Title of Invention

POLYESTER FIBER AND METHOD OF MANUFACTURING THE SAME

Abstract

The present invention relates to a method of manufacturing polyester fiber raw yarn for processing by drawing and false twist texturing, comprising the steps of cooling polyester yarn melt-spun from spinning nozzle with cooling wind flow on an upstream side of take-up rollers to a temperature not higher than a glass transition temperature while taking-up the polyester yam by the take-up rollers at a speed not lower than 3000 m/min, and then heating the polyester yam on the a downstream side of the take-up rollers to a temperature higher than the glass transition temperature while the polyester yarn is taken up from the take-up rollers to a take-up unit for packaging, said polyester fiber formed has physical property values including a double refraction L\n of not higher than 0.07, an elongation at break of 80 to 140% and a specific gravity of 1.35 to 1.385 g/cm3.

Full Text

POLYESTER FIBER AND METHOD OF MANUFACTURING THE SAME Technical Field: This invention relates to polyester fiber and a method of manufacturing the same, and more particularly to polyester fiber used as raw yarn for drawing and false twisting and a method of manufacturing the same. Background Art: For the polyester fiber used as raw yarn for drawing and false twisting, semi-drawn yarn called "POY", which is an abbreviation of partially oriented yarn, is used widely. This semi-drawn polyester yarn is manufactured by spinning polyester in a molten state from a spinning nozzle into yarn, and cool-curing the spun yarn with cooling wind while taking up the yarn at a high speed of not smaller than 3000 m/min. The semi-drawn polyester yarn obtained by such a high-speed spinning and taking-up operation has values of physical properties including not lower than 80% in elongation at break, not higher than 0.07 in double refraction An and lower than 1.35 g/cm3 in specific gravity. In a related art method, a typical kind of 75d-36f (83dt x -36f) of drawn and false twisted yarn is manufactured by using this semi-drawn polyester yarn as raw yarn for drawing and false twisting, under the conditions including a processing speed of 900 to 1000 m/min and processing tension of 40 to 50 cN, and bulky processed yarn having residual elongation of about 20 to 25% was obtained. The residual elongation of the bulky processed yarn thus attains a comparatively high level in a range of about 20 to 25%- Therefore, even when this yarn is used in a high-order step, such as a cloth weaving step and a knitting step, a stable yarn manufacturing operation could be carried out without causing the breakage of yarn to occur. However, when the stretching and false twisting speed is increased to not lower than 1200 m/min for the purpose of further improving the' productivity with the processing tension set not lower than 55 cN for the purpose of eliminating the nonuniformity of the tension during a high-speed processing operation, nothing but bulky processed yarn having residual elongation of lower than 20% could be obtained when semi-stretched polyester yarn of the above-mentioned physical property values is used as raw yarn for drawing and false twisting. Because the residual elongation of the bulky processed yarn is lower than 20%, when this yarn is used in a high-order step. such as a cloth weaving step and a knitting step, the breakage of yarn occurs frequently, and carrying out a stable yarn manufacturing operation becomes impossible. Also, the quality of woven and knitted materials lowers * Disclosure of the Invention: The present invention provides polyester fiber as raw yarn for drawing and false twisting which is adapted to obtain bulky processed yarn of high residual elongation even when the drawing and false twisting conditions including a processing speed and processing tension are set higher than those for a similar related art drawing and false twisting process. The present invention also provides a method of manufacturing polyester fiber for raw yarn to be drawn and false twisted which is adapted to obtain bulky processed yarn of high residual elongation even when the drawing and false twisting conditions including a processing speed and processing tension are set higher than those for a similar related art drawing and false twisting process. The present invention further provides polyester fiber as raw yarn for drawing and false twisting which is made capable of obtaining bulky processed yarn of residual elongation of not lower than 20% even when a processing speed is set not lower than 1200 m/min with processing tension set not lower than 55 cN, and a method of manufacturing the same. The polyester fiber according to the present invention has values of physical properties including not higher than 0.07 in double refraction An, not lower than 80% in elongation at break and not lower than 1.35 g/cm3 in specific gravity. The method of manufacturing such polyester fiber according to the present invention includes cooling with cooling wind polyester yarn melt-spun from a spinning nozzle to a temperature not higher than a glass transition temperature while taking up the polyester yarn by take-up rollers