Epoxy-primary amine reaction

Recently, FTIR spectroscopy studies have been reported which support the above observations. Moacanin et al 3) concluded that two reactions dominate the TC3fDA/DDS cure epoxy-primary amine addition is the principal reaction occurring during the early stage of cure followed by the epoxy-hydroxyl addition reaction. Indeed they find that the rate of epoxy-hydroxyl addition is at least an order of magnitude slower than for the epoxy-primary amine reaction at 177 C. Furthermore, Morgan et al (4) report that the epoxysecondary amine addition and epoxy-epoxy homopolymerization reactions also occur at 177°C but at rates that are approximately 10 and 200 times slower, respectively, than the epoxy-primary amine react ion. 

Figure 3. Plot of Ca/(5a l) versus cure time to calculate rate constant (k ) for epoxy-primary amine reaction. (Reproduced from Ref. 4. Copyright 1986 American Chemical Society.)...

In the well-recognized epoxy-aliphatic amine reaction, the primary or secondary amine adds to the epoxy ring, forming a tertiary amine, as shown in Fig. 5.2 (top). The formed hydroxyl groups accelerate the amine curing, and with excess epoxy present, the secondary hydroxyl groups can also add to the epoxy ring, as shown in Fig. 5.2 (bottom). 

Pg. 4 - Para. 2. Is it true that the aromatic primary amines reaction with epoxy (the selective reaction in this instance) is catalyzed by acid Salicylic acid in conjunction with an alcoholic hydroxyl carrier (benzyl elcoholi nonyl phenol, furfuryl alcohol, etc.) may do it. But I doubt it if CTBN will. Perhaps I m wrong. But this needs explaining. 

The initial increase in the heat capacity signal corresponds to the reaction heat capacity or the change in heat capacity from reactants to products (see arrow in Rg. 2.112). A thermodynamic analysis of the epoxy-aromatic amine reaction revealed that the primary amine-epoxy reaction contributes less to the increase in reaction heat capacity than does the secondary epoxy-amine reaction (Swier and van Mele 2003b). Information specific to the different steps in the reaction mechanism can therefore be deduced from the heat capacity signal, in contrast to the global conversion evolution obtained from the total heat flow signal. 

By contrast with tertiary amines used in catalytic quantities, primary and secondary amines or acid anhydrides may be used to bring about the cure of epoxy resins by reaction in stoichiometric proportions. A typical amine curing agent used at this level is diaminodiphenylmethane (DDM), which reacts with an individual epoxy-group in the way shown in Reaction 4.17. 

For TGDDM/DDS (<35 wt% of DDS), Morgan and Mones [57] affirmed that all primary groups were consumed at 177°C after 2.5 hours of reaction. The epoxy-amine reactions dominate the early stages of cure and, hence, the composite processing conditions. 

Table S. Change in the enthalpies of the reactions of the epoxy compounds with primary amines...

TA can be effetive additives for controlling the reaction rate in the curing of epoxy compounds with primary amines. 

To attain the requirements of an epoxy matrix utilized in filament-wound C-fiber-epoxy composites we have considered the characteristics required of the amine curing agent molecule. To ensure long gel times at 23 °C requires that the primary amine-epoxide (P.A.-E) reaction rate is considerably greater than the rate of the secondary amine-epoxide (S. A.-E) reaction, and that the S.A. reaction does not occur at low temperatures. Furthermore, to attain low 23 °Cr s and low post-cure temperatures... 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

If, on the other hand, primary and secondary amine hydrogens react at different rates, it is necessary to use a kinetic scheme to obtain the evolution of the concentration of different fragments along the reaction. The epoxy -amine reaction may take place both by a noncatalytic path (specific rate constant k ) and by a reaction catalyzed by OH groups (specific rate constant k). The secondary amine hydrogen is usually less reactive than the primary amine hydrogen. Specific rate constants for the secondary amine... 

Equation (5.1) becomes a rigorous equation only in the case of stepwise polymerizations with equal initial reactivities, absence of substitution effects, and following a single reaction path. It may be also used when the reactivity ratio does not vary with temperature. This is, fortunately, the case of epoxy-amine reactions where the reactivity ratio of the secondary to the primary amine is approximately constant in a broad temperature range. But, even in this case, the parallel polyetherification of epoxy groups must be negligible to keep a single reaction path. 

The best way to elucidate the reaction path is to follow the evolution of as many independent species and functional groups as possible. For example, analysis of the epoxy-amine reaction following the simultaneous evolution of epoxy and primary amine groups by near infrared spectroscopy (NIR) simultaneous determination of the conversion of double bonds belonging to unsaturated polyester (UP) and styrene (S) using FTIR, as shown in Fig. 5.13 (Yang and Lee, 1988) determination of the evolution of the concentration of free radicals using ESR, as shown in Fig. 5.14 (Tollens and Lee, 1993). 

The primary and secondary amines are discussed in this section. The secondary amines are derived from the reaction product of primary amines and epoxies. They have rates of reactivity and crosslinking characteristics that are different from those of primary amines. The secondary amines are generally more reactive toward the epoxy group than are the primary amines, because they are stronger bases. They do not always react first, however, due to steric hindrance. If they do react, they form tertiary amines. Tertiary amines are primarily used as catalysts for homopolymerization of epoxy resins and as accelerators with other curing agents. 

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