Polyurethane polyamide self-crimping conjugate fiber

Abstract: A polyurethane polyamide self-crimping conjugate fiber having eccentric conjugate form, comprises a high hardness polyurethane (1) having a Shore hardness D of 58 to 75 and a polyamide (2) having a melting point of at least 200 o C, which has a excellent extension stress properties and excellent heat resistance. The conjugate fiber is suitable for hosiery products and tricot products, having excellent transparency of fabric and excellent close-fitting properties.

Description: The present invention relates to a self-crimping conjugate fiber comprising a polyurethane and a polyamide. Particularly, the invention relates to a polyurethane polyamide conjugate fiber which exhibits excellent recovery stress properties and heat resistance and which is particularly useful as a fiber material for hosiery with excellent close-fitting properties and transparency. Self-crimping conjugate fibers comprising polyurethanes and polyamides which are eccentrically combined with each other can be formed into fabrics having good stretchability and transparency and are thus highly valued as filament materials for making high-quality stockings. Examples of known polyurethane elastomer components that may be used in such conjugate fibers include polyurethanes obtained by reaction between diisocyanates and polyols, and then chain extension using a low-molecular weight glycol and/or low-molecular weight diamine such as hydrazine or ethylenediamine. Useful polyols are, for example, polyethers comprising polyalkylene oxides and polytetrahydrofuran; polylactones obtained by ring opening polymerization of ε-caprolactone; polyesters obtained by condensation polymerization of acids such as adipic acid, glutaric acid and glycols such as ethylene glycol, propylene glycol, and polycarbonates. It is considered that, of these polyurethane components, polycarbonate-urethanes having excellent resistance to separation from polyamide components and relatively excellent heat resistance are preferable, when either used alone or together with other polyurethanes such as polyester-urethanes or polyether-urethanes, which are described in Japanese Patent Publication Nos. 55-22570 and 57-34370. In addition, it has been generally considered that such polyurethanes must have Shore hardness A within the range of 90 to 100, which is measured in accordance with the measurement method described as method A in JIS K6301. That is, it has been considered that, since polyurethanes having Shore hardness A over 100 exhibit a lower degree of extension than that of polyurethanes having Shore hardness A of 100 or less, polyurethane polyamide conjugate fibers obtained by using such polyurethanes having Shore hardness A over 100 exhibit poor crimping properties. Furthermore, it has been thought that the viscosity of polyurethanes having Shore hardness A over 100 cannot be easily stabilized during melt spinning, and thus yarns cannot be easily formed by using such polyurethanes. This has lead to a situation in which it has been substantially impossible to use such polyurethanes in an industrial spinning process such as that described in JP-A-50-71918 and JP-A-62-156314. Although polyurethane polyamide conjugate fibers having excellent coil-like crimps can be formed even by using polyurethanes with Shore hardness A of 100 or less, it cannot be said that the stretch fabric products such as stockings that are thereby produced have satisfactory close-fitting properties. There has therefore been a demand for fabric products exhibiting improved recovery stress properties and superior close-fitting properties and transparency. It is also necessary for polyurethanes to have a certain level of heat resistance for composite melt-spinning with polyamides. It is therefore preferable to use polyurethanes containing polycarbonate-urethanes, as described above. In the case of a polyurethane containing a polycarbonate-urethane with a Shore hardness A of 100 or less, the polyurethane exhibits a significantly lower level of heat resistance than that of a polyamide. There has therefore been a problem in that the stretch products so formed cannot be subjected to sufficient heat setting, because heat setting can be effected only at a relatively low temperature whthout heat deterioration of the polyurethane component. Further, in some cases, the stretchability and high degree of the product of strength and elongation of the products may deteriorate even if heat setting is performed at a relatively low temperature. The present invention provides a polyurethane polyamide conjugate fiber with which the abovementioned difficulties may be minimized or overcome. In particular, a polyurethane polyamide conjugate fiber embodying the present invention is very useful for obtaining