Note: Descriptions are shown in the official language in which they were submitted.
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
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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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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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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
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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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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
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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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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
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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
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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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Table 3
Profile
5 Mechanically Resistant Decrease
Material [mm] Wear Type
1.6 + 0.3 ---
Diadur~ 1.4 + 0.3 B
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.
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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
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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
19
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.
: ` :
