JPH0514327B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a polyester film that provides excellent running properties, electromagnetic conversion characteristics, and abrasion resistance when used as a magnetic recording medium. More specifically, the present invention relates to a base film suitable for high-density magnetic recording, which generates very little scratches or abrasion particles during tape running, and suppresses the occurrence of dropouts. [Prior Art and Problems to be Solved] Polyester films have excellent physical and chemical properties and are widely used as industrial base materials. In particular, biaxially oriented polyethylene terephthalate films are particularly excellent in mechanical strength, dimensional stability, flatness, etc., and are now indispensable as base films for magnetic recording media. Incidentally, in recent years, improvements in magnetic recording media have been made at a rapid pace, and as a result, demands on base films have become even more severe. For example, in products that require high-density recording such as video tape, the magnetic layer is made thinner, but in this case, the unevenness of the base film itself is reflected on the surface of the magnetic layer, and if the degree of unevenness is large, the electromagnetic conversion characteristics This results in a decrease in performance and frequent dropouts. Therefore, it is desirable that the surface of the base film be as flat as possible, but as is well known, when the film surface becomes flat, it not only deteriorates the runnability of the film, but also causes problems between the film and the base material, such as guide pins. The friction and abrasion between the parts increases, causing scratches on the film and generation of abrasion powder. In other words, if the abrasion resistance of the polyester film is insufficient, abrasion powder will be generated between the film and various rolls or guide pins during the magnetic layer manufacturing process or after it is made into a product, which will also affect the electromagnetic characteristics and dropout. become inferior in terms of Furthermore, if the scratch resistance of the film is insufficient, the film is likely to be scratched before and after applying the magnetic layer, and these scratches are reflected on the surface of the magnetic layer, resulting in poor electromagnetic properties. Furthermore, the electromagnetic properties are often deteriorated by the presence of white powder-like material that has been scraped off. As a means of improving surface properties such as runnability and abrasion resistance of polyester films, conventional methods include the presence of inert fine particles in the film to appropriately roughen the film surface, specifically the so-called precipitated particle method and addition. The particle method is known. The precipitated particle method is a method in which catalyst residues are precipitated into polyester as fine particles during the polyester manufacturing process.It is easy to operate and can produce polymers at low cost, and the life of the filter during film formation is long. It has the advantage of Furthermore, it generally has excellent affinity with polyester, has little detachment of particles from the film, and has relatively excellent abrasion resistance. However, since the main component of the particles is an organic component, they are inferior in scratch resistance. On the other hand, the additive particle method, which is contrasted with the precipitated particle method, is a method in which calcium carbonate, kaolin, silica, etc. are added as they are or after being made into fine particles, and then added during polyester production or molding. Not only is the chipping and abrasion resistance insufficient, but unnecessary large particles are often mixed in, creating rough protrusions on the film surface and impairing film quality. In this way, attempts have also been made to use both in combination in order to obtain effects that cannot be achieved by each alone. For example, Japanese Patent Publication No. 55-20496 or Japanese Patent Publication No. 55-40930 describes the combined use of the precipitated particle method and the additive particle method.
Even if these methods are employed, it has not been possible to achieve high levels of running performance, abrasion resistance, and scratch resistance. [Means for solving the problem] In view of the above-mentioned circumstances, the inventors of the present invention conducted intensive studies on polyester films for magnetic recording media, and as a result, discovered that certain particles obtained by the precipitated particle method and The inventors have found that the above-mentioned problems can be easily solved industrially by using a combination of additive particles having a specific particle size and hardness, and have thus arrived at the present invention. That is, the gist of the present invention is to provide inert material particles with an average particle size of 0.1 to 5 μm precipitated in the polyester manufacturing process.
