에폭시 몰딩 컴파운드의 열반응 특성

여만경, 이철호, 김상욱*

화인폴리머(주), *서울시립대학교 화학공학과

Thermal Reaction Characterization of

an Epoxy Molding Compound

M.G. Lu, C.H. Lee and S.W. Kim*

Fine Polymer Co. Ltd., ．Dept. of Chem. Eng., The Univ. of Seoul

Introduction

Recently, epoxy molding compounds have been widely used in the electronic industry and encapsulation of integrated circuits by means of transfer molding. Only limited information, however, may be found in the literature concerning the cure and degradation characterization of these molding compounds. Therefore, kinetic characterization of thermoset systems is necessary for a better understanding of structure-property relationships.

In this paper, an iso-conversional analytical model was proposed, where f(□) has the same form regardless of the temperature for the same degree of conversion □. The kinetic parameters of cure and degradation of epoxy molding compounds, and the dependence of activation energy on conversion were described on the basis of DSC and TGA measurements.

Experimental

The resin used in this study is a commercial mineral filled DGEBA-based epoxy compound (MG6-0330, The Dexter Corporation, USA), which contains acid anhydride hardeners, about 72% of fused silica and 4% of carbon black, and other additives, according to the supplier. The samples for analysis of thermal degradation were processed by transfer molding at 135-177℃ and post-cured at 150℃ for 2 hours.

Dynamic curing were performed in a Solomat DSC 4000 at different heating rates from 3 to 20℃/min with 10-15 mg of samples. A Instrumental Specialists Incorporated TGA-1000 Thermogravimetric Analyzer is used to obtain weight loss data for cured samples. Constant heating rates of 5, 10, 20 and 50 ℃/min are used. The sample weights are in the range of 20-50 mg in all cases and purged with nitrogen gas at a flow rate of 50 ml/min.

Results and Discussion

The plots of -ln (□i/T□,i2) vs T□,i-1 are given in Fig. 1, resulting in the very good linear relations for the whole curing process. These results confirm that the basic idea of iso-conversional analysis is appropriate to describe the dynamic cure for the observed system. The values of activation energy, Ea, for curing reaction, at different conversions can be calculated from the slopes of the straight lines and have been listed in Table 1. The dependence of Ea on □ for the epoxy curing was given in Fig. 4. The shape is somewhat close to the result discovered in our previous work for a isothermal cure process. It is clear that the values of activation energy are not a constant, as supposed in many works throughout the whole cure process. At the early stage of cure reaction, the activation energy takes a maximum value, then decreases, and finally remains practically constant.

Fig.  Iso-conversional plots for EMC         at different conversions.

a: 0.1, b: 0.2, c: 0.3, d: 0.4, e: 0.5,

f: 0.6, g: 0.7, h: 0.8, I: 0.9.

Fig.   Activation energy of curing as a function of conversion.

The value of activation energy of 125 kJ/mol at □→0 (Fig. 2) should be ascribed to the anhydride-epoxide reaction initiated by HA impurities. The value is markedly higher than the results reported by some researchers during the early stage curings (63 kJ/mol, ＠12% conv. by Peyser, 95 kJ/mol, 2-22% conv. by Mohler). However, in all the above cases, the curing process was considered as a single step so that the activation energies obtained were supposed to be constant throughout the entire reaction interval. It does not correspond to the autocatalytic mechanism of the process. As mentioned above, the hydroxyl groups formed during the cure facilitate ring opening. So it is reasonable to expect a decrease (from ∼125 kJ/mol at □→0 to about  80 kJ/mol) in the activation energy of cure reaction as compared to the very beginning of the process.

The pre-exponential factor can be calculated using the method given in ASTM E 698. It assumes the extent of the reaction at the exotherm peak, □p, is constant and independent of heating rate. Based on the obtained linear relationship between the reciprocal of exotherm peak temperature (Tp) and the logarithm of the heating rate (ln□/Tp2), the activation energy can be obtained from the slope of the straight line, and pre-exponential factor be calculated.

The data from ASTM E698 are given in Table 1. As can be seen, the activation energy obtained by this method seems to be little lower than that by the iso-conversional analysis. It is not surprising since the conversions at exotherm peak are not a constant as supposed, and ASTM E698 method treats the complex cure process as a single-step reaction of the first order. Compared to the results of iso-conversional analysis, the activation energy evaluated by this method is a single point to the dependence of E on □ at □=□p.

Table 2. Calculation of Arrhenius parameters by ASTM E698-79 method.

□, Kmin-1	Tp, K	□p	10-8A, min-1	Ea,p, kJmol-1
3	397.5	0.438	5.07	82.2
5	403.8	0.427	5.83
7	410.7	0.488	5.49
10	416.7	0.485	5.56

A number of experimental indices are often used to characterized the process of thermal decomposition, including the onset temperature of decomposition (Tonset), the temperature of the maximum rate of decomposition (Td,max), and the activation energy of decomposition (Ed). Those values are given in Table 2.

Table 2. The characteristic temperatures of materials by TGA analysis.

Heating rate, K/min	Tonset, ℃	Tmax, ℃	Residual, %
5	312.3	402.5	79.04
10	339.3	412.7	78.39
20	325.3	431.6	78.27
50	412.6	471.7	78.29

Fig.   Activation energy of thermal degradation

as a function of conversion.

The iso-conversional analysis also can been used to characterize the degradation kinetics of thermosets as mentioned above. The decomposition activation energies at different conversions obtained are plotted in Fig 3. They are in the range from about 120 to 135 kJ/mol. It is worthy to note herein that the activation energy values of cured compounds are not a constant, as supposed in many works throughout the whole degradation process. Unlike the case in curing, it increases with increasing conversion. Detailed mechanisms are still not clear. Perhaps it is related to the fact that the samples do not reach the completed curing under isothermal conditions, and aromatization occurs during degradation.

Conclusions

The cure and thermal degradation behavior of epoxy molding compounds can be characterized using an iso-conversional model that does not require the knowledge of reaction rate equation f(□). The kinetic results show that the activation energy values for both curing and degradation processes are dependent on the degree of conversion and the cure process is auto-catalytic. The cured materials exhibit very good thermal stability and the activation energy value of decomposition increases with increasing conversion.

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