Reaction parameters, epoxy curing

To manufacture reproducible C-fiber-TGDDM-DDS epoxy composites with well-defined lifetimes in service environment requires a knowledge of the parameters that affect composite processing conditions and the resultant structure of the epoxy within the composite. The cure reactions directly control the composite processing and the final epoxy network structure and mechanical response. Hence, it is important to understand the cure reactions and the variables that affect such reactions. 

Figure 6.5 Storage G and loss G" moduli as a function of frequency, at different cure times, for the same epoxy-diamine system as that represented in Figs 6.3 and 6.4. T, = 90°C. The parameter is the reaction distance from... 

An identical mathematical description of the kinetics of curing of reactants different in chemical nature and that obtained on the basis of fundamentally different experimental methods allows us to assume that this apparent selfacceleration course of some rheokinetic parameters is common to the processes of formation of materials with a crosslinked structure. It should be emphasized once more that the self-acceleration" effect must not be identified with the self-catalysis of the reaction of interaction between epoxy monomers and diamines which is studied in detail on model compounds [116, 117]. For each particular curing process the self-acceleration effect is influenced by the mechanism of network formatic, namely, chemical self catalysis [118], the appearance of local inhomogeneities [120], the manifestation of gel eff t [78], parallel course of catalytic and noncatalytic reactions [68]. It is probably true that the phenomena listed above may in one form or another show up in specific processes and make their contribution into self-acceleration of a curing reaction. 

Thus, the stated above results have demonstrated that both scaling Eq. (86) of Chapter 1 and fractal Eq. (27) of Chapter 1 (or Eq. (61) of Chapter 2) describe well to an equal extent haloid-containing epoxy polymer 2DPP+HCE/DDM curing reaction kinetics at different curing temperatures. In virtue of this circumstance there exists intercoimection between parameters included into the indicated equations. The fractal Eq. (61) of Chapter 2 introduces in the kinetics problem consideration reaction products structure (in the given case structure of microgels and condensed state after gel formation point), characterized by its fractal dimension D, that makes this conception physically more informative [34]. 

Actually, the model of a consecutive reaction corresponds better to the epoxy curing, namely, A —> B and B C with B an intermediate compound. With this model, the epoxy curing can be satisfactorily described. The fit is perfect for all three heating rates (Figure 9.18). The kinetic parameters determined this way read (private communication) as follows ... 

Lee, J.H., Lee, J.W., 1994. Kinetic parameters estimation for cure reaction of epoxy based vinyl ester resin. Polymer Engineering Science 34, 742—749. 

Table 14.10 Summary of the kinetic parameters for the cure reaction of epoxy and the chemorheological model parameters...

In Figure 3.14 the dependences In (100 - a) on the parameter corresponding to Equation 3.9 at d = 3 are adduced for the same epoxy system. It was again not possible to obtain linear correlations and this means that despite the essential density fluctuations (see Figure 3.13) Equation 3.9 does not describe the curing reaction of system EPS-4/DDM. 

The first attempt to describe theoretically the processes of phase separation during the reaction of formation of semi-IPNs has been done in the works [296,297]. Semi-IPNs based on PS and a reactive epoxy monomer based on DGEBA with a stoichiometric amount of 4,4 -methylenebis(2,6-diethylaniline) were studied experimentally. Thermodynamic analysis of the phase separation proceeding during the curing reaction was performed that considered the composition dependence of the interaction parameter x(T, 2) (where T is the temperature and 2 is the voliune fraction of PS) and the polydispersity of both polymers. The latter is especially important, hi this analysis, x(T,)]. For the initial mixture (before the reaction) the cloud point curves showed upper critical solution temperature behavior and the dependence x(r, >2) on the composition was determined from the threshold point. 

Han et al. [191] found that the rate of cure of a resin is greatly influenced by the presence of fibers and the type of fibers employed. The rate of reaction for resin-fiber system can be 60 percent different from that of neat resin, after a 10-min cure. A similar conclusion was presented by Mijovic and Wang [192] for graphite-epoxy composites based on TGDDM/DDS (33phr). They verified large differences (see Table 2.5) in the kinetic parameters when considering an autocatalytic model. 

Because of all these minor components (e.g., catalysts and inhibitors, added to major ones) the cure of vinyl ester resins is very complex, involving many competitive reactions. There are some new variables to account for, such as the inhibitor and initiator concentrations and induction time. Several papers [81,96,200,201] use the mechanistic approach, claiming that the phenomenological models do not explicitly include these facts, resulting in a new parameter characterization after each change in resin formulation [96]. Despite these arguments, the phenomenological approach is the most widely used and is based on an autocatalytic model which has been successfully applied to epoxy resins. Many authors [30,34,74,199,202,203] proposed the Equation 2.30 to describe the cure kinetic of unsaturated polyesters ... 

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