WO2002042528A1

WO2002042528A1 - A sea-island typed composite fiber used in warp knitting, and a process of preparing for the same

Info

Publication number: WO2002042528A1
Application number: PCT/KR2001/001979
Authority: WO
Inventors: Young-Nam Hwang; Joon-Young Yoon; Yoeng-Beek Choi
Original assignee: Kolon Industries, Inc
Priority date: 2000-11-21
Filing date: 2001-11-20
Publication date: 2002-05-30
Also published as: EP1370718A1; BR0115677A; CN1277963C; AU2002224186A1; CN1440470A; EP1370718A4

Abstract

The present invention relates to a sea-island type composite fiber. The sea-island type composite fiber used in warp knitting which is prepared by the direct spin draw method and by using alkali soluble copolymer polyester as a sea component and polyester mainly consisting of polyethylene terephthalate of more than 90 mole% as an island component, wherein the sea-island type composite fiber is characterized in that it satisfies the following physical properties at the same time: an initial shrinkage starting temperature the composite fiber: 55∩90 °C; a maximum thermal stress temperature the composite fiber: 130∩160 °C; and a maximum thermal stress per denier the composite fiber: 0.150∩0.250 g. The sea-island type composite fiber prepared by the present invention has good thermal shrinkage properties and excellent raising property and improves the appearance and touch of warp knit fabric.

Description

A SEA-ISLAND TYPED COMPOSITE FIBER USED IN WARP KNITTING, AND A PROCESS OF PREPARING FOR THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sea-island type composite fiber used in warp knitting and a process of preparing the same. More particularly, the present invention relates to a sea-island type composite fiber used in warp knitting and a process of preparing the same which improve the quality of raised warp knif fabric as an end product, because of its excellent physical properties of yarn of an island component after dissolving a sea component.

2. Description of the Related Art In a warp knitting process for preparing a warp knit fabric by using a sea-island type composite fiber, the yarn of a high quality, particularly, the smoothness of yarn is required because of a high knitting speed.

In addition, the post treating process for preparing a warp knit fabric requires a large number of complex steps such as weight loss, raising, dyeing and the like, it is necessary to control the internal stress and thermal shrinkage property of yarn appropriately.

Specifically, a thermal shrinkage stress specified according to the internal structure of each a sea component and an island component of the sea- island type composite fiber is very important in preparing a warp knit fabric. The sea-island type composite fiber is prepared by using alkali soluble polymer as a sea component and fiber forming polymer as an island component and by conjugated-spinning them inlO a sea-island type, which is produced mainly for the purpose of preparing a fine denier fiber. In other words, after preparing the sea-island type composite fiber, the sea component of alkali soluble polymer is dissolved by treating the sea-island type composite fiber with an alkali solution, thus to prepare a fine denier fiber composed of only island components.

In this way, the method for preparing the fine denier fiber from the sea- island type composite fiber is advantageous in that the finer denier fiber of an excellent workability of spinning and drawing can be obtained as compared to the method for preparing the finer denier fiber by direct spinning, while it requires a process for dissolving and removing the sea component polymer, with an organic solvent in the finishing process after weaving or knitting. Generally, as the sea component polymer used for the sea-island type composite fiber used in warp knitting, alkali soluble copolymer polyester is mainly used. The reason of which is because it is possible to dissolve the sea component from an alkali solution and weight loss facilities widely applied in the weight loss processing of general polyester fabric without using a special apparatus and the organic solvent requiring a high recovery cost.

If the island component polymer is nylon, the dissolution speed of the sea component is not so important because the extent that the nylon is penetrated by the alkali solution is very low in dissolving the sea component. While, if the island component is polyester, the island component is penetrated before the sea component is completely dissolved in a case that the dissolution speed of the sea component is low because the polyester is weak to alkali, for thereby abruptly degrading the physical properties of yarn after the dissolution. Resultantly, the raising property becomes poor and it is difficult to gain desirable appearance and touch of the end product.

