CA1050222A

CA1050222A - Carbon fibre production

Info

Publication number: CA1050222A
Application number: CA196,938A
Authority: CA
Inventors: Roger Moreton; William Watt
Original assignee: National Research Development Corp of India
Current assignee: National Research Development Corp of India
Priority date: 1973-04-06
Filing date: 1974-04-05
Publication date: 1979-03-13
Also published as: GB1455724A; FR2224405A1; DE2416674A1; DE2416674C2; JPS5052323A; FR2224405B1; US3904716A

Abstract

ABSTRACT OF THE DISCLOSURE
A process for the preparation of polyacrylonitrile precursor fibres and their subsequent conversion to carbon fibres, in which at least the fibre spinning process and oxygen permeation process are carried out under conditions in which particles, and so far as the spinning solution is concerned air bubbles, are excluded from the liquids employed in the spinning process and from the gases in which such process and the oxygen permeation process take place is described.
Carbon fibres produced by the above process have an ultimate tensile strength which increases as the final heat treatment temperature is increased, over the whole range of final heat treatment temperatures used, for example up to about 3,000°C.

Description

)ZZ2 The pr~ent invention i6 concerned with the produotion of carbon fibres.
Proce~6es for the production of carbon fibres are known, for eYample, UK Specification No 1,110,791 disclones the conversion of polyacrylonitrile to carbon fibre by heating at a temperature in the range 200-250C in an oxidi6ing at sphere for a time suffi¢ient to permit complete permeation of oxygen followed by carbonisation at a temperature of at least 1,~00C wherein the fibre iB 6ubjected to ten6ion at least at some stage in its conversion to carbon fibre. ~he proce~s disclosed in this specification also contemplates a further heat treatment at a temperature of up to 3,000C. The process of oxygen permeation iB frequently ter=ed o~idation; carbonisation and ~urther heat treatment may be separate or the fibre m~y be passed as a continuou~ tow from one furnaoe to another at the appropriate temperatures.
~¦ Carbon fibres produ¢ed bg thi~ prooess and by difi¢ations of thi~ proce~s as hereinbefore de~cribed are di~closed in, for example UK Specifications 1 168 619~ 1 166 252, 1 166 251~ show a Young's i Modulus which increase6 as the final heat tre~tment temperature i~
`I increased but the ultimate tensile strength show~ a maximum~ generally ¦ in the region of 1,500C- It i8 therefore impossible to obtain a 1~ oarbon fibre by thess prooesses having simultaneously maximum values of both Young'- Modulus and ultimate tensile strength.
I In aocordance with the present invention a prooes~ for the preparation of polyaorylonit~ile preoursor fibres and thelr sub~equent conversion to carbon fibres whi¢h in¢ludes the steps of 8pinning the j polya¢rylonitrile precursor fibres from solution, heating the pre¢ur~or fibres at a temperature in the range aoo - 300& in an oYidising at D sphere for a time sufficient to per it complete permeation of ~ . '.
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oxygen while the natural shrinkage of the polyacry~onitrile precursor fibre is at least restrained, followed by carbonisation and further heat treatment at temperatures of up to 3,000C includes the improvement ~herein the spinning process and oxygen permeation process are carried out under conditions in which particles, and in the case of the spinning solution, also gas bubbles, are excluded from the liquids . " , . .
employed in the spinning process and from the gases in which such process and the oxygen permeation process take place, whereby carbon fibres are produced having an ultimate tensile strength which increases as the final heat treatment temperature is increased over the whole range of final heat treatment temperatures.
Advantageously the c~rbonisation and also the further heat treatment are also carried out under conditions in which particles are excluded.
Generally the particles are excluded by filtering the said liquids and gases through filters capable of removing any particles having a size greater than 3 microns.
Preferably the filtering applied to the liquids is such that the size of particles excluded is the smallest commensurate with ease of filtration. The less viscous the liquid being filtered the smaller the particles which can be easily removed. For example with spinning solutions which are relatively viscous it had been found convenient to use a filter capable of excluding any particles having a size greater than 1.5 microns, whereas less viscous coagulation bath liquids and wash liquids can be conveniently filtered through a filter, -~
capable of excluding particles having a size greater than 0.25 micron t~
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1~)5~)222 ~ he air or gas eupplied to air or gas spaoes around the apparatue i~
ad~antageously passed through laminar air flow filters capable of meeting Cla~n 100 clean room conditions as set foxth in UB Federal Stendard 209A~ that i8~ not more than 100 particles per cubic foot of a size greater than 0.5 microns and none greater than 5 microne~ and preferably such gas or air does not contain more than 10 particle~ per cubic foot of a size greater than 0.5 micron~.
Preferably where a stage of the proces~, eg the o~ygen permeation step, carbonisation or further heat treatment is oarried out in a stream of gas~that gas i8 passed through a 0.05 micron filter before contact with the fibre under heat treatment.
In the present spe¢ification carbonisation means heating in vacuo, or in an inert or reducing atmosphere with respect to carbon, at a temperature at which volatile materials are driven off from the polyacrylonitrile fibres leaving ~ carbon residue which ~ay oontain ~` a minor proportion of other elements, eg up to 5~0 by weight of nitrogen.
The higher the o~rbonisation temperature, the lower the nitrogen content of the finally produced carbon fibres i8- For example, at 1000C about ~ by weight of nitrogen remains while at 1500C oub-stantially all the nitrogen is driven of~. Carbonisation take~ place at temperatures broadly within the range 800 to 1200C although temperatures of up to 1500C may be included.
Further heat treatment may be an extension of the carbonisation process in which the temperature is raised to the de~cribed final temperature or it may be a separate step or steps.
i Ihe term polyacrylonitrile as u~ed in the present ~pecification includes within its scope copolymer~ or te~polymers of acrylonitrile . , .
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lnso222 ~ .-with not more than 15% and preferably less than 10% by weight of other monomers, for example, methyl methacrylate, methyl acrylate or vinyl acetate, either alone or to which have been added polymers compatible with them.
It has been found that carbon fibres having a length of less than 5 cm, produced in accordance with the present invention do not show a significant change in tensile strength as the gauge length of the test specimen is reduced.
The spinning of polyacrylonitrile precursor fibres and their conversion to carbon fibres in accordance with the present invention will now be described by way of example only, together with the spinning and conversion of polyacrylonitrile fibres as a control and with reference to the accompanying drawings of which Figure 1 is a graph having final heat treatment -temperature as the abscissa and tensile strength as the ordinate, and Figure 2 is a graph showing tensile strength as ordinate against gauge length of test samples as abscissa.
The spinning apparatus was a laboratory spinning apparatus. The apparatus includes a reservoir for the spinning ~;
solution pressurised by argon and a stainless steel spinneret.
After extrusion the fibre passed sequentially through a ;
coagulation bath, 1.20 metres in length, a wateruash bath, a steam stretch tube, 0.60 metres long, a further water wash bath, a traversing device, and was finally taken up on a fused silica collecting frame. All the baths were contained in ~
polyethylene coated stainless steel tanks. ~-A bank of laminar air flow filters directed a flow 3Q of clean air towards the apparatus. The flow was directed in a direction parallel to the longitudinal axis of the spinning apparatus with the mechanism of the spinning apparatus and the , .

