Natural rubber accelerated

Reinforcement of SBR with carbon black leads to vulcani2ates which resemble those of natural rubber, and the two products are interchangeable in most applications. As with natural rubber, accelerated sulfur systems consisting of sulfur and an activator comprising a metal oxide (usually zinc oxide) and a fatty acid (commonly stearic acid) are used. A conventional curing system for SBR consists of 2.0 parts sulfur, 5.0 parts zinc oxide, 2.0 parts stearic acid, and 1.0 part N-r-butylbenzothiazole-2-sulfenide (TBBS) per 100 parts polymers. 

Dimeihylamine, C2H7N, (CH3)2NH. Colourless, inflammable liquid with an ammoniacal odour, mp -96" C, b.p. 7°C. Occurs naturally in herring brine. Prepared in the laboratory by treating nitrosodimetbyl-aniline with a hot solution of sodium hydroxide. Dimethylamine is largely used in the manufacture of other chemicals. These include the solvents dimethylacetamide and dimethyl-formamide, the rocket propellant unsym-metrical dimethylhydrazine, surface-active agents, herbicides, fungicides and rubber accelerators. 

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 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. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

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. 

FIGURE 14.3 Development of accelerated sulfur vulcanization of natural rubber (NR). (From A.Y. Coran, Chem. Tech., 23, 106, 1983.)... 

FIGURE 14.5 Comparison of accelerator classes in natural rubber (NR). (From B.H. To, Rubber Technology, Hanser Verlag, Munich, Germany, 2001.)... 

Oxidation of Natural Rubber (NR) Pale Crepe at 46°C, Accelerated by Ultraviolet (UV) Light... 

An example of a nonlinear polymer derived by cross-linking an initially linear polymer is afforded by vulcanized natural rubber. In the usual vulcanization procedure involving the use of sulfur and accelerators, various types of cross-linkages may be introduced between occasional units (about one in a hundred) of the polyisoprene chains. Some of these bonds are indicated to be of the following type ... 

These conclusions have been confirmed by Wood and Roth, who carried out measurements at both constant lengths and at constant elongations using natural rubber vulcanized with sulfur and an accelerator. Their results at constant elongation, to be considered later in connection with the thermodynamics of rubber elasticity at higher elongations, are summarized in Fig. 89. 

Fig. 89.—The total force of retraction at 25°C and dE/dL)T,v obtained from the force-temperature intercepts at constant elongation for natural rubber gum-vulcanized using an accelerator. (Wood and Roth. )...

Deformulation of vulcanised rubbers and rubber compounds at Dunlop (1988) is given in Scheme 2.3. Schnecko and Angerer [72] have reviewed the effectiveness of NMR, MS, TG and DSC for the analysis of rubber and rubber compounds containing curing agents, fillers, accelerators and other additives. PyGC has been widely used for the analysis of elastomers, e.g. in the determination of the vulcanisation mode (peroxide or sulfur) of natural rubbers. 

Also known as hexamine, hexa or HMT, a once-popular aldehyde-amine type of accelerator for natural rubber, now used as a vulcanising agent for polyacrylate rubbers. 

The process, now obsolescent, of permitting the coagulum of natural rubber to mature in the wet state before washing or smoking. Due to bacterial activity various natural accelerators are produced and the resulting rubber has a faster rate of cure than that prepared by the conventional method. MBI... 

Organic compounds having four alkyl groups attached to nitrogen. A quaternary ammonium salt is employed in reversing the electric charge on natural rubber latex quaternary ammonium dithiocarbamates are ultra-accelerators for rubber vulcanisation. 

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

SBR can be cured by the use of sulphur, sulphur donor systems and peroxides. Sulphur cures generally require less sulphur (1.5-2.0 phr) and more accelerator than is normally required to cure natural rubber. 

Uses Removing carbon dioxide and hydrogen sulfide from natural gas in emulsifiers, hair waving solutions, polishes softening agent for hides agricultural sprays pharmaceuticals, chemical intermediates corrosion inhibitor rubber accelerator nonionic detergents wool treatment. 

The presence of a comonomer has, in certain cases, 9 marked influence on polymerization rate. For example, the mastication of natural rubber in the presence of maleic anhydride, even with small concentrations of the latter, about 5%, leads to accelerated polymerization of styrene monomer (11) either because of its high reactivity in the propagation step of heterochain copolymerization and/or because of a hardening effect. This reaction is discussed later. 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutylene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural rubber or SBR, cure system design generally requires higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, their attributes, and principal applications have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl rubber as shown in Table 7 (21). 

Types of Latex Compounds. For comparison with dry-rubber compounds, some examples of various latex compounds and the physical properties of their vulcanizates are given in Table 23. Recipes of natural rubber latex compounds, including one without antioxidant, and data on tensile strength and elongation of sheets made from those, both before and after accelerated aging, are also listed. The effects of curing ingredients, accelerator, and antioxidant are also listed. Table 24 also includes similar data for an SBR latex compound. A phenolic antioxidant was used in all cases. 

Rubber. The rubber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural rubber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfurless and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useful as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

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