JP3103023B2 - Method for producing composite sewing thread - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a loop,
The present invention relates to a method for producing a low-cost filament composite sewing thread having slack and excellent sewability.

[0002]

2. Description of the Related Art Sewing yarns currently used are broadly classified into filament sewing yarns composed of long fiber multifilament yarns and spun sewing yarns composed of short fiber spun yarns.

[0003] Filament sewing threads are high in strength and excellent in seam appearance, but are susceptible to thermal effects due to a low porosity between the needle holes, resulting in a decrease in the strength retention after sewing. There's a problem.

[0004] On the other hand, spun sewing threads have fluff and a high porosity, and therefore have excellent strength retention after sewing.
Although excellent in sewability, there is a disadvantage that the seam appearance is inferior to that of the filament sewing thread.

Therefore, as a sewing thread having both advantages of a filament sewing thread and a spun sewing thread, a core yarn sewing thread having a multifilament yarn as a core yarn and a spun yarn wrapped around the core yarn has been proposed. For example,
As a core component, a synthetic filament multifilament yarn is used as a core component, and a spun yarn of vegetable short fiber such as cotton is enveloped as a sheath component, or a polyester filament yarn is used as a core component,
A core yarn sewing thread having a polyester suf as a sheath component (Japanese Patent Publication No. 62-38457) is known.

[0006] However, in these core yarn sewing threads, the short fibers on the sewing thread surface are cut or dropped due to the tension or friction applied to the sewing thread during sewing, so that the needle portion of the sewing machine can be used.
It adheres to the hook part, the holding part, etc., and the sewing machine oil etc. infiltrates into it, causing the contaminated material to be sewn.Furthermore, it is difficult to uniformly cover the spun yarn around the filament yarn. There is a drawback in that the spun yarn of the sheath component is lifted due to a difference in shrinkage from the spun yarn, which causes skipping and thread breakage during sewing.

In order to eliminate these drawbacks of the core yarn sewing yarn, a multifilament yarn having a high elongation (a spontaneously extensible polyester multifilament yarn) and a multifilament yarn having a high tenacity and a low elongation are used. A composite sewing thread having a loop and a slack at the surface layer portion of the sewing thread has been proposed (JP-A-5-106134 and JP-A-6-128834).

However, in this composite sewing thread, multifilament yarns having different characteristics are prepared in advance,
Since the yarn is mixed and mixed later, there is a problem that the number of manufacturing steps increases and the cost increases.
A part where the multifilament yarn of the core component and the multifilament yarn of the sheath component were mixed together was formed, and in this part, the formation of loops and slacks on the surface layer of the sewing thread was insufficient, and it was possible as expected. There is also a problem that the sewability is not improved. In addition, the thickness of the multifilament yarn prepared in advance has its own limit, and it is difficult to obtain a very thin multifilament yarn. There is also a problem that the obtained composite sewing thread becomes thicker and a sewing thread with a small count cannot be obtained.

[0009]

SUMMARY OF THE INVENTION Accordingly, the present invention solves the above-mentioned problems of the conventional sewing thread and manufactures a sewing thread which is excellent in sewability and can be finely counted at a low cost. It is an object to provide a method.

[0010]

In order to solve the above problems, the present inventor has made various studies, and as a result, has conceived of adopting a cospun method for simultaneously spinning two or more kinds of multifilament yarns having different characteristics. Thus, the present invention has been completed.

That is, according to the present invention, (1) a multifilament yarn having a high birefringence is disposed substantially at the core portion, and a multifilament yarn having a low birefringence is disposed substantially at the sheath portion. The core-sheath type composite filament yarn having a birefringence difference of 0.01 or more is spun by the cospan method, and is heat-treated after or without stretching, so that a loop and a slack are developed on the surface layer of the sewing machine thread. (2) The method for producing a composite sewing thread according to (1), wherein the multifilament yarn is a polyester multifilament yarn, and (3) a spinning liquid guide rod protruding from a discharge surface. The method for producing a composite sewing thread according to the above (1) or (2), wherein the discharge hole is spun using a co-spun spinneret arranged on the center side, (4) a discharge hole having a discharge cross-sectional area that is continuously increased. To The method for producing a composite sewing machine yarn according to the above (1) or (2), wherein the multi-filament yarn having a low birefringence is arranged in a sheath portion, wherein the multifilament yarn is arranged in a sheath portion. The method for producing a composite sewing thread according to any one of the above (1) to (4), wherein the composite sewing thread is used in an amount of 5 to 40% by weight based on the total weight of the sewing thread.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, as a polymer which is spun by a cospun method to form a core-sheath type composite filament yarn, various thermoplastic polymers which can be melt-spun can be used. For example, polyethylene terephthalate can be used. And polyesters such as polybutylene terephthalate; polyamides such as nylon 6 and nylon 66; and polyolefins such as polyethylene and polypropylene. Among them, polyester is preferred from the viewpoint of cost and versatility, and particularly, polyester whose main repeating unit is an ethylene terephthalate unit is preferred. A small amount of the third component other than the terephthalic acid component and / or the ethylene glycol component may be copolymerized. Further, a mixture of a small amount of another polymer may be used.

