. Fibers have been drawn from concentrated solutions. Commercial proteins (84) include zein—a water-insoluble protein from corn (maize). Zein is thermoplastic and can be processed with extrusion and injectionmolding techniques. It forms strong, grease-resistant, and dyeable fibers. Starch has been blended with zein to increase water resistance; natural fibers were added for increased modulus (85). Other commercial proteins include wheat protein, soy protein, and casein. Epoxidized soybean oil has been polymerized with citric acid and used to coat kraft paper for biodegradable agricultural mulching covers (86); the coating extends the lifetime of the paper mulch to a practical length (Figa. 10). Other plant oils that are being studied include castor seed oil, sunflower seed oil, corn (maize) oil, and rapeseed oil. Lignin (qv) is an abundant renewable natural resource; it accounts for over 20% of all organic matter and is second in abundance only to cellulose. Approximately 46 million metric tons are isolated in the United States each year as a by-product of wood pulp processing. Its underutilization has often been described,

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Fig. 10. Biodegradable mulch—kraft paper coated with a polyester made from a reaction product of epoxidized soybean oil and citric acid. The coating reduces the degradation rate and increases wet strength so the mulch can inhibit weed growth for more than 10 weeks. Courtesy of R. Shogren, U. S. D. A.

and the significance of its large-scale potential use in thermoplastics or thermosets has been noted (87). The development of lignin-containing plastics is a compelling objective because it would raise lignin to the status of a high value commodity material. Lignin bioproducts could also provide a stabilizing component to composts: lignin degrades at a slower rate than most other biopolymers, and in composts it would provide an important soil stabilizer, helping to maintain organic content and moisture level. Lignin has been studied as a component in many nonbiodegradable thermoplastics materials: as a filler in lowdensity polyethylene and polypropylene

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(88,89), in compression-molded unplasticized poly(vinyl chloride)–lignin blends (90), in blends of organosolv lignin and poly(vinyl chloride-co-vinyl acetate) (91), and in solvent-cast films of hydroxypropyl lignin–caprolactone copolymers blended with poly(vinyl chloride) (92). Lignin and lignin esters have also been blended with various biodegradable thermoplastics: poly(ε-caprolactone)–lignin blends prepared by solvent casting (93), kraft lignin–poly(vinyl alcohol) blends (94), lignin–poly(ethylene oxide) blends (95), methylated lignin–poly(ethylene oxide) blends (95), and lignin and lignin esters blended with cellulose acetate butyrate, with poly(hydroxybutyrate), and with a starch-poly(ε-caprolactone) copolymer blend (96,97). Derivatized kraft lignin has been incorporated at a level of 85% into poly(vinyl acetate) to produce a thermoplastic material that can be extrusionmolded (98). Alkylated 100% kraft lignin has been shown to give thermoplastic material that possesses tensile properties similar to polystyrene; pyrrolidinone was used as a solvent, diethyleneglycol dibenzoate and indene were used as plasticizers (99). Lignin has also been used in thermosetting resins as additions to polyurethanes, epoxy resins, acrylic resins, and phenol resins. For example, IBM has explored the use of epoxidized lignin in producing thermosetting resins for circuit board applications (100).

How Green are Green Plastics? There is a hope that LCA (8) of plastics will be developed to the point that meaningful product environmental profiles can be produced, whereby the environmental impacts of various plastics can be compared with one another. Initial LCA development has focused, in particular, on energy consumption and carbon dioxide release, but even then the dependence of carbon dioxide release on disposal method, for example, is not treated uniformly. In the coming years LCA methodologies are likely to be improved. Certain trends, however, are beginning to emerge. A review of 20 LCA studies of biodegradable polymers (101,102) indicates that thermoplastic starch, the major component of approximately 75% of green plastics production, offers important environmental benefits compared to conventional polymers. Compared to starch, the environmental benefits of poly(lactic acid), currently accounting for 10–15% of production, and of biodegradable polymers made from nonrenewable resources, eg, poly(vinyl alcohol) and poly(ε-caprolactone), accounting for approximately 10% of production, seem to be smaller, but still greater than that of conventional polymers. For microbial polyhydroxyalkanoates, which currently make up a very small part of total green plastics production, the environmental advantage seems to be small (or perhaps nonexistent), but the fermentation technologies for producing microbial polyhydroxyalkanoates are among the most recently developed, and both the production method and the scale of production can influence evaluations of the overall environmental balance. One LCA that was published in the popular literature and received much attention (103) was limited to poly(lactic acid) and microbial polyhydroxyalkanoates; thermoplastic starch was not considered.

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Future Prospects The future of green plastics will be determined by the cost–performance ratio, where performance will include environmental attributes as well as physical properties. More than 90% of current conventional plastics are environmentally nondegradable, but at the present time they have a significant cost advantage. Within the group of degradable/biodegradable/compostable plastics, activated polyolefins, on account of the cost advantage they now enjoy and in spite of remaining questions related to their environmental fate and impact, will likely find continued market acceptance, especially in third-world countries where environmental considerations are perforce relegated to secondary status, and where interim solutions are being sought pending optimization of the bioplastics industry. Hybrid materials, containing biodegradable, but nonrenewable, petroleumbased polymers combined with renewable biopolymers (mainly starch), constitute an environmental compromise, but hybrids provide improved physical properties relative to the biopolymers alone; they are biodegradable; and, in many cases, they are compostable. Hybrids satisfy many of the growing environmental concerns of the major industrialized regions of North America, Europe, and Asia. Developing bioplastics, ie, biodegradable and compostable plastics that are derived entirely or almost entirely from renewable raw materials, faces the greatest technological challenge. Reducing cost will be a critical factor. Most surveys indicate that for green plastics consumers might be willing to pay a premium of 10–20% above the cost of conventional plastics. The cost of green plastics now exceeds that amount, partly because of unoptimized technologies and limited production. One can envision two, not mutually exclusive, directions for the future. The usefulness of biomass polymers (starch, cellulose, and others) will continue to be extended through chemical modification, novel formulations, and processing innovations to produce an increasing number and variety of materials and applications. Concurrently, biomass polymers will be transformed into an increasing array of polymerizable monomers, using enzymatic and conventional means, followed by polymerization, also using both enzymatic and conventional means, to produce polymer materials having sequences and molecular weights required for the targeted applications. As the “green chemistry” movement presses the polymer industry toward sustainability and low environmental impact, technologies that produce shifts in those directions will continue to emerge and develop. Significantly, the new technologies are being aided with help from governments that are impelled by environmental concern, need for reduced dependence on foreign crude oil, and desire for increased agricultural markets. The allure of a sustainable industry producing commodity plastics for an increasing world population will no doubt persist. It remains to be seen at what rate the required technologies will develop.

BIBLIOGRAPHY 1. E. S. Stevens, Green Plastics. An Introduction to the New Science of Biodegradable Plastics, Princeton University Press, Princeton, 2002.

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E. S. STEVENS Binghamton University

ENZYMATIC POLYMERIZATION.

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