On the other hand, if the dissolution speed of the sea component is high, the occurrence of the above problems can be prevented and the alkali concentration, the dissolution temperature and time can be reduced, thereby decreasing the dissolution cost and increasing the productivity. To increase the dissolution speed of the sea component, the content of a copolymer compound should be increased. However, if the content of the copolymer compound is excessively increased, the sea component becomes an amorphous polymer with no melting point but only softening point while the dissolution is improved, thus making spinning difficult. The prior art techniques for preparing alkali soluble polyester used in preparing the sea-island type composite fiber includes the following methods: 1 ) method for copolymerizing dimethyl-5-sulfoisophthalate sodium salt (hereinafter, "DMIS") or polyalkyleneglycol (hereinafter, "PAG") of a low molecular weight in a polyester polymerization process; 2) method for blending polyester with PAG of a high molecular weight; and 3) method for blending polyester polymer with PAG of a high molecular weight.

In the case of preparing the sea-island type composite fiber by spinning, drawing and false-twisting the above-described alkali soluble polyester of the prior art as the sea component and the polyester as the island component, the flat property(smoothness) of yarn is degraded and the knitting property becomes poor.

More specifically, since a false-twisted yarn has a tendency of bulky, the knitting property is degraded in a high speed warp knitting. In addition, since the thermal shrinkage property of yarn is poor, the raising property is degraded and the appearance and quality of a raised warp knit fabric become worse in the subsequent raising process after the warp knitting.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a sea-island type composite fiber which is useful particularly for a yarn used in warp knitting because of its excellent smoothness (hereinafter, "flat property") and thermal shrinkage property of the yarn. The fait property of yarn improve the processing property by reducing the friction between the yarn and needle of knitter.

It is another object of the present invention to provide a sea-island type composite fiber having an initial shrinkage starting temperature of an appropriate level of the yarn, the maximum thermal stress temperature and the maximum thermal stress per denier in order to improve the raising property and the sea component and the island component section forming property in the process of dissolving and raising.

It is another object of the present invention to provide a sea-island type composite fiber which is useful particularly for the yarn used in warp knitting. In accordance with this present invention to achieve the above objects, a sea-island type composite fiber used in warp knitting which is prepared by the direct spin draw method by using alkali soluble copolymer polyester as a sea component and polyester mainly consisting of polyethylene terephthalate of more than 90 mole% as an island component, wherein the sea-island type composite fiber is characterized in that it satisfies the following physical properties at the same time:

- an initial shrinkage starting temperature the composite fiber : 55~90°C,

- a maximum thermal stress temperature the composite fiber : 130~160°C, and - a maximum thermal stress per denier the composite fiber : 0.150~0.250g.

In addition, the present invention provides a method of preparing a sea- island type composite fiber which is useful particularly for yarn used in warp knitting, which is manufactured by the direct spin draw method and by using alkali soluble copolymer polyester as a sea component and polyester mainly consisting of polyethylene terephthalate of more than 90 mole% as an island component, wherein the method is characterized in that it satisfies the following physical properties at the same time:

- a speed (Vi) of the first Godet roller : 1 ,000m/min ~ 5,000m/min,

- a speed (V₂) of the second Godet roller : 1 ,500m/min ~ 6,000m/min, - a crystallinity of the composite fiber on the first Godet roller : 8.5~25%, and

- a birefringence (Δn) of the island component in the composite fiber on the second Godet roller : 0.10-0.20.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. Firstly, in the present invention, alkali soluble copolymer polyester as a sea component and polyester mainly consisting of polyethylene terephthalate of more than 90 mole% as an island component are used, and they are conjugated-spun by a conjugated spinning spinneret 1. In the present invention, the raising property and sea-island shaping property are improved by properly adjusting the melt viscosity of the sea component and the island component.

