Colour Quality of CO

Laser-Treated Denim Fabric

C. W. Kan and K. K. Law

Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kwoloon, Hong Kong

Keywords: Denim Fabric, CO

Laser, Colour Quality.

Abstract: CO

laser processing was used in this study for engraving denim fabric. By controlling the laser process

parameters, i.e. pixel time (µs) and resolution (dot per inch), laser with different powers can be produced

which were used for engraving a square pattern in denim fabric. After laser engraving, the shade and

related colour properties of the denim fabric were measured and compared with the untreated one.

1 INTRODUCTION

laser treatment has been applied to different

areas of textile industry in recent years (Dascalu et

al., 2000); (Ozguney, 2007); (Tarhan and Sariisik,

2009). By altering the laser power, various amount

of the surface fibres and dye molecules from fabric

surface can be removed causing change in fabric

colour quality values. Therefore, laser treatment can

have the ability of being applied to textile material

for achieving colour fading effect. In recent denim

fabric finishing, colour fading shares one of the most

important finishing effect. Generally speaking, the

colour fading effect can be achieved by using mainly

chemical such as pre-washing, rinsing, stone

washing, sand washing, snow washing, stone

washing with enzymes and bleaching. However,

these treatments may cause pollution problem in the

effluent and inconsistent surface outcomes (Dascalu

et al., 2000); (Ozguney, 2007); (Tarhan and Sariisik,

2009). In addition, the chemical treatments are time-

consuming and product quality is quite difficult to

control. In order to cope with these problems, a

novel treatment that does not have the drawbacks

involved in the conventional chemical colour fading

technologies should be adopted for treating the

denim fabric. The laser treatment, being a dry

treatment, will be an alternative to the conventional

technologies. With the selection of laser intensity

precisely, it is easy to apply certain design effect

onto the textile surface with desired colour quality

values (Esteves and Alonso, 2007); (Kamata and

Suzuki, 2004); (Naruse and Suzuki, 2004). In the

present study, laser engraving will be applied to the

denim fabric in order to evaluate its effect on fabric

shade and colour related properties.

2 EXPERIMENTAL

2.1 Material

Blue indigo-dyed denim fabric was used. The fabric

weight was 384g/m

with warp density 20 ends/cm

(80tex) and weft density 20 picks/cm (60tex). The

denim fabric was conditioned under standard

atmosphere of 65±2% relative humidity and 20±2°C

before further treatment.

2.2 CO

Laser Processing

The laser process was conducted with a CO

source

laser (wavelength: 10.6µm) engraving machine

(GFK, Spain) which is computer-controlled. A

square pattern of size 200mm x 200mm as shown in

Figure 1 was input into the computer system. During

the laser processing, the square pattern was

transferred to denim fabric by laser engraving. The

resolution of the computer-controlled laser beam

was set to 30, 60, 80 and 100 dot per inch (dpi) with

pixel time of 110, 160, 220 and 300µs. Totally, 16

combinations were made.

Figure 1: Square pattern with size 200mm x 200mm.

129

W. Kan C. and K. Law K..

Colour Quality of CO2 Laser-Treated Denim Fabric.

DOI: 10.5220/0004264601290133

In Proceedings of the International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2013), pages 129-133

ISBN: 978-989-8565-44-0

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

2.3 Laser Power Measurement

In order to investigate the relationship between

resolution (dpi) and pixel time (µs) to give the laser

power density, a 842-PE hand-held Optical

Power/Energy Meter was used for measuring the

laser power energy of the 16 parameter

combinations.

2.4. Colour Measurement

Colour measurement was performed by a

spectrophotometer of GretagMacbeth Color-

Eye7000A. D

Daylight with a 10° standard

observer was used during colour measurement.

Totally, four measurements were done for each

sample. The samples were conditioned at 20 ± 2°C

and the relative humidity of 65±2% before taking

the measurements. Reflectance curves, K/S

sum

and

CIE L*a*b* values were obtained.

