Scientists are developing new plastics so that shopping bags, packaging materials and other products retain their strength for a specific period of time and then disintegrate.

Existing plastics take 200 to 400 years to degradate. Degradable plastics, by contrast, would disappear within a few months or years."The impact on the consumer already is being felt," said Ramani Narayan of Purdue University's laboratory for renewable resources engineering.

"Within the next year we'll see more and more biodegradable plastic because of legislative action and public concern over the condition of our solid waste disposal system."

Narayan was among dozens of scientists who reported progress toward the enduring dream of practical controlled-life plastics this week. The reports were presented here at an American Chemical Society meeting.

Environmentalists have long urged laws to ban non-degradable plastic products such as fast-food packaging; certain wrappings for meat, cheese and fruit; yokes that hold 6-packs of beer and soft drinks together; egg cartons; liquor bottles; and grocery bags.

The corn industry also is lobbying for greater use of degradable plastics, partly as a means of creating a whole new market for surplus corn.

Some degradable plastics are made by incorporating about 6 percent corn starch into the polymer backbone of the plastic. When the starch is consumed by soil bacteria, the plastic weakens and begins to break apart.

One of the advances was reported by Gerald Griffin, who developed the starch-plastic technology used in more than 500 million degradable plastic shopping bags in Europe since 1974.

"I have now greatly improved this technology to give a reliable `clock' action that embrittles the material and renders it accessible to biological breakdown," Griffin said. He is director of Ephron Industries Ltd., a British firm that develops and produces degradable plastics.

Griffin said that the controlled-life plastics, which are starch-polymer composites, retain virtually all of their strength during their designed "working period." Then there is an abrupt loss of physical strength, with the plastic breaking into particles that decompose like vegetable or wood wastes.

Marketing rights for the new plastics have been sold to an American firm, Archer Daniels Midland Co., he said. The new plastics would cost about 10 percent more than conventional plastics. But Griffin said their use in packaging, for example, would result in only slight increases in the cost of a product.

Plastics have numerous advantages - strength, durability, lightness, flexibility and low cost - that have led to their widespread use. The United States alone produces about 50 billion pounds of plastics each year. Plastic packaging materials account for about 25 percent of the total, 12 billion pounds.

Many of the advantages of plastics also contribute to problems with their disposal as litter or municipal waste.

Plastic litter is unsightly and in the ocean poses a growing environmental problem. Marine biologists have reported as many as 46,000 pieces of plastic litter per square mile at sea. About 10 miles of plastic netting is left floating in the oceans each day from Asian commercial fishing operations alone.