열중량 분석에 의한 SAN의 열분해 속도론

이현배, 돈윤승．, 심미자．．, 김상욱

서울시립대학교 화학공학과, ．제일모직(주) 화성연구소,

．．서울시립대학교 생명과학과

Thermal Degradation Kinetics of SAN Estimated

by Thermogravimetry

Hyun-Bae Lee, Yoon-Seung Don．, Mi-Ja Shim．． and Sang-Wook Kim

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

．R ＊ D Center, Cheil Industries Inc.

．．Dept. of Life Sci., The Univ. of Seoul

Introduction

With the increasing demands for polymer materials in the high temperature applications, it is important to predict the thermal degradation characteristics, thermal stability, limited temperature, etc before their applications. Especially, thermal degradation, considered as any modification of a polymer involving the main-chain backbone and side groups, is significant for many fields such as polymer waste management and the processing operations. Numerous differential or integral methods have been described in obtaining kinetic parameters with TGA(thermogravimetric analyzer) which is a powerful instrument to study complex degradation processes. Representative methods[1] are differential methods proposed by Kissinger, Friedman, and Freeman ＊ Carroll and integral methods by Coats ＊ Redfern, Reich ＊ Levi and Flynn ＊ Wall. The thermal degradation is required to break the valence bonds of polymeric chain, leading to lower molecular weights. So its characteristics are affected by not only the chemical composition of the polymer but also the physical parameters such as molecular weight, molecular weight distribution, crystallinity, chain flexibility, cross-linking, chain conformation branching, and so on.

In this study, the effect of molecular weights on thermal degradation brought about by thermal stress was investigated by using the weight loss curves obtained by TGA instrument. SAN(styrene-acrylonitrile-copolymer), which possesses transparency, high heat deflection properties, and excellent gloss and chemical resistance was used as material and Flynn ＊ Wall's equation and Kissinger equation were applied to obtain the kinetic parameters of thermal degradation.

Experiment

The SAN polymer used in this study was powder type with various molecular weights, 9200, 119000, 180000 and 319000 prepared from Cheil Industries Inc. Its acrylonitrile content which contributes to heat resistance, high strength and chemical resistance was in the range of 24.6 to 28.5%. Its MI(melting index) was 8.5, 3.0, 0.8 and 0.3 g/10min and molecular weight distribution was 1.84, 1.97, 1.85 and 2.28, respectively. TGA(ATI CAHN, TG121) was operated in order to measure the weight loss curves. SAN powders of 3 to 7 □ were placed on the balance located in the furnace of TGA and then were heated in the temperature range of 50 to 600℃ at the heating rates of 5, 10, 15 and 20 ℃/min, respectively. To prevent the oxidation of the sample, nitrogen gas was flown at the rate of 100 □/min. Above procedure was repeatedly carried out in case of each molecular weight.

Results and Discussion

Fig. 1 plots the relationship between TG and temperature for SAN, having weight-average molecular weight of 92,000 at the heating rates of 5, 10, 15 and 20℃/min, respectively. The TG curves moved right side with the increase of heating rate. The temperature of initial degradation, defined as that of 5% weight loss, was 359, 371, 378 and 384℃ at the heating rate of 5, 10, 15 and 20 ℃/min, respectively. From these results, it is clear that the heating rates took an effect on this shift, or the change of thermal properties.

Fig.  Plots of TG and temperature for SAN having molecular weight of 92,000 at various heating rates.

There are many methods for the kinetic evaluation of the TG thermal degradation data as shown in Fig. 1. The isoconversional method known as the most reliable was applied in this study of degradation kinetics of SAN. This technique which is essentially the same as that of Ozawa[2] represents a relatively simple method from weight loss vs. temperature data obtained at several heating rates. This method is based upon the following expression, which is an approximate solution of the integrated rate equation having several assumptions[3].

