ENVIRONMENTAL EFFECTS OF BIODEGRADABLE POLYMERS

Plastic shopping bags have advantages and disadvantages when compared to alternatives such as paper bags. All disposable bags are problematic from an energy use and disposal perspective.

Advantages

The durability, strength, low cost, water and chemicals resistance, welding properties, lesser energy and heavy chemicals requirements in manufacture, fewer atmosphere emissions and light weight are advantages of plastic bags. Many studies comparing plastic versus paper for shopping bags show that plastic bags have less net environmental effect than paper bags, requiring less energy to produce, transport and recycle; however these studies also note that recycling rates for plastic are significantly lower than for paper. Plastic bags can be incinerated in appropriate facilities for waste-to-energy. Plastic bags are stable and benign in sanitary landfills. Plastic carrier bags can be reused as trash bags or bin bags.

Disadvantages

Our main concerns with plastic shopping bags:

1.                  Plastic bag littering and associated indiscriminate waste disposal and consumer behavior.

2.                  Resource consumption issues, including reduction, reuse and recycling.

3.                  Plastic degradability issues relating to littering and resource use.

4.                  Social issues, community education and awareness, and consumer perceptions.

5.                  Plastic bags are made of petrochemicals, a nonrenewable resource.

6.                  Plastic bags are flimsy and often do not stand up as well as paper or cloth.

7.                  When disposed of improperly, they are unsightly and represent a hazard to wildlife.

8.                  Conventional plastic bags are not readily biodegradable under any normal circumstance.

9.                  Plastic bags can cause unsupervised infants to suffocate.

DISADVANTAGES OF BIODEGRADABLE POLYMERS

1.        Most of the biodegradable polymers are not commercially viable due to their higher cost.

2.        Major applications of biodegradable polymers are in medical field.

3.        Biodegradable polymers are too unstable for long term industrial use.

4.        There is a public opposition against genetically modified foods.

5.       Manufacture of biodegradable polymers consumes more energy than the conventional polymers.

6.        Mode of the degradation is mainly by hydrolysis which in turn pollutes the water.

7.     Migration of plastic degradation by-products such as residual pigments, catalyst residues and isocyanate via run-off and leachate from landfills and composting facilities to groundwater and surface water bodies is resulted.

8.   Trauma and death of marine species resulting from only partial or slow degradation of biodegradable plastic products in marine environments is caused.

9.        They increase the littering as the people believe that biodegradable plastics will disappear quickly.

10.      It increase the soil and crop degradation resulting from the use of compost that may have unacceptably high organic and or metal contaminants derived from biodegradable plastic residuals, additives and modifiers such as coupling agents, plasticizers, fillers, catalysts, dyes and pigments.

11.      They do not disappear completely and leave visible trace and toxic residues.

12.       They do not possess the strength and storage comparable to that of the conventional polymers.

ADVANTAGES OF BIODEGRADABLE POLYMERS

1.        Compost derived in part from biodegradable plastics increases the soil organic content.

2.        The water and nutrients are retained.

3.        The chemical inputs and plant diseases are suppressed.

4.        Biodegradable shopping and waste bags disposed in a landfill can increase the rate of organic waste degradation in landfills.

5.        Methane harvesting potential is also enhanced.

6.        The landfill space usage is decreased. Biodegradable landfill covers can extend landfill life considerably.

7.        Energy required to synthesize and manufacture biodegradable plastics is much lower for most biodegradable plastics than for non-biodegradable plastics.

8.        Biodegradable plastics also offer important environmental benefits through the use of renewable energy resources and reduced greenhouse gas emissions.

MECHANISM OF POLYMER DEGRADATION

Polymer degradation is broadly of two types:

Chain degradation: Here the degradation starts from the chain ends resulting in successive release of monomer units. It is reverse of chain propagation hence can be called de polymerization.

Random degradation: It occurs at any random point along the polymer chain. It is reverse of poly condensation process. Here the polymer degrades to lower molecular weight fragments and practically no monomers are released.

TYPES OF POLYMER DEGRADATION:

Degradation usually involves the chemical modification of polymer by its environment. Degradation of  polymer may be brought about by:

THERMAL DEGRADATION

Traces of transition metals accelerate thermal oxidative process by inducing hydro peroxide decomposition.

MECHANICAL DEGRADATION

Stretching, grinding, milling and any type of polymer shearing process produce free radicals as a result of main chain fracture. Upon warming; these radicals attack the polymer matrix and lead to further scission reaction through radical rearrangement reactions. In the melt, it is difficult to separate the combined degradative effects of torque, time and temperature.

