DE69421169T2 - Composite fibers made from potassium hexatitanate and titanium dioxide - Google Patents

Description

Composite fibers made of potassium hexatitanate and titanium dioxide

The present invention relates to composite fibers of potassium hexatitanate crystals and titanium dioxide crystals having excellent characteristics in terms of abrasion resistance, heat resistance, heat insulation, heat resistance properties and reinforcing properties, and is suitable for friction materials for braking devices of automobiles, railroad cars, aircraft, industrial machines, etc.

BACKGROUND OF THE INVENTION

Friction materials containing chrysotile asbestos fibers have been conventionally used for braking devices. However, the friction material containing chrysotile asbestos has low thermal resistance and its friction coefficient decreases rapidly at relatively low temperatures, causing fading, while the friction material wears out significantly at high temperatures. Thus, there has been a desire to solve these problems. In addition, there is a tendency to restrict the use of asbestos due to its carcinogenic property from the standpoint of environmental hygiene, so there is a strong demand for the development of substitutes for chrysotile asbestos fibers.

In order to meet the demand, attempts have been made to use potassium titanate fibers represented by the general formula K₂O nTiO₂ in various fields for technical applications, of which potassium hexatitanate fiber (K₂Ti₆O₁₃), potassium tetratitanate fiber (K₂Ti₄O₉) and potassium octatitanate fiber (K₂TigO₁₇) are typical as substitutes for asbestos fibers. Among them, potassium hexatitanate fiber is effective in preventing fading due to its high heat resistance, since it has a Mohs hardness of about 4, is therefore less likely to cause abrasion to the adjacent materials and is therefore suitable for preventing accidental brake operation, for example, since it has a lower It has hygroscopicity and does not react with water. Due to these properties, this fiber has been proposed for use in friction materials for automobile brake pads, etc. (Unexamined Japanese Patent Publications SHO 61-191599 and HEI 1-294553).

However, it has been found that although the friction material containing the potassium hexatitanate fibers has an increased abrasion resistance, the friction coefficient thereof at low temperatures (up to about 250ºC) is rather lower than that of the friction material containing the chrysotile asbestos fibers. With the increase in the speed of vehicles, a higher friction coefficient of the friction material for braking devices is required in recent years, and it is desired to provide friction materials which maintain a high friction coefficient over a low to high wide temperature range.

US-A-3 779 784 describes a composite fiber with a core made of a single crystal K₂Ti₆O₁₃ which is enclosed by a cladding made of TiO₂.

JP-A-04 321 517 describes polycrystalline potassium hexatitanate particles with an average diameter of 1-100 µm and a thickness of 0.01-2 µm and their production.

JP-A-50 09 462 describes a friction material obtained by binding potassium titanate fibers with an inorganic or organic binder and molding the mixture.

The present invention provides a fiber material which satisfies the above requirement, i.e., a polycrystalline fiber having a composite phase or structure consisting of potassium hexatitanate crystals and titanium dioxide crystals mixed therewith.

Incidentally, rutile priderite-potassium hexatitanate composite fiber is known as a polycrystalline fiber having a composite structure comprising potassium hexatitanate crystals and other crystals (unexamined Japanese Patent Publications SHO 60-34617 and SHO 60-259627), whereas This composite fiber differs from the composite fiber of the present invention in that it contains priderite. The composite fiber containing priderite not only exhibits lower strength but also has a problem that it turns completely brown.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method for producing a composite fiber of polycrystalline structure according to claim 1, a composite fiber of polycrystalline structure obtained by this method according to claim 3, and a friction material according to claim 6.

The composite fiber of the present invention is a polycrystalline fiber having a composite phase in which potassium hexatitanate crystals and titanium dioxide crystals exist in a mixed state. The composite phase is a structure obtained by melt-making processes in which, when potassium is removed from a mass or lump of potassium dititanate fibers while loosening the lump in a washing liquid, K+ ions are dissolved out until the potassium content of the fibers decreases to 1 to 13.5 wt%, causing the titanium content of the fibers to exceed the titanium content of potassium hexatitanate, potassium hexatitanate crystals are formed by a heat treatment, and the excess titanium simultaneously forms into titanium dioxide crystals.

