DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a novel tire cord fabric or strip comprising alternating parallel polyester cords having different twists. Such a novel tire cord fabrics or strips improve high speed durability, tread separation resistance and impact resistance when used as zero degree spirally wound cap ply on belt package in pneumatic radial tires.

BACKGROUND OF THE INVENTION

Under high speed conditions, the outer diameter of the tire increases due to the centrifugal forces generated by steel cord belt package and tread. Such a diameter increase or tire growth increases the pantographic movements of the belt edge cords leading to the crack initiations, crack propagations and at the end belt edge separations.

On the other hand, the temperature rise at belt edges under high speed conditions might cause local adhesion degradation of the textile cap ply strip and leads to local cap ply- tread separations which may subsequently result in belt edge separations in a short time due to non-unifom stress distributions. The cap ply layer wound on belt package circumferentially prevents excessive tire growth under high speed conditions by applying compressive forces (restraining force) on heavy belt package made of cross-ply steel cord layers. In order to enhance the restraining force, high cord count (epdm) cap ply strips are usually used (e.g. PET 1100x2, l lOepdm etc.). Currently, most widely used cap ply materials are nylon 6,6 and aramid/nylon hybrid cords which are spirally wound on belt package at 0 to 5 degrees to equatorial plane of the tire.

PET (polyethylene therephthalate) cords have higher modulus and higher in-tire cord tensions compared to nylon 6.6 cords as cap ply reinforcements. Due to less active sites (functional groups) on fiber surface compared to nylon 6,6; PET is more sensitive to thermal degradation of the adhesion with rubber at high temparatures. That's why, the tread-cap ply separation resistance of PET is less than nylon 6.6.

It is well known the hybrid cords comprising high and low modulus yarns having bi-elastic tensile behaviour are also used as cap ply in high speed tires. The low modulus component of hybrid cord enables easy belt package lifting without excessive tight cord formation due to its high extensibility and the high modulus component becomes effective in service conditions. By using hybrid cords as cap ply, the total thickness of the cap ply layer and rubber content can be decreased, and the high modulus component of the hybrid cord enhances the restraining force and improves the high speed durability.

US Patent No. 7,584,774 describes a polyethylene terephthalate (PET) cord as belt reinforcing layer (cap ply) spirally wound on belt package in circumferential direction. Said PET cord has an elastic modulus not less than 2.5mN/dtex.% under a load 29.4N at 160°C.

The major drawback with conventional PET cap plies in high speed radial tires, is the necessity of a robust dynamic adhesion to delay the crack formations between cords (due to shear forces). Under high speed conditions, temperature of belt edge area becomes higher, and modulus of PET cap ply cord goes down. That's why high epdm strips are preferred to use as single layer cap ply. But high epdm cap plies have a higher tendency for crack formations, which can lead to ply separations. SUMMARY OF THE INVENTION

The present invention relates to a novel tire cord fabric or strip made (woven) of alternating polyester cords having different twists. Such a novel tire cord fabrics or strips improve high speed durability, tread separation resistance and impact resistance when used as zero degree spirally wound cap ply in pneumatic radial tires.

Due to inferior dynamic adhesion of polyester (PET), it is preferred to be used as single layer, high epdm and narrow rivet cap ply on belt packages in high speed radial tires (Figure- 1).

On the other hand, the cord-to-cord distance (rivet area) in such fabrics or strips are too narrow which makes the rubber penetration difficult between the cords without scorch. Additionally, the rubber cracks can easily initiate between the cords having narrow rivet under dynamic conditions due to high shear stresses.

According to the invention in the cap ply strips, the PET cords having different twists have different modulus or LASE, and different thermal shrink force values. During lifting (expansion) in the curing process, low twist PET cords (high modulus, low extensible cords) are loaded much higher than that of high twist cords (low modulus, high extensible cords). In addition to higher cord tensions, low twist nylon cords under higher loading generate higher thermal shrink force (contraction force) at curing temperature. As a result of higher cord loading and higher thermal shrink force, low twist nylon cords penetrate the skim compound more than high twist cords.

As a result of different penetration levels of low and high twist PET cords, the mono-ply cap strip layer becomes two layer cap strip (Figure-2).

Definitions: Cord: The reinforcement element formed by twisting together two or more plied yarns. Denier: The gram weight of yarn having 9,000 meter length. Dtex: The gram weight of yarn having 10,000 meter length. Equatorial plane: Plane passing through the circumferential center line in tire LASE: Load At Specified Load

7% LASE: Load At 7% Elongation in load elongation curve Linear density: Weight per unit length as g/dtex or g/d (denier) PET: Polyethylene terephthalate

Restraining force: Force applied by cap ply on belt package during driving to prevent tire growth

Total linear density: The sum of the nominal linear densities of the ply yarns of the cord

Two-ply cord: Cord prepared by twisting together two plied yarns Three-ply cord: Cord prepared by twisting together three plied yarns Twist: Number of turns per meter (t/m or tpm)

Warp: The set of yarn or cord in all woven fabrics, that runs lengthwise and parallel to the selvage

and is interwoven with the filling.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure- 1 is a cross-sectional view of conventional (prior art) polyester (PET) cap ply on cross-ply steel cord belt package after curing process. a is cross ply steel cord belt package b is conventional cap ply on belt package c is tread

Figure-2 is a cross-sectional view of polyester (PET) cap ply according to the invention on cross-ply steel cord belt package after curing process. a is cross ply steel cord belt package b is cap ply on belt package according to invention c is tread