DEGRADATION BY IONIZING RADIATION

The interaction of high energy radiation with polymers generates free radicals and produce defected products. Aliphatic polymers are damaged largely by post irradiation thermal oxidation.

METAL CATALYZED DEGRADATION

Polymers contain metallic compounds, as impurities or deliberately incorporated additives induce both photo and thermal stability problems.

OXIDATIVE DEGRADATION

Oxygen is present in the amorphous domains of all polymers, crystallites of some polymers and in fast quenched polyolefin. The oxidative chain reaction is initiated by any process capable of generating free radicals.

SOLAR DEGRADATION

Photo catalytic decomposition of organic dyes in aqueous solution has been carried out with nano size TiO_2. Solar degradation of aqueous wastes are carried out by out door exposure to sun on rotating plastic disc fitted with TiO₂   coated plastic sheet.

HYDROLYTIC DEGRADATION

Hydrolysis of the polymer backbone requires water and can be considered a bimolecular reaction.

ULTRASONIC WAVE AND HIGH ENERGY RADIATION DEGRADATION

Polymers are subjected to Ultrasonic waves, uv and g radiation during polymer processing to reduce the bacterial contamination. Radiation effects occur at random, throughout a polymer.

PHOTO DEGRADATION

Photo degradation occurs when polymers are exposed to sunlight during their outdoor service. Pigments protect against or sensitize photo degradation.

CHEMICAL DEGRADATION

It occurs by introducing hydrolysable or oxidative functional group into the polymer backbone. The polymer chains become labile to an aqueous environment and thus, chemical degradation initiates polymer erosion.

BIO DEGRADATION

It implies the degradation that is mediated by a biological system. It is a mass loss of monomers, oligomers. Chemical reactions describing biodegradation of a hydrocarbon polymer in aerobic and anaerobic environment can be expressed as:

AEROBIC ENVIRONMENT:

Polymer + O₂      →     CO₂   +    H₂O + Biomass + Residue

ANAEROBIC ENVIRONMENT:

Polymer →     CO₂/CH₄   +    H₂O + Biomass + Residue

BIODEGRADABLE POLYMER TERMINOLOGY

Many polymers that are claimed to be ‘biodegradable’ are in fact ‘bioerodable’, ‘hydrobiodegradable’ or ‘photo-biodegradable’. These different polymer classes all come under the broader category of ‘environmentally degradable polymers’.

Various classes of biodegradable plastics, considered in terms of the degradation mechanism are:

1.         Biodegradable

2.         Compostable

3.         Hydro- biodegradable

4.         Photo- biodegradable

5.         Bioerodable

6.         Degradable  polymers

7.         Degradation

8.         Disintegration

9.         Elimination

10.       Erosion

BIODEGRADABLE

Biodegradation is degradation caused by biological activity, particularly by enzyme action leading to significant changes in the materials chemical structure. In essence, biodegradable plastics should break down cleanly, in a defined time period, to simple molecules found in the environment such as carbon dioxide and water.

COMPOSTABLE

Compostable biodegradable plastics must be demonstrated to biodegrade and disintegrate in a compost system during the composting process (typically around 12 weeks at temperatures over 50°C).

HYDRO-BIODEGRADABLE

Hydro-biodegradable polymers are broken down in a two-step process - an initial hydrolysis followed by further biodegradation.

PHOTO-BIODEGRADABLE

Photo-biodegradable polymers are broken down in a two-step process - an initial photo-degradation stage, followed by further biodegradation.

BIO-ERODABLE

Many polymers are  ‘bioerodable’. They degrade without the action of micro-organisms. This is also known as abiotic disintegration, and include processes such as dissolution in water, ‘oxidative embrittlement’ (heat ageing) or ‘photolytic embrittlement’

DEGRADABLE POLYMERS

A material is called degradable if it undergoes degradation to a specific extent within a given time measured by specific Standard Test methods.

DEGRADATION

It is an irreversible process leading to a significant change of the structure of a material, typically characterized by a loss of properties (e.g. integrity, molecular weight, structure or mechanical strength) and/ or fragmentation.

DISINTEGRATION

Disintegration means the falling apart into very small fragments of packaging or packaging material caused by degradation mechanisms.

ELIMINATION

It is the excretion and metabolism of polymer and erosion product from mammals.

EROSION

It is the mass loss of a polymer matrix which can be due to the loss of monomers, oligomers or even pieces of non degraded polymers. Erosion can be the result of biological, chemical or physical effect.