What Makes Green Plastics Green?
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24-27 2/9/04 8:56 AM Page 24 BACKBONES OF POLYMER SCIENCE WHAT MAKES GREEN PLASTICS 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 polymers — disappear in the form of envi- ety of factors perhaps is the extremely en- Gronmentally acceptable degradation prod- vironmentally stable polyolefins: polyethy- ucts. How they function and disappear is lene, polypropylene, poly(vinyl chloride), both petroleum- largely a matter of their chemistry. Several and polystyrene. (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 plastic products als. This second part de- petroleum-based feed- being marketed scribes the chemical stocks. Polyethylene (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 ethylene properties, and by processing. Polymers monomer in the polymer chain. Single are long chained molecules 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). 24 BIOCYCLE MARCH 2003 24-27 2/9/04 8:56 AM Page 25 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 polyester citrant (resistant) with respect to environ- by virtue of containing the ester 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 enzymes 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 starch (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- copolymer. 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 polymerization 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 plastic mulch, 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) BIOCYCLE MARCH 2003 25 24-27 2/9/04 8:56 AM Page 26 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.