The Cross-linking Reactions

Diglycidyl ethers of Bisphenol A cannot be cross-linked through heating alone. Chemical cross-linking agents must be added. Most commonly used compounds are tertiary amines, polyfunctional amines, and acid anhydrides. Lewis acid, phenols, and compounds like dicyandiamide, however, are also used. 

The reactions between tertiary amines and epoxy groups result in formations of quaternary bases  

The product reacts with hydroxyl compounds to form anions  

The anions in turn initiate polymerizations of the epoxy groups  

Because the monomer is a diepoxide, a three-dimensional lattice results. A similar three-dimensional product forms from reactions with other cross-linking materials, like boron trifluoride-etherate, boron 

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The free radicals initially formed are neutralized by the quinone stabilizers, temporarily delaying the cross-linking reaction between the styrene and the fumarate sites in the polyester polymer. This temporary induction period between catalysis and the change to a semisoHd gelatinous mass is referred to as gelation time and can be controUed precisely between 1—60 min by varying stabilizer and catalyst levels. 

The cross-linking reaction mechanism is also influenced by the presence of other monomers. Methyl methacrylate is often used to improve the uv resistance of styrene-based resins. However, the disparate reaction rates of styrene and methacrylate monomer with the fumarate unsaturation not only preclude the use of more than 8% of the methacrylate monomer due to the significant slowing of the cross-linking reaction but also result in undercured products. 

Some fabrication processes, such as continuous panel processes, are mn at elevated temperatures to improve productivity. Dual-catalyst systems are commonly used to initiate a controlled rapid gel and then a fast cure to complete the cross-linking reaction. Cumene hydroperoxide initiated at 50°C with benzyl trimethyl ammonium hydroxide and copper naphthenate in combination with tert-huty octoate are preferred for panel products. Other heat-initiated catalysts, such as lauroyl peroxide and tert-huty perbenzoate, are optional systems. Eor higher temperature mol ding processes such as pultmsion or matched metal die mol ding at temperatures of 150°C, dual-catalyst systems are usually employed based on /-butyl perbenzoate and 2,5-dimethyl-2,5-di-2-ethyIhexanoylperoxy-hexane (Table 6). 

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

Retarders were originally arenecarboxylic acids. These acidic materials not only delay the onset of cross-linking but also slow the cross-linking reaction itself. The acidic retarders do not function weU in black-fiUed compounds because of the high pH of furnace blacks. Another type of retarder, A/-nitroso diphenylamine [86-30-6] was used for many years in black-fiUed compounds. This product disappeared when it was recognized that it trans-nitrosated volatile amines to give a several-fold increase in airborne nitrosamines. U.S. production peaked in 1974 at about 1.6 million kg. 

Studies by and nmr of the reactions of ethyl Hnoleate with oxygen ia the presence of cobalt driers iadicate that the cross-linking reactions... 

The principal mbbers, eg, natural, SBR, or polybutadiene, being unsaturated hydrocarbons, are subjected to sulfur vulcanization, and this process requires certain ingredients in the mbber compound, besides the sulfur, eg, accelerator, zinc oxide, and stearic acid. Accelerators are catalysts that accelerate the cross-linking reaction so that reaction time drops from many hours to perhaps 20—30 min at about 130°C. There are a large number of such accelerators, mainly organic compounds, but the most popular are of the thiol or disulfide type. Zinc oxide is required to activate the accelerator by forming zinc salts. Stearic acid, or another fatty acid, helps to solubilize the zinc compounds. 

The cross-linking reaction is carried out after the resin has been applied to the glass fibre. In practice the curing is carried out either at elevated temperatures of about 100°C where press mouldings are being produced, or at room temperature in the case of large hand lay-up structures. 

Induction period. The curatives react with themselves in preparation for the cross-linking reaction. This period allows the ingredients to be safely mixed avoiding premature curing ( scorch ). 

Cross-linked xylan-based microparticles are produced by the emulsification of an alkaline solution of xylan with a lipophilic phase formed by a mixture of chloroform and cyclohexane by using 5% (w/v) sorbitan triesterate as the surfactant. Subsequently, the cross-linking reaction is carried out for 30 minutes with 5% (w/v) terephthaloyl chloride in order to yield a hard and rigid polymeric shell (Nagashima et al., 2008). 

The acrylic compounds, maleimides, dimaleimides, and thiols are known as direct cross-hnk promoters, since they enter directly into the cross-linking reaction and become interconnecting molecular links [243]. 

Viscosity measurements of the extracted polysaccharides cross-linked at increasing concentrations showed a clear increase in relative viscosity, when the cross-linking reaction took place at concentrations higher than 0.5 % (Fig. 4). At a concentration higher than 1.5 % a gel was formed. 

When the butyl rubber was compounded with up to 30 percent of polyisobutylene, which, lacking the unsaturated isoprene units, did not enter into the cross-linking reaction, the tensile strengths were, of course, considerably reduced. They were found nevertheless to be accurately represented by the same equation, (53), provided merely that Sa is taken as the fraction of the composite specimen consisting of network chains subject to orientation. Thus, in this case... 

The rheology of hydroxypropylguar is greatly complicated by the cross-linking reactions with titanium ions. A study to better understand the rheology of the reaction of hydroxypropylguar with titanium chelates and how the rheology depends on the residence time, shear history, and chemical... 

Efficiency of the cross-linking reaction can be defined as the ratio of effective cross-link density, px, to the theoretical cross-link density, p(. Theoretical cross-link density assumes that all of the cross-linker added to the formulation created elastically effective cross-links however, it cannot be defined for nonchemical methods. Thus the theoretical cross-link density, p is given as... 

Buffers also are used to maintain the proper pH for the cross-linking reaction to occur at an optimum rate. Sodium bicarbonate and sodium carbonate are used to attain basic pH while weak acids acetic, fumaric, formic, and adipic, are generally used to obtain acidic pH values. 

Figure 43 The cross-linking reaction of the boron-containing Novolac resin (66) (obtained by the modification of the commercially available Novolac resin) with bis(benzo-l,3,2-dioxaborolanyl)oxide. (Adapted from ref. 91.)...

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