0.01 to 1% by weight of aluminum oxide or silicon carbide with an average particle size of less than 1 μm and a Mohs hardness of 6 or more
0.01 to 3% by weight of a biaxially oriented polyester film for magnetic recording media. The present invention will be explained in more detail below. The term "polyester" used in the present invention refers to a polyester obtained using an aromatic dicarboxylic acid such as terephthalic acid or 2,6-naphthalene dicarboxylic acid or its ester, and ethylene glycol as the main starting materials, but may contain other third components. It doesn't matter if you stay there. In this case, the dicarboxylic acid component includes, for example, isophthalic acid, phthalic acid, 2,6-naphthalene dicarboxylic acid, terephthalic acid, adipic acid, sebacic acid, and oxycarboxylic acid component such as p-oxyethoxybenzoic acid or Two or more types can be used. As the glycol component, one or more of diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, etc. can be used. In any case, the polyester of the present invention refers to a polyester in which 80% or more of repeating structural units have ethylene terephthalate units or ethylene-2,6-naphthalene units. Further, the polyester film of the present invention refers to a biaxially oriented polyester film using such a polyester as a starting material, and any known method can be used for its production. For example, it is usually melt-extruded into a sheet at 270-320°C, then cooled and solidified at 40-80°C to form an amorphous sheet, then biaxially stretched in the longitudinal and transverse directions sequentially or simultaneously, and then stretched at 160-250°C. Methods such as heat treatment (for example, the method described in Japanese Patent Publication No. 30-5639) can be used. When stretching in the longitudinal and transverse directions, each may be stretched in one stage, or if necessary, stretching may be performed in multiple stages, or a heat treatment section for orientation relaxation may be provided between the multiple stages of stretching. Moreover, after biaxial stretching, it is also possible to stretch again before subjecting to the next heat treatment step. This re-stretching can be performed in either the longitudinal or transverse directions, or in both directions. In the present invention, the inert substance particles precipitated in the reaction system are particles that are precipitated in the reaction system by, for example, polymerizing a system using an alkali metal or alkaline earth metal compound as a transesterification catalyst by a conventional method. . Alternatively, precipitation may be carried out by adding terephthalic acid during transesterification or polycondensation reaction. In these cases, phosphoric acid,
One or more phosphorus compounds such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, acidic ethyl phosphate, phosphorous acid, trimethyl phosphite, triethyl phosphite, and tributyl phosphite may be present. . Furthermore, even when an esterification step is performed, inert material particles can be precipitated by these methods. For example, an alkali metal or alkaline earth metal compound is made to exist before or after the completion of the esterification reaction, and the polymerization reaction is carried out in the presence or absence of a phosphorus compound. In any case, the fine precipitated compounds produced during the polyester production reaction in the present invention contain one or more elements such as calcium, lithium, antimony, and phosphorus. For example, when calcium acetate is used as a transesterification catalyst, a calcium compound estimated to have the following composition consisting of calcium terephthalate and a calcium salt of a polyester oligomer is precipitated during the polymerization reaction. Furthermore, when a phosphorus compound is used in combination with this system, it is thought that, in addition to the above formula, a portion of a compound presumed to have the following structural formula is precipitated. When a lithium compound is used instead of a calcium compound, a compound estimated to have the following structure will precipitate during polycondensation. When a phosphorus compound is used in combination with this system, the following compounds are thought to precipitate in addition to the formula. Furthermore, when a calcium compound and a lithium compound are used together, it is thought that complex compounds represented by the above formula and the following formula will precipitate. Furthermore, if a phosphorus compound is used in combination with this system, it is presumed that in addition to the above formulas, complex compounds such as those represented by the following formula will be precipitated. In any case, in the present invention, these precipitated particles must be contained in the polyester film in an amount of 0.01 to 1.0% by weight, preferably 0.05 to 0.5% by weight. If the content of precipitated particles is less than 0.01% by weight, the slipperiness of the film will be insufficient, while if this amount exceeds 1.0% by weight, the slipperiness will no longer be improved; Coarse protrusions occur frequently due to aggregation. Further, the average particle size of the precipitated particles must be 0.1 to 5 μm. This is because, if the average particle size is less than 0.1 μm, the effect of developing slipperiness is insufficient and the runnability is not satisfied, and the wear resistance also deteriorates. On the other hand, if the average particle size exceeds 5 μm, the life of the filter during film formation will be shortened even though the particles are precipitated particles, and the electromagnetic properties will deteriorate when used as a magnetic recording medium. The average particle diameter of the precipitated particles is particularly preferably 0.3 to 3 μm. Since there are a wide variety of methods for producing precipitated particles that can be used in the present invention, it is not possible to discuss all of them in detail, but the following examples can be cited as specific examples. In other words, a lithium compound is used as a transesterification reaction catalyst for all acid components constituting polyester.
