**3. Dyeing procedure**

180 Textile Dyeing

Fig. 6. Effect of energy input W/F (RF power/C2H2 flow) on the deposited film.

Fig. 7. Effect of gas ratio on wicking property of plasma-coated PET

The wicking property of plasma-coated fabrics was tested according to the water climbing time in the fabric at a certain height (strips size: 15 cm x 2 cm). The wicking times were recorded at fixed 2 cm water climbing height. Surface hydrophilicity was greatly enhanced, as shown in Fig. 7, due to the addition of polar functional groups (Hossain et al., 2007c). The

**2.1.2 Wicking property improvement** 

The dyeing of plasma treated samples was carried out in a laboratory-scale machine manufactured by Mathis, Switzerland (LABOMAT–8, Type BFA–8). Hydrophilic functional a-C:H:N films of the PET substrate were dyed with hydrophilic acid dyes (C.I. Acid Blue 127:1). The light-shade dyeing was performed using 0.5 % owf acid dye and 5 % owf sodium sulphate salt for exhaustion and the pH of the dye bath was adjusted to 4.5-5.0 by adding ammonium sulphate. The liquor to fabric ratio in dyeing was 1:100 and the following dyeing conditions were adopted: initial temperature 25 °C, followed by temperature gradient of 1.5 °C min-1 up to 80 °C, then the dye bath temperature was maintained at 80 °C for 60 min. After dyeing, the dyed fabrics were washed with soap (Ultravon W) at 60 °C for 30 min (L:R = 1:100), then rinsed with warm and cold water and dried at room temperature. Substrate independent plasma dyeing mechanism is described in the Fig. 8. Due to the presence of sulfonic acid groups (-SO3H), acid dye is water-soluble which is transported to the coating on the fiber by the motion of dye-liquor or the textile or simultaneous movement of both in the exhaustion dyeing process. By adsorption process, dye molecule comes on the coated surface; it is then diffused into the nanoporous coating. The basic amino group of coating is decisive important for acid dyeing which can be protonated in the acid medium and becomes fiber-cation (ammonium group). On the other hand, dye-molecule dissociates in the Na2SO4/H2O solution and which gives rise to dye anions, as a consequence they interact with the ammonium groups of the coated fiber mainly by the formation of ionic bonds. Secondary bonds such as dispersion, polar bonds and hydrogen bonds can also additionally be formed between fiber and dye. As a result, dye molecule is fixed in the plasma coating which is independent from the substrate materials.

#### Fig. 8. Film/dye interaction

CIELAB color values of the dyed fabrics were determined using a Datacolor Spectraflash interfaced to a PC. Each fabric was folded twice so as to give four thicknesses, and an

Substrate Independent Dyeing of Synthetic Textiles Treated with Low-Pressure Plasmas 183

detect the amount of amine groups inside the coatings. It should be mentioned that untreated PET textiles were not dyeable with acid dye, since they do not contain

The gaseous mixture (suitable NH3/C2H2 ratio ~ 1.25 and NH3/C2H4 ratio ~ 1.0) helped to build up voids in the deposition resulting in a nanoporous plasma polymer coating which was accessible to dye molecules throughout the entire film volume (Freiere et al., 1995; Hammer et al., 2001). Higher ammonia flow to monomer gases indicates an increase nitrogen content, as can be seen in Fig. 10. Moreover, NH3/C2H2 plasma yields a lower nitrogen content compared to NH3/C2H4 plasma owing to an increased carbon content , which can be seen for equal gas ratios (ammonia/monomer = 1.25). The previous study also confirmed the chemical composition of the a-C:H:N deposited films discussing the XPS spectra elaborately (Hossain et al., 2007a). It can be assumed that higher NH3/C2H4 produce more amides, cyanides, imines etc. within the coating rather than simple amine functional groups. Highly reactive ammonia, on the other hand, generates a high number of surface free radicals at higher nitrogen content (i.e. high NH3/C2H2 = 2.70) as compared to a lower ratio (NH3/C2H2 = 1.70). These reactive radicals contribute to form oxygen-containing polar functional groups by post-plasma reactions in the atmosphere. As a consequence, superhydrophilic coatings can be achieved with high ammonia content by incorporation of polar functional groups into the surface within a cross-linked hydrocarbon network. Likewise, the highest nitrogen content reveals the lowest color difference due to a lower number of amines (Table 1). The color difference was compared to non-plasma treated fabric. Thus, the dyeability of the plasma coatings was found to be low at a gas ratio > 2.50. Thus, coloration does not depend on the surface wettability, but on the density of amine-end group. The color intensity in different positions for each dyed sample was measured and the value was found to be almost identical and thus confirming

functional groups in their structure needed for ionic bond (dye-fiber interaction).

