WO2007015617A1

WO2007015617A1 - Method for preparing polyester multifilament yarn for reinforcement of rubber and polyester multifilament yarn prepared by the same method

Info

Publication number: WO2007015617A1
Application number: PCT/KR2006/003013
Authority: WO
Inventors: Eun-Lai Cho; Sang-Kon Lee; Sang-Kun Seok
Original assignee: Hyosung Corporation
Priority date: 2005-08-03
Filing date: 2006-08-01
Publication date: 2007-02-08
Also published as: KR100808803B1; KR20070016469A

Abstract

Disclosed is a method of producing a polyester multifilament yarn for reinforcing rubber, and a polyester multifilament yarn produced using the method. The method includes a) adding a polymerization catalyst containing 150 to 250 ppm of an antimony compound based on an antimony metal to a terephthalic acid (TPA) and ethylene glycol (EG) to perform poly¬ merization, b) solid state polymerizing a polymer obtained in step (a), c) melt spinning a solid state polymer obtained in step (b), d) winding melt spun filaments at a speed of 1500 m/min or more to produce an undrawn yarn having a density of 1.338 g/cm or more, and e) stretching the undrawn yarn obtained in step (d) to produce a polyester multifilament yarn that has intrinsic viscosity of 0.83 or more and tenacity of 8.3 g/d or more and includes 20 microequivalents/g or less of a carboxyl end group and 1.2 wt% or less of diethylene glycol. The polyester mul¬ tifilament yarn is useful as a rubber reinforcing material to produce a treated cord, such as a tire cord.

Description

METHOD FOR PREPARING POLYESTER MULTIFILAMENT

YARN FOR REINFORCEMENT OF RUBBER AND POLYESTER

MULTIFILAMENT YARN PREPARED BY THE SAME

METHOD Technical Field

[1] The present invention relates to a method of producing a polyester multifilament yarn, and a polyester multifilament yarn produced using the same. More particularly, the present invention pertains to a method of producing an industrial polyester multifilament yarn that is useful as a rubber reinforcing material and has high modulus and low shrinkage, and a polyester multifilament yarn produced using the method.

[2]

Background Art

[3] A typical method of producing industrial polyester multifilament yarns having high modulus and low shrinkage includes high speed spinning at 1,500 m/min or more, multi-stage drawing 1.5 to 2.5 times, and heat setting at high temperatures. However, since multifilament yarn that is produced using the above-mentioned method have a sparse microstructure as compared to multifilament yarn produced using low speed spinning at 1500 m/min or less, there are disadvantages due to hydrolysis and pyrolysis in view of the chemistry aspect.

[4] Accordingly, U.S. Pat. No. 4,751,143 discloses multifilament yarn having 18 mi- croequivalent/g or less of a carboxyl end group (CEG). Additionally, Japanese Patent No. 2822503 discloses multifilament yarn having 1.3 wt% or less of diethylene glycol (DEG) and 25 microequivalent/g or less of a carboxyl end group (CEG).

[5] Meanwhile, in order to improve workability in the production of industrial multifilament yarn using high speed spinning and physical properties required in a final rubber reinforcing material, various technologies, such as distribution of the draw ratio, control of the drawing temperature, control of the heat setting temperature, selection of spinning finishes, and drawing using steam jet, may be used in the course of producing the multifilament yarn. However, even though the above-mentioned technologies are used, if the polyester polymer does not have excellent heat resistance and cannot provide excellent strength, it is difficult to assure desirable physical properties, such as high modulus, low shrinkage, and high tenacity, which are required in rubber reinforcing materials, such as tire cords.

[6] With respect to examples of polymerization catalysts used to produce polyester fibers for reinforcing rubber until now, Japanese Examined Patent Application Publication No. 37-5821 discloses the use of manganese acetate, antimony trioxide, and phosphoric acid catalyst systems. Japanese Unexamined Patent Application Publication No. 55-12781 discloses the use of calcium acetate, antimony trioxide, and a hypophosphoric acid. Japanese Unexamined Patent Application Publication No. 51-134789 discloses the use of lithium acetate, antimony trioxide, and a hypophosphoric acid. The process of producing the polyester fibers for reinforcing rubber using the above-mentioned catalysts adopts a catalyst technology used before the development process of producing industrial polyester multifilament yarns having high tenacity, high modulus, and low shrinkage using high speed spinning. Accordingly, in the above-mentioned catalyst technology, it is difficult to perform spinning at a high speed of 1,500 m/min or more due to generation of a large amount of foreign substances caused by agglomeration of polymerization catalyst particles.

