Polyamide-polyester composite fiber and process for producing same

Abstract

 Polyamide-polyester side-by-side type composite fibers useful for bulky nonwoven fabrics, which comprise a polyamide filamentary segment and a polyester copolymer filamentary segment comprising an aromatic dicarboxylic acid component comprising 2.0 to 10.0 molar% of 5-sodiumsulfo-isophthalic acid and the balance consisting of terephthalic acid and a glycol component comprising at least on C₂-C₁₀ alkylene glycol, and have a crimp number of 2 crimps/25 mm or less in water or under a wet condition at 100°C or less and of 5 crimps/25 mm or more under an equilibrium condition at a temperature of 20°C at 65% RH.

Description

BACKGROUND OF THE INVENTION

1) Field of the Invention

[0001] The present invention relates to a polyamide-polyester composite fiber and a process for producing same. More particularly, the present invention relates to a side-by-side type polyamide-­polyester composite fiber which has a latent crimping property, can be evenly dispersed in water, and is useful for producing a nonwoven fabric having a high bulkiness and strong elastic recovery.

2) Description of the Related Arts

[0002] Currently, there is an increasing interest in and demand for nonwoven fabrics. Especially, in the fields of printing materials, filtering materials, cloth materials, sanitation materials, living materials and construction and building materials, nonwoven fabrics having a uniform basis weight and thickness are in strong demand.

[0003] The nonwoven fabrics, especially those made by a wet paper-forming method, are advantageous in that they can be produced not only from organic fibers, for example, rayon fibers, water-insolubilized polyvinyl alcohol fibers, polyamide fibers, polyacrylic fibers, polyester fibers, aramide fibers and polyolefin fibers, but also from inorganic fibers, for example, glass fibers and ceramic fibers. Nevertheless, the wet method nonwoven fabrics are disadvantageous in that, since the starting fibers must be dispersed in water and then subjected to the wet paper-forming process, the resultant nonwoven fabrics always have a paper-like stiff touch. Due to this disadvantage, the conventional nonwoven fabrics are not practical for uses such as medical protective gowns, drops, and sanitation top sheets and bottom sheets, which are brought into contact with the human body, and for resin-coated paper sheets and roofing sheets, which must be produced by being impregnated with various resinous treating materials.

[0004] To overcome the above-mentioned disadvantages, attempts were made to increase the bulkiness of the nonwoven fabrics by preliminarily imparting mechanical crimps to the starting fiber for the nonwoven fabrics. In these attempts, however, the crimped fibers were entangled with each other when dispersed in water, and the resultant nonwoven fabrics contained a number of entangled fibers in the form of irregular bundles and thus had a low quality.

[0005] In other attempts to impart a high bulkiness to the nonwoven fabrics, side-by-side type composite fibers which had a latent crimping property and did not exhibit crimps even when immersed in water were dispersed in water, and then subjected to a wet paper-forming process to provide a nonwoven fabric. Therefore, the resultant wet nonwoven fabric was dried or heat-treated to allow the composite fibers to create crimps therein. In this side-by-side type composite fibers, two polymer filamentary segments having different shrinkages are bonded to each other in a side-by-side relationship, and accordingly, those two polymers must have a satisfactory compatibility with each other. For this reason, the two polymers are usually of the same type but having different shrinkages. The above-mentioned type of conventional composite fibers, however, have a relatively poor crimp-forming property.

[0006] Also, to prevent the generation of crimps in the composite fibers before the wet paper-forming process, a heat treatment, which effectively stabilizes the fine structure in the composite fibers, should not be applied to the composite fibers. Therefore, even if the nonwoven fabrics are heat-treated to allow the composite fibers therein to be crimped, the resultant crimps exhibit a poor stability, and thus the bulkiness of the resultant nonwoven fabrics is also very low.

[0007] As stated above, the conventional side-by-side type conventional composite fibers composed of two polymers, which are the same type but have different shrinkages, suffer from restriction in the processing method and conditions thereof, and thus exhibit a limited crimp-forming property.

[0008] The inventors of the present invention carried out research into combinations of different types of polymer used for making composite fibers, and into the bonding mechanism of the different polymer segments in the composite fibers, and as a result, disclosed, in Japanese Examined Patent Publication No. 45-28728, a side-by-side type composite fiber in which a polyamide filamentary segment and a metal sulfonate group-­containing polyester filamentary segment are firmly bonded to each other and which exhibits a high crimp-­durability.

