Effect of CO 2 laser irradiation on the properties of cotton fabric

Abstract

In this paper, a CO₂ laser was used for treating cotton fabric. By controlling the laser process parameters, resolution (40, 50 and 60 dpi) and pixel time (100, 110 and 120 µs), the fabric properties changed accordingly. Fabric weight and strength decreased with increase of resolution and pixel time. However, yellowing was observed in laser treated cotton fabric but the extent of this yellowing was nearly the same under different laser process parameters. Scanning electron microscopy images showed that the surface morphology of cotton fiber was modified and it was believed that the low stress mechanical properties of the fabric were also modified. The low stress mechanical properties were evaluated by the Kawabata Evaluation System for Fabric (KES-F). The results were thoroughly evaluated and discussed. Dyeing performance of the laser treated fabric was evaluated in terms of dyeability, rate of exhaustion and color fastness. According to the reflectance curves, laser treatment reduced the amount of direct dye absorbed by the fabric and resulted in a higher reflectance. K/S values showed a decreased color strength when the resolution and pixel time were increased. The exhaustion curves indicated that laser treatment could cause color fading on cotton and a slight increase in the time of half dyeing (t_½) compared with untreated sample. In terms of color fastness to washing, laser treated samples performed the same as the untreated sample, irrespective of the resolution and pixel time. Laser treated samples had relatively poor crocking results when compared with the untreated sample.

Keywords

Laser cotton fabric low stress mechanical properties surface morphology dyeing direct dye color fastness

Introduction

Clean and dry physical treatments provide more advantages than traditional chemical wet processing. By using dry and clean processes to finish textile materials, consumption of water, chemicals and energy can be reduced significantly or even eliminated completely.¹ Computer-controlled laser treatment is one of the established dry and physical methods being used as a reliable surface finishing method. CO₂ laser irradiation is able to modify different materials or substrates, such as polymers, metals and semiconductors, in which physical and chemical properties of the material surface can be changed.²^–⁶ When a CO₂ laser was applied on cotton material, the color fading effect was the main property studied.^5,6

In order to obtain a better understanding of the effect of CO₂ laser irradiation on cotton fabric, evaluations of different low stress mechanical properties of laser treated cotton were carried out. The degree of the engraving effect of laser on treated fabrics was measured by fabric weight loss and yellowness index.⁷^–⁹ Breaking and tearing strength measurements were also conducted for evaluating performance of the textile materials. Appearance and handle characteristics such as roughness, stiffness and drape are important to textile fabrics; measurement of low stress mechanical properties is one of the ways of assessing these properties¹⁰ which can be achieved using the Kawabata Evaluation System for Fabric (KES-F).

In recent years, faded textiles have become increasingly popular. For example, blue jeans, after color fading, can have different visual effects in terms of design and texture.¹¹ However, fading effects produced by conventional technologies involve a large consumption of water and generation of highly contaminated effluents containing chemicals applied in the process.¹² Apart from the associated environmental problems, conventional methods such as stone-washing and bleaching also have problems of relatively low reproducibility and it is difficult to apply design processes to different fabrics. The time-consuming and non-standard procedures of conventional methods are a significant barrier to mass production and cause increased costs. Recently, the dry process of using a CO₂ laser has shown that it is possible to create reproducible patterns and eliminate drawbacks of conventional chemical washing methods. Laser is able to transfer graphics of a desired variety, size and intensity on textile surfaces, including knitted or woven fabrics, with less water consumption, process flexibility, precision and reproducibility of designs.¹²

According to some researchers, the dyeing performance of laser treated synthetic fibers is different from the normal.^10,13,14 In the case of dyed cotton fabric, the desired color fading effect can be achieved by controlling the CO₂ laser process parameters (i.e. resolution and pixel time).¹⁵ However, no study seems to have reported the dyeing performance of CO₂ laser treated cotton fabric. The CO₂ laser system used in this study is computer-controlled and is able to modify the surface of cotton fabric by using different resolutions and pixel times. After laser treatment, cotton fabrics were dyed with direct dyes in order to examine the effect of laser irradiation on fabric dyeability. Direct dye was used due to the low overall dyeing cost, ease of application, good light fastness, good water solubility and shorter dyeing cycle.^16,17 Due to the inadequate wet color fastness of direct dyes, dyed cotton fabrics were after-treated with a cationic fixing agent.¹⁸ The effect of laser treatment on fabric dyeability, rate of exhaustion, color fastness to washing and crocking was evaluated.

Experimental

All measurements were taken within the 95% confidence level.

Sample preparation

100% bleached cotton fabric (density: 60 ends/inch and60 picks/inch; yarn count: 20 × 20 tex; weight: 128.8 g/m²) was used. The fabric was first cleaned with 10% acetone and was then dried and conditioned at 21 ± 1°C and 65 ± 2% relative humidity for at least 24 hours, before experiments and evaluations.

