Rubber composition

A rubber composition which contains, in a matrix consisting mainly of rubber, irregularly shaped particles having high hardness, and subjected to surface treatment for improving their affinity for the matrix, or their adhesion to it.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a rubber composition which is particularly 
suitable for making the treads of tires and the soles of shoes. 
2. Description of the Prior Art 
The treads of tires on the wheels of a vehicle have a by far smaller 
friction force on a frozen road surface than on a normal dry road surface. 
Spiked tires or tire chains are used to improve the friction of the treads 
on a frozen road surface and thereby the safety of the vehicle which is 
driven on such a surface. Both the spiked tires and the tire chains, 
however, damage the road surface when the vehicle is caused to make a 
sharp turn, or a sudden start or stop. The damaged road surface forms dust 
which is scattered by wind when the surface has dried. Moreover, the 
spikes or chains on the tires make noise as they hit the road surface when 
the vehicle is running. 
A number of technical approaches have recently been made to improve the 
friction on ice of the rubber which is used for making the treads of 
tires. They are classified into two main groups of methods. One of the 
first group of methods comprises foaming rubber to form closed cells in 
it, as proposed in Japanese patent application laid open to the public 
under No. 89547/1988. This method gives rubber having a surface covered 
with a multiplicity of pores which exert a sucking effect on ice, exhibits 
a water-absorbing effect, and undergo a microscopic behavior causing the 
loss of energy, so that the rubber surface may produce a large amount of 
friction on ice. Studless tires formed from such rubber are commercially 
available. 
Another attempt belonging to the first group that has been proposed 
comprises incorporating various kinds of materials into rubber, so that 
when the tires formed from such rubber and mounted on a vehicle are 
rotating, the incorporated material may drop from the tires and thereby 
form pores in the surfaces thereof. Sand, chaff, or other natural 
materials are, for example, incorporated into rubber. This method also 
gives a tread surface which exhibits a high degree of friction on ice. 
The second group of methods comprises incorporating various kinds of 
materials having a high degree of hardness into rubber, so that the 
incorporated material may exert a scratching effect on ice to enable a 
tread surface to exhibit a high degree of friction on a frozen road, as 
proposed in Japanese patent publication No. 31732/1971, Japanese patent 
application laid open to the public under No. 147803/1976 and Japanese 
patent publication No. 52057/1981. These methods are definitely based on a 
mechanism which differs from that on which the first group of methods 
relies. As a matter of fact, it is often likely that the incorporation of 
a greater amount of any such material may give rubber exhibiting a higher 
degree of friction on ice. 
It is, however, to be noted that not only friction, but also high wear 
resistance is essentially required of any rubber used to make a tread, or 
anything else that will be brought into frequent contact with a road 
surface. No rubber exhibiting a high degree of friction on ice is suitable 
for making a practically acceptable tread, if it is low in wear 
resistance. None of the known methods as hereinabove described can, 
however, produce rubber exhibiting both a high degree of friction on ice 
and a high degree of wear resistance. Rubber having both of the two 
properties is very difficult to obtain even by the incorporation of any 
hard material as hereinabove mentioned. It has hitherto been believed that 
the two properties as hereinabove stated cannot be attained at the same 
time. 
SUMMARY OF THE INVENTION 
Under these circumstances, it is an object of this invention to provide a 
rubber composition which exhibits both a high degree of friction on ice 
and a high degree of wear resistance. 
This object is attained by a rubber composition which comprises a mixture 
of materials forming a matrix of rubber, and irregularly shaped particles 
having high hardness and subjected to surface treatment for improving 
their affinity for the matrix. 
Although the rubber composition of this invention is prepared by a method 
which may basically appear to belong to the second group as hereinbefore 
described, the method can definitely be distinguished therefrom by the 
surface treatment of the hard particles employed for reinforcing the 
rubber matrix. 
It is well known that the theoretical achievement of any function, 
particularly of the dynamic nature, by a mixture of materials calls for 
the complete adhesion of its components to one another. It is no 
exaggeration to say that the degree of their adhesion determines the 
properties of the mixture. 
