Degradable Polymers

By Gerald D. Biby, The University of Nebraska-Lincoln

We make almost 100% of today's plastics from oil or natural gas. As concern mounts about the potential effects of petroleum-based plastics on the environment and the increased dependence on oil and gas imports, degradable polymers could become an important piece in the solution puzzle. One option may be to make plastics directly from corn. One hundred percent of the people who have tested prototype cards made of a corn product called "Mazin" could not tell they were made from corn instead of from a petroleum product.

Waste disposal issues

According to a study by the U.S. Environmental Protection Agency (EPA), plastics alone account for roughly 21% (by volume) of the nearly 200 million tons of municipal waste generated each year in the USA. Integrated waste management practices including recycling, source reduction of packaging materials, composting of degradable wastes, and incineration may help bring waste disposal under control. However, this will not solve the importation of petroleum products.

Instead of making plastics from conventional petroleum products, they can be made from lactic acid. Lactic acid is produced (via starch fermentation) as a co-product of corn wet milling, which can be converted to polyactides (PLA), the material from which Mazin is made. Or it can be produced using the starch from food wastes, cheese whey, fruit or grain sorghum. By using lactic based plastics, the U.S. could save 50 - 90 trillion Btu¹s per year. This is equivalent to 9 - 15 million barrels of oil. Other estimates have placed the savings as high as 600 trillion Btu's to 32 million barrels of oil per year.

Some plastics need to be durable like the parts in a car. Yet, there are many plastics that are only used once or have a limited life before being thrown into a landfill or incinerator. Plastics, unlike most organic polymers, are poorly degraded (if at all) by microbes. Environmentally degradable polymers are one potential solution to replacing petroleum-based polymers. Potential uses for these polymers are plastics intended for one-time or limited use, for example those used as fast-food wrappers and water-soluble polymers in detergents and cleaners, and for use in the printing industry.

The question of how best to dispose of domestic waste is now becoming increasingly important. In Europe and Japan there are few sites left that can be used for landfill. Since the main bulk of domestic waste is made up of plastics there is a great deal of interest in recycling plastics and in producing plastic materials that can be safely and easily disposed of in the environment. Current degradable polymers are designed to degrade either biologically, photolytically, or chemically, depending on the disposal environment that they will encounter after use.

Ideally, degradation pathways should ultimately lead to the bio conversion of the polymer into carbon dioxide (aerobic) or carbon dioxide/methane (anaerobic) and biomass. The goal is to reduce the environmental loading of polymeric waste through biological recycling as the polymers degrade and reduce the need to import petroleum products.

Environmental laws and regulations and consumer demands for environmentally friendly products are beginning to have an impact on the use of degradable polymers. As a result degradable polymers, when combined with other degradable plastics, will begin playing a crucial role in helping to solve our waste disposal problems and reducing petroleum imports.

Properties of degradable polymers

These new polymers are truly degradable, and may be used in many applications. Some are impervious to water and retain their integrity during normal use but readily degrade in a biologically rich environment. However, full biodegradability can occur only when these materials are disposed of properly in a composting site or landfill. Today, there are three major degradable polymers groups that are either entering the market or are positioned to enter the market. They are polyactides (PLA), polyhydroxybutyrate (PHB) and starch-based polymers.

First-generation degradable polymers, which were largely commercialized in the 1980's, did not satisfy the public's view of complete degradation. Second-generation polymers began being introduced during the last five years. Although they are degradable, the industry has not promoted them. One reason is these new polymers are higher priced than the commodity polymers typically in use in plastics applications. However, producers are currently working toward bringing down the price of degradable polymers by increasing production capacity and improving process technology. Five years ago PLA and PHB sold for more than $25.00 USD per pound. Today PLA, depending on quantities, is between $1.50 USD and $3.00 USD per pound and PHB, in large quantities is near $4.00 USD per pound.

