High strength polyester filamentary yarn

Abstract

A polyester multi-filamentary yarn having a novel internal microstructure, polyester tire cord produced from the yarn and a process for production thereof are provided. The polyester yarn has a novel three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions which are changed to a structure having crystalline and amorphous portions during formation of a tire cord. The polyester resin contains at least 90 mol. % polyethylene terephthalate. The resin is melt-spun and solidified by quenching to produce an undrawn yarn having a birefringence of 0.03 to 0.08, which is then drawn at a total draw ratio of 1.4:1 to 2.2:1 and thermally treated and relaxed. The resulting filamentary yarn is dipped in a rubber solution to produce a tire cord which exhibits excellent dimensional stability and fatigue resistance when it is incorporated into the rubber matrix of a tire.

Description

This application is a Continuation of application Ser. No. 07/989,366, filed on Dec. 11, 1992, abandoned.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an industrial polyester filamentary yarn and a tire cord formed from this filamentary yarn. More particularly, it relates to a polyester filamentary yarn, a tire cord having improved fatigue resistance due to increased thermal dimensional stability and strength, as well as a process for production of this yarn.

(2) Description of the Prior Art

In general, nylon, rayon, polyester etc. fibers are typical examples of the fibers which have been used as reinforcements in rubber tires.

Nylon tire cord has higher strength and toughness than the other materials due to the inherent properties of nylon fiber and has been generally used in bias tires for trucks, buses etc. Rayon tire cord provides a low degree of shrinkage and has good thermal and dimensional stability due to the inherent properties of rayon fiber and has been generally used in high speed radial travelling ires for passenger cars.

Unfortunately, nylon tire cord has poor dimensional stability due to its low modulus characteristics and high shrinkage and further exhibits flat spots due to its low glass transition temperature. Rayon tire cord also has low modulus characteristics and exhibits a sharp decrease in strength after the fibers have been formed into a tire cord.

In view of these defects found in both nylon and rayon tire cords, polyester tire cord has been widely used.

Prior art polyester fibers that have been used in tires have benzene rings in their molecular structure, and a rigid molecular chain. Accordingly, tire cord formed from polyester yarn has a good elastic modulus, good fatigue resistance, provides few flat spots, excellent creep resistance and excellent endurance. For these reasons, polyester tire cord has been widely used in radial tires for passenger cars.

However, in spite of the above described merits, conventional polyester tire cords do have a problem: they undergo substantial variation in their properties with temperature due, it is thought, to hysteresis effect. In particular conventional industrial high strength polyester fibers generally exhibit substantial shrinkage when heated.

Also, when industrial polyester fibers have been incorporated into a rubber matrix of a tire, as the tires rotate during use the fiber is stretched and relaxed during each tire rotation. Further, the internal tire air pressure stresses the fiber, and tire rotation while axially loaded or stressed causes repeated stress variations, particularly on unsmooth surfaces.

Since more energy is consumed during the stretching of a fiber than is recovered during its relaxation, the difference of energy dissipates as heat. This is termed hysteresis or work loss. Significant temperature increases have been observed in rotating tires during use which are attributable at least in part to this fiber hysteresis effect.

The variation in properties caused by heat generation occurs due to moisture and amines contained in conventional rubber solutions used in rubber treatments for producing tire cord, and the observed variation tends to be increased when the content of carboxyl group is high, leading to a significant lowering of strength and fatigue resistance.

In recent years, as radial tires having high performance have been widely developed and used, the demand for polyester tire cords with superior properties, especially properties superior to those obtained with nylon or rayon tire cord, has been increasing. Therefore, research into development of a polyester tire cord having improved fatigue resistance by minimizing the heat generated due to the hysteresis effect has been undertaken.

Prior art methods for improving fatigue resistance of polyester fibers have focused on a chemical method for increasing stability by reducing the content of carboxyl groups in the polyester and a method wherein highly-oriented undrawn yarn produced using a polyester with a relatively low I.V. (intrinsic viscosity), or produced by employing a high-speed spinning process, is drawn.

References directed towards increased chemical stability are Japanese patent laid-open No. Sho. 54-132696 and 54-132697 which disclose the inhibition of deterioration due to thermal decomposition resulting from heat generation by reducing the carboxyl group content of the polyester. By reducing the content of terminal carboxyl groups via copolymerization with or melt-blending in an aliphatic polyester. The increased mobility of amorphous regions effects a reduction in heat generation leading to a reduction in thermal decomposition which effects improved fatigue resistance. But in this method, high crystalline polyester fiber cannot be obtained and the tenacity and initial elastic modulus of the material is always low. Thus, the shrinkage of the resulting fiber is increased and the product obtained is not a high quality tire cord yarn. Also, reducing the content of terminal group by adding a blocking agent has a disadvantage in that the degree of polymerization is lowered and the cost is increased.

References directed to a method of increasing thermal stability by high speed drawing are U.S. Pat. No. 4,101,525 and 4,195,052 which disclose an improvement in fatigue resistance by increasing the mobility of the molecular chains in the amorphous region high-speed spinning. In this process, the fatigue resistance is improved but the molecular chain length is irregular and long, and the relaxed molecular chains coexist so that the loss of tenacity is high. Also, a difference in properties between the inner and outer layer of fiber is effected so that the drawability deceases. The resultant variation in physical properties within these regions of the fiber is severe due to the presence of a defective microstructure.

Prior art processes for producing tire cord from yarn include, for example, Japanese patent laid-open No. Sho 61-12952 which discloses a tire cord having a tenacity of at least 7.0 g/d, an absorption peak temperature in the amorphous region of 148°-154°C, a shrinkage of 3.3-5% by spinning a polyester polymer having an intrinsic viscosity of 1.0, a diethylene glycol content of 1.0 mol %, a carboxyl group content of 10 eq/10⁶ g at a spinning speed of 2,000-2,500 m/min to obtain undrawn yarn, drawing the undrawn yarn at about 160°C thermally treating at 210°-240°C and dipping the obtained yarn in a conventional rubber solution.

In addition, U.S. Pat. No. 4,101,525 and 4,195,052 disclose a polyester tire cord produced by a process comprising drawing highly oriented undrawn yarn prepared from a high-speed spinning process to obtain highly oriented drawn yarn, specifically multi-drawn yarn comprising 85 mol % polyethylene terephthalate having a denier per filament of 1 to 20 and a work loss at 150°C of 0.004-0.02, and dipping the multi-drawn yarn in a rubber solution.

In the above methods, tie molecules, which have an important effect upon dimensional stability (especially shrinkage) are oriented. This leads to residual internal stress and finally causes a lowering of the fatigue resistance of the tire cord. In most of the conventional polyester yarns for tire cord, internal stress produces a temperature rise which induces a continuous increase of thermal stress. This finally results in poor tire cord fatigue resistance because after the tire cord conversion process (or dipping process) comprising dipping the cord in a rubber solution and thermally treating, an internal stress of about 0.5 g/d usually remains in the tire cord.

