Factors affecting biodegradability of plastic
Plastic biodegradation is the function of microbes to convert polymers (organic substrates) into small molecular weight fragments that can be further degraded to water and carbon dioxide (Ahmed et al. 2018). Among the various factors affecting plastic biodegradation, physical and chemical propexties of polymers and environmental coxxditions are ofpiinxe importance (Figure 3).
Figure 3. Factors affecting biodegradability of plastics.
7.1.1 Molecular weight
The molecular weight plays a critical role in biodegradation of plastic as microbial colonization depends on surface features that allow microbes to establish a locus from which they expand their growth. Molecular weight is inversely related to biodegradation as polymer biodegradability decreases with increase in molecular weight (Tokiwa et al. 2009).
Compounds which are amphiphilic in nature and produced on living surface are called biosurfactants. The biodegradation process of plastic is enhanced by the addition of biosurfactants due to the presence of specific functional groups (Auras et al. 2004).
7.1.3 Shape and size
The polymers’ properties like shape and size play critical role in the biodegradation process. The polymers having large surface area degrade faster than small surface area. Certain types of standards are fixed for various types of polymers having different shape and size (Tokiwa et al. 2009).
Polymer additives are usually low molecular weight organic compounds that provide help to microbes to form colonization due to their ease of biodegradation. Non-polymeric contaminants such as dyes (waste of catalysts used for the polymerization and additives conversion products) or filler materials affect the biodegradation ability of microbes (Ahmed et al. 2018). It is well known that thermal stability would be reduced when lingo-cellulosic filler increases in the sample. The major factors responsible for the thermal stability of the composite system are the dispersal and interfacial adhesion between the lingo-cellulosic filler and the thermoplastic polymer system (Yang et al. 2005). Likewise, metals serve as excellent pro-oxidants in polyolefin manufacturing of polymers sensitive to thermo-oxidative degradation.
7.1.5 Polymer crystallinity
Polymer crystallinity can play a sound role as it has been observed that colonization of microbes to the surface of polymer occurs and utilizes polymer substances in amorphous sections of the polymer sinface.
Moisture is the most important factor responsible for the growth and multiplication of microorganisms, so it plays a major role in the biodegradation of polymers (Ahmed et al. 2018). Sufficient quantity of moisture is needed for activation of microbes and the hydrolytic capacity of microbes is also enhanced with increase in moisture content (Iram et al. 2019).
The rate of hydrolysis reaction of polymer can alter through changing the acidic or basic condition. Change in the pH also affects the growth and multiplication of microorganism and ultimately the biodegrading of polymers.
Biodegradability of polymers is significantly influenced by the polymer’s softening temperature. Polymers having higher melting point are less prone to biodegradation. Efficient enzymatic biodegradation occurs with polymers having low melting point (Tokiwa 2009). For example, purified lipase of R. delemar capably hydrolyzed polyesters like PCL which were showing low melting points (Tokiwa and Calabia 2004). Different enzymes showing unique active sites have the ability to biodegrade various types of polymer substrates, for example, straight chain polyesters, obtained from diacid monomers with 6 to 12 C-atoms, have been degraded faster by enzymes produced by fungal species A. flavtts and A. nigeras compared to straight chain polyesters produced by other monomer (Kale et al. 2007). Polymers made from the petro chemical sources, because of their hydrophobicity and 3D structure, camiot be easily degraded in the environment (Yamada-Onodera et al. 2001).
Now, it’s clear that plastic use in our daily life is inevitable and their demand is an ever increasing trend. So there is urgent need to formulate the bio-based biodegradable polymer to maintain the environmental health. In addition to bio-based biodegradable polymer, new and potential microbial isolates to degrade these bio or fossil based polymer need to be characterized with possible mechanism to exploit them to the maximum. The technology of biodegradation of plastics employing microbial enzymes such as cutinases, lipases, hydrolases, etc. should be fine-tuned and commercialized at the earliest. Moreover, the higher use of biodegradable plastics in various industries and other sectors must be positively correlated with proper waste management and littering control mechanism in order to achieve higher environmental safety and sustainability. Alternative to synthetic plastic, there is an urgent need to generate bioplastics having propexties such as lower durability and faster decomposability in order to create a cleaner and safer environment.