Referring particularly to the composition of the type to which this 
invention pertains, therefore, the adhesion of the hard particles to the 
rubber matrix is very important on both the frictional property and wear 
resistance of the composition. 
According to this invention, the improved adhesion of the particles to the 
matrix is achieved by the surface treatment thereof, and in the event that 
no satisfactory improvement should be achieved by the surface treatment, 
the particles are irregularly shaped so as to be physically held in the 
matrix by an "anchor" effect and thereby attain a satisfactorily improved 
adhesion to it. 
The rubber composition of this invention is a very good material which can 
be used to make, for example, the treads of tires or the soles of shoes. 
Other features and advantages of this invention will be apparent from the 
following description and the accompanying drawing.

DETAILED DESCRIPTION OF THE INVENTION 
The rubber composition of this invention comprises a matrix of rubber, and 
irregularly shaped particles having high hardness and subjected to surface 
treatment for improving their affinity for the matrix. 
The particles preferably have a Vickers hardness number (H.sub.v) of at 
least 20. They may be of an inorganic or organic material. More 
specifically, it is possible to use a wide range of materials including 
ceramics such as Al.sub.2 O.sub.3, ZnO, TiO.sub.2, SiC, SiO.sub.2, 
ferrites, zirconia and MgO; metals such as Fe, Co, Al, Ca, Mg, Na, Cu and 
Cr, or alloys thereof such as brass and stainless steels, or compounds 
thereof such as nitrides, oxides, hydroxides, carbonates, silicates, and 
sulfates; glass, carbon, carborundum, micas, zeolites, kaolin, asbestos, 
montmorillonite, bentonite, graphite, silica, sand, silica sand, wood, 
Syras balloon (one kind of foamed volcanic glass), coal, rock and stone; 
and plastics. 
Examples of suitable plastics are thermoplastics such as polystyrene, 
polyethylene, polypropylene, ABS, polyvinyl chloride, polymethyl 
methacrylate, polycarbonate, polyacetal, nylon, chlorinated polyether, 
polytetrafluoroethylene, acetyl cellulose and ethyl cellulose; 
thermosetting plastics such as phenolic, resorcinol, urea, unsaturated 
polyester, epoxy, alkyd and melamine resins; and, modified, blended or 
polyblended (polymer-alloyed) resins aforementioned. 
It is preferable to use particles having a Vickers hardness (H.sub.v) of at 
least 20, more preferably at least 30, still more preferably at least 50, 
and most preferably at least 500. 
It is preferable to use particles having a size which is over 1000 mesh, 
but under 50 mesh (i.e., 50 to 1000 mesh), more preferably a size of 60 to 
800 mesh, and still more preferably a size of 100 to 500 mesh. 
The particles may be solid or hollow, or foamed (or porous). They are not 
spherical, but are irregularly shaped. It is necessary to use irregularly 
shaped particles, so that they may adhere more strongly to the rubber 
matrix and may also scratch a frozen surface more effectively. 
The irregularity in shape of the particles which are employed in the 
composition of this invention is defined by the following formula: 
##EQU1## 
where lmax is the maximum distance between the surface of the particle 1 
and its center P of gravity, and lmin is the minimum distance 
therebetween, as shown in FIG. 1. 
The irregularity R in shape of the particles is preferably in the range 
shown as R.gtoreq.1.10, more preferably R.gtoreq.1.20, and still more 
preferably R.gtoreq.1.30. 
The surfaces of the irregularly shaped particles are treated so as to have 
an improved affinity for the rubber matrix. A variety of methods can be 
employed for that purpose. According to one method, the particles are 
coated with an adhesive which adheres strongly to both the particles and 
the matrix. According to another method, the particles are coated with 
brass, copper or cobalt by plating, plasmatic evaporation, or sputtering. 
In this case, a still higher adhesive strength is obtained if the matrix 
has a high sulfur content. 
It is also effective to coat the particles with an adhesive rubber 
material. The material may be equal in composition to the matrix, or may 
be different from it. In the latter case, the material can be prepared by 
employing as its rubber component an ordinary solid rubber, a latex of 
emulsion thereof, or a thermoplastic rubber. If the material has a high 
viscosity, it can be dissolved in a solvent to form a solution which can 
be applied to the particles. 