Generally, a shear flow is occurred to fiber by a pressure applied to a spinneret from an extruder during a spinning process, and the flow rate and shear rate are low in the extruder while they are very high in the spinneret.

The shear stress according to such a shear rate is called the melt viscosity (MV), which is different with the polymer properties.

However, with respect to the sea-island type composite fiber prepared by conjugated spinning more than two kinds of polymers, since the melt viscosities of sea components are different from each other, there occurs a difference between their shear stresses, resultantly affecting the sea and island shaping of the composite fiber and the physical properties of the composite fiber.

Therefore, to obtain the physical properties required for a uniform sea- island cross section formation and use, it is necessary to select sea-island component polymer having a proper melt viscosity.

Particularly, in the case of a raised warp knit fabric product, in order to express the raising property and the appearance and touch of fine yarn, it is necessary that polymers used for the sea-island type composite fiber keep their relative viscosity properly, rather than their melt viscosity. In the present invention, the difference (hereinafter, "ΔMV_9|0oo") between the melt viscosity of island component polymer and the melt viscosity of sea component polymer at a shear rate of 9,000(1/s) is 20-70% of the difference (hereinafter, "ΔMV500") between the melt viscosity of island component and the melt viscosity of sea component at a shear rate of 500(1/s). That is, the value of ΔMV at the spinneret should be smaller than the value of ΔMV at the extruder.

In other words, the difference (ΔMV) in melt viscosity between the island polymer and the sea component polymer according to an increase of the shear rate should be reduced. Otherwise, the orientation property of the island component is decreased, the knitting property is degraded because a sufficient drawing is difficult, and raised fibers are entangled with one another in warp knit fabric.

Fig. 4 is a graph illustrating a change in ΔMV according to an increase of a shear rate. In Fig. 4, it is found that the difference in melt viscosity between the island component and the sea component is decreased gradually as the shear rate is increased.

In addition, it is preferred that the melt viscosity (hereinafter, "MVs") of the sea component at a shear rate of 500-9, 000(1/s) is lower than the melt viscosity (hereinafter, "MVi") of the island component (MVs ≤ Mvi). Fig. 3 is a graph illustrating the correlation between the melt viscosity and shear rate for each component.

If the melt viscosity (MVs) of the sea component is larger than the melt viscosity(MVi) of the island component polymer, the cross section shaping of the sea-island type fiber might be difficult. This causes a decrease in number of island components or the wreck of uniform formation of island components, and thusly the raising property becomes poor in raising process and it is made difficult to express the appearance and touch of an end product. Moreover, it is preferred that the difference in melt viscosity (hereinafter,

"ΔMV") between the sea component and the island component is lower than 1 ,000 poises. If the difference (ΔMV) in melt viscosity between the sea and island components is more than 1 ,000 poise, the island components become adhesive to one another during spinning and there may be generated unseparated fibers in which the island components are not separated even after the dissolution. Due to this, there is a risk that the fiber raising state becomes non-uniform in warp knit fabric, the appearance becomes unclean, the writing effect becomes weak and the touch becomes rough.

The melt viscosity of the island component polymer can be adjusted by an -intrinsic viscosity, and the melt viscosity of the sea component polymer can be adjusted by properly controlling the kinds of copolymer, content of copolymer and copolymerization conditions.

As an example of adjusting the melt viscosity of the sea component polymer, DIMS of 3-15 mole is copolymer into polyethylene terephthalate, to which polyethyleneglycol of 4-20 weight % having average molecular weight more than 8,000 can be added.

Continuously, in the present invention, the sea-island type composite fiber is prepared by drawing a spinning yarn between the first Godet roller 2 and the second Godet roller 3, and winding up the same by a winder 4. In other words, in the present invention, the sea-island type composite fiber is prepared by the direct spin draw method in which spinning and drawing are performed in the same process.

