Note: Descriptions are shown in the official language in which they were submitted.
62-220,697 comb.
PNEUMATIC TIRES
This invention relates to pneumatic tires, and
more particularly to a pneumatic tire having consider-
ably improved durability, ln which wear resistance is
durable in practical use without damaging the corneriny
05 performances and heat durable performance in summer
season as well as spring and autumn seasons (hereinafter
referred to as summer season), and also ice and snow
performances such as driving performance, braking
performance, cornering performance and the like on ice
and snow road surface in winter season are improved and
further dynamic compression permanent deformation (so-
called flattening resistance) and separation resistance
are improved.
In the conventional pneumatic tires, in order to
ensure the driving performance, braking performance and
cornering performance during the running on ice and snow
road surface (hereinafter referred to as ice-snow
performances, simply), a chain is attached to the tire,
or a spike tire obtained by striking spike pins into a
surface portion of a tire tread is frequently used~
However, microparticles produced by wearing of the chain
and spike pins or the wearing of the road are scattered
to cause dust nuisance, or injury of road is caused by
~ 3 ~
the spike pins, which comes into a large social problem.
For this end, a so-called studless tire using no spike
pin is recently proposed, and various investigatlons on
the tread pattern and rubber material for such a
05 studless tire are made. However, the ice-snow
performances of the studless tire are not yet developed
up to a level equal to those o the spike tire.
Particularly, there are an attempt that in order to
ensure rubbery elasticity at a low temperature, a
10 polymer having a low glass transition point is used as
the rubber material for the tread, an attempt that a
softening agent having a low melting point is used for
ensuring the friction coefficient on road surface at a
low temperature, and the like, but these attempts are
1~ still insufficient in the ice-snow performances.
On the other handr tires using an expanded
rubber in the tread are proposed in ~apanese Patent
Application Publication No. 40-4,641, Japanese Patent
laid open No. 56-154,304 and US Patent 4,249,588.
In Japanese Patent Application Publication No. 40-4,641,
however, a synthetic rubber having a large hysteresis
loss such as high-styrene rubber is used in the tread,
which raises the glass transition point of rubber to
increase the hardness of rubber at a low temperature, so
that it is unfavorable in view of the Ice-snow
performances.
~ .
~ 3 ~ L
Further, Japanese Patent laid open
No. 56-154,304 discloses a light wei~ht tire by using an
expanded rubber so as to provide a hardness equal to
that of nonexpanded rubber, which can not improve the
OS ice-snow perormances.
~ oreover, when the tire using the expanded
rubber in the tread is used over a long period on dry
road surface and ice and snow road surface, the wearing
of the tread rubber is fast and the wear resistance is
not sufficient. In this connection, the improvement of
the expanded rubber for solving the above problem has
previously been proposed in Japanese Patent ~pplication
No. 61-235,921, but it has been confirmed that the
flattening resistance is not yet sufficient in practical
use.
It is, therefoxe, an object of the invention
to provide a pneumatic tire durable in practical use in
which a tread rubber is composed of an expanded rubber
containin~ a particular rubber component and a
particular high-reinforcing caxbon black and having
specific expansion ratio and cell diameter distribution
to cause no flattening phenomenon and lessen the change
of hardness of the expanded rubber with the lapse of
time even if the tire is used on ice and snow road
surface over a long period.
The inventors have made various studies in order
~ 3 ~
to solve the above problems and found the following
points.
That is, in the tire tread having the expanded
rubber layer, it has been found that a particular rubber
composition obtained by compounding a proper amount oE
05 high reinforcing carbon black and a particular amount or
less of an oil with a particular rubber component is
effective as an expanded rubber in order to prevent the
flattening phenomenon of blocks and maintain the wear
resistance.
Further, it has been found that in the expanded
rubber comprised of the above particular rubber composi-
tion, the optimization of cell diameter distribution in
the expanded rubber is more effective for the solution
of the above problems.
1~ The inventors have made further investigations
and as a result the invention has been accomplished.
According to the invention, there is provided a
pneumatic tire comprising a tire casing and a tread
covering a cro~n portion of said casing/ characterized
in that said tread is provided at its surface side with
an expanded rubber layer in a volume corresponding to at
least 10% of a total volume of said tread; and said
expanded rubber layer is comprised of an e~panded rubber
containing not less than 50 parts by weight of at least
2~ one rubber component selected from natural rubber,
~ 3 ~
polybutadiene rubber and styrene butadiene copolymer
rubber having a glass transition temperature of not
higher than -45C, a high reinforcing carbon black
having an N2SA of 90~1~0 m2/g and not more than 20 parts
05 by weight of an oil per 100 parts by weight of said
rubber component; and said expanded rubber contains
closed cells at an expansion ratio IVs) of 5~50% so that
the number of closed cells having a cell diameter of
5~30 ~m is not less than 50% of total number of closed
cells having a cell diameter of not less than 5 ~m per
unit area as a whole of said expanded rubber.