[0009] After completing the invention disclosed in the above-mentioned Japanese Publication, several attempts were made to use polyamide-polyester composite fibers. For example, Japanese Examined Patent Publication No. 57-55806 discloses a method of providing a composite fiber having a latent spiral crimp-forming property, comprising the steps of drawing an undrawn composite fiber in a single step, applying a mechanical crimping procedure to the drawn composite fiber, and heat-treating the crimped composite fiber in a dry relaxed condition. Also, Japanese Examined Patent Publication No. 57-55807 discloses another method of providing a composite fiber having a latent spiral crimp-forming property, comprising the steps of drawing an undrawn composite fiber in a single step, heat-­treating the drawn composite fiber under tension, and then applying a mechanical crimping procedure to the heat-treated composite fiber.

[0010] In those methods, however, it is difficult to produce composite fibers completely free from spiral crimps therein, and thus the above conventional composite fibers are not suitable as starting fibers for producing nonwoven fabrics by the wet paper-forming method, in which the starting fibers must be evenly dispersed in water, and therefore, must be free from crimps.

[0011] In still other attempts, Japanese Examined Patent Publication Nos. 63-44843 and 63-44844 disclose a moisture-sensitive crimping composite fiber. A degree of crimping of this fiber can vary in response to the humidity of the ambient atmosphere, but this Japanese Publication does not disclose a method of completely eliminating spiral crimps from the fiber.

[0012] U.S. Patent No. 4,118,534 and Japanese Unexamined Patent Publication No. 59-116417 disclose composite fibers useful for woven or knitted fabrics, and Japanese Examined Patent Publication No. 52-30628 discloses a process for producing a nonwoven fabric from split fine fibers, but these publications do not teach composite fibers free from spiral crimps. Japanese Unexamined Patent Publication No. 63-92721 discloses a composite fiber composed of a nylon 46 segment and a metal sulfonate group-containing polyester segment, but does not teach or suggest how to make the spiral crimps completely latent in the composite fiber.

[0013] As stated above, the conventional composite fibers having a latent crimping property are still provided with a certain number of spiral crimps, and when converted to spun yarns, the productivity of the carding procedure thus becomes very low. Therefore, the industrial utilization of the conventional crimp-forming composite fibers is still restricted.

[0014] Japanese Examined Patent Publication No. 52-30628 discloses a method of producing a nonwoven fabric in which, after the composite fibers are passed through a carding procedure, the composite fibers in the card are divided into segment fibers. This method, however, does not teach or suggest how to completely eliminate the spiral crimps.

SUMMARY OF THE INVENTION

[0015] An object of the present invention is to provide a polyamide-polyester composite fiber which is substantially free from crimps when subjected to a fabric-forming process in a wet or dry condition and in which crimps can be created by applying a crimping treatment, for example, a heat-treatment, to the fabric.

[0016] Another object of the present invention is to provide a polyamide-polyester composite fiber which can be formed into a uniform-nonwoven fabric by a wet paper-forming process without creating crimps therein, and then spiral crimps having a satisfactory durability can be created therein by applying a drying treatment or heat treatment to the nonwoven fabric, to provide a nonwoven fabric having a high bulkiness.

[0017] The above-mentioned objects can be attained by the polyamide-polyester composite fiber of the present invention comprising a polyamide filamentary segment; and a polyester copolymer filamentary segment comprising a copolymerization product of an aromatic dicarboxylic acid component comprising 2.0 to 10.0 molar% of 5-sodiumsulfo-isophthalic acid and the balance consisting of terephthalic acid with a glycol component comprising at least one type of alkylene glycol having 2 to 10 carbon atoms,
the polyamide filamentary segment and the polyester copolymer filamentary segment extending in parallel to each other along the longitudinal axis of the composite fiber and being bonded together in a side-by-side relationship, and
the composite fiber having a crimp number of 2 crimps/25 mm or less in water or under a wet condition at a temperature of 100°C or less, and of 5 crimp/25 mm or more under an equilibrium condition of a temperature of 20°C and a relative humidity of 65%.

[0018] The above-mentioned polyamide-polyester composite fiber can be produced by the process of the present invention comprising the steps of: preparing an undrawn composite fiber which comprises a polyamide filamentary segment and a polyester copolymer filamentary segment comprising a copolymerization product of an aromatic dicarboxylic acid component comprising 2.0 to 10.0 molar% of 5-sodiumsulfo-isophthalic acid and the balance consisting of terephthalic acid with a glycol component comprising at least one type of alkylene glycol having 2 to 10 carbon atoms, and in which the polyamide filamentary segment and the polyester copolymer filamentary segments extend in parallel to each other along the longitudinal axis of the undrawn composite fiber and are bonded to each other in a side-by-side relationship, by a melt-spinning procedure; drawing the undrawn composite fiber at a draw ratio corresponding to 88% to 98% of the ultimate draw ratio thereof; and restrictively relaxing the drawn composite fiber in hot water at a temperature of 80°C to 90°C to an extent such that the drawn composite fiber is allowed to shrink to a length thereof corresponding to 85% to 98% of that of the drawn composite fiber, to provide a composite fiber having a crimp number of 2 crimps/25 mm or less in water or under a wet condition at a temperature of 100°C or less, and of 5 crimps/25 mm or more under an equilibrium condition of a temperature of 20°C and a relative humidity of 65%.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In the polyamide-polyester composite fiber of the present invention, a polyamide filamentary segment and a polyester copolymer filamentary segment extend in parallel to each other along the longitudinal axis of the composite fiber, and are bonded together in a side-by-side relationship or in the form of a bi-metal, to form a composite fiber.

