Rubber sulphur cured

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. 

The sulphur vulcanisation of NR generally requires higher added amounts of sulphur, and lower levels of accelerators than the synthetic rubbers. Sulphur contents of 2-3 phr, and accelerator levels of 0.2-1.0 phr are considered to be conventional cure systems. 

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. 

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. 

If we take a rubber-sulphur ratio of 68 32 in an ebonite compound and then cure it at 155°C, the vulcanization coefficient increases practically to a constant value of 47 after about five hours and the uncombined sulphur decreases during the first four hours. This state may be called a full cure in the chemical sense. There is also a reduction in volume of about 6%. It is known that after the combination of the first few percent of sulphur, the material passes through a leathery stage with low strength and poor resistance to oxidation and with time passing a true ebonite is formed with increased impact strength. 

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 ...

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... 

Unvulcanized rubber (sulphur-crosslinked) mixes are freshly coated with a 20% solution of 4,4, 4"-triphenylmethane triisocyanate (Desmodur R) or methylene chloride. After drying, the rubber is vulcanized in contact with a cleaned metal surface and a strong bond results. The reverse procedure is also satisfactory. For example, by this technique heat-, fatigue-, impact-, oil- and solvent-resistant bonds can be obtained between metals and elastomers by press or hot-air curing the green freshly milled or calendered... 

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...

The determination of cross-link density or of Me by a chemical method is virtually impossible with an accelerated sulphur-cured diene rubber vulcanizate because of the complexity of the system and the formidable obstacles to analysis. Certain vulcanizing systems do however appear to be simpler and capable of such treatment. If for such a system Me is determined by a physical method then it is to be expected that it might be possible to obtain a meaningful calibration between the two values of Me obtained. It is then reasonable to assume that this calibration may then enable, in the case of a sulphur-vulcanized diene rubber, an equivalent of the chemically... 

Thus the original conclusion that cross-linking occurred between alpha-methylene groups alone could not be sustained simply on the basis of experiments with methyl iodide. Methyl iodide has nevertheless been of value in at least one respect. Peroxide-induced crosslinks in natural rubber compounds have been found to withstand prolonged treatment with methyl iodide whilst conventional sulphur-cured counterparts lose most of their cross-links. This observation (Moore, 1958) provided strong evidence that the number of C—C cross-links in an accelerated sulphur-natural rubber vulcanizate was negligible (a conclusion supported by later work using model systems by Watson and Moore (c. 1%7). 

If a study of vulcanization chemistry is to become part of a routine technological investigation it is necessary for a technique to be developed that will give very rapidly information on the distribution of rubber-sulphur reaction products for a specified set of vulcanization conditions (e.g. sulphur level, accelerator level, cure time, cure temperature). A most useful contribution in this direction has been made by Lautenschlaeger (1977). The model compound 2-methyl-2-pentene was heated with typical curing systems in Pyrex tubes from 10 to 100 minutes over a range of temperatures from 100 C to 150 C. The reaction products were then subjected to gas... 

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