**2.1 Deposition of plasma polymer films**

176 Textile Dyeing

chemically and physically react with the polymer surfaces, thus altering the surface

The setup for RF excitation is well-established. In the case of a capacitively coupled RF discharge, two electrodes are mounted in a vacuum chamber as shown in Fig. 1. A process gas with a typical pressure of a few pascal is introduced while working in LPP conditions. When the RF voltage exceeds a certain value in the range of some hundred volts, depending on gas, pressure and reactor geometry, the discharge ignites. The energy coupling in RF plasmas via the electrons is well-defined enabling highly uniform discharges, a trait that is critical in treating irregularly shaped and large objects. RF plasmas are characterized by higher ionization efficiencies and can be sustained at lower gas pressures than DC discharges. Finally, in the case of RF discharge, the energy of the ions bombarding the sample is controlled by the (positive) plasma potential and the (negative) bias potential,

Fig. 2. RF plasma set up and illustration of active species present in oxygen plasma

plasma processes can be subdivided into two main categories:

non-film-forming plasma

adhesion with textile surfaces.

Based on a wide range of applications and mechanism involved in plasma technology,

plasma modification referring to surface cleaning, activation, and surface etching >>

Plasma activation generates radicals mainly by hydrogen abstraction from the polymer chain during collisions of reactive species with the polymer surfaces. Electrons, UV radiation or ion bombardment can generate radicals by C-C bond scission of the polymer. Non-polymer forming inorganic gases are used in this plasma activation process (Hossain et al., 2006a). Surface activation ranges from surface cleaning, radical formation and atom implantation to surface etching; it depends on different process parameters such as for example energy input. Surface cleaning is commonly used prior to other processing steps such as polymerization, metallization, dyeing, lamination etc. in order to increase the

properties and surface morphology in the topmost layers (Hossain, 2008).

which can be adjusted over a wide range of values.

In plasma depositions from the gas phase, which is commonly known as plasma polymerization or plasma enhanced chemical vapor deposition (PECVD), a very thin polymer layer (nm to μm) is deposited on the substrate surface. The layer is formed through polymerization of a monomer yielding film growth directly on the surface activated by the plasma (both in gas phase and surface reactions). In contrast to classic polymerization, plasma polymerization can use every monomer gas or vapor and is not limited to its reactivity. It is well known that plasma polymerization could be performed for almost any kind of monomer and it is mainly the elemental composition of the monomer, which is fed into the reaction that is important. The growth rate, mainly determined by gas flow rate and power input, varied depending on the monomer structure even if polymerized films showed similar characteristics (Morita et al., 1985).

In plasma polymerization, the monomer is fragmented under plasma conditions and builds up a plasma polymer. The plasma polymer does not contain regular repeating units; the chains are branched and randomly terminated with a high degree of crosslinking. Thus, it has a highly crosslinked and disordered structure without repeating units, as shown in Fig. 3. Structural preservation and gradients, with increasing degree of crosslinking over film thickness, can be controlled through process parameters, such as gas pressure, gas flow, and applied electric voltage, so that one can also construct so-called gradient layers. It is thus possible to obtain ultra-thin films with very useful properties for technological applications (Bismarck et al., 1999). A combination of polymerizable gases with non-polymerizable gases allows for the deposition of a variety of plasma polymer layers with many different functional groups possible. Thus, depending on the selection of the gas, monomer, process parameters, these thin coatings can be deposited with various physical and chemical characteristics. Consequently, functionalized surfaces with special properties can be obtained.

Fig. 3. Illustration of conventional polymer (left) and crosslinked plasma polymer (right)

The plasma treatments were carried out in a pilot-plant reactor, as shown in Fig. 4, in order to demonstrate the feasibility for industrial up-scaling. The reactor is described in more detail in the literature (Hossain et al., 2007b). The fabric samples were kept on the cylindrical electrode (65 cm width). The RF power (13.56 MHz) was connected to the electrode and the

Substrate Independent Dyeing of Synthetic Textiles Treated with Low-Pressure Plasmas 179