excellent hosiery products having a high level of close-fitting properties and excellent transparency. Such a fiber may have improved recovery stress properties as compared with known eccentric conjugate fibers comprising polyamide and polyurethane elastomers which have been subjected to a treatment for crimp development. Furthermore, such a polyurethane polyamide conjugate fiber may be capable of improving the heat resistance of and preventing any deterioration of the characteristics of products during heat treatment. The invention provides, according to one aspect, a polyurethane polyamide self-crimping conjugate fiber having eccentric conjugate form and comprising a polyurethane having a Shore hardness D of from 58 to 75 inclusive and a polyamide having a melting point of at least 200°C. Such a conjugate fiber may have a crimping property such that it shows a spring constant of at least 14 g/cm. The invention will now be described in more detail with reference to the accompanying drawings in which: Figs. 1 and 2 are respective cross sectional views of fibers which are illustrated as examples of a conjugate fiber structure in accordance with the present invention. In Figs. 1 and 2, the polyurethane 1 and the polyamide 2 together provide the eccentric conjugate form. In a conjugate fiber in accordance with one aspect of the invention a particularly important characteristic lies in the use of a polyurethane having a Shore hardness D of at least 58 as a polyurethane component of a polyurethane polyamide conjugate fiber. By using such a polyurethane component it is consequently possible to provide a polyurethane polyamide conjugate fiber having such a high level of recovery stress properties that the spring constant of the crimped fiber after a crimp development treatment is at least 14. The Shore hardness D of the polyurethane is a value obtained by measurement in accordance with the method described in ASTM-D-2240, using a type D durometer hardness tester. There is a certain correlation between the Shore hardness D and the above-described Shore hardness A, the Shore hardness D of 58 being substantially at the same level as a Shore hardness A of 101. The Shore hardness D is used for indicating Shore hardness A within the hardness range above 100 which cannot be easily measured by using Shore hardness A, and is thus used in measuring the Shore hardness of fibers embodying the present invention. Although a polyurethane homopolymer, polyurethane copolymer or polyurethane mixture may be used as the polyurethane in a fiber embodying the present invention, it is important that the level of hardness is as high as at least 58 in terms of Shore hardness D, particularly preferably a Shore hardness D of at least 60. The higher the value of the Shore hardness D, the better are the recovery stress properties and the heat resistance of the conjugate fiber. However, if the Shore hardness D is too high, melt spinning itself becomes difficult, and the degree of elongation will then significantly decrease. The practical limit of Shore hardness D is thus about 75, preferably about 70 or less. As the Shore hardness of the polyurethane does not substantially change during melt spinning and/or heat treatment, the value of the Shore hardness can be ascertained from a polyurethane polymer before spinning or from the polyurethane component in the conjugate fiber before or after crimp development or after further heat setting. The hardness level of the polyurethane can be easily set by changing the ratio of the crystal forming portion (hard segment) to the essentially amorphous portion (soft segment), the polymer viscosity, the number of cross-linking points and the kind of polyol component. For example, the hardness of the polyurethane is effectively increased by increasing the proportion of the crystal forming portion (hard segment) in the polyurethane, the polymer viscosity, the crosslinking points in the polymer, using a rigid polyol such as polycarbonate and/or reducing the molecular weight of the polyol. The ratio of the crystal forming portion (hard segment) of the polyurethane can be increased by raising the content of a chain extender such as a low-molecular weight diol and/or diamine. It is particularly preferable that the weight ratio between the hard segment, which is formed from a chain extender such as a low-molecular weight diol and/or diamine, and the soft segment, which is formed from a polyol component, is within the range of 17 : 83 to 25 : 75 in terms of ratio by weight. Any of such known polyurethanes as polycarbonate-urethanes, polyester-urethanes, polylactone-urethanes and polyether-urethanes may be used as the polymer which forms the polyurethane component in the form of a homopolymer or copolymer of polyurethane or a mixture thereof. Of these