Laser irradiation

Laser irradiation was performed using a commercial pulsed CO₂ laser (GFK Marcatex FLEXI-150) under atmospheric conditions. The samples were irradiated directly by the laser beam. The pulse energy of the laser beam was in the range of 5 mJ to 230 mJ and wavelength was 10.6 µm. Parameters of resolution (40, 50 and 60 dpi) and pixel time (100, 110 and 120 µs) were varied to study the effects of different conditions as shown in Table 1.

Table 1.

Different parameters used for laser treatment

Sample No.	Resolution (dpi)	Pixel Time (µs)
D1	-	-
D2	40	100
D3	40	110
D4	40	120
D5	50	100
D6	50	110
D7	50	120
D8	60	100
D9	60	110
D10	60	120

Scanning electron microscopy (SEM)

Surface morphology of cotton specimens was investigated by the JEOL JSM-6490 scanning electron microscope (SEM) with magnification up to 3000×.

Fabric weight loss

Fabric weight loss was evaluated based on weight of fabric conditioned at 21±1°C and 65 ± 2% relative humidity for 24 hours before and after laser treatment. After laser treatment, the fabric was weighed and percentage change of weight was calculated by equation (1):

(1)

where W₀ is the initial weight and W is the weight after treatment. Positive change indicates a gain in weight and negative change implies weight loss.

Yellowness index

The yellowness index of cotton fabrics caused by laser treatment was measured according to the ASTM E313 standard using spectrophotometer (GretagMacbeth Colour-Eye 7000A) under a light source (illuminant D65) with a 10° observer.

Breaking strength

Strength of cotton fabrics was measured by a tensile testing machine (Instron 4411) in accordance with standard ASTM D 5034-08.

Tearing strength

The ASTM D1424-07 test method was used to evaluate the tear strength of cotton fabric samples, measured by Elmatear Digital Tear Tester (James H. Heal Co. Ltd.).

Low stress mechanical properties

The Kawabata Evaluation System for Fabric (KES-F) was used to measure low stress mechanical properties (tensile, shearing, bending, compression and surface) of cotton fabric. The size of the specimen was 20 × 20 cm (which could pass through the measuring system), and the samples were conditioned at 21 ± 1°C and 65 ± 2% relative humidity for 24 hours before testing. The results of these measurements were described in Table 2.

Table 2.

Low stress mechanical properties measured by KES-F.⁷

Properties	Symbol	Definition	Unit
Tensile energy	WT	Energy consumed in extending the fabric to 500 cN/cm width	cN.cm/cm²
Tensile resilience	RT	Percent energy recovery from tensile deformation	%
Extensibility	EMT	Percent extension at the maximum applied load of 500 cN/cm, of specimen width	%
Shear rigidity	G	Average slope of linear regions of the shear hysteresis curve to ± 2.5° shear angle	cN/cm. degree
Shear stress at 0.5°	2HG	Average width of the shear hysteresis loop at ± 0.5° shear angle	cN/cm
Shear stress at 5°	2HG5	Average width of the shear hysteresis loop at ± 5° shear angle	cN/cm
Bending rigidity	B	Average slope of linear regions of the bending hysteresis curve to 1.5 cm⁻¹	cN.cm²/cm
Bending moment	2HB	Average width of the bending hysteresis loop at 0.5 cm⁻¹ curvature	cN.cm/cm
Fabric thickness at 0.5 cN/cm² pressure	T_o	Fabric thickness at 0.5 cN/cm² pressure	mm
Fabric thickness at 50 cN/cm² pressure	T_m	Fabric thickness at 50 cN/cm² pressure	mm
Compressional energy	WC	Energy consumed for compressing fabric under 50 cN/cm²	cN.cm/cm²
Compressional resilience	RC	Percent energy recovery from lateral compression deformation	%
Compressibility	EMC	Percent reduction in fabric thickness resulting from an increase in lateral pressure from 0.5 cN/cm² to 50 cN/cm²	%
Coefficient of friction	MIU	Coefficient of friction between fabric surface and standard contactor	-
Geometrical roughness	SMD	Variation in surface geometry of the fabric	mm

Dyeing

A commercial direct dye (Solophenyl Blue FGL-01 165%) (Chanson & Co. Ltd) was used, without further purification, in the dyeing experiment. Bleached cotton fabric was dyed with direct dye to different depths, 0.1%, 2% and 5% (o.w.f., on weight of fabric) at a liquor-to-goods ratio of 50:1. Common salt (20% o.w.f.) and soda ash (0.5% o.w.f.) were added to the dyebath and dyeing was started at 40°C. The temperature was increased to 95°C and the fabric was dyed for 45 minutes. The dyeing curve is shown in Figure 1. After dyeing, the fabrics were treated with 2% (o.w.f.) of cationic fixing agent (Albafix E) with a liquor-to-goods ratio of 50:1 over a period of 20 minutes at 70°C. Then the treated samples were rinsed and oven dried, and finally conditioned at 21 ± 1°C and 65 ± 2% relative humidity for 24 hours before further experiments.

