Highly oriented undrawn polyester fibers and process for producing them

Abstract: Highly oriented undrawn polyester fibers show a diffraction peak, due to crystals of the polyester, observable in a wide angle x-ray diffraction image of the fibers and having a birefringence of 0.015 to 0.05. A process for preparing highly oriented undrawn core-sheath type conjugated polyester fibers which may have such diffraction and birefringence properties comprises the step of spinning a polyester as the sheath polymer and a polymer having a gradient of elongational viscosity with temperature larger than that of the sheath polymer, at a spinning speed of 4000 to 12000 m/min. The fibers may be falsetwisted to obtain a textured yarn.

Description: The present invention relates to highly oriented undrawn polyester fibers, in particular, to fibers ideal for clothing and industrial materials such as flat yarn, twisted yarn or draw-falsetwist textured yarn. Polyester fibers are widely used for clothing and industrial materials because of their various excellent mechanical and other properties. Among them, polyethylene terephthalate (PET) is a typical polyester used for general purposes. In recent years, a high speed spinning method for obtaining practical PET fibers in one step at a high take-up speed of 5000 m/min or more without drawing has been industrially adopted. Since the productivity in the step of spinning greatly depends on the quantity discharged per unit time, a higher spinning speed can achieve a higher productivity in the one-step method. However, for example, PET fibers show practically preferable mechanical properties when they are spun at a high speed of 6000 to 7000 m/min, but at higher speeds the strength and elongation of the fibers are lowered. Therefore, there is a limit to the spinning speed which can sufficiently exhibit the effect of productivity improvement. In this regard, it has been proposed to blend a small amount of a polymer incompatible with the matrix polymer for spinning. For example, JP-A-58-098414 and JP-A-60-209015 disclose spinning methods for controlling the molecular orientation by adding 0.1 to 10 wt% of a polymer incompatible with the matrix polymer. Furthermore, JP-A-57-011211 discloses a method of adding a liquid crystal polymer. Moreover, JP-A-56-091013, JP-A-57-047912 and JP-A-62-021817 disclose methods for controlling the molecular orientation by adding a small amount of a polyolefin based polymer to a polyester. However, in these methods, the added polymer exerts adverse influence even though the molecular orientation is controlled. For example, when a polymer having low softening temperature such as polystyrene is added, the added polymer existing in the surface layer of fibers may cause the fibers to adhere to each other during the falsetwist texturing under a high temperature. In addition, the coloring of dyed fibers may be poor. Moreover, it is very difficult to blend homogeneously a small amount of a different polymer with a polyester. The lack of homogenity in the blend may cause frequent fiber breakage, and the fabric thus obtained may not be sufficient in color uniformity after dyeing. In application for clothing, falsetwist textured yarns are usually used to provide moderate bulk to a fabric. Recently, draw texturing has become a common process where highly oriented undrawn fibers, for example, those of a now conventional so-called "partially oriented yarn" or "POY" (but which in any event are regarded as highly oriented) are simultaneously drawn and falsetwisted. To improve the process stability and processing speed in the draw texturing process, the ballooning of the yarn along the thread line must be stabilized. It is known that this stabilization can be achieved at a higher twist tension, and/or higher drawing ratio and the use of more highly oriented fibers. However, if the drawing ratio is increased to an excessively high value or if excessively highly oriented fibers are used, fluff and frequent fiber breaking occur, to lower the quality of the textured yarns obtained, and also to inconvenience the operation disadvantageously. Therefore, in general, the upper limit of the spinning speed for the undrawn yarn which can be textured with a moderate twist tension is about 4000 m/min. Furthermore, in the melt spinning of a polyester, it is known that at higher than a critical spinning speed, neck-like deformation occurs on the spinning line, to cause orientation and crystallization, and that fibers similar to conventional drawn fibers can be obtained. For example, it is well known that, in the case of PET, this critical spinning speed is about 4500 to 5000 m/min. Since the fibers obtained at higher than the critical spinning speed do not provide the so-called POY, they cannot be processed by draw texturing under a draw ratio of, for example from 1.2 to 2.0, even though they can be textured without being drawn. A feature of the draw texturing process is that highly oriented undrawn yarn with a large denier, selected in dependence upon the drawing ratio to be employed in the draw texturing can be spun at a high speed. Therefore, a high productivity in spinning can be realized. However, conventionally, polyester fibers spun at a spinning speed higher than 4000 m/min, could not be stably processed even by draw texturing. The conventional POY is only little crystallized as can be seen from its boil off shrinkage of higher than 50% and the absence of wide angle X-ray diffraction peak, which would have indicated the presence of crystals of the polyester. For this reason, the fiber structure is unstable and changes under storage conditions. Especially to maintain uniformity in dyeing, the storage