**2. Plasma treatments**

174 Textile Dyeing

textiles, home textiles, the automobile industry and to lesser extent in apparel textiles due to their special characteristics such as resistance to chemicals, low density etc. The dyeing of PP is difficult and the fastness properties are not good due to the highly crystalline molecular structure. However, dyeing of PES blends is a problem due to damage/reduction in the strength of natural fibers (cellulose degradation etc.) at high dyeing temperatures. Due to the hydrophobic nature of PES, its compact structure and cristallinity dyeing is limited to water-insoluble dyes such as disperse dyes, vat dyes etc. (De Girogi et al., 2000). The low and finite water solubility of these dyes is also a critical factor in determining levelling properties and dyeing rate (Kulkarni et al., 1986). Conventional polyester dyeing processes additionally require dispersing agent, dye carriers, and surfactants to obtain dye solubility in water. Due to dye reduction and migration, the fastness to washing was found to be on the satisfactory level when dyeing with these dyes (Son et al., 2004). The rate of diffusion of these dyestuffs in the fiber is relatively low. Therefore, the rate of diffusion in the conventional PES dyeing commercially may be raised either by working at higher temperature in the region of 130 °C and/ or in the super atmospheric pressure in the

presence of accelerating agent or carrier or surfactants or dyeing auxiliaries.

the fraction of light reflected from the treated surfaces.

Recently, research on the use of different surface modification techniques such as low pressure plasma (LPP) and atmospheric pressure plasma (APP) in order to improve the dyeability of polyesters has grown in interest, since they are environmentally efficient. An increase in hydrophilicity and dyeability of knitted PES textiles after the plasma modification in an O2/CF4 mixture was obtained, as reported by Aubrecht et al. (2006). The dyeability with basic dye is enhanced on PET/cotton blends by in situ polymerization of acrylic acid and water (Öktem et al., 2002). Sarmadi et al., 1993 observed that the dyeability with basic dye (dye bath temperature 100 °C and 2 hours dyeing time) can further be improved by an increase in the time it is exposed to CF4 cold plasma, and found K/S values between 0.50-1.51 for a 2% owf (on the weight of fabric) dark shade dyeing with hydrophobic basic dye. A higher color yield was obtained on acrylic acid grafted PET fiber induced by Ar plasma. Barani et al. (2010) compared the dyeability of PES microfiber fabrics using various pre- and post-treatments. They reported that alkali, sol-gel and oxygen plasma treatments are able to enhance the color strength. Continuous modification of PES was carried out using DBD (dielectric barrier discharge) plasma at atmospheric pressure with a Ar/O2 ratio of 10:1 (Zhongfu et al., 2007). It was reported that the spectral value (K/S) dyed with blue disperse dyes was found to be increased by 50% in comparison with that of untreated samples due to the formation of –COOH groups during plasma treatment. The work of Ferrero et al. (2004) has shown that the fastness to washing with basic dye on PET by in situ polymerization of acrylic acid using low temperature plasma was found to be unsatisfactory probably because of an unstable bond between grafted acrylic acid and dye molecules. Anti-reflecting coating layers have been deposited with organo-silicon compounds using APP plasma, which enhanced the color intensity on PET surfaces as explained by Lee et al. (2001). Okuno et at. (1992) studied the correlation between the crystallinity and dyeability of PET fibers using non-film-forming gases by cold plasma. They found that plasma-treated samples significantly reduced the dyeability due to the etching of macromolecules in the dyeable amorphous phase. Recently, Addamo et al. (2006) reported that the color depth of air LPP plasma treated PET fibers is related to their topographical characteristics and to their chemical surface composition. They observed that the color strength with disperse dye at a dyeing temperature of 100 °C can be increased by decreasing

A gas discharge allows the acceleration of free electrons when driven by an external source (e.g. RF generator). As a consequence, highly reactive and activated molecular species such as chemical radicals, ions, metastables, electrons etc. can be created by ionization, fragmentation (dissociation), excitation, UV radiation, etching reactions etc. These species

Substrate Independent Dyeing of Synthetic Textiles Treated with Low-Pressure Plasmas 177

plasma modification referring to surface chemistry restructuring via deposition >>

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

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

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

film-forming plasma, as described in the following section.

**2.1 Deposition of plasma polymer films** 

showed similar characteristics (Morita et al., 1985).

obtained.

chemically and physically react with the polymer surfaces, thus altering the surface properties and surface morphology in the topmost layers (Hossain, 2008).

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, which can be adjusted over a wide range of values.

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

Based on a wide range of applications and mechanism involved in plasma technology, plasma processes can be subdivided into two main categories:

 plasma modification referring to surface cleaning, activation, and surface etching >> non-film-forming plasma

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 adhesion with textile surfaces.

 plasma modification referring to surface chemistry restructuring via deposition >> film-forming plasma, as described in the following section.
