CA2043262A1 - Pneumatic studless tire - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A pneumatic studless tire has a tread formed from a cellular rubber and short fibers distributed therein.
All-weather tire performance is ensured by a specified blend of a blowing agent and a urea-based additive, a specified average cellular area X over a sectional surface of the tread, a specified variation coefficient K of the cellular area X, a specified cellular occupancy rate and a specific orientation of short fibers in the tread.

Description

~fl32~2 BACKG~O~ND OF THE INVENTION
Field of the Invention This invention relates to pneumatic tires suitable Eor use in automotive vehicles and more particularly to such a tire capable of stable driving not only on dry and wet roadway 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 a tire chain. Such antiskid devices, when brought into severe abrasive contact with a paved road surface, would tend to damage the road, kicking up dusts or surface debris which eventually poses environmental pollution.
Studless tires have oE late become prominently popular for their improved braking and ride qualities and are beginning to take the place of conventional spiked or chained tires.
There have already been introduced a number of studless tires as disclosed for example in Japanese Patent Laid-Open Publication Nos. 62-283001 and 63-90402, which studless tires Eeature the use oE cellular rubber oE a closed-cell structure Eor the tread oE the tire. Such prior studless tires are satisfactory in terms of frictional force directed onto icy or snowy roadway, but not quite satisfactory in edging and draining effects so that their wear-resistant property and driving performance on normally dry or wet roadway tend to decline.

~3'~2 There are known two forms oE frictional Eorces exerted on the tire during its rolling along an icy or snowy road;
one is called a plowing frictional force and the other an adhesive frictional force. Rubber blends therefore hold an important role in obtaining a maximum e~fect of these frictional forces. A relatively high rigidity of rubber block in the circumferential direction of the tire is required to provide increased edging effect. On the other hand, rigidity radial of rubber block facing at right angles to an icy road surface is required to be rather low so as to obtain sufficient adhesive frictional force. Attempts have been made to cope with this problem by incorporating short fibers in cellular rubber for the tread to increase its hardness as disclosed for example in Japanese Patent Laid-Open Publication No. 63-~9547. However, due to short fibers being randomly distributed in the rubber, the block rigidity tends to increase uniformly throughout the tread and does not increase more circumferentially than radially oE the tire. Therefore, no appreciable adhesive effect or on-ice frictional force of the tire is anticipated.
SUMMARY OF THE INVENTION
It is therefore a primary object oE the present invention to provide a studless pneumatic tire which has improved all-weather driving performance under dry, wet, icy and snowy road conditions.
This object is achieved according to the invention by the provision of a studless pneumatic tire having a tread ~3~2 with blocks which is formed Erom a cellular rubber and short fibers distributed therein, characterized in that the cellular rubber contain a urea-based additive blended in less than equivalent ratio with a blowing agent; the cellular rubber has an average cellular area X over a sectional surface of the tread in the range of 500 - 6,000 ~m2; the cellular rubber has a variation coefficient K of its cellular area in the range of 0.5 - 0.8; the cellular rubber has a cellular occupancy rate in the tread in the range of 5% - 40%; and the short fibers are oriented circumferentially of the tire along the ground contacting surface and side walls of each block.
Other advantages and features of the invention will appear apparent from the following detailed description taken in connection 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 tire constructed in accordance with the invention;
FIG. 2 is a diagrammatic plan view of a tread portion of the tire; and FIG. 3 is a cross-sectional view taken on the line III
- III of FIG. 2.
DETAILED DESCRIPTION OF TIIE INVENTION
Referring to the drawings and FIG. 1 in particular, there i5 shown a studless pneumatic tire lO0 embodying the invention, the tire 100 comprising a pair of spaced beads ~32~

101, 101, a pair oE side walls 102, 102 extending radially inward to join the respective beads 101, 101, a tread 103 interposed between the side walls 102, 102, a carcass 10~
extending between the beads 101, 101 and a belt structure 105 surrounding the inner peripheral wall of the tread 103.
Designated at 106 -is the outer surface of the tread 103.
The present invention relies on the following combination features which constitute the essential requirements of the inventive studless tire for achieving the aforesaid object.
(1) The tread 103 is formed from a cellular rubber and short fibers, the rubber having a urea-based additive blended in less than equivalent ratio with a blowing agent.
It has been found that a closed-cell structure in cellular rubber greatly contributes to enhanced edging and draining efEects of the tire partialarly on the ice which assumes a pseudoliquid phase at about 0C~ It has now been further found that these effects are pronounced with use of a relatively hard cellular rubber for the tread, contrary to the conventional notion that on-ice or on-snow frictional force of the tread can be improved by the use of rubber of lower hardness at lower temperature. Cellular rubber is known to decline considerably in hardness compared to non-cellular rubber. Therefore, matrix rubber must be chosen with higher hardness typically for example by using increased amounts oE carbonblack and other reinforcing ~3~6~