In a preferred embodiment of the invention, the
tread consists of an outer layer portion comprised of
the expanded rubber layer and an inner layer portion and
l~ is provided near to a shoulder thereof with a rubber
reinforcing layer covering a first joint end part
between the outer layer portion and the inner layer
portion and a second joint end part between the inner
layer portion and a sidewall portion.
The invention will be described with reference
to the accompanying drawing, wherein:
A single f igure is a partial section view of an
embodiment of the pneumatic tire according to the
invention.
2~ ~ccording to the invention, it is necessary that
at least one of natural rubber, polybutadiene rubber and
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1 3 .~
styrene-butadiene copolymer rubber is contained in an
amount of not less than 50 parts by weight as a rubber
component. Preferably, the amount of polybutadiene
rubber is not less than 20 parts by weight but not more
05 than 50 parts by welght. The reason why the rubber
component is limited to the aforementioned rubbers is
due to the fact that the tread containing such rubber
has a sufficient rubber elasticity even at a low
temperature.
Further, the reason why the N2SA of the high
reinforcing carbon black is limited to 90~130 m2/g is
based on the fact that when the N2SA is less than
90 m2/g or more than 180 m2/g, the effect of the
invention can not be developed. Moreover, the value of
15 N2SA is measured according to a test method of ASTM
D3037-B. The amount of the high reinforcing carbon
black is preferable to be 30~70 parts by weight per
100 parts by weight of the rubber component. When the
amount is less than 30 parts by weight, the wear
resistance is degraded, while when it exceeds 70 parts
by weight, the processabiliky is degraded. As the high
reinforcing carbon black, carbon blacks of ISAF and SAF
grades are desirable, and particularly carbon black SAF
is preferable.
The oil is added in an amount of not more than
20 parts by weight, preferably not more than 10 parts by
~L 3 ~
weight, more particularly not more than 5 parts by
weight per 100 parts b~ weight of the rubber component.
In this case, the expanded rubber may contain no oil.
When the amount of the oil exceeds 20 parts by weight,
05 the wear resistance is degraded.
Furthermore, the expanded rubber layer is
necessary to have a volume corresponding to at least 10%
of total volume o~ the tread. The volume of the
expanded rubber layer is preferably within a range of
10 10~70~, particularly 40~60%. When the volume of the
expanded rubber layer is less than 10%, the effect of
improving the ice-snow performances is small.
According to the invention, the whole of the
tread may be comprised of only the expanded rubber layer
1~ (i.e. 100% expanded rubber layer).
The expansion ratio Vs of the expanded rubber is
represented by the following e~uation:
~s = {(P~~Pg)/(Pl~Pg) - 1} x 100 (%)...... (1~
where Pl is a density of the expanded rubber (g/cm3)~ pO
20 is a density of solid rubber phase portion in the
expanded rubber (g/cm3) and pg is a density of gas
portion in cells of the expanded rubber (g/cm ). That
is, the expanded rubber is comprised of solid rubber
phase portions and cavities formed by the solid rubber
2~ phase portion (closed cells) or gas portions in the
cells. Since the density pg of the gas portion is very
small and substantially approaches to zero and also i5
very small to the density Pl of the solid rubber phase
portion, the above equation (l) is approximately equal
to the following equation:
OG Vs = (Po/Pl ~ 1) x 100 (%) .. (2)
~ ccording to the invention, the expansion ratio
Vs is within a range of 5~50%, preferably 5~30%. When the
expansion ratio is less than 5~, the flexibllity of the
expanded rubber is not developed at a low temperature,
10 while when it exceeds 50%l the wear resistance is
degraded and becomes practically insufficient on ice and
snow road surface as well as dry road surface.
