Ozone reaction with diene rubbers

This article reviews literature on ozone reaction with diene rubbers. Physical and chemical phenomena of rubber ageing by ozone and mechanisms of ozone reaction with rubbers are described. Possibilities of improving ozone resistance of rubbers and vulcanisates made from... 

Antiozonents protect the rubber surface by the formation of a protection layer, the ozonides on the surface of rubber by reaction of the antiozonents with ozone. Certain polymers also provide good ozone protection. The use of 10-20 parts of EPDM, a low diene rubber, in natural rubber compound significantly increases ozone resistance. 

All antiozonants react with ozone at a much higher rate than does the rubber which they protect. For example, selected antiozonants have been shown to react preferentially with ozone in rubber films as well as in solution. Antiozonant-ozone reaction rates are typically one to two orders of magnitude higher than the rates for diene rubbers. 

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. 

SOME NEW ASPECTS OE OZONE AND ITS REACTIONS WITH DIENE RUBBERS... 

Some New Aspects of Ozone and Its Reactions with Diene Rubbers 3... 

As mentioned in Section 18.2.3, natural rubber (in common with other diene elastomers) is readily attacked by ozone. The mechanism of the reaction is probably the same as that established for simple olefins [8]. In this case, the... 

Although the full significance of the reactions of ozone with diene rubbers was not to be appreciated until the mid-1940s its importance was recognized in part, by the early years of this century, with the ozonolysis techniques developed by Harries (see Chapters 1 and 2). Subsequently the reactions between olefins and ozone were studied by various workers and much of the experimental data obtained can be accounted for by a series of mechanisms proposed by Criegee (1951, 1954, 1955). The essential proposals are summarized below. 

As mentioned in section 20.2.3, natural rubber (in common with other diene elastomers) is readily attacked by ozone. The mechanism of the reaction is probably the same as that established for simple olefins [10]. In this case, the initial product is a -complex which cleaves to form an aldehyde or ketone and a zwitterion. Several subsequent reactions may then occur, depending on the nature of the reactants and conditions. The zwitterion may dimerize or polymerize or react with the carbonyl compound to form an ozonide ... 

Many types of rubber which resist ozone were obtained by copolymerization of ethylene and propylene with asymmetric dienes such as 1,4-hexadiene, dicyclopen-tadiene, or ethylidenenorbornene. The addition to the reaction mixture of a certain amount of diene allows preparation of a very stable polymer, because the formation of a linear polymer requires breaking of one double bond while the other bond is utilized to net the copolymer which takes place during the polymerization process. Vanadium... 

The main advantages of these polymers, in common with SBR and natural rubber, are that they are inexpensive, may be cross-linked by standard vulcanizing systems and, in many cases, remain rubbery at lower temperatures than is common with many synthetic rubbers. Like natural rubber and SBR, the synthetic diene homopolymer rubbers have limited oxygen and ozone resistance whilst chemical reactions resulting from the existence of a carbon-carbon double bond can, in some instances, lead to unwanted cleavage or scission. Furthermore as hydrocarbons they do not have good swelling resistance to petrol and other hydrocarbon liquids. 

Acid, acid chloride and -OH functionalized telechelics have been described. Although this does not represent the most convenient way to synthesize such materials, the reaction of polybutadiene with controlled amounts of ozone has been used to synthesize hydroxy-terminated telechelic polybutadienes. The oxidative cleavage of copolymers of isobutylene with dienes such as piperylene, isoprene (butyl rubber) and butadiene is probably one of the most-studied cleavage reactions. " In particular, the preparation of carboxy-terminated telechelics has been extensively studied. ... 

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