**1. Introduction**

260 Textile Dyeing

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Synthetic fibers, mainly polyethylene terephthalate (PET), polyamide (PA), polyacrylonitrile (PAN) and polypropylene (PP), are the most widely used polymers in the textile industry. These fibers surpass the production of natural fibers with a market share of 54.4%. The advantages of these fibers are their high modulus and strength, stiffness, stretch or elasticity, wrinkle and abrasion resistances, relatively low cost, convenient processing, tailorable performance and easy recycling. The downside to synthetic fibers use are reduced wearing comfort, build-up of electrostatic charge, the tendency to pill, difficulties in finishing, poor soil release properties and low dyeability. These disadvantages are largely associated with their hydrophobic nature. To render their surfaces hydrophilic, various physical, chemical and bulk modification methods are employed to mimic the advantageous properties of their natural counterparts. This review is focused on the application of recent methods for the modification of synthetic textiles using physical methods (corona discharge, plasma, laser, electron beam and neutron irradiations), chemical methods (ozone-gas treatment, supercritical carbon dioxide technique, vapor deposition, surface grafting, enzymatic modification, sol-gel technique, layer-by-layer deposition of nano-materials, micro-encapsulation method and treatment with different reagents) and bulk modification methods by blending polymers with different compounds in extrusion to absorb different colorants.

Nowadays, the bulk and surface functionalization of synthetic fibers for various applications is considered as one of the best methods for modern textile finishing processes (Tomasino, 1992). This last stage of textile processing has employed new routes to demonstrate the great potential of nano-science and technology for this industry (Lewin, 2007). Combination of physical technologies and nano-science enhances the durability of textile materials against washing, ultraviolet radiation, friction, abrasion, tension and fading (Kirk–Othmer, 1998). European methods for application of new functional finishing

Surface and Bulk Modification of Synthetic Textiles to Improve Dyeability 263

determined by the ratio of negatively charged groups of the dye molecule to positively

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

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

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

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

techniques and will be discussed in this chapter (Textor et al., 2003).

technique with some examples on their resultant properties.

charged end groups in the fiber.

**2.1 Physical methods** 

**2.1.1 Corona discharge** 

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 agents of textiles with emphasis on dyeability enhancements.

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 materials (Gubitz & Cavaco-Paulo, 2008).

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

#### **1.1 Fiber-forming synthetic polymers**

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

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 determined by the ratio of negatively charged groups of the dye molecule to positively charged end groups in the fiber.
