Epoxy system, isothermal kinetic

Figure 1. Isothermal Kinetic Behavior - Accelerated Epoxy System,...

In fact, nonisothermal temperature cure is a different process from isothermal cure. The reaction kinetics, total reaction order, and even the reaction energy for epoxy systems may not be constant and same, but process dependent. Therefore, modifications need to be made to reflect the effects of such a difference. 

As can be seen, the enthalpies of different apoxy-amine systems, according to different authors, lie in a rather narrow range (100-118 kJ per mole of epoxy groups, i.e. close to the heat of the epoxy ring opening). These data confirm the above conclusion as to the small total contribution of the donor-acceptor interactions in the epoxyamine systems to the observed integrated value of the heat release and the possibility of the application of the isothermal calorimetry method to the reaction kinetic studies. 

Other authors observed the same inconsistent results for other epoxy-anhydride-tertiary amine systems. For example, Peyser and Bascom (1977) observed first-order kinetics under isothermal and dynamic conditions however, the activation energy for dynamic runs was E = 104.2 kJ mol-1, much larger than the value for isothermal runs, E = 58.6 kJ mol-1. 

The experiments described above for the cure studies of epoxy resins (a typical three-dimensional network) were restricted to isothermal experiments (or quasi-isothermal experiments in the case of MDSC). As discussed later, these are the most reliable methods for generating kinetic information, but suffer from the length of time taken to generate data and, furthermore, in some systems the heat flow may be vanishingly small. 

The kinetics of curing of a poly(phenylene ether)/epoxy resin system has been investigated by an advanced iso-conversional method. Curing experiments with different PPE/EP ratios were carried out using non-isothermal differential scanning calorimetry. It was shown that the curing mechanism of this system is very complicated [21],... 

The ejq)erimental MTDSC observations on anhydride-cured and amine-cured epoxies, described in the previous section, will now be modelled to illustrate the benefits of the technique to obtain a quantitative law of cure kinetics for such thermosetting systems. Because cure kinetics are often complicated by diffusion limitations and/or mobility restrictions, the effect of diffusion has to be incorporated into the overall reaction rate law. For this purpose, both heat capacity and non-reversing heat flow signals for quasi-isothermal and non-isothermal cure experiments are used. 

Isothermal Method 1. This method capitalizes on the ability of DSC to simultaneously monitor both the conversion and the rate of conversion over the entire course of the cure reaction. This allows direct use of derivative forms of the rate equation, such as Eq. (2.86), which are necessary for kinetic analysis of autocatalytic reactions such as epoxy-amine. Experimentally this method is well suited to autocatalytic reactions that do not reach maximum rate until later in the reaction after the instrument has achieved thermal equilibrium. Even so, at high temperatures a significant portion of the reaction can take place before the calorimeter equilibrates and go unrecorded. Widmann (1975) and Barton (1983) have proposed a means to correct for such unrecorded heat by rerunning the experiment on the reacted sample, under the same conditions, to obtain an estimate of the true baseline and the unrecorded heat that should be added to the measured heat, as illustrated in Fig. 2.68. Note that this system appears to follow nth-order kinetics where the maximum reaction rate occurs at f = 0. For the sample shown, Widmann reports that 5% of goes... 

The chemistry for a stoichiometrically balanced reaction suggests that m = 1 and n = 2 in Eq. (2.86). For real systems, values are often close to these values but not identical. In the epoxy-amine reaction the alcohol, which may be present initially in small concentrations but is also a product of the reaction, catalyzes further reaction, resulting in autocatalysis. Since there are four unknowns ki, k2, m, and n) nonlinear regression analysis must be employed, although ki can be evaluated independently as the extrapolated reaction rate at a = 0. Autocatalytic kinetics are usually evaluated by the derivative form of the autocatalytic rate equation [Eq. (2.86)] with data coUected by isothermal method 1 measurements. Activation energy E and preexponential factor A are measured from the Arrhenius equation... 

Dynamic mechanical analysis is quite useful to observe the result of chemical reactions of polymer chains (e.g., transesterification) as evidenced by Figs. 3.12 and 3.13 [26]. The DMA method can be applied isothermally to determine crystallization kinetics (modulus versus time measurements) [13, 27] and reaction rate of thermosetting materials (e.g., epoxy) [28]. For reaction rate determination of liquid systems, the torsional braid analyzer is most appropriate as the braid can be saturated with the prepolymer liquid. A cellulose blotter could be used for the torsion pendulum, and a section of nylon hosiery could be used for forced vibration studies (both supports saturated with liquid prepolymer). 

Bmardic, M. Ivankovic, H. Ivankovic, and H. J. Mercer. Isothermal and nonisothermal cure kinetics of an epoxy/poly(oxypropylene)diamine/octadecyl-ammonium modified montmorillonite system. Journal of Applied Polymer Science, 100 (2006), 1765 1771. 

An investigation using differential scanning calorimetry was carried out under both isothermal and dynamic curing conditions to determine the cure kinetics of four epoxy/ amine resins. Various cure kinetic models were used to compare them with the results of the DSC results. Good fits were found, in good agreement with the experimental results for the resin systems. 22 refs. 

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