CA3103194A1

CA3103194A1 - A tread for winter and endurance

Info

Publication number: CA3103194A1
Application number: CA3103194A
Authority: CA
Inventors: Kazutaka YOKOKAWA; Tenko Hayashi
Original assignee: Compagnie Generale des Etablissements Michelin SCA
Current assignee: Compagnie Generale des Etablissements Michelin SCA
Priority date: 2018-06-15
Filing date: 2018-06-15
Publication date: 2019-12-19
Also published as: EP3807104A4; JP2021534027A; WO2019239570A1; CN112351895A; EP3807104A1

Abstract

Present invention provides a tread having a contact face, the tread being provided with a plurality of grooves and a plurality of contact elements, each the plurality of contact elements having a top face, the tread comprising two rubber layers, a cap rubber layer configuring the contact face when the tread being brand new and a base rubber layer being provided radially inward of the cap rubber layer, the cap rubber layer and the base rubber layer being composed of different rubber compositions each other, the base rubber layer being located totally radially inward of radially innermost part of the plurality of grooves, a rubber composition constituting the cap rubber layer comprises solid particles, and a rubber composition constituting the base rubber layer is devoid of said solid particles, and a shore A hardness of the rubber composition constituting the base rubber layer is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer.

Description

Description Title of Invention: A TREAD FOR WINTER AND ENDURANCE
Technical Field [0001] The present invention relates to a tread for a tire, in particular to a tread for a tire provides improvement on wintry performance while maintaining satisfactory tire endurance.
Background Art

[0002] A tire so-called "studless" tire is suitable for driving on ice covered and/or snow covered wintry surface. In many cases, the "studless" tire has a wear limit relative to its remaining groove depth in which the tire can be used as a winter tire.

[0003] A rubber composition visible on a contact face after such the wear limit is, often the case, not suitable for use on wintry surface, thus there is a desire to be able to drive safely on wintry surface which may occur suddenly even after such the wear limit.

[0004] In order to maintain performance on wintry surface until close to tire life, it is known to produce whole the tread with a rubber composition suitable for use on wintry surface is effective. It is also known that such the way impacts negatively performance on non-wintry surface and tire endurance mainly due to a softness of the rubber com-position suitable for use on wintry surface.

[0005] JP59-199307 discloses a studdable pneumatic tire having a tread comprising a sheet rubber layer whose hardness being harder below 5 C and being equal or lower above 30 C than that of a tread rubber, the sheet rubber layer being placed continuously in circumferential orientation, and as to embed a flange of a stud or as to touch with a bottom face of a flange of a stud.

[0006] KR20160026426 discloses a tire having a tread comprising a rubber layer made of at least one of a first rubber composition having relatively lower rolling resistance or a second rubber composition having relatively better braking performance, the first rubber composition or the second rubber composition comprising a plurality of particles for preventing an irregular wear while maintaining good performance on both rolling resistance and braking performance.
Citation List Patent Literature

[0007] PTL 1: JP59-199307 PTL 2: KR20160026426

[0008] However with the solutions disclosed in these documents, maintenance of good per-formance on wintry surface until close to tire life while maintaining performance on non-wintry surface and tire endurance are not satisfactory, and there is a desire to further maintenance of these performance simultaneously until close to tire life.

[0009] Therefore, there is a need for a tread for a tire which provides maintenance of good performance on wintry surface, on non-wintry surface and tire endurance simul-taneously until close to tire life.

[0010] Definitions:

[0011] A "radial direction/orientation" is a direction/orientation perpendicular to axis of rotation of the tire. This direction/orientation corresponds to thickness orientation of the tread.

[0012] An "axial direction/orientation" is a direction/orientation parallel to axis of rotation of the tire.

[0013] A "circumferential direction/orientation" is a direction/orientation which is tangential to any circle centered on axis of rotation. This direction/orientation is perpendicular to both the axial direction/orientation and the radial direction/orientation.