[7] Japanese Patent No. 2822503 discloses that the metal antimony, which is formed by reducing an antimony compound used as a polymerization catalyst, gives adverse effection to tenacity and toughness, and also discloses one example of the use of 60 to 150 ppm of the antimony compound as the polymerization catalyst based on the antimony metal. However, with respect to the above-mentioned case, the patent discloses that a thin film polymerization process, which includes increasing the ratio of the molten surface area to the volume, so as to reduce a polymerization time during the polymerization, must be performed or the antimony metal must be used along with other polymerization catalysts when 130 ppm or less of antimony metal is used to assure a commercial polymerization effect.

[8] U. S. Patent No. 4,867,936 discloses the use of 300 to 400 ppm of an antimony compound and 0.5 to 1.5 ppm of a titanium compound as polymerization catalysts which are required to produce industrial polyester multifilament yarns having high tenacity, high modulus, and low shrinkage. In the U. S. patent, 250 to 350 ppm of the antimony compound is used as the polymerization catalyst based on antimony metal to produce industrial polyester multifilament yarn products for reinforcing rubber having high tenacity. However, in the case of when 250 to 350 ppm of the antimony compound is used as the polymerization catalyst based on the antimony metal, the antimony metal that is separated during the polymerization attaches to a pack filter for melt spinning, thus increasing pressure applied to the pack. Accordingly, there are adverse effects in that an exchange cycle of the pack is reduced and stretchability is deteriorated.

[9] With respect to this, U. S. Patent No. 5,997,789 discloses a method of coating a filter using SiO , TiO , ZrO , Al O , SiC, TiN, TiCN, and TiC to prevent antimony metal from being attached to the filter, thereby lengthening the exchange cycle of the pack. Further, the patent discloses the addition of 200 to 400 ppm of TiO to a polyethylene terephthalate polymer. However, the metal that is added to the polymer deteriorates physical properties, for example tenacity or heat resistance, of tire cords.

[10]

Disclosure of Invention Technical Problem

[11] The present inventors have conducted studies into a method of producing polyester multifilament yarns having high modulus and low shrinkage that are to be used for a rubber reinforcing material. As a result in the findings, a solid state polymerization speed was not reduced even though 150 to 250 ppm of an antimony compound was used as a polymerization catalyst for polymerization using a terephthalic acid (TPA) as a raw material and the subsequent solid state polymerization based on antimony metal in comparison with a known art using 250 to 350 ppm of the antimony compound based on the antimony metal. Furthermore, in the case of when 150 to 250 ppm of the antimony compound is used as the polymerization catalyst based on the antimony metal, spinnability and stretchability of the polymer are improved as compared to a known method, thus increasing multifilament yarn production efficiency. Furthermore, when the treated cords are produced using the polyester multifilament yarn, strength manifestation (strength retention) and heat resistance are excellent, and high modulus, low shrinkage, and high strength are obtained. As such, the yarn becomes desirable as a rubber reinforcing material.

[12] Therefore, an object of the present invention is to provide a method of producing a polyester multifilament yarn that is useful as a rubber reinforcing material and a polyester multifilament yarn produced using the method.

[13]

Technical Solution

[14] The present invention provides a method of producing a polyester multifilament yarn. The method includes a) adding a polymerization catalyst containing 150 to 250 ppm of an antimony compound based on an antimony metal to a terephthalic acid (TPA) and ethylene glycol (EG) to perform polymerization, b) solid state polymerizing a polymer obtained in step (a), c) melt spinning a solid state polymer obtained in step (b), d) winding melt spun filaments at a speed of 1500 m/min or more to produce an undrawn yarn having a density of 1.338 g/cm or more, and e) drawing the undrawn yarn obtained in step (d) to produce a polyester multifilament yarn that has intrinsic viscosity of 0.83 or more and tenacity of 8.3 g/d or more and includes 20 microe- quivalents/g or less of a carboxyl end group and 1.2 wt% or less of diethylene glycol.

