The Greening of Plastics

Article excerpt

Landfills are clogged with the detritus of unusable or just unwanted plastic consumer goods. Why not, asked fiber scientist Anil Netravali, make a plastic that decomposes? A plastic that, in fact, could end up in the compost heap instead of the landfill?

By early next century, if Anil Netravali has his way, we may be putting broken or outmoded pieces of the family car - the cracked tape deck holder, the broken window handle - out on the compost pile, along with last night's onion peels and apple cores. Netravali, an associate professor in the Department of Textiles and Apparel, is investigating the properties of several natural fibers that, when imbedded in a moldable, degradable matrix, may become the reinforced plastic of the future - one that's biodegradable, or a "green" composite.

Remember the scene from The Graduate when a winking uncle advised the graduate that plastics were the future? To a large extent that fictional uncle was right. Plastics did become a large part of the history of the second half of the twentieth century, both for consumers and scientists. Without plastics and other new lightweight composites, the space program would have been all but impossible. In fact, much of the research leading to the new materials and fibers of the twentieth century focused on flight and space exploration and eventually filtered down to more mundane consumer items.

The problem is that those now ubiquitous plastics tend to hang around a very, very long time, even after the product is defunct. Landfills are clogged with the detritus of unusable or just unwanted plastic consumer goods. Recycling is one way of reducing this stream of solid waste, though recycling plastic brings its own problems and environmental issues.

Why not, asked Netravali, make composites that decompose? Plastics that, in fact, could end up in the compost heap instead of the landfill? Enter green composites, as Netravali terms this new and needed material for the manufacture of consumer goods. While small groups of scientists in Europe are also investigating more environmentally friendly plastics, Netravali probably is the only researcher whose ultimate goal is a totally biodegradable plastic.

"Green composites could eventually become inexpensive composites for mass-produced items," Netravali says. "And they would be environmentally friendly because they would biodegrade."

Netravali came to Cornell, "for one year," he explains, as a postdoctoral associate in 1984, and he has been here since then as a faculty member in the college's Fiber Science Program. The green composite program began officially in 1996.

The substance that nonscientists call plastic is part of a larger group of manufactured substances technically known as polymers. Polyurethane foam, nylon, polyester, and spandex are examples. Reinforced polymer composites are versatile and provide a high strength-to-weight ratio, making them invaluable in, for instance, the space program and aviation. Voyager, the first aircraft to circle the globe without refueling, was made of a high-strength, lightweight composite. Green composites, made of biologically and chemically active rather than inert materials would, of course, be weaker and less durable. But there are many other applications for such products that are noncritical, where a biodegradable composite could be as feasible as a tougher, nonbiodegradable alternative.

"You wouldn't use a green composite in an application where, if it broke, it would be a significant loss," Netravali says. "You don't want an airplane wing to break, for instance. 'But there are many other instances of use when if a piece of plastic breaks, it is just inconvenient, not critical. Those uses would be suitable for green composites."

Current research in the fiber science laboratory at Cornell focuses on finding the most advantageous combination of fiber and resin - one that would provide a finished product that is as strong as possible, durable, affordable, and biodegradable - and the most efficient method of combining the two substances. Netravali's research includes fibers obtained from various parts of plants that have already been used by some populations, mostly in Southeast Asia, to make textiles: sugar cane and kenaf stems, pineapple leaves, and banana stems.

"Exotic, but nice," Netravali describes them. "They are strong and degradable." Silk is not used because it is too expensive and involves too much processing. Cotton fibers are short and weaker than those of the pineapple and banana plants, which produce longer fibers.

"We need to incorporate long fibers in composites to obtain maximum advantage of their strength. The ends of the fibers are not load bearing, so shorter fibers with more ends weaken the composite," Netravali explains. Each fiber end, each break, represents a weak link in a chain, and the fewer breaks the better.

