WO2003066353A1

WO2003066353A1 - Belt for a pneumatic tire belt made of differente rubber compositions

Info

Publication number: WO2003066353A1
Application number: PCT/US2003/003211
Authority: WO
Inventors: Georg A. Bohm; Walter Tomaszewski
Original assignee: Bridgestone Corp
Current assignee: Bridgestone Corp
Priority date: 2002-02-04
Filing date: 2003-02-03
Publication date: 2003-08-14
Anticipated expiration: 2004-08-04
Also published as: AU2003208965A1

Abstract

A tire belt (18) for a pneumatic tire comprising a rubber strip (20) and reinforcement cords (22) embedded in the rubber strip (20). The strip (20) comprises two or more portions made of different compounds optimized respectively towards different desired belt properties. These portions can comprise an annular coating (24) surrounding the reinforcement cords (22) and a ribbon (26) surrounding the annular coating (24). The annular coating (24) can be made of a compound optimized towards one set of desired belt properties (e.g., good rubber-to-cord adhesion) and the ribbon (26) can be made a different compound optimized towards another set of desired belt properties (e.g., good thermal aging stability, cut growth performance, etc.).

Description

TIRE BELT MADE OF DIFFERENT RUBBER COMPOSITIONS

FIELD OF THE INVENTION The present invention relates to a reinforcement belt for a pneumatic tire {i.e., a tire belt) and, more particularly, to a tire belt which includes a rubber strip and reinforcement cords embedded in the rubber strip.

BACKGROUND OF THE INVENTION

A pneumatic tire commonly comprises a tread, a supporting carcass, sidewalls, and a tire belt positioned between the tread and the carcass. A tire belt usually comprises a plurality of reinforcement cords embedded within a rubber strip. The cords can be, for example, twisted arrangements of steel, glass, nylon, aramid and/or polyester monofi laments.

The tire belt typically includes at least two rows of reinforcement cords. When the tire belt includes two rows, it is usually constructed from two rubber layers, each having reinforcement cords embedded within the rubber layer. The cords are usually diagonally arranged relative to the layer, with the diagonal direction of one layer's cords being opposite that of the other layer's cords. The layers are fused together to form the rubber strip, with the two layers of diagonal cords providing a "criss-crossing" grid of cords in the tire belt. When the tire belt includes more than two rows, it can be constructed in a similar manner.

A tire belt is used to control or limit the outward expansion or deformation of the tire in service, and to stiffen and strengthen the interface between the carcass plies and the tread rubber, all of which positively influence important tire performance criteria such as cornering, wear, etc.

Strong adhesion between the cords and the rubber strip is a prerequisite design parameter in the manufacture of tire belts. Without such adherence, the cords would separate from the rubber strip and the tire belt would be rendered useless. This cord-to-rubber adhesion can be accomplished by coating steel cords with brass prior to embedding them in the bulk rubber to form the rubber strip. Good rubber-to-cord bonding is generated by the reaction between sulfur in the bulk rubber and the copper in the brass coating.

Accordingly, the bulk rubber must contain not only the necessary amount of sulfur to ensure proper vulcanization, but an extra amount of sulfur is needed to generate the desired reaction with the brass. Also, the bulk rubber may need to contain other appropriate cord-to-rubber adhesion-promoting additives, such as cobalt salts, resorcinol and resins. However, high concentrations of sulfur can negatively affect some of the performance parameters that the bulk rubber strives to achieve, such as high strength, good cut growth resistance, and sufficient resistance against thermal aging. Moreover, the above-mentioned and other adhesion-promoting additives can be relatively expensive.

SUMMARY OF THE INVENTION The present invention provides a tire belt for a pneumatic tire, comprising a rubber strip and reinforcement cords embedded in the rubber strip. The strip comprises two or more portions made of different compounds {i.e., stocks having a different combination of ingredients) optimized respectively towards different desired properties.

The strip can comprise an annular coating surrounding the reinforcement cords and a ribbon surrounding the coated cords. The annular coating can be made of a compound A optimized towards one set of desired belt properties, and the ribbon can be made of a different compound R optimized towards another set of desired belt properties.

