IE911504A1 - A composite material comprising mechanically resistant¹particles - Google Patents

Abstract

Composite materials comprising an elastomer matrix, in particular based on rubber, and included, rounded, edge-free hard particles coated with an adhesion promoter, have high wear resistance and good non-skid properties on smooth surfaces. They are particularly suitable for use in the tread of vehicle tyres. Furthermore, a process is described for the production of rounded, edge-free hard particles with high compressive strength by mechanical treatment of starting particles of any desired shape.

Description

This invention relates to a composite material comprising an elastomer matrix with embedded mechanically resistant particles, the use thereof for transmission of frictional forces, particularly in vehicle tyre treads, rounded compression-proof mechanically resistant particles required for this purpose, and a method of manufacturing them.

Elastomeric materials, more particularly those based on natural and/or synthetic rubber, are widely used in industry for transmitting frictional forces, owing to their special mechanical properties. Large amounts are used in the manufacture of vehicle tyres of all kinds, particularly tyres for motor vehicles. The tyre treads must reliably transmit not only the weight of the vehicle but also and particularly the driving, braking and side forces to the road. The quality and nature of the road may vary considerably, with varying adverse effects on the transmission of force in - 2 individual cases. Usually the greatest difficulties occur when the road is covered with ice or snow or is made of a material having a smooth surface such as basalt or granite, or is covered with a film of water, possibly containing oil.

To a limited extent, the static friction on an icy or wet road can be improved by adapting the elastomer mixture and the tread cross-section to these special conditions. However the first of these methods, in particular, can adversely effect wear rates, i.e. it can reduce the attainable tread life.

A considerable improvement in adhesion to ice can be obtained by incorporating hard-alloy pins (or spikes) in the tyre tread, one end of each spike projecting slightly from the surface and pressing into the ice when the tyre moves, thus for an instant producing a sort of positive connection. This technical solution, however, had such serious disadvantages that the use of such tyres on public streets has been banned in most countries. Hard-alloy pins are relatively heavy and, when the wheel rotates at speed, they can come loose and fly off in an uncontrolled manner and cause injury - 3 and damage. More importantly, even at moderate speeds, the pins act like drill bits on the road surface and can cause unacceptably severe wear.

In the past there have been numerous proposals for improving the static friction of vehicle tyres, simply by mixing mechanically resistant particles such as corundum or silicon carbide abrasive grains in the rubber tread mixture.

For example FR-PS 1,365,406, dated 1964, proposes incorporating abrasive grains of corundum, silicon carbide, boron carbide, emery, aluminium oxide, quartz or the like in the tread or parts thereof.

At first sight this method appears simple, but does not provide the desired results. Firstly, the mechanically resistant particles are not compatible with the rubber mixture, i.e. adhesion between the mechanically resistant material and the rubber is so low that if the particles are on the surface of the tread (and this is the only place where they can exert the desired effect) they are rapidly tom from the composite material by mechanical forces and thus become ineffective. This - 4 part of the problem has been satisfactorily solved by suitably coating the surfaces of the particles.

Coatings of this kind are described in published PCT application WO 89/06670. However, tests on vehicle tyres manufactured by using the aforementioned coated mechanically resistant particles have shown that good adhesion between the mechanically resistant material and the rubber mixture does not by itself ensure an adequate service life for the tyres. Flexing stress on the tread can cause the sharp edges of the particles to gradually cut through the rubber and begin to migrate. In the most favourable case, the result may be only a loss of particles. The consequences are more serious if individual particles migrate inwards, since this can result in gradual destruction, and, in the particular case of tubeless tyres, can result in punctures or even a burst tyre. In addition, particles containing faults such as large pores, inclusions, cracks, grain boundaries or relatively large faults in their crystal structure, are easily crushed by the constant impact stress when striking the road surface. The resulting broken pieces usually have particularly sharp edges, and the new fracture surfaces are not - 5 coated with primer, so that the aforementioned problems are exacerbated.