As compared to the method for preparing a bulky sea-island type composite fiber by the spin-draw-false twist method, the present invention can prepare yarns having a flat property, so it is more advantageous in high speed warp knitting.

At this time, the speed (Vi) of the first Godet roller is 1 ,000~5,000m/min, more properly 1 ,000~3,000m/min, and the speed (V₂) of the second Godet roller is set to 1 ,500~6,000m/min. If the speed of the first Godet roller 1 and the second Godet roller 2 is lower than the above-mentioned range, the yarn is not sufficiently oriented and crystallized, thereby making cutting non-uniform in a after raising process and making it impossible to control a weight loss in a weight loss process. In addition, if the speed of the first Godet roller and the second Godet roller exceeds the range, the birefringence and crystallinity of the island component in the yarn are degraded for thereby unsatisfying the physical properties of the yarn and accordingly degrading the workability of warp knitting.

Besides control the speed of the first Godet roller and the second Godet roller, the physical properties of the yarn at each stage of the spinning process can be adjusted by properly adjusting the drawing temperature, drawing ratio, cooling condition, melting viscosity of sea-island type polymer and the like.

Firstly, a preparing condition is set such that the crystallinity of yarn on the first Godet roller, that is, the crystallinity of yarn passing through the first Godet roller is 8.5-25%. If the crystallinity is higher than the above range, it is difficult to perform drawing in a drawing zone, it is made impossible to proceed the process. If the crystallinity is too lower than the above range, an excessive drawing is required for obtaining desirable physical properties of yarn. This causes an excessive drawing tension for thereby increasing the deviance of the physical properties and generating a problem in the process.

Additionally, a preparing condition is set such that the birefringence (Δ n) of the island component in the composite fiber on the second Godet roller is 0.10-0.20. If the birefringence is deviated from the above range, winded yarn contains excessively high mechanical properties to thus be made unsuitable for apparel, thereby causing a fatigue phenomenon, to which polymer is non- resistant, being accumulated onto the yarn and sharply degrading the physical properties of the yarn.

To obtain more properd sea island type composite fiber, it is more preferred to control the properties of yarn at each stage of spinning process as below. But the method of this invention is not restricted by below mentioned condition.

In addition, it is more preferred that the yarn on the first Godet roller has a modulus of 5~35g/d. If the modulus of the yarn on the first Godet roller is deviated from the above range, the stress applied on the yarn becomes excessive and a filament bundle is bursted or cut of yarn is generated, for thereby degrading the workability of spinning.

In addition, a preparing condition is set such that the birefringence (Δn) of the island component in the yarn on the first Godet roller, that is, the birefringence (Δn) of the island component in the yarn passing through the first Godet roller, is 0.005-0.090. If the birefringence is greater than the above range, a mechanical stress becomes too large, thus generating a fracture phenomenon. If it is smaller than the above rage, a modulus, strength and elongation of yarn are too low, thereby making spinning impossible.

In addition, a preparing condition is set such that the yarn on the first Godet roller has a modulus of 60~90g/d. If the modulus is lower than the above range, the density of the yarn becomes lower and the elongation degree thereof becomes too high, the mechanical strength becomes higher and a raised pile becomes rough, thereby weakening the effect of the sea-island type fine yarnr

In addition, a preparing condition is set such that the crystallinity of the yarn on the second Godet roller is 25-45%. If the crystallinity is lower than the above range, passing property of yarn become worse due to low tension, thereby making a spinning process difficult and degrading the physical properties of the yarn. If it is higher than the above range, the filament becomes stiff with too excessive orientation and crystallization to be made unsuitable for the yarn used in warp knitting.

A variety of physical properties of the yarn on the first Godet roller are measured by the method to be described later. The sampling method of the yarn on the first Godet roller are as below. Firstly cutting the spinning yarn at the front end portion of the first Godet roller by means of capturer, and almost simultaneously with cutting the spinning yarn at the rear end portion of the first Godet roller by means of capturer, and then sampling the yarn wound up on the surface of the first Godet roller as soon as it is cut.

