Semi-EV cure systems

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. 

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

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). 

Figure 5.24 Tearing energy versus tear rate at 23 °C of vulcanized NR latex filmfperoxide pre-vulcanized), black-filled NR (semi-EV sulfur system) and peroxide-cured unfilled NR (SMR L).

Cure System Design. There are three generally recognized classifications for sulfur vulcanization conventional, efficient (EV) cures, and semiefficient (semi-EV) cures. These differ primarily in the type of sulfur cross-links that form as shown in Figure 12, which in turn significantly influences the vulcanizate properties (33). The term efficient refers to the number of sulfur atoms per cross-link (32). 

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. 

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). 

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. 

Eig. 8. Sulfiir-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, monosulftde cross-links C, disulfide cross-links D, polysulftde cross-links... 

Eig. 12. SBR cure systems (A = conventional, B = semi-EV, C = EV) relative to conventional cure at 100 where I I shows aged elongation retained ,... 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

Fig. 11. Aging properties of cured natural rubber for 70 hours at 70°C. A is the conventional, B the semi-EV, and C the EV system where U shows tensile...

Reviewed earlier, a semi-EV system is a compromise designed to produce, in structural terms, a vulcanizate containing a balance of monosulfldic and polysulfidic crosslinks at a defined optimum cure state. If polysulfidic crosslinks are to persist over extended periods, new ones must be created to replace those lost through reversion. With use of normal acceleration systems, there is limited opportunity for such events. Maintenance of a polysulfidic network through the curing process thus dictates utilization of... 

Material Conventional cure system, phr Semi-EV (semiefficient vulcanization system, phr EV (efficient vulcanization system), phr... 

Fig. 13. Aging properties of cured natural rubber. A conventional cure system, B semi-EV, C EV system, tensile strength loss (%), fatigue life, andhardness change.

Vulcanization System Components. Tire compounds are almost exclusively cured (cross-linked) with sulfur. Sulfenamides, thiazoles, thiurams, guanidine, and carbamates are the most popular choices to accelerate curing. Efficient vulcanization (EV), semiefficient (semi-EV), and the conventional curing systems... 

Although acceptable cure characteristics are obtained with a conventional system based on 2 5 phr sulphur and 0 5 phr sulphenamide, this formulation is not recommended for ENR as the vulcanizates have poor ageing characteristics compared with unmodified NR. The cause of this is discussed in Section 6.9. Semi-EV or EV-type cure systems have been found to be the most satisfactory for ENR. Examples of such systems are recorded in Table 1. The need to compound ENR with a base affects the vulcanization characteristics, and one of the factors to be considered in the choice of base is the processing safety of the mix. " Stronger bases can markedly reduce scorch delay by base catalysis of the vulcanizing system and should only be employed at low levels, whereas the less basic compounds can be employed at higher concentrations (Fig. 9). 

In nitrile rubber, peroxide cure systems provide the best in heat and compression set resistance compared with semi-EV and Efficient Vulcanization combinations. There are quite a few peroxides available as given in Table 2.23, but the most commonly used ones in NBR are Di-Cup 40KE (Trigonox BC-40K), Peroximon F40 (Varox 802-40KE), and Varox 130-XL (Trigonox 145-45B) [15]. Peroxides function best... 

In addition, 1.5-2.5 phr of activators or coagents such as HVA 2, TAC, TAIC are commonly used with peroxides to achieve faster cures and higher modulus. In addition, TMPT, SR365, and other di- and tri-acrylate resins may be used at the 5 phr level. The acrylate resins may be used up to 40 phr in which case the hardness will increase approximately 8 points for every 10 phr of resin. The resin also acts as a plasticizer, which is very beneficial in keeping the compound viscosity low while achieving a high hardness vulcanizate. Table 2.25 illustrates the difference an EV, semi-EV, peroxide, and peroxide with SR-297 cure system. 

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