PROCESS FOR PRODUCING FIBER AGGREGATE

Abstract: A process for producing fiber aggregate with centripetally oriented fibers, comprising a dispersion step of dispersing fibers such as short fibers into a dielectric fluid; an orientation step of placing the dielectric fluid containing the fibers dispersed therein in a space between a core electrode and an annular peripheral electrode across which a high voltage is applied, whereby causing individual fibers in the dielectric fluid to statically orient, with one end pointing to either of the core electrode or peripheral electrode and the other end pointing to the other electrode, the core electrode having the electrode surface on the external circumference thereof and the peripheral electrode having the internal circumference opposite to the external circumference of the core electrode; and an aggregating step of aggregating the statically oriented fibers while keeping the oriented state.

Description: The present invention relates to a process for producing fiber aggregate, and more particularly, it relates to a process for producing fiber aggregate in which most fibers are centripetally oriented. Fiber aggregate of short fibers or whiskers has been produced by the following conventional methods. Japanese Patent Application Laid-Open No.65200/1985 discloses a centrifugal forming method which employs a centrifugal forming apparatus, as shown in Fig.7. According to this method, 7, an aqueous suspension of silicon carbide whiskers or the like is fed through a supply pipe 24 to a porous cylindrical vessel 23 which is lined with a filter film 25 and disposed in an outer cylinder 21. A hollow fiber aggregate 26 is formed by centrifugal action. Water is discharged form an outlet 22. A conventional suctional forming method employs a suctional forming apparatus, for example, shown in Fig. 8. According to this method, a prescribed amount of fiber-containing fluid 34 is fed to a cylinder 31, and pressure is applied to the fluid 34 by a plunger 32 arranged above a cylinder 31. At the same time, the filtrate is removed by suction through a filter 33 disposed at the bottom of the cylinder 31. Thus the fibers in the fluid are oriented and aggregated. Other conventional methods include a papermaking method and a spraying method. The fiber aggregate formed by the centrifugal method or suction method is not composed of centripetally oriented fibers, but is composed mainly of two- or three-dimensionally oriented fibers. The term "centripetally oriented" means that fibers are mostly oriented in the direction toward the axis (or an equivalent thereof) in the sectional plane perpendicular to the direction of the axis and also in the direction perpendicular to the direction of the axis in the longitudinal sectional plane including the axis and being parallel to the direction of the axis. This definition applies not only the fiber aggregate but also to the orientation step mentioned later. The fiber aggregate produced by the conventional methods has a disadvantage that it does not provide a sufficient strength in the desired one-dimensional direction when incorporated into fiber-reinforced metal (referred to as FRM hereinafter). Additional disadvantages are the low volume ratio of fiber and the excessive spring back at the time of compression molding. According to the conventional methods, it was impossible to produce centripetally oriented fiber aggregate and it was only possible to produce two- or three-dimensionally oriented fiber aggregate. The present invention was completed to overcome the above-mentioned disadvantages. It is an object of the present invention to provide a process for producing fiber aggregate in which most fibers are centripetally oriented. The fiber aggregate produced by the process of this invention has a high fiber volume ratio and a low degree of spring back. When incorporated into FRM, it provides FRM having a high strength in the desired one dimension. A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Fig.1 is a schematic longitudinal sectional view illustrating the process for producing fiber aggregate in Example 1, the process including the step of filtering the dielectric fluid through a porous filter; Fig.2 is a schematic sectional view illustrating the production process shown in Fig.1; Fig.3 is an illustrative sectional view showing the process for producing fiber aggregate with a peripheral electrode composed of a plurality of plates disposed at certain intervals in the circumferential direction; Fig.4 is an illustrative sectional view showing the process for producing fiber aggregate with a core electrode of octagonal rodlike shape and a peripheral electrode of octagonal cylindrical shape; Fig.5 is a partly perspective view showing a portion of the longitudinal sectional plane containig the axis; Fig.6 is a longitudinal sectional view