Peroxide-cured rubber

DCP is relatively inexpensive compared to many other peroxides that can also be used to cure rubber. Peroxide cures, in general, impart to a rubber compound advantages over conventional sulfur cures in terms of heat resistance and compression set. 

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... 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Figure 8 shows the SEM images with a low level of strain (50%). It is clear that even with a low-strain level defects are initiated in the sulfur cured system with the formation of large cracks at the boundary layer between the two phases. However, in the peroxide cured system the mechanism of crack initiation is very different. In the latter case the NR-LDPE interface is not the site for crack initiation. In this case, stress due to externally applied strains is distributed throughout the matrix by formation of fine crazes. Furthermore, such crazes are developed in the continuous rubber matrix in a direction... 

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.)... 

Comparison of Properties of Sulfur- and Peroxide-Cured Ethylene-Propylene-Diene Rubber (EPDM)... 

Peroxide cure, silicone network preparation via, 22 562-563 Peroxide-cured dipolymers, 20 697-698 Peroxide-cured EPDM, 22 804t. See also Ethylene- propylene-diene monomer (EPDM) rubber... 

Peroxide cure systems, in rubber compounding, 22 793-794 Peroxide decomposers, 3 111-114 Peroxide decomposition, 24 279-280 Peroxide formation, by VDC, 25 694. See also Hydrogen peroxide Peroxide initiators, 23 379-380 worldwide producers of, 24 303 Peroxide-ketazine process, 23 582-583 flow sheet for, 23 582 versus Raschig process, 23 583 Peroxide linkages, in VDC polymer degradation, 25 713... 

Thinner coatings on fabrics and fibers were prepared by dipping the fabric or fibers Into a solution containing 500 ml hexane, 50 g polybutadiene and 0.02 g dlcumyl peroxide. Excess solution was squeezed off by passing the fabric or fibers between rollers, the hexane was removed by evaporation, and the rubber was cured for 2 hours In a vacuum oven at 150°C. 

Since EPDM rubber is costly and the price fluctuates widely, high filler and oil loadings are resorted to, to achieve relatively low cost compounds. But these compounds tend to have poor strength and abrasion resistance. High quality stocks, especially those with peroxide cures, are considerably more expensive than NR, BR or SBR stocks. EPDM is used in belts and hoses meant for conveying hot materials because of its excellent heat resistance. Its excellent chemical resistance makes it a preferred rubber for specialty belts, chemical hoses and pump liners. 

Oh, S. J. and Koenig, J. L. (2000) Studies of peroxide curing of polybutadiene/zinc diacrylate blends by fast FTIR imaging. Rubber Chem. Technol. 73(1), 74-9. 

Rubbers rely on fillers (both reinforcing and nonreinforcing) to obtain their properties. The curing system also produces a dirty-colored material. To color a rubber is difficult, and only a few basic colors are used. To obtain a transparent rubber, special latex or synthetic cis-polyisoprene must be used, and the use of a peroxide cure is normal. Polyurethanes can be colored any color, but the yellowing of aromatic systems must be taken into account. Aliphatic systems can give transparent nonyellowing systems. 

For peroxide cross-linking, organic peroxides, such as dicumyl, di-t-butyl, and benzoyl peroxides, are used in amounts 1 to 3 phr (parts per hundred parts of rubber). Typical cure cycles are 5 to 10 min at temperatures 115 to 170°C (239 to 338°F), depending on the type of peroxide used. Each peroxide has a specific use. A postcure is recommended to complete the cross-linking reaction and to remove the residues from the decomposition of peroxide. This improves the long-term heat aging properties.62... 

Figure 1.8 EPDM rubber with peroxide curing agent, detection of cumyloxy fragments...

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