time of POY must be controlled to be constant. Furthermore, since the structural change of the fibers during storage is different between the inner portion and the outer portion of each package, the quality is disadvantageously uneven. Therefore, to maintain uniform quality, heat treatment at a certain elevated temperature must be applied. The present invention addresses the problem of providing highly oriented undrawn polyester fibers which at most exhibit only a small change in fiber structure during storage, which due to their stable structure have good process stability and processability in draw texturing and which are therefore capable of contributing to improvement of productivity. The invention also addresses the problem of providing highly oriented undrawn polyester fibers capable of contributing to the improvement of productivity by increase of output and of providing a falsetwist textured yarn obtained from the highly oriented undrawn polyester fibers, and a process for producing them. The above mentioned problems addressed by the present invention can be solved by highly oriented undrawn polyester fibers capable of exhibiting a diffraction peak, due to crystals of the polyester, observable in wide angle X-ray diffraction, and having a birefringence, due to orientation of the polyester, of 0.015 to 0.05, and a process for producing them. Thus, according to a first aspect, the present invention provides a highly oriented undrawn core-sheath type polyester fiber in which the sheath polymer comprises a polyester and the core polymer is a polymer having a gradient of elongational viscosity with temperature larger than that of the sheath polymer and is conjugated, with the sheath polymer, in an amount of from 1 to 15 wt% based on the weight of the entire fiber and without the core polymer being exposed on the surface of the fiber, which fiber is crystalline such that the fiber is capable of exhibiting a diffraction peak observable in the equatorial intensity distribution of a wide angle X-ray diffraction image of the fiber and at least the polyester portion of the fiber cross section has a birefringence of 0.015 to 0.05. According to a second aspect, the invention provides a process for preparing a highly oriented undrawn core-sheath type conjugated polyester fiber, the process comprising: spinning a polyester as the sheath polymer and, as the core polymer, a polymer having a gradient of elongational viscosity with temperature larger than that of the sheath polymer, at a spinning speed of 4000 to 12000 m/min, such that the core polymer is conjugated with the sheath polymer in an amount of from 1 to 15 wt% based on the weight of the entire conjugated fiber and without the core polymer being exposed on the surface of the fiber, the fiber is capable of exhibiting a diffraction peak observable in the equatorial intensity distribution of a wide angle X-ray diffraction image of the fiber, and at least the polyester portion of the cross section of the fiber obtained has a birefringence of 0.015 to 0.05. Aspects of the invention, and embodiments thereof, will now be described in detail with reference to the accompanying drawings in which Fig. 1 is a diagram showing intensity distributions in the equatorial direction of wide angle X-ray diffraction images of the fibers of the present invention. Fig. 2 is a diagram showing the relation between birefringence and % boil off shrinkage of fibers obtained using polystyrene (Styron 685) as the core polymer (closed circles) and the fibers obtained using PET only as the polymer (open squares). Fig. 3 is a drawing typically showing the method of measuring the birefringence of the polyester portion. According to the first aspect of the present invention a highly oriented undrawn polyester fiber shows a wide angle X-ray diffraction peak due to crystals of the polyester. The structure containing such crystals may be regarded as a plurality of microcrystallites distributed throughout a generally amorphous polyester. Such a fiber structure is stable during storage, and the drawing ratio in the draw texturing of the fibers can be as large as that conventionally employed in POY production. Furthermore, a high twist tension can be employed. Especially when the polyester is PET, a proportion of the main diol and/or acid component of the polyester can be substituted by another copolymerizable component up to 15 mol% of total polyester. The polyester can also contain such additives as a delustering agent, flame retarding agent, antistatic agent and a pigment. There have been many studies concerning the high speed spinning of polyesters, especially of PET. For example, according to Shimizu, et al. (Journal of Textile Machinery Society, (1985), 38 , 243), up to a spinning speed of 4000 m/min, the fiber is not deformed to form a neck on the spinning line, and wide angle X-ray diffraction analysis of the fiber shows only an amorphous structure. The fiber spun at a speed higher than 5000 m/min at which neck-like deformation occurs shows the diffraction peak due to crystals of the polyester. Therefore, it can be said that since the spinning speed of conventional PET POY is less than 4000 m/min, the fiber is amorphous as evaluated by wide angle X-ray diffraction. On the contrary, the highly oriented undrawn fiber of the present invention shows a peak corresponding to the diffraction angle of PET crystals in the equatorial intensity distribution of its wide angle X-ray diffraction image (Fig. 1). That is, it is shown that oriented crystals of PET exist. However, since the peak is weak in intensity compared to the peak of the conventional drawn fibers, it is estimated that the amount of crystals is small. The