materials or reduced amounts of oil and other softening ayents. This will however invite rubber processing difEiculty and aggrevated heat generation. Noting that urea-based additives have a contributory efEect upon the increase in the crosslin~ing density oE a starting rubber, extensive investigation has been made to indicate that such contributory effect can be much more enhanced by using a urea-based additive in combination with a blowing agent than introducing urea alone into the rubber composition. It has now been found that urea-based additives when added in specified blend ratios with blowing agents to prepare a cellular rubber will suppress a decline in the hardness of the rubber due to blowing and ensure a hardness comparable with non-cellular rubber without adverse effect upon rubber processing or heat generation. The use of a urea-based additive is also effective in that it serves as an acceptor for malodorous formaldehyde which is formed during decomposition of a blowing agent such as nitroso compounds.
As already mentioned hereinabove, with cellular rubber having a uniform rigidity distribution it is difficult to provide an improvement in both edging effect and adhesive frictional force of'the tread relative to the ground. It is known that the elastic modulus of rubber increases in a direction parallel to the orientatio,n oE short fibers unidirationally laid in the rubber but does not appreciably vary in a direction at right angles to the fiber orientation. This anisotropy of fiber reinforced rubber can ~32~2 be thus utilized in controlling the orientation oE short fibers to be directed in a direction parallel to the surEace of the rubber block so that rigidity will decrease in a direction at right angles to the block surface and conversely increase in a direction parallel to the block surface, thereby achieving a maximum eEfect of edging and adhesive frictional force both at the same time.
The urea-based additive according to the invention is used preferably in an amount of 30 - 90 weight percent based on the blowing agent. Amounts of the urea-based additive greater than those of the blowing agent lead to saturation of the desired effect, hence would be only economically infeasible and would furthermore invite undue reduction of the decomposition temperature depending upon the type of blowing agents used, most likely resulting in unvulcanized rubber blown in the mixing and extruding operation.
Conventional vulcanization processes may be employed in preparing the tire of the invention, in which instance the cellular rubber has a glass transition temperature or brittle point of below -30C such that the tire is free from cracking when used in cold season.
The blowlng agent used in the invention may be an organic compound such as benzenesulfonyl hydrazide, dinitroso-pentamethylene tetramine anr~ azodicarbonamide, or an inorganic compound such as sodium bicarbonate, ammonium carbonate and ammonium nitrite.
The urea-based additive according to the invention may ~32~2 be urea per se or coagulation-inhibitors or moisture-inhibitors such as acid and urea compounds typically including Celpaste M3, Ks and 101 tradenamed and manuEactured by ~iwa Chemicals Industry Ltd.
There may be used other additives such as carbonblack, softening agents, processing agents, ageing inhibitors, waxes, vulcanizers, vulcanization accelerators, etc. in a manner well known in the art.
The blowing agent is added preferably in an amount of 0.5 - 20 weight parts per 100 parts of starting rubber.
It may be desirable to use two-ply or three-ply treads such as a cap tread/base tread structure to provide better all-weather driving performance.

(2) An avera e cellular area X over a sectional surface oE
g ~ ~
the tread is in the ran~e of 500 - 6,000 ~m2.
The cellular rubber is of a closed-cell structure having an average cellular area of 500 - 6,000 ~m2, preEerably 1,000 - ~1,000 ~m2. Cellular areas less than 500 ~m2 provide insufficient improvement in on-ice or on-snow tire performance, whereas more than 6,000 ~m2 give rise to deterioration of wear resistant property and driving performance.

(3) A variation coefficient K of the cellullar area X in the tread is in the range of 0.5 - 0 8 It has now been found that a relatively narrow distribution width of the cellular structure, a cellular shape and a maximized cellular occupancy rate hold an ~3~

