CA2043663A1

CA2043663A1 - Composite material with particles of mechanically resistant material

Info

Publication number: CA2043663A1
Application number: CA002043663A
Authority: CA
Inventors: Reiner Kunz; Rudolf Strittmatter
Original assignee: Reiner Kunz; Rudolf Strittmatter; Lonza Ltd
Current assignee: Lonza AG
Priority date: 1990-05-31
Filing date: 1991-05-31
Publication date: 1991-12-01
Also published as: FI912566A0; IE911504A1; NO912092L; NO912092D0; EP0461469A3; EP0461469A2; FI912566A; JPH04227937A; BR9102130A

Abstract

ABSTRACT OF THE DISCLOSURE

Composite materials are disclosed which consist of an elastomer matrix, especially rubber based, and embedded rounded, edge-free particles of mechanically resistant material coated with an adhesive. The materials exhibit a great wear resistance and good anti-skid properties or smooth surfaces. They are especially suitable for use in the treads of vehicle tires. A process is also disclosed for the production of rounded, edge-free and pressure-resistant particles of mechanically resistant material by mechanical treatment of initial particles of any shape.

Description

This invention relates to a composite material made from an elastomer matrix with embedded particles of mechanically resistant material. The material is used for transmission o~ frictional forces, especially in the tread of vehicle tires. The invention further ralates to the pressure-resistant rounded particles of mechanically resistant material necessary for this purpose and includes a process for the production thereof.
Elastomer materials, especially those based on natural and/or synthetic rubber, because of their special mechanical properties are used in the art to a great extent for transmission of frictional forces. The most important use by far is for the production of vehicle tires o~ all types, especially for motor vehicles. The treads of these vehicle tires not only bear the weight of the vehicle, but also, especially, transmit the driving, braking and lateral forces reliably to the road. ~n this connection, the quality and condition of the road can vary quite considerably and, as a result, the ~orce transmission in any individual case can be more or less adversely affected.
The greatest difficulties generally occur if the road is covered with ice or snow or consists of a material with a smooth surface, for example basalt or granite, and is coated with a water film, possibly also with lubricating fillers.
To a limited extent, an improved static friction on an icy or wet road can be achieved by matching the elastomer mixture and the tread profile to these special conditions. But especially the first-named measure generally worsens ~he wear characteristics, i.e. reduces the achievable tread performance.
A considerable improvement of tire tread adherence on ice was achieved by studded tires, in which hard metal spikes or studs were used in the tread of the tires, whereby one end of the spike in each case projected slightly from the surface of the tread. When the tire `, :, ~ `' ` : ~ .
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rolled on the ice, the spike was capable of penetratin~ it and thus to a certain extent for a moment produced a positive connection. However, this technical solution exhibited serious drawbacks, so that the use of such tires on public streets was prohibited in most countries after a few years. ~he hard metal spikes have a relatively great weight. As a result, they can come loose at high rotational speeds of the wheel and then fly about in an uncontrolled manner and possibly causing harm to persons and objects. More commonly and thus more importantly, at moderate speeds the spikes act like chisels on the road surface and lead thereby to an unacceptably high wear and tear of road surfaces.
Proposals have not been lacking in the past to improve the static friction of vehicle tires by a simple incorporation of particles of mechanically resistant material, such as corundum or silicon carbide abrasive grain, in the rubber mixture of the tread. Thus, for example, in French Patent No. 1,365,~06 (1964), it was proposed to incorporate abrasive grains of corundum, silicon carbide, boron carbide, emer~, aluminum oxide, quartz, etc., into the tread or parts of it.
However, such measure, appearing simple at first glance, did not lead to the hoped-for results. First, these particles of mechanically resistant material are not compatible with the rubber mixture, i.e. the adhesion between the mechanically resistant material and rubber is so slight that the particles, once they are on the surface of the tread -- and only there can they perform the desired action -- are quickly torn out of the material compound by the mechanical action of force and thus become ineffective.
This partial problem was satisfactorily solved by an appropriate surface coating of the particles. Such a coating is described in published PCT Application W089/06670. However, testing of vehicle tires which were produced using such coated particles of mechanically ' ~ 3~