[15] Furthermore, the present invention provides a polyester multifilament yarn that is produced according to the above-mentioned method, has intrinsic viscosity of 0.83 or more and multifilament yarn tenacity of 8.3 g/d or more, and includes 20 microe- quivalents/g or less of a carboxyl end group and 1.1 wt% or less of diethylene glycol. Best Mode for Carrying Out the Invention

[16] Hereinafter, the present invention will be described in detail.

[17] In a case of when an antimony compound is used as a polymerization catalyst during the production of polyester multifilament yarns, a small amount of antimony metal is reduced during the polymerization. In connection with this, it is known that the reduced antimony metal aggravates the spinning process condition, for example, the reduced antimony metal clogs a pack filter. Additionally, the reduced metal reduces stretchability and negatively affects physical properties of treated cords when using the produced multifilament yarn. As described above, in a known art, a very small amount of antimony compound is used as a polymerization catalyst. However, there is a problem in that an additional polymerization catalyst must be used or a special polymerization method, such as a thin film polymerization method, must be used. Particularly, unlike the production of yarn for cloths, it is necessary to perform solid state polymerization as well as melt polymerization using a terephthalic acid (TPA) in order to produce industrial yarn having high intrinsic viscosity, such as yarn for reinforcing rubber. In this case, it is known that it is necessary to use more polymerization catalysts.

[18] However, in the present invention, it is found that the solid state polymerization speed is not reduced even though 150 to 250 ppm of an antimony compound is used as the polymerization catalyst based on the antimony metal if the polymerization condition is controlled in the course of producing the polyester multifilament yarn. When the antimony compound is used in the above-mentioned amount range, it is possible to set the reduced antimony metal content of the polymer to 5 ppm or less. Thus, it is possible to improve spinnability and stretchability, to produce the polyester multifilament yarn having high modulus and tenacity and low shrinkage, and to significantly improve strength manifestation and heat resistance during the production of treated cords.

[19] Hereinafter, steps constituting the method of producing the polyester multifilament yarn according to the present invention will be described in detail.

[20] In step (a) of the method of producing the polyester multifilament yarn according to the present invention, 150 to 250 ppm of the antimony compound is added to the terephthalic acid (TPA) and ethylene glycol (EG) based on antimony metal to perform polymerization. In step (a), the antimony compound is the polymerization catalyst, and the content of the antimony compound is 150 to 250 ppm, and preferably 150 to 220 ppm, based on the antimony metal. If the content of the antimony compound is more than 250 ppm based on the antimony metal, a filter of a spinning pack is clogged due to precipitation of the reduced metal antimony. Thus, an exchange period of the pack is significantly reduced, workability is reduced due to reduced stretchability, and strength retention is reduced after the multifilament yarn is immersed in an adhesive (RFL) and heat treated for rubber reinforcement. If the content of the antimony compound is less than 150 ppm based on the antimony metal, it is impossible to control a polymerization speed by only adjusting a polymerization temperature and a vacuum, and it is difficult to achieve commercial production due to the significantly reduced polymerization speed.

[21] In step (a), additional catalysts may be used along with the antimony compound as the polymerization catalyst, and examples of the additional catalysts include a Ti-based compound and an Al-based compound. In connection with this, it is preferable that the polymerization catalyst added include 80 wt% or more of the antimony compound in view of efficiency. Furthermore, in step (a), metal compounds, such as titanium dioxide and silica, may be added for improvement in spinnability and stretchability or for other purposes, in addition to the polymerization catalyst. Preferably, the content of the added metal compound is set so that heat resistance and strength manifestation of the multifilament yarn produced according to the present invention are not negatively affected. For example, the metal compound may be added in the amount of 150 ppm or less.

[22] It is preferable that the molar ratio of ethylene glycol/terephthalic acid is 1.1 to 1.2 in step (a). In the present invention, the molar ratio of ethylene glycol/terephthalic acid is set so that the polymer produced in step (a) include 50 ppm or less of an unreacted terephthalic acid. If the polymer includes the unreacted terephthalic acid of more than 50 ppm, since drawing properties of the undrawn yarn are reduced, it is difficult to produce multifilament yarn having high tenacity.