Figure 1.

Dyeing curve.

Rate of dyeing

The dyebath exhaustion rate was measured by extracting the dye liquor from the dyeing process of 2% (o.w.f.) depth dyebath after different dyeing times (5, 10, 15, 25, 35, 45, 60, 75, 90, 120, 180 and 240 minutes). The concentration of the dye liquor was measured at the maximum adsorption wavelength (λ_max) using a Spectronic 20 Genesys spectrophotometer. The percentage of exhaustion (%E) was calculated according to equation (2):

(2)

where A₀ is absorbance of dye at 0 minute, A_t is absorbance of dye at time t, and %E is the percentage of exhaustion at time t.

Color measurement

Color strength (K/S) values (sum of K/S values of wavelengths from 400 to 700 nm) of dyed samples based on the reflectance curve were determined by equation (3). The higher the K/S value is, the better is the color strength.

The reflectance curves were measured by using aspectrophotometer (GretagMacbeth Colour-Eye 7000A).

(3)

where K is the absorption coefficient (depending on the concentration of the dye); S is the scattering coefficient (caused by the dyed substrate); and R is the reflectance of the colored sample.

Color fastness

Color fastness to laundering and crocking of dyed cotton fabrics were assessed by the AATCC Test Method 61-2007 (Colorfastness to Laundering: Accelerated) and AATCC Test Method 8-2007 (Colorfastness toCrocking: AATCC Crock meter Method), respectively.

Results and discussions

Surface morphology

Figure 2(a) shows an SEM image of the untreated sample. It is seen that cotton fiber contains some grooves and wrinkles but no pores or cracks are observed. The untreated cotton fiber surface can bedescribed as a smooth fiber surface. Figure 2(b) shows an SEM image (3000×) of laser treated cotton fabric under irradiation condition of 40 dpi and 100 µs pixel time. Laser irradiation results in pores of various sizes on the cotton fiber, causing a sponge-like structure.¹⁹ As the CO₂ laser used is a kind of pulsed laser, a large number of pores were formed.

Figure 2.

(a) Untreated cotton; (b) Laser treated cotton at 40 dpi and 100 µs.

Fabric weight loss

When cellulose is heated, it undergoes a series of interrelated physical and chemical changes, such as loss of weight. As shown in Figure 3, there is a significant weight loss in all samples after treatment with a laser. It is clear that the weight loss increased with an increase of resolution and pixel time. For example, samples treated at 60 dpi suffered more prominent weight loss than samples treated at 40 dpi. Weight loss of samples increased from 7% at 40 dpi and 100 µs, to 14% at 60 dpi and 100 µs. Besides, weight loss increased from 7% to 8% when the pixel time was increased from 100 to 120 µs at 40 dpi. Weight loss increase with laser irradiation was due to the engraving effect induced by the laser under different resolutions and pixel times.⁹ Also, weight loss after laser treatment could be due to thermal degradation of cotton.²⁰ During the laser process, thermal energy would be absorbed by the fiber. The evolution of gaseous products such as water vapor and/or carbon dioxide would also take place simultaneously leading to the increment of the internal volume of the fiber rapidly. As a result, swelling and expansion effects of fiber occurred resulting in a sponge-like structure.¹⁹ The cotton materials then suffer a chemical reaction at high temperatures that causes elimination of the water inside the fiber, formation of carbonyl/carboxyl groups, and at the same time evolution of carbon monoxide and carbon dioxide, and finally generation of a charred residue.

Figure 3.

Percentage of weight loss due to laser treatment.

Yellowness index

Yellowness indexes of untreated and laser treated cotton fabric under various parameters are shown in Table 3. The yellowness index can be used to measure the change in yellowing of cotton fabrics caused by CO₂ laser treatment. Table 3 shows that when the pixel time was prolonged, the degree of yellowness increased slightly and also the yellowness grew steadily with resolution. The untreated fabric sample had the lowest yellowness index, while the sample treated with laser at 60 dpi and 120 µs had the highest yellowness index value. The main reason for yellowness on laser treated fabric samples is that oxidation occurs on the surface of the fabric after it is subjected to laser treatment.^21,22 The oxidation caused by a laser on the cotton surface is due to the elimination of hydroxyl groups and the formation of oxidation products, such as carbonyl groups, and finally, the generation of degradation residues.²⁰

Table 3.

