Study on Curing Kinetics of Heat-resistant Flexible Polyamide Modified Epoxy Resin Adhesive

In order to study the effects of numerous variables affecting the reaction rate of heat-resistant flexible modified epoxy resin adhesive, the curing kinetics of polyamide modified epoxy resin was studied. The heatresistant flexible modified epoxy resin adhesive cured at room-temperature was prepared with epoxy resin, polysulfide rubber and organosilicone as adhesive component, polyamide as main curing agent and addition of different modified filler and the curing agent containing benzene ring structure. The curing kinetics of polyamide modified epoxy resin was studied by Differential Scanning Calorimetry (DSC) at different heating speeds and the characteristic temperatures of the curing process were analyzed and confirmed. the kinetics parameters of activation energy was calculated using Flynn-Wall-Ozawa equation and Kissinger equation, respectively, then the kinetic model of curing reaction was built as = 4.38×10 7 exp (-57740/RT) (1-α) 0.93 , the results show that the twoparameter model is adequate to represent the curing reaction process, the model can well describe the curing reaction process of the studied resin. The DSC curves obtained using the experimental data show a good agreement with that theoretically calculated. The research results will provide theoretical basis for the choice of manufacturing process and the optimization of processing window.


INTRODUCTION
Epoxy resins are widely used in diverse areas such as molding compounds, surface coating and painting materials, microelectronic encapsulated materials, printed circuit boards and adhesives owing to their satisfactory mechanical properties, including suitable weather, chemical and thermal resistance (Horie et al., 1970;Acitelli et al., 1971).The glass transition temperature, weigh loss temperature and char yield of cured system, the curing rates and activation energy are important kinetic parameters in the curing reaction process.Therefore, study on the curing kinetics of modified epoxy resin have both practical and theoretical value (Horie et al., 1970;Acitelli et al., 1971;Zhao et al., 2006;Niu et al., 2008).
In this study, the curing kinetics of polyamide modified epoxy resin was studied by Differential Scanning Calorimetry (DSC) at different heating speeds and the characteristic temperatures of the curing process were analyzed when the heating speed is zero.The kinetics parameters of activation energy was calculated using Flynn-Wall-Ozawa equation and Kissinger equation respectively, then the kinetic model of curing reaction was built (Xia et al., 2010;Wang et al., 2011).The research results will provide theoretical basis for the choice of manufacturing process and the optimization of processing window.

METHODOLOGY
Reagents and instruments: Epoxy Resin E51, E20 (Hangzhou Wuhuigang Adhesive Co., Ltd., China), polysulfide rubber 121 (Tianjin Kailis Chemical Co., Ltd., China), organosilicone Ltd.,China), Ltd.,China) Preparation of the modified epoxy resin: Take out of the adhesive component epoxy resin E51 (100% Wt), E20 (10% Wt), a certain amount of organotin, DBP, KH560 and silicone resin (20% Wt), then mixed them together and stirred for 2 h at 140-160°C.And then the mixtures were cooled down to room temperature.Subsequently and polysulfide rubber was added in and then heated to 120°C for 1 h.After that, modified adhesive component is prepared.

Preparation of curing agent:
The curing component were prepared by adding m-phenylenediamine, dicyandiamide, DMP30, KH560 etc., to polyamide and stirred for 30 min at room temperature and maintained at this temperature until used and then curing component is obtained.
Experimental method: Take out of the adhesive component (100% Wt), the curing component (60% Wt) and mixed them together, then dried them in vacuum at 65°C for 30 min to remove little water.Take out 5 mg samples accurately in a crucible and the kinetics was studied by non-isothermal DSC analysis with the DTA-TG thermal analyzer at the different heating rates of 5, 10, 15 and 20°C/min, respectively.The input rate of nitrogen with high purity is 20 mL/min.

