Epoxy resins polyfunctional amines

The majority of the currently used coating systems is based on copolymers prepared from epoxy resins, polyfunctional amines, and partially blocked isocyanates (84-87). The resins are generally solubilized by the formation of cationic derivatives of amines by aminolysls of the carbamate in the partially blocked isocyanate. The principal applications of these cationic electrocoating systems are as automotive primers, in appliances, as primers and one-coat system.s, and in various other industrial applications. The main... 

For some years, this type of vehicle was used in anionic electro deposition primers. However for automobiles cationic electro deposition primers are now preferred. The vehicles for cationic primers are proprietary but probably consist of the reaction product of epoxy resins and polyfunctional amines solubilized by the conversion of amine groups into salts using an acid such as lactic acid (2-hydroxypropanoic acid) [50-21-5] Alcohol-blocked... 

Polyfunctional aliphatic resins have exhibited high reactivity and degrees of cure with amines but problems of toxicity have diminished their usehilness and commercial interest. SoHd epoxy resins can be prepared by the taffy process or the advancement process. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

Table 14.4 presents formulation information for bisphenol A and polyfunctional epoxy resin emulsions that are cured with an aliphatic amidoamine curing agent. Adhesive performance data are also provided for substrates common to the automotive industry. Both formulas are based on a 1 1 epoxy-amine stoichiometry and they are reduced to 45 percent nonvolatiles with water. The working life of each system is several hours at room temperature. 

Finally, the Mannich reaction may be applied to the. synthesis of reactive amines used as crosslinking agents of epoxy resins. The process requires the availability of molecules possessing more than two NH groups, which are obtained by reaction of polyfunctional substrates with polyfunctional primary amines (oligomeric polyalkyle-ncamines, diamino cyclohexane, etc.) - (see also 422, Chap. Ill, C). 

Eor adhesive applications, it is more convenient to have the two components of the epoxy resin system react at a reasonable rate at room temperature. Replacing the anhydride component by a suitable, more reactive polyfunctional amine enables this to occur at room temperature. Several aliphatic and aromatic polyfunctional amines are used for this purpose (Eq. 21.28). 

In contrast to the phenolic, urea and furan resinous cements, the liquid epoxy resin in the epoxy cements is cured by a reaction with a polyfunctional amine, such as polyethyleneamine or an active polyamide (Versamid) which is dispersed in the carbon or silica filler as shown in the following equation. ... 

Higher molecular weight epoxy resin hardeners such as fatty polyamides do not have the problems associated with amines and amine adducts. These reaction products of dimer and trlmer acids with polyfunctional amines provide films with fewer surface discontinuities. Polyamide cured epoxy coatings are more forgiving than amine cured epoxy coatings since they require less demanding surface preparation. Also, mix ratios are less critical for poly-amide/epoxy coatings (56). 

To convert the epoxy resins into cross-linked structures, it is necessary to add a curing agent. Most of the curing agents in common use can be classified into three groups tertiary amines, polyfunctional amines, and acid anhydrides. 

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. 

Curing an epoxy resin with a polyfunctional amine... 

Paints, varnishes and lacquers based on epoxy resins are used in various industrial applications because of their strength and durability. Two-component epoxy paints that cure at room temperature need a hardener added before their use. One-component epoxy paints that are heat-cured contain a hardener which can be activated only by heating. Polyfunctional aliphatic amines, aromatic amines, solid polyamides and anhydrides can be used as curing agents. Epoxy-ester-resin paints are formed by reacting epoxy resin with... 

Despite the complexity of the reaction mechanism involved in the cure of epoxy resins by polyfunctional amines, the overall reaction rate can be expressed by means of a relatively simple kinetic equation. Under the assumption that the heat flow relative to the instrumental baseline is proportional to te reaction rate, the fractional conversion i s expressed by equation 1 ... 

Modifiers that may also be reactive as curing agents (such as polysulphides, triphenyl phosphite, and some polyamides) can be used. Polysulphide polymers used on their own will react slowly with expoxies. One to three parts of an active catalytic amine or amine salt can be used to accelerate cure. Triphenyl phosphite reduces viscosity and lowers cost. Ratios up to 2 5 phr appear to have no severe effect on physical properties at room temperature. Although reactive with epoxy, triphenyl phosphite is not an effective curing agent by itself it needs also a polyfunctional amine. About 75% of the normal stoichiometric amount of amine gives optimum results when 2 5 phr triphenyl phosphite is used with a resin such asDER331. 

Both aliphatic and aromatic compounds having at least three active hydrogen atoms present in primary and/or secondary amine groups are widely used as curing agents for epoxy resins. Examples of such polyfunctional amines are... 

Reaction between a bisphenol A-epichlorhydrin epoxy resin and a polyfunctional amine thus results in a cross-linked polymeric structure. The process is illustrated by the reaction with triethylenetetramine (which has six reactive hydrogen atoms) ... 

Polyfunctional amines which are commonly used as curing agents for epoxy resins but which are of a rather different kind to those described above are the so-called fatty polyamides. These polymers, which are described in Section 9.3, are of low molecular weight (2000—5000) and are prepared by treating dimerized and trimerized fatty acids with ethylenediamine or diethylene triamine. Fatty polyamides are used to cure epoxy resins where a more flexible product is required, particularly in adhesive and coating applications. 

Crosslinking can then be accomplished by using a polyfunctional acid or epoxy resin or through the use of curatives such as aromatic and aliphatic amines, dicyandiamide, or Lewis acids. Curing can take place at ambient temperature to 177°C but the most highly toughened adhesives are generally cured at 120°C. 

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