In this case, the intrinsic viscosity of the polyester is particularly
Although not limited, heat resistance, abrasion resistance and strength
A higher limiting viscosity is desirable. However, the intrinsic viscosity is large
This increases the production cost,
Intrinsic viscosity of lament yarn [η] _FIs in the range of 0.6 to 1.0
It is appropriate to be inside.

Further, the cross-sectional shape of the multifilament yarn is not particularly limited, but a polygon such as a triangle is preferable when gloss is desired on the sewing thread, and a round shape is desired when the sewing thread is not inconspicuous because gloss is not desired. preferable. In addition, a multifilament yarn having a high birefringence to be arranged in the core,
The cross-sectional shape may be different between the multifilament yarn having a low birefringence and arranged in the sheath.

In the present invention, the multifilament yarn having a high birefringence is substantially arranged in a core portion, and the multifilament yarn having a low birefringence is spun by a cospun method so as to be substantially arranged in a sheath portion. Thus, it is important to obtain a core-sheath type composite filament yarn.

By making such a core-sheath type composite filament, the high birefringence multifilament yarn of the core part shrinks more than the low birefringence multifilament yarn of the sheath part by the subsequent heat treatment. , The low birefringence multifilament yarn of the sheath loops at the surface layer of the sewing thread,
It forms sagging and improves the sewability of the sewing thread.Simply blending a multi-filament yarn with a high birefringence index and a multi-filament yarn with a low birefringence index is not sufficient for the surface layer of the sewing thread. No loops or slacks can be formed, and the sewability is not improved.

In the present invention, since the core-sheath type composite filament yarn is spun by the cospan method, multifilament yarns having different characteristics are prepared in advance, and then combined with the conventional method of mixing and blending. In comparison, the number of manufacturing processes is significantly reduced, cost can be reduced,
Furthermore, there is no occurrence of a part where the multifilament yarn of the core part and the multifilament yarn of the sheath part are mixed together, and a sufficient loop and slack are formed on the surface layer of the sewing thread, and the sewability is improved. improves.

Further, according to the cospun method, the spun core-sheath type composite filament yarn can be used as it is without the necessity of twining and blending, so that it can be used with the minimum thickness that can be melt-spun. This makes it possible to reduce the number of sewing threads.

Further, in the present invention, spinning is performed so that the birefringence difference between the high birefringence multifilament yarn of the core portion and the low birefringence multifilament yarn of the sheath portion becomes 0.01 or more. is necessary. The difference between the birefringences is 0.
If it is less than 01, the expression of loops and slacks in the surface layer of the sewing thread due to the heat treatment will be insufficient, and the desired sewing thread with excellent sewability will not be obtained.

In the cospun method used in the present invention, a core-sheath composite filament yarn in which a multifilament yarn having a high birefringence is disposed in a core and a multifilament yarn having a low birefringence is disposed in a sheath is spun. It is a method of obtaining all at once in the process,
For example, (1) a method of spinning using a spinneret having two or more types of discharge holes having different hole diameters, (2) a bundle of spun filaments, which is combined with a focusing guide, a metering oil, an interlace, and the like to form a bundle. A method for generating a spinning tension difference due to an air resistance difference between the group and the unfocused group,
(3) A method in which the ejection surface of the polymer is made different and a projection die provided with a cooling difference is used.

In particular, a method of spinning a discharge hole having a spinning liquid guide rod protruding from a discharge surface by using a spinneret for cospan arranged on the center side, and a method of forming a discharge hole having a discharge cross-sectional area which continuously increases. The method of spinning using a co-spun spinneret arranged on the center side is preferably used because the birefringence difference between the two multifilament yarns can be increased.

