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What Makes Green Plastics Green?

What Makes Green Plastics Green?

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BACKBONES OF SCIENCE WHAT MAKES GREEN GREEN?

REEN plastics carry high ex- under all conditions. pectations. They are expected Although the rate of plastics degradation to perform their intended func- depends on the environment in which it is An understanding tion as bags, packages or film placed, it also strongly depends on the barriers and then, within a rea- chemical nature of the polymer. A suitable of the chemistry sonable timeframe, essentially starting point for understanding the vari- of — disappear in the form of envi- ety of factors perhaps is the extremely en- Gronmentally acceptable degradation prod- vironmentally stable : polyethy- ucts. How they function and disappear is lene, , poly(vinyl chloride), both petroleum- largely a matter of their chemistry. Several and . (Note: By standard con- products, with varying chemical skeletons, vention, polymer chemists enclose the derived and from are being promoted as green plastics to the monomer name in parentheses when it con- composting and recycling community. sists of two words.) natural sources The first part of this article, “How Green Are Green Plastics?” (December 2002), gave POLYOLEFINS – THE STANDARD OF STABILITY — provides a an account of terminology and standards re- Polyolefins are at the top of the list of lated to green plastics, and surveyed the commodity polymers, accounting for road map to commercial products used in manufactur- almost 90 percent of all plastics manufac- ing collection bags for compostable materi- tured. They are manufactured from products als. This second part de- petroleum-based feed- being marketed scribes the chemical stocks. (Fig- nature of polymers in the Figure 1. Chemical composition and ure 1), for example, is as compostable. plastic and how the chem- structure of low-density polyethylene polymerized from the istry strongly influences (LDPE) monomer compound ethy- the degradability of the lene, CH2=CH2 where the plastic. = symbol indicates a dou- E.S. Stevens ble bond. Double bonds OF MONOMERS are shorter and stronger AND POLYMERS in a thermodynamic sense In general, the proper- but they are more chemi- ties of plastics are deter- cally reactive compared to mined by the constituent C=carbon atom, H=hydrogen atom, n represents a single bond. When ethy- polymers that are their the number of repeated units and can be as lene is polymerized the main ingredient, by addi- large as many thousands double bond is replaced tives introduced to im- with two single bonds, one prove sometimes otherwise poor physical of which attaches to another properties, and by processing. Polymers monomer in the polymer chain. Single are long chained composed of bonds between carbon atoms are especially multiple and repeated units of one or more difficult to break (i.e. they are stable). In monomers (a single identifiable chemical part, polyethylene owes its stability to this compound). Polymer chemists and engi- uninterrupted string of carbon-carbon sin- neers can come up with a plastic that meets gle bonds. Polyolefins are generally inex- almost any application requirement by pensive and their physical properties, such varying the chemical nature of the poly- as melting point, strength, and resistance mer, the mix of additives, and the method to water (hydrophobicity), are useful for a of processing. Tradeoffs sometimes enter wide range of applications. It is their favor- the picture, as in the case of plastic collec- able cost-performance ratio that makes tion bags for compostable materials. Those them the commodity leaders. bags must be strong and durable enough Polypropylene (PP) (Figure 2) differs for the collection process but then degrade chemically from polyethylene only in hav- during composting. These “programmed- ing a side chain attached to every other car- degradable” plastics have received atten- bon atom; in the case of PP, the side chain tion relatively recently; the main efforts of is a methyl group (CH3). (The side chain polymer scientists for many years were di- adds specific performance characteristics to rected at making plastics maximally stable the polymer, e.g. makes it more pliable).