An object of the invention is to make a composite material from an elastomer matrix, preferably rubber-based, which includes embedded mechanically resistant particles, the material being free from the aforementioned disadvantages, simple and inexpensive to manufacture, and suitable particularly for use in vehicle tyres.

This object is achieved by the present invention which provides, in a first aspect a composite material comprising an elastomer matrix with embedded primer coated mechanically resistant particles, wherein the mechanically resistant particles are substantially free from sharp comers and edges and strength-reducing structural faults and at least 90% by weight of the mechanically resistant particles have a Krumbein roundness of at least 0.3.

In a second aspect, the present invention provides rounded compression-proof mechanically-resistant particles which are substantially free from sharp - 6 corners and edges and strength reducing structural faults, wherein at least 90% by weight of the particles have a Krumbein roundness of at least 0.3.

It was surprising to find that the properties, more particularly the service life, of the composite materials known from the prior art and comprising an elastomer matrix can be considerably improved by embedded mechanically resistant particles, if the particles used are substantially free from sharp edges and have a rounded shape. Substantially free from sharp edges means that there are substantially no convex edges formed by acutely-angled convergent cutting surfaces, and the radius of curvature of the remaining comers and edges is large, preferably at least 20% of the particle diameter. Ideally the particles have an approximately spherical or ellipsoidal shape. If the particles are directly manufactured from irregularly shaped particles obtained e.g. by breaking up lumps of material, the ideal shape cannot be completely obtained. - 7 Although the term roundness is direct and descriptive, the degree of rounding of irregularly shaped particles is difficult to describe quantitatively. An introduction to this problem is given e.g. by M.H. Pahl, G. Schadel and H. Rumpf in Aufbereitungstechnik, 1973 , pages 754-764. Although there are a few exactly defined measures of roundness, they have been unsuccessful in practice owing to the expense of determining them in individual cases. The Krumbein method, which is adequate for most practical requirements, simple to perform and therefore widely used, is based on a visual comparison between the particles under investigation, Suitably magnified if necessary, and standard shapes having exactly defined roundness. This method however is not quite satisfactory in the case of rounded particles which have subsequently been crushed and which, in spite of their substantially rounded surface, have some sharp edges near their fracture surfaces.

Advantageously, therefore, the mechanically resistant particles used according to the invention are characterised by their Krumbein roundness with the - 8 additional specificaton that they are free from sharp edges as previously described.

Advantageously at least 90% of the total weight of particles have a Krumbein roundness of at least 0.3, and preferably at least 80% have a roundness of at least 0.5. Particles having a roundness of 0.6 or above are particularly preferred.

According to the invention, mechanically resistant particles having the required properties can be manufactured if mechanically resistant particles of arbitrary shape are subjected to mechanical treatment, which gives the particles a rounded surface substantially free from edges and which also eliminates particles having major structural faults such as pores, inclusions, cracks or the like.

To this end and according to the invention the particles, which usually initially have irregular shapes with sharp edges imported to them during manufacture, particularly when obtained by breaking up coarse material, are subjected to intensive frictional and impact stress in a liquid medium. - 9 Preferably the particles are treated in an agitator mill, an annular gap mill, an attritor or similar device. These devices are known per se; they are usually loaded with grinding elements and are used for very fine grinding or for breaking up agglomerates, e.g. of ceramic powders or pigments.

No grinding elements are needed for the method according to the invention; preferably grinding elements are not used, so that the particles strike and rub against one another during treatment. As a result the particles are rounded and are also given a microscopically roughened surface which improves their adhesion to the elastomer matrix. During treatment, particles of inadequate strength are crushed to smaller fragments which, together with the fine powder abraded from the other particles, can easily be removed by screening or sedimenting after treatment and used for other purposes. Preferably treatment is such that the particles in the agitator mill or attritor are just covered by the liquid when at rest. - 10 The liquid is preferably water, which is not only cheap but also has the desired viscosity and a high heat capacity.

Particularly advantageously, the particles are partly rounded in a preceding step, so that less fine powder is produced in the method according to the invention and the treatment can be speeded up. The aforementioned partial rounding, basically restricted to the breaking of projecting edges and comers, can be carried out e.g. by the method described in EP-PS 0,082,816.

