**2.1.4 Other physical methods**

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

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 et al., 2005).

Surface and Bulk Modification of Synthetic Textiles to Improve Dyeability 269

time =20 minutes, O2 flow = 50 L· hr-1). It should also be noted the concentration of peroxide

Fig. 5. Typical set-up for ozonolysis of substrates. Redrawn from (Fujimoto, Takebayashi,

Fig. 6. Decrease in contact angle of PE and PU films by ozone oxidation and plasma exposure. Ozone (60V, 50 Lhr-1): (○) PE, (●) PU. Plasma (Ar, 24 W): (∆) PE, (▲) PU. Virgin:

() PE, (■) PU. From (Fujimoto et al., 1993).

decreases with polymer rigidity and nonpolar constituents (C-F).

Inoue, & Ikada, 1993)

Critical temperature and pressure describes a fluid at temperatures and pressures higher than those at which the liquid and gaseous states of the given substance would have the same density. Prorokova et al. showed modification of the surface of PET fabrics by application of a water-repellent coating in supercritical carbon dioxide medium. It was believed that in treating PET fiber materials with a solution of low-molecular-weight poly(tetrafluoroethylene) in supercritical carbon dioxide, an ultrathin layer of fluoropolymer is formed on the surface. This layer gives the fabric a high degree of water repellency (Prorokova et al., 2009).

Another potentially attractive approach would take advantage of UV light's ability to transform chemistry of the polymer surface. Zhu and Kelley modified the surface of PET by deep UV (172 nm) irradiation. The main effects were increased conversion of the C=O moiety to carboxylic acid with negligible change to fiber topography. Other studies revealed that surface chemical composition, morphology, adhesion, thermomechanics, and stiffness/modulus of PET are strongly influenced by UV irradiation in the presence of bifunctional media (Zhua & Kelley, 2004; Gao et al., 2005).

#### **2.2 Chemical methods**

The methods discussed in the following sections include ozonolysis, supercritical carbon dioxide, chemical vapor deposition, "grafting to", enzymatic modification, sol-gel deposition, layer-by-layer, micro-surface modification by alkaline or acidic means, and macro-encapsulation of dye molecules. In many instances, the best performance is obtained from a combination of physical and chemical methods, especially for poly(propylene).

#### **2.2.1 Ozone gas treatment**

Hydrophilicity of synthetic polymer surfaces can be achieved with functionalization of peroxide molecules via exposure to ozone (O3). An advantage of ozone treatment is the uniform coverage of the reactive molecules on 3-D structures. It is also well-known that ozone will treat not only the surface but diffuse through the polymer bulk (Fujimoto et al., 1993; Ko et al., 2001; Kulik, Ivanchenko, Kato, Sano, & Ikada, 1995). The process is often done in the gas phase but aqueous treatment has also been employed with good results (Gu, Wu, & Doan, 2009). In an early study by Fujimoto et al., the surface oxidation of polyurethane (PU) and polyethylene (PE) films were evaluated by both ozone and plasma treatment(Fujimoto et al., 1993). The process conditions for ozone treatment followed typical gas reactor set-up as illustrated in **Figure 5**. Of the process variables to control, gas mass flow rate, voltage and time are the most common to control ozone concentration. The formed polymeric peroxides on the surface and in the bulk were quantified by iodide (Frew, Jones, & Scholes, 1983), DPPH (Frew et al., 1983), and peroxidase spectrophotometric methods (Suzuki, Kishida, Iwata, & Ikada, 1986). Additional surface analysis to confirm reactivity of the ozone included wettability measurements, Fourier infrared spectroscopy in the attenuated reflectance mode (FTIR-ATR), and X-Ray Photoelectro Spectroscopy (XPS/ ESCA). Subsequent to oxidation was the graft polymerization of acrylamide which will be discussed in Section 2.2.4. The surface oxidation efficiency was evaluated based on water contact angle (*cf.* **Figure 6**) and peroxide concentration. Of interest is that the wettability of the plasma exposed films is much higher than the ozonated films but the peroxide concentration in the plasma treated polyurethane film (Power = 24W, exposure time = 20 seconds) is lower than its ozone counterpart (Voltage =100V, exposure

