EV cures

There are three generally recognized classifications for sulfur vulcanization conventional, efficient (EV) cures, and semiefficient (semi-EV) cures. These differ primarily ki the type of sulfur cross-links that form, which ki turn significantly influences the vulcanizate properties (Eig. 8) (21). The term efficient refers to the number of sulfur atoms per cross-link an efficiency factor (E) has been proposed (20). 

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. 

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

However, conventional systems ia natural mbber do provide better flex life than EV cures, and this is one of the limitations of EV curiag. The short monosulftde bonds are less able to rearrange to reheve localized stresses which build duriag flexing, whereas the longer S bonds can. This abiUty for stress rehef is thought to be the mechanism for the superior flex life of conventional cures. 

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. 

R. Mehnert A. Pincus and I. Janovsky, Eds., Joint Protocol on Improved Condition of Use of UV-Technology UV EV Curing Technology Equipment, John WUey, 1998. 

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. 

Sulfur cures do not always use elemental sulfur. Sometimes with special EV (efficient vulcanization) cure systems no elemental sulfur is used at all. Instead a sulfur donor chemical is used. The most common sulfur donor is dithiodimorpholine (DTDM), which donates two sulfur atoms from the center of its molecule to participate in the sulfur vulcanization process. These EV cures are more expensive than conventional sulfur cures based on elemental sulfur. This is because sulfur donors such as DTDM are more expensive per pound than sulfur itself. Elowever, the EV cure will usually impart better air aging resistance than that of a conventional sulfur cure system using a significant concentration of elemental sulfur. 

DTDM is sometimes preferred as a sulfur donor because its use does not greatly shorten the scorch safety time of the compound, thus causing scrap. A sulfur donor such as DTDM is often needed in an EV cure in order to achieve better aging properties. 

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 Donor or Semi-EV cures for nitrile rubber are given in Table 2.21 [13]. When selecting a cure system to obtain good heat and compression set resistance, this type of cure should be selected. Process safety and cure rate need to be considered in order to satisfy factory conditions and economics. Although dynamic properties are not as good as with normal sulfur cures, some combinations such as sulfur 0.5,... 

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). 

Whereas the crosslink density of conventionally cured SBR vulcanisates increases on ageing at elevated temperatures (e.g. 110 °C) the EV-cured material has a very stable crosslink density at the same temperature. 

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