3 RESULTS AND DISCUSSION

3.1 Laser Power Measurement

The laser power (expressed in W/cm

) of the

corresponding combinations of resolutions and pixel

time were described in Figure 2. For pixel time, it is

a unique parameter in computer graphical file to

control the time for laser head positioning in each

image point in µs. The long pixel time means more

energy focused on the fabric causing a higher degree

of engraving effect. Resolution (in term of dpi) is a

parameter to control the intensity of laser spot in a

particular area; the higher dpi means a higher

resolution. However, too high resolution may cause

the fabric burnt. A steady increased

trend of the

power density was observed with the prolonged

pixel time and high resolution.

Figure 2: Power density of different parameters.

The laser power plays an important role in the CO

laser treatment on the denim fabric which is closely

related to the indigo removal process of textile. The

physical phenomena involved in the indigo removal

process will be the vapourisation process. The

material removal by laser may often be a simple

vapourisation process with absorption of the laser

energy at a continually treated surface. As the laser

energy increases, the material reaches vapourisation

conditions more rapidly (Dascalu et al., 2000).

3.2 Reflectance Measurement

Figure 3: Reflectance of laser-engraved denim fabrics at

30dpi.

Figure 4: Reflectance of laser-engraved denim fabrics at

60dpi.

Figures 3 to 6 present the reflectance curves of

different laser-engraved denim fabrics. The

reflectance values of denim fabrics increased as the

resolution and pixel time were enhanced when

compared with the control fabric. At 30 dpi as

shown in Figure 3, one peak representing the

reflectance curve of all the selected pixel time were

located at the wavelength 400-430nm (indicating

blue shade). When the resolution and pixel time

increased as shown in Figures 4 to 6, there was no

sharp peak appeared with a band shifting to longer

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wavelength at around 460-560nm (indicating green-

yellow shade).

Figure 5: Reflectance of laser-engraved denim fabrics at

80dpi.

Figure 6: Reflectance of laser-engraved denim fabrics at

100dpi.

In addition, the reflectance curve provides the

information of the shade of the material in the

visible spectrum. When the reflectance value is

large, a pale shade will be and vice versa. It is clear

that the shorter the pixel time, the darker the shade

of the laser-engraved fabric will be. It had been

proved that the short pixel time would have a lower

laser power for the engraving process such that a

fewer amount of dyes could be removed from the

denim fabric surface resulting in a darker shade.

Similarly, when the resolution was taken into

consideration, the reflectance values increased

accordingly with the enhanced resolution. As shown

in Figure 2, the higher the resolution, larger the laser

power will be and resulting in more dye removal and

paler shade. Although a band shifting was noted,

there was no obvious change in the overall shape of

the reflectance curves.

3.3 Colour Yield

The colour yield of the denim fabrics after laser

treatment were expressed as K/S value, i.e. K/S = (1-

/2R. Since there was a slight band shifting in the

reflectance curves, thus the K/S

sum

value (which is

the summation of K/S values over the visible

spectrum from 400 nm to 700 nm) was then used for

evaluating the effect of laser engraving process on

the colour yield of the denim fabric. Figure 7

represents the K/S

sum

value of the laser-engraved

denim fabrics in which the higher the K/S

sum

value,

the more the dye uptake would be. It was very clear

to find out that the K/S

sum

values decreased in each

resolution category with respect to different pixel

times. The K/S

sum

value of the untreated fabric was

396.32 but the K/S

sum

values dropped significantly

when compared with different laser-engraved denim

fabrics. As a result, the colour yield of the denim

fabrics decreased due to dye removal in laser

engraving process.

Figure 7: Colour yiled of differently laser-engraved denim

fabrics.

3.4 CIE L*a*B* Values

CIE L* value indicates the lightness of the sample.

The higher the CIE L* value, the lighter the shade of

sample will be. The CIE L* value increased with the

pixel time and resolution resulting in a lighter

surface appearance as shown in Table 1. The CIE L*

value of the control denim fabric value was 21.63.

