Rubber composition for treads of studless tires

A rubber composition suitable for use in tread of tires, particularly studless tires having increased digging and adhesive frictions to improve characteristics on icy roads of the tires without substantial deterioration of the durability of the tires or with minimizing the deterioration, which comprises a diene rubber component, 3 to 30 parts by weight of an ebonite powder and 0.1 to 3 parts by weight of a silylating agent, respectively, per 100 parts by weight of the rubber component.

BACKGROUND OF THE INVENTION 
The present invention relates to a rubber composition suitable for use in 
tire treads, and more particularly to a rubber composition having 
excellent characteristics on icy and snow-covered roads and suitable for 
use in treads of studless tires. 
Spiked tires which grip icy roads by spike pins, have been popularly used 
as pneumatic tires suitable for running on icy roads. However, the spiked 
tires cause dust pollution because roads are scraped off by the pins if 
automobiles with spiked tires are run on usual roads. The use of spiked 
tires is in some countries legally prohibited in order to prevent dust 
pollution from spiked tires. Thus, in recent years, studless tires attract 
attention as pneumatic tires which can grip icy roads without the use of 
spike pins. However, the characteristics on icy roads of studless tires 
are still insufficient as compared with spiked tires which directly grip 
icy roads by means of the spike pins. Thus, in order to improve the 
characteristics on icy roads of studless tires, there have been made 
various attempts to increase the coefficient of friction of the tread 
surface to icy roads. 
Among frictions between a tread and a road, digging friction and adhesive 
friction are known as main frictions to secure the driving force and 
braking force. It is necessary to increase these frictions in order to 
improve the grip characteristics on icy roads of studless tires. 
For example, it is proposed to produce the tread of studless tire from an 
expanded rubber, wherein it is contemplated to increase the digging 
friction by means of edges resulting from cells of expanded rubber and to 
increase the adhesive friction by the action of the edges to remove a 
water film formed between the tread surface and an icy road. Improvement 
in digging friction and adhesive friction requires increasing the 
proportion of cells, namely expansion ratio, but this results in softening 
of the tread, and in its turn brings about deterioration of stability, 
controllability and abrasion resistance in a non-snowy area. 
Japanese Patent Publication Kokai No. 2-167353 proposes a pneumatic tire 
having a tread made from a rubber composition incorporated with a powdery 
product of a cellulose material such as chaff or wood flour. Such a 
cellulose powder does not form any chemical bonding with a rubber 
component. Thus, the cellulose powder exposed onto the surface of tread 
will fall off during running, and open pores resulting from falling off of 
the cellulose powder from the tread may contributes to increase in digging 
friction and adhesive friction in the same manner as cells of an expanded 
rubber. On the other hand, since the cellulose powder is present as an 
additive in the rubber before it is exposed to the tread surface and falls 
off therefrom, softening of the tread rubber as encountered by the 
expanded rubber does not occur, thus this tread rubber does not bring 
about deterioration of the durability in a non-snowy area. 
However, the cellulose powder has many OH groups in the cellulose molecule 
and shows a hydrophilic property, so its hardness decreases due to 
absorption of water before it is exposed to the tread surface and falls 
off therefrom. Thus, there is a limit in increasing the digging friction 
by incorporation of the cellulose powder. 
It is an object of the present invention to provide a rubber composition 
suitable for use in treads of studless tires, which has improved digging 
friction and adhesive friction without deteriorating the stability and 
controllability and the durability thereof. 
A further object of the present invention is to provide a rubber 
composition having a good durability such as abrasion resistance and 
capable of exhibiting excellent grip characteristics even on an icy road. 
These and other objects of the present invention will become apparent from 
the description hereinafter. 
SUMMARY OF THE INVENTION 
It has been found that the digging friction and adhesive friction of a 
tread rubber can be successfully improved with minimized deterioration of 
the durability of the tread rubber by incorporating an ebonite powder into 
the tread rubber, while the adhesive friction is further improved by 
incorporation of a silylating agent. 
In accordance with the present invention, there is provided a rubber 
composition for a tire tread comprising a diene rubber component, 3 to 30 
parts of an ebonite powder having an average particle size of not more 
than 100 .mu.m and 0.1 to 3 parts of a silylating agent, said parts all 
being parts by weight per 100 parts by weight of said diene rubber 
component. 
The rubber composition of the present invention is suitable for use in tire 
treads and provides studless tires having an excellent durability and 
improved grip characteristics on icy roads as well as snow covered roads 
to exhibit excellent acceleration and braking characteristics on icy 
roads. 
DETAILED DESCRIPTION 
Diene rubbers are used as the rubber component of the composition of the 
present invention without particular restriction in the kind thereof. 
