Polysulphides vulcanization

An increase of Av to a constant value was observed in the course of vulcanization of polysulphide rubbers in the presence of sodium dichromate. Rates of vulcanization were determined from the time dependence of Av, as the reciprocal time at which Av reaches the maximum constant value. It is found that the rate of vulcanization linearly increases with the oxidizer concentration37. ... 

Spin-spin relaxation dynamics were also used in the study of the kinetics of the vulcanization of polysulphide rubbers. The T2 values decrease with the course of the reaction and the time dependence of log (T2/T ), where corresponds to the time equal to zero, exhibits an inflection. The inflection point is attributed to gel formation, and the reaction rate constants for the two separate processes are determined from the T2 data. It was also observed that the addition of carbon black reduces T2 by a factor of 2 or 3, because vulcanization occurs both through the thiol groups and by the chemical reaction between the polymer and carbon black 37>. 

Fig. 22. Relative effect of atomic oxygen on a variety of polymers 1, polysulphide 2, formaldehyde polymer 3, polypropylene 4, low density polyethylene 5, polyethylene glycol terephthalate) 6, polystyrene 7, polytetrafluoroethylene 8, sulphur-vulcanized natural rubber [268].

The third step in the vulcanization sequence is considered to be the formation of polysulphide cross-links. The most probable route for the conversion of the rubber-bound intermediate into cross-linked polysulphides is by disproportionation reactions, involving cleavage of S—S bonds these reactions may be initiated by mercaptide ions (XS ) derived from the zinc mercaptide (cf., Section 17.2) ... 

The changes depicted in Fig. 18.2 may continue to occur after the formal vulcanization process is complete. The reactions may continue whilst the vulcanizate is in service, particularly if elevated temperatures are encountered. Monosulphide cross-links are thermally stable and hence vulcanizates of Type (i) (Fig. 18.2) show relatively little change on aging. On the other hand, polysulphide cross-links are thermally unstable and vulcanizates of Type (ii) undergo reversion (loss of cross-links) and main chain modification with corresponding changes in physical properties. Polysulphide cross-links are also susceptible to interchange reactions, initiated by nucleophilic species such as... 

As a result of these reactions, the vulcanizate will exhibit reduced resistance to compression set and creep. It may be noted that although vulcanizates with predominantly monosulphide cross-links show better heat resistance, they may not have the excellent overall physical properties of predominantly polysulphide cross-linked networks. For this reason, a compromise is usually aimed for in practice and so-called semi-efficient vulcanizing systems are used. 

Vulcanization of polysulphides proceeds via terminal groups so that it is important to know the nature of such groups. 

Fig. 7.1. Relative effect of atomic oxygen on different polymers (a) polysulphides (b) polyformaldehyde (c) polypropylene (d) low density polyethylene (e) poly(ethyl-ene glycol terephthalate) (f) polystyrene (9) polytetrafluoroethylene and (h) sulphur-vulcanized natural rubber [907]. (Reproduced with permission from [907] pubUshed by John Wiley Sons, Inc., 1965.)...

V.S. Minkin, R. Ja. Deberdeev, F.M. Paljutin, Ju. N. Khakimullin. Promyshlennye Polisul fidnye Oligomery Sintez, Vulkanizacija, Modifikacija (industrial Polysulphide Oligomers Synthesis, Vulcanization, Modification). - Kazan Novoe Znanie , 2004 (in Russian). 

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