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-- lns~222 operators downstream of the apparatus so that any contamination generated by them was carried away from the apparatus.
In the vicinity of the steam stretch tube the laminar air flow was directed downwards and an extractor was provided below the surface on which the steam stretch tube was supported.
It is contemplated that stretching in hot glycerol at temperatures above 100C, eg 150C, could be substituted for steam stretching.
The laminar air flow filters were nominally capable of providing a clean zone covering the apparatus up to Class 100 conditions as set forth in US Federal Standard 209A;
that is less than 100 particles of size greater than 0.5 ;
microns per cubic foot and none of 5 microns and a check of the apparatus showed that air delivered to the clean area did not contain more than 10 particles of a size greater than 0.5 microns per cubic foot.
STARTING MATERIAL
The material used in this work was polyacrylonitrile which included 6% by weight of methyl acrylate as comonomer - 20 and had a number average molecular weight of 52500.
The spinning solution was prepared by dissolving 14 weight % of the polyacrylonitrile/methyl acrylate copolymer in a 50 weight % aqueous sodium thiocyanate solvent at a temperature of 90-95C. The viscous copolymer solution was stirred for about an hour and while still hot was passed j through a 1.5 micron filter.
The solution was then de-aerated by warming to a temperature of about 60C, and centrifuging in 3 inch tubes, in an 8 inch diameter centrifuge at 4000 evolutions per minute.