Moreover, yarns which are highly oriented drawn yarns before undergoing the tire cord conversion process have a definite two-phase structure with both crystalline and amorphous regions. When it is dipped in a rubber solution and thermally treated, deterioration of the crystalline regions occurs and leads to a lowering of strength.

In addition, Japanese patent laid-open No. Sho. 61-146876 discloses a process for producing a polyester tire cord by spinning yarn with a small mass flow rate per spinneret capillary to attain a relatively high Spin-Draw Ratio, thereby obtaining a highly oriented undrawn yarn at a relatively low spinning rate, producing a high strength yarn followed by dipping it in a rubber solution and thermally treating it at a temperature higher than 220°C This process has a disadvantage in that the beneficial properties of the polyester yarn are lost in the twisting and dipping process due to deterioration of the crystalline portions of the yarn by heat, and the final dipped cord has rather poor properties.

Japanese Patent laid-open No. Sho. 54-77794 discloses a process which comprises treating polyester drawn yarn with an epoxy resin compound prior to dipping in a rubber solution but this process did not solve the above-described problems.

The present invention has been developed to solve the above described problems of the prior art. According to the present invention, the two problems of i) lowering of fatigue resistance due to residual internal stress by high-speed spinning and ii) lowering of strength due to deterioration of the crystalline portions on dipping in a rubber solution can be solved based upon the points described below.

Polyester yarn having a high crystallinity undergoes a high degree of thermal hysteresis and, accordingly, has a high thermal stress. Thus, it tends to undergo a lowering of strength, elastic modulus or strength conversion efficiency due to formation of folded crystals and in particular, from unconstrained molecular chains in the amorphous regions present during recrystallization which subsequent heat treatments, such as dipping process etc., cause. In addition, though a high crystalline polyester yarn itself may exhibit good thermal stability, dimensional stability and fatigue resistance, the yarn has a definite two phase microstructure which may effect a rapid growth of crystal size or long period growth upon subsequent heat treatment so that the fatigue resistance initially exhibited by the yarn itself can not be obtained after it is twisted and subsequently heat-treated.

Conventional tire cords which have been used reinforcers in tire production exhibit a shrinkage of at least 10% when subjected to a high temperature. Moreover, when they have been incorporated into the rubber matrix of a tire, the repeated fatigue movements such as stretching, compression and flexing lower the inherent properties of the fiber such as strength, elastic modulus and toughness. Furthermore, the poor fatigue resistance results in bad tire uniformity.

The present inventors have directed their research towards improving the prior art methods of producing a polyester yarn for tire cord which has excellent overall physical properties like strength, and, at the same time, which has a high strength conversion efficiency and excellent dimensional stability lusting to excellent fatigue resistance when used even after having been subjected to a cord conversion process and then incorporated into a rubber matrix. As a result of this research, the present invention has been achieved. Thus, whereas most of the prior art methods comprise producing a polyester yarn having a stable two-phase structure of crystalline and amorphous portions or regions and then simply dipping it into a rubber solution to obtain a final tire cord, the present invention comprises producing a polyester yarn having a three-phase structure of crystalline, amorphous and mesomorphous portions and thereafter subjecting it to recrystallization during a dipping process to obtain a tire cord having a stable two-phase structure.

As the mesomorphous portions present in the yarn are crystallized while being subjected to heat in a dipping process, crystals with a 10% smaller size than the crystals obtained in prior art methods are produced and the present yarn in cord form provides dimensional stability by developing a network structure with uniformly formed crystalline and amorphous portions, and has a high elastic modulus due to the minimization of the formation of folded crystals during recrystallization thereby increasing the content of the strained tie molecular chains which interlink crystals.

Moreover, the present inventors have discovered particular spinning and drawing processes which achieve the above characteristics. Consequently, the process conditions necessary to produce an excellent polyester filamentary yarn have been designed. In more detail, a undrawn yarn is produced which has highly oriented molecular chains in amorphous state such that crystalline diffraction by x-ray is not observed, thereafter the undrawn yarn is drawn at a low draw ratio and a low temperature (below the crystallization temperature) so as to minimize the strain of molecular chains in amorphous regions induced by drawing, and then subjected to thermal treatment and relaxing at a low temperature so that no further crystallization proceeds. The filamentary yarn is then dipped into a rubber solution and thermally treated at certain temperature and tension conditions enabling recrystallization to occur, thereby obtaining a final polyester tire cord.

OBJECTS OF THE INVENTION

The first object of the present invention is to provide a polyester filamentary yarn which exhibits excellent fatigue resistance and dimensional stability both before and after it has been incorporated in a rubber matrix even under the conditions where it is subjected to repeated fatigue behavior at high temperatures (at least 210°C), and a process for production thereof.

The second object of the present invention is to provide a tire cord comprising a polyester filamentary yarn exhibiting excellent dimensional stability and fatigue resistance useful as a reinforcement in rubber.

The third object of the present invention is to provide a tire exhibiting significantly improved fatigue resistance and dimensional stability even under the conditions of repeated fatigue behavior at high temperatures.

To achieve the above objects, the present invention provides a polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3 to 5 denier per filament, and possessing a three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions, characterized in that the proportion of said mesomorphous portion is 5 to 15 percent base upon the total amount of crystalline, mesomorphous and amorphous portions.

The present invention additionally provides a polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3 to 5 denier per filament, characterized in that said yarn satisfies the following characteristics

i) a crystalline orientation function (fc) of at most 0.94,

ii) an amorphous orientation function (fa) of at least 0.60,

iii) fa X (1-Xc)>0,330 (where, Xc is the percent crystallinity, 0.30-0.45); and

iv) a long period value of at most 155 Å.

The present invention additionally provides a polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3 to 5 denier per filament, characterized in that said yarn satisfies the following characteristics:

i) a crystallinity of 30-45 percent by weight;

ii) a crystallite face size [(105) plane] of at most 65 Å; and

iii) a crystal volume of 0.5×10⁵ ų -1.54×10⁵ ∈³.

The present invention additionally provides a polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3 to 5 denier per filament, characterized in that said yarn has a maximum thermal stress of at most 0.5 g/d in the temperature range of 60° to 250°C and exhibits a decrease of thermal stress beyond 210°C

Also, the present invention provides a process for producing a polyester filamentary yarn from a polyester resin comprising at least 90 mol % polyethylene terephthalate and having an intrinsic viscosity of at least 0.85 by melt-spinning, drawing, thermally treating and relaxing, characterized in that:

1) said polyester resin is spun at a spinning speed of 2,500-4,000 m/min and then solidified by quenching at a quench air temperature of 25° C.-Tg of the polymer to produce a undrawn yarn;

2) said undrawn yarn is drawn at a drawing temperature of Tg of the polymer-120°C and a total draw ratio of 1.4:1-2.2.1;

3) the obtained drawn yarn is thermally treated at a temperature of 100°-210°C; and

4) the thermally treated yarn is relaxed at a temperature of at most 140°C and a relax ratio of 3 to 6%.