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The backbone chains of the two polymers polymers that are biodegradable and com- are the same. postable. One example is poly(-caprolac- Polyethylene and polypropylene are recal- tone) or PCL (Figure 3). PCL is a citrant (resistant) with respect to environ- by virtue of containing the group of Polymer chemists mental degradation; even in a compost en- atoms, COO, in its repeating unit. The pres- and engineers can vironment they can last many years. ence of the oxygen heteroatom in the back- Polyethylene and polypropylene do degrade bone makes the polymer susceptible to come up with a in the environment by oxidation. Natural degradation by hydrolysis (i.e. chemical re- daylight can accelerate the oxidation, giving action with water). PCL is biodegradable plastic that meets rise to photo-oxidation (photodegradation). through the action of nonspecific The carbon-carbon chains are broken, and in including the esterases found abundantly in almost any time the plastic will be- soil. The low melting tem- come brittle and eventual- perature of PCL (60°C) application ly fragment. The rate in limits applications but it is requirement by any case, however, is very Figure 2. Chemical composition and often used in combination slow (nonetheless, anti-ox- structure of polypropylene (PP) with other polymers, in- varying the idation stabilizers still are cluding (see below). added to polyethylene and The properties of poly- chemical nature of polypropylene to prolong mers can be modified by us- their useful lifetime). ing more than one type of the polymer, the Perhaps the single most monomer in the polymer- important reason for the ization, to produce a mix of additives extreme stability of poly- . Polymer scien- For brevity, the individual C-H bonds in the CH3 and the method of olefins is that they contain side chain groups are not shown tists, aiming to achieve only carbon atoms in their application-specific prop- processing. backbone (Figs. 1 and 2), erties, have studied innu- and each carbon atom is merable combinations of bonded to four other Figure 3. Chemical composition and monomers. A variety of atoms. The carbon-carbon structure of poly(-caprolactone) biodegradable copolyesters single bond is very stable. have been produced from As will be seen, introducing petroleum-based mono- a non-carbon atom (a het- mers. eroatom) such as oxygen One such copolyester into the polymer backbone commercially available for significantly reduces envi- collection bags is the East- ronmental stability. ar Bio® copolyester, manu- C=carbon atom, H=hydrogen atom, factured by Eastman HELPING PETROLEUM-BASED O=oxygen atom Chemical (Figure 4). The POLYMERS DEGRADE monomers from which it is “Activated” polyolefins are polyolefins, produced are butanediol, adipic acid, and usually polyethylene, that have been modi- terephthalic acid. The oxygen-containing fied, either during the initial linkages are responsible for the polymer’s or afterwards during processing, so as to in- biodegradability, as with PCL. But the ad- crease the rate of oxidative degradation. The ditional chemical compositional elements in chains fragment and the plastic becomes a the copolyester lead to very satisfactory friable powder. Eventually, it is known, the physical properties during use; for example, chain fragments become so short that they the terephthalate component improves can be converted by microorganisms in the chain rigidity. Eastar Bio® biodegrades to environment to carbon dioxide (CO2) and the extent of 80 percent in 150 days in a com- water (H2O). What is not known, and is the post environment, according to the manu- subject of much current research, is the ex- facturer, and satisfies the BPI-USCC com- act time scale for that complete conversion, post label requirements. although it is known that activated poly- Somewhat similar products, also olefins do not meet the current Biodegrad- copolyesters, are Ecoflex® manufactured by able Polymers Institute-U.S. Composting BASF and Biomax® manufactured by Council (BPI-USCC) composting label re- DuPont. These products may be marketed in quirements (see Part One of this article). the United States for use in film products Another unknown is the effect of accumu- such as compost feedstock collection bags in lating residual polyethylene fragments on the near future. agricultural productivity should such plastics be used for applications like agricultural , or collection bags Figure 4. Schematic representation of a copolyester, that are intended to remain in poly(butylene adipate-co-terephthalate the compost. Some people mistakenly think of synthetic polymers polymerized from petroleum feedstocks as necessarily being nondegradable. But there are For brevity, the individual C-H bonds are not shown, and the ring structure is an abbrevi-

synthetic petroleum-based ation for the phenyl group (C6H6)

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tapioca (cassava), rice, and some other plants with annual world production well Figure 5. Chemical composition and structure of Figure 6. Chemical composition over 32 million metric tons. Approximately amylose, a major component of starch and structure of the polyester, half the total is produced in the United poly(lactic acid) (PLA) States, mainly from corn, but also from potatoes, wheat and a few other sources. As a raw material, starch is the only biopoly- mer competitive with polyethylene in terms of price. The major polymer components of starch are amylose and amylopectin. The chemi- cal structure of amylose is shown in Figure 5. In amylopectin, there are branch points in the chain where segments of chain, iden- tical in chemical structure to the main In the figure, the ring structures are simplified; everywhere that there chain, are attached to the main chain are four bonds coming from a single point, the point is meant to rep- resent a carbon atom. where the –CH2OH group is located. The ratio of amylose to amylopectin varies with source plant and affects the physical prop- NATURAL POLYMERS erties of the starch. Many polymers are also found abundant- The biodegradability of starch stems ly in nature. Natural polymers tend to be mainly from the oxygen atom connecting degradable because organisms have successive ring structures, and the oxygen evolved enzymes to attack them. Attention atom within each ring. Starch interacts has reasonably turned to such polymers as strongly with water (it is hydrophilic) and potential feedstocks for compostable plas- degrades by hydrolysis. tics. These manufactured are Moreover, starch can be softened by heat- inherently biodegradable, and as they are ing and shaped into articles by extrusion or made from renewable resources they have molding; i.e., it is . It can the additional benefit of not depleting fossil therefore be processed using the conven- resources. tional processing methods of the plastics in- Chief among the biopolymers is starch, a dustry. A significant drawback, however, is carbohydrate polysaccharide. Starch is pro- that its physical properties are not suitable duced from corn (maize), potatoes, wheat, for many applications, and its hydrophilic nature makes its physical properties (e.g., strength) dependent on relative humidity. There are many strategies for developing practical applications for starch-based plas- tics, but the strategy that has, to date, been most successful commercially has been to combine it with other compatible, biodegradable (but petroleum-derived) poly- mers to improve properties. An example of a successfully commercial- ized starch-based blend is MaterBi®, manu- factured by Novamont S.P.A., Italy. The Z- class of MaterBi products contains starch combined with the petroleum-derived poly- mer, poly(-caprolactone) (Fig. 3). suitable for, among other things, com- posting feedstock collection bags is current- ly being marketed in the United States and abroad. MaterBi satisfies the BPI-USCC compost label requirements. MaterBi is a hybrid in the sense that it consists of both a renewable component (starch) and a nonrenewable petroleum- based component (PCL). It is also a hybrid in the sense of consisting of a natural poly- mer (starch) and a synthetic polymer (PCL). These features illustrate that there is no simple correlation among the terms natural- synthetic, renewable-nonrenewable, or com- postable-noncompostable. Poly(lactic acid), or PLA (Figure 6), pro- vides yet another example. It is a synthetic polymer because it is not found in nature — even though the monomer starting materi- al, lactic acid, is found in nature. PLA is produced commercially in large-scale biore- actors through fermentation. Microrgan-

