Polyolefins ozone attack

Reactions of ozone with polyolefins at room temperature produce oxidized surface layers that act as jAiysical barriers to further attack. Bie layers are too thin (affect mechanical properties but the products are capable of initiating thermal and photooxidations. 

The y-irradiation of samples in air occurs in the presence of some ozone formed by the radialysis of the air itself. (The smell of ozone is always detectable when the Gamma cell is opened.) Ozone can attack saturated polyolefins to generate radicals and initiate oxidative chains (12). The small but reproducible retardation of the oxidation of the PP films intimately wrapped in a natural rubber film is then consistent with an 03 component to the oxidation, the unsaturated rubber shielding the PP film to some extent from 03 attack by means of the rapid 0,-unsaturation reaction. As well as free radical generation, the 03-PP reaction may also lead to unstable ozonide and peroxide intermediates. 

The effect of ozone on saturated polymers, polyolefins (PO) and polystyrene (PS) in particular, is underrated. Ozone is involved in the initiation phase of their oxidation. Its attack results in formation of R, RO, ROO, ROOH and I CO species in PO, and free radicals and polymeric peroxides in PS. Moreover, ozone decomposes hydroperoxides into oxygen-centred radicals RO, ROO and HO. ... 

In the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which through a complex series of reactions result in polymer degradation. Oxidative degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet radiation (photooxidation). Unsaturated polyolefins are particularly susceptible to attack by oxygen or ozone (Equation 9.6). 

It is proposed that this is due to attack of carbonyl oxides, in their biradical form, on the rubber double bonds. Typical diene rubbers (polyisoprene and polybutadiene) have rate constants several orders of magnitude greater than polymers having a saturated backbone (polyolefins). Other unsaturated elastomers having high reaction rates with ozone include styrene-butadiene (SBR) and acrylonitrile-butadiene (NBR) rubbers. As an example, Polychloroprene (CR) is less reactive than other diene rubbers, and it is therefore inherently more resistant to attack by ozone. 

Polymers may be attacked by molecular oxygen, ozone, or by indigenous free radicals in the polymer. Thermal-oxidative degradation of polyolefins in air is autocatalytic, i.e., the rate is slow at first but gradually accelerates to a constant value. According to the three-step mechanism outlined below, the RO2 peroxy radicals formed (Step 1) are sufficiently reactive to attack some primary CH bonds of the chain R H (Step 2). The peroxy radical RO2 is thus reformed (Step 3) and can attack another CH bond. This chain reaction continues until termination occurs (Step 4) [1-11]. 

Polyolefin thermoplastic elastomers are characterized by low cost, good mechanical properties and excellent resistance to oxygen, ozone and polar solvents they are attacked by non-polar and chlorinated solvents, especially... 

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