In addition, the present invention provides a tire cord formed from a polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate, characterized in that said cord satisfies the following characteristics:

i) a strength at 10% elongation of at least 100 Newtons,

ii) a shrinkage, S, of at most 3.5% obtained upon dry heat treatment at 177°C during 2 minutes under a dead weight loading of 20 g,

iii) a strength at 10% elongation after the treatment as described in the ii) above, L, of at least 65 Newtons, and

iv) a coefficient of dimensional stability, L/S, of at least 20.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diffractogram obtained from the X-ray diffraction patterns for a polyester filamentary yarn of the present invention.

FIG. 2 illustrates a diffractogram obtained from the X-ray diffraction patterns for a polyester filamentary yarn of the prior art.

FIG. 3 illustrates a graph plotting the thermal stress values of polyester filamentary yarns vs. temperature.

FIG. 4 illustrates a graph plotting the thermal stress values of polyester tire cords vs. temperature.

=(b² -B²)^1/2 β=(b² -B²)^1/2

(b: the width at half-height of peak, B: instrument constant)

The crystal volume may be calculated from the following formula

Crystal volume=(crystal size of a axis)×(crystal size of b axis)×(long period)×(crystallinity)

Furthermore, the present filamentary yarn is characterized by a terminal modulus of at most 20 g/d.

In general, it is known that the higher the initial modulus or terminal modulus, the more severe the lowering of strength upon twisting and dipping process. However, the present inventors have found that such lowering of strength is more affected by terminal modulus than by initial modulus. But in the case where significant crystallization has progressed in the yarn, the lowering of strength is severe though the terminal modulus is low. Thus, by a high relaxing ratio or strong heat treatment the terminal modulus may be lowered even to a minus(-) value, but in this case the lowering of strength upon twisting and dipping cannot be avoided owing to a high crystallinity.

Another characteristic of the present yarn is thermal stress behaviour as if depends on applied temperature.

A yarn used for reinforcing a rubber is required to have excellent mechanical properties and thus strong tension in the face of drawing and the application of high heat yields accumulated stress.

To isolate most of the stress, a relaxing process has been used in the production of yarn. But the inventors have found as a result of their research that there is a limit in isolating the stress when employing a relaxing process. This is because the accumulated stress is mostly the stress caused by heat generated from drawing, thermal treatment etc.

Therefore, in a conventional yarn producing process the stress isolation inevitably has been limited to an extent.

In addition, another factor to limit the stress isolation present in the methods to isolate stress employing a relaxing process is that even if the degree of orientation is lowered to less than 0.6 as in U.S. Pat. No. 4,101,525 and U.S. Pat. No. 4,195,052, the constraint of the amorphous molecular chains cannot be sufficiently released owing to the folded molecular chains on the crystal surface and a high amount of defects on the crystal interface, and it is not easy to obtain high elastic properties due to the decrease of the proportion of tie molecules.

Therefore, the present yarn is characterized in that the maximum thermal stress is at most 0.5 g/d in the temperature range of 60°C to 250°C

All after-treatment temperatures employed to transform a polyester yarn into a reinforcement for rubbers usually exceed 210°C The conventional polyester yarns for reinforcing a rubber, particulary a tire, normally do not exhibit high thermal stress but do exhibit fairly high thermal stress at high temperatures greater than 210°C On the contrary, the present yarn has a characteristic where the thermal stress drops gradually beyond 210°C and the final cord may have a thermal stress less than 0.1 g/d. Accordingly, heat-generation and dimensional properties can be improved significantly so that a cord with excellent fatigue resistance can be produced.

Consequently, the present yarn itself exhibits a high shrinkage of about 8 to 15% in an oven at 150°C during 30 minutes under zero tension but is a polyester filamentary yarn for reinforcing a rubber with excellent dimensional stability and fatigue resistance owing to the factors such as the above described microstructural characteristics, the compact macrostructure, the network structure of crystals and the significant decrease of thermal stress at high temperatures.

The characteristics of the polyester filamentary yarn will now be described by way of the thermal stress curves in FIG. 3.

FIG. 3 illustrates the thermal stress curves obtained from testing the polyester filamentary yarns of 1,000 denier. Curve (a) represents the thermal stress behaviour of a polyester yarn for a tire cord according to the prior art (Comparative examples 8) and curve (b) represents that of a prior art polyester yarn with slightly improved dimensional stability (Comparative example 9) and curve (c) represents that of a polyester yarn of the present invention (Example 5). The testing method used was the measurement of the thermal stress at from ambient temperature to 265°C with a heating rate of 2.5°C/sec under an initial load of 50 g by a thermal stress analyzer manufactured by KANEBO Co. Japan. The figure demonstrates that the thermal stress of the present yarn decreases beyond 210°C

A tire cord of the present invention produced from polyester filamentary yarn of the present invention as described above exhibits excellent properties as follows:

i) a strength at 10% elongation (L10) of at least 100 Newton.

ii) a shrinkage, S, of at most 3.5% obtained upon dry heat treatment at 177°C during 2 minutes under a dead weight loading of 20 g;

iii) a strength at 10% elongation after the treatment in the above ii), L, of at least 65 Newton; and

iv) a coefficient of dimensional stability (L/S) of at least 20.

In addition to the above characteristics, the present tire cord has a maximum thermal stress of at most 0.1 g/d as can be seen in FIG. 4.

FIG. 4 illustrates the thermal stress curves obtained from testing the polyester tire cords produced by subjecting the polyester filamentary yarns similar to those used in FIG. 3.

Curve A represents the thermal stress behaviour of a prior art polyester tire cord (Comparative example 17), curve B represents that of a prior art polyester tire cord (Comparative example 15) and curve C represents that of a tire cord according to the present invention (Example 24). The thermal stress was measured at ambient temperature to 300°C with a heating rate of 2.5°C/sec under an initial load of 50 g by KANEBO thermal stress analyzer.

From the figure, it can be seen that the present tire cord exhibits a significantly lower thermal stress compared to the tire cords of the prior art.

The dipped cord of the present invention exhibits excellent thermal stability, leading to a low shrinkage, because the network structure comprising the uniformly distributed crystalline portions develops well in the cord.

The present inventors have achieved a means to increase the dimensional stability with a lowering of the shrinkage by producing a dipped cord having a network structure wherein the crystalline and amorphous portions are uniformly distributed.

