MANUFACTURE OF CONTROLLED SURFACE RESISTANCE CARBON FIBRE SHEET PRODUCTS

Abstract: A method of manufacturing controlled electrical resistivity carbon fibre sheet products from a carbonizable fibre starting material, comprises heating and drawing the starting material (if required), oxidizing and stabilizing the starting material at an elevated temperature of the order of 220°C to effect molecular aromatic rearrangement thereof, and partially carbonizing the oxidized and stabilized starting material. The carbonizable fibre starting material may be formed into sheet product form at any convenient stage in the manufacturing sequence, either before or after the selective partial carbonization. The selective partial carbonization is achieved at an elevated temperature in an oxygen free atmosphere within a furnace. The furnace temperature or temperaure range controls the degree of carbonization and the final resistivity of the sheet material. The oxidized and stabilized starting fibre material should be heat-soaked at from 370°C to 1250°C for a predetermined period of time to provide the preselected electrical resistivity to the carbonized material.

Description: TECHNICAL FIELD This invenition relates to a novel method of manufacture of controlled surface resistance carbon fibre sheet products composed of partially carbonized carbon fibres. BACKGROUND PRIOR ART The terms 'surface resistance' and 'surface resistivity' are synonymous. 'Surface resistivity' is defined in McGraw-Hill's 'Dictionary of Physics and Maths' as 'the electrical resistance of the surface of an insulator, measured between the opposite sides of a square on the surface; the value in ohms is independent of the size of the square and the thickness of the surface film'. In this Specification the term 'surface resistance' is used exclusively with the above meaning, so as to avoid any confusion with 'resistivity' which is the volume resistivity of an individual fibre and which has units of ohm.cms. Both woven and non-woven fibre mats employing carbon fibres have been fabricated in the past for a variety of purposes such as in the electromagnetic interference (EMI) shielding of radios in automobiles. Such known prior art products and a method of manufacture thereof are described in an article entitled 'Conductive Fibre Mats as EMI Shield for SMC' (sheet molding compounds), by J.R. Quick and Z. Mate appearing in 'Modern Plastics' - published May, 1982 pages 68-71. In this article, a number of SMC products employing panels moulded from non-woven carbon fibre mats is described wherein the non-woven carbon fibre mats employ either 100% carbon fibres, 50% carbon fibres and 50% glass fibres or 33% carbon fibres and 67% glass fibres in their makeup. Similar arrangements are also known in the art wherein the fabric being formed is woven by known weaving techniques, knitting techniques or the like employing varying percentages of carbon fibre filaments and glass fibre filaments. From one of the tables included in the article it is clear that the electrical surface resistance measured in ohms per square inch increases with decreasing carbon filament content and increasing glass fibre filament content. However, this method of interspersing glass filaments with carbon filaments to control the resulting surface resistance of the resulting sheet product at best can only achieve stepped increases in the electrical resistivity. As is well known to those skilled in the art, the surface resistance of a product is in inverse relationship to its conductivity. Thus, where it is desired finely to control the electrical surface resistance (conductivity) of a given sheet product, the technique of interspersing glass fibre filaments with conductive filaments of either carbon, aluminium or the like to achieve a desired surface resistance (conductivity) is at best a gross technique requiring much experimentation and adjustment and more often than not resulting in a product having less than optimum values of electrical surface resistance (conductivity) which are not uniformly dispersed over the surface of the sheet. EP-A-218235 discloses a process for producing a partially carbonized polymeric fibrous material having a predictable electrical resistivity by oxidizing fibres and subsequently heating them during a partial carbonization step to a maximum temperature in the range 600°C to 1150°C. The fibres can be processed into a woven or paper sheet product. The book 'Carbon Fibres in Composite Materials' by R. M. Gill, Iliffe Books, London (1972) pages 53 to 57, discusses polyacrylonitrile as a suitable starting material for producing carbon fibres, and US-A-3497318 and US-A-3285696 describe methods of production of such fibres. GB-A-1261288 illustrates apparatus for heat treating a length of material such as polyacrylonitrile. None of these prior art documents describes a method of controlling the electrical surface resistance of a sheet product. SUMMARY OF INVENTION The present invention is based on a realisation that a greater control of the surface resistance of the carbon fibre sheet product is obtained by controlling not the proportion of carbon fibres in the sheet product but the electrical resistivity of the carbon fibres themselves. The inventor has found