JP2000144518A - Production of conjugate fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a conjugate fiber having few unevenness of the fiber thickness while having excellent crimp-generating properties, and hardly causing peeling at the conjugated face at the time of use, and unevenness in dyeing in the time of dyeing. SOLUTION: A hole of a spinning nozzle composed of two circular arc slits A and B, satisfying the inequalities B1<A1, 1.1<=SA/SB<=1.8, 0.4<=(SA+SB)/ SC<=10.0 and d/A1<=3.0, and having the center point of the discharge side eccentric to the highly viscous polymer side based on the center point at the introduction side of the hole of the spinning nozzle, is used, and the melt spinning is carried out while adjusting the highly viscous polymer so as to be mainly discharged from the slit A side to produce the objective conjugate fiber of two kinds of polymers which have the different viscosities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a side-by-side conjugate fiber having excellent crimp development performance and a method for producing the same. More specifically, the present invention relates to a side-by-side type conjugate fiber which has excellent crimp development performance, has less unevenness in the thickness of a fiber, and is less likely to cause peeling of a bonded surface during use or unevenness during dyeing, and a method for producing the same.

[0002]

2. Description of the Related Art Various methods have been proposed for producing a composite fiber having a latent crimp developing ability by composite spinning two kinds of fiber-forming thermoplastic polymers having different viscosities. However, if the difference in melt viscosity between the polymers is increased to provide better latent crimp development ability, the viscosity difference causes the composite flow of the polymer discharged from the spinneret to bend, a so-called kneeling phenomenon. In addition, there is a problem that not only single yarn breakage occurs frequently and spinning cannot be performed stably, but also the quality of the obtained composite fiber such as unevenness in thickness and unevenness in dyeing at the time of dyeing is deteriorated. Was.

[0003] In order to solve the problem caused by the difference in melt viscosity, conventionally, a thickener is blended with a polymer having a low viscosity or a viscosity reducer is blended with a polymer having a high viscosity. A method to reduce the difference, a method in which each polymer is separately discharged from the spinneret, and a method in which the two discharge streams are immediately joined by collision, or a low-viscosity side polymer stream is inserted from the side into the high-viscosity polymer stream A method has been proposed in which the linear velocities of both polymer streams are made uniform. However, the method of adding a thickener or a thickener has a problem that the obtained conjugate fiber is colored and the ability to develop latent crimp is reduced. In addition, in the method of joining both polymer streams immediately after discharge, it is necessary to punch the spinning nozzle hole obliquely, which not only increases the die cost but also reduces the cross-sectional shape after joining into a constricted shape like a dumpling. Therefore, there is a problem that it is easy to peel off at the joint. Further, in the method of inserting a low-viscosity polymer stream from the side into a high-viscosity polymer stream, the effect of suppressing the kneeling is still insufficient.

[0004] As another method, a method of forming two side-by-side type hollow composite fibers of two fiber-forming thermoplastic polymers having different viscosities to improve the crimp developing ability (Japanese Patent Publication No. 47-17089), At this time, a method of suppressing the occurrence of the kneeling phenomenon by specifying the slit shape of the spinning nozzle hole and also specifying the positional relationship of the composite polymer flow discharged from the slit (Japanese Patent Publication No. 5-65604, Japanese Patent Laid-Open No. -226008) has been proposed. However, any of these methods has a problem that the fluctuation of the hollow ratio and the fluctuation of U% are large, and accordingly, there is a problem that a variation in crimp developing ability and a spotted spot at the time of dyeing become large, and the quality is made more uniform. There is a need for the development of composite fibers.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned prior art, and has as its object the advantage of excellent crimp development performance, and the occurrence of unevenness in thickness due to the fact that the kneeling phenomenon is unlikely to occur during yarn production. An object of the present invention is to provide a side-by-side type conjugate fiber which has a small amount of peeling at a bonding surface during use and a spot at the time of dyeing.

