IE47375B1

IE47375B1 - Improvements to motor vehicle tyres

Info

Publication number: IE47375B1
Application number: IE1910/78A
Authority: IE
Original assignee: Pirelli
Priority date: 1977-09-23
Filing date: 1978-09-22
Publication date: 1984-03-07
Also published as: IE781910L; BE870399A; LU80253A1; DK421778A; TR20493A; NO783219L; ES473886A1; CA1095822A; IT1087461B; FI782357A; SE7809823L; AR218931A1; DE2841132A1; GR73083B; NL7807613A; FI58890B; AU3944078A; FR2403901B1; JPS5457706A; BR7804806A

Abstract

A pneumatic tire for a motor vehicle has a tread divided into first and second annular contiguous zones or portions. The first zone is free from annular grooves and has a substantially smooth surface with circumferentially spaced generally laterally extending short cuts or grooves inclined with respect to the circumferential line of the tire. The elastomer in the composition forming the first zone has a second order glass transition temperature of from -10.degree.C. to-25.degree.C. The second zone has laterally spaced annular grooves and transverse grooves forming a tread pattern. The elastomeric composition of the second zone contains an elastomer having a second order glass transition temperature of -30.degree.C. to -55.degree.C.

Description

The present invention relates to pneumatic tyres and in particular relates to motor vehicle tyres, having treads able to provide good traction on roads covered with ice or snow and also on roads free from ice and snow.

It is known that the tread of a pneumatic tyre is designed in such a way as to comply with various requirements, and first to provide good road holding both at normal and at high speed, during straight, steady travel as well as on cornering and braking. It is desirable that this road holding should be obtained in any road condition, namely on dry, slippery and wet roads, and on snowy and icy roads. However, these requirements are in opposition to one another and hitherto it has been thought to be impossible to meet them in any one tyre, and therefore separate types of tyres have been made for winter use. In tyres intended to be prevailingly used on snowy and/or icy roads, the tyre tread is of the so-called “winter type, namely it has a pattern divided into small zines by deep grooves and, if desired, it is provided with spikes or studs in the blocks defined by said grooves in order to afford a better grip on ice. Moreover, the elastomeric compositions of which such tyres are formed have such physical properties as to remain unaffected even at the lowest temperatures. On the other hand, for tyres intended to be prevailingly used on dry ground, the tread pattern is not so finely divided and is provided with shallow grooves to offer the largest possible area in contact with the ground; moreover, the elastomeric composition used to form it is in general of the high 2. hysteresis loss type which is more appropriate to ensure a good road holding in said conditions.

It is clear that the above practice is inconvenient since pneumatic tyres of both types should be available for any car and since one type has to be changed for the other whenever the conditions of the road change to or from wintery weather.

There has now been devised, according to the present invention, a pneumatic tyre which obviates the disadvantage referred to above and which is suitable for use on roads covered with snow or ice and on roads free from snow and ice.

Accordingly, the present invention provides a pneumatic tyre comprising a carcass and a tread, said tread comprising first and second annular circumferentially-extending tread portions contiguous with each other, the radially outer surface of said first circumferential portion having a configuration different from that of the radially outer surface of 3aid second portion, and said first portion being of an elastomeric composition having a second order transistion temperature in the range from minus 30°C to minus 55°C, and said second portion being of an elastomeric composition having a second order transition temperature in the range from minus 10°C to minus 25°C.

Thus, the present invention provides a tread for motor vehicle tyres of a type divided into two annular circumferentially-extending portions,contiguous to each other, having surface 3. configurations and made of different elastomeric said compositions, in which/second portion is able to provide good adherence on dry and/or wet ground and said first portion is able to provide the good adherence on snowy and/or icy ground, with the result of a good road holding of the tread in any road condition.

The values of the second order transition temperatures of the compositions comprising said first and second portionspreferably differ from each other by at least °C.

The expression second order transition temperature (or glass transition temperature) means, as known, the temperature at which a given cured composition based on an elastomeric polymer passes from a rubber-like state to a glass-like state. Said temperature can be measured using the apparatus known as Sehirieder & Wolf torsional pendulum and according to DIN 53445 Standard, and the values for second order transition temperatures given in the claims and description of this specification are those obtained using that apparatus.

We have ascertained that the selection of elastomeric compositions, each having a second order transition temperature falling in the above indicated limits, to form the portions constituting the tread of motor vehicle tyres, gives good adherence of each circumferential portion on a ground in the condition for which it is intended, balancing therefore the poorer adherence which, in said condition, is shown by the other 4. circumferential portion of the tread.