resistant material showed that a good adhesion between mechanically resistant material and a rubber mixture, by itself, still does not guarantee an adequate life of the tires. On the one hand, with passage of time/ the sharp edges of the particles by the flexing stress of the tread, cut the rubher and beqin to miyrate, which in the most favourable case leads only to the loss of the particles.
The consequences are more serious if individual particles migrate inward, for this can lead to gradual destruction of o the tire, and especially with tubeless pneumatic tires to leakage or even blowout of the tire. In addition, by the constant impact stress in striking the road surface, especially those particlss that exhibit defects, such as large pores, inclusions, cracks, grain boundaries, lattice defects and dislocations, are easily destroyed. The resultant fragmented pieces generally exhibit especially sharp edges and are not provided on the new fracture surfaces with the coupling coating so that the above-named problems increase even more.
The main object of the invention is to provide a composite material from the elastomer matrix, especially rubber-based, with embedded particles of mechanically resistant material, which does not exhibit the above-mentioned disadvantages, which can be produced simply and economically and which is especially suitable for use in v~hicle tires.
Accordingly, one aspect of the invention provides a composite material comprising an elastomer matrix having embedded therein particles of mechanically resistant material coated with an adhesive, the particles of mechanically resistant material being substantially free from sharp corners and edges and strength-reducing structural defects and at least 90 percent of the total mass of particles of mechanically resistant material being particles with a roundness according to Krumbein of at least 0.3.

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~ 3~ $~ 3 Another aspect of the invention provides a process for the production of rounded particles of mechanically resistant material resistant to pressure, comprising subjecting particles o~ a mechanically resistant material of any shape in a liquid medium to a combined friction and impact stress until they are essentially free of sharp corners and edges and strength-reducing structural defects and at least 90 percent of the total mass of the particles exhibit a roundness according to Krumbein oP at least 0.3.
Thus, the invention involves a composite material of an elastomer matrix with embedded particles of mechanically resistant material coated with an adhesive.
The particles of mechanically resistant material are substantially free from sh~rp corners and edges and strength-reducing structural defects. At least 90 percent of the total mass of the particles of machanically resistant material is accounted for by particles with a roundness according to Krumbein of at least 0.3. As is wçll known in the art, a variety of different methods may be used to measure the roundness of particles, e.g. as described by Wadell in Journal of Geoloqy, Vol. 40, pp. 443 to 451, 1932. Preferably, however~ the rapid method of measuring the roundness described by Krumbein in "Measurement and Geological Significance of Shape and Roundness of Sedimentary Particles" in Journ~al_ of Sedimentary Petrology, Vol. ll, No. 2, pages 64 to 72, August, 1941 is utilized. Hereinafter, roundness measurements obtained under this method will be referred to as measurements 1'according to Krumbein".
The invention also involves the rounded particles of mechanically resistant material resistant to pressure, obtainable according to the process of the invention.
It was surprisingly found that the properties, especially the life, of composite materials known from the prior art and made of an elastomer matrix with embedded 3 ~ ~ 3 particles of mechanically resistant material, can be considerably improved by using particles of mechanically resistant material which are basically free from sharp edges and have a rounded shape. Free from sharp edges, in this case, means that there are basically no convex outer edges formed by surfaces inters~ecting at an acute angle.
The radius of curvature of the remaining corners and edges is as large as possible, preferably at least 20 percent of the particle diameter. Ideally, the particles have an almost spherical or ellipsoidal shape. If the particles are produced by starting with irregularly shaped particles, as may be obtained, for example, by breaking of lumpy material, this ideal shape cannot be fully achieved. The degree of roundness of irregularly shaped particles, despite the immediate descriptiveness of the term "roundness", can be quantitatively described only with difficulty. M.H. Pahl, G. Schaedel and H. Rumpf, for example, give an introduction to this problem in Aufbereitungs-Technik, (1973), pages 759 to 764. Although there are some exactly defined methods for making roundness measurements, they have not been generally accepted in practice because of the high expense which is necessary for their use in individual cases. A preferred method which is su~ficient for most practical requirements and which is simple to perform and therefore in very widespread use is that of Xrumbein (see Journal of Sedimentary Petrology, Vol. 11, No. 2, pages 64 to 72, August 1941, which was cited above) which is based on visual comparison of the particles to be examined, optionally with appropriate enlargement, with standard images of exactly determined roundness. By this comparative method of Krumbei , a reliable roundness value may quickly be assigned to the particle being examined. However, this process with rounded and subsequently broken particles, which, despite a largely rounded surface, have some sharp edges in the area of the fracture surface, is not entirely satisfactory.

