Switchgrass: A Bioplastic Factory
In the past, cheap oil spurred the development of petroleum-based consumer products such as plastics. Today high oil prices are driving research and development away from fossil fuel-based processes to those using renewable feedstocks. The markets for these new bioproducts are growing and companies such as Massachusetts-based Metabolix Inc. are cashing in by engineering bioenergy crops that also serve as factories for bioplastics.
Since conventional plastics originate from fossil-fuel-based feedstocks, they're built to last, clogging landfills and persisting in other environments such as rivers and oceans. With volatile oil prices, alternatives to petroleum-based plastics are becoming a real option-again.
For hundreds of years, humans have depended on natural, biological sources for the production of everyday materials ranging from fibers, dyes and waxes to coatings, lubricants and detergents. Plants and animals continue to serve as sources for the large-scale production of numerous products including wood, cork, paper, leather, cotton, hemp, wool and silk. A naturally produced, biodegradable form of plastic was first characterized in the mid-1920s by French researchers. The molecule is called polyhydroxybutyrate, more commonly referred to as PHB. It is produced by many different types of bacteria. As they grow on carbon food sources such as cornstarch or cane sugar, the bacteria store PHB as an energy reserve much like the fat that is stored in human cells. One such soil bacterium is called Ralstonia eutropha. The chemical reactions that lead to the production of PHB in R. eutropha are well understood, and scientists have figured out how to tweak growth conditions in such a way that encourages yields of PHB to reach 80 percent of the dry cell weight of the microbe.
PHB belongs to a family of polyesters called polyhydroxyalkanoates or PHAs. In general, these molecules can be produced with properties resembling their nonrenewable plastic counterparts, specifically polypropylene, which is the type of plastic used to make syrup bottles, yogurt tubs and diapers (the resin code of polypropylene, which is found on the bottom of plastic containers, is number 5). The one striking difference between PHAs and petroleum-based plastics is that PHAs are biodegradable. When these plastics are disposed of in environments populated by organisms such as bacteria, fungi and algae, PHAs are broken down to their essence-carbon dioxide and water-and recycled by the natural metabolic processes of these microbes.
The first U.S.-based company to evaluate whether PHAs (in particular PHB) could be produced from microbes on a commercial scale was W.R. Grace. The company was issued several patents for its efforts in the late 1950s and early 1960s but interest died and wasn't renewed for more than a decade. At that time-in the mid-1970s-the UK-based company Imperial Chemical Industries, which is now part of AkzoNobel, a leading global supplier of specialty chemicals, began a research and development program for PHB production through microbial fermentation. In the late 1980s, the company began commercializing a family of PHB polymers under the trade name Biopol. Although the production of Biopol was not cost-competitive with plastics derived from petroleum products, the bioplastic was used in shampoo bottles and consumers who wanted all-natural, high-end products accepted the price.
In 1990, the agriculture and pharmaceutical business of ICI was spun off as Zeneca Ltd., and in 1996, Monsanto Co. acquired the Biopol business from Zeneca. In an attempt to produce PHAs cost-competitively with conventional plastics, Monsanto aimed to use plants rather than microbes as the factories for biodegradable plastics production. This new direction for research and development was partly inspired by the work of scientists from Michigan State University led by Chris Somerville. In 1992, Somerville's team reported in the journal Science that PHB could be produced in the leaves of a plant called Arabidopsis thaliana. To accomplish this, the researchers modified two genes from the bacterium R. eutropha and engineered the plant to express these genes and produce PHB. The plant was able to grow and develop normally and accumulate PHB to as much as 14 percent of the plant's dry weight. This form of PHB, however, was brittle and not useful for most applications but the research provided a proof of concept that other scientists have since expanded upon.
Some of these "other scientists" including researchers at Monsanto in the late 1990s devised a pathway for producing Biopol in several different plants including Arabidopsis and rapeseed. In 2001, Metabolix Inc. purchased Monsanto's Biopol assets. The company had already developed a range of PHA products from microbial fermentation processes and was interested in producing them directly in plant crops. These efforts were recognized in 2005 when the company was awarded a U.S. EPA Small Business Award through the agency's Presidential Green Chemistry Challenge Awards Program, which provides a competitive incentive for the creation of environmentally friendly chemicals and processes.
The company then teamed with Archer Daniels Midland Co. in April 2007 to commercialize Mirel bioplastic through a joint venture called Telles, which is the name for the Roman goddess of the Earth. The plastics can be used in a variety of applications including compostable bags, business equipment, packaging, consumer products such as cosmetics and gift cards, and in agriculture horticulture, marine and water applications. The first commercial-scale plant for the annual production of 110 million pounds of Mirel plastics is now being built adjacent to ADM's wet corn mill in Clinton, Iowa.
In addition, the company recently announced the results of greenhouse trials of switchgrass plants engineered to produce significant amounts of PHA bioplastics in leaf tissues. "Metabolix has been developing technology to produce PHA polymer in switchgrass for over seven years," says Oliver Peoples, the company's chief scientific officer.
Switchgrass is an attractive feedstock option for the production of cellulosic ethanol because it is a tall-growing, nonfood crop, and the prospect of additional revenue from biodegradable plastics should make it even more appealing. "A key corporate goal has been to develop value-added industrial crops such as oilseeds, sugarcane and switchgrass," explains Richard Eno, president and chief executive officer of the company. "This proof of concept in switchgrass is an important milestone as we develop commercialization strategies for our plant science activities." A detailed description of the research is forthcoming in Plant Biotechnology Journal.
There are hurdles to overcome, however, including: how to harvest the plastic from the plant; the cost of recovering plastic from plant leaves; and how to best dispose of these plastics in composting or other bioconversion facilities. But the incentive for breaking down these barriers is sweetened by the market outlook for bioplastics; worldwide consumption of biodegradable polymers increased from 31 million pounds in 1996 to an estimated 150 million pounds in 2001, and since 2004, the consumption of bioplastics has increased three- to four-fold. In Europe alone, the potential for bioplastics is estimated to be at least 2 million tons per year and global production capacity is expected to exceed 750,000 tons each year by 2010.
The numbers suggest that the future is bright for bioplastic suppliers such as Metabolix in the United States, Biomer in Germany and Natureplast in France. Likewise, biomass-to-ethanol producers may find something to cheer about in Metabolix's research, which opens the possibility for another revenue stream from cellulosic ethanol production. "This result validates the prospect for economic production of PHA polymer in switchgrass, and demonstrates for the first time an important tool for enhancing switchgrass for value-added performance as a bioenergy crop," Peoples says.
Jessica Ebert is a freelance writer for Biomass Magazine. Reach her at firstname.lastname@example.org.