polyurethanes, polycarbonate-urethanes or polyurethanes containing polycarbonate-urethanes of at least 10 percent by weight as copolymer components or mixture components are preferable for increasing the degree of adhesion to polyamides. The polyurethanes may contain other polymers such as polyesters, polyisocyanates, or low-polymerization compounds (molecular weight about 500 to 3000) having urethane groups, as long as they are present in small amounts (for example, 20% by weight or less, preferably 10% by weight or less), which do not inhibit such characteristics as resistance to separation from polyamides, thermal plasticity, heat stability, and a high degree of strength, elongation and elasticity. Examples of low-polymerization compounds having urethane groups include diisocyanate compounds such as diphenylmethane diisocyanate, tolylenediisocyanate, lysineisocyanate and the like. Examples of polyols that may be used for obtaining the above-described polycarbonate-urethanes include aromatic polycarbonates obtained from 4,4′-dioxydiphenyl-2,2′-propane (bisphenol A) and aliphatic polycarbonates obtained by reaction between aliphatic bivalent alcohols and phosgene. The molecular weights of the polycarbonate-polyols are preferably about 600 to 5000. Examples of polyols that may be used for obtaining the polyether-urethanes include poly(oxyethylene) glycol, poly(oxypropylene) glycol, poly(tetramethylene) glycol and the like. The molecular weights of the polyether-polyols are preferably about 600 to 4000. Examples of polyols that may be used for obtaining the polyester-urethanes include polyesters with molecular weights of about 600 to 4000, which are obtained by condensation reaction between acids such as adipic acid, glutaric acid, sebacic acid, and glycols such as ethylene glycol, 1,4-butylene glycol, 1,3- or 2,3-butanediol, and 2,5-hexanediol. Further, the molecular weight of the polycarbonate-polyols is preferably 1 to 6 times that of the polyols other than polycarbonate-polyols, more preferably 1 to 3. If the molecular weight ratio is less than 1, a conjugate yarn with sufficiently good heat resistance and close-fitting properties is difficult to obtain. Examples of diisocyanates that may be used for obtaining polyurethanes include diphenylmethane diisocyanate, tolylenediisocyanate, naphthalenediisocyanate, isophoronediisocyanate, and lysineisocyanate. Examples of chain extenders include low-molecular weight glycols, hydrazine, ethylenediamine and bis-β-hexanone The molar ratio (-NCO/-OH) between the -NCO terminal regroups and -OH terminal groups in the material for polymerization may be about 1.00 to 1.10. This polymerization material may be subjected to polymerization using a conventional polyurethane polymerization method such as a one-shot process or prepolymer process. The polyurethane thus obtained may be subjected to polymer mixing and additive mixing to form a polyurethane component to be used for composite spinning to provide a fiber in accordance with the present invention. Although such a high-hardness polyurethane has a tendency to display deviations in viscosity during melt spinning, this tendency can be suppressed by controlling the degree of polymerization of the polyurethane used to remain within an appropriate range corresponding to the polyurethane composition. The degree of polymerization of the polyurethane can be controlled to remain within an appropriate range by adjusting its melt viscosity, and it is generally preferable that the melt viscosity is between about 3500 and 35000 poise. It is also preferable in terms of stabilizing the viscosity during melt spinning that the viscosity of the polyurethane is within the range of 1.60 to 3.00 relative to dimethylacetamide, more preferably within the range of 1.70 to 2.80. The value of viscosity relative to dimethylacetamide is closely related to the stability during melt composite spinning with the polyamide component and spinning properties such as yarn breakage during the spinning and drawing process. The abovementioned stability during melt spinning refers, for example, to the thermal stability in a spinning pack, and the extent of yarn breakage just after spinning out. The high-hardness polyurethane having a Shore hardness D of at least 58 can therefore be stably subjected to melt composite spinning on an industrial scale by controlling the value of the viscosity to stay within an appropriate range. When the viscosity of the polyurethane relative to dimethylacetamide is over 3.00, significant deterioration in the fluidity caused by an increase in the viscosity during melt spinning causes gelation to be promoted and thermal decomposition to easily occur, resulting in the deterioration in the stability during melt spinning and yarn-making properties. On the contrary, when the viscosity