Figure-3a is a cross- sectional view of nylon cap ply according to invention on cross-ply steel cord belt package, (1) before curing process, (2) after curing process

A: First Polyester (PET) cord with higher twist or higher twist multiplier having lower modulus compared to B

B: Second Polyester (PET) cord with lower twist or lower twist multiplier having higher modulus compared to A

H: Cord penetration difference in rubber matrix

Figure-3b is a top view of PET cap ply strip made of A (higher twist, lower modulus, lower tension) and B (lower twist, higher modulus, higher tension) cords

Figure-4 is a cross-sectional view of PET cap ply with side-by-side paired cords according to invention on cross-ply steel cord belt package, (1) before curing process, (2) after curing process

Figure-4b is a top view of side-by-side (paired) A (higher twist, lower modulus, lower tension) and B (lower twist, higher modulus, higher tension) cords in cap ply strip

Figure-5 shows the cord-to-cord distance changes after curing process (1) before curing process, (2) after curing process SI: Cord-to-cord distance between A and B before process expansion and curing.

S2: Cord-to-cord distance between A and B after process expansion and curing

S3: Cord-to-cord distance between A and A after process expansion and curing

S4: Cord-to-cord distance between B and B after process expansion and curing

Figure-6 shows 1+2+1 cord arrangements. (1)- A+B+B+A+B+B+A+... cord arrangement before process expansion and curing

(2)- A+B+B+A+B+B+A+... cord arrangement after process expansion and curing

Figure-7 shows 2+1+2 cord arrangements. (1)- A+A+B+A+A+B+A+A+... cord arrangement before process expansion and curing

(2)- A+A+B+A+A+B+A+A+... cord arrangement after process expansion and curing

DETAILED DESCRIPTION OF THE INVENTION

According to invention, the spirally wound cap ply fabrics or cap ply strips on belt package of the pneumatic radial tires in circumferential direction, having alternate polyester (PET) warp cords with different twist levels;

-improves the high speed durability due to high modulus of low twist PET cords, and increased cord-to-cord distances(less shear stresses) between cords(cord shiftings in vertical direction, two layer or zig-zag formation), (Figures-3a, 3b and 5). -improves the impact resistance of belt package due to higher energy absorption of high twist, and high extensible PET cords

-and also improves the tread-cap ply separation resistance due to the wavy surface structure of the cap ply. The zig-zag surface enhances also mechanical bonding between cap ply and tread compound. The first PET cord (A) and second PET cord (B) in cap ply strip are two and/or three-ply cords.

The angle of spirally wound cap ply strips to circumferential center line (or equatorial plane of the tire) is 0 to 5°.

In order to obtain the advantages mentioned above, the twist factor of high twist PET cord (A) determined according to the formula (1) given below is at least 13,000 and less than 17,000.

Twist factor = Cord twist (tpm) x (total linear density of cord as dtex)½ (1)

According to invention, the twist factor of the second (lower twist) PET cord (B) is at least 15% and preferably 25% less than the twist factor of the first (high twist) PET cord (A).

If the twist factor of the second (lower twist) PET cord (B) is less than 15% of the twist factor of the first (higher twist) PET cord (A), the LASE , modulus or extensibility difference between them becomes insignificant. Under those conditions, the wavy cap ply surface can not be created in tire. The total linear densities of the cords are minimum l,000dtex and maximum 5,000dtex. The cap ply cords having less than l,000dtex are too thin and LASE values are too low which can not provide enough restraining force even with very high cord counts (epdm). Besides this drawback, during process lifting and curing, they can cut the skim compound of the belt layer and contact to steel cords. The cap ply cords having higher than 5,000dtex are too thick and requires too much rubber for coating. The potential drawback for such cords in tire is increased rolling resistance and heat build-up in crown area.

The total linear density difference between the first PET cord (A) and the second PET cord (B) is less than 15%. Preferably, the total linear densities of the first PET cord (A) and the second PET cord should be the same. According to the invention, the sequence of the first PET cord (A) and the second PET cord (B) is parallel to each other in a cap ply strip is in an alternating form as A+B+A+B+A+B+... and so on, wherein A is the first cord with higher twist, and B is the second cord with lower twist compared to A. Such a cap ply strip creates a uniform wavy surface after curing process, which bonds to tread compound strongly improving tread separation resistance under high speed conditions (Figure- 3 a)

According to the invention, the sequence of the first PET cord (A) and the second PET cord (B) is parallel to each other in a cap ply strip is in an alternating form as AB+AB+AB+... and so on, as paired cords, wherein A is the first cord with higher twist, and B is the second cord with lower twist compared to A (Figure-4a).

According to the invention, the sequence of the first PET cord (A) and the second PET cord (B) is parallel to each other in a cap ply strip is in an alternating form as A+B+B+A+B+B+A +B+B+... and so on, ,wherein A is the first cord with higher twist, and B is the second cord with lower twist compared to A (Figure-6). According to the invention, the sequence of the first PET cord (A) and the second PET cord (B) is parallel to each other in a cap ply strip is in an alternating form as follows A+A+B+A+A+B+A +A+B+... and so on, wherein A is the first cord with higher twist, and B is the second cord with lower twist compared to A (Figure-7).

According to the invention, the cord count in strip is minimum 70 epdm (ends per decimeter). In case of cord counts lower than 70epdm, the effectiveness of surface waviness is not enough for mechanical bonding to tread. According to the invention, the width of the cap ply strips is 8 to 25mm, preferably 10 to 15 mm.