at a speed of not lower than 3000 m/min, and then heating the resultant polyester yarn to a temperature higher than the glass transition temperature while the polyester yarn is taken up from the take-up rollers to a take-up unit to be formed a package so that the resultant polyester yarn has values of physical properties including not higher than 0.07 in double refraction An, not lower than 80% in elongation at break and not lower than 1.35 g/cm3 in specific gravity. Since the polyester fiber according to the present invention has specific gravity of not lower than 1.35 g/cm3 as mentioned above, the polyester fiber has a tensile strength high enough to prevent the occurrence of breakage of yarn even when the crystallization of the molecules in the fiber progresses to cause a drawing and false twisting speed and processing tension to increase. In addition to the high specific gravity, the double refraction An of not higher than 0-07 and elongation at break of not lower than 80% enable the increasing of the residual elongation of bulky processed yarn which has been subjected to a drawing and false twisting process to not lower than 20% to be attained. Therefore, even when this bulky processed yarn is subjected to a high-order step, such as a cloth weaving step, a knitting step and the like, a stable yarn manufacturing operation can be carried out without causing the breakage of yarn to occur. Since the breakage of yarn does not occur in a high-order step, a high-quality knitted material can be obtained. Brief Description of the Drawings: Fig. 1 is schematic diagram illustrating an apparatus for practicing the method of manufacturing polyester fiber according to the present invention; Fig. 2 illustrates a heating unit used in the apparatus of Fig. 1, and is a figure drawn in the direction of the arrowed line B-B in Fig, 3; Fig. 3 is a figure drawn along the arrowed line A-A in Fig. 2; Fig. 4 is a schematic diagram illustrating another mode of the apparatus for practicing the method of manufacturing polyester fiber according to the present invention; and Fig, 5 is a schematic diagram illustrating a drawing and false twisting apparatus; Mode for Carrying Out the Invention: The polyester fiber according to the present invention is formed of semi-drawn yarn (POY) having values of physical properties including not higher than 0.07 in double refraction An in the axial direction of the fiber, not lower than 80% in elongation at break and not lower than 1.35 g/cm3 in specific gravity. It is preferable that the elongation at break be 80 to 140%, and that the specific gravity be 1.35 to 1.385 g/cm3. The polyester fiber according to the present invention having such physical properties is suitable, especially, as raw yarn for drawing and false twisting, and can be turned into bulky processed yarn, the residual elongation of which is in a range of high levels of 20 to 25%, even when the drawing and false twisting conditions including a processing speed and processing tension are heightened. Although the polyester used in the present invention is not specially limited as long as it has fiber forming characteristics, it is preferably polyester in which a main cyclic unit is formed of ethylene terephthalate, more preferably linear polyester containing an ethylene terephthalate unit at not lower than 85%, and specially preferably linear polyester containing an ethylene terephthalate unit at not lower than 95%. The dicarboxylic acids copolymerized in the polyester and put to practical use include aromatic dicarboxylic acids, such as isophthalic acid, 2, 6-naphthalene dicarboxylic acid, diphenyl-4, 4-dicarboxylic acid and diphenoxyethane dicarboxylic acid and the like and functional derivatives thereof, aliphatic dicarboxylic acids, such as adipic acid, sebacic acid, succinic acid, glutaric acid and the like and functional derivatives thereof, and alicyclic dicarboxylic acids, such as cyclohexane dicarboxylic acid and the like and functional derivatives thereof. The glycols copolymerized in the polyester and put to use include aliphatic glycols, such as diethylene glycol, trimethylene glycol, tetramethylene glycol, neopentyl glycol and the like, alicyclic glycols, such as cyclohexane dimethanol and the like, and aromatic glycols, such as bisphenol A, alkylene oxide additives of bisphenol A. The characteristics of the polyester fiber according to the present invention reside in that the double refraction An is not higher than 0.07; elongation at break not lower than 80%; and specific gravity not lower than 1,35 g/cm3. Namely, in the related art semi-drawn polyester yarn, physical properties, i.e• the double refraction An and elongation at break have substantially the same values as those of the polyester fiber according to the present invention but the specific gravity of the former is lower than 1,35 g/cm3. Therefore, the degree of crystallization of the polyester resin according to the present invention is higher than that of the related art semi-drawn polyester yarn, and the former polyester yarn has a high tensile strength. Accordingly, even when the drawing and false twisting speed in the case of, for example, 75d-36f (83dtx-36f) which is