The compound A can be optimized to provide good cord-to-rubber adhesion (for brass-coated cords) by including a relatively high content of sulfur, such as more than about 3 phr sulfur, more than about 4 phr sulfur, more than about 5 phr sulfur, between about 2 phr and about 9 phr, and/or between about 3 phr and about 7 phr sulfur. The compound R used in the ribbon surrounding the coated cords {i.e., the bulk rubber) can have a very low level of sulfur (e.g. less than about 3 phr sulfur and/or less than about 2 phr sulfur), as it is not required to provide cord-to-rubber adhesion-enhancing qualities. In this manner, the compound R can be optimized to provide other different properties, such as good tear strength, good aging stability and/or good cut growth performance. The compound A can be chosen to provide a modulus of elasticity M_A, which is greater than the modulus of elasticity M_R of the bulk rubber and less than the modulus of elasticity M_c of the cord material. For example, the ratio M_A/M_R can be greater than about 1.1 and/or can be greater than about 2.0. To this end, the compound A can include an increased amount of reinforcement filler (e.g., N339 and/or N220 carbon black, silica) and/or the addition of nano- elements (e.g., nano-fibers, nano-tubes). The reinforcement filler concentration can be greater than about 20 phr, greater than about 30 phr, greater than 40 phr, greater than 50 phr, and/or between about 40 phr and about 50 phr. The nano-elements can be greater than about 5 phr, greater than about 6 phr, greater than about 7 phr, greater than about 8 phr, greater than about 9 phr, about or greater than 10 phr, and/or between about 2 phr and about 10 phr. In this manner, the annular coating provides a gradual transition in stiffness between the very rigid cord and the relatively soft bulk rubber.

The deformation of a tire during service causes significant stresses and strains to be encountered in the tire belt and particularly at the belt edge. On prolonged use of the tire, and particularly in hot climates, such strains may cause the formation of micro-cracks, particularly at the belt edge. It is thus important to select belt constructions and rubber compounds which avoid the formation and growth of micro-cracks in a tire. In the presently used tire belts, the rubber compound contains ingredients such as cobalt salts, high sulfur levels, etc., which provide good adhesion to brass coated steel cords. However, the compound also has to have good stability to thermal aging and good cut growth resistance, which is an equally important criteria for a well-performing tire belt. The need for both often leads to compromises in performance. For example, some of the ingredients used to promote the adhesion to the cord are detrimental to compound stability on heat aging, etc. This problem is avoided by this invention. Here, compound A of the annular coating can be optimized in performance separately from the optimization of the bulk layer (R). For example, compound A can be designed for optimum adhesion to the cord, whereas compound R is designed for good tear strength, good aging stability and good cut growth resistance. Alternatively or additionally, compound A may contain more reinforcing filler or special fillers such as short fibers (i.e. nanofibers), which will increase the stiffness of the annular coating layer and provide for a gradient in stiffness between the very stiff cord and the low modulus bulk layer. The use of one or more coating layers providing such a stiffness gradient has been shown to be very effective in avoiding the formation and growth of micro-cracks. The strip can comprise a first section made from a compound R1 optimized towards one set of desired belt properties and a second section made from a different compound R2 optimized towards a different set of desired belt properties.

Accordingly, the present invention provides a tire belt in which critical performance criteria may be improved independently from each other and without the compromises conventionally encountered. The present invention provides these and other features hereinafter fully described and particularly pointed out in the claims. The following description and drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative of, however, but a few of the various ways in which the principles of the invention can be employed.

DRAWINGS Figure 1 is a cross-sectional view of a tire according to the present invention, the tire including a tread, a carcass, side walls, and a tire belt positioned between the tread and the carcass.

Figure 2 is an isolated cross-sectional view of the tire belt according to one embodiment of the invention, the belt comprising reinforcement cords, an annular coating surrounding the reinforcement cords, and a bulk rubber surrounding the annular coating and defining the strip geometry of the belt. Figure 3 is a graph schematically showing the stiffness transition between the cord and the bulk rubber.

Figure 4 is an isolated cross-sectional view of the tire belt according to another embodiment of the invention, wherein two annular coatings surround the reinforcement cords to provide a more gradual stiffness transition.

Figure 5 is an isolated cross-sectional view of the tire belt according to another embodiment of the invention, wherein the bulk rubber surrounds the reinforcement cords and comprises two sections.

Figure 6 is a view similar to Figure 5, with the bulk rubber comprising two sections.

DETAILED DESCRIPTION Referring now to the drawings in detail, and initially to Figure 1 , a pneumatic tire 10 according to the present invention is shown. The tire 10 includes a tread 12, a carcass 14, side walls 16 and a belt 18 positioned between the tread 12 and the carcass 14.