that the necessary control of resistivity of the individual fibres can be achieved accurately and reproducibly by only partially carbonizing the fibres in the carbonization step of manufacture, rather than achieving total carbonization as in the past. The invention provides a method of manufacturing a predetermined controlled surface resistance carbon fibre product from a carbonizable fibre material that has been pre-oxidized and pre-stabilized to effect molecular aromatic rearrangement of the fibres; which method comprises carbonizing the pre-oxidized and pre-stabilized fibre starting material by subjecting it to a heat soak treatment at a temperature lying within the range 370°C to 1250°C for a predetermined time in an oxygen-free atmosphere within a furnace to obtain incomplete carbonization of the fibres, and either before or after the carbonizing step, processing the fibres into a desired sheet product having the form of non-woven paper or woven or knitted fabric sheet products or mats, CHARACTERIZED BY HAVING a first step of establishing and recording a relationship between the maximum temperature during the heat soak treatment, the resistivity of the resulting individual fibres and the surface resistance of sheet products or mats made therefrom; and by having a second step of selecting from the recorded relationship the maximum temperature for the heat soak treatment corresponding to the desired surface resistance of the product. The desired sheet product is preferably a non-woven paper or a woven or knitted fabric sheet. The optimum duration of the heat soak treatment may be determined empirically. It should be long enough to carry the carbonization to the limit established by the heat soak temperature, but once a stable degree of carbonization is achieved there is no merit in continuing the heat soak treatment. The duration will therefore depend on the size of the fibres or fibre bundles and whether or not they have been performed into a tow, yarn or fabric prior to carbonization. An optional step in the method of the invention comprises first heating and drawing the carbonizable starting material: if the initial starting material is a carbonizable material either in continous tow or continous yarn form of a small dpf (denier per filament) of the order 1.5 dpf (1.65 dtex), then the initial step of heating and drawing the starting material can be eliminated. The carbonizable fibre starting material is preferably polyacrylonitrile (PAN), in which case the oxidized and stabilized material is typically composed of about 62% carbon, 22% nitrogen, 11% oxygen and 5% hydrogen and preferably has a density of about 1.36 grams per cubic centimetre. It is desirable to use for the carbonization a furnace which has a continuously increasing temperature profile over the duration of the carbonization step. Although this may be achieved using a batch furnace, it is more readily achieved using a continuous line carbonizer which is divided into different temperature zones within the range of 370°C to 1250°C, and continuously moving the carbonizable material therethrough at a relatively low rate of travel so as to assure a prescribed temperature-time treatment of the order of 10 to 20 minutes if it is the oxidized and stabilized yarn being carbonized, rising to 7 to 10 hours if it is a made-up fabric sheet being carbonized. This heat-soak provides the preselected electrical resistivity to the resulting partially carbonized fibres, as a function of the maximum heat-soak temperature in the furnace. If the resulting sheet product is to be in the form of a paper, then the partially carbonized starting material resulting from the carbonizing treatment described above is chopped into bundles of fine carbon fibres having a length of from 0.125 to 1 inch (0.3 to 2.5 cm) which then are supplied to a mixer for mixing with water in copious quantities to form a slurry. The non-aqueous components of the slurry are about 83% by weight of the partially carbonized fibres, 14% by weight of a binder or dispersant fibre such as cellulose, 2% by weight polyvinyl alcohol (PVA) and the remainder a resin. Preferably the constituents of the slurry exclusive of the water amount to about 0.12% by weight of the overall slurry solution including the water. The overall slurry solution then has ammonia added to adjust the pH to from 8 to 9. The pH-adjusted slurry solution is then subjected to a wet lay paper formation process to form carbon fibre paper in wet sheet form. The wet carbon fibre paper is then conveyed to a series of dryer cans and then taken up continuously on a take-up roll for storage and use. If the initial carbonizable material is in the form of 1.5 denier per filament (dpf) (1.65 dtex per filament) yarn or less, then as noted earlier the optional initial step of heating and drawing the starting material can be elminated, and the step of forming the carbonized material into a desired end sheet product may comprise weaving the partially carbonized yarn into a carbonized sheet product having a preselected desired surface electrical resistance. If the carbonizable fibre starting material is in the form of a tow, it can advantageously be formed into the sheet product after the oxidation and stabilization step but before the