a typical kind of yarn for drawing and false twisting is increased from 1000 m/min which has heretofore been employed in general to not lower than 1200 m/min with processing tension increased from a level of around 49 cN to not lower than 55 cN, it is possible to obtain bulky processed yarn having residual elongation of not lower than 20%, especially, a level of 20 to 25% with the executing of a stable drawing and false twisting process ensured without causing the breakage of the yarn to occur. Thus, the productivity of the yarn is improved owing to the increased drawing and false twisting speed, and the variation of tension thereof can be held down owing to the highly increased tensile strength, so that high-quality bulky processed yarn can be obtained- Furthermore, since the residual elongation of the bulky processed yarn obtained by a drawing and false twisting process is high, the bulky processed yarn does not cause the breakage thereof to occur even when the yarn is subjected to a high-order step, such as a cloth weaving step and a knitting step, and a stable yarn manufacturing operation can be carried out. Moreover, products of a high quality can be obtained in the cloth weaving and knitting steps. The above-described method of manufacturing polyester fiber includes spinning polyester in a molten state and sending out spun yarn from a spinning nozzle, cooling with cooling wind the polyester yarn melt-spun and sent out from the spinning nozzle to a temperature not higher than a glass transition temperature while taking up the polyester yarn by take-up rollers at a speed of not lower than 3000 m/min, and then heating the resultant polyester yarn to a temperature higher than the glass transition temperature while the polyester yarn is taken up from the take-up roller to a take-up unit to be formed a package so that the resultant polyester yarn has values of physical properties including not higher than 0.07 in double refraction An, not lower thane 80% in elongation at break and not lower than 1.35 g/cm3 in specific gravity. The take-up rollers are devices for restricting a take-up speed by transferring the spun yarn so as to wind around surfaces thereof, the take-up roller being driven positively by a motor. Although a single take-up roller may be provided, two take-up rollers may be preferably provided. The yarn may be wound around the respective take-up rollers alternately at a winding angle of not smaller than 90°. In the method of manufacturing polyester fiber according to the present invention, a method of cooling the melt-spun polyester yarn to a temperature not higher than a glass transition temperature is not specially limited as long as the method uses cooling wind. Such yarn cooling methods include, for example, a method of blowing cooling wind upon one side of melt-spun yarn, a method of blowing cooling wind centripetally, i.e., upon the whole circumference of melt-spun yarn toward the center thereof and the like. In the former one-side blowing method, the melt-spun yarn advances in the direction which is at right angles to the cooling wind blowing direction, and air resistance occurs. Therefore, the tension of the spun yarn being cooled is liable to increase, so that the take-up speed of the take-up rollers is preferably in the range of 3000 to 4500 m/min. This take-up speed enables the double refraction An of not higher than 0.07 and elongation at break of not lower than 80% to be attained. In the case of the latter cooling method in which the cooling wind is blown upon the whole circumference of the melt-spun yarn, the yarn can be made to travel from the spinning nozzle to the take-up rollers substantially straight* When a region in which the cooling wind is made to flow in parallel with the spun yarn is provided on the downstream side of the cooling region by utilizing this straight traveling condition of the yarn, a stress relaxing effect of reducing the tension, by the parallel flow of the cooling wind, of the spun yarn cooled to a temperature not higher than a glass transition temperature can be given thereto. Owing to this stress relaxation, the double refraction An and elongation at break can be controlled to be not higher than 0.07 and not lower than 80% respectively even when the take-up speed of the take-up roller is increased to 5000 m/min. When any of these cooling methods are employed, the polyester fiber according to the present invention is obtained by heating again the spun yarn, which was cooled to a temperature not higher than the glass transition temperature, to a temperature higher than the glass transition temperature in a subsequent step in which the yarn is taken up from the take-up rollers by a take-up unit. Heating the spun yarn to a temperature higher than the glass transition temperature in this manner enables the crystallization of the molecules in the polyester fiber to be progressed, and the specific gravity of the same fiber to be increased to not lower than 1.35 g/cm3. The methods of obtaining such polyester fiber without using a heat treatment in a step after the step in which the take-up rollers are used also include a method in which the degree of crystallization of the spun