The coagulant bath contents, a 10% by weight aqueous sodium thiocyanate solution, and the distilled water used in the wash ~ -~
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baths and in the steam generators were filtered through 0.25 micron filters using a peristaltic pump to provide the driving force.
SPINNING
The spinning solution, obtained as described above, at room temperature was spun through a five hole spinneret having 75 micron holes in a 10% by weight aqueous sodium thiocyanate coagulation bath at an extrusion rate of 0.30 metres/minute and the speed at the first roller was 0.60 metres/minute. The ...
temperature of the first wash bath was 50C, the steam stretch ' ratio was 14 and the final wash bath temperature was 30C.
; CONVERSION TO CARBON FIBRES
The polyacrylonitrile precursor fibre was secured to the collecting frame-so that it could not shrink during oxidation and was oxidised at a temperature of 220C for 8 hours -' ' . .
n a glass vessel in the clean zone. Oxygen was passed into the oxidation rig through a 0.05 micron filter.
Further processing to carbon fibre was carried out '~, stepwise, first carbonisation at 1000C in a nitrogen atmosphere ' ' ! 2Q while the fibres were still on the fused silica collecting frame '~
then further heat treatment to 1400C in a vacuum furnace or to '~
2500C in an argon atmsophere in a carbon tube furnace. ,' The oxidised fibre while still on the fused silica collection frame was placed in a fused silica tube with a tightly fitting cap while in the clean zone to prevent con-~', tamination. The tube was then transferred to a furnace and the oxidi,sed fibre heated to 1000C for a period of 1/2 hour to carbonise the fibres. During the heating a stream of filtered nitrogen was passed over the fibres. ' ~ -At this state the sample was split in two and each sample placed in a close fitting carbon tube. One~a~p~e-~a~ - ', , heat treated at 1400C in a vacuum furnace for 1/2 hour and the : ~: ' - 7 - '~"
.'' ' lns~)22z other at 2500C in a carbon tube fuxnace for 1/2 hour in a steam filtered argon.
The carbonising and heat treatment furnaces were not in the clean zone but all transfers were carried out in the clean zone to prevent, or at least minimise, surface contamination during transfer. The streams of nitrogen and argon used were filtered by passing them through 0.05 micron filter.
Control samples, using the same filtered spinning -solution were spun on similar apparatus to the same parameters, ~-but not in the clean zone and were then carbonised and further heat treated in exactly the same way as described above. -TESTING OF CARBON FIBRES
The properties of the carbon fibres produced, measured as the average of 20 determinations on 5 cm gauge lengths in each case, are given in Table 1 below and illustrated in Figure 1.

Fibre -Clean ~one Control -~
treatmen*s ProPerties fibres fibres Polyacrylonitrile Diameter, microns 15.9 15.0 fibres Elongatian, % 10 6 11 6 as Youngs modulus, psi 1.45 x 103 1-88 x 103 spun Tensile strength, psi 79.7 x 10 92.2 x 10 Coefficient of varia-tion of strenqths, % 8 15 _ _ _ Carbonized Diameter, microns 8.2 7.5 in nitrogen Youngs modulus, psi 26.9 x 106 24.9 x 106 1/2 hour at Tensile strength, psi 318 x 103 282 x 103 100C Coefficient of varia- -tion of strengths, % 15 32 .
Heat treatment Diameter microns 7.8 6 6 1400C Youngs modulus, psi 31.0 x 10 29.0 x 10 in vacuum Tensile strength, psi 349 x 103 190 x 103 : 1/2 hour at Coefficient of varia-1400C tion of strengths, % 14 33 Heat Treatment Diameter microns 7.5 6 6.3 6 in argon Youngs modulus, psi 55.1 x 10 53.1 x 10 1/2 hour at Tensile strength, psi 400 x 103 245 x 103 -2500C Coefficient of varia-tion of strength-, % 27 33 : ~
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~SOZZ2 In the accompanying Figure 1 the line 1 represents carbon fibres produced by the process of the present invention whereas line 2 represents carbon fibres produced by an identical process except that the fibres, although not deliberately contaminated, were not spun in clean conditions. The dotted lines represent the 95% confidence limits of the quoted results. Line 2 clearly shows a maximum in ultimate tensile strength, which is normally found whereas line 1 shows that the carbon fibre produced in accordance with the present ~

invention has an ultimate tensile strength which increases ~ -as the final heat treatment temperature increases.
, In interpreting these results and in particular in comparing them with prior art results it should be noted that the careful exclusion of contamination the form of particles from the spinning solution and liquids used therein and from ', the oxygen permeation, carbonisation and further heat treatment processes applied to the control fibres in these experiments has not been general practice in the prior art. It should also be noted that there is evidence for the existence of a scale effect by which improved absolute results are obtained by increasing .. . .
the quantities of fibre treated. For example, the fibres for the present experiments were spun from a spinneret having 5 ~.i .
holes and 0.4g were treated and the control fibres showed ' a tensile strength maximum in the region of 1000C. Commercial - '~

