1995

ISSN 1229-9197 (print version)

ISSN 1875-0052 (electronic version)

Fibers and Polymers 2015, Vol.16, No.9, 1995-2002

Alkaline Dissolution and Dyeing Properties of Sea-island Type Polyethylene

Terephthalate Ultramicrofiber Knitted Fabrics

Hodong Kim, Hyun Sung Kim, Young Ki Park, and Jung Jin Lee*

Department of Fiber System Engineering, Dankook University, Yongin 16890, Korea

(Received March 31, 2015; Accepted July 20, 2015)

Abstract: Sea-island type polyethylene terephthalate ultramicrofibers with a single fiber diameter of ca. 600 and 800 nmhave been produced over the last 4 years. Alkaline dissolution behavior and dyeing properties of the ultramicrofiber fabricswere investigated. It was found that the dissolution ratio was dependent upon temperature in the alkali treatment. When thefabric was treated with 1 wt% sodium hydroxide solution at 95 oC, the dissolution ratio reached the theoretical value in about30 min. Alkali dissolution behavior could be monitored indirectly by the cationic dye staining method and observed directlyby SEM analysis. The color yield of the ultramicrofiber fabric dyed with disperse dye was dependent on the dyeingtemperature and K/S value decreased as the dyeing temperature increased. Build-up property was generally good and K/Svalue of dyed fabric from ca. 800 nm fiber was higher than that from ca. 600 nm fiber. Wash fastness was poor to moderatefor ca. 800 nm fabric and poor for ca. 600 nm fabric. Light fastness was very poor.

Keywords: Sea-island type polyethylene terephthalate, Ultramicrofiber, Alkaline dissolution, Cationic dye staining, Disperse dye

Introduction

Polyethylene terephthalate (PET) fibers, which show great

chemical resistance, wash and wear properties, and heat

stability, are used in many applications such as clothing and

tire cords [1]. Microfibers refer to synthetic fibers finer than

about 1 denier per filament (dpf) [2]. Microfiber fabrics are

used in various applications, for example, athletic wear,

suede product, wiping cloth, and industrial filter because of

their specific characteristics including large surface area,

enhanced drape, luster, softness, and water or oil absorbency

[3,4].

Direct spinning and conjugate spinning are two methods

for producing microfibers. Microfibers produced from direct

spinning, in which single component filaments are extruded,

are highly uniformed, although the fibers have limited

fineness, typically in the range of 0.2-0.9 denier [4,5]. In the

case of conjugate spinning, two technologies, namely

separation and dissolution, have been devised. In the separation

technique, bicomponent filaments, typically comprising

polyamide and polyester, are extruded through spinnerets.

After weaving or knitting with filaments of this type, the two

different components separate by physical or chemical

treatment giving microfiber fabrics. Dissolution technique

also involves the spinning of bicomponent filament. The

bicomponent filament contains several ‘island’ components

embedded within a ‘sea’ component. After the filaments are

assembled to a fabric by weaving or knitting, microfiber

fabrics are obtained by dissolving and removing the ‘sea’

component such that only ‘island’ component remains [6-8].

These sea-island type microfibers are known to have finer

denier (<0.01 dpf) than microfibers from other types of

technique.

In the case of sea-island type polyester microfiber, the

regular polyester is commonly used as an ‘island’ while

alkali-soluble polyester is used as ‘sea’ component. When

the alkali-soluble polyester is dissolved out by the treatment

with alkaline solvent, the polyester island remains and forms

the very fine fibers [6]. It is important and difficult to

suitably remove sea component and reveal island component

by alkaline treatment [9].

Several studies on alkaline dissolution and dyeing properties

of sea-island type microfiber have been reported on wide

range of fineness of microfibers (0.01-0.2 dpf). Park et al.

[1] reported the effect of alkaline dissolution on physical

change of 0.01 denier microfiber. Koh et al. [10] or Cho and

Lee [11] reported alkaline dissolution and dyeing behavior

of microfibers, the fineness of which was ranged from 0.2 to

0.01 dpf. In early 2010s, sea-island type PET ultramicrofibers

with the diameter of a single fiber being about 800 nm

(0.007 dpf) or 600 nm (0.004 dpf) have been produced. As

these ultramicrofibers are known to contain more than 330

(800 nm) or 630 (600 nm) island components respectively, it

is expected that the dissolution of the sea component from

the fibers should become more difficult than common

microfibers. For example, Cho and Lee [11] reported that

0.06 and 0.01 dpf sea-island fibers contained 36 and 169

islands respectively. In the previous studies [12,13], we

reported alkaline dissolution behavior and dyeing properties

of knitted fabrics manufactured from sea-island type PET

ultramicrofiber of ca. 800 nm. Up to date, little research has

been reported for the ultramicrofiber of ca. 600 nm.

