Cross-linking systems

Epoxy novolac resins are produced by glycidation of the low-molecular-weight reaction products of phenol (or cresol) with formaldehyde. Highly cross-linked systems are formed that have superior performance at elevated temperatures. 

Cross-linking cotton Cross-linking systems Cross ozomdes Cross-section shapes Crotarbital [1952-67-6]... 

Fig. 24. Representative cross-linking systems employed in negative tone CA resists, (a) Epoxy polymers requiring organic solvent development, (b) PHOST-based cross-linking systems requiring aqueous development, (c) Monomeric cross-linking agents used in PHOST matrix polymers.

Fig. 36. Representative bilayer resist systems. Both CA and non-CA approaches are illustrated (116—119). (a) Cross-linking E-beam resist, 193-nm thin-film imaging resist (b) acid-cataly2ed negative-tone cross-linking system (c) positive-tone CA resist designed for 193-nm appHcations and (d) positive-tone...

Another approach to durable press—flame retardancy uses a combination of a cross-linking system, antimony(III) oxide, and a bromine-containing reactive additive, namely dibromoneopentyl glycol, to achieve dual properties (120,121). 

Organic cross-linkers, which include glyoxal (48) and formaldehyde (qv), have also been used. Use of hypohaUte salts (49) and epichlorohydrin (50) promotes gel stabiUty. Phenol—formaldehyde cross-linking systems have been used to produce stable acrylamide copolymer gels at temperatures above 75°C and brine hardness levels above 2000 ppm (51). 

Copolymers of sodium acrylate with sodium 2-acrylamido-2-methylpropane sulfonate (AMPS) or /V, /V- dim ethyl acryl am i de (52) have been used to prepare cross-linked systems at high temperatures and salinity. Chromium cross-linked gels, prepared from a 3 1 blend of partially hydrolyzed... 

In general, however, the vulcanizates suffer from poor low temperature crystallization performance compared to a conventional sulfur cure, and also have inferior tensile and tear properties. Urethane cross-linking systems (37), eg, Novor 950 (see Table 3) are also extremely heat resistant, but exhibit inferior tensile and dynamic properties compared to conventional sulfur-cured vulcanizates. One added virtue is that they can be used in conjunction with sulfur systems to produce an exceUent compromise according to the ratios used (38). 

Nonformaldehyde Finishing. The concern for formaldehyde release prompted interest in the development of cross-linking systems that did not contain formaldehyde. A number of systems were investigated but generally these systems seemed to fall short in performance (106,107). For example, l,3-dimethyl-4,5-dihydroxyethyleneurea (DMeDHEU) (5) has been used in Japan since 1974. This same agent has been marketed in the United States and elsewhere, but generally the level of smooth-dry performance is substantially lower than the level achievable with DMDHEU. The cost of dimethylurea also raises the overall cost of DMeDHEU above that of DMDHEU. 

The physical properties of polyurethanes are derived from their molecular stmcture and deterrnined by the choice of building blocks as weU as the supramolecular stmctures caused by atomic interaction between chains. The abiHty to crystalline, the flexibiHty of the chains, and spacing of polar groups are of considerable importance, especially in linear thermoplastic materials. In rigid cross-linked systems, eg, polyurethane foams, other factors such as density determine the final properties. 

It has also been found that there can be interactions between hydrolytic degradation and photochemical degradation. Especially in the case of melamine-formaldehyde cross-linked systems, photochemical effects on hydrolysis have been observed. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

Ziabicki, A Walasek, J, Topological Structure and Physical Properties of Permanently Cross-Linked Systems. 1. s-Functional, Homogenerous, Gaussian Systems, Macromolecules 11, 471, 1978. 

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