Science / Medicine : The Greening of Plastic Bags : Environment: New efforts to make plastic products decompose more readily hold promise. But environmentalists are concerned about the possible byproducts of such decay.
Take a stroll through the supermarket aisles and check out the section with garbage bags. Look at the disposable diapers and don’t miss the shopping bags the checkers load your groceries into. You can find at least one brand among all these plastic products claiming to be “biodegradable,” or as the advertisers say: These products are “environmentally friendly.”
What does this mean? The scientists who are designing these new plastics hope they will decompose at a far quicker rate than their traditional counterparts (without, of course self-destructing on the way home from the store).
Manufacturers have quickly embraced the degradable concept, hoping to capitalize on the new environmental consciousness. And researchers say that products on the shelves today represent only the beginning of a new technology. In the lab, they’re taking the concept to its limits, developing a new twist to the plastic revolution of the post-war era.
But these advances are triggering their own set of concerns among environmentalists who worry about the byproducts of decomposing plastic.
When World War II ended, technology began to advance at a dizzying rate and scientists refined a new wonder product--plastic. It was strong; it was durable; it would last forever, it seemed. The material steadily gained popularity. But today these exact qualities which allowed it to capture the market are now being partly blamed for our mounting garbage crisis.
Microorganisms, which readily degrade food and paper, rarely penetrate plastics. Because these synthetic products, like their parent compound petroleum, repel water, they provide an inhospitable environment for bacteria.
The synthetic polymers, as plastics are called, are composed of exceedingly long chains of carbon and hydrogen. For example, polyethylene, one of many variations on the plastic theme, strings together 100,000 carbon-hydrogen molecules. When bacteria finally begin decomposing the materials, they attack only at the ends of a chain and not throughout the length of it. Because of this inefficient mode of operation, the microbes progress at a painstakingly slow rate.
With these facts in mind, scientists devised techniques to accelerate the breakdown process. First, in the mid-1980s, they introduced cornstarch into traditional petroleum-based plastics to accelerate the breakdown process. Cornstarch serves as a type of bait for the bacteria, enticing them into the synthetic matrix.
But cornstarch, like water, does not mix readily with these oil-based substances. Ramani Narayan, from the Michigan Biotechnology Institute in Lansing and professor of chemical engineering at Michigan State University, has developed one method for combining the two substances. He has synthesized what he calls a “graft copolymer,” a molecule containing both starch and plastic chemically linked together.
Using this method in the laboratory, Narayan has been able to synthesize plastic composed of 15% to 30% starch, an increase over the commercial products, which contain 6% starch. Consumers should soon see plastics available with more than 6% starch, Narayan said.
In addition to adding starch, scientists have incorporated catalysts into degradable plastics. The compounds, made from iron, nickel, manganese or other metals, initiate degradation by breaking the carbon-hydrogen chains into several smaller fragments.
Consequently, this creates additional chain ends for microbes to attack. Narayan says it’s like “taking a long rope and slowly nibbling away at one end of the rope. It’s going to take a long while before you can chew up everything. But if you cut it into four or five pieces, then each piece can now be chewed up and . . . and chewed up faster.”
What keeps the catalyst from acting before the plastics are discarded? Some products on the market today have a short-acting stabilizing agent, which should allow the material to remain intact long enough to serve its purpose. Essentially, however, the products begin degrading as soon as they are synthesized.
But Narayan has developed an alternative still in the testing stage. He uses what he calls an environmental trigger. He hides the catalyst in the starch. Once the plastic has been disposed of in a compost pile or other biologically active environment, microbes from the compost release the catalyst as they degrade the starch.
With cornstarch and catalysts at work accelerating the decomposition process, plastics are no longer granted immunity from decay. That sounds like an environmentalist’s dream, but ironically, while television commercials boast the ecological merits of degradable plastics, environmental groups are fighting their development. Several issues have caught their attention.
In particular, many people pose questions about the byproducts of degradation. What do these plastics break down into? “There’s the $64,000 question,” responds Michael Gould, research leader for biopolymer research at the U.S. Department of Agriculture in Peoria, Ill. “I think there isn’t an answer at this point or there are a lot of answers.”
Gould says very little data exists about the byproducts. He is concerned that the new plastics have been marketed prematurely. If you put biodegradable plastics into the environment, Gould points out, “the first thing they do is disintegrate. . . . The bacteria come along and eat up the starch, and the plastic disintegrates into little pieces, At that point you can’t gather it back up again. You have this plastic dust everywhere and it’s continuing to undergo chemical and biological degradation and no one knows for sure what the products are.”