relative to dimethylacetamide is less than 1.60, the polyurethane exhibits unsatisfactory properties of fiber formation and thus poor properties of yarn making, and thus conjugate fibers which are fit for practical use cannot be easily obtained. The viscosity of the polyurethane relative to dimethylacetamide is measured by the following method: 0.25 g of a polyurethane sample is dried under reduced pressure at 50 °C for 16 hours and then dissolved in 25 ml of dimethylacetamide at room temperature by a shaking method for 2 to 5 hours. The relative viscosity of the resultant solution is measured using an Ostwald viscometer at 25 °C under the condition that the falling time is 40 seconds. The viscosity of the polyurethane relative to dimethylacetamide can be adjusted by appropriately selecting methods and conditions of polymerization, melting and spinning,which are, for example, a method of re-melting and pelletizing a polymer (pellet) and a method of adjusting the melt spinning temperature corresponding to the level of viscosity of the polymer used. It is necessary that the polyamide component used in the present invention has a melting point of at least 200 °C. Examples of polyamides having a melting point of at least 200 °C include nylon 6, nylon 66, nylon 46 and nylon 6 10. Although many polyamide copolymers have melting points less than 200 °C, some polyamide copolymers have melting points of at least 200°C and such copolymers may be also used. Since the conjugate fibers obtained from polyamides having excessively low melting points exhibit poor physical properties such as an inappropriate degree of extension, poor wear resistance and so on, it is difficult to obtain fibers which are fit for practical use from such polyamides. On the other hand, it is undesirable to use polyamides having excessively high melting points for composite spinning with polyurethanes, and it is preferable from the viewpoint of practical use that polyamides have melting points of at most about 300 o C. Of these polyamides, particularly, polyamides essentially formed from nylon 6 or nylon 66 are more preferable. The degree of polymerization of the polyamide component may be a value corresponding to relative viscosity ηr which is generally employed for clothing fibers, for example, relative viscosity to sulfuric acid of 2.0 to 2.8. The polyamide component may contain general additives such as a heat-resisting agent, a light-resisting agent, a delustrant agent and so forth. The above-described high-hardness polyurethane and polyamide may be subjected to melt composite spinning using a method which is basically the same as that used in conventional melt composite spinning of polyamides and polyurethanes. For example, these polymers are supplied to a normal melt composite spinning machine and separately melted therein, and then subjected to composite spinning using a composite spinneret heated at about 230 to 290 o C. The polyamide component is then subjected to crystal orientation using a normal method to produce a conjugate fiber with latent crimping properties. Examples of fiber-making methods include a two step method in which yarns are wound up at a low speed to form undrawn yarns and after then drawn with or without heat-treatment; a direct spinning drawing method in which yarns are taken up at a low speed, drawn and then subjected to heat treatment using a means such as a hot roller, steam treatment or the like; and a high-speed spinning method in which yarns are wound up at a high speed, without drawing or with some drawing of a relatively low degree. The high-speed spinning method employs such conditions that the take-up speed is at least 3500 m/min., the degree of drawing is at most 2.5 times, and the wind-up speed is at least 4000 m/min. Some heat treatment during the yarn-making process is effective to decrease the fiber-shrinkage in boiling-water, so that conjugate yarn with low shrinkage useful for stockings can be obtained. The conjugate fiber structure may be an eccentric conjugate structure which allows the attainment of latent crimping properties that allow coil-like crimps to be produced by the crimp developing treatment. For example, the eccentric sheath-core conjugate structure such as shown in Fig. 1 is preferable, but the side-by-side conjugate structure shown in Fig. 2 may be used. These conjugate structures can be subjected to composite spinning using ordinary composite spinnerets. Although the optimum value of the compounding ratio of the polyurethane component and the polyamide component depends upon the conjugate structure used, the compounding ratio is generally about 80/20 to 20/80, preferably about 70/30 to 30/70. It is also preferable that at least half of the external peripheral surface of the fiber is occupied by the polyamide, and preferably 80% or