US3904716A

US3904716A - Carbon fibre production

Info

Publication number: US3904716A
Application number: US458035A
Authority: US
Inventors: Roger Moreton; William Watt
Original assignee: National Research Development Corp UK
Current assignee: BTG International Ltd
Priority date: 1973-04-06
Filing date: 1974-04-04
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09
Also published as: GB1455724A; FR2224405A1; DE2416674A1; CA1050222A; DE2416674C2; JPS5052323A; FR2224405B1

Abstract

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 up to 3000*C.

Description

United States Patent [191 Moreton et al.

[ CARBON FIBRE PRODUCTION [75] Inventors: Roger Moreton, Church Crookham; William Watt, Farnborough, both of England National Research Development Corporation, London, England 221 Filed: Apr. 4, 1974 21 Appl. No.: 458,035

[73] Assignee:

[30] Foreign Application Priority Data Apr. 6, 1973 United Kingdom 16546/73 AVERAGE TENSILE STRENGTH X|O |bs./sq.in 3

[451 Sept. 9, 1975 Primary Examiner-Jay H. Woo Attorney, Agent, or Firm-Cushman, Darby & Cushman [5 7] ABSTRACT 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 up to 3000C.

12 Claims, 2 Drawing Figures FIG.I.

HEAT TREATMENT TEMPERATURE C CARBON FIBRE PRODUCTION The present invention is concerned with the production of carbon fibres.

Processes for the production of carbon fibres are known, for example, UK Specification No 1,110,791 discloses the conversion of polyacrylonitrile to carbon fibre by heating at a temperature in the range 200250C in an oxidising atmosphere for a time sufficient to permit complete permeation of oxygen followed by carbonisation at a temperature'of at least 1,000C wherein the fib e is subjected to tension at least at some stage in its conversion to carbon fibre. The process disclosed in this specification also contemplates a further heat treatment at a temperature of up to 3,000C. The process of oxygen permeation is frequently termed oxidation; carbonisation and further heat treatment may be separate or the fibre may be passed as a continuous tow from one furnace to another at the appropriate temperatures.

Carbon fibres produced by this process and by modifications of this process as hereinbefore described are disclosed in,for example UK Specifications 1,168,619, 1,165,252, 1,166,251, show a Youngs Modulus which increases as the final heat treatment temperature is in creased but the ultimate tensile strength shows a maximum, generally in the region of 1,500C. It is therefore impossible to obtain a carbon fibre by these processes having simultaneously maximum values of both Youngs Modulus and ultimate tensile strength.

In accordance with the present invention 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 temperature in the range 200300C in an oxidising atmosphere for a time sufficient to permit complete permeation of oxygen while the natural shrinkage of the polyacrylonitrile precursor fibre is at least restrained, followed by carbonisation and further heat treatment at temperature of up to 3,000C includes the improvement wherein the spinning process and oxygen permeation process are carried out under conditions in which particles, and also in the case of the spinning solution, 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 carbonisation and also the further heat treatment are also 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 exeluding particles having a size greater than 0.25 microns.

The air or gas supplied to air or gas spaces around the apparatus is advantageously passed through laminar air flow filters capable of meeting Class clean room conditions as set forth in U.S. Federal Standard 209A, that is not more than 100 particles per cubic foot of a size greater than 0.5 microns and none greater than 5 microns, and preferably such gas or air does not contain more than 10 particles per cubic foot of a size greater than 0.5 microns.

Preferably where a stage of the process, eg the oxygen permeation step, carbonisation or further heat treatment is carried out in a stream of gas, that gas is passed through a 0.05 micron filter before contact with the fibre under heat treatment.

In the present specification carbonisation means heating in vacuo, or in an inert of reducing atmosphere with respect to carbon, at a temperature at which volatile materials are driven off from the polyacrylonitrile fibres leaving a carbon residue which may contain a minon proportion of other elements, eg up to 5% by weight of nitrogen. The higher the carbonisation temperature, the lower the nitrogen content of the finally produced carbon fibres is. For example, at 1000C about 5% by weight of nitrogen remains while at 1500C substantially all the nitrogen is driven off. Carbonisation takes 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 described final temperature or it may be a separate step or steps.

The term polyacrylonitrile as used in the present specification includes within its scope copolymers or terpolymers or acrylonitrile 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.

The spinning apparatus was a laboratory spinning apparatus. The apparatus included 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 water wash bath, a steam stretch tube, 0.60 metres long, a filrther 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 of clear 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 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, e.g. 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 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 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 9095C. The viscous co polymer solution was stirred for about an hour and while still hot was passed 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 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 at extru sion rate of 0.30 metres/minute and the speed at the first roller was 0.60 meter/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 5 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 in a glass vessel in the clean zone. Oxygen was passed into the oxidation rig through a 0.05 micron 0 filter. 1::

Further processing to carbon fibre was carried out stepwise, first carbonisation at 1000C in a nitrogen atmosphere while the fibres were still on the fused silica collecting frame then further heat treatment to 1400C 15 in a vacuum furnace or to '2500C in an argon atmosphere 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 contamination. The tube was then transferred to a furnace and the oxidised fibre heated to 1000C for a period of /2 hour to carbonise the fibres. During the heating a stream of filtered nitrogen was passed over the fibres.

At this stage the sample was split in two and each sample placed in a close fitting carbon tube. One sample was heat treated at 1400C in a vacuum furnace for /2 hour and the other at 2500C in a carbon tube furnace for /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 FIG. 1.

