7.2 Ageing

The rate of deterioration of materials depends on their nature, for the hardest rocks, the time scale stretches to millions of years, whereas for some organic polymers, major modifications can be induced by exposure of only a few days. Ageing is often used somewhat interchangeably with the term degradation.

The ageing process of polymers occurs in a wide variety of environments and service conditions, and very often limits the service lifetime [67]. Ageing is the adverse or detrimental change in a desired physical or chemical property. This process is primarily caused by the climatic stresses of sunlight, pollutants, temperature and water (dew, humidity, rain, snow). To serve satisfactorily, polymers, like most materials, must meet the more stringent requirements of general use in terms of sustaining their function with a minimum of change in properties. They have to be maintained in a wide range of situations including use under rigorous weather, that is the case for some plastics used in construction [68, 69].

Polymer degradation can be caused by chemical factors (oxidation-degradation, hydrolysis), heat (thermal-degradation), light (photo-degradation), ionising radiation (radio-degradation), mechanical action (mechanical-degradation), or by fungi, bacteria, yeast, algae, and their enzymes (biodegradation).

In principle, there are a few ways to control the ageing process, such as through the use of stabilisers, by avoiding unnecessary thermal exposure, and by excluding oxygen and water as much as possible [70]. Stabilisation is the procedure of slowing down the rate of degradation. Free radical stabilisation techniques commonly used include: UV...