Why do polymers break down?

Polymer degradation is a feature both of the intrinsic chemical structure of the material itself and of environmental parameters. For a given material, a number of external and environmental factors can influence the rate of degradation. These include:

• the effects of oxygen,
• the effects of moisture in the atmosphere, leading to hydrolytic breakdown,
• heat, leading to thermal degradation,
• mechanical stress, leading to loss in tensile properties, for example,
• the effects of light,
• radiative breakdown,
• chemical breakdown due to atmospheric pollutants,
• biological breakdown - the growth of fungi, mould etc on the surface of polymer materials,
• ultrasonic breakdown.

The action of all those factors upon a particular polymer results in a number of property changes that can be divided into two groups, physical and chemical changes.

• Physical changes include a decrease in molecular weight and hence loss in tensile strength for the polymer system, loss of gloss, elongation, distortion, etc.
• Chemical changes are those that result in discoloration of the system, production of new groups within the polymer system itself.

The same changes happen in motion picture film, which is essentially composed of a polymeric support. It does not matter whether it is cellulose nitrate, cellulose triacetate, or polyester, and the same is true for magnetic media, and for emulsion containing the silver image and gelatine or a urethane binder and magnetic oxide. In all cases, the binders are polymeric and the support materials are polymeric.

I said earlier that the stability of a given polymer will be dependent upon both the innate structural features and external environmental parameters. The structural features of most polymers are:

• the main polymer unit itself, the form in which it’s present, its physical form, whether it’s present as a film, whether it’s present in bulk, or whether it’s present as a powder - all these influence stability;
• the presence of impurities within the system from manufacturing catalysts, for example metal trace ions that it has picked up from washing procedures;
• the presence of incorporated additives, such as plasticizers (which are generally added to polymers to render them flexible), pigments, antistatic agents and flame retardants.