[0014] A "tire" means all types of elastic tire whether or not subjected to an internal pressure.

[0015] A "tread" of a tire means a quantity of rubber material bounded by lateral surfaces and by two main surfaces one of which is intended to come into contact with ground when the tire is rolling.

[0016] A "groove" is a space between two rubber faces/sidewalls which do not contact between themselves under usual rolling condition connected by another rubber face/
bottom. A groove has a width and a depth.

[0017] An "incision", also referred to as a "sipe", is a narrow cutout formed toward radially inwardly from a surface of a tread made by, for example a thin blade having a shape like a knife blade. A width of the incision at the surface of the tread is narrower than a groove, for example less than or equal to 2.0mm. This incision may, different from the groove, be partly or completely closed when such the incision is in a contact patch and under usual rolling condition.

[0018] A "contact patch" is a footprint of a tire mounted onto its standard rim as identified in tire standards such as ETRTO, JATMA or TRA, and inflated at its nominal pressure and under its nominal load.

[0019] It is thus an object of the invention to provide a tread for a tire which provides maintenance of good performance on wintry surface, on non-wintry surface and tire endurance until close to tire life.
Summary of Invention

[0020] The present invention provides a tread for a tire having a contact face intended to come into contact with ground during rolling, the tread being provided with a plurality of grooves of a depth D opening to the contact face and a plurality of contact elements being delimited by the plurality of grooves, each the plurality of contact elements having a top face constituting a part of the contact face, the tread comprising two rubber layers laminated in radial orientation of the tire, a cap rubber layer configuring the contact face when the tread being brand new and a base rubber layer being provided radially inward of the cap rubber layer, the cap rubber layer and the base rubber layer being composed of different rubber compositions each other, the base rubber layer being located totally radially inward of radially innermost part of the plurality of grooves, a rubber composition constituting the cap rubber layer comprises solid particles, and a rubber composition constituting the base rubber layer is devoid of said solid particles, and a shore A hardness according to ASTM D2240 of the rubber composition constituting the base rubber layer is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer.

[0021] This arrangement provides a tread for a tire which provides maintenance of good per-formance on wintry surface, on non-wintry surface and tire endurance simultaneously until close to tire life.

[0022] Since the tread comprising two rubber layers laminated in radial orientation of the tire, a cap rubber layer configuring the contact face when the tread being brand new and a base rubber layer being provided radially inward of the cap rubber layer, the cap rubber layer and the base rubber layer being composed of different rubber com-positions each other, the base rubber layer being located totally radially inward of radially innermost part of the plurality of grooves, it is possible to make the cap rubber layer which is composed of a rubber composition suitable for use on wintry surface always visible on the contact face until close to tire life. Therefore it is possible to maintain good level of performance on wintry surface until close to tire life.

[0023] Since the rubber composition constituting the cap rubber layer comprises solid particles, it is possible to provide high level of performance on wintry surface as the solid particles would create roughness on the top face of the contact element by fall out and/or wear of the solid particles for trapping snow particles and/or for evacuating water film generated between wintry ground and the contact element. Also at the same time such the roughness would increase edges in micro scale to be able to scrape snow and/or ice covered surface. Therefore it is possible to further improve performance on wintry surface until close to tire life.

[0024] Since the rubber composition constituting the base rubber layer is devoid of the solid particles and the shore A hardness according to ASTM D2240 of the rubber com-position constituting the base rubber layer is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer, the base rubber layer acts as supporting layer of the contact element totally made of the cap rubber layer. Also at the same time the base rubber layer acts as protection layer protecting tire internal construction from damages. Therefore it is possible to maintain non-wintry performance and tire endurance until close to tire life.

[0025] If this gap of shore A hardness between the rubber composition constituting the cap rubber layer and the rubber composition constituting the base rubber layer is less than 6 points, there is a risk of degradation on non-wintry performance and/or tire endurance as the base rubber layer cannot acts neither supporting layer nor protection layer. By setting the shore A hardness of the rubber composition constituting the base rubber layer is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer, it is possible to maintain non-wintry per-formance and tire endurance until close to tire life.