Yellowness index of cotton fabric samples

Treatment conditions	Yellowness index	Change in %
Untreated cotton	98.72	-
40 dpi, 100 µs	98.88	0.17
40 dpi, 110 µs	98.92	0.21
40 dpi, 120 µs	98.94	0.22
50 dpi, 100 µs	98.94	0.22
50 dpi, 110 µs	98.94	0.22
50 dpi, 120 µs	98.96	0.24
60 dpi, 100 µs	98.96	0.25
60 dpi, 110 µs	98.98	0.26
60 dpi, 120 µs	98.98	0.26

Fabric strength

Table 4 shows fabric strength vis-à-vis laser parameters (i.e. resolution and pixel time). Reduction in tensile strength increased with increase of resolution and pixel time, as laser power density increased when resolution and pixel time increased.²³ Laser treatment at higher resolution and pixel time resulted in a more severe engraving and lower tensile strength. Fabric strength was reduced by 68% at 60 dpi resolution and 120 µs pixel time, which was the highest decrease. Fabric samples treated at 40 dpi and 100 µs pixel time suffered the least reduction in strength. The reduction in strength was due to some of the weak points created on the fabric after laser treatment.

Table 4.

Percentage loss of fabric strength at different resolutions and pixel times

Resolution	Pixel time (µs)
	100	110	120
40 dpi	13%	22%	29%
50 dpi	45%	46%	48%
60 dpi	55%	63%	68%

Laser treatment resulted in reduced tearing strength for all samples (Figure 4); laser treatment at 60 dpi had the most detrimental effect, followed by 50 dpi and 40 dpi. In respect of pixel time also, the higher the pixel time, the lower was the tearing strength. However, the effect of resolution was relatively more prominent than that of pixel time. Laser treatment has the ability to alter surface roughness of the cottonfiber and mobility of yarn at the same time, as shown in the SEM image in Figure 2(b).^24,25 Friction between fibers on the rough surface of laser treated cotton fabric increases, resulting in reduced yarn mobility and a lower force being required to tear the fabric.²⁶ Similar to tensile strength, the tearing strength also deteriorated more significantly by an increase of resolution rather than by pixel time.

Figure 4.

Effect of laser resolution and pixel time on tearing strength.

Low stress mechanical properties

The low stress mechanical properties of laser treated cotton fabric samples measured through KES-F are shown in Table 5.

Table 5.

Low stress mechanical properties of cotton

Properties		Control	Laser parameters
			100 µs			110 µs			120 µs
			40 dpi	50 dpi	60 dpi	40 dpi	50 dpi	60 dpi	40 dpi	50 dpi	60 dpi
Tensile	WT	10.82	10.40	10.40	10.37	10.35	10.35	10.35	10.33	10.33	10.28
	RT	39.46	39.60	39.73	39.88	39.72	39.88	39.91	39.85	39.90	40.13
	EMT	6.20	5.79	5.77	5.65	5.58	5.51	5.47	5.33	5.20	5.08
Shearing	G	1.09	1.81	1.91	2.03	2.12	2.57	2.71	2.97	3.25	3.35
	2HG	2.98	4.19	4.28	4.53	4.41	5.23	5.41	5.87	6.31	6.69
	2HG5	5.34	7.06	7.10	7.28	7.44	7.65	7.73	7.76	7.88	7.90
Bending	B	0.06	0.08	0.08	0.08	0.09	0.09	0.10	0.11	0.11	0.12
	2HB	0.06	0.09	0.09	0.09	0.09	0.09	0.09	0.09	0.09	0.10
Compression	T₀	0.90	0.96	0.93	0.92	0.84	0.81	0.79	0.78	0.76	0.75
	T_m	0.51	0.54	0.53	0.53	0.52	0.52	0.51	0.50	0.50	0.50
	WC	0.31	0.35	0.34	0.33	0.28	0.27	0.26	0.24	0.23	0.23
	RC	32.65	26.43	27.92	28.87	30.06	30.59	30.65	32.71	33.99	34.37
	EMC	42.55	43.81	42.75	42.61	38.32	35.68	35.37	36.51	34.47	33.92
Surface	MIU	0.21	0.26	0.31	0.38	0.43	0.46	0.59	0.47	0.47	0.51
	SMD	5.08	9.62	6.26	5.86	5.89	5.84	5.37	4.82	4.46	4.17

Tensile properties

Tensile properties, as shown in Table 5, can be expressed by tensile energy (WT), tensile resilience (RT) and extensibility (EMT). Tensile energy is defined as the energy required to stretch the fabric to a certain width; this parameter indicates the ability of the fabric to resist external force when being extended. Lower values of WT mean that lower energy is required for deformation. Since various sizes of pores and even cracks were created by the laser, as shown in Figure 2(b), some weak points may also have been created^27,28 and thus the laser treated fabric is slightly weakened compared with untreated fabric.

A higher value of RT implies better recovery of the fabric from tensile deformation. In other words, laser treatment can cause a slight improvement in tensile resilience. Extensibility is the percent extension after applying a certain load on the fabric. The greater the EMT value, the greater the elongation of the fabric is. There is a reduction in EMT after laser treatment, and this reduction is further enhanced when pixel time and resolution are increased. This may be due to the increase in the interaction force between fibers and yarns, which may impede movement of fibers and yarns, thereby restraining elongation of the fabric. The laser treatment introduced pores to the smooth fiber surface. The pores will reduce the overall fiber strength but the pores will make the fiber surface become rougher when compared with the untreated fiber surface. Hence, during the tensile properties testing process, the roughened surface will increase the inter-yarn and inter-fiber friction which may be a result of increase contact between fiber and yarn. As a result, the fabric elongation was restrained.