[23] In step (a), the polymerization temperature is not limited, but preferably 275 to

288⁰C. Furthermore, it is preferable to control the polymerization temperature and time so that esterification reactivity is increased to 98% or more. If the esterification reactivity is not 98% or more, a large amount of unreacted terephthalic acid remain. Accordingly, processability of multifilament yarn is significantly reduced and it is difficult to produce multifilament yarn having high tenacity.

[24] Additionally, during the latter half of polymerization in step (a), the polymer is maintained in a final polymerization reactor at a high vacuum of 2.5 torr or less for 1.5 hours or more, and preferably for 2 hours or more, to efficiently perform polymerization using a relatively small amount of polymerization catalyst. In the final polymerization reactor, if the vacuum is more than 2.5 torr or if the residence time is less than 1.5 hours, reduced metal antimony and the unreacted terephthalic acid are insufficiently removed. Thus, the amount of residual metal antimony is more than 5 ppm and the amount of residual unreacted terephthalic acid is more than 50 ppm. Accordingly, it is difficult to produce multifilament yarn having high tenacity, and to assure tenacity manifestation and heat resistance required in the rubber reinforcing material.

[25] The polymer that is obtained by polymerization of step (a) performed under the above-mentioned condition is a polyethylene glycol terephthalate polymer including 95% or more of the ethylene glycol terephthalate repeating unit, 5 ppm or less of the reduced metal antimony, and 50 ppm or less of the unreacted terephthalic acid. The polymer has intrinsic viscosity (LV.) of 0.6 to 0.8.

[26] According to the method of the present invention, the polymer that is produced in step (a) is subjected to solid state polymerization in step (b). The solid state polymerization may be performed using a process known in the art, but is not limited. For example, the solid state polymerization may be performed at 18O⁰C to the melting temperature, and preferably 220 to 24O⁰C, in a vacuum or in a nitrogen atmosphere. The solid state polymer that is produced using the solid state polymerization has intrinsic viscosity of 0.9 or more, and preferably 1 to 1.15, such that the produced multifilament yarn has high tenacity of 8.3 g/d or more, which is required when the multifilament yarn is used as industrial yarn.

[27] According to the method of the present invention, the solid state polymer that is obtained in step (b) is melt spun in step (c). The melt spinning may be performed using a method known in the art, but is not limited thereto. In the melt spinning of the present invention, the water content of the solid state polymer is set to 25 ppm or less, and the temperature of spun beam is set to 300⁰C or less so that the temperature is as low as possible, thereby preventing the polymer from being thermally decomposed. Additionally, the solid state polymer is extruded and melt spun using an extruder and a spinneret. In connection with this, the spinning may be performed while a spinning rate is set to 1,500 m/min or more and other spinning conditions are appropriately controlled. The undrawn yarn that is obtained in this step has intrinsic viscosity (LV.) of 0.83 or more and density of 1.338 g/D or more.

[28] In the present invention, the undrawn yarn that is obtained after the extrusion and melt spinning of step (c) may be passed through a delay cooling zone directly under the spinneret and a rapid cooling zone using cooling air, thus being solidified. The delay cooling zone may be a heating type or a non-heating type.

[29] In the present invention, the undrawn yarn that is wound while a spinning finish is provided at a speed of 1500 m/min or more after the yarn is extruded and melt spun during step (c) is stretched during step (d) to produce the polyester multifilament yarn. For example, the undrawn yarn is drawn through multi stages at a stretch ratio of 1.5 to 2.5 to perform the drawing during step (d). After the drawing, it is preferable to heat set the drawn yarn at high temperatures, thereby stabilizing the yarn.

