Plastic Degradability and Agricultural Product Utilization - ACS

Chapter 5

Plastic Degradability and Agricultural Product Utilization

Downloaded by CORNELL UNIV on September 27, 2016 | http://pubs.acs.org Publication Date: July 13, 1990 | doi: 10.1021/bk-1990-0433.ch005

J. Edward Glass Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105

Factors important in the commercialization of a process and in product acceptance in the areas of waste product disposal and increased use of agricultural materials are discussed. The relationships of market volumes to cost, achievement of cost performance through blending and the pa.adox of good compatibility and film properties with domain accessibility in the scenario of achieving degradability in commodity packaging are discussed. Greater use of agricultural products in which the thrust is not based on low cost secondary products is suggested, and an example of the role of noneconomic factors in the success of a new process and product is given in the concluding section. Several years ago during the "oil shortage", when serious efforts to resolve the complexities of applying enhanced oil recovery processes were being made, a crude oil sample recovered from Chevron's Red Wash basin in Utah was analyzed as ca. 5% low molecular weight polyethylene. That initial analysis was hard for many on the project to believe, but several well equipped and staffed analytical departments confirmed its accuracy. Polyethylene is but one of several large volume commodity polymers produced above ground in billion pound quantities each year. It is very unlikely that the subterranean Utah polymer arose from pollution, but it does highlight the resistance of an all carbon backbone polymer to environmental degradation. The stability of commodity polymers is now recognized as an environmental problem in "advanced societies" and is one of the problems addressed in this text. The second topic addressed is that of the American farmer. The remarks of one farmer at the Cora Utilization Conference in November, 1988 (Columbus, Ohio) defined the situation: "The soil and rainfall on my farm in Colorado are not good enough to compete with those in South America; we have to find new applications for agricultural products". Perhaps because he was not a traditional farmer (he had entered fanning as a result of land acquisitions and resigned from a marketing position with an electronics company), he clearly saw the marketing need. One of the objectives of the annual Corn Utilization O097-6156/90A)433-O052$06.00/0 © 1990 American Chemical Society

Glass and Swift; Agricultural and Synthetic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Conference is to review progress in sponsored research for solution of both pollution and the farmer's problems by blending corn starch polymers with polyethylene (1,2). There are several problems in blending these two polymers of very different polarities.


Degradable Plastics Market Restrictions. To evaluate the viability of commercializing carbohydrate/synthetic polymer blends, an understanding of the three laws of industrial polymer science must be appreciated. Any academic would find these laws, relative to the three laws of thermodynamics, repulsive; anyone with greater than five years of industrial experience knows their utility well. The three laws of industrial polymer science are: 1. The market volume of any polymer is inversely related to its price. 2. Avoid making a new polymer, keep blending the one with a long production history with other available materials, until the desired properties are obtained. 3. The way to improve the physical properties of any polymer is to decrease its price a few pennies a pound. Blending of the lowest price commodity polymers from synthetic and carbohydrate polymer families [e.g., poly(ethylene) and starch] would appear to follow these laws. Although each polymer class is produced in large volume (first law), the production rate for corn starch/synthetic polymer blends is much lower than that for the synthetic polymer; this slower extrusion rate directly affects the final cost. Ignoring this limitation, the film properties of the blend are significantly poorer than those of the synthetic polymer film. Both deficiencies are related to the poor thermoplastic properties of water-soluble polymers such as corn-starch. Technical Impositions. When chemically similar polymers of very low polarities, such as polyethylene and polypropylene, are mixed, there is no enthalpic interaction between the two nonpolar macromolecules, and phase separation occurs due to entropie restrictions (5). A compatibilizer, such as an ethylene/propylene block copolymer for the above blend, aids in compatibilization by residing at the interface of the two phases. The need for a compatibilizer was discovered early in the blending of corn starch and polyethylene (2); an ethylene/acrylic acid copolymer compatibilizing agent is generally used. The first alarm sounded in these studies was that this compatibilizer added too much to the cost of polyethylene/starch blends. The significant loss in production rate of such blends is mentioned infrequently, but the economics imposed by this parameter are significantly greater than the cost of the compatibilizer. If economic restraints were not a limitation, there would still be the consumer acceptance factor to consider. A polyethylene/starch blend does not have the film properties (e.g., tensile strength) of a polyethylene film. One can approach biodegradable packaging through the inclusion of weak linkages (e.g., degradable to sunlight) in the hydrocarbon backbone, without the complication of adding starch. However, an ultraviolet unstable linkage serves no purpose if the material is buried in a landfill. The performance of polyolefins in the various consumer markets is dependent on their molecular weight and molecular



