Development of Packaging Film Using Microcrystalline Cellulose and Pro-Oxidative Additive Using Blown Film Technique

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Chapter 2.8 Biodegradation:

Biodegradation is one of the several ways of polymer may degrade in the environment. This process are also interpreted by the general public as the same as other processes of polymer degradation such, as photo degradation, oxidation and hydrolysis, though they lead to very different end products. It is often conceived that the breakdown of a plastic into small, invisible fragments is biodegradation, when in reality these fragments may remain in the environment over a significant period. Biodegradable polymers when placed in bioactive environments, such as compost, will break down to carbon dioxide and water under the action of bacteria and fungi. There are two major steps in the biodegradation process. The first one involves the depolymerisation or chain cleavage of the polymer to oligomers, and the second step is the resulting mineralization of these oligomers. The depolymerisation step normally occurs outside the microorganism and involves both endo and exoenzymes. Endo-enzymes cause random scission on the main chain, while exo-enzymes cause sequential cleavage of the terminal monomer in the polymer main chain. Once depolymerized, sufficiently small-sized oligomeric fragments are formed. These fragments are transported into the cell where they are mineralized. Mineralization is defined as the conversion of the polymers into biomass, minerals, water, CO2, CH4 and N2. There are several standard test methods available to evaluate the biodegradability of plastics as listed in. Most of these test methods measure the percent conversion of the carbon from the designed biodegradable plastics to CO2 and CH4 (plus some CO2) in aerobic and anaerobic environments, respectively. The absence of polymer and residue in the environment indicates complete biodegradation process, whereas incomplete biodegradation may leave polymer and/or residue as a result of polymer fragmentation or metabolism in the biodegradation process.
2.9 Factors Affecting Biodegradation:

Polymeric materials were subjected to degradation by biological, chemical and/or physical actions in the environment. Generally, biodegradation involves successive chemical reactions, such as hydrolysis, oxidation with/without the aid of enzymes in living organisms. The rate of biodegradation was found to be affected by several factors. Polymer's environment, organisms utilized and the nature of the polymeric materials are three main factors affecting biodegradation. All microorganisms have an optimum temperature, at which maximum growth rate occurs and thus highest enzyme kinetics exist. Discovered that an increase in the temperature of sewage in a waste water treatment plant, correlated with the increase in the rate of biodegradation of poly (hydroxylalkanoates) being tested. However, if the temperature in the environment becomes higher than the optimum temperature of a microorganism, then the denaturing of enzymes and other proteins in the microorganism takes place. In this case, the rate of biodegradation is reduced. An optimum pH value also will affect the rate of biodegradation. A microorganism also needs a certain amount of nutrients from its environment to allow it to grow. Therefore, the concentration of nutrients is essential to the rate of biodegradation. Oxygen and moisture concentration also have considerable effect on rates of biodegradation in terrestrial environments. One of the main problems in landfill sites is that there is lack of oxygen and moisture in the environment. If there is not enough moisture and oxygen in the environment, the microorganisms cannot growth. Nature of polymer substrate also affects the rate of biodegradation. Increased branching in polymeric materials will reduce the rate of degradation. Maximizing the linearity of the molecule reduces stearic hindrance facilitates the maximum susceptibility of the molecule to enzymatic attack and promotes microorganism assimilation. Low molecular plastics are susceptible to degradation, due to the ability to transport into a microbial cell. List of factors that affecting the rate of biodegradation are shown in Fig 2.2.
Chapter 2.10 Mechanism of Biodegradation:

The production of biodegradable polymers is now rapidly increasing, and new biodegradable polymeric materials have been developed based on various factors, such as polymer structure, chemical/enzymatic modification, blending and mechanical treatments. Polymeric materials were subjected to degradation by biological, chemical and/or physical (mechanical) actions in the environment. Polymeric materials generally undergo these factors concurrently in the environment. Typical examples related to biodegradation are biological hydrolysis by hydrolase enzymes and oxidation by oxidoreductase enzymes. The hydrolase enzyme is responsible for the hydrolysis of ester, carbonate, amide and glycosidic linkages of the hydrolysable polymers producing the corresponding low molecular weight oligomers. The oxidoreductase enzyme is responsible for the oxidation and reduction of ethylenic, carbonate, amide, urethane, etc. Hydrocarbons such as polyethylene, natural and polyisoprene rubbers, lignin and coal are first subjected to biological oxidation by oxidoreductase, such as oxygenases, hydroxylases, monooxygenases, peroxydases and oxidases in the biodegradation process. However, the degradation process precedes both by abiotic and biotic actions in the environment. Structure of the main chain polymer and the specific example of the related enzyme are shown in Table 2.3. Biodegradable polymers are generally degraded through two steps of primary degradation and ultimate biodegradation.
Primary degradation is the main chain cleavage forming low molecular weight fragments (oligomers) that can be assimilated by the microbe's hydrolysis or oxidative chain scission. Hydrolysis occurs using environmental water with the aid of an enzyme or under non-enzymatic conditions (abiotics). Oxidative scission occurs mainly by oxygen, a catalytic metal, UV light or an enzyme. Polymer chain can also be cleaved by mechanical strain such as bending, pressing or elongation. The low molecular weight fragments produced were incorporated into microbial cells for further assimilation to produce carbon dioxide and microbial cells, metabolic products under aerobic conditions. Under anaerobic conditions, methane is mainly produced in place of carbon dioxide and water. [.].
Polymer chain scission is one of the degradation phenomena in biodegradable. This process occurs in two ways, depolymerisation (exogeneous) scission and random (endogeneous) scission. In the former the polymer chain is cleaved from the terminal of the chain. A water soluble oligomer is generally liberated into the reaction media and the rate of the molecular weight reduction of the residual polymer is small. In the latter way the polymer chain is randomly cleaved. In this case, the molecular weight of the remaining polymer quickly decreased. At the same time, the mechanical properties of the remaining polymer are also quickly decreased. The addition of these two types of polymer chain scission causes degradation at the weak link. The polymer chain is cleaved at the relatively weak bond by the various physico-chemical actions. Polyesters, polyanhydrides, polycarbonates and polyamides are mainly degraded by hydrolysis into low molecular weight oligomers at the primary degradation with subsequent microbial assimilation in the biodegradation process. Some other degradation mechanisms include oxidative cleavage by a radical mechanism. Oxidative degradation is the main mechanism for nonhydrolyzable polymers, such as polyolefins, natural rubber, lignins and polyurethanes. For many polymers, hydrolysis and oxidation occur simultaneously in the environment. Surface degradation and bulk degradation are examples of polymer degradation mechanisms depending on the main degradation site. The C-C polyvinyl type polymer containing side groups, such as short alkyl Groups and phenolic groups are generally resistant to biodegradation. PVA is readily biodegradable by environmentally occurring microbes.
Polyethylene (PE) with a low molecular weight of less than 1,000 is biodegradable. Biodegradation of the low molecular weight PE involves oxogenase elimination by the action of oxidoreductases, such asoxygenase, dehydrogenase and oxydase, forming a fatty acid with subsequent oxidation. The mechanism shows similarities with the typical oxidation of fatty acids and n-alkanes. For PE degradation, an initial abiotic oxidation of the polymer chain is also a necessary step. Once hydro peroxides have been introduced, a gradual increase in the ketone groups of the polymers is followed by a decrease in the ketone groups when short chain carboxylic acids are release as degradation products [23]. The combined effect of an abiotic oxidative step with consequent biotic action will be a slow but definite and progressive mineralization.