yarn is heightened by quenching the yarn with the cooling wind blown thereupon in a region which is away from the spinning nozzle by not lower than 30 mm and not higher than 150 mm. However, this method is liable to reduce the surface temperature of the spinning nozzle as compared with the above-described methods, so that the elongation at break readily decreases or varies. Since the tensile strength of the polyester fiber increases by setting the specific gravity thereof to not lower than 1.35 g/cm3, a stable yarn manufacturing operation can be carried out without causing the breakage of the yarn to occur even when the drawing and false twisting speed and processing tension of raw yarn for drawing and false twisting are increased to not lower than 1200 m/min and 55 cN respectively. Also, bulky processed yarn of residual elongation of not lower than 20%, especially, 20 to 25% can be obtained. The heat treatment for the yarn in a step after the step in which the take-up rollers are operated may be conducted in any position as long as the position is between the take-up rollers and take-up unit, A heating device (heater) may be a direct heating device with which the yarn is brought into contact directly, or an indirect heating device for indirectly heating the yarn with radiation heat or heating atmosphere. For example, a heating device may also be formed of heaters contained in take-up rollers and adapted to heat treat the yarn simultaneously with a yarn take-up operation, or a heater provided between front and rear take-up rollers and adapted to heat the yarn directly or indirectly. Fig. 1 illustrates an apparatus for practicing the method of manufacturing polyester fiber according to the present invention. Referring to Fig. 1, a reference numeral 1 denotes a spinning block, 2 a cooling unit, 3 take-up rollers, and 4 a take-up unit. The spinning block 1 is provided therein with a metering pump and a spinning pack 6, and a spinning nozzle 7 is fitted in the spinning pack 6, the spinning nozzle 7 being provided with a plurality of discharge ports 7a. The metering pump 5 is adapted to supply polyester, which is melted by a melt-extruder (not shown), in a predetermined quantity at a time to the spinning pack 6, and sent out as a plurality of filaments f from the plural discharge ports 7a of the spinning nozzle 7, A cooling body 8 of the cooling unit 2 is disposed so that the cooling body extends along one side of the filaments f spun and sent out from the spinning nozzle 7, and cooling wind is supplied in a predetermined quantity each time from a blower 9 to the cooling body 8- The cooling body 8 is provided at a front side portion thereof with a blowout port 10 having current setting plates, and the cooling wind blows out of this blowout port 10 so that the cooling wind distributes uniformly, to cool the spun filaments f. The cooling unit 2 is adapted to cool the plural spun filaments f with the cooling wind to a temperature not higher than the glass transition temperature of polyester. The plural cooled spun filaments f are given oily medicine by an oily medicine application guide 11f and collected into one piece of yarn Y, which is then taken up around the take-up rollers 3. The take-up rollers 3 include front and rear rollers 3a, 3b, and are adapted to take up the spun yarn Y at a take-up speed of not lower than 3000 m/min. Owing to this high-speed take-up operation, the double refraction An and elongation at break of the spun polyester yarn Y are controlled so that the levels thereof become not higher than 0.07 and not lower than 80% respectively. In order that the take-up speed of the spun yarn Y supplied from the spinning nozzle 7 be stabilized at a constant level, the yarn is taken up around a lower surface of the first take-up roller 3a of the take-up rollers 3 at a take-up angle of not smaller than 90°, and then around an upper surface of the second take-up roller 3b at the same take-up angle of not smaller than 90°. Between the front and rear take-up rollers 3a, 3b, a heating unit 12 is provided. This heating unit 12 is adapted to heat again the spun yarn Y, which is cooled to a temperature not higher than the glass transition temperature and thereafter travels between the two take-up rollers 3a, 3b as mentioned above, to a temperature higher than the glass transition temperature. Owing to this heat treatment, the molecules in the spun yarn Y (polyester fiber) are crystallized, by which fiber of a high tensile strength is obtained. The spun yarn Y which has been heated is taken up so as to be formed into a package P as the yarn radiates heat into the surrounding air while the yarn travels. The take-up unit 4 is adapted to support the package P on a spindle 13, and take up the spun yarn Y while rotating the package P with the surface of the package pressed by a contact roller 14, A reference numeral 15 denotes a traverse mechanism for traversing the spun yarn Y while reciprocatingly moving the same in the axial direction of the package P. Fig. 2 and Fig. 3 illustrate the heating unit 12 provided between the two take-up rollers 3a, 3b. The heating unit 12 is formed by providing electrodes 22a, 22b on both of longitudinal