Oxo-biodegradable Plastics in the Production and Distribution of Food

Gerald Scott

Introduction

The practical usefulness of degradable polyolefins is now recognised in the growing of foodstuffs by the protection of growing plants by mulching films and tunnels, which both increases yields and conserves irrigation water (Scott 1999).The technology is also widely used in the food packaging and retailing industries. Polyolefins are widely used in both applications because they are highly cost-effective. However, the main benefits of the use of plastics were recognised in the second half of the last century to be their Achilles heel after discard in the environment, where they appear as persistent litter. This led to a search, particularly during the 1970s, for a mechanism to induce biodegradability into commercial plastics.

The Science of Oxo-biodegradation

Commercial plastics are normally stabilised after manufacture against oxidation by atmospheric oxidation by antioxidants and light stabilisers to make them fit for purpose (Scott 1999a). However, the rate at which they biodegrade in the environment is not sufficient to obviate the nuisance of their persistence in the environment.

Many natural materials, such as straw, leaves, and wood biodegrade by a process involving atmospheric oxygen to give humus, which is vital to the fertility of the soil. Research at the University of Aston in 1971 (Eggins et al. 1971) showed that commodity polyolefin plastics could be made to oxidise rapidly in the environment at the end of a programmed life-time, giving rise to low molar mass biodegradable products. Subsequent academic research has shown (Arnaud et al. 1994, Chiellini et al. 2003, Jakubowicz 2003) that the slowest step in oxo-biodegradation is the initial oxidation process by atmospheric oxygen. Soil bacteria rapidly assimilate the low molar mass oxidation products that are chemically the same as those produced in the bioassimilation of nature’s wastes. Photo-oxidation (weathering) is a faster process than thermal oxidation (ageing) and occurs primarily at the surface of the polymer, leading to rapid embrittlement of the plastic, with little change in the average molar mass. Bioerosion results in the ultimate conversion of the polymer to carbon dioxide and water and this process continues at ambient temperatures so long as oxygen is present. It can be predicted from laboratory testing at various temperatures how long it will take for weathered polyethylene to be fully bioassimilated under soil burial conditions. Antioxidant-controlled biodegradation of carbon-chain polymers has been discussed in a number of reviews (Scott 1995, 1997, 1999b, 2001) and will not be discussed further here.

Some concern has been expressed by environmentalists about the potential eco-toxicity of the oxidation products of oxo-biodegradable polyolefins. However fragmented mulching films produced annually in the same fields for up to five years cause no reduction in yields of crops (Yang and Wu 1999). No dangerous metals are used in the manufacture of oxo-biodegradable plastics and the ones that are used commercially (iron, cobalt, manganese and nickel) are abundant in agricultural soils from which they are absorbed by rain water and act as required trace-elements in drinking water (UK Food Standards Agency 2003).

Renewability or Sustainability?

Polyethylene was originally manufactured from bioethanol and is again being manufactured from sugar-based ethylene in Brazil. However, the concept of “renewability” must take into account “sustainability”, and little has so far been published on the eco-efficiency of bio-based raw materials in the polymer industry. There are serious doubts as to whether the demands placed on land and water resources by agriculturally-produced polymers and biofuels can be sustainable in the long-term (Scott 1999b). Moreover, as new oil fields are exploited and alternative renewable energy resources become available, it will be far into the future before the manufacture of plastics from oil is totally superseded.

The concept of renewability is superficially persuasive, but at present scientifically unconvincing, since the biodegradability of the hydrocarbon polymers has less to do with the chemical structure of the plastic than with the additives that are added to protect against atmospheric oxidation. Polyolefins are hydrophobic hydrocarbon polymers that are highly resistant to hydrolysis and cannot hydro-biodegrade, but the use of prooxidant additives leads to hydrophilic surface modification, friendly to microorganisms that are able to bioassimilate the low molar mass oxidation products. Lemaire, Scott and co-workers (Arnaud et al. 1994) found that microorganisms can utilise oxidised polyethylene as the sole source of carbon, leading to bioerosion of the polymer surface while leaving the molecular weight essentially unchanged.

A second question concerning renewable polymers is that their production creates competition between food supply and bioplastics manufacture. This will intensify if the demand for bio-based plastics increases, leading to escalation in the cost of staple foods.

Recycling of Oxo-biodegradable Plastics

The key to the stability of oxo-biodegradable plastics lies in the correct selection of the stabilisation system, and provided this is done, oxo-biodegradable plastics may be re-used, recycled or incinerated with energy recovery. The product may be reprocessed after first life application in a closed-loop system (Scott 1999a) when its provenance is known. Equally importantly, when recovered as part of a mixed domestic or commercial waste stream, the stabilisers provide stability to the whole recyclate (Al-Malaika et al. 1995). However, for long-term durability in second-life, as most re-processors are already aware, additional stabilisers must be added, even in the case of “non-degradable”plastic wastes.

