Carbon fibres from a SO2 treated PAN precursor

Curbon Vol. 27. No. 6. pp 777-783. Prmted I” Great Brikun.

1989

0008-6223:X9 $3.(10 + .(I(1 e 1989 Pergamon Press plc

CARBON

FIBRES FROM A SO, TREATED PAN PRECURSOR

S. BKAZEWICZ Academy of Mining and Metallurgy, Cracow, Poland (Received 2 November

1988; accepted in revised form 30 January 1989)

Abstract-The paper describes the mechanism of stabilization and conversion of PAN fibres preliminarily oxidized with SO, to carbon fibres. Elementary analysis, IR and mass spectrographic analysis are used to determine the development of PAN structure at various stages of stabilization. The effect of the stabilization conditions in continuous processing upon the tensile strength of carbon fibres has been shown. Shrinkage measurements during the stabilization and carbonization have been taken. Thus, a relationship between the tensile strength and shrinkage produced during the carbonization has been demonstrated. Key Word-PAN

fibres. stabilization. carbonization,

1. INTRODUCTION

In the production of carbon fibres, the oxidative stabilization of PAN fibres is, so far, the speed-limiting step. The influence of various oxidative compounds on PAN-based precursor during stabilization has been studied in several works[1,2,3]. One of the most important requirements is to speed this step up. Proper conditions of the rate of heating and the temperature of heating should be established for the optimum stabilization for each precursor. To resolve this problem many ways have been attempted. One possibility is to modify the chemical composition of the PAN fibres or its structure[4,5,6]. Another way is to optimize the stabilization conditions to obtain suitably stabilized PAN fibres[7,8,9,10]. Our previous paper[l] has been concerned with the results of studies on the structure changes due to preliminary treatment of the PAN precursor fibres with SO,. Later, our attempts were made to optimize the stabilization rate of PAN fibres treated with SO,. We report here a part of these results.

tensile strength.

speed of the yarns. The stabilized fibres were then subjected to thermal pyrolysis in an argon atmosphere up to 13OO”C, held under tension, without maintaining it at the final temperature. All experiments were made under various conditions of stabilization (temperature profiles, speed of yarns). The IR spectroscopic studies of selected samples of stabilized fibres were carried out on a FTS-14 DIGILAB INTERFEROMETER in the range of 400-4000 cm-‘. The volatile compounds released during the outgassing of SO2 treated PAN fibres up to 1600°C have been identified by means of a mass spectrograph with a Mettler-kiln. Chemical analyses TG, DTA were determined for the samples obtained under various conditions. The samples were analyzed for carbon, hydrogen, nitrogen and sulphur. The oxygen content was calculated from the difference assuming the presence of only the mentioned elements in the fibres. The mechanical properties were determined by using single fibres and an Instron machine.

3. RESULTS AND DISCUSSION 2. EXPERIMENTAL

A precursor with comonomer (6 wt% methyl acrylate and 1 wt% itaconic acid) was used in the experiments. The fibres were stabilized and carbonized in the laboratory system in a continuous form. The yarns of PAN fibres containing 3000 filaments (2 drs) were passed continuously through a furnace for stabilization divided into 3 zones with different temperature profiles. Fibres were stabilized in SO2 atmosphere. During this process, the fibres were restrained from shrinking by feeding and removing the yarns at the same speed from the tubular furnace. The stabilization was carried out at 200 to 300°C and the time required for this process depended on the

Results of our preliminary experiments were carried out in order to check the influence of the heating rate on exothermal effect during stablization in SO?. Data of DTA of the varied stabilization conditions of PAN fibres are shown in Fig. 1. It can be seen that along with the increase of the heating rate increases, the height of exothermal effect and a maximum value is shifted to a higher temperature. This phenomena can also occur during the stabilization step of the commercial stabilization process. This indicates that fibre tow temperature in a reactor can increase to levels found in DTA measurements. For example, for a heating rate of lS”C/mn an observed thermal effect reaches the value of 6°C. The second

S. E~A~WICZ

778

15" c/mn

IF

7”C/mn

f

50

100

150

S’C/mn

300 250 200 TEMPERATURE,"C

400

350 t

Fig. 1. Exotherms obtained during stabilization treatment of PAN fibres in SO,.