The fibers will be imbedded in a matrix made of resin, just as straw was mixed into mud to make the first bricks. The fibers provide strength, the resin provides form. The resins Netravali uses in his research are polyvinyl alcohol and biopol, a commercial resin made from microorganisms. Because the fibers and resins are both degradable and contain no oil or hydrocarbons, once in the compost pile they'll break down as easily as the morning's orange peel or last night's potato peels. That's the theory. The trick is to find the perfect combination. Getting there is a series of painstaking steps that must determine, among other things, how much load a single fiber can bear after it has been treated with resin.

To demonstrate, Netravali and his graduate research assistant, Shuiyuan Luo, have set up the lab to determine the fiber interface and composite strength. Interfacial shear strength is a critical factor in obtaining better composite properties such as toughness and transverse strength. The long, whitish-blond pineapple fiber is already being used commercially as a geotextile. But how does pineapple fiber respond to resin? What is the interface strength between resin and fiber?

"In a composite, the fiber provides the strength," Netravali explains. "It, not the resin, takes the load."

First, the thread of pineapple fiber must be measured. Natural fibers vary greatly in thickness, unlike synthetic ones such as acrylic, which are made to a uniform diameter. The measurements are taken with a vibroscope, an instrument that vibrates the fiber and measures the frequency at which it resonates to determine diameter. After measuring diameter, the thread is stretched taut in an Instron until it breaks, which gives a measure of tensile strength. Next, a bead of resin is applied to the fiber and pulled out. The test is slow, measuring electronically in microscopic increments to what extent the fiber can be stretched and how much load it can bear before the bond between resin and fiber breaks down.

"We repeat these tests, twenty, thirty times," Netravali say, s, "then take an average from the different test results." By summer, the testing will have advanced beyond individual fiber and resin bonds to laminate composites. Ultimately, the fibers must be completely imbedded in resin, and five to ten layers laminated on top of each other to produce a laminate green composite.

"It will feel like, look like, and have the strength and other mechanical properties of other plastics, but it will be biodegradable," Netravali says.

The green composite Netravali envisions as the end result of his research will not just be a replacement for current plastics, which use increasingly scarce supplies of petroleum and end up clogging landfills. Green composites could also be a needed substitute for wood in many products such as packaging materials and crates.

"Wood takes twenty-five years to generate," Netravali says. "A pineapple stem takes a year. Green composites would help preserve forests."

Perhaps before the end of this century, Netravali speculates, we'll have reached an understanding of the properties of products made from green composites, and even have some products in the marketplace. "This is probably too optimistic. The main thing is how much effort we put into this program. Time is the only challenge," he says.

Well, perhaps not the only challenge. A major problem Netravali anticipates is convincing industry and consumers of the product's usefulness and the practicality of goods made from them. To some people, biodegradable and plastic are a contradiction of terms.

"We'll have to prove that green composites are worth trying, worth consideration. We'll have to overcome that perception of use," Netravali says.

And initially, products made of green composites may be more expensive than nonbiodegradable plastics. But as green composites gain acceptance and their production increases, the marketplace rule that as production increases costs decrease will help eventually to make the green composites less expensive. Graphite fibers, for example, one of the most common fibers now used for reinforced composites, cost $180 a pound when they were first developed; now the price is down to about $10 a pound.

Netravali has already opened communications - and stirred interest in his research - with several industries, including one of the world's largest chemical companies, Monsanto. "They are being very supportive," he says.

Even if demand and consumer interest are initially slow, Netravali is convinced that green composites will be a large part of the future of plastics simply because we need them. We need to cut back on use of petroleum in industry, reduce plastics in the solid waste stream, and find other natural substitutes for wood, because, as Netravali says, quoting Carl Sagan, "The world is not disposable."

"So plastics should be," he concludes.

For more information, contact Anil Netravali Cornell University Department of Textiles and Apparel 205 MVR Hall Ithaca, NY 14853 607-255-1875 ann2@cornell.edu