Shearing properties

Shearing properties, namely, shear rigidity (G), shear stress of 0.5° (2HG) and 5° (2HG5) are summarized in Table 5. After treating with a laser, there is a remarkable increase in value of G, 2HG and 2HG5. The increase became greater when resolution and pixel time were increased.

Shear rigidity (G) of a fabric depends mainly on mobility of warp/weft threads in the fabric.²⁹ A lower value of G reflects the better handle of the fabric.³⁰ Theremarkable increase in G after treatment with a laser suggests that the fabric has become more difficult to tailor and would provide less comfort in wearing. Inaddition, a laser treated fabric’s recovery ability worsened, as 2HG and 2HG5 values increased obviously. Itis believed that the interaction of yarns in cotton fabrics was increased due to the effect of the laser, and thus shear rigidity also increased.²⁷

After laser treatment, the surface roughness of the fiber was increased as reflected by the SEM image. The surface roughness would increase the frictional force between fibers and yarns. When the inter-yarn frictional force increased, the movement ofthe yarns will be restrained, hence the mobility of the yarns during shearing will be decreased.

Bending properties

Table 5 shows bending properties, including bending rigidity (B) and bending movement (2HB) of laser treated fabric. Bending rigidity of the fabric depends upon the bending rigidity of threads and the mobility of the warp/weft yarn in the fabric.²⁹ Laser treatment resulted in the overall values of B and 2HB increasing slightly. Since the interaction of yarns in the fabric was increased by the laser, the mobility of the yarn decreased and higher values of B and 2HB were obtained. In other words, the fabric’s tailorability and drape were also lowered.

Compression properties

Table 5 shows results of compression properties, including fabric thickness at 0.5 cN/cm² pressure (T₀) and 50 cN/cm² pressure (T_m), compressional energy (WC), compressional resilience (RC) and compressibility (EMC). Thickness values (T₀ and T_m), compressional energy (WC) and compressibility (EMC) of fabric treated with the laser at 40 dpi and 100 µs pixel time were greater than the untreated fabric and other laser treated fabrics. The increase in T₀, T_m and WC values reflect that the fabric sample had a fuller feeling and a greater raising effect.

At lower resolution and pixel time, the laser engraving effect and damage were less severe. The increase in fullness may be due to the milder laser effect at 40 dpi and 100 µs pixel time; a relatively low amount of fibers are etched away and roots of the remaining etched fibers remain standing on the fabric surface, thereby resulting in higher thickness and a fluffier feeling. However, these values of thickness and compressibility decreased with increase in resolution and pixel time. Since laser engraving power increased with resolution and pixel time, laser damage became more severe and a greater amount of fibers was etched away. As a result, other laser treated samples suffered a remarkable reduction in T₀, T_m, WC and EMC, with values lower than even the untreated fabric.

In addition, compressional resilience (RC) was used to indicate recoverability of the fabric after compression induced deformation.²⁷ The higher values of RC reflect the better ability for retention from deformation. After laser treatment, there was a decrease in RC values first and then an increase gradually with resolution and pixel time. The initial reduction was due to the surface raising effect caused by laser and hence increased the cohesive force between the yarns so that the yarns could not move freely and easily leading to lower RC values. With the increment of resolution and pixel time, more surface fibers were removed and hence the cohesive force between yarns were increased leading to higher RC values. Therefore, values of RC first decreased and then increased gradually when pixel time and resolution were increased.

Surface properties

Results of surface properties were combined into coefficient of friction (MIU) and geometrical roughness (SMD) of fabric surface (Table 5). MIU indicates smoothness, roughness and crispness; the higher value of MIU represents a rougher surface. Laser treatment makes fabric surface less smooth and rougher than the untreated fabric.

SMD reflects evenness characteristics of surface of the fabric. A higher value of SMD indicates a lower evenness of the fabric surface. SMD values of fabric treated with a laser at 40 dpi and 100 µs pixel time indicate that fabric surface structure became highly uneven after laser treatment. However, surface evenness of laser treated fabric increased gradually with an increase of resolution and pixel time. As shown in Table 6, laser induced pores became denser and were evenly developed when resolution and pixel time were increased. As a result, SMD increased first and then decreased along with an increase of laser parameters. Also, the increase in the resolution and pixel time would help to remove more surface fibers and therefore the evenness of the fabric surface was improved leading to a decrease in SMD values.

Table 6.