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weight distribution, which are determined in large part by the organometallic catalyst used in production. The incorporation of weak linkages such as one sensitive to ultraviolet radiation would necessitate definition of new catalysts and associated scaleup studies. The processability (primarily stability) at comparable times and temperatures of the weak linked" polyolefin to the polyolefin also would need verification. A second approach to biodegradable packaging is to blend polyethylene with a second synthetic polymer with polar repeating units that are capable of degradation, such as ester linkages (chapter 12). Poly(caprolactone) represents such a class of polymer, which has a long history of compatibility (4) with a variety of polymers and degradability (5); recently, improved miscibility and film properties have been reported when poly(caprolactone) is blended with commodity plastics (6). Given a resolution of the major deficiencies, production rate and film properties, a third restriction with respect to polymer blends becomes evident: the film produced would degrade in proportion to the fraction of the poly(caprolactone) present and the extent of degradation would decrease with improving miscibility. The kinetics of degradation are influenced by the limited domain accessibility as compatibility of the polyolefin with the poly(caprolactone) increases. Domain accessibility is discussed for polyethylene/starch blends in chapter 8. There are no realistic answers in this area nor any promising approaches on the horizons. The definition of an optimum polymer blend, however, is only the beginning. Whatever the degrading moieties, they function in a highly complex system. A proper study of the matrix interactions must be defined before a viable scenario is realized for resolution of the environmental problems. Such a matrix approach, in the absence of a polymer blend, is addressed in chapter 2. This type of system approach is not a standard technique in classical academic studies, but it is the standard operational approach in most industrial research efforts. The contributions of insect symbionts (chapter 3) and recycling of plastics (chapter 4) will make an impact, but without a breakthrough improvement in the degradability of plastic packaging it is unlikely that long-standing progress on the waste disposal problem will be realized. The breakthrough improvement, noting the deficiencies cited above, would seem to dictate a totally degradable plastic. The blending of degradable natural polymers with nondegradable polyolefins is not just objectionable in packaging. The ecological threat of such blends has been debated in disposable diapers and recently in a new total package, the "juice box," which is a natural/commodity plastic blend that also includes a metal layer. Utilization of Agricultural Products Historical Perspective. During the 1930s each new synthetic polymer found a ready application and the manufacturer profited from its production. In the late forties studies in the copolymerization of different monomers accelerated and in thefifties,the remaining unpolymerizable, low cost alpha-olefins were converted to macromolecules by Ziegler-Natta organometallic catalysts. In the late fifties, the realities of the three laws of industrial polymer science began to surface, which were reflected in the middle sixties by significant reductions in industrial staffs. Intermingled with synthetic polymer acceptance, a few carbohydrate polymers were accepted in commodity applications.



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Nonnutrition Application Anas. With food consumption demands approached through increased agricultural productivity worldwide, the utilization of agricultural products in plastic applications is considered by many as a future avenue for American farm products. In the "shortage situations" of the seventies, the Japanese very competently demonstrated the usefulness of a variety of fermentation carbohydrate polymers as commodity plastic substitutes, but agricultural products outside of nutritional areas have not received wide acceptance. In many cases one encounters the biodégradation goal in plastic waste a shortcoming; the commercial use of nature's products (e.g., Chapter 21) are often found too biodegradable in many applications. In the third section of this book, it is evident that agricultural products are directed at low cost segments of a given market for two reasons. The thrust is to replace an established synthetic product, and the agricultural material is almost always a by-product of an established farm product grown for food applications. These conditions demand that the secondary farm products be low cost. There are good contributions from three USDA laboratories in this book and some of the past USDA studies have made significant contributions to the use of agricultural products. The use, however, is nowhere near its potential. If this trend is to be broken, the growth of a specific product for a specific market appears to be a requirement. The question of the biodegradability of agricultural products (compared to petroleum based products) and the reliability, both in terms of availability (again when compared to petroleum based products), and uniformity of compositions appear to be some of the primary determinants in agricultural product growth outside of nutritional areas. Both degradable packaging and the greater use of agricultural material require a process for the product's production. This topic is considered in the next section. Realization of a Commercial Process Will environmental factors bring about the goals of the chapters addressed in this book? It is unlikely, for it is unwise to make business decisions on environmental factors. The following is an appropriate example. In the midsixties, emphasis was placed on the production of powder coatings by many industrial organizations, some of whom were major solvent suppliers to the coatings industry. They were concerned about the California Rule 66 pollution control law on solvent emissions. As the seventies were approached the research efforts in powder coatings became nonexistent among solvent suppliers. As we approach the mid-nineties, powder coatings is "the fastest growing segment of the coatings industry" but this segment is still a minor percentage of the total coatings business. It might be argued that the latex technology developed in the forties took 25 years of incubation to achieve commercial reality in latex coatings in the sixties, and that success will be realized in powder coatings in the nineties. Is profitability an inducement to realizing commercialization? Not necessarily, as illustrated by another example of a reaction to environmental influences in the coatings industry. Lead was removed from alkyd coatings over a decade ago. In the seventies, removal of mercuric biocides from aqueous latex coatings was advocated. The use of phenylmercuric acetate inhibited the enzymic degradation of hydroxyethyl cellulose. A sequential process (7,8) to provide greater substitution at the 0-2 carbon position of the repeating glucopyranosyl units of cellulose negated the need for mercuric biocides in latex coatings. The