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2 ~ 3 The particles of mechanically resistant material used according to the invention therefore are advantageously characterized by their roundness according to Krumbein, with the additional proviso that they do not exhibit any sharp fracture edges as described above.
Suitably, at least 90 percent of the total mass of particles exhibits a roundness according to Krumbein of at least 0.3, preferably at least 80 percent of at least 0.5.
Especially preferred are particles with a roundness of 0.6 and greater.
According to the invention, particles of mechanically resistant material with the necessary properties can be produced by subjecting particles of mechanically resistant material of any shape to a mechanical treatment. such treatment gives the particles a rounded and basically edge-free surface and, at the same time, eliminates those particles which exhibit coarser structural defects, such as, pores, inclusions, cracks and the like.
This is achieved according to the invention by subjecting the particles, which initially exhibit mostly an irregular shape with sharp edges caused during production, especially if they are obtained by the breaking of coarse material, to an intensive friction and impact stress in a liquid medium.
Advantageously, the particles are treated ~or this purpose in a stirred ball mill, a ring gap mill or attrition mill or a similar davice. These devices, which are known in the art, are usually fed with grinding media and are used for pulverizing and also for deagglomerating materials such as ceramic powders or pigments.
Grinding media is not necessary for the process according to the invention and preferably it is performed without grinding media so that the particles to be treated themselves strike and rub against one another. In this way, the particles are not only rounded, but also ~ t~

