Epoxy-Amine System

Curing of Epoxy-amine Systems at Nonisothermal Conditions. 139... 

Low-molecular-weight model compounds such as phenylglycidyl or other mono-glycidyl ethers as well as primary, secondary and tertiary amines have been used for the study of the kinetics, thermodynamics and mechanism of curing. To reveal the kinetic features of network formation, results of studies of the real epoxy-amine systems have also been considered. Another problem under discussion is the effect of the kinetic peculiarities of formation of the epoxy-amine polymers on their structure and properties. 

These results indicate that studies of the donor-acceptor interactions on the model systems are quite justified. This study is the only possible approach to quantitative characterization of all the numerous complexes appearing in the epoxy-amine system. Today, we are making but initial efforts in the thermodynamic studies of the epoxyamine systems. For the time being, we have only managed to estimate the effective thermodynamic characteristics in such systems. The development of an algorithm for both the experimental and the theoretical approach to the study of similar systems still remains an important task. 

Table 5 presents reaction enthalpies measured by the calorimetric method for a number of epoxy-amine systems of stoichiometric composition 29,49 52). 

The above consideration indicates that at present, the researchers face large difficulties in the physical interpretation of the effective reaction rate constant associated with the absence of detailed quantitative information on the thermodynamics of formation of H-complexes and their reactivities. Further studies of the donor-acceptor interactions in the epoxy-amine systems will shed more light on this problem. 

The reaction of curing the epoxy-amine system occurring in the diffusion-controlled mode has little or no effect on the topological structure of the polymer 74> and on its properties in the rubbery state. However, the diffusion control has an effect on the properties of glassy polymers 76 78). 

The problem of the reaction kinetics and structure of the resulting polymer has many facets and at present it is far from being solved 2 6,15>6164 65 70>71 74 78 80>87-97)-It should be noted that the epoxy-amine systems turned out to be the most convenient for experimental and theoretical studies of the process of formation of the topological structure of networks. In many cases their topology in the rubbery state agrees with the theoretical predictions 61, M 80,87,88>. 

It is obvious from the two previous paragraphs that the above-mentioned processes cannot proceed independently. The kinetic features of the processes of curing of the epoxy-amine systems with the mixtures of primary and tertiary amines as well as the mechanism of the simultaneously occurring polymerization and polycondensation u 461 will be discussed next. 

Substitution effects produce only a slight shift in the gel conversion from 0.577 to 0.618 in the limit of R21 -> 0. Most of the reported experimental values of gel conversion in stoichiometric epoxy-amine systems are close to 0.60. Within the experimental error of the measurement of gel conversion, it is difficult to quantify the value of the reactivity ratio from this single determination. 

The limiting case of R21 —> oo may also be considered. Although this situation does not occur for an epoxy-amine system, it is of interest to determine the effect of an instantaneous activation of the second available... 

The solution of the precursors of the thermosetting polymer (mixture of monomers or oligomers with or without initiators, catalysts and different additives) is usually a liquid at room temperature e.g., unsaturated polyester styrene, some epoxy anhydride and epoxy amine formulations, cya-nate esters, one-stage phenolics, etc. Cooling any of these solutions below room temperature leads to a glass. The temperature at which the glass-liquid transition of the initial formulation takes place is denoted as Tg0. Some other particular formulations, such as two-stage phenolics (novo-lac-hexa mixtures), some epoxy-amine systems, etc., exhibit a Tg0 above room temperature. 

However, for several epoxy-amine systems, the simple kinetic model expressed by the set of Eqs (5.18) (5.21) does not provide a good fitting with experimental results. Reaction mechanisms, including the formation of different kinds of complexes, have been postulated to improve the kinetic description (Flammersheim, 1998). Also, a more general treatment of the kinetics of epoxy-amine reactions would have to include the possibility of the homopolymerization of epoxy groups in the reaction path. Sets of kinetic equations including this reaction have been reported (Riccardi and Williams, 1986 Chiao, 1990 Cole, 1991). 

For the particular epoxy-amine system of Fig. 5.7, experimental curves could be fitted with the following equation ... 

The use of model compounds is a convenient starting point to determine the reaction path, particularly for stepwise polymerizations. For epoxy-amine systems, a monofunctional epoxide such as phenyl glycidyl ether (PGE) is often used for these studies (Verchere et al., 1990 Mijovic and Wijaya, 1994). Figure 5.10 shows the reaction scheme for the curing of a monoepoxide with a diamine. 

Many dielectric studies of epoxy-amine systems have been reported. It appears that the change in dielectric properties during cure may be due to three main contributions ... 

Figure 6.9 represents the variations of the product s soco versus reaction time at several frequencies for two epoxy-amine systems. The s SoCO... 

However, for some other epoxy-amine systems, experimental monotonous decrease from epoxy-rich to amine-rich formulations (Yamini and Young, 1980 Won et al., 1990). This means that the proportionality constant between ay and (Tg — T) must vary along the series (P relaxations and physical aging may be invoked as some of the possible factors accounting for the observed trends). For these series, Bowden s theory, with the Burger vector increasing with the amount of hardener, provided a very good lit of experimental data. 

TABLE 3.7 Effect of Curing Agents on the Shrinkage of Highly Filled Epoxy-Amine Systems during Cure16... 

Boey, F. Y. C., et al., Microwave Curing of Epoxy Amine System—Effect of Curing Agent on Rate Enhancement, Polymer Testing, vol. 18, no. 2, 1999. 

Sheppard, Jr., N. F. Dielectric analysis of the cure of thermosetting epoxy/amine systems, Ph.D. Thesis, Massachusetts Institute of Technology, 1985, unpublished... 

Fig. 6a and b. Heat capacity ACP(TS) and cubic thermal expansion coefficient jumps AP(Tg) for some epoxy-amine systems.. Experimental results O DGER-mPhDA networks of different P, completely cured and annealed (T = 0.5°/min) DGEBA-mPhDA networks of different P, completely cured and annealed DGEBA-mPhDA unreacted mixture (P = 1) DGER-mPhDA unreacted mixture (P = 1) solid lines — the best fit... 

Another proof against inhomogeneous cure in simple epoxy-amine and other systems has been supplied by gel point measurements. The critical conversion at the gel point (cf. Sect. 4) is a sensitive function of any inhomogeneity. For epoxy-amine systems, the gel point conversion has been found to agree well with the prediction of the theory assuming uniform distribution of reactive groups throughout the volume The deviation does not exceed 1 %. In contrast, for free-radical... 

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