EV cure systems

In contrast, the EV cure systems employ much lower levels of free sulfur (0.1—1.0 phr) or they use sulfur donors such as TMTD or DTDM combkied with higher accelerator levels. The short mono- and disulfide cross-links that form often do not exhibit the excellent physical properties of the conventional systems but they do retain thek properties much better after aging. 

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems ia a simple, carbon black-filled natural mbber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found ia Figure 9 (24). 

Table 7. Conventional and Semi-EV Cure Systems for Butyl Rubber ...

Thermo-oxidative stability is primarily a function of the vulcanization system. Peroxide vulcanization or cure systems tend to perform best for reversion resistance as a result of the absence of sulfur and use of carbon-carbon crosslinks. Efficient vulcanization (EV) systems that feature a low sulfur level (0.0-0.3 phr), a high acceleration level, and a sulfur donor similarly show good heat stability and oxidation resistance. Such systems do, however, have poor resistance to fatigue because of the presence of predominantly monosulfidic crosslinks. Conventional cure systems that feature a high sulfur level and low accelerator concentration show poor heat and oxidation resistance because the polysulfidic crosslinks are thermally unstable and readily oxidized. Such vulcanization systems do, however, have better fatigue resistance. Semi-EV cure systems, which are intermediate between EV and conventional systems, are a compromise between resistance to oxidation and required product fatigue performance. 

Table 19 illustrates formulas for conventional, semi-EV, and EV cure systems in a simple, carbon black-filled natural rubber compoimd cured to optimum cure (t90). 

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. 

TMTD is used as a sulfur donor in EV cure systems to improve the rubber compound s aging properties. TMTD imparts less scorch safety time than dithiodimorpholine (DTDM) when used as a sulfur donor in an EV cure. Also, TMTD is commonly used as a very fast accelerator in conventional sulfur cures. It is also used as a secondary accelerator (a kicker ) in conjunction with a conventional primary accelerator. 

Sulfur is difficult to disperse in NBR and particularly in soft compounds resulting in orange peal or dimpled surfaces or sulfur spots hence sulfur dispersions added at the beginning of the mixing cycle are recommended. It is even better, as shown in a 50 phr DIDP extended NBR in Table 2.31, to use sulfurless cures [20]. Some of the EV cure systems match the physical properties of the sulfur and semi-EV cures and exhibit improved scorch safety. 

EV Cure Systems versus Sulfur and Semi-EV in Plasticizer-Extended NBR... 

Zeon Chemicals L.P. Semi-EV and EV Curing Systems for Zetpol 1020 (Z5.1.2) [Brochure]. Louisville, KY. 

Examples of a regular sulfur and an efficient vulcanization (EV) sulfur less cure in nitrile mbber are shown in Table 13.8. Compared to the normal sulfur control, the EV cure system provides comparable initial physical properties, lower (better) compression set, and less change in properties during high temperature aging. 

The replacement of a conventional cure by an EV curing system also increases the monosulphide content with SBR, in this case to a value about twice that for a natural rubber EV system (Table 3). 

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... 

HYBRID, SEMI-EV CURE SYSTEM DESIGN FOR GREATER PHYSICAL AND DYNAMIC PROPERTIES STABILITY EVA NATURAL RUBBER BUSHING COMPOUND... 

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