Dienes ozonation

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

Ozonc-rcsjstant elastomers which have no unsaturation are an exceUent choice when their physical properties suit the appHcation, for example, polyacrylates, polysulfides, siHcones, polyesters, and chlorosulfonated polyethylene (38). Such polymers are also used where high ozone concentrations are encountered. Elastomers with pendant, but not backbone, unsaturation are likewise ozone-resistant. Elastomers of this type are the ethylene—propylene—diene (EPDM) mbbers, which possess a weathering resistance that is not dependent on environmentally sensitive stabilizers. Other elastomers, such as butyl mbber (HR) with low double-bond content, are fairly resistant to ozone. As unsaturation increases, ozone resistance decreases. Chloroprene mbber (CR) is also quite ozone-resistant. 

Blends of diene and backbone-saturated mbbers are frequently used in appHcations where discoloration by chemical antiozonants caimot be tolerated, yet where cost is stiH a primary consideration (eg, white sidewalls of tires). Disadvantages are that physical properties have to be compromised and the two mbbers usually differ greatly in their rates of vulcanization. Usually, at least a 25% replacement by the ozone-resistant mbber is needed for an appreciable enhancement in ozone protection (6). 

This situation is identical to the previous one and occurs for example when paraffin wax is mixed into rubber above the melting point of the wax. On cooling, the wax starts to crystallise, some of it forming a bloom on the rubber surface. Such a bloom assists in protecting a diene rubber from ozone attack. 

When diene rubbers are exposed to ozone under stressed conditions cracks develop which are perpendicular to the direction of stress. Whilst ozone must react with unstressed rubber no cracking occurs in such circumstances nor when such rubber is subsequently stressed after removal of the ozone environment. For many years such rubbers were protected by waxes which bloomed on to the surface of the rubber to form an impermeable film. This was satisfactory for static applications but where the rubber was operating under dynamic conditions the wax layer became broken and hence less effective. 

At one time it was widely expected that EPDM rubbers would become of significance in the tyre sector. For a variety of reasons but primarily because of the high cost of the diene monomers this expectation has not been realised. On the other hand these rubbers have become widely accepted as moderately heat-resisting materials with good weathering, oxygen and ozone resistance and they find extensive use in cars, domestic and industrial equipment. 

In 1980 the Goodyear company announced copolymers of cyclopentadiene, cyclo-octene or cyclo-octa-1,5-diene with the Diels-Alder addition product of hexachlorocyclopentadiene and cyclo-octa-1,5-diene. This material has been proposed as an alternative to the polychloroprenes, with lower ( 5°C), and superior ozone resistance... 

The double bond present in the diene part of the elastomer is generally more susceptible to thermal and oxidative degradation. The selective hydrogenation of olefmic unsaturation in NBR imparts significant improvements in resistance to degradation and other properties, such as permeability, resistance to ozone and chemicals, and property retention at high temperature. 

The most prevalent approach to achieve long-lasting and nonstaining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The ozone-resistant polymer must be used in sufficient concentration (minimum 25 phr) and must also be sufficiently dispersed to form domains that effectively block the continuous propagation of an ozone-initiated crack through the diene rubber phase within the compound. Elastomers such as ethylene-propylene-diene terpolymers, halogenated butyl mbbers, or brominated isobutylene-co-para-methylstyrene elastomers have been proposed in combination with NR and/or butadiene rubber. 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

With isoprene, 2,3-dimethylbuta-1,3-diene and cyclopentadiene, if the ozone concentration in the ozone/oxygen mixture exceeds a certain limit (not stated), the medium immediately combusts when incorporating this mixture at -78°C. 

See Dinitrogen oxide or Halogens, both above Oxygen, below Calcium hypochlorite Acetylene Nitric acid Acetylene, Mercury(II) salts Nitrogen oxide Dienes, Oxygen Ozone Acetylene... 

See Dienes, above, also 2-Methyl-l,3-butadiene Ozone Nitrogen... 

The unsaturated structure of the diene hydrocarbon rubbers makes them susceptible to attack by both oxygen and ozone. Oxidative degradation of all rubbers, irrespective of their structures, is inevitable as the energy associated with incident natural light is approximately three times that of a typical carbon-carbon or carbon-hydrogen bond. 

Ozone attack occurs mainly at the olefinic double bond of a diene rubber and, if not protected against, will result in loss of physical integrity for thin sectioned articles and surface cracking on larger mass products. 

This route to the a-nitroso derivatives of the 7r-deficient heterocycles has permitted an exploration of their chemistry. They are extremely reactive and condense readily with 1,3-dienes to give 3,6-dihydro-l,2-oxazines (e.g. 99), and with aromatic amines in the presence of acid to give azo compounds (Scheme 86). This latter reaction is particularly useful in view of the instability of the corresponding 2-pyridinediazonium salts referred to above, which limits conventional access. The a-nitroso heterocycles are oxidized by ozone or sodium hypochlorite to the a-nitro compounds (Scheme 86) (82JOC553). 

---