It is generally known that a tire cord having a network structure as described above generates a lot of heat, when it has been incorporated in a tire and subjected to deformation power such as elongation and compression, because a high activation energy is required for the molecular chains existing in the amorphous portions to move, and, as a result, the interior temperature of tire will be increased and accordingly the tire cord will have poor fatigue resistance and a short lifetime. But in practice the opposite phenomenon is observed.

From "Effects of Structure on the Tensile, Creep and Fatigue Properties of Polyester Fibres" by A. R. Bunsell, appearing in Journal of Material Science, 1987, Vol. 22, p 4292-4298, it is known that fibers having a network structure wherein the crystals with a small size are uniformly distributed exhibits good fatigue resistance. The present inventors also have verified by experiments that the above described network contributes to excellent fatigue resistance. This is because the fatigue mechanism of tire cord is attributed to the chemical deterioration much more than to physical deterioration.

From "Research for Deterioration of Polyester Tire Cord in Tire" in Journal of Japanese Rubber Associates, 1991, Vol. 64, p260-266, it is known that about 80 percent of the deterioration due to fatigue is caused by the hydrolysis and aminolysis of ester bonds in the polyester molecular chains and the remainder is caused by physical deformation.

If a tire cord in tire has a structure wherein a network develops well, movement of the amorphous molecules by exterior elongation, compression and flexing deformation is difficult so that a high amount of heat is generated to increase the temperature and thus to increase the physical fatigue, but it is very small. On the contrary, the present yarn has a high degree of orientation in the amorphous portions which makes the penetration of water and amines difficult, thereby decreasing the chemical deterioration to obtain excellent fatigue resistance.

Now, the process of the present invention will be described in detail.

The polyester used as the starting material may be a polyester with a high polymerization degree such that its intrinsic viscosity is at least 0.85.

The intrinsic viscosity (η) may be calculated from the following equation by determining the relative viscosity (ηr) of a solution of 8 g of sample in 100 ml of ortho-chlorophenol at 25°C using an Ostwald viscometer.

η=0.0242 ηr+0.634

where ##EQU7## where t: dropping time of solution (in seconds)

to: dropping time of orthochlorophenol (in seconds)

d: density of solution (in g/cc)

do: density of orthochlorophenol (in g/cc)

The degree of polymerization is very important with respect to ultimate dimensional stability and fatigue resistance. In particular, a polymer with a low molecular weight may be used advantageously for dimensional stability but a high molecular weight is preferred for fatigue resistance. In the present invention, optimization of the whole of the properties and a lowering of fatigue resistance can be achieved by selecting a polymer with an intrinsic viscosity of at last 0.85, preferably at least 1∅

A high-speed spinning process is performed to obtain a highly oriented undrawn yarn with a birefringence of 0.03-0.08, preferably 0.05-0.08. It is important to produce the undrawn yarn exhibiting unique shrinkage behaviour in the prior step before producing the present yarn having a three-phase structure.

When the birefringence is less than 0.03, an excessive drawing is required in the drawing process to get a sufficient strength and modulus of elasticity for rubber-reinforcing fiber.

Accordingly, the degree of orientation is rapidly increased so that the excessive drawing tension yields high residual stress leading to a high shrinkage of the fiber. On the contrary, when it is more than 0.08, the undrawn yarn is already in the state wherein the crystalline and amorphous portions exist in the mixed fashion so that the elongation phenomenon, which occurs when the oriented amorphous portions are crystallized, does not occur and the lowering of strength by high-speed spinning cannot be avoided and thus the final dipped cord has poor properties, especially a very low strength.

The birefringence of the undrawn yarn is proportional to the magnitude of tension which the extruded yarn is subjected to upon reaching the glass transition temperature by cooling with quench air. The magnitude of a tension depends upon the spinning speed, the discharge quantity per opening and the temperature of the quench air. In general, the orientation of the undrawn yarn is effected on the point that the extruded yarn from the spinneret reaches a temperature below the glass transition temperature by cooling with quench air. In the present invention, the birefringence is controlled to be 0.03-0.08 with a high tension in the solidification point by heightening the spinning speed to increase the speed of tensile deformation of the extruded yarn or by fixing the spinning speed and increasing the temperature of quench air or reducing the discharge quantity per opening. At this time, to heighten the tension in the solidification point, it is advantageous that the bundle of filaments is slowly cooled so that the solidification point moves as far as possible from the spinneret.

In the case that a high spinning speed is employed (2500-4000 m/min, preferably 3,000-3,600 m/min) to produce a highly oriented undrawn yarn as in the present process, the quench rate becomes different between the inner and outer layer of the polymer stream and thus the spinning stress also becomes different, leading to production of a filamentary yarn having a skin-core structure wherein the inner and outer layer of the filament have different structures from each other. To prevent formation of such a yarn, in the high-speed spinning process it is important to melt-extrude the polymer through an orifice with a ratio of length to diameter (L/D) of 2-4 to form a multifilamentary yarn.

When L/D is less than 2, the temperature difference becomes severe between the inner and outer layer of the filament, which results in formation of the skin-core structure, leading to poor properties. On the contrary, when L/D exceeds 4, the pack pressure rapidly increases so that the operation and handling become difficult and the pack period is to be reduced.

The lowering of strength owing to the temperature difference between the inner and outer layer of the filament can be deceased by increasing the quench temperature to a range of from 25°C to the glass transition temperature of the polymer, desirably from 40°C to 60°C in order to decease the temperature difference between the inner and outer layer of the filament at the solidification point in a high-speed spinning process. When the temperature is less than 25° C., the filament may be too quickly quenched and thus the tension at the solidification point may be decreased so that it may be difficult to obtain a highly oriented undrawn yarn. On the contrary, when exceeding Tg, the filaments may be insufficiently quenched, so it may be impossible to process further.

Varying the discharge quantity per opening may have a great influence upon the mechanical properties of the yarn. It is advantageous to maintain the fineness of the yarn after drawing within 3 to 5 denier by controlling the spinning conditions and preventing a ununiform quench.

The present process is characterized by drawing at a low draw ratio and at a temperature below the crystallization temperature of the undrawn yarn.

Multi-step drawing of two or more step is preferably used in the present process. The crystallization temperature of a highly oriented undrawn yarn produced by a high-speed spinning process is usually lower by more than 10°C than that of an undrawn yarn by a low-speed spinning process.

Thus, the drawing temperature is controlled to be in the range of from the glass transition temperature to 120°C, desirably 80° to 90°C When the drawing temperature exceeds 120°C, fine crystals are already formed before the orientation of the molecular chains and accordingly the drawability is degraded. At temperatures below 80°C, the molecular chains lose their mobility, whereby the efficiency of drawing is low.