highly oriented undrawn polyester fiber has a cross-section at least a polyester portion of which comprises (and preferably consists of) the polyester, at least which portion (and preferably the entire fiber) has a birefringence of 0.015 to 0.05. If the birefringence is lower than 0.015, it is difficult to string up the yarn at the start of draw texturing and the fibers obtained tend to adhere to each other. On the other hand, if higher than 0.05, the twist tension is so high as to cause, undesirably fluff and frequent fiber breaking. A preferred range of birefringence values is 0.02 to 0.045. The birefringence of at least the polyester portion of the highly oriented undrawn fibers of the present invention is 0.05 or less, about the same as or lower than that of the conventional POY. Therefore, the fibers of the present invention are estimated to have a new structure not previously suggested, in which the amorphous POY structure is dotted with a very small amount of crystals. The birefringence referred to here is the birefringence due to orientation of the polyester. The percentage boil off shrinkage of the fibers in boiled water is preferably 10 to 50%. In this range, since oriented crystallization does not progress so much, the drawability and thermosettability in the draw texturing process are good. Furthermore, if the shrinkage percentage is in this range, the polyester crystals as a feature of the present invention are properly developed, to stabilize the fiber structure, maintaining its change during storage small. The boil off shrinkage is more preferably 20 to 50%. In general, amorphous fibers high in their degree of orientation show a high boil off shrinkage corresponding to the degree of orientation. If the degree of orientation becomes so high as to initiate the formation of crystal nuclei, the boil off shrinkage reaches a ceiling, and at a higher degree of orientation with further crystallization caused, the boil off shrinkage declines suddenly. As described here, the crystallinity of fibers is well reflected by the boil off shrinkage. The relationships between the birefringence and the boil off shrinkage of ordinary PET fibers and fibers embodying the present invention are shown in Fig. 2. In the case of ordinary PET, the boil off shrinkage declines suddenly when the birefringence is in a range from about 0.075 to 0.085, but in the case of the fibers embodying the present invention, the boil off shrinkage percentage begins to decline greatly at a birefringence of about 0.015, and further suddenly declines in a range from about 0.035 to 0.045. This shows that the fibers of the present invention begin to be crystallized already when the birefringence is still low, compared with ordinary PET. This agrees with the fact that a wide angle X-ray diffraction peak due to crystals of the polyester is observed. In the conventional fibers, the degree of crystallinity is almost 0% at a birefringence of 0.05, but in the highly oriented undrawn fibers of the present invention, the degree of crystallinity is 10% at a birefringence of 0.033 (Experiment No. 2). This shows that crystals are produced already at a low degree of orientation and again agrees with the fact that a wide angle X-ray diffraction peak, due to crystals of the polyester is observed. The "degree of crystallinity" of a polyester referred to herein is the value determined by Raman spectroscopy. It is stated in J. Polym. Sci., (1972), 10 , 317 that the half value width of the peak at 1730 cm -1 due to the carbonyl portion of a polyester is inversely proportional to the density of the polyester. The half value width of the peak at 1730 cm -1 was obtained by Raman spectroscopic analysis, and from the density value determined by it, the degree of crystallinity was calculated. As described above, the fibers of the present invention have a feature that crystals coexist in an amorphous structure as large as the conventional POY in the degree of orientation. That is, it is surmised that a stable network structure is formed, in which amorphous sea is dotted with a very small amount of crystals. Because of this, it is estimated that the change of the fiber structure during storage is small, and that the twist tension becomes higher than that of the conventional POY during deformation in the draw texturing process. Thus, the process stability and processability in the draw texturing process is improved, and the processing speed can be raised. The elongation at break of the highly oriented undrawn fibers of the present invention is preferably 100 to 250%. In this range, it is easy to string up the fibers in the draw texturing process, and non-untwisted spots due to adhesion, fluff and fiber breaking do not occur. Furthermore, the drawing ratio can be set preferably as high as that of the conventional POY. Moreover, the draw textured yarn obtained is less deformed in the cross section, and is free from rough feeling and shows moderate luster. A more preferable elongation range is 100 to 200%. Highly oriented undrawn fibers with such high drawing potential can enhance productivity in spinning. A second aspect of the present invention is a process for producing highly oriented undrawn polyester fibers each having a cross section a polyester portion of which comprises (and preferably consists of) the polyester and another polymer portion of which has a gradient of elongational viscosity with temperature greater than that of the polyester portion. Such fibers have the form of core-sheath type conjugated polyester fibers. The sheath component is the polyester portion. The core component is the polymer having a gradient of elongational viscosity with the temperature larger than that of the