Process Economics Program Report 115
Environmentally Degradable Polymers
Published: August 1977
In recent years concerns over the environmental impact of the generation, collection, and disposal of solid waste have prompted much discussion of the topic and brought forth some legislation. The problems and solutions have had different emphasis in different locations, but have been broadly similar in Europe, Japan, and North America. Attention has often centered on plastic products, which form a highly visible though relatively small fraction of that waste. Because plastics are a class of materials whose properties can be uniquely tailored for specific applications, one natural response to the problem at the technological end has been to provide the option of producing, on a wide scale, plastic products with "controlled lifetimes" or "built-in environmental degradability." It is with this option that the present report is primarily concerned.
The plastic products under consideration are identified essentially by their common potential/ to become litter, although a somewhat wider question than just the litter problem is addressed. Plastic products with a high propensity for becoming litter include packages and containers for consumer goods (predominantly food and drink); containers such as carrier (shopping) bags and garbage bags; protective containers and sheets used in agriculture and construction; and the range of throwaway dishes and cutlery. Such plastics are sometimes loosely identified as "packaging" plastics, since packaging constitutes the major end use. Examination of the degradability option as it relates to solid waste disposal must therefore be centrally concerned with its application to low cost plastics available in commodity form, namely the common thermoplastics such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride, or others with similar potential. At the margin lie the products in which a specific form of degradability constitutes the prime requirement such as water-soluble food additives, tapes for precision planting of seeds, or implants for slow-release drugs in medicine. Allied in a technical sense, but excluded from the present scope are such applications as photocuring of paints and inks, and photoimaging (photoresists).
The term degradability has, in general, been used very loosely. In the present report, degradability, if unqualified, should be taken to mean the physical deterioration of an end-use plastic article, and its breakup under environmental erosive forces within a period ranging from a few days to a year.
Degradable polymers which have reached semicommercial status are next reviewed by geographical location (i.e., Europe, Japan, and North America), and company or individual principally concerned with their development. A feel for the prospects and current market status is thus evolved. It is seen that a number of degradable products which were early contenders for commercialization have since been withdrawn from the market. Their lack of success is attributed principally to lack of cost competitiveness. In this light, two aspects were selected for detailed cost evaluation. First, the manufacture and potential use of a specific type of degradable end product, namely agricultural mulch film.
Second, a manufacturing process for a base resin. The reasons for examining the case of agricultural mulch were twofold. It is an application for degradability which is technically demanding and thus highlights some of the problems. Also, it is an application which was reputed to be cost effective. The evaluation confirms this. Also the cost factors generated in this specific instance can be used to broadly gauge the prospects both for other types of degradable products and various other uses.
The resin manufacturing process selected for evaluation is that for high molecular weight isotactic polybutene-1 (PB-l).* This polymer has been claimed to have the potential to become both a superior bulk thermoplastic and to be an ideal candidate for degradable applications. It can thus be considered to represent the borderline between the commodity polymers and the specialty ones. Examination of the costs at this margin was felt to provide a good basis for making some general judgments regarding the prospects for degradables. Evaluation of the basic processes for the commodity polymers themselves are, of course, available in other PEP reports.