Cross-linking sites

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

The stmctural architecture of siUcone polymers, such as the number of D, T, and Q sites and the number and type of cross-link sites, can be deterrnined by a degradative analysis technique in which the polymer is allowed to react with a large excess of a capping agent, such as hexamethyidisiloxane, in the presence of a suitable equiUbration catalyst (eq. 38). Triflic acid is often used as a catalyst because it promotes the depolymerization process at ambient temperature (444). A related process employs the KOH- or KOC2H -catalyzed reaction of siUcones with excess Si(OC2H )4 (eq. 39) to produce ethoxylated methylsiUcon species, which are quantitatively deterrnined by gc (445). 

Butyl Rubber. Butyl mbber was the first low unsaturation elastomer, and was developed ia the United States before World War II by the Standard Oil Co. (now Exxon Chemical). It is a copolymer of isobutylene and isoprene, with just enough of the latter to provide cross-linking sites for sulfur vulcanization. Its molecular stmcture is depicted ia Table 1. 

Roughly 65% of the substituents are trifluoroethoxy, and 35% are telomer alcohols prepared from tetrafluoroethylene and methanol. About 0.5 mol % of an aHyhc substituent is used as a cross-link site. The substituent pattern is beUeved to be stricdy statistical. 

Another elastomer to find use is the substitution product of phenol and -ethylphenol along with an aHylic monomer to provide cross-link sites (5). This is trademarked EYPEL-A elastomer [66805-77-4] by Ethyl Corp., and designated PZ by ASTM. The substitution ratio is roughly 52 mol % phenol, 43% j )-ethylphenol, and 5% aHylic substituent. 

In consequence ethylene-propylene rubbers were introduced with a small amount (3-8%) of a third, diene, monomer which provided a cross-link site for accelerated sulphur vulcanisation. Such ethylene-propylene-diene monomer ternary copolymers are designated as EPDM rubbers. 

Provision of more active cross-link sites. 

Fig. 6-5 Effect of distance between TS cross-linked sites on compression properties.

G(scission) = G(S) = Number of polymer chain scissions per 100 eV absorbed. G(cross-linldng) = G(X) = Number of polymer cross-link sites per 100 eV absorbed. 

The same relation is found for the end-linking of molecules of low functionality if = 3 or 4) and for the vulcanization of long molecular chains. The second-moment average number of cross-linking sites along the chain, f2, is defined as... 

Detection/Labeling Possibilities for Cross-Linked Sites. 184... 

Fig. 13. The secondary structure of the E. colt ribosomal 16 S RNA, showing protein-binding sites, RNA—protein cross-link sites, and intra-RNA cross-link sites. The relations between a, b, and c are represented as in Figs. 2 and 3. Reproduced with permission from Brimacombe et al. (1983). 

LAGA is a monocarbonyl derivative of isosorbide. In contrast to isosorbide, however it is not hydroscopic and easily opens its lactone ring with the formation of potential cross-linking sites. 

The presence of cross-linked phosphates may be recognized by their ready hydrolysis, which leads to a rapid drop in the viscosity of the solution and a parallel decrease in its pH. Aqueous solutions of all cross-linked phosphates are hydrolyzed after twenty hours. In contrast to the hydrolysis of normal P—O—P bonds in meta- and polyphosphates, that of the cross-linking sites is practically independent of concentration, pH, ionic strength and the nature and concentration of added salts. It does, however, follow a first-order law, as for normal P—O—P bonds, and is strongly temperature dependent. Activation energies of 18.9 and 15.4 kcal/mole have been... 

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