As is best seen by referring additionally to Figure 2, the tire belt 18 comprises a rubber strip 20 and a plurality of reinforcement cords 22 embedded in the rubber strip 20. In the tire belt 18 shown in Figure 2, the rubber strip 20 comprises two rows (or levels) of cords 22, with the diagonal direction of one row's cords being opposite that of the other row's cords to form a "crisscrossing" grid of cords 22 in the tire belt 18. However, tire belts with a different number of cord rows (e.g., 4 rows) and/or differently oriented rows are possible with, and contemplated by, the present invention.

The cords 22 can comprise a twisted arrangement of a plurality of filaments (e.g., four in the illustrated embodiment). Cords having more or less filaments, non-twisted arrangements and/or single filament constructions are possible with, and contemplated by, the present invention. The cords 22 can be made of any suitable material, such as steel, glass, nylon, aramid and/or polyester monofilaments. An annular coating 24 surrounds each of the reinforcement cords 22, and a ribbon 26 surrounds the annular coating 24 to form the rectangular profile of the strip 20. The annular coating 24 is made of a compound A optimized primarily towards rubber-to-cord adhesion properties, and the ribbon 26 is made of a different compound R optimized towards other (e.g., non-cord-adhesion) parameters. In this manner, the compromises conventionally necessary to meet both adhesion-promoting requirements and non-adhesion-related performance parameters when formulating a bulk rubber are significantly reduced.

According to one embodiment of the invention, the cords 22 are brass- coated steel cords, and the compound A includes a relatively high content of sulfur. More particularly, the compound A could include more than 3 phr (parts- per-hundred rubber) sulfur, more than about 4 phr sulfur, more than about 5 phr sulfur, between about 2 phr and about 9 phr, and/or between about 3 phr and about 7 phr sulfur. The compound A could additionally include adhesion- promoting additives such as resins, resorcinol, and/or cobalt salts, as well as other ingredients such as carbon black, zinc oxide, resins tackifier, anti-oxidents, anti-ozonents, and/or accelerators.

With high content of sulfur in the compound A, there can be a significant reduction of sulfur in the bulk rubber (e.g., less than 3 phr sulfur, less than 2 phr sulfur and/or less than 1 phr sulfur), resulting in the bulk rubber having a higher tear strength, greater resistance against thermal-aging, and/or improved cut growth resistance when compared to conventional bulk rubber. Also, there can be a dramatic decrease or even elimination of cord-to-rubber adhesion- promoting additives (e.g., resins, resorcinol, and/or cobalt salts), translating into cost savings, as these additives can be relatively expensive.

According to another embodiment of the invention, the compound A has a modulus of elasticity (M_A) greater than the modulus of elasticity (M_R) of the compound R and less than the modulus of elasticity (M_C0RD) of the cord material. For example, the ratio M_A/M_R can be greater than about 1.1 and/or greater than about 2.0. In this manner, the annular coating provides a gradual transition in stiffness between the relatively rigid cord and the bulk rubber, as is shown schematically in Figure 3. To accomplish the desired transition, the compound A can include an increased amount of reinforcement filler (e.g., N339 and/or N220 carbon black, silica) and/or additional nano-elements. For example, the reinforcement filler concentration can be greater than about 20 phr, greater than about 30 phr, greater than 40 phr, greater than 50 phr, and/or between about 40 phr and about 50 phr. The nano-elements can be greater than about 5 phr, greater than about 6 phr, greater than about 7 phr, greater than about 8 phr, greater than about 9 phr, about or greater than 10 phr, and/or between about 2 phr and 20 phr. The nano-elements can constitute nano-fibers or nano-tubes (single- walled, doubled-walled, or greater-walled). These elements typically have a cross-sectional diameter D greater than about 10 nanometers and a length-to- diameter ratio L/D between about 10 to about 1000 or more. In other words, the length of the element is much greater than its diameter. During the coating of the cords 22, the nano-elements tend to align themselves in the direction of the cords 22, whereby the length of the nano-elements is substantially parallel to the length of the cords 22. This parallel orientation (as opposed to, for example, a perpendicular orientation) introduces a desired anisotropic stiffness in the annular coating 24. It may be noted that if both reinforcement fillers and nano-elements are used, the amounts may be inversely adjusted depending on the ratio therebetween. For example, if a high concentration of reinforcement fillers is used (e.g., greater than about 50 phr), a lesser amount of nano-elements (e.g., less than 10 phr) may be used. Conversely, if a lower concentration of reinforcement fillers is used (e.g., less than about 50 phr), a greater amount of nano-elements (e.g., greater than 10 phr) may be used.

As stated earlier, if the cords are brass-coated steel cords, the compound A could additionally include rubber-to-cord adhesion-promoting additives such as resins, resorcinol, and/or cobalt salts, in addition to the basic compound ingredients such as carbon black, zinc oxide, tackifiers, anti-oxidants, anti- ozonants, and/or accelerators, etc. For other types of cords, other appropriate additives/ingredients could be used to promote rubber adhesion to such cords.