Critical temperature and pressure describes a fluid at temperatures and pressures higher than those at which the liquid and gaseous states of the given substance would have the same density. Prorokova et al. showed modification of the surface of PET fabrics by application of a water-repellent coating in supercritical carbon dioxide medium. It was believed that in treating PET fiber materials with a solution of low-molecular-weight poly(tetrafluoroethylene) in supercritical carbon dioxide, an ultrathin layer of fluoropolymer is formed on the surface. This layer gives the fabric a high degree of water repellency

Another potentially attractive approach would take advantage of UV light's ability to transform chemistry of the polymer surface. Zhu and Kelley modified the surface of PET by deep UV (172 nm) irradiation. The main effects were increased conversion of the C=O moiety to carboxylic acid with negligible change to fiber topography. Other studies revealed that surface chemical composition, morphology, adhesion, thermomechanics, and stiffness/modulus of PET are strongly influenced by UV irradiation in the presence of bi-

The methods discussed in the following sections include ozonolysis, supercritical carbon dioxide, chemical vapor deposition, "grafting to", enzymatic modification, sol-gel deposition, layer-by-layer, micro-surface modification by alkaline or acidic means, and macro-encapsulation of dye molecules. In many instances, the best performance is obtained from a combination of physical and chemical methods, especially for poly(propylene).

Hydrophilicity of synthetic polymer surfaces can be achieved with functionalization of peroxide molecules via exposure to ozone (O3). An advantage of ozone treatment is the uniform coverage of the reactive molecules on 3-D structures. It is also well-known that ozone will treat not only the surface but diffuse through the polymer bulk (Fujimoto et al., 1993; Ko et al., 2001; Kulik, Ivanchenko, Kato, Sano, & Ikada, 1995). The process is often done in the gas phase but aqueous treatment has also been employed with good results (Gu, Wu, & Doan, 2009). In an early study by Fujimoto et al., the surface oxidation of polyurethane (PU) and polyethylene (PE) films were evaluated by both ozone and plasma treatment(Fujimoto et al., 1993). The process conditions for ozone treatment followed typical gas reactor set-up as illustrated in **Figure 5**. Of the process variables to control, gas mass flow rate, voltage and time are the most common to control ozone concentration. The formed polymeric peroxides on the surface and in the bulk were quantified by iodide (Frew, Jones, & Scholes, 1983), DPPH (Frew et al., 1983), and peroxidase spectrophotometric methods (Suzuki, Kishida, Iwata, & Ikada, 1986). Additional surface analysis to confirm reactivity of the ozone included wettability measurements, Fourier infrared spectroscopy in the attenuated reflectance mode (FTIR-ATR), and X-Ray Photoelectro Spectroscopy (XPS/ ESCA). Subsequent to oxidation was the graft polymerization of acrylamide which will be discussed in Section 2.2.4. The surface oxidation efficiency was evaluated based on water contact angle (*cf.* **Figure 6**) and peroxide concentration. Of interest is that the wettability of the plasma exposed films is much higher than the ozonated films but the peroxide concentration in the plasma treated polyurethane film (Power = 24W, exposure time = 20 seconds) is lower than its ozone counterpart (Voltage =100V, exposure

(Prorokova et al., 2009).

**2.2 Chemical methods** 

**2.2.1 Ozone gas treatment** 

functional media (Zhua & Kelley, 2004; Gao et al., 2005).

time =20 minutes, O2 flow = 50 L· hr-1). It should also be noted the concentration of peroxide decreases with polymer rigidity and nonpolar constituents (C-F).

Fig. 5. Typical set-up for ozonolysis of substrates. Redrawn from (Fujimoto, Takebayashi, Inoue, & Ikada, 1993)

Fig. 6. Decrease in contact angle of PE and PU films by ozone oxidation and plasma exposure. Ozone (60V, 50 Lhr-1): (○) PE, (●) PU. Plasma (Ar, 24 W): (∆) PE, (▲) PU. Virgin: () PE, (■) PU. From (Fujimoto et al., 1993).