Pore size and density of laser treated cotton samples at various pixel times and resolutions

	Pixel time (µs)
	100	110	120
Pore size (µm)	1.68	2.04	2.82
	Resolution (dpi)
	40	50	60
Pore density (no. of pores/µm²)	0.013	0.019	0.022

Fabric dyeability

Figures 5 to 7 illustrate reflectance curves of dyed cotton fabric samples treated with a laser under varying conditions with different dyeing depths, namely, 0.1%, 2% and 5% o.w.f., respectively. Blue color direct dye was used and, therefore, the samples look blue as they reflect most of the blue light and absorb less light than other wavelengths in the reflectance curves. It was found that the untreated fabric sample had the lowest reflectance percentage, while fabric samples treated with a laser at 60 dpi attained the highest reflectance. As reported in extant literature, the more light they absorb or the lower the reflectance factor is, the deeper is the dyeing achieved and the darker the fabric looks.^13,14 Therefore, laser treated cotton fabric samples had lighter shades compared with the untreated fabric sample.

Figure 5.

Reflectance curves of laser treated fabrics dyed with 0.1% o.w.f.

Figure 6.

Reflectance curves of laser treated fabrics dyed with 2% o.w.f.

Figure 7.

Reflectance curves of laser treated fabrics dyed with 5% o.w.f.

Reflectance decreased when dyeing depth increased (Figures 5 to 7). The measured reflectance curve is like a fingerprint which describes the kind and amount of colorants used. The lower the reflectance, the greater is the amount of colorant absorbed by the samples. In other words, fabric samples dyed with 5% o.w.f. had the lowest reflectance values and, therefore, had the darkest shade. In addition, laser treatment reduced the amount of direct dye absorbed by the fabric and resulted in higher reflectance. Last but not least, no wavelength shift occurred on laser treated fabric samples. Hence, laser treatment could not induce a chromaticity change or any change in color of cotton fabric samples.

Figure 8 shows the color strength of dyed laser treated cotton fabric at different dyeing depths, i.e. 0.1%, 2% and 5%. By increasing the dyeing depth, the K/S value increased; fabric dyed at 0.1% o.w.f. had the lowest K/S value and samples dyed at 5% o.w.f. had the highest K/S value. There was a decrease in the overall K/S values of fabric samples compared with the untreated sample, irrespective of dyeing depth. All laser treated samples (D2–D10) had relatively low color strength compared to fabric without laser treatment (D1).

Figure 8.

Color yield of laser treated cotton fabric dyed with different dye concentrations.

Table 7 shows the color yield of samples laser treated at different resolutions and pixel times. Color strength decreased when resolution and pixel time increased. The fabric sample not subjected to laser irradiation had the highest K/S value after dyeing at 0.1%, 2% and 5% o.w.f., while dyed fabric samples laser treated with 60 dpi had the lowest K/S value. Moreover, color strength of dyed fabric with laser treatment at 100 µs was higher than that of the sample treated at 120 µs. Wickability of laser treated samples was highly improved, mainly due to the capillary action of the porous structure.³¹ However, in the dyeing process, laser treated samples were subjected to dynamic action, such as mechanical agitation, and also the dyeing results depend not only on physical properties of the fiber, such as wickability, but also on chemical properties such as available dyesite. In accordance with K/S values of dyed samples, absorption of dyes by laser treated samples was not enhanced by the increase in wickability. It is believed that the reduction of K/S values was caused by other factors. As shown in the SEM image in Figure 2, laser induced pores and engraving effects increased at higher resolution and pixel time; laser induced fragments of outer layers of cotton fibers were removed and thus the number of dyesites was lowered. Besides, the rapid heating caused by the absorption of energy from the laser beam led to breaking of some of the fibers, forming some fragments which remained adhered to the fiber surface.³² As a result, the amount of dyes absorbed by laser treated samples was decreased and resulted in a lower color strength when resolution and pixel time increased.

Table 7.

K/S values of laser treated fabrics dyed with direct dye at various depths

	Depth of dyeing
	0.1%			2%			5%
	100 µs	110 µs	120 µs	100 µs	110 µs	120 µs	100 µs	110 µs	120 µs
Untreated	19.62			136.05			207.46
40dpi	15.06	15.08	14.78	84.95	84.28	82.43	129.20	122.80	121.93
50dpi	14.26	13.87	13.90	75.41	75.67	75.77	113.94	110.12	110.36
60dpi	13.47	13.15	13.36	74.51	72.15	72.46	108.72	107.58	105.05

On the other hand, oxidation occurred on the fabric surface after irradiation by the laser; oxidation products were reflected by the FTIR-ATR analysis and in the yellowness index. The elimination of hydroxyl groups, oxidized into carbonyl/carboxyl groups, diminished the attraction of direct dyes towards cotton. Since direct dyes are retained by the fiber through hydrogen bonds and van der Waal’s force, the loss of hydroxyl groups results in less dye absorption and lower color strength. Thus, reduction in color yield of laser treated fabric samples occurred due to oxidation of hydroxyl groups by the laser.^31,33

Figures 9 to 11 show that the laser treated side of the fabric has a lower color strength compared with the other. Both sides of the untreated fabric samples had approximately equal K/S value after dyeing with direct dye. However, the laser treated fabric samples had two different K/S values after dyeing; the laser treated side of the fabric obtained lower color strength and the other side, not subjected to laser treatment, had higher color strength. This result confirms that laser irradiation can modify only the fiber surface and the other side of the fabric has insignificant modification, such that the bulk of the dyeing properties of the samples were not affected by laser treatment.²

Figure 9.