Diene rubbers which have good low temperature characteristics are 
generally used, e.g. natural rubber (NR), butadiene rubber (BR), isoprene 
rubber (IR) and styrene-butadiene rubber (SBR). The diene rubbers may be 
used alone or in admixture thereof. 
Ebonite is known as a hard rubber produced by adding a large amount of 
sulfur to a rubber such as natural rubber, styrene-butadiene rubber or 
nitrile rubber, e.g. 30 to 50 parts by weight of sulfur per 100 parts by 
weight of a rubber, and curing the rubber for a long time. Ebonite has a 
water repelling property. A powder of ebonite prepared from any of source 
rubbers can be used in the present invention. 
The ebonite powder has a hardness equal to or higher than a cellulose 
powder, and the ebonite powder exposed to the tread surface can exhibit 
the digging friction on icy roads. Also, since the ebonite powder has a 
water repellency and serves to facilitate removal of a water film present 
between the tread surface and an icy road, it contributes to increase in 
adhesive friction on icy roads also before falling off from the tread 
surface. Further, since in the ebonite the unsaturated bouble bonds of a 
rubber are almost completely saturated by sulfur, the ebonite powder 
incorporated in a rubber composition does not form a chemical bonding with 
a matrix rubber of the composition even by the curing of the composition. 
Thus, the ebonite powder exposed to the tread surface can fall off 
therefrom during running. After the ebonite powder has fallen off, the 
pores formed thereby on the tread surface serve in the same manner as 
cells of an expanded rubber to contribute to increase in digging friction 
and adhesive friction. Moreover, before the ebonite powder appears at the 
tread surface and fails off therefrom, it can be present as an additive in 
the rubber without softening the tread rubber and adversely affecting the 
durability, even if the ebonite powder is incorporated in a rubber 
composition in an amount necessary and sufficient to improve the grip 
characteristics. 
The average particle size of the ebonite powder used in the present 
invention is not more than 100 .mu.m. If the particle size is more than 
100 .mu.m, the local difference in hardness of the tread surface becomes 
large to decrease the adhesive friction, since the ebonite powder is a 
hard materials. Also, the pores formed on the tread surface by falling off 
of the powder become too large to deteriorate the abrasion resistance. 
Further, incorporation of the ebonite powder having such a large particle 
size may deteriorate the grip characteristics on general roads and is apt 
to become a cause of chipping of the tread rubber. The ebonite powder 
having an average particle size within the range of 30 to 90 .mu.m is 
usually employed in the present invention. 
The ebonite powder is used in an amount of 3 to 30 parts by weight, 
preferably 5 to 30 parts by weight, more preferably 7 to 15 parts by 
weight, per 100 parts by weight of the diene rubber component. If the 
amount of the ebonite powder is less than 3 parts by weight, desired 
effects are not sufficiently exhibited. If the amount is more than 30 
parts by weight, the number of ebonite particles appearing at the tread 
surface (the proportion of the ebonite particles in the tread surface) 
increases, thus resulting in increase of the pores formed by falling off 
thereof, to deteriorate the abrasion resistance of the tread surface. 
Incorporation of the silylating agent into a rubber composition is 
effective for increasing the adhesive frictional force of the tire tread. 
The silylating agent easily reacts with compounds having an active 
hydrogen, such as OH group, NHR group, COOH group, CONH group or SH group, 
present in the rubber composition to replace the active hydrogen with a 
silyl group such as --SiR.sub.3 group. Thus, hydrophilic groups in a tread 
rubber composition are decreased and changed to hydrophobic to impart a 
water repellency to the tread rubber, whereby sticking of water or water 
film onto the tread rubber surface can be restrained and water droplets 
sticking onto the tread rubber surface can be easily removed. 
Known silylating agents which have been used in silylation of organic or 
inorganic compounds, namely organic silicon compounds which are capable of 
introducing a silyl group into organic or inorganic compounds having 
active hydrogen by reaction with the active hydrogen, can be used in the 
present invention. The silylating agent may include, for instance, a 
chlorosilane compound, especially a chlorosilane compound of the formula 
R.sup.1.sub.4-n SiCl.sub.n wherein R.sup.1 is an alkyl group or phenyl 
group and n is 1, 2 or 3, an alkoxysilane, especially an alkoxysilane of 
the formula: (R.sup.2 O).sub.m --Si--R.sup.3.sub.(4-m) wherein R.sup.2 is 
an alkyl group, R.sup.3 is an alkyl group or phenyl group and m is 1, 2 or 
3; and a disilazane compound, especially a hexaalkyl disilazane compound. 
Representative examples of the silylating agent are, for instance, a 
chlorosilane compound such as tert-butyldimethylchlorosilane, 
phenyltrichlorosilane or diphenyldichlorosilane, preferably a di or 
trichlorosilane compound; an alkoxysilane compound, preferably a di or 
trialkoxysilane such as phenyltriethoxysilane, isobutyltrimethoxysilane or 
diphenydimethoxysilane; and a silazane compound such as 
hexamethyldisilazane. Other silylating agents such as 
N,O-bis(trimethylsilyl)acetamide and N,N'-bis(trimethylsilyl)urea are also 
applicable to the present invention. The silylating agents may be used 
alone or in admixture thereof. 
