Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
This invention is directed to pneumatic tires suitable
for use in automotive vehicles and more particularly to such
a tire capable of stable driving not only on dry and wet
roads but also on ice and snow.
Description of the Prior Art
In snowy and icy season automobile cars have been
assembled usually with a spiked tire or with a tire chain.
Such antiskid means tends to be brought into abrasive
contact with the road surface during running of the tire.
This literally leads to road marring, eventually posing
environmental pollution in dry season.
Studless tires have of late become prominent to cope
with the trend of safety drivability and pollution
protection. They are reputed for high receptivity of
friction on ice and snow without need for antiskid means.
In Japanese Patent Laid-Open Publication No.
55-135149, No. 58-199203 and No. 60-137945 there are
disclosed certain studless tires which are rendered less
hard at a tread portion at low temperature and hence
resistant to skidding on ice. These tires are provided with
a tread portion formed from a rubber mix in which are
incorporated a large amount of a softener or plasticizer.
Such additive, though adequate in running on ice, is
susceptible to insufficient braking and hence instable
running on wet roads at high temperature.
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Alternatively, it is known that cellular rubber can be
used for tread formation as taught in Japanese Patent
Laid-Open Publication No. 62-283001 and No. 63-90402. Such
prior rubber, however, is not wholly satisfactory as it is
too small in hardness to produce edging and draining effects
arising from its close cells. This leaves the problem that
the resulting tire tread will invite skidding on ice and
snow. Low hardness is responsible for objectionable
wear on dry and wet roads.
According to the disclosures of Japanese Patent
Laid-Open Publication No. 63-89547 and No. 64-63401 it has
been proposed that tire treads be made sufficiently hard by
the use of a cellular rubber blended with short fibers,
thereby attaining safe running on dry and wet roads. A
shoulder portion disposed adjacent to the tread has also
been reinforced to improve the ultimate tire in its
structural strength. There is left much to be desired for
practical application.
S~MMARY OF THE INVENTION
The present invention turns on the finding that tread
rubber when formulated to contain close cells in a specific
distribution is allowed to fully exhibit its edging and
draining effects at low temperature with the results that
braking and driving capabilities can be improved on ice and
snow, while drivability on dry and wet roads is held at a
high level. Although heretofore preferred to be less hard,
cellular rubber has now been found to show a unique behavior
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at specific rubber hardnesses and in specific cell
distributions.
The invention therefore seeks to provide a novel
pneumatic tire which excels in frictional forces, i.e.
braking and driving qualities, on ice and snow and even in
running on dry and wet roads, thus ensuring safe and
efficient drivability in all seasons.
More specifically, the invention provides a pneumatic
tire comprising a tread portion formed of a cellular rubber
of a closed-cell structure, the cellular rubber meeting the
following physical requirements of (a) a hardness of from 60
to 70 at 0C as measured by the JIS procedure, (b) an
average area of cells in the range of from 500 to ~,000 ~m2
as determined on a section surface of the tread portion, (c)
a variation coefficient of the area of cells in the range of
0.5 to 0.8 as determined on the section surface of the tread
portion and as defined by the equation K=S/~ where K is a
variation coefficient, X is an average area of cells in ~m2,
and S is a standard deviation of the area of cells in ~m2,
and ld) a space factor of cells in the range of from 10 to
40% as determined on the section surface of the tread
portion and as defined to be an area of cells per unit area
of the cellular rubber.
DETAILED DESCRIPTION OF THE INVENTION
Pneumatic tires accordlng to the present invention are
contrived to have a tread portion formed of a selected class
of cellular rubbers of a closed-cell structure.
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Cellular rubbers used herein may be produced by
blending a blowing agent with a rubber composition commonly
employed for tread formation and subsequently by vulcanizing
the blend in conventional manner. Base polymers for use in
the cellular rubber may preferably have a glass transition
temperature of lower than -30C to prevent the finished tire
tread against cracking during driving on ice and snow. The
blowing agent is not specifically restricted, but may be
suitably chosen from many, organic or inorganic, compounds.
Typical examples include organic blowing agents such as
benzenesulfonylhydrazide, dinitrosopentamethylene tetramine,
azodicarbonamide and the like, and inorganic blowing agents
such as sodium bicarbonate, ammonium carbonate, ammonium
nitrite and the like.
Various other additives may be incorporated, as is
known in the art, which are selected from carbon blacks,
softeners, antioxidants, waxes, vulcanizing agents,
vulcanizing accelerators and the like.
Tire treads are feasible, in implementing the
invention, preferably in a cap-base construction having two
or more layers integrally laminated. This greatly
contributes to tire performance both under icy and snowy
conditions and under dry and wet conditions.
In accordance with one important aspect of the
invention, the cellular rubber should meet the following
physical requirements.