ends of a cross-sectionally U-shaped reflecting plate 20, and fixing a halogen lamp 21 between the electrodes 22a, 22b. The radiation heat occurring in the halogen lamp 21 directly heats the yarn Y traveling on an opened side of the reflecting plate, and the radiation heat reflected on the reflecting plate 20 indirectly heats the same, by which the yarn Y is heated to a temperature higher than the glass transition temperature. Although the illustrated heating unit 12 is a non-contacting type heating unit, a contacting type heating unit, such as a sheathed heater and the like may, of course, be used. Fig* 4 illustrates another mode of apparatus for practicing the method of manufacturing polyester fiber according to the present invention. The apparatus for manufacturing polyester fiber of Fig. 4 differs from the apparatus illustrated in Fig. 1 only in the construction of a cooling unit, and in a structure having a stress relaxation unit and provided on a lower portion of the cooling unit, and the remaining part of the former apparatus is made of the same structure as that of the latter apparatus. Referring to Fig. 4, a cooling unit 42 the construction of which is different from that of the cooling unit of Fig. 1 is formed cylindrically at a cooling body 48 thereof. Inside the cooling body 48, a cylindrical blowout port 50 formed by current setting plates is provided so that a filaments f spun from the spinning nozzle 7 pass through a central portion of this cylindrical blowout port 50. The cooling wind is supplied in a predetermined quantity at a time from a blower 49 to the cooling body 48, and blown from the cylindrical blowout port 50 uniformly upon the whole circumferences of the spun filaments d:. A stress relaxation unit 51 for the spun yarn Y is provided on a lower portion of the cooling unit 42. The stress relaxation unit 51 is provided with an inner cylinder 52 for air current extending from the blowout port 50 of the cooling unit 42 in the downward direction, and an outer cylinder 53 for air current is fixed on an outer side of the inner cylinder 52 coaxially therewith via an annular space. In addition, a stress relaxation body 54 is provided so as to surround the outer cylinder 53 for air current, to which stress relaxation body 54 the cooling wind is supplied in a predetermined quantity at a time from the blower 56* According to the apparatus of Fig. 4, the filaments f sent out from the spinning nozzle 7 are cooled at the whole circumferences thereof with the cooling wind blown out from the cylindrical blowout port 50 of the cooling unit 42, and the temperature of the filaments becomes not higher than a glass transition temperature. When the spun filaments f passes through the stress relaxation unit 51, the annular space between the inner cylinder for air current 52 and outer cylinder 53 for air current serves as an ejector mechanism, in which the cooling wind turns into a jet current flowing in the direction in which the filaments f move, the cooling wind thus contacting the filaments in parallel therewith. Accordingly, the filaments receive such a stress relaxing effect that causes the tension of the filaments which have a temperature not higher than the glass transition temperature to be lowered. Therefore, even when a take-up speed of take-up rollers 3 is increased to 5000 m/min, the double refraction An and elongation at break of the yarn Y can be maintained at not higher than 0.07 and not lower than 80% respectively. Besides the matter, the operations and effects of the take-up rollers 3 and a heating unit 12 are the same as those in the apparatus of Fig. 1. When the polyester fiber according to the present invention is used as raw yarn for drawing and false twisting, the productivity of drawn and false twisted yarn is improved, and an excellent effect is produced in the obtainment of bulky processed yarn of a high quality. The applied drawing and false twisting method is not specially limited. All of the known drawing and false twisting methods can be applied. Fig. 5 illustrates a drawing and false twisting apparatus to which the polyester fiber according to the present invention is applied as raw yarn for drawing and false twisting. In this drawing and false twisting apparatus, a heating unit 62, a cooling unit 63 and a false twister 64 are provided in the mentioned order between supply rolls 61 and drawing rollers 65, and, furthermore, a take-up unit 6 6 on the downstream side of drawing rollers 65. In this example, a disc type friction false twister is used as a false twister-While the polyester fiber Y is withdrawn from a package P by the supply rolls 61 and thereafter drawn by the drawing rollers 65, a twisting force is given thereto by the false twister 64, and the twisted state of the polyester fiber is set by the heating unit 62 and cooling unit 63. The polyester fiber is untwisted on the downstream side of the false twister 64, and turned into bulky processed yarn Y'. This bulky processed yarn Y' is taken up around the take-up unit 66. Embodiments 1 to 4: The same apparatus for manufacturing polyester fiber as was shown in Fig. 1 was used, and polyester fiber was manufactured with the number of