Sulfur-cured EPDM systems

In sulfur-cured EPDM systems, the primary amino- and mercaptofunctional silanes can participate in the cure mechanism to a far greater extent than the vinylsilane as shown by the higher modulus values. In the peroxide-cured EPDM system, all silanes promote significant improvements in modulus but in different degrees depending on their relative reactivity. The methacryloxy functional silane... 

Coated materials are evaluated in S-SBR and in 50 50 blends of S-SBR and EPDM rubbers. In blends, the partitioning of fillers and curatives over the phases depends on differences in surface polarity. In S-SBR, polythiophene-modified silica has a strong positive effect on the mechanical properties because of a synergistic reaction of the sulfur-moieties in the polythiophene coating with the sulfur cure system. In S-SBR/EPDM blends, a coating of polyacetylene is most effective because of the chemical similarity of polyacetylene with EPDM. The effect of... 

Cure characteristics of EPDM/PP blends were investigated by Sengupta and Konar (27). They calculated the state of cure in blends containing conventional sulfur curing system under variable time-temperature conditions. They found that the activation energy for the cross-linking is almost similar for the virgin EPDM and EPDM/PP mixtures. Cross-link densities in TPVs can be analyzed by swollen-state... 

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown (Isayev et al., 1996b) that the revulcanization process of devulcanized SBR was essentially different from those of the virgin SBR. The induction period is shorter or absent for revulcanization of the devulcanized SBR. This is also true for other unfilled and carbon black-filled mbbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules resulting in crosslinking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber (Levin et al., 1997a). 

Since EPR rubber molecules do not contain unsaturation, they can be vulcanized only by organic peroxide curing systems. If a third monomer is added during the polymerization, i.e., a diene monomer (wherein only one of the two double bonds takes part in the polymerization), unsaturation can be introduced into the molecule, and it can then be vulcanized by accelerated sulfur curing systems. A chemical structure for ethylene-propylene-diene-monomer (EPDM) rubbers can be expressed as follows ... 

Halobutyl rubbers are compatible with EP(D)M in all proportions and can be covulcanized using zinc oxide sulfur cure systems. The properties of the resulting blends are intermediate between those of the participating elastomers, taking into account their ratio. The addition of halobutyl rubbers is of special benefit in blends of EPDM and general-purpose rubbers. In such compounds, EPDM imparts ozone resistance, but it is not sufficiently compatible with dienic elastomers to yield satisfactory dynamic properties. The addition of halobutyl rubbers produces a ternary blend with greatly improved dynamic performance, ozone resistance, and flex resistance, while maintaining other desirable properties of these blends. 

TABLE 26 Typical sulfur cure systems for EPDM... 

It was found that a high-impact strength is obtained in PP-EPDM blends by slow curing with sulfur. Thiuram disulfide N-(cyclohexylthio)phthalimide was used as an inhibitor of curing, and its effect on the impact strength of dynamically cured PP-EPDM blends was studied (Table 6). It was also found that the one-step method of blend preparation also has a favorable effect on the impact strength of the resultant blend system. 

Mechanical and Physical Properties of EPDM and EPDM-SBR Blends Cured with Peroxide and Sulfur Co-Agent Cure System... 

The curing systems used to vulcanize specialty elastomers such as EPDM, CR, HR, and NBR are different than those used to cure NR, SBR, BR, and its blends. The former elastomers are less unsaturated and therefore need high ratio of accelerator to sulfur. 

Peroxides vulcanization of EPDM is growing in popularity because of enhanced aging resistance. A comparison of sulfur- and peroxide-cure system is shown in Table 14.31 [53]. 

Vulcanisation of EPDM with sulfur systems was studied by H-NMR using ethylidene norbornane (ENBH) as a model of ENB [62]. The use of ENBH was also effective to elucidate the curing reaction of EPDM with phenol-formaldehyde resin [63]. Similarly, halogenation reaction of HR was studied by H-NMR using 2,2,4,8,8-pentamethyl-4-nonene as a model [64],... 

The effect of partitioning of curatives on the crosslinking reactions in NR/BR blends was explored in more detail in a later paper [105]. The ultimate extent of curing observed in the individual phases of the blends was identical to that obtained for the pure components, however, for the blend faster curing was initially observed in the BR phase. This was related to the greater affinity of sulfur and accelerator for BR compared with NR. Other systems examined include blends of epoxidised NR and c/s-BR [106], NR blended with ds-BR [107] and NR blended with EPDM [108]. The work of Tinker and co-workers has been discussed at length by Cook [109]. 

General curing system for EPDM rubbers will be a thiazole (mercaptobenzothiazole or dibenzothiazole disulfide) accelerator with a thiuram and/or a dithiocarbamate. For high heat exposure condition in a process industry, sulfur donor types like tetra methyl thiuram disulfide may replace a larger part or all of the sulfur. 

NR/modified EPDM, in which EPDM was modified by pendant sulfur, exhibited improved endurance to repeated stress over that of covulcanized EPDM-NR mbber blends (68). The effects of ethylene and diene contents in EPDM, blend ratio, dicumyl peroxide curing system on the physical properties, interfacial adhesion force, and dynamic crack growth were examined (69). As the ethylene and diene contents in EPDM increased, the physical properties, such as dynamic cut growth, adhesion to other component were also increased. The mechanical properties of the blends are compared to those of the pure components in Table 15.3 (56). The ultimate tensile strength of noncompatibilized blends is lower than that of pure NR, as expected since these blends are incompatible. 

Over the years, much of the research on accelerated-sulfur vulcanization was done by using natural rubber as a model substrate. Natural rubber was the first elastomer and therefore the search for the understanding of vulcanization originated with work on natural rubber. Most of the work cited in the previous sections is related to natural rubber. However, some rather early studies have been directed to the vulcanization of butadiene 1,4-polymers (Skinner and Watson, 1969 Wolfe et al, 1329 Gregg and Katrenick, 1970). More recent is the work of Pellicioli and coworkers. Early basic work on the vulcanization of ethylene-propylene-diene-monomer rubber (EPDM) has been carried out (van den Berg et al., 1984a,b). Recently, Kuno and coworkers did basic work on EPDM networks. They found that, essentially, the vulcanizate properties depend only on the crosslink density, not on the type of curing system (Dijkhuis et al., 2009). 

Methods to prevent this diffusion gradient include preparing a masterbatch of dithiocarbamate accelerators in the polymer (HR, EPDM) and grafting accelerators onto the polymer (EPDM). One advantage of a cure system mismatch is improved ozone crack resistance. In any case, cure system location control is critical. Figure 4.10 shows the diffusion of sulfur from natural rubber to SBR. Two rubber samples are placed in contact with each other and heated. The sulfur concentration is then measured at increasing distances from the interface. In this case, considerable transfer occurs and significant concentrations build up in the SBR phase. 

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