Fig. 10. Nitrogen and carbon contents of a-C:H:N films in different gas ratios

a level dyeing.

average of six readings was taken for each measurement. The reflectance (R %) value of the dyed fabrics was measured over the wavelength range of 360-750 nm. The illuminant type was D65 and the observer angle was 10°. The color strength values (K/S values at 490 nm) of the fabrics were calculated from Kubelka-Munk equation (1), where K is the absorption coefficient, S is the scattering coefficient, and *R* is the decimal fraction of the dyed fabrics.

$$\mathbf{K}/\mathbf{S} = (\mathbf{1} \cdot \mathbf{R})^2 / 2\mathbf{R} \tag{1}$$

#### **3.1 Dyeing of plasma films**

In order to obtain nanoporous thin films, different modifications were employed by varying the important plasma parameters such as discharge power and gas flow ratios. It was found that a gas ratio of NH3/C2H4 around 0.70 - 1.0 and NH3/C2H2 around 1.0 - 1.25 and a discharge power of around 500 - 600 W yield the optimum regarding amine functionalities (Hossain et al., 2007b). The amine functionalities were accessible by dye molecules. Fig. 9 demonstrated the dyeability of plasma-treated PET textiles with different coating thicknesses. The relative color strength value was increased gradually with the increase in coating thickness (i.e. plasma exposure time) (Hossain et al., 2007b; Balazs et al., 2007). It is noteworthy that with increasing plasma process time the penetration of reactive plasma species yielding plasma polymerization goes deeper into the textile structure, even in the inter-filament or inter-yarn spaces, resulting in better dyeability, i.e. number of amine-end groups. On the other hand, the reduced film thickness at very low flow rates led to a lower K/S value of the PET fabrics. Thus, dye molecules are able to penetrate into the nanoporous films and facilitate the dyeing of a-C:H:N thin films by forming chemical bonds mainly with amino groups within the plasma coatings (Hossain et al., 2009; Siow et al., 2006). Since the dyestuff used specifically binds with amine groups, dyeing can be used as a specific chemical tracer to

Fig. 9. Relative color strength values (K/S) depending on film thickness and exposure time (light-shade dyeing with 0.5% owf acid dyes).

average of six readings was taken for each measurement. The reflectance (R %) value of the dyed fabrics was measured over the wavelength range of 360-750 nm. The illuminant type was D65 and the observer angle was 10°. The color strength values (K/S values at 490 nm) of the fabrics were calculated from Kubelka-Munk equation (1), where K is the absorption coefficient, S is the scattering coefficient, and *R* is the decimal fraction of the

In order to obtain nanoporous thin films, different modifications were employed by varying the important plasma parameters such as discharge power and gas flow ratios. It was found that a gas ratio of NH3/C2H4 around 0.70 - 1.0 and NH3/C2H2 around 1.0 - 1.25 and a discharge power of around 500 - 600 W yield the optimum regarding amine functionalities (Hossain et al., 2007b). The amine functionalities were accessible by dye molecules. Fig. 9 demonstrated the dyeability of plasma-treated PET textiles with different coating thicknesses. The relative color strength value was increased gradually with the increase in coating thickness (i.e. plasma exposure time) (Hossain et al., 2007b; Balazs et al., 2007). It is noteworthy that with increasing plasma process time the penetration of reactive plasma species yielding plasma polymerization goes deeper into the textile structure, even in the inter-filament or inter-yarn spaces, resulting in better dyeability, i.e. number of amine-end groups. On the other hand, the reduced film thickness at very low flow rates led to a lower K/S value of the PET fabrics. Thus, dye molecules are able to penetrate into the nanoporous films and facilitate the dyeing of a-C:H:N thin films by forming chemical bonds mainly with amino groups within the plasma coatings (Hossain et al., 2009; Siow et al., 2006). Since the dyestuff used specifically binds with amine groups, dyeing can be used as a specific chemical tracer to

Fig. 9. Relative color strength values (K/S) depending on film thickness and exposure time

(light-shade dyeing with 0.5% owf acid dyes).