partial carbonization step. A preferred procedure is as follows: the starting tow is if desired heated and drawn. The heated and drawn starting material is then oxidized and stabilized at an elevated temperature of the order of 220°C to effect aromatic rearrangement of the molecules of the starting material and thereby form a stabilized tow. The stabilized tow is then stretch-broken and formed into sliver comprising large bundles of discontinuous filaments of the starting material. The sliver is then aligned and the ends thereof joined into a roving preferably having a slightly twisted condition. The roving then is spun and formed into yarn which then is plied or twisted. The plied or twisted spun yarn then is woven or knitted into a fabric. The fabric thus formed is then subjected to the partial carbonization treatment, preferably with a heat-soak time of the order of seven to ten hours, and is then preferably force cooled back to ambient to provide a preselected surface electrical resistance to the carbonized fabric. Brief Description of the Drawings Figure 1 is a functional block diagram depicting the essential and certain alternative steps employed in practising the method of manufacturing controlled surface resistance carbon fibre sheet products according to the invention; Figure 2 is a schematic functional block diagram of an alternative spun yarn operation which can be used in conjunction with the initial processing steps of the system and method of practising the invention shown in Figure 1 to result in a controlled surface resistance carbon fibre sheet product in fabric form manufactured according to the invention; Figure 3 is a temperature-resistivity curve plotted in log form and showing the electrical resistivity of a single carbonized fibre filament treated according to the invention; and Figure 4 is a composite curve showing both the resistivity versus heat treatment temperature characteristics of a single carbonized fibre filament and the resulting surface resistance of a carbonized sheet product fabricated according to the invention and illustrates data with which the temperature-resistivity of the single carbonized fibre is translated into the surface resistance of the resultant carbonized fibre sheet product produced with such fibres. Best Mode of Practising the Invention Referring first to Figure 1, the method of manufacturing controlled surface resistance carbonized fibre sheet products according to the invention includes a source of carbonizable precursor material comprising PAN (polyacrylonitrile) as shown at 11. The precursor material generally used is in plaited tow form as shipped from the supplier. The precursor material is supplied to a commercially available heating and drawing stage shown at 12 where the material is heated to a temperature of about 150°C and drawn at a ratio dependent upon the desired size of the output tow. For example, a tow having dimensions ranging from 160,000 filament bundles times 3 denier per filament (3.3 dtex per filament) would be drawn down to a tow of 1.5 dpf (1.65 dtex per filament) or less having 160,000 filaments per bundle dependent upon the draw ratio. From the heating and drawing operation at 12, the carbonizable tow is subjected to an oxidation operation 14 where it is stabilized by being heated in atmospheric oxygen to a temperature of about 220°C. This results in the aromatic molecular rearrangement of the material. In the case of PAN, the resulting oxidized tow has a composition of about 62% carbon, 22% nitrogen, 11% oxygen and 5% hydrogen with a density of about 1.36 grams per cubic centimetre. The resulting oxidized tow is sold under the trademark 'PYRON'. During the oxidation phase the tow changes colour from white or off-white to black and undergoes a change in density although the carbon content remains essentially the same. The time required for the stabilization is about two or three hours. Ovens for this purpose are commercially available. For certain types of operations, it may be desirable to start with a PAN precursor material which is initially supplied in continuous yarn form having a filament of 1.5 dpf (1.65 dtex per filament) or less and a filament count of up to 20,000 filaments per bundle. For such operations, it is not necessary to include the heating and drawing operation and hence this operation will be bypassed as indicated by the line 13 in Figure 1. This is particularly advantageous for the production of sheet products in fabric form. The oxidized tow produced by the heating and drawing and oxidation operations 12 and 14 as described above and marketed as 'PYRON' is employed in the further processing required to form sheet products. If the sheet product desired is to be in the form of fabric as in woven or knitted fabric, the 'PYRON' is supplied as an input tow to an oxidized spun yarn operation indicated at 15 and to be described more fully hereinafter with relation to Figure 2 of the drawings. However, if the sheet product to be produced is in paper form, the 'PYRON' tow is supplied directly to the input of the continuous line carbonizer 16. The line carbonizer 16 comprises an on-line, extended length furnace having a temperature profile which gradually increases from the input to the output end thereof