showing the fiber aggregate produced Example 1; Fig.7 is a partly cutaway sectional view of the conventional centrifugal forming apparatus; Fig.8 is an illustrative sectional view of the conventional suctional forming appratus. The process of the present invention for producing fiber aggregate with most fibers centripetally oriented comprises: a dispersion step of dispersing fibers in the form of short fibers, whiskers, or a mixture thereof into a dielectric fluid; an orientation step of placing the dielectric fluid containing said fibers dispersed therein in a space between a core electrode and an annular peripheral electrode across which a high voltage is applied, whereby causing individual fibers in the dielectric fluid to statically orient, with one end pointing to either of the two electrodes and the other end pointing to the other electrode, the core electrode having the electrode surface on the external circumference thereof and the peripheral electrode having the internal circumference opposite to the external circumference of the core electrode; and an aggregating step of aggregating the statically oriented fibers while keeping the oriented state, wherein the aggregating step is performed by filtering the dielectric fluid containing the fibers which have been oriented in the orientation step, in the direction perpendicular to the direction of the orientation of the fibers so that the oriented fibers are collected on a filter disposed over the filtration plane, whereby producing fiber aggregate in which most of fibers are oriented in the centripetal direction. The fibers used in the dispersion step are short fibers, whiskers or a mixture thereof. Short fibers and whiskers of any kind can be used. They are not specifically limited in diameter and length. Also, they are not limited in material so long as they are capable of static orientation in the dielectric fluid when a high voltage is applied across the positive and negative electrodes. The material of the fiber includes, for example, alumina, silica, alumina-silica, beryllia, carbon, silicon carbide, glass, and metals. Either fibers of single material or a mixture of fibers of different materials may be used. The dielectric fluid means a fluid which exhibits the dielectric properties upon application of a high voltage. Examples of the dielectric fluid include carbon tetrachloride, fluorine- and chlorine-substituted hydrocarbon, n-hexane, and cyclohexane. Preferable among them is carbon tetrachloride. Fluorine- and chlorine-substituted hydrocarbons are preferable from the standpoint of handling safety. Fibers of some kind or state may need surface treatment to loosen fibers sticking together. To facilitate the dispersion of fibers, a proper amount of surface active agent, especially a nonionic surface active agent should be added to the dielectric fluid. The orientation step is accomplished by placing a dielectric fluid containing fibers dispersed therein in a space between a core electrode and a peripheral electrode across which a high voltage is applied, whereby causing individual fibers in the dielectric fluid to statically orient, with one end pointing to one electrode and the other end pointing to the other electrode. (This orientation state is referred to as the centripetally oriented state.) One of the features of the present invention resides in the shape of the electrode employed. The core electrode has such a shape that its external circumference functions as the electrode surface. This electrode may be of cylindrical shape or rodlike shape. The cylinder may be round or octagonal (as shown in Fig. 4), and the rod may has a round or octagonal cross section. Their external circumferential configuration should be selected according to the internal circumferential configuration of the desired fiber aggregate. In addition, a proper selection should be made for the outside diameter and length of the core electrode and the inside diameter and wall thickness of the cylinder, according to the object and application of the fiber aggregate. Moreover, the core electrode may be constructed of a plurality of plates annularly disposed at certain intervals. The peripheral electrode is of annular shape and it has internal circumference opposite to the external circumference of the core electrode. The shape of the peripheral electrode should be properly selected according to the shape or the like of the external circumference of the desired fiber aggregate. For example, the peripheral electrode may be a cylindrical one disposed outside the core electrode or an octagonal cylinder disposed outside the core electrode as shown in Fig. 4. Alternatively, the peripheral electrode may be composed of a plurality of plates (9a) or rods disposed at certain intervals in the circumferential direction outside the core electrode (8a), as shown in Fig. 3. The