In this study, alkaline dissolution and dyeing properties of

knitted fabric from sea-island type PET ultramicrofiber of

ca. 600 nm or 800 nm were investigated. Alkaline dissolution

behavior was assessed by dissolution ratio measurement,

cationic dye staining method, and scanning electron microscopy

(SEM) analysis. Effect of dyeing temperature on color yield*Corresponding author: jjlee@dankook.ac.kr

DOI 10.1007/s12221-015-5250-9

1996 Fibers and Polymers 2015, Vol.16, No.9 Hodong Kim et al.

was discussed and build-up of 600 nm or 800 nm ultra-

microfiber was compared. Wash and lightfastness were also

evaluated.

Experimental

Materials

Two circular knitted fabrics (interlock) prepared using

100 % sea-island type PET ultramicrofiber were obtained

from KMF Co., South Korea. The composition of each

fabric is shown in Table 1.

Three disperse dyes, Foron Yellow Brown RD-S, Foron

Rubine S-WF, and Foron Blue RD-E were obtained from

Clariant, Co. The cationic dye, Doracryl Orange R 400 %

(C.I Basic Orange 22) (Scheme 1), was obtained from

M.Dohmen Korea, Ltd. and used without purification. All

chemical reagents for alkaline dissolution or dyeing were of

general laboratory grade.

Alkaline Dissolution

Using a 50:1 liquor ratio, a 100 ml alkali bath containing

sodium hydroxide (NaOH, concentration 1 wt%) and

penetrating agent (1 wt%) was prepared. The ultramicrofiber

fabric samples were alkali treated at 95 or 100 oC for 10-

60 min in a laboratory IR-dyeing machine (DTC-6000,

DaeLim Starlet Co., Korea). Each beaker was removed from

the IR-dyeing machine at 10 min intervals and the fabrics

were neutralized with acetic acid and rinsed (Figure 1). After

drying at room temperature for 24 h, the fabrics were

weighed before and after alkali treatment. The dissolution

ratio was calculated by means of equation (1).

Dissolution ratio (%) = (a − b)/a × 100 (1)

where a: weight of fabric before alkali treatment (g) and b:

weight of fabric after alkali treatment (g)

Scanning Electron Microscopy

Scanning electron microscopy (SEM) of the ultramicro-

fiber fabrics before and after alkali treatment with 1 wt%

NaOH aqueous solution at 95 oC was carried out using a

JSM-6700F instrument (JEOL, Japan). The samples were

cryogenically fractured in liquid nitrogen and then coated

with gold by vapor deposition using a vacuum sputter before

SEM observation.

Cationic Dyeing

Dyebath was prepared with the cationic dye (1 % o.w.f)

and buffered at pH 5 with sodium acetate (0.05 M)/acetic

acid. The PET ultramicrofiber fabrics untreated or alkali

treated with 1 wt% NaOH aqueous solution at 95 oC for 10-

Figure 1. Alkaline dissolution profile of sea-island type PET

fabrics.

Scheme 1. Structure of C.I Basic Orange 22.

Table 1. Yarn composition and theoretical dissolution ratio of sea-

island type PET ultramicrofiber fabrics

Sample Yarn composition

Sea-island

ratio

(Co-PET:PET)

Theoretical

dissolution

(%)

N800 Sea-island type PET, 70d/18f,

331 islands, 0.007 dpf

40 : 60 40

N600 Sea-island type PET, 50d/12f,

631 islands, 0.004 dpf

40 : 60 40

Figure 2. Dyeing profile of ultramicrofiber PET fabric with

cationic dye.

Alkaline Dissolution and Dyeing of PET Ultramicrofiber Fabric Fibers and Polymers 2015, Vol.16, No.9 1997

60 min, were dyed in the prepared dyebath with a liquor-to-

goods ratio of 20:1. Dyeing was performed at 80 oC for

20 min in a laboratory dyeing machine (Figure 2). After

dyeing, all the samples were soaped with soaping agent

(Protesol DSL, 1 g/l), rinsed and dried at room temperature.

