WO2018124985A1

WO2018124985A1 - Polyester cap ply cord

Info

Publication number: WO2018124985A1
Application number: PCT/TR2016/050552
Authority: WO
Inventors: Mehmet Sadettin Fidan
Original assignee: Kordsa Teknik Tekstil Anonim Sirketi
Priority date: 2016-12-28
Filing date: 2016-12-28
Publication date: 2018-07-05
Also published as: EP3512719A1

Abstract

The present invention relates to a novel polyester tire cord which has different tensile and thermal properties at top and bottom side of the the cord. Such a novel bi-elastic polyester tire cord improves processability, high speed durability, eliminates flatspotting and enables high liftings during tire building and curing processes when used as spirally wound zero degree cap strip in pneumatic radial tires.

Description

POLYESTER CAP PLY CORD

FIELD OF THE INVENTION

The present invention relates to a novel polyester tire cord which has different tensile and thermal properties at top and bottom side of the cord. Such a novel bi- elastic polyester tire cord improves processability, high speed durability, eliminates flatspotting and enables high liftings during tire building and curing processes when used as spirally wound zero degree cap strip in pneumatic radial tires.

BACKGROUND OF THE INVENTION

Under high speed conditions, the outer diameter of the tire increases due to 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.

The cap ply layer which is circumferentially wound on belt package 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.

Currently, most widely used cap ply materials are nylon 6.6 and hybrid cords which are spirally wound on belt package at 0 to 5 degrees to equatorial plane of the tire. In addition to nylon 6.6 and aramid/nylon 6.6 hybrid cords, there are also some polyester (polyethylene terephthalate, PET) applications as cap ply. But due to high initial modulus of PET, there is a risk of tight cord formation after lifting which means contact with steel cords of the top belt layer leading to cord breaks under dynamic conditions.

Nylon cords have excellent fatigue resistance under bending and axial compression, and bi-elastic tensile characteristic enabling easy processing during tire building. Additionally, shrink force generation with increasing service temperature under high speed conditions improves belt edge separation resistance and high speed durability. But low glass transition temperature of nylon causes flatspot problems in tire during parking after high speed driving. Other potential drawback of multilayer nylon cap ply assembly is its high rubber gauge which cause to increase the rolling resistance of the tire by hysteresis (heat build up). As mentioned above, 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 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 reduced, and the high modulus component of the hybrid cord enhances the restraining force and improves the high speed durability. But existence of nylon causes also some flatspot. As cap ply, hybrid cord containing aramid is an expensive solution.

On the other hand, cap ply cords made of ultra high modulus yarns having high twist have been also used as tire reinforcement to eliminate flatspot and improve high speed performance, but high level of cord twist results in drastic modulus and breaking strength (tenacity) losses.

US Patent No. 6,799,618 describes a textile cap ply structure which is superimposed radially outwardly to the belt assembly is reinforced with cords being made of materials including nylon and aramid. At initial elongation of the cords, the primary load bearing yarns are nylon yarns, and after appreciable elongation the primary load bearing yarns are the aramid yarns.

US Patent No. 7,584,774 describes a belt reinforcing layer (cap ply) disposed outside the belt in the radial direction, characterized in that the belt reinforcing layer is formed by continuously and spirally winding a polyethylene terephthalate (PET) cord in a circumferential direction of the tire, and this cord has an elastic modulus of not less than 2.5mN/dtex.% under a load of 29.4N measured at 160°C. Due to high initial modulus of the PET cord, lifting must be less than 2% in this applications to prevent excessive tight cord formations.

US Patent No. 2013/0025758 describes a high performance pneumatic radial tire for passenger cars which uses a hybrid cord composed of two ply aramid yarn and single ply nylon yarn having different twist counts for the first twist and different twist counts for the second twist, as a cap ply, and thereby has improved high speed durability and steering stability.