FIG. 1 is a sectional view showing an example of a spinneret for cospan in which a discharge hole having a spinning liquid guide rod protruding from a discharge surface is arranged on the center side. And a discharge hole 5 in which a spinning liquid guide rod 4 protruding from the discharge surface 3 is fitted. Reference numeral 6 denotes a blade for fixing the spinning liquid guide rod 4 in the discharge hole 5.

When the thermoplastic polymer is melt-spun from the spinneret 1A at the same spinning temperature and the same spinning speed, the filament yarn discharged from the discharge hole 5 having the spinning liquid guide rod 4 disposed at the center side is The birefringence becomes higher than that of the filament yarn discharged from the normal discharge hole 2A disposed on the outer peripheral portion, and the multifilament yarn having a high birefringence is substantially disposed in the core portion and has a low birefringence. A core-sheath composite filament yarn in which the multifilament yarn is substantially arranged in the sheath is obtained. By changing the projection length L of the spinning liquid guide rod 4 from the discharge surface 3, the birefringence of the spun high birefringence filament yarn can be adjusted. In addition,
Japanese Patent Publication No. 2-20725 discloses a spinneret provided with a discharge hole having a spinning liquid guide rod protruding from the discharge surface.
This is described in detail in Japanese Patent Publication No. 3-43364.

FIG. 2 is a cross-sectional view showing an example of a spinneret for co-span in which a discharge hole whose discharge cross-sectional area is continuously increased is provided on the center side. A hole 2B is provided, and a discharge hole 7 whose discharge cross-sectional area is continuously increased is provided on the center side.

When the thermoplastic polymer is melt-spun from the spinneret 1B at the same spinning temperature and the same spinning speed, the filament yarn discharged from the discharge hole 7 provided on the center side and having a discharge cross-sectional area which continuously increases. Receives a large spinning draft and has a higher birefringence than a filament yarn discharged from a normal discharge hole 2B disposed in the outer peripheral portion, and a multifilament yarn having a high birefringence is substantially disposed in the core portion. And
A core-sheath type composite filament yarn in which a multifilament yarn having a low birefringence is substantially disposed in a sheath portion is obtained.

The birefringence of the spun high birefringence filament yarn is changed by changing the extent of the cross-sectional area of the discharge hole 7, that is, the diameter D of the large diameter portion, the diameter d of the small diameter portion, and the nozzle length l. Can be adjusted. The details of the discharge hole whose discharge cross-sectional area is continuously increased are described in JP-A No. 4-19 / 1992.
No. 4007 and JP-A-5-132810.

FIG. 3 shows a spinneret as described above.
FIG. 2 is a schematic diagram illustrating an example of an apparatus for performing cospun spinning, in which a filament yarn 12 discharged from a spinneret 11 passes through a hot zone 14 heated by a heater 13, is cooled by a cooling air blowing device 15, After the oil agent is applied by the oil agent applying device 16, the oil agent is wound around the undrawn yarn package 20 by the winding machine 19 via the godet rollers 17 and 18.

If the spinning speed is several thousand m / min or more and the yarn has sufficient strength and elongation characteristics as a sewing thread without stretching, the yarn is looped on the surface layer of the sewing thread without stretching. Heat treatment may be performed to develop sag, but when the spinning speed is low, heat treatment is performed after stretching is further performed.

FIG. 4 is a schematic view showing an example of an apparatus for performing this drawing. The undrawn filament yarn 21 unwound from the undrawn yarn package 20 is, for example, 85.degree.
The film is stretched at a predetermined magnification between the preheating roller 22 heated to 20 ° C. and the stretching roller 23. The stretching roller 23 is heated to, for example, 180 ° C. to 240 ° C., and after the stretched filament is thermally set on the stretching roller 23, the filament is relaxed between the godet roller 24 and
The yarn is wound around a drawn yarn package 26 by a winder 25.

When the undrawn yarn is drawn, it is preferable to draw it at a draw ratio of about 85% of the maximum draw ratio of the high birefringence multifilament yarn. When the drawing is performed at such a draw ratio, for example, when the birefringence difference is 0.01, the low birefringence multifilament yarn is drawn to about 65% of its maximum draw ratio, and the drawn composite filament yarn is the same. When heat-set under the conditions, the low birefringence multifilament yarn of the sheath component has higher elongation, lower strength, and lower shrinkage (compared to the higher birefringence multifilament yarn of the core component drawn under a higher load). Or self-elongation), and a sufficient loop and slack are formed in the surface layer of the sewing thread in the subsequent heat treatment step.

