Curing with amines

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. 

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

Different behaviors and mechanisms were clearly recognized between these resins. Epoxy resin cured with amine showed no degradation during immersion because of its stable crosslinks. Epoxy resin cured with anhydride showed the uniform corrosion with the softening and dissolution of the surface and also behaved similar to the oxidation corrosion of the metal at high temperature obeying linear law. 

Schlack [2] and Castan [3,4] are credited with the earliest U.S. patents describing epoxy resin technology. Greenlee [5] further emphasized the use of bisphenols and their reaction with epichlorohydrin to yield diepoxides capable of reaction with crude tall oil resin acids to yield resins useful for coatings. The use of diepoxide resins that are cured with amines was reported by Whittier and Lawn [6] in a U.S. patent in 1956. 

It is evident that relaxation studies in the solid state can look at the motions which are responsible for the mechanical properties of the cured epoxy systems 43). Therefore, Garroway, Moniz and Resing continued to do relaxation studies 61). Garroway, et al. looked at four epoxy polymers based on the DGEBA resin. Two of the epoxy resins were cured with amines and the other two were cured with anhydrides. Proton enhanced spectra of the epoxy systems were generated. The solid state spectra were compared to the solution spectra of the unreacted epoxy. The< epoxy resin of interest was again DGEBA which was reacted with ... 

The epoxy-oligomer curing with amines has an autocatalytic character due to the accumulation of hydroxyl groups during the reaction [cf. Scheme (1)]. 

Thermodynamics of the epoxy compound curing with amines is interesting from two points of view. It concerns all the numerous elementary reactions resulting in the formation of H-complexes in the reaction system and the process as a whole, as well. 

In practice the epoxide-amine cure is often accelerated by the addition of catalysts such as boron trifluoride complexes, and the boron trifluoride-ethylamine adduct (BFE) is widely used for this purpose. In addition to catalysing the epoxide-amine reactions, BFE can initiate homopolymerisation of epoxide. The accelerating effect of BFE is illustrated by DSC scans for the TGDDM/DDS/BFE system in Figure 12. The multiple-peaked exotherm associated with the BFE-catalysed TGDDM/DDS cure indicates that the kinetics of this system are more complex than those for the cure with amine alone. For this system the overall heat of reaction was found to decrease with increasing BFE concentration 89). For DDS alone Q0 was about 110 kJ per mole epoxide while the value for BFE alone was 75 kJ/mole, and the DDS/BFE values were between these limits. It appears that the proportion of epoxide homo-polymerisation relative to amine or hydroxyl addition increases with increasing BFE concentration. 

Blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines.18,19 These blocked prepolymers will initially react with the amines to form amine-terminated prepolymers that crosslink the epoxy resin. Urethane amines are also offered commercially for use with epoxy resins to develop hybrid adhesive systems.20... 

The dynamic mechanical properties of the cured resin are very similar to those of resins cured with amines or acid anhydrides (Fig. II)44. ... 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

The system can be formulated with fillers, pigments and auxiliaries. They cure with aminic hardeners at room temperature according to the two-pack process. Specific construction applications with the above development are as follows ... 

Epoxy resins cured with amines and other nucleophiles... 

It is well known that polymers with condensed imide rings exhibit high thermal stability. Properties of the imide ring containing ACEC LXII (Scheme 78), cured with amines and with an acid anhydride, are given in Table 20. Acid anhydride cured epoxyimides reach the heat resistance of 190 °C, temperature of the start of decomposition above 265 °C and flexural strength above 70 MPa. 

Epoxy paints (see also Section 2.10) are usually applied as two-pack systems. Curing with amines is preferred in high-solids systems. Epoxy resins may also be used as coreactants or reactive diluents in low-solvent paints based on acrylic, polyester, or alkyd resins. 

Epoxy primers cured with amine adducts or polyamides, pigmented with zinc, zinc chromate and potassium chromate, strontium chromate, or, more recently zinc phosphate and calcium phosphate... 

Epoxy primers cured with amine adducts, containing inert pigments (usually iron oxide)... 

More recent efforts have focused on developments that create true hybrids. For example, blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines [68-70]. These blocked prepolymers will react initially with the amines to form amine-terminated prepolymers that cross-link the epoxy resin. Several blocked isocyanates are commercially available. The DESMOCAP (Bayer) llA and 12A products are isocyanates (believed to be blocked with nonylphenol) used as flexibilizing agents for epoxy resins. ANCAREZ (trademark. Pacific Anchor, Inc.) 2150 is a blocked isocyanate epoxy blend used as an adhesion promoter for vinyl plastisols. A one-package, heat-cured hybrid adhesive was reported consisting of isophorone diisocyanate, epoxy resin, and a dispersed solid curative based on the salt of ethylenediamine and bisphenol A [71]. Urethane amines are offered commercially that can be used with epoxy resins to develop hybrid adhesive systems [72]. 

The copolymerization of acrylic esters with 5%-15% acrylonitrile or 2-chloroethyl vinyl ether produces elastomers which are more heat and oxidation resistant than butadiene/acrylonitrile copolymers because of the absence of double bonds. For the same reasons, these materials are also suitable for the manufacture of gaskets and membranes for use with high-sulfur-content industrial oils (for example, crankshaft gaskets in the automobile industry). Since the side groups have poor resistance to hydrolysis, steam curing is not possible. Curing with amines can be subsequently carried out. 

The mechanism of epoxy-prepolymer curing with amine-boron trifluoride complexes is controversially discussed in the literature. It k often postulated that the BFj-amine complex reacts at its dissociation temperature [83,86], but this dissociation temperature has not been determined experimentally. Arnold [87] postulated that a proton k released in the dissociation of the amine complex,... 

Two basic forms are used for composites. A di-functional form is used for general purpose applications, which may be either cured with an anhydride or an amine. The use of aliphatic amines as accelerators/ catalysts allows cure to occur at low temperatures. The tetra-functional form is generally used for aerospace applications often requiring high temperatures, and is generally cured with amines. 

Phenoplasts and epoxy resins cured with amines... 

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