Standards for Biodegradable Plastics

The standards that have so far been published on the composting, of synthetic polymers if applied to nature’s ligno-cellulosic wastes would exclude straw, leaves and wood from the category of biodegradable. In essence, unrealistically short times are required in these Standards for conversion of the material to carbon dioxide. For example European Standard 13432 for the industrial composting of packaging polymers stipulates that they must be substantially (>90%) converted to carbon dioxide within the composting time scale. The application of this Standard in its present form would result in almost all the carbon in the plastic being sent to the atmosphere as greenhouse gas and hence contributing nothing to the fertiliser value of the compost. By contrast, polyethylene, like lignin, humic acid and tannic acid, oxo-biodegrade relatively at a rate similar to nature’s waste and thus act as fertilisers before final conversion to carbon dioxide and water. Consequently, materials compliant with EN 13432 and ASTM D6400 and their equivalents are not ‘recoverable’ in the sense required by the European Waste Framework Directive (1991) since carbon dioxide is not a useful product and contributes to degradation of the environment.

These standards are causing a great deal of confusion in the packaging industry since biodegradation has become conflated with composting. In an attempt to obviate this confusion, standards are being independently developed to certify the biodegradability of oxo-biodegradable plastics in the environment. Typical of these are ASTM 6954 and the draft British Standard BS 8472 “Packaging –Methods for determining the degradability, biodegradability and phytotoxicity of oxo-biodegradable plastics”. Neither mandates abiotic degradation within a specific time-frame because the loss of mechanical properties relates to the rate of oxidation and is a matter for agreement between the manufacturer and user.

References

Al-Malaika, S, Chohan, S, Coker, M, Scott, G, Arnaud, R, Dabin, P, Fauve, A and Lemaire, J (1994) Recycling of biodegradable polyethylene. J. Macromol. Sci. Pure App. Chem. A32 (4): 731.

Arnaud, R, Dabin, P, Lemaire, J, Al-Malaika, S, Chohan, S, Coker, M, Scott, G, Fauve, A and Maaroufi, A (1994) Photooxidation and biodegradation of commercial photodegradable polyethylenes. Polym. Deg. Stab. 46: 211-224.

Chiellini, E, Corti, A and Swift, G (2003) Biodegradation of thermally oxidised, fragmented low-density polyethylenes, Polym. Deg. Stab. 81: 341-35.

Eggins, HOW, Mills, J, Holt, A and Scott, G (1971) Biodegradation of synthetic polymers. In Microbial Aspects of Pollution (Sykes, G and Skinner, FA, eds). London: Academic Press; 267-277.

Jakubowicz, I (2003) Evaluation of biodegradable polyethylene (PE). Polym. Deg. Stab., 80: 39-43.

Scott, G (1995) Photo-biodegradable plastics. In Degradable Polymers: Principles and Applications (Scott, G and Gilead, D, eds). Chapter 9. London: Chapman & Hall.

Scott, G. (1997) Antioxidants in Science, Technology, Medicine and Nutrition, Chichester, UK : Albion Chemical Science Series.

Scott, G (1999a) Biodegradable polymers. In Polymers and the Environment. Chapter 5. London: Royal Society of Chemistry.

Scott, G. (1999b) Antioxidant control of polymer biodegradation. In Degradability, Renewability and Recycling; 5th International Scientific Workshop on Biodegradable Plastics and Polymers, Macromolecular Symposia. (Albertsson, A-C, Chiellini, E, Feijen, J, Scott, G and M.Vert, M, eds). Weinheim: Wiley-VCH; 113-125.

Scott, G (2001) Environmentally degradable polyolefins: When, why and how. In Expert Group Meeting on Environmentally Degradable Plastics, Present Status and Perspectives. Trieste: ICS-UNIDO; 37-48.

[UK] Food Standards Agency (2003) Expert Group on Vitamins and Minerals. Part 3. Risk Assessment. London: FSA; 213-286.

Yang, S-R and Wu, C-H (1999) Degradable plastic films for agricultural applications in Taiwan. In Degradability, Renewability and Recycling; 5th International Scientific Workshop on Biodegradable Plastics and Polymers. Macromol. Symp. 144 (Albertsson, A-C, Chiellini, E, Feijen, J, Scott, G and Vert, M, eds), Weinheim: Wiley-VCH; 101-112.

Gerald Scott, DSc, FRCS, FIMMM is Emeritus Professor in Chemistry and Polymer Science of Aston University, Birmingham, UK; E-mail: scott2rogat@yahoo.com