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3 Biodegradability, Its Mechanism and Methods of Biodegradation

3 Biodegradability, Its Mechanism and Methods of Biodegradation

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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.

16.4.1 Packaging

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.

16.4.2 Agriculture

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 [131].

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.

16.4.5 Food

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


B.S. Kaith et al.

continuing to manipulate biopolymers into food products. The eventual goals have

improved physical characteristics such as foaming, gelling, and water- or fatbinding abilities [132].

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.


Environmental Impact

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 [135].


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 [136]. 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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