The gradual gradient of stiffness can be expanded by including a second annular coating 28 as shown in Figure 4. The annular coating 28 is made of a compound B selected so that it provides a modulus of elasticity (M_B) less than M_A and greater than M_R. It might also be desirable for the innermost coating 24 to include a high sulfur content if brass-coated steel cords are being used.

The following table sets forth possible compound formulations for the cord-coating rubber (Compound A) and the bulk rubber (Compound R):

The tire belt 18 shown in Figures 2 and 4 can be made by first forming a strip component comprising one row of cords 22. This forming can be accomplished by extruding annular coatings onto the reinforcing cords, and then embedding these coated cords with the compound R by calendering, a secondary extrusion operation, or co-extrusion with the annular coating. By way of example only, the cord-coating step can be accomplished with conventional wire coating extrusion equipment. The embedding step can be accomplished with the same equipment used to produce tire belts with uncoated reinforcement cords, such as the equipment shown in U.S. Patent No. 4,274,821 , the entire disclosure of which is hereby incorporated by reference. Other belt forming processes could be used as well.

If the strip components are formed in the above-described manner, the cords 22 will be aligned in the direction of extrusion. The strip component is then cut, repositioned, and subsequently spliced to form a continuous belt component strip in which the cords are oriented in a desired diagonal direction. The belt component strip is then applied to a cylindrical drum in two separate layers so that the respective rows of cords criss-cross each other. Usually, the inner-most layer {i.e., the layer closest to the drum) will be wider than the outer layer. A narrow rubber strip can be positioned between the belt component layers above the circumferential edges of the lower layer (belt insert) and/or a rubber strip can be positioned to cover the circumferential edges of both the inner and outer belt component layers. When the green tire is vulcanized, the belt component layers, the belt inserts and the rubber edge strip fuse together to form the strip 20. In addition or as an alternative to the annular coating(s) 24/28, the ribbon

26 can comprise sections 30 and 32 as shown in Figures 5 and 6. The first section 30 can be made from a compound R1 optimized towards one set of desired belt properties, and the second section 32 can made from a different compound R2 optimized towards a different set of desired belt properties. In the illustrated embodiment, the sections 30 and 32 comprise radial layers of the tire belt 18, thereby allowing the tread-adjacent section 30 to be optimized towards certain properties while the carcass-adjacent section 32 is optimized towards other desired properties. However, non-layered ribbon sections and/or non- radial ribbon sections are also possible with and contemplated by the present invention. One can now appreciate that the present invention provides a tire belt in which critical performance criteria may be improved independently from each other and without the compromises conventionally encountered. Although the invention has been shown and described with respect to certain embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. The present invention includes all such alterations and modifications and, moreover, is limited only by the scope of the following claims.

For the purposes of the United States, this application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/354,159, filed on February 4, 2002. The entire disclosure of this earlier application is hereby incorporated by reference.

Claims

1. A tire belt (18) for a pneumatic tire comprising a rubber strip (20) and reinforcement cords (22) embedded in the rubber strip (20); wherein the strip (20) comprises two or more portions (24/26 and/or 30/32) made of different compounds optimized respectively towards different desired belt properties.

2. A tire belt (18) as set forth in claim 1 , wherein the strip (20) comprises an annular coating (24) surrounding the reinforcement cords (22) and a ribbon (26) surrounding the annular coating (24); and wherein the annular coating (24) is made of a compound A optimized towards one set of desired belt properties, and the ribbon (26) is made of a different compound R optimized towards another set of desired belt properties.

3. A tire belt (18) as set forth in claim 2, wherein the compound A is optimized to provide good adhesion to the reinforcement cords (22).

4. A tire belt (18) as set forth in either claim 2 or claim 3, wherein the compound A includes a higher content of sulfur than the compound R.

5. A tire belt (18) as set forth in any of claims 2-4, wherein the compound A has more than about 3 phr sulfur.

6. A tire belt (18) as set forth in the preceding claim, wherein the compound A has more than about 4 phr sulfur.

7. A tire belt (18) as set forth in the preceding claim, wherein the compound A has more than about 5 phr sulfur.

8. A tire belt (18) as set forth in any of claims 2-4, wherein the compound A has between about 3 phr and about 7 phr sulfur.

9. A tire belt (18) as set forth in the preceding claim, wherein the compound A has between about 3 phr and about 7 phr sulfur.