The mechancially resistant particles, in principle, can be formed from any materials which are sufficiently hard and not too brittle. The preferred substances are oxides, carbides, nitrides and borides of metals or metalloids or mixtures of such compounds with one another or with metals (cermets). Examples of such substances are aluminium oxide (corundum), aluminium oxide and zirconium oxide (zirconium corundum), silicon carbide, boron carbide, titanium carbide, tantalum carbide, tungsten carbide, silicon nitride, titanium nitride, tantalum nitride, boron nitride and titanium - 11 boride. The compounds can be in pure form or contain normal impurities and/or adjuvants such as sintering aids or binders.

The method according to the invention is particularly suitable for mechanically resistant materials which are manufactured in the form of solidified melts or coarse crystalline substances, such as corundum 6r zirconium corundum or silicon carbide, because these substances give particularly irregular, sharp-edged particles after initial comminution.

Mechanically resistant particles rounded and of use according to the invention can also be made by synthetic processes, e.g. by granulation and sintering of ceramic powders, optionally including admixed sintering aids and/or temporary and/or permanent binders, or by drying and sintering of gel beads.

These methods are known per se.

Advantageously, before being incorporated in an elastomer matrix, the rounded mechanically resistant particles according to the invention are coated in known manner, the coating being in one or more layers. Ε 911504 - 12 It may be advantageous if the surface is not absolutely flat but has microscopic roughness, thus improving the adhesion of the coating.

The multi-layer coatings described in published PCT patent application WO 89/06670 are particularly suitable for incorporation in a rubber-based matrix as used in the manufacture of vehicle tyres.

The coated mechanically resistant particles are incorporated in the material of the elastomer matrix in a known manner, e.g. by mixing and kneading. The rounded shape of the particles is an advantage in this case also, since the equipment is subjected to much less wear them it would be if sharp-edged particles are used.

The equipment, however, may limit the maximum size of particles that can be used. If, for example, a calender or cylinder mill is used, the ratio of the nip width to the particle size must be greater them 2:1, preferably greater than 2.5:1, to prevent crushing of particles and/or damage to the rolls. - 13 During tyre manufacture also, the particles should be smaller than the details of the press moulds, to prevent damage to the mould or jamming of particles in gaps and slots in the mould.

The mechanically resistant particles according to the invention are preferably included in the treads of vehicle tyres of all kinds. The term vehicle in this connection includes e.g. aircraft or machines movable on elastomer-tyred wheels or chains with elastomeric supports. However, the invention includes all other uses in which frictional forces are transmitted by an elastomer-based article, e.g. shoe soles, conveyor belts, non-slip resilient floors or linings such as loading surfaces of transport equipment, non-slip substrates for stationary objects such as furniture or machines or the like. Natural and/or synthetic rubber, optionally in chemically modified form, or other elastomers, e.g. polyurethane-based, can be used depending on the application. The size and composition of the mechanically resistant particles are also advantageously adapted to the application, as is the quantity of matrix relative to embedded particles. - 14 As already mentioned, the maximum size of the particles is limited by the dimensions of the processing equipment. The proposed use may impose other limiting values. Excessively small particles measuring less than about 0.2 mm act only as fillers. Excessively large particles result in non-uniform properties, e.g. in excessive noise and increased wear on the road in the case of vehicle tyres.

The sizes of individual particles in an individual application should not be too variable, because small particles in the presence of considerably greater particles do not substantially contribute to the effect according to the invention. However, it is not necessary for all particles to be of equal size.

Advantageously the quantity of mechanically resistant particles relative to matrix material is adjusted so that the proportion by volume of mechanically resistant particles is between 1 and 35%. The proportion by volume relates to the part of the composite material actually containing the mechanically resistant particles, if the particles are not uniformly distributed throughout the material. - 15 If the proportion by volume is below 1%, too few particles are at the surface to have a satisfactory effect, whereas if the proportion by volume is over 35% the resilience of the material is greatly reduced.

