Elsevier

Thermochimica Acta

Volume 371, Issues 1–2, 26 April 2001, Pages 45-51
Thermochimica Acta

Effect of modified rubber compound on the cure kinetics of DGEBA/MDA system by Kissinger and isoconversional methods

https://doi.org/10.1016/S0040-6031(00)00771-1Get rights and content

Abstract

The cure kinetics of diglycidyl ether of bisphenol A (DGEBA)/4,4′-methylene dianiline (MDA) system with various contents of MDA-endcapped carboxyl-terminated butadiene acrylonitrile (CTBN) were studied by Kissinger and isoconversional methods. With increasing MDA-endcapped CTBN content, the exothermic heat decreased due to the diffusion control induced by rubber domain produced in the epoxy matrix, which disturbed the diffusion of functional groups, and the maximum exothermic peak value and the activation energy by Kissinger equation decreased due to the increasing content of amine group in rubber compound, which reacted with epoxy group and formed a hydroxyl group acted as a catalyst. In the isoconversional method, the activation energy decreased until minimum value in the initial stage and increased after that value. The decreasing in the initial stage was due to the autocatalytic cure reaction and the increasing after the minimum value was due to the increasing crosslink density and rubber domain.

Introduction

The applications of the cured epoxy resins have been expanded in various fields such as coatings, adhesives, electrical insulators, matrices for fibrous composites, etc. However, the cured epoxy resins having high crosslink density are so brittle that they are easily broken by an instant impact. Therefore, many researchers have studied to enhance the toughness of the cured epoxy resins and one of the most well-known methods is to incorporate various amounts of reactive liquid rubbers [1], [2] and in this study, carboxyl-terminated butadiene acrylonitrile (CTBN) rubber was introduced to the epoxy system of diglycidyl ether of bisphenol A (DGEBA)/4,4′-methylene dianiline (MDA). When epoxy resin together with liquid rubber is cured, the rubber-rich domains are separated from the epoxy-rich matrix due to the incompatibility between epoxy resin and liquid rubber and the rubber domain improve the toughness by absorbing the impact energy [2].

Many useful methods and techniques are proposed to estimate the cure rate of epoxy system by differential scanning calorimetry (DSC) with the assumption that the heat evolved during the cure reaction is proportional to the monomer conversion [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Kissinger equation [3], [4], isoconversional equation [5], [6], autocatalytic cure rate equation [7], [8], fractional-life equation [9], [10], etc. are well-known, and in this study the following Kissinger (Eq. (1)) and isoconversional (Eq. (2)) equations were used. Kissinger equation isEa=−Rd(ln(q/TP2))d(TP−1)where q is the heating rate, TP the maximum temperature of exothermic peak, Ea the activation energy, and R the gas constant. The relationship between −ln(q/TP2) and 1/TP was obtained from the dynamic DSC curves of different heating rates, and activation energy could be calculated from the slope.

While only an overall activation energy could be obtained from Kissinger equation, a more complete accession of activation energy throughout the entire conversion could be calculated by isoconversional equation and it isEa=−Rd(lnq)d(T−1)where Ea, q and R are the same terms of Kissinger equation and T is the temperature to be a selected conversion, α at each heating rate. From the slope, activation energies could be obtained and the average activation energy from isoconversional equation was compared with the overall activation energy from Kissinger equation.

Section snippets

Experimental

Epoxy formulation was the complex mixture of DGEBA (Epon 828, Shell), MDA (Fluka Chemie AG) and CTBN (Hycar 1300×8, B.F. Goodrich). To enhance the compatibility between epoxy and CTBN, MDA-endcapped CTBN was synthesized by the reaction between MDA and CTBN at 160°C for 1 h [13].

DGEBA and MDA-endcapped CTBN (0, 10 and 20 phr) were vigorously mixed at 100°C for 5 min, and cooled to 70°C. Then, MDA (30 phr) was added and well-mixed at 70°C for 2 min, and the samples were stored at −5°C. The unit “phr”

Results and discussion

Fig. 1 shows DSC curves for DGEBA/MDA system without MDA-endcapped CTBN at four different heating rates. The cure reaction took place in one stage regardless of heating rate and the total exothermic heat was 117.0±1.8kJ/mol of epoxide group. The base line was taken as the tangent to the DSC curve at the locations proceeding and following the exotherm. The exothermic curve started from about 50°C for all curves, and the temperature of the maximum peak value increased with increasing heating rate

Conclusions

With increasing MDA-endcapped CTBN content, the exothermic heat decreased due to the diffusion control induced by rubber domain produced in the epoxy matrix, which disturbed the diffusion of functional groups, and the maximum exothermic peak value and the activation energy by Kissinger equation decreased due to the increasing content of amine group in rubber compound, which reacted with epoxy group and formed a hydroxyl group acted as a catalyst. In the isoconversional method, the activation

Acknowledgements

This work was supported by Han Yang Petrochemical Co., Ltd. in Korea.

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