[0026] In another preferred embodiment, a thickness of the base rubber layer measured per-pendicular to the contact face at a center of the tread is at least equal to 0.8 mm.

[0027] If this thickness of the base rubber layer measured perpendicular to the contact face at the center of the tread is less than 0.8 mm, there is a risk that the base rubber layer cannot act sufficiently as the protection layer thus tire endurance would be degraded.
By setting this thickness of the base rubber layer measured perpendicular to the contact face at the center of the tread at least equal to 0.8 mm, it is possible to maintain tire endurance until close to tire life.

[0028] This thickness of the base rubber layer measured perpendicular to the contact face at the center of the tread is preferably at least equal to 1.0 mm, and more preferably at least equal to 1.2 mm.

[0029] In another preferred embodiment, a size of the solid particles is from 0.1 mm to 1.0 mm.

[0030] If this size of the solid particles is less than 0.1 mm, there is a risk that a roughness created via the solid particles is too small thus performance on wintry surface cannot be improved. If this size of the solid particles is more than 1.0 mm, there is a risk that a roughness created via the solid particles is too large thus performance on non-wintry surface would be degraded. By setting this size of the solid particles from 0.1 mm to 1.0 mm, it is possible to maintain good performance on wintry surface and on non-wintry surface simultaneously until close to tire life.

[0031] This size of the solid particles is preferably from 0.1 mm to 0.9 mm, more preferably from 0.2 mm to 0.8 mm and still more preferably from 0.3 mm to 0.8 mm.

[0032] In another preferred embodiment, the rubber composition constituting the base rubber layer is devoid of silica as reinforcing filler.

[0033] According to this arrangement, it is possible to bring less water to internal mixes which directs better adhesion, thus endurance performance would further be improved.

[0034] In another preferred embodiment, the rubber composition constituting the base rubber layer comprises at least equal to 70 phr of natural rubber or isoprene rubber or mixture of natural rubber and isoprene rubber.

[0035] According to this arrangement, it is possible to improve further tire endurance as natural rubber and/or isoprene rubber show better mechanical properties over synthetic rubber.

[0036] Preferably the rubber composition constituting the base rubber layer comprises 100 phr of natural rubber or isoprene rubber or mixture of natural rubber and isoprene rubber.

[0037] In another preferred embodiment, the shore A hardness of the rubber composition constituting the cap rubber layer is at most equal to 60.

[0038] If this shore A hardness of the rubber composition constituting the cap rubber layer is more than 60, there is a risk that the cap rubber layer cannot follow smoothly a roughness on ground and decreases an area of contact between ground and the tread thus performance on both wintry and non-wintry performance would be degraded.
By setting this shore A hardness of the rubber composition constituting the cap rubber layer at most equal to 60, it is possible to maintain good performance on wintry surface and on non-wintry surface simultaneously until close to tire life.

[0039] In another preferred embodiment, an amount of the solid particles in the cap rubber layer is from 3 to 40 volume %.

[0040] If this amount of the solid particles in the cap rubber layer is less than 3 volume %, there is a risk that a roughness created after wear would be not enough thus per-formance on wintry surface cannot be improved. If this amount of the solid particles in the cap rubber layer is more than 40 volume %, there is a risk that a roughness created after wear decreases too much contact area performance on non-wintry road and tire endurance would be degraded. By setting this amount of the solid particles in the base rubber layer from 3 to 40 volume %, it is possible to maintain good performance on wintry surface, on non-wintry surface and tire endurance simultaneously until close to tire life.

[0041] This amount of the solid particles in the cap rubber layer is preferably from 3 to 35 volume %, more preferably from 5 to 30 volume % and still more preferably from to 30 volume %.

[0042] In another preferred embodiment, the solid particles are crumb rubber particles.

