Diene rubbers oxidation

Ethylene—Propylene (Diene) Rubber. The age-resistant elastomers are based on polymer chains having a very low unsaturation, sufficient for sulfur vulcanization but low enough to reduce oxidative degradation. EPDM can be depicted by the following chain stmcture ... 

Most accelerators used in the accelerated sulfur vulcanization of other high diene rubbers are not applicable to the metal oxide vulcanization of CR. An exception is the use of so-called mixed-curing system for CR, in which metal oxide and accelerated sulfur vulcanization are combined. Along with the metal oxides, TMTD, DOTG, and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

A high diene rubber can also be vulcanized by the action of a dinitrosobenzene, made in situ by the oxidation of a quinonediooxime (Figure 14.13) [56-60] incorporated into the rubber together with the vulcanizing agent lead peroxide. 

Because of the many double bonds present, diene rubbers are sensitive to air oxidation unless antioxidants are added to inhibit oxidation. 

Phenanthroline in the presence of heavy metals acts as an activator of the polymerization of vinyl compounds - and other olefins. " It also assists the dimerization of olefins in the presence of titanium catalysts. - It enhances the metal catalyzed oxidation of ascorbic acid and dimethyl sulfoxide. On the other hand, on its own it can inhibit several polymerization processes. - It also stabilizes butadiene and isoprene and prevents their dimerization. It prevents peroxide formation in ether, inhibits the vinylation of alcohol and stabilizes cumyl chloride. It accelerates the vulcanization of diene rubbers and copolymers. 1,10-Phenanthroline catalyzes the autooxidation of linoleic and ascorbic acids in the absence of metals. ... 

Figure 15.2 Reactions of bis(diisopropyI)-thiophosphoryI disulfide (DIPDIS) with ethylene-propylene-diene rubber (EPDM) and zinc oxide. (From Reference 32 with permission from John Wiley Sons.)...

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. 

Diene rubbers such as natural rubber, SBR, and BR can be vulcanized by the action of phenolic compounds [65-68], which are (usually di-)substituted by -CH2-X groups where X is an -OH group or a halogen atom substituent. A high-diene rubber can also be vulcanized by the action of a dinitrosoben-zene that forms in situ by the oxidation of a quinonedioxime [69-73] that had been incorporated into the rubber along with the oxidizing agent, lead peroxide. 

Blending methyl methacrylate-butadiene-styrene copolymer with poly(vinyl chloride) for instance was shown to decelerate the dehydrochlorination (leading to discoloration). The gel content, surface energy, and the spectroscopic characteristics of the blend was altered by the presence of the seccHid polymer [158]. In ethylene-propylene-diene rubber EPDM where the third monomer is ethylene-2-norbomene (NB), the photo-oxidation rate as measured by the accumulation of typical products such as hydroperoxides, varied linearly with the NB content [159]. The same held true for peroxide-crosslinked compounds of the same EPDM except that the linear relationship was found between the relative carbonyl absorbance on photoxidation and the amoiuit of peroxide used to crosslink the material... 

Rubbers, often based on poly diene rubbers or else copolymers of dienes like 1,3-butadiene, were the first successful toughening additives, and they are effective partly because they have a low modulus, 100 to 500 times lower than that of most thermoplastic polymers. Unfortunately polydienes introduce chemical double bonds which are susceptible to UV, thermal and oxidative degradation. Hydrogenation removes some of them. Acrylic compounds and ethylene copolymers are also popular impact modifiers, and they do not necessarily introduce double bonds. 

Compounding, to a large extent, follows the rules for conventional diene rubbers with some notable exceptions. For example, zinc oxide and stearic... 

Butyl rubber, unlike the diene rubbers, is substantially free of olefinic linkages after vulcanization and hence resistant to oxidation and weathering. Butyl, one of the major sjmthetic rubbers, was manufactured in recent years in at least six countries for the total world production (exclusive of USSR manufacture) of approximately 400,000 metric tons annually. In 1979, butyl was the third most important synthetic rubber, volume-wise, in the United States (Table I). 

Antiozonant protection is required for most rubber products, which are based on diene rubbers. Ozone attack is different to oxidative ageing in that specific conditions are required for it to occur. These conditions are ... 

The nitroso compounds may be added directly or, preferably in order to reduce toxic hazards, formed in situ either by reduction of a nitro compound or by oxidation of an oxime. Vulcanizing agents of this type are not known to be in current use with diene rubbers but have found some application with butyl rubber. They are therefore considered in more detail in Chapter 11. 

REACTIVITY OF DIENE RUBBERS—HI OXIDATION AND RELATED PROCESSES... 

There are many formal resemblances between the oxidation of saturated hydrocarbons including polymers such as polyethylene, diene rubbers such as natural rubber and low molecular weight analogues such as squalene. The principal features common to these systems are as follows ... 

---