When 0.1 to 0.5 mol% is used to complete the polycondensation reaction, 0.05 to 0.3 precipitated particles with an average particle size of 0.5 to 2μ
In this case, after the transesterification reaction is complete, adding 0.5 to 1.5 times the mole of phosphoric acid to the lithium compound and performing the same operation will result in precipitation with an average particle size of about 0.3 to 1μ. A polyester containing 0.05-0.2% by weight of particles is obtained. In addition, when a trialkyl ester such as trimethyl phosphate, triethyl phosphate or tributyl phosphate is used in place of phosphoric acid, the amount of precipitated particles becomes 0.1 to 0.5% by weight. Calcium compounds as transesterification catalysts
When the polycondensation reaction is completed using 0.05 to 0.15 mol%, the precipitated particles with an average particle size of about 2 to 5 μ
A polyester containing 0.4% by weight can be obtained, and if terephthalic acid is added after the transesterification reaction in this operation, the average particle size of the precipitated particles can be reduced.
It will be about 0.5 to 2μ. Note that, as a matter of course, the particle size and amount of the precipitated particles also depend on the polymerization conditions, particularly the temperature and pressure, and the examples shown above are only some of the results. The characteristics of the polyester film of the present invention are such specific particle size and specific amount of precipitated particles;
The purpose is to use it in combination with inorganic particles of a specific hardness. That is, by using precipitated particles, running properties can be improved and abrasion resistance can also be improved to a considerable extent, but scratch resistance remains insufficient. The present invention takes this point into consideration, and as a result, if 0.01 to 3% by weight of aluminum oxide or silicon oxide with an average particle size of less than 1 μm and a Mohs hardness of 6 or more is used in addition to the precipitated particles, the characteristics of the precipitated particles can be improved. The present invention was completed based on the discovery that scratch resistance can be imparted while the material is still in use. When these particles with high Mohs hardness are present in the film, the abrasion resistance and scratch resistance, especially the scratch resistance, are significantly improved. However, if the amount of particles is less than 0.01% by weight based on the polyester film, or if the Mohs hardness is less than 6, the effect will sharply decrease. Incidentally, even if aluminum oxide or silicon oxide is present in an amount exceeding 3% by weight, the scratch resistance will no longer be further improved, and on the contrary, coarse protrusions will occur frequently, which is not preferable. Particularly preferred amount range is 0.1-2% by weight
It is. In addition, the average particle size of such particles used in the present invention is
It must be less than 1 μm. If this value exceeds 1 .mu.m, the particles often separate from the film surface, resulting in a worsening of the abrasion resistance. The average particle size of the additive particles is preferably 0.5 μm or less, particularly 0.1 μm.
m or less, and at the same time desirably smaller than that of the precipitated particles. An example of particles that meet these requirements and can be particularly suitably used in the present invention is aluminum oxide produced by a so-called thermal decomposition method. These particles are usually produced by flame hydrolysis using anhydrous aluminum chloride as a raw material, and have a particle size of about 10 mμ to 100 mμ.
In the present invention, aluminum oxide particles produced by an alkoxide hydrolysis method can also be suitably used. In this case, usually Al (OC ₃ H ₇ ) or Al
(OC ₄ H ₉ ) is used as a starting material, and fine particles of 1 μm or less are obtained by appropriately selecting hydrolysis conditions.
Of course, in this case, after adding acid to the synthesized slurry to obtain a transparent sol and then gelling this sol,
It is also possible to adopt a method such as forming a sintered body by heating to 500°C or higher. Alternatively, aluminum oxide fine powder obtained by another method, ie, adding methyl acetate or ethyl acetate to a sodium aluminate solution and stirring to obtain AlOOH, and then heating this may be used. In any case, in the present invention, aluminum oxide having an average particle size of less than 1 μm is particularly preferably used. In the present invention, as with other additive particles, it is preferable to use such aluminum oxide particles by completely dispersing them to the primary particles, but as long as it does not adversely affect the surface condition of the film, they may aggregate to some extent as secondary particles. It's okay to behave. However, in this case as well, the apparent average particle size must be less than 1 μm.