We have further ascertained that a further contribution to the achievement of such a result is given by the adoption, for said second portion, that is the one intended to grip dry and/or wet ground, of a radially outer portion completely devoid of grooves in lieu of a pattern conventionally adopted for the treads directed to this purpose. In said case, which represents a preferred form of the present invention, said second portion whose surface is completely devoid of grooves has an axial width in the range from 28% to 53% (for example from 40% to 50%) of the total width of the tread. The total tread width” as used herein refers to the width of a radial section of the tyre between the tyre shoulders. Correspondingly, said first circumferential portion intended to provide a high adherence on snowy and/or icy ground has an axial width in the range from 47% to 72% (for example from 50% to 60%) of said total width of the tread. In its radially outer surface said first portion has a pattern of conventional type, that is, a tread pattern having many grooves defining protruding blocks, preferably, but not necessarily arranged in circumferential lines. In said first portion, the solid/hollow ratio is preferably in the range 0.43 to 0.46.

The vehicle results obtained by adopting a tread configuration devoid of grooves for said second portion are particularly surprising when it is considered that high adherence of that portion is also obtained on wet road surfaces.

. It is known that a tread having a completely smooth surface has a road holding which is very high on a dry road, but is minimal on a wet road, owing to the so-called aquaplane phenomenon caused by a water layer present between the ground and the tread in the impression area, which excludes the tread from any cor tact with the ground). Although it is not intended that the present invention is limited according to any particular theory, it is believed that the good road holding shovzn by smooth tread of said second portion on wet ground is due to the fact that the limited area of the smooth tread being substantially less than the total tread area, minimises the aquaplane phenomenon and that, at the same time, the extremely subdivided pattern of the surface of said first portion (which, by itself, would not be very satisfactory in said ground condition) facilitates the drainage of the water layer which would otherwise tend to accumulate under the smooth ground-contacting area of said second portion. Therefore, it is preferable for said first portion to have at least a proportion of its grooves oriented so that they are inclined with respect to the mid-circumferential plane of the tyre and end at the lateral (axial) extremity of the tread belonging to said portion, facilitating in this way the elimination of water from the ground-contacting area of the other portion. If desired, in order greatly to facilitate the road holding of a smooth second .portion, said portion can be provided at said surface with short cuts. Such cuts 6. 7 3 7 5 can, for example have a width in the range from 1 to 5 mm and a length in the range from 5 to 25 mm and a depth in the range from 6 to 9 mm. The cuts can be arranged along the circumference at intervals between 45° and 180° according to the tyre size and inclined at an angle between 45° and 90° with respect to the mid-circumferential plane of the tyre. Preferably, the present invention applies to motor vehicle tyres of the radial carcass type, although it can be adopted for tyres having crossed ply carcass.

The elastomeric compositions having second order transition temperatures as indicated above and intended to form the one or the other of the portions constituting the tread are based on natural and/or synthetic rubber. The elastomeric compositions intended to form the first portion, namely that able to grip snowy and/or icy ground, can be based, for example, on a polymer or polymers selected from natural rubber, polyisoprene, polybutadiene, and butadiene-styrene copolymers having a styrene content in the range from 18% to 23.5% by weight of the total weight of styrene and butadiene.

The elastomeric compositions intended to form the second portion, namely that able to grip dry and/or wet ground, can be based, for example, on a butadiene-styrene copolymer having a styrene content in the range from 23.5% to 40% by weight of the total weight of styrefle and butadiene, and can contain other polymers .

Such other polymer, for example, can be a rubbery polymer, for instance a rubbery styrene-butadiene polymer. In one instance, 7. described below, the elastomeric composition comprising said second portion includes a minor quantity of a rubbery polymer included in said first portion.

One embodiment of the invention, a motor car radial tyre of 5 size LR 78.15, will now by described by way of example with reference to the accompanying drawings of which Figure 1 is a partial plan view of the tyre tread; and Figure 2 is a radial section of the tyre having a tread shown in Figure 1.

EXAMPLE Two tyre tread compositions, Compositions A and B, were obtained by blending together the following ingredients, quantities shown being parts by weight. These compositions, A and B, were used respectively to form the first and second annular circumferentially15 extending portions 2,6 of the tyre tread 1.

