Biodegradable plastics are those that are disintegrated by microorganisms into water, carbon dioxide (or methane during anaerobic process) and biomass under specified conditions. Universal standards have been implemented, new materials have been developed, and a compostable logo has been introduced to identify Biodegradable plastics.
Mechanism Of Biodegradation
Biodegradation of polymers involves following steps:
- Attachment of the microorganism to the surface of the polymer.
- Growth of the microorganism, using the polymer as a carbon source.
- Ultimate degradation of the polymer.
Microorganisms are able attach to a polymer’s surface, as long as the latter is hydrophilic. Once the organism is attached to the surface, it is able to grow using the polymer as its carbon source. In the primary degradation stage, the extracellular enzymes secreted by the organism cause the main chain to cleave, leading to the formation of low-molecular weight fragments, like oligomers, dimers or monomers. These low molecular weight compounds are further used by the microbes as carbon and energy sources. Small oligomers may also diffuse into the organism and get assimilated in its internal environment.
Various additives are added during compounding for this purpose. Eg.Starch.
Factors Affecting Biodegradation Of Plastics
The biodegradability of a polymer is essentially determined by the following physical and chemical characteristics:
- The availability of functional groups that increase hydrophobicity (hydrophilic degradation is faster than hydrophobic).
- The molecular weight and density of the polymer (lower degrades faster than higher).
- The morphology of polymer – amount of crystalline and amorphous regions (amorphous degrades faster than crystalline).
- Structural complexity such as linearity or the presence of branching in the polymer.
- Presence of easily breakable bonds such as ester or amide bonds.
- Molecular composition (blend).
- The nature and physical form of the polymer (e.g., films, pellets, powder or fibers).
- Hardness (soft polymers degrade faster than hard ones)
An Overview On Biodegradation Of Synthetic Plastics
Polyethylene is a stable polymer that consists of long chains of ethylene monomers; it cannot be degraded easily by microorganisms. However, it has been reported that lower molecular weight PE (MW=600–800) can be partially degraded by Actinobacter spp. upon dispersion, while high molecular weight PE could not be degraded.The biodegradation of PE is a very slow process. A wide variety of Actinomycetes like Streptomyces strain and fungi like Aspergillus and Penicillium have been used in research to facilitate this process.
The biodegradation of polyethylene is known to occur by either Hydro-biodegradation and Oxo-biodegradation. These two mechanisms can be used because of two additives, starch and pro-oxidant, used in the synthesis of biodegradable polyethylene. Starch blended polyethylene has a continuous starch phase that makes the material hydrophilic, and therefore allows it to be catalyzed by amylase enzymes. Microorganisms can easily access, attack and remove this section, thus the polyethylene with the hydrophilic matrix continues to be hydro-biodegraded. If a pro-oxidant additive was used, biodegradation occurs following photodegradation and chemical degradation. If the pro-oxidant is a metal compound, after transition-metal catalyzed thermal peroxidation, biodegradation of low molecular weight oxidation products occurs sequentially.
Polypropylene is a thermoplastic that is commonly used for plastic mouldings, stationary folders, packaging materials, plastic tubs, non-absorbable sutures, diapers, etc. It can be degraded by exposure to ultraviolet radiation from sunlight, and it can also be oxidized at high temperatures. The possibility of degrading PP with microorganisms has also been investigated. Even though PP is a polyolefin, and thus prone to oxidative degradation like PE, the substitution of methyl for hydrogen in the ß position makes it more resistant to microbial attacks. The decreasing order of susceptibility of polymers to degradation in soil mixed with municipal refuse was PE > LDPE > HDPE. Studies reported on biodegradation of PP, many microbial communities such as certain fungal species like Aspergillus niger and bacteria such as Pseudomonas and Vibrio have been reported to biodegrade PP.
Polystyrene is a synthetic hydrophobic polymer with a high molecular weight. It is recyclable, but not biodegradable; and although it has been reported that PS film was biodegraded by an Actinomycetes strain, the degree of this biodegradation was very low.
Various investigations on the biodegradation of Polystyrene with many species of fungi capable of degrading plastics found low decomposition rates, even though the addition of cellulose and minerals increased these decomposition rates significantly
Polyvinyl chloride is a strong plastic that resists to different factors such as abrasion and chemicals, and has low moisture absorption; there are many studies about thermal and photo degradation of PVC, but only a few reports on its biodegradation. According to one report, PVC’s low molecular weight can be exposed to biodegradation by white rot fungi.
Polyethylene terephthalate has different properties. It is a semi crystalline polymerand chemically and thermally stable .The molecular weight of this polymer ranges from 30,000 to 80,000 g/mol.
Various studies on the degradation of PET transparency sheets by microbes and Esterase enzyme showed important chemical changes of polymeric chains by X-ray photoelectron spectroscopy (XPS) analysis. Microbial degradation affect crystalline structure and a presence of microbes inside the polyethylene terephthalate were seen as well.