More specifically, the yarn positioned on the surface of filament layer wound up to the first Godet roller is sampled as soon as it is cut in order to prevent a change in the physical properties due to the temperature of the first Godet roller.

A variety of physical properties of the yarn on the second Godet roller are measured by the method to be described later. The sampling method of the yarn on the second Godet roller are as below. Firstly cutting the spinning yarn almost simultaneouly at the front and rear end portions of the second Godet roller by means of capturer, and then sampling the yarn wound up onto the surface of the second Godet roller.

The resultant sea-island type composite fiber(4 in Fig 1 ) used in warp knitting of the present invention has the following characteristics.

Firstly, the initial shrinkage starting temperature of the composite fiber is 55~90°C: The yarn is drawn between the first Godet roller and the second

Godet roller and it is moved from a low orientation low crystallization state to a high orientation high crystallization state, resultantly reducing the initial shrinkage starting temperature.

Therefore, if the initial shrinkage starting temperature is lower than 55°C, this is a state in which crystal orientation is too excessive. If the initial shrinkage starting temperature is higher than 90°C, this is a state in which crystal orientation is insufficient, thus making the composite fiber unsuitable for the yarn used in warp knitting.

In addition, the maximum thermal stress temperature of the composite fiber is 130~160°C. The thermal shrinkage power of the filament is best at the maximum thermal stress temperature. Moreover, most of the post treatment processes of warp knit fabric are performed in the above range.

Thusly, if the maximum thermal stress temperature is lower than the above range, an excessive shrinkage is generated in the initial process of the post treatment and it is made difficult to control the post treatment process. If the maximum thermal stress temperature is higher than the above range, a sufficient shrinkage is not occurred in the post treatment and thus the volume and density of warp knit fabric is degraded, resultantly making the appearance and touch of an end product worse.

In addition, the maximum thermal stress per denier of the composite fiber is 0.150~0.250g. The thermal stress is related to a level of thermal treatment applied to the yarn in process. If the maximum thermal stress per denier is lower than the above range, this causes an insufficient crystal orientation and an elongation degree becomes higher. If it is higher than the above range, this causes an excessive crystal orientation and the yarn becomes rough. That is, if the maximum thermal stress, per denier is deviated from the above range, the post processability becomes worse and the appearance and touch of the fabric itself are degraded. As described above, the thermal shrinkage properties of the sea-island type composite fiber of the present invention harmonizes with one another. As a result, the shaping property of filament cross section, raising property and touch are excellent, and the degradation of the physical properties of the composite fiber is minimized in dissolving a sea component and raising process. Resultantly, the sea-island type composite fiber of the present invention is particularly suitable for the yarn used in preparing raised warp knit fabric.

The composite fiber of the present invention has the finesse of mono filament of 2-5 deniers before dissolving the sea component, and has the finesse of mono filament of 0.001-0.3 denier after dissolving the sea component. The modulus of the composite fiber after dissolving sea component is 25~60g/d. It is preferred that the total finesse of the composite fiber before dissolving the sea component is 50-150, but it is not limited thereto. A warp knit fabric is knitted by the sea-island type composite fiber of the present invention, and then the sea component is dissolved by an alkali solution, and passing through raising process, thereby preparing raised warp knit fabric. The dissolution of the sea component is performed by processing the sea-island type composite fiber in a sodium hydroxide solution with a concentration of 1 % (bath ratio = 10:1 ) at 95°C for 30 minutes.

The sea-island type composite fiber of the present invention has a strength maintenance rate of more than 82% after the dissolution of the sea component.

With respect to the sea-island type composite fiber of the present invention, the degradation of the yarn modulus is minimized in the above sea component dissolution process or raising process due to the dissolubility of the sea component and the physical properties of the yarn. As a result, the raising property is improved and the appearance and touch of warp knit fabric, an end product, becomes excellent. In the present invention, the physical properties of the composite fiber are evaluated as below.