Color yield of laser treated side and untreated side of cotton fabrics (0.1% o.w.f.).

Figure 10.

Color yield of laser treated side and untreated side of cotton fabrics (2% o.w.f.).

Figure 11.

Color yield of laser treated side and untreated side of cotton fabrics (5% o.w.f.).

Furthermore, the side that was not laser treated suffered a slight decrease in K/S value when the resolution and pixel time were increased. The main reason was probably the engraving effect induced by laser irradiation. It has been stated earlier that the weight of fabric after treating with a laser decreases as parts of cotton fibers are etched away by the laser beam. Thus the available dyesites for the dyes to adhere on the cotton were reduced and the side not treated with laser also suffered a small decrease in color strength. However, no oxidation occurred on the untreated side and hence K/S values decreased to a lesser extent than the laser treated side. Although there was an increase in depth of coloration, the reduction in K/S values was not affected, or improved.

Rate of exhaustion

In Figures 12 and 13, the reduction in percentage of exhaustion (%E) is more prominent after change in resolution than after change of pixel time; dyed fabric treated with laser at 60 dpi had the lowest percentage of exhaustion at equilibrium (Figure 13). However, the time for achieving equilibrium of exhaustion was not changed much by laser treatment. As a result, it is found that laser treatment can be used as a pretreatment of cotton fabric by varying the combination of laser process parameters of resolution and pixel time to provide specific desired fading effects more precisely, before the dyeing process.

Figure 12.

Dyebath exhaustion of laser treated cotton fabric at different pixel times.

Figure 13.

Dyebath exhaustion of laser treated cotton fabrics at different resolutions.

Table 8 shows the dye bath exhaustion rate for dyed cotton fabric samples with laser pretreatment at various conditions. Dyed fabric without laser treatment had the highest equilibrium dye uptake, indicating that laser treatment could cause color fading since available dyesites on fabric surface are decreased by laser treatment; hydroxyl groups are reduced by oxidation. When resolution and pixel time are increased, the degree of oxidation is increased. As a result, the final percentage of exhaustion is reduced with increased resolution and pixel time.

Table 8.

The time of half dyeing and %E of laser treated cotton at different conditions

Sample	t_½ (minutes)	%E at equilibrium
D1	3.33	49.93
D2	3.40	48.51
D3	3.40	48.32
D4	3.40	47.93
D5	3.37	46.71
D6	3.37	46.14
D7	3.37	44.74
D8	3.27	44.00
D9	3.27	43.00
D10	3.27	40.60

The time of half dyeing (t_½) means the time required to achieve half the equilibrium, which can be used to quantify the rate of dyeing in the dyeing process.¹⁰ In general, the effect of laser treatment on t_½ is not prominent as the range of t_½ was changed from 3.40 to 3.27 minutes. Samples treated with a laser at 40 and 50 dpi suffered a slight increase in t_½ over the untreated sample, probably due to the total surface area of fibers having been increased by the laser induced sponge-like structure, implying more contact area for dyes molecules to diffuse.¹⁰ However, t_½ decreased when resolution was increased to 60 dpi. Since there were laser induced fragments and debris adhering on the fiber surface and their formation increased with resolution, diffusion of dyes to the fiber surface was reduced and t_½ decreased, as resolution increased.

Color fastness to washing

The AATCC Test Method 61-2007 Color Fastness to Laundering: Accelerated test was used to evaluate color change and staining of dyed laser pretreated cotton fabric samples. In Table 9, only results of staining of cotton in multi-fiber fabric are presented as other materials suffered no change in shade after the washing test. Table 9 shows that laser untreated samples and laser pretreated samples have the same color change and staining results. In other words, laser technology does not cause significant effect in the color fastness to washing test. Laser pretreated samples perform the same as the control sample, irrespective of resolution and pixel time. In addition, color fastness to washing decreased with increase of the amount of dyes used. Since color fastness depends to some degree on the amount of dye on the fiber, color fastness to washing decreases as concentration of dye increases.

Table 9.

