**2. Classification of methods used for synthetic textiles modification**

Synthetic fibers have relatively high levels of orientation and crystallinity that impart the desired properties. These same characteristics contribute to their structural resistance to coloration by dye compounds and finishing with various materials necessitating enhancement of the fiber surface for improved dye receptivity. In this sense, a large body exists to improve fiber dyeability via physical methods (corona discharge, plasma, laser, electron beam and neutron irradiation functionalizations), chemical methods (enzymatic modification grafting of different monomers, utilization of supercritical carbon dioxide as the solvent carrier for disperse dyes, sol-gel technique, layer-by-layer deposition and treatment with different reagents) and bulk modification methods using various additives during fiber processing. In last decade, traditional methods that consume high amounts of energy and water are under pressure for replacement due to high manufacturing costs and negative environmental impact. In addition, some processing negates the bulk properties of fibers, require harsh process conditions, and produce undesirable side effects and or waste disposal problems. Recent methods address these challenges and deficiencies of traditional techniques and will be discussed in this chapter (Textor et al., 2003).

#### **2.1 Physical methods**

262 Textile Dyeing

materials must meet high ethical demands for environmental-friendly processing (Fourne, 1999). For this purpose the process of textile finishing is optimized by different researchers in new findings (Elices & Llorca, 2002). Application of inorganic and organic nano-particles have enhanced synthetic fibers attributes, such as softness, durability, breathability, water repellency, fire retardancy and antimicrobial properties (Franz, 2003; McIntyre, 2005; Xanthos, 2005). This review article gives an application overview of various physical and chemical methods of inorganic and organic structured material as potential modifying

The composition of synthetic fibers includes polypropylene (PP), polyethylene terephthalate (PET), polyamides (PA) or polyacrylonitrile (PAN). Synthetic fibers already hold a 54% market share in the fiber market. Of this market share, PET alone accounts for almost 50% of all fiber materials in 2008 (Gubitz & Cavaco-Paulo, 2008). Polypropylene, a major component for the nonwovens market accounts for 10% of the market share of both natural and synthetic fibers worldwide (INDA, 2008 and Aizenshtein, 2008). It is apparent that synthetic polymers have unique properties, such as high uniformity, mechanical strength and resistance to chemicals or abrasion. However, high hydrophobicity, the build-up of static charges, poor breathability, and resistant to finishing are undesirable properties of

Synthetic textile fibers typically undergo a variety of pre-treatments before dyeing and printing is feasible. Compared to their cotton counterparts, fabrics made from synthetic fibers undergo mild scouring before dyeing. Nonetheless, these treatments still create undesirable process conditions which can result in increased waste production, unpleasant working conditions and higher energy consumption. Therefore reducing pollution in textile production is becoming of utmost importance for manufacturers worldwide. In coming years, the textile industry must implement sustainable technologies and develop environmentally safer

Fibers comprising at least 85% by weight of a substituted aromatic (or aliphatic) carboxylic acid ester are termed polyesters. The most important representative of this category is polyethylene terephthalate or PET (BISFA, 2009). PET is a hydrophobic fiber with maximum moisture regain of only 1% at 100% relative humidity. Until the development of disperse dyes, dyeing of polyester was difficult. Disperse dyes with very low water solubility can sublime into PET fibers by heat through Thermosol and/or thermofixation processes. It can be applied with heat, pressure or via carriers by an exhaust process (Cavaco-Paulo & Gubitz, 2003). Alkali treatment can be used to etch the PET surface to increase the hydrophilicity of the fiber resulting in better dyeability. However, the rate of hydrolysis is

In generic terms, aliphatic polyamides (PA) are called nylons and aromatic PAs are called aramids. The first important PA was Nylon 66 as produced by the reaction of adipic acid and hexamethylene diamine monomer (Cavaco-Paulo & Gubitz, 2003). Several structural modifications with differing temperature capabilities have become commercially available, including Nylon 46, 610, 612, 6, 11, and so on. Polyamides have applications in many areas, the most important being in the production of fiber-based materials (BISFA, 2009). Nylons are dyeable with disperse dyes or with acid dyes under mild acidic conditions. Aqueous acids (below pH 3) as well as bases cause the rupture of the polymer backbone. In the case of acid dyeing, dye molecules only attach to available amino end groups, thus shade depth is

methods for textiles processing to remain competitive (Agrawal et. al., 2008).

very low without a catalyst and surface-limited (BISFA, 2009).

agents of textiles with emphasis on dyeability enhancements.

synthetic materials (Gubitz & Cavaco-Paulo, 2008).

