Curative sulphur

In order to achieve the direct crosslinking between the dip latex rubber and the matrix polymer, it is, of course, necessary for the curatives, sulphur and accelerator (or active groupings derived from them), to migrate from the matrix into the dip film. Consequently, the actual formulation of the curing system can have significant effects on the levels of... 

Newer polyacrylate rubbers can be cured with certain amines and are more responsive to a broad range of curative systems, e.g., alkali metal stearate/sulphur or sulphur donor, methyl zimate and ammonium adipate. 

Project A involves the synthesis of novel biologically active materials for testing in the preservation of the latex used in the manufacture of paint. From past experience, it is known that the chemicals that are likely to be active in this environment will be those based on heterocycles containing sulphur. Therefore for this Project the particular Skill Requirement is the ability to design and synthesise sulphur-containing heterocycles that will show bioactivity in this industrial outlet. The two people who will be required for this task are available in the cur-... 

Detoxification procedures from mustard gas are difficult because of its insolubility and also because of the drastic effects it can have on lung epithelial tissue following inhalation. During World War I, physicians had no curative means of treating the victims of mustard gas exposure. The only method of detoxification that was known involved a rather extreme oxidation procedure using superchlorinated bleaches, such as 5% sodium hypochlorite. Today, several novel methods of detoxification have been developed to counter the effects of mustard gas and these include the use of sulphur-amine solutions and magnesium monoperoxyphthalate. The most effective method to date employs peroxy acids, because they are able to react quickly with the mustard gas. 

The next link in the rubber to metal bond is between the primer and adhesive interface. The curative present in the adhesive layer migrates or diffuses into the primer layer during vulcanisation and forms a chemical bond between the primer and adhesive. The polymeric film former present in the primer diffnses and knits with the adhesive layer and further strengthens the bond between primer and adhesive becanse of its compatibility with polymers present in the adhesive layer. The final link in the rnbber to metal bond is the adhesive to rnbber interface. The curative present in the adhesive layer also diffuses into the rubber during vulcanisation and forms a chemical bond between the adhesive and the rubber. These bonds which span across the layers in the assembly are called crossbridges to differentiate them from the crosslinks which occnr within the rnbber itself. In addition, sulphur from the rubber compound diffnses into the adhesive layer and helps to form additional crossbridges between the rnbber and the adhesive. 

The choice of vulcanisation system for the rubber can have a dramatic effect on adhesion. Typically sulphur cured rubbers are easier to bond to than sulphur-free or peroxide cured rubbers. This is believed to be due to the interaction of sulphur with key curative materials in the adhesive. The more sulphur that is present, the more interactions that are available, and hence the better the chance of getting good adhesion. SEV (semiefficient vulcanisation) and EV (efficient vulcanisation) cure packages are typically more difficult to bond because of their lower free sulphur contents. EV refers to cure systems which give predominantly monosulphidic or disulphidic crosslinks whereas conventional sulphur cure systems produce mostly polysulphidic crosslinks. SEV systems fall somewhere between EV and conventional systems in the type of crosslinks produced. Vulcanisation proceeds at different rates and with different efficiencies in different types of polymers, so the amount of sulphur needed to produce an EV cure system will also vary. For example, in NR, an EV system will generally contain between 0.4 and 0.8 phr of sulphur, while in NBR the sulphur level will generally be less than 0.3 phr of free elemental sulphur. 

The capacity or service life of a lithium-sulphur dioxide cell at various discharge rates and temperatures is shown in Figure 30.55. The data are normalized for one cell and presented in terms of hours of service at various discharge rates. The linear shape of these curs es, except for the fall-off at high current levels and low temperatures, is again indicative of the capability of the lithium-sulphur dioxide battery to be efficiently discharged at these extreme conditions. These data are applicable to the standard cells and can be used in several ways to calculate the performance of a given cell or to select a cell of a suitable size for a particular application. 

A good example of this situation exists in the case of the curatives (e.g. sulphur) and accelerators that are used in the large number of rubber products. These cure system species undergo the following types of reaction with the sample ... 

The advantages are outlined of the use of peroxide vulcanising agents in place of sulphur curing for elastomers. The chemistry of both types of vulcanisation systems is explained. Since the chemical mechanisms of each are very different, mbber compounders, experienced in the use of sulphur are reported to often find difficulties in formulating new products which make use of peroxide curatives. An overview is presented of the types of peroxides available and their properties. A description is included of the chemistry by which peroxides vulcanise rubber, followed by a discussion of the potential chemical interferences which may face rubber compounders converting from sulphur vulcanisation to peroxide curing. USA... 

The results are reported of a study of accelerated sulphur vulcanisation using the above compounds. 2,3-Dimethyl-2-butene, was utilised as a model compound for polyisoprene and residual curatives, intermediates and products produced by heating the mixes for various times at 150C in sealed tubes were analysed using high-performance liquid chromatography. A synergistic effect was observed between bis(2-mercaptobenzothiazole) zinc(II) and 2-bisbenzothiazole-2,2 -disulphide. 23 refs. SOUTH AFRICA... 

The cure and tensile properties of sulphur vulcanised styrene-butadiene rubber filled with a conventional furnace black or a fluorinated black have been determined. Compositions with the fluorinated black and normal curative levels exhibit retarded cure compared to corresponding ones with the furnace black. This is due to a reaction between the sulphenatnide accelerator and the fluorinated black. Notwithstanding, a fluoro-filled composition with no curatives substantially crosslinks when moulded at 150 deg.C. Thus, fluoro-black filled specimens have competing effects toward crosslinking. On the one hand, crosslinking is inhibited by reaction with the accelerator, while on the other, the fluoro-black itself can cause crosslinking. 2 refs. 

The disodium salt of hexamethylene bisthiosulphate (HTS) was evaluated as an accelerator in the zinc oxide mediated vulcanisation of tyre sidewall compounds containing a brominated isobutylene-p-methylstyrene copolymer. Rheometer profiles of blends with polybutadiene cured with sulphenamide, sulphur, zinc oxide and HTS showed a characteristic delayed action fast cure kinetic profile, while the same blends cured without HTS exhibited undesirable cure characteristics, with long early cure and fast late cure. Many vulcanisation characteristics and vulcanisate properties were largely dominated by a single cure component. State of cure was dominated by sulphur level, scorch delay and early cure time by HTS loading, and the time to reach t90 by the sulphenamide level. Time of flight secondary ion mass spectroscopy imaging showed that HTS favourably affected the distribution of curatives in blends with SBR. 11 refs. 

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