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3 Biodegradability, Its Mechanism and Methods of Biodegradation
Environment Benevolent Biodegradable Polymers
grown polymers and the fiber reinforcements are of agro-based origin. Microorganisms are able to consume such materials leaving behind carbon dioxide and water as
by-products. In fact microbial attack on a material is dependent on the framework
structure of the polymer. Moreover, in polymer materials from a scientific standpoint, certain ingredients like starch and cellulose must be present in order for
biodegradation to occur.
The organism type determines the appropriate degradation temperature, which
usually falls between 20 and 60 C. The disposal site must contain oxygen, moisture, and mineral nutrients along with neutral or slightly acidic pH. Biodegradation
of materials involves the digestible macromolecules, which join to form a chain and
experience a direct enzymatic scission. This is followed by metabolism of the split
portions, leading to a progressive enzymatic dissimilation of the macromolecules
from the chain ends. Oxidative cleavage of the macromolecules leading to metabolization of the fragments may take place. Thus, the chain fragments become short
enough to be degraded easily. Biodegradable polymers begin their lifecycle as
renewable resources, usually in the form of starch or cellulose. Many biopolymers
are designed to be discarded in landfills, composts, or soil. The materials are broken
down by the microorganisms in presence of appropriate pH [118–120].
In the case of materials where starch is used as an additive to a conventional
plastic matrix, the polymer in contact with the soil and/or water is easily attacked by
the microbes. The microbes digest the starch, leaving behind a porous sponge-like
structure with a high interfacial area and low structural strength. After the depletion
of starch the polymer matrix begins to be degraded by an enzymatic attack.
Enzymatic reaction results in the scission of the molecular chain thereby, slowly
reducing the weight of the matrix till the entire material gets digested. Another
method of microbial degradation of biopolymers involves growing microorganisms
for the specific purpose of digesting polymer materials. But this process is more
intensive and costs more. Moreover, it circumvents the use of renewable resources
as biopolymer feedstocks. Microorganisms under consideration are designed to target
and breakdown petroleum-based plastics. Though this method reduces the volume of
waste, it does not aid in the preservation of nonrenewable resources [120–125].
Polyolefins are the polymers susceptible to photodegradation and addition
of additive-like benzophenone further accelerate the photochemical degradation.
Modification of the composition of the polymer with the incorporation of more
UV absorbing groups accelerates the rate of photo degradation. Synthesizing new
polymers with light-sensitive groups are the examples of photodegradable polymers.
Applications of Biodegradable Polymers
Biopolymers are used in diversified sectors including medicine, packaging, agriculture, and automotive industry. With advancement, choice of materials, and environmental awareness, some of the materials are replaced whereas the others are
complimented. Biodegradable plastic films may be used as garbage bags, disposable
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cutlery and plates, food packaging, and shipping materials. Depending upon the
application, biodegradable materials can be classified in different categories.
It is estimated that approximately two-fifths of the total plastics is used in packaging
and about 50% of that is used in food industry as a packaging material. Since large
volume of the inert materials is disposed of as waste-land fillers, therefore, people
all over the world are trying to use the biodegradable packaging materials so as to
reduce the volume of waste materials.
At the end of the life cycle, the biopolymers are used in agriculture as soil regenerators. For example, ecoflex as a thin film can be used to cover frost susceptible
plants during winter season and at the end of life cycle gets mixed up with the soil
as a nutrient-rich fertilizer. Biopolymers which are compostable supplement the
current nutrient cycle in the soil and are important from agriculture point of view.
Application of a plastic mulch cover for less than 40 days, immediately after seeding
is found to increase the yield of spring wheat and is an ideal material as a crop
mulcher. Applications of biopolymers are not limited to film covers as mulchers,
other areas of more interest are plant pots, disposable composting containers,
and bags. The plants along with pots are seeded directly into the soil where the
biodegradable plastic breaks down as the plant begins to grow. In order to avoid the
disposal problem, material scientists are also carrying out research on biodegradable fertilizer and chemical storage bags [125, 126].
16.4.3 Medical Applications
The classification of bioactive materials includes all biopolymers used for medical
applications. BASF, a world leader in the chemical and plastic industry, introduced
ecoflex as a fully biodegradable material in 2001. The material has been found to
be resistant toward water and grease, making it suitable for its application as
a hygienic disposable wrapping material and at the end it gets decomposed in
normal composting systems. Polyvinyl alcohol is designed for extrusion, injection
molding, and blow molding possessing features user-controlled solubility in water.
Dissolution of this product occurs at a preset temperature, thereby, allowing its
applications in diversified fields like hospital laundry bags, disposable food service
items, agricultural products, and catheter bags. Biodegradable loose-fill packaging
Environment Benevolent Biodegradable Polymers
materials can be developed from renewable material like starch. The starch is
treated by an acetylation process followed by other chemical treatments and postextrusion steaming. Mechanical properties of such materials have been found to be
adequate and true biodegradability can be achieved.
Since today’s medical world is constantly and rapidly changing, therefore, the
materials required also need recurrent adjustments. The biopolymers used in medical
applications must have tissue compatibility. Moreover, they may or may not be
expected to break down after a given time period. Researchers working all over
the world on tissue engineering are attempting to develop organs from polymeric
materials, which could be transplanted into humans. The plastics with growth factor
injections are required for growth of cell and blood vessel in the new organ.
Biopolymers with adhesion sites that act as cell hosts in giving shapes and can
mimic different organs have been developed, e.g., artificial bone material which
adheres and integrates onto bone in the human body. The most commonly employed
material is bioglass. Other application of biopolymers is in controlled release delivery of medications. The bioactive materials release drugs at a rate determined by
its enzymatic degradation over a long period. PLA materials have been developed
as medical devices such as resorbable screws, sutures, and pins. These materials
are neutral and reduce the risk of tissue reactions to the devices and have short
recovery times and decreased number of doctor visits needed by patients [127–130].
