Tread for a tire

Abstract

An asymmetric tread for a snow tire comprising a rubber composition, comprising a tread surface intended to be in contact with the ground when the tire is running, and comprising a sequence of basic patterns arranged in the circumferential direction, each extending over at least 80% of the width (W) of the tread, each comprising a plurality of raised elements provided with sipes opening onto the tread surface, each sipe having a width of less than 1 mm and a depth of at least 3 mm. For each basic pattern, a sipes orientation level (NO) is defined that corresponds to $\frac{\underset{i}{.Math.}.Math.\mathrm{li}*i.Math.}{P*\mathrm{Wm}}$
where i is the number of sipes in the pattern, li is the length of the i.sup.th sipe on the tread surface, P is the pitch of the basic pattern, Wm is the width of the basic pattern and i is the positive or negative angle formed on the tread surface by the i.sup.th sipe with the transverse direction, where |i|45 degrees, said orientation level being greater than or equal to 1.5 degrees/mm. The rubber composition comprises at least one diene elastomer, a reinforcing inorganic filler, and a plasticizing system comprising a liquid plasticizing agent being a vegetable oil in a content B of between 10 and 60 phr.

Claims

1. An asymmetric tread for a snow tire comprising a rubber composition which comprises: at least one diene elastomer, a reinforcing inorganic filler, and a plasticizing system comprising a liquid plasticizing agent in a content B of between 10 and 60 phr, said liquid plasticizing agent being a vegetable oil, wherein the tread comprises: a tread surface intended to be in contact with the ground when the tire is running, a sequence of basic patterns arranged in a circumferential direction (X), wherein each basic pattern has a width (Wm) that extends in a transverse direction over at least 80% of the width (W) of the tread, each basic pattern has a pitch (P) extending a distance in the circumferential direction, and each basic pattern comprises a plurality of raised elements, one or more of which are provided with sipes opening onto the tread surface, wherein each sipe has a width of less than 1 mm and a depth of at least 3 mm, wherein, for each basic pattern, a sipes orientation level (NO) is defined that corresponds to $\frac{\underset{i}{.Math.}.Math.\mathrm{li}*i.Math.}{P*\mathrm{Wm}}$ where i is the number of sipes in the pattern, li is the length of the i.sup.th sipe on the tread surface, P is the pitch of the basic pattern, Wm is the width of the basic pattern and i is the positive or negative angle formed on the tread surface by the i.sup.th sipe with the transverse direction, where |i|45 degrees, said sipes orientation level being greater than or equal to 1.5 degrees/mm, and wherein the tread comprises a median dividing the tread into an inboard portion and an outboard portion, and the plurality of the raised elements on the inboard portion are provided with sipes at a steeper angle relative to the transverse direction than the plurality of the raised elements on the outboard portion, and the sipe orientation level for all the plurality of the raised elements in the inboard portion is greater than 2.

2. The tread according to claim 1, wherein said rubber composition comprises 20 to 100 phr of a diene elastomer bearing at least one SiOR function, R being hydrogen or a hydrocarbon radical.

3. The tread according to claim 1, wherein the reinforcing inorganic filler comprises from 50% to 100% by weight of silica.

4. The tread according to claim 1, wherein said rubber composition comprises 100 to 160 phr of said reinforcing inorganic filler.

5. The tread according to claim 1, wherein the plasticizing system further comprises a hydrocarbon resin in a content A of between 10 and 60 phr.

6. The tread according to claim 5, wherein the total content A+B is greater than 50 phr.

7. The tread according to claim 1, wherein the vegetable oil is a sunflower oil.

8. The tread according to claim 1, wherein the tread has a sipes density (D) defined as $\frac{\underset{i}{.Math.}\mathrm{li}}{P*\mathrm{Wm}},$ said sipes density (D) being greater than or equal to 60 m/mm.sup.2.

9. The tread according to claim 1, wherein the tread has a steering pull criterion (CT) for the basic pattern defined as $.Math.\frac{\underset{i}{.Math.}\mathrm{li}*i}{\underset{i}{.Math.}.Math.\mathrm{li}*i.Math.}.Math.,$ said steering pull criterion being less than or equal to 0.2.

10. The tread according to claim 1, wherein all or some of the raised elements of the basic patterns comprise at least one chamfer, said chamfer belonging to an edge of the raised elements making an angle at most equal to 45 with the transverse direction.