[0006]

According to the study of the present inventors, the above-mentioned problem has been solved in the case of melt-spinning two kinds of fiber-forming thermoplastic polymers having mutually different viscosities into a side-by-side type; The center point of the spinning nozzle hole on the discharge side is biased toward the high-viscosity polymer flow side with respect to the center point of the spinning nozzle hole on the side where the side-by-side type joint composite flow of the two fiber-forming thermoplastic polymers is introduced. The melt spinning is performed from the centered spinning nozzle hole, and (ii) the opening of the spinning nozzle hole is constituted by two arc-shaped slits A and B arranged on the substantially same circumference at an interval (d). , The area S of the arc-shaped slit A
A, the slit width A ₁ , the area SB of the arc-shaped slit B, the slit width B ₁ , and the area SC surrounded by the inner peripheral surfaces of the arc-shaped slits A and B are spinning nozzle holes that simultaneously satisfy the following expressions: And B ₁ <A ₁ 1.1 ≦ SA / SB ≦ 1.8 0.4 ≦ (SA + SB) /SC≦10.0 d / A ₁ ≦ 3.0 (iii) Fiber forming property with higher viscosity The composite fiber is characterized in that the thermoplastic polymer is melt-spun from the arcuate slit A side, while the fiber-forming thermoplastic polymer having the lower viscosity is discharged mainly from the arcuate slit B side. It has been found that this can be achieved by the "production method".

Another object of the present invention is to provide a composite fiber in which two fiber-forming thermoplastic polymers having different viscosities are joined in a side-by-side type at a weight ratio of 40/60 to 60/40. Total crimp rate (TC) of the composite fiber
Is not less than 4.0% and the thickness unevenness (U%) is not more than 0.8%. "

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention will be described below in detail with reference to the drawings. FIG. 1 is an example showing an opening shape of a spinning nozzle hole constituted by two arc-shaped slits A and B used in the present invention. The fiber-forming thermoplastic polymer having higher viscosity is a circle. From the side of the arc-shaped slit A, the fiber-forming thermoplastic polymer having the lower viscosity is the arc-shaped slit B
It is mainly discharged from the side. FIG. 2 shows the positional relationship between the high-viscosity polymer flow and the low-viscosity polymer flow flowing in the spinning nozzle hole, and the spinning nozzle hole cut at a plane perpendicular to the joining surface of the polymer composite flow. It is the schematic which showed the cross section typically.

In FIG. 1, SA and A ₁ respectively represent the area and slit width of the arc-shaped slit A, SB and B ₁ represent the area and slit width of the arc-shaped slit B, respectively, and SC represents the arc-shaped slit A and A, respectively. B represents the area surrounded by the inner peripheral surface, and d represents the interval between the arc-shaped slits A and B. In the present invention, it is important that these satisfy the following formulas at the same time. B ₁ <A ₁ 1.1 ≦ SA / SB ≦ 1.8 0.4 ≦ (SA + SB) /SC≦10.0 d / A ₁ ≦ 3.0

In the present invention, the above formula is particularly important. When the ratio is less than 0.4, the obtained composite fiber has a high hollow ratio, and the joining of both polymers is not possible. It is not preferable because it becomes sufficient to easily cause peeling (hollow cracking), and the fluctuation of the hollow ratio, the variation of crimping ability, the U% unevenness, and the like increase. On the other hand, if it exceeds 10.0, the effect of suppressing the kneeling phenomenon is insufficient, and the spinning process condition is deteriorated, so that the U% of the obtained composite fiber and the spots at the time of dyeing are undesirably increased. A particularly preferred range is 1.5
６6.0.

On the other hand, if the width A _{1 of the} slit A is smaller than the width B _{1 of the} slit B, the high-viscosity polymer flows into the low-viscosity side and the effect of suppressing the kneeling phenomenon becomes insufficient, which is not preferable. Among them, A ₁ / B ₁ is 1.2 to 1.
When it is in the range of 6, it is more preferable because it becomes easy to perform the divided discharge between the slits.

When the slit area ratio SA / SB is less than 1.1, the effect of suppressing the kneeling phenomenon becomes insufficient.
On the other hand, when it exceeds 1.8, the spinnability deteriorates and it becomes impossible to spin stably, which is not preferable. A particularly preferred range of the area ratio is 1.2 to 1.6.

Further, when the interval d between the arc-shaped slits A and B exceeds three times the width A ₁ of the arc-shaped slit A,
It is not preferable because the joining of the polymer streams discharged from both slits becomes insufficient and the polymer stream is easily separated. Particularly preferred d / A ₁ range is 0.2 to 2.0.

