Polysulfide cross-links

Sulfur Donors. MBSS, DPTH, and the thiuram disulfides (see Table 2) ate examples. The morpholine disulfide and caprolactam disulfide examples in Table 4 can also donate one atom of sulfur from their molecular stmcture for cross-linking purposes. Monosulfide cross-links provide better thermal stabiUty than the sulfur—sulfur bonds in di- and polysulfide cross-links, which predominate when elemental sulfur is used. 

Sulfur. Low sulfur stocks and EV sulfur-accelerated systems have better aging resistance. Normally, the oxidation rate increases with the amount of sulfur used in the cure. The increased rate may be due to activation of adjacent C—H groups by high levels of combined sulfur. Saturated sulfides are more inert to oxidation than aHyUc sulfides. Polysulfidic cross-links impart excessive hardening of SBR as compared to more stable monosulfidic cross-links. 

The Scission of Polysulfide Cross-Links in Rubber Particles through Phase-Transfer Catalysis... 

Fig. 8. Sulfur-based cure system designs where conventional systems are polysulfidic, EV systems are mono-/disulfidic, and semi-EV systems are clean polysulfidic. A shows pendent sulfide groups terminated by accelerator B, monosulfide cross-links C, disulfide cross-links D, polysulfide cross-links...

Semi-EV cures represent a compromise between conventional and EV cures. Although semi-EV cures do yield polysulfide cross-links, they tend to minimize formation of inefficient moieties such as sulfur bridging with itself, accelerator-terminated sulfur linkages, etc. This cleaner usage of sulfur is the reason for their compromise properties between conventional and EV cures. 

Devulcanization in a sulfur cured rubber is defined as the cleavage of the mono-, di-, and polysulfidic cross-links formed during vulcanization (Fig. 1). The vulcanization process is irreversible and additional heating induces changes in the network with a shift toward shorter cross-links but does not devulcanize the compound. Other methods are therefore needed to induce devulcanization. 

FIGURE 1.31 Typical chemical groupings in a sulfin-vulcanized natural rubber network, (a) Monosulfide crosslink (b) disulfide cross-link (c) polysulfide cross-link (x=3-6) (d) parallel vicinal cross-links ( = 1-6) attached to adjacent main-chain atoms (e) cross-links attached to common or adjacent carbon atoms (f) intrachain cyclic monosulfid (g) intrachain cyclic disulfide (h) conjugated diene (i) pendant sulfide group terminated by moietyZ derived from accelerator. 

Vulcanization is the most important NR chemical reaction. Most applications require cross-linking via vulcanization to increase resiliency and strength. Exceptions are crepe rubber shoe soles and rubber cements. There are a number of methods for sulfur vulcanization, with certain methods producing polysulfidic cross-linking and other methods producing more monosulfidic cross-links. 

Sketch showing the difference between a monosulfidic (left portion) and a polysulfidic cross link (right portion). In the latter case, the chains of sulfur atoms may act as additional, elastically effective chains in what is essentially a bimodal network. 

Sharaf, M. A. Mark, J. E., Elastomers with Polysulfidic Cross Links Viewed as Bimodal Networks. J. Macromol. Sci., Chem. 1991, A28, 67-73. 

Since Activator 73 A is a mixture of zinc salts of linear aliphatic and aromatic carboxylic acids (50), it would be expected that the mechanism of action would be similar to zinc stearate, which can extract sulfur from polysulfidic cross-links and reuse that sulfur to generate more cross-links (51). Ultimately this would result in a network structure high in monosulfidic content (42). 

In contrast, EV cure systems employ much lower levels of free sulfur (0.1 to 1.0 phr) or they use sulfur donors such as TMTD or DTDM combined with higher accelerator levels. The short mono- and disulfide cross-links that form often do not exhibit the physical properties afforded by the conventional systems, but they better retain properties after aging. Semi-EV cures represent a compromise between conventional and EV cures. Although semi-EV cures do yield polysulfide cross-links, they tend to minimize formation of inefficient moieties such as sulfur bridging with itself, accelerator-terminated sulfur linkages, etc. This cleaner usage of sulfur is the reason for their affording a compromise properties between conventional and EV cures. 

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