[30] The drawn yarn has intrinsic viscosity (LV.) of 0.83 or more, and preferably 0.88 or more. Additionally, the drawn yarn includes 20 microequivalents/g or less, and preferably 18 microequivalents/g or less, of the carboxyl end group (CEG), and 1.2 wt% or less, and preferably 1.1 wt% or less, of diethylene glycol (DEG). Tenacity of the multifilament yarn is 8.3 g/d or more, and preferably 8.5 g/d or more. The multifilament yarn produced according to the present invention has strength retention (tenacity of the tire cord/tenacity of the multifilament yarn) and heat resistance that are relatively higher than those of a known multifilament yarn having tenacity of 8.3 g/d or more when the yarn of the invention is used as the rubber reinforcing material, such as a tire cord. The polyester multifilament yarn that has the above-mentioned physical properties is useful for reinforcement of rubber. Further, if density of the multifilament yarn is 1.38 to 1.3865 g/D, better strength manifestation (strength retention) is assured when the multifilament yarn is heat treated during the post-process in the course of producing the treated cord.

[31] The diethylene glycol (DEG) and carboxyl end group (CEG) contents, and intrinsic viscosity of the multifilament yarn directly affect the tenacity of the multifilament yarn, strength manifestation (strength retention) of the tire cord, and heat resistance. For example, if the diethylene glycol content is more than 1.2 wt% or if the carboxyl end group (CEG) content of the multifilament yarn is more than 20 microequivalents/ g, strength manifestation (strength retention) and heat resistance of the rubber reinforcing material that becomes the final product are reduced. If the molar ratio of ethylene glycol/terephthalic acid is not in the range of 1.1 to 1.2, the diethylene glycol (DEG) and carboxyl end group (CEG) contents are not in the above-mentioned range, and the yarn is poor in views of tenacity of the multifilament yarn, strength manifestation of the treated cord (strength retention), and heat resistance.

[32] A yarn cutting ratio of the polyester multifilament yarn that is produced by the method of the present invention is 0.8 times/ton, and it can be seen that the yarn cutting ratio is significantly improved as compared to a yarn cutting ratio, that is 3.0 times/ton, of the multifilament yarn that is produced using 250 to 350 ppm of an antimony compound as a known polymerization catalyst based on antimony metal and has the same tenacity, for example, 8.5 g/d. Additionally, strength retention at the maximum drawing ratio (Max. DRt.) is significantly improved by 2% or more in comparison with a multifilament yarn using the polymer having the same intrinsic viscosity.

[33] The polyester multifilament yarn of the present invention may be converted into the treated cord using a typical treating process. For example, two plies of drawn yarns of 1000 deniers are plied and cabled at a ratio of 440 turns/m (the standard twist number of a typical polyester treated cord) to produce cord yarns, immersed in an adhesive solution for rubber (RFL solution), stretched 2.0 to 5.0%, and heat set at a temperature of 230 to 245⁰C for 1.5 to 2.5 min to produce the treated cord. In comparison with a known high modulus and low shrinkage multifilament yarn having the same multifilament yarn tenacity, the treated cord (1000 deniers, two plies, and plying and twisting at the ratio of 440 turns/m) that is the rubber reinforcing material has strength availability that is improved by 2% or more, improved heat resistance, high modulus, and low shrinkage when the treated cord is subjected to a post-process, for example, when the treated cord is heat treated during treatment using a rubber adhesive.

[34] The treated cord that is produced using the polyester multifilament yarn of the present invention has dimensional stability (E + FS)(E is elongation at 2.25 g/d and FS is free shrinkage) of 5.5 to 8.0%, and tenacity of 6.8 g/d or more, and preferably 6.9 to 8.5 g/d.

[35] According to the present invention, since an increase in pack pressure due to metal accumulated on the spinning filter is avoided in the process of producing the multifilament yarn having high modulus and low shrinkage, the exchange cycle of the filter is lengthened 50% or more, the maximum draw ratio is increased 10% or more, and the multifilament yarn tenacity at the maximum draw ratio is improved 5% or more in comparison with use of a known polymer. Thereby, physical properties and productivity are improved. Furthermore, it is possible to produce multifilament yarn that has high tenacity and modulus and low shrinkage and is used to produce a treated cord having excellent strength retention.

[36]

Mode for the Invention

[37] A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

[38] Physical properties of the multifilament yarns and the treated cords produced according to examples and comparative examples of the invention were evaluated using the following methods.