Though recent advances in production technology have helped lower prices of some degradable resins, prices are still higher than for petroleum-based plastics. This suggests that in the short term, companies making degradable polymers will continue to focus on niche markets. The long-term outlook for marketing these products is still uncertain, but is likely to depend on global regulatory developments and continued improvements in cost-reducing technologies. We expect future prices to fall to roughly $1 USD per pound as production capacity increases.

Niche application areas

Due to their high prices, most current applications for degradable polymers are in niche areas with unique environmental considerations. Although these amounts are high when compared with conventional resins like PVC or styrene, they can degrade in the environment. Another important feature is that they do not typically contribute to environmental pollution during their manufacture.

In 1993, total demand for degradable polymers (including cellulosic) in the United States, Western Europe and Japan reached 25 million pounds (11 thousand metric tons) valued at $50 million USD. Consumption in the United States and Western Europe was nearly equal, while Japan accounted for less than 5% of 1993 demand. We project U.S. demand to increase to 1.6 billion pounds in the year 2000, driven by improved new properties, emerging industry-wide standards and declining prices.

Future growth

Several factors, besides cost, will be important in determining the future growth of degradable polymers. One major obstacle is a lack of a composting infrastructure. Large-scale composting would provide the ideal disposable environment for spent degradable. Western Europe has made progress toward developing a composting infrastructure, but the infrastructure is lacking in the United States and Japan. In the past, legislation in Western Europe, and to a lesser extent in the United States, has helped to spur demand for degradable materials. Future legislation will depend not only on the environmental awareness of politicians but also on their perceptions of how degradable polymers may affect the development of plastics recycling.

Many biobased resins can be processed on conventional plastic molding equipment and, depending on the properties of the specific resin, can be converted into many types of plastic products. These include, but are not necessarily limited to single use items like: compost bags (lawn and leaf); disposable food-service items (e.g., cutlery, plates, cups); packaging materials (e.g., loose fill, films); but they also include more durable products like: coatings (e.g., laminations, paper coatings); and other injection molded and sheet extruded products (phone and other cards and sheet printed plastics). The desired properties of the product generally determine the relative amounts of additives used in the resin.

About Mazin and PLA polymers

PLA is a hard material, similar in hardness to acrylic plastic with a hardness on the Rockwell H Scale of more than 60. Therefore, when we extrude a pure PLA sheet and a die is used to cut out the product being printed, the cutting edge of the die wears out rapidly. In addition, due to the hardness, the PLA fractures along the edges creating a product that cannot be used. To overcome these limitations PLA has to be compounded with materials to adjust the hardness and eliminate the fractures when the material is die cut. Mazin¹s hardness (which can be altered easily) is approximately 20.6 on the Rockwell H Scale. Printers whom have worked with it have found the stiffness of the card acceptable and die wear almost eliminated.

Specifically, Mazin is composed of PLA (similar to those used in surgical sutures) and an additive polymer created from starch and other degradable monomers. Both comply with the positive list in the "ECC Commission Directive of 23 February 1990 relating to the Plastic Materials and Articles intended to come in Contact with Foodstuffs" - 90/128/EEC, published in the official Journal of the European Communities of 21 March 1990. More specifically, the base polymer contains substances included in Annex 2 - Section A therein, or by their mixture, or by substances deriving from monomers included in Annex 2 - Section A, and by a component included in Annex 2 - Section B.

The additive polymers also are certified to comply with the requirements of European Standard EN71 - "Safety of Toys" - Part 3: "Chemical Properties." Moreover, the polymer is free of the following chemical substances, which are not used in the manufacture of the base polymer: 2-naftilamina and its salts (CAS: 91-59-8), 4-aminodifenile and its salts (CAS: 92-67-1), Benzidina and its salts (CAS: 92/87-50) and 4 Nirodifenile (CAS: 92-93-3). To the best of our knowledge, Mazin has no legal or toxicological problems related to its use, manufacture or disposal.

