CA2057435A1

CA2057435A1 - Pneumatic tire with all-weather driving performance

Info

Publication number: CA2057435A1
Application number: CA002057435A
Authority: CA
Inventors: Shingo Midorikawa
Original assignee: Individual
Current assignee: Yokohama Rubber Co Ltd
Priority date: 1990-12-21
Filing date: 1991-12-11
Publication date: 1992-06-22
Also published as: DE4142367A1; JPH04221206A; KR920011778A; JP3026024B2

Abstract

ABSTRACT OF THE DISCLOSURE
A pneumatic tire has a tread formed from orienting short fibers of a selected polyamide resin of specified molecular weight into a selected rubber. The tread has specified dynamic modulus characteristics which contribute to enhanced driveability on ice and snow as well as on dry and wet roads.

Description

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BACKGROUND OF THE INVENTION
Technical Field This invention relates to pneumatic tires for use in automotive vehicles and more particularly to such a tire having enhanced driveability under all-weather conditions.
Prior Art Several automotive tires are known which are designed to improve driving performance on an icy road. A typical example of these tires is constructed, as disclosed for instance in Japanese Patent Laid-Open Publication Mo.
63-34206, with a tread portion in which short fibers of a metallic material are uniformly distributed. Such prior tire is prone to grow unduly hard at the tread and hence decline in its frictional force relative to ice. Another drawback is that upon progressive wearing the tire tends to scatter the metallic fibers during running on contact with a paved road surface, normally dry or wet in warm season, thus posing a dust pollution~problem.
Alternatively, attempts have been made to fabricate a tread ~ortion with use of cellular rubber of a closed~cell structure and also of short fibers distributed randomly circumferentially of the closed cell as taught in Japanese Patent Laid-Open Publication No. 63-89547. However, due to random distribution of the fibers in the rubber, no appreciable improvement can be expected in the frictional or gripping power of the tread on icy roadway.
SUMMARY OF THE INVENTION

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With the foregoing drawbacks of the prior art in view, the present invention has for its primary object to provide a pneumatic tire which has improved braking and driving capabilities on ice and snow, ensuring enhanced tire performance under all-weather conditions.
This object is achieved by the provision of a pneumatic tire comprising a tread portion formed from a rubber composition and having a plurality of tread blocks, each of the tread blocks including a ground contacting surface, an inner central region and a pair of side wall surfaces opposite to adjacent tread blocks, and a multiplicity of short fibers Eormed from a polyamide-based resin of more than 5,000 in number average molecular weight and distributed in the tread portion. ~ majority of the short fibers are oriented to extend internally circumferentially of the tread portion and along the ground contacting surface of and the side wall surfaces of the tread block. The tread has the dynamic modulus characteristics set forth in the following equations (I) and (II) 1.03 < El/E2 ............... (I) 3 MPa < E2 < 20 MPa ........ (II) where El is the dynamic ~oung's modulus of the ground contacting surface of the tread block, and E2 is the dynamic Young's modulus of the inner central region of the tread block.
Many other features and advantages of the invention 2~5~3~

will be better understood from -the following description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view taken in half along the meridian line of a pneumatic tire constructed in accordance with the present invention.
FIG. 2 is a diagrammatic plan view of a tread po~tion of the tire.
FIG. 3 is a cross-sectional view taken on the line III
- III of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION
With reference to the drawings, FIG. 1 in particular shows a pneumatic tire 100 embodying the present invention.
The tire 100 comprises a pair of spaced beads 101, 101, a pair of sidewalls 102, 102 extending radially inwardly to join the respecti~e beads 101, 101, a tread 103 lying between the sidewalls 102, 102, a carcass 104 extending between the beads 101, 101 and a belt structure 105 surrounding the tread 103 along its inner peripheral wall.
Designated at 106 is a tread constituting block. The tread block 106 is formed from a selected class of rubbers and provided with, as better seen in FIG. 3, a ground contacting surface 106a and a pair of side wall surfaces 106b, 106b opposed to adjacent blocks.
Rubbers useful in the invention for tread formation are blends of natural rubber (~R) and a selected class of different diene rubbers. Specific examples of diene rubbers ~7~