SUMMARY OF THE INVENTION

The invention provides a two or more ply polyester cord reinforcement having different tensile and thermal properties at top and bottom side of the cord. Such cords show bi-elastic tensile properties, because each ply within this cord comprises alternately high and low modulus parts. The production principle of the transformation of more or less linear tensile behaviour of the polyester cord to bi-elastic characteristic (initial modulus reduction and enhancing initial extensibility with low forces) is based on the thermal relaxation of one side of the pre-stretched polyester cord along its axis. That means, the polyester cord according to the invention comprises high modulus top side and low modulus bottom side (or vice versa) along its axis. Each of the single plies of such a polyester cord contains intermittent zones (parts) having high and low modulus with bi-elastic tensile behaviour.

Definitions:

Cord: The product formed by twisting together two or more plied yarns

Cord ply: Plied single yarns within cord.

Dtex: The gramm weight of yarn having lO.OOOmeter length.

Flatspotting: Cords in tire having low Tg and high thermal shrikforce is subjected to shrinkage at footprint. When cooled in this position, the cord maintains flatspot until it again reaches its Tg in use.

Greige cord: Twisted cord before dipping and heat- setting

Heat-setting: The process of conferring dimensional stability and heat resistance to the

yarns, cords or fabrics by means of either moist or heat.

Heat-set cord: Cord exposed to high temperature(e.g. 120°C to 260°C under tension)

Linear density: Weight per unit length as g/dtex or g/d(denier)

TASE at 3% elongation: Stress at 3% elongation as cN/dtex

Tenacity: Breaking force(N) divided by linear density(dtex)

Tg: Glass transition point of polymer

Thermal relaxation: Thermal shrinkage causing modulus reduction

Total nominal cord dtex: Sum of nominal yarn linear densities (3340 dtex for 1670x2 cord)

Twist: Number of turns about its axis per meter of a yarn or cord (t/m or tpm) BRIEF DESCRIPTION OF THE DRAWINGS

Figure- 1 is a lateral view of conventional (prior art) two ply polyester cords and its individual plies in which,

1- Lateral view of the conventional two-ply polyester cord

2- First ply of polyester cord in separated(isolated) form

3- Second ply of polyester cord in separated(isolated) form

Figure-2 is a lateral view of a two-ply polyester cord under tension during heat- setting process (prior art process),

4- Cord tension(F) applied to polyester cord during heat-setting process 5- Oven temperature (Tl) to which polyester cord exposed uniformly from all directions during heat-setting process.

Figure-3 is a lateral view of a pre-stretched two-ply polyester cord under tension during heat- setting process according to invention

4- Cord tension (F) applied to polyester cord during heat-setting process 5-Oven temperature (T2) to which the polyester cord exposed at the bottom side

6-Oven temperature (Tl) to which the polyester cord exposed at the top side

Figure-4 is a lateral view of partially relaxed two-ply polyester cord

2a- The top part of the first ply of the polyester cord exposed to the temperature Tl during

heat- setting process

3a- The top part of the second ply of the polyester cord exposed to the temperature Tl

during heat-setting process

2b-The bottom part of the first ply of the polyester cord exposed to the temperature T2

during heat-setting process. 3b-The bottom part of the second ply of the polyester cord exposed to the temperature

T2 during heat-setting process

Figure-5 The separated plies of the polyester cord heat-set according to the invention

2a-The top part of the first ply of the polyester cord exposed to the temperature

Tl during

heat-setting process having high modulus (3% TASE)

during heat-setting process having high modulus (3% TASE)

during heat-setting process having lower modulus (3% TASE), relaxed part of the ply

with higher extensibility

3b-The bottom part of the second ply of the polyester cord exposed to the temperature

T2 during heat-setting process having lower modulus (3% TASE), relaxed part of the

ply with higher extensibility.

DETAILED DESCRIPTION OF THE INVENTION

PET with its high modulus and low thermal shrinkage is a dimensionally stable material. In twisted form as two or three-ply cord, it can be used as carcass and cap ply reinforcement in radial passenger and light truck tires.

The cord twisting improves bending and compression fatigue resistance of the PET, but at the same time reduces the modulus and strength too. The current tire process technology for radial passenger, SUV and light truck tires requires bi-elastic cap ply for zero degree belt reinforcement applications.

In zero degree cap ply applications in pneumatic radial tires, the high modulus is needed for restraining force to prevent belt edge separations under high speed conditions, but initial extensibility with low forces (initial low modulus) is also needed for processability during lifting of belt package in tire building and curing processes to avoid cord cuttings through the belt skim compound.