microscopically roughened sur~aces are obtained which favour the adhesion of the ela~stomer matrix. Moreover, particles with insufficient strength in this case are smashed to smaller fragments, which, together with the grit of the other particles after completion of the treatment, can easily be separated by screening or settling and therPafter otherwise used. The treatment is preferably performed so that the particles in the stirred ball mill or the attrition mill in the state of rest are just covered with the liquid. Preferably water is used as the liquid since it is not only cheap, but also exhibits a favourable viscosity and a high heat capacity.
Especially advantageously, the particles are partially rounded in a preceding step so that, in the process according to the invention, less grit accumulates and the treatment time can be shortened. Such a partial rounding, in which basically only projecting edges and corners are broken, can be performed, for example, according to the process described in European Patent No.
0,082,816.
Suitable materials for use as particles of mechanically resistant material, include basically all materials which have a sufficient hardness and are not too brittle. Include~ here are basically oxides, carbides, nitrides and borides of metals or semimetals or mixtures of such compounds with one another or with metals (cermets).
To these classes belong, for example, aluminum oxide (corundum), aluminum oxide~xirconium oxide (zirconium corundum), silicon carbide, boron carbide, titanium carbide, tantalum carbide, tungsten carbide, silicon nitride, titanium nitride, tantalum nitride, boron nitride and titanium boride. The compounds can be present in pure form or contain the usual impurities and/or auxiliary agents, such as sintering additives or binders.
The process according to the invention is especially suitable ~or those mechanically resistant ~ .. ; .
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materials which~ because of production, occur as solidified melts or coarse crystal masses, such as corundum and zirconium corundum or silicon carbide, since these materials after the necessary comminution yield especially irregular and sharp edged particles.
Particles of mechanically resistant material usable according to the invention can also be produced by synthesizing processes, for example by granulating and sintering of ceramic powders, optionally with the addition of sintering additives and/or temporary or permanent binders, or by drying and sintering of gel beads. Such processes are known in the art.
The particles of mechanically resistant material, rounded according to the invention, before their incorporation in an elastomer matrix, are suitably provided with a coating (in a manner known in the art~, which coating can be built up in one or more layers. In this case, it can be advantageous if the particle surfaces are not absolutely smooth, but exhibit a microscopic roughness and thus facilitate the adhesion of the coating.
For their incorporation into a rubber-based matrix, as used in the production of vehicle tires, the multilayer coatings which are described in published PCT
Patent Application No. W089/06670 are especially suitable.
25The incorporation of the coated particles of mechanically resistant material into th~ material of the elastomer matrix may take place in a manner known in the art, for example by mixing and kneading. In this case, the rounded shape of the particles is also advantageous, since, in comparison with the sharp-edged particles, the devices used are subjected to considerably less wear.
I'he devices used can set a maximum limit on the usable particle size. If a calender or roller frame is used, the ratio of nip width: particle size should be greater than 2:1, preferably greater than 2.5:1 to avoid crushing the particles and/or damaging the rollers.

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Moreover, in tire production, the particles should be smaller than the details of the vulcanizing mold to prevent damage of the mold or jamming of particles in gaps and slots.
The particles of mechanically resistant material according to the invention can not only be used in the treads of vehicle tires of all types, including aircraft tires and those of machines moving on elastically tired wheels or chains with elastic supports, but can also be used for all other uses in which friction forces can be transmitted by an object on an elastomer base. All of the above uses are within the framework of the invention and include, for example, shoe soles, conveyor belts, non-skid elastic floors or linings such as loading areas of transport means and non-skid bases for stationary objects such as furniture or machines and the like.
Depending of the intended use, besides natural and synthetic rubbers, optionally in chemically modified form, other elastomers can also be used, for example polyurethane-based elastomers. Also, the size of the particles of mechanically resistant material as well as their composition are advantageously matched to the US8, as is also the ratio between the matrix and the amount embedded particles.
The maximal size of the particles, as already mentioned, is limited by the dimensions of the devices used in the processing. The plannsd use can set other limit values. Particles which are too small with a size of less than about 0.2 mm act only as filler. Particles which are too large lead to irregular properties and, for example with vehicle tires, to increased noise generation and increased wear of the road surfaces.
The sizes of the individual particles in individual uses should not be too different, since small particles in the presence of considerably larger particles can make no substantial contribution to the effect achieved :' .

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by the invention. But it is not necessary that all particles ~e of equal size~
The ratio between the amount of the particles of mechanically resistant material and the matrix material is advantageously selected so that the volume of the particles of mechanically resistant material is between 1 and 35 percent, based on the overall composite material. The volume in this case relates to the part of the composite material actually intermingled with the particles of mechanically resistant material, if the particles are not uniformly distributed in the entire material. At volumes below 1 percent, too few particles come to the surface to be able to achieve a satisfactory effect, while at volumes above 35 percent, the elasticity of the material is greatly reduced. Preferably, the volume of the particles of mechanically resistant material is between 5 and 20 percent. For the tread mixture of vehicle tires, volumes of 6 to 12 percent are especially preferred.
Depending on the use, it can be advantageous to locate the particles either only in the near-surface part of the object or to distribute them uniformly in it.
For use in vehicle tires, silicon carbide particles are especially preferred because of their great hardness and favourable price. The size of the particles in this case is prefera~ly between 1 and 5 mm. In the case of silicon carbide, larger particles consist mostly of several crystallites and therefore exhibit less strength.
In the accompanying drawings:
- Figure 1 is a microphotograph of the product of Example 1;
Figure 2 is a microphotograph of the product of Example l;
Figure 3 is another microphotograph regarding Example 3; and Figure 4 is a diagrammatic representation of wear types.

