10-17 OXIDATION RESISTANCE

10-17 OXIDATION RESISTANCE

Many commercially important polymers are susceptible to oxidation when exposed to atmospheric oxygen either during manufacturing processes or during the use. Oxidation is a chain reaction initiated and propagated by free radicals which, in the air, readily combine with molecular oxygen to form a polymer peroxy radical (POO ⋅ ). Most polymers are prone to oxidation and photoinitiated degradation. Molecular weight is changed by chain scission or by crosslinking, causing deterioration of physical properties such as softening or brittleness. The greater the amount of unsaturation (carbon-to-carbon double bond) the more susceptible is the material to oxidation and subsequent degradation. Among saturated polymers those containing a tertiary carbon hydrogen bond such as polypropylene (PP) are more easily oxidized than polymers that do not show such bonding. In PP, oxidation causes scission of the main polymer chain, attacking the carbon—carbon bond. In certain high-density polyethylenes (HDPE) the crosslinking reaction predominates. In polyvinyl chloride (PVC) breakdown occurs by dehydrochiorination with evolution of a hydrogen chloride molecule (HCl). Polyesters, polyurethanes, polyamides and polycarbonates usually discolor under oxidation with little change in physical properties. However, at higher temperatures or under prolonged exposure to UV radiation, the physical properties may deteriorate. Polymers also differ in the effects of oxidation on their properties. Depending on the polymer and the type of reactions that take place during polymer breakdown, changes may occur that adversely Many commercially important polymers are susceptible to oxidation when exposed to atmospheric oxygen either during manufacturing processes or during the use. Oxidation is a chain reaction initiated and propagated by free radicals which, in the air, readily combine with molecular oxygen to form a polymer peroxy radical (POO ⋅ ). Most polymers are prone to oxidation and photoinitiated degradation. Molecular weight is changed by chain scission or by crosslinking, causing deterioration of physical properties such as softening or brittleness. The greater the amount of unsaturation (carbon-to-carbon double bond) the more susceptible is the material to oxidation and subsequent degradation. Among saturated polymers those containing a tertiary carbon hydrogen bond such as polypropylene (PP) are more easily oxidized than polymers that do not show such bonding. In PP, oxidation causes scission of the main polymer chain, attacking the carbon—carbon bond. In certain high-density polyethylenes (HDPE) the crosslinking reaction predominates. In polyvinyl chloride (PVC) breakdown occurs by dehydrochiorination with evolution of a hydrogen chloride molecule (HCl). Polyesters, polyurethanes, polyamides and polycarbonates usually discolor under oxidation with little change in physical properties. However, at higher temperatures or under prolonged exposure to UV radiation, the physical properties may deteriorate. Polymers also differ in the effects of oxidation on their properties. Depending on the polymer and the type of reactions that take place during polymer breakdown, changes may occur that adversely

Certain rubbers, especially those that contain a double bond, like natural rubber, styrene—butadiene, neoprene, and acrylonitrile rubber, are very sensitive to oxidation. The attack is enhanced by the presence of ozone in the atmosphere which under normal

conditions may range from 0 to 20 parts per 100 million. Ozone (O 3 ), unlike molecular oxygen (O 2 ), adds directly to a double bond, forming ozonides which further decompose to peroxides and ketones. In an unstressed rubber a silvery film is formed “on” the surface especially in a humid atmosphere. When rubber is under stress, cracks appear perpendicular to the direction of stress which may lead to failure of the material. Light, especially UV radiation, enhances the action of oxygen, causing deterioration of the physical strength and the phenomenon of crazing, and alligatoring at the surface occurs. When the film is washed away filler particles are exposed which are easily rubbed off.

To prevent or to reduce oxidation effects on polymers a number of antioxidants and antiozonants are mixed with the polymer during processing operations. Antioxidants are compounds that inhibit or retard atmospheric oxidation and the effects on a polymer system. There are three main classes of antioxidants used: (1) secondary amine bodies

R 2 NH, which react with chain-propagating radicals that intercept either R ⋅ or RO 2 ⋅ , (2) phenolic bodies R(OH) x , and (3) phosphites (RO 3 )P that decompose peroxide into non- radical and stable products.