액정 에폭시 수지의 열분해 특성 연구

여만경, 김상욱

서울시립대학교 화학공학과

Thermal degradation Behaviour of LC Epoxy Thermosets

M. G. Lu and S. W. Kim

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

Introduction

It is impossible to understand the properties of polymers if the transitions that occur in such materials and specially the temperatures at which these occur are not known. The main transitions are the glass-rubber transition, the crystalline melting point and heat degradation temperatures.

It is well known that LC polymers containing large aromatic repeating units have good thermal resistance in general. A number of analytical methods have been developed for the determination of the thermal stability and thermal decomposition kinetics of polymers.

Comparing with the isothermal method, the case of dynamic degradation is theoretically more difficult for the phenomenological kinetic model due to the complex temperature dependence of rate constant and the peculiar feature of decomposition, although it is a case which more closely simulates the factual process.

In this present paper, TGA data obtained at different heating rates are used to provide an alternative model of kinetics of LC epoxy thermosets degradation. The kinetic parameters from various published models are compared and used to elucidate the thermal behavior.

Experimental

Materials

The LC epoxy monomer  was a diglycidylether of biphenol (DGEBP). The preparation was described previously. The diepoxide was cured with methylene dianiline (MDA) and 4,4'-aminophenoxy hexane (APOH) at 140℃ for 5 h and 180℃ for another 30 min. The resulting polymers are used  for TG analysis.

Instruments

A Setaram TG/DTA 92 simultaneous thermogravimetric/differential thermal analyzer is used to obtain weight loss data for the degradation of cured LC thermosets. For the thermal degradative runs, the samples are purged with argon gas at a flow rate of 50 ml/min. Constant heating rates of 5, 10, 15 and 20 ℃/min are used. The sample weights are 5-15 mg in all cases.

Results and Discussion

1. Thermal degradation

The thermogravimetric (TG) curves for the thermal degradation of two LC thermoset powders are shown in Figures 1 and 2.  The shift of onset to higher temperature with increasing □ is due to the shorter time required for a sample to reach a given temperature at the faster heating rates. The initial decomposition temperature Td,0  and the temperature of half decomposition Td,1/2 for both thermosets  conducted at various heating rates are listed in Table 1.

Table 1. The characteristic temperatures of degradation for both systems

Sample		Td,0	Td,1/2	Td, max
DGEBP/MDA	5 ℃/min	297	498	436
	10℃/min	313	521	455
	20℃/min	342	563	475
DGEBP/APOH	5 ℃/min	284	403	370
	10℃/min	297	417	396
	20℃/min	321	429	402

It has been reported that the measured values of the main indices appear to be interrelated:

Td,0 □ 0.9 Td,1/2                          (1)

Obviously, the empirical relationship in eq. (1) is invalid here for LC polymer systems. Especially, the values of Td,0 are much less 0.9 Td,,1/2. On the other hand, the LC thermosets degrades more slowly. It is attributed to the local orientation in LC polymers. Some researchers have pointed out that, as LC epoxy was curing, the transverse properties increased. The mesophases formed persist by the crosslinked network until decomposition. Therefore, differing from the linear LC polymers, the thermal degradation in LC thermosetting polymers not only includes the cleavage of network bonds, but also have to destroy the ordered LC domains. It is resonable for LC thermosets to exhibit the excellent high temperature resistance.

FIG 1. TG thermogram of DGEBP /MDA at various heating reates.

FIG 2. TG thermogram of DGEBP /APOH at various heating rates.

