We are all familiar with the fact that rubber articles undergo, over more or less prolonged periods of time, a series of changes in their properties leading to a partial or total deterioration of the article (excluding cases where there is a direct mechanical action, we speak in such cases of abrasion or wear).
This deterioration is known as rubber aging and is manifested by changes in appearance (color, surface cracking, etc.) and/or changes in mechanical properties (hardness, tensile strength, modulus, etc.) that may make the article unsuitable for the function for which it was designed and require replacement (whether we like it or not, there is a parallel with what happens to people with the passage of time).
TYPES OF AGING
One of the first questions that comes to mind when dealing with this subject is: what causes rubber to age? There is no single cause and there are a number of factors that cause the deterioration of rubber, and usually two or more of these factors act at the same time, the interrelationship between them being very complex:
Just as the rate of chemical reactions normally increases with temperature, so do the chemical reactions of aging. In some cases the heat source is external (articles used in high temperature environments) or internal (articles subjected to repeated deformations that generate heat by hysteresis).
We have all noticed at some time that the properties of a rubber compound change over time (in some cases they harden, in others they soften) and this phenomenon is particularly noticeable if the rubbers are of the dienic type (i.e. with double bonds in the main chain), such as NR, SBR, NBR, BR, etc. This phenomenon is aggravated if service temperatures are high. Some become rigid and brittle (case of SBR and NBR). On the other hand, if chain breaking reactions predominate, the article becomes soft and even sticky (NR, IR and IIR). The speed of these oxidation reactions increases markedly with temperature and this explains why the above-mentioned rubbers cannot be used above a certain temperature, as they would age very rapidly.
Ozone (O3) is formed in the atmosphere by the action of electrical discharges and/or UV radiation. Although the percentage of ozone in the air is very low, its exceptional reactivity makes its effect noticeable through the formation of typical cracks. These cracks propagate perpendicular to the direction of the tensile or bending stress and deepen as the ozone attack progresses. It is very important to know which areas of the article are subject to deformation (static or dynamic), since these areas will be the focus of O3 attack and the deformation itself helps to expose (as the cracks grow) new areas susceptible to attack.
Articles subjected to repeated deformations (belts, belting, tires, etc.) are more prone than those in static use to the appearance of cracks on their surface. These cracks grow as a result of repeated deformations and can render the article unusable. Fatigue is the aging of a rubber article subjected to cyclically varying stresses. This is a typical situation for items such as belts, conveyor belts, tires, engine mounts, etc. This type of aging is made visible by the appearance of cracks, which grow with the time of use of the product, eventually rendering it unusable. The two basic mechanisms of crack growth are:
Articles exposed to light receive ultraviolet (UV) radiation, which promotes the initiation of oxidation reactions. Since carbon black absorbs much of the UV radiation, articles containing carbon black are better protected from this effect.
There are two types of situations:
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