In the expanded rubber according to the
invention, the number of closed cells having a cell
diameter of 5~30 lum, preferably 5~25 ~um is not less than
50% of total number of closed cells ha~ing a cell
diameter of not less than 5 ~um per unit area as a whole
of the e~panded rubber. The reason why the number of
the closed cells having a cell diameter of 5~30 ~m is
limited to not less than 50% of the total numker of the
closed cells per unit area is due to the fact that when
the cell diameter is less than 5 lum, the braking
performance on ice road surface is degraded, while when
it exceeds 30 ~m, the wear resistance is degraded and
2~ the flattening phenomenon is caused to degrade the
appearance of the tire.
g
~ 3 ~
In general, when the tire as in the invention is
used in a vehicle running on ice and snow road, it is
liable to cause crack growth at a state of applying
tension to the tire surface because the expanded rubber
as is soft. That is, cut damage may be entered into the
vicinity oE the tread shoulder through broken stones or
pebble stones. If the tire is run after the attachment
of a chain, damage or the like may be entered into the
shoulder portion. From these damages, cracks may grow
10 and finally separation failure may be caused in the
interior of the tire.
In this connection, the inventors have made
various studies with respect to action of road surface
to tread during the running, relation between function
1~ Of each part o~ the tread and properties thereof,
occurrence of cut damage in the vicinity of the
shoulder, position of generating separation failure,
state of proceeding such failure and the like. As a
result, it has been found that the tread is rendered
into a two-layer structure of an outer layer portion and
an inner layer portion to thereby divide the function
thereof and also the balance in the properties between
the outer layer portion and the inner layer portion is
taken by adjusting the volume of the outer layer portion
2~ composed of the expanded rubber and making the modulus
of elasticity of the inner layer portion large, whereby
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1 3 ~
the cornering per~ormance and other per~ormances are
improved. Further, it has been found that the
occurrence and growth oE the failures can be prevented
by coverin~ a join-t part between different rubbers in
05 the vicinity of the shoulder with a rubber reinorciny
layer having adequate properties.
That is, in the preferred embodiment of the
invention, the tread consists o~ an outer layer portion
comprised of the e~panded rubber layer and an inner
layer portion and is provided near to a shoulder thereo
with a rubber reinforcing layer coverin~ a first joint
end part between the outer layer portion and the inner
layer portion and a second joint end part between the
inner layer portion and a sidewall portion, whereby the
1~ function o~ the tread is divided and also the shoulder
portion is reinforced to enhance the ice-snow per~orm-
ances in winter season, various performances in summer
season and the durability.
The reason why the tread is rendered into the
two-layer structure of outer layer portion and inner
layer portion is based on the fact that the hardness of
the inner layer portion is made harder than ~hat of the
outer layer portion to increase the rigidity of blocks as a
surface pattern in the outer layer portion of the tread,
2~ whereby the performances on snow in winter season,
cornering stability and braking performance on ice are
~,
~ 3 ~
improved and the performances between summer season and
winter season can be simultaneously established.
Further, the hardness of the expanded rubber in the
outer layer portion of the tread is within a range of
o~ 35~59, preferably 43~53. When the tread rubber is
made softer, the true ground contact area of the tread
can be increased to enhance the performances on ice, but
the rigidity of the blocks in the tread is reduced to
degrade the cornering stability on snow.
On the other hand, the reason why the first and
second joint end parts are covered with the rubber
reinforcing layer is due to the fact that a non-expanded
skin layer of about 0.3 ~m in thickness is usually
exi~tent in the outer surface of the expanded rubber and
1~ the boundary between the expanded rubber and the other
rubber. Further, the expanded rubber is poor in the
resistance to crack growth under a tension as compared
with the ordinary rubber, so that crack is particularly
apt to be caused from a side face of the block located
in the shoulder of the tread. That is, in the
conventional tread of the two-layer structure, the
expanded rubber having a skin layer of about 0.3 ~m is
existent in the vicinity of the shoulder being most
liable to be subjected to cut damage and the like,
resulting in the degradation of separation resistance
and cut resistance from the side face of the tread.
1 3 ~
on the contrary, in the invention, the rubber
reinforcing layer is arranged to cover the skin layer of
the expanded rubber in the shoulder and the joint end
parts, whereby the cut resistance and separation
o~ resistance are improved in the vicinity of the shoulder.
The expanded rubber in the tread of the
pneumatic tire according to the invention is formed by
adding an expanding agent to the usual rubber composi-
tion and curing them according to the usual tire
building process. ~s the expandiny agent for the
expanded rubber, use may be made, for example, of
azodicarbon amide, dinitrosopentamethylene tetramine,
azobisisobutyronitrile, benzene sulphonyl hydxazide,
oxybis-benzene sulphonyl hydrazide, microcapsule of high
1~ boiling hydrocarbon resin and the like. Preferably, the
sulphonyl hydrazide compounds, particularly oxybis-
benzene sulp~onyl hydrazide are used in view of fine
expansion.