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The following Examples illustrate embodiments of the invention.
Example 1 (Pretreatment of particles according to the prior art) Silicon carbide grains (Carsilon~ 9899, LONZA
Werke) were rounded according to Example 1 of European Patent No. 0,082,816 and then graded by size. A grain size range of 1.55 to 4.0 mm was used for the following examples. Figure 1 shows the typical particle shape of the untreated silicon carbide grain; and Figure 2 depicts the shape after pretreatment. The figures clearly show the sharp edges as well as the projections and recesses of the untreated grain and the rough rounding by pretreatment, which leaves the strength-reducing recesses and pores basically unchanged. The roundness according to Kxumbein of the untreated particles was about 0.1; that of the treated was in the range of 0.2 to 0.5.
Example 2 7.2 kg of the particles pretreated according to Example 1 were placed with 2 liters of water in an attrition mill (Netsch Company, Model PR15). The particles are attrited for a total of 28 hours, and the water was changed after 10 and 20 hours to remove the grit. After the end of the attriting process, the particles were washed with water, dried at 200C and those particles with a size less than 1.55 mm were removed by screening. The yield of particles with a size above 1.55 mm averaged 60 percent of the original amount. The roundness according to Krumbein of the treated particles was basically in the range o~ 0.4 to 0.7; paxticles with a roundness below 0.3 were no longer present.
Example 3 A stirred ball mill (Drais Company, Model PM
12.5) was charged with 20 kg of the particles pretreated according to Example 1 and 50 liters of water. The mill was operated for 3 hours, and the water was continuously .

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S~ ~ ~ 3 circulated between storage container and grind container.
Then the particles were washed in the grind container with fresh water. Drying and screening took place as in Example 2. The yield of particles above 1.55 mm in size averaged about 55 percent of the original amount. The typical particle shape after the treatment according to Example 2 or 3 is depicted in Figure 3. In their shape, the particles look somewhat like potatoes in that they have no outside edges, exhibit a dull lightly roughed surface and no longer exhibit deep holes and pores. The roundness according to Krumbein of the particles thus treated was basically between 0.5 and 0.8.
Example 4 (Determination of compressive strength) Method of measurement: A defined amount (about 2 g) of graded silicon carbide grains (2.00 to 2.36 mm in size) was put in a cylindrical mold consisting of a die (bore diameter 13.5 mm), a fixed lower punch and a mobile upper punch and was shaken for about 1 minute to obtain a compact grain bed. After positioning of the upper punch, the mold was inserted in a strength testing machine (Swi¢k Company, Universal Testing Machine Model 1478) and with a feed rate of 1 mm/min was loaded to a final force of 0.8 kN
or 1.5 kN. After reaching the final force, the feed was cut off and the pressure drop was determined after 1 minute. As a measurement of compressive strength, the comminuted portion (smaller than 2.00 mm) of the grain bed separable by screening was determined. The measurement results are summarized in Table 1. By comparison with pretreatment according to the prior art, a considerable reduction of the portion of crushed particles from 26 to 15 percent by weight is shown at high pressure~

