Agricultural and Synthetic Polymers - ACS Publications - American

Chapter 1

Degradability of Commodity Plastics and Specialty Polymers An Overview

Downloaded by BALL STATE UNIV on April 27, 2016 | http://pubs.acs.org Publication Date: July 13, 1990 | doi: 10.1021/bk-1990-0433.ch001

Graham Swift Rohm and Haas Company, 727 Norristown Road, Spring House, PA 19477

This overview is an attempt to briefly cover the history and recent developments in environmentally degradable commodity and specialty polymers and plastics. Degradation pathways are mentioned, polymer types, including blends, are reported and the limitations of current testing protocols raised. The chapter concludes with generalizations on structural requirements for degradable polymers. The review is not meant to promote degradable plastics, but rather to highlight them as one possible route to a cleaner environment. In many applications, recycling of polymers or incineration of waste products may offer more protection to the environment. The final decision for any application requires careful deliberation and not emotional judgements.

For the purpose of this overview, commodity plastics are packaging and agricultural products based largely on polyolefins, polyvinyl chloride), and polyethylene terephthalate). Specialty polymers represent a wider range of applications for generally lower volume products; they find use, for instance, in the home in detergents and superabsorbent polymers, e.g. poly(alkylene oxides) and poly (aery lie acid), in industry, e.g. polyvinyl alcohol), and in medicine for sutures and drug delivery, e.g. polyesters. Degradability refers to their stability in a given environment, including in vivo for mammalian implants used in medicine. Any or all of the following degradation mechanisms may be operative in the degradation process; with biodégradation being the ultimate desirable goal with complete removal from the environment. Biodégradation is promoted by enzymes and may be either aerobic or anaerobic. Operative in all environments, burial, surface exposure, waterways, in vivo, etc., and may lead to complete removal from the environment.

O097-6156/90/0433-O002$06.00/0 © 1990 American Chemical Society

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

SWIFT

Degradability of Commodity Plastics and Specialty Polymers Photodegradation is promoted by irradiation, e.g. sunlight, and is restricted to surface environment exposure. It rarely leads to complete removal, though fragments produced may biodegrade. Environmental Erosion is promoted by the elements - wind, rain, temperature - and by larger animals. Usually, a surface environment phenomenon, and fragments produced may biodegrade.


Chemical Degradation is promoted by chemical reaction through additives in the plastic, e.g. metals, functional groups and is operative under all conditions. Fragments produced may biodegrade. Commodity plastics and specialty polymers should either have predictable life times and then degrade on exposure to the chosen release environment, or be nondegradable and recycled or incinerated. Research on (bio)degradable synthetic polymers is not new. Several reviews have appeared over the last twenty years(1-7). However, beginning in the early 1980's, the recognized lack of biodegradability of many large volume commercial polymers, particularly commodity plastics, used in packaging, e.g. fast food industry, and in agricultural films, focussed public attention on a potentially huge environmental accumulation and pollution problem that could persist for centuries. Plastics have reached about 7 weight percent and 30 volume percent of solid waste in the U.S.A(8)and Japan(9), a large proportion of which are commodity plastics referred to above. Their lack of degradability is impacting significantly on the rate of depletion of landfill sites and their persistence also adds to the growing water and land surface litter problem. This has prompted intense research activity world-wide to either modify current products to promote degradability(10,11 ) or to develop new, suitable, alternatives that are degradabie by the mechanisms mentioned. Specialty polymers represent a broad compositional and use range. Some enter the environment as landfill, along with commodities; e.g. poly(acrylic acid) as super absorbents in diapers, poly (aery lie acid) and poly(alkylene oxides) enter streams in detergent effluent and others from industrial effluents, e.g. polyvinyl alcohol) from paper and textile mills. Some of these polymers are biodegradable and of little concern; others are not, and will receive attention over the course of time as priorities change. Medical polymers are a unique case of specialty polymers. These have been designed to degrade in vivo where they perform such functions as controlled drug release and temporary supports as sutures in postoperation recovery. They are widely accepted and used. Degradabie plastics do not represent the only route to an improved environment. Plastics can be collected after use for recycle and/or incineration.(8) Collection looms as a problem for both approaches, and separation of plastic types is needed for recycling to be successful. Without debating the 'pros' and 'cons' of recycling, incineration, and degradabie plastics, it does seem likely that all three will play an important part in a cleaner environment. Each will offer particular advantages for certain applications of plastics. This overview, for simplicity of presentation, is divided into addition and step-growth polymers rather than commodity plastics and specialty polymers. It