The total draw ratio is controlled to be in the range of about 1.4:1 to 2.2:1, desirably 1.4:1 to 1.8:1 and 1.5:1 to 1.7:1. When this ratio is less than 1.4, the fiber attains insufficient strength and in the case exceeding 2.2, high modulus values and low shrinkage cannot be achieved and the percentage of lowering of strength may be high.

The reason why multi-step drawing comprising two or more steps is preferably used in the present process is as follows: if the drawing is performed in one step by drawing to achieve about 70 percent of total draw ratio in the first drawing zone, the period of time taken is not enough for the tangled molecular chains to attain a fibrillar structure so that part of the molecular chains remain in the tangled state. This causes a defect of structure and, accordingly, shrinkage by heat may be increased.

According to the present invention, the shrinkage of the dipped cord can be greatly decreased by use of a highly oriented undrawn yarn produced by the high-speed spinning process such that it is transformed into a liquid-like form rather than undergoes shrinkage when it is subjected to heat after drawing it under specific conditions.

In "Fundamental Aspects of Stress, Deformation and Phase Transitions in Crystallizable Polymers: Experiments with Poly (ethylene terephthalate) in Uniaxial Stress Fields" by P. Desai and A. S. Abhiraman in Journal of Polymer Science: Part B, Vol. 26, 1988, p 1657-1675, it is reported that as a result of experiments wherein the initially oriented amorphous polymer was maintained at a temperature between the glass transition temperature and melting temperature and then its behaviour under stress was observed, it is proved that shrinkage is originated from twisted molecular chains in the oriented amorphous portion and transformation to liquid-like form by elongation occurs as the degree of orientation increases when a stress higher than the shrinkage power is applied.

Therefore, the elongation and shrinkage behaviour upon application of heat can be considered to be a phenomenon originated from the difference of elongation power due to crystallization of the oriented amorphous molecular chains.

Accordingly, in the present invention, the mechanism of elongation-shrinkage behaviour is put to use so that the shrinkage can be minimized.

The present inventors have found that in order to maximize the elongation behaviour like a liquid, crystallization by heat should not occur during drawing. Accordingly, the drawing should be carried out at a low draw ratio and at a temperature below the crystallization temperature of the undrawn yarn. That is to say, when crystallization by heat already has occurred in the drawing process, because the oriented amorphous portion has been transformed into a crystalline portion, the elongation transformation occurring as the oriented amorphous portion is changed to oriented crystals no longer occurs. The shrinkage behaviour only occurs by disorientation of the amorphous molecular chains existing in the amorphous region, which leads to a high shrinkage value.

The present process is characterized in that the thermal treatment of the resulting drawn yarn is carried out at a temperature within the whole range of 100° to 210°C with effective intermediate temperature including 120°, 140°, 160°, 180° and 200°C, for example.

When the temperature exceeds 210°C, the crystalline and amorphous portions may be previously defined in the yarn. Accordingly, the network structure characteristic of the present invention wherein the development of intermicrofibrillar tie molecules cannot be achieved and the orientation of the crystalline portions is greatly increased and the orientation of the amorphous region is decreased. Therefore, the lowering of strength due to abnormal crystal growth in the subsequent dipping process can not be minimized.

The temperature is one of the important factors to determine the structure of the yarn because in this thermal treatment a yarn with nearly completed orientation is treated. The temperature is required to be in the range of 100° to 210°C, desirably 100° to 180°C to produce the present polyester yarn for tire cord.

In general, the undrawn yarn before drawing will gain the characteristic properties of the final yarn as it undergoes a drawing process wherein the crystallization and orientation of molecular chains occur by the heat employed in drawing. The orientation in drawing occurs concurrently in the crystalline and amorphous portions, and drawing tension of the amorphous portion is higher than the crystalline portions. Thus, when a yarn for tire cord having such a microstructure is subjected to twisting or to a dipping process to be formed into a tire cord, the mechanical properties of the yarn are often seriously degraded.

In the present invention, this problem is solved by subjecting, the yarn after the drawing process to a relaxing treatment at a relax ratio of 3 to 6 percent at a temperature below 140°C, which is the temperature at which the amorphous molecular chains initiate mobilization, that is, the loss tangent (tan δ) is at a maximum.

When the relaxing temperature is greater than 140°C, the yarn will initiate the creation of defects in the crystalline structure or destruction of the same upon the application of heat in the subsequent dipping process.

When the relax ratio is less than 3 percent, the wind-up tension may be increased and the removal of residual drawing stress may be insufficient, thereby making process difficult. When the relax ratio is more than 6 percent, the strength efficiency may be deceased and the resulting lowering of shrinkage may be so small that the effect cannot remain in the dipped cord.

The resulting polyester filamentary yarn of the present invention produced by the above process is then subjected to a cord conversion process to produce a tire cord of the present invention.

The cord conversion process is described in detail hereinafter. The present yarn is subjected in sequence to dipping in a rubber solution, drying, thermal treatment and normalizing thereby to produce a tire cord. In the above thermal treatment a tension in the range of 0.2 to 0.6 g/d, a temperature in the range of 220° to 250°C is suitably employed.

If the tension exceeds 0.6 g/d or the temperature exceeds 250°C, a stress much higher than the elongation power originated from the crystallization of the oriented amorphous molecular chains may be applied against the yarn and then it may remain as a residual stress finally in the dipped cord, leading to an increase in shrinkage. When the tension is less than 0.2 g/d, the shrinkage may be decreased but the strength is lowered due to the undesirable growth of the amorphous molecular chains owing to disorientation and folding of the chains. And if the above temperature is less than 220°C, the adhesion of the rubber solution may be insufficient, the shrinkage may increase and, further, it may be impossible to obtain a tire cord with a high crystallinity.

Generally, filaments of in total 1,000 denier, are twisted in more than two strands and formed into a fabric, then this fabric is dipped into a conventional rubber solution and dried. Subsequently, the fabric is thermally treated at the above described temperature and tension, and normalized to obtain a cord fabric, from which the dipped cord of the present invention is obtained.

The thus obtained dipped cord exhibits a strength at 10% elongation of at least 100N measured by the use of an instron type tensile strength tester, and a shrinkage (S) of at most 3.5 percent after thermally treating it in a dry-heat oven at 177°C under a dead weight loading of 20 g during 2 minutes. In addition, the tire cord after measuring the shrinkage (S) exhibits a high value of strength at 10% elongation (L) of at least 65N.

According to the present invention, a tire cord with a coefficient of dimensional stability (L/S) of at least 20 obtained by multiplying the value (L) by a reciprocal of the shrinkage (S) can be produced.

Now, the present invention will be described in more detail by the Examples and Comparative examples which are illustrative only and are not intended to limit the scope of the invention.