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Table l Final Pressure Comminuted pressure drop portion 5 Specimen [MPa] [MPa] [% by weight]
.. . .. . _ _ . _ . .. . _ untreated 5.5g 0.22 46.5 according to:
Example 1 5.59 0 0 Example 2 5.59 o o 15 Example 1 10.48 0.27 26 Example 2 10.48 0.20 15 _ _ .
Example 5 Analogously to Example 3, silicon carbide grains of Carsilon~ 9899 were treated for 2, 4 and 6 hours and examined analogously to Example 4 for their compressive strength. Deviating from Example 4, the mold die had a bore diameter of 29 mm, the weight of e~ch specimen was 10 g and the final force was 20 kN. Moreover, the bulk density in each case for the grain size range 1.55 to 3.0 mm was determined. Table 2 shows the measured values in comparison with untreated particles and particles pretreated according to the prior art (Example 1).
Table 2 Comminuted Bulk Density Portion Specimen ~kg/l] [% by weight]

35 untreated 1.50 64 according to:
Example 1 1.59 59 40 Example 3 2 hrs. 1.87 31 Example 3 4 hrs. 1.88 27 Example 3 6 hrs. 1.89 23 .. . . _ .. .

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2~3~3 Example 6 (Coating of the particles) The 1.55 to 3.35 mm fraction was screened out from the silicon carbide particles treated according to Example 3 and further processed as follows:
I. 57 g of Chemosil~ 211 (Henkel) was added to 1 kg of sic particles, distributed uniformly on the particle surface in a rotary table and dried with hot air at about 80C. Then the particles were passed through a screen (mesh size 4 mm) to break up possible resulting agglomerates.
II. The SiC particles were again put into the rotary table and coated analogously to Step I with 136 g of Chemosil~ 221 (Henkel). In this case, after brief initial drying (about 5 to 10 minutes), large agglomerates (about 2 to 10 cm in diameter) resulted. These agglomerates were mechanically broken up starting from the surface. At the same time the particles were dried by feeding of hot air.
For control, the particles were then again passed through a screen (mesh size 4 mm).
III. The SiC particles were again wetted with 300 g of a rubber solution and then dried with hot air (about 80)o The rubber solution consisted of 15 percent by weight of rubber type V2/30 (Nuova Piovanelli Gomma, Milan) in heptane/toluene (50 percent by volume each).
Finally, the SiC particles were again passed through a screen (mesh size 4 mm).
Example 7 (Procluction of vehicle tires) The tread mixture (type V2/30, Nuova Piovanelli) was homogenized in a water-cooled kneader (T less than 80C) for about 5 minutes and then transferred to a calender. In the calender the particles of mechanically resistant material, coated according to Example 6, where incorporated into the rubber compound, i.e., homogeneously : ~ . : . : . . ~ , -:: . - : ` ~ : .

distributed by multiple repetition of the calendering process. Then the calender strip was cut to a size corresponding to the size of the tire to be produced. The strip thickness was about 8 mm, the proportion of the SiC
particles, based on tha rubber mixture, was about 8 percent by volume. The strip cut to size was vulcanized on a basic tire (Michelin) prepared for recapping at 150~ ~ 3C and 12 + 0.5 bars with a holding time of 29 + 0.5 minutes. In this case, the tire was acted on from the inside with pressure and pressed against the rigid mold. The profile of the mold and thus the produced tread profile was of the type as Goodyear Ultragrip~ 2. The finished tire was then either run over a distance of 1,000 to 2,000 km, or treated on the tread with a steel wire brush to expose partially the embedded particles of mechanically resistant material, so that they were able to fully display their action.
Examples 8 t_ 13 (Wear tests) Tires of size 175 R 14 were pxoduced as described in Example 7, but their treads were not pretreated. The tires were mounted on steel rims, electronically balanced and put on the driving axle of a delivery truck of the model Renault Traffic Van~. A distance of about 12,000 km was then covered with this vehicle, and predominantly (about 70 to 80 percent) expressways and highways were used. The maximum vehicle speed was about 130 km/h and, in each case, was maintained over rather long distances. A
negative influence on the driving qualities or an increase of the tread noise was not observed.
After 12,000 km distance, the tires were taken off and the wear (decrease of the profiled depth in mm) in the tire center was measured at several places and averaged. The wear tests were performed with tires without particles of mechanically resistant material (Example 8, comparison example), with different tires with particles of mechanically resistant materials without mechanical , .