11. A snow tire comprising a tread according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will become apparent from the following description, given by way of nonlimiting example, with reference to the attached drawings in which:

(2) FIG. 1 schematically represents a partial view of a tread of a tire in accordance with the invention;

(3) FIG. 2 illustrates how the axial edge of a tread is determined according to a first determination method;

(4) FIG. 3 illustrates how the axial edge of a tread is determined according to a second determination method;

(5) FIG. 4 more specifically represents a basic pattern of the tread of FIG. 1.

(6) In the description that is to follow, elements that are identical or similar will be denoted by identical references.

(7) FIG. 1 partially depicts the tread surface of a tread 1.

(8) The tread 1 comprises a sequence of n basic patterns 3, 5 arranged in the circumferential direction X, where n is a natural integer greater than or equal to 1. To make FIG. 1 easier to understand, only a first basic pattern 3 and a second basic pattern 5 have been represented.

(9) Each basic pattern 3, 5 extends in the circumferential direction X at a determined pitch P.

(10) The pitch of the first basic pattern 3 here is identical to the pitch of the second basic pattern 5. As an alternative, the pitches of the basic patterns are different.

(11) Each basic pattern 3, 5 extends over at least 80% of the width W of the tread. In the example of FIG. 1, the width Wm of the basic pattern 3, 5 here is substantially identical to the width W of the tread.

(12) The width W of the tread corresponds to the distance between a first axial edge 7 and a second axial edge 9 of the tread.

(13) The way in which the width W of a tread is determined is illustrated in FIGS. 2 and 3, each of which show the partial profile of a tread 1 and the part of a sidewall 8 adjacent to it. In certain designs of tire, the transition from the tread to the sidewalls is very clear cut, as in the case represented in FIG. 2, and the first axial edge 7 and the second axial edge (not represented) can be determined intuitively.

(14) However, there are tire designs in which the transition between the tread and the sidewalls is continuous. An example is represented in FIG. 3.

(15) In this FIG. 3, the first axial edge 7 and the second axial edge of the tread are determined as follows. On a radial section through the tire, the tangent to the tread surface at every point on said tread surface in the region of transition toward the sidewall is plotted. The first axial edge 7 is the point at which the angle (beta) between said tangent and an axial direction is equal to 30. When there are a number of points for which the angle between said tangent and an axial direction is equal to 30, the point adopted is the radially outermost one. The same procedure is followed in order to determine the second axial edge of the tread.

(16) FIG. 4 more particularly depicts the first basic pattern 3 of FIG. 1.

(17) The first basic pattern 3 here comprises 11 raised elements, respectively termed the first raised element 11a, second raised element 11b, third raised element 11c, fourth raised element 11d, fifth raised element 11e, sixth raised element 11f, seventh raised element 11g, eighth raised element 11h, ninth raised element 11i, tenth raised element 11j, eleventh raised element 11k.

(18) The first basic pattern 3 is provided with i sipes 13i opening onto the tread surface, where i is a natural integer greater than 1.

(19) The i sipes are distributed across the various raised elements 11a-11k.

(20) For each sipe 13i it is possible to determine a sipe length li corresponding to the length of the line of said sipe on the tread surface.

(21) An angle i (alpha i) is also determined for each sipe 13i, where |i|45 degrees. The angle i corresponds to the angle formed by the i.sup.th sipe 13i with the transverse direction Y.

(22) The way in which the angles of the sipes are defined is explained later on in the description.

(23) The sipe angle is said to be positive with respect to the transverse direction Y if the rotation that brings said transverse direction Y onto the line of the sipe on the tread surface is in the counterclockwise direction.

(24) Conversely, the sipe angle is said to be negative with respect to the transverse direction Y if the rotation that brings said transverse direction Y onto the line of the sipe on the tread surface is in the clockwise direction.

(25) Thus, the first element 11a and the second element 11b have sipes that form an angle that is zero overall with the transverse direction Y.

(26) Likewise, the third element 11c, the fourth element 11d, the fifth element 11e, the sixth element 11f, the eighth element 11h and the tenth element 11j have sipes that form a positive angle with the transverse direction Y.

(27) Finally, the seventh element 11g, the ninth element 11i and the eleventh element 11k have sipes that form a negative angle with the transverse direction Y.

(28) The sipes belonging to one and the same raised element are oriented here with the same angle.

(29) As a variant, it is possible to have sipes of different orientation in one and the same raised element.

(30) A sipes density D, a sipes orientation level NO and a steering pull criterion CT are also defined for the basic pattern 3.