[0020] The polyamide filamentary segment preferably comprises at least one member selected from the group consisting of nylon 4, nylon 46, nylon 6, nylon 66 and nylon 12, and copolymers of the above-mentioned polymers. Those polymers may contain a small amount of another polymer.

[0021] Nylon 6 and nylon 66 are most preferably employed for the present invention. Also, the polyamide usable for the present invention preferably has a melt viscosity close to that of the polyester copolymer, to ensure a smooth production of the composite fiber by a melt-spinning process. Preferably, the polyamide has a limiting viscosity [η] of 1.0 to 1.4 determined in m-cresol at a temperature of 30°C.

[0022] The polyester copolymer usable for the present invention comprises a copolymerization product of an aromatic dicarboxylic acid component comprising 2.0 to 10.0 molar%, preferably 2.0 to 5.0 molar%, of 5-sodiumsulfo-isophthalic acid and the balance consisting of terephthalic acid with a glycol component comprising at least one type of alkylene glycol having 2 to 10 carbon atoms, for example, ethylene glycol, propylene glycol, and butylene glycol.

[0023] When the content of 5-sodiumsulfo-isophthalic acid in the aromatic dicarboxylic acid is less than 2 molar%, the resultant polyester copolymer filamentary segment exhibits an unsatisfactory bonding property to the polyamide filamentary segment and thus, in the resultant composite fiber, the polyester copolymer filamentary segment is sometimes peeled from the polyamide filamentary segment when the composite fiber is drawn.

[0024] Also, if the content of 5-sodiumsulfo-isophthalic acid is more than 10.0% by mole, the resultant polyester copolymer exhibits a poor melt-mobility, and thus is difficult to smoothly melt-spin to thereby provide the composite fiber. The polyester copolymer preferably has a limiting viscosity (η) of 0.35 to 0.70 determined in 0-chlorophenol at a temperature of 25°C. In the polyester copolymer, the principal polyester chains preferably consist of polyethylene terephthalate or polybutylene terephthalate. The polyester copolymer optionally contains a small amount of copolymerized another component or is mixed with another polymer. One or both of the polyamide segment and the polyester copolymer segment optionally contains an additive comprising at least one member selected from, for example, delustering agents, coloring agents, and anti-static agents.

[0025] In the polyamide-polyester composite fiber of the present invention, the polyamide filamentary segment and the polyester copolymer filamentary segment are present preferably in a volume ratio to each other of 35:65 to 65:35.

[0026] Also, the polyamide-polyester composite fiber of the present invention preferably has a denier of 0.1 to 15.0 (a d tex of 1/9 to 150/9) and a length of 3 to 30 mm.

[0027] There is no limitation to the cross-sectional profile of the polyamide-polyester composite fiber of the present invention, but a preferable cross-sectional profile is a circle.

[0028] When the composite fiber has a crimp number of substantially zero per 25 mm length of the fiber, the composite fiber having a length of 3 to 30 mm can be uniformly dispersed in water. If the crimp number is 1 crimp/25 mm, the length of the composite fiber is preferably 3 to 10 mm, and if the crimp number is 2 crimp/25 mm, the length of the composite fiber is preferably 3 to 5 mm.

[0029] The polyamide-polyester composite fiber of the present invention has the following characteristic properties.

1. When dispersed in water, the composite fibers are substantially straightened and have a satisfactory hydrophilic property due to the polyamide filamentary segment, and a suitable rigidity due to the polyester copolymer filamentary segments.

2. The composite fibers are evenly dispersed in water without entanglement with each other, to provide a uniform aqueous slurry useful for a wet paper-forming process.

3. When dried, a number of spiral crimps are created in the composite fibers, and thus the resultant nonwoven fabric has a high bulkiness and does not have an undesirable paper-like stiff touch.