3 ~ ~ 3 pretreatment (Examples 9 and 11), with tires with particles rounded with an impact process (Example 10), with tires with particles pretreated according to Example 1 (Example 12) and with tires with particles treated according to the invention according to Example 3 (Example 13).
All particles of mechanically resistant material used were coated according to E~ample 6 and had a ~ize of 1.55 to 3.55 mm. As particles of mechanically resistant material, besides those mentioned in Examples 1 and 2, there were used: Diadur~ (zirconium corundum, LONZA-Werke, Example 9) and Abradux~ Tl (normal corundum, LONZA, Example 10).
Besides the decrease of the profile depth, in each case the wear of the particles of mechanically resistant material was determined by inspectioll of the tread. For this purpose, four types of wear were distinguished:
Type A: the particles are intact and in unloaded state partially project from the profile surface.
20Type B: the particles are intact and in unloaded stata are flush with the profile surface.
Type C: the particles are intact but lie deeper than the profile surface. But with rotating and loaded tires khey come in direct road contact by the centrifugal force and khe resilience of the rubber matrix on the contact surface.
Type d: the particles are disintegrated - the fragments have partially migrated into the matrix or ~allen out.
30The different wear types are represented diagrammatically in Figure 4: the test results are summarized in ~rable 3.

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Abradux~ T1 1.2 + 0.3 A, B
15 Carsilon~ 9899 untreated 1.6 ~ 0.3 C, D
Carsilon~ 9899 according to Example 1 1.1 + 0.3 A, C, D
Carsilon~ 9899 according to Example 3 0.9 + 0.3 A
Examples 14 to 16 (Driving tests on ice~
Tires of size 155 R 13 with different volume portions of silicon carbide particles treated according to Examples 3 and 6 were produced according to Example 7 and mounted on all four wheels of a vehicle of the model Volkswagen Rabbit CL.
After a test distance of 3,000 km each, the following tests were performed in an ice stadium:
A stretch in the form of an "8" was marked (length about 150 m, width about 7 m), which had to be travelled in the shortest possible time. In each case, an ;average time of 10 laps was determined. In each case, the temperature of the ice was -5C at 2 cm depth.
Tires with proportions by volume of 0 percent (Example 14, comparison example), 8 percent (Example 15), and 12 percent (Example 16) of SiC particles in the tread mixture were tested.
The results of the road tests are summarized in Table 4.

, ~' ! , ' ' , Table 4 Portion by volume Average Time of sic particles [%] per lap [s]

0 39 + 2.4 8 34 + 1.9 12 32 -~ 1.9 ~ . . . _ _ . _ . _ _ _ Example 17 (Use of pressureless sintered round silicon carbide particles) Approximately spherical (i.e., roundness according to Krumbein larger than or equal to 0.9) particles from pressureless sintered silicon carbide with a diameter of 2 to 2.36 mm were used as particles of mechanically resistant material. The particles were produced by granulation of ultrafine silicon carbide powder with sintering auxiliary agents in a fluid bed spray granulator and subsequent pressureless sintering in a loose bed; they are commercially available from the Saechsische Ingenieurkeramik GmbH, D-0-8273 Coswig. The bulk density of the particles was 1.75 kg/l. An examination of the compressive strength under the conditions descri~ed in Example 5 (specimen amount 10 g, diameter 29 mm, final force 20 kN) yielded a comminuted portion of only 8 percent by weight. The silicon carbide particles were coated analogously to Examples 6 and 7 and incorporated in vehicle tires. Wear tests were performed analogously to Examples 8 to 13. The profile decrease after 12,000 km run was 0.9 + 0.3 mm; the wear of the tread was of Type A.
Example 18 (Use of sintered round silicon nitride particles) Approximately spherical ~i.e., roundness according to Krumbein, larger than or equal to 0,9) particles from sintered silicon nitride with a diameter of about 2 mm were used as particles of mechanically resistant .; , .