SBR/1712 is an oil-extended styrene-butadiene copolymer formed from styrene, butadiene and extending oil in the ratio by weight 23:77:37 and it contains an antioxidant. Synpol 8401 is an oilextended styrene-butadiene copolymer formed from styrene, butadiene and extending oil in the ratio by weight 40:60:37. 1st circumf. 2nd circumf. portion portion Composition A Composition B SBR/1712 70.000 25.000 cis 4-polybutadiene 30.00 - SBR (Synpol 8401) - 75.000 Zinc oxide 2.500 3.500 Sulphur 1.500 2.000 CBS 1.000 1.500 Anti-ozone agent 0.750 1.500 1st portion 2nd portion Composition A Composition B Antioxidant 0.750 1.000 DPG 0.150 - Stearic acid 2.000 1.000 Paraffin 2.000 1.000 Aromatic oil 16.000 40.000 Carbon black N-375 71.000 75.000 2nd order transistion temperature - 46°C - 15°C The tyre tread is constituted by first and second annular circumferentiallyextending portions 2, 6, contiguous to each other, in which portion 2 (the portion which, when the tyre is mounted on a car, will be directed towards the inside of the latter) shows five series or bands of circumferential grooves 3 defining four circumferential series or bands of projecting blocks 4; each block is divided from the adjacent blocks belonging to the same series by means of inclined grooves 5. On the other hand, the portion 6 is completely devoid of grooves and shows only short cuts 7, having a width and a length of 2mm and 20 mm, respectively, and a depth not greater than that of the grooves 3 and 5; said cuts are arranged at an angle of 45° with respect to the mid-circumferential plane of the tyre and in such a circumferential sequence that they are mutually spaced at intervals corresponding to 60°.

The width L* of said portion 6 is equal to 45% of the total width L of the tread; as it can be noted, said width L is represented by the length separating the lateral (axial) extremities of the tread, the latter measured at the points where the tread is radiused to the tyre shoulders. Consequently, the width of the portion 2 corresponds to of the total width L of the tread.

The portions 2 and 6 are formed, respectively, of the previously described elastomeric compositions A and B, whose second order transition temperatures are minus 46°C and minus 15°C, respectively.

Figure 2 represents'in radial section the tyre 8 provided with the tread 1 shown in Figure 1, which is substantially constituted by a radial carcass 9 and by a reinforcing structure 10 (the so-called breaker) positioned between the carcass and the tread. The tread is constituted by the previously described portions 2 and 6.

Claims

1. A pneumatic tyre comprising a carcass and a tread, said tread comprising first and second annular circumferentiallyextending tread portions contiguous with each other, the radially outer 5 surface of said first portion having a configuration different from that of the radially outer surface of said second portion, and said first portion being of an elastomeric composition having a second order transition temperature in the range from minus 30°C to minus 55°C, and said second portion being of an elastomeric composition Ιθ having a second order transition temperature in the range from minus 1O°C to minus 25°C.

2. A pneumatic tyre according to Claim 1, in which said first and second elastomeric compositions have values for their respective second order transition temperatures which differ from each 15 other by at least 10°C.

3. A pneumatic tyre according to Claim 2, in which said first portion, at its radially outer surface, has a configuration of grooves and blocks according to a solid/hollow ratio in the range from 0.43 to 0.46, the radially outer surface of said second portion being 20 completely devoid of grooves.

4. A pneumatic tyre according to Claim 1, 2 or 3, in which the width of said second portion is in the range from 282 to 53% of the total width of the tread.

5. A pneumatic tyre according to Claim 3 or 4, 25 in which the radially outer surface of said second portion is provided with cuts having a width in the range 1 to 5mm, a length in the range from 5 to 25 mm and a depth not greater than that of the grooves present in the radially outer surface of said first portion, said cuts being inclined, with respect to the mid-circumferential plane of the tyre, at an angle in the range from 45° to 90° and being arranged circumferentially of the tyre at an angular spacing in the range from 45° to 180°.

6. A pneumatic tyre according to any of the preceding claims, in which the elastomeric composition forming said first portion is based on a rubber comprising a butadiene-styrene copolymer having a styrene content in the range from 18% to 23.5% by weight of the total weight of styrene and butadiene.

7. A pneumatic tyre according to any of the preceding claims, in which the elastomeric composition forming said second portion is based on a rubber comprising a butadiene-styrene copolymer having a styrene content in the range from 23.5% to 40% by weight of the total weight of styrene and butadiene.

8. A pneumatic tyre according to any of the preceding claims, which has a radial carcass.

9. A pneumatic tyre according to any of the preceding claims in which the axial width of said first portion is in the range from 47% to 72% of the total width of the tread and the axial width of said second portion is in the range from 53% to 28% of the total width of the tread.

10. A pneumatic tyre according to Claim 1, substant12. ially as described herein with reference to the accompanying drawings.

11. A pneumatic tyre substantially as described in the Example with reference to the accompanying drawings.