When composite fibers of the present invention are used for a friction material for braking devices, it is desirable that the composite fibers be included in an amount of 3 to 50% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 to 4 are photomicrographs showing composite fibers of the invention;

Fig. 5 is a photomicrograph showing a conventional single-phase fiber made of Ka lium hexatitanate;

Figure 6 is a graph showing the measurements of the coefficient of friction obtained for disk pad samples by a constant speed friction and abrasion test;

Figure 7 is a graph showing the wear rate measurements obtained for the disk pad samples by the test;

Figure 8 is a graph showing the measurements of the coefficient of friction obtained for disk pad samples by a constant speed friction and abrasion test;

and Fig. 9 is a graph showing the measurements of the wear rate obtained for the disk pad samples by the test.

DETAILED DESCRIPTION OF THE INVENTION

The composite fiber of the present invention is characterized in that the fiber is a polycrystalline fiber having a composite structure consisting essentially of potassium hexatitanate crystals (K₂Ti₆O₁₃) and titanium dioxide crystals (TiO₂) which are in a mixed state. An advantageous feature of the polycrystalline composite structure of the fiber according to the present invention can be obtained by a melting process.

The raw material used in the melting process is a mixture of a titanium compound convertible to titania or titanium dioxide (TiO₂) when heated and a potassium compound convertible to potassium oxide (K₂O) when heated in a ratio to thereby obtain a TiO₂/K₂O molar ratio of 1.5 to 2.5. The melting process comprises the steps of heating the mixture to about 950 to about 1100°C to produce a melt, cooling the melt to obtain a lump of potassium dititanate fibers, treating the fiber clump in a washing liquid to extract K+ ions until the potassium content of the fibers decreases to 1-13.5 wt.% (this percentage is based on fibers in an anhydrous state) while loosening the fibers, and heat treating the resulting fibers at a temperature of 900 to 1300ºC.

The titanium compound used as one of the raw materials is a purified anatase, a purified rutile or the like. The reason is that since naturally occurring anatase sand or rutile sand contains large amounts of impurity elements (Fe, Si, Cr, Al, Zr, V and the like), numerous cracks may occur in the fibers by the heat treatment and it is difficult to keep the fibers at the appropriate size. Consequently, it is desirable to use purified anatase or purified rutile with a relatively high purity (at least about 98%). The potassium compound mixed with the titanium compound is typically potassium carbonate (K2CO3). Potassium hydroxide, nitrate, etc. are also usable.

The mixing ratio of the titanium compound and the potassium compound is 1.5 to 2.5 in terms of the TiO₂/K₂O molar ratio in order to cause the melt thereof to form fibers of potassium dititanate crystals (K₂Ti₂O₅) as the primary phase (the phase appearing at the beginning) during cooling-solidification processing. The resulting lump of potassium dititanate fibers can be freed of potassium and loosened relatively easily due to its crystal structure. From the standpoint of easier dispersion in water, the molar ratio is preferably 1.7 to 2.0.

The fiber lump obtained by cooling the molten mixture for solidification is a bundle-shaped aggregate of potassium dititanate fibers grown as primary phase fibers along a temperature gradient in the cooling-solidification process.

The amount of K+ ions to be dissolved out by the potassium removal-loosening processing is adjusted so that the potassium content of the fibers is 1 to 13.5 wt.%. The TiO₂/K₂O molar ratio of the fibers (hydrated potassium titanate) is greater than 6. The amount of K+ ions to be dissolved out is adjusted in this way because no titanium dioxide crystals formed by heat treatment if this is lower.

Although water or hot water is usable as the washing liquid for the potassium removal treatment, it is desirable to use an aqueous acid solution adjusted to an appropriate concentration such as 0.01 to 1% aqueous solution of sulfuric acid, 0.01 to 1% aqueous solution of hydrochloric acid or 0.1 to 2% aqueous solution of acetic acid to ensure efficient treatment. If necessary, the treatment is carried out with stirring using a propeller, mixer or the like. The amount of K+ ions to be extracted is adjustable, for example, by using a different washing liquid or by varying the amount of the washing liquid, the stirring intensity or the treatment time.

The fibers taken out of the washing liquid (polycrystalline fibers of hydrated potassium titanate) have a chemical composition corresponding to a mixed phase of potassium hexatitanate crystals and titanium dioxide crystals, but retain certain characteristics of the precursor, i.e. potassium dititanate crystals, in terms of their crystalline structure, so that the fibers are heat treated and thereby formed into potassium hexatitanate crystals and titanium dioxide crystals.

The heat treatment is accomplished by keeping the fibers at a temperature of 900 to 1300ºC as mentioned above. The lower limit of the treatment temperature is 900ºC because if the temperature is lower (especially not higher than 800ºC), the obtained composite fiber has poor crystallinity and is chemically unstable. The upper limit is 1300ºC to avoid melting of the potassium hexatitanate crystals.

