Peroxide curing vulcanizate

In addition to dicumyl peroxide (DCP), in two different batches zinc oxide (ZnO) or a conventional organic accelerator (ZDMC) were used. Figure 34 depicts the corresponding XRD pattern. In both cases, the peak positions are almost the same as that of the pure peroxide-cured vulcanizates. However, the intensity of the XRD pattern was significantly reduced in the case of ZDMC, and there is only a little effect of ZnO. Obviously, the sulfur-containing zinc salt influences and promotes dispersion and reorientation of the layered... 

Figure 1. Shows compression set for a peroxide cure vulcanizate.

Peroxide-induced cross-links are stable to oxidation, whereas sulfur cross-links may oxidize, leading to cross-link rupture. Peroxide-cured vulcanizates exhibit higher compression set resistance than their counterpart sulfur-cured vulcanizates. Peroxide vulcanizates generally have better low-temperature flexibility than do sulfur vulcanizates. 

Now, the gel fractions of peroxide-cured-vulcanizates were measured to estimate the degree of vulcanization. 

Figure 8 shows the effect of TAIC on gel fractions of peroxide-cured-vulcanizates prepared from polymer B and C. 

Figure 9 shows the effect of peroxide on gel fractions of peroxide-cured-vulcanizates prepared from polymers B and C. The gel fractions of vulcanizates prepared from monoiodide-copolymer are smaller than those from di-iodide-copolymer even it was cured using a great quantity of peroxide. Analyses of extracts were done to investigate the cause of difference of gel fractions. Consequently, it was found that the extracts were lov/ molecular weight copolymers which have no iodine. Thus it seems the mono-iodide-copolymer contains about 10-15% iodine-free-copolymer by weight. 

Fig. 9. Effect of peroxide on gel fraction of peroxide cured vulcanizates.

In spite of the objections to the various methods of measuring scission events, good 1 1 correlations are found between the scission efficiency values obtained by the various methods in the cases of natural rubber latex, uncross-linked natural rubber, uncross-linked gutta percha and peroxide-cured vulcanizates. On the other hand divergent results are obtained using the stress-relaxation technique with sulphur-cured vulcanizates. 

In the case of the purified rubber the trisulphide retarded oxidation but with the smoked sheet (which because of the natural antioxidants present oxidized at a lower rate) the trisulphide was a pro-oxidant. A similar pro-oxidant effect was obtained with a peroxide-cured vulcanizate from purified natural rubber but this contained both a trisulphide and a fairly conventional amine antioxidant (N-isopropyl N -phenyl-p-phenylene diamine—actually better known as an anti-ozonant). Monosulphides (e.g. AiSAi) were less antagonistic in their effect. 

The most widely used plasticizers are paraffinic oils. Por appHcations that specify high use temperatures, or for peroxide cures, paraffinic oils of low volatihty are definitely recommended. However, since paraffinic oils exude at low temperatures from EPDM vulcanizates, or from high ethylene EPDMs, they are often blended with naphthenic oils. On the other hand, naphthenic oils interfere with peroxide cures. Aromatic oils reduce the mechanical properties of vulcanizates, and they also interfere with peroxide cures. Therefore, they are not recommended for EPM/EPDM. 

The resistance to heat and aging of optimized EPM/EPDM vulcanizates is better than that of SBR and NR. Peroxide-cured EPM can, for instance, be exposed for 1000 h at 150°C without significant hardening. Particularly noteworthy is the ozone resistance of EPM/EPDM vulcanizates. Even after exposure for many months to ozone-rich air of 100 pphm, the vulcanizates will not be seriously harmed. EPM/EPDM vulcanizates have an excellent resistance to chemicals, such as dilute acids, alkaUes, alcohol, etc. This is in contrast to the resistance to aUphatic, aromatic, or chlorinated hydrocarbons. EPM/EPDM vulcanizates swell considerably in these nonpolar media. 

FIGURE 5.15 Failure envelope of various mixes A, natural rubber-polyethylene (NR-PE) vul-canizate (peroxide cured) , NR-PE vulcanizate (sulfur cured) , NR-PE vulcanizate with CPE as compati-bilizer V, EPDM-PE vulcanizate o, EPDM-PP vulcanizate (sulfur cured) NR-ENR-PE -PE. (Erom Roy Choudhury, N. and Bhowmick, A.K., J. Mat. Sci., 25, 161, 1990. With permission from Chapman HaU.)... 

Resistance to steam of FKM-based vulcanizates increases with fluorine content. Peroxide cures are superior to diphenol and diamine cures. Compounds based on AFLAS and particularly on KALREZ surpass FKM in this respect.48... 

Fluoroelastomer vulcanizate properties cure improved by oven post curing. This is true for diamine, bis-phenol, and peroxide cures. For example in Table 3 both bis- tenol and peroxide cured black stocks of a VFi/WE/HFP terpolyner show a 50% increase in MlOO and TB, a 50% drcrease in elongation at break, and a substanti d improvement in ccn ression set resistance. 

One of the essentied functions of the oven cure in the nucleophilic diamine and bis-phenol vulcanizates is the removal of water, a by-product of the cure whose presence prevents full development of the diamine cure and causes reversion of the bis-phenol cure. This is not an important factor in the peroxide cure. 

Funatsu et al. examined the change in free radical concentration of sulfur and peroxide cured cA-l,4-polyisoprene rubber (IR) vulcanizates during tensile deformation. Figure 25.32 shows the first derivative ESR curves determined... 

Peroxide vulcanization is also suitable for BUR. While HR degrades rapidly when heated in the presence of organic peroxides, BUR can be cured with peroxides in combination with a coagent. Vulcanizates with unusually low compression set, heat resistance and excellent ozone resistance can be produced with peroxide cures. 

The methods by which antioxidants function were outlined in Section 9.2. Subsequent sections however amply demonstrated that the behaviour of an antioxidant in an accelerated-sulphur cured diene rubber vulcanizate can be very different from its behaviour in a simple hydrocarbon like petrol or polyethylene or even a peroxide-cured natural rubber. Antioxidant combinations which in some circumstances behave synergistically may in other cases be antagonistic to each other. As was demonstrated in Fig. 9.1 an increase in the antioxidant loading may decrease the degree of pro-... 

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