In this case, a portion of the aluminum oxide, for example less than 30% by weight, may be replaced with oxides such as Si, Ti, Fe, Na and K. In any case, in the present invention, together with the precipitated particles, the average particle size is less than 1 μm and the Mohs hardness is 6.
Although it is characterized by using a specific amount of aluminum oxide or silicon oxide as described above, the particle size distribution of the inorganic particles is not particularly limited. However, in the present invention, when integrating particles with a sharper particle size, specifically starting from the smaller particle size, the particles with a weight fraction of 75% and 25% have a particle size of 2.0 or less,
Preferably, those having a value of 1.5 or less are suitably used. Furthermore, there are no particular restrictions on the shape of these particles, but those that are more lumpy or spherical than usual are preferably used. Specifically, for example,
The volume shape defined in Publication No. 53-14583 is 0.1~
Particles having a diameter of π/6, preferably between 0.2 and π/6 are preferred. Furthermore, there is no particular restriction on the specific surface area of the particles used. For example, average particle size 0.5 μ, density 2 g/cm ³
True spherical particles have a specific surface area of about 6 m ² /g, but porous particles of about 500 m ² /g can also be suitably used. Note that the surfaces of these particles may be modified with various surface treatment agents. These surface treatment agents, such as so-called silane coupling agents and titanium coupling agents, which are normally used for the purpose of improving the affinity with ethylene glycol and polyester, are generally applied in an amount of 5% by weight or less based on the particles. These inorganic particles used in the present invention are blended into polyester after being subjected to operations such as pulverization, classification, and sieving as necessary. For the crushing process, for example, a rod mill, a ball mill, a vibrating rod mill, a vibrating ball mill, a palm mill, a roller mill, an impact mill, an agitation mill, a fluid energy mill, etc. can be used, and for the classification process, for example, a semi-free spiral type can be used. , forced whirlpool method, hydrocyclone method, centrifugal separation method, etc. can be adopted. As for the method of blending the particles with polyester, it is usually preferable to add the particles as an ethylene glycol slurry to the reaction system at the initial stage of polyester production from the viewpoint of dispersibility, but of course it is preferable to blend the particles directly into the polyester before film formation. You may also use the method of As described above, two types of particles having different properties are used in the present invention, namely, precipitated particles and additive particles. In order to blend these into a polyester film, for example, the following method is adopted. A film is formed after blending a polyester containing precipitated particles and a polyester containing additive particles. A polyester containing both precipitated particles and additive particles is obtained and used to form a film. After blending the polyester containing precipitated particles and inorganic particles, a film is formed. In addition, in the present invention, particles other than the two types of particles essential to the present invention or other compounding agents may be included as long as the purpose is not impaired. For example, for the purpose of finely adjusting surface roughness and slipperiness, additive particles with a Mohs hardness of less than 6 such as kaolin, talc, calcium carbonate, lithium fluoride, mordenite, calcium fluoride, zinc oxide, calcium silicate, zeolite, calcium phosphate, etc. You may use them together. Further, the effects of the present invention are fully exhibited even in systems containing weathering agents, light stabilizers, antistatic agents, lubricants, light shielding agents, antioxidants, heat stabilizers, and the like. As described above, the present invention has a specific precipitated particle, an average grain size of less than 1 μm, and a Mohs hardness of 6.
By using the above-mentioned aluminum oxide or silicon oxide in combination, it is possible to achieve high runnability, abrasion resistance and scratch resistance, and finally by using the polyester film of the present invention. It becomes possible to obtain a magnetic recording medium that also has excellent electromagnetic properties. [Examples] The present invention will be explained in more detail by Examples below, but the present invention is not limited to the following Examples unless the gist of the invention is exceeded. In the examples and comparative examples, "parts" indicate "parts by weight." Furthermore, the measurement method used in the present invention is as follows. (1) Average particle size In the case of precipitated particles, the polyester containing the particles was sandwiched between slides, melted, cooled, and then measured using a microscope. In the present invention, the average particle size refers to the particle size (diameter) at a point where the volume fraction of the equivalent sphere is 50%, regardless of its shape. In the case of additive particles, the particle size is also measured using a microscope.