Moreover, it is very effective to coat the particles with a resin. A 
variety of resins can effectively be used. They include polyesters. 
hydroxylated polyesters, polyether polyols, polycaprolactone polyols, 
hydroxylated polyester polyisocyanates, epoxy resins, acrylic resins, 
ethylene-vinyl acetate copolymers, phenolic resins, tolylene diisocyanate, 
glycidyl ether of bisphenol A, polysiloxanes, silicone resins, PVA 
(polyvinyl alcohols), PMMA (polymethyl methacrylates), polyvinyl acetate, 
polyacrylic acids, pitch, methyl methacrylate, and styrene. 
It is also effective to coat the particles with polybornene, thermoplastic 
rubber, or any other resin having shape retaining property. This type of 
resin holds the particles firmly at a freezing temperature, and softens at 
an elevated temperature to exhibit good affinity for the matrix to achieve 
a greatly improved result. 
The surface treatment of the particles with a silane, titanate, chromium, 
or aluminum coupling agent, or polyalkylene oxides is also effective for 
improving their adhesion to the matrix. 
The coating which is formed on the surfaces of the particles by any of a 
variety of materials as hereinabove mentioned, has a thickness (l) which 
is preferably smaller than the diameter of the particles (l.sub.0 ; the 
diameter of a sphere which is equal in volume to the average particles), 
or represented as l.ltoreq.l.sub.0, or more preferably 
l.ltoreq.1/2l.sub.0, or still more preferably l.ltoreq.1/4l.sub.0. 
The surface treatment of the particles may be carried out by employing one 
of the methods as hereinabove described, or two or more thereof, so that 
in the latter case, the particles may be coated with two or more 
materials. 
The composition of this invention may contain one kind of particles, or two 
or more kinds of particles which differ from one another in material, 
size, shape, or the coating formed thereon. 
It is beneficial from the standpoint of friction on ice to increase the 
proportion of the particles in the composition, but its wear resistance 
drops sharply as the proportion of the particles is increased. Therefore, 
the composition of this invention preferably contains 2 to 20% by volume 
(V.sub.f) of particles. A V.sub.f range of 3 to 16% is more preferable, 
and a range of 4 to 13% is most preferable. 
There is no particular limitation to the materials forming the matrix in 
the composition of this invention. The matrix may comprise an ordinary 
rubber composition which can be prepared by adding ordinary additives, 
such as a filler, oil and a vulcanizing agent, to rubber. The composition 
of this invention can be manufactured if a kneaded mixture of an ordinary 
rubber composition and the particles as hereinabove defined is vulcanized. 
The method which is employed in accordance with this invention to achieve a 
high degree of friction may be classified into the second group of 
methods, rather than the first group, as hereinbefore stated. This 
invention does, however, not preclude the use in combination of the first 
and second groups of methods which are based on different mechanisms. The 
use in combination thereof is, on the other hand, rather effective. When 
preparing the composition of this invention, it is effective to form pores 
in the matrix by foaming it, or incorporating any material failing to 
adhere well to it, if the pores do not have any adverse effect on the wear 
resistance of the composition. The pores are formed after the hard 
irregularly shaped particles have been incorporated into the matrix. 
The invention will now be described more specifically with reference to 
several examples thereof and comparative examples. 
EXAMPLES 1 TO 4 AND COMATIVE EXAMPLES 1 TO 4 
Rubber compositions were prepared by adding particles of materials as shown 
in TABLE 1 below, which had been subjected to surface treatment as shown 
in TABLE 1, in proportions as shown in TABLE 1 to a matrix having the 
composition shown below. The composition of COMATIVE EXAMPLE 1 did not 
contain any such particles, but consisted solely of the matrix. The 
coefficient of friction (.mu.) on ice of the product of each example and 
the relative amount of its wear were determined by the methods as 
hereinafter described, respectively. The results are shown in TABLE 1. 