more, more preferably substantially the entire external peripheral surface of the fiber is occupied by the polyamide. That is, since the exposure of the polyurethane component from the external peripheral surface of the fiber easily causes deterioration in the spinning properties and after processing properties, if possible, it is preferable that no polyurethane component is exposed from the external peripheral surface of the fiber. It is preferable for obtaining good crimping properties that the single fiber fineness of the polyurethane polyamide conjugate fiber of the present invention is at most 40 denier, more preferably about 3 to 40 denier. The yarn fineness and the number of filaments depend upon the desired end use. For example, the yarn fineness and the number of filaments for a leg portion of stockings, and tights are preferably 10 to 40 denier and 1 to 12 filaments, and 30 to 70 denier and 1 to 24 filaments, respectively. Since an increase in the hardness of a polyurethane generally causes deterioration of its stretching properties, the hardness of the polyurethane for an elastic fiber formed from polyurethane alone cannot be significantly increased, so the shore hardness D of the known elastic polyurethane fibers was always less than 58. In the case of a conjugate fiber comprising a polyurethane and polyamide, it was generally considered that such a conjugate fiber must have a level of hardness equal to that of polyurethane, which is substantially the same as that of an elastic fiber formed from polyurethane alone, for the purpose of obtaining sufficient self-crimping properties. However, when a conjugate fiber is actually produced using a high-hardness polyurethane, although there was a tendency for the stretching properties to deteriorate as the hardness increases, no critical deterioration in elasticity was actually observed. It was rather found that the recovery stress properties and heat resistance are improved as the hardness increases, and crimped fibers extremely useful for stockings can be formed owing to the significantly improved close fitting properties and heat resistance of the fabric products. It is thought that this is because, in the case of the conjugate fiber, the elastic properties possessed by the conjugate fiber which was subjected to crimp developing treatment are mainly attributed to coil-like crimps, which were produced as a result of the difference in shrink properties between the polyamide and the polyurethane, and hardly depend upon the stretching properties possessed by the polyurethane component. The conjugate fiber formed by eccentrically compounding the high-hardness polyurethane and the polyamide are subjected to crimp developing treatment using a normal method to exhibit the same elastic properties as those of a coil-like crimped fiber. Such a coil-like crimped fiber has such a high level of stretch recovery stress that the spring constant is 14 g/cm or more and such a fiber had, to date, not been obtained. Since the fiber has a high spring constant, the 60% recovery stress and 70% stretch stress of the stretch fabric product obtained are significantly increased, as well as the close-fitting properties thereof being significantly improved. The spring constant (K) of a coil-like crimped fiber is the value obtained by the following method: A fiber yarn sample having latent crimping properties is treated with boiling water at 98 °C for 30 seconds to develop coil-like crimps. One end of the coil-crimped yarn sample is fixed, and a load (W mg) of 35 mg/d is applied to the other end so as to stretch the yarn sample. The length (σ mm) of one coil pitch in the stretched yarn sample and the length (σ₀ mm) of that in the unstretched yarn are measured. The spring constant (K) is determined by using the following equation: K = [W/(σ-σ 0 )] X 10 -2 (g/cm) A conjugate fiber embodying the invention is also excellent in its heat resistance. For example, the retention of the product of strength and elongation (refer to the examples described below) after the fiber has been subjected to the crimp developing treatment using boiling water and then to heat setting at 110 o C is as high as 70 percent or more. Since the fiber has excellent heat resistance, the deterioration of the physical properties owing to the crimp developing treatment and heat setting is suppressed, and the strength-elongation properties of the fibers used in the stretch fabric product are significantly improved as compared with conventional polyurethane polyamide fibrous fabrics. In additon, since the high-hardness polyurethane present in a fiber embodying the present invention exhibits a relatively high melting point and excellent heat resistance, it is possible to use as polyamide components relatively high-melting point polyamides such as nylon 66 and