Disperse Dyeing

The PET ultramicrofiber fabrics alkali treated with 1 wt%

NaOH aqueous solution at 95 oC for 40 min, were dyed with

three disperse dyes in a laboratory dyeing machine. The

dyebaths were prepared with disperse dye (0.5-5.0 % o.w.f.)

and dispersant (1 g/l), and buffered at pH 5 with sodium

acetate (0.05 M)/acetic acid. Liquor-to-goods ratio was 20:1.

Dyeing was commenced at 40 oC. The dyebath temperature

was raised at a rate of 1 oC/min to 130 oC, maintained at the

temperature for 40 min and cooled (Figure 3). Instead of

130 oC, different dyeing temperature such as 110 or 120 oC

was applied to investigate the effect of dyeing temperature

on color yield. The dyed fabrics were then reduction-cleared

at 80 oC for 20 min with NaOH (2 g/l) and sodium hydrosulfite

(Na2S2O4, 2 g/l).

Measurement of Color Yield and Fastness

The K/S values of the dyed fabric were measured on a

spectrometer (Coloreye 3100, Gretag-Macbeth, USA) with

D65 standard illuminant and a 10 o standard observer.

According to the Kubelka-Munk theory, K/S value from

surface reflectance of the maximum absorption wavelength

is given by equation (2).

K/S = (1 − R)2/2R (2)

where K: absorption coefficient, S: scattering coefficient,

and R: reflectance (0<R≤1).

The dyed fabrics were heat-set at 180 oC for 60 s and

tested for fastness to washing (ISO 105-C06/C1S) and light

(ISO 105-B02). The shade change, together with the staining

of adjacent fabrics, was rated according to the appropriate

ISO grey scale and light fastness was rated using ISO blue

scale.

Results and Discussion

As shown in Table 1, the sea-island type ultramicrofiber in

the present study is composed of regular PET (60 %) as an

island component and alkali-soluble PET (40 %) as a sea

component. The alkali-soluble PET, or Co-PET, is a copolymer

containing 10-15 wt% sulfonated isophthalate, as shown in

Scheme 2. The sulfonate group is considered to assist the

dissolution of polymer by NaOH aqueous solution and

disrupt the crystallinity of the polymer by the bulky group,

which results in easier dissolution when compared to the

Figure 3. Dyeing profile of PET fabric with disperse dye.

Scheme 2. Structure of alkali-soluble PET.

Figure 4. Cross-section of sea-island type PET ultramicrofibers;

(a) N800 and (b) N600.

1998 Fibers and Polymers 2015, Vol.16, No.9 Hodong Kim et al.

regular PET [10,14,15].

The cross-section of the sea-island type PET ultramicrofiber

is shown in Figure 4. The filament of N800 and N600

contained 331 and 631 island components, respectively.

Dissolution Ratio

The effect of alkali treatment temperature on the dissolution

ratio of N600 is shown in Figure 5. The dissolution ratio of

N600 at 100 oC increased more rapidly with increasing

treatment time than the dissolution ratio at 95 oC. It is well

known that alkali hydrolysis of the sea component readily

occurs at higher temperature. The theoretical dissolution

ratio required for the completion of dissolution of the sea

component in N600 is 40 % as given in Table 1. The

dissolution ratio of alkali-treated N600 at 100 oC reached its

theoretical value between 20 and 30 min, while that of

alkali-treated N600 at 95 oC reached in 30 min. The practical

dissolution ratio continued to increase after reaching the

theoretical values in both cases. This suggests that the island

component should also have been dissolved in the excess

treatment time. Again, it is important to find an optimum

condition of alkaline treatment in order not only to obtain

well-revealed island ultramicrofiber but also to avoid

undesirable damage to the island component.

The alkaline dissolution behavior of N600 was found to be

similar to that of N800 from the previous report [13]. Thus,

dissolution ratio of alkali-treated ultramicrofiber fabric was

dependent on alkaline treatment temperature as well as

treatment time.