It is to be noted that a so-called straight drawing method in which the spun filament yarn is continuously drawn without being once wound up, may be employed.

The core-sheath type composite filament yarn (single yarn) thus obtained preferably has a low heat shrinkage from the viewpoint of production of a sewing thread, and has a boiling water shrinkage of 3.0% or less. desirable.

The single yarn obtained by the above method is twisted as required.

In the present invention, there is no particular limitation on the twist structure and the number of twists of the sewing thread. For example, a single twisted sewing thread (also referred to as a single twist sewing thread) twisted into a single thread, an S twist (primary twist) ), A twin-thread sewing thread in which two single yarns are Z-twisted (up-twisted),
The present invention is applied to, for example, a triple thread sewing thread in which three single yarns subjected to S-twist (primary twist) are Z-twisted (up-twisted).

In this case, the ratio of the multifilament yarn having a low birefringence constituting the sheath portion to the total weight of the sewing thread is preferably 5 to 40% by weight, more preferably 1 to 40% by weight.
0 to 30% by weight. If the ratio is too low, the sewability is difficult to improve, while if it is too high, the strength of the sewing thread tends to decrease.

In the present invention, the single yarn may be used for all the single yarns of the sewing thread, or one single yarn of the double thread sewing thread or one single yarn of the triple thread sewing thread. It can also be used for some single yarns, such as used for yarn or two single yarns.

Next, dyeing is performed. In the case of a polyester sewing thread, it is usually performed at about 130 ° C., and may be dyed in a cheese shape or a scab shape.

As for the heat treatment for developing a loop and a slack in the surface layer portion of the sewing thread, for example, in the case of a polyester sewing thread, the heat treatment may be performed at a temperature of 80 ° C. to 150 ° C. while allowing shrinkage. Later, it can be performed at any stage. In particular, in the case of a polyester sewing thread, since a loop and a slack are sufficiently developed at a dyeing temperature (around 130 ° C.), it is convenient to simultaneously develop a loop and a slack at the time of dyeing.

[0039]

The present invention will be described in more detail with reference to the following examples. The evaluation of the sewing thread was performed by the following method.

(1) High-speed sewability When three T / R surges are sewn with a sewing machine needle # 14 at a speed of 4000 rpm using a lockstitch single-needle sewing machine, and when the sewing thread is frequently cut, x: The cutting was very slight, but was indicated by 場合 when there was no practical problem, and ◎ when not cut at all.

(2) Back stitch sewability One T / R surge of 30 cm in the back direction with a sewing machine needle # 11 at a speed of 2000 rpm using a lockstitch single needle sewing machine.
If the sewing machine thread was frequently cut after sewing, the cross was indicated by x, and the cut was very slight. However, there was no problem in practical use.

(3) Puckering One T / R broad was sewn with a sewing machine needle # 11 at a speed of 3500 rpm using a lockstitch single-needle sewing machine. The case where the occurrence was large is indicated by x.

(4) Sewing thread surface loop The number of raised loops and slacks on the surface of the sewing thread having a length of 20 cm and about 0.2 to 1.0 times the apparent diameter of the sewing thread is visually counted. , Pieces / cm.

[Examples 1 to 4, Comparative Examples 1 and 2] Polyethylene terephthalate having an intrinsic viscosity of 0.80 was prepared by using a spinning apparatus shown in FIG. 3 and a spinneret for cospan shown in FIG.
Spin at a spinning temperature of 295 ° C. and a spinning speed of 1000 m / min.
Sixteen high birefringence filaments are placed in the core and 4
A core-sheath type undrawn composite filament yarn in which the low birefringence filaments were arranged in the sheath portion was obtained.

In the spinneret for cospan shown in FIG.
The nozzle diameter of the normal discharge hole 2A is 0.25 mm, the nozzle length is 0.5 mm, the nozzle diameter of the discharge hole 5 is 1.0 mm,
The nozzle length is 1.0 mm, and the diameter of the spinning solution guide rod 4 is 0.5
mm. Further, the projecting length L of the spinning liquid guide rod 4 from the discharge surface 3 was changed as shown in Table 1, and the birefringence of the filament spun from the discharge hole 5 was changed.