the like, which are generally considered to be subjected only with difficulty to composite spinning together with polyurethanes on an industrial scale. EXAMPLE 1 A polyurethane polymer was formed by polymerization by a normal one-shot process using a mixed polyol containing a polycarbonate (average molecular weight, 3000) and a polycaprolactone (average molecular weight, 1000) in a ratio of 5:5, 1,4-butylene glycol as a chain extender, and diphenylmethane diisocyanate as a diisocyanate. The thus-formed polymer was chopped into flakes, melt-extruded by using an extruder and then pelletized. The molar ratio (-NCO/-OH) of the -NCO groups to the -OH groups in the raw material used for polymerization was 1.04. The molar ratio between 1,4-butylene glycol and the mixed polyol was 5.5 so that polyurethanes having Shore hardness D of 63 were obtained. These were used as polymer A. The Shore hardness D, viscosity relative to dimethylacetamide, ratio between the amounts of hard segment and soft segment polyurethane, and degree of elongation of the thus-obtained polyurethanes were measured. The results obtained are shown in Table 1. The above-obtained polyurethane and a polycapramide having viscosity relative to 98 percent sulfuric acid of 2.50 were separately rendered molten at 230°C and 260°C and then supplied to a composite spinning machine. Both polymers were then compounded together and spun out in an eccentric form having a core and a sheath in a ratio of 50/50 using a composite spinneret heated to 250°C, and then cooled by a conventional method. Spinning oil was supplied to the cooled filaments, which were then wound up at 600 m/min. The as-spun filaments were then drawn at a ratio of 4.0 times without heat-treatment, to form a conjugate filament yarn with latent crimping properties, which has two filaments and 18 denier. The thus-obtained filament yarn had a conjugate structure in an eccentric form having a core and a sheath, as shown in Fig. 1. A stocking was formed by knitting the thus-formed yarns by a conventional method and then subjected to a heat setting treatment at 110°C to produce a stocking product. The strength-elongation properties, the spring constant after crimp developing treatment of the conjugate filament yarn without being knitted, the physical properties of the coil-like crimped yarn in the stocking product, and the elastic properties and elongation recovery stress properties of the stocking product were measured. The results obtained are also shown in Table 1. The above-described physical properties were respectively measured by the following methods: Spring constant; measured after the conjugate filament yarn has been subjected to crimp developing treatment in the above-mentioned manner without being knitted. Retention of product of strength and elongation; The product of strength and elongation is calculated from the value of yarn strength (g/d) and yarn elongation (percent), which are measured : in the usual manner. The product = strength (g/d) X [elongation (%)/100 +1] and the ratio (percent) of the product of the fiber after being heat-set to that of a fiber before being heat-set, is calculated. Crimping properties of stockings; A sample obtained by folding a stocking product in two was subjected to a tension test using a constant extension-type tensile tester (manufactured by Shinko Tsushin Kogyo Co., Ltd.). The stretched length (L1) of the sample which was subjected to a load of 2 Kg was measured. Then, the stress value (g) at a point of extension of 75 percent of L1 was read from the hysteresis curve which was formed by effecting extension of 75 percent of L1 and recovery, and the stress value (g) at a point of recovery of 60 percent of L1 was read from the recovery curve. These values were divided by 2 and respectively shown as values of 75 percent extension stress (75 % SP) and 60 percent recovery stress (60 % BP). These values are indexes which indicate the close fitting properties of stockings, and the higher the values, the more excellent the fitting properties. The fitting properties were evaluated by tests which were performed by actually wearing the stockings. EXAMPLE 2 Yarns were formed by essentially the same method as in EXAMPLE 1 with the exception that the molecular weight of the polyols, ratio of mixed polyols, and the molar ratio between 1,4-butylene glycol and the mixed polyol of the polyurethane supplied to composite melt-spinning were changed. That is, the average molecular weight of the polycarbonate was 2000, that of polycaprolactone was 2000, the ratio of mixed polyols as between a polycarbonate and a polycaprolactone was 6:4, and the molar ratio as between 1,4-butylene glycol and the mixed polyol was 6.0, 5.5, 5.0 or 4.0 so that four types of polyurethane having different levels of Shore hardness, were