and through which the 'PYRON' tow is passed continuously. The continuous line carbonizer 16 typically may be about 80 feet (24 metres) in length and is divided into four temperature zones whose temperatures gradually increase from about 370°C for the first zone at the entrance to the oven to 650°C for the second zone, 790°C for the third zone and the last zone going up to 1250°C as required for the particular carbonizing operation being conducted. The carbonization takes place in an inert gas atmosphere such as nitrogen or argon. The rate of travel of the tow through the line carbonizing furnace 16 is adjusted such that it is heat soaked at the elevated temperatures indicated for an overall period through all of the temperaure zones of about ten to twenty minutes. The operation is designed to achieve pyrolysis of the tow continuously passing through the furnace. Suitable continuous-line carbonizers for use as furnace 16 are commercially available. Figure 3 is a graph showing the volume electrical resistivity of a single carbon fibre filament as a function of heat treatment of the 'PYRON' tow in the carbonizing operation achieved in the line carbonizing furnace 16 as described above. The change in volume resistivity of a carbon fibre filament with increasing temperature previously has been reported in an East German publication entitled 'Plaste Und Kautschuk', Volume 27, No. 6, 1980, pages 309-313, Brehmer Pinnow and Ludwig - Published by the Institute of Polymer Chemistry - Academy of Sciences of the German Democratic Republic - Teltow - Seehof. In Figure 3, the maximum temperature within the furnace is plotted as the abcissa and the resistivity of the carbonized fibre filament is plotted as the ordinate in ohm centimetres on a logarithmic scale. As illustrated in Figure 3, the portion of the curve extending from 1250°C upward flattens out so that any change in resistivity induced in a single carbon fibre filament at the higher temperatures is negligible in comparison with the considerable increases in temperature required to drive them to the points in question. However, in that portion of the curve extending from about 670°C to 1250°C quite a wide range of electrical resistivities can be achieved for the single carbon fibre filament over this range of temperature values by appropriate selection of a temperature-time soaking period dependent upon the total denier of the incoming 'PYRON' tow. For example, an incoming 'PYRON' tow having a filament count of 320,000 times 1.5 dpf (1.65 dtex per filament) would require a residence time within the furnace of about fifteen to twenty minutes. An incoming 'PYRON' tow of 160,000 times 1.5 dpf would have a residence period of ten to twelve minutes. If it is desired to produce a carbon fibre paper sheet product having a preselected electrical surface resistance, the carbonized tow produced at the output of the line carbonizer 16 is supplied to a chopping apparatus 17 where the fibres are chopped into lengths which may extend from 0.125 to 1 inch (0.3 to 2.5 cm) but are preferably of the order of 0.25 inch (0.6 cm). Suitable chopping equipment for this purpose is sold commercially. The chopped fibres are supplied to a mixer 18 along with copious amounts of water to form a slurry whose composition, exclusive of the water, is about 83% by weight of the chopped partially carbonized fibres from the line carbonizer furnace 16, 14% by weight cellulose or other binder or dispersant fibres, 2% by weight polyvinyl alcohol (PVA) and the remainder a viscosity modifier resin. The mixer 18 thoroughly mixes all these constitutents into an extremely dilute slurry solution wherein the constitutents listed above consitute about 0.12% by weight of the overall slurry solution including the water. Before treating the slurry solution further as described hereafter, the pH of the solution is adjusted to a pH value of 8 to 9 by the addition of ammonia. Satisfactory mixers for use as mixer 18 are manufactured and sold commercially. From mixer 18, the dilute slurry is subjected to a wet paper formation process using equipment 19 such as that described in a textbook entitled 'Synthetic fibres and Paper Making' - edited by O. A. Battista and published by Interscience Publishers, a division of John Wylie and Sons, Inc., New York, N.Y. - copyrighted 1964 by John Wylie and Sons, Inc. - Library of Congress Catalog Card No. 64-13211. The wet paper processing equipment 19 produces wet paper stock that is transported by a conveyor 21 to a series of driers comprising heated cans 22 and then supplied to a take-up roll 23 for storage and subsequent use. Figure 4 is a characteristic cuvre showing the heat treatment temperature plotted as the abcissa and both the log of the resistivity in ohm centimetres for a single carbon fibre filament and the logarithm of the electrical surface resistance measured in ohms per square plotted as ordinates on scales indicated to the left in Figure 4. The surface resistances for two different weight carbon fibre sheet products produced according to the method illustrated in Figure 1 are shown plotted by a solid line curve A for a one half ounce per square yard (17 g / sq.m) sheet and a dash-dot curve B for a one ounce per square yard (34 g / sq.m) sheet, over