Introduction
A plastic material is a manmade organic solid that if used to manufacture products that are used in various industries. Plastics are among the novel materials that have been manufactured and used by man for the last one hundred years or so. Plastics have become important in the current lifestyle because they have gained wide usage. Among the reasons why polymers have gained wide usage is because they are cheaper. Also, their versatility has allowed them to be used in a wide range of application. Their low cost have thus, had lifesaving consequences like in drought-prone regions where plastics have replaced clay containers to enable water to be carried for longer distances.
Chemical nature of plastics
The essential ingredients of plastics are polymers. A polymer is a chemical substance which has molecules that have the monomeric units repeated in its structure. (Stevens, E., 31). Polymers differ chemically from one another in the types of atom they contain. They also differ in the number of atoms and also in their chemical structure of their molecules. Polymers are mostly composed of carbon, hydrogen and oxygen. In addition, some polymers may also contain chlorine, fluorine, nitrogen, or sulfur atoms. It is also important to note that polymers are organic substances that contain organic elements such as carbon. They may also contain other elements in small quantities that combine with the organic carbon atoms.
Degradation of plastics
Degradation of plastics is any change that may have a diverse effect on its properties and functions. The chemical and physical changes of a polymer are regarded as the most important factors in degradation of plastics by most industrial polymer chemists (Chiellini, E, & Solaro, R. 69). Degradation may occur during two phases in the life cycle of plastics. During manufacture, plastics are subjected to high temperatures under moulding and extruding. This process provides a good environment for thermal and oxidative degradation (Platt, D. 11). The other way in which plastic degrades is through their use. During use, plastics are exposed to air and moisture, light and heat. This process also known as environmental weathering is responsible for changing the chemistry of the polymers (Shashoua, Y., 151).
It is important to note that polymer degradation occurs mainly through scission of the main chain or side chains of the same molecule. Naturally, polymer degradation occurs through the thermal activation, hydrolysis, biological activity, oxidation, photolysis or radiolysis (Nampoothiri, M., Nair, N. & John, R. 5). In many cases, polymer degradation is referred to as environmental degradation. There are so many processes that are associated with environmental degradation. These processes may be physical, chemical and biological. It is important to note that environmental factors not only influence polymer degradation but also have a critical role in the determination of microbial population and the activity of the different microorganisms themselves. Environmental factors such as humidity, temperature, pH, salinity, and presence/absence of oxygen and the supply of different nutrients have important effects on microbial degradation of polymers. These conditions must therefore be considered when the biodegradability of plastics is tested. Chemical and physical characteristics of polymers such as diffusivity, mechanical strength, thermal tolerance, and resistance to electromagnetic radiation also affect polymer degradation process.
Degradation of polymers is also dependent on molecular weight, crystallinity, purity, temperature, pH, presence of terminal carboxyl or hydroxyl groups, water permeability, and additives acting catalytically that may include enzymes, bacteria or inorganic fillers (Nampoothiri, M., Nair, N. & John, R. 5). The end chain degradation of a polymer may occur in such a way that the intramolecular sterification, leads to a ring formation, and the polymer is shortened by hydrolysis of the resultant lactide. Intramolecular degradation occurs by the random alkaline attack on the carbon of the ester group followed by hydrolysis of the ester link. This results into formation of new molecules with low molecular weight. However, in acidic conditions, the protonation of the hydroxyl end group forms an intramolecular hydrogen bond.
Biodegradation of natural plastics
It is important to note that microorganisms that produce and store polyhydroxyalkanoates (PHA), in conditions that have limited nutrients may degrade and metabolize it when the nutriens are added (Shah, A. et al, 254). It is also important to note that the ability to store PHA does not necessarily guarantee the ability to degrade it in the environment. There are a number of aerobic and anaerobic microorganisms that degrade PHA in various environments. These microorganisms are particularly fungi and bacteria.
Table 1: various polymer degradation routes
Bioplastics
Bioplastics are also referred to as biodegradable plastics. They can be degraded in landfills, composters or sewage treatment plants. Their mode of degradation occurs by way of the action of naturally occurring microorganisms (Mooney, B. 219). One of their characteristics is that bioplastics do not leave any toxic, visible or distinguishable residues after the degradation process. They are thus different from other petroleum based plastics which are essentially indestructible in a biological context. Biodegradable plastics are actually being considered due to the fact that they are environmentally friendly and are easier to work with in terms of waste management. Thus, they reduce environmental pressure and carbon footprints. The above factors are some of the reasons that have enhanced the development of plant based natural polymers. Naturally, polymers are produced by plants in form of rubber. Other forms in which plants can produce polymers are starch, cellulose, and storage proteins. All these forms of natural polymers have been exploited for biodegradable plastic production. Other alternatives for producing bioplastics include transgenic plants although there is still some stigma associated with these kinds of plants.