Preferably the proportion by volume of mechanically resistant particles is between 5 and 20%. Proportions between 6 and 12% are particularly preferred for a vehicle tyre tread mixture.

Depending on the application, it may be advantageous either to incorporate the particles in the regions near the surface of the article, or to distribute them uniformly therein.

Silicon carbide particles are preferred for use in vehicle tyres, owing to their great hardness and advantageous price. The particle size is preferably between 1 and 5 mm. Larger silicon carbide particles usually consist of a number of crystallites and are therefore less strong. - 16 The following examples illustrate the working of the invention.

Exaaple 1 (Known pretreatment of the particles) Silicon carbide grains (Carsilor^ 9899, LONZA Werke) were rounded as per Example 1 of EP-PS 0,082,816 and then graded. In the following Examples, a particle size range of 1.55 to 4.0 mm was used.

The typical shape of a silicon carbide particle before treatment is shown in Fig. 1, whereas the shape after pretreatment is shown in Fig. 2. The photographs clearly show the sharp edges, projections and recesses in the untreated particle and the way in which it is roughly rounded after pretreatment, which does not substantially alter the strength-reducing recesses and pores.

The Krumbein roundness was about 0.1 for the untreated particles and in the range from 0.2 to 0.5 after treatment. - 17 Example 2 7.2 kg of particles pretreated as in Example 1 and 2 1 water were placed in an attritor (Messrs Netsch, Type PR15). The particles were attrited for a total of 28 h, and the water was changed after 10 and 20 hours to remove the fine powder. At the end of the attrition process the particles were washed with water and dried at 200°C and particles larger than 1.55 mm were removed by screening.

The yield of particles larger than 1.55 mm was on average 60% of the original amount.

The Krumbein roundness of the thus-treated particles was mainly in the range from 0.4 to 0.7. There were no particles with a roundness of less than 0.3.

Example 3 An agitator mill (Messrs Drais, Type PM 12,5) was loaded with 20 kg of particles treated as in Example 1 and 50 1 water. The mill was operated for 3 hours, during which time the water was continuously circulated - 18 between a storage container and the milling container. Next, the particles in the milling container were washed with fresh water. Drying and screening were carried out as in Example 2.

The average yield of particles greater than 1.55 mm was about 55% of the original amount.

The typical shape of a particle after treatment as per 10 Example 2 or 3 is shown in Fig. 3. The particles are shaped like potatoes. They do not have any outer edges but have a dull slightly rough surface and do not show any deep holes or pores.

The Krumbein roundness of the thus-treated particles was mainly between 0.5 and 0.8.

Example 4 (Determining the compressive strength) Method: A defined quantity (about 2 g) of graded silicon carbide particles (2.00 to 2.36 mm) was placed in a cylindrical press mould comprising a die (bore diameter 13.5 mm) a fixed bottom ram and a movable top ,E 911504 - 19 ram, and was vibrated for about 1 minute in order to compact the grains. After the top ram had been mounted, the press mould was placed in a strength-testing machine (Messrs Zwick, Universalprufmaschine Type 1478) and was loaded at a feed rate of 1 mm/min up to a final force of 0.8 kN or 1.5 kN. When the final force had been reached, the feed was switched off and the pressure drop after 1 minute was determined. The proportion of comminuted grains (< 2.00 mm) separated after screening was used as a measure of the compressive strength.

The results are shown in Table 1.

In contrast to particles produced by the known pretreatment method, those produced in accordance with the present invention suffer far less comminution (from 26 to 15 wt.%).

T»blt Jt Final pressure Pressure drop Comminuted proportion Sample_fMPal_LMEsJ_LstuSJ- Untreated 5.59 0.22 48.5 As per Example 1 5.59 0 0 As per Example 2 5.59 0 0 Ae per Example 1 10.48 0.27 26 As per Example 2 10.48 0.20 15 - 20 Example 5 In the same way as in Example 3, grains of CarsiloiR 9899 silicon carbide were treated for various periods (2, 4 and 6 h) and tested for compressive strength as per Example 4. In contrast to Example 4, the press die had a bore diamter of 29 mm, the amount of sample was 10 g in each case, and the final force was 20 kN.

