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Biodegradable Polymer, Biopolymer, Agro-polymer, Bioplastic, Biomaterial, Compostable Packaging

 
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Biodegradable polymers (Biopolymers)

 

It is more and more widely accepted that the use of long-lasting polymers for short-lived applications (packaging, catering, surgery, hygiene,) is not entirely adequate. This is not justified when increased concern exists about the preservation of ecological systems. Most of today’s synthetic polymers are produced from petrochemicals and are not biodegradable. Theses persistent polymers are a significant source of environmental pollution, harming wildlife when they are dispersed in the nature. For example, the effects of plastic bags are well known to affect the sea-life [1]. Besides, plastics have a large part on waste management, and the collectivities (municipalities, regional or national organizations) are becoming aware of the significant savings that the collection of compostable wastes would provide. Besides, valorising the plastics wastes presents some issues. Energetic valorisation yields some toxic emissions (e.g., dioxin). Material valorisation implies some limitations linked to the difficulties to find accurate and economically viable outlets. In addition, material valorisation shows a rather negative eco-balance due to the necessity, in nearly all cases, to wash the plastic wastes and to the energy consumption during the process phases (waste grinding and plastic processing).
For these different reasons, reaching the conditions of conventional plastic replacements by degradable polymers, particularly for packaging applications, is of major interest for the different actors of the socio-economical life (from the plastic industry to the citizen).
The potential of biodegradable polymers and more particularly that of polymers obtained from agro-resources such as the polysaccharides (e.g., starch) has long been recognized. However, to this day, these agro-polymers largely used in some applications (e.g., food industry) have not found extensive applications in the packaging industries to replace conventional plastic materials, although, they could be an interesting way to overcome the limitation of the petrochemical resources in the future. The fossil fuel and gas could be partially replaced by greener agricultural sources, which should also participate to the reduction of CO2 emissions [1].

BIODEGRADABLE POLYMERS

                        Concepts: Biodegradability and Renewability
                                   Biodegradability and compostability

According to ASTM standard D-5488-94d, biodegradable means capable of undergoing decomposition into carbon dioxide, methane water, inorganic compounds, or biomass in which the predominant mechanisms is the enzymatic action of micro-organisms that can be measured by standard tests, over a specific period of time, reflecting available disposal conditions. There are different media (liquid, inert or compost medium) to analyse biodegradability. Compostability is material biodegradability using compost medium.   Biodegradation is the degradation of an organic material caused by biological activity - mainly microorganisms' enzymatic action. This leads to a significant change in the material chemical structure. The end-products are carbon dioxide, new biomass and water (in the presence of oxygen: aerobia) or methane (oxygen absent: anaerobia), as defined in the European Standard EN 13432:2000. Unfortunately, depending on the standard used (ASTM, EN), different composting conditions (humidity, temperature cycle) must be realised to determine the compostability level [2]. Then, it is difficult to compare the results using different standard conditions. We must also take into account the amount of mineralization as well as the nature of the residue left after biodegradation [3]. The accumulation of contaminations with toxic residues and chemical reactions of biodegradation can cause plant growth inhibition in these products, which must serve as fertilizers. Actually the key issue is to determine for these by-products, the environment toxicity level which is called the eco-toxicity [4].
Some general rules enable the estimating of the biodegradability evolution. An increase of parameters such as the hydrophobic character, the macromolecular weight, the crystallinity or the size of spherulites decreases biodegradability [5]. On the contrary, the presence of polysaccharides (blends) favours biodegradation.

                                   Renewability and sustainable development

Renewability is linked to the concept of sustainable development. The UN World commission on “Environment and Development in our Future” defines sustainability as the development, which meets the needs of the present without compromising the ability of future generations to meet their own needs. According to Narayan (2001) [1], the manufactured products e.g., packaging, must be designed and engineered from “conception to reincarnation”, the so-called “cradle-to-grave” approach. The use of annually renewable biomass, like wheat, must be understood in a complete carbon cycle. This concept is based on the development and the manufacture of products based on renewable and biodegradable resources: starch, cellulose … By collecting and composting biodegradable plastic wastes, we can generate much-needed carbon-rich compost: humic materials. These valuable soil amendments can go back to the farmland and reinitiate the carbon cycle. Besides, composting is an increasingly key point to maintain the susbstainability of the agricultural system by reducing the consumption of chemical fertilizers.

            Biodegradable Polymers Classifications

Figure 1: Classification of the biodegradable polymers.

Classification 


Biodegradable polymers are a growing field [6-8]. A vast number of biodegradable polymers have been synthesised or are formed in nature during the growth cycles of all organisms. Some microorganisms and enzymes capable of degrading them have been identified [6, 9-10].
Depending to the evolution of the synthesis process, different classifications of the different biodegradable polymers have been proposed. Figure 1 shows an attempt at classification. We have 4 different categories. Only 3 categories (a to c) are obtained from renewable resources:

  1. polymers from biomass such as the agro-polymers from agro-resources (e.g., starch, cellulose),
  2. polymers obtained by microbial production, e.g., the polyhydroxy-alkanoates,
  3. polymers conventionally and chemically synthesised and whose the monomers are obtained from agro-resources, e.g., the poly(lactic acid),
  4. polymers whose monomers and polymers are obtained conventionally, by chemical synthesis.

We can also classify these different biodegradable polymers into two main families: the agro-polymers (category a) and the biodegradable polyesters (categories b to d).

References:

  1. Narayan, R. Drivers for biodegradable/compostable plastics and role of composting in waste management and sustainable agriculture; Report Paper. Orbit Journal 2001, 1(1), 1-9.
  2. Steinbuchel, A.  Biopolymers, Volume 10: General Aspects and Special Applications. Wiley-VCH: Weinheim (Germany), 2003, 516 pp.
  3. Avella, M.; Bonadies, E.; Martuscelli, E.; European current standardization for plastic packaging recoverable through composting and biodegradation. Polymer testing 2001, 20. 517-521.
  4. Fritz, J.; Link, U.; Braun, R. Environmental Impacts of biobased/biodegradable Packaging. Starch 2001, 53; 105-109.
  5. Karlsson, R.R.; Albertsson, A-C.; Biodegradable polymers and environmental interaction. Polymer Eng & Sci. 1998, 38(8), 1251-1253.
  6. Kaplan, D.J.; Mayer, J.M..; Ball, D.; McMassie, J.; Allen, A.L.; Stenhouse, P. Fundamentals of biodegradable polymers. In Biodegradable polymers and packaging; Ching, C., Kaplan, D.L., Thomas, E.L.; Eds.; Technomic publication: Basel, 1993, 1-42.
  7. Van de Velde, K.; Kiekens, P. Biopolymers: overview of several properties and consequences on their applications. Polymer Testing. 2002, 21, 433-442.
  8. Rouilly, A.; Rigal, L. Agro-materials: a bibliographic review. J. Macomol. Sci.-Part C. Polymer Reviews 2002, C42(4), 441-479.
  9. Chandra, R.; Rustgi, R. Biodegradable polymers. Prog Polym Sci 1998, 23, 1273-1335.
  10. Kaplan, D.L. Biopolymers from renewable resources. Springer Verlag: Berlin, 1998, 414 pp.

See also, biodegradable polyesters or agro-polymers (starch, )

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