No free lunch.
Bacteria are living organistas that need a constant food supply. Like humans, when food is abundant they will store the excess for leaner times. We store the excess in the form of fat, but certain bacteria store it as polyhydroxybutyrate. This is a type of polyester that can be extracted from the bacteria and processed into items ranging from containers and wraps to fibres and sutures. It has properties similar to polypropylene, a widely used petroleum-based plastic. There are, however, two major differences. While polypropylene floats, PHB sinks, and while polypropylene is environmentally persistent, PHB can be gobbled up by microbes that convert it to water and carbon dioxide. A cup tossed into the ocean will sink and be degraded in the sediment on the ocean floor. Scientists have known about bacterial production of PHB since 1926, but efforts to exploit it as a commercial material earnestly began only in the 1970s in response to increasing oil prices. Much of the pioneering work in this area was carried out by R. H. Marchessault, FCIC. "Biopol," the first version of PHB to be marketed, was extracted from cells of the bacterium Alcaligenes eutrophus, which had been nurtured on sugar. The extraction process is not simple and does rely on using methylene chloride, a solvent that is not exactly environmentally friendly.
By about 1990, shampoo bottles and utensils made of PHB began to appear on store shelves, but sales did not really take off. Why didn't consumers flock to a biodegradable material made from a renewable resource? Simple. Price! Items made from PHB were far more expensive than those made from polypropylene. While many people have emotional ties to the environment, they have stronger ties to their wallets. Could there be a cheaper way to produce polyhydroxybutyrate? An interesting idea was first described by researchers at MIT in a patent application back in 1989. How about taking the genes from bacteria that give the instructions for converting excess nutrients into PHB, and through recombinant DNA technology, introducing these genes into a plant? Rather than oil refineries, farms could then produce the material needed to make plastics.
The idea turned out to be workable. Cress plants fitted with bacterial genes cranked out PHB, although the yields were poor. Metabolix, an American company is experimenting with tobacco and switch grass. Wouldn't it be something ii tobacco could redeem itself by supplying us with a useful substance? It seems though that switch grass, a very hardy, fast-growing plant, is more likely to be commercially viable. This is the same plant that President Bush mentioned in his 2006 State of the Union Address as a candidate for producing large quantities of biomass that can be fermented into ethanol. Imagine growing a plant capable of supplying us with fuel and plastics at the same time! Still, we have to remember that farming on such a large scale still requires the use of fossil fuels for producing fertilizers, pesticides, and for running farm equipment. There is no free lunch.
While the potential for generating plastics from plants is exciting, Metabolix is currently using another technology. The genes that code for the production of PHB, instead of being inserted into plants, are inserted into E. coli bacteria. These bacteria, which can be made to multiply very quickly, then yield large amounts of polyhydroxybutyrate. Using E. coli as little factories to produce commercial substances is not a novel idea. Drugs such as insulin for diabetes, and tissue plasminogen activator for dissolving blood clots after a heart attack are now routinely made by introducing the appropriate genes into E. coli. bacteria.
Metabolix is certainly not the only company interested in exploiting the potential of plants to furnish plastics. Soyol has focused on polyurethane, one of the most versatile plastics. Polyurethane can be used to make flexible foams for pillows, solid wheels for roller blades, varnishes for furniture, glues, surfboards, insulation for walls and fridges, side panels for farm machinery, tires, and a host of other products. Like other plastics, polyurethanes are polymers, meaning they are giant molecules composed of individual units, like a chain is composed of links. In this case the links are compounds called diisocyanates and polyols, which are made from petroleum products. We will at some point run out of petroleum, but we will not run out of soybeans. And the major component of polyurethanes, the polyols, can be made from soy oil. A simple chemical reaction can convert soy oil into "epoxidated soy oil," which in turn can readily be changed into the required polyols. Using these to make polyurethane products is not only a theoretical possibility, it is a practical reality. Molded seats for tractors, panels for combines, office furniture, carpet backing, pillows, and foam insulation are already being produced from soy oil. Furthermore, these soy polyols cost less than the ones that derive from petroleum, and require less energy to produce. Eventually they have the potential to replace petroleum polyols in all polyurethanes. The real beauty is that the resource is renewable. Obviously, chemical ingenuity can solve some of our problems. And luckily, that is also a renewable resource.
Popular science writer, Joe Schwarcz, MCIC, is the director of McGill University's Office for Science and Society. He hosts the Dr. Joe Show every Sunday from 3:00 to 4:00 p.m. on Montreal's radio station CJAD. The broadcast is available on the Web at www.CJAD.com. You can contact him at email@example.com.
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|Publication:||Canadian Chemical News|
|Date:||Jun 1, 2006|
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