Epoxy-amino cross-linking reaction

It has been shown that the majority of the polar moitles in epoxy-diamine networks, such as hydroxyl groups generated by the cross-linking reaction and residual amino hydrogens, take part in inter- or intramolecular hydrogen bonds (6, 7 ). Indeed, at ambient temperatures, the infrared hydroxyl stretch frequency of epoxy-diamine systems are characteristically shifted from its free value of 3600 cm-1 to a hydrogen bonded value of 3440 cm 1 (8). 

Thermosets are plastics that form large molecules by an irreversible chemical cross linking reaction. Figure 9.3 illustrates the formation of a thermoset. In this case, phenol is reacted with formaldehyde to produce the thermoset called bakelite, with water as a by-product. This was the first synthetic plastic developed. It was invented by L. H. Bakeland in 1906. Thermosets cannot be melted and cannot be recycled. Other thermosets are epoxies, alkyds, polyesters, aminos, polyimides, and silicones. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

Nonetheless, for the more than 50 years since the first publication in this field, NIPUs still do not have sufficiently broad application. This can be explained by certain features of these materials. Cyclic carbonate (CC) groups interact with aliphatic and cycloaliphatic polyamines at ambient temperatures more slowly than isocyanates with hydroxyl groups. The rate of this reaction is comparable to the rate of curing epoxy resins (ER) with amines. At the same time, the CCs react only with primary amino groups, in contrast to the ERs, which react with primary and with secondary amino groups. This results in a decrease in cross-linking density of the polymer network. 

Their pendent amine groups react readily with terminal epoxy groups to form cross-linking 3-hydroxy amino and polyether reaction products. These reactions are illustrated in the following equations ... 

Unsubstituted and Heat Reactive. The first class, the unsubstituted, heat-reactive resins, are made by using phenol, cresols, and xylenols. They are multifunctional and thus can be cross-linked to form films. They are soluble in alcohols, ketones, esters, and glycol ethers and insoluble in aromatic and aliphatic hydrocarbons. They will tolerate some water in their solvents and, in some cases, are completely water soluble. They are compatible with polar resins such as amino resins, epoxies, polyamides, and poly(vinyl butyral), though compatibility on curing is dependent on reaction between the two resins. Less polar resins such as alkyds and drying oils are incompatible. 

Many other cross-linking processes exist that are not widely adopted or have only recently been developed. Cross-linking of epoxy-containing acrylates with amino resins and reactions with polysulfonazides have been reported [2.57]. Reactions of unsaturated acrylic groups with ketimines and reaction of acetoacetate-containing polymers with ketimines should also be mentioned [2.52], [2.61 a]. 

Alkylated melamine-, urea-, and benzoguanamine-formaldehyde resins are the principal cross-linking agents in many industrially applied baked coatings. They are combined with acrylic, alkyd, epoxy, and polyester resins. The amide, hydroxyl, or carboxyl groups of these backbone polymers are used as functional sites for reaction with the amino resin. 

Dicyandiamide (see p. 168) and its derivatives cross-link at 180° C by reaction of the amino and imino (=NH) groups with the epoxy ring ... 

Epoxy resins proper may be cross-linked by reaction with chemicals containing amino groups. The finishes produced may be classified as follows ... 

We have not evaluated the rate of the addition of secondary amino group to epoxy r ing. This reaction does not lead to cross-linking and, therefore, is not so critical in respect to the adhesive performance. 

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