Surface and Bulk Modification of Synthetic Textiles to Improve Dyeability 271

In recent years, waterless dyeing in supercritical carbon dioxide (scCO2) fluid has been gaining much interest to textile chemists. This process is deemed an environmentally safe solvent as opposed to the traditional solvent of choice, water. Application of these techniques can result in reducing waste and cost for the entire dyeing process of synthetic

1. Plasticizing effect due to CO2 on textiles decreasing the glass transition temperature.

3. Elimination of chemicals, such as leveling agents, pH regulations and dispersants, to

5. Diffusion of dyes and penetration of voids within the fiber structure in the fluid is higher/ faster due to zero surface tension between air and scCO2 carbon dioxide. 6. Generation of effluents due to dyeing or recycling of contaminated gas streams does not

8. For polyester, elimination of reduction clearing process, short dyeing times, and high

**Figure 8** shows the supercritical carbon dioxide apparatus which is usually used for dyeing of synthetic fibers. Fabric and dye are put in the container before starting the process. The apparatus is then sealed and heated to a pre-selected dyeing temperature and CO2 is pumped simultaneously to the set pressure. The dyeing is carried out on textile depending on the type of fiber and then the pressure is slowly reduced to atmospheric at isothermal

Fig. 8. The supercritical carbon dioxide apparatus used for dyeing of synthetic fibers 1. Liquid CO2, 2. Pump, 3,5,9. Pressure-control valves, 4. Manometer, 6.Autoclave, 7.

Bach et al. dyed PP fibers in scCO2 with different disperse dyes. They showed that disperse azo dyes with a naphthalene moiety gave much deeper colors on PP versus benzo-azo or anthraquinone dyes. They stated that improvement in dyeability is due to the changes in the crystal network of PP by the treatment in CO2 as contrasted with PP dyed in water or air

Temperature sensor, 8. Dyepot, 10. Adjust valve (Li-qiu et al., 2005)

**2.2.2 Supercritical carbon dioxide technique** 

textiles (Kikic & Vecchione, 2003).

conditions (Li-qiu et al., 2005).

exist.

(Bach, 1998).

The possible advantages of this process are

solubilize disperse dyes in medium.

2. Elimination of contaminated waste water streams.

4. Controllable solubilities of disperse dyes via pressure.

7. Energy consumption is low for heating up dyeing liquor.

diffusivities resulting in high extraction/ reaction rates.

Ozone self-decomposes rapidly in water producing free radicals, a stronger oxidant than ozone itself (Hoigne & Bader, 1976). This property was utilized to produce hydrophilic and highly reactive high-density polyethylene (HDPE) films (Gu et al., 2009). The O2 gas rate in this study was about twelve times higher with exit ozone concentrations ranging between 1-3.7 weight percent.

**Figure 7** illustrates the difference of ozone treatment in the aqueous phase versus the gas phase. While initially there is no apparent difference in the media treatment, the peroxide generation (as measured by the iodometric method (Kokatnur & Jelling, 1941)) is greater and faster for ozonation in the aqueous phase. For all samples, it was demonstrated that the stability of the generated peroxides lasted at least 15 days with no change in concentration. The subsequent grafting of acrylamide on the aqueous ozone treated samples was successful but its performance in terms of surface energy was not contrasted to acrylamide graft polymerization on gas-phase ozone treated samples.

Fig. 7. Peroxides generated after different ozonation times in different media (Gu et al., 2009)

Specific to fabrics, chemical modification via gas-phase ozone treatment was performed on nylon 6 taffeta, polyester taffetas (Lee et al., 2006), cationic dyeable polyester (CDP) and poly(butylene terephthalate) fibers (Lee et al., 2006, 2007). The surface analysis via ESCA was very comparable to the plasma studies by others (Fujimoto et al., 1993), the reader is referred to previous methods for process conditions (Wakida et al., 2004). Notable is that the processing time was only 10 minutes with similar surface chemistries for operations at atmospheric pressure and 0.1 MPa. The ozone-modified fabrics were treated with Disperse Red 60 and Disperse Blue 56 dyes in batch immersion at 100oC for up to 120 hours. The authors found that the internal structure of the fibers increased in crystallinity (as measured by a density gradient column and X-ray diffraction), wettability and moisture uptake upon ozone treatment. These characteristics of the modified fibers were attributed to the increase in dye uptake rate, especially for polyester fibers. The equilibrium dye uptake increased for PBT fibers, polyester taffeta and nylon 6 taffeta but remained unchanged for CDP fibers.
