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