the range of heat treatment temperatures shown. To produced a sheet product having a desired value surface resistance, the value of surface resistance obtained from either of the two curves A and B for the given weight sheet product there plotted, or a corresponding curve for any given weight carbonized fibre sheet product, the temperature to which the sheet product must be driven can be translated into a corresponding temperature that must be provided to the single fibre filament 'PYRON' tow passing through the line carbonizer 16. In this manner the carbonizing temperature of the line carbonizer 16 can be adjusted to provide a resulting carbon fibre paper sheet product having a preselected surface resistance. As mentioned earlier, the carbonized 'PYRON' tow appearing at the output of the line carbonizing furnace 16 and supplied to the fibre chopping apparatus 17 for use in the carbon paper sheet product formation operation, is in the form of a carbonized continuous filament tow called 'PANEX' tow. If desired, and provided that the initial starting PAN precursor material being used is in the form of a highly oriented 1.5 dpf (1.65 dtex) or less continuous filament yarn, then the 'PANEX' yarn output 24 from the line carbonizing furnace 16 will be in a form that may be woven into a carbon fibre fabric by a weaving operation shown at 25. The carbon fibre fabric sheet product weaving operation and the carbon fibre sheet product formation are mutually exclusive process steps due to the commonly used line carbonizer 16 so that only one or the other can be run at any particular time with the system shown in Figure 1. Should the demand for such products occasion the need, separate manufacturing process lines can be set up by providing separate input front ends comprising the precursor starting material source 11, the heating and drawing operations 12 (where required), the oxidation processing 14 and the separate line carbonizing furnace 16 for each of the respective production lines. At an earlier point in the description of Figure 1, it was indicated that the oxidized tow (known as 'PYRON') produced at the output of the oxidation operation 14 could be supplied either to the line carbonizer 16 as described above or, alternatively, it could be supplied to a spun yarn operation 15 shown in block diagram form in Figure 2. Referring to Figure 2, it will be seen that the stabilized 'PYRON' tow is first supplied to a stretch-break machine 31 which also is referred to as a tow-to-top converter in the art and is commercially available. In the stretch-break operation, the stabilized 'PYRON' tow is passed through a series of stretching rollers which are spaced apart gradually reducing distances and then broken into lengths of about 6 to 8 inches (15.2 to 20.3 cm). From the stretch-break operation 31 the discontinuous length filaments of stabilized tow are supplied to a sliver preparation operation 32 where they are joined together in large bundles of discontinuous filaments in an untwisted condition. Suitable machinery for performing the sliver preparation operations are available commercially. The sliver is then supplied to a roving machine 33 of a type commercially available where the sliver is processed into a roving. Roving produced at the output of the machine 33 is then supplied to a spinning machine 34 also commercially available. The spinning machine 34 converts the roving into a spun yarn produced at the output of the spinning machine 34. The spun yarn is supplied to a commercially available plying and twisting machine 35 where it is plied or twisted to prepare it for a weaving, knitting or other similar operation. The plied or twisted spun yarn then is supplied to a fabric weaving operation 36, or a knitting or other fabric forming operation which provides a fabric of desired characteristics for an intended end application. From the fabric forming operation, the fabric is then supplied to a batch carbonization treatment furnace 37 where the fabric is carbonized at an elevated temperature in a vacuum within the furnace whose temperature increases from ambient up to a selected temperature within the range of from 370 to 1250°C and then is force cooled back to ambient over a period of about three to four days. The fabric is soaked at the elevated temperatures pursuant to the temperature-surface resistance treatment schedule depicted in Figure 4 of the drawings in order to provide the fabric with a preselected electrical surface resistance. After carbonization in the above described manner, the output carbonized fabric sheet product is then accumulated as on a take-up roll shown at 38. The resulting carbon fibre sheet products formed either by the manufacturing steps illustrated and described with relation to Figure 1 or those shown in Figure 2 can be supplied with electrical surface resistances of any value within the range of values depicted in Figure 4. Because the temperature-surface resistance treatment schedule provides a substantially linearly changing electrical surface resistance for each incremental increase in temperature during the soak period, carbon fibre sheet products having precise and evenly distributed electrical surface resistiances can be manufactured in accordance with the invention.