Next, the stretching device shown in FIG.
Stretching at a speed of 0 m / min, heat setting at 210 ° C.
% Relax to give a core-sheath drawn composite filament yarn of 50 denier / 20 filaments. In this case, the draw ratio was adjusted so that the elongation of the core / sheath drawn composite filament yarn was about 20%. The tensile elongation of the obtained drawn composite filament yarn was as shown in Table 1.

The obtained core-sheath type drawn composite filament yarn is twisted at 900 T / m in the S direction to obtain a ply-twisted single yarn. To 80
After performing the twisting of 0 T / m, the cheese is dyed at 130 ° C. for 40 minutes, and at the same time as the dyeing, a loop and a slack are developed on the surface layer of the sewing machine thread. Obtained.

The evaluation results of the obtained sewing thread are as shown in Table 2, and the difference in birefringence between the high birefringence multifilament yarn at the core and the low birefringence multifilament yarn at the sheath is 0.1%. In the case of less than 01 (Comparative Examples 1 and 2), the expression of loops and slacks in the surface layer of the sewing thread due to the dyeing became insufficient, and the desired sewing thread excellent in sewability was not obtained. When the refractive index difference is 0.01 or more (Examples 1 to
In 4), a sewing thread having sufficient loops and slacks in the surface layer of the sewing thread, excellent in sewability, and having little occurrence of puckering was obtained.

[0049]

[Table 1]

[0050]

[Table 2]

[Comparative Example 3] In Example 2, four ordinary discharge holes 2A were provided at the center of the spinneret 1A shown in FIG. Stick 4
And four normal discharge holes 2A are inserted.
In this case, an undrawn composite filament yarn in which 16 high birefringence filaments and 4 low birefringence filaments were randomly mixed was obtained.

The undrawn composite filament yarn was drawn under the same conditions as in Example 2, laid and twisted, and then dyed to obtain a sewing thread.

The evaluation results of the sewing thread obtained were as follows: high speed sewability, back stitch sewability, and puckering were all "x", loops and sagging were insufficient, and the effect of improving sewability was poor. I was not able to admit.

[Examples 5 to 8, Comparative Examples 4 and 5] 5 mol%
The polyethylene terephthalate having an intrinsic viscosity of 0.47 obtained by copolymerizing 5-sodium sulfoisophthalic acid of FIG.
Are spun at a spinning temperature of 300 ° C. and a spinning speed of 2000 m / min using a spinning apparatus shown in FIG. 2 and a spinneret for cospan shown in FIG. A core-sheath type undrawn composite filament yarn having a birefringent filament disposed in a sheath portion was obtained.

In the spinneret for cospan shown in FIG.
The nozzle diameter of the normal ejection hole 2B is 0.25 mm, the nozzle length is 0.5 mm, and the small diameter d of the ejection hole 7 is 0.2 m.
m. Further, the diameter D of the large diameter portion of the discharge hole 7 is the same as that of the eighth embodiment.
The length of the filament spun from the discharge hole 7 was changed by changing the nozzle length l as shown in Table 3 by changing only the length to 10.0 mm and the others to 5.0 mm.

Next, the stretching device shown in FIG.
Stretching at a speed of 0 m / min, heat setting at 200 ° C.
% Relax treatment to obtain a core-sheath type drawn composite filament yarn of 50 denier / 20 filaments. In this case, the draw ratio was adjusted so that the elongation of the core / sheath drawn composite filament yarn was about 20%. The tensile elongation of the obtained drawn composite filament yarn was as shown in Table 3.

The obtained core-sheath type drawn composite filament yarn was subjected to ply twisting at 900 T / m in the S direction to obtain a ply twisted single yarn, and then the three ply twisted single yarns were aligned to form a Z direction. To 70
After giving a twist of 0 T / m, the cheese is dyed with cheese at 130 ° C. for 60 minutes. At the same time as the dyeing, a loop and a slack are developed on the surface layer of the sewing machine thread. Obtained.

The evaluation results of the obtained sewing threads are as shown in Table 4, and the difference in birefringence between the high birefringence multifilament yarn of the core and the low birefringence multifilament yarn of the sheath is 0.1%. In the case of less than 01 (Comparative Examples 4 and 5), the appearance of loops and slacks in the surface layer of the sewing thread due to dyeing became insufficient, and the desired sewing thread with excellent sewability was not obtained. When the refractive index difference is 0.01 or more (Examples 5 to 5)
In the case of 8), a sufficient sewing loop was formed on the surface layer portion of the sewing thread, and a sewing thread excellent in sewability and having little puckering was obtained.

