Curing, rubber peroxide vulcanization

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

Surface acidity is controlled by the hydroxyl groups on the surface of the silica and is intermediate between those of P-OH and B-OH. This intrinsic acidity can influence peroxide vulcanization, although in sulfur curing, there is no significant effect. Rubber-filler interaction is affected by these sites. 

It is known that sulfur-vulcanized Natural Rubber (NR) can be completely recycled at 200 to 225°C by using diphenyldisulphide [41]. Recently, the effi-ciacy of various disulphides as recycling agents for NR and EPDM vulcan-izates were reported [42]. While complete de vulcanization was observed on sulfur-cured NR at 20b°C, a decrease in crosslink density of 90% was found when EPDM sulfur vulcanizates with diphenyldisulphide were heated to 275°C in a closed mold for 2 hours. At the same time, EPDM cured by peroxide showed a decrease in crosslink density of about 40% under the same conditions. 

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown [93] that the revulcanization process of devulcanized SBR was essentially different from that of the virgin SBR. The induction period is shorter or absent for re vulcanization of the devulcanized SBR. This is also true for other unhlled and carbon-black-filled rubbers 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 the 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 crosshnking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber [93]. 

Uncured ethylene-propylene copolymers are soluble in hydrocarbons and have rather poor physical properties useful technological properties are developed only on vulcanization. As mentioned above, the saturated copolymers are vulcanized by heating with peroxides whilst the terpolymers are vulcanized by conventional sulphur systems. The peroxide-cured rubbers have somewhat better heat aging characteristics and resistance to compression set but sulphur-cured rubbers are more convenient to process and allow greater compounding freedom. 

The copolymer ethylene propylene rubber (EPM) is manufactured directly from only the two monomers, ethylene and propylene. Since this polymer contains no unsaturation, it must be cured with a peroxide vulcanizing agent. 

Rubber maker s sulfur is by far the most commonly used vulcanizing agent for curing rubber. Elemental sulfur cures impart advantages in flex fatigue, scorch safety time, and cost versus other vulcanizing agents such as peroxides. 

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... 

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