include butadiene (s~) and diene rubbers containing more than 50~ by weight of BR, typically such as isoprene ~IM), styrene-butadiene rubber (SBR) and ethylene-propylene-diene rubber (EPDM). Particularly preferred is the use of BR
alone as the diene rubber. To attain various important qualities necessary for on-ice driving, such diene rubber may preferably be not higher than 30C in glass transition temperature.
The ratio of natural rubber to diene rubber is in the range of 50:50 to 75:25. The diene rubber if smaller ratios than 25 would adversely affect performance on icy and snowy roads or on dry and wet roads and if greater ratios than 50 would make the resulting tread physically weak.
According to one important aspect of the invention, short fibers of a specific structure of a polyamide-based resin are distributed in the composition of tread rubber.
Such fibers should be oriented to extend internally circumferentially of the tread and along the ground contacting surface of and the side wall surfaces of the tread block. Also importantly, the tread should have the dynamic moduli of the equations (I) and (II) l.Q3 < El/E2 ............... (I) 3 MPa < E2 < 20 MPa ........ (II) To meet the above specified requirements enables the resulting rubber composition to be adequately soft and sufficiently rigid and thus produces a tread block having achieved a maximum level of friction and adhesion both at ~7~

the same time. This greatly contributes to improvements in on-ice tire performance.
In implementing the invention, polyamide-based short fibers of more than 5,000 in number average molecular weight, preferably above 8,000, may be well dispersed and properly oriented in a tread rubber on molding by extrusion.
Less than 5,000 in that molecular weight would make the polyamide resin difficult to become sufficiently fibrous and get adequately oriented. Furthermore, the short fibers according to the invention have a melting point preferably in the range of 150 to 260C.
Short fibers to be used in the invention are set at from ]. to lO0 ~m in average length, preferably 100 to 5,000 ~m, more preferably l,000 to 3,000 ~m, and from 0.05 to 0.8 ~m in average diameter and may be further treated with a silane coupling agent typified by y-aminopropyltrimethoxy silane. The length to diameter ratio is preferably in the range of lO to 1,000. Short fibers if too short or too long in the average length and in the average diameter would leads to insufficient dispersion or otherwise random orientation in the tread rubber, resulting in a tread block of unacceptable rigidity and adhesion and eventually of poor tire performance on ice and snow and even on dry and wet roads.
Polyamide resins for use in the short fibers under contemplation include nylon 6, nylon 66 and the like.
The polyamide-based short fibers are admixed in an 2~1~7~3~

amount of 1 to 15 parts by weight per 100 parts by weight of the rubber used. Below 1 part would not be effective in improving on ice driving, whereas above 15 parts would be responsible for insufficient resistance to abrasion.
Moduli outside the equation (I), namely El/E2 values smaller than 1.03 (El/E2 < 1.03), are responsible for the short fibers being not subject to orientation along the ground contacting surface of and the side wall surfaces of the tread block. Failure to orient the fibers in that way causes a decline in on-ice performance. To make the short fibers sufficiently blendable or compatible with the tread rubber, El/E2 is preferably equal to or smaller than 5.0 (El/E2 < 5.0). Blending is difficult to attain in the case of El/E2 > 5O-The distribution of polyamide-based short fibers 108 in a tread rubber 107 is shown in FIGS. 2 and 3 in which the fibers 108 are oriented circumferentially in the direction of E - E and along the ground contacting surface 106a of and the side wall surfaces 106b of the tread block 106. This fiber orientation gives a softer tread rubber with smaller amounts of carbon black, thereby providing great adhesive and frictional effects with respect to an icy road, and further ensures retention o high rigidity of the tread block. The illustrated orientation of the fibers 10 can be obtained during extrusion molding of the tread 103 owing to the inherent tendency of the fibers 103 to follow the flow of rubber. Such a fiber flow takes place also 2 ~ 3 ~

while in vulcani~ation of a green tire in a mold in which the tread rubber flows.
Various other additives may be incorporated in the rubber composition according to the invention for tread formation. They include fillers such as stearic acid, zinc oxide and the like, antioxidants, oils, waxes, carbon blacks, sulfurs, vulcanizing accelerators and the like.
EXAMPLES
The following examples are given to further illustrate the present invention.
Performance evaluation was made of different test tires sized to 1~5/70 R13 85Q and assembled with varying treads of the tabulated compositions. Testing was conducted with an FF-type, 1600-cc car and under a set of conditions indicated below.
l) Braking Effect on Ice A braking distance was measured upon braking after the car was started at 30 km/hr on an ice board. A reference value of 100 for a control tire used was taken for index comparison. Braking performance is better the greater the index.
2~ Driving Effect on Snow Snow on paved roadway was compressed and made slipperly by repeated brak1ng of the car. On such slipperly road the car was caused to climb at a slope of 5~ (2.9).
Acceleration time was measured by a zero start method over a travel distance of 30 m and indexed against the control 20~3~

tire. Driving performance is higher the greater the index.
3) Driving Stability on Dry Road ~ 10-point procedure by a panel of five drivers was followed with the results averaged and adjudged by index comparison as in (1) above. Driveability is more stable the larger the index.