discharge ports of the spinning nozzle, a discharge rate of polymer per one discharge port, and a temperature of the heating unit set to 36, 1.2 g/min and l8OºC respectively and the take-up speeds of the take-up rollers and take-up unit changed to 3000 m/min, 3500 m/min, 4000 m/min and 4500 m/min. The size (dtx), elongation at break (%), double refraction An and specific gravity (g/cm3) of the polyester fiber yarn obtained under each condition were measured, and the results shown on Table 1-a were obtained. Table 1-a Embodiment Take-up Size Elongation Double Specific speed (dtx) at break refraction gravity (m/min) (%) (An) (g/cm3) 1 3000 143 132 0.048 1.350 2 3500 125 111 0.057 1.360 3 4000 109 97 0.066 1.369 4 4500 97 84 0.070 1.377 The four types of polyester fiber obtained in the above-described manner and having physical properties prescribed in the present invention were used as raw yarn for drawing and false twisting. Drawing and false twisting processes were carried out by using the same drawing and false twisting apparatus as is shown in Fig. 5 with a processing speed set to 1400 m/min and a draw ratio set to a level at which processing tension became stable. The results are shown on Table 1-b. On Table 1-b, processing tension (stable processing tension) which enabled each drawing and false twisting process to be carried out stably was also shown. As is clear from the results shown on Table 1-b, when the polyester fiber having physical properties of the present invention is used as raw yarn for drawing and false twisting, it is understood that bulky processed yarn having residual elongation of not lower than 20% can be obtained stably without causing the breakage of yarn to occur even when the processing speed is set as high as 1400 m/min. Table 1-b Embodiment Processing tension Residual elongation (cN) (%) 1 60 20.1 2 58 22.5 3 60 25.2 4 59 26.4 Embodiments 5 to 7: The same apparatus for manufacturing polyester fiber as was shown in Fig. 4 was used, and polyester fiber was manufactured with the number of discharge ports of the spinning nozzle, a discharge rate of polymer per one discharge port, a speed of a parallel air current in the stress relaxation unit and a temperature of the heating unit set to 36, 1.2 g/min, 1200 m/min and l8OºC respectively and with the take-up speeds of the take-up rollers and take-up unit changed to 4000 m/min, 4500 m/min and 5000 m/min. The size (dtx), elongation at break (%), double refraction An and specific gravity (g/cm3) of the polyester fiber yarn obtained under each condition were measured, and the results shown on Table 2-a were obtained. properties prescribed in the present invention were used as raw yarn for drawing and false twisting, and drawing and false twisting processes were carried out under the same drawing and false twisting conditions as those in Embodiment 1- The results are shown on Table 2-b. The processing tension (stable processing tension) which enabled each drawing and false twisting process to be carried out stably was also shown on Table 2-b. When these drawing and false twisting processes are carried out at processing tension lower than that shown on Table 2-b by decreasing the respective draw ratios, the variation of tension increased to cause the breakage of yarn to occur, and, moreover, dyeing specks to occur in the false twisted yarn. Conversely, when such processes are carried out at tension higher than that shown on Table 2-b, by increasing the draw ratio, the amount of plumes ascribed to the breakage of single yarn increased. As is clear from the results shown on Table 2-b, when the polyester fiber having the physical property values of the present invention is used as raw yarn for drawing and false twisting, it is understood that bulky processed yarn having residual elongation of not lower than 20% is obtained without causing the breakage of yarn to occur even when the processing speed is set as high as 1400 m/min. Polyester fiber was manufactured under the same conditions as those in Embodiments 5 to 7 except that the speed of parallel air current in a stress relaxation unit was changed to 2400 m/min. The size (dtx), elongation at break (%), double refraction An and specific gravity (g/cm3) of the polyester fiber yarn obtained under each condition were measured, and the results shown on Table 3-a were obtained. this manner and having the physical property values prescribed in the present invention were used as raw yarn for drawing and false twisting, and drawing and false twisting processes were carried out under the same drawing and false twisting conditions as those in Embodiment 1, The results are shown on Table 3-b. Processing tension (stable processing tension) which enabled each drawing and false twisting process to be carried out stably was also shown on Table 3-b. As is clear from Table 3-b, when the polyester fiber having physical property values of the present invention is used as raw yarn for drawing and false twisting, it is understood that bulky processed yarn having residual elongation of not lower than 20% can be obtained without causing the breakage of yarn to occur even when the processing speed is set as high as 1400 m/min. Polyester