Vulcanisation system

In the case of polychloroprene the chlorine atom so deactivates both the double bond and the a-methylenic group that a sulphur-based vulcanisation system is ineffective and special techniques have to be employed. 

In addition to the components of the vulcanising system several other additives are commonly used with diene rubbers. As a general rule rubbers, particularly the diene rubbers, are blended with many more additives than is common for most thermoplastics, with the possible exception of PVC. In addition the considerable interaction between the additives requires the rubber compounder to have an extensive and detailed knowledge concerning the additives that he employs. 

Report 79 Rubber Compounding Ingredients - Need, Theory and Innovation, Part I Vulcanising Systems, Antidegradants and Particulate Fillers for General Purpose Rubbers, C. Hepburn, University of Ulster. 

A term apphed to vulcanisation systems in which sulphur or a sulphur donor is used very efficiently for crosslinking the rubber. EV systems produce vulcanisates with crosslinks that are mainly monosulphidic, which are thermally and mechanically stable. 

Benzothiazole sulphenamide accelerators are suitable for semi-efficient and efficient vulcanisation systems. 

In many sulphur vulcanisation systems there is an optimum peak for physical properties. 

Crosslinking systems based on peroxides are usually simple. Unlike with sulphur vulcanisation systems, the addition of ZnO and stearic acid is not necessary, so crosslinking is achieved in many cases simply using a crosslinking peroxide. However, the addition of a coagent may be useful (see below). In some other cases, a combination of various types of crosslinking peroxides may be suitable. 

Sulphur vulcanisation of rubber is catalysed by the presence of alkali materials. This activation of the vulcanisation system can result in unwanted short scorch times. The addition of weak acids to the rubber compound results in retardation of the crosslinking mechanism. The common materials used for retardation are... 

TV-Cyclohexylthiophthal i m idc (CTP) functions as an effective retarder of vulcanisation for accelerators in the sulphenamide classes. It produces scorch retardation without effect on compound modulus. It is not effective, however, in other classes of vulcanisation systems, notably those based on thiazoles, thiurams and dithiocarbamates. 

Microwave units are available for both continuous vulcanisation systems, for preheating of moulding blanks for compression and transfer moulding, and for heating large bales of frozen rubber prior to mixing. 

STUDIES ON VULCANISATION SYSTEMS USED IN THE MANUFACTURE OF LATEX PRODUCTS... 

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. 

Accelerated sulfur formulations are the most common vulcanisation systems used in commercial and industrial applications. Therefore, research on both the fundamental and applied aspects of accelerated sulfur vulcanisation is ongoing. Several reviews of the chemistry and/or physics of accelerated sulfur-vulcanisation of elastomers have been published [13, 14, 22, 23]. 

High resolution MAS techniques of 13C, DEPT, correlated spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear chemical shift correlation (HETCOR) were used to examine conventional CBS and efficient TMTD vulcanisation of polybutadiene [37]. In conventional CBS vulcanisation, the major vulcanisate 13C NMR peak occurred at 44.9 ppm and was assigned to a trans allylic structure (-C=C-C-Sx with X=3 or 4). The efficient TMTD vulcanisation yielded as main product a 13C NMR peak at 54.0 ppm and was assigned to a cis allylic vulcanisate (-C=C-C-Sx x=l). While cyclic sulfur by-products were observed in both vulcanisation systems, the CBS formulations gave rise to a higher percentage postulated to be formed via a episulfide intermediate. 

It should be recognised that appreciable shifts in properties are sometimes made possible by special compounding variations. For instance, the heat resistance of natural rubber vulcanisates may be improved considerably by variation of the vulcanising recipe. The normal sulfur vulcanisation system is capable of many variants which will govern the chemical nature of sulfur crosslinks, i.e., whether it is essentially a mono, di or polysulfide linkage. The nature of sulfur crosslinks can have considerable influence on the heat and chemical resistance of vulcanisates. 

Vulcanisation system Accelerator (e.g. sulphonamide, dithiocarbamate, thiuram) 1.5... 

One such contaminant associated with sulphur-based vulcanisation systems is the organic accelerator 2-mercaptobenzothiazole (2-MCBT) and associated derivatives. In 1981 (Petersen et al., 1981) the presence of 2-(2-hydroxyethylmercapto) benzothiazole (HEB) was detected in the contents of a disposable hypodermic syringe. It was identified that the extractant was a reaction product formed between a 2-MCBT derivative and ethylene oxide used for sterilisation. Subsequently the oxidation product of HEB, 2-(carboxymethylthio)benzothiazole (CMB), was detected in the serum of premature babies receiving prolonged intravenous therapy (Meek and Pettit, 1985). So-called modem vulcanisation systems do not use sulphur as the cross-linking agent nor use 2-MCBT or derivatives, and are consequently free from this particular problem. These new vulcanisation systems show a considerable reduction in aqueous extractable matter and are often described as having low water extractables. 

The original crosslinking process for natural rubber, called vulcanisation, involved mixing in 2-3% of sulphur plus an accelerator. On heating to 140 °C the sulphur reacts with C=C bonds on neighbouring polyisoprene chains to form sulphur crosslinks C—(S) —C. Typically, 15% of the crosslinks are monosulphide [n = 1), 15% are disulphide and the rest are polysulphide with n > 2. The polysulphide crosslinks are partially labile, which means that they can break and reform with other broken crosslinks when the applied stresses are high. This leads to permanent creep in compressed rubber blocks. To avoid such permanent set, efficient vulcanisation systems have been developed that produce only monosulphide crosslinks. 

H. Nabil, H. Ismail, A.R. Azura, Optimisatioii of accelerators and vulcanising systems on thermal stability of natural rubber/recycled ethylene—propylene-diene-monomer blends. Materials Design, ISSN 0261-3069 53 (January 2014) 651-661. http //dx.doi. org/10.1016/jjnatdes.2013.06.078. 

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