(31) The sipes density D corresponds to the equation

(32) $\frac{\underset{i}{.Math.}\mathrm{li}}{P*\mathrm{Wm}}.$
Remember that i is the number of sipes in the basic pattern, li is the length of the i.sup.th sipe on the tread surface, P is the pitch of the basic pattern and Wm is the width of the basic pattern.

(33) The sipes orientation level NO corresponds to the equation

(34) $\frac{\underset{i}{.Math.}.Math.\mathrm{li}*i.Math.}{P*\mathrm{Wm}}.$
Remember that i is the positive or negative angle formed on the tread surface by the i.sup.th sipe with the transverse direction Y and |i|45 degrees.

(35) The steering pull criterion CT for the basic pattern corresponds to the equation

(36) $.Math.\frac{\underset{i}{.Math.}\mathrm{li}*i}{\underset{i}{.Math.}.Math.\mathrm{li}*i.Math.}.Math..$

(37) The number of sipes 13i in the basic pattern 3, the length of the sipes and the angle of the sipes are determined so that the sipes density is greater than or equal to 60 m/mm.sup.2, the sipes orientation level is greater than or equal to 1.5 degrees/mm and the steering pull criterion for the pattern is less than or equal to 0.2.

(38) In this way a low degree of lateral pull of the tire when running on dry ground is guaranteed, while at the same time maintaining a good level of grip for this tire on snowy ground.

(39) Advantageously, the steering pull criterion for the pattern is less than 0.1.

(40) In another variant, the steering pull criterion for the pattern is less than 0.05.

(41) By way of example, the characteristics of the sipes present in the various raised elements of FIG. 4 have been listed in the table below. In this table, the first row references the various raised elements of the basic pattern 3, the second row details the angles of the sipes associated with the various raised elements, and the third row details the total length of the sipes present in the various raised elements.

(42) TABLE-US-00001 Raised elements 11a 11b 11c 11d 11e 11f 11g 11h 11i 11j 11k Angle of 0 0 4.5 4.5 12 20 30 35 30 18 30 the sipes in degrees Total 50 50 50 50 250 245 170 70 90 160 220 length of sipes (mm)

(43) By adopting a basic pattern pitch of the order of 50 mm and a pattern width Wm of 230 mm, a sipes density D of the order of 122 m/mm.sup.2, an orientation level NO of 2.44 degrees/mm, and a steering pull criterion CT of 0.025 can be calculated.

(44) It will be noted here that the shape of the raised elements 11a-11k and their arrangement on the tread are determined in such a way that the tread pattern design thus formed is asymmetric. The tire therefore has a predetermined direction of fitting so that one sidewall of the tire is always on the outside of the vehicle irrespective of where (right or left) on the vehicle it is mounted. These tires generally bear a marking (outside or inside) indicating to the user which sidewall of the tire has to face towards the outside of the vehicle, which hereinafter will be termed the outboard sidewall, and which sidewall of the vehicle has to face towards the inside of the vehicle, hereinafter termed the inboard sidewall. According to the markings on the tire, it is possible to determine in FIG. 4 an inboard half-tread 17 and an outboard half-tread 15. Specifically, the inboard half-tread is the half-tread adjacent to the inboard sidewall and the outboard half-tread is the half-tread adjacent to the outboard sidewall. It will be noticed that a median plane 19 separates the inboard half-tread 17 from the outboard half-tread 15.

(45) In FIG. 4, the inboard half-tread comprises the seventh element 11g, the eighth element 11h, the ninth element 11i, the tenth element 11j, the eleventh element 11k. Likewise, the outboard half-tread comprises the first element 11a, the second element lib, the third element 11c, the fourth element 11d, the fifth element 11e and the sixth element 11f.

(46) It will be noted that the inboard half-tread plays an important role in the functioning of the tire because this is the part of the tread that is most heavily involved in ensuring grip on snowy ground, particularly during cornering. Thus, in order to improve this cornering grip, the sipes of the inboard half-tread are inclined more steeply with respect to the transverse direction. In particular, measures are taken to ensure that the orientation level NO of the sipes on the raised elements 11g-11k are greater than 2 degrees/mm. In the example of the table described hereinabove, and considering an inboard half-tread width of Wm/2, an orientation level of these sipes of the order of 3.43 degrees/mm is thus determined.

(47) In one embodiment variant, all or some of the raised elements of the basic patterns comprise at least one chamfer, said chamfer belonging to an edge of the raised elements making an angle at most equal to 45 with the transverse direction.