fiber was manufactured by using the same apparatus for manufacturing polyester fiber as shown in Fig- 1 which was not provided with a heating unit, with the number of discharge ports of a spinning nozzle and a discharge rate of polymer per one discharge port set to 36 and 1.2 g/min respectively and take-up speeds of take-up rollers and take-up unit changed to 3000 m/min# 3500 m/min and 4000 m/min. The size (dtx), elongation at break (%), double refraction An and specific gravity (g/cm3) of the polyester fiber yarn obtained under each condition were measured, and the results shown on Table 4-a were obtained. this manner and not provided with the physical property values prescribed in the present invention were used as raw yarn for drawing and false twisting, and drawing and false twisting processes were carried out under the same drawing and false twisting conditions as those in Embodiment 1. The results are shown in Table 4-b. The processing tension in each drawing and false twisting process was also shown on Table 4-b. As is clear from the results shown on Table 4-b, when polyester fiber not having the physical property values of the present invention was used as raw yarn for drawing and false twisting, the breakage of yarn occurred frequently at a processing speed of 1400 m/min, and bulky processed yarn having residual elongation of not lower than 20% could not be obtained. fiber according to the present invention is not lower than 1.35 g/cm3 as mentioned above, the crystallization of the molecules in the fiber progresses, and the drawing and false twisting speed increases, so that the fiber has such tensile strength that does not cause the breakage of the yarn to occur even when the processing tension is increased- In addition to the high specific gravity, the double refraction An and elongation at break are not higher than 0.07 and not lower than 80%, so that the increasing of the residual elongation of the bulky processed yarn obtained after the drawing and false twisting operation to not lower than 20% can be done. Therefore, even when this bulky processed yarn is subjected to a high-order step, such as a cloth weaving step, a knitting step and the like, a stable operation can be carried out without causing the breakage of yarn to occur. Since the breakage of yarn does not occur in a high-order step, a high-quality knitted material can be obtained. Industrial Applicability: The present invention can be utilized for the production of polyester fiber in the fiber industries, and more particularly for the development of raw yarn for drawing and false twisting. CLAIMS 1. Polyester fiber characterized by having physical property values of double refraction An of not higher than 0,07, elongation at break of not lower than 80% and specific gravity of not lower than 1.35 g/cm3. 2. Polyester fiber according to Claim 1, wherein the elongation at break is 80-140%; and specific gravity 1.35 to 1,385 g/cm3. 3. Polyester fiber according to Claim 1 or 2, wherein the polyester fiber is raw yarn for drawing and false twisting. 4. A method of manufacturing polyester fiber, comprising the steps of cooling polyester yarn, which has been melt-spun from a spinning nozzle, with cooling wind to a temperature not higher than a glass transition temperature while taking up the polyester yarn by take-up rollers at a speed not lower than 3000 m/min, and then heating the polyester yarn to a temperature higher than the glass transition temperature while the polyester yarn is taken up from the take-up rollers to a take-up unit to be formed a package so that the values of physical properties of the polyester yarn including double refraction An, elongation at break and specific gravity become not › higher than 0.07, not lower than 80% and not lower than 1,35 g/cm3 respectively, 5. A method of manufacturing polyester fiber according to Claim 4, wherein the melt-spun polyester yarn is cooled with cooling wind blown upon one side thereof with the take-up speed of the take-up rollers set to 3000 to 4500 m/min. 6. A method of manufacturing polyester fiber according to Claim 4, wherein the melt-spun polyester yarn is cooled with cooling wind blown upon the whole of the circumference thereof, the take-up speed of the take-up rollers being set to 3000 to 5000 m/min as a stress relaxing force is given to the polyester yarn by making the cooling wind flow on the downstream side in parallel with the direction in which the polyester yarn advances. 7. A method of manufacturing polyester fiber according to Claim 4, 5 or 6, wherein the method is directed to the manufacturing of raw yarn for drawing and false twisting. 8. Polyester fiber substantially as herein described with reference to the accompanying drawings. 9. A method of manufacturing polyester fiber substantially as herein described with reference to the accompanying drawings. Dated this 28 day of May 2002

Documents:

in-pct-2002-793-che-abstract.pdf

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in-pct-2002-793-che-correspondnece-others.pdf

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in-pct-2002-793-che-description(complete)filed.pdf

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