[0059]

[Table 3]

[0060]

[Table 4]

[Comparative Example 6] In Example 6, four normal discharge holes 2B are arranged at the center side of the spinneret 1B shown in FIG. 2, and the discharge cross-sectional area continuously increases on the outer peripheral side. An undrawn composite filament yarn in which four discharge holes 7 and twelve normal discharge holes 2B are arranged, and 16 high birefringence filaments and four low birefringence filaments are randomly mixed. I got

The undrawn composite filament yarn was drawn under the same conditions as in Example 6 and twisted and then dyed to obtain a sewing thread.

The evaluation results of the obtained sewing thread were as follows: high speed sewability, back stitch sewability, and puckering were all ×, loops and sagging were insufficient, and the effect of improving sewability was poor. I was not able to admit.

[Examples 9 to 13] In Example 2, the number of high birefringence filaments arranged in the core and the number of low birefringence filaments arranged in the sheath were changed as shown in Table 5. Thus, the ratio of the low birefringence multifilament yarn arranged in the sheath portion to the total weight of the sewing machine yarn was changed. The strength and elongation of the obtained drawn composite filament yarn were as shown in Table 5.

Further, the evaluation results of the obtained sewing thread are as shown in Table 6, wherein the ratio of the low birefringence multifilament thread arranged in the sheath to the total weight of the sewing thread is 5%. -40% by weight (Examples 9 to 13)
Particularly preferable results were obtained, and when the content was 10 to 30% by weight (Examples 10 and 11), more preferable results were obtained.

[0066]

[Table 5]

[0067]

[Table 6]

[0068]

According to the present invention, a sewing thread which is excellent in sewability and can be reduced in count can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an example of a spinneret for cospan used in the present invention.

FIG. 2 is a longitudinal sectional view showing another example of a spinneret for cospun used in the present invention.

FIG. 3 is a schematic diagram showing an example of an apparatus for performing cospun spinning in the present invention.

FIG. 4 is a schematic view showing an example of an apparatus for drawing an undrawn composite filament yarn that has been cospun spun in the present invention.

[Explanation of symbols]

 1A, 1B Spinneret for cospan 2A, 2B Normal discharge hole 3 Discharge surface 4 Spinning liquid guide rod 5 Discharge hole with spinning liquid guide rod protruding from discharge surface 6 Blade 7 Discharge hole whose discharge cross-sectional area increases continuously DESCRIPTION OF SYMBOLS 11 Spinneret 12 Filament yarn 13 Heater 14 Hot zone 15 Cooling air spraying device 16 Oil application device 17, 18 Godet roller 19 Winding machine 20 Undrawn yarn package 21 Undrawn filament yarn 22 Preheating roller 23 Drawing roller 24 Godet roller 25 Winding machine 26 Drawing yarn package

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-92743 (JP, A) JP-A-2-216233 (JP, A) JP-A-5-106134 (JP, A) 57039 (JP, A) JP-A-4-194007 (JP, A) JP-A-62-104907 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) D01D 4/02 D01D 5 / 08-5/098 D01F 6/62 303 D02G 1/00-3/48

Claims (5)

(57) [Claims] 1. A multifilament yarn having a high birefringence is substantially disposed in a core portion, and a multifilament yarn having a low birefringence is substantially disposed in a sheath portion. A core-sheath type composite filament yarn having a diameter of 0.01 or more is spun by the cospan method, and is heat-treated after or without drawing, so that a loop is formed on the surface layer of the sewing machine thread, and a slack is developed. Production method. 2. The method according to claim 1, wherein the multifilament yarn is a polyester multifilament yarn. 3. The method for producing a composite sewing thread according to claim 1, wherein a discharge hole having a spinning liquid guide rod protruding from a discharge surface is spun using a spinneret for cospan arranged at a center side. 4. The method for producing a composite sewing thread according to claim 1, wherein the discharge hole whose discharge cross-sectional area is continuously increased is spun using a spinneret for cospan arranged on the center side. 5. The production of a composite sewing thread according to claim 1, wherein the multifilament thread having a low birefringence disposed in the sheath portion is used in an amount of 5 to 40% by weight based on the total weight of the sewing thread. Method.