The following examples are given in illustration
2~ f the invention and are not intended as limitations
thereof. In these examples, the properties of the
expanded rubber and tire performances were evaluated by
the following test methods.
(1) Expansion ratio Vs
The expansion ratio Vs of the expanded rubber
was determined according to the aformentioned equation
1 3 ~
(2) after a block-like specimen was cut out from the
expanded rubber layer in the tread of the test tire and
the density Pl (g/orn ) of the specimen was measured and
the density pO of non-expanded rubber (solid phase
o~ rubber) of the tread was measured.
(2) Cell diameter and number of closed cells
The cell diameter and number of closed cells in
the expanded rubber were determined as follows. That
is, a block-like specimen was cut out from the expanded
rubber layer in the tread of the test tire and the cut
surface of the specimen was photographed by means of an
optical microscope with a magnification of 100~400, from
which the cell diameter of the closed cell was measured.
Then, the number of cells having a cell diameter of not
1~ less than 5 ~m was measured over a total area of 4 mm2
or more, from which the number of closed cells per unit
area of l mm2 was calculated.
(3) Flattening appearance (appearance of dynamic
compression permanent deformation)
~fter two test tires were mounted onto a driving
shaft of a passenger car having a displacement of
l,500 cc and run on a general road over a distance of
20 r 000 km, the deformation quantity of each block in the
block pattern of the tread was measured. The deforma-
tion quantity of block was represented by a ratio (%) of
displacement b of outer edge of the block in a certain
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deformation direction to height a ranging from groove
bottom of the block to outer surface of the block,
wherein a case that the ratio b/a is 0~10% was ~, a case
of 10~20% was l a case of 20~30% was ~ and a case of
05 more than 30% was x. The flattening phenomenon became
larger and degraded in this order.
(4) Wear resistance
After two test tires were mounted onto a driving
shaft of a passenger car having a displacement of
1~ l,500 cc and run on concrete road surface of a test
course at a given speed, the variation of groove depth
was measured. The wear resistance was represented by an
index on the basis that the comparative tire was lO0.
The larger the index value, the better the wear
t
1~ resls ance.
(5) Braking performance on ice
After four test tires were attached to a
passenger car having a displacement of l,500 cc, the
braking distance was measured on ice at a temperature of
-SC. The value was represented by an index when the
comparative tire was 100. The smaller the index value,
the better the braking performance.
(6) Change of hardness with lapse of time
~s to the ice-snow performances such as braking
2S performance on ice, hill climbing performance on snow
and the like, the change of hardness of rubber block in
~3~a~
the surface portion of the tire was measured with the
lapse of time. That is, after the tire was trained over
a distance of 100 Icm, the hardness of the bloc~ (initial
hardness Hd) was measured, and then the hardness after
o~ the running over a distance of 5,000 km was measured.
Moreover, the measurement of the hardness was performed
according to ~IS K6301, type A at 25C.
Then, the examples of the invention will be
described with reference to the drawing.
(Examples 1~3, Comparative Examples 1~5)
A single figure shows a first embodiment of the
pneumatic tire according to the invention (Example 1).
As shown in the single figure, the pneumatic
tire 1 (tire size: 165 SR13) comprises a tire casing 2,
1~ and a tread 3 covering a region between shoulders 4 of a
crown portion 2a of the casing 2. The casing 2
comprises a pair of bead portions 5, a carcass portion 6
composed of a rubberized cord ply extending between the
bead portions 5 and containing cords substantially
arranged in the radial direction, and a belt portion 7
superimposed about a crown of the carcass portion 6 and
arranged substantially in the circumferential direction
of the tire. In the casing 2, sidewalls 8 are arranged
on both outer sides of the carcass portion 6 in the
axial direction of the tire.
The tread 3 consists of an outer layer portion
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:L 3 ~
3a contacting with ground surface and an inner layer
portion 3b located inside the outer layer portion 3a.
The volume of the outer layer portion 3a is 60% of the
total volume of the tread 3.
o~ In the vicinity of the tire shoul.der 4 are
formed a first joint end part A of contact line between
the outer layer portion 3a and the inner layer portion
3b in the clrcumferential direction of the tread 3 and a
second joint end part B of contact line between the
inner layer portion 3b and the sidewall 8 in the
circumferential direction toward the bead portion.