**1.1 Fiber-forming synthetic polymers** 

Traditional transformation of a hydrophobic polymer such as poly(dimethylsiloxane) (PDMS) to a hydrophilic state has been achieved via techniques such as corona and plasma treatments (Ferguson et al., 1993; Owen, 2005). Corona, plasma, irradiation, and laser technologies are ideal for textile surface modifications due to the energy-efficient dry-state processing, continuous on-line applicability, and minimal precursor quantity requirements. In addition, surface modification does not ingress to the bulk fiber's mechanical properties although physical alterations are generally realized on the fiber's outermost surface layer. Depending on the treatment duration, the changes can propagate several microns below the surface. X-ray photoelectron spectroscopy (XPS) analysis on oxygen plasma treated samples demonstrated a rapid substitution of carbon atoms by oxygen atoms, which led to the formation of hydrophilic surfaces (Hillborg & et al., 2000). This treatment will propagate several hundred nanometers below the surface with irreversible chemical changes at the near-surface region (Hillborg & Gedde UW, 1999; Hillborg & et al., 2000; Owen & Smith, 1994). The various physical methods all have their merits with some processes, such as corona, more applicable to simple on-line installation. This section will highlight each technique with some examples on their resultant properties.

#### **2.1.1 Corona discharge**

Corona discharge is the breakdown of a gas between two electrodes. When the gas is anything but air, it is termed plasma modification to be discussed in the next section. The generation of the initial hydrophilic surface is the same in both modifications with just different outcomes based on the introduced gas stream. **Figure 1** (Borcia, 2006) depicts a typical schematic for dielectric discharge modification. When the high voltage or electric field is applied in a gas layer exposed to a polymer surface, the gas molecules, in this case air, breakdown to ions to conduct electricity. It is the bluish air glow from the electric source that is termed corona. This

2010).

a) b)

Fig. 2. AFM images of polypropylene woven fabric a) before and b) after corona discharge

treatment (Brzezinski, Kaleta, Kowalczyk, Malinowska, & Gajdzicki, 2010).

Surface and Bulk Modification of Synthetic Textiles to Improve Dyeability 265

A recent study evaluated not just how corona discharge affects conversion to hydrophilicity in terms of enhanced dyeability of synthetic fibers but also in how the fiber topography changes as a result of physical treatment ultimately influencing dyeability as well (Brzezinski et al., 2010). The study postulated that without fully understanding topography, specifically fiber roughness, correlating the varying results of the many conflicting studies of fiber dyeing with respect to corona treatment is too convoluted. To this extent, three synthetic fibers in separate woven mats were evaluated 1) polyester (PET), 2) polyamide (PA6) and 3) polypropylene (PP). The corona generator was unique in that its construction afforded a high degree and uniform surface modification to the fibers. This was by designing a multi-segmented electrode system where continuous low energy doses equated the larger required total energy dose for adequate surface modification. The process conditions previously determined for each fiber resulted in total activation energies *Ej* of 75.6 J/cm2 for PET, *Ej* of 18.9 J/cm2 for PA6, and *Ej* of 22.7 J/cm2 for PP fabrics. **Figure 2** depicts the difference via atomic force microscopy (AFM) before and after modification for the PP fabric sample where clear disruption of the fiber surface is apparent. In addition to increased roughness, the change in free surface energy was approximately 10, 4, and 30 J/cm2 for PET, PA6, and PP respectively. Dyeability for the modified samples was assessed with two techniques; 1) exhaustion and 2) the preferred Thermosol continuous method. The results between unmodified and modified samples showed little difference in the dyeing attributes via the exhaustion methods, as measured by degree of dye exhaustion, E or color difference, and dyeing fastness. This was attributed to the fact that corona discharge does not modify the entire fiber depth but rather only the first 200nm from the surface of the approximately 6500nm diameter fiber. Therefore in the dyeing techniques, such as exhaustion, where the entire fabric is immersed into the bath batch-wise, the dye pick-up rate is minimal. In contrast, dyeing methods that are applied only to the surface of the fiber, as in the case with the Thermosol method, surface hydrophilicity and roughness are far greater parameters to gauge the receptivity of the dye. In this case, the dye intensity for the corona-modified samples was significantly enhanced as measured by the color difference E for the two dyes studied, C.I. Disperse Blue 73 and C.I. Disperse Red 54 (Brzezinski et al.,

