first approach, which turns sugar from corn and other plants into a plastic called polylactide (PLA). Microorganisms transform the sugar into lactic acid, and another step chemically links the molecules of lactic acid into chains of plastic with attributes similar to polyethylene terephthalate (PET), a petrochemical plastic used in soda bottles and clothing fibers.
Looking for new products based on corn sugar was a natural extension of Cargill's activities within the existing corn-wet-milling industry, which converts corn grain to products such as high-fructose corn syrup, citric acid, vegetable oil, bioethanol and animal feed. In 1999 this industry processed almost 39 million tons of corn—roughly 15 percent of the entire U.S. harvest for that year. Indeed, Cargill Dow earlier this year launched a $300-million effort to begin mass-producing its new plastic, Nature-Works™ PLA, by the end of 2001 [see box on page 40].
Other companies, including Imperial Chemical Industries, developed ways to produce a second plastic, called polyhy-droxyalkanoate (PHA). Like PLA, PHA is made from plant sugar and is biodegradable. In the case of PHA, however, the bacterium Ralstonia eutropha converts sugar directly into plastic. PLA requires a chemical step outside the organism to synthesize the plastic, but PHA naturally accumulates within the microbes as granules that can constitute up to 90 percent of a single cell's mass.
In response to the oil crises of the 1970s, Imperial Chemical Industries established an industrial-scale fermentation process in which microorganisms busily converted plant sugar into several tons of PHA a year. Other companies molded the plastic into commercial items such as biodegradable razors and shampoo bottles and sold them in niche markets, but this plastic turned out to cost substantially more than its fossil fuel-based counterparts and offered no performance advantages other than biodegradability. Monsanto bought the process and associated patents in 1995, but profitability remained elusive.
Many corporate and academic groups, including Monsanto, have since channeled their efforts to produce PHA into the third approach: growing the plastic in plants. Modifying the genetic makeup of an agricultural crop so that it could synthesize plastic as it grew would eliminate the fermentation process altogether. Instead of growing the crop, harvesting it, processing the plants to yield sugar and fermenting the sugar to convert it to plastic, one could produce the plastic directly in the plant. Many researchers viewed this approach as the most efficient—and most elegant—solution for making plastic from a renewable resource. Numerous groups were (and still are) in hot pursuit of this goal.
In the mid-1980s one of us (Slater) was part of a group that isolated the genes that enable the bacteria to make
Corn or other plants grown, harvested and delivered to factory plastic. Investigators predicted that inserting these enzymes into a plant would drive the conversion of acetyl coenzyme A—a compound that forms naturally as the plant converts sunlight into energy— into a type of plastic. In 1992 a collaboration of scientists at Michigan State University and James Madison University first accomplished this task. The researchers genetically engineered the plant Arabidopsis thaliana to produce a brittle type of PHA. Two years later Monsanto began working to produce a more flexible PHA within a common agricultural plant: corn.
So that plastic production would not compete with food production, the researchers targeted part of the corn plant that is not typically harvested—the leaves and stem, together called the stover. Growing plastic in stover would still allow farmers to harvest the corn grain with a traditional combine; they could comb the fields a second time to remove the plastic-containing stalks and leaves. Unlike production of PLA and PHA made by fermentation, which theoretically compete for land used to grow crops for other purposes, growing PHA in corn stover would enable both grain and plastic to be reaped from the same field. (Using plants that can grow in marginal environments, such as switchgrass, would also avoid competition between plastic production and other needs for land.)
The Problem: Energy and Emissions
Researchers have made significant technological progress toward increasing the amount of plastic in the plant and altering the composition of the plastic to give it useful properties. Although these results are encouraging when viewed individually, achieving both a useful composition and high plastic content in the plant turns out to be difficult. The chloroplasts of the leaves have so far shown themselves to be the best location for producing plastic. But the chloroplast is the green organelle that captures light, and high concentrations of plastic could thus inhibit photosynthesis and reduce grain yields.
The challenges of separating the plastic from the plant, too, are formidable. Researchers at Monsanto originally viewed the extraction facility as an adjunct to an existing corn-processing plant. But when they designed a theoretical facility, they determined that extracting and collecting the plastic would require large amounts of solvent, which would have to be recovered after use. This processing infrastructure rivaled existing petrochemical plastic factories in magnitude and exceeded the size of the original corn mill.
Given sufficient time and funding, researchers could overcome these technical obstacles. Both of us, in fact, had planned for the development of biodegradable plastics to fill the next several years of our research agendas. But a greater concern has made us question whether those solutions are worth pursuing. When we calculated all the ener-
gy and raw materials required for each step of growing PHA in plants—harvesting and drying the corn stover, extracting PHA from the stover, purifying the plastic, separating and recycling the solvent, and blending the plastic to produce a resin—we discovered that this approach would consume even more fossil resources than most petrochemical manufacturing routes.
In our most recent study, completed this past spring, we and our colleagues found that making one kilogram of PHA from genetically modified corn plants would require about 300 percent more energy than the 29 megajoules needed to manufacture an equal amount of fossil fuel-based polyethylene (PE). To our disappointment, the benefit of using corn instead of oil as a raw material could not offset this substantially higher energy demand.
Based on current patterns of energy use in the corn-processing industry, it would take 2.65 kilograms of fossil fuel to power the production of a single kilogram of PHA. Using data collected by the Association of European Plastics Manufacturers for 36 European plastic factories, we estimated that one kilogram of polyethylene, in contrast, requires about 2.2 kilograms of oil and natural gas, nearly half of which ends up in the final product. That means only 60 percent of the total—or 1.3 kilograms—is burned to generate energy.
Given this comparison, it is impossible to argue that plastic grown in corn and extracted with energy from fossil fuels would conserve fossil resources. What is gained by substituting the renewable resource for the finite one is lost in the additional requirement for energy. In an earlier study, one of us (Gerngross) discovered that producing a kilogram of PHA by microbial fermentation requires a similar quantity— 2.39 kilograms—of fossil fuel. These disheartening realizations are part of the reason that Monsanto, the technological leader in the area of plant-derived PHA, announced late last year that it would terminate development of these plastic-production systems.
The only plant-based plastic that is currently being commercialized is Car-gill Dow's PLA. Fueling this process requires 20 to 50 percent fewer fossil resources than does making plastics from oil, but it is still significantly more energy intensive than most petrochemical processes are. Company officials anticipate eventually reducing the energy requirement. The process has yet to profit from the decades of work that have benefited the petrochemical industry. Developing alternative plant-sugar sources that require less energy to process, such as wheat and beets, is one way to attenuate the use of fossil fuels. In the meantime, scientists at Cargill Dow estimate that the first PLA manufacturing facility, now being built in Blair, Neb., will expend at most 56 megajoules of energy for every kilogram of plastic—50 percent more than is needed for PET but 40 percent less than for nylon, another of PLA's petrochemical competitors.
The energy necessary for producing plant-derived plastics gives rise to a second, perhaps even greater, environmental concern. Fossil oil is the primary resource for conventional plastic production, but making plastic from plants depends mainly on coal and natural gas, which are used to power the corn-farming and corn-processing industries. Any of the plant-based methods, therefore, involve switching from a less abundant fuel (oil) to a more abundant one (coal). Some experts argue that this switch is a step toward sustainability. Missing in this logic, however, is the fact that all
Growing PHA in corn stover would enable both grain and plastic to be reaped from the same field.
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