Nitrile rubber compounding

The two nitrile rubber compounds were suggested by the main UK supplier at the time to represent a general purpose material (P) and a good ageing formulation (R). 

It is claimed that the development of a nitrile rubber-based compound for battle tank track pads could save the US Army million of dollars in maintenance costs. Researchers at the Army Materials Research Laboratory and Belvoir Research Development Engineering Centre developed a highly saturated nitrile rubber compound in 1987 in an attempt to make the pads, currently moulded in SBR, wear longer. Details are given. 

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). 

A representative TGA trace of a polymer product (a nitrile rubber compound) is shown below in Figure 6.1. 

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). 

Because nitrile rubber is an unsaturated copolymer it is sensitive to oxidative attack and addition of an antioxidant is necessary. The most common practice is to add an emulsion or dispersion of antioxidant or stabilizer to the latex before coagulation. This is sometimes done batchwise to the latex in the blend tank, and sometimes is added continuously to the latex as it is pumped toward further processing. PhenoHc, amine, and organic phosphite materials are used. Examples are di-Z fZ-butylcatechol, octylated diphenylamine, and tris(nonylphenyl) phosphite [26523-78-4]. All are meant to protect the product from oxidation during drying at elevated temperature and during storage until final use. Most mbber processors add additional antioxidant to their compounds when the NBR is mixed with fillers and curatives in order to extend the life of the final mbber part. 

Nitrile (Buna-N) is a rubber compound popular in most household plumbing applications. It s a basic plumbers o-ring seal, and handles most household liquids and chemicals. Because industry pumps so much water, this elastomer may be the single most popular o-ring seeondary seal in the world. Its service range is from -30° F to +250° F (-34° C. to +120 C). 

The rubbers may be vulcanised by conventional accelerated sulphur systems and also by peroxides. The vulcanisates are widely used in petrol hose and seal applications. Two limiting factors of the materials as rubbers are the tendency to harden in the presence of sulphur-bearing oils, particularly at elevated temperatures (presumably due to a form of vulcanisation), and the rather limited heat resistance. The latter may be improved somewhat by Judicious compounding to give vulcanisates that may be used up to 150°C. When for the above reasons nitrile rubbers are unsatisfactory it may be necessary to consider acrylic rubbers (Chapter 15), epichlorohydrin rubbers (Chapter 19) and in more extreme conditions fluororubbers (Chapter 13). 

During the past four decades phenolic resins have become of increased significance in rubber compounding. For example, the resin based on cashew nut shell liquid, which contains phenolic bodies such as anacardic acid (Figure 23.23), may, when blended with hexamine, be incorporated into nitrile rubber (butadiene-acrylonitrile rubber). 

In a particular application involving a typical O type mount, the nitrile rubber (NR) compound causes a resonance frequency of 28 Hz, whereas resonance was previously tolerable only well below 20 Hz. In this case, a blend of NR and bromobutyl rubber is more suitable. A comparison is shown in Table 3. 

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. 

Infrared spectroscopy is a major tool for polymer and rubber identification [11,12]. Infrared analysis usually suffices for identification of the plastic material provided absence of complications by interferences from heavy loadings of additives, such as pigments or fillers. As additives can impede the unambiguous assignment of a plastic, it is frequently necessary to separate the plastic from the additives. For example, heavily plasticised PVC may contain up to 60% of a plasticiser, which needs to be removed prior to attempted identification of the polymer. Also an ester plasticiser contained in a nitrile rubber may obscure identification of the polymer. Because typical rubber compounds only contain some 50% polymer direct FUR analysis rarely provides a definitive answer. It is usually necessary first... 

An organic (ester) compound used as a plasticiser in nitrile rubbers and in polyvinyl chloride. A/,AT-Dibutylthiourea DBTU, accelerator. 

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. 

The EVAs make it possible to produce compounds with high filler levels and are compatible with many elastomers, plastics and other materials SBR, butyl rubber, EPDM, nitrile rubber, Hypalon, thermoplastic elastomers, epoxies, PVC, PVDC, bitumen. 

The original route to acrylonitrile was the catalytic reaction of HCN with acetylene. That was a combination of two compounds that together had all the characteristics youd like to avoid—poisonous, explosive, corrosive, and on and on. But during World War II, acrylonitrile became very important as a comonomer for synthetic rubber (nitrile rubber). Later, the growth for acrylonitrile came from synthetic fibers like Orion, Acrylon, and Dynel. 

Aromatic compounds are used as plasticizers and components in the processing of certain rubber products. Rubber products, such as nitrile rubber, are used to manufacture fuel system seals. Conventional diesel fuels containing aromatic compounds will act to swell these seals and prevent fuel system leakage. 

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