EXAMPLE 1 TO 20 AND COMPARATIVE EXAMPLE 1 TO 10

A polyethylene terephthalate polymer having an intrinsic viscosity of 1.0 to 1.1 and a terminal carboxyl group content of about 15 eq/10⁶ g was used as a starting material. The polymer was melt-spun at 305°C

In the melt spinning, a spinneret containing 192 250 holes and a diameter of 0.5 mm was used in the extrusion. The L/D (ratio of length to diameter) of the orifice was varied within the range of from 2 to 4. A shroud with 200 mm length maintained at 330°C was placed directly under the spinneret and below the shroud quenching and solidification was carried out with quench air at a temperature less than 80°C

The other process conditions employed to produce the polyester filamentary yarns are described in Table 1 and 2 which follow.

The flow rate was controlled to produce a final polyester yarn with a fineness of 1,000 total denier. The properties of the obtained yarns are described also in Table 1 and 2 which follow.

EXAMPLE 21 TO 28 AND COMPARATIVE EXAMPLE 11 TO 20

The drawn filamentary yarn produced in the above Examples and Comparative examples was subjected to twisting consisting of first twist of 49 times/10 cm in Z direction and second twists of 49 times/10 cm is S direction and two times of doubling and then formed into a fabric. The resulting fabric was dipped in a rubber treatment solution comprising resorcinol formalin latex and epoxy isocianate and then dried at 160°C during 60 seconds.

Thereafter, the dipped fabric was thermally treated under the conditions described in Table 3 and 4 which follow, relaxed at 1.5 percent, and normalized at 245°C for 60 seconds, to finally obtain a polyester tire cord.

The properties of the so obtained tire cord are also described in Table 3 and 4 which follow.

The tests of the properties appearing in Table 1 to 4 were performed according to the following methods.

1. tenacity and elongation: in accordance with JIS-L1017 method.

Instrument: low-speed elongation type tensile strength tester from Instron Co., Ltd., tensile speed: 300 mm/min, length of sample: 250 mm, atmospheric conditions: 25°C, 65% RH.

2. medium elongation

The elongation value at a load of 4.5 g/d in the elongation load curve obtained in accordance with JIS-L1017 method using an instrument same as that used in above 1).

3. shrinkage of yarn:

The value (Δδ, in percent) calculated from the following equation wherein L₀ is the length of a sample measured under a load of 20 g after it has been placed at 25°C, 65% RH during more than 24 hours, and L₁ is the length after it has been placed in the oven at 150°C during 3 minutes under zero load. ##EQU8##

4. shrinkage of cord

The value calculated from the following equation wherein l₀ is the length of a cord sample taken from a cord fabric measured under a dead weight loading of 20 g after it has been placed at 25°C, 65% RH during more than 24 hours, and 11 is the length of same measured after it has been placed in an oven at 177°C during minutes under a dead weight loading of 20 g. ##EQU9##

5. Percentage of retained strength of cord: in accordance with ASTM D 885 method.

The value obtained from the following formula by measuring the strength of a cord sample taken from a tire, before and after hours rotation at an inner tube pressure of 3.5 kg/cm², rotation speed of 850 rpm and tube angle of 80°C ##EQU10##

TABLE 1
__________________________________________________________________________
Parameters   Example No.
and          Example
Example
Example
Example
Example
Example
Example
Example
Example
Example
properties   1    2    3    4    5    6    7    8    9    10
__________________________________________________________________________
I.V. of polymer
1.1  1.0  1.0  1.0  1.0  1.0  1.1  1.0  1.0  1.0
L/D of orifice
2    2    2    3    3    3    3    3    3    3
quench air temp. (°C)
25   40   25   60   50   40   40   40   50   60
spinning speed (m/min)
2500 3500 3800 3400 2500 3600 3000 3200 4000 2700
birefringence
0.032
0.060
0.066
0.069
0.040
0.070
0.060
0.064
0.080
0.044
temp. of 1st draw zone
90   90   100  70   80   80   90   80   90   90
(°C)
draw ratio of 1st draw
1.80 1.64 1.65 1.65 1.85 1.76 1.70 1.65 1.60 1.80
temp. of 2nd draw zone
100  90   100  90   110  90   90   90   90   100
(°C)
draw ratio of 2nd draw
1.2  1.1  1.06 1.1  1.11 1.05 1.06 1.06 1.03 1.06
total draw ratio
2.08 1.75 1.60 1.76 1.97 1.79 1.74 1.68 1.58 1.84
temp. of heat
200  180  190  180  160  170  170  180  170  180
treatment (°C)
relax temp. (°C)
30   100  80   80   30   140  130  120  130  30
relax ratio (%)
3.5  3.2  3.0  3.0  4.0  3.0  3.6  4.0  4.0  3.5
tenacity (g/d)
8.5  8.0  7.0  7.3  8.3  8.6  7.4  7.3  7.2  7.6
elongation (%)
12.0 12.6 15.2 13.2 12.2 12.1 13.5 14.2 15.0 14.0
medium elongation (%)
4.5  5.0  7.2  7.0  5.2  4.4  6.2  6.3  7.2  6.2
shrinkage (%)
12.0 11.0 10.3 11.2 13.0 12.6 11.4 11.2 10.0 11.3
maximum thermal stress
0.48 0.46 0.29 0.43 0.50 0.50 0.40 0.38 0.28 0.34
(g/d, 60-250°C)
thermal stress
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
behavior beyond 210°C
long period (Å)
154  150  153  140  148  155  140  150  139  135
crystallite size of
65   64   65   60   62   63   58   64   64   63
(lo5) plane (Å)
crystal volume
1.45 1.43 1.42 1.40 0.92 1.32 0.54 1.30 0.62 0.54
(× 105 Å3)
crystalline  0.924
0.921
0.911
0.901
0.924
0.938
0.920
0.911
0.912
0.919
orientation function
(fc)
amorphous orientation
0.645
0.634
0.622
0.621
0.632
0.652
0.610
0.611
0.600
0.620
function (fa)
terminal modulus (g/d)
19.5 18.5 16.4 16.9 18.9 19.9 16.6 17.0 16.5 17.1
crystallinity (wt %)
45   42   43   40   38   35   42   43   41   40
fa (1 - Xc)  0.355
0.368
0.355
0.373
0.392
0.424
0.354
0.348
0.354
0.372
7.8  11.0 10.5 12.6 13.3 13.9 11.2 10.7 11.9 12.2
__________________________________________________________________________
Parameters   Example No.
and          Example
Example
Example
Example
Example
Example
Example
Example
Example
Example
properties   11   12   13   14   15   16   17   18   19   20
__________________________________________________________________________
I.V. of polymer
1.1  1.0  1.1  1.1  1.1  1.1  1.1  1.0  1.1  1.1
L/D of orifice
2    2    2    3    2    3    3    4    4    4
quench air temp. (°C)
25   25   40   40   60   60   50   60   30   60
spinning speed (m/min)
2700 2900 3400 3800 3000 2900 3500 3200 4000 3600
birefringence
0.042
0.046
0.065
0.075
0.057
0.052
0.068
0.065
0.080
0.072
temp. of 1st draw zone
90   90   90   80   80   80   80   80   100  70
(°C)
draw ratio of 1st draw
1.90 2.05 1.70 1.50 135  1.50 1.65 1.60 1.40 1.70
temp. of 2nd draw zone
100  100  90   90   90   120  90   80   100  90
(°C)
draw ratio of 2nd draw
1.14 1.05 1.03 1.05 1.32 1.33 1.04 1.07 1.14 1.08
total draw ratio
2.10 2.07 1.67 1.53 1.71 1.93 1.63 1.63 1.53 1.76
temp. of heat
120  160  180  180  160  170  180  180  160  170
treatment (°C)
relax temp. (°C)
30   30   120  120  100  100  30   30   140  140
relax ratio (%)
3    4    4.5  3    4    3.5  5    5    4    4
tenacity (g/d)
7.7  8.4  7.6  7.2  7.7  8.0  7.5  7.4  7.0  8.0
elongation (%)
12.8 13.0 14.8 15.1 14.9 12.5 14.2 14.1 15.4 11.3
medium elongation (%)
5.4  4.9  6.8  7.2  6.7  5.2  7.0  6.5  7.2  4.6
shrinkage (%)
13.0 12.4 11.4 10.2 11.2 12.5 11.2 11.2 10.4 12.6
maximum thermal stress
0.48 0.42 0.36 0.24 0.36 0.43 0.40 0.38 0.36 0.46
(g/d, 60-250°C)
thermal stress
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
behavior beyond 210°C
long period (Å)
153  152  150  151  140  154  142  152  153  152
crystallite size of
61   65   63   64   60   63   60   60   63   64
(lo5) plane (Å)
crystal volume
1.51 1.49 1.43 1.44 1.32 1.52 1.43 1.40 1.45 1.48
(× 105 Å3)
crystalline  0.924
0.935
0.929
0.911
0.928
0.938
0.919
0.921
0.909
0.936
orientation function
(fc)
amorphous orientation
0.639
0.632
0.621
0.601
0.620
0.609
0.611
0.609
0.601
0.650
function (fa)
terminal modulus (g/d)
19.6 19.4 18.0 17.5 18.0 18.8 18.6 18.2 17.0 19.7
crystallinity (wt %)
32   38   42   40   35   36   41   44   39   33
fa (1 - Xc)  0.445
0.392
0.360
0.361
0.403
0.390
0.360
0.341
0.367
0.436
##STR1##    14.9 13.7 11.1 12.5 14.2 13.8 11.8 10.2 12.4 14.8
__________________________________________________________________________

