Curing, rubber accelerators structure

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

With bromobutyl/butyl rubber blends, elastomer structures are essentially the same, but the different reactive functionalities provide different vulcanization chemistry. Since bromobutyl rubber has greater cure reactivity, accelerators that will over-cure the bromobutyl rubber phase should be avoided. Briefly... 

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. 

Captax (Structure 15.21) is used to the extent of 1% with hevea rubber and accounts for the major part of the over 30,000 t of accelerators used annually in the United States. Other accelerators widely used include 2-mercaptobenzothiazole sulfenamide (Santocure Structure 15.22), used for the vulcanization of SBR dithiocarbamates and thiuram disulfides. Thiuram disulfide (Structure 15.23) is a member of a group called ultra-accelerators, which allow the curing of rubber at moderate temperatures and may be used in the absence of sulfur. 

The substances listed in Table 28 correspond to the basic structure of elastomeric closures. The other components in rubber formulations are curing or vulcanizing agents, accelerators, activators, antidegradants, plasticizers, fillers, and pigments. The most common additives used to compound rubber for the pharmaceutical industry are listed in Table 29. The amount of each component may vary from rubber to rubber, and, depending on the component, the amount can reach more than 50% of the total mass of a formulation. While accelerators are used in amounts of around 1%, fillers may make up more than 50% of the formulation mass. 

It is theorized that between the complex network structure of the unaccelerated system and the simpler network structure of the accelerated system, structures made up of the two models represent natural-rubber vnlcani7ares made at various times and temperatures of cures, with different reactant concentrations, and showing the effects of other variants. 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

Sulfidation of the brass surface is not due to its interaction with elemental sulfur, but it is the result of the interaction between the brass surface and accelerator-sulfur reaction products, which can be represented by the general structure, Ac-S -Ac and Ac-Sy-H, where Ac is an accelerator-derived moiety (e.g., benzothiazolyl group). The value of the subscript, y, increases with the ratio of the concentration of sulfur to the concentration of accelerator used in the curing system. Generally, high sulfur levels and high ratios of sulfur concentration to accelerator concentration favor good rubber-to-brass adhesion. 

This figure also illustrates the three main regions of rubber vulcanization. The first regime is the induction period, or scorch delay, during which accelerator complex formation occurs. The second time period is the cure period, in which the network or sulfurization structures are formed. The network structures can include crosslinks, cyclics, main chain modification, isomerization, etc. The third regime is the overcure, or reversion regime. 

Since EPR rubber molecules do not contain unsaturation, they can be vulcanized only by organic peroxide curing systems. If a third monomer is added during the polymerization, i.e., a diene monomer (wherein only one of the two double bonds takes part in the polymerization), unsaturation can be introduced into the molecule, and it can then be vulcanized by accelerated sulfur curing systems. A chemical structure for ethylene-propylene-diene-monomer (EPDM) rubbers can be expressed as follows ... 

Both of these curatives are added in the lower temperature, final mixing stage. HMTA must be isolated from the other rubber curatives during storage and batch preparation since its basicity can cause premature decomposition of the rubber cure accelerators and can accelerate the conversion of insoluble sulfur into the soluble form. The structure of HMTA and the reaction with resorcinol are illustrated in Scheme 4.1. Classical chemical studies indicate that as much as 75% of nitrogen remains chemically bonded to the rubber though some ammonia is released during the cure of the resin and the rubber, which can have detrimental effects on rubber composites reinforced with brass coated steel cords. 

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