When the heat treatment is carried out at lower temperatures (up to about 970ºC) in the above range, the form of titanium dioxide is anatase, while when the heat treatment is carried out at higher temperatures (at least about 1050ºC) in the above range, the form of titanium dioxide is rutile. When heat treated in a medium temperature range (about 1000ºC), the form of titanium dioxide is a mixed phase of anatase and rutile.

Although the transformation of the crystal structure by the heat treatment in the above-mentioned temperature range can be carried out for several minutes, the treatment time affects the densification of crystal grains as well as the fiber strength, so that the heat treatment is carried out for at least 10 minutes, preferably 0.5 to 3 hours, thereby achieving increased fiber strength.

As previously mentioned, according to the manufacturing process described, the heat treatment (for converting the crystal structure of fibers) provides for the formation of a polycrystalline composite structure in which potassium hexatitanate crystals are in a state of being mixed with titanium dioxide crystals. The crystallization of titanium dioxide is due to the potassium removal-loosening process that precedes the heat treatment, by which K+ ions are dissolved out until the potassium content of the fibers has decreased to 13.5% by weight or less. It should be noted that this percentage is based on fibers in an anhydrous state. When the chemical composition of the fibers (hydrated potassium titanate crystals) obtained after K+ ions are dissolved out from the primary phase fiber lump (potassium dititanate crystals) is expressed as TiO2/K2O molar ratio, these fibers containing up to 13.5 wt% of potassium are expressed as "TiO2/K2O > 6" and thus exceed potassium hexatitanate fibers (TiO2/K2O = 6) in titanium content. Consequently, potassium hexatitanate crystals are formed by the heat treatment, and the excess titanium is also crystallized into titanium dioxide crystals, so that the composite phase of potassium hexatitanate crystals and titanium dioxide crystals mixed therewith is formed. Depending on the temperature of the heat treatment, the titanium dioxide crystals are formed into a rutile phase or anatase phase.

The proportion of titanium dioxide crystals in the potassium hexatitanate crystal-titanium dioxide crystal composite fiber, although not strictly dependent on the required characteristics of the fiber or its use, is preferably at least 1 vol% (about 6 mol%) in order for the fiber to fully exhibit the effect of its mixed phase.

The composite fiber of the present invention is produced in the form of a plate-shaped polycrystalline fiber having a diameter of about 20 to about 50 µm and a length of about 100 to about 400 µm, although the fiber size depends on the structural conditions, particularly on the treatment conditions of the potassium removal-loosening process for the primary phase fiber lump.

As mentioned above, the titanium dioxide crystals exist as rutile phase, anatase phase or mixed phases thereof with a specific crystal structure depending on the potassium content of the fibers or the heat treatment temperature. The rutile phase has a higher hardness than the anatase phase (rutile phase about 7.0 to 7.5, anatase phase about 5.5 to 6.0 Mohs hardness), so that the composite fiber exhibits a higher abrasion resistance when it contains titanium dioxide crystals as rutile phase than when it contains the titanium dioxide crystals as anatase phase.

Compared with the single phase fiber of potassium hexatitanate, the polycrystalline composite fiber of the present invention consisting of potassium hexatitanate crystals and titanium dioxide crystals has higher abrasion resistance and friction coefficients and improved strength than the effects of a two-phase mixture. In addition, the two-phase mixture imparts minute protrusions or recesses to the fiber surface, which provide increased bonding strength at the interface when the fiber is bonded to other material.

A friction material in which the composite fibers of potassium hexatitanate and titanium dioxide having the above-mentioned characteristics are incorporated therein has a higher friction coefficient than the friction material using the single-phase fiber of potassium hexatitanate. The composite fiber has a high thermal stability of its friction coefficient and maintains the high friction coefficient over a high to low, wide temperature range. At low to high temperatures, the friction material of the present invention also has a higher abrasion resistance than the friction material containing chrysotile asbestos fiber.

The composite fiber of the present invention is of polycrystalline structure and therefore has local voids at the interfaces of the crystal grains. The voids have a dampening function during use, so that the friction material comprising the present composite fiber has the additional advantage of abrasion of the adjacent materials to a lesser extent.

When the composite fiber of the present invention is used for friction materials, it is desirable that the friction material contains the composite fiber in an amount of 3 to 50% by weight. The lower limit of the content is 3% by weight because if it is present in smaller amounts, the fiber cannot fully exhibit its effect, while the upper limit is 50% by weight because if this limit is exceeded, the effect of imparting improved friction and abrasion characteristics is almost canceled out.