T,,,b=l3OO"C T

"E 3.5z‘ u x-3.04 g 2.5%

12O”Umin

1.0’

Fig. 2. Influence

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260 270 280 290 300 310 320 MAXIMUM OF STABILIZATION TEMPERATURE,"C of heating rate and final stabilization temperature carbonized fibres.

on the tensile strength

of

Conversion of PAN to carbon fibres

groups of experiments were carried out directly in the laboratory system. The fibres passed continuously through the furnace at various speeds. The temperature profiles inside the furnace were established with gradually increasing temperature from the room temperature to the maximum of the stabilization temperature. The influence of the stabilization condition on the final tensile strength (1300°C) of manufactured carbon fibres is shown in Fig. 2. The quantitative differences observed here are due to both examined parameters. The tensile strength of carbon fibres goes through a maximum value with stabilization temperature and it also changes along with the rate of heating. The increase of the rate of heating (the speed of yarn along the furnace) diminishes the total time of stabilization. Experimental evidence supplied by the elementary analysis shows a strong influence of the time of stabilization on the sulphur and oxygen content in the stabilizing fibres (Fig. 3). The amount of sulphur is directly related to the stabilization time while an oxygen content in the fibre shows a maximum. The relation between the tensile strength of carbon fibres and the sulphur content in the stabilized fibres is shown in Fig. 4. An initial increase of sulphur content of up to about 8% in stabilized fibres results in an increase of tensile strength of carbon fibres. However, surpassing this value of sulphur content doesn’t produce an apparent change in the tensile strength of the examined fibres. The shrinkage produced during the stabilization and the following carbonization is plotted against the final stabilization temperature in Fig. 5. The total shrinkage (free shrinkage conditions) of stabilization

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is caused by the reorientation of polymer chains (entrophy shrinkage) in the initial stage of the process and in the second stage by chemical reaction associated with stabilization (chemical shrinkage). In addition, the changes in tensile strength have been presented as a function of the final temperature of stabilization of PAN fibres. The IR spectra of PAN fibres stabilized at different heating rates conditions provide further experimental evidence (Fig. 6). In spite of the short duration of the total treatment, the spectra show a deep development of structure changes. Our previous data concerning the IR analysis of SO> stabilized PAN fibres in optimal isothermal treatment (2h, 270°C) are very similar to those obtained from the very short time of non-isothermal heating. These observations appear to suggest that fast heating leads to cumulate of the heat resulting in the increasing of the yarn temperature in the oxidative reactor. The height of absorbance in the range of 2000-3000 cm-’ increases significantly with the increase of the heating rate, indicating the formation of an aromatic structure. The maximum of the observed value of the tensile strength of carbon fibres corresponds with the maximum of oxygen content in stabilized fibres at about 300°C (see Fig. 3). It suggests that it is the oxygen that is particularly responsible for observed relationships. Evidence for this is based on IR, elemental and mass spectrographic analysis. Results of analysis of compounds volatilized during pyrolysis of SO, treated fibres (Fig. 7) indicate that sulphur is removed as hydrogen sulphide. Besides hydrogen sulphide, hydrogen is removed as water vapour and hydrogen cyanide. The variation of hydrogen content in the fibres with heat treatment during stabilization and carbonization is presented in Fig. 8. These

-m-w-I"" L 'STAB.-

T

SULPHUR

IO

20 30 40 50 60 TOTAL STABILIZATION TIME,min

Fig. 3. Influence of stabilization time on the elements content in PAN fibres.

