Structural Properties of Plastics and Its Uses
Plastics can be manufactured to produce a myriad of products ranging from the nose cone of an airplane to the lining of a disposable diaper. Each type of plastic boasts a unique blend to suit its purpose. Fundamentally, plastics are composed of long chains of repeating hydrocarbons, called polymers. These polymers are so tightly bound that they are impenetrable to the many different microbes and weather conditions which would usually breakdown other less durable materials.
These polymers are essentially what give plastic its strength and flexibility. There has been some success at emulating this structure with the addition of starch and other organic materials to allow biodegradation by microorganisms (Burgess - Cassler, A., Imam, S. H., Gould, J. M. 1990). Thermoplastic starch polymer materials have been used to manufacture products such as drug capsules, packaging foam pellets and mulch films. Thermoplastic is prepared using pressure extrusion and molding and is mainly used in the manufacturing of drug capsules because of its unique ability to disintegrate in water (Glenn, G. M. 2007).
More stable forms of starch-based biodegradable plastics have also been manufactured commercially. One company Novamont boasts an annual output of about 36,300 metric tons of packaging pellets, one of their main biodegradable products, to a number of packaging companies. Novamont utilises four main starch-based plastic compounds that vary in its additives for the desired purpose. Collectively trademarked as Mater-Bi, these biodegradable plastics are used to manufacture a whole range of products such as garbage bags, diapers and organic food packaging (Bastioli, C. 1998). Starch-based polymers also have many surgical applications because of their porous nature and biocompatibility. This material has proven particularly useful in support braces for tissue-bone regeneration because it allows blood vessel proliferation during the natural healing process (Salgado, A. J., Coutinho, O. P. & Reis, R. L. 2004). In addition, they do not inhibit cell growth and did not produce severe inflammatory responses when tested on rats; this furthers their potential for further applications (Marques, A. P., Reis, R. L. and Hunt, J. A., 2005).
Are Starch-Based Polymers Truly Degradable?
Strictly speaking, the term degradable plastics is misleading as it implies that normal petroleum-based plastics are not biodegradable, which is untrue. For practical reasons, petroleum-based plastics are regarded as non-biodegradable because they are extremely durable and take centuries to breakdown. Thus degradable plastics, either biodegradable or photodegradable, are plastics that have been engineered to rapidly decompose in the natural environment. In order to classify a particular starch polymer blend as ‘biodegradable’, comprehensive tests have to be conducted and criteria have to be met. These tests include simulating different environments and calculating the rate of decomposition. Confounding factors such as the starch simply separating from the plastic component or mechanical forces in nature i.e. running water must be excluded to determine that the decomposition is due to biodegradation and not other causes (Arevalo-Nino, K 1996).
The main objective in the degradation of plastics, regardless of its base material, is to break the long polymer chains allowing the embrittlement and subsequent fragmentation of the polymers by natural forces. Landfills are the largest source of discarded plastic and decomposition is slow due to the lack of an abundant water source.
Decomposition of starch-based polymers involves two key steps. First, starch granules within the polymer are digested until the granules are completely consumed. This has two purposes; the other polymer structure is weakened and the surface area of the plastic is also increased. The granules closest to the surface of the polymer act as ‘stepping stones’ for the microorganisms to reach the granules that are embedded deeper within the matrix (Griffin, G. J. L. 1976).
The second step in the process involves the formation of peroxides. The plastic, or rather the auto-oxidants present in the plastic blend, react with metal salts present in water or soil. This reaction allows the auto-oxidants to be incorporated into the plastic matrix. The resulting production of peroxides facilitates the breakdown of the plastics. These peroxides act on the polymer chains, breaking them into units of lower molecular weight. This process weakens the material and also enhance breakdown by microbes if the particles are small enough. Many factors including the environment, the abundance and type of microorganisms, and the physical properties of the polymer, affect the rate of degradation. Therefore, large scale implementation is difficult (Griffin, G. J. L. 1976).
Conclusions
Starch-based polymers are a promising alternative to the current petroleum-based, non-biodegradable plastics. Over the past fifty years, a lot of research has been generated to find a suitable alternative to oil-based plastics but much is still unknown. Issues such as the byproducts of the biodegradation process and their exact chemical composition have yet to be formally studied. Although small and seemingly insignificant, these byproducts could be poisonous and without proper identification, these products could contaminate water supplies and have a major affect on the environment they had originally been invented to protect. Unfortunately, economic pressures prevent money from being pumped into the development of biodegradable forms of plastic as companies would have to recuperate the millions already invested into the research and development of these plastics. Although biodegradable polymers are valuable in specialty areas such as healthcare and agriculture, large scale commercial implementation of biodegradable plastics into already stable oil-based plastic markets, such as packaging, is still under development as until prices become more idealistic. It might still be a long time before biodegradable, starch-based plastics replace petroleum-based plastics in the market.
References
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Salgado, A. J., Coutinho, O. P. and Reis, R. L. (2004) Novel starch-based scaffolds for bone tissue engineering: cytotoxicity, cell culture, and protein expression. Tissue Eng. 10, 465–474