The central angle of the arc-shaped slit A is determined by the area ratio of the arc-shaped slits A and B, the viscosity difference of the polymer used,
The weight ratio may be appropriately changed depending on the composite weight ratio, etc., but the hollow portion having a hollow ratio of 8% or less, preferably 1 to 6% toward the high-viscosity polymer component is provided when the composite weight ratio is in the range of 40/60 to 60/40. When the central angle is increased so as to form a film, the crimp development performance is particularly improved. On the other hand, when the central angle is reduced, the viscosity of the low-viscosity side polymer is selected (lower viscosity) to thereby improve the crimp development performance. It is preferable because a solid composite fiber having particularly excellent uniformity can be obtained.

[0015] In addition to the above requirements, the spinning nozzle hole of the present invention is a center point of the spinning nozzle hole on the side where a composite flow in which two kinds of fiber-forming thermoplastic polymers having different viscosities are joined side-by-side is introduced. On the other hand, the center point of the opening of the spinning nozzle hole on the side to be discharged needs to be eccentric to the high-viscosity polymer side. In particular, as shown in FIG.
In the vertical sectional view of the spinning nozzle hole, it is preferable that the side on which the higher viscosity fiber-forming thermoplastic polymer flows is substantially straight. As shown in FIG. 2 (b) or (c), when the center point of the opening of the spinning nozzle hole is not eccentric or eccentric toward the low viscosity polymer, the kneeling phenomenon can be suppressed. No, the object of the present invention cannot be achieved.

In the present invention, as described above, a composite stream of two fiber-forming thermoplastic polymers having different viscosities, which are joined side-by-side using the above-described spinning nozzle holes, are mixed with one another having a higher viscosity. It is necessary to melt-spin so that the fiber-forming thermoplastic polymer is discharged from the arc-shaped slit A side, and the lower viscosity fiber-forming thermoplastic polymer is mainly discharged from the arc-shaped slit B side. Preferably, the high-viscosity polymer is discharged from the arc-shaped slit A side. Also, by appropriately selecting the viscosity difference of the fiber-forming thermoplastic polymer and the dimensions of each part of the spinning nozzle hole, the joint surface shape of both polymers is made substantially flat, which is a point of crimp development performance. Is preferred.

The fiber-forming thermoplastic polymer used in the present invention is not particularly limited as long as it is a polymer capable of forming fibers by melt spinning.
Any thermoplastic polymer such as polyamide and polyolefin can be mentioned, and among them, a fiber-forming polyester is preferable.

As a specific example of such a fiber-forming polyester, a polyethylene terephthalate-based polyester whose main repeating unit is ethylene terephthalate, or a polybutylene terephthalate-based polyester whose main repeating unit is butylene terephthalate is preferable. Excellent conjugate fibers can be obtained.

The above-mentioned polyethylene terephthalate-based or polybutylene terephthalate-based polyester is
If necessary, it may have a small amount (usually less than 30 mol%) of a copolymer component. Examples of the copolymer acid component include isophthalic acid, diphenyldicarboxylic acid, naphthalenedicarboxylic acid, 5-sodium sulfoisophthalic acid, Adipic acid, sebacic acid, etc., as the oxycarboxylic acid component, parahydroxybenzoic acid, p- (β-hydroxyethoxy) benzoic acid, etc., and as the diol component, trimethylene glycol, hexamethylene glycol,
Examples thereof include neopentyl glycol, bisphenol A (a phenolic hydroxyl group to which ethylene oxide is added), polyethylene glycol, polytetramethylene glycol, and the like. In particular, when isophthalic acid and / or bisphenol A is copolymerized with 5 to 25 mol%, preferably 8 to 15 mol%, of the polyester having higher viscosity, the crimp development performance of the obtained composite fiber is further improved. It is desirable.

Specific examples of polyamide and polyolefin include nylon-6, nylon-66, polypropylene and the like, and those containing a small amount of a copolymer component.

In the present invention, two different viscosities are used.
The above fiber-forming thermoplastic polymers (high-viscosity polymer, low-viscosity polymer) are used in combination, but the adhesion between the high-viscosity polymer and the low-viscosity polymer is excellent, and peeling occurs. The combination of the same type of polymer is preferable because it becomes difficult.