[0043] According to this arrangement, it is possible to sustain the advantageous effect of solid particles longer as crumb rubber particles adhere with rubber composition con-stituting the cap rubber layer better than the other solid particles thus difficult to fall off, and eventhough crumb rubber particles wear faster than the rubber composition constituting the cap rubber layer, crumb rubber particles wear gradually compared with the other solid particles. Thus it is possible to further maintain performance on wintry surface until close to tire life.

Advantageous Effects of Invention

[0044] According to the arrangements described above, it is possible to provide maintenance of good performance on wintry surface, on non-wintry surface and tire endurance until close to tire life.
Brief Description of Drawings

[0045] Other characteristics and advantages of the invention arise from the description made hereafter in reference to the annexed drawings which show, as nonrestrictive examples, the embodiment of the invention.

[0046] In these drawings:
[fig.11Fig. 1 is a schematic plan view of a tread according to a first embodiment of the present invention;
[fig.21Fig. 2 is a cross sectional view taken along line II-II in Fig. 1;
[fig.31Fig. 3 is a schematic cross sectional view of a tread according to a second em-bodiment of the present invention;
[fig.41Fig. 4 is a schematic cross sectional view of a tread according to prior art.
Description of Embodiments

[0047] Preferred embodiments of the present invention will be described below referring to the drawings.

[0048] A tread 1 for a tire according to a first embodiment of the present invention will be described referring to Figs. 1 and 2.

[0049] Fig. 1 is a schematic plan view of a tread according to a first embodiment of the present invention. Fig. 2 is a cross sectional view taken along line II-II in Fig. 1. The tread 1 shown in the figures 1 and 2 is a half of a portion of the tread divided with respect to a center line C-C' extending in circumferential orientation.

[0050] The tread 1 is a tread for a tire having dimension 205/55R16 and comprises a contact face 2 intended to come into contact with the ground during rolling, and a plurality of grooves 3 of a depth D (shown in Fig. 2) opening to the contact face 2 and extending generally in circumferential orientation that is top-bottom orientation or in axial ori-entation that is left-right orientation in Fig. 1. The plurality of grooves 3 is delimiting a plurality of contact elements 4 in a form of a rib or a block. A top face 41 of each the plurality of contact elements 4 constitutes part of the contact face 2. Each the plurality of contact elements 4 is provided with at least one incision 5 opening to the top face 41 and extending radially inward of the contact elements 4.

[0051] As shown in Fig. 2, the tread 1 comprising two rubber layers laminated in radial ori-entation of the tire, a cap rubber layer 6 configuring the contact face 2 (and also the top face 41 of the contact elements 4) when the tread being brand new and a base rubber layer 7 being provided radially inward of the cap rubber layer 6, the cap rubber layer 6 and the base rubber layer 7 being composed of different rubber compositions each other, the base rubber layer 7 being located totally radially inward of radially innermost part of the plurality of grooves 3, thus the base rubber layer 7 is not visible while the plurality of grooves 3 is available on the tread 1. A carcass 10 which is a main body of the tire and a ply 9 are disposed radially inward of the tread 1, as typical construction of radial tire.

[0052] As shown in Fig. 2, the base rubber layer 7 is placed above the ply 9 in a form of rubber sheet, and a width of the base rubber layer 7 in axial orientation is a little narrower than a width of the ply 9 in the same orientation. In this first embodiment, a thickness of the base rubber layer 7 measured perpendicular to the contact face 2 at a center of the tread 1 is 1.0 mm.

[0053] A rubber composition constituting the cap rubber layer 6 comprises solid particles, and a rubber composition constituting the base rubber layer 7 is devoid of the solid particles. In this first embodiment, a size of the solid particles is in a range of 0.3 ¨ 0.7 mm, an amount of the solid particle in the cap rubber layer 6 is 8 volume %, and the solid particles are crumb rubber particles.