1. Hardness in the range of 60 to 70 at 0C as
measured by the JIS procedure.
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2. Average area of cells in the range of 500 to 6,000
~m2 as determined on a section surface of a tread
portion.
3. Variation coefficient of the area of cells in the
range of 0.5 to 0.8 as defined in a manner to be
mentioned.
4. Space factor of cells in the range of 10 to 40% as
determined on the tread section surface and as
defined to be an area of cells per unit area of
cellular rubber.
The hardness at 0C should range from 60 to 70,
preferably from 63 to 68. Less than 60 would show no
appreciable rise in braking and driving capabilities on ice
and snow and further lead to objectionable abrasion and
inadequate driving on dry and wet roads. More than 70 would
produce a tire of instable driving on ice and snow.
The average area of cells should be from 500 to 6,000
~m2, preferably from 1,000 to 4,000 ~m2. This parameter if
smaller than 500 ~m2 would not be effective to improve tire
performance on ice and snow and if greater than 6,000 ~m2
would cause severe wear while in running under dry and wet
conditions.
The variation coefficient of the area of cells should
be between 0.5 and 0.8. Below O.S would induce insufficient
edging effect, hence instable running both on ice and snow
and on dry and wet roads. Above 0.8 would lead to small
draining effect, meaning safe drivability on dry and wet
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roads but to an extent to reduce braking and driving
capabilities on ice and snow.
^The above specified range of variation coefficients
has been determir.ed as a result of research efforts made
with behaviors of cells varied in cellular rubbers. Thus
the cell shape and space factor have turned out to be
important determinants in gaining a good balance of
drivability at both low and high temperatures.
The variation coefficient defined herein is expressed
by the following equation.
K = S/~
where K : variation coefficient
- X : average area of cells (~m2)
S : standard deviation of area of cells ~m2)
The space factor of cells should not depart from a
range of 10 to 40~. Smaller factors would be ineffective
for quality improvement on ice and snow, whereas greater
factors would make the resultant tire tread weary on dry and
wet roads. This parameter is defined to be an area of cells
per unit area of the cellular rubber.
The following examples are provided for a better
understanding of the invention.
Different pneumatic radial tires of a 185/70R13 85Q
size were produced which were reinforced with two steel
belts and built with a block pattern. Performance
evaluation was made of the test tires under the conditions
given below and with the results as per tabulated.
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Avera e Area of Cells, Variation Coefficient of Area of
Cells and Space Factor of Cells
A specimen was cut out of a tread portion of each of
the test tires and forced into flat form, followed by image
processing to a magnification of 165 on a processor (NEXUS
6400, Kashiwagi Kenkyusho Co.). N=5 measurements were
averaged.
Hardness
JIS K-6301 was followed in measuring the hardness at
0C of a specimen cut out of each tire tread.
Braking on Ice
Each test tire was allowed to run on ice at an initial
speed of 30 km/hr. Determination was made by the distance
required upon braking and as an index with a tire of
Comparative Example 1 taken as a control of 100. The
greater index, the better braking.
Traction on Snow
Snow on the road was slicked with a passenger car
being alternately accelerated and braked. Ascent running
was effected on the slippy snow at a slope of 5% (2.9).
The time of acceleration required for ascending at a
distance of 30 m by a zero-startup procedure. The greater
index, the better driving.
Running Stability on Dry Road
Drivability was adjudged by a five-driver panel test
with a full mark of 10 points. The greater index, the more
stably the tire runs.
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Abrasion Resistance on Dry Road
Wear was measured after each test tire was run on a
dry road at a distance of 20,000 km under the working load
and air pressure conditions stipulated by JATMA (Japan
Automobile Tire Manufacturers Association). The greater
index, the smaller abrasion.
As evidenced from the tabulated results, the tires of
Inventive Examples 1 to 4 have been proved quite
satisfactory in respect of all the test qualities.
Too low a hardness was unacceptable, as is apparent
from Comparative Example 2, in regard to traction capability
on snow and running stability and abrasion resistance on dry
roads. By contrast, too high a hardness in Comparative
Example 3 revealed insufficient braking on ice.
An average area of cells outside the scope of the
invention was encountered with unacceptable braking on ice
as demonstrated by Comparative Example 4.
Departures of variation coefficients from the
specified range, Comparative Examples 5 and 6, involved
inadequate braking capability in an icy state and
insufficient abrasion resistance in a dry state,
respectively.
Too great a space factor, though acceptable in braking
on ice and in traction on snow, was instably runnable and
severely abrasive on a dry road as is clear from Comparative
Example 7.
The foregoing parameters of cell distribution and
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rubber hardness are critical to achieving superior tire
performance at both low and high temperatures. A lack of
either one of those parameters should be avoided to preclude
little parity in quality.
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