[0030] The polyamide-polyester composite fiber of the present invention can be produced by a process comprising the steps of, preparing an undrawn composite fiber, drawing the undrawn composite fiber, and restrictively relaxing the drawn composite fiber.

[0031] The undrawn composite fiber is prepared from a polyamide and a polyester copolymer, by a usual composite fiber-melt spinning method.

[0032] The resultant undrawn composite fiber comprises a polyamide filamentary segment and a polyester copolymer filamentary segment comprising a copolymerization product of an aromatic dicarboxylic acid component with a glycol component. The aromatic dicarboxylic acid component comprises 2.0 to 10.0 molar% of 5-sodiumsulfo-isophthalic acid and the balance consisting of terephthalic acid. The glycol component comprises at least one type of alkylene glycol having 2 to 10 carbon atoms.

[0033] The polyamide filamentary segment and the polyester copolymer filamentary segment in the undrawn composite fiber extend in parallel to each other along the longitudinal axis of the undrawn composite fiber, and are bonded to each other in a side-by-side relationship or in the form of a bi-metal.

[0034] The resultant undrawn composite fiber is subjected to a drawing procedure, which is carried out by using a usual drawing machine for the production of polyester staple fiber, at a specific draw ratio corresponding to 88% to 98% of the ultimate draw ratio at which the drawn composite fiber is broken. In this drawing step, the polyamide and polyester copolymer filamentary segments are oriented with a high degree of orientation.

[0035] Preferably, the drawing procedure is carried out in hot water at a temperature of 65°C to 75°C. The draw ratio is determined in consideration of the drawing temperature, the cooling conditions, the melt-spinning speed, the drawing speed, and the thickness of the undrawn composite fiber.

[0036] If the draw ratio is less than 88% of the ultimate draw ratio of the undrawn composite fiber, the resultant composite fiber is unsatisfactory in that, when the resultant composite fiber is immersed in water, the crimps in the fiber are not substantially removed. Also, if the draw ratio is more than 98% of the ultimate draw ratio of the undrawn composite fiber, the composite fiber is partly broken, and thus the resultant drawn composite fiber tow has fluffs and contains undrawn composite fibers.

[0037] In the restrictive relaxing step, the drawn composite fiber is restrictively relaxed in hot water at a temperature of 80°C to 90°C, to an extent such that the drawn composite fiber is allowed to shrink to a length thereof corresponding to 85% to 98% of the length of the drawn composite fiber.

[0038] If the relaxing temperature is less than 80°C, or more than 90°C, the resultant composite fiber is unsatisfactory in that, even when the resultant composite fiber is immersed in water, the crimps created in the fiber cannot be substantially removed.

[0039] Also, if the length of the relaxed composite fiber is more than 98% of the corresponding length of the drawn composite fiber, i.e., the shrinkage is very small, the crimps in the fiber cannot be removed by treating the fiber with water. If the length of the relaxed composite fiber is less than 80% of the corresponding length of the drawn composite fiber, the relaxed composite fiber often becomes wound around a delivery roll in the drawing machine, and thus the production efficiency of the composite fiber is lowered.

[0040] In the drawing step and the relaxing step, the composite fiber is conveyed at a speed of 80 to 150 m/min. This speed does not strongly affect the property of the resultant composite fiber.

[0041] Optionally, the resultant drawn and relaxed composite fiber is heat-set, preferably at a temperature of 90°C to 150°C. The heat-setting procedure can be carried out by using a heating roll or a heating oven.

[0042] The resultant drawn and relaxed composite fiber is oiled with a predetermined amount of a hydrophilic oiling agent, which effectively increases the dispersing property of the composite fiber in water, dried at a temperature close to room temperature, preferably 40°C or less over a time of about 30 minutes or more to an extent such that the content of water in the composite fiber is decreased to about 30% by weight, and then drawn to a predetermined length.

[0043] Preferably, the hydrophilic oiling agent comprises at least one member selected from, for example, nonionic oiling agents, for example, polyethyleneglycol and anionic oiling agents, for example, sulfate compounds, sulfonate compounds and phosphate compounds of polyethyleneglycol-copolymerized polyesters.

[0044] Surprisingly, it was discovered, for the first time, by the inventors of the present invention that, when immersed in a large amount of water, the polyamide-polyester composite fiber of the present invention loses its crimps and is straightened. Also, the polyamide-polyester composite fiber of the present invention exhibits a satisfactory rigidity or stiffness, due to the polyester copolymer filamentary segment, and a high hydrophilic property due to the polyamide filamentary segment, and therefore, shows an excellent uniform dispersing or suspending property in water.