The friction material of the present invention may further include at least one of various fibrous fillers known as fibers used for friction materials, if necessary, in addition to the composite fiber of potassium hexatitanate and titanium dioxide. Examples of such fillers are synthetic organic fibers (aramid fiber, etc.), metal fibers (steel fiber, stainless steel fiber, copper fiber, brass fiber, etc.), carbon fiber, glass fiber, alumina-silica fiber, silica fiber, rock wool, slag wool, and wood pulp. These fillers are used in an appropriate amount (e.g., 1 to 60% by weight) together with the composite fiber of potassium hexatitanate and titanium dioxide, for example, for the purpose of reinforcing the friction material.

If necessary, these fiber fillers are used in surface-treated form with a coupling agent, for example to obtain improved dispersibility and adhesion to the binder. Examples of useful coupling agents are silane coupling agents (such as vinylsilane, aminosilane, epoxysilane, methacryloxysilane and mercaptoxysilane) and titanate coupling agents (such as isopropyl tristearoyl titanate and di(dioctyl pyrophosphate)ethylene titanate).

The friction material of the present invention may further contain various additives which are generally used as friction and abrasion modifiers. Examples of useful additives are organic powders such as vulcanized or unvulcanized, natural or synthetic rubber powders, cashew resin particles or granules, resin dust and rubber dust; inorganic powders such as natural or artificial graphite, molybdenum disulfide, antimony trisulfide, barium sulfate, calcium carbonate and xonotlite; flaky inorganic fillers such as mica and vermiculite; metallic powders such as copper, aluminum, zinc, iron and stainless steel powders; metal oxide powders such as alumina, silica, chromium oxide, titanium oxide, iron oxide, antimony trioxide, copper oxide powders; etc. These additives are used individually or at least two of them are used in combination in an appropriate amount (e.g., 20 to 80% by weight) according to the required friction characteristics of the product such as friction coefficient, abrasion resistance and vibration characteristics. Also usable are corrosion inhibitors, abrasives and similar aids in an appropriate amount (e.g., up to 60% by weight) if required. Therefore, the friction material is prepared in the form of a composition containing such additives or agents, as is the case with conventional friction materials.

Examples of suitable binders are those used for friction materials of the type described, such as resins containing epoxy resin, phenolic resin, formaldehyde resin, polyester resin, polyamide resin, alkyd resin, silicone resin, polyimide resin and similar thermosetting resins, as well as these resins in their modified form; and rubbers, including natural rubber, styrene-butadiene rubber and nitrile rubber.

Like conventional friction materials, the friction material of the present invention can be prepared by dispersing the fibers in a resin serving as a binder to form a starting composition with friction and abrasion modifiers and other additives optionally included therein, and molding the composition with a die or the like under heating and applying pressure to solidify it. Alternatively, the friction material can be prepared by dispersing the starting composition, e.g., in water, positioning the dispersion over a screen, molding the dispersion into a sheet by squeezing out water, and pressing the sheet under heating at an elevated pressure for solidification. The product thus formed or designed can be machined or mechanically processed or ground in a suitable manner.

EXAMPLES Production of fibres (1) Composite fiber of potassium hexatitanate and titanium dioxide (examples of the invention) a. Production of raw material

Purified anatase powder (99.8% in purity) and potassium carbonate powder (99.5% in purity) for industrial use were mixed together in a ratio giving a TiO₂/ K₂O molar ratio of 1.8.

b. Heating to melt

The raw material was placed in a platinum crucible and melted by heating to 1050ºC for 40 minutes.

c. Cooling to solidify

The melt was poured into a bowl-shaped container (made of copper) to obtain a lump of potassium dititanate fibers.

d. Potassium removal loosening treatment

The fiber lump was placed in an aqueous solution of sulfuric acid, which was prepared by adding sulfuric acid to water in an amount 150 times (by weight) of the fiber lump. K+ ions were caused to dissolve out of the fibers by stirring with a propeller while loosening the fibers.

e. Heat treatment

The loosened fibers were removed from the solution, dewatered, dried (in air) and then heat-treated in an alumina crucible for two hours.

(2) Single phase fiber of potassium hexatitanate (comparative example)

A plate-shaped, polycrystalline fiber of potassium hexatitanate single phase was prepared under the same treatment conditions as above, except that the aqueous sulfuric acid solution for the potassium removal-loosening treatment was replaced by water (150 times the amount of the fiber lump).

