Plastics extensively used in packaging of products like food,

Plastics
are manmade long chain polymeric molecules (Scott, 1999).Synthetic polymers has
substituted natural materials in almost every area and nowadays plastics have
become an essential part of our life. With time, stability and durability plastics
have been improved continuously and it has become resistant to many
environmental influences (Shah et al 2008). The word plastic comes from the
Greek word “plastikos”, which means ‘able to be moulded into different shapes’
(Joel, 1995). The plastics are made from inorganic and organic raw materials,
such as carbon, silicon, hydrogen, nitrogen, oxygen and chloride. The basic
materials used for making plastics are extracted from oil, coal and natural gas
(Seymour, 1989). Plastics are resistant against microbial attack, because there
is no microbial enzyme found so far, that has the capability to degrade the
polythene completely (Mueller, 2006). Synthetic plastics are extensively used
in packaging of products like food, pharmaceuticals, cosmetics, detergents and
chemicals.  The commercial success of
plastics as a packaging product is due to combination of flexibility, strength,
lightness, stability, impermeability and ease of sterilization. “Plastic
Packaging” is the fastest emerging trend. Plastic today form the foundation of
our “convenience consumer culture”. Approximately 42%of the plastics are used
in India for packaging applications.

 

The
most widely used plastics in packaging are polyethylene (LDPE, MDPE, HDPE and
LLDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC),
polyurethane (PUR), polyethylene terephthalate (PET), poly butylene
terephthalate (PBT), nylons. Plastic waste is a widespread and persistent
global challenge with negative impacts on the environment, economy, human
health and aesthetics (Reina M. Blair et al., 2017).There are different methods
of disposal of plastics such as incineration, recycling and landfills (Sharma
& Sharma, 2004). Plastic waste is released during all stages of production
and post consumption every plastic product is a waste (Sabir, 2004). Some synthetic
plastics like polyester polyurethane, polyethylene with starch blend, are
biodegradable, although most commodity plastics used now are either
non-biodegradable or even take decades to degrade.

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This
issue has raised the concern about the development of polymers that are naturally
degradable or to develop alternatives that are degradable by any or all of the
following mechanisms: Biodegradation, Photodegradation, Environmental erosion
and Thermal Degradation (Kawai, 1995). In 1980?s, research in the field of
designing of plastics that are easily degraded by microorganisms has started.  Biodegradable plastics opened the way for new
consideration of waste management strategies since these materials are designed
to degrade under environmental conditions or in municipal and industrial
biological waste treatment facilities (Augusta et al., 1992; Witt et al., 1997).
In the past years, several biodegradable plastics have been introduced into the
market. However, none of them is efficiently biodegradable in landfills. At
present, biodegradable plastic represents just a tiny market as compared with
the conventional petrochemical material. Hence, there is an urgent need to
develop efficient microorganisms and their products to solve this global issue
(Kathiseran, 2003). Biodegradation plays a key role in reducing the molecular
weight of the polymer by naturally occurring microbes like bacteria, fungi and
actinomycetes isolated from different environments (Gu, 2003; Sivan et al
2006). It has been found that the nanoparticles enhance growth cycle,
mechanical and physiochemical stability of microorganisms along with
biodegradable property (Pandey et al., 2015).

 

Biodegradation
of plastics

Microorganisms
such as bacteria and fungi are involved in the degradation of both natural and
synthetic plastics (Gu et al.,2000). Polymers especially plastics are potential
substrates for heterotrophic microorganisms (Glass and Swift, 1989). Polymer
biodegradation is governed by different factors that include polymer
characteristics, type of organism, and nature of pre treatment. The polymer
characteristics such as its mobility, tacticity, crystallinity, molecular
weight, the type of functional groups and substituents present in its
structure, and plasticizers or additives added to the polymer all play an
important role in its degradation (Artham and Doble, 2008; Gu et al., 2000).

 

The
biodegradation of plastics proceeds actively under different soil conditions
according to their properties, because the microorganisms responsible for the
degradation differ from each other and they have their own optimal growth
conditions in the soil (Shah et al., 2008). The process in which a complex
community of microorganisms is established on a surface is known as “microfouling”
or formation of biofilm. During degradation the polymer is first
converted to its monomers, then these monomers are mineralized. Most
polymers are too large to pass through cellular membranes, so they must first
be depolymerized to smaller monomers before they can be absorbed and
biodegraded within microbial cells. The initial breakdown of a polymer can
result from a variety of physical and biological forces (Swift, 1997). Physical
forces, such as heating/cooling, freezing/thawing, or wetting/drying, can cause
mechanical damage such as the cracking of polymeric materials (Kamal and Huang,
1992). The growth of many fungi can also cause small-scale swelling and
bursting, as the fungi penetrate the polymer solids (Griffin, 1980).

 

Generally,
an increase in molecular weight results in a decline of polymer degradability
by microorganisms. In contrast, monomers, dimers, and oligomers of a polymer’s
repeating units are much easily degraded and mineralized. High molecular
weights result in a sharp decrease in solubility making them unfavourable for
microbial attack because bacteria require the substrate to be assimilated
through the cellular membrane and then further degraded by cellular enzymes. At
least two categories of enzymes are actively involved in biological degradation
of polymers: extracellular and intracellular depolymerases (Doi, 1990; Gu et
al., 2000). During degradation, exoenzymes from microorganisms break down
complex polymers yielding smaller molecules of short chains, e.g., oligomers,
dimers, and monomers, that are smaller enough to pass the semi-permeable outer
bacterial membranes, and then to be utilized as carbon and energy sources. The
process is called depolymerization. When the end products are CO2,
H2O, or CH4, the degradation is called mineralization
(Frazer, 1994; Hamilton et al., 1995). It is important to note that biodeterioration
and degradation of polymer substrate can rarely reach 100% and the reason is
that a small portion of the polymer will be incorporated into microbial
biomass, humus and other natural products (Alexander, 1977; Atlas and Bartha,
1997; Narayan, 1993). Dominant groups of microorganisms and the degradative
pathways associated with polymer degradation are often determined by the environmental
conditions. When O2 is available, aerobic microorganisms are mostly
responsible for destruction of complex materials, with microbial biomass, CO2,
and H2O as the final products. In contrast, under anoxic conditions,
anaerobic consortia of microorganisms are responsible for polymer
deterioration. The primary products will be microbial biomass, CO2,
CH4 and H2O under methanogenic (anaerobic) conditions
(Barlaz et al., 1989; Gu et al., 2000, 2001; Gu and Mitchell, 2001) (e.g.
landfills/ compost).