[0045] When the aqueous slurry containing the polyamide-­polyester composite fibers of the present invention is subjected to a wet paper-forming process in a TAPPI paper-forming machine, and the resultant wet nonwoven fabric is dried in a hot air-circulating oven, for example, at a temperature of 150°C for 10 minutes, the resultant dry nonwoven fabric having, for example, a basis weight of 50 g/m², exhibits a high bulkiness. This is due to the fact that, in the drying procedure, a number of spiral crimps are created in the composite fibers. In the wet paper-forming procedure, however, the composite fibers are uncrimped and straightened, and therefore uniformly dispersed in water, and the oiling agent is washed out from the composite fibers.

[0046] The mechanism of the crimp-formation and the crimp-elimination is not completely clear, but it is assumed that these phenomena are derived from the highly sensitive water-absorption and -desorption of the polyamide filamentary segment and from the very delicate combination of the water-absorption and desorption with a high crystallization speed any crystal form of the polyamide filamentary segment. Accordingly, the drawing step and the restrictive relaxing step must be very strictly controlled.

[0047] In a preferable embodiment of the process of the present invention, the drawing procedure is carried out at a draw ratio corresponding to 90% to 98% of the ultimate draw ratio of the undrawn composite fiber, and the resultant composite fiber has a crimp number of substantially zero per 25 mm length of the fiber when immersed in water or placed in a wet condition at a temperature of 0°C to 100°C, and of 20 crimps/25 mm or more in an equilibrium condition of a temperature of 20°C and a relative humidity of 65°C.

[0048] The polyamide-polyester composite fibers of the present invention can be utilized for providing a wet process-produced nonwoven fabric which comprises 20 to 90% by weight of the polyamide-polyester composite fibers of the present invention and the balance consisting of at least one type of other fibers. This wet process-produced nonwoven fabric is advantageous in that it has a high dimensional stability, and a satisfactory bulkiness and stretchability.

[0049] When the content of the polyamide-polyester composite fibers is less than 20% by weight, the resultant nonwoven fabric has an unsatisfactory bulkiness. Also, if the content of the polyamide-­polyester composite fibers is more than 90% by weight, the resultant nonwoven fabric exhibits an unsatisfactory mechanical strength.

[0050] The other fibers to be blended with the polyamide-­polyester composite fibers of the present invention are preferably selected from drawn and undrawn polyester fibers. The polyester fibers are preferably selected from polyethylene terephthalate fibers, 5-sodiumsulfo­isophthalic acid-copolymerized polyethylene terephtha­late copolymer fibers. The content of 5-sodiumsulfo­isophthalic acid in the polyester copolymer is preferably in the range of from 2 to 10 molar%. Alternatively, the other fibers may be selected from nylon 6, nylon 66, nylon 4, nylon 46 and nylon 12 fibers.

EXAMPLES

[0051] The present invention will be further explained by way of specific examples, which in no way limit the scope of the invention.

[0052] In the examples, the measurements for the number of crimps and residual percentage crimp were carried out in accordance with Japanese Industrial Standard (JIS) L1015-(1981), in the following manner.

1). Number of crimps
A specimen was held at the two ends thereof by clamps of a crimp tester, and tensed at a load of 2 mg/denier, and then the distance (mm) between the clamps and the number of crimps in the specimen between the clamps were measured.
The number of crimps per 25 mm length of the specimen was determined.
The crimp number of the specimen in a wet condition was determined by applying the above-mentioned measurement to the specimen after immersion in water for 1 hour, without drying the specimen.
The crimp number of the specimen in a dry condition was determined by applying the above-mentioned measurement after the specimen had been left to stand in a chamber at a temperature of 20°C and a relative humidity of 65%.

2) The thickness of a nonwoven fabric was measured in accordance with JIS P8118-(1976).

3) The tensile strength and ultimate elongation of a nonwoven fabric were measured in accordance with JIS P8113-(1976), using a specimen having a width of 15 mm.

4) The softness of a nonwoven fabric was measured in accordance with JIS L1096-(1979), 45 degree cantilever method.

5) The number of pills composed of a plurality of fibers entangled with each other in a nonwoven fabric having a surface area of 10 m² or more was counted by the naked eye, and the counted number of the pills was converted to the number of the pills per 1 m² of the nonwoven fabric area.

Examples 1 to 9 and Comparative Examples 1 to 4

[0053] In each of Examples 1 to 9 and Comparative Examples 1 to 4, side-by-side type undrawn composite filaments were prepared from a nylon 66 resin having a limiting viscosity [η] of 1.17 determined in m-cresol at 30°C, and a polyester copolymer resin composed of polyethylene terephthalate copolymerized with 4.5 molar% of 5-sodiumsulfo-isophthalic acid and having a limiting viscosity [η] of 0.37 determined in o-chlorophenol at 25°C, by a usual melt-spinning process for side-by-side type composite fibers, at a spinning temperature of 285°C and a taking off speed of 1,100 m/min. The volume ratio of the polyamide filamentary segment to the polyester copolymer filamentary segment in each undrawn filament was 50:50.