sheath polyester portion. The spinning speed in such a process the present invention is from 4000 to 12000 m/min. The gradient of elongational viscosity with temperature can be compared, for example, as described below. The polymers to be compared are spun separately under the same spinning conditions (spinning machine, pack, nozzle hole diameter, number of filaments, cooling condition, spinning speed, etc.), to be the same in final fiber diameter, and the respective fiber speeds or fiber diameters are measured along the spinning line. The polymer which is deformed more upstream (closer to the nozzle face) in the spinning line can be judged to have a larger gradient of elongational viscosity with temperature. Polymers, the gradient of elongational viscosity with temperature of which are larger than that of the polyesters to be used, for example, PET, include, for example, polystyrene, polymers of styrene derivatives such as α-methylstyrene, p-methoxystyrene and chlorostyrene, copolymers with styrene, polystyrene based polymers such as styrene-acrylonitrile copolymer, polyacrylate based polymers such as polymethyl methacrylate, polyethyl methacrylate, polypropyl methacrylate, bolybutyl methacrylate and polyethylhexyl methacryalte, acrylate-styrene copolymers such as copolymers of these polyacrylate based polymers and polystyrene based polymers, polymethylpentene and polymethylpentene based polymers obtained by copolymerizing methylpentene and an olefin, etc. Among them, polystyrene, polymethyl methacrylrate and polymethylpentene are preferable having regard to handling convenience and the orientation control effect described later. Polystyrene is especially preferable. Considering the balance of the fiber properties, the amount of the core-polymer in the conjugated fiber of the present invention is from 1 to 15 wt%. If the amount is more than 15 wt%, the influence of the core polymer may be exerted to make the fiber poor in mechanical properties. If the amount is less than 1 wt%, the residence time in the spinning machine and pack may need to be so long that the polymers thermally deteriorate. A more preferable range is 2 to 7 wt%. In the present invention, as the core polymer, any of the above polymers can be used singly or blended with the polyester used as the sheath polymer or any other polymer. When any polymer is blended with a specific polymer having a larger gradient of elongational viscosity with temperature, the content of the specific polymer in the blend is preferably 30 to 70 wt%. It is preferable to conjugate so that the content of the specific polymer in the entire conjugated fiber becomes 1 to 15 wt%. The conjugated amount in the blend refers to the amount based on the entire conjugated fiber. The cross sectional form of the fiber and the conjugated form of sheath and core are not especially limited. Plural cores may be adopted to form a structure of sea and island. However, usually, considering the decline of spinnability caused by the bending of the discharged fiber, it is preferable to avoid eccentricity and to arrange the core substantially symmetrically with reference to the fiber axis. What is important in a process aspect of the present invention is that the core polymer is conjugated as the core in the sheath, without being exposed on the surface of the fiber. JP-C-43-023879 discloses core-sheath type conjugated fibers consisting of a thermoplastic amorphous polymer as the core and a thermoplastic crystalline polymer as the sheath. However, it simply shows a yarn of core/sheath = amorphous polymer/crystalline polymer. It does not state or suggest anything about the combination of polymers different in their respective gradients of elongational viscosity with temperature. In addition, the technique is characterized by cold-drawing a fiber spun at a low speed, for partially cutting the core polymer, and there is no control of molecular orientation by high speed spinning. Furthermore, the amount of the conjugated core polymer is 20 wt% or more based on the weight of the entire conjugated fiber. If the technique is applied to a polyester, the spun fiber obtained is smaller than 0.015 in birefringence and larger than 50% in boil off shrinkage. Thus, the subject matter of the present invention is clearly distinguished, and addresses problems different from, the disclosure in JP-C-43-023879. Considering the processability in the draw texturing process, it can be considered to be better that the fibers obtained are oriented to some extent. For this reason, the spinning speed is 4000 m/min to 12000 m/min, more preferably 4000 m/min to 9000 m/min, further more preferably 5000 m/min to 9000 m/min. If the spinning speed is higher than 12000 m/min, the fiber obtained may decline in residual elongation, to cause inconvenience during such operations as winding. If the core-sheath type conjugation spinning is effected, the orientation of the sheath polyester is controlled, and the highly oriented undrawn fiber can be obtained even by high speed spinning. The orientation control mechanism is considered to be as described below. Since the core polymer has a larger gradient of elongational viscosity with temperature, it is prone to become finer earlier (more upstream in the spinning line) that the sheath polyester. Thus, the sheath polyester is deformed to follow the deformation of the core polymer. That is, compared with the case of spinning the sheath polyester alone, the sheath polymers is forcibly deformed at a higher temperature (when the elongational viscosity is lower), and the spinning stress during deformation is lower than that in the case of spinning the polyester itself without core polymer. Since the spinning