CA2030087A1 - Pneumatic tire for all-weather use - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Pneumatic tires are made up essentially of a tread portion formed of a selected class of cellular rubbers of a closed-cell structure. The cellular rubber has specified magnitudes of hardnesses, average areas of cells, variation coefficients of areas of cells and space factors of cells.
Braking and driving capabilities on ice and snow and running stability and abrasion resistance on dry and wet roads are greatly enhanced.

Description

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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 " 2~3~

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 ~3~$~i 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.

~` 2~3~7 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.

2~3~7 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 ~3~$7 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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Claims (2)

1. A pneumatic tire comprising a tread portion formed of a cellular rubber of a closed-cell structure, said cellular rubber meeting the following physical requirements:
(a) a hardness of from 60 to 70 at 0°C as measured by the JIS procedure;
(b) an average area of cells in the range of from 500 to 6,000 m2 as determined on a section surface of said 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 said tread portion and as defined by the equation K= where K is a variation coefficient, ? is an average area of cells in m2, and S is a standard deviation of the area of cells in m2; and (d) a space factor of cells in the range of from 10 to 40% as determined on the section surface of said tread portion and as defined to be an area of cells per unit area of said cellular rubber.

2. A pneumatic tire according to claim 1 wherein said tread portion has a multi-layered construction.