^• Initial shrinkage starting temperature, maximum thermal stress temperature and maximum thermal stress per denier of yarn. They are measured by a thermal stress tester of KANEBO Company.

Specifically, a loop sample(composite fiber) with a length of 10cm is latched to upper and lower end hooks and a predetermined tension (total denier of sample

2 x — g) is applied thereto. In this state, the temperature is increased at a

predetermined speed (150°C/min). At this time, a change in stress according to a change in temperature is illustrated by a chart as shown in Fig. 2 and then each of the physical properties are obtained. The initial shrinkage starting temperature of the composite fiber has the same meaning as galss transition temperature(Tg) of the composite fiber and it is obtained by the temperature of x portion of Fig. 2, and the maximum thermal stress temperature is a temperature at which the composite fiber receives the largest stress and it is obtained by the temperature of the y portion of Fig. 2. In addition, the maximum thermal stress, per denier of the composite fiber is calculated by obtaining a maximum thermal stress value (z portion of Fig. 2) in the chart and then substituting the same by the following formula.

J - , . j , . j . Maximum thermal stress

Maximum thermal stress per denier— Total denier of sample X 2

• strength/modulus The average value is obtained by measuring strength/modulus by a tension tester of Instron company by 10 times (sample length: 5cm, elongation speed: 30cm/min). Here, the modulus represents an initial modulus.

^• density (p )

The sea-island type composite fiber is inputted into a densimeter (Product of Shibayama Company, Japan, Model name : Model SS) consisting of a mixed solvent of normal heptane and carbontetrachloride and then is left as it is at 23 °C for one day, and then the density of the integrated sea and island components of a bulky state is measured.

^• crystallinity fXc(%)1 The crystallinity is obtained by using the theoretical density value (p c =

1.457g/cm3) of a complete crystal region of polyester and the density value (1.336g/cm3) of a complete amorphous region thereof based on the above density (p ).

Crystallinity [Xc(%)] = ^{p ~ Pa} x 100

^• birefringence (Δn) of island component in composite fiber

The birefringence is measured by an interference microscope (product of Karl Zeiss Company, Model name: JENAPOL - UINTERPHAKO). The birefringence is obtained by the following formula.

R + S

Birefringence (Δn) =

1000 xD

Here, R represents compensator retardation, S represents retardation of quartz shim, and D represents fiber diameter. In addition, the unit of R and S is nm, and the unit of D is μm. • Retention of yarn strength after reduction of sea component By the above-described method, the yam(composite fiber) strengths before and after the reduction of the sea component are obtained by Instron, and then the retention of yarn strength after the dissolution of the sea component is obtained by substituting the yarn strengths before and after the dissolution of the sea component by the following formula. The dissolution of the sea component is performed by processing the sea-island type composite fiber in a sodium hydroxide solution with a concentration of 1% [bath ratio(solution : yarn) = 10:1] at 95 °C for 30 minutes. Retention of yarn strength after reduction of sea component =

Yarn strength after reduction of sea component

X lUU

Yarn strength before reduction of sea component

Herein yarn strength mean tenacity(g/d) of yarn. ^• melt viscosity

The melt viscosity according to a shear rate is measured by applying a shear stress to a sample(chip) by using a capillary rheometer (spec: L=25.38mm, D=0.762mm, L/D=33.31). At this time, the melting viscosity of polymer is 290 ^°C, the shear rate is continuously changed in the ranges from 500(1 /s) to 9,000(1 Is), and the drying condition of the island component poiymer(chip) and sea component polymer(chip) is set to 150°C^χ 5 hours in the vacuum state.