The viscosity difference between the high-viscosity polymer and the low-viscosity polymer is suitably in the range of 100 to 1800 poise, preferably 300 to 900 poise at the temperature during melt spinning, and this viscosity difference is 100 poise. If it is less than 1, the latent crimp developing ability tends to decrease, while if it exceeds 1,800 poise, the effect of suppressing the kneeling phenomenon of the present invention cannot be sufficiently exhibited.

A particularly preferred combination of a high-viscosity polymer and a low-viscosity polymer is an ethylene terephthalate copolymer in which the high-viscosity polymer is copolymerized with isophthalic acid or bisphenol A in an amount of 5 to 25 mol%, preferably 8 to 15 mol%. A combination of a polymerized polyester and a polyester in which the low-viscosity polymer consists essentially of ethylene terephthalate. At that time, it is preferable that the intrinsic viscosity has a difference of 0.1 to 0.5, particularly 0.15 to 0.3, and the intrinsic viscosity of the high-viscosity polyester is in the range of 0.4 to 0.8. It is preferable that the intrinsic viscosity of the low-viscosity polyester is in the range of 0.3 to 0.7. According to such a combination, the object of the present invention can be highly achieved. Here, the intrinsic viscosity is measured at a temperature of 30 ° C. in an orthochlorophenol solvent.

In the present invention, the high-viscosity polymer and the low-viscosity polymer are formed into a conjugate fiber joined in a side-by-side manner, and the composite weight ratio of the high-viscosity polymer / low-viscosity polymer is 60/40 to 40/40. 40/60, preferably 55 / 45-4
A range of 5/55 is appropriate. If the proportion of the high-viscosity polymer is too large, the crimp developing ability of the obtained composite fiber tends to decrease, while if the proportion of the low-viscosity polymer is too large, the crimp developing ability decreases. Not only do
The mechanical properties also tend to decrease, and the yarn production stability tends to decrease.

The melt spinning method, spinning apparatus, spinning conditions, drawing conditions and the like for producing the conjugate fiber of the present invention are conventionally adopted as long as they satisfy the above-mentioned essential requirements of the present invention. The method, apparatus, and conditions used can be adopted.

The fiber-forming thermoplastic polymer used in the present invention may contain, if necessary, a coloring inhibitor, a heat stabilizer, a flame retardant, and a fluorescent enhancer within a range not to impair the object of the present invention. One or more of whitening agents, matting agents, coloring agents, antistatic agents, hygroscopic agents, antibacterial agents, inorganic fine particles and the like may be added.

Next, the total crimp ratio (TC: see the column of Examples for the measuring method) of the composite fiber of the present invention produced by the above-mentioned production method or the like is 4.0% or more, preferably 5.0 to 15%. .0
%, And if it is less than 4.0%, it is not preferable because crimping performance becomes insufficient and it becomes difficult to obtain a product such as a fabric having excellent stretchability. It is important that the U% representing the thickness unevenness (see the column of Examples for the measuring method) is 0.8% or less, preferably 0.5% or less.
If it exceeds 8%, staining spots are likely to occur during staining,
In addition, there is a large variation in the crimp that has been revealed by heat treatment of the potential crimp performance, and the quality of the product such as a woven fabric deteriorates, which is not preferable.

Further, when a hollow portion having a hollow ratio of 8% or less is present in the conjugate fiber, and preferably a hollow portion having a hollow ratio of 5% or less, particularly 3% or less, is present in the high-viscosity polymer component. Since it has excellent crimping performance and small thickness unevenness (fineness unevenness and hollowness unevenness), it is possible to obtain a fabric excellent in quality and stretchability. On the other hand, when the hollow portion is not present in the conjugate fiber, the uniformity of the crimping performance is particularly improved, so that it is possible to obtain a high-quality fabric, which is preferable.

If the monofilament fineness of the conjugate fiber of the present invention is too small, the crimp development performance is reduced. On the other hand, if it is too large, the flexibility tends to decrease and the feeling tends to deteriorate. A range of 10 to 10 deniers, particularly 2.0 to 6.0 deniers, is suitable. The number of filament counts is not limited, but is suitably about 10 to 50.

When the elongation at break is less than 20%, the process condition such as breakage of a single yarn is liable to be deteriorated in the spinning process.
On the other hand, if it exceeds 50%, stain spots tend to occur,
A range of 20 to 50%, especially 25 to 40% is suitable.

