Cross-linking bridge

As we have mentioned, there are various types of keratins, and this brings us to one final aspect of sulfur cross-link bridges. The a-keratins can be classified as soft or hard by their sulfur content The low sulfur content keratins of wool and hair are much more flexible than the hard, high sulfur... 

The reactive additives applied in this procedure are typically vinyl monomers and/or oligomers, with one or more double bond, capable of radiation initiated chain reactions. The reactive additives — applied in moderate concentrations, — may form short side-chains and this way cross-linking bridges between the components to be compatibilized. The whole process may also be considered as promoted cross-linking, or graft - cross-linking. 

Plastics featuring wide-meshed cross-linking (elastomers) do swell, since their cross-linking bridges prevent complete dissolution. Swelling capacity is reduced as the degree of cross-linking increases. 

Finally, the unsaturated polyester is free-radically cross-linked by copolymerization with, for example, styrene or methyl methacrylate. Mixtures of the actual unsaturated polyester with these monomers are commercially known as unsaturated polyester resins. The properties of the thermosets can be matched to the application by variations in the acids, glycols, or vinyl monomers. Copolymerization with electronegative comonomers such as styrene or vinyl acetate leads, for example, to alternating copolymers, that is, to short cross-link bridges and therefore, to more rigid thermosets. Alternatively, electropositive comonomers such as methyl methacrylate form long methyl methacrylate bridges between the polyester chains and so produce more flexible polymerizates. 

Figure 30-2. Representation of the chain conformation of the enzyme precursor chymotrypsinogen. The molecule has five disulfide cross-link bridges. The histidine and serine residues, drawn in black, form the active center of the molecule. The molecule transforms into the active enzyme, chymotrypsin, when the chain is broken at the position of the black rings to the left in the diagram. (After H. Neurath.)...

The resulting mixture has a quite complicated structure (Table 37-2). Normally, the amounts of additives are given in terms of parts per hundred parts of rubber, phr. The rubber provides the elastomer with the required viscoelastic properties. Sulfur forms the cross-link bridges between molecular chains the cross-linking shifts the property spectrum from viscous towards elastic. Carbon black as filler fulfills two functions to cheapen the product as a classical filler, and to increase the mechanical strength as what is known as... 

Here some of the cis double bonds convert to trans double bonds. A cross-link bridge occurs for every approximately 50 sulfur atoms added. The hot vulcanization is accelerated by compounds such as zinc oxide, 2-mercaptobenzthiazole, or tetramethyl thiuram disulfide in industry. The mode of action of these compounds is not completely established however, they increase the sulfur utilization from 1/50 to about 1/1.5. 

Cold vulcanization occurs at room temperature. It leads to monosulfide cross-link bridges when disulfur dichloride is used ... 

Novolaks are soluble resins that melt without chemical change (no heat curing). They are polynuclear phenols with methylene cross-link bridges and molecular weights of 600-1500. Although they can be converted into insoluble resins by means of curing agents such as formaldehyde, heat alone is not effective. 

Here, Y can be O-alkyl, NH-alkyl, S-alkyl, etc., or a CH-acid compound (aromatics ). Since the activation energy for the formation of a —CH2— cross-link bridge is 59 kJ/mol and that of a —CH2—O—CH2— bridge 113 kJ/mol, Bakelite C is preferentially cross-linked via methylene bridges. The formation of —CH2— bridges is also favored by the ease with which ether groups are eliminated in acid media. 

Urea-formaldehyde resins and similar aminoplast precondensates form the greatest proportion of all the resins used as additives. Mono-methylated and dimethylated ureas are used, as are the analogous condensation products of formaldehyde with melamine. The monomeric compounds penetrate into the intermicellar space in the cellulose in aqueous solution, and there harden with heat to form insoluble resins (cf. also Section 28.2). Since the formation of mono- and dimethylated urea is reversible, CH2O occurs in equilibrium. Formaldehyde can form methylene cross-link bridges between the individual chains. In addition, longer cross-linking... 

Cross-linking bridges extend from an amino-group of... 

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