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Composition of the Matrix (phr) 
______________________________________ 
Natural rubber (NR) 
100 
Carbon (HAF) 60 
Stearic acid (StA) 3 
Zinc oxide (ZnO) 5 
Sulfur (S) 1.5 
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Determination of Coefficient of Friction (.mu.) on Ice 
A sample 2 of the product of each example was rotatably held against a 
fixed block of ice 4 by a bolt 3, and was rotated about the bolt 3, as 
shown in FIG. 2. The resulting force of friction was measured by a load 
cell, and the corresponding coefficient of dynamic friction, .mu.was 
calculated from it. 
The measurement was carried out by holding each sample under a pressure of 
5 kg/cm.sup.2 against the ice and rotating it at a speed giving a 
peripheral velocity of 85 cm/sec. at the temperature of -2.degree. C. at 
which a thin film of water was formed on the surface of the ice and 
reduced the friction between the contacting surfaces to a minimum. 
Determination of Relative Amount of Wear 
The wear of each sample was tested by Lambourn abrasion tester BS903 at 
room temperature by holding the sample under a pressure of 7 kg/cm.sup.2, 
employing a slip ratio of 100% and rotating it at a speed giving a 
peripheral velocity of 85 cm/sec. The results are shown in TABLE 1, in 
which the amount of wear which occurred to each sample, except that of 
COMATIVE EXAMPLE 1, is shown by the value relative to the amount of 
wear on the sample of COMATIVE EXAMPLE 1 as taken to be 100. 
TABLE 1 
__________________________________________________________________________ 
Example of the invention 
Comparative Example 
Example 1 2 3 4 1 2 3 4 
__________________________________________________________________________ 
Compo- 
Material Stainless 
SiO.sub.2 
SiO.sub.2 
SiO.sub.2 
-- Stainless 
SiO.sub.2 
Iron 
sition steel steel 
(part) 
Vickers hardness 
150 1100 1100 1100 -- 150 1100 200 
(Hv) 
Size (mesh) 
100.about.300 
150.about.300 
150.about.300 
150.about.300 
-- 100.about.300 
150.about.300 
90.about.100 
Shape irregularity 
2.5 2.1 2.1 2.1 -- 2.5 2.1 1.02 
(R) 
Surface 
Coating material 
Adhesive 
Rubber 
Polyester 
Epoxy resin 
-- -- -- Cobalt 
treatment 
Coating thickness 
1/10 1/8 1/8 1/8 -- -- -- 1/20 
(I/I.sub.0) 
Proportion by volume of 
8.0 6.0 6.0 6.0 -- 8.0 6.0 10.0 
hard particles (V.sub.f (%)) 
Coefficient of friction on ice 
0.105 
0.095 
0.095 
0.095 0.05 
0.065 
0.065 
0.070 
(.mu.) 
Relative amount of friction 
130 140 128 145 100 
200 280 180 
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Attention is drawn to TABLE 1. The product of COMATIVE EXAMPLE 1 
consisting solely of the rubber matrix showed a coefficient of friction, 
.mu., on ice of 0.05. The product of EXAMPLE 1 of this invention contained 
8.0% by volume of stainless steel particles which had been coated with an 
adhesive, and was superior in both the coefficient of friction on ice and 
wear resistance to the product of COMATIVE EXAMPLE 2 which was 
identical in composition to it, but in which the particles had not been 
given any surface treatment. The products of EXAMPLES 2 to 4 containing 
SiO.sub.2 particles coated with different materials were also outstanding 
in both of the two properties. The product of COMATIVE EXAMPLE 3 was 
identical to the products of EXAMPLES 2 to 4, except that the particles 
had not been coated with any material, and was inferior in both of the 
properties. These results confirm that the surface treatment of the 
particles in the composition of this invention are very effective for 
improving the frictional property and wear resistance of the composition. 
The product of COMATIVE EXAMPLE 4 contained iron particles coated with 
cobalt by plating, but was low in both of the properties, apparently 
because the particles had a very low degree of irregularity in shape and 
were almost spherical. 
It is, therefore, obvious from the results of these examples that the use 
of satisfactorily irregularly shaped and surface treated particles of a 
hard material is essential for making a rubber composition which is 
satisfactory in both of frictional property and wear resistance, as 
proposed by this invention.