If the breaking strength of the conjugate fiber is too small, the process condition in the spinning process is liable to deteriorate, while if it is too high, it is difficult to obtain a soft feeling when woven into a woven fabric.
A range of 5.0 g / de, particularly a range of 3.0 to 4.0 g / de, is appropriate.

[0032]

The present invention will be described more specifically with reference to the following examples. In addition, each evaluation item in an Example was measured according to the following method.

<Hollow crack> The hollow composite fiber (24
Take any section (50 filaments)
場合 indicates no hollow cracks, ○ indicates 5% or less, and × indicates more than 5%.

<Hollow Ratio> An arbitrary section (50 pieces) of the obtained hollow conjugate fiber (24 filaments) was taken, and the hollow ratio was determined from a micrograph thereof.

<U%> Using an Evenness tester, a sample was supplied to the detection end at a supply speed of 100 m / min while twisting at 3000 r / m to measure.

<Total crimp rate (TC)> A sample is wound around a stencil frame with a tension of 50 mg / denier, and is wound for about 3,000 d.
Make a case of e. After making the scallop, 2 mg / d
e + Apply a load of 200 mg / de and measure the length L ₀ (cm) after 1 minute. Then, 200 mg / de
In a boiling water at 100 ° C. with the load of
Process for a minute. After the boiling water treatment, the load of 2 mg / de is removed, and the product is air-dried in a free state for 24 hours. A load of 2 mg / de + 200 mg / de is applied again to the naturally dried sample, and the length L ₁ (cm) after one minute has elapsed is measured.
Then removed a load of 200 mg / de, the length L ₂ measured after a lapse of 1 minute was calculated on the total crimp ratio in the following formula. TC (%) = [(L ₁ −L ₂ ) / L ₀ ] × 100 The measurement was performed 10 times, and the average value was obtained.

<Spinning Condition> Spinning was performed at 4 positions for 5 days with a winding time of 2.5 hours and a winding amount of about 7 kg. The spinning condition was calculated as the number of broken yarns / number of yarns.

<Stretching condition> 5 days at 24 positions
The film was stretched so that the winding amount was about 2 kg in a stretching time of 52 hours. The stretching condition was calculated as the number of broken yarns / number of yarns.

<Hand of knitted fabric> The drawn yarn was knitted into a knitted knit, and the obtained knitted fabric was evaluated in the following three grades by a sensory test by 50 persons. 35 or more people are judged as having good texture: ○ 34 to 15 people are judged as having good texture: △ 14 or less people are judged as having good texture: ×

<Stains> The above-mentioned knitted fabric is dyed, and there is no spots: 、, when there is a single-pitch shade spot of about 5 mm in part and there is a thin spot entirely: Δ, dark shades When there is a spot: represented by x.

[Examples 1 to 6, Comparative Examples 1 to 6] FIG.
10 mol% of an isophthalic acid component was copolymerized as a high-viscosity polyester as a high-viscosity polyester from a spinneret having an opening shape shown in (a) and having 24 spinning nozzle holes each having the dimensions described in Table 1 in Table 1. Polyethylene terephthalate copolymer polyester having an intrinsic viscosity of 0.63, and polyethylene terephthalate having an intrinsic viscosity of 0.36 as a low-viscosity polyester were mixed side-by-side at a composite weight ratio of 50/50 to form a composite melt spinning (melt spinning temperature: 28
5 ° C., melt viscosity difference: 700 poise), winding speed 145
It was wound at 0 m / min. The positional relationship between the high-viscosity polymer flow and the low-viscosity polymer flow flowing in the spinning nozzle hole is as follows.
Comparative Example 1 is as shown in FIG. 2 (b), Comparative Example 2 is as shown in FIG. 2 (c), and others are as shown in FIG. 2 (a). The discharge amount is such that the total fineness of the obtained drawn yarn is 100 denier. I made it.

The obtained undrawn yarn is subjected to a drawing temperature of 85 ° C., a drawing speed of 600 m / min, and an elongation of the obtained drawn yarn of about 30%.
The film was stretched at the following magnification. Table 2 shows the obtained results.

[0043]

[Table 1]

[0044]

[Table 2]

[Examples 7 to 10] Composite spinning and drawing were performed in the same manner as in Example 1 except that the high-viscosity polyester and low-viscosity polyester described in Table 3 were melt-spun at a composite weight ratio as shown in Table 3. Thus, a composite fiber was obtained. Table 4 shows the results.