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sequential process had a few drawbacks in that a threefold increase in reaction time was required and the increased degree of substitution produced a more hydrophilic surface, which created minor problems in product recovery and storage. These deficiencies were more than off-set by significant increases in adduct addition efficiencies and solvent and product recovery costs (#), but that did not result in a new process modification. Business team decisions are dominated by marketing and sales personnel. Their annual performance ratings are highlighted by new market penetrations, not necessarily from implementation of research contributions. The new process is now a commercial realization (9), not directly related to a greater profit and a more environmentally sound process, but because it regained market share lost from not producing the less biodegradable cellulose ether. The Non-Cost Factor There are many examples where commercial success of a new product and process are not achieved even when all the elements of an industrial business team are pulling together. The latest appears to be Group Transfer Polymerization. This is unfortunate for it is a beautiful piece of technology, but it is understandablefromthe laws of industrial polymer science presented previously. An example of the complexity in achieving a commercial success, more related to the environmental theme of this book is the Q-resin technology developed for the production of porous poly(vinyl chloride) [PVC] particles by a suspension process. This technology (10) was developed to produce polymer particles with a porous surface to aid rapid penetration of the particle by commercial plasticizers when "dry-blended". Particles by a conventional process contain pericellular skins that in most cases retarded plasticizer penetration in "dry-blend" (11) processes, and in thefinalfilm,nonplasticized "fisheyes" are observed. During the time of its commercial run, the carcinogenic nature of the vinyl chloride monomer was declared. Limits on residual monomer in a given production polymer were imposed; a lower limit that a more porous particle could help facilitate. The Q-resin process failed to achieve real commercial success because of the mind set of industrial formulators:fisheyeswould be present if the particle size was above 200 microns. The Q-resin technology produced particles that were essentiallyfisheyefree after application of the PVC/plasticizer dry-blend, but the resin's initial median size was slightly greater than 300 microns. The best perspective on the efforts of both thrusts of this book are reflected in the TV response of a McDonalds representative. Under questioning he responded that McDonalds was going to fast food containers that were thinner. He then raised a question and volunteered an answer. "Is going to thinner packaging part of a public relations effort? Maybe, but we are trying". The contributions in this text will not resolve the two major problems addressed, but the efforts are serious ones that hopefully will catalyze support for additional studies that may provide more substantive answers. Conclusions Many factors are involved in determining a commercial process and product's acceptance. The chapters that follow should be read with this understanding. The technical contributions andfinancialsupport in areas addressed by this book are much less than that expended in development of enhanced oil production in


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our "shortage years." If I had a choice for an illustration for this text, it would be a photo on display in the London Museum of Natural History in 1989 taken by a young American. A bald eagle is photographed atop a mound of garbage bags covered with a second type of transparent commodity plastic. The mountains of Alaska are in the background. Plastic waste and agricultural product utilization are not seen as major crises. In the perspective of other social problems, they are not. The crisis efforts in the shortage years of the seventies did not lead to a technology that would have delivered us if the oil crisis had really existed. It is hoped that the topics covered in this text will stimulate a sustained effort in the areas covered so that progress can be realized for a critical situation that may develop in the future.

Literature Cited 1.

Proceedings of the First Annual Corn Utilization Conference, June 11 and 12, 1987, National Corn Growers Association, St. Louis, M O . 2. Proceedings of the Corn Utilization Conference II, November 17 and 18, 1988, National Corn Growers Association, St. Louis, M O . 3. Olabisi, O.; Robeson, L. M.; Shaw, M.T. Polymer-Polymer Miscibility, Academic Press: New York, NY, 1979. 4. Paul, D.R.; Barlow, J. W. J. Macromol.Chem. 1980, C18(1), 109-168. 5. Potts, J. E . Plastics Engr. Tex. Pap. 1975, 217. 6. Private communication from one of the chapter reviewers of this text. 7. Glass, J. E. ; Buettner, A . M. ; Lowther, R. W. ; Young, C. S. ; Cosby, L. A. Carbohyd. Res. 1980, 84, 245-263. 8. Glass, J. E. ; Lowther,R. G . , U.S. Patent 4 084 060, 1978. 9. Cellosize Enzyme, Resistant Hydroxyethyl Cellulose, 1984 Product Bulletin, Union Carbide Corp., Old Ridgebury Road, Danbury, C T 06817. 10. Nelson, A . R.; Floria, V . E . Br. Patent 1 195 478, 1970. 11. Glass, J. E . ; Fields, J. W. J. Appl. Polym. Sci. 1972, 16(9), 2269-2290. RECEIVED March 29, 1990


Plastic Degradability and Agricultural Product Utilization - ACS

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