(48) Table 1 below describes the nature of a conventional composition C1 that can be used to form a winter tire tread, based on polybutadiene and SBR copolymer (SBR1).

(49) In this control composition, the two elastomers used are in particular free of SiOR; the content of reinforcing inorganic filler is less than 100 phr, and the content A+B of plasticizing system is less than 50 phr, formed of polylimonene plasticizing resin (20 phr), sunflower vegetable oil (15 phr) and MES oil (5 phr) as liquid plasticizing system.

(50) The composition C2, in accordance with the invention, is characterized by the presence of at least 20 phr of a diene elastomer bearing a silanol function, at least 100 phr of a reinforcing inorganic filler, more than 50 phr of a plasticizing system formed of plasticizing resin (polylimonene) and of liquid plasticizing system (sunflower vegetable oil) at contents respectively between 10 and 60 phr. The elastomer SBR2 of composition C2 contains a mixture of 85% of an SBR bearing a dimethylsilanol function at a chain end and 15% of SBR star-branched to tin and having the same microstructure.

(51) TABLE-US-00002 TABLE 1 Composition No. C1 C2 BR (1) 40 IR (2) 50 SBR1 (3) 60 SBR2 (4) 50 Carbon black (5) 5 5 Silica (6) 90 115 Coupling agent (7) 7.2 9.2 Liquid plasticizer (8) 5 Liquid plasticizer (9) 15 40 Resin (10) 20 35 Total plasticizer 40 75 Stearic acid 3 3 Antiozone wax 1.5 1.5 Antioxidant (11) 2 2 DPG (12) 2.1 2.1 ZnO 1.2 1.2 Accelerator (13) 1.6 1.6 Sulphur 1.4 1.4 (1) BR with 4% of 1,2-units and 93% of cis-1,4-(Tg = 106 C.); (2) synthetic polyisoprene (SKI-3S sold by Nizhnekamsk); (3) SBR1: SBR with 27% of styrene units and 57% of 1,2-units of the butadiene part (Tg = 24 C.); (4) SBR2: SBR (Sn star-branched) with 27% of styrene units and 24% of 1,2-units of the butadiene part bearing a silanol function as end of the elastomer chain (Tg = 48 C.); (5) ASTM N234 grade (Cabot); (6) Zeosil 1165 MP silica from Rhodia of HDS type; (7) TESPT (Si69 from Degussa); (8) MES oil (Catenex SNR from Shell); (9) Sunflower oil containing 85% by weight of oleic acid, Lubrirob Tod 1880 from Novance; (10) C5/C9 resin (Escorez ECR-373 from Exxon); (11) N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine, from Flexsys; (12) Diphenylguanidine (Perkacit DPG from Flexsys); (13) N-dicylohexyl-2-benzothiazole sulphenamide (Santocure CBS from Flexsys).

(52) These two compositions were extruded in the forme of a tread in order to then be tested.

(53) Compositions C1 and C2 are used as treads for conventionally manufactured radial carcass passenger vehicle winter tires, denoted respectively P1 (control tire) and P2 (tire in accordance with the invention), having dimensions 225/45 R17. The control tire P1 is a commercial snow tire of conventional tread pattern design. Tire P2 has a tread pattern design in accordance with the invention. Tires P1 and P2 thus differ by their rubber composition and their tread pattern design.

(54) Tires P1 and P2 were then subjected to straight-line braking tests on wet ground and on snowy ground.

(55) In order to test the braking on wet ground, the tires were fitted to a motor vehicle of Audi make and A4 model, equipped with an ABS braking system and the distance needed to go from 80 km/h to 10 km/h was measured during sudden braking on sprayed ground (bituminous concrete).

(56) In order to test the braking on snowy ground, the tires were fitted to a motor vehicle of Volkswagen make and Golf model, equipped with an ABS braking system and the distance needed to go from 50 km/h to 5 km/h was measured during emergency braking on snow.

(57) It is observed that the snow tire in accordance with the invention P2 has grip on wet ground that is improved by around 5% compared to the control tire P1. It is also noted that this result is obtained without being at the expense of the performance on snowy ground. On the contrary, the snow tire P2 has grip on snowy ground that is improved by around 4% compared to the control tire P1.

(58) Additional braking tests were carried out on a winding circuit under wet conditions. It was observed that the snow tire P2 also has improved transverse grip on wet ground, that is to say curve grip, compared to the control tire P1.