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

According to the production method of the present invention, while having high crimping performance, the unevenness of the thickness of the fiber is small, and the side-by-side surface is less likely to cause peeling of the bonding surface during use and unevenness of dyeing during dyeing. The type composite fiber can be manufactured with high productivity. In addition, the conjugate fiber of the present invention is suitable for manufacturing woven or knitted fabrics, and can provide a fabric having good stretchability, excellent wearing comfort, and good drapeability.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of an opening shape of a spinning nozzle hole used in the present invention.

FIG. 2 shows a positional relationship between a high-viscosity polymer flow and a low-viscosity polymer flow flowing in a spinning nozzle hole, and a cross section when the spinning nozzle hole is cut along a plane perpendicular to a joining surface of a polymer composite flow. It is the schematic diagram which showed the figure typically.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4L041 AA07 AA20 BA02 BA05 BA09 BA42 BA59 BC17 BC20 BD20 CA06 CA12 DD01 DD04 DD15 4L045 AA05 BA03 BA21 BA36 BA60 CA25 CB09 CB16 CB19 CB40 DA42 DC02

Claims (7)

[Claims] 1. A method for producing a composite fiber by melt-spinning two types of fiber-forming thermoplastic polymers having different viscosities into a side-by-side type, wherein (i) the side-by-side of the two types of fiber-forming thermoplastic polymers. With respect to the center point of the spinning nozzle hole on the side where the mold-joining composite flow is introduced, the center point of the spinning nozzle hole on the discharge side is melt-spun from the spinning nozzle hole eccentric to the high-viscosity polymer side, and (ii) The opening of the spinning nozzle hole is composed of two arc-shaped slits A and B arranged on the substantially same circumference at an interval (d). The area SA of the arc-shaped slit A, the slit width A ₁ , The area SB of the arc-shaped slit B, the slit width B ₁ , and the area SC surrounded by the inner peripheral surfaces of the arc-shaped slits A and B are spinning nozzle holes satisfying the following expressions at the same time, and B ₁ <A ₁ 1.1 ≦ SA / SB ≦ 1.8 .4 ≦ (SA + SB) /SC≦10.0 d / A 1 ≦ 3.0 (iii) higher fiber-forming thermoplastic polymer of viscosity from the circular slit A side, whereas lower viscosity fiber A method for producing a conjugate fiber, wherein the forming thermoplastic polymer is melt-spun so as to be mainly discharged from the arc-shaped slit B side. 2. The method for producing a conjugate fiber according to claim 1, wherein the vertical cross-sectional shape of the spinning nozzle hole is straight on the side where the higher viscosity fiber-forming thermoplastic polymer flows. 3. The fiber-forming thermoplastic polymer having two viscosities different from each other is a polyester.
A method for producing the composite fiber according to the above. 4. The high-viscosity polyester is an ethylene terephthalate copolymer polyester obtained by copolymerizing isophthalic acid or bisphenol A in an amount of 5 to 25 mol%, while the low-viscosity polyester is a polyester substantially consisting of ethylene terephthalate. Intrinsic viscosity difference is 0.1 ~
0.5, and their composite weight ratio is 40 / 60-
The method according to claim 3, wherein the ratio is 60/40. 5. A composite fiber in which two kinds of fiber-forming thermoplastic polymers having different viscosities are joined in a side-by-side type at a weight ratio of 40/60 to 60/40, the total crimp rate of the composite fiber ( (TC) is 4.0% or more, and the thickness unevenness (U%) is 0.8% or less. 6. The conjugate fiber according to claim 5, wherein the conjugate fiber has a hollow portion having a hollow ratio of 8% or less toward the high-viscosity polymer component or is solid. 7. The fiber-forming thermoplastic polymer having a higher viscosity is an ethylene terephthalate copolymer obtained by copolymerizing isophthalic acid or bisphenol A in an amount of 5 to 25 mol%, and the fiber-forming thermoplastic polymer having a lower viscosity is used. The conjugate fiber according to claim 5 or 6, wherein the thermoplastic polymer is a polyester substantially composed of ethylene terephthalate, and has a difference in intrinsic viscosity of 0.1 to 0.5.