A rubber reinforcing layer 10 of a triangular shape in
section is a~ranged over a region ranging from each side
face of the tread to the sidewall 8 to cover the first
1~ joint end part A and the second joint end part B as well
as a part of side face of the outer layer portion 3a, a
side face of the inner layer portion 3b and a part of
side ~ace of the sidewall 8.
The outer layer portion 3a of the tread 3 is
composed of an expanded rubber having a composition
shown in the following Table 1. The hardness of the
expanded rubber is 52. On the other hand, the i~ner
layer portion 3b has a hardness Hd of 60 which is higher
than that of the outer layer portion 3a and is composed
2~ of a rubber composition containing 80 parts by weight of
carbon black and 15 parts by weight of oil based on
~ 3 ~
lO0 parts by weight of natural rubber and further
containing usual additives. The sidewall 8 is a usual
rubber having an excellent flexural resistance.
The rubber reinforc.ing layer 10 is a rubber having
o~ excellent flexural resistance and cut reslstance.
Z~
- 18-
~ 3 ~
__ _ _ _ _ _ _
~0 ~ ~o, o In o Ln o t~ c~i o o ~
U ____ _ _ _ _ _
~3~ o~ ~ o ~ o u~ o o o
U __ _ _ _ _ _
~3~ ~ ~o o ~ o ul o ~ ~ o o Ln
b ~ ____ _ _ __
~ ~ ~ O In U~ O U~ O O U~
_ , _ _ _ _
U~O O ~ O In O ~, O I~ In O O O
O O O O O O ~1 Ll~ O O N 11~ 1~1 O
U _ _ _ _ _ _
~3~ o ~ ~ o o o o ~ ~ ~ o
_ _ _ _ ~ _
~ ~ ~r r~ o t~ o Ln co o o o
r ~
U .4 t Z Z O ~- E tt5 = .C Z Z
~ t I 3 ~ ~ ~ ~ o ~ a o ~ t~ ~
S~ ~ ~ .,, ~ ta o c~ ~ ~ _
.,~s~ ~ ~ o ~1 tt5 ~ ta ~ æ z
J~ tt5 ~) Q ~~rl N \~
ta ~ .~ c~ .~:: ,~ ,1 ....
O t~ s~ ~ ~: ~ .,~ 3 ~: ~ ~1
h ~ ~a ta ~ tt5 ~ ~:: ~ ,
::5 ~ ~-,~ R ~i tl5 c) ~) 4~ tt5 ~: ~c #
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- 19 -
~ 3 ~
The above expanded rubber corresponds to a
rubber composition (composition ~) as shown in ~able 1,
i.e. a rubber composition containing 50 parts by weight
o~ high reinEoraing carbon black NllO based on 100 parts
05 by weight o rubber component consisting of 60 parts by
weight of natural rubber (glass transition temperature:
-72C) and 40 parts by weight of polybutadiene rubber
(glass transition temperature: -100C) and containing no
aromatic oil. This rubber composition further contains
usual additives and an expanding agent (3 parts by
weight of each of dinitrosopentamethylene tetramine and
urea). Furthermore, the high reinorcing carbon black
N110 has an N2SA of 143 m~/g. This rubber composition
is built according to the usual tire manufacturing
1~ process and cured at a curing pressure index of 125
higher by 25% than the usual pressure in the curing,
whereby closed cells 11 are formed.
The e~panded rubber (composition 4) used in the
tire of Example 1 contains closed cells 11 at an
expansion ratio Vs of 23% as shown in the following
Table 2, wherein the number o~ closed cells having a
cell diamPter of 5~30 ~m is 70% of total number of closed
cells having a cell diameter of not less than 5 ~m per
unit area as a whole of the expanded rubber.
The structure other than the tread 3 and the tire
manuacturing method are the same as in the usual
- 20-
.~
~L 3 ~
pneumatic tire, so that the detalled description is
omitted here.
Then, the test results on the performances of
the pneumatic tires according to the invention will be
0~ described below.
By using each of the rubber compositions shown
in Table 1, there are prepared 8 test tires (Examples
1~3, Comparative Examples 1~5). The tire of Example 1 is
a tire shown in the single figure as previously
10 mentioned. In Examples 2 and 3 and Comparative Examples
1 to 5, the outer layer portion of the tread is formed
by using the rubber composition shown in ~able 1 other
than the composition 4. In Example 3, the number of
closed cells having a cell diameter of 5~30 um is 90% of
1~ the total number of closed cells. In Comparative
Example 1, the expanding agent is not used.