[39] (1) Viscosity

[40] In compliance with ASTM D 4603, 0.1 g of a sample was dissolved in a reagent containing phenol and 1,1,2,2-tetrachloroethane mixed at a weight ratio of 6:4 (9O⁰C) for 90 minutes so that the concentration was 0.4 g/100 ml, and the solution was transferred to an Ubbelohde viscometer and then maintained in a thermostat at 3O⁰C for 10 minutes. The number of solution drops per second was obtained using a viscometer and an aspirator.

[41] The number of solvent drops per second was obtained using the same method, and the R. V. value (relative viscosity) and the LV. value (intrinsic viscosity) were calculated according to the following equations:

[42]

[43] <Equation 1>

[44] R. V. = the number of sample drops per second / the number of solvent drops per second

[45]

[46] <Equation 2>

[47] LV. = 1/4 x [(R.V.-1)/C] + 3/4 x (lnR.V./C)

[48]

[49] In the above equation, C denotes the concentration (g/100 ml) of the sample in the solution.

[50] (2) Analysis of a carboxyl end group (CEG) content

[51] The sample was dissolved using benzyl alcohol, and then analyzed using acid-base neutralization titration. In detail, about 0.2 g of the sample was prepared, and 10 ml of benzyl alcohol was added to the sample. The resulting sample was heated in a heating block at 200⁰C for 10 min to melt, and then cooled in a water bath for 1 min. 10 ml of chloroform, and a few drops of an indicator, such as phenol red, phenolphthalene, or bromothymol blue, were added to the cooled sample, and titration was then performed using 0.1 N KOH (or NaOH).

[52] (3) Analysis of diethylene glycol (DEG) content

[53] Aminolysis was performed using monoethanolamine, and analysis was conducted using the GC. In detail, 1 g of the PET sample was prepared, 3 ml of monoethanolamine was added to the sample, and the resulting sample was completely decomposed by heating on the hot plate that is provided with a cooler. After the cooling, 20 ml of MeOH containing the internal standard (1,6-hexanediol) and 10 g of a terephthalic acid (TPA) were added to the sample, and analysis was conducted using the GC. The DEG standard quantitative curves were made using MeOH solutions that included the same internal standard and had DEG contents of 0, 0.05, 0.1, and 0.15%.

[54] (4) Analysis of an unreacted terephthalic acid (TPA) content

[55] The TPA was selectively dissolved using reprecipitation, and quantitatively analyzed using the HPLC. In detail, about 0.1 g of sample was prepared, and 1 ml of HFIP (Hexafluoro iso-propanol) was added to the sample to completely dissolve the sample. 5 ml of MeOH was added to selectively reprecipitate the polymer, the liquid was filtered, and the analysis was performed using the HPLC. The TPA standard quantitative curves were made using 1% ammonia water, and the HPLC analysis was performed under the same condition as the sample solution.

[56] (5) Content of reduced metal antimony

[57] The polymer was dissolved in 500 ml of ortho-chlorophenol, centrifuged (12,000 rpm x 2 hrs), washed, and dried. The spectrum of the particles that were obtained using centrifugal precipitation was measured using an X-ray diffraction device, and metal antimony was quantitatively analyzed using the spectrum.

[58] (6) Tenacity and elongation

[59] Measurement was performed using Instron 5565 (Instron, USA) according to

ASTM D 885 in a standard state (2O⁰C, 65% relative humidity) under the condition of the sample length of 250 mm, the tensile rate of 300 mm/min, and 80 turns/m.

[60] (7) Density

[61] Density was measured using a xylene/carbon tetrachloride density gradient tube at

23⁰C. The density gradient tube was manufactured according to ASTM D 1505 so as to measure density of 1.33 to 1.41 g/D.

[62] (8) Shrinkage

[63] After the sample was left in the standard state where the temperature was 2O⁰C and the relative humidity was 65% for 24 hours or more, the sample of 0.1 g/denier was weighed to measure a length (L ). Subsequently, the sample was treated in a dry oven at 15O⁰C for 30 min in a tensionless state, left for 4 hours or more, and weighed to measure a length (L). The shrinkage was calculated using Equation 3.