Claims: A method of manufacturing a predetermined controlled surface resistance carbon fibre product from a carbonizable fibre material that has been pre-oxidized and pre-stabilized to effect molecular aromatic rearrangement of the fibres; which method comprises carbonizing the pre-oxidized and pre-stabilized fibre starting material by subjecting it to a heat soak treatment at a temperature lying within the range 370°C to 1250°C for a predetermined time in an oxygen-free atmosphere within a furnace to obtain incomplete carbonization of the fibres, and either before or after the carbonizing step, processing the fibres into a desired sheet product having the form of non-woven paper or woven or knitted fabric sheet products or mats, CHARACTERIZED BY HAVING a first step of establishing and recording a relationship between the maximum temperature during the heat soak treatment, the resistivity of the resulting individual fibres and the surface resistance of sheet products or mats made therefrom; and by having a second step of selecting from the recorded relationship the maximum temperature for the heat soak treatment corresponding to the desired surface resistance of the product. A method according to claim 1, wherein the furnace used for the carbonization has a continuously increasing temperature profile over the duration of the carbonization step. A method according to claim 2, wherein the furnace is a continuous line carbonizer divided into different temperature zones. A method according to claim 3, wherein the continuous line carbonizer is divided into four temperature zones having operating temperatures of about 370°C for the first zone, 650°C for the second zone, 790°C for the third zone and a temperature not exceeding 1250°C for the fourth zone, the temperature of the fourth zone being selected to control the electrical resistivity of the resulting partially carbonized fibres and thus the electrical surface resistance of the final sheet product. A method according to any preceding claim, wherein the carbonizable fibre starting material comprises polyacrylonitrile. A method according to any preceding claim, wherein the carbonizable fibre starting material is heated and drawn before the oxidation and stabilization step. A method according to any preceding claim, wherein the final sheet product is a carbon fibre paper and the fibres are processed into a paper after the partial carbonization step by chopping the carbonized fibre material into bundles of fine fibres having lengths of from 0.125 inch to 1 inch (0.3 to 2.5 cm); supplying the chopped carbonized fibres to a mixer for mixing with water to form a slurry of about 99.88% by weight water and 0.12% by weight of a mixture comprising about 83% by weight of the chopped carbonized fibres, 14% by weight of a binder or dispersant fibre such as cellulose, 2% by weight polyvinyl alcohol and the remainder a resin; adjusting the pH of the slurry to from 8 to 9 by the addition of ammonia; supplying the pH-adjusted slurry to a wet lay paper formation apparatus to form carbon fibre paper in wet sheet form; conveying the wet carbon fibre paper to a series of drying cans; and taking up the dried carbon fibre paper continously on a take-up roll for storage and use. A method according to any of claims 1 to 6, wherein the carbonizable fibre starting material is a continuous filament yarn of about 1.5 denier (1.65 dtex) per filament, and the fibres are processed into a woven fabric sheet product after the partial carbonization step by weaving the carbonized continuous filament yarn into fabric. A method according to any of claims 1 to 6, wherein the carbonizable fibre starting material is a tow: the oxidized and stabilized carbonizable fibre tow after the oxidation and stabilization step is formed into a sheet product by the steps of stretch-breaking the oxidized and stabilized tow, forming the resulting filaments into sliver comprising large numbers of discontinuous filaments in untwisted condition, converting the sliver into roving, spinning the roving into spun yarn, plying or twisting the spun yarn and weaving or knitting the plied or twisted spun yarn into fabric; and the partial carbonization treatment is performed on the fabric.

Inventors: BOYD, GEORGE PHILIP, JR.
Applicants: STACKPOLE FIBERS COMPANY INC.
Application year: 1987
Main classification: D01F9
IPC codes: D01F9, H05B3, D21H13