phenomenon starts with a few stray electrons colliding with other gas molecules. The collision rapidly generates several multiple electrons, positive ions, and excited molecules. The unstable excited molecules decompose to radicals, ions and photons, i.e. reactive species. When the gas is oxygen (or air), the reactive species are elemental oxygen (O), ozone (O3) and activated oxygen (O2\*). **Scheme 1** shows the general reactions of polyethylene terephthalate (PET) in the presence of either UV or corona as the mechanisms are believed to be the same. The terminal phenols and hydroxy phenolics are rapidly formed on the surface of corona-exposed PET between the reactive oxygen species and moisture in the air (Owens, 1975; Valk, Kehren, & Daamen, 1970; Zhang, Sun, & Wadsworth, 1998). Other polar moieties that are formed during corona treatment include carbonyls (-CO) and carboxyls (-COO). This change in surface polarity has been widely studied for increases in adhesion, wettability, printing, and as the subject of this chapter, dyeing. Several excellent reviews are referenced here that detail the specific and mechanisms of the treatment (Nitschke, 2008; Podhajny, 1987; Zhang, Sun, & Wadsworth, 1998}).

Fig. 1. Typical setup for corona discharge modification to a polymeric surface (Borcia, 2006).

Scheme 1. Phenolic hydroxy groups in PET after exposure to physical treatment such as corona or oxygen plasma (Owens, 1975).

phenomenon starts with a few stray electrons colliding with other gas molecules. The collision rapidly generates several multiple electrons, positive ions, and excited molecules. The unstable excited molecules decompose to radicals, ions and photons, i.e. reactive species. When the gas is oxygen (or air), the reactive species are elemental oxygen (O), ozone (O3) and activated oxygen (O2\*). **Scheme 1** shows the general reactions of polyethylene terephthalate (PET) in the presence of either UV or corona as the mechanisms are believed to be the same. The terminal phenols and hydroxy phenolics are rapidly formed on the surface of corona-exposed PET between the reactive oxygen species and moisture in the air (Owens, 1975; Valk, Kehren, & Daamen, 1970; Zhang, Sun, & Wadsworth, 1998). Other polar moieties that are formed during corona treatment include carbonyls (-CO) and carboxyls (-COO). This change in surface polarity has been widely studied for increases in adhesion, wettability, printing, and as the subject of this chapter, dyeing. Several excellent reviews are referenced here that detail the specific and mechanisms of the treatment (Nitschke, 2008; Podhajny, 1987; Zhang, Sun, &

Fig. 1. Typical setup for corona discharge modification to a polymeric surface (Borcia, 2006).

Scheme 1. Phenolic hydroxy groups in PET after exposure to physical treatment such as

corona or oxygen plasma (Owens, 1975).

Wadsworth, 1998}).

