Nitrile cure systems

Nitrile mbber compounds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from —30 to 125°C. The compounds are also plasticized using polar ester plasticizers. The main dilemma is the selection of a heat-stable, nonfugitive plasticizer that also gives good low temperature properties. 

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems ia a simple, carbon black-filled natural mbber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found ia Figure 9 (24). 

Double-Bond Cure Sites. The effectiveness of this kind of reactive site is obvious. It allows vulcanization with conventional organic accelerators and sulfur-based curing systems, besides vulcanization by peroxides. Fast and controllable vulcanizations are expected so double-bond cure sites represent a chance to avoid post-curing. Furthermore, blending with other diene elastomers, such as nitrile mbber [9003-18-3] is gready faciUtated. 

Hydrogenated nitrile rubbers were introduced in the mid-1980s as Therban by Bayer. The initial grade had an acrylonitrile content of only 17% instead of approx. 34% in conventional NBR. Whilst non-sulphur-curing systems such as the use of peroxides with triallyl cyanurate or isocyanurate are necessary, the saturated rubber has a number of advantages over NBR. These include improved... 

High Sulfur-Cure Systems for Nitrile Rubber ... 

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

In addition to the normal sulphur cure systems, metal oxides can be used to cure the carboxylated nitriles. 

Fig. 13. Relationship of nitrile rubber cure systems where DCP is dicumyl peroxide MBTS, benzothiazyl disulfide ZnDMD, zinc dirnethyldithiocarbamate MBT, mercaptobenzothiazole TMTM, tetramethylthiuram mono sulfide TMTD, tetramethylthiuram disulfide and CBTS, iV-cyclobexyl-2-benzothiazole...

Because of the diene component, nitrile rubbers can be vulcanized with sulfur. A conventional curing system consists of 2.5 parts sulfur, 5.0 parts zinc oxide, 2.0 parts stearic acid, and 0.6 parts N-t-butylbenzothiazole-2-sulfenamide (TBBS) per 100 parts polymer. 

Nitrile and Acrylic Rubber. Nitrile rubbers are made by the emulsion copolymerization of acrylonitrile (9-50%) and butadiene (21) and are abbreviated NBR (eq. 11). The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties and the nature of the pol5nners. As the ACN content increases, the oil resistance of the poljnner increases (14). As the butadiene content increases, the low temperature properties of the polymer are improved. Nitrile rubber is much like SBR in its physical properties. It can be compoimded for physical strength and abrasion resistance using traditional fillers such as carbon black, silica, and reinforcing clays. The primary benefit of the polymer is its oil and solvent resistance. At a medium ACN content of 34% the volume swell in IRM 903 oil at 70°C is typically 25-30%. Nitrile rubber can be processed on conventional rubber equipment and can be compression, transfer, or injection molded. It can also be extruded easily. Nitrile rubber compoimds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from -30 to 125°C. The compounds are also plasticized nsing polar ester plasticizers. 

Several rubbers may be crosslinked using divalent metal oxides, usually zinc oxide. There are a limited number of polymers that utilise this method, which is used with halogenated polymers such as polychloroprene [8], chloro- and bromobutyl, and chlorosulfonated polyethylene and carboxylated nitrile rubbers. The system may utilise the metal oxide alone or in combination with the organic accelerators used with sulfur-curing systems. In the case of halogenated polymers, magnesium oxide may be added to act as an acid scavenger. 

Thus a conventional nitrile seal may perform adequately in some situations but fail badly in others. Even fluoroelastomers can fail in some cases if not based upon the correct polymer type or curing system. 

The failure of rubber parts used in domestic water applications has been reported flem several US states. The complaints involved swelling and cracking and are related to nitrile and ethylene-propylene copolymer type compoimds. The failure of the rabber parts coincides with the implementation and use of chloramines as a method of water purification. This investigation was designed to determine if EPDM composition variables influenced resistance to chloramines. Also evaluated were peroxide vs sulphur cure systems, the effect of antioxidants and whether practical compoimds can be developed for water systems. 5 refs. 

Figure 6.4 shows a dynamic headspace examination of the volatiles from a nitrile rubber compound. Early eluting peaks were identified as dimethylamine from the cure system (1.08 minutes), acrylonitrile monomer (1.47 minutes), carbon disulfide from the cure system (1.57 minutes) and diethylamine from the cure system (1.83 minutes). 

The major considerations in designing an NBR compound, in the order of importance, are the proper nitrile mbber the plasticizo- the cure system the antioxidant/antiozonants the filler and the economics, once the sawice conditions are defined and the factory process is established. 

Sulfur Donor or Semi-EV cures for nitrile rubber are given in Table 2.21 [13]. When selecting a cure system to obtain good heat and compression set resistance, this type of cure should be selected. Process safety and cure rate need to be considered in order to satisfy factory conditions and economics. Although dynamic properties are not as good as with normal sulfur cures, some combinations such as sulfur 0.5,... 

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... 

Carboxylated Nitrile Rubber (XNBR), which contains carboxyl groups ranging from approximately 1% to 8%, offers much higher abrasion, hardness, modulus, tensile, and tear resistance. In order to achieve the best properties the compound must contain zinc oxide or preferably zinc peroxide as well as sulfur or peroxide cure systems. The various grades available in North America are given in Tables 2.9 and 2.10. 

Polysulhdes often are blended with other elastomers such as nitrile rubber, NBR, or neoprene, CR, for improved physical properties and factory processing. Traditionally, Thiokol FA is blended with neoprene for improved strength and processing for rollers, at some sacrihce in solvent resistance. Table 11.9 has information on Thiokol FA blends. If minimal loss in chemical resistance is indicated, then a blend with a high ACN nitrile is employed. A cure system that is compatible with both rubbers needs to be used in all cases. The zinc peroxide-cure system is NBR specihc, hence it is important that the recommended one be used or others be tested since many NBRs do not cure with zinc peroxide. Best results are obtained with blends if separate masterbatches are made with the individual polymers, which are then blended in the... 

---