TABLE 2
__________________________________________________________________________
comp. example No.
Parameters   comp.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
and          example
example
example
example
example
example
example
example
example
example
properties   1    2    3    4    5    6    7    8    9    10
__________________________________________________________________________
I.V. of polymer
1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0
L/D of orifice
2.5  2.5  2.5  3    3    3    4.5  3    3    3
quench air temp. (°C)
25   25   25   25   40   40   40   25   40   40
spinning speed (m/min)
1800 1800 2500 3000 3000 4800 Handling
600  3050 3500
birefringence
0.020
0.020
0.03 0.036
0.040
0.098
was  0.003
0.04 0.052
temp. of 1st draw zone
90   90   90   130  100  80   impossible
110  110  110
(°C)
draw ratio of 1st draw
2.15 2.15 2.10 1.80 1.80 1.80      4.0  1.5  1.3
temp. of 2nd draw zone
100  100  100  150  130  90        220  220  220
(°C)
draw ratio of 2nd draw
1.07 1.07 1.05 1.03 1.03 1.04      1.4  1.1  1.03
total draw ratio
2.22 2.22 2.17 1.78 1.72 1.28      5.26 1.58 1.30
temp. of heat
190  245  220  200  220  180       240  240  240
treatment (°C)
relax temp. (°C)
150  140  230  130  240  140       180  180  180
relax ratio (%)
3.5  3.5  4    4    7    5         6    4    3
tenacity (g/d)
8.9  8.8  8.5  Handling
8.0  6.2  (Pack
8.9  8.14 7.95
elongation (%)
11.0 12.1 13.4 was  14.9 15.8 pressure
14.1 13.7 14.4
medium elongation (%)
4.0  4.4  5.0  impossible
6.5  8.4  was  4.8  6.6  6.5
shrinkage (%)
11.3 7.4  7.0       5.0  5.2  increased)
10.5 5.9  5.6
maximum thermal stress
0.56 0.49 0.42      0.30 0.24      0.65 0.54 0.51
(g/d, 60-250°C)
thermal stress
increase
increase
increase  increase
increase  increase
increase
increase
behavior beyond 210°C
long period (Å)
153  157  159       161  156       155  142  142
crystallite size of
64   66   67        69   68        68   65   65
(lo5) plane (Å)
crystal volume
1.50 1.55 1.57      1.58 1.54      2.0  1.57 1.55
(× 105 Å3)
crystalline  0.946
0.932
0.941     0.943
0.911     0.932
0.944
0.945
orientation function
(fc)
amorphous orientation
0.620
0.502
0.501     0.468
0.511     0.435
0.500
0.510
function (fa)
terminal modulus (g/d)
23.1 19.6 18.4      16.4 18.4      34.1 16   14
crystallinity (wt %)
47   55   51        54   48        50   53   53
fa (1 - Xc)  0.329
0.226
0.245     0.215
0.266     0.217
0.235
0.240
##STR2##    4.8  2.9  3.6       2.7  4.5       3.9  3.4  3.2
__________________________________________________________________________

TABLE 3
__________________________________________________________________________
Example No.
Example
Example
Example
Example
Example
Example
Example
Example
21   22   23   24   25   26   27   28
__________________________________________________________________________
example no. of the
1    4    8    9    12   15   18   19
used yarn
temperature (°C)
240  245  230  240  220  240  250  240
tension (g/d)
0.6  0.3  0.5  0.3  0.6  0.3  0.2  0.5
strength (L10; Newton)
104  106  102  105  107  106  102  101
shrinkage (S; %)
3.1  3.1  2.9  3.0  3.3  3.2  3.0  3.2
strength after dry-heat
72.1 72.8 71.5 72.5 74.2 73.0 70.9 70.0
(L; Newton)
L/S        23.2 23.4 24.6 24.1 22.5 22.8 23.6 21.9
maximum thermal stress
0.07 0.08 0.06 0.06 0.08 0.09 0.07 0.09
(g/d)
percentage of retained
96   96   98   97   95   95   97   94
strength of cord (%)
__________________________________________________________________________