A recent study evaluated not just how corona discharge affects conversion to hydrophilicity in terms of enhanced dyeability of synthetic fibers but also in how the fiber topography changes as a result of physical treatment ultimately influencing dyeability as well (Brzezinski et al., 2010). The study postulated that without fully understanding topography, specifically fiber roughness, correlating the varying results of the many conflicting studies of fiber dyeing with respect to corona treatment is too convoluted. To this extent, three synthetic fibers in separate woven mats were evaluated 1) polyester (PET), 2) polyamide (PA6) and 3) polypropylene (PP). The corona generator was unique in that its construction afforded a high degree and uniform surface modification to the fibers. This was by designing a multi-segmented electrode system where continuous low energy doses equated the larger required total energy dose for adequate surface modification. The process conditions previously determined for each fiber resulted in total activation energies *Ej* of 75.6 J/cm2 for PET, *Ej* of 18.9 J/cm2 for PA6, and *Ej* of 22.7 J/cm2 for PP fabrics. **Figure 2** depicts the difference via atomic force microscopy (AFM) before and after modification for the PP fabric sample where clear disruption of the fiber surface is apparent. In addition to increased roughness, the change in free surface energy was approximately 10, 4, and 30 J/cm2 for PET, PA6, and PP respectively. Dyeability for the modified samples was assessed with two techniques; 1) exhaustion and 2) the preferred Thermosol continuous method. The results between unmodified and modified samples showed little difference in the dyeing attributes via the exhaustion methods, as measured by degree of dye exhaustion, E or color difference, and dyeing fastness. This was attributed to the fact that corona discharge does not modify the entire fiber depth but rather only the first 200nm from the surface of the approximately 6500nm diameter fiber. Therefore in the dyeing techniques, such as exhaustion, where the entire fabric is immersed into the bath batch-wise, the dye pick-up rate is minimal. In contrast, dyeing methods that are applied only to the surface of the fiber, as in the case with the Thermosol method, surface hydrophilicity and roughness are far greater parameters to gauge the receptivity of the dye. In this case, the dye intensity for the corona-modified samples was significantly enhanced as measured by the color difference E for the two dyes studied, C.I. Disperse Blue 73 and C.I. Disperse Red 54 (Brzezinski et al., 2010).

Fig. 2. AFM images of polypropylene woven fabric a) before and b) after corona discharge treatment (Brzezinski, Kaleta, Kowalczyk, Malinowska, & Gajdzicki, 2010).

Surface and Bulk Modification of Synthetic Textiles to Improve Dyeability 267

synthetic fibers can be modified by this technique to a regular, roll-like structure, which has a striking effect on adhesion of particles and coatings, wetting properties and optical appearance (Knittel & Schollmeyer, 1998; Watanabe & Takata, 1996; Ondogan et al., 2005;

Yip et al. applied a 193 nm argon fluoride excimer laser on polyamide (nylon 6) fabrics. Micrometer-sized ripple like structures were developed on the surface of irradiated fabric and chemical analysis indicates that carbonization has occurred. It is believed that the laser treatment breaks the long chain molecules of nylon, thus increasing the number of amine

Kan stated that properties such as wettability and air permeability of polyester were positively affected by laser while fiber weight and diameter, tensile strength, yarn abrasion and bending were adversely affected (*.cf* **Figure 4**). In this study, laser irradiation was not found to affect the bulk properties of polymer due to its low penetration depth (Kan, 2008a;

Exposing fibers to a stream of high-energy electrons is another method for surface modification. The dyeability of hydrophobic polypropylene fibers was enhanced by Kim and Bae using electron beam irradiation and sulfonic acid incorporation. The color strength of polypropylene fibers after irradiation was examined according to the dyeing conditions including the pH of the dye bath, absorbed doses, and the introduction of a functional group to the fiber substrate. The best dyeing result was obtained with cationic dyes at alkaline

Neutron irradiation significantly changes the material properties by displacement of lattice atoms and the generation of helium and hydrogen by nuclear transmutation. Mallick et al. considered the shift in some of the Raman peak positions to a higher value with the development of micro-stresses due to neutron irradiation of synthetic fibers. The defects due to irradiation were confirmed by SEM micrographs of virgin and irradiated fibers (Mallick

Fig. 4. Surface structure of polyester fiber before and after laser treatment

Shaohua et al., 2003).

2008b).

(Kan, 2008a)

et al., 2005).

**2.1.4 Other physical methods** 

conditions (Kim & Bae, 2009; Alberti et al. 2005).

end-groups (Yip et al., 2002).

#### **2.1.2 Plasma functionalization**

As mentioned, when the dielectric discharge occurs in environments besides air, the technology is referred to as plasma treatment; it has been studied at both vacuum and atmospheric pressures. The various gases employed include oxygen, nitrogen, argon, ammonia and reactive monomers. The hydrophilicity of poly(ethylene terephthalate) (PET) was greatly improved with the plasma method where the discharge barrier occurred in argon, nitrogen and air (Hsieh & Chen, 1985). The surface wettability enhancements were due to two reactions; 1) direct reaction (i.e. oxidation) of reactive gases (oxygen plasma) and 2) free radical formation and their subsequent reactions such as degradation and crosslinking (Hsieh & Chen, 1985). In this study, it was determined that a nitrogen atmosphere with a glow discharge of 30W provided the most durable and wettable surface finish for PET. The optimum power level of 30W was chosen from experiments in air at power levels ranging from 10-30W. **Figure 3** illustrates the differences of wettability at the optimum power level.