In Comparative Examples 2 and 3, the number of closed
cells having a cell diameter of 5~30 ~um is less than 50
of the total number of closed cells. In Comparative
20 Example 4, the amount of carbon black is outside the
range defined in ~he invention. In Comparative Example
5, the number of closed cells having a particular cell
diameter is less than 50~ of the total number of closed
cells and the closed cells having a cell diameter of not
2~ more than 5 ~um are existent.
AS the performances of the pneumatic tire, the
- 21 -
braking performance on ice, flattening appearance, wear
resistance and change of hardness of the expanded rubber
during the running with the lapse of time were evaluated
by the aforementioned test methods. The test results
06 are also shown in Table 2.
1~
.~
- 22-
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E ~ E 3 D Ln ~ o c~ ~) In Ln Ln
.
~ I ~
E ~ E 3 Ln o N a~ O L Ll') Ln
~1 ~ __ _
I ~
ra ~31 r Ln ~ o oo O ~ Ln Ln
c~ u~
_ _
I ~
N~ E ~ ~ E 3rr) o ~ ~ co <I L ~ Ln
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E~ I a) ~ I ~
0 D E E 1) N o N ~ __ X O O
~ r~ ~o r~ o l r~ O o Ln ~
~ VU~ .
U~~1 ~ ~ S~
P~1 ~C rl a~ aJ
o~ v~ ~ a~ ~ Q O
~_ .,, a~ s-- s-~ " a) ~s ~s
X ~ U~ (~ 0-,1 S~ ~: O h Q) CJ
a~ ~ O 4~ -~ ~ ~ ~1 s~ ~:
s~ rcs s~ ,~ ~ s-~ ~ ~ s~ ~ 4~
as ~: o v ~:: d~ Q ~ -l s~ ., S ~1 O ~:
~ _ . ~ O ~ ~ ~s~ ~ ~ ~_~ ~Q ul 1 s~ s
s~ a) u~ ~ ~, s_ s~ o as s~ s~ ~ ~ ~ ~ Y
s~ ~: O a~ ~ .,, ~ ~IS a~ .,, ~ ~ ~: s~
5- o ~: a) ~ O S~ ~ .~ h a~ ~ U as o
s u~ p~,_ ~ ~ ~ ~ ~ c) ~ s~ ~ s~ .,~ s~ s~ o ~ o
a~ ~ tn ~ O~o ~a as. ~ I s~ ~ ~ Q, a~ 1 s:: w 1 i~ ~ ~ o
3 Q as--s, s ~ m ~ ~ w ~ 3 ~ ~ o ~ S Q ~ Ln
.,, I ~ a) ~ o
o u~ ~ h o~ h
o ~: a) ~ ~ o u~
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1 ~3 S ~ O a) QI Q h t) h
rl O ::~ ~rl X ~ O ~
?~ _ 1 4 ~) a~ h P~
- 23 -
i31~
As seen from Table 2, the braking performance on
ice in the tires of Examples 1 to 3 are largely lmproved
as compared with those of Comparative Examples 1 to 5.
Furthermorel in Examples 1 to 3, the flattening
05 appearance of the blocks after the running over a long
period are largely improvedl and also the wear
resistance is a sufficiently practicable level
substantially equal to that of the conventional tire
(Comparative Example 1). Moreoverl in the tires of
10 Examples 1 -to 3, there are caused no occurrence of
damage in the vicinity of the tread shoulderl separation
failure between the first and second joint end parts/
cut failure of the expanded rubber and side damage due
to the chain or the likel so that the tire durabllity is
1~ considerably improved. And alsol the change of hardness
in the tread after the running is considerably small as
compared with those of Comparative Examples 1 to 5
As mentioned abovel according to the invention
the outer layer portion of the tread is composed of an
expanded rubber containing a particular rubber component
and particular high reinforcing carbon black and oil and
the number of closed cells having a particular cell
diameter in the expanded rubber is restricted to a
certain range of the total number of closed cells,
2~ whereby the flattening phenomenon can largely be reduced
in use on ice and snow road surface over a long period
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l ~ L ~
of time and the braking performance on ice and snow road
surface can considerably be improved. Furthermore, the
wear reslstance is sufficient for practical use.
Moreover, the rubber reinforcing layer is
OB arranged in the vicinity of the tread shoulder so as to
cover the joint end part between outer and inner layer
portions and joint end part between inner layer portion
and sidewall, whereby the function of the tread is
divided and the shoulder portion is reinforced and
10 consequently the ice-snow performances in winter season
can be improved together with the performances in summer
season and tire durability.
1~
2~
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