Nitrile rubber highly crosslinked

Polymers can be modified by the introduction of ionic groups [I]. The ionic polymers, also called ionomers, offer great potential in a variety of applications. Ionic rubbers are mostly prepared by metal ion neutralization of acid functionalized rubbers, such as carboxylated styrene-butadiene rubber, carboxylated polybutadiene rubber, and carboxylated nitrile rubber 12-5]. Ionic rubbers under ambient conditions show moderate to high tensile and tear strength and high elongation. The ionic crosslinks are thermolabile and, thus, the materials can be processed just as thermoplastics are processed [6]. 

Commercially important elastomeric thermoplastic alloys are dynamically vulcanized blends of polypropylene with high volume fractions of EPDM, polybutadiene rubber, nitrile rubber, and butyl rubber (Santoprene , Vyram , Geolast and Trefsin ) all currently sold by Advanced Elastomer Systems, a joint venture of Monsanto and Exxon. Another recent member of the commercial dynamically cured elastomeric thermoplastic alloys is the blend of PVC and a crosslinked ethylene copolymer (Alcryn , DuPont). The current consumption of all the elastomeric thermoplastic alloys in the USA is over 23 kton/y, with the EPDM/PP blend (Santoprene ) assuming about 90% of the market share. 

We might expect that nitrile rubber would also be vulcanized with efficiencies greater than 1.0 however, though the double bonds in nitrile rubber are highly accessible, the crosslinking efficiency is somewhat less than 1.0 (Loan, 1963). 

The highly crosslinked nitrile rubber is S5mthesized according to the invented method (fully disclosed in the patent). Nitrile mbber is dispersed in hot DOP (25-80°C) before PVC and its stabilizer are added. The rubber containing plastisol was designed with the manufacture of gloves, shoes, and boots in mind. 

The mechanism responsible for the hardening of hydrogenated nitrile rubber automotive seal compounds on prolonged exposure to automatic transmission fluids at high temperature was investigated. The increase in hardness was found to result primarily from chemical attack at the acrylonitrile sites. This chemical attack first resulted in the formation of isocyanide or isonitrile ions. The proposed mechanism involved crosslinking at the acrylonitrile sites through the formation of isonitrile ions and the subsequent formation of imides. 

Commercial nitrile rubber polymerizations are typically run to final conversions (of monomer to polymer) between 75 and 90%. As conversion increases, the resulting polymer is more highly branched and crosslinking or gel formation is increased. If a gel-free or clear cement is desired, lower conversions need to be employed, since the solubility of the nitrile copolymer is dependent on the ultimate conversion and molecular weight. 

In a recent review of the properties of cured epoxy resins, Kaelble has pointed out that even simple formulations of diepoxide monomer and polyfunctional crosslinking agent reflect in the cured state the separate chemistries and structures of the co-reactants. When the molecular weight of both co-reactants in cured resin are high enough to provide more than four chain atoms between junction points, the resultant network begins to display the properties of a three-dimensional block copolymer. Creation of elastomeric microphases in epoxy structural adhesives has been recently identified with in situ block copolymerization between carboxy terminated nitrile (CTBN) rubber and the diepoxide.The adsorption-interdif-... 

There are basically six types of crosslinking mechanisms for NBR elastomers, very high sulfur for a special ebonite, normal sulfur, semi-EV (semi-efficient vulcanization or low sulfur systems), EV (efficient vulcanization with sulfur donors alone), peroxide and zinc oxide/peroxide for carboxylated nitrile. The innumerable combinations that can and are used are as numerous as there are rubber chemists in the world, but with care they all work in any given application. 

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