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4


is not all encompassing, rather a skim across the field. A brief review of testing protocols is included, for appreciation of the difficulties involved in establishing (bio)degradability. Fate analysis deserves special comments since it does bear strongly on the environmental issues being discussed with regard to degradabie plastics. Unless total fate analysis is done on the newly developed degradabie plastics, it will be extremely difficult to pronounce them environmentally friendly. In the course of degradation, small molecules are produced (from polymers) which are considered to be biodegradable. Without identification and testing, they may equally be considered non-biodegradable. Since they are produced in oxidation processes, they may be expected to be polar and water soluble (to some extent) and could enter water tables and rapidly become part of our ecosystems. Questions that should and must be raised, therefore, relate to toxicity, both as the small molecules produced and with any heavy metal affinity they may possess. All forms of life should not be jeopardized by a precipitous but well meaning switch from an inert to a degradabie polymer. Testing Protocols A testing protocol should include an environment that fairly represents that to which the substrate polymer will be exposed. This has not always been the case; many polymers that are disposed of in landfills have been tested for (bio)degradation under aerobic conditions when anaerobical is probably closer to the condition that pertains. Another shortcoming in tests that have been reported on commercial polymers is that no indication as to the degree to which degradation has occurred is reported. Results are based merely on whether fungal or bacterial life is supported, or whether physical change occurs on exposure to the environment, e.g. U.V., from sunlight. Many 'biodegradable' plastics, based on polyolefin blends will disintegrate in the environment, but the recalcitrant polyolefin remains. Can this be called biodégradation, and is it acceptable? In spite of these objections, much of the work that has been done is valuable and sets the tone for future developments, as we recognize these deficiencies. Biodégradation testing with plastics and polymers falls into two categories: water soluble and water insoluble. Testing of water soluble polymers is simpler and benefits from the testing that has occurred with water soluble detergent additives over many years. These include biological oxygen demand (BOD), carbon dioxide evolution, and semi-continuous activated sludge tests (SCAS) (12), which are reviewed by R.D. Swisher in his monumental work on surfactant degradation.(13) Many workers have extended these tests to analyze metabolites and identify the mechanism of degradation. Labelling with C has played an important role. 1 4

Water insoluble polymers are more difficult to evaluate. Early tests were subjective and based on a relative growth rating of bacterial and fungal colonies.(14,15) They ignored low molecular weight species and additives present, both of which could make positive contributions to growth. Extension to physical testing after the bacterial and fungal growth gave some indication of property loss,(16,17 18), but again, no distinction was made for the contribution of impurities, making it difficult to compare different samples of the 'same' polymer or plastic. Loss of physical form(19) and weight(20) have also been used to measure the biodégradation of plastics buried in soil; some measure of loss f


1. SWIFT

Degradability of Commodity Plastics and Specialty Polymers

of polymer can be obtained in these tests. Finally, C labelled materials have been used to follow C 0 evolution as a measure of biodégradation and its rate. 1 4

1 4

2

As of this time, there is no real satisfactory test for degradation in any polymeric system. Complete fate analysis is needed. Definitions have to be established and degree of analysis accepted for many different polymers, something that will take time. Tests will be required for each type of degradation mentioned earlier.


Addition Polymers Early work by Potts and coworkers(21 ) indicated that high molecular weight polymers did not support fungal growth. He tested polymers such as poly(methyl methacrylate), polyethylene, polystyrene, polyvinyl chloride), poly(vinylidene chloride) and polyvinyl acetate). In the few instances where some growth occurred, he attributed this to low molecular weight fragments or the presence of plasticizers. Potts(22) also demonstrated that low molecular weight linear polyethylene was biodegradable by fungal growth support, whereas branching stopped biodégradation as shown in Table I. Table I

Microbialfirowthin Hydrocarbons Compound

Substitution

MM

growth

(Methyl) n-Dodecane

4

-

170

2,6,11

212

-

226

2,6,1 1,15

283

n-Dotriacontane

-

451

n-Hexatriacontane

-

507

Dodecane n-Hexadecane Hexadecane

(0 = no growth;

0 4 0 4 0

4 = maximum growth)