^• sea-island cross section shaping property

500 samples are prepared by sampling a composite fiber section, and the uniformity and seperation of a sectional form are observed and evaluated by a microscope. Specifically, if the sectional form is uniform and the number of unseparated island components is less than 2, the shaping property is evaluated to be excellent, if the sectional form is not uniform and the number of unseparated island components is two or less, the shaping property is evaluated to be good, if the sectional form is uniform and the number of unseparated island components is 3-4, the shaping property is evaluated to be moderarate, and if the number of unseperated island components is more than 5, the shaping property is evaluated to be poor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic view of a process of the present invention;

Fig. 2 is an example of a chart created by a thermal stress; Fig. 3 is a graph illustrating the correlation between the melt viscosity and shear rate for each component constituting a sea-island type composite fiber of the present invention; and Fig. 4 is a graph illustrating a change in the difference (ΔMV) in melt viscosity between a sea component and an island component constituting the sea-island type composite fiber of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, the present invention will be described in more detail through examples and comparative examples, but it is not limited thereto. Example 1

Alkali soluble polymer with MV500 of 700 poise and MV9000 of 300 poise is prepared by blending polyethyleneglycol of 8 weight% having a number average molecular weight of 8,500 with copolymer polyester in which dimethyl- 5-sulfoisophthalate sodium of 4 mole% is copolymer. The prepared alkali soluble polymer is used as a sea component and polyethylene terephthalate (MV500: 1 ,200 poise, MV9000: 500 poise) having an intrinsic viscosity of 0.65 is used as an island component. They are spun by a conjugated spinning spinneret having 36 island components at 288 °C

Continuously, the spun yarn is drawn between the first Godet roller of 80^°C at a speed of 1 ,500m/min and the second Godet roller of 125°C at a speed of 4,200m/min. Then, it is wound up at a winding speed of 4,150m/min, thereby preparing a sea-island type composite fiber of 75 denier / 24 filament.

At this time, cooling conditions under the spinning spinneret are set such that a relative humidity is 100%, a temperature of cooling air is 20 °C and a speed of cooling air is 0.4m/sec. In addition, preparing conditions are set such that the crystallinity of the yarn on the first Godet roller is 8.8% and the birefringence of the sea component in the composite fiber on the second Godet roller is 0.137.

Moreover, the modulus of the yarn on the first Godet roller is set to 10g/d, the birefringence of the island component in the composite fiber on the first Godet roller is set to 0.015, the crystallinity of the yarn on the second Godet roller is set to 30% and the modulus of the yarn on the second Godet roller is set to 70g/d. After dissolving the thusly prepared sea component by processing the sea-island type composite fiber in 1 % NaOH solution of 95 °C for 30 minutes, and then evaluating the physical properties of the yarn after the dissolution of the sea component by the above-described method. The finess of mono filament after dissolving the sea component is 0.06 denier. Table 2 show the evaluating results.

Continuously prepare the raw warp knit with density of 23C/CM by using the sea-islane type composite fiber, as a yarn of the front surface layer, and then using copolyester yarn with mono filament of 5 denier and shrinkage rate of boiling water of 28%(high shrinkage yarn) as a yarn of the rear surface layer. At this time, content of the yarn of the rear surface layer is 26% in weight to the total weight of processed warp knit.

Next, treat the manufactured raw warp knit by raising machine untill the shrinkage of the warp knit is reached 50%. And then, after heat setting the warp knit at 190°C preliminarily, dipping the warp knit in NaOH solution(1% concentration) during 30 minutes at 98 "C in other to remove the extraction component of composite fiber. And then prepare a processed warp knit by dyeing(with disperse dyes), buffing and heat setting at 180°C finally the above mentioned warp knit. The processed warp knit has excellent touch and quality. Examples 2 and 3 and Comparative Examples 1 and 2 Except that the preparing conditions are changed as shown in Table 1 , the sea-island type composite fiber of 75 denier / 24 filament is prepared under the same process and conditions as those in Example 1. At this time winding speed is set to 99% of the second Godet roller speed.