Gould is not convinced this “plastic dust” is dangerous, he simply believes adequate testing should have taken place before, rather than after, the products hit the market.
Though Narayan doesn’t dispute the need for testing, he thinks Gould advocates unnecessary caution. He says standard biochemical mechanisms should break polyethylene into short carbon-hydrogen chains which are identical to the fatty acids found in food.
While there is minimal data to prove Narayan’s theory, Michael Tempesta, a researcher at the University of Missouri, has produced some evidence supporting that notion. Studying degradation in a simulated sewage plant, Tempesta found that plastics decomposed into waxy materials, similar to the fatty acids described by Narayan. However, sewage plants differ from landfills and it is not certain that the plastics would react in the same way when placed under different conditions.
Those fighting the development of biodegradable plastics also voice concern that these products cannot be recycled. “The reason that you can’t do it,” Gould says, “is that there has never been a need to do it before, so no technology exists.”
Joining the environmentalists in the battle against the new plastics are representatives from Keep America Beautiful, the national anti-litter organization. They fear that consumers will read the word biodegradable and think they can throw a plastic bag out the window and it will simply disappear. Over a long period, it will ultimately decompose--but in the meantime, it contributes to the unsightly litter on the roadways.
Indeed, the very use of the word degradable has come under fire. Seven states, including California, recently filed suit against Mobil Chemical Co., the maker of Hefty plastic trash bags, for allegedly using false advertising to sell its so-called degradable bags. The company denied that its labeling is misleading but has agreed to remove claims of degradability from its packaging until common industry and government guidelines can be established.
How long before that plastic bag does disintegrate? No one knows the precise answer. Today manufacturers can claim degradability of their product without evidence to back it up, because no standard definition for the word exists.
Anything and everything will degrade--eventually. In a landfill, where the majority of trash ends up, very little biological activity takes place, because of inadequate air, fluids, light and circulation of microorganisms. Practically nothing, including such biodegradable materials as paper, food and yard waste, decomposes in the landfill, and plastic is not an exception.
Gould believes that the commercial manufacturers of these new plastics are leading the public down the wrong path by promoting them as a major solution to environmental problems. “I think it’s a mistake to tie it to an environmental issue,” he says. Rather, Gould says, these plastics can be used in specific cases to contribute to enhancing the overall environmental consciousness.
For example, when replanting a forest, plastic planter pots are often used. Someone needs to go back and collect these pots, adding cost to the operation. A biodegradable container would alleviate this additional expense.
Mulch films for farming could also be made to degrade over the lifetime of a single crop. And if garbage bags for handling yard waste biodegraded, the entire package could be composted, without having to remove the bag.
Aside from the biodegradable advantages, Gould points out why he began researching cornstarch plastics in the first place: to reduce dependence on imported oil. “Half the oil we use in this country is imported,” he says. If we can make a dent in that, that’s an enormously important national goal. Everybody forgets the ‘70s and OPEC . . . when our research on this started.”
Narayan feels the issues have been muddled. “You have two things here,” he says. “One is concept and one is technology. You may critique the technology.” But first it must be determined if the concept is right or wrong. Both Narayan and Gould are very enthusiastic about the concept of biodegradable plastics when used in the appropriate situations. The technology, they say, must still be developed and advanced.
“Ultimately . . . the consumers will find out the truth,” says Gould.
“What we’re worried about is if people make promises about degradable plastics and their function in landfills and they don’t deliver, the (public will) turn thumbs down on it and that will be it. I think it’s too good of a technology to just throw away.”
Breaking the chain: Making plastics more degradable
Making plastic:
Polyethylene, the compound used in plastic bags, is made of small hydrocarbon molecules thatare formed into long strings by a process called polymerization.
Ethylene--C2H4: * Colorless, orderless gas * Most versatile of the basic hydrocarbon molecules. 1. Catalysts force double bond to open. 2. Molecules realign 3. Carbon atoms link in long single-bonded chain
Polyethylene--a long and lasting plastic: Polyethylene chains consist of about 100,000 ethylene molecules Bacteria decompose the very long chains from the ends, very slowly
Adding starch to speed decomposition: Cornstarch is ‘grafted’ into the long molecule chains to speed up decomposition Bacteria eat the starch, breaking down the long molecucle chains, so that the plastic falls apart