Plant based biodegradable plastics
There has been growing interest in the use of plants to produce biodegradable plastics. This growing interest has led to an interest in using plants for a number of applications. Most of the applications of polymers are based upon crop plants that may be for production of biofuels. The biofuels include ethanol and biomass. Another reason why there has been growing interest in polymer science is to explore there use in power generation and production of novel compounds such as chemicals and medicines. Plant materials, such as rubber, can also be harvested and used directly for plastic production. Otherwise, plant polymers can be derivatized to produce plastics. In addition, plants can also be used indirectly in bioreactors. In this case, the plant material acts as a source of nutrients for the bioreactors. The nature of these plants allows them to be modified genetically. The genetic modification can thus enable them to directly synthesize novel (Mooney, B. 219).
In nature, some structural and carbon reserve polymers are produced directly by plants. Polysaccharides are estimated to constitute approximately 70% of all organic substances. Cellulose constitutes approximately 40% of the total organic substances while lignin accounts for approximately 15-25 % of typical woody plants. Another major component of global biomass is starch. Rubber, a polymer of isoprene, is extracted from plants, and is also the most widely used natural polymer. Natural rubber production has continued to increase with countries like China and Vietnam being some of the leaders. Polymers produced from rubber are mostly polymerized from the isoprenyl units. This polymerization is catalyzed by the rubber elongation or particle-bound rubber transferase. This is an enzyme that adds Isoprenyl units from IPP to form the polymer. Also, magnesium cations are responsible for the regulation of the activity of penyltransferase process of the polymerization.
Another source of polymers is proteins. Proteins can be considered to be polymers of amino acids. They combine in various associations that function depending on their side chain structure. Proteins do also function on relative to the arrangement of the amino acid monomers that exist within a protein. This characteristic function enables them to form a tertiary or quaternary structure. Currently, plants are being used to synthesize proteins in various ways. This happens in addition to the existence of a number of plants that produce naturally occurring proteins. For instance, proteins from wheat, corn, and soy beans have been used as the basis for biodegradable polymers (Mooney, B. 222).
Gluten is a natural polymer that is protein in nature. It is extracted from glutenin and gliadin and can easily be harvested from seed by washing away soluble components. The washing results in production of an essentially pure protein isolate. Gluten exhibits elastic properties on dough because of the presence of disulfide-linked glutenin chains. Thus, glutenin coated with zein has been used to produce a biodegradable plastic whose polymers originate from protein substances. The strength of this kind of plastic is almost similar to that of polypropylene. Zein is a protein that is soluble in alcohol. It is extracted from corn gluten meal.
Cellulose is also another naturally occurring polymer that is used to produce plastic by way of derivatization (Mooney, B. 223). Cellulose is a linear polymer that cannot be thermally processed to produce plastics. This is because its hydrogen-bonded structure usually decomposes when subjected to heat. However, derivative cellulose has been used to produce plastics. A good example is when cellulose, extracted from cotton fibers, is reacted with nitric acid and ethanol to solubidize. The ‘collodin’, as it was called could be cast in sheets or pressure moulded. The plastic is made more flexible by the addition of camphor plasticizer. This plasticizer also makes the plastic less likely to fracture.
Another naturally occurring polymer is starch. Starch is one of the cheapest and most abundant agricultural products. It is also completely degradable in many environments. The main methods of producing thermoplastic starch are by temperature and pressure extrusion. During the process, moulding of the plastic is also done in order to complete the production process. Starch is usually mixed with vinyl alcohols to produce various thermoplastic starch composites. Starch based plastics have gained their usage in the medical industry. For instance, scaffolds for bone-tissue engineering are a good example of starch based plastics. This is because starch polymers are biocompatible, biodegradable, and are also porous in nature. All the three characteristics combined allow blood vessel proliferation during bone growth.
Bioconversions of plant polymers
The use of plant carbon reserves for plastics has been addressed in two ways. The first way is by production of bioethanol while the second way is by fermentation using various strains of bacteria. Fermentation using different strains of bacteria has been used to produce different types of polymers. They include Polylactic acid (PLA), and polyhydroxyalkanoates (Mooney, B. 224). PLA is a polymer of lactic acid used mainly in the medical industry. Its usage in medicine is mainly in treatments concerned with sutures and soft tissue implants. It is also used in packaging and is produced naturally by the lactic acid bacteria of the genus Lactobacillus.
