Curative system

Although these curative systems may also be used with the polyepichlorohydrin elastomers containing AGE, the polymers were developed to be cured with conventional mbber curatives, sulfur, and peroxides. These polymers containing the pendent aHyl group are readily cured with a typical sulfur cure system such as zinc oxide, and sulfur along with the activators, tetramethylthiuram mono sulfide [97-74-5] (TMTM) and... 

Newer polyacrylate rubbers can be cured with certain amines and are more responsive to a broad range of curative systems, e.g., alkali metal stearate/sulphur or sulphur donor, methyl zimate and ammonium adipate. 

An advantage of the autoclave for smaller components, is that a large volume of the same or different products can be vulcanised at the same time, provided that the curative systems in use do not interact between the different compounds. It is usual to support extrusions and low hardness products in talc. Products formed on mandrels and sheeting, for example, are cloth wrapped to prevent distortion. 

The system Cl-butyl-cis-polybutadiene has been studied in some detail because it was suitable for the developed differential swelling technique and because this system of blends vulcanized with zinc oxide, sulfur, and thiuram disulfide first revealed the presence of interfacial bonds. This curative system has the feature of a flat cure —i.e.y the two homophases are vulcanized rapidly, and the crosslinked density does not increase radically as vulcanization time is prolonged. This is observed in Table IV by swelling and extractable levels of a series of crosslinked networks cured at increasing times and swollen in a common solvent, cyclohexane. 

For polysulfides at 176°F (SO C) the relaxation constant is shown for different curative systems. (See Table II). This ability of polysulfides to relieve stress is valuable in maintaining adhesion in joints where movement must be accommodated. Polysulfide polymers which are based on bis chloroethyl formal show no evidence of crystallization at low temperatures and have a glass transition temperature of -74°F (-59 C). 

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). 

Figure 10.3 Tensile strength for unvulcanized, HVA-2 and sulfur curative system of the HDPE/NR/TPS blends.

The bisphenol-onium systems achieved considerable market penetration during the mid-1970s. The normal approach was for the rubber to be supplied with the curative system already incorporated into the polymer and such compounds have been widely used for O-rings and other applications requiring low compression set at elevated temperatures. The development of these systems has been the subject of a review by Hill (1975). 

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