Natural rubber cross-reactivity

Vulcanisation of rubber Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity, it Is soluble in non-polar solvents and Is non-resistant to attack by oxidising agents. To improve upon these physical properties, a process of vulcanisation is carried out. This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K. On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened. 

This type of cross linking could be readily brought about by vulcanization. This process requires the heating of natural rubber with sulfur, thus forming sulfur bridges between different chains. This is possible because the double bonds in the rubber molecule provide reactive allylic hydrogens. 

Accelerated-sulfur vulcanization is the most widely used method. For many applications, it is the only rapid cross-linking technique that can, in a practical manner, give the delayed action required for processing, shaping, and forming before the formation of the intractable vulcanized network. It can be used to vulcanize natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), butyl rubber (HR), chlorobutyl rubber (CIIR), bromobutyl rubber (BUR), and ethylene-propylene-di-ene-monomer rubber (EPDM). The reactive moiety for all of these elastomers can be represented by... 

Alenius H, M nen-Kiljunen S, Ahlroth M, Turjanmaa K, Reunala T, Palosuo T (1996c) Cross-reactivity between allergens in natural rubber latex and banana studied by immunoblot and immunoblot inhibition methods. Clin Exp Allergy 26 341-348... 

There are two opposing requirements concerning the chemical reactivity of a rubber. On one hand it should be sufficiently reactive to allow desirable reactions such as cross-linking, whilst on the other hand it should not react during service in any adverse manner. Natural rubber, and indeed all the hydrocarbon diene rubbers, provide excellent examples of this. In such polymers the double bond provides a site for vulcanization. At the same time it is a vulnerable point for attack by oxygen, ozone and other agents which can, in some circumstances, have a catastrophic effect. 

Polymer reactivity differs from the reactivity of simple molecules in two important respects. The first is the considerable influence of groups present in only trace amounts. Some of these are weak links being either thermally unstable or undesirably reactive and lead to premature degradation. Others, such as the trace aldehydic groups noted in natural rubber can cause undesired cross-linking. 

This chapter has considered the application of several well-known reactions of alkenes to diene polymers. Whilst the basic reactions are generally predictable from a knowledge of alkene reactivity they are influenced by the fact that the double bond is part of a very long chain molecule consisting of double bond-containing repeat units. In particular the tendency to cyclize has been noted, for example in the case of chlorinated natural rubber and with rubber hydrochloride whilst in other cases degradation or cross-linking has occurred. It may be noted here that the ability to produce cyclized natural rubber is a direct consequence of the affinity of a carbonium ion for a double bond when activated in a polymeric environment. 

Siler, D. X, Cornish, K., Hamilton, R. G. Absence of cross-reactivity of IgE antibodies from subjects allergic to Hevea brasiliensis with new sources of natural rubber latex from guayule (Parthenium argentatum). J. Allergy Clin. Immunol. 1996, 98, 895-902. 

The sulfur vulcanization properties of guasuile rubber reflect the low level of reactive substituents. Because of natural vulcanization activators not present in guayule rubber, Hevea rubber has a higher cure rate and attains a higher level of cross-linking (111,112). However, adjusting the cure system can reduce these performance differences (112). 

The chemical reactivity of a commercial rubber compound will depend on the nature of the initial polymer, the nature of the cross-links introduced during polymerization and, to some extent, the additives that may be present. It is stating the obvious to say that the first two are dependent on the nature of the chemical bonds present. Nevertheless it is useful to make some general observations about the various groups and bonds that occur in rubbers by reference to the following tabulated list of examples ... 

---