electrodes 8a and 9a cause most fibers to centripetally orient at the addendums but not at the dedendums as shown in Fig. 3. Therefore, in the case of fiber aggregate produced by using such electrodes, fibers are centripetally oriented at the addendums but not at the dedendums. Such fiber aggregate can be advantageously applied to the production of FRM gears which have a high strength at the addendums and a high resistance to flexural stress at the dedendums. The shape of the core electrode and peripheral electrode and the disposition of the electrodes should be properly selected according to the shape of fiber aggregate to be produced. For examples the two electrodes may be disposed coaxially or non-coaxially. In the orientation step, a high voltage is applied across the two electrodes. The voltage may be DC or AC; usually a DC voltage is used. Where a DC voltage is applied, either of the two electrodes may be a positive electrode. In the orientation step, usually an electric field of about 0.5 to 5 kV/cm is generated between the two electrodes. An electric field weaker than 0.2 kV/cm is not enough for the static orientation of fibers; and an electric field stronger than 10 kV/cm disturbs the dielectric fluid and interferes with the orientation of fibers. Preferred electric field is about 1 to 2 kV/cm. It is suitable for static orientation of fibers with a minimum disturbance of the dielectric fluid. The intensity of electric field should be properly established according to the dielectric properties of the fibers and dielectric fluid to be used and the thickness of the fiber aggregate to be produced and the like. The individual fibers which have been statically oriented as mentioned above are mostly strung to one another in the centripetal direction and the stringing fibers settle downward. The stringing fibers settle faster than discrete fibers. In the third step of the process of the invention, the statically oriented fibers are aggregated while keeping the oriented state to make fiber aggregate. The aggregating step can be accomplished by permitting the oriented fibers to spontaneously settle, with the drain cock 62 on the drain pipe 61 kept closed as shown in Fig. 1. In the case where an apparatus as shown in Fig. 1 is used, the aggregating step is performed by keeping the drain cock 62 on the outlet 6 open, so that the dielectric fluid containig the oriented fibers is filtered in the direction perpendicular to the orientation direction of the fiber. Thus the oriented fibers 1a are collected on the filter 10. This aggregating method by filtration shortens the time for aggregation. In addition, the filtration may be promoted by applying suction or vacuum. If the spontaneous settling or filtration is performed with the filter disposed all over the filtration plane, it is possible to discharge the dielectric fluid with a minimum of disturbance. Thus the resulting fiber aggregate maintains the centripetal orientation undisturbed. Incidentally, the filter may be made of porous ceramics or the like. The above-mentioned dispersing step, orientation step, and aggregatinhg step may be accomplished continuously. The fiber aggregate formed by the aggregating step may have a variety of shape according to the desired shape of the two electrodes. For examples, the fiber aggregate may have a comparatively large cylindrical shape, or the same height of comparatively small mat shape, or else, much thinner film shape. The centripetally oriented fiber aggregate thus obtained is discharged from the appratus, and it is used as such or after cutting or piling to a desired shape, as the reinforcement for FRM. The apparatus used for the process of the present invention is schematically shown in Fig. 1. It is made up of an orientation vessel 7 including a supply part 4 placed above which receives a dielectric fluid 2 in which are dispersed fibers 1 such as short fibers and supplies the dielectric fluid downward, a discharge part 6 which discharges the dielectric fluid 6 downward, and an orientation part 5 through which the dielectric fluid moves downward from the supply part 4 to the discharge part 6; a core electrode 8 and an annular peripheral electrode 9 installed in the orientation part 5 of the orientation vessel 7, the core electrode 8 having its external circumference as the electrode surface and the peripheral electrode 9 having the internal circumference opposite to the external circumference of the core electrode 8; and a high voltage source unit (not shown) to apply a high voltage across the electrodes 8 and 9. The appratus may be constructed such that a supply unit for the fiber-dispersed dielectric fluid is disposed above the supply part 4. The process of the present invention for producing fiber aggregate includes placing a dielectric fluid containing fibers dispersed therein in a space between a core