Fig. 5. 3-D AFM Imaging of a a-C:H:N plasma coating (W/F = 144 J/cm3, NH3/C2H2 = 1.2) Previously, we examined the mass deposition rates for a wide range of NH3/hydrocarbon ratio (Hegemann et al., 2005). It was found that plasma deposition, in particular radicalpromoted plasma polymerization, was governed by the composite parameter: power input per monomer flow W/F. At moderate energy input (120 ≤ W/F ≤ 144 J/cm3) a maximum in deposition rate can be achieved for a high NH3/C2H2 ratio of 5:1, as can be seen in Fig. 6. Increasing the C2H2 content in the gas mixture leads to an increase of hydrocarbon radicals in the active plasma zone resulting in a gradual increase in a-C:H film character and enhanced deposition rates (Hegemann et al., 2005). Increasing the energy input yields more fragmentation and thus also more hydrocarbon radicals. However, modifications in film growth such as densification, degradation reactions of polymer chains, chemical and physical etching, and some temperature effects can be observed at higher specific energies (W/F > 144 J/cm3) yielding a reduced deposition rate due to the transition between film growth and erosion regime (Hossain et al., 2007a; Hegemann et al., 2005). Moreover, higher energy input avoids the formation of voids, with high crosslinking of the amorphous network, and raises film rigidity due to the hybridization state of carbon atoms in a-C:H:N system. The presence of clustered carbon atoms decreases in the amorphous network connectivity. As a consequence, at high energy input inhomogeneous porous coating causes declination of dye molecules penetration into the coating during plasma dyeing process (Hossain et al., 2007a). In addition, at very low flow rates the film growth is limited by the availability of monomer supply (Waldman et al., 1995). Commonly, in LPP plasma more energetic particles and long-living radicals are generated compared to APP plasma. As a consequence etching effects can play a major role in low pressure plasma. However, the application of ammonia/acetylene plasma is very beneficial in avoiding strong etching effects during plasma polymerization, while coherent etching yield the formation of voids within the growing films (Hegemann et al., 2007; Jang et al., 1992). It can be concluded that the optimum chemical modification can be attained at moderate energy input, and suitable NH3/hydrocarbon ratio (around 1.0) to obtain accessible amine-functional groups within a-

C:H:N coatings (Hossain et al., 2007b).

glow discharges were carried out for the required power (500-750 W), gas flow (NH3/monomer ratio = 0.71-4.0), and duration (10-60 min), while the pressure was kept at 10 Pa for all experiments.

Fig. 4. Set-up of web coater

#### **2.1.1 Nanoporous functionalized films**

In this study, ammonia/acetylene and ammonia/ethylene gaseous discharges were performed in order to obtain nanoporous functionalized coatings. A RF plasma generator was used for the deposition of a-C:H:N films on PET textiles. In addition, since the plasma treatment is largely independent of the substrate material, this leads to the possibility to use a universal coating process instead of optimizing surface modification processes and plasma parameters for each different substrate material. However, special care should be taken in particular for textile substrates due to their 3-D structure and manufacturing residuals.

The structural modification of a-C:H films by the addition of nitrogen to the hydrocarbon precursor yielded hydrophilic functional sites (mainly amine functionalities) in a-C:H:N coatings (Hossain et al., 2007c). The a-C:H:N films become more graphitic and the density of voids increases with the incorporation of nitrogen and/or nitrogen functionalities in the coating (Cuong et al., 2005; Freire et al., 1995). As shown in Fig. 5, the AFM image indicates that the interconnected voids in the coating are below 25 nm. The dye molecules are thus small enough (about a few nanometer) to diffuse easily through the interconnected voids of the nanoporous structure into the plasma-polymer matrix and form dye-film bonds. Sufficiently large nanopores with a porosity of 10-20% strongly increase the specific surface area and provide a high functionality to attach molecules such as dyestuff.

glow discharges were carried out for the required power (500-750 W), gas flow (NH3/monomer ratio = 0.71-4.0), and duration (10-60 min), while the pressure was kept at

In this study, ammonia/acetylene and ammonia/ethylene gaseous discharges were performed in order to obtain nanoporous functionalized coatings. A RF plasma generator was used for the deposition of a-C:H:N films on PET textiles. In addition, since the plasma treatment is largely independent of the substrate material, this leads to the possibility to use a universal coating process instead of optimizing surface modification processes and plasma parameters for each different substrate material. However, special care should be taken in particular for textile substrates due to their 3-D structure and manufacturing

