Modes Of Degradation Of Polymers 15u7

Discuss the various environmental effects causing degradation in the properties of polymers.

Introduction A polymer is a macromolecule composed of a large number of repeating units called monomers connected by covalent bonds. Examples of natural polymers are cellulose, rubber, wool, resins, silk and proteins. Man-made polymers include plastics such as polyethylene, polystyrene, nylon, Perspex and synthetic rubbers. Polymer degradation is a change in the properties- tensile strength, colour shape of a polymer under theinfluence of one or more environmental factors, such as heat, light, chemicals and in some cases, galvanic action.

The chemistry and technology of polymer degradation relates to the engineering issues:  Material durability - mechanical and aesthetic qualities  Polymer stabilization - antioxidant and UV stabilizer choices  Recycling - breakdown reactions of packaging and vulcanizates

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Oznolysis Cracks can be formed in many different polymers and elastomers by ozone attack. It is characterized by form of cracks in mostly vulnerable rubber (tires) or rubber like plastic. Ozone cracks form in products under tension, but the critical strain is very small. Such cracks are very dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow.

Susceptible elastomers: Tiny traces of ozone in air will attack double bonds in rubber chains. Most susceptible elastomers are natural rubber, polybutadiene , styrene and nitrile rubber.

Thermal degradation At high temperature, polymers such as poly(methyl methacrylate) become thermally unstable, leading to degradation by depolymerisation to yield a mixture of monomer and polymer. The components of the long chain backbone of the polymer separate (molecular scission) and react with one another to change the properties of the polymer. Characteristics of thermal degradation; mechanical failure i.e. possible loss in mechanical properties; ductility, malleability and brittleness is altered. Other properties altered: colour change, crack formation and molecular weight distribution.

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Ways of polymer thermal degradation 

Random scission

The backbone breaks down randomly at any possible position of the backbone, forming chain molecule radicals reacting with one another in a disproportionate manner.

Random scission parts



Side-group chain elimination.

When the polymer is being heated, the side groups are stripped off from the backbone into small monomers, e.g. PVC.

Side groups being eliminated

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Chlorine-induced cracking Chlorine gas is highly reactive and attacks susceptible polymers such as acetal resin and polybutylene pipe work. Essentially the gas attacks sensitive parts of the chain molecules (especially secondary, tertiary carbon atoms), oxidising the chains and ultimately causing chain cleavage. Sometimes chlorine reduces the lining of the pipes ts carrying water thereby causing leakage & domestic flood..

Radical attacks Radical from external, foreign to the structure polymer attacks the polymer chains .initiation starts followed by propagation and ends with termination. Radical chemistry is very disorder chemistry. So, it leads to unknown possibilities of change in the polymer properties.

Hydrolysis Some polymers such as nylon is sensitive to degradation by acids, a process known as hydrolysis, and nylon mouldings will crack when attacked by strong acids.

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Oxidative degradation Polymer degradation is almost always faster in the presence of oxygen (air), due primarily to the auto accelerating nature of reactions between oxygen and carbon centered radicals; especially at elevated temperatures encountered during processing to shape.

Plastic degradation can occur aerobically or anaerobically. In this case aerobically is considered. It is characterized by physical disintegration and drastic reduction in molecular weight. Plastic becomes like flakes .Conversion of carbon into monomers and carbon dioxide gradually happens.

Degradation of polymers by micro organisms Degradation of polymers by micro organisms leading to biological attacks .polyethylene degrades by a variety of bacterium (rhodocucus rubber)(C208).

New Plastic

plastic degraded

prolonged exposure

by bacteria

to the bacteria.

later on

Other agents of degradation include micro organism leading to biological attack & ionizing radiation leading to radiolysis.

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Photo-oxidation Photo-oxidation is the oxidation of a polymer surface in the presence of oxygen or ozone that is facilitated by radiant energy such as UV or artificial light. This process is often the most significant component of weather degradation of polymers. Photo-oxidation results in chemical changes and normally a reduction in the polymer's molecular weight. Consequently, the material becomes more brittle coinciding with a reduction in tensile strength, impact strength, elongation at break. Often discoloration and loss of gloss accompany photo-oxidation. High temperature and stress concentration are significant factors to increase reaction rate of photooxidation. 

Graph of hardness v/s deepness

Material becomes less ductile at surface compared to inside the material cracks can be encouraged as a result on the surface hardness.

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Degradation by u.v light. Most vulnerable plastics, thermoplastics and some thermosetting plastics degrade in the presence of u.v light. These are characterized by cracks formation, fading in colour and yellowing discoloration. T here is loss in mechanical integrity due to cracks that occurred. As a result, u.v light attacks increases because of greater surface to volume ratio. Severity of the impact of increased UV-B levels on the outdoor lifetimes of materials depend on both the geographic location of exposure and the susceptibility of the particular material to UVB radiation. While the higher latitudes will experience the high levels of ozone depletion, the high ambient temperatures in the near-equator regions will tend to severely magnify the effect of even a very marginal increase in solar UV-B in these regions.

Exposure to humid and wet environments to damp and wet environments, tends to reduce the performance of composite backbones. Polymer network composites soak up water which may cause plasticization and bulge of the matrix. This therefore induces reductions of Young’s modulus and glass transition temperatures of the polymer matrix. Moisture swelling of the matrix may alter the state of residual stresses introduced during processing of the composite. Moisture may also affect the fibres in a composite. The material becomes weak after exposure to damp environment. However Glass fibres do not absorb water, although when subject to water or humidity, they are susceptible to leaching of soluble oxides reflected in strength reductions.

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