stress during deformation decides the orientation of the polymer, the orientation of the sheath polymer is controlled as a result. The degree of orientation control effect depends on the difference between the energy required for the core polymer to deform at a given temperature and the energy required for deforming the sheath polyester. Therefore, if the gradient of elongational viscosity with temperature of the core polymer and its absolute value are higher than those of the sheath polyester, the deformation is caused when the sheath polyester is lower in elongational viscosity, i.e., higher in temperature. Moreover, since large energy is given to the sheath polyester as a result, the orientation control effect is large. Furthermore, if the amount of the conjugated core polymer is larger, the orientation control effect is larger. Thus, the highly oriented undrawn fiber of the present invention can be obtained in a far higher spinning speed range of 4000 to 12000 m/min than the conventional POY. It is also an advantage of the present invention that the so called POY can be produced at an efficiency of about double that of the conventional POY. The highly oriented undrawn fiber of the present invention can also be obtained by ultrahigh speed spinning at higher than 12000 m/min by selecting the polymer adopted as the core and its conjugated amount to increase the orientation control effect as described above, but since the equipment such as an ultrahigh speed winder suitable for winding the undrawn fiber at a speed higher than 12000 m/min is costly, the higher productivity will be decreased. Moreover, since the polyester fibers with the new structure obtainable utilising the present invention are formed under the orientation control mechanism as described above, it is desirable properly to select the amount of the conjugated core polymer, for obtaining an optimum orientation control effect at the intended spinning speed by the polymers used. It is reported in Sen-i-Gakkaishi, (1995), 51 , 408 that the core-sheath type conjugated fibers consisting of PET as the core and 50 wt% of polystyrene as the sheath are lowered in their orientation of PET. However, the technique is different from that of the present invention, since the wide angle X-ray diffraction photos do not show the existence of crystals. Moreover, the technique does not state the percentage boil off shrinkage of the conjugated fibers obtained or drawing at all. Furthermore, if the conjugated fibers with polystyrene as the sheath are simultaneously drawn and falsetwisted at a high temperature, fusion occurs in the polystyrene portion, not allowing the object of the present invention to be achieved. In addition, it is very difficult to draw polyester fibers with a large amount of polystyrene conjugated, as is used in that technique. For example, according, for example, to JP-A-50-157617, if undrawn polyester fibers with 30 wt% of polystyrene as the core are drawn, the core polystyrene is partially cut and becomes uneven in thickness. Therefore, if undrawn conjugated polyester fibers with polystyrene as the sheath are drawn, the sheath is broken, and no satisfactory fibers can be obtained. In this regard, in the present invention, since the specific polymer is small in amount and confined as the core component of the fibers, they can be drawn like the conventional POY, and such problems as fusion do not arise during false twisting. If the highly oriented undrawn polyester fibers obtained in the present invention are draw textured, the process stability and processability are improved advantageously as described before. Furthermore, since the twist tension can be set at a high value, the processing speed can be raised, to also improve the productivity in the draw texturing process. Moreover, thus obtained textured yarn shows crimp characteristics as good as the conventional textured yarn, and it is advantageously smaller in density, lighter in weight and higher in heat resistance, because it has a melting point higher than that of the conventional POY. Polyester fibers embodying the present invention can be ideally used for clothing as a flat yarn, twisted yarn or textured yarn. They can also be used for industrial materials. Embodiments of the present invention will now be described in more detail with reference to the following Examples. The methods of measuring the polymer and filament properties employed in the Examples are as described below. A. Intrinsic viscosity [η] Measured in orthochlorophenol at 25°C. B. Tensile strength and elongation at break According to JIS L 1013, a load-elongation curve was obtained at a sample length of 50 mm at a cross head speed of 50 mm/min using a tensile testing machine produced by Orienteck. Then, the load was divided by the initial denier (thickness) of the fiber, to be expressed as the strength, and the elongation was divided by the initial sample length, to be expressed as the elongation. C. Boil off shrinkage A yarn in skein form was immersed in 98°C boiled water for 15 minutes, and the lengths before and after the immersion were measured, to calculate the boil off shrinkage from the following formula: Boil off shrinkage=[(Length before immersion-Length after immersion)/Length before immersion]x100. D. Wide angle X-ray diffraction Model 4036A2 X-ray generator produced by Rigaku Denki was used for measuring in the equatorial direction with CuKα rays (using an Ni filter) as the ray source. The output was 40 kV and 20 mA, and the slit system was 2 mm in diameter. The integrating time was 2 seconds. The diffraction intensity curve obtained was processed to be made smooth according to Savitzky and Golay's smoothing method (Analytical