[0054] The undrawn composite filaments were drawn and then restrictively relaxed in the manner as shown in Table 1.

[0055] In the restrictive relaxing procedure, the shrinkage (S) of the composite filaments was determined in accordance with the following equation:

wherein S_h represents a shrinkage of the filaments, S_f represents a peripheral speed of a feeding roll for feeding the filaments to the relaxing step, and S_d represents a peripheral speed of a delivery roll for delivering the filaments from the relaxing step.

[0056] The resultant drawn and relaxed filament tow was cut to a length of 51 mm in a water-wet condition, to provide short composite fibers. The crimp number of the short composite fibers was measured in a wet condition, and after drying at room temperature. The results are shown in Table 1.

[0057] The individual short composite fibers had an average thickness of about 2.5 denier.

[0058] The drying procedure at room temperature was carried out by storing the cut wet composite fibers in a closed room at a temperature of 20°C and a relative humidity of 65%, for 24 hours.

[0059] The test results are shown in Table 1.

Note: (*)1 ... The ultimate draw ratio was in the range of from 3.95 to 4.06.

Examples 10 to 14 and Comparative Examples 5 to 6

[0060] In each of Examples 10 to 14 and Comparative Examples 5 and 6, the same procedures as those in Example 1 were carried out except that the drawing and restrictive relaxing steps were carried out in the manner as shown in Table 2.

[0061] The test results are shown in Table 2.

Examples 15 to 18 and Comparative Examples 7 to 10

[0062] In each of Examples 15 to 18 and Comparative Examples 7 to 10, the same procedures as those in Example 1 were carried out, with the following exceptions.

[0063] The melt-spinning procedure was carried out by using a composite filament spinneret having 100 orifices at an extruding rate of 40 ml/min.

[0064] The resultant undrawn composite filaments had an ultimate draw ratio of 3.1.

[0065] The undrawn composite filaments were subjected to drawing, restrictive relaxing, and heat-setting proce­dures in the manner as indicated in Table 3.

[0066] The wet crimp number and the dry crimp number of the resultant composite filaments were measured.

[0067] The results are shown in Table 3.

Example 19 to 22 and Comparative Example 11 to 15

[0068] In Example 19 to 22, the heat-set composite fibers mentioned in Examples 15 to 18 were employed to prepare nonwoven fabrics, respectively. In Comparative Exam­ples 11 to 15, the heat-set composite fibers mentioned in Comparative Examples 7 to 10 were employed to prepare nonwoven fabrics, respectively. In Comparative Example 15, comparative side-by-side type polyester-­polyester composite fibers were employed to prepare a nonwoven fabric. This comparative composite fiber is composed of 50% by volume of a polyethylene terephtha­late filamentary segment and 50% by volume of a poly­ethylene terephthalate copolymer filamentary segment containing 3.0 molar% of copolymerized 5-sodiumsulfo­isophthalic acid. Undrawn polyethylene terephthalate filaments were prepared by melt-spinning a polyethylene terephthalate resin having a limiting viscosity [η] of 0.64 through a spinning orifices having a circular cross-section at a take-up speed of 1000 m/min. The resultant individual undrawn polyester filaments had a denier of 1.2.

[0069] A portion of the undrawn polyester filaments were drawn to provide drawn polyester filaments having a denier of 0.5.

[0070] The above-mentioned composite filaments, the undrawn polyester filaments and the drawn polyester filaments were cut to a length of 5 mm.

[0071] In each of Examples 19 to 22 and Comparative Examples 11 to 15, the composite fibers, the undrawn polyester fibers and the drawn polyester fibers were evenly blended in a blend ratio of 40:30:30 by weight and dispersed in water in a beater. The resultant fiber slurry was subjected to a wet paper-forming process by a cylinder paper machine. The resultant wet sheet was dehydrated and dried in a dryer at a temperature of 120°C. The resultant nonwoven fabric had a basis weight of 25 g/m².

[0072] The properties of the nonwoven fabric are shown in Table 4.

[0073] Each of the nonwoven fabrics of Examples 19 to 22 and Comparative Examples 11 and 15, which had the very small number of defective fiber pills, was subjected to a fuse-bonding treatment in which the nonwoven fabric was pressed by an embossing metallic roll having 400 embossing patterns per 25.4 mm x 25.4 mm area at a temperature of 190°C under a pressure of 60 kg/cm². Then a polyamide adhesive agent was applied in the form of a number of dots regularly distributed at predeter­mined intervals on one surface of the fuse-bonded nonwoven fabric, to provide a bonding padding cloth.