[64]

[65] <Equation 3>

[66] ΔS (%) = (L₀ - L)/L x 100

[67]

[68] (9) Elongation at specific load

[69] The elongation of the grey yarn was measured at a load corresponding to 4.5 g/d

(based on 1000 deniers) using the strength elongation S-S curve, and the elongation of the treated cord (1000 deniers, two plies, and plied and twisted yarn) was measured at the load of 2.25 g/d.

[70] (10) Dimensional stability (E +FS)

[71] The dimensional stability of the treated cord is a physical property that relates to a side wall indentation (SWI) of a tire and handling, and defined as the high modulus at the given shrinkage. E (elongation at 2.25 g/d) + FS (free shrinkage) is useful as an index regarding the dimensional stability of the treated cord subjected to different heat treatment processes. The lower the E + FS value is, the higher the dimensional stability is.

[72] (11) Strength retention

[73] Strength retention was calculated with Equation 4 using tenacity measured in (6). [74]

[75] <Equation 4>

[76] Strength retention (%) = (tenacity of the treated cord/tenacity of the multifilament yarn) x 100

[77]

[78] (12) Heat resistance in rubber

[79] The treated cord was added to a rubber compound for testing adhesion strength that was manufactured by Hankook Tire Co. Ltd., vulcanized at 15O⁰C for 6 hrs, and evaluated in terms of strength retention. Strength retention was designated by ©, O, and x when strength retention is 70% or more, 60 to 70%, and less than 60%.

[80]

[81] EXAMPLE l

[82] Ethylene glycol and a terephthalic acid were melted at a molar ratio of 1.15: 1, 190 ppm of antimony trioxide was added based on antimony metal (based on the weight of added ethylene glycol and the terephthalic acid), and polymerization was performed at the polymerization temperature of 280 to 288⁰C. The resulting substance was maintained in the final polymerization reactor at a vacuum of 1.3 torr for 2 hours at the latter half step of polymerization. The polyester polymer that contained 3 ppm of reduced antimony metal, 35 ppm of the unreacted residual terephthalic acid, 0.85 wt% of diethylene glycol(DEG), and 24 microvalents/g of carboxyl end group (CEG), and had intrinsic viscosity (LV.) of 0.65 was produced using the polymerization.

[83] Subsequently, the polymer was subjected to solid state polymerization at 237⁰C for

8 hours to produce a solid state polymerized polyethylene terephthalate chip having intrinsic viscosity (LV.) of 1.05.

[84] The chips having the moisture content of 20 ppm or less that were solid state polymerized as described above were provided into an extruder in a nitrogen atmosphere, and then melt spun through a spinning nozzle using the extruder and the spinning pack (manufactured by Natzuron Co. in Japan) having two non- woven fabric 16 micron-filters at a spinning temperature of 288⁰C and a discharging rate of 580 g/ min so that monofilament fineness of the final stretched yarn was 4.0 deniers.

[85] Subsequently, after the spun yarn was passed through a hot zone and a cooling zone

(cooling air was blown at a speed of 0.5 m/sec) that were directly under the nozzle to be solidified, the water-based spinning finish having the original solution content of 15% was provided, and winding was performed at the spinning rate of 2200 m/min to produce an undrawn yarn. Subsequently, the undrawn yarn was passed through godet rollers to perform three-stage drawing, heat set by the heat-set roller (GR ) at 21O⁰C, slackened 2.0%, and wound. Thereby, the final drawn yarn (multifilament yarn) of 1000 denier that had the carboxyl end group (CEG) of 15 microequivalents/g was produced.

[86] Two plies of multifilament yarns were cabled and plied at the ratio of 440 turns/m to produce the cord yarn, drawn 2.0 to 2.5% using a two bath process (dipping twice) where a Pexul solution was used in a first bath and a RFL (Resorcinol-Formalin-Latex) solution was used in a second bath, and heat set at 24O⁰C for 2 min. The amount of the attached RFL was controlled to 4%, thereby producing the treated cord.

[87] Physical properties of the undrawn yarn, the drawn yarn, and the treated cord thusly produced were evaluated, and the results are described in the following Tables 1 and 2.