Chemistry, (1964), 36(8 ), 1627). E. Birefringence of polyester The birefringence of the polyester portion was obtained as described below (Fig. 3), using a BH-2 polarization micrometer produced by Olympus. From the retardation Γ near the interface between the sheath polyester and the core polymer and the optical path length d of the polyester portion, the birefringence of the polyester portion was calculated as Γ/d. The optical path length d was calculated at the retardation measuring position located at a distance b from the surface of the filament. In the case of a filament made of a polyester alone, it was obtained from Γ at the center of the filament and the filament diameter. F. Degree of crystallization of polyester For the degree of crystallization(c) of the polyester portion, the half value width (Δv) at 1730 cm -1 was obtained from Raman spectropic analysis, and the density (ρ) was determined from it. Then, the degree of crystallization was calculated using the following equation. ρ=(305-Δv)/209 c=100x(ρ-1.335)/(1.455-1.335) In the above, the density of the perfectly amorphous polyester was assumed to be 1.335 g/cm 3 , and the density of the perfectly crystalline polyester, 1.455 g/cm 3 .The Raman spectrum was analyzed by applying a laser beam onto the lateral side of the filament with Ar + laser (514.5 nm) as the light source by Jobin Yvon Ramanor T-64000. G. Melting point The melting point was measured by differential scanning calorimetry (DSC). The measurement steps were as follows. The sample was chopped fine, allowed to shrink freely and measured by DSC-2C produced by Perkin Elmer. The heating rate was 16°C/min, and the weight of the sample was 10 mg. H. Crimp rigidity (CR) The yarn was wound 5 turns on a spool to make a skein, and it was allowed to freely shrink in 90°C water for 20 minutes. Then, the skein was taken out of the water and dried in air overnight. Subsequently in 20°C water, an initial load and an additional load were applied to the skein, and two minutes later, the length of the skein was measured (L 0 ). The additional load was immediately removed, and a further two minutes later, the length of the skein was measured in water (L 1 ). From the following formula, the crimp recovery rate was calculated as CR value. CR=(L 0 -L 1 )/L 0 ×100(%) The initial load was (Number of deniers of filament)x5x2/25 grams, and the additional load was (Number of deniers of filament)x2 grams. I. Color uniformity after dyeing The textured yarn was knitted into a fabric which was dyed by a blue disperse dye, and the color uniformity was visually judged. J. Density of textured yarn The density of the textured yarn as a whole was measured by density gradient tubes of aqueous sodium bromide solutions at 25°C. Example 1 PET of 0.63 in intrinsic viscosity and polystyrene (Styron 685 produced by Asahi Chemical Industry Co., Ltd.) as a polymer having a larger gradient of elongational viscosity with temperature than that of PET were selected. These two polymers were made molten separately, being filtered by a stainless steel nonwoven fabric filter of 10 µm in absolute filtration diameter. Then, the polystyrene and PET were discharged from a die with 36 concentric holes for conjugating polystyrene as the core and PET as the sheath. The amount of polystyrene as conjugated in this case was 5 wt%. The spinning temperature was 295°C, and the discharged amount was adjusted to be 90 g/min as total of core and sheath. The discharged filaments were cooled, oiled, interlaced, and wound by a winder through a take-up roller according to conventional methods. The speeds of the take-up roller are shown as spinning speeds in Table 1 (Nos. 1 and 2). The birefringences, boil off shrinkages and elongations of the samples are shown in Table 1. Furthermore, the relation between the birefringence of the PET portions and the boil off shrinkage is shown in Fig. 2. In both the cases, the wide angle X-ray diffraction peak due to the crystals was observed in the halo, to show that PET crystals exist. The diffraction intensity curve in the equatorial direction of the fibers spun at a speed of 6000 m/min (No. 2) is shown as curve (a) in Fig. 1. The degree of crystallization was 10% in the case of the fibers spun at a speed of 6000 m/min (No. 2) while that of the conventional POY (No. 15) was 0%. Also from this, it can be confirmed that the fibers were crystallized, and the crystals stabilized the fiber structure. Even though crystals existed, the birefringence of the PET portion was low, to show that orientation did not progress, as can be confirmed from Table 1. Therefore, the fibers obtained are highly oriented undrawn fibers having a large elongation at break and capable of being drawn. It can be seen that the so called POY can be produced by high speed spinning for contribution to the enhancement of productivity. Example 2 Fibers were melt spun under conditions similar to those shown in Example 1, except that polystyrene used was Denka Styrol MT-2 produced by Denki Kagaku Kogyo K.K. and that the spinning speed was as shown in Table 1 (Nos. 3 to 6). The birefringences, elongations and boil off shrinkages are shown in Table 1. At every spinning speed, the wide angle X-ray diffraction peak due to crystals of the PET was observed in the halo, to show that PET crystals existed. Even though crystals existed, the birefringence of the PET portion was low, to show that orientation did not progress, as can be confirmed from Table 1. Example 3 Fibers were melt spun under conditions similar to those shown in Example 1, except that the amount of polystyrene and spinning speed were changed as shown in Table 1 (Nos. 7 to 9). The birefringences, elongations and boil off