Epoxy-Aminosilane/MDA계의 경화반응 속도론

여만경, 김상욱

서울시립대학교 화학공학과

Cure Kinetics of Epoxy-Aminosilane/MDA System

M. G. Lu and S. W. Kim

Dept. of Chem. Eng., Seoul City Univ.

Introduction

Silane coupling agents have an important role at the interface for improving the performance of composite materials based on polymer matrices reinforced with glass fibers or mineral fillers. The silanes are also used in some adhesive formulations or as substrate primers, giving higher strength of adhesives joints.

The coupling agent studied here is the □-aminopropyltriethoxysilane (□-APS). It is one of the most commonly used coupling agents. During composite processing, it is frequently reacted with an epoxy prepolymer based on diglycidylether of bisphenol A (DGEBA). Despite its interest, few studies have examined the □-APS-DGEBA interaction, especially the effect on cure behavior of epoxy resin as a additive as it is usually used. In this paper, differential scanning calorimetry (DSC) has been used to investigate the cure kinetics of epoxy resin containing □-APS. The aim of this study is to try to provide some of the basic data.

Experimental

Epoxy resin (Epon 828, Shell Chemicals, epoxy equivalent=184, viscosity=11,000∼14,000 cps at 25℃) and □-APS (Fluka Chemie, AG) are mixed stoichiometrically with 4, 4-methylene dianiline (MDA, Fluka Chemie, m.p. 30∼32℃) under good stirring at 80℃. All samples are stored in a freezer at -20℃ and under desiccation prior to use. Samples consisting of 3-6 mg mixture are placed in aluminum DSC pans and run on a SEIKO I-5000 series. The kinetic data are obtained from the dynamic DSC curves at the different heating rate of 5, 10, 15 and 20 ℃/min.  Isothermal  curves  are  carried out  at  five temperatures between 80-120℃ in order to obtain both the cure rate and extent of cure as a function of time. The carrier gas is nitrogen at a flow rate of 50 ml/min.

Results and Discussion

For the DGEBA-□-APS system, there is a competition between epoxy-amine and epoxy-silanereactions. It can be defined by two stages. During the first stage, there is only epoxy-amine reactions. The second stage consists of alkoxy-silane reactions without any interference between the two stages. Therefore, the effects of □-APS on the cure process of epoxy resin  can be attributed to the interactions between epoxy group and amino-hydrogen in silane.

For the exploitation of cure kinetic, we used Kissinger expressions:

[eqn: -ln { PHI } over { { T}`_{p } ^{2 } } = { E_a} over {R } CDOT { 1} over {T_p } - ln( {AR } over {E_a } )]                                (1)

[eqn: {E_a } over { n}= { R { T}`_{p } ^{2 } } over {{(1- alpha }_{p } ) PHI } · {({ d alpha } over {dt}) }_{p } =~{R { T}`_{p } ^{ 2} } over {( { H}_{0 } - { H}_{p } ) PHI }·{({dH } over {dt }) }_{p }]              (2)

where □ is heating rate, Tp, Hp are the temperature, reaction heat where maximum conversion rate occurs on DSC curve, Ea is curing activation energy, n is a reaction order, H0 is the mole exothermic heat, A is pre-exponential factor, and R is gas constant.

Fig.1 shows the results of the temperature scans at the different heating rates.   The  temperature  at  peak  gradually  increases  when the  heating  rate is  from 5∼20 ℃/min. From these curves, we can determine the temperature at the peak, Tp, and Tp is plotted against -ln □/Tp2 (Fig.2). Linear representations were obtained with good correlation coefficients, which indicates that noncatalytic mechanism is valid.

Table 1 shows the values of A, Ea and the relations between k and T. From those data, we can find that the reaction rate constant 10 times increases when temperature is from 50℃ to 100℃, and also increases with increasing the content of □-APS because it has a similar reactivity to those other primary aliphatic amines in the reaction with epoxy group, the presence of alkoxylsilane groups and the possibility of intermolecular hydrogen bonds postulated by Plueddemann will not reduce thhis reactivity. But the activation energy has only little change with the increase of □-APS content. It is also the reason why the cure rate increases and the temperature at peak decreases with increasing of □-APS (see Fig. 3).

Table 1. Kinetic parameters of DGEBA-□-APS/MDA system with

the different □-APS contents

□-APS cont. (%)	Ea (KJ/mol)	A (S-1)	k(S-1) 50℃ 100℃
0	44.4	1.0	6.64×10-5 6.05×10-4
1	47.7	2.28	4.41×10-5 4.76×10-4
5	49.6	6.15	5.89×10-5 6.96×10-4
10	48.8	9.82	12.6×10-5 14.4×10-4

The results of isothermal experiment were shown in Fig. 4. The degree of cure obviously increases with raise of temperature. The sigmoidal shape of those curves also suggested the noncatalytic reaction mechanism. There is no serious autocatalysis even if upto 50% of degree of cure.  In fact, the catalytic mechanism involving epoxy-amino-hydrogen and hydroxyl is negligible at the higher temperature due to the difficulty of forming the ternary transition complex. So the noncatalytic mechanism involving epoxy and amino-hydrogen groups subsists alone.

Fig. 5 shown  the effect of □-APS content on the degree of cure. The cure reaction can be greatly enhanced when □-APS reaches 5% of content, especially in  the  late stages  of  polymerization. It  may be attributed the higher concentration  of active hydrogen in DGEBA-□-APS system which keeps the persistent high   reaction rate of polymerization, and is also possible that there is a catalytic effect of the tertiary amine. This effect is not taken into account in the mechanism used for the calculation. It is negligible in experiments where the amine concentration is in excess with respect to the epoxy concerntration as in the case where the extent of reaction is far from unity. It needs to be studied further for confirmation.

Conclusions

The following conclusions are suggested from the experimental results and analysis:

(1). The cure process of epoxy resin consisting of □-APS is mainly controlled by noncatalytic mechanism.

(2). □-APS can enhance the cure due to its high reactivity.

(3). The concentration of □-APS has little effect to the reactive energy, but results in the remark increase of reaction rate.

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