Cationic Dye Staining

Sea component of sea-island type PET ultramicrofiber is a

copolymer containing sulfonated isophthalate. This sulfonate

anionic group has a substantivity to cationic dyes so that the

sea component can be dyed or stained with cationic dyes by

electrostatic attraction. On the other hand, the island component,

which is a 100 % hydrophobic PET homopolymer, is hardly

dyed by cationic dyes. Therefore, it is possible to monitor

the dissolution behavior of the sea component by cationic

dye staining after alkali treatment [6]. Untreated or alkali-

treated PET ultramicrofiber fabric with 1 wt% NaOH aqueous

solution at 95 oC for 10-60 min, were dyed with a cationic

dye. Figure 6 shows the K/S values of N600 according to

alkali treatment time. The untreated fabrics exhibited high

K/S values by cationic dye staining, which suggested that

there should be lots of sea component in the fabric. The K/S

values decreased as alkali treatment time increased, and

finally levelled off after ca. 30 min. The decline in K/S

values suggests that the sea component would be dissolved

out by alkali treatment and only a small amount of cationic

dye could be adsorbed to the fiber. There was no marked

decrease in K/S value after 30 min which might indicate that

only the island component should remain, with almost all of

the sea component being removed from the fiber. The

levelling-off point was ca. 30 min, which coincided with the

optimum time of alkaline treatment at 95 oC from dissolution

ratio measurement in Figure 5. Thus, the cationic dye staining

method was found to be a good method for monitoring alkali

dissolution behavior of sea-island type ultramicrofiber fabric.

Surface and Cross-sectional Morphology

Figure 7 shows SEM images of ultramicrofiber fabrics

(N600) before and after alkali treatment with 1 wt% NaOH

at 95 oC. As the alkali treatment time increased, the island

components were gradually separated from one another.

After 20 min of alkali treatment, several island components,

especially outer part of the sea-island fiber, were separated

in the cross-sectional view, which should be attributed to

dissolution of the sea component. However, inside of the

fiber, a large portion of sea and island components remained

Figure 5. Effect of alkali treatment temperature on the dissolution

ratio of N600.Figure 6. Cationic dye staining of PET ultra-microfiber fabric

(N600) as a function of alkaline treatment time.

Alkaline Dissolution and Dyeing of PET Ultramicrofiber Fabric Fibers and Polymers 2015, Vol.16, No.9 1999

Figure 7. SEM images of N600 before and after alkaline treatment with 1 wt% NaOH at 95 oC; (a) untreated, (b) 20 min treatment, (c) 30 min

treatment, and (d) 40 min treatment.

2000 Fibers and Polymers 2015, Vol.16, No.9 Hodong Kim et al.

without separation. More island components were separated

after 30 min, although still some island components appeared

not to be separated from a surface and cross-sectional view.

From the results of dissolution ratio and cationic dye

staining, optimum alkaline treatment time was 30 min when

using 1 wt% NaOH at 95 oC. However, sea components

were not found to be completely dissolved from SEM data.

It seems that there are so many and highly packed island

components (631 islands) that the alkaline solution cannot

permeate the inside of every single fiber and dissolve all the

sea component in 30 min at this condition. After 40 min,

much more island components were separated when compared

to 30 min. But island components did not seem to be fully

separated. In case of N800 which has less island components

(331 islands) than N600, island components was completely

split after 40 min with the same alkaline treatment condition

(1 wt% NaOH at 95 oC) as shown in Figure 8.

Dyeing Properties

In order to investigate the dyeing properties, the sea-island

type PET ultramicrofiber fabrics were firstly alkali treated

with 1 wt% NaOH aqueous solution at 95 oC for 40 min, and

then dyed with commercial disperse dyes. Figure 9 shows

the K/S values of the ultramicrofiber fabric, N600, from

different dyeing temperature. It was found that K/S values

slightly decreased as dyeing temperature increased from 110

up to 130 oC. Generally, the higher dyeing temperature is,

the more dye migration or dye diffusion from the surface

into the core part of the fiber will occur. At 110 oC, it seems

that a large portion of dye molecules would exist at the

surface rather than the core part of the fiber owing to the

very small diameter. When dyed at higher temperature such

as 130 oC, more dye molecules might diffuse within the fiber

which would result in low K/S values. Another explanation

can be made by considering that dyeing is thermodynamically

exothermic process so that dye-fiber adsorption will preferably

occur at relatively low temperature while reverse reaction or

desorption might occur at a very high temperature.