The silylating agent is used in an amount of 0.1 to 3 parts by weight, 
preferably 0.5 to 2 parts by weight, per 100 parts by weight of the rubber 
component. 
The tread rubber composition of the present invention can contain additives 
for rubbers usually employed in the production of tires, for example, 
curing agent such as sulfur, curing accelerator, antioxidant, carbon 
black, softener, activator such as zinc oxide or stearic acid, and the 
like. 
The rubber composition of the present invention can be prepared and cured 
in a usual manner. 
The present invention is more specifically described and explained by means 
of the following examples wherein all parts are by weight. It is to be 
understood that the present invention is not limited to the examples.

EXAMPLE 1 
Rubber compositions were prepared according to the recipes shown in Table 
1, wherein ebonite powder was one having an average particle size of 80 
.mu.m, carbon black was N220, silylating agent was phenyltriethoxysilane 
(KBE103 made by Shin-Etsu Chemical Co., Ltd. ), cellulose powder was a 
chaff powder having an average particle size of 120 .mu.m (trade mark 
"Sumicelluco", product of Sumitomo Bussan Kabushiki Kaisha), and the oil 
was used in an amount to provide rubber compositions having the same 
hardness. 
No ebonite powder was incorporated into the composition of Run No. 1, and a 
cellulose powder was incorporated instead of the ebonite powder into the 
composition of Run No. 2. 
There were prepared pneumatic tires having a size of 185/70R14 and having a 
tread made from the rubber compositions of Run Nos. 1 to 11. The 
characteristics on icy roads and abrasion resistance of the tires were 
evaluated as follows: 
Characteristics on Ice 
The tires were attached to a car. The car was run at 15 km/hour on an 
ice-covered road and braked, and the distance required for stopping the 
car was measured. The result was represented as an index of the reciprocal 
of the braking distance value measured to the result of the tire of Run 
No. 2 regarded as 100. The larger the index, the better the braking 
characteristics on ice. 
Abrasion Resistance 
The amount of abrasion of a tire tread was measured by a Lambourn abrasion 
tester. The result was represented by as an index to the result of the 
tire of Run No. 2 regarded as 100. The larger the index, the better the 
abrasion resistance. The value 78 is permissible. 
TABLE 1 
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Run No. 1 2 3 4 5 6 7 8 9 10 11 
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Ingredients (part) 
Natural rubber 
100 100 100 100 100 100 100 100 100 100 100 
Carbon black 
50 50 50 50 50 50 50 50 50 50 50 
Stearic acid 
2 2 2 2 2 2 2 2 2 2 2 
Zinc oxide 4 4 4 4 4 4 4 4 4 4 4 
Sulfur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 
Curing accelerator 
0.75 
0.75 
0.75 
0.75 
0.75 
0.75 
0.75 
0.75 
0.75 
0.75 
0.75 
Oil 5 10 6 8 11 17 6 8 10 17 11 
Ebonite powder 
-- -- 3 7 15 30 3 7 15 30 15 
Cellulose powder 
-- 10 -- -- -- -- -- -- -- -- -- 
Silylating agent 
-- -- -- -- -- -- 1 1 1 1 0.5 
Characteristics on ice 
90 100 104 109 126 130 108 112 130 133 128 
Abrasion resistance 
110 100 99 98 95 78 99 98 95 78 95 
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It can be seen from the results shown in Table 1 that the rubber 
compositions of Run Nos. 3 to 11 containing an ebonite powder have 
superior characteristics on icy roads as compared with the rubber 
compositions of Run Nos. 1 and 2, particularly the rubber composition 
containing a cellulose powder of Run No. 2 according to prior art, and 
that the characteristics on icy roads are increased with increasing the 
amount of the ebonite powder. The abrasion resistance is decreased with 
increasing the amount of the ebonite powder, but can be tolerated so long 
as the ebonite powder is used in an amount of at most 30 parts by weight 
per 100 parts by weight of the rubber component. 
It can be also seen from comparison between the results of Run Nos. 3 to 6 
and the results of Run Nos. 7 to 10 that the characteristics on icy roads 
are further increased by incorporating a silylating agent. Therefore, it 
is possible to improve the characteristics on icy roads while restraining 
the amount of ebonite powder, in other words, while restraining the 
decrease in abrasion resistance, by using the ebonite powder in 
combination with a silylating agent. 
In addition to the ingredients used in the Examples, other ingredients can 
be used in the Examples as set forth in the specification to obtain 
substantially the same results.