10. A tire belt (18) as set forth in any of claims 2-9, wherein the reinforcement cords (22) have a modulus of elasticity M_cord, the annular coating (24) has a modulus of elasticity M_A, and the ribbon (26) has a modulus of elasticity M_R, and wherein M_cord is greater than M_A and M_A is greater than M_R.

11. A tire belt (18) as set forth in the preceding claim, wherein M_A/M_R is greater than about 1.1.

12. A tire belt (18) as set forth in the preceding claim, wherein M_A/M_R is greater than about 2.0.

13. A tire belt (18) as set forth in any of claims 10-12 wherein the compound A comprises a greater concentration of reinforcement fillers than the compound R.

14. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement filler comprises carbon black and/or silica.

15. A tire belt (18) as set forth in either of the preceding claims, wherein the reinforcement filler in compound A is greater than about 20 phr.

16. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement filler in compound A is greater than 30 phr.

17. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement filler in compound A is greater than 40 phr.

18. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement filler in compound A is greater than 50 phr.

19. A tire belt (18) as set forth in any of claims 10-18 wherein the compound A includes nano-elements.

20. A tire belt (18) as set forth in the preceding claim, wherein the nano-elements comprise nanofibers.

21. A tire belt (18) as set forth in either of the preceding claims, wherein the nano-elements comprise nanotubes.

22. A tire belt (18) as set forth in any of claims 19-21 , wherein the nano-elements are greater than about 5 phr.

23. A tire belt (18) as set forth in the preceding claim, wherein the nano-elements are equal to or greater than about 10 phr.

24. A tire belt (18) as set forth in the preceding claim, wherein the nano-elements are between about 2 phr and about 10 phr.

25. A tire belt (18) as set forth in any of claims 2-24, wherein the compound R has less than 3 phr sulfur.

26. A tire belt (18) as set forth in any of claims 2-25, wherein the compound R is optimized to provide good tear strength, good aging stability and cut growth performance.

27. A tire belt (18) as set forth in any of claims 2-26, wherein the strip

(20) comprises a second annular coating (28) surrounding the first annular coating (24), and wherein the second annular coating (28) is made of a compound B different from compound A.

28. A tire belt (18) as set forth in the preceding claim, wherein the strip (20) comprises a second annular coating (28) surrounding the first annular coating (24), and wherein the second annular coating (28) is made of a compound B different from compound A, and wherein the second annular coating (28) has a modulus of elasticity M_B which is greater than M_R but less than M_A.

29. A tire belt (18) as set forth in any of claims 2-28, wherein the strip (20) comprises a plurality of annular coatings (24,28) surrounding the reinforcement cords (22) and a ribbon (26) surrounding the annular coatings (24,28).

30. A tire belt (18) as set forth in the preceding claim, wherein the modulus of elasticity of the annular coatings (24,28) is greater than the modulus of elasticity of the reinforcement cords (22) and less than the modulus of elasticity of the ribbon (26), and wherein the modulus of elasticity of the annular coatings (24, 28) decreases from the innermost coating (24) to the outermost coating (28).

31. A method of making the tire belt (18) set forth in any of claims 2- 30, comprising the steps of extruding the annular coating (24) onto the reinforcing cords (22) and then combining these coated cords with the compound R by calendering or extrusion.

32. A tire belt (18) as set forth in claim 1 , wherein the ribbon (26) comprises a first section (30) made from a compound R1 optimized towards one set of desired belt properties and a second section (32) made from a different compound R2 optimized towards another set of desired belt properties.

33. A tire belt (18) as set forth in any of the preceding claims, wherein the reinforcement cords (22) are made of steel, glass, nylon, aramid and/or polyester.

34. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement cords (22) comprise steel cords coated with brass.

35. A tire belt (18) as set forth in any of the preceding claims, wherein the reinforcement cords (22) comprise monofilaments.

36. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement cords (22) comprise twisted monofilaments.

37. A tire belt (18) as set forth in any of the preceding claims, wherein the reinforcement cords (22) are arranged in a plurality of rows.

38. A tire belt (18) as set forth in the preceding claim, wherein the reinforcement cords (22) are arranged in at least two rows.

39. A tire belt (18) as set forth in either of the two preceding claims, wherein a first row runs in one diagonal direction and a second row runs in an opposite diagonal direction to form a criss-cross grid of cords.

40. A pneumatic tire (10) comprising a tread (12), a supporting carcass (14), side walls (16) and the tire belt (18) set forth in any of the preceding claims positioned between the tread (12) and the carcass (14).