electrode and an annular peripheral electrode across which a high voltage is applied, the core electrode having the electrode surface on the external circumference thereof and the peripheral electrode having the internal circumference opposite to the external circumference of said core electrode; whereby causing individual fibers in the dielectric fluid to statically orient, with one end pointing to one electrode and the other end pointing to the other electrode, and an aggregating step of aggregating the statically oriented fibers while keeping the oriented state. Therefore, the process provides fiber aggregate in which most fibers are oriented in the centripetal direction. Thus it is possible to produce FRM having an extremely high strength in a certain direction with the fiber aggregate. The fiber aggregate especially produced from short fibers, whiskers or mixtures thereof according to the process of the present invention provides FRM having the same properties as those of FRM reinforced with long fibers. Short fibers are less expensive than long fibers, and whiskers provide FRM of higher strength. The fiber aggregate obtained according to the process of the present invention is oriented such that fibers are mostly oriented centripetally, namely in the direction toward the axis in the sectional plane perpendicular to the direction of the axis and also in the direction perpendicular to the direction of the axis in the longitudinal sectional plane including the axis and being parallel to the direction of the axis. The fiber aggregate thus obtained has a minimum of fiber entanglement and consequently has a high fiber volume ratio. Such fiber aggregate provides FRM having a high strength. Furthermore, the fiber aggregate produced according to the process of the present invention has a low degree of spring back because it contains only few fiber entanglements. Such fiber aggregate provides FRM having a high precision. Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. The invention is now described with reference to the following examples. (Example 1) The static orientation apparatus as shown in Fig. 1 was made ready. It is made up of an orientation vessel 7 including a supply part 4 placed above which receives a dielectric fluid 2 in which are dispersed fibers 1 such as short fibers and supplies the dielectric fluid downward, a discharge part 6 which discharges the dielectric fluid 6 downward, and an orientation part 5 through which the dielectric fluid moves downward from the supply part 4 to the discharge part 6; a cylindrical core electrode (negative electrode) 8 and a peripheral electrode (positive electrode) 9, which is disposed coaxially with the core electrode 8 and outside the external circumference of the core electrode 8 and has an inside diameter greater than the outside diameter of the core electrode 8; and a high voltage source unit (not shown) to apply a high voltage across the electrodes 8 and 9. The distance between the two electrodes is 20 mm. Alumina short fibers (having an average diameter of about 3 µm and a length of 10 to 500 µm) without surface treatment were dispersed by stirring together with a small amount of nonionic surface active agent into carbon tetrachloride as the dielectric fluid. Carbon tetrachloride was placed in the space between the electrodes in the static orientation apparatus. A DC voltage of about 1 kV was applied across the electrodes. The dielectric fluid 2 into which the fibers 1 were dispersed was slowly poured into the supply part 4 of the apparatus from the beaker. The fibers 1 thus supplied underwent induction polarization in the dielectric fluid 2 and static orientation, with one end of the fiber pointing to the core electrode 8 and the other end pointing to the peripheral electrode 9. The statically oriented fibers 1a became strung while they were settling, and the strung fibers settled in the centripetally oriented state in the direction across the positive and negative electrodes. With the strung fibers 1a kept in oriented state, the drain cock 62 on the drain pipe 61 was opened to discharge the dielectric fluid through the filter 10. In this way, the fibers were aggregated on the filter 10 in the centripetally oriented state. The above-mentioned dispersion step, orientation step, and aggregating step were carried out consecutively until the fiber aggregate reached a desired thickness. The dielectric fluid remainig in the apparatus was removed through the drain pipe 61. Thus there was obtained the fiber aggregate 3. The resulting fiber aggregate has a cylindrical shape as shown in Fig. 5, in which most fibers are centripetally oriented, in other words, most fibers are oriented in the direction toward the axis in the plane of cross-section perpendicular to the axis and in the direction perpendicular to the axis in the cross section including the