Fig. 3. Advancing water contact angles for PET modified as a function of plasma exposure time in air (■), argon (▲), and nitrogen (●). The bar at time zero is the contact angle for virgin PET. Replotted from data in (Hsieh & Chen, 1985).

Plasma treatment has also been performed on PET fiber prior to coating with a PDMS surfactant. In this case the efficacy of the surfactant was greatly enhanced with plasma activation at 1kW, 10 kHz, in the presence of air. It was determined by scanning electron microscopy that the plasma treated fibers were rougher allowing increased PDMS deposition via observation of a smoother coating, less moisture regain after coating with PDMS which is hydrophobic in nature, better drape recovery and more wrinkle resistance (Parvinzadeh & Ebrahimi, 2011).

#### **2.1.3 Laser treatment**

Laser induced surface modification of polymers provides a unique and powerful method for the surface modification without any changes in their bulk properties. The smooth surface of

As mentioned, when the dielectric discharge occurs in environments besides air, the technology is referred to as plasma treatment; it has been studied at both vacuum and atmospheric pressures. The various gases employed include oxygen, nitrogen, argon, ammonia and reactive monomers. The hydrophilicity of poly(ethylene terephthalate) (PET) was greatly improved with the plasma method where the discharge barrier occurred in argon, nitrogen and air (Hsieh & Chen, 1985). The surface wettability enhancements were due to two reactions; 1) direct reaction (i.e. oxidation) of reactive gases (oxygen plasma) and 2) free radical formation and their subsequent reactions such as degradation and crosslinking (Hsieh & Chen, 1985). In this study, it was determined that a nitrogen atmosphere with a glow discharge of 30W provided the most durable and wettable surface finish for PET. The optimum power level of 30W was chosen from experiments in air at power levels ranging from 10-30W. **Figure 3** illustrates the differences of wettability at the

0123456

Exposure Time (minutes)

Fig. 3. Advancing water contact angles for PET modified as a function of plasma exposure time in air (■), argon (▲), and nitrogen (●). The bar at time zero is the contact angle for

Plasma treatment has also been performed on PET fiber prior to coating with a PDMS surfactant. In this case the efficacy of the surfactant was greatly enhanced with plasma activation at 1kW, 10 kHz, in the presence of air. It was determined by scanning electron microscopy that the plasma treated fibers were rougher allowing increased PDMS deposition via observation of a smoother coating, less moisture regain after coating with PDMS which is hydrophobic in nature, better drape recovery and more wrinkle resistance

Laser induced surface modification of polymers provides a unique and powerful method for the surface modification without any changes in their bulk properties. The smooth surface of

**2.1.2 Plasma functionalization** 

optimum power level.

0

virgin PET. Replotted from data in (Hsieh & Chen, 1985).

10

20

30

40

Contact Angle of Water (degrees)

(Parvinzadeh & Ebrahimi, 2011).

**2.1.3 Laser treatment** 

50

60

70

synthetic fibers can be modified by this technique to a regular, roll-like structure, which has a striking effect on adhesion of particles and coatings, wetting properties and optical appearance (Knittel & Schollmeyer, 1998; Watanabe & Takata, 1996; Ondogan et al., 2005; Shaohua et al., 2003).

Yip et al. applied a 193 nm argon fluoride excimer laser on polyamide (nylon 6) fabrics. Micrometer-sized ripple like structures were developed on the surface of irradiated fabric and chemical analysis indicates that carbonization has occurred. It is believed that the laser treatment breaks the long chain molecules of nylon, thus increasing the number of amine end-groups (Yip et al., 2002).

Kan stated that properties such as wettability and air permeability of polyester were positively affected by laser while fiber weight and diameter, tensile strength, yarn abrasion and bending were adversely affected (*.cf* **Figure 4**). In this study, laser irradiation was not found to affect the bulk properties of polymer due to its low penetration depth (Kan, 2008a; 2008b).

Fig. 4. Surface structure of polyester fiber before and after laser treatment (Kan, 2008a)