16.4.4 Automotive Sector
This sector is constantly responding to societal and governmental demands for
environmental responsibility. Research and development activities in the area of
automobiles using natural fibers as reinforcing materials in plastic parts continues
to be enthusiastic, especially in European countries. Bio-based cars are lighter with
better mileage, making them a more economical choice for consumers. Being
biodegradable natural fibers are the best substituents for glass fibers as reinforcement materials in plastic parts of automobiles and commercial vehicles .
At the end of life cycle such biodegradable polymer materials can be composted.
Natural fibers like flax, jute, ramie, and hemp are being used in interior parts as
reinforcing agents. Since the components do not need load-bearing capacities and
the dimensional stability is more important, the application of natural fibers as
reinforcing materials is more effective.
One of the novel applications of biopolymers, which do not fit into any of the
previous categories, is its use in modifying the food textures, e.g., gelatin-based
biopolymer starch fat replacers possess fat-like characteristics with smoothness
and short plastic textures that remain highly viscous after melting. Research is
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continuing to manipulate biopolymers into food products. The eventual goals have
improved physical characteristics such as foaming, gelling, and water- or fatbinding abilities .
16.4.6 Other Applications
Nowadays, biopolymer materials are finding their applications in other fields such
as adhesives, paints, engine lubricants, and construction materials. Biodegradable
golf materials and fishing hooks are also available.
Biopolymers are of great importance in environment-friendly management because
of their applications in diversified fields. For example, formulation of biodegradable mulching films to be used for agriculture crops and such films do not need to be
taken off the fields as they do not have any environmental impact. Moreover, these
bioplastic films possess specific mechanical and optical properties similar to those
of the traditional plastics used in films, agriculture, e.g., like polyethylene and
poly[ethylene-co-(vinylacetate)]. However, biodegradable mulch films are made to
be biodegraded in soil at the end of crop cycle, therefore, durability cannot be
compared with traditional mulch films. In case of agriculture, resistance to photooxidation is more important characteristic of films so as to enhance the durability of
the materials. Researchers have processed a biodegradable polymer in the presence
of small amount of UV stabilizing systems, added during the melt process, which
could improve the photo-oxidation resistance [133, 134].
16.5.1 Recycling Impact
In order to protect the environment, scientists are attempting to integrate environmental considerations directly into material selection processes. The use of renewable resources in the production of biopolymers achieves it through: the feed-stocks
being used can be replaced either through natural cycles or through intentional
intervention by humans or use of renewable feed-stocks for biopolymer development is biodegradable as the end product and can prevent the potential pollution
which can be due to the disposal of equivalent volume of traditional plastics. At the
end of their life cycle, biopolymeric materials are generally used as land fillers or
are composted. In case of plastic materials recycling is encouraged but effective
recycling is very less, e.g., in United States, less than 10% of plastic products are
recycled at the end of their life cycle. For material development, recycling has to be
recognized as a disposal technique but not as a final goal. Since the return on
Environment Benevolent Biodegradable Polymers
investment in recycling is positive under economic situation, therefore, in underdeveloped countries plastics are almost completely recycled.
No doubt it appears to be positive at the onset but the open systems by which
the plastics are recycled emits toxic gases in the environment. Though recycling
appeared to be a viable way to reduce pollution and environmental damage when
it was first introduced as a waste reduction technique but the emission of toxic
gases in the environment has diverted the attention of researchers and environmentalists toward the use of plastic-based renewable feed-stocks that are biodegradable and the end products are organic matter. In this way toxic emission
can be avoided and the growth of easily compostable and biodegradable plastics
is encouraged .
Microorganism Degradation of Plastics
Microbial degradation of polymers is a two-step process. Initially, microorganisms
bind to the polymer substrate followed by the catalyzed hydrolytic cleavage.
Degradation of polymers can be monitored through roughening of the surface,
formation of holes or cracks, defragmentation or change in color. Various characterization techniques used for the confirmation of different stages of biodegradation
are scanning electron microscopy (SEM), atomic force microscopy (AFM), FT-IR,
and thermal analysis . CO2 evolution/O2 consumption is the laboratory
method for the measurement of biodegradation of polymers. Anaerobic microorganisms produce predominantly a mixture of CO2 and methane as an extracellular
product of their metabolic reactions. Use of biodegradable plastic goods in daily life
is common and is increasing day by day in the developed countries. Now the trend
is going to change and dependency is increasing on biodegradable plastics in
comparison to synthetic polymeric materials because of their complete biodegradability in the natural environment after disposal. Moreover, selected strains can be
characterized and used in defined degradation tests. Biological degradation of
polymers is generally influenced by a number of factors, such as, the kind of
organisms involved in the biodegradation and the environmental conditions.
Biopolymers are being used in diversified fields like packaging, agriculture, medicals, automobile sector, and food industry. Because of environment friendliness,
biopolymers are replacing the synthetic polymers at a fast pace. Biodegradation of
plastic materials is of prime importance and is a process occurring through the
intervention of bacteria and other living organisms like fungi, yeasts, and insects.
Biodegradation of biopolymers is regarded as a green process because it leaves CO2
and H2O molecules along with organic materials as the end products, thereby
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adding no pollution load to the environment. Moreover, the amount of CO2 released
at the end of life cycle is much lower than the amount of CO2 consumed by the
plants throughout the life cycle. Rate of use of biopolymers is higher in case of
developed and developing countries, whereas underdeveloped countries are still
dependent upon synthetic plastics because of their cost-effectiveness.
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