Aging of Polymers

the irreversible change in the properties of polymers under the action of, among other factors, heat, oxygen, sunlight, ozone, and ionizing radiation. Depending on the factor, the aging will be of the thermal, oxidative, light, ozone, or radiation type. Aging occurs during the storage and treatment of polymers, as well as during the storage and use of articles made from polymers. Under actual conditions, several factors act on polymers concurrently; for example, in atmospheric aging, oxygen, light, ozone, and moisture all affect the polymer. An important factor accelerating aging is mechanical stress that develops during the treatment of polymers and under certain operating conditions of polymer articles.

Aging is caused by chemical transformations of macromole-cules, which lead to the degradation of the macromolecules and the formation of branched or three-dimensional structures (cross-linking). There are various mechanisms of aging. For example, degradation in oxidative aging is related to a chain oxidation reaction of the polymer accompanied by the formation and decomposition of hydroperoxides. The rate of aging depends on the sensitivity of the polymer to the factors mentioned above, on the intensity of these factors, and on the composition of the polymer material. Carbon-chain polymers whose macromolecules contain unsaturated bonds, in particular, certain rubbers (natural rubber, synthetic isoprene rubbers), are most susceptible to aging.

Aging manifests itself in a deterioration of the mechanical characteristics of polymers, in the appearance and growth of cracks on the surface that sometimes result in destruction of the polymer, and in a change in color. The resistance of polymers to aging in many cases determines the storage period and, sometimes, the service life of polymer articles. Stabilization is an effective method for preventing aging in polymers.