[0074] The touch and hand of the resultant padding cloth was compared with that of a conventional padding cloth composed of a nylon 6 nonwoven fabric. Also, the resultant padding cloth was adhered to a polyester fabric at a temperature of 150°C. The bonding property of the padding cloth was compared with that of the conventional padding cloth.

[0075] The results are shown in Table 5.

[0076] From Table 4 and 5, it is clear that the nonwoven fabrics made from the polyamide-polyester composite fibers of the present invention had a satisfactory bulkiness, evenness, and softness. Also, the nonwoven fabrics of the present invention had a satisfactory thermal stability, and therefore, can be smoothly fuse-bonded without curling.

Examples 23 to 25 and Comparative Examples 16 and 17

[0077] In each of Examples 23 to 25 and Comparative Examples 16 and 17, the same procedures as in Example 1 were carried out with the following exceptions.

[0078] The polyester copolymer consisted of a copolymeri­zation product of an aromatic dicarboxylic acid component consisting of 3.5 molar% of 5-sodiumsulfo­isophthalic acid and 96.5 molar% of terephthalic acid with a glycol component consisting of 5 molar% of tetramethylene glycol and 95 molar% of ethylene glycol, and had a limiting viscosity [η] of 0.50.

[0079] The undrawn composite filaments were drawn and restrictively relaxed in the manner as indicated in Table 6. The wet and dry crimp numbers of the resultant composite fibers are shown in Table 6.

Examples 26 to 28 and Comparative Examples 18 to 20

[0080] In each of Examples 26 to 28 and Comparative Example 18 to 20, a nonwoven fabric was prepared from composite fibers, wood pulp and polyolefin fibers each having a length of 5 mm, by a wet paper-forming process.

[0081] In Examples 26 to 28, the composite fibers men­tioned in Examples 23 to 25 were respectively employed. In Comparative Example 18 and 20, the composite fibers mentioned in Comparative Examples 16 and 17 were respec­tively employed. In Comparative Example 20, the same polyester-polyester copolymer composite fibers as mentioned in Comparative 15 were used.

[0082] The composite fibers, the wood pulp and the polyolefin fibers were blended in a blending weight ratio of 50:30:20 and dispersed in water in a beater. The wood pulp had a freeness (Canadian) of 600 ml and polyolefin fibers had a denier of 3 and a length of 5 mm, and were available under the trademark of ES fibers from Chisso K.K.

[0083] The wet paper-forming process was carried out by using a cylinder paper machine and the resultant wet sheet was dried at a temperature of 115°C. The dried nonwoven fabric was heat treated in a hot air dryer at a temperature of 130°C for 30 seconds. The resultant nonwoven fabric had a basis weight of 40 g/m².

[0084] The properties of the nonwoven fabric are shown in Table 7.

Examples 29 to 33 and Comparative Examples 21 and 22

[0085] In each of Examples 29 to 33 and Comparative Examples 21 and 22, the same procedures as those in Examples 15 and 19 were carried out except that the composite fibers, the undrawn polyester fibers and the drawn polyester fibers were blended in the blending weight ratio as indicated in Table 8.

[0086] The properties of the resultant nonwoven fabric are shown in Table 8.

[0087] Table 8 clearly shows that the nonwoven fabrics of the present invention exhibit a satisfactory bulkiness and mechanical strength, whereas in Comparative Example 21 in which the composite fibers were used in a small content of less than 20% by weight, the resultant nonwoven fabric has a poor bulkiness and in Comparative Example 22 in which the composite fibers were used in a large amount of more than 90% by weight, the resultant nonwoven fabric exhibited a poor mechanical strength.

Examples 34 to 40 and Comparative Example 23 to 27

[0088] In each of Examples 34 to 40 and Comparative Example 23 to 27, the same procedures as those in Example 15 were carried out except that the drawing procedures, the restrictive relaxing procedures and the heat setting procedures under tension were carried out in the manner as indicated in Table 9.

[0089] The resultant composite fibers had the wet crimp number and the dry crimp number as indicated in Table 9.

Examples 41 to 47 and Comparative Examples 28 to 33

[0090] In each of Examples 41 to 47 and Comparative Examples 28 to 33, a nonwoven fabric was produced as follows.

[0091] In Examples 41 to 47, the composite fibers as mentioned in Examples 34 to 40 were respectively employed, in Comparative Examples 28 to 33, the com­posite fibers as mentioned in Comparative Examples 23 to 26 were respectively employed, and in Comparative Example 33, the same polyester-polyester copolymer composite fibers as mentioned in Comparative Example 15 were used.

