Amine cross-linking epoxy resin

In contrast to tertiary amine hardeners, which, as shown, cross-link epoxide resins by a catalytic mechanism, polyfunctional primary and secondary amines act as reactive hardeners and cross-link epoxy resins by bridging across epoxy molecules. 

Ester and ketone solvents, which react with amines, must not be present. The adduct is a polyamine and can therefore be used to cross-link epoxy resins. It has advantages in that it is odourless (simple amines are unpleasant to handle), part-reacted (which speeds up drying) and free from simple amines (which cause exudation and are toxic). It can be seen now why it is an advantage to age paints cured by simple amines amine adducts are formed during the aging. 

Epoxy Resins. Epoxy resins (qv) are used to cross-link other resins with amine, hydroxyl, and carboxyHc acid (or anhydride) groups. The epoxy group, properly called an oxkane, is a cycHc three-membered ether group. By far the most widely used epoxy resins in coatings are bisphenol A (BPA) (4,4 -(l-methylethyHdene)bisphenol) [80-05-7] epoxy resins. 

Cationic epoxy resin based systems can be prepared by reacting quaternary amine salts with epoxy resins to introduce the cationic functions. The polymers can then be cross-linked with blocked isocyanates at reasonable baking temperatures (34. 35). 

Solvent-free epoxy systems must be applied within an extremely short time (30-60 min), which presents problems, particularly in hot countries. Such systems are also frequently applied with heatable two-component spraying equipment. Keti-mines give a longer pot life. They are produced by condensing ketones and polyamines (Epikure H 3, Shell). When applied as a thin layer, they react with atmospheric moisture and are converted back into the ketone and polyamine. The ketone evaporates from the film and the amine cross-links the epoxy resin. Ketimines can only be used if a sufficiently high ambient temperature (>20 C) and high atmospheric moisture (>75% R. H.) can be guaranteed. 

Epoxy resins form a durable protective coating only when polymerized with amine, polyamide, or esterified fatty acids. Coal-tar epoxies are modified with coal-tar fiUer to improve moisture resistance. Coatings highly resistant to solvents, acids, and alkahs are based on phenoHc cross-linked epoxies. These coatings are used to protect process equipment. When the phenolformaldehyde constituent of phenol is used to cross-hnk epoxy resin instead of amine, the coating has improved resistivity to alkahs. 

The effect of hardener structure on heat resistance of a cross-linked DGEBA resin is shown in Table 19 (140). Thermal stability is affected by the structin-e of the hardener. The heat resistance of aliphatic amine cimed epoxy is low as measured by TGA. The nitrogen atoms are oxidized by atmospheric oxygen to amine oxides, which attack the polymer backbone. Anhydride systems tend to spht off the anhydride at temperatures well below their decomposition point at about 390°C. The ether segments formed by 2-MI and phenohc cured epoxies have the highest thermal stability. 

Kortaberria, G., et al.. Curing of an Epoxy Resin Modified with Poly(Methylmethacrylate) Monitored by Simultaneous Dielectric/Near Infrared Spectroscopies. Europ. Polym, J., 2004.40 129-136. Mijovic, J., et al.. Interplay of Segmental and Normal Mode Dynamics in Polymer Networks Undergoing Chemical Cross-Linking. Epoxy/Amine-Terminated Linear and Stai PPO Eormnlations. Macmmolecules, 2003. 36 4589-4602. 

Some commercial durable antistatic finishes have been Hsted in Table 3 (98). Early patents suggest that amino resins (qv) can impart both antisHp and antistatic properties to nylon, acryUc, and polyester fabrics. CycHc polyurethanes, water-soluble amine salts cross-linked with styrene, and water-soluble amine salts of sulfonated polystyrene have been claimed to confer durable antistatic protection. Later patents included dibydroxyethyl sulfone [2580-77-0] hydroxyalkylated cellulose or starch, poly(vinyl alcohol) [9002-86-2] cross-linked with dimethylolethylene urea, chlorotria2ine derivatives, and epoxy-based products. Other patents claim the use of various acryUc polymers and copolymers. Essentially, durable antistats are polyelectrolytes, and the majority of usehil products involve variations of cross-linked polyamines containing polyethoxy segments (92,99—101). 

The linear polymer formed is cured hy cross-linking either with an acid anhydride, which reacts with the -OH groups, or hy an amine, which opens the terminal epoxide rings. Cresols and other hisphenols are also used for producing epoxy resins. 

Consist of a range of chemicals which promote cross-linking can initiate cure by catalysing ( catalysts , hardeners, initiators), speed up and control cure (activators, promoters) or perform the opposite function (inhibitors) producing thermosetting compounds and specialised thermoplastics (e.g. peroxides in polyesters, or amines in epoxy formulations). The right choice of a cure system is dependent on process, process temperature, application and type of resin. 

Other possible reactions, such as homopolymerization (epoxide+epoxide) and epox-ide+hydroxyl group (in the latter stages of cure), can be neglected when the ratio of epoxide to amine is stoichiometric and in the absence of catalyst or accelerator [194], For TGDDM/DDS resins, the homopolymerization reaction may be neglected at cure temperature below 180°C [84], At temperatures between 177°C and 300°C, dehydration and/or network oxidation occur, which results in formation of ether cross-linkings with loss of water. Decomposition of the epoxy-OH cure reaction can also take place, which results in propenal... 

The traditional cross-linking technologies utilized in the coatings market range from hydroxyl functional binders (acrylics or polyesters) cross-linked with melamines (IK) or isocyanates (2K) to drying oils that cure through oxidation of unsaturation in the backbone, to epoxy resins cross-linked with amino resins (amido amines, amines, etc.). 

---