[eqn: log( beta )=-0.4567 E_ad /RT+log(E_ad A/R)-2.315-logF( alpha )]

where, [eqn: beta `] is heating rate(℃/min), [eqn: E_ad`] is apparent activation energy(kJ/mol), R is gas constant(8.3136 J/mol·K), [eqn: T`] is temperature in degree absolute, [eqn: A`] is pre-exponential factor(min-1) and [eqn: alpha `] is degree of conversion. With these curves in Fig. 1, the reciprocal temperatures in Table 1 were obtained at each degree of conversion with various heating rates. The degradation activation energy [eqn: E_ad`] was calculated from the slopes in the plot of [eqn: log beta `] against [eqn: 1/T`] for the conversion degree of 0.1 to 0.9 as shown in Fig. 2. In addition, the values of [eqn: log A`] were found from the intercept on the [eqn: log beta `] axis.

Table  log□ and 1/T×103 Values for the Calculating of Kinetic Parameters through Flynn ＊ Wall's Equation.

Mw	□ log□	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
92,000	0.699	1.561	1.533	1.521	1.512	1.504	1.496	1.489	1.480	1.467
	1.000	1.533	1.510	1.495	1.485	1.476	1.468	1.460	1.451	1.482
	1.176	1.518	1.495	1.482	1.472	1.463	1.455	1.446	1.437	1.424
	1.301	1.502	1.479	1.465	1.456	1.448	1.439	1.431	1.421	1.406

Table  Kinetic Parameters Determined from Isoconversional Plots according to Flynn ＊ Wall's Method ; #1 : 119,000, #2 : 319,000, #3 : 92,000, #4 : 180,000.

□ SAN		0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
#1	Ead	177.58	189.04	189.04	191.68	190.06	185.56	180.30	178.72	168.68
	logA	15.90	16.95	17.03	17.30	17.21	16.90	16.53	16.44	15.70
#2	Ead	185.81	194.11	201.70	199.81	199.19	196.96	195.68	191.66	188.40
	logA	16.63	17.42	18.08	17.97	17.96	17.81	17.74	17.46	17.23
#3	Ead	188.95	205.68	200.20	199.97	199.50	196.43	192.16	189.36	183.78
	logA	16.89	18.31	17.92	17.96	17.96	17.75	17.45	17.26	16.85
#4	Ead	167.69	175.85	188.53	187.12	186.12	182.47	181.47	177.39	171.73
	logA	15.12	15.92	17.00	16.94	16.90	16.66	16.62	16.33	15.93

The calculated activation energies and [eqn: log A`] values are shown in Table 2 with various molecular weights. Similarly, from the temperature of maximum conversion rate on the DTG curves at several heating rates, the kinetic parameters were obtained through the simple Kissinger equation[4,5]. In case of Flynn and Wall's method, activation energies of degradation were 205.68, 189.04, 175.85 and 194.11 kJ/mol and pre-exponential factors were 2.04×1018, 8.91×1016, 8.32×1015 and 2.63×1017 min-1 for molecular weights of 319000, 119000, 180000 and 92000, respectively, at a fixed degree of 0.2 conversion. From Kissinger equation, activation energies were 175.80, 169.63, 168.08, and 177.52 kJ/mol at 180000 to 319000 respectively. The pre-exponential factors were 6.76×108, 2.20×108, 1.66×108 and 8.84×108 min-1, respectively.

Fig.  The plots of log□ vs. the reciprocal absolute temperature at fixed conversions of the degradation for SAN, 92,000.

References

1. Stivala, S. S., Kimura, J. and Gabbay, S. M., Degradation ＊ Stabilization of

Polyolefins, Ch. 3, ed. by N. S. Allen, Applied Sci. Publishers, London ＊ New York,

(1983).

2. Ozawa, T., Bull. Chem. Soc. Japan, 38, 1881 (1965).

3. Day, M., Cooney, J. D. and Wiles, D. M., J. of Appl. Polym. Sci., 38, 323 (1989).

4. Cooney, J. D., Day, M. and Wiles, D. M., J. of Appl. Polym. Sci., 28, 2887 (1983).

5. Song, Y. W., Shim, M. J. and Kim, S. W., Korean J. of Mater. Res., 7, 386 (1997).