important bearing upon all-weather tire performance.
The term variation coefficient K as used herein is derived from the formula K = S/X
where X is an average cellular area (~m2) and S is a standard deviation.
The variation coefficient K should be in the range of 0.5 ~ 0.8. Smaller values than 0.5 would result in reduced edging effect of the tire, while larger than 0.8 would lead to reduced draining efEect.
t4) A cellular occupancy rate is in the range of 5~ - 40~.
The term cellular occupancy rate is used to designate a rate of cellular area per unit area of the rubber.
Departures downward from 5% and upward from 40~ would result in reduced edging effect and reduced wear resistance and normal driving qualities, respectively.
t5) A majority oE short fibers are oriented along the ground contacting surface and side wall surfaces of a tread block 107.
Reference to FIGS. 2 and 3 shows the distribution of short fibers 108 in a celluar rubber 109 in which the fibers 108 are oriented circumferentially in the direction of E - E
along the ground contacting surface,107a and the side wall surfaces 107b of the tread block 107. The depicted orientation of the fibers 108 can be obtained during vulcanization of the tire in the mold due to the inherent ~3~2 tendency of the fibers 108 following the flow oE rubber.
However, care must be taken to choose a proper fiber length as too short fibers tend to move objectionably randornly.
Therefore, the short fibers 108 to be used in the invention are preferably greater than 100 ~m in average length, more specifically in the average length range of 100 - 5,000 ~m, preferably 1,000 - 3,000 ~m. The average diameter of the short fibers 108 is preferably greater than 1 ~m. The length/diameter ratio of the fibers is preferably from 10 to 1000. The short fibers 108 may be those of cotton, silk and other natural fibers, or cellulose, polyamide and other chemical synthetic fibers.
The invention will be further described by way of the following examples.
Inventive Examples 1 - 2 and Comparative Examples 1 - 5 Various sample tires of 185/70R13 85Q having the constituent compositions shown in Table 1 were subjected to the following tests conducted on 1600 cc FF car.
Avera~e cellular area X, variation coefficient K and cellular occupancy rate A test piece was cut out from each sample tire tread and projected to an image magnified 165 times by NEXUS6400 of Kashiwagi Research Institute. Tes~ results were averaged out of ten (10) such test pieces.
on-ice braking performance A braking distance was measured upon braking after the 2~43262 car was started at 30 kg/hr. on ice board. A reference value of 100 for conventional tire (control) was taken as an index. Braking effect i9 better the larger the index value.
On-snow driving performance Snow on paved road was compressed and made slippery by applying a brake repeatedly on the car. Climbing test was conducted on such slippery road at 5~ (2.9) slope and acceleration time was measured from zero start over a travel distance of 30 meters. Control tire was taken as a reference for index display of the test results on each sample tire. Driving performacne is better the larger the index value.
Steering stability (on dry road) Five test drivers assessed the steering quality of the test car mounted with each set of sample tires. Their ratings were indexed against control tire. Steering stability is better the larger the index value.
Wear resistance (on dry road) After a travel of the test car for 20,000 killometers under JATMA standard load and air pressure conditions, the sample tires were checked for wear. The amount of wear for each sample tire was indexed against control tire. Wear resistance is better the larger the~index value.
DYnamic Young's modulus MPa Test piece was cut out from both the outer surface and interior of each sample tread block circumferentially of ; :
':
:

:

the tire. The test piece measuring 5 mm wide, 2 mm thick and 20 mm in interchuck length was tested under conditions of ~0 Hz frequency, 10~ initial strain, -~ 2% dynamic strain and 0C temperature. Rigidity of the tread is higher the greater the measured value.
In Table 1, the control tire is a conventional studless tire without cellular rubber and short fibers.
Inventive Examples 1 and 2 respectively represent the tires oE the invention which incorporate the specified combination cellular rubber/short fiber tread structure having satisfactory all-weather tire performance. The tire of Comparative Example 1 incorporates cellular rubber but no short fibers and is satisfactory Eor on-ice and on-snow performance but not for normally dry and wet road performance. The tire of Comparative Example 2 has both cellular rubber and short fibers, but these fibers are too short and randomly distributed in the rubber so that rigidity of the tread becomes nearly equalized both circumferentially and radially of the tire. Therefore, on-ice or on-snow tire performance is not satisEactory.
Comparative Example 3 represents a tire with both cellular rubber and short fibers but lacks the urea-based additive, with the result that the dynamic ~oung's modulus (rigidity) declines with deteriorated tire performance. A cellular rubber/short Eiber combination tread in the tire oE
Comparative Example 4 is not satisEactory in terms of on-ice and on-snow performance because the average cellular area is ~3~

too small. A similar tire in Comparative Example 5 is not satisfactory in terms of normally dry and wet road performance because the cellular occupancy rate is too large.

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ote: *l ... Santoweb D (cellulose short fibers manuEactured by Nihon ~lonsant) *2 ... Carbon short fibers, 5 ~m in average length, 1 ~m in average diameter *3 ... Regimen 3520 (hexa-methoxy-methyl melamine manuEactured by Nihon Monsant) *4 ... Cellular D (dinitroso-pentamethylene tetramine manufactured by Eiwa Chemical) *5 ... Neocellbon N~lOOOSW (benzenesulfonyl hydrazide manufactured by Eiwa Chemical) *6 ... Cellpaste K5 (urea compound manuEactured by Eiwa Chemical)

Claims (5)

1. A pneumatic studless tire having a tread with blocks which is formed from a cellular rubber and short fibers distributed therein, characterized in that said cellular rubber contain a urea-based additive blended in less than equivalent ratio with a blowing agent; said cellular rubber has an average cellular area X over a sectional surface of said tread in the range of 500 - 6,000 µm2; said cellular rubber has a variation coefficient K of its cellular area in the range of 0.5 - 0.8; said cellular rubber has a cellular occupancy rate in said tread in the range of 5% - 40%; and said short fibers are oriented circumferentially of said tire along the ground contacting surface and side walls of each block.

2. A tire according to claim 1 wherein said urea-based additive is added in an amount of 30 - 90 weight parts per 100 weight parts of said blowing agent.

3. A tire according to claim 1 wherein said urea-based additive is one selected from the group of urea, coagulation inhibitors and acid/urea compounds.

4. A tire according to claim 1 wherein said variation coefficient K is derived from the formula K = S/X
where X is an average cellular area (µm2) and S is a standard deviation (µm2).

5. A tire according to claim 1 wherein said short fibers have an average length of 100 - 5,000 µm, an average.

diameter of greater than 1 µm and a length/diameter ratio of 10 - 1000.