액정 에폭시 수지의 경화반응 특성

유한영, 이홍기*, 김상욱

서울시립대학교 화학공학과, *우석대학교 화학공학과

Characterization of Curing Reaction for LC Epoxy Resin

H. Y. Yu, H. K. Lee*, and S. W. Kim

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

*Dept. of Chem. Eng., Woosuk Univ.

1. Introduction

In the presence, liquid crystalline thermosets(LCTs) have gained extensive interest from many workers because of their marked advantage in the unique properties and the excellent performance of thermosets: thermal stability, mechanical strength, high heat resistance, low coefficient of thermal expansion and dimensional stability. Therefore, it can be applied in many industrial fields such as adhesive, matrix for advanced composite, and electronic packaging material[1-5].

There exist various LCTs reported earlier by many reserch groups. LC epoxy, LC cyanate ester, LC bismaleimide, and LC acrylate have been widely  investigated. Also, various aromatic types of liquid crystalline epoxy resins have been synthesized by endcapping mesogenic units with an epoxy group. It is well known that the curing reaction plays an important role in deciding the final crosslinking structure of the thermosets[6].

In this research, characterization of cure reaction and kinetic parameters for a cured LC epoxy system were investigated using the Kissinger equation as follows:

[eqn: -ln( {q } over { { T}`_{p } ^{2 } } ) = {{E}_{ a} } over {R } } □ { 1} over { { T}_{p } } - ln( { AR} over { { E}_{a } } )]

where, q is the heating rate, Tp the temperature at which maximum cure rate takes place on the differential scanning calorimeter(DSC) curve, Ea the activation energy of curing, R the gas constant and A the pre-exponential factor. In the graph for the relationship between -ln(q/Tp2) and 1/T□103, activation energy and pre-exponential factor could be calculated from the slope and y-intersect, respectively.

2. Experimental

Materials

The liquid crystalline epoxy monomer used in this study was synthesized according to the method reported in the literatures[7-8]. 4,4'-Diaminobiphenyl (DABP) used as curing agent was purchased from Aldrich and was used  without further purification. Their chemical structures are shown as follows:

LCE

4, 4'-diaminobiphenyl

DSC Characterization

After mixing LCE and DABP at the ratio of 1:1, 1:2 and 2:3, thermal analysis was performed using Solomat TA differential scanning calorimeter (DSC) 4000 under the dynamic condition. Around 5-6 mg of well-mixed sample was placed in aluminum pan on the DSC heating stage and cured at the heating rates 5, 7, 10 and 20□/min, respectively. Nitrogen purge gas was flowed at 50ml/min. The residual exothermic heat obtained from a dynamic run in the range of 30□ and 300□.

3. Results and Discussion

Fig. 1 shows the result of the DSC trace for the synthesized monomer    measured from 30□ to 300□ at the heating rate 10□/min. This LCE monomer started to melt around 120□. In succession, a storng endothermic peak represented around 137□. Also, the exothermic reaction appeared to start at 257□ and the maximum temperature of exothermic peak appeared at270□. The dynamic DSC curve points out that the synthesized monomer is stable until 257□.

Fig. 2 shows DSC curves for LCE/DABP (molar ratio of 1/1, 2/3 and 1/2) at the heating rate 7□/min. The melting points were observed around 80□   regardless of the molar ratios. The first exothermic peak temperature of these systems have a maximum in the range of 101~116□ and the temperatures decrease with increment of curing agent. The second exothermic peak was weakly showed in the range of 222~223□. The former peak may be associated with the primary and secondary amine addition and the latter peak may effect the homopolymerization of residual epoxide. The melting temperature for the mixture of LCE and curing agent is lower than LCE monomer because of its low polarity with introduction of flexible spacer.

Fig. 3 shows DSC curves for LCE/DABP (1/2) system at various heating rates.  In general, the cure temperature is so low that curing reaction occurs before the mixture melts. The initial curing temperature increased as the heating rate increased. When the heating rate is high, the increase of the initial temperature becomes highly significant so that the melting of LCE/DABP system occurs before cure. The weak endothermic peak showed at low heating rates 5 and 10□/min and at the same time there were appeared weak exothermic peak. On the other hand, the steep endothermic peak and  exothermic peak appeared at high heating rate 20□/min. This system is started to melt at the temperature range of 76~87□ and it was influenced by the melting of DABP. The first temperatures of their respective exothermic peaks are 95, 101, 107, and 123□. As heating rate increased, the maximum cure rate appeared at higher temperature due to the rapid cure reaction in a short time.

Table 1. Kinetic Paramaters by Kissinger Equation

Molar ratio	1/1	2/3	1/2
Linear expression	Y=12.940X-22.941	Y= 7.959X-11.312	Y=8.495X-12.795
Ea (kJ/mol)	1.08□105	6.62□104	7.06□104
A (sec-1)	1.19□1014	6.51□108	3.06□109

To obtain kinetic parameters by Kissinger equation, the relation between 1/T□103 and -ln(q/Tp2) for LCE/DABP (1/2) system was displayed in Fig. 4.  The activation energy was calculated from the slope and the pre-exponential factor from the y-intersection of the straight line. All the results were listed in Table 1. The activation energy of LCE/DABP systems were 1.08□105, 6.62□104, and 7.06□104 kJ/mol for the molar ratio of 1/1, 2/3 and 1/2, respectively.

4. Acknowledgement

This work was supported through grant No. 1999-2-308-001-2 of the research program of the Korean Science & Engineering  Foundation.

References

1) H. Korner, A. Shiota, and C. K. Ober, Chem. Mater., 9, 1588 (1997).

2) A. Furukawa, and R. W. Lenz, Macromol. Chem., Macromol. Symp., 2        (1996).

3) S. Jahromi, W. A. G. Kuipers, B. Norder and W. J. Mijs, Macromol. Chem.     Phys., 28, 2201 (1995).

4) D. J. Broer, J. Lub, and G. N. Mol., Macromolecules., 26, 1244 (1993).

5) Q. H. Lin, A. F. Yee, H. J. Sue, J. D. Earls and R. E. Hefner, J. Polym.        Sci., B: Polym. Phys., 35, 2363 (1997).

6) J. Y. Lee, and J. S. Jang, J. Polym. Sci., A: Polym. Chem., 37, 419 (1999).

7) W-F. A. Su, J. Polym. Sci., A: Polym. Chem., 31, 3251 (1993).

8) W. Mormann and M. Brocher, and P. Schwarz, Macromol. Chem. Phys.        198, 3615 (1997).