Other Crosslinking Processes

Many other crosslinking reactions are used in commercial applications. A variety of halogen-containing elastomers are crosslinked by heating with a basic oxide (e.g., MgO or ZnO) and a primary diamine [Labana, 1986 Schmiegel, 1979]. This includes poly(epichlorohydrin) (Sec. 7-2b-6) various co- and terpolymers of fluorinated monomers such as vinylidene fluoride, hexafluoropropene, perfluoro(methyl vinyl ether), and tetrafluoroethylene (Sec. 6-8e) and terpolymers of alkyl acrylate, acrylonitrile, and 2-chloroethyl vinyl ether (Sec. 6-8e). 

Crosslinking involves dehydrohalogenation followed by addition of the diamine to the double bond (Eq. 9-27) with the metal oxide acting as an acid acceptor. Some vulcanizations 

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Finally, a new water-soluble polyphosphazene was recently synthesized that has the structure shown in 36 (46). This polymer has two attributes as a biomedical macromolecule. First, the pendent carboxylic acid groups are potential sites for condensation reactions with amines, alcohols, phenols, or other carboxylic acid units to generate amide, ester, or anhydride links to polypeptides or bioactive small molecules. Second, polymer forms ionic crosslinks when brought into contact with di- or trivalent cations such as Ca or Ai3+. The crosslinking process converts the water-soluble polymer to a hydrogel, a process that can be reversed when the system... 

Other authors assume that network strands are incorporated with equilibrium configurations and maintain the same configurations throughout the crosslinking process, /0 being therefore unity for networks formed and tested under the same set of conditions. 

Disposal of the remainders of finish baths should be done in compliance with local regulations. Some catalysts used with these finishes contain zinc or other metals whose presence in waste water may be restricted. One side benefit of cellulose crosslinking finishes, especially the formaldehyde-containing ones, is increased resistance of the fabric to microbial attack. The effects of wrong condensation conditions for the common dry crosslinking process and steps to their optimisation are listed in Table 5.10. ... 

On the other hand, Oiwa [32] has carried out theoretical studies of the reaction kinetics of polymerization involving chain cyclization and multiple crosslinking processes. In addition, the sterically reduced reactivity of pendant allyl groups in the polymer chain was considered. 

Vulcanization refers to crosslinking of long linear chains that start out strongly overlapping each other. This process is well described by the mean-field percolation model, discussed in Section 6.4. Condensation and vulcanization are two closely related processes. By increasing the fraction of bifunctional alcohol monomers in the first process (i.e., increasing the... 

Crosslinking of polyethylen and its copolymers EPM, EPDM, and EVA is mainly carried out by radicals generated from organic peroxides. Other crosslinking methods like electron-beam radiation and grafting of vinylsUanes assume a smaller role in this process. 

In terms of environmental conditions, the cure rate is dependent on the temperature and relative humidity during the cure process. A silicone sealant used at -45°C will take several days to become tack free, while the same sealant takes 30 minutes at 23 C, 50% relative humidity and 8 minutes at 38°C, 100% relative humidity. The time it takes to develop a tack-free surface will also vary with the type of crosslinker utilized. With other crosslinkers mentioned earlier, sealants may develop a tack free surface in as little as six minutes or as long as one hour, given similar cure conditions. 

The original crosslinking process for natural rubber, called vulcanisation, involved mixing in 2-3% of sulphur plus an accelerator. On heating to 140 °C the sulphur reacts with C=C bonds on neighbouring polyisoprene chains to form sulphur crosslinks C—(S) —C. Typically, 15% of the crosslinks are monosulphide [n = 1), 15% are disulphide and the rest are polysulphide with n > 2. The polysulphide crosslinks are partially labile, which means that they can break and reform with other broken crosslinks when the applied stresses are high. This leads to permanent creep in compressed rubber blocks. To avoid such permanent set, efficient vulcanisation systems have been developed that produce only monosulphide crosslinks. 

Experimentally, piezoelectricity in cholesteric and chiral smectic C phases was reported for liquid-crystalline networks [140-147]. Multidomain lightly cross-linked systems were synthesized, then the orientation is obtained by mechanical strain [140] or by poling [147]. In other samples this orientation is performed prior to the crosslinking process [144, 146]. Macro-scopically oriented samples were subjected to either a static or a periodically varying strain. Open circuit voltages across the samples were measured that are linear functions of the applied strain [140-142, 144, 145],... 

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