Also, the bulk density for the particle size range 1.55 to 3.0 mm was determined in each case.

Table 2 shows the results in comparison with untreated particles or particles pretreated as in the prior art (Example 1).

Table 2 Sample Bulk density Comminuted [kg/1] proportion [wt.X] Untreated 1.50 64 As per Example 1 1.59 59 As per Example 3 2 h 1 .87 31 As per Example 3 4 h 1.88 27 As per Example 3 6 h 1 .89 23 - 21 Example 6 (Coating of particles) The 1.55 to 3.35 mm fraction of silicon carbide particles treated as per Example 3 was selected by screening and given the following additional treatment: 1. 57 g of Chemosi^ 211 (Henkel) was added to 1 kg of SiC particles and was uniformly distributed over the particle surface on a rotary table and dried in hot air at about 80°C. The particles were then run through a screen (4 mm) in order to break up any agglomerates which formed. 2. The SiC particles were returned to the rotary table and coated with 136 g of Chemosiji^221 (Henkel) as in step 1. This process, after brief drying (about 5-10 min), resulted in large agglomerates (about 2-10 cm in diameter). These agglomerates were mechanically broken up, starting from the surface, and the particles were simultaneously dried in a supply of hot air. As a check, the particles were then again run through a screen (mesh width 4mm). - 22 3. The SiC particles were again wetted with 300 g of a rubber solution and then dried in hot air (about 80°C).

The rubber solution consisted of 15 wt.% rubber type V2/30 (Nuova Piovanelli Gomma, Milan) in heptane/toluene (50 vol.% in each case).

At the end, the SiC particles were again run through a screen (mesh width 4 mm).

Example 7 (Manufacture of vehicle tyres) The tread mixture (type V2/30, Nuova Piovanelli) was homogenised in a water-cooled kneader (T < 80°C) for about 5 minutes and then transferred to a calender. In the calender, mechanically resistant particles coated as per Example 6 were then incorporated in the rubber, i.e. homogeneously distributed by multiple repetition of the calendering process. The calendered web was then cut to the size of the tyres to be manufactured. The web thickness was about 8 mm and the proportion of SiC particles relative to the rubber - 23 mixture was about 8 vol. %. The cut strips were vulcanised at 150 ± 3°C and 12 ± 0.5 bar and for a holding time of 29 ± 0.5 min on to a tyre base (Michelin) prepared for retreading. In the process, the tyres were pressurised from the interior and pressed against the rigid mould. The cross-section of the press mould, and consequently the resulting tread cross-section, were of the Goodyear UltragrijP^ 2 type. The finished tyre was then either testrun over a distance of 1000 to 2000 km or the tread was treated with a steel-headed brush in order partly to expose the embedded mechanically resistant particles so that they could have their full effect.

Examples 8-13 (Wear tests) lyres size 175 R 14 were manufactured as described in Example 7 but without treating their treads. The tyres were mounted on steel rims, electronically counterbalanced and attached to the driving axle of a Renault Trafic Varrttype delivery vehicle. - 24 The vehicle was then driven for a distance of about 12000 km, mainly using motorways and main roads (about 70-80%). The maximum speed was about 130 km/h and was maintained over relatively long distances in each case.

There was no observable adverse influence on driving properties or increase in the noise of the tyres.

After travelling for 12000 km, the tyres were removed 10 and the wear (the decrease in depth of cross-section in mm) was measured in the middle of the tyre at a number of places and averaged.

The wear tests were made on tyres as follows: without 15 mechanically resistant particles (comparative Example 8), with various mechanically resistant substances without mechanical treatment (Examples 9 and 11), with particles rounded by an impact process (Example 10), particles pretreated as in Example 1 (Example 12) and particles treated according to the invention as in Example 3 (Example 13). - 25 All the mechanically resistant particles used were coated as per Example 6 and were between 1.55 and 3.55 mm in size. The following mechanically resistant particles were used in addition to those mentioned in Examples 1 and 2s Diaduj® (zirconium corundum, LONZA-Werke, Example 9) and AbradingT^ (normal corundum, LONZA, Example 10).