obtained, which were respectively used as polymers B, C, D and E. The obtained conjugate yarns were evaluated in the same manner as in EXAMPLE 1, and the results are also shown in Table 1. As can be seen from Table 1, the conjugate fibers comprising polyurethanes having a Shore hardness D of 58 or more, as Samples A to D, exhibited low degrees of elongation of raw yarns, as compared with the conjugate fiber (E) comprising a polyurethane having a Shore hardness D less than 58, but they exhibited significantly improved heat resistance and extension stress properties after crimp development and thus could be formed into stockings having excellent fitting properties and strength-extension properties. EXAMPLE 3 Yarns were formed by essentially the same method as in EXAMPLE 2 (Test C) with the exception that the molecular weight of the polyol of the polyurethane supplied to composite melt-spinning used in EXAMPLE 1 was changed to the values shown in Table 2, and then evaluated. The results are shown in Table 2. As can be seen from Table 2, the conjugate fibers comprising polyurethane having a ratio of average molecular weight as between polycarbonate and caprolactone of at least 1 exhibited more excellent fitting properties than that having a ratio of less than 1. EXAMPLE 4 The as-spun yarn obtained in Tests C and E of EXAMPLE 2 were drawn at a ratio of 4.0 times using a hot plate of 30, 60, 80, or 100 °C, to form heat-treated filament yarns with latent crimping properties. The strength and elongation of the obtained heat-treated filament yarns were measured in the same manner as in EXAMPLE 1, and the results are shown in Table 3. As can be seen from Table 3, the conjugate fiber (No. C) embodying this invention exhibited significantly improved heat resistance, so that a conjugated fiber having low shrinkage and good strength was obtained by heat-treatment, which is useful for production of stockings. On the other hand, the strength of the conventional conjugate yarn (No. E) having a Shore hardness D of less than 58, was decreased by heat-treatment, so that a heat-treated conjugated yarn with good properties was not obtained. EXAMPLE 5 A polyurethane polymer was formed by a conventional one-shot process in the same manner as Test C in EXAMPLE 2. The thus-formed polyurethane polymer was then chopped into flakes, ground, heated by hot air at 45 °C for 14 days, melt-extruded by an extruder (cylinder temperature; 195 to 210 °C) and then pelletized. The thus-obtained polyurethane was used as Polymer J. The polymer flakes, after the heat-treatment with hot air, were melt-extruded by an extruder, wherein the cylinder temperature of the extruder was changed to 200 to 215 °C, or 205 to 225 °C to form polyurethane elastomers which were respectively used as polymers K and L. The above-obtained polymer J was again melt-extruded through a cylinder having temperature of 165 to 200 °C or 185 to 205 °C and then pelletized, respectively to form polymers M and N. Each of the polyurethane elastomers and a polycapramide with viscosity relative to 98 percent sulfuric acid of 2.30 were supplied to a composite spinning process. Each polyurethane and the polycapramide were separately melted at 230 °C and 250 °C, respectively, compounded together and cospun out in an eccentric form having a core and a sheath in a ratio of 50/50 by using a composite spinneret heated at 240 °C, and then cooled by a conventional method. Spinning oil was then supplied to the fibers which were then wound up at 600 m/min. The fibers were then 4.0 times cold-drawn to obtain a conjugate filament yarn with 20 denier comprising 2 filaments. The results of melt-spinnability, the state of occurrence of gel in a spinning pack, and the viscosity relative to DMAc of the polyurethane components, are shown in Table 4. The results of the spinning operation and the state of occurrence of gel in a spinning pack were evaluated using the degree of coloring (yellowing) of each polymer caused by modifiers. As can be seen from Table 4, the foaming and gelation of the polyurethane during melt spinning were suppressed by controlling the viscosity of the polyurethane component used relative to DMAc to remain within the range of 1.60 to 3.00, as shown in polymers K to M. Furthermore, the melt-spinning stability and yarn-making properties could be significantly improved. On the other hand, the use as a polyurethane elastomer of polymer J having a relative viscosity to DMAc of over 3.00 exhibited poor spinning and stretching properties and caused the occurrence of gel during melt-spinning, which was mixed as brown foreign matter in the fibers. In addition, the use as a polyurethane elastomer of polymer N having a viscosity less than 1.60 relative to DMAc caused the deterioration of the spinning and drawing properties owing to the