After dissolving the thusly prepared sea component by processing the sea-island type composite fiber in 1% NaOH solution of 95 °C for 30 minutes, and then evaluating the physical properties of the yarn after the dissolution of the sea component by the above-described method. The finess of mono filament after dissolving the sea component is 0.06 denier. Table 2 show the evaluating results. [Table 1 ] preparing conditions

[Table 2] Results of evaluation of physical properties of composite fiber

INDUSTRIAL APPLICABILITY

The sea-island type composite fiber of the present invention has very excellent thermal shrinkage properties, so the raising property becomes excellent in preparing warp knit fabric and it is possible to prepare warp knit fabric, with excellent appearance and touch. By such an effect, the sea- island type composite fiber of the present invention is particularly useful in preparing raised warp knit fabric.

Claims

What is claimed is:

1. A sea-island type composite fiber used in warp knitting which is prepared by the direct spin draw method by using alkali soluble copolymer polyester as a sea component and polyester mainly consisting of polyethylene terephthalate of more than 90 mole% as an island component, wherein the sea- island type composite fiber is characterized in that it satisfies the following physical properties at the same time:

- an initial shrinkage starting temperature the composite fiber : 55-90 °C , - a maximum thermal stress temperature the composite fiber : 130-160 °C, and

- a maximum thermal stress per denier the composite fiber : 0.150~0.250g.

2. The sea-island type composite fiber of claim 1 , wherein the finesse of mono filament before dissolving the sea component is 2-5 deniers, and the finesse of mono filament after dissolving the sea component is 0.001 -0.3 denier.

3. The sea-island type composite fiber of claim 1 , wherein the modulus of the composite fiber after dissolving the sea component is 25~60g/d.

4. The sea-island type composite fiber of claim 1 , wherein the retention of yarn strength of the composite fiber after dissolving the sea component is more than 82%.

5. The sea-island type composite fiber of claim 1 , wherein the total finesse of the sβa-island type composite fiber after dissolving the sea component is 50-150 deniers.

6. A method of preparing a sea-island type composite fiber which is useful particularly for yarn used in warp knitting, which is manufactured by the direct spin draw method and by using alkali soluble copolymer polyester as a sea component and polyester mainly consisting of polyethylene terephthalate of more than 90 mole% as an island component, wherein the method is characterized in that it satisfies the following physical properties at the same time:

- a speed (Vι) of the first Godet roller : 1 ,000m/min - 5,000m/min,

- a speed (V₂) of the second Godet roller : 1 ,500m/min - 6,000m/min,

- a crystallinity of the composite fiber on the first Godet roller : 8.5-25%, and

7. The method of claim 6, wherein the difference (ΔMVg.ooo) between the melt viscosity of island component polymer and the melt viscosity of sea component polymer at a shear rate of 9,000(1/s) is 20-70% of the difference (Δ MV₅oo) between the melt viscosity of island component and the melt viscosity of sea component at a shear rate of 500(1 Is).

8. The method of claim 7, wherein the melt viscosity (MVs) of the sea component at a shear rate of 500-9,000(1 /s) is lower than the melt viscosity (MVi) of the island component .

9. The method of claim 7, wherein the difference in melt viscosity (ΔMV) between the island component and the sea component is lower than 1 ,000 poises.

10. The method of claim 6, wherein the crystallinity of the composite fiber on the second Godet roller is 25-45%.

11. The method of claim 6, wherein the modulus of the composite fiber on the first Godet roller is 5~35g/d.

12. The method of claim 6, wherein. the speed (Vi) of the first Godet roller is 1 ,000~3,000m/min.

13. The method of claim 6, wherein the birefringence of the island component in the composite fiber on the first Godet roller is 0.005-0.090.

14. The method of claim 6, wherein the modulus of the composite fiber on the second Godet roller is 60~90g/d.

15. A warp knit fabric knitted by using the sea island type composite fiber of claim 1.