In addition to the decrease in the depth of 10 cross-section, the wear on the mechanically resistant particles was determined in each case by inspection of the tread. For this purpose, four types of wear were distinguished: Type A: The particles are intact and, when unloaded, project partly from the surface of the cross-section.

Type B: The particles are intact and, when unloaded, are flush with the surface of the cross-section. - 26 Type C: The particles are intact but below the surface of the cross-section. When the tyres are rotating under load, however, the particles come into direct contact with the road owing to the centrifugal force and the yielding of the rubber matrix on the contact surface.

Type D: The particles are crushed and the fragments in the matrix have partly migrated or fallen out.

The various kinds of wear are shown diagrammatically in Fig. 4, and the results are in Table 3.

Table 3 Mechanically Decrease in Type of resistant substance cross-section wear ;[mm] Di adurR 1.6 + 0.3 B 1.4 + 0.3 Abradux” Ti 1.2 + 0.3 A, B Carsilon” 9899 Untreated 1.6 + 0.3 C, D Carsilon” 9899 As per Example 1 1.1 + 0.3 A, C, Carsilon” 9899 As per Example 3 0.9 + 0.3 A - 27 Examples 14-16 (Driving tests on ice) lyres size 155 R 13 were manufactured as in Example 7 with varying proportions by volume of silicon carbide particles treated as in Examples 3 and 6, and were mounted on all four wheels of a VW Golf CL type vehicle.

TO After a test run of 3000 km in each case, the following tests were carried out in an ice stadium: A course in the shape of an 8 (length about 150 m, width about 7 m) was marked out and had to be travelled in the shortest possible time. The average time for 10 laps was measured in each case. The temperature of the ice in each case was -5°C at a depth of 2 cm.

The tyres tested contained 0% by volume (comparative example 14), 8% (Example 15) and 12% (Example 16) of SiC particles in the tyre mixture.

The driving-test results are given in Table 4.

Table 4 Proportion by volume Average time per lap of SiC particles [X]LsJ0 39 ± 2.4 8 34+1.9 32 + 1.9 - 28 Example 17 (Use of rounded silicon carbide particles sintered without pressure) The mechanically resistant particles used were approximately spherical (Krombein roundness > 0.9) particles of silicon carbide sintered without pressure and 2 to 2.36 mm in diameter.

The particles were produced by granulation of ultra-fine silicon carbide powder in a fluidised-bed spray granulator using sintering aids, followed by pressure-less sintering in bulk. These particles are commercially obtainable from Messrs Sachsische Ingenieurkeramik GmbH, D-0-8273 Coswig.

The bulk density of the particles was 1.75 kg/l.

When the compressive strength was tested under the conditions described in Example 5 (amount of sample 10 g, diameter 29 mm, final force 20 kN), the comminuted proportion was only 8 wt.%. - 29 The silicon carbide particles were coated as per Examples 6 and 7 and incorporated in vehicle tyres. Tests on wear were made as in Examples 8 to 13. The decrease in cross-section after travelling for 12000 km was 0.9 ± 0.3 mm, and the wear on the tread was type A.

Example 18 (Use of sintered round silicon nitride particles) The mechanically resistant particles were approximately spherical (Krumbein roundness > 0.9) particles of sintered silicon nitride about 2 mm in diameter. The particles are commercially obtainable from Messrs Nippon Kagoku Togyo Co. Ltd., Osaka, Japan, reference SUN-11. The bulk density of the particles was 1.80 kg/1.

When the compressive strength was tested under the conditions described in Example 5 (amount of sample 10 g, diameter 29 mm, final force 20 kN), no material was comminuted. - 30 The silicon nitride particles were coated as in Examples 6 and 7 and incorporated in vehicle tyres.