[0092] In each of those examples and comparative examples, the composite fibers were blended with the same undrawn polyester fibers and the drawn polyester fibers as those mentioned in Example 19 in the blending weight ratio of 40:30:30, and the blend was converted to a nonwoven fabric in the same manner as in Example 19.

[0093] The properties of the resultant nonwoven fabric are shown in Table 10.

Claims

1. A polyamide-polyester composite fiber, com­prising:
a polyamide filamentary segment; and
a polyester copolymer filamentary segment comprising a copolymerization product of an aromatic dicarboxylic acid component comprising 2.0 to 10.0 molar% of 5-sodiumsulfo-isophthalic acid and the balance consisting of terephthalic acid with a glycol component comprising at least one type of alkylene glycol having 2 to 10 carbon atoms;
said polyamide filamentary segment and said polyester copolymer filamentary segment extending in parallel to each other along the longitudinal axis of the composite fiber and being bonded together in a side-by-side relationship, and said composite fiber having a crimp number of 2 crimps/25 mm or less in water or under a wet condition at a temperature of 100°C or less, and of 5 crimps/25 mm or more under an equilibrium condition of a temperature of 20°C and a relative humidity of 65%.

2. The composite fiber as claimed in claim 1, which has a denier of 0.1 to 15.0 and a length of 3 to 30 mm.

3. The composite fiber as claimed in claim 1, wherein the polyamide filamentary segment comprises at least one member selected from the group consisting of nylon 4, nylon 46, nylon 6, nylon 66 and nylon 12.

4. The composite fiber as claimed in claim 1, wherein the polyamide filamentary segment comprises at least one polyamide having a limiting viscosity [η] of 1.0 to 1.4 determined in m-cresol at a temperature of 30°C.

5. The composite fiber as claimed in claim 1, wherein the polyester copolymer filamentary segment comprises at least one polyester copolymer having a limiting viscosity [η] of 0.35 to 0.70 determined in o-chlorophenol at a temperature of 25°C.

6. The composite fiber as claimed in claim 1, wherein the polyamide filamentary segment and the polyester copolymer filamentary segment are in a volume ratio to each other of 35:65 to 65:35.

7. The composite fiber as claimed in claim 1, wherein the crimp number is substantially zero per 25 mm of the composite fiber in water or under a wet condition at a temperature of 0°C to 100°C and 20 crimps/25 mm or more under the equilibrium condition of a temperature of 20°C and a relative humidity of 65%.

8. A process for producing a polyamide-polyester composite fiber, comprising the steps of:
preparing an undrawn composite fiber which comprises a polyamide filamentary segment and a polyester copolymer filamentary segment comprising a copolymerization product of an aromatic dicarboxylic acid component comprising 2.0 to 10.0 molar% of 5-sodiumsulfo-isophthalic acid and the balance con­sisting of terephthalic acid with a glycol component comprising at least one type of alkylene glycol having 2 to 10 carbon atoms, and in which the polyamide fila­mentary segment and the polyester copolymer segment extend in parallel to each other along the longitudinal axis of the undrawn composite fiber and are bonded together in a side-by-side relationship, by a melt-­spinning procedure;
drawing the undrawn composite fiber at a draw ratio corresponding to 88% to 98% of the ultimate draw ratio thereof; and
restrictively relaxing the drawn com­posite fiber in hot water at a temperature of 80°C to 90°C to an extent such that the drawn composite fiber is allowed to shrink to a length thereof corresponding to 85% to 98% of that of the drawn composite fiber, to provide a composite fiber having a crimp number of 2 crimps/25 mm or less in water or under a wet condition at a temperature of 100°C or less, and of 5 crimps/25 mm or more under an equilibrium condition of a temperature of 20°C and a relative humidity of 65%.

9. The process as claimed in claim 8, wherein the drawing procedure is carried out in hot water at a temperature of 65°C to 75°C.

10. The process as claimed in claim 9, wherein the drawing procedure is carried out at a draw ratio corresponding to 90% to 98% of the ultimate draw ratio of the undrawn composite filament, and the resultant composite fiber has a crimp number of substantially zero per 25 mm length of the composite fiber in water or under a wet condition at a temperature of 0°C to 100°C and 20 crimps/25 mm or more under the equilibrium condition of a temperature of 20°C and a relative humidity of 65%.

11. A wet process-produced nonwoven fabric comprising 20 to 90% by weight of the polyamide-­polyester composite fibers as claimed in any of claims 1 to 7 and the balance consisting of at least one type of other fibers.