[88] COMPARATIVE EXAMPLE 1 [89] A solid state polymerized polyethylene terephthalate chip was produced using the same method as example 1, with the exception of 120 ppm of antimony trioxide being added based on the antimony metal. In example 1, the solid state polymerization time was 8 hours. However, in comparative example 1, since the solid state polymerization time was 16 hours which meant a very long time, the chip of comparative example 1 was difficult to apply to commercial production.

[90] EXAMPLE 2 [91] The undrawn yarn, the drawn yarn, and the treated cord were produced using the same method as example 1, with the exception of 50 ppm of titanium dioxide being further added to the polymer at an initial polymerization step, and physical properties thereof were evaluated. The results are described in the following Tables 1 and 2.

[92] [93] Table 1 Improvement in undrawn yarn and stretchability

[94] [95] Table 2 Physical properties of the drawn yarn and the treated cord

[96] [97] EXAMPLES 3 TO 5 AND COMPARATIVE EXAMPLES 2 TO 5 [98] The multifilament yarn was produced using the same method as example 1, with the exception of the condition of the following Table 3 being used, and physical properties thereof were evaluated. The results are described in Tables 4 and 5.

[99] Table 3

[100]

[101] Table 4

Characteristics of the polyethylene terephthalate polymer (raw chip)

[102]

[103] Table 5

Physical properties of the drawn yarn and the treated cord

[104]

Industrial Applicability

[105] In a method of producing a polyester multifilament yarn according to the present invention, when multifilament yarn processability and stretchability are improved, production efficiency of the multifilament yarn is significantly improved. Further, the polyester multifilament yarn has excellent modulus, shrinkage, tenacity, strength retention, heat resistance, and etc.. Accordingly, the yarn can be used as an industrial polyester multifilament yarn having high tenacity for reinforcing rubber which has high modulus and low shrinkage to produce treated cords, such as tire cords.

Claims

[1] A method of producing a polyester multifilament yarn for reinforcing rubber, the method comprising the steps of: a) adding a polymerization catalyst containing 150 to 250 ppm of an antimony compound based on an antimony metal to a terephthalic acid (TPA) and ethylene glycol (EG) to perform polymerization; b) solid state polymerizing a polymer obtained in step (a); c) melt spinning a solid state polymer obtained in step (b); d) winding melt spun filaments at a speed of 1500 m/min or more to produce an undrawn yarn having a density of 1.338 g/cm or more; and e) drawing the undrawn yarn obtained in step (d) to produce a polyester multifilament yarn that has intrinsic viscosity of 0.83 or more and tenacity of 8.3 g/d or more and includes 20 microequivalents/g or less of a carboxyl end group and 1.2 wt% or less of diethylene glycol.

[2] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein an antimony compound content is 80 wt% or more based on the polymerization catalyst used in step (a).

[3] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein a molar ratio (EG/TPA) of ethylene glycol (EG) and the terephthalic acid (TPA) is 1.1 to 1.2 in step (a).

[4] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein a metal compound is added as an additional catalyst other than the polymerization catalyst, and an amount of the metal compound added is 150 ppm or less in step (a).

[5] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein a polymerization temperature is set to 275 to 288⁰C and esterification reactivity is 98% or more in step (a).

[6] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein the polymer is maintained at 2.5 torr or less for 1.5 hours or more in a latter stage of the polymerization in step (a).

[7] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein the polymer obtained in step (a) includes 95 % or more of a polyethylene glycol terephthalate repeating unit, 50 ppm or less of the unreacted terephthalic acid, and 5 ppm or less of the reduced antimony metal.

[8] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein the solid state polymer obtained in step (b) has intrinsic viscosity of 0.

9 or more. [9] The method of producing a polyester multifilament yarn for reinforcing rubber as set forth in claim 1, wherein the unstretched yarn produced in step (d) has intrinsic viscosity of 0.83 or more.

[10] A polyester multifilament yarn that is produced according to the method of any one of claims 1 to 9, the polyester multifilament yarn has intrinsic viscosity of 0.83 or more and multifilament yarn tenacity of 8.3 g/d or more, and includes 20 microequivalents/g or less of a carboxyl end group and 1.2 wt% or less of diethylene glycol.

[11] A treated cord produced using the polyester multifilament yarn of claim 10.

[12] The treated cord as set forth in claim 11, wherein the treated cord is a tire cord.