Calculate the average particle size in the same way as for precipitated particles. If precipitated particles and additive particles are included at the same time, the average particle size of each is calculated by depolymerizing and separating only the precipitated particles using the method described in Japanese Patent Publication No. 55-20496. . (2) Measurement of amount of precipitated particles Add o-chlorophenol to 100g of polyester
After adding 1.0 and heating at 120°C for 3 hours, centrifugation was performed at 30,000 rpm for 40 minutes using a Beckman ultracentrifuge L3-50, and the resulting particles were vacuum-dried at 100°C. When the particles are measured using a scanning differential calorimeter, if a melting peak corresponding to the polymer is observed, o-chlorophenol is added to the particles, and after heating and cooling, centrifugation is performed again. When the melting peak is no longer observed, the particles are defined as precipitated particles. Normally, two centrifugation operations are sufficient. (3) Surface roughness of the film Expressed by center line average roughness (Ra), this value is measured using a surface roughness measuring instrument manufactured by Kosaka Institute (SE-
3FK) was calculated as follows. The tip radius of the stylus was 2 μm, and the load was 30 mg. The standard length L (2.5
mm) is extracted, and when the center line of this extracted portion is the X axis and the vertical magnification direction is the Y axis, the roughness curve y=f(x) is expressed, and the value given by the following formula is μm It is expressed as Ra=1/L∫ ^L ₀ |f(x)|dX Note that the cutoff value is 0.08 μm. (4) Abrasion resistance Using the equipment shown in Figure 2, the film was
After running for m, the amount of white powder adhering to the fixing pin was visually judged, and the wear resistance was ranked into three ranks: A (excellent, with very little adhesion) to C (poorly practical, with too much adhesion). Divided into. (5) Running properties Evaluated based on the slipperiness of the film. The slipperiness was measured using the apparatus shown in FIG. In other words, the film is wrapped around a fixed hard chrome-plated metal roll (diameter 6 mm) at an angle of 135° or 2.356 rad (θ).
A load of 53 g (T ₂ ) was applied to one end, the object was run at a speed of 1 m/mm, the resistance force (T ₁ , g) at the other end was measured, and the coefficient of friction (μd) during running was calculated using the following formula. ) was sought. μd=1/θlnT ₁ /T ₂ =0.424lnT ₁ /53 (6) Scratch resistance First, a magnetic tape was manufactured. That is, the following magnetic paint was applied to a polyester film to form a magnetic layer having a thickness of 2 μm after drying.
That is, 200 parts of magnetic fine powder, 30 parts of polyurethane resin,
10 parts of nitrocellulose, 10 parts of vinyl chloride-vinyl acetate copolymer, 5 parts of lecithin, 100 parts of cyclohexanone, 100 parts of methyl isobutyl ketone and 300 parts of methyl ethyl ketone in a ball mill.
Polyisocyanate compound 5 after 48 hours of mixing and dispersion
This was applied to a polyester film, magnetically oriented before the paint was fully dried and solidified, and then dried. Further, this coated film was subjected to surface treatment using a super calender, and was slit into 1/2 inch width to make a videotape. Next, wrap the magnetic tape around a hard chrome-plated metal pin (diameter 6 mm, surface roughness 3S).
The base film surface was rubbed at a running speed of 4 m/sec by contacting at 135° and a tension of 50 g. Next, aluminum was vapor-deposited on the scratched surface of the magnetic tape, and the degree of scratches was visually judged and divided into the following five ranks. Rank 1: There are many scratches and often deep scratches. Rank 2: The amount of damage is relatively large and there are deep scratches in places. Rank 3: The degree of damage is relatively small and there are only a few deep scratches. Rank 4: There are some scratches, but the level is satisfactory. Rank 5 Almost no damage. (7) Electromagnetic properties The electromagnetic properties of the above magnetic tape were
Measurements were made using an NV-3700 video deck. ΓVTR head output Measure the VTR head output at a measurement frequency of 4 MHz using a synchroscope,
The blank was set to 0 decibels and the relative value was expressed in decibels. Number of Γ dropouts: Play back a videotape containing a 4.4 MHz signal, and measure the number of dropouts using an Okura Industry Co., Ltd. dropout counter.
