Natural rubber oxidative degradation

As with c -polyisoprene, the gutta molecule may be hydrogenated, hydro-chlorinated and vulcanised with sulphur. Ozone will cause rapid degradation. It is also seriously affected by both air (oxygen) and light and is therefore stored under water. Antioxidants such as those used in natural rubber retard oxidative deterioration. If the material is subjected to heat and mechanical working when dry, there is additional deterioration so that it is important to maintain a minimum moisture content of 1%. (It is not usual to vulcanise the polymer.)... 

The author is unaware of any commerical polymers that are specifically designed to degrade oxidatively, although oxidation may be involved in association with hydrolytic and biological degradation. It may be of interest to note that before World War II products known as rubbones were produced by degrading natural rubber with cobalt linoleate in the presence of cellulosic materials to produce low molecular weight, fluid oxidised natural rubber (Section 30.4). 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

The action of certain metals (e.g., copper) on unsaturated rubbers, primarily natural rubber, is to catalyse the oxidative degradation of the polymer. The metal must be in an ionic form, i.e., straightforward contact with the metal such as a seal with a copper pipe will not promote such degradation. 

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. 

The most thoroughly investigated oxidative degradation is that of natural rubber. In 1943 Farmer and Sundralingham found that in the photochemical oxidation of this polymer a hydroperoxide is formed, the number of double bonds in the chain remaining constant. The oxygen was found to act on an activated methylene group, not on a double bond, as had previously been assumed. 

Oxidative degradation can also occur in other polymers including natural rubber, polystyrene, polypropylene, nylons, polyurethanes, and most natural and naturally derived polymers. With the exception of fluo-ropolymers, most polymers are susceptible to oxidation, particularly at elevated temperature or during exposure to ultraviolet hght. Oxidation usually leads to increasing brittleness and deterioration in strength. 

Protects against thermo-oxidative degradation in natural and synthetic rubbers as well as plastics. It is commonly used in applications such as tire carcasses, wire breaker retreads, apex, belts, hoses, seals, mechanical goods, footwear and wire. 

Natural rubber is a polymer of isoprene units. As a naturally occurring polymer, it is subject to microbial attack especially in hot humid conditions. The microbial degradation may be accentuated by chemical oxidation. 

In the same manner as blends of thermoplastics or thermoplastic/ elastomer, irradiated blends of elastomers can undergo chain scission due to degradation or cross-linking, depending especially on the dose range. For example, Zurina et al. i measured tanS versus temperature for 50/50 epoxidized natural rubber (ENR-50)-EVA blends by DMA. At 60 kGy the irradiation-induced cross-link enhanced the Tg of the blend, whereas at higher dose (100 kGy), the Tg decreased due to the occurrence of oxidative degradation that broke the cross-link structure. 

Zinc oxide is commercially the most important compound of zinc and it is used to a considerable extent in the rubber, ceramic and paint industries. It is utilized to activate the organic accelerator of the vulcanization process for natural rubber and for most synthetic rubbers. Also zinc oxide serves as the accelerator for some types of elastomers. Further, it provides reinforcement to the rubber, it improves its heat conduetivity, it limits the degradation by UV radiation and it improves adherence. Typically, rubber eontains 2 % zinc oxide. A very broad range of other compounds are of extensive interest in chemistry, biology and physics and they make great practical contributions to the chemical, ceramic, fertilizer, paint, plastics, textile and electronics industries. 

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