The structural modification of a-C:H films by the addition of nitrogen to the hydrocarbon precursor yielded hydrophilic functional sites (mainly amine functionalities) in a-C:H:N coatings (Hossain et al., 2007c). The a-C:H:N films become more graphitic and the density of voids increases with the incorporation of nitrogen and/or nitrogen functionalities in the coating (Cuong et al., 2005; Freire et al., 1995). As shown in Fig. 5, the AFM image indicates that the interconnected voids in the coating are below 25 nm. The dye molecules are thus small enough (about a few nanometer) to diffuse easily through the interconnected voids of the nanoporous structure into the plasma-polymer matrix and form dye-film bonds. Sufficiently large nanopores with a porosity of 10-20% strongly increase the specific surface area and provide a high functionality to attach molecules

10 Pa for all experiments.

Fig. 4. Set-up of web coater

residuals.

such as dyestuff.

**2.1.1 Nanoporous functionalized films** 

Fig. 5. 3-D AFM Imaging of a a-C:H:N plasma coating (W/F = 144 J/cm3, NH3/C2H2 = 1.2)

Previously, we examined the mass deposition rates for a wide range of NH3/hydrocarbon ratio (Hegemann et al., 2005). It was found that plasma deposition, in particular radicalpromoted plasma polymerization, was governed by the composite parameter: power input per monomer flow W/F. At moderate energy input (120 ≤ W/F ≤ 144 J/cm3) a maximum in deposition rate can be achieved for a high NH3/C2H2 ratio of 5:1, as can be seen in Fig. 6. Increasing the C2H2 content in the gas mixture leads to an increase of hydrocarbon radicals in the active plasma zone resulting in a gradual increase in a-C:H film character and enhanced deposition rates (Hegemann et al., 2005). Increasing the energy input yields more fragmentation and thus also more hydrocarbon radicals. However, modifications in film growth such as densification, degradation reactions of polymer chains, chemical and physical etching, and some temperature effects can be observed at higher specific energies (W/F > 144 J/cm3) yielding a reduced deposition rate due to the transition between film growth and erosion regime (Hossain et al., 2007a; Hegemann et al., 2005). Moreover, higher energy input avoids the formation of voids, with high crosslinking of the amorphous network, and raises film rigidity due to the hybridization state of carbon atoms in a-C:H:N system. The presence of clustered carbon atoms decreases in the amorphous network connectivity. As a consequence, at high energy input inhomogeneous porous coating causes declination of dye molecules penetration into the coating during plasma dyeing process (Hossain et al., 2007a). In addition, at very low flow rates the film growth is limited by the availability of monomer supply (Waldman et al., 1995). Commonly, in LPP plasma more energetic particles and long-living radicals are generated compared to APP plasma. As a consequence etching effects can play a major role in low pressure plasma. However, the application of ammonia/acetylene plasma is very beneficial in avoiding strong etching effects during plasma polymerization, while coherent etching yield the formation of voids within the growing films (Hegemann et al., 2007; Jang et al., 1992). It can be concluded that the optimum chemical modification can be attained at moderate energy input, and suitable NH3/hydrocarbon ratio (around 1.0) to obtain accessible amine-functional groups within a-C:H:N coatings (Hossain et al., 2007b).

section.

**3. Dyeing procedure** 

which is independent from the substrate materials.

Fig. 8. Film/dye interaction

Substrate Independent Dyeing of Synthetic Textiles Treated with Low-Pressure Plasmas 181

wettability strongly depends on gas ratios rather than energy input etc. Higher NH3/monomer ratios (> 2.50) were found to yield the lowest wicking times due to the formation of polar functionalities on the plasma polymer surface. Free radicals created on the plasma polymer produced polar oxygen functionalities, when the treated substrates were exposed to atmosphere, although no oxygen was used during plasma polymerization. On the other hand, higher gas ratios yield lower coloration, which is discussed in the following

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

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

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

#### **2.1.2 Wicking property improvement**

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

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

wettability strongly depends on gas ratios rather than energy input etc. Higher NH3/monomer ratios (> 2.50) were found to yield the lowest wicking times due to the formation of polar functionalities on the plasma polymer surface. Free radicals created on the plasma polymer produced polar oxygen functionalities, when the treated substrates were exposed to atmosphere, although no oxygen was used during plasma polymerization. On the other hand, higher gas ratios yield lower coloration, which is discussed in the following section.