4) Abrasion Resistance on Dry Road Wear was checked of each tire after being traveled for 20,000 km under the standard load and air pressure conditions stipulated by Japan Automobile Tire Manufacturers Association (JATMA). The amount of wear was indexed as in (1). Abrasion resistance is better the larger the index.

5) Dynamic Young's Modulus, MPa Test specimens were cut out cirucmerentially o~ the tire from both the ground contacting surface and the inner central portion of each tread block. Each specimen of 5 mm in width, 2 mm in thickness and 20 mm in interchuck length was tested, on a viscoelasticity spectrometer (Toyo Seiki Co.), at 20 Hz in frequency, at 10~ in initial strain, at +2% in dynamic strain and at 0C in temperature. Rlgidity is higher in tread rubber the greater the measured value.

6) Processability of Tread Rubber Each tread rubber was inspected for blending, sheeting, rolling and extruding by five panelists each with a full mark of 5 points. The results were averaged and compared with the control rubber taken as a reference point , 2~57~3~

of 5. Processing is better the larger the point.
As appears clear from the tabulated results, Inventive Examples 1 and 2 represent the tires of the invention having assembled treads of a specified structure. These inventive tires produce satisfactory driving perormance under all-weather conditions.
Control is a conventional studless tire with a tread free from short fibers.
The tire of Comparative Example 1 has a tread incorporating carbon short fibers in random distribution.
It is unacceptable in on-ice and on-snow driving due to the tread block being alike in elastic modulus both at the ground contacting surface and at the inner central portion.
The tire of Comparative Example 2 has a tread containing aramid short fibers of too large an average length in regular orientation, but suffers from poorer driving performance and lower tread processability than the control tire. The tire of Comparative Example 3 is constructed with a tread in which aramid short fibers of a larger average length are oriented in a cellular rubber.
This comparative tire is comparable ln performance with but worse in processability than the control tire.
The tire of Comparative Example 4 is directly compared to that of Inventive Example l except that the tread is made soft but without short fibers. This comparative tire, though acceptably driveable on ice and snow, is unsatisfactory in regard to driving performance on dry and wet roads.

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Notes to Table:
*l) master batch of a blend of 100 parts by weight of NR rubber and 50 parts by weight of nylon 6 short fibers of 20 ~m in average length and 0.3 ~m in average diameter, the short fibers having a number average molecular weight of 30,000 and a melting point of 221C and treated with y~aminopropyltrimethoxy silane *2) carbon fibers of 5 ~m in average length and 1 ~m in average diameter *3) aramid fibers of 4,000 ~m in average length and 5 ~m in average diameter *4) dinitrosopentamethylene tetramine (Cellular D, Eiwa Chemical Co.) *5) urea compound (Cellpaste K5, Eiwa Chemical Co.)

Claims

1. A pneumatic tire comprising a tread portion formed from a rubber composition and having a plurality of tread blocks, each of the tread blocks including a ground contacting surface, an inner central region and a pair of side wall surfaces opposite to adjacent tread blocks, and a multiplicity of short fibers formed from a polyamide-based resin having a number average molecular weight of more than 5,000 and distributed in the tread portion a majority of the short fibers being oriented to extend internally circumferentially of the tread portion and along the ground contacting surface of and the side wall surfaces of the tread block, the tread having the dynamic modulus characteristics set forth in the following equations (I) and (II) 1.03 ? El/E2 ............... (I) 3 MPa ? E2 ? 20 MPa ........ (II) where El is the dynamic Young's modulus of the ground contacting surface of the tread block, and E2 is the dynamic Young's modulus of the inner central region of the tread block.

2. The tire according to claim 1 wherein the polyamide-based resin is nylon 6 or nylon 66.

3. The tire according to claim 1 wherein each of the short fibers has an average length of 1 to 100 ?m and an average diameter of 0.05 to 0.8 ?m.

4. The tire according to claim 1 wherein each of the short fibers is treated with a silane coupling agent.

5. The tire according to claim 4 wherein the silane coupling agent is y-aminopropyltrimethoxy silane.

6. The tire according to claim 1 wherein the rubber composition comprises as a starting rubber a blend of natural rubber and a different rubber.

7. The tire according to claim 1 wherein the short fibers are used in an amount of 1 to 15 parts by weight per 100 parts by weight of the starting rubber.

8. The tire according to claim 6 wherein the diene rubber is butadiene rubber or a combination of butadiene rubber with isoprene rubber, styrene-butadiene rubber or ethylene-propylene-diene rubber.