[0054] The rubber composition constituting the base rubber layer 7 comprises at least equal to 70 phr of natural rubber or isoprene rubber or mixture of natural rubber and isoprene rubber, and is devoid of silica as reinforcing filler. In this first embodiment, the rubber composition constituting the base rubber layer 7 comprises 100 phr of natural rubber.

[0055] A shore A hardness according to standard ASTM D 2240 of the rubber composition constituting the base rubber layer 7 is at least equal to 6 points higher than a shore A
hardness of the rubber composition constituting the cap rubber layer 6, and the shore A
hardness of the rubber composition constituting the cap rubber layer 6 is at most equal to 60. In this first embodiment, the shore A hardness of the rubber composition con-stituting the base rubber layer is 64, and the shore A hardness of the rubber com-position constituting the cap rubber layer is 54.

[0056] Since the tread 1 comprising two rubber layers laminated in radial orientation of the tire, a cap rubber layer 6 configuring the contact face 2 when the tread being brand new and a base rubber layer 7 being provided radially inward of the cap rubber layer 6, the cap rubber layer 6 and the base rubber layer 7 being composed of different rubber compositions each other, the base rubber layer 7 being located totally radially inward of radially innermost part of the plurality of grooves 3, it is possible to make the cap rubber layer 6 which is composed of a rubber composition suitable for use on wintry surface always visible on the contact face 2 until close to tire life.
Therefore it is possible to maintain good level of performance on wintry surface until close to tire life.

[0057] Since the rubber composition constituting the cap rubber layer comprises 6 solid particles, it is possible to provide high level of performance on wintry surface as the solid particles would create roughness on the top face 41 of the contact element 4 by fall out and/or wear of the solid particles for trapping snow particles and/or for evacuating water film generated between wintry ground and the contact element 4.
Also at the same time such the roughness would increase edges in micro scale to be able to scrape snow and/or ice covered surface. Therefore it is possible to further improve performance on wintry surface until close to tire life.

[0058] Since the rubber composition constituting the base rubber layer 7 is devoid of the solid particles and the shore A hardness according to ASTM D2240 of the rubber com-position constituting the base rubber layer 7 is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer 6, the base rubber layer 7 acts as supporting layer of the contact element 4 totally made of the cap rubber layer 6. Also at the same time the base rubber layer 7 acts as protection layer protecting tire internal construction, ply 9 for example, from damages.
Therefore it is possible to maintain non-wintry performance and tire endurance until close to tire life.

[0059] If this gap of shore A hardness between the rubber composition constituting the cap rubber layer 6 and the rubber composition constituting the base rubber layer 7 is less than 6 points, there is a risk of degradation on non-wintry performance and/or tire endurance as the base rubber layer 7 cannot acts neither supporting layer nor protection layer. By setting the shore A hardness of the rubber composition constituting the base rubber layer 7 is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer 6, it is possible to maintain non-wintry performance and tire endurance until close to tire life.

[0060] Since a thickness of the base rubber layer 7 measured perpendicular to the contact face 2 at a center of the tread 1 is at least equal to 0.8 mm, it is possible to maintain tire endurance until close to tire life.

[0061] If this thickness of the base rubber layer 7 measured perpendicular to the contact face 2 at the center of the tread 1 is less than 0.8 mm, there is a risk that the base rubber layer 7 cannot act sufficiently as the protection layer thus tire endurance would be degraded.

[0062] This thickness of the base rubber layer 7 measured perpendicular to the contact face 2 at the center of the tread 1 is preferably at least equal to 1.0 mm, and more preferably at least equal to 1.2 mm.

[0063] Since a size of the solid particles is from 0.1 mm to 1.0 mm, it is possible to maintain good performance on wintry surface and on non-wintry surface simultaneously until close to tire life.

[0064] If this size of the solid particles is less than 0.1 mm, there is a risk that a roughness created via the solid particles is too small thus performance on wintry surface cannot be improved. If this size of the solid particles is more than 1.0 mm, there is a risk that a roughness created via the solid particles is too large thus performance on non-wintry surface would be degraded.