poor straight chain properties, i.e., poor properties of fiber formation. No. A F C G H Polyurethane Molecular weight of Polycarbonate 3000 2000 2000 1000 1000 Molecular weight of Polycaprolactone 1000 1000 2000 2000 3000 ratio of molecular weight 3 2 1 0.5 0.33 Stocking 60 % Recovery Stress (g) 160 122 140 112 120 75 % Extention Stress (g) 890 680 710 650 650 No. J K L M N Polyurethane (pellet) Relative Viscosity in DMAc 5.32 4.80 3.30 2.33 2.08 Shore D 64 64 63 63 63 Polyurethane (in filament) Relative Viscosity in DMAc 3.10 2.95 2.60 1.98 1.58 Melt Spinnability bad no good good good bad Gelation in spinning pack some existence a little existence no existence no existence no existence The use of a polyurethane having Shore hardness D of at least 58 enables the polyurethane polyamide conjugate fiber in accordance with the present invention to exhibit significantly improved recovery stress properties of a coil-like crimped fiber after crimp development. Thus, stretch fabric products with more improved fitting properties can be produced. In addition, since the heat resistance is improved, it is possible to prevent the deterioration in quality during crimp developing treatment and heat setting and improve the strength-extension properties of stretch fabric products. The conjugate fiber in accordance with the present invention can therefore be used in the same way as conventional self-crimping conjugate fibers and is particularly useful for fiber products which are required to possess a high level of fitting properties. For example, it is useful for hosiery such as stockings, socks, and tricot products. The conjugate fiber of the present invention can be formed into a fiber finer than conventional covered elastic yarns which comprise polyurethane elastic filament covered with polyamide fibers and which are widely used in stocking products with high levels of stretchability and fitting properties. The conjugate fiber can therefore be used in stocking products with high levels of stretchability and fitting properties, as well as a high level of transparency of the fabrics.

Claims: A polyurethane polyamide self-crimping conjugate fiber having eccentric conjugate form and comprising a polyurethane, and a polyamide having a melting point of at least 200°C, characterised in that the polyurethane has a Shore hardness D of 58 to 75. A polyurethane polyamide self-crimping conjugate fiber according to claim 1, wherein the polyurethane has a Shore hardness D of at least 60. A polyurethane polyamide self-crimping conjugate fiber as defined in claim 1 or 2, wherein said polyurethane is a polyurethane selected from a polycarbonate-urethane and a polyurethane containing a polycarbonate-urethane of at least 10 percent by weight as a copolymer component or a mixture component. A polyurethane polyamide self-crimping conjugate fiber as defined in claim 3, wherein the molecular weight of the polycarbonate-polyol in said polyurethane is 1 to 6 times the molecular weight of any other polyol present in the polyurethane or mixture. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein the polyurethane has respective hard and soft segments in a weight ratio, between the hard segment and the soft segment, of 17:83 to 25:75. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein the relative viscosity of said polyurethane to dimethylacetamide is 1.60 to 3.00. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein said eccentric conjugate form is an eccentric sheath-core conjugate structure, said sheath being eccentrically formed from said polyamide. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein the compounding ratio between said polyurethane and said polyamide is 80/20 to 20/80. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein a single said conjugate fiber has at most 40 denier. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein the retention of the product of strength and extension after heat treatment at 110°C for 30 seconds is at least 70%. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein said conjugate fiber is subjected to heat treatment at at least 60°C after melt-spinning without substantial crimp development. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein the crimping property of said conjugate fiber shows a spring constant of at least 14 g/cm. A polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim, wherein said polyamide is formed essentially from polycapramide having relative viscosity to sulfuric acid of 2.0 to 2.8. A hosiery product comprising a polyurethane polyamide self-crimping conjugate fiber as defined in any preceding claim.

Inventors: KOBAYASHI HIROFUMI, TAKEDA TOSHIYUKI, KOBAYASHI, HIROFUMI, TAKEDA, TOSHIYUKI
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1989
Main classification: D01F8
IPC codes: D01F8