There are different environments in which different microorganisms can degrade PHA. In soil for example, Pseudomonas lemoignei is one of the microorganisms that are known as a decomposition agent of PHAs (Shah, A. et al, 254). Other microorganisms are found in sludge, seawater, and lake water. Each environment has its own distinct microorganism responsible for degradation (Unmar, G. & Mohee, R. 6740). There is also degradation that occurs through enzyme action. There are at least two categories of enzymes that are actively involved in the biological action. They are intracellular and extracellular depolymerases. The extracellular depolymerases are secreted from various microorganisms and also play a critical role in polyhydroxybutirate (PHB) metabolism in the environment (Mittal, V. 38).
Biodegradation of synthetic plastics is usually a slow process especially when it happens in nature. It involves environmental factors followed by wild microorganism action (Moore, G. & Saunders, S. 4). Oxidation is the main mechanism through which synthetic plastic biodegrade. Hydrolysis can also play a big role in this process. During the oxidation and hydrolysis process, the main chains of polymer are degraded resulting in polymers of low molecular weight and feeble mechanical properties. This makes the weakened polymers more accessible for further microbial assimilation. Examples of synthetic polymers that biodegrade include poly (vinyl alcohol), poly (lactic acid), aliphatic polyesters, polycarprolactone, and polyamides.
Recycling plastics
Recycling of plastics requires a comprehensive waste management program that will take good care of environmental degradation process. Waste plastics can be recovered when they are diverted from landfills or littering (Hopewell, J., Dvorak, R. & Kosior, E. 2116). Waste management program also includes the reduction of the amount of material going into the waste-management system. This is done through actions that decrease the use of materials in the products. The process is therefore achieved through using lighter packaging formats instead of the heavy packaging formats. It can also be achieved through downgauging of packaging. Designing products that allow for reuse and repairing also helps in the process of waste management. This is because it will result in fewer products going into the waste stream (Song, J. et al 3).
Recycling is the process by which recovered materials from waste are used as raw materials to manufacture a new product. It is worth noting that calorific value of the plastic material is also utilized by controlled combustion as a fuel during the polymer recycling process. It is also possible for the same polymer to cascade through multiple stages that begin from manufacturing the polymer into a reusable container. The processes that follow are collection of the used container, recycling it, and returning it to waste after use. After it is thrown to waste, the plastic materials can then be recovered for energy.
Waste management may include the use of landfills. However, land fillw require a waste management program with huge chunks of land. This makes the process of land filling inappropriate in countries that do not have enough land resources. In this method, none of the material resources used to produce the plastic is recovered. The material flow is thus linear and not cyclic. Incineration as well as energy recovery is also another method by which waste management programs can utilize. This reduces the need for landfill of plastics waste. This method can be used so as to recover the calorie content in some of the polymers. Another method of waste management is downgauging. Downgauging is the reduction of the amount of packaging material for each product. As a result, waste volumes are reduced. Re-using plastic packaging is also another method of waste management. Post-consumer packaging in the form of glass bottles and jars can be reintroduced in the contemporary business operations (Maynard, E. 190). Another way in which waste management can be done is by recycling the plastic materials. The use of alternative materials to solve waste management issues is also another way of good waste management. The use of biodegradable plastics also has a great potential of solving this problem in this case (Janssen, L. & Moscicki, L. 73).
Production, application and usage
As discussed in the preceding paragraphs, bacteria can synthesize a wide range of polymers. These plastics are the biodegradables that serve diverse biological functions and have material properties that are suitable for diverse applications (Rehm, B. 578). Polysaccharides are some of the kinds of polymers produced by bacteria. Other polymers produced by microorganisms include polyamides and polyesters. Polysaccharides produced by bacteria can be subdivided into the exopolysaccharides, capsular polysaccharides, and the intracellular polysaccharides. Exopolysaccharides can have relevant material properties that are attractive for particular industrial and medical uses. These properties include the ability to form viscous soluble solutions and the ability to exhibit a pseudoplastic material nature (Rehm, B. 580). Capsular polysaccharides are secreted but remain attached to the cell. It also functions as a major surface antigens and virulence factor. Storage polysaccharides are also another type of natural polymer that includes glycogen.