[0065] This size of the solid particles is preferably from 0.1 mm to 0.9 mm, more preferably from 0.2 mm to 0.8 mm and still more preferably from 0.3 mm to 0.8 mm.

[0066] Since the rubber composition constituting the base rubber layer 7 is devoid of silica as reinforcing filler, it is possible to bring less water to internal mixes which directs better adhesion, thus endurance performance would further be improved.

[0067] Since the rubber composition constituting the base rubber layer 7 comprises at least equal to 70 phr of natural rubber or isoprene rubber or mixture of natural rubber and isoprene rubber, it is possible to improve further tire endurance as natural rubber and/
or isoprene rubber show better mechanical properties over synthetic rubber.

[0068] Preferably the rubber composition constituting the base rubber layer comprises 100 phr of natural rubber or isoprene rubber or mixture of natural rubber and isoprene rubber.

[0069] Since the shore A hardness of the rubber composition constituting the cap rubber layer 6 is at most equal to 60, it is possible to maintain good performance on wintry surface and on non-wintry surface simultaneously until close to tire life.

[0070] If this shore A hardness of the rubber composition constituting the cap rubber layer is more than 60, there is a risk that the cap rubber layer cannot follow smoothly a roughness on ground and decreases an area of contact between ground and the tread thus performance on both wintry and non-wintry performance would be degraded.

[0071] Since an amount of the solid particles in the cap rubber layer 6 is from 3 to 40 volume %, it is possible to maintain good performance on wintry surface, on non-wintry surface and tire endurance simultaneously until close to tire life.

[0072] If this amount of the solid particles in the cap rubber layer 6 is less than 3 volume %, there is a risk that a roughness created after wear would be not enough thus per-formance on wintry surface cannot be improved. If this amount of the solid particles in the cap rubber layer 6 is more than 40 volume %, there is a risk that a roughness created after wear decreases too much contact area performance on non-wintry road and tire endurance would be degraded.

[0073] This amount of the solid particles in the cap rubber layer 6 is preferably from 3 to 35 volume %, more preferably from 5 to 30 volume % and still more preferably from to 30 volume %.

[0074] Since the solid particles are crumb rubber particles, it is possible to sustain the ad-vantageous effect of solid particles longer as crumb rubber particles adhere with rubber composition constituting the cap rubber layer 6 better than the other solid particles thus difficult to fall off, and eventhough crumb rubber particles wear faster than the rubber composition constituting the cap rubber layer 6, crumb rubber particles wear gradually compared with the other solid particles. Thus it is possible to further maintain per-formance on wintry surface until close to tire life.

[0075] The base rubber layer 7 may have various thicknesses along axial orientation.

[0076] A tread 21 according to a second embodiment of the present invention will be described referring to Fig. 3. Fig. 3 is a schematic cross sectional view of a tread according to a second embodiment of the present invention. The construction of this second embodiment is similar to that of the first embodiment other than the ar-rangement shown in Fig. 3, thus description will be made referring to Fig. 3.

[0077] As shown in Fig. 3, the tread 21 comprising two rubber layers laminated in radial ori-entation of the tire, a cap rubber layer 26 configuring the contact face 22 (and also the top face 241 of the contact elements 24) when the tread being brand new and a base rubber layer 27 being provided radially inward of the cap rubber layer 26, the cap rubber layer 26 and the base rubber layer 27 being composed of different rubber com-positions each other, the base rubber layer 27 being located totally radially inward of radially innermost part of the plurality of grooves 23, thus the base rubber layer 27 is not visible while the plurality of grooves 23 is available on the tread 21. A
carcass 210 which is a main body of the tire and a ply 29 are disposed radially inward of the tread 21, as typical construction of radial tire.

[0078] As shown in Fig. 3, the base rubber layer 27 is placed above the ply 29, and a width of the base rubber layer 27 in axial orientation is wider than a width of the ply 29 in the same orientation and substantially equal to a width of the cap rubber layer 26 in the same orientation. In this second embodiment, a thickness of the base rubber layer 27 measured perpendicular to the contact face 22 at a center of the tread 21 is 1.5 mm.