2~3~3 material. The particles are commercially available from Nippon Kagoku Togyo Co., Ltd., Osaka, Japan, under the designation SUN-ll. The bulk density of the particles was 1.80 kg/1. In an examination of the compressive strength under the conditions described in Example 5 (specimen about 10 g, diameter 29 mm, final force 20 kN), no formation of comminuted material was determined. The silicon nitride particles were coated analogously to Examples 6 and 7 and incorporated into vehicle tires. ~ear tests were performed analogously to Examples 8 to 13. The profile decrease after 12,000 km run was 0.9 + O.3 mm; the observed wear of the tread was of Type A.

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Claims

1. A composite material comprising an elastomer matrix having embedded therein particles of mechanically resistant material coated with an adhesive, the particles of mechanically resistant material being substantially free from sharp corners and edges and strength-reducing structural defects and at least 90 percent of the total mass of particles of mechanically resistant material being particles with a roundness according to Krumbein of at least 0.3.

2. A composite material according to claim 1, wherein at least 80 percent of the total mass of the particles of mechanically resistant material is accounted for by particles with a roundness according to Krumbein of at least 0.5.

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

4. A composite material according to claim 1 or 2, wherein the proportion by volume of the particles of mechanically resistant material in the total volume of the composite material is from 5 to 20 percent.

5. A composite material according to claim 1, wherein the elastomer matrix is formed from an elastomer based on natural and/or synthetic rubber, optionally with addition of the usual fillers and auxiliary agents.

6. A composite material according to claim 5, wherein the particles of mechanically resistant material consist of oxides, carbides, nitrides or borides of metals or semimetals or mixtures of such compounds with one another or with metals.

7. A composite material according to claim 6, wherein the particles of mechanically resistant material consist of silicon carbide, silicon nitride, corundum and/or zirconium corundum.

8. A composite material according to claim 7, wherein the particles of mechanically resistant materials consist of silicon carbide having an average particle size of 0.2 to 5 mm.

9. A composite material according to claim 1, 2, 5, 6, 7 or 8, wherein the adhesive layer consists of at least two layers.

10. A process for transmission of friction forces, comprising using the composite material according to claim 1.

11. A process comprising using the composite material according to claim 1 in the treads of vehicle tires for use on wet and/or slippery roads.

12. A process for the production of rounded particles of mechanically resistant material resistant to pressure, comprising subjecting particles of a mechanically resistant material of any shape in a liquid medium to a combined friction and impact stress until they are essentially free of sharp corners and edges and strength-reducing structural defects and at least 90 percent of the total mass of the particles exhibit a roundness according to Krumbein of at least 0.3.

13. A process according to claim 12, wherein the friction and impact stress is exerted by treatment in a stirred ball mill, a ring gap mill or an attrition mill.

14. A process according to claim 13, wherein no additional grinding media are used besides the particles of mechanically resistant material to be treated.

15. A process according to any of claims 12 to 14, wherein water is used as liquid medium.

16. A process according to any of claims 12 to 14, wherein particles of mechanically resistant material which have already been partially rounded in another way are used as initial material.

17. Rounded particles of mechanically resistant material resistant to pressure, which have been obtained according to the process of claim 12.

18. Particles of mechanically resistant material according to claim 17, which consist of oxides, carbides, nitrides or borides of metals or semimetals or mixtures of such compounds with one another or with metals.

19. Particles of mechanically resistant material according to claim 18, which consist of silicon carbide, silicon nitride, corundum and/or zirconium corundum.

20. Particles of mechanically resistant material according to claim 19, which consist of silicon carbide and have an average particles size of 0.2 to 5 mm.

21. A process comprising using the particles of mechanically resistant material according to claim 17, 18, 19 or 20, for the production of composite materials with an elastomer matrix.