PLA polymers are generally derived by fermenting carbohydrate crops such as corn, wheat, barley, cassava, and sugar cane. The process involves the fermentation of sugars to produce lactic acid, which is converted to PLA through low-cost, high-yield catalytic polymerization. PLA-based polymers are completely degradable under compost conditions. Although PLA is not water soluble, microbes in marine environments can also degrade it into water and carbon dioxide. PLA-based resins can be modified to adapt to many applications, from disposable food-service items, sheet extrusion, or coatings for paper.

The largest producer of PLA based resins is Cargill, Inc., a privately held company with 75,000 employees, in Minneapolis, Minnesota. Their PLA-based resins, called EcoPLA, form the backbone of Mazin. One other U.S. company is also producing PLA. Commercial scale production capacity is nearing 12 million pounds annually. Subject to demand, both companies have stated they will be increasing production as markets develop.

Other degradable polymers

There are two additional degradable polymers groups that need to be identified. Two are made from starch. Mater-Bi, available from Novamont, is manufactured primarily from corn or potato starch, along with smaller amounts of food-grade additives (not intended for human consumption). This resin is suitable for manufacturing injection molded pieces, films (for bags) and a starched based loose fill packaging material. Novamont resins degrade in an active biological environment like PLA. However, unlike PLA they can require a year or more to degrade. Current sole production capacity is in Italy and imported into the United States. Prices range from $2.25 USD to $2.90 USD per pound. Commercial scale production is available at this time.

Novon International also produces Novon, a starch-based resin that contains performance enhancing additives, such as synthetic linear polymers, plasticizers, and compounds that trigger or accelerate degradability. Novon is intended to be mixed with synthetic polymers to create a plastic product, while making the product more degradable than traditional synthetic plastics. A typical product would contain about 43 % starch, 50 % synthetic polymer, and 7 percent proprietary ingredients. Typical application is for agricultural mulch films. Current pricing for Novon is about $1.60 USD to $1.70 USD per pound. Current sole production capacity is in New Jersey. Commercial scale production is available at this time.

About Biopol

The third major type of commercially available biobased polymer consists of polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) polymers, which were initially produced by Zeneca Bio Products (now Monsanto) by fermenting a sugar feedstock (glucose is currently being used) with a naturally occurring microorganism. Biopol is technically a family of linear polyesters of three hydroxybutyric and three hydroxyvaleric acids produced in nature from the fermentation of sugars by the bacterium Alcaligenes Eutrophus.

Biopol is stable when stored in air and is quite stable when stored even in humid conditions. Degradation to carbon dioxide and water will occur only when they expose the polymer to microorganisms found naturally in soil, sewage, river bottoms , and other similar environments. The rate of degradation is dependent on the material thickness and the amount of bacteria present. Landfill simulations over a 19 week period show test bottles experienced a weight loss ranging from 30% with oxygen present to 80% with no oxygen present. The fact that Biopol decomposes more rapidly without oxygen present is significant because oxygen is not present in modern landfills.

Monsanto's resulting Biopol resins can be converted into various types of plastic products, depending on the physical properties of the resin used. The first major product, a degradable shampoo bottle, was developed about 5 years ago. However, because Biopol resin prices ranging from $4 USD to $6 USD per pound (somewhat higher than prices for other degradable resins) the number of markets for Biopol may be limited. According to Monsanto, major target products are likely to be plastic films and coatings. With environmental regulations in several European countries, particularly Germany, favoring degradable products, the principal markets for Biopol are in Europe and to a limited extent, Japan. Current sole production capacity is in Europe and is estimated at 300 metric tons annually.

Presently, there are many scientists around the world who have created laboratory samples of other degradable polymers. However, for this time the resins that we list above represent, to the best of our knowledge, the only commercially available degradable polymers with the potential for widespread use.

ABOUT THE AUTHOR: Gerald Biby is Technical Assistance Coordinator for the Industrial Products Center at the University of Nebraska-Lincoln. He can be contacted at tel: 402-472-8187; fax: 402-472-6338; e-mail: bsen090@unlvm.unl.edu