[0079] Since the width of the base rubber layer 27 in axial orientation is wider than the width of the ply 29 in the same orientation, it is possible to further increase it is possible to further maintain performance on non-wintry surface and tire endurance until close to tire life as the base rubber layer 27 can act effectively as a supporting layer and a protection layer while increasing manufacturing efficiency.

[0080] In a region between the ply 29 and the carcass 210, a rubber composition other than both the rubber compositions constituting the cap rubber layer 26 or the base rubber layer 27 may be provided for making a form of the base rubber layer 27 as a sheet rubber.

[0081] The invention is not limited to the examples described and represented and various modifications can be made there without leaving its framework.

[0082] Fig. 4 is a schematic cross sectional view of a tread according to prior art. In this Fig.
4, a tread 101 having a contact face 102 intended to come into contact with ground during rolling, the tread 101 being provided with a plurality of grooves 103 of a depth D opening to the contact face 102 and a plurality of contact elements 104 being delimited by the plurality of grooves 103, each the plurality of contact elements 104 having a top face 1041 constituting a part of the contact face 102, the tread comprising two rubber layers laminated in radial orientation of the tire, a cap rubber layer 106 configuring the contact face 102 when the tread 101 being brand new and a base rubber layer 107 being provided radially inward of the cap rubber layer 106, the cap rubber layer 106 and the base rubber layer 107 being composed of different rubber compositions each other, the base rubber layer 107 being exposed partly radially outward of radially innermost part of the plurality of grooves 103.

[0083] A rubber composition constituting the cap rubber layer 106 may comprise solid particles, and a rubber composition constituting the base rubber layer 107 may be devoid of said solid particles. A shore A hardness according to ASTM D2240 of the rubber composition constituting the base rubber layer 107 may be at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer 106.

[0084] A carcass 1010 which is a main body of the tire and a ply 109 are disposed radially inward of the tread 101, as typical construction of radial tire.
Examples

[0085] In order to confirm the effect of the present invention, one type of tire of Example to which the present invention is applied, other type of tire of Reference and other type of tire of Comparative Example were prepared.

[0086] The Example was a tire as described in the above first embodiment; a tread of the Example comprised two rubber layers, a cap rubber layer in which a rubber com-position constituting the rubber layer included 8 volume % of crumb rubber particles as solid particles of a size in a range of 0.3 ¨ 0.7 mm, and a base rubber layer in which a rubber composition constituting this layer was devoid of solid particles.
The rubber composition constituting the base rubber layer comprises 100 phr of natural rubber and devoid of silica as reinforcing filler. A shore A hardness of the rubber composition constituting the cap rubber layer was 54 and that of the case rubber layer was 64. A
thickness of the base rubber layer measured perpendicular to the contact face at a center of the tread was 1.0 mm. The Reference was a tire as described in the above prior art. The Comparative Example was a tire having the same construction as the Example but without a base rubber layer.

[0087] Wintry surface performance test:

[0088] The friction coefficient measurements on snow were carried out with the above Examples, References and Comparative Examples all at worn state (remaining groove depth = 4.0 mm) sliding at a given condition (a displacement: 0 to 0.03 m, a speed: 0 to 0.5 m/s, and an acceleration: 5 m/s2) over a hard pack snow track, set at about -10 C
with a CTI penetrometer reading of about 90 in accordance with Standard ASTM
F1805, with an imposed normal stress (about 300 kPa). The forces generated in a direction of travel (Fx) of each of the Examples, References and Comparative Examples and in another direction perpendicular to the travel (Fz) were measured. The Fx/Fz ratio determines the friction coefficient of each of the Examples, References and Comparative Examples on the snow. This test, the principle of which is well known to a person skilled in the art (see, for example, an article entitled "Investigation of rubber friction on snow for tires" written by Sam Ella, Pierre-Yves Formagne, Vasileios Koutsos and Jane R. Blackford (38th LEEDS-Lyons Symposium on tribology, Lyons, 6-9 Sep. 2011)) makes it possible to evaluate, under representative conditions, the grip on ice/snow which would be obtained after a running test on a vehicle fitted with tires whose tread is composed of the same contact elements.