Advantages and disadvantages of polymers
The advantages of plastics are based mainly on their costs, range of applications and their ease of use. They are economical and durable (Andrady, A. 158). Plastics are also easy to cut hence they can be used easily in the manufacture of secondary products, and are also cheaper than most ray materials (Utracki, L. 1186). It is worth noting to know that plastics are easier to work with and are also easy to clean. They are impermeable to water, resistant to rust and can be used underground. The disadvantages of plastics are based on their physical attributes in relation to their wide applications. Thus, plastics are not resistant to heat (U.S. Department of Interior 8). Some of the plastic products are brittle and break easily. In most cases, plastic products cannot be repaired. Also the finished product may not last as long as equivalent wooden or glass products. The finished product is in most cases, inferior to equivalent glass, wooden or steel products where real beauty and outstanding workmanship are required (Bekker, C. 54).
Conclusion
Plastics are among the novel materials that have been manufactured and used by man for the last one hundred years or so. The essential ingredients of plastics are polymers. Degradation of plastics is any change that may have a diverse effect on its properties and functions. The chemical and physical changes of a polymer are regarded as the most important factors in degradation of plastics by most industrial polymer chemists. In nature, some structural and carbon reserve polymers are produced directly by plants. Polysaccharides are estimated to constitute approximately 70% of all organic substances. Cellulose constitutes approximately 40% of the total organic substances while lignin accounts for approximately 15-25 % of typical woody plants. Another major component of global biomass is starch.
Works cited
Andrady, Anthony. Plastics and the Environment. Hoboken John Wiley & Sons: Inc., 2003.
Bekker, C. F. J. Building Science N3, Issue3. Johannesburg: Maskew Miller Longman, 1998.
Chiellini, Emo, & Solaro, Roberto. Biodegradable Polymers and Plastics. New York: Plenum Publishers, 2003.
Hopewell, Jefferson, Dvorak, Robert, & Kosior, Edward. Plastics Recycling: challenges and opportunities. Journal of the Royal Society, 2009.
Janssen, Leon & Moscicki, Leszek. Thermoplastic Starch: Green Material for Various Industries. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA., 2009.
Maynard, Elliott. Transforming the Global Biosphere: Twelve Futuristic Strategies. USA.: Watchmaker Publishing, 2003.
Miller, George & Spoolman, Scott. Living In the Environment: Principles, Connections, and Solutions. Belmont: Yolanda Cossio, 2009.
Mittal, Vikas. Nanocomposites with Biodegradable Polymers: Synthesis, Properties and Future Prospective. New York: Oxford University Press Inc, 2011.
Mooney, Brian. The Second Green Revolution? Production of Pant-Based Biodegradable Plastics. Biochem J. Columbia: University of Missouri, Interdisciplinary Plant Group, 2009.
Moore, G.F. & Saunders, S. M. Advances in Biodegradable Polymers. Report 98. Vol 9. Shropshire: Rapra Technology Ltd., 1997.
Song, J.H., Murphy, R.J., Narayan, R., & Davies, G.B. Biodegradable and Compostable Alternatives to Conventional Plastics. Royal Society Publishing. 2011.
Nampoothiri, Madhava, Nair, NImisha, & John, Rojan. An overview of the recent developments in polylacttide (PLA) Research. Biotechnology division, national institute for interdisciplinary science and technology (NIIST), CSIR, India: Thiruvananthapuran 695019, Elsevier, 2010.
Platt, David. Biodegradable Polymers: Market Report. Shropshire, Smithers Rapra Limited., 2006.
Rehm, Berm. Bacterial polymers: biosynthesis, modifications and applications. Institute of molecular biosciences, Massey University, Palmerston North. New Zealand: Macmillan Publishers Limited, 2010.
Shah, Aamer, Hasan, Fariha, Hameed, Abdul, & Ahmed, Safia. Biological degradation of plastics: a comprehensive review. Department of microbiology, Quaid-i-Azam University, Islamabad. Pakistan: Elsevier Inc., 2008.
Shashoua, Yvonne. Conservation of plastics: materials science, degradation and preservation. Burlington: Elsevier Ltd., 2008.
Stevens, Eugene. Green plastics: an introduction to the new science of biodegradable plastics. Oxfordshire: Princeton University Press, 2002.
Unmar, G. & Mohee, R. Assessing the Effect of Biodegradable and Degradable Plastics on The Composting of Green Wastes And Compost Quality. Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Mauritius, Mauritius, 2008.
U.S. Department of Interior. New Materials Society, Challenges and Opportunities: New Material Science and Technology. Volume 2. Bureau of mines. Diane Publishing Company, 1993.
Utracki, L. A. Polymer Blends Handbook, Volume 1. Dordrecht: Kluwer Academic Publishers, 2002.