axis and parallel to the axis. The fiber volume ratio is greater in the internal circumference than in the external circumference. In the above-mentioned procedure, the dielectric fluid of the about uniform concentration was fed about uniformly to the supply part 4; however, for example, in an alternative procedure the dielectric fluid of higher concentration is fed to the outer side of the supply part 4 so that the fiber aggregate has about a uniform fiber volume ratio. In this example, the centripetally oriented strung fibers settle and aggregate as shown in Figs. 1 and 2 and the dielectric fluid is discharged through the filter 10 disposed all over the bottom of the discharge part 6 of the orientation part 5. Thus no turbulence occurs at the time of filtration. Therefore, in this example, it is possible to produce fiber aggregate in which most fibers are centripetally oriented. The fiber aggregate has a greater fiber volume ratio in its internal part. Therefore, it provides FRM having an internal part with improved abrasion resistance. Such FRM will be conveniently used as a bearing and the like. The fiber aggregate in this example may be combined with fiber aggregate in which fibers are randomly oriented in two dimensions, for the production of FRM 11 as shown in Fig. 6. (The former is disposed inside and the latter outside.) The resulting FRM has the outside part 11b reinforced against centrifugal stress and the inside part 11a superior in abrasion resistance. It is useful as the outer ring running at a high speed. (Example 2) Centripetally oriented fiber aggregate was produced in the same manner as in Example 1, except that (1) a high AC voltage was applied across the two electrodes, (2) the dielectric fluid was freon (C Cl F (Du Pont CO., LTD.)), and (3) the voltage applied was about 0.8 kV. (The distance between the two electrodes was 20 mm which is the same as in Example 1.) The above-mentioned voltage was experimentally selected from the range of about 0.25 to 1.5 kV for the appropriate static orientation and the prevention of convection. As in Example 1, there was obtained fiber aggregate in which most fibers are centripetally oriented.

Claims: A process for producing fiber aggregate comprising: a dispersion step of dispersing fibers in the form of short fibers, whiskers, or a mixture thereof into a dielectric fluid; an orientation step of placing said dielectric fluid containing said fibers dispersed therein in a space between a core electrode and an annular peripheral electrode across which a high voltage is applied, whereby causing individual fibers in the dielectric fluid to statically orient, with one end pointing to either of the core electrode or peripheral electrode and the other end pointing to the other electrode, said core electrode having the electrode surface on the external circumference thereof and said peripheral electrode having the internal circumference opposite to the external circumference of said core electrode; and an aggregating step of aggregating the statically oriented fibers while keeping the oriented state wherein the aggregating step is performed by filtering the dielectric fluid containing the fibers which have been oriented in the orientation step, in the direction perpendicular to the direction of the orientation of the fibers so that the oriented fibers are collected on a filter disposed over the filtration plane, thereby producing fiber aggregate in which said fibers are mostly oriented in the centripetal direction. A process for producing fiber aggregate as claimed in Claim 1, wherein the orientation step is performed by applying a DC high voltage across the core electrode and peripheral electrode, either of which is a positive electrode and the other is a negative electrode. A process for producing fiber aggregate as claimed in Claim 1, wherein both of the core electrode and peripheral electrode are of cylindrical shape. A process for producing fiber aggregate as claimed in Claim 1, wherein the core electrode is of cylindrical shape and the peripheral electrode is composed of a plurality of plates disposed at certain intervals in the circumferential direction of the peripheral electrode. A process for producing fiber aggregate as claimed in Claim 1, wherein the fiber is one which is made of alumina, silica, alumina-silica, beryllia, carbon, silicon carbide, glass, or metal. A process for producing fiber aggregate as claimed in Claim 1, wherein the dielectric fluid is carbon tetrachloride, fluorine- and chlorine-substituted hydrocarbon, n-hexane, or cyclohexane.

Inventors: GOMI, FUKUO C/O K.K. TOYODA JIDOSHOKKI SEISAKUSHO, ISOMURA, RENICHI K.K. TOYODA JIDOSHOKKI SEISAKUSHO, ITO, TOMOHITO K.K. TOYODA JIDOSHOKKI SEISAKUSHO
Applicants: KABUSHIKI KAISHA  TOYODA JIDOSHOKKI SEISAKUSHO
Application year: 1987
Main classification: B22F3
IPC codes: B22F3, C22C47