780

S. B~AZEWICZ

T-300°C

3.0 "E t ?2.5z is aotz W

@ l.sif 1.0 -

CONTENT OF SULPHUR,%w/o Fig. 4. Influence of the sulphur content in the stabilized fibres on the tensile strength of carbon fibres.

results have been used to calculate the amount of hydrogen removed in form of hydrogen sulphide. Due to SO, decomposing, the dehydrogenation process results from oxygen and sulphur reacting with hydrocarbon groups of the polymer. Hydrogen is eliminated in form of water as a volatile compound with oxygen and H,S as a volatile compound of sulphur. During the stabilization, almost 42% of the total content of polymer hydrogen is removed in such a way. The rest of the hydrogen is evolved during the carbonization and the amount (40-50%) is dependent on the final carbonization temperature. As-

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suming H,S as the main volatile compound of sulphur during the carbonization, the limit of the hydrogen evolved in such a way can be estimated to be about 8% of the total hydrogen content in stabilized PAN precursor. The residual amount of hydrogen is removed from the stabilized polymer in form of H,O,HCN and also CH,. The carbonized fibres still retain 10% of the total amount of hydrogen. The estimated amount of hydrogen evolved H,S suggests a stronger influence of oxygen from decomposed SO, on the dehydrogenation process. The argument in favour of the proposed assignment comes

SH. OF CARBONIZATION

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of shrinkage of fibres formed during stabilization and carbonization the stabilization temperature.

as a function of

Conversion of PAN to carbon fibres

800

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1600

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Fig. 6. IR spectra of PAN fibres stabilized in SO, at different heating rates a) 120”C/mn, b) 30”Cimn, c) 15”Cimn.

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600 800 TEMPERATURE

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Fig. 7. Intensities of gas evolution from SO, treated PAN fibres during carbonization in vacuum.

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from the comparison of the spectra of PAN fibres preliminarily stabilized in SO, as well as in the air[l]. It is apparent from the spectra that the overall trend of the spectral features is quite similar. Thus, the SO, treatment produces a similar structure to these observed after the air oxidation. The spectra of SO, treated PAN fibres yielded no evidence of the existence of chemical bondings involving bridging sulphur in the structure. The range of H,S evolving from stabilized fibres during carbonization at 200-

L 200

)

600 800 1000 1200 TEMPERATUREPC

Fig. 8. Variations of hydrogen content in the fibres with heat treatment carbonization.

50'

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during stabilization

and

400°C indicates a relative low temperature dehydrogenation process in which sulphur takes part. The double role of SO, in stabilization and pyrolysis of PAN fibres consists of facilitating the dehydrogenation of polymer due to H,O and H2S formation. Thanks to this mechanism, it is likely that carbonhydrogen volatile compounds may be reduced. Typical thermograms of PAN pyrolysis of samples pretreated in air and in SO, are shown in Fig. 9. The weight loss of SO, stabilized PAN fibres is

, , I I 400 600 800 1000 1200 TEMPERATURE."C

Fig. 9. TG of PAN fibres during carbonization preliminarily stabilized: a) in air, b) in SC&.

783

Conversion of PAN to carbon fibres

lower in comparison with the air stabilized PAN fibres. It is clear that the final yield of carbon after the carbonization has a strong influence on mechanical properties of PAN based carbon fibres. distinctly

released and carbon-hydrogen volatile compounds may then be reduced. The optimum of tensile strength of carbon fibres is found to be related to the minimum of shrinkage produced during carbonization.

4. SUMMARY

These results have shown that the stabilization of PAN fibres in SO, facilitates the dehydrogenation process which results in the increase of carbon yield after further carbonization. The elementary analysis, IR and mass spectrographic analysis indicate that, besides sulphur, oxygen formed from SO, also takes part in the stabilization process. Relative low temperature of H,S evolving during the carbonization suggests that the main percentage of the sulphur in the stabilized fibres is elemental in nature. The removal of hydrogen as water and H2S is an important reaction in the decomposition of polymer. This mechanism of dehydrogenation takes place directly during stabilization and also during further carbonization. Thus, most of hydrogen of C-H bonding is

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