K/S = (1-R)2/2R (1)

dyed fabrics.

**3.1 Dyeing of plasma films** 

detect the amount of amine groups inside the coatings. It should be mentioned that untreated PET textiles were not dyeable with acid dye, since they do not contain functional groups in their structure needed for ionic bond (dye-fiber interaction).

The gaseous mixture (suitable NH3/C2H2 ratio ~ 1.25 and NH3/C2H4 ratio ~ 1.0) helped to build up voids in the deposition resulting in a nanoporous plasma polymer coating which was accessible to dye molecules throughout the entire film volume (Freiere et al., 1995; Hammer et al., 2001). Higher ammonia flow to monomer gases indicates an increase nitrogen content, as can be seen in Fig. 10. Moreover, NH3/C2H2 plasma yields a lower nitrogen content compared to NH3/C2H4 plasma owing to an increased carbon content , which can be seen for equal gas ratios (ammonia/monomer = 1.25). The previous study also confirmed the chemical composition of the a-C:H:N deposited films discussing the XPS spectra elaborately (Hossain et al., 2007a). It can be assumed that higher NH3/C2H4 produce more amides, cyanides, imines etc. within the coating rather than simple amine functional groups. Highly reactive ammonia, on the other hand, generates a high number of surface free radicals at higher nitrogen content (i.e. high NH3/C2H2 = 2.70) as compared to a lower ratio (NH3/C2H2 = 1.70). These reactive radicals contribute to form oxygen-containing polar functional groups by post-plasma reactions in the atmosphere. As a consequence, superhydrophilic coatings can be achieved with high ammonia content by incorporation of polar functional groups into the surface within a cross-linked hydrocarbon network. Likewise, the highest nitrogen content reveals the lowest color difference due to a lower number of amines (Table 1). The color difference was compared to non-plasma treated fabric. Thus, the dyeability of the plasma coatings was found to be low at a gas ratio > 2.50. Thus, coloration does not depend on the surface wettability, but on the density of amine-end group. The color intensity in different positions for each dyed sample was measured and the value was found to be almost identical and thus confirming a level dyeing.

Fig. 10. Nitrogen and carbon contents of a-C:H:N films in different gas ratios

Substrate Independent Dyeing of Synthetic Textiles Treated with Low-Pressure Plasmas 185

The dyeability was found to strongly depend on the dye bath temperature in the plasma dyeing process, as shown in Fig. 12; this is also a common phenomenon in traditional dyeing processes. Results show that the amount of dye absorbed on the coating decreases with decreasing dye-bath temperature. The low reduction in dyeability at lower temperature (< 80 °C) is probably due to dye aggregation and a low degree of adsorption. The differences, however, became very small especially at higher temperatures within the range of 80-120 °C. By increasing the temperature, dye uptake can be enhanced because of the increased solubility and mobility of dye molecules in water. There may be another reason for that, which is commonly seen in traditional synthetic dyeing processes: the enhanced color yield, which gradually increases with increasing temperature, can be attributed to a corresponding increase in the amount of accessible volume available for dye diffusion (Burkinshaw et al., 1995). The highest color yields were found to be at ~100 °C implying that the dye uptake reached the "saturation level" resulting in maximum acid-base intermolecular interaction between dyes and amine functionalities. At this level, the dye molecules occupied most of the incorporated amine functionalities in the film volume yielding maximum dyeability. Thus, substrates such as PES, PP, aramid, glass textiles etc. can be dyed at low temperatures similar to wool dyeing, since good coloration was obtained

Fig. 12. Effect of dyeing temperature on dyeability (dyeing time 60 min) (600 W, NH3/C2H4 = 0.84, 20 min) and light-shade dyed PET (0.5% owf)

affinity of dyes to the film resulting in weak dye-film interaction.