More recent work on hydrocarbon oligomers by T. Suzuki established the biodégradation of oligomers of cis-1,4-polyisoprene,(23) polybutadiene,(24) polyethylene,(25) and polystyrene(26). The most fundamental work on polyethylene degradation is being done by A. C. Albertsson in Sweden(27,28,29,30,31) using elegant science with C labelled polymers to measure the rate and quantity of carbon dioxide evolution from buried high and low density polyethylene films. Degradation occurs at about 1 4


AGRICULTURAL AND SYNTHETIC POLYMERS

6

0.5% per year; powders which have larger surface area than film degrade slightly faster; irradiation prior to burial also slightly enhances degradation rate. Albertsson observed slightly enhanced degradation for HOPE versus LDPE, which she attributes to less branching and probably slightly lower molecular weight. Polyethylene degradation rate enhancement has been claimed for blends with mercaptans,(32) ketones,(32) ethers,(32) and specifically by photodegradation in the presence of benzophenone,(33) and metal promoted oxidations.(34 ) Copolymers with carbon monoxide are also subject to photodegradation.


Major contributors to this area of 'doped' polyolefins are G. J. C. Griffin, J. E. Guillet, and G. Scott, and many of the blossoming degradabie packing polymers are based on their pioneering work. Polyvinyl alcohol is unique in being the only biodegradable carbon-carbon backbone polymer.(35,36) The degradation is oxidative, followed by hydrolytic cleavage, Suzuki claims acids and ketones as metabolites, and Watanabe acids and alcohols. This difference is shown below, schematically.

-C-C-C-C-C-

I

OH Suzuki /

c c - c - f - c

W

ο

C- aliphatic branched » aliphatic phenyl, and the polyether degrades, as described earlier. Polymer Blends Polymer blends, particularly olefins with biodegradable polymers, are gaining popularity as an approach to degradabie packaging plastics. The materials are at best only partially biodegraded, but will lose form and bulk as the plastic disintegrates. This may be sufficient in landfill as volume diminishes, leaving room


1. SWIFT


for more solid waste. It should be understood, though, to date, only the biodegradable part of the blend has been shown to be removed from the environment.


This work began at the U.S.D.A. with Otey,(70) where he developed blends, based on starch and ethylene/acrylic acid copolymers. Potts also has patents in this area with olefin/polycaprolactone blends(71). Starch is recognized as a cheap filler for polyolefins which enhances the biodegradability, or apparent biodegradability; depending on the interpretation of results,e.g. starch/poly(vinyl alcohol),(72) starch/polyurethane,(73) and starch/poly(methyl methacrylate) (74) also biodegrade. Polyethylene has also been blended with metals(75) and metals plus autoxidizable substrates76 in attempts to enhance degradation by U.V. irradiation and/or oxygen. Other uses of blends include controlled rate of fertilizer re lease (77) based on ethylene/vinyl acetate/carbon monoxide polymers which is U.V. sensitive, polyolefin blends with any biodegradable polymers,(78) and polyolefins blended with metals and autoxidizable substrates.(79) Doane and co-workers(80) at the U.S.D.A. have used grafted starches in many applications, including soil stabilization. Starch, being cheap, will continue to be an attractive substrate(81) unless the blends and grafts are determined to be unacceptable substitutes for packaging materials.

Medical Polymers This field has been well reviewed by B. J. Tighe.(82) The polymers, for the most part, are polyesters. Poly(glycolic acid) (83) is widely used in sutures under the trade name of DEXON. Poly(lactic acid) is also used.(84) A copolymer of 92/8 mole percent poly(glycolic acid)/poly(lactic acid) (85,86) is another alternative. Recent advances have seen both Tighe(87) and Lenz(90)developing procedures for carboxylated polyesters. Tighe has evaluated biologically produced polymers.(88) American Cyanamid Company continues to modify poly(glycolic acid) by copolymerizing ethylene oxide.(89)

generalizations From this brief overview, several generalizations can be established concerning (bio)degradable polymers. 1.

Addition polymers with C-C backbones do not biodegrade to any significant extent.

2.

Non-branched oligomers of C-C polymers (M.W. ethers > amides > urethanes molecular weight, lower is faster morphology, amorphous degrades faster than crystalline hardness, (Tg), softer degrades faster hydrophilicity, more hydrophilic degrades faster


5.

Recalcitrant polymers may be rendered susceptible to degradation by incorporation of weak linkages which are hydrolytically unstable (esters), photo reactive (ketones), etc. blending with biodegradable additives (starch) addition of activators to promote oxidative degradation. References

1. 2. 3.

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Agricultural and Synthetic Polymers - ACS Publications - American

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