Measurements were made for 20 minutes and converted into the number of dropouts per minute. Example 1 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, 0.10 parts of calcium acetate hydrate, and 0.18 parts of lithium acetate dihydrate were placed in a reactor, heated to raise the temperature, and methanol was distilled off to perform a transesterification reaction to start the reaction. It took about 4 hours to reach 230°C, and the transesterification reaction was substantially completed. Next, 0.31 part of triethyl phosphate and 0.04 part of antimony trioxide were added to this reaction mixture, followed by polymerization according to a conventional method to obtain polyethylene terephthalate having an intrinsic viscosity of 0.66 after 4 hours. (Polyester (A)) Many uniform and fine precipitated particles were observed in the polyester, and the particle size was 0.8 μm. The amount of precipitated particles was measured according to the method described in the text and was found to be 0.37% by weight based on the polyester. On the other hand, 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and magnesium acetate tetrahydrate
0.09 part was placed in a reactor and the temperature was raised while methanol was distilled off, taking about 4 hours after the start of the reaction.
The temperature was reached to 230°C to complete the transesterification reaction. Then, the average particle size 0.02μ obtained by pyrolysis method
After adding 1.0 part of aluminum oxide, 0.04 part of triethyl phosphate and antimony trioxide were added.
0.04 part was added and a polycondensation reaction was carried out in a conventional manner to obtain polyethylene terephthalate having an intrinsic viscosity of 0.66. (Polyester (B)) Aluminum oxide was extremely uniformly dispersed in the polyester, and substantially no aggregated particles were observed. Next, add polyester (A) and polyester (B) to 70:
The mixture was mixed at a weight ratio of 30%, dried, and then extruded into a sheet from an extruder at 290°C to obtain an amorphous sheet. At this time, a so-called electrostatic cooling method was adopted in which an electrostatic charge was applied to the sheet. Then, the sheet is rolled vertically by 3.5
The film was stretched 3.4 times in the transverse direction and heat treated at 225°C to obtain a biaxially stretched film with a thickness of 15 μm. Next, a magnetic layer was applied to the polyester film, and its scratch resistance and electromagnetic properties were evaluated. Example 2 A polyethylene terephthalate film and the film were used in the same manner as in Example 1, except that the amount of precipitated particles in the polyester film and the average particle size and blending amount of added particles were changed as shown in Table 1. A magnetic tape was obtained. Example 3 In the production of polyester (A) in Example 1, triethyl phosphate was used instead of 0.31 part of triethyl phosphate.
A polyester containing precipitated particles with an average particle size of 0.5 μm was obtained in the same manner as in Example 1, except that 0.26 parts of ethyl phosphate and 0.05 parts of acidic ethyl phosphate were used. A polyethylene terephthalate film containing the particles shown in Table 1 was obtained by mixing the polyester and a polyester containing aluminum oxide particles, and then evaluated as a magnetic tape. Example 4 A polyethylene terephthalate film containing the particles shown in Table 1 was obtained by using the precipitated particles obtained in Example 3 in combination with silicon carbide particles, and then evaluated as a magnetic tape. Comparative Examples 1 to 4 A biaxially compounded film with a thickness of 15 μm was obtained by forming a film in the same manner as in Example 1, except that the particles contained in the polyethylene terephthalate film were changed as shown in Table 1. Then, it was made into a magnetic tape and its properties were evaluated. evaluated. The results obtained above are summarized in Table 1.

〔Effect of the invention〕

As detailed above, the present invention improves various properties necessary for a magnetic recording medium, particularly scratch resistance, which has been desired in recent years, by using a combination of specific precipitated particles and additive particles. This has been achieved and its industrial value is high.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the running system of the device for evaluating the coefficient of dynamic friction with metal;
φSUS-420-J2 fixing pin, indicates the inlet tension meter, indicates the outlet tension meter,
The wrapping angle θ is 135°. FIG. 2 is a schematic diagram showing the running system of the apparatus for evaluating wear resistance. In the figure, the 6 mmφ SUS-420-J2 fixing pin represents a tension meter, and the wrapping angle θ is 135°.

Claims (1)

[Claims] 1 Average particle size precipitated in the polyester manufacturing process
0.01-1% by weight of inert material particles of 0.1-5 μm;
A biaxially oriented polyester film for magnetic recording media, characterized in that it contains 0.01 to 3% by weight of aluminum oxide or silicon carbide having an average grain size of less than 1 μm and a Mohs hardness of 6 or more.