Table 1 shows the concentration of the acid solution, the heat treatment temperature, the crystal phase of the final fiber product, etc. according to the examples of the invention and the comparative example. Table 1

Annotation:

6 TK: potassium hexatitanate crystals

(A) : Anatase phase of titanium dioxide crystals

(R) : Rutile phase of titanium dioxide crystals

(*): The K content of the fibres was determined the heat treatment and indicates the value in the fibres in their water-free state.

Nos. 1 to 6 are examples of the invention, and No. 11 is a comparative example. Each of the fiber final products is in the form of a plate-shaped polycrystalline fiber measuring about 30 µm in average fiber diameter and about 200 µm in average fiber length (determined by reference to photographs by the scanning electron microscope).

Figs. 1 to 5 show images (magnification: 2000) under a scanning electron microscope of the fiber end products obtained in the respective examples (6TK: potassium hexatitanate crystals, T: titanium dioxide crystals).

Figs. 1, 2, 3 and 4 show the composite fibers of Examples No. 1, No. 3, No. 4 and No. 5 of the invention, respectively, and Fig. 5 shows the single-phase potassium hexatitanate fiber of Comparative Example No. 11.

Although the final fibers of the examples of the invention and the comparative example are not particularly different in shape or size, the fiber of the comparative example is a single phase fiber of only potassium hexatitanate crystals, whereas those of the examples of the invention have a composite phase comprising potassium hexatitanate crystals and titanium dioxide crystals mixed with the aforementioned crystals in a dispersed state.

Reference is further made to Figures 1 to 4 which show the fibers of the invention. A comparison between Figures 2, 3 and 4 (the potassium-removed fibers shown all have a potassium content of 9% while the heat treatment temperature is 950°C in Figure 2, 1050°C in Figure 3 or 1150°C in Figure 4) shows that the grain size of the composite fiber increases with an increase in the heat treatment temperature. A comparison between Figs. 1 and 3 (the heat treatment temperature is 1050°C in both cases, while the potassium content is 4.4% in Fig. 1 or 9.0% in Fig. 3) reveal that the amount of titanium dioxide crystal phase increases with a decrease in potassium content. These results demonstrate that the type of composite phase of the composite fiber embodying the invention can be controlled as desired by varying the amount of potassium dissolved out by the potassium removal process and the temperature of the heat treatment process.

By the way, larger crystal grains are advantageous because the fiber then has a higher density and shows greater strength.

Strength of fibers

Some of the composite fibers of the invention and the single phase fiber of potassium hexatitanate were consolidated into samples using a phenolic resin binder and then tested for their flexural strength, bending at break and deformation of the fibers.

The samples were prepared from fibers No. 4, No. 5 and No. 11 as listed in Table 1 by the following procedure. The fiber (70 wt%) and 30 wt% phenolic resin were mixed together and the mixture was compacted (under a pressure of 30 MPa at room temperature for 1 minute) and molded (under a pressure of 15 MPa at a temperature of 170°C for 5 minutes) into a solidified piece. The molded piece was heat treated (at 180°C for 3 hours). The sample thus prepared was 7 x 15 x 70 mm in size.

The obtained samples were subjected to a 3-point bending test (span: 40 mm, crosshead speed: 0.5 mm/min). Table 2 shows the results. Table 2

The results in Table 2 reveal that Samples No. 4 and No. 5, each comprising the composite fiber of the invention, have higher bending strength than Sample No. 11, which comprises the single-phase fiber of potassium hexatitanate.

Friction and abrasion characteristics (Abrasion resistance and friction coefficient) example 1

A friction material (disc lining) was prepared from some of the composite fibers of the invention and the single-phase fiber made of potassium hexatitanate, which was then tested for its friction characteristics.

The disc pad for the tests was prepared by the following procedure. The below mixture was compacted, molded and heat treated under the same conditions as for the preparation of the samples for the above-mentioned fiber strength test, further followed by grinding. In this way, the test pads A1, A2, B and C were obtained.

Fiber 30 wt.%

Binder (phenolic resin) 20 wt.%

Friction and abrasion modifier (barium sulfate) 50 wt.%

The fibres used for test coverings A1, A2 and B are No. 4, No. 5 and No. 11 in Table 1, respectively, and the main fibre used for test covering C is chrysotile asbestos fibre (class 6).