Table 1 indicates clearly that sample DGEBP/MDA shows the higher initial decomposition temperature because of its higher content of aromatic ring. However, the values of Td,0 of DGEBP/APOH seem to be as low as expected when the flexible linkage was introduced. It may be associated with its higher degree of orientation. As reported previously, the introduction of flexible spacer into main chain plays an important role in determining transition temperatures, mesomorphic range and

the nature of the LC phase. The increase in molecular segment length drive the formation of liquid crystalline phase. In fact, the addition and chemical bonding of several rigid units via short flexible spacers effectively increase the "Kuhn length". As a consequence, the LC phae is stabilized by geometric constraints during curing. In some cases, with extensive chain alignment and good chain reglarity, crystallization even proceeds so quickly that results in the transition from smectic phase to crystal. However, DGEBP/APOH still exhibits a lower Td,1/2 because the methylene linkage exists as weak bonds and the active species of thermal decomposition.

FIG 3. Coats-Redfern plots for DGEBP/APOH at n=1, 2, and 3.

FIG 4. Kissinger plots for DGEBP/ MDA at different conversions.

2 Kinetic Analysis of Thermal Degradation

Random thermal degradation can usually be described as a first-order reaction (loss of weight as a parameter) if the decomposition products are volatile. Although there are several methods for calculating kinetic parameters as reviewed before, it is common believable that non-isothermal methods can describe the degradation process better. In here the integral methodologies are used to deduce the activation energies, including the methods of Kissinger and Ozawa, Coats-Redfern and Horowitz and Metzger.

The kinetic parameters for both systems are analyzed in the 40-90% degradation range. The initial 10% degradation zone is not considered to avoid errors to an overlap from the previous degradation zone. The Coats-Redfern equation is used for evaluation of n values. The kinetic plots of DGEBP/APOH at 5 K/min are shown in Fig. 3 for n=1, 2, and 3, respectively. It is unexpected that the linearity of the plots seems to be insensitive to the magnitude of n, 1 or 2. However, for DGEBP/MDA system, only n=1 can fit the linearity of plots. Although it is not clear in            Table 2. The vaules of activation energy from multiple constant heating rates

Conversion	Activation Energy, kJ/mol
	DGEBP/MDA		DGEBP/APOH
	Kissinger	Osawa	Kissinger	Osawa
0.9	207	144	68	74
0.8	153	164	72	74
0.7	151	164	65	86
0.6	115	127	54	89
0.5	101	114	47	72

mechanism, as is obvious from Fig. 3, the values of the activation energy and to some extent Z so calculated will change with the n chosen. The calculated decomposition activation energies are listed in Table 2. We can find that the activation energy of thermal degradation for DGEBP/MDA is much higher than that for DGEBP/APOH system. However, we can not assert that the linear sensivity of plot to n was only determined by Ed. The low decomposition activation energies for DGEBP/APOH may be affected by the flexible structure of curing agent. As mentioned previously, the weak methylene linkage makes the decomposition easier as the degradation starts. Besides, in these rapid decomposition zones the thermal degradation process is not affected significantly by the heating rates. On the other hand, the kinetic parameter can be given by a single constant heating rate.

Figs. 4 show Kissinger's plots. The decomposition activation energies at different conversions are obtained and listed in Table 2. The values are in good agreement with that given by the methods described above. Similar results can also been obtained by the method of Ozawa. However, comparing with the results from one constant heating rate, the Kissinger and Ozawa methods exhibit the higher values of activation energy. The difference between both systems observed by Ozawa method is that there is no marked decrease tendency in activation energy with increasing conversion.

Conclusions

The thermal degradation experiment shows that the LC epoxy resins have high temperature resistance. However, it can be decreased as the resin is cured with the aliphatic methylene containing curing agent in which the lower decomposition activation energy is observed. The kinetic analytical methods from one constant heating rate are insensitive to magnitude of reaction n in our obervation, although the activation energies obtained are in agreement with that from the multiple processes describedd by Kissinger and Ozawa.

References

1. Li, X. G. et al., Angew. Makromol. Chem., 1998, 256, 9

2. Rao, M. P. et al., Polym. Degrad. Stab., 1997, 61, 283

3. Gonzalez-Romero, Polym. Eng. Sci., 1989, 29, 302

4. Ozawa, T., Polymer, 1971, 12, 150

5. Kissinger, H. E., Anal. Chem., 1957, 29, 1702