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Table 1.Origin, degradability, and properties of some plastic film products

Poly “Activated” Poly Co- Starch/Poly Polylactic (Hydroxybutyrate- Polymer Polyethylene Polyethylene (-caprolactone) Starch (-Caprolactone) Acid co-Hydroxyvalerate)

Synthetic (polymerized from nonrenewable monomer feedstocks) X X X X X Synthetic (polymerized from renewable monomer feedstocks) X Natural (obtained directly from plants) X X Natural (obtained by fermentation of renewable monomer feedstocks) X Nondegradable X Degradable/not compostable X Degradable/compostable X X X X X X Favorable film properties (strength, melting temperature, water resistance) X X X X X X

isms are fed sugar feedstocks; the microor- monomers (PHBV) (Figure 7). By varying ganisms convert the sugar feedstocks to lac- the ratio of sugar feedstocks, the relative tic acid (CH3CHOHCOOH). PLA is synthe- amounts of the two components in the sized from the isolated lactic acid by copolymer can be controlled, producing a Whatever the origin conventional means, first to produce a low- range of physical properties from brittle to molecular weight polymer. That polymer is nearly rubber-like. Examples of commercial of the polymer or then depolymerized to produce a cyclic products are Biopol® and other PHAs man- how it is made, its dimer form (lactide) which is repolymerized ufactured by Metabolix. using metal catalysts to produce a high Nodax, under development by Proctor & degradability in a molecular weight polymer. PLA is also Gamble, consists of a family of of called polylactide. hydroxybutyrate and one or more hydrox- particular PLA degrades mainly by hydrolysis even yalkanoates having a longer side chain, in the absence of microorganisms. It is com- where the side chain has anywhere from environment is postable, but the rate of biodegradability in three to 20 carbon atoms. Nodax can be con- a composting environment depends on the verted to films, sheets, molded articles, more related to its size and shape of the article (e.g. a bag, cut- foams, fibers, and nonwoven fabrics, and is final chemical both biodegradable and compostable. Table 1 summarizes some of the makeup than to its Figure 7. Schematic representation of poly(hydroxybutyrate- features of the plastics described co-hydroxyvalerate) (PHBV) here. It illustrates how both natural origin. and synthetic polymers can be biodegradable and compostable. Whatever the origin of the polymer or how it is made, its degradability in a particular environment, such as a composting facility, is more related to its final chemical makeup than to its origin. As described in the first part of this lery). The main applications of PLA to date article, the various degradable/biodegrad- have been in fiber products and clear pack- able/compostable plastics featured here are aging containers, but Biocorp North Ameri- important for both the industry ca has announced plans for combining Na- and the composting industry. Plastics man- tureWorks PLA (a product of the Cargill ufacturers continue to improve their prod- Dow Company) and MaterBi in blends to be ucts and to produce new products. With re- used for compost bags. spect to compostable plastics, many Fermentation is also involved in the pro- strategies are being followed, having the duction of polyhydroxyalkanoates (PHAs), a aim of combining superior physical proper- family of polyesters produced naturally by ties with low cost. It can be said that devel- microorganisms. In the case of PHAs, mi- opment of the nascent bioplastics industry is croorganisms produce the polymer directly well under way. from supplied sugar feedstocks. Depending on the sugar feedstock provided, and the mi- E.S. Stevens is Professor of Chemistry at State croorganism used, one or another of the fam- University of New York at Binghamton. His ily of polyesters is produced. The polymer is book, Green Plastics, An Introduction to the then isolated, purified, and processed. The New Science of Biodegradable Plastics, was re- most significant PHA produced at the pre- cently published by Princeton University Press. sent time is a copolyester comprised of hy- His web site is at greenplastics.com. He can be droxybutyrate and hydroxyvalerate reached at [email protected].

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