TABLE 4
__________________________________________________________________________
comp. example No.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
comp.
example
example
example
example
example
example
example
example
example
example
11  12  13  14   15  16  17  18  19  20
__________________________________________________________________________
example no. of the
1   8   15  19   comp.
comp.
comp.
comp.
comp.
comp.
used yarn                   example
example
example
example
example
example
1   3   5   8   9   10
temperature (°C)
210 240 200 250  240 245 230 240 250 240
tension (g/d)
0.4 0.1 0.1 0.7  0.6 0.3 0.5 0.3 0.2 0.5
strength (L10; Newton)
104 95  96  The cord
82  79  83  82  80  80
was cut
shrinkage (S; %)
4.1 3.5 3.4      4.0 3.9 4.9 4.0 3.5 4.7
strength after dry-heat
70.2
65.1
66.2     50.4
52  56  54.2
53.5
52
(L; Newton)
L/S        17.1
18.6
19.4     12.6
13.3
11.4
13.6
15.3
11.1
maximum thermal stress
0.18
0.09
0.08     0.25
0.22
0.29
0.19
0.15
0.20
(g/d)
percentage of retained
90  93  92       85  88  90  87  86  87
strength of cord (%)
__________________________________________________________________________

From the result in Table 3, it can be seen that the present tire cord has a shrinkage less than 3.5 percent at 150°C over 30 minutes under zero tension and a coefficient of dimensional stability of at least 20 and thus exhibits excellent dimensional stability. Also, it can be seen that the present tire cord has a strength at 10% elongation of at least 100N and a strength at 10% elongation after the dry-heat treatment of at least 65N accordingly it has good fatigue resistance. Though the present invention has been described by the preferred embodiments, it is understood that the variation and modification apparent to the skilled in this art is possible. Such variation and modification should be considered to be included within the spirit and scope of the present claims which follow.

Claims

1. A polyester filamentary yarn comprising at least 90 mol. % polyethylene terephalate and having a fineness of 3-5 denier per filament wherein said polyester has a crystalline orientation function (fc) of at most 0.94, percent crystallinity (Xc) of 0.320-0.45, and has a three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions, the proportion of the mesomorphous portion is 5-15 percent based upon the total amount of crystalline, amorphous and mesomorphous portions of said polyester, and wherein the amount of the mesomorphous portion and the total amount of crystalline, amorphous and mesomorphous portions are determined by the equation

percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyester calculated from the X-ray intensity of the peak for the (010) crystal plane in a quantitative equatorial diffraction pattern of the polyester and I total is the sum of the crystalline, mesomorphous and amorphous portions of the polyester calculated from said quantitative equatorial X-ray diffraction pattern.

2. The polyester yarn of claim 1, wherein said yarn has:

i) a crystalline orientation function (fc) of at most 0.94,

ii) an amorphous orientation function (fa) of at least 0.60,

iii) a long period value of at most 155 Å, where fa(1-Xc)>0,330 where Xc is the percent crystallinity and is a value of 0.30-0.45.

3. The polyester yarn of claim 2, wherein said polyester has a three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions, the proportion of the mesomorphous portion is 5-15 percent based upon the total amount of crystalline, amorphous and mesomorphous portions of said polyester, and wherein the amount of the mesomorphous portion and the total amount of crystalline, amorphous and mesomorphous portions are determined by the equation

percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyester calculated from the X-ray intensity of the peak for the (010) crystal plane in a quantitative equatorial diffraction pattern of the polyester and I total is the sum of the crystalline, mesomorphous and amorphous portions of the polyester calculated from said quantitative equatorial X-ray diffraction pattern.

4. The polyester yarn of claim 2, wherein said yarn has a terminal modulus of at most 20 g/d.

5. The polyester yarn of claim 1, wherein said yarn has a terminal modulus of at most 20 g/d.

6. The polyester yarn of claim 1, where said yarn has a crystal volume of 0.5-1.54×10⁵ ų.

7. A polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3-5 denier per filament, wherein said yarn is:

i) a crystallinity of 30-45 percent by weight,

ii) a crystallite size in the of at most 65 Å

iii) a crystal volume of 0.5×10⁵ ų -1.54×10⁵ ų.

8. The polyester yarn of claim 7, wherein said yarn has:

i) a crystalline orientation function (fc) of at most 0.94,

ii) an amorphous orientation function (fa) of at least 0.60,

iii) a long period value of at most 155 Å, where fa(1-Xc) 0.330 where Xc is the percent crystallinity and is a value of 0.30-0.45.

9. The polyester yarn of claim 8, wherein said polyester has a three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions, the proportion of the mesomorphous portion is 5-15 percent based upon the total amount of crystalline, amorphous and mesomorphous portions of said polyester, and wherein the amount of the mesomorphous portion and the total amount of crystalline, amorphous and mesomorphous portions are determined by the equation

percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyester calculated from the X-ray intensity of the peak for the (010) crystal plane in a quantitative equatorial diffraction pattern of the polyester and I total is the sum of the crystalline, mesomorphous and amorphous portions of the polyester calculated from said quantitative equatorial X-ray diffraction pattern.

10. The polyester yarn of claim 8, wherein said yarn has a terminal modulus of at most 20 g/d.

11. A polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3-5 denier per filament, wherein said yarn has a maximum thermal stress of at most 0.5 g/d in the temperature range of from 60°C to 250°C and exhibits decreasing slope above 210°C in a thermal stress versus temperature curve.

12. The polyester yarn of claim 11, wherein said yarn has:

i) a crystalline orientation function (fc) of at most 0.94,

ii) an amorphous orientation function (fa) of at least 0.60,

iii) a long period value of at most 155 Å, where fa(1-Xc)>0.330 where Xc is the percent crystallinity and is a value of 0.30-0.45.

13. The polyester yarn of claim 12, wherein said polyester has a three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions, the proportion of the mesomorphous portion is 5-15 percent based upon the total amount of crystalline, amorphous and mesomorphous portions of said polyester, and wherein the amount of the mesomorphous portion and the total amount of crystalline, amorphous and mesomorphous portions are determined by the equation

percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyester calculated from the X-ray intensity of the peak for the (010) crystal plane in a quantitative equatorial diffraction pattern of the polyester and I total is the sum of the crystalline, mesomorphous and amorphous portions of the polyester calculated from said quantitative equatorial X-ray diffraction pattern.

14. The polyester yarn of claim 12, wherein said yarn has a terminal modulus of at most 20 g/d.

15. A polyester filamentary yarn comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3-5 denier per filament, wherein said yarn has:

i) a crystalline orientation function (fc) of at most 0.94,

ii) an amorphous orientation function (fa) of at least 0.60,

ii) a long period value of at most 155 Å

and where said yarn is characterized in that fa(1-xc)>0.330 where Xc is the percent crystallinity and is a value of 0.30-0.45.