shrinkages are shown in Table 1. Also in these cases, the wide angle X-ray diffraction peak due to crystals of the PET was observed in the halo, to show that PET crystals existed. Even at spinning speeds of 10000 m/min or more, fibers embodying the present invention could be obtained. Example 4 Fibers were melt spun under conditions similar to those as shown in Example 1, except that polymethyl methacrylate (Sumipex LG produced by Sumitomo Chemical Co., Ltd.) was used instead of polystyrene (No. 10). The birefringence, elongation and boil off shrinkage are shown in Table 1. The wide angle X-ray diffraction intensity curve in the equatorial direction is shown as curve (b) in Fig. 1. Even if polymethyl methacrylate were used as the core component, the wide angle X-ray diffraction peak due to crystals of the PET was observed, to show that fibers embodying the present invention could be obtained. Example 5 Fibers were melt spun under conditions similar to those shown in Example 1, except that polymethylpentene ("TPX" RT18 produced by Mitsui Petrochemical Industries, Ltd.) was used instead of polystyrene, and that its amount conjugated was 3 wt% (No. 11). The birefringence, elongation and boil off shrinkage are shown in Table 1. Also in this case, the wide angle X-ray diffraction peak due to crystals of the PET was observed in the halo, and it can be seen that even if polymethylpentene were used, fibers embodying the present invention can be obtained. Comparative example 1 Fibers were melt spun under conditions similar to those shown in Example 1, except that the polymer used was only the PET used in Example 1 (Nos. 12 to 17). At every spinning speed, properties of typical PET fibers are shown. At spinning speeds of 5000 m/min or higher, remarkable oriented crystallization occurred, and the boil off shrinkage suddenly declined. The birefringences, boil off shrinkages, and residual elongations are shown in Table 1. The wide angle X-ray diffraction curve in the equatorial direction of the fibers obtained at a spinning speed of 3500 m/min (No. 14) is shown as curve (c) in Fig. 1., from which it can be seen that no diffraction peak was observed. Likewise, Sample Nos. 12, 13 and 15 (also spun at low spinning speed) showed no diffraction peak. Furthermore, the relation between the birefringence and the boil off shrinkage is shown in Fig. 2. The degree of crystallization at a spinning speed of 3500 m/min (No. 14) was 0% and agreed with the fact that only an amorphous halo was observed in its wide angle X-ray diffraction curve. Comparative Example 2 Fibers were melt spun under conditions similar to those shown in Example 1, except that polyethylene (Sumikathene L produced by Sumitomo Chemical Co., Ltd.) having a gradient of elongational viscosity with temperature smaller than that of PET was used instead of polystyrene (No. 18). Fully oriented crystallization occurred, and the wide angle X-ray diffraction peak due to crystals of the PET was observed. However, the birefringence was as high as 0.085, and fibers embodying the present invention could not be obtained. Example 6 Fibers were melt spun under conditions similar to those as shown in Example 1, except that the amount of polystyrene (Styron 685 produced by Asahi Chemical Industry Co., Ltd.) was changed to amounts in a range from 1 to 10 wt% and that the spinning speed was 6000 m/min. The birefringences, elongations and boil off shrinkages are shown in Table 1. When the amount of polystyrene was larger, the effect of controlling the orientation of PET was higher. The relation between the birefringence and the boil off shrinkage is shown in Fig. 2. When the amount of polystyrene is in this range, the wide angle X-ray diffraction peak due to crystals of the PET was observed in the halo, to show that PET crystals existed. From the results, it can be seen that when the amount of polystyrene is in this range, fibers embodying the present invention can be obtained. Comparative example 3 Fibers were melt spun under conditions similar to those shown in Example 1, except that the amount of polystyrene (Styron 685 produced by Asahi Chemical Industry Co., Ltd.) was 0.5 or 17 wt%, and that the spinning speed was 6000 m/min (Experiment Nos. 22 and 23). The birefringences, elongations and boil off shrinkages are shown in Table 1. The relation between the birefringence and the boil off shrinkage is shown in Fig. 2. When the amount of 0.5 wt%, fully oriented crystallization occurred, and the wide angle X-ray diffraction peak due to crystals of the PET was observed, even though the birefringence was as high as 0.098. On the other hand, when the amount of 17 wt%, no diffraction peak was observed. From the results, it can be seen that when the amount of polystyrene is too large or too small, fibers embodying the present invention cannot be obtained. Example 7 The highly oriented undrawn fibers obtained in Example 1 (No. 2 at a spinning speed of 6000 m/min) were draw textured at a heater temperature of 215°C, at a twister speed of 6800 rpm and at a drawing ratio of 1.8 times. The results obtained at a processing speed of 700 m/min are shown in Table 2. The results obtained at a processing speed of 1500 m/min are shown in Table 3. As can be seen from the tables, if the highly oriented undrawn fibers embodying the present invention are used, a textured yarn low in density and high in heat resistance can be obtained. In addition, since the twist tension was high, the process stability and processability were good and the color uniformity of the textured yarn obtained after dyeing were also good even if the processing speed were raised. Thus, the present invention could contribute to the enhancement of productivity