t~ fibre tows have in general many more dilaments. For example ', carbon fibres from 10,000 filament Courtelle* show a tensile , strength ~axi- ~ in the region of 1500C. However the trends ~ .
< of tensile strength are not af-~ecte-d by scale only the absolute ,' values~. '' ~ ' A s~r~es~o test~ u~sg ~a-,uge~l~ngt-~s of 2.5-~m and 1.0 I c,m ~ere caxried out on the fibres produced by the process of the ,, present invention and on the control fibres ' * Trademark of Courtaulds . _ 9 _ , : .. ' . :. ' ' ' . : , , ' . ' ', . ' ' . ' ', ' , ' . , , ., -, . ! ,. :., . , ... . . .' , .: . . -. , . :: -', ' ', ' ' ' ' ' ' " , ' . ; .. . . .' , ~ .. : : ' . ':: . , : "' .,: ' .': ,.. ' :' ' ' " ' ' ' ' ': , . ' ., ' . . ';

)Z22 and the results are given in Table 2 below and plotted as a graph in accompanying Figure 2.

Clean zone fibers Control fibres ... ,, . :
Coeffici- Coef-Gauge Strength ent Strength ficient length psi 3 varia- Diameter psi 3 of vari-cm Diameter x 10 tion ~ microns x 10 ation 1.0 7.9 397 13 6.3 318 25

2.5 7.5 400 18 6.3 259 32 5.0 7.5 399 27 6.3 245 33 In accompanying Figure 2 line 1 represents the results obtained from carbon fibres obtained by the process of the present invention, line 2 represents the control fibres and line 3 represents results obtained from carbon fibres obtained from a proprietary precursor. These last results are taken from a paper, "The effect of gauge length ,on the tensile strength of carbon fibres" by R Moreton appearing in Fibre Science Technology, 1, 4, 273(1969).
It is believed that breakages in tensile test samples are caused by random faults in the fibres. The theory has it that the longer the gauge length the greater the liklihood of including a fault and therefore the lower the average tensile strength. This is borne out by lines 2 and 3 in Figure 2 but line 1 may be interpreted as an indication that gross faults of the type causing failure in the control fibres do not occur sufficiently frequently in carbon fibres produced by the process of the present invention to show any gauge length effect.

~t will of course be reali~d that although the .

~present invention is specifically described in this specifi-. .
cation in terms of discrete-steps, the invention, the, ::
; subject of the present apFIication, is readily adapted to , proceases for the continuous production of carbon fibres.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the preparation of polyacrylonitrile precursor fibres and their subsequent conversion to carbon fibres which includes the steps of spinning the polyacrylonitrile precursor fibres from solution, heating the precursor fibres at a tempreature in the range 200-300°C in an oxidising atmos-phere for a time sufficient to permit complete permeation of oxygen while the natural shrinkage of the polyacrylonitrile precurosr fibre is at least restrained, followed by carbonisation and further heat treatment at temperatures of up to 3000°C
which includes the improvement wherein the spinning process and oxygen permeation process are carried out under conditions in which particles, and in the case of the spinning solution, also gas bubbles, are excluded from the liquids employed in the spinning process and from gases in which such process and the oxygen permeation process take place, whereby carbon fibres are produced having an ultimate tensile strength which increases as the final heat treatment temperature is increased, over the whole range of final heat treatment temperatures.

2. A process as claimed in claim 1 in which the carbonisation and further heat treatment are also carried out under conditions in which particles are excluded.

3. A process as claimed in claim 1 or claim 2 in which the particles are excluded by filtering the said liquids and gases through filters capable of removing any particles having a size greater than 3 microns.

4. A process as claimed in claim 1 or claim 2 wherein the filters are capable of removing any particles having a size greater than 1.5 microns.

5. A process as claimed in claim 1 or claim 2 wherein the filters, with the exception of the filters used in conjunction with the spinning solution are capable of removing particles having a size greater than 0.25 microns.

6. A process as claimed in claim 1 or claim 2 in which the said gases are filtered through filters capable of removing any particles having a size greater than 0.05 microns.

7. A process as claimed in claim 1 or 2 in which the polyacrylonitrile contains less than 10% by weight of other monomers.