When compared, the laser engraving process could

increase the CIE L* of the treated denim fabrics

significantly. As indicated in Figure 2, the laser

power level was related closely to the level of pixel

time and resolution. The increment in pixel time

together with the resolution would provide more

laser power in the laser engraving process. As a

result, more fibre materials together with the blue

indigo dyes could be removed from the fabric

ColourQualityofCO2Laser-TreatedDenimFabric

131

surface, leading finally to a paler shade.

CIE a* value represents redness and greenness of

a sample. The more positive the value of CIE a* is,

the redder the shade of the sample will be and vice

versa. In Table 1, the laser engraved denim fabrics

resulted in all negative values of CIE a* with the

magnitude of the values tending to increase. The

CIE a* value of the untreated denim fabric was 0.50,

which indicating that the original denim fabric was

redder than the laser-engraved fabrics. Upon the

laser treatment with different pixel time and

resolution, the pixel time of 110 µs gave the least

reduction in CIE a* followed by 160, 220 and 300

µs respectively. Since the yarns in denim fabric were

blue indigo ring-dyed, the core of the yarn remained

white in colour. In laser engraving, the fibre

together with the blue indigo dyes in the sheath of

yarn were removed from fibre surface leaving a

paler blue colour remaining in the yarn. Owing to

the thermal effect occurred during laser engraving,

thermal oxidation would occur resulting in yellow

surface colour in the yarn. The yellowing in cotton

yarn together with the remaining blue indigo dye

remained in the fabric surface contributes to a

greenish effect on the fabric surface. Since the laser

power increases correspondingly with the increment

of pixel time and resolution, thus the enhanced

thermal oxidation effect contributing to more yellow

surface colour would be obtained resulting in

increased greenish effect.

Table 1: CIE L*a*b* values of differently laser engraved

denim fabrics.

Sample

CIE L* CIE a* CIE b*

Control 21.63 0.50 -4.45

Pixel time

(µs)

dpi

110

31.89 -0.77 -4.05

42.56 -1.34 -1.55

51.99 -1.49 3.20

100

52.45 -1.55 5.44

160

35.59 -1.03 -3.65

42.78 -1.38 -0.23

53.76 -1.53 5.06

100

54.91 -1.65 6.30

220

35.89 -1.13 -3.06

47.42 -1.49 3.65

54.15 -1.57 5.39

100

54.98 -2.23 6.73

300

39.29 -1.26 -2.49

51.48 -1.59 3.70

55.23 -2.00 5.76

100

55.70 -2.36 6.82

Table 1 shows CIE b* values of the denim fabrics

after the laser engraving with different process

parameters. Generally speaking, CIE b* value

describes the yellowness and blueness of a sample.

The higher the positive the value of b*, the more the

yellowish of the sample will be and vice versa. The

CIE b* value of the untreated denim fabric was -

4.45 but after laser treatment, the CIE b* values

increased. The laser engraved fabrics tended to have

a yellowish shade with the prolonged pixel time and

increased resolution. According to Figure 2, the

increase in pixel time and resolution would increase

the laser power in the engraving process. Thus,

more fibres containing the blue dye could be

removed from the fabric surface resulting in a paler

blue shade with increasing CIE b* values. In

addition, the thermal oxidation effect imparted onto

the fibre could give yellowing effect on the fabric

surface also increase the CIE b* values.

4 CONCLUSIONS

Denim fabrics were engraved by laser with different

process parameters, i.e. resolution and pixel time.

Experimental results revealed that the increased

resolution and pixel time would increase the laser

power density accordingly. Under the influence of

high laser power, a paler surface appearance was

noted on the denim fabric. At the same time, the

K/S

sum

values were reduced accordingly, indicating

that the amount of blue dye on denim fabric was

decreased significantly. Meanwhile, the

measurement results of CIE L*a*b* values revealed

that a lighter shade was obtained for the laser-

engraved denim fabrics and also a green-yellow look

was observed for the denim fabrics. In conclusion,

with the suitable selection of various laser process

parameters, the laser engraving process could be

technically used as a novel dry surface treatment for

creating different shading effects on denim fabric.

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Ferreo, F., Testore, F., 2002. Autex Research Journal.

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Kamata, K., Suzuki, A., 2004. Journal of Applied Polymer

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