not only in the spinning step but also in the draw texturing step. Comparative example 4 The highly oriented undrawn fibers of PET only, obtained in Comparative Example 1 (No. 14 at a spinning speed of 3500 m/min), were falsetwisted under the same conditions as in Example 7. The results obtained at a processing speed of 700 m/min are shown in Table 2. The results obtained at a processing speed of 1500 m/min are shown in Table 3. Since the twist tension was low at a drawing ratio of 1.8 times, ballooning was not stabilized in the draw textured zone, showing poor process stability. The textured yarn obtained had extremely darkly dyed portions and streaks, showing lack of color uniformity. When the drawing ratio was raised to 1.9 times, to increase the twist tension, fluff and frequent fiber breaking occurred, to reduce considerably the processability. No Core polymer Amount of core polymer (wt%) Spinning speed (m/min) Strength Elongation Boil off shrinkage Birefringence Example 1 1 Polystyrene (Styron 685) 5 5000 2.1 173 43 0.020 2 6000 2.2 150 34 0.033 Example 2 3 Polystyrene (Denka Styrol MT-2) 5 6000 2.3 191 40 0.024 4 7000 2.3 165 40 0.033 5 8000 2.4 132 38 0.040 6 9000 2.6 115 35 0.044 Example 3 7 Polystyrene (Styron 685) 10 8000 2.0 190 45 0.020 8 14 10000 2.0 195 44 0.020 9 12000 2.0 180 40 0.025 Example 4 10 Polymethyl methacrylate 5 6000 2.5 130 28 0.038 Example 5 11 Polymethypentene 3 6000 2.1 110 21 0.045 Comparative example 1 12 PET - 2000 1.6 280 62 0.017 13 3000 2.5 190 62 0.033 14 3500 2.7 145 62 0.050 15 4000 3.0 120 56 0.059 16 5000 3.4 71 4 0.086 17 6000 3.7 53 4 0.103 Comparative example 2 18 Polyethylene 5 4000 3.1 55 6 0.085 Example 6 19 Polystyrene (Styron 685) 1 6000 2.9 103 11 0.45 20 2 2.6 116 20 0.039 21 10 1.6 215 49 0.015 Comparative example 3 22 0.5 6000 3.7 58 4 0.098 23 17 1.0 320 62 0.005 Twist tension cN Strength cN/dtex Elongation % Boil off shrinkage% CR % Melting point °c Density g/cm 2 Example 7 41 4.1 24 5 46 258 1.369 Comparative example 4 32 4.1 23 5 47 256 1.396 Drawing ratio Twist tension cN Process stability and passability Dye mottles of textured yarn Example 7 1.8 44 good good Comparative example 4 1.8 34 Unstable ballooning, fluff and frequent fiber breaking Darkly dyed portions and streaks 1.9 44

Claims: A highly oriented undrawn core-sheath type polyester fiber in which the sheath polymer comprises a polyester and the core polymer is a polymer having a gradient of elongational viscosity with temperature larger than that of the sheath polymer and is conjugated, with the sheath polymer, in an amount of from 1 to 15 wt% based on the weight of the entire fiber and without the core polymer being exposed on the surface of the fiber, which fiber is crystalline such that the fiber is capable of exhibiting a diffraction peak observable in the equatorial intensity distribution of a wide angle X-ray diffraction image of the fiber and at least the polyester portion of the fiber cross section has a birefringence of 0.015 to 0.05. A fiber according to claim 1, having a boil off shrinkage of 10 to 50%. A fiber according to claim 1 or 2, having an elongation at break of 100 to 250%. A fiber according to any preceding claim, wherein the amount of the core polymer, conjugated with the sheath polymer, is 1 to 10 wt%. A fiber according to any preceding claim, wherein the core polymer is at least one polymer selected from polystyrene based polymers, polyacrylate based polymers, acrylate-styrene copolymers and polymethylpentene based polymers. A process for preparing a highly oriented undrawn core-sheath type conjugated polyester fiber, the process comprising: spinning a polyester as the sheath polymer and, as the core polymer, a polymer having a gradient of elongational viscosity with temperature larger than that of the sheath polymer, at a spinning speed of 4000 to 12000 m/min, such that the core polymer is conjugated with the sheath polymer in an amount of from 1 to 15 wt% based on the weight of the entire conjugated fiber and without the core polymer being exposed on the surface of the fiber, the fiber is capable of exhibiting a diffraction peak observable in the equatorial intensity distribution of a wide angle X-ray diffraction image of the fiber, and at least the polyester portion of the cross section of the fiber obtained has a birefringence of 0.015 to 0.05. A process according to claim 6, wherein the amount of the core polymer conjugated with the sheath polymer is 1 to 10 wt% based on the weight of the entire conjugated fiber. A process according to claim 6 or 7 wherein the spinning speed is 4000 to 9000 m/min. A process according to any one of claims 6 to 8, wherein the fiber obtained has a boil off shrinkage of 10 to 50%. A process according to any one of claims 6 to 9, wherein the fiber obtained has an elongation at break of 100 to 250%. A process according to any one of claims 6 to 10, wherein the sheath polyester is polyethylene terephthalate, and the core polymer is at least one polymer selected from styrene based polymers, acrylate based polymers, acrylate-styrene copolymers and methylpentene based polymers. A textured polyester yarn comprising the highly oriented undrawn polyester fibers according to any one of claims 1 to 5. A process for preparing a textured polyester yarn, comprising falsetwisting highly oriented undrawn polyester fibers obtained by a process according to any one of claims 6 to 11.

Inventors: FUKUHARA MOTOTADA, KIKUTANI TAKESHI, KISHIRO AKIRA, OCHI TAKASHI, TANIGUCHI ATSUSHI, FUKUHARA, MOTOTADA, KIKUTANI, TAKESHI, KISHIRO, AKIRA, OCHI, TAKASHI, TANIGUCHI, ATSUSHI
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1996
Main classification: D01F8
IPC codes: D01F8