Crosslinking polysulphide

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. 

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. 

It has been demonstrated previously that such additions can increase the strength of rubber-brass adhesion considerably. In this research it was established that polysulphides are only weak crosslinking agents for unsaturated rubber by themselves. In the presence of sulphenamide accelerators, such as OBTS, polysulphides, in amounts of 0.5 - 1 phr, activate the sulphur vulcanisation. However, the reversion process (crosslink breakdown) is not accelerated. A favourable effect on the physicomechanical properties of the vulcanisate was also reported. 

Purified natural rubber (PNR) is of interest because of its potentially lower toxicological effects than whole natural rubber (WNR) due particularly to the reduction in protein content. Improved dynamic mechanical properties have also been reported. Comparison of both gum and filled compounds, vulcanised using conventional cure systems (CV) and efficient vulcanisation systems (EV) prepared from PNR and WNR indicate that generally properties of the PNR are poorer than WNR. The exception is in the flex cracking resistance and the heat build up in the filled samples where PNR shows an improvement. Using the EV cure system on filled PNR gives properties almost comparable to WNR. A study of the distribution of the types of sulphur crosslink in both pNR and WNR vulcanisates indicates a more uniform distribution of monosulphidic (S), disulphidic (S2) and polysulphidic (Sx) crosslinks in the PNR samples (38.7/25/36.4 in PNR compared to 64.1/29.7/6.6 in WNR respectively). This is believed to be the reason for the better dynamic properties of PNR vulcanisates. 7 refs THAILAND... 

A study was conducted to differentiate between the crosslinking reactions of bismaleimides and biscitraconimides in squalene as well as in NR. Bismaleimides were found to participate in the crosslinking reaction in the absence of curing compounds such as accelerators and sulphur. Biscitraconimides took part in the crosslinking reaction when polysulphidic crosslinks were under the process of degradation. Sulphur and accelerator were required for the formation of polysulphidic crosslinks and hence the presence of sulphur and accelerator were essential for biscitraconimide crosslinking. The differences in the reactivity and the chemistry of crosslinking are studied and discussed. Some application data are provided in order to elaborate on the differences. 17 refs. 

It is shown that the role of zinc stearate in vulcanisation formulations is to promote the reaction of accelerator terminated polysulphidic pendant groups with neighbouring polymer chains, thereby resulting in higher crosslink densities. Cyclisation reactions may also be reduced, thus increasing stress induced crystallisation under load and thereby further enhancing the physical properties of the vulcanisate. 6 refs. 

Reversion of sulphur-based crosslinks continues to a problem for NR compounders. Reversion is the thermal degradation of polysulphidic crosslinks leading to a reduction of crosslink density and an introduction of main chain modifications. In practice, reversion leads to a decline in compound physical properties. Reversion occurs when compounds are overcured or when vulcanisates are exposed to anaerobic ageing. Ideally, a... 

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