Analysis of the kinetic DSC curves:
The DSC curves of polyamide as curing agents at different heating rates was shown in Fig. 1.We can see from Fig. 1 and that the initiative curing temperature T i , the peak top temperature T p and the finish curing reaction temperature T f increase with the increase of the heating rate.The main reason is that the samples have enough time for the cured reaction at a relative low temperature when the heating rate is small.If the heating rate raises instead, the heat production per unit time increases, the greater temperature difference will yield in a short time and the exothermic peak of reaction shift forward to high temperature range.

Characteristic temperature of the curing process:
The characteristic temperatures of the curing process can be found from Fig. 1 and they are listed in Table 1.
The curve of heating rate β and the characteristic temperature of T i , T p and T f was drawn and shown in Fig. 2.
In the polyamide system, the linear regression equation which describe the relations of β with T i , T p and T f is y = 0.9334x + 47.625, y = 1.7488x + 86.19 and y = 1.7766x + 120.03 and their linear correlation coefficients are R i = 0.952, R p = 0.996 and R f = 0.953, respectively.When the heating rate β is zero, the curing characteristic temperatures can be obtained by extrapolation and they are T i0 = 57.58°C,T p0 = 94.43°C and T f0 = 126.96°C,respectively.
In the Eq. ( 1), "A" is frequency factor, "β" is heating rate of the cured reaction, R = 8. 314 J/mol•K, is ideal gas constant and E α , T and F (α) are activation energy, temperature and the integral form of kinetic mechanism function.The curve of l g β ~ 1/T p was drawn according to the Eq. ( 1) and shown in Fig. 3.
The equations of the curing systems are obtained through linear regression, they are y = -3.2604x+ 9.60282 and the correlation coefficients are R = 0.9987.According to its slope, we can get that E α = 59.35 kJ/mol.The activation energy of cured reaction can also be obtained by Kissinger equation.The Kissinger equation can be expressed as follows: The curve of ln (β/T 2 p ) ~ 1/T p was drawn according to the Eq. ( 2) and shown in Fig. 4.
We can see through linear regression that, the equations of the curing systems are y = -6.7498x+ 8.2369 and the correlation coefficients are R = 0.9984.According to its slope, we can get that E α = 56.12kJ/mol.
The activation energies obtained by the above two kinds of different equations are approximate to each other and the activation energy should be obtained by averaging the above calculating values, therefore the activation energy of each curing system are E α = 57.74kJ/mol.In Eq. (3), because of E α /nR>>2T p , the Eq.(3) can also be expressed as follows: The curve of ln β~1/T p was drawn according to the Eq. ( 4) and shown in Fig. 5.
We can see through linear regression that, the equation of the curing system is y = -7.5073x+ 22.111 and the correlation coefficient is R = 0.9987.According to Eq. ( 4), we can see that the reaction order "n" is 0.93, so it is the first order reaction, which is consistent with the single peak in DSC curve.
The pre-exponential factor in Flynn-Wall-Ozawa equation can be obtained by Kissinger equation: The different β and T p was substituted in Eq. ( 5), "A" is solved by averaging the calculating values, we can obtain that A = 4.38×10 7 , substituted these kinetic parameters in the kinetic equation, the kinetic equations of curing reaction of polyamide can be expressed as follows:

CONCLUSION
The curing kinetics of polyamide/epoxy resin system was studied by the method of non-isothermal DSC.When the heating rate is zero, the curing characteristic temperatures of the cured products are T i0 = 47.63°C,T p0 = 86.19°Cand T f0 = 120.03°C.The apparent activation energy of the curing reaction is that E α = 57.74kJ/mol, pre-exponential factor "A" = 4.38×10 7 and the reaction order is 0.93.Therefore, the kinetic equations of curing reaction can be expressed as follows:

Fig. 2 :
Fig. 2: The curve of characteristic temperature T and heating rate β of polyamide ■: T i ; •: T p ; ▲: T f

Table 1 :
The characteristic temperatures of polyamide at different Fig. 1: Kinetic DSC curves of polyamide