Tests on wear were made as in Examples 8 to 13. The 5 decrease in cross-section after travelling 12000 km was 0.9 ± 0.3 mm, and the observed wear on the tread I was Type A.

Claims (5)

1. A composite material comprising an elastomer matrix with embedded primer coated mechanically 5 resistant particles, wherein the mechanically resistant particles are substantially free from sharp corners and edges and strength-reducing structural faults and at least 90% by weight of the mechanically resistant particles have a Krumbein roundness of at least 0.3.

2. A composite material according to claim 1, wherein at least 80% by weight of the mechanically resistant particles have a Krumbein roundness of at least 0.5.

3. A composite material according to claim 1 or 2, wherein the proportion by volume of mechanically resistant particles in the total volume of material is from 1 to 35%.

4. A composite material according to claim 3, wherein the proportion by volume of mechanically resistant particles in the total volume of material is from 5 to 20%. - 32 5. A composite material according to one or more of claims 1 to 4, wherein the elastomer matrix includes a natural or synthetic rubber and, optionally, conventional fillers and adjuvants. 6. A composite material according to one or more of claims 1 to 5, wherein the mechanically resistant particles consist of oxides, carbides, nitrides or borides of metals or metalloids or mixtures of such 10 compounds with one another or with metals. 7. A composite material according to one or more of claims 1 to 6, wherein the mechanically resistant particles consist of silicon carbide, silicon nitride, 15 corundum, zirconium corundum or a mixture of any of these. 8. A composite material according to one or more of claims 1 to 7, wherein the mechanically resistant 20 particles consist of silicon carbide and the average particle size is 0.2 to 5 mm. -33 9. A composite material according to one or more of claims 1 to 8, wherein the primer coating consists of at least two layers. 5 10. Use of the composite material according to any of claims 1-9 for transfer of frictional forces. 11. Use of the composite material according to any of claims 1-9 in vehicle tyre treads, more particularly 10 for use on wet and/or smooth roads. 12. A method of manufacturing rounded compression-proof mechanically-resistant particles, wherein mechanically resistant particles of arbitrary 15 shape are subjected to combined frictional and impact stress in a liquid medium until they are substantially free from sharp corners and edges and strength-reducing structural faults and until at least 90% of the total weight of the particles has a Krumbein roundness of at 20 least 0.3. 13. A method according to claim 12, wherein the frictional and impact stress is exerted by treatment in an agitator mill, an annular gap mill or «σι attritor. - 34 14. A method according to claim 13, wherein no additional grinding elements are used in addition to the mechanically resistant particles during treatment in the agitator mill, annular gap mill or attritor. 15. A method according to any of claims 12 to 14, wherein the liquid medium used is water. 16. A method according to one or more of claims 12 10 to 15, wherein the starting material consists of mechanically resistant particles which have already been partly rounded by other means. 17. Rounded compression-proof mechanically 15 resistant particles, obtainable by the method according to any of claims 12-16. 18. Mechanically resistant particles according to claim 17, wherein said particles consist of oxides, 20 carbides, nitrides or borides of metals of metalloids or mixtures of such compounds with one another or with metals. -35 _ 19. Mechanically resistant particles according to claim 18, wherein said particles consist of silicon carbide, silicon nitride, corundum, zirconium corundum or mixtures of any of these. 20. Mechanically resistant particles according to claim 19, wherein said particles consist of silicon carbide and have an average size of 0.2 to 5 mm. 10 21. Use of the mechanically resistant particles according to claim 17 for manufacturing composite materials with an elastomer matrix. 22. A composite material substantially as 15 hereinbefore described in the non-comparative examples 23. A method of forming rounded compression-proof mechanically resistant particles substantially as hereinbefore described in the non-comparative examples 24. A method of forming a composite material substantially as hereinbefore described in the non-comparative examples. .36 25. A vehicle tyre comprising a composite material as claimed in any of claims 1-9 and 22. 26. A vehicle tyre comprising a composite material

5. Including particles as claimed in any of claims 17-20.