Cross-linking catalyst formaldehyde resin

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

Hexamethylolmelamine can further condense in the presence of an acid catalyst ether linkages can also form (see Urea Formaldehyde ). A wide variety of resins can be obtained by careful selection of pH, reaction temperature, reactant ratio, amino monomer, and extent of condensation. Liquid coating resins are prepared by reacting methanol or butanol with the initial methylolated products. These can be used to produce hard, solvent-resistant coatings by heating with a variety of hydroxy, carboxyl, and amide functional polymers to produce a cross-linked film. 

However, the most important furan resins are those produced with 2-furfuryl alcohol, for example, the 2-furfuryl alcohol-formaldehyde-based resins, which are normally synthesized by a condensation reaction catalyzed by acidic sites and promoted by heat [224] or the poly(furfuryl alcohol) thermosetting resin that is usually synthesized by the cationic condensation of its monomer 2-furfuryl alcohol, which polymerizes exothermically in the presence of a catalyst such as acid and iodine in methylene chloride, producing black, amorphous, and branched and/or cross-linked structures [225],... 

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes sympy, then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, then show how further condensation leads to cross-linking. (Hint The carbonyl group lends acidity to the N—H protons of urea. A first condensation with formaldehyde leads to an inline, which is weakly electrophilic and reacts with another deprotonated urea.)... 

CNC REZ LFR-C is a modified cyclic cross-linking resin containing a latent catalyst. High quality wash and wear and durable press finishes can be produced by pad or foam apparatus. When properly cured, free formaldehyde on fabric is 100-150 ppm. 

Urea (NH2CONH2) reacts with formaldehyde similarly to phenol to produce methylol derivatives that then condense further to yield a cross-linked network (Scheme 1). Actually, at a mole ratio of 1.5-2 mol of formaldehyde to urea and a pH of 7.5, a mixture of the monomethylol, dimethylol, trimethylol, and tetramethylol ureas are formed. For further extensive condensation to take place, the pH of the system must be made acidic. Thus, it is possible to concentrate the initial resin solution or spray-dry it to a soluble powder that can be dissolved and mixed with an acid catalyst at the time of application to induce the curing reaction. The ratio of formaldehyde to urea used in commercial resins varies with the manufacturer, but is always less than 2 1. 

Use Catalysts, resin reactions, formaldehyde substitute, cross-linking agents, and tanning agents. 

The novolac resins are prepared by using acidic catalysts and a deficiency of formaldehyde. Because this type of resin is less reactive, cross-linking is accomplished by the addition of a curing agent or catalyst. The most common is hexamethylenetetramine or "hexa." The curing agent serves as a latent source of formaldehyde. As in the case of the resoles, volatiles are emitted during the cure. The chemistry of the phenolic resin is old but complex and well documented in the literature (10). 

With higher proportions of formaldehyde and the para-position vacant, branching is possible. An alkaline catalyst is used to get a slower rate of reaction, thus making it easier to stop the reaction before cross-linking occurs. This type of phenolic resin is called a resole. 

The copolymerization of ethylene with larger amounts of dicyclopenta-diene with, for example, vanadium trisacetyl acetonate/AIR3 as catalyst, leads to polymers with isolated double bonds. They oxidize at room temperature to insoluble cross-linked films. They can be cross-linked with phenol/ formaldehyde resins and blended with them. 

A large proportion of the amino resin produced is used as compression-molded plastics, with or without carrier materials such as cellulose. Amino resins are less colored and less light sensitive then phenolic resins, but they are more sensitive to humidity and temperature. Urea formaldehyde resins can be employed up to temperatures of 90° C and melamine formaldehyde resins up to 150°C. The urea formaldehyde resins are particularly suitable for rapidsetting compression molding materials. Of course, aniline formaldehyde resins must be used as pre-cross-linked products, since no aromatic nucleus condensation occurs in the absence of acid catalysts and a postcuring with... 

Thus, urea formaldehyde resins are particularly suitable as rapid-setting (curing, hardening) molding materials. Aniline formaldehyde resins are hardened with compounds such as paraformaldehyde, hexamethylene tetramine, or furfurol, since as no polycondensation reactions occur in the absence of acid catalysts, pre-cross-linked products have to be used for this purpose. Aniline resins therefore cannot be used as rapid-setting molding materials (cf. Section 12.2.2). 

The catalyst or add source can consist of ammonium phosphate or polyphosphate salts, phosphoric add-derived amides or alkyl or halo-alkyl phosphates. Charring agents are based on molecular structures that can form cross-linked networks such as pentaerythritol, sorbitol, melamine, and phenol-formaldehyde resins. Other polymeric systems capable of intumescence are some polyamides and polyurethanes. Blowing agents help form a porous structure in the char and can fadlitate its formation. Common blowing agents are based on urea and urea-formaldehyde resins, melamines, and polyamides that can liberate moisture. 

The can and coil coatings, generally, are cross-linked with phenol, melamine, or urea-formaldehyde condensation products at elevated temperatures (150-200°C) with acid catalysts. Normal epoxy-amino resin weight ratios are epoxy-urea, 70 30 epoxy-benzoguanamine, 70 30 epoxy-melamine, 80 20, and 90 10. Increasing cross-linker levels give improved thermal and chemical resistance at the sacrifice of coating flexibility and adhesion. 

The fibers are prepared from a high molecular weight novolak resin. Uncured fibers are prepared by melt-spinning the novolak. These fibers are then immersed in aqueous formaldehyde solution containing an acidic catalyst. As heat is ap-phed, curing commences and the novolak resin is transformed into a cross-linked... 

Resole PF resins are produced by the reaction of phenol with excess formaldehyde (P F molar ratio 1 1.8 to 1 2.2) in the presence of an alkali catalyst (Fig. 3). Because resole resins contain reactive methylol groups, they are self-curing resins that can condense with active sites on the phenol rings to form a cross-linked network in the presence of heat even without additional hardener. Resole resins have a very branched structure (Fig. 3) and are by far the more important of the two types of PF resins for wood composites. 

Conversely, novolac PF resins are produced by the reaction of excess phenol with formaldehyde (P F molar ratio 1 0.8 to 1 1) in the presence of an acid catalyst. Unlike resole resins, novolac PF resins have a more linear structure (Fig. 4) and do not self-cure because they lack the residual reactive methylol groups of resoles. Therefore, an external curing agent such as hexamethylenetetraamine must be added to novolac resins to yield a cross-linked structure. Novalac PF resins cured under acidic conditions are not recommended for wood composites for long-term structural applications. 

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