Test pieces were cut from the test linings and subjected to constant speed friction and abrasion tests (disk friction surface: FC250 gray cast iron, surface pressure: 1 MPa, friction speed: 7 m/s) prescribed in JIS D4411, "Motor Vehicle Brake Linings" and tested for their friction coefficient and abrasion rate. The measurements of the friction coefficient are shown in Fig. 6 and those of the abrasion rate in Fig. 7).

Fig. 6 shows that test pads A1 and A2 of the invention maintain a higher coefficient of friction than test pad B (which includes the single phase fiber of potassium hexatitanate) over the entire low to high temperature range. Furthermore, test pads A1 and A2 have a higher coefficient of friction at higher temperatures than test pad C (which includes chrysotile asbestos fiber).

Fig. 7 shows that the test coatings A1 and A2 of the invention have a low abrasion rate and have a high abrasion resistance at higher temperatures.

Example 2

Under the same conditions as in Example 1, the following mixture was compacted, molded and heat treated to produce test pads A3, B3 and C3, followed by grinding.

Fibre-1 16 wt.%

Fibre-2 3 wt.%

(Aramid fiber: Kevlar pulp, 2 mm length, Toray Industries, Inc.) Binder (phenolic resin) 9 wt.%

Organic additives (cashew dust etc.) 9 wt.%

Other 63 wt.%

(graphite and similar lubricants, barium sulfate and similar inorganic powders, metal powders, oxide powders)

With respect to this example, the components of the test pads other than Fiber-1 are typical of those currently used in automotive brake pads. The Fiber-1 used for Test Pads A3 and B3 are No. 5 and No. 11, respectively, as shown in Table 1. The fiber used for Test Pad C3 is chrysotile asbestos fiber (Class 6).

Test pieces were cut from the test pads and subjected to constant speed friction and abrasion testing in the same manner as in Example 1. The test results are described in Figs. 8 and 9.

Figure 8 shows that the test pad A3 of the invention maintains a higher coefficient of friction than the test pads B3 and C3 over the entire range from low to high temperatures.

Figure 9 shows that the test coating A3 of the invention has a lower abrasion rate at higher temperatures and is more abrasion resistant than the test coating C3.

The potassium hexatitanate-titanium oxide composite fiber of the present invention has greater hardness, abrasion resistance, shear strength, etc. than the single-phase potassium hexatitanate fiber as effects of the presence of titanium dioxide crystals. When the present composite fiber is incorporated into the friction materials for automobiles and the like braking devices, improved friction and abrasion characteristics can be imparted to the sliding surface.

The composite fiber of the present invention is useful in place of the single phase fiber of potassium hexatitanate for other applications, such as as a filler for heat-resistant, abrasion-resistant coating compositions, as a reinforcing fiber for reinforced plastics, and for heat-resistant or heat-insulating materials to impart improved material characteristics.

The preceding description The embodiments and examples are intended to illustrate the present invention and should not be construed as a limitation of the invention defined in the appended claims or as a narrowing of its scope. The present invention is therefore not limited to the described embodiments and examples, but can be modified in various ways by a person skilled in the art.

Claims (6)

1. A process for producing a composite fiber having a polycrystalline structure consisting of potassium hexatitanate crystals and titanium dioxide crystals mixed therewith by a melting process, the process comprising the steps of heating, as a starting material, a mixture of a titanium compound convertible into titanium dioxide by heating and a potassium compound convertible into potassium oxide by heating in a ratio resulting in a TiO₂/K₂O molar ratio of 1.5 to 2.5 to produce a melt, cooling the melt to obtain a lump of potassium dititanate fibers, treating the lump of fibers in a washing liquid to remove K⁺ ions while loosening the fibers until the potassium content of the fiber decreases to 1 to 13.5 wt.% based on an anhydrous state of the fiber, and heat treating the resulting fibers at a temperature of 900 to 1300°C. 2. The method according to claim 1, wherein the heat treatment is controlled at a temperature of 1,050°C or higher. 3. A composite fiber having a polycrystalline structure consisting of potassium hexatitanate crystals and titanium dioxide crystals mixed therewith, obtained by the process according to claim 1. 4. The composite fiber according to claim 3, wherein the potassium content contained in the fiber is 1 to 13.5 wt%. 5. The composite fiber according to claim 3, wherein the potassium content included in the fiber is 4.4 to 11.2 wt%. 6. Friction material in which fibers are solidified with a resin binder, the friction material being characterized in that it contains polycrystalline composite fibers according to at least one of claims 3 to 5 in an amount of 3 to 50% by weight.