Nylon and aramid/nylon hybrid cords fulfill bi-elasticity requirement which is needed due to high lifting ratios during tire building process.

As well-known aramid/nylon hybrid cords are expensive and their transition points from low modulus to high modulus should conform with lifting process parameters. Otherwise the hybrid cords may cause tight or loose cords in cap ply.

Two or three-ply nylon 6.6 cords are well known reinforcements as cap ply in radial passenger car and light truck tires, but flatspot and low level of modulus are their major drawbacks.

The conventional polyester cords are uniformly heat-set in high temperature ovens having constant temperature along the oven (Tl) under a given tension (F) (Figure 1 and 2). Such cords have uniform and constant physical properties in radial and axial directions.

According to the present invention, two or more ply polyester cords without any low modulus component ply like nylon, can be produced with bi-elastic tensile properties. Such a novel bi-elastic polyester cords can be used as zero degree cap ply in radial tires to improve high speed durability and do not show any significant flatspotting.

According to invention, the basic production principle of the bi-elastic polyester cord is to relax one side (e.g. bottom side) of the cord at high temperature while maintaining the original high modulus at another side (e.g. top side). The relaxed parts of the polyester cord become more extensible under a given force, and the rest of the cord maintaining the original high modulus keeps its low extensibility (Figure- 3)

According to invention, , a polyester cord having relaxed side (e.g. bottom side) means, the individual cord plies comprise high and low modulus parts (zones) as repeated units alternately along its length axis. Such a cord elongates initially more than the original non-relaxed high modulus cord upon subjecting to a load. That means it has bi-elastic tensile behaviour. In order to obtain a bielastic tensile characteristics, the already hot-stretched (pre- hot stretched) polyester cord is exposed to high temperature from the bottom side under a tension which is lower than the thermal shrink force at that temperature which leads to partial relaxation at the bottom side of the polyester cord. In another words, each of the cords plies are relaxed (lowered modulus) at the bottom side, while maintaining non-relaxed state (high modulus) at the top side. The polyester cord comprising plies with high and low modulus parts shows bi-elastic tensile behaviour which is required for cap ply applications (Figure-4 and 5)

The tensile bielasticity characteristic of the polyester cord can be changed with the degree of relaxation.

The 3% TASE value is measured in accordance with ASTM D885-16 and this value for the polyester cord according to the invention is less than 1.5cN/dtex.

According to invention, the twist factor of the cord is higher than 10,000 and less than 20,000 which is determined based on the following formula;

Twist factor = cord twist (tpm)x square root of total nominal cord dtex (1) The cords with lower than 10,000 twist factor have insufficient fatigue resistance under bending and the cords with higher than 20,000 twist factor have significant modulus reductions.

According to invention, the total nominal cord linear density is higher than 3000dtex and less than 8,000 dtex.

The cords having less than 3000 dtex can not be partially relaxed and the cords having higher than 8,000 dtex are too thick.

The preferable polyester polymer types are polyethylene terephthalate and polyethylene naphthlate.

Claims

1- At least two ply, dipped and heat- set polyester cap ply cord which is spirally wound on the top belt layer of a radial pneumatic tire is characterized in that,

- said polyester cap ply cord has 3% TASE value less than 1.5cN/dtex,

- each of the said polyester cap ply cord comprises alternately repeated parts having different 3% TASE values

- and the ratio of lower 3% TASE/higher 3%TASE is less than 0.70 and higher than 0.30.

2- A polyester cap ply cord according to claim 1 which is characterized in that, twist factor of the said polyester cap ply cord is higher than 10,000 and lower than 20,000 which is calculated according to the following formula (1);

Twist factor = cord twist (tpm)x square root of total nominal cord dtex.

3- A polyester cap ply cord according to claim 1 which is characterized in that, total nominal linear density of the said polyester cap ply cord is higher than 3000dtex and lower than 8,000dtex.

4- A polyester cap ply cord according to claim 1 which is characterized in that the said polyester is polyethylene terephthalate.

5- A polyester cap ply cord according to claim 1 which is characterized in that the said polyester is polyethylene naphthalate.