Polyisoprene, natural degradable

To manufacture chlorinated rubber (CR) natural or synthetic rubber such as polyethylene, polypropylene or polyisoprene is degraded to low molecular mass compounds by mastication or addition of radical formers and dissolved in carbon tetrachloride (CTC). Chlorine contents are typically 64-68 wt%. Chlorine gas is introduced into this solution and reacts with the raw material to form CR. The solution is then introduced into boiling water. The CR is precipitated, and the solvent vaporizes. The CR is separated from water, rinsed, dried and ground to form a white powder which is the saleable product. After removal of the water, chlorine, hydrochloric acid and other impurities the solvent is reused. 

Comparative studies on the thermal degradation of polyisoprene, natural rubber (cis-polyisoprene) and gutta percha (t r tns-polyisoprene) all of which have the same chemical composition have shown that they differ in their thermal degradation characteristics. Studies under vacuum at 290-380 "C have shown that the decomposition of natural rubber (NR) is initiated at comparatively low temperatures at a considerable rate, whereas its decomposition rate at higher temperatures (above 330 "C) is to some extent slower than that of gutta percha and polyisoprene (Figure 2.1). The half-life temperature for synthetic polyisoprene is 320 C [1, 2]. 

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.)... 

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. 

While polymers that contain sites of unsaturation, such as polyisoprene and the polybutadienes, are most susceptible to oxygen and ozone oxidation, most other polymers also show some susceptibility to such degradation including NR, PS, PP, nylons, PEs, and most natural and naturally derived polymers. 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

During high-energy irradiation in vacuo, for example, from a Co source, some main-chain degradation of natural rubber and other polyisoprenes occurs ... 

Figure 2.1 Dependence of the degree of thermal degradation of natural rubber (cis-polyisoprene) (1), gutta percha (tr jns-polyisoprene) (2), and polyisoprene (3) on temperature. Source Author s own files...

Thermo-oxidative studies have also been reported on polyisoprenes and on natural rubber. In the latter case, the effect of relative molecular weight on thermo-oxidation as evidenced by carbonyl group formation was investigated. A mechanism for formation of isoprene during thermal degradation of natural rubber has been proposed. It involves a cyclic intermediate. 

The oxidation of diene rubbers proceeds under the influence of atmospheric oxygen even at room temperature, and results in the hardening and fragility of the surface layer. In its initial stages, the oxidative degradation of natural rubber is characterised by softening of the material and the appearance of stickiness rubber elasticity then decreases and it cracks [2]. Non-vulcanised synthetic polyisoprene is oxidised extensively even at room temperature [47]. The oxidation of rubber proceeds at its double bonds and at the single bond a to the tertiary carbon atom. 

Piya-areetham P, Rempel GL, Prasassarakich P. Hydrogenated nanosized polyisoprene as a thermal and ozone stabilizer for natural rubber blends. Polym Degrad Stab. 2014 102 112-121. 

Linos, A., Reichelt, R., Keller, U., and Steinbiichel, A., 2000, A Gram-negative bacterium, identified as Pseudomonas aeruginosa AL98, is a potent degrader of natural rubber and synthetic cis-l,4-polyisoprene. FEMSMicrobiol. Lett. 182 155-161. 

Nature provides a chemically diverse variety of degradable polymers. These polymers were the first materials used by humankind. Early men dressed themselves in hides (proteins, polysaccharides), later in cotton (polysaccharide), silk and wool (proteins). Early men used wood (polysaccharides, polyphenols) for tools and construction materials. Where available natural rubber (polyisoprene) was used for a variety of daily-life fimctions, from construction to water-proofing storage containers. 

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