Natural vulcanisation

To complete the modified view of geomacromolecule formation, the process of natural vulcanisation (Fig. 4. 10) has been proposed to play a major role under suitable conditions (e g. Sinninghe Damste etal. 1989a, 1990 de Leeuw and Sinninghe Damste 1990). Many marine sediments contain high-molecular-weight... 

By comparison, temperatures as high as 150°C are often required for mold-enclosed hard natural mbber compounds, where mold plattens are directly heated by steam or electricity. Synthetic latex mbber compounds, however, can be vulcanised at temperatures higher than those for natural mbber neoprene and acrylonitrile—butadiene can be vulcanised at as high as 135°C. 

ISO 1658 Natural Bubber - Test Recipes and Evaluation of Vulcanisation Characteristics, International Oiganization foi Standaidization, Geneva, Switzerland, 1973. 

Processings and Properties. Polybutadiene is compounded similarly to SBR and vulcanised with sulfur. The high cis-1,4 type crystallizes poorly on stretching so it is not suitable as a "gum" stock but requires carbon black reinforcement. It is generally used for automotive tires in mixtures with SBR and natural mbber. Its low T (—OS " C) makes it an excellent choice for low temperature tire traction, and also leads to a high resilience (better than natural mbber) which ia turn results ia a lower heat build-up. Furthermore, the high i j -polybutadiene also has a high abrasion resistance, a plus for better tire tread wear. 

The vulcanisation of natural rubber, a long chain polyisoprene, with sulphur involves a similar type of cross-linking. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

The proximity of the methyl group to the double bond in natural rubber results in the polymer being more reactive at both the double bond and at the a-methylenic position than polybutadiene, SBR and, particularly, polychlor-oprene. Consequently natural rubber is more subject to oxidation, and as in this case (c.f. polybutadiene and SBR) this leads to chain scission the rubber becomes softer and weaker. As already stated the oxidation reaction is considerably affected by the type of vulcanisation as well as by the use of antioxidants. 

Because of the excellent properties of its vulcanisates under conditions not demanding high levels of heat and oil resistance, natural rubber commands a premium price over SBR, with which it vies for top place in the global tonnage... 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Over the past 40 years there have been a number of developments that have resulted in the availability of rubbery materials that are thermoplastic in nature and which do not need chemical cross-linking (vulcanisation or setting) to generate elastomeric properties (see also Section 11.8 and 31.2). This approach has been extended to the fluoroelastomers. 

Another approach has been adopted by the Du Pont Company with Adiprene C. This is a urethane-type polymer with unsaturated groups in the polymer. Because of the unsaturation the polymer may be vulcanised with sulphur, the standard vulcanising agent of the rubber industry. This is a clear-cut example of a product being modified to suit the processor rather than that of a processor adapting himself to meet new products. Whereas Adiprene C has poor tensile strength when unfilled, the use of carbon black leads to appreciable reinforcement (as is the case with SBR and to some extent natural rubber. 

Hard products may also be made by vulcanising rubber (natural or synthetic) using only about two parts of sulphur per 100 parts of rubber. In these cases either the so-called high-styrene resins or phenolie rubber compounding resins are ineorporated into the formulation. These compounds are processed using the methods of rubber technology but, like those of ebonite, the produets are more akin to plastics than to rubbers. Examples of the usage of these materials are to be found in battery boxes, shoe heels and ear washer brushes. 

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.)... 

Oil resistance demands polar (non-hydrocarbon) polymers, particularly in the hard phase. If the soft phase is non-polar but the haid phase polar, then swelling but not dissolution will occur (rather akin to that occurring with vulcanised natural rubber or SBR). If, however, the hard phase is not resistant to a particular solvent or oil, then the useful physical properties of a thermoplastic elastomer will be lost. As with all plastics and rubbers, the chemical resistant will depend on the chemical groups present, as discussed in Section 5.4. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

Standard butyl rubber, which is a copolymer of isobutylene with about 2% of isoprene vulcanises in the same manner as natural rubber but, as it only contains a small proportion of polyisoprene, the cross-link percentage is much reduced. It is therefore not possible to make ebonite from a butyl rubber. The same vulcanisation chemistry, with some modifications, applies to ethylene-propylene terpolymers and brominated butyl rubber. 

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations. 

In the past chemical cure linings have been employed on a wide scale. These linings, usually based on natural rubber or acrylonitrile-butadiene rubber consist of a standard lining compound with a chemical activator such as dibenzylamine incorporated in the formulation. Prior to the application of the lining to the substrate, the individual sheets of rubber are dipped or brush coated with carbon disulphide or a solution of a xanthogen disulphide in a solvent. The carbon disulphide or xanthogen disulphide permeates the rubber and combines with the dibenzylamine to form an ultra-fast dithiocar-bamate accelerator in situ, and thus the rubber rapidly vulcanises at ambient temperature. 

In more recent years, lining compounds have been developed that vulcanise at ambient temperatures. Most polymers can be used for such compounds, although most materials are based on natural rubber, acrylonitrile-butadiene rubber and polychloroprene. These compounds contain accelerators which usually give rise to a material which has a delay in the onset of vulcanisation with a subsequent rapid rise in cross-link formation to give full vulcanisation in 6 to 8 weeks. Such materials, unless to be used within a few days of manufacture, are refrigerated to arrest the sel f-vulcanisation. 

Modern bonding systems usually consist of a primer coat, often with a secondary tie coat, plus a tacky solution to assist in the application of the rubber. The bonding systems currently in use are usually suitable both for autoclave vulcanisation and vulcanisation at 100°C with atmospheric pressure steam or hot water. Ambient vulcanisation bonding systems have to be chemically active at the lower temperatures and are therefore specialist in nature. 

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