Color fastness to washing

	0.1% o.w.f.		2% o.w.f.		5% o.w.f.
Cotton Sample	Color change	Staining on cotton fabric	Color change	Staining on cotton fabric	Color change	Staining on cotton fabric
D1	4–5	4–5	4–5	4	4–5	3–4
D2	4–5	4–5	4–5	4	4–5	3–4
D3	4–5	4–5	4–5	4	4–5	3–4
D4	4–5	4–5	4–5	4	4–5	3–4
D5	4–5	4–5	4–5	4	4–5	3–4
D6	4–5	4–5	4–5	4	4–5	3–4
D7	4–5	4–5	4–5	4	4–5	3–4
D8	4–5	4–5	4–5	4	4–5	3–4
D9	4–5	4–5	4–5	4	4–5	3–4
D10	4–5	4–5	4–5	4	4–5	3–4

Color fastness to crocking

Table 10 shows color fastness to crocking results. All laser pretreated samples have relatively poor crocking results compared with the control sample. Surface morphological modifications induced by laser irradiation shown in SEM images (Figure 2) include cracks and fiber fragments that form the sponge-like structure. Outer surfaces of samples were damaged and some weak points were created;¹⁵ outer parts of fibers may be more easily rubbed off, resulting in lower dry and wet crocking results. However, the change in laser treatment condition did not cause any variation in wet crocking and dry crocking results. Apart from this, wet crocking results were poorer than dry crocking.

Table 10.

Color fastness to crocking

	Wet crocking			Dry crocking
Cotton sample	0.1% o.w.f.	2% o.w.f.	5% o.w.f.	0.1% o.w.f.	2% o.w.f.	5% o.w.f.
D1	4–5	3–4	3	5	4–5	4–5
D2	4–5	3	2–3	5	4	4
D3	4–5	3	2–3	5	4	4
D4	4–5	3	2–3	5	4	4
D5	4–5	3	2–3	5	4	4
D6	4–5	3	2–3	5	4	4
D7	4–5	3	2–3	5	4	4
D8	4–5	3	2–3	5	4	4
D9	4–5	3	2–3	5	4	4
D10	4–5	3	2–3	5	4	4

Conclusion

In conclusion, laser induces a sponge-like surface structure on cotton fabric as shown in the SEM image. Results of fabric weight loss and tensile and tearing strength of laser treated fabric were indicative of the effect of laser treatment. It was found that the fabric surface turned yellow after laser treatment, compared with untreated fabric, but the effect was not significant visually. Also there was an enhancement of yellowness when resolution and pixel time of the laser beam were increased. The reduction in fabric weight and breaking and tearing strength decreased with an increase of resolution and pixel time. Through controlling resolution and pixel time of the laser beam, desired laser effects on cotton can be obtained. Low stress mechanical properties, namely, tensile, shearing, bending, compression and surface properties were evaluated through KES-F. Tensile energy of laser treated fabric was slightly reduced, but the recovery from tensile deformation became better. Elongation of laser treated fabric was restricted by the increase of interaction between fibers. Since yarn mobility was decreased by laser irradiation, reduction in shearing properties was observed. Both bending rigidity and recovery from bending of laser treated fabric were lowered. The results also show that laser treatment provided a fuller hand and feel. Finally, fabric surface irradiated by laser became less smooth but had better evenness. As shown in reflectance curves, there was no chromaticity change or any change in color. However, reflectance of dyed samples pretreated with the laser was reduced and a lighter shade was obtained. Moreover, the color yield of laser treated samples was lower than the untreated sample. The reduction in K/S values was enhanced by increasing resolution and pixel time. Also, the laser treated side suffered a greater decrease in K/S valuethan the other side. On the other hand, color fastness to washing results did not vary significantly with different laser treatment conditions. There was no deterioration or improvement in color fastness to washing because of laser treatment. However, color fastness to washing became worse when dyeing depth increased. Last but not least, results of color fastness to crocking were deteriorated by laser treatment because of weak points on the cotton fabric; weakened outer parts of fabric could be easily rubbed off and both wet and dry crocking results were worsened. Based on this work, laser treatment of the fabric, followed by dyeing, resulted in paler shades. Thus, the color fading effect can be easily (pre)determined by controlling laser treatment parameters whereas in conventional methods such as enzyme washing, the color fading effect depends greatly on experience and trial and error.

Footnotes

Funding

This research received no specific grant from any fundingagency in the public, commercial, or not-for-profit sectors.

Acknowledgements

The authors want to express the gratitude to the Hong Kong Polytechnic University for the financial assistance.

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Sample No.	Resolution (dpi)	Pixel Time (µs)
D1	-	-
D2	40	100
D3	40	110
D4	40	120
D5	50	100
D6	50	110
D7	50	120
D8	60	100
D9	60	110
D10	60	120

Sample No.	Resolution (dpi)	Pixel Time (µs)
D1	-	-
D2	40	100
D3	40	110
D4	40	120
D5	50	100
D6	50	110
D7	50	120
D8	60	100
D9	60	110
D10	60	120

Sample No.	Resolution (dpi)	Pixel Time (µs)
D1	-	-
D2	40	100
D3	40	110
D4	40	120
D5	50	100
D6	50	110
D7	50	120
D8	60	100
D9	60	110
D10	60	120