The pH value of the dye bath was found to be very important in order to achieve level dyeing and to increase dye affinity to the functionalized films. The optimum level dyeing and color strength were obtained at a pH in the range of 4.5-5. However, the color intensity was increased at pH 2.5-3.0 due to improved exhaustion, but uneven shade was observed. On the other hand, dyeability was reduced at pH 6.0-7.0 due to low substantivity or low

**3.1.2 Temperature and pH effects** 

at levels as low as 80 °C.


Table 1. Color difference of dyed a-C:H:N films at different plasma power, film thickness, and nitrogen concentration (NH3/C2H4 plasma).

#### **3.1.1 Rate of dye-uptake**

Uptake of acid dyes onto the plasma coated fabric enhanced remarkably even in a very short dyeing time (5 min), as shown in Fig. 11. It is very interesting to see that the dye uptake remained similar from a short dyeing time (5 min) to a very long dyeing time (120 min), since no significant difference in dye uptake was found at longer time periods. The observed enhancement of dye uptake can be attributed to the deposition of nanoporous thin films which provide amine groups easily accessible to dye molecules within a short time. This demonstrates that the dye diffusion coefficients are quite high; in general these values are quite low when traditional dyeing processes are used (Banchero et al., 2005). In addition, since ultrathin films were dyed, the dye uptake reached to "dyeing equilibrium" very fast. Moreover, the uniform shade obtained certainly proved the regular distribution of dye molecules throughout the entire film thickness. The low internal diffusion time seems to be responsible for preventing non-uniformity problems along the film thickness. In fact, a high dye uptake rate represents a certain advantage since the total dyeing time can be shortened ten times over using plasma-enhanced dyeing, whereas, faster dye uptake kinetics is apt to cause non-uniform dye distribution in the final product when dyeing is done traditionally.

Fig. 11. Effect of dyeing time on dyeability (dyeing temperature 80 °C) (600 W, NH3/C2H4 = 0.84, 20 min) and light-shade dyed PET (0.5% owf)

#### **3.1.2 Temperature and pH effects**

184 Textile Dyeing

650 52 13 33.0 600 54 20 43.0 500 30 30 26.0 Table 1. Color difference of dyed a-C:H:N films at different plasma power, film thickness,

Uptake of acid dyes onto the plasma coated fabric enhanced remarkably even in a very short dyeing time (5 min), as shown in Fig. 11. It is very interesting to see that the dye uptake remained similar from a short dyeing time (5 min) to a very long dyeing time (120 min), since no significant difference in dye uptake was found at longer time periods. The observed enhancement of dye uptake can be attributed to the deposition of nanoporous thin films which provide amine groups easily accessible to dye molecules within a short time. This demonstrates that the dye diffusion coefficients are quite high; in general these values are quite low when traditional dyeing processes are used (Banchero et al., 2005). In addition, since ultrathin films were dyed, the dye uptake reached to "dyeing equilibrium" very fast. Moreover, the uniform shade obtained certainly proved the regular distribution of dye molecules throughout the entire film thickness. The low internal diffusion time seems to be responsible for preventing non-uniformity problems along the film thickness. In fact, a high dye uptake rate represents a certain advantage since the total dyeing time can be shortened ten times over using plasma-enhanced dyeing, whereas, faster dye uptake kinetics is apt to cause non-uniform dye distribution in the final product when

N-content in % [N/(N+C)]

Color difference (ΔE)

Film thickness (nm)

Fig. 11. Effect of dyeing time on dyeability (dyeing temperature 80 °C) (600 W, NH3/C2H4 = 0.84, 20 min) and light-shade dyed PET (0.5% owf)

Power input (watt)

**3.1.1 Rate of dye-uptake** 

dyeing is done traditionally.

and nitrogen concentration (NH3/C2H4 plasma).