TABLE 1 Fibre Properties Clean zone Control treatments fibres fibres Diameter, microns 15.9 15.0 Polyacrylonitrile Elongation, 1O 1 l fibres Youngs modulus, psi 1.45 X 10 1.88 X l0 as Tensile strength, psi 79.7 X 10" 92.2 X 10" spun Coefficient of variation of strengths, 8 15 Diameter, microns 8.2 7.5 carbonized Youngs modulus psi 26.9 X 10" 24.9 X 10 in nitrogen Tensile strength, psi 318 X 10 282 X 10" V2 hour at Coefficient of variation 1000C of strengths, 15 32 Heat treatment Diameter microns 7.8 7.0 1400C Youngs modulus, psi 31.0 X 10' 29.0 X 10 in vacuum Tensile strength, psi 349 X 10'' X 10" V2 hour at Coefficient of variation 1400C of strengths, 7r 14 33 Heat Treatment Diameter microns 7.5 6.3 in argon Youngs modulus, psi 55.1 X l0 53.1 X 10 V2 hour at Tensile strength, psi 400 X l0" 245 X 10" 2500C Cocfficicnt of variation 27 33 of strengths, 71

In the accompanying FIG. 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, where 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 existance 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 holes and 0.4g were treated and the control fibres showed a tensile strength maximum in the region of I000C. Commercial fibre tows have in general many more filaments. For example carbon fibres from 10,000 filament Courtelle show a tensile strength maximum in the region of I500C. However the trends of tensile strength are not affected by scale only the absolute values.

A series of tests using gauge lengths of 2.5 cm and 1.0 cm were carried out on the fibres produced by the pro cess of the present invention and on the control fibres and the results are given in Table 2 below and plotted as a graph in accompanying FIG. 2.

TABLE 2 invention to show any gauge length effect.

It will of course be realised that although the present invention is specifically described in this specification in terms of discrete steps, the invention, the subject of the present application, is readily adapted to processes for the continuous production of carbon fibres.

What we claim is:

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 temperature in the range 200300C in an oxidising atmosphere for a time sufficient to permit complete permeation of oxygen while the natural shrinkage of the fibre is at least restrained, followed by carbonisation and further heat treatment at temperatures of up to 3000C 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, 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 over the whole range of final heat treatment temperatures.

2. A process as claimed in claim 1 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.

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

4. A process as claimed in claim 3 in which the said gases are filtered through filters capable of removing any particles having a size greater than 0.05 microns.

5. A process as claimed in claim 4 and in which the Clean zone fibres Control fibres Gauge Diameter Strength Coefficient Diameter Strength Coefficient length psi of microns psi of cm X 10" variation X It) variation L0 7.9 397 I3 6.3 3l8 25 2.5 7.5 400 18 6.3 259 32 5.0 7.5 399 27 6.3 245 33 In accompanying FIG. 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 of the tensile strength of carbon fibres by R Moreton appearing in Fibre Science Technology, I, 4, 273(1969).

It is believed that breakages in tensile test samples are caused by random faults in the fibres. The theory hasit 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 FIG. 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 carbon fibres shown no significant gauge length effect when subjected to tensile strength testing at lengths of 5 cm or less.

6. A process as claimed in claim 5 in which the polyacrylonitrile contains less than 10% by weight of other monomers.

7. 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.

8. A process as claimed in claim 7 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.

9. A process as claimed in claim 8 wherein the filters are capable of removing any particles having a size greater than 1.5 microns.

10. A process as claimed in claim 9 in which the said gases are filtered through filters capable of removing cm or less.

sting at lenths of 12. A process as claimed in claim 11 in which the polyacrylonitrile contains less than 10% by weight of other monomers.

UNITED STATES PATENT OFFICE Q TEFICATE OF QORRECTEN PATENT NO. 1 3,904 ,716

DATED September 9 1975 INVENTOMS) 1 Roger Moreton and William Watt it is certified that error appears in the above-identified patent and that said Letters Patent Q are hereby corrected as shown below:

Column 1, Line 54 between "also" and "out read carried..

0 Column 6, Line 50 read "shown" as show--.

Etigncd and fizalcd this v t twenty-third Of March 1976 Q [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Aflesfing ff (ummr'ssr'rmer uflalenls and Trademarks

Claims

1. A PROCESS FOR THE PREPARATION OF POLYACRYLONITRILE PRECURSOR FIBRES AND THEIR SUBSEQUENT CONVERSION TO CARBON FIBRES WHICH INCLUDES THE STEPS OF SPINNING THE POLYACRYLONTRILE PRECURSOR FIBRES FROM SOLUTION, HEATING THE PRECURSOR FIBERS AT A TEMPERATURE IN THE RANGE 200#-300#C IN AN OXIDISING ATMOSPHERE FOR A TIME SUFFICIENT TO PERMIT COMPLETE PERMEATION OF OXYGEN WHILE THE NATURAL SHRINKAGE OF THE 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, 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 OVER THE WHOLE RANGE OF FINAL HEAT TREATMENT TEMPERATURES.

5. A process as claimed in claim 4 and in which the carbon fibres shown no significant gauge length effect when subjected to tensile strength testing at lengths of 5 cm or less.

10. A process as claimed in claim 9 in which the said gases are filtered through filters capable of removing any particles having a size greater than 0.05 microns.

11. A process as claimed in claim 10 and in which the carbon fibres show no significant gauge length effect when subjected to tensile strength testing at lenths of 5 cm or less.

12. A process as claimed in claim 11 in which the polyacrylonitrile contains less than 10% by weight of other monomers.