polymer degradation

THEORY OF POLYMER DEGRADATION
Mrs.G.D.SHAH (M.E. POLYMER TECH.)
I/C HEAD OF PLASTICS ENGG.DEPT.
GOVT. POLYTECHNIC, AHMEDABAD
GUJARAT, INDIA.
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ABSTRACT


Polymers are known for their versatile qualities of strength, durability, light weight, low cost etc. Today polymer industry is facing environmental problem regarding its degradability and its durability. Environmentalists object the use of plastic packaging because they are non degradable. In fact all polymers are degradable in longer or shorter period and that to without imposing any harm to the environment. There are many ambiguities regarding the term degradability. The present paper emphasizes on the term degradability, various types of degradability, various modes through which polymers tend to degrade, factors affecting the degradability and mechanism of degradability. Also the future research scope is shortly described.

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Polymers have become inevitable material for our daily routine life, quality and comfort. This is due to the versatile qualities of polymer with respect to their strength, lightness, durability, protection and low cost. Plastics are used in almost all industries and have virtually glass, paper and wood in the packaging and building industry. The other bulk industries are consumer goods, electrical/electronics, automobile agriculture, medical, pharmaceutical, aeronautical and space application and so on…
Synthetic polymers were originally developed for their durability and resistance to all forms of degradation, including biodegradation. These virtues of polymer have now become a greatest problem. It would be highly beneficial from the environmental stand point, if these films were biodegradable, decomposing almost completely under the attack of microorganism over a period of say few months or even they can break into fragments so that they do not become harmful to the environment.
DEGRADATION:
It is the process of polymer chain scission by the cleavage of bonds between the monomers in the polymer backbone. Hence degradation leads to a size reduction of the polymer chain.
DEBORAH NUMBER (D) is the measure of degradability.
D = TIME OF DEGRADATION / HUMAN LIFE TIME
Degradable polymers will have small values of D.
All polymers degrade in one way or other and hence all polymers can be considered degradable. But in practice if a polymer does not degrade within human life time is usually considered non-degradable.
TYPES OF DEGRADATION:
Polymer degradation is broadly of two types:
1. Chain degradation: Here the degradation stars from the chain ends resulting in successive release of monomer units. It is reverse of chain propagation hence can be called depolymerization.
2. Random degradation: It occurs at any random point along the polymer chain. It is reverse of polycondensation process. Here the polymer degrades to lower molecular weight fragments and practically no monomers are released. Almost all polymers undergo random degradation. The polymer chain need not carry any active site. The polymer molecules break up while degrading. Hence a sudden drop in the molecular weight occurs.
VARIOUS MODES OF POLYMER DEGRADATION:
1. Thermal degradation
2. Mechanical degradation
3. Ultrasonic wave degradation
4. High energy radiation degradation
5. Photo degradation
6. Chemical degradation
7. Bio degradation
Thermal and mechanical degradation occur during polymer processing. It is important when polymers are used for providing a mechanical function where they are subject to stress.
Ultrasonic wave and high energy radiation degradation occur when polymer is subjected to sterilization. Polymers are subjected to uv and  radiation during polymer processing to reduce the bacterial contamination.
Photo degradation occurs when polymers are exposed to sunlight during their outdoor service.
Chemical degradation 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. Different types of chemically degradable polymer bonds have different velocities at which they hydrolyze.
BIODEGRADABILITY implies the degradation that is mediated by a biological system. It is a mass loss of monomers, oligomers.
FACTORS AFFECTING DEGRADABILITY:
Synthetic polymers are inherently resistant to biological attack. But susceptibility to biodegradation varies and is affected by:
1. Additives
2. Plasticizers
3. The type of chemical bond
4. Water uptake
5. Crystallinity and molecular weight
6. pH
7. Copolymer composition
8. Enzymatic degradation
1. ADDITIVES: Various compounding ingredients may have nutritive value for microorganisms. Hence may invite microbial attack. Most of the plasticizers, lubricants, thickening agents, starch and cellulose fillers are susceptible to microbial attack.
2. PLASTICIZERS: Plasticizers are occasionally used as processing aids in highly cross linked polymers. Plasticizers appear to function in polar, mainly by masking polar sites in the chain and there by reducing hydrogen bonding. In all polymers, plasticizer tends to force the chain apart, giving them greater freedom of movement and also reducing van Der Waals forces between the chains. There are no plasticizers that are completely free from fungal or bacterial attacks. Susceptibility of microbial attack increases as the plasticizer level increases.
3. THE TYPE OF CHEMICAL BOND: Various chemically degradable polymer bonds are: polycyanoacrylates, polyanhydrides, polyketals, polyorthoesers, polyacetals, poly (2-hydroxy-esters), poly (E-caprolactone), polyphosphazenes, polyB-Hydroxyesters, polyamino carbonates, polypeptides, polycarbonates, polyphosphate esters.
4. WATER UPTAKE: The hydrolysis of the polymer backbone requires water and can be increased by raising the concentration of either partner. The water uptake can be controlled by altering the lipophilicity of the system. Degradation rates increase when the hydrophilic component contents are increased.
5. CRYSTALLINITY AND MOLECULAR WEIGHT: Crystalline polymers degrade slower than amorphous polymers. High molecular weight causes increase in glass transition temperature. Hence leads to slower degradation. Higher molecular weight increases the chain length and hence more bonds have to be cleaved in order to generate water soluble oligomers or monomers to allow degradation.
6. pH: pH changes can modify hydrolysis rates by orders of magnitude. Also the degradation products of many degradable polymers change pH by their acid functionality.
7. COPOLYMER COMPOSITION: The presence of variety of functional groups affects degradability. e.g. A polymer built up of two monomers contains four types of bonds: A-A, B-B, A-B and B-A. This has some significance when these bonds have different hydrolysis rates.
8. ENZYMATIC DEGRADATION: Biodegradable polymers can be hydrolyzed passively or actively via enzymatic catalysis. The fastest process controls the overall degradation mechanism. Enzymatic degradation is mainly effective for natural polymers. However with non hydrolysable, water insoluble polymers such as PE, enzymatic degradation may occur. Experiments with microbes even PE was found to be enzymatically degradable, however at a low rate. Synthetic polymers with functional groups have higher chances of non specific enzymatic degradation. The combinations of enzymatically degradable and non degradable materials are useful in drug targeting and gene therapy.
MECHANISM OF BIODEGRADATION:
There are three mechanisms for the biodegradation.
1. Cleavage of crosslinks: It involves water soluble polymers that are crosslinked by covalent bonds to make them insoluble. Hydrolytic cleavage of either the crosslink or the backbone yields water soluble polymers or polymers fragments whose size depends on the density of the bonds being hydrolyzed.
2. Hydrolysis, Ionization or Protonation of pendent group: It involves water insoluble polymers that become soluble when pendent groups are hydrolyzed, ionized or protonated.
3. Backbone Cleavage: Here cleavage of the hydrolysable bonds in the polymer backbone produces low molecular weight water soluble fragments.

Actual biodegradation may be combination of all these three mechanism.

RESEARCH SCOPE:
1. Various modes of polymer degradation can be assessed and the quantity of degradation can be evaluated. The degradation procedure can be framed and should be reproducible.
2. Various factors affecting degradability can be evaluated in terms of their type, methods of incorporation in the base polymer, mode of degradation, concentration/quantity, consistency towards degradation, processing ability, cost effectiveness, their impact on the environment i.e. the effect of residue and degradation fragments on the environments
3. The degree of degradation can be quantified and analytical methods can be developed.
4. Degradation mechanism can be established and a cost effective degradable polymer can be developed.