Curing nitrile rubbers

Chiou and Bradley [81] conducted hydraulic burst and stress rupture tests on 1.28mm thick (58v/o 87/ 35/87° hoop filament wound) tubes made from E-glass fibre/Brunswick LRF-571 DGEBA epoxy resin. There were 6% voids in the laminate. A co-cured nitrile rubber liner was employed, partly to keep the inner surface dry and partly to ensure that pressure could still be maintained if the GRP cracked during the tests. The tests followed 6 months immersion in static simulated sea water (Aquarium Systems Instant Ocean, p = 1023 kgm, pH = 8.2). The tubes had a high (1.5%) moisture uptake, although some of this might have been free water in the voids, but saturation was not reached. 

The stress-strain curves of sulfur-cured nitrile rubber having 29wL% acrylonitrile (NBR-S), NBI, NBV-1, NBV-2 and NBV-3 are shown in Fig.8. From Fig.9 it is to be noted that tensile strength, Tg, and tensile modulus at 100% elongation, M qO depend strongly on the number of BP in a structural unit of the ionene, m. 

Resin curing of SBR and BR imparts excellent cut growth and abrasion resistance. Resin cured nitrile rubber shows high fatigue life and high relaxation, while resin-cured butyl rubber shows outstanding ozone and age resistance (409). 

The mechanical properties and cross-linked network structure of a peroxide-cured nitrile rubber containing magnesium methacrylate are investigated and discussed. Some typical properties of the material are compared with those of carbon black reinforced nitrile rubber. 8 refs. CHINA... 

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). 

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... 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

Blends of carboxylated nitrile rubber (XNBR) with EPDM are likely to provide an attractive combination of properties including oil resistance, heat and ozone resistance, high tensile strength, modulus, and hardness. However, the polar curing ingredients often diffuse from the nonpolar to polar component, thereby producing cure rate mismatch and inferior properties. Three different measures have been used to overcome the cure rate mismatch [29] ... 

High Sulfur-Cure Systems for Nitrile Rubber ... 

Nandanan et al. [35] reported the utilization of linseed oil as an MFA in nitrile rubber vulcanizates. Linseed oil not only acted as a plasticizer but also as the fatty acid component of the activator in the NBR vulcanizates. Use of linseed oil gave appreciable increase in properties like tensile strength, tear resistance, etc. while the viscosity of the compound was marginally lower than that of the control compound (which used di-octyl phthalate as the plasticizer). The vulcanizates containing linseed oil also exhibited increased cure rate as well as reduced leachability compared to the control at a dosage of 2-5 phr. This loading was seen to replace 6 phr DOP and 2 phr stearic acid in conventional NBR vulcanizates thereby reducing compound costs. 

Low molecular weight liquid nitrile grades are available and these can be used as compatible plasticisers in the compounding of nitrile rubber. Such plasticisers can be partially crosslinked to the main chain during cure, and hence exhibit low extractability. 

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. 

Carboxylic functional polyester curing agents, 10 401-406 Carboxyl ion system, 19 193 Carboxyl-terminated butadiene nitrile, rubbers and, 10 376-375 Carboxymethyl cellulose (CMC),... 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

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...

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