Build-up property of N600 with three disperse dyes was

evaluated and compared to that of N800 (Figure 10). When

dyeing temperature was fixed to 130 oC, K/S values of both

ultramicrofiber fabrics continuously increased as the dye

concentration increased, suggesting that build-up properties

of both fabric should be generally good. The result might be

attributed to the larger surface area of ultramicrofiber and

bigger capability of dye adsorption when compared to the

regular fiber. On the other hand, overall K/S values of N800

or N600 are not so high and maximum K/S value of N800

was not more than 10 when using 5 % o.w.f of Foron Blue

RD-E. K/S value of N600 was always lower than that of

N800. These results are due to the low linear density of the

ultramicrofiber. It is well known that the specific surface

area increases markedly with decreasing filament linear

density, which results in reduced depth of shade of dyed

fabric. Finer fiber would appear lighter in shade than coarser

fiber because greater amount of incident light would be

scattered or reflected at the surface of the dyed substrate

resulting from larger surface area [4].

Figure 8. SEM images of N800 after alkaline treatment with 1 wt% NaOH at 95 oC for 40 min.

Figure 9. Effect of dyeing temperature on color yield of N600

dyed with disperse dyes (3 % o.w.f).

Alkaline Dissolution and Dyeing of PET Ultramicrofiber Fabric Fibers and Polymers 2015, Vol.16, No.9 2001

Fastness Properties

Table 2 shows the results of wash and light fastness test for

ultramicrofiber fabric dyed with commercial disperse dyes

(3 % o.w.f.). In wash fastness of rubine and blue dyes,

staining of adjacent acetate, nylon, or PET for N800 was

poor to moderate and that for N600 was poor. During the

heatsetting, dye molecule would migrate from interior to the

surface of the fiber. This thermomigration would occur more

easily in finer fiber resulting in lower wash fastness.

Light fastness of N800 or N600 was very poor. This is

Figure 10. Build-up properties of N800 and N600 dyed with disperse dyes; (a) Foron Yellow Brown RD-S, (b) Foron Rubine S-WF, and

(c) Foron Blue RD-E.

Table 2. Wash and light fastness of dyed ultramicrofiber fabric

Sample Dye

Wash fastness

LightChange

Staining

Acetate Cotton Nylon PET Acrylic Wool

N800

Yellow browna 4 3 4-5 4 4 4-5 4-5 2

Rubineb 4 2 4 3 3-4 4-5 4 1

Bluec 4 2 3 3 3-4 4-5 4 1

N600

Yellow browna 4 3 4-5 3-4 4 4-5 4-5 1

Rubine b 4 2 3-4 2 2 4 3-4 1

Bluec 4-5 2 3 2 2 4 3-4 1aForon Yellow Brown RD-S,

bForon Rubine S-WF, and

cForon Blue RD-E.

2002 Fibers and Polymers 2015, Vol.16, No.9 Hodong Kim et al.

similar to the previous result in which light fastness of fabric

containing 800 nm ultramicrofiber was very poor (grade 1)

while that of microfiber fabric was poor to moderate (grade

2-3) [12]. It seems that, in ultramicrofiber having larger

surface area, the more dye molecules should be exposed to

external light than in coarser fiber.

Conclusion

Alkaline dissolution and dyeing properties of knitted

fabric from sea-island type PET ultramicrofiber of ca. 600 nm

or 800 nm were investigated. The dissolution ratio of fabric

from ca. 600 nm ultramicrofiber was found to be dependent

upon temperature in the alkali treatment. The dissolution

ratio increased more rapidly with increasing treatment time

at higher temperature. When the fabric was treated with

1 wt% NaOH aqueous solution at 95 oC, the dissolution ratio

reached the theoretical value in 30 min. Alkali dissolution of

the sea component could be monitored by the cationic dye

staining method. While the K/S value of untreated fabric was

high by cationic dyeing, that of alkali-treated fabric decreased

as the treatment time increased, and finally levelled off.

Morphological change of sea-island type ultramicrofiber before

and after alkali treatment was observed directly by SEM

analysis. Although island components with a diameter of ca.

800 nm could be completely separated when treated with

1 wt% NaOH at 95 oC for 40 min, those with a diameter of

ca. 600 nm could not.

The color yield of the disperse dye on the ultramicrofiber

fabric was found to be dependent on the dyeing temperature.

K/S values decreased as dyeing temperature increased from

110 up to 130 oC. K/S value of dyed fabric from ca. 600 nm

ultramicrofiber was lower than that from ca. 800 nm

ultramicrofiber. The wash fastness of ca. 800 nm fabric was

poor to moderate and that of ca. 600 nm fabric was poor.

The light fastness of both fabrics was very poor. Efforts to

improve the wash and light fastness of the ultramicrofiber

are needed.

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