[0089] The results are shown in table 1. In this table 1, results are represented by an index of 100 for Reference, higher the number indicates better the performance.

[0090] Endurance test:

[0091] New test tires were mounted onto all four wheels of a vehicle and travelled on a road over which stones of suitable size having sharp cutting edges were scattered.
The tires were then disassembled and number of impacts reached to a ply was counted by a well-trained investigator.

[0092] The results are also shown in table 1. In this table 1, results are represented by an index of 100 for Reference, higher the number indicates better the performance. Within 15 points difference on this comparison could be considered as equivalent.

[0093] [Table 11 Comparative Example Reference Example Wintry surface performance (index) 109 109 100 Endurance performance (index) 90 60 100

[0094] As seen from table 1, the Example shows maintenance of good performance on wintry surface, on non-wintry surface and tire endurance until close to tire life.
Reference Signs List

[0095] 1, 21 tread 2, 22 contact face 3, 23 groove 4, 24 contact element 41, 241 top face incision 6, 26 cap rubber layer 7, 27 base rubber layer 9, 29 ply 10, 210 carcass

Claims

Claims [Claim 11 A tread (1) for a tire having a contact face (2) intended to come into contact with ground during rolling, the tread (1) being provided with a plurality of grooves (3) of a depth D opening to the contact face (2) and a plurality of contact elements (4) being delimited by the plurality of grooves (3), each the plurality of contact elements (4) having a top face (41) constituting a part of the contact face (2), the tread (1) comprising two rubber layers laminated in radial ori-entation of the tire, a cap rubber layer (6) configuring the contact face (2) when the tread being brand new and a base rubber layer (7) being provided radially inward of the cap rubber layer (6), the cap rubber layer (6) and the base rubber layer (7) being composed of different rubber compositions each other, the base rubber layer (7) being located totally radially inward of radially innermost part of the plurality of grooves (3), the tread being characterized in that a rubber composition constituting the cap rubber layer (6) comprises solid particles, and in that a rubber composition constituting the base rubber layer (7) is devoid of said solid particles, and in that a shore A hardness according to ASTM
D2240 of the rubber composition constituting the base rubber layer (7) is at least equal to 6 points higher than a shore A hardness of the rubber composition constituting the cap rubber layer (6).
[Claim 21 The tread (1) according to Claim 1, wherein a thickness of the base rubber layer (7) measured perpendicular to the contact face (2) at a center of the tread (1) is at least equal to 0.8 mm.
[Claim 31 The tread (1) according to Claim 1 or Claim 2, wherein a size of the solid particles is from 0.1 mm to 1.0 mm.
[Claim 41 The tread (1) according to any one of the Claims 1 to 3, wherein the rubber composition constituting the base rubber layer (7) is devoid of silica as reinforcing filler.
[Claim 51 The tread (1) according to any one of the Claims 1 to 4, wherein the rubber composition constituting the base rubber layer (7) comprises at least equal to 70 phr of natural rubber or isoprene rubber or mixture of natural rubber and isoprene rubber.
[Claim 61 The tread (1) according to any one of the Claims 1 to 5, wherein the shore A hardness of the rubber composition constituting the cap rubber layer (6) is at most equal to 60.

[Claim 71 The tread (1) according to any one of the Claims 1 to 6, wherein an amount of the solid particles in the cap rubber layer (6) is from 3 to 40 volume %.
[Claim 81 The tread (1) according to any one of the Claims 1 to 7, wherein the solid particles are crumb rubber particles.
[Claim 91 A tire having a tread according to any one of the Claims 1 to 8.