Sulphur curing

Hydrogenated nitrile rubbers were introduced in the mid-1980s as Therban by Bayer. The initial grade had an acrylonitrile content of only 17% instead of approx. 34% in conventional NBR. Whilst non-sulphur-curing systems such as the use of peroxides with triallyl cyanurate or isocyanurate are necessary, the saturated rubber has a number of advantages over NBR. These include improved... 

Hydrated or slaked lime Ca(OH)2 is an inorganic accelerator used in the curing of fluoroelastomers. In conventional sulphur cured polymers it counteracts the retardation of cure due to the presence of acidic substances in a rubber compound. Quicklime (CaO) dispersed in mineral oil or in wax/oil is used as a dessicant to reduce porosity in vulcanisates, particularly in fluid bed curing. 

In addition to the normal sulphur cure systems, metal oxides can be used to cure the carboxylated nitriles. 

Due to the low level of unsaturation in the main chain, sulphur cures require the more active thiuram and dithiocarbamate accelerators to achieve an adequate state of cure. 

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. 

Bloom can be a problem in sulphur cures, so selection of the accelerator system is important. 

Resin cures utilise the same resins that are used for butyl rubber, but more resin (ca. 10-12 phr) and a halogen donor (10 phr), typically bromobutyl, or polychloroprene, are required. Although heat stability is slightly improved by resin curing when compared to sulphur cures, the effect is not as marked as in the resin curing of butyl. 

The carcinogenicity of polycyclic aromatic compound-rich tyre extender oils has lead to the proposal of a legislative ban on their use in Europe. The suitability of naphthenic oils as non-toxic plasticisers in tyre treads is discussed and results are presented of experimental studies of the use of these plasticisers in SBR, EPDM, sulphur-cured EPDM and peroxide-cured EPDM. Despite their low aromatic content, the naphthenic plasticisers are shown to give good results in SBR, probably as a result of the contribution to solvent characteristics of the naphthenic molecular structure. The use of naphthenic oils is expected to increase worldwide as they are said to be one of the best alternatives to aromatic extracts with regard to solvent properties, compatibility, performance and availability. 

Figure 9.2 Stack plots of 10% sulphur-cured NR for different times of cure. B butadiene-like species, P polysulfidic crosslinks Carbon numbers in 1,4-isoprene unit in NR have been designated as follows -Cl-C2(-C5)=C3-C4- or -Cy-Ca(-Ce)=C 3-C8-...

In this chapter the word elastomer is used to describe the base polymer and rubber to describe the fully compounded finished component. A rubber formulation is a complex blend of ingredients, and a typical high extract sulphur cured natural rubber formulation is given in Table 12.4. 

Table 12.4 Typical sulphur cured natural rubber formulation ...

Compound PIO NBR (28% ACN) 100 (pphr) Mineral fillers 100 Silica 25 Zinc oxide 10 Stearic acid 1 Antioxidants Sulphur cure system... 

Sulphur-curing grades are usually preferred by the rubber industry as they make use of established technology, but two other vulcanization techniques are also well established as they produce elastomers with certain superior properties these are crosslinking with peroxide or diisocyanate. Peroxide curing can be universally used as it has a free radical-type... 

Sulphur-cured urethane elastomers have the least resistance to hydrolysis (possibly the typical amine-based sulphur-accelerator combination used accelerates hydrolysis) (see Table 13.5). Carbodiimides (e.g. Staboxal PCD by Bayer) are effective in stabilizing all polyester urethanes against hydrolytic attack they are not effective when used with polyether-based urethane elastomers. In general it is true that the longer the hydrocarbon chain of the glycol portion of a polyglycol adipate the more resistant is the polyester to hydrolysis. 

Hydroxy cure Amine cure Sulphur cure... 

Manshaie et studied the physico-mechanical properties of NR/SBR blends cured by electron beam irradiation and sulphur. They showed that the irradiated blends have better mechanical properties than those cured by sulphur system. They also showed that the irradiation cured samples exhibited better heat stability than the sulphur cured samples. 

Uncured ethylene-propylene copolymers are soluble in hydrocarbons and have rather poor physical properties useful technological properties are developed only on vulcanization. As mentioned above, the saturated copolymers are vulcanized by heating with peroxides whilst the terpolymers are vulcanized by conventional sulphur systems. The peroxide-cured rubbers have somewhat better heat aging characteristics and resistance to compression set but sulphur-cured rubbers are more convenient to process and allow greater compounding freedom. 

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. 

In sulphur cured rubbers, accelerators are generally used to reduce the dependency on sulphur in order to achieve more efficient vulcanisation, to improve heat and flex resistance due to the presence of more monosulphidic crosslinks, and to increase the cure rate of the rubber and improve production capacity. Two accelerators which have been shown to enhance bondability of rubbers are 2-mercaptobenzothiazole (MBT) and mercaptobenzothiazole disulphide (MBTS). An accelerator which is known to negatively impact on adhesion is tetramethyl thiuram disulphide (TMTD). 

Figure 5.3 Surface bloom of sulphur cured natural rubber Reprinted from A. D. Roberts, Wear, 1997, 42, 119, Figure 7, with permission...

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