The dyeability was found to strongly depend on the dye bath temperature in the plasma dyeing process, as shown in Fig. 12; this is also a common phenomenon in traditional dyeing processes. Results show that the amount of dye absorbed on the coating decreases with decreasing dye-bath temperature. The low reduction in dyeability at lower temperature (< 80 °C) is probably due to dye aggregation and a low degree of adsorption. The differences, however, became very small especially at higher temperatures within the range of 80-120 °C. By increasing the temperature, dye uptake can be enhanced because of the increased solubility and mobility of dye molecules in water. There may be another reason for that, which is commonly seen in traditional synthetic dyeing processes: the enhanced color yield, which gradually increases with increasing temperature, can be attributed to a corresponding increase in the amount of accessible volume available for dye diffusion (Burkinshaw et al., 1995). The highest color yields were found to be at ~100 °C implying that the dye uptake reached the "saturation level" resulting in maximum acid-base intermolecular interaction between dyes and amine functionalities. At this level, the dye molecules occupied most of the incorporated amine functionalities in the film volume yielding maximum dyeability. Thus, substrates such as PES, PP, aramid, glass textiles etc. can be dyed at low temperatures similar to wool dyeing, since good coloration was obtained at levels as low as 80 °C.

Fig. 12. Effect of dyeing temperature on dyeability (dyeing time 60 min) (600 W, NH3/C2H4 = 0.84, 20 min) and light-shade dyed PET (0.5% owf)

The pH value of the dye bath was found to be very important in order to achieve level dyeing and to increase dye affinity to the functionalized films. The optimum level dyeing and color strength were obtained at a pH in the range of 4.5-5. However, the color intensity was increased at pH 2.5-3.0 due to improved exhaustion, but uneven shade was observed. On the other hand, dyeability was reduced at pH 6.0-7.0 due to low substantivity or low affinity of dyes to the film resulting in weak dye-film interaction.

Substrate Independent Dyeing of Synthetic Textiles Treated with Low-Pressure Plasmas 187

on a-C:H:N deposited plasma polymers exhibit acceptable fastness to laundering and rubbing. From these results, it is concluded that a nanoporous and functional plasma polymer enables permanent dye-fiber bonding with hydrophilic dyestuff. The coating was well adhered with the textile surface. No damages of the coating were detected after 60,000 rubbing cycles.

A considerable amount of basic research has been devoted mostly in laboratory scales to incorporate functional groups on the textile surfaces by plasma modification. Besides plasma parameters, the reactor geometry complicates the process scaling-up (Hegemann et al., 2007). Another fundamental problem at this moment is the lack of adapted plasma systems and the transfer of the laboratory findings into textile industry. Textilveredelung Grabher GmbH/Austria is using LPP plasma reactor (vol. 11 m3) in obtaining permanent hydrophilic and hydrophobic surface modification on textiles. The roll-to-roll system has a 15 m long plasma passage and is able to treat substrates up to 1.60 m in width (Fig. 14). Sefar AG/Switzerland is one of the pioneering companies in high-performance filtration solutions and uses the world largest APP systems, consisted of a high voltage power supply, for industrial applications (Fig. 15). The machine is installed inline textile processing and is capable of treating textiles, nonwovens, polymer webs etc. up to 4.0 m wide and materials speed up to 25 m/min (depending on the hydrophilization effect). The developed processes offer a range of cost-effective, and environmentally sound solutions to problems faced in screen printing. As a process gas He/Ar, and CO2 etc. have been successfully used in their

Fig. 14. Industrial plasma reactor at Textilveredelung Grabher GmbH/Austria

**4. Commercialization** 

system at modifying textiles.

Fig. 13. Effect of dye bath pH on dyeability (dyeing time 60 min and dyeing temperature 80 °C) (600 W, NH3/C2H4 = 0.84, 20 min) and light-shade dyed PET (0.5% owf).

#### **3.1.3 Fastness properties**

Table 2 shows summarized results of plasma-dyed PES that were assessed using an ISO test method (ISO 105-X12 for color fastness to rubbing and EN ISO 105-C06 for color fastness to washing). Under D65 illumination color changes, staining and rub were evaluated using grey scales: ISO-105-A02 grey scale for assessing change in color; ISO-105-A03 grey scale for staining and rub. All plasmas show almost comparable fastness properties after washing at 60 °C for ammonia/acetylene and ammonia/ethylene plasmas dyed PET. The acid dyeing


Table 2. Wash (at 60 °C) and rub fastness of a-C:H:N films deposited on PET fabrics

on a-C:H:N deposited plasma polymers exhibit acceptable fastness to laundering and rubbing. From these results, it is concluded that a nanoporous and functional plasma polymer enables permanent dye-fiber bonding with hydrophilic dyestuff. The coating was well adhered with the textile surface. No damages of the coating were detected after 60,000 rubbing cycles.
