**4. Technical approach**

The coloration of cellulosic fibers with vat dyes is well established [31]. However, hydrophobic fibers such as PP are not dyeable by the conventional, aqueous-based methods which are successful for cellulosic fibers in which the alkaline leuco, dianion form (completely water soluble) of vat dyes is utilized. Theoretically, acid leuco vat dyeing analogous to the disperse dyeing of polyester fiber may provide a method to aqueous color unmodified PP fiber. Etters et al. [29] among others [23, 24, 27] reported the acid leuco vat dye route as a potential path to aqueous dyed PP fiber in its unmodified form.

Commercially Adaptable Coloration Processes for Generic Polypropylene Fiber 163

determined using the general hydrosulfite/hydroxide reaction. The Fedors group contribution method was applied after structural analysis of the acid leuco vat dyes to compute the solubility parameter. The table of atomic and group contributions to energy of

Table 5.1 shows the calculated solubility parameters (SPs) for oxidized and reduced vat dyes in which Vat Red 15, Vat Orange 7, Vat Yellow 2, Vat Blue 18 and Vat Orange 1 have the lowest values. The solubility parameter of PP was calculated to be 8.1 (cal/cm3)1/2. Since Vat Red 15 and Vat Orange 7 are cis- and trans- isomers, respectively, the two molecules yielded

Vat Red 1 73360 13.9 16.0 Vat Yellow 2 67300 13.7 15.0 Vat Blue 6 69825 15.3 18.6 Vat Orange 1 59105 13.0 14.6 Vat Blue 1 (Indigo) 73000 13.9 16.7 Vat Brown 1 70800 15.5 19.3 Vat Red 15 71100 14.2 14.4 Vat Yellow 33 65429 14.3 16.0 Vat Red 13 70320 13.4 14.8 Vat Orange 7 71105 14.2 14.4 Vat Yellow 4 59100 13.6 15.6 Vat Violet 1 60010 12.1 14.8 Vat Violet 13 68700 14.9 16.6 Vat Black 27 69005 15.1 17.5 Vat Green 8 71050 15.3 19.1 Vat Orange 2 59705 13.4 14.8 Vat Blue 18 59815 13.1 14.5 Vat Green 1 59825 13.5 14.9 Vat Red 10 67000 15.1 18.4 Vat Orange 15 69025 14.6 16.7 Table 5.1. Calculated Solubility Parameters of Vat Dyes Using Fedors' Method [36]

The four stages of reduction of C. I. Vat Orange 1 dye (Fig. 5.1) indicate that excess sodium hydrosulphite acts as an acid in addition to being a reducing agent for the carbonyl groups, and the chemical thus converts the alkaline leuco form of vat dye (II) into first the monoioninc leuco form (III) and, with further addition of sodium hydrosulphite, into the

Calculated SPs (cal/cm3)1/2 for Keto Form

Calculated SPs (cal/cm3)1/2 for Acid Leuco Form

vaporization and molar volumes from Fedors was used in this computation [36].

equal solubility parameters.

C.I. Name of Dye C. I. Constitution

**5.2 Chemistry of acid leuco dyeing** 

nonionic (acid leuco) form (IV):

Number

Recently, researchers [32, 33] investigated the process of dyeing polyester with indigo vat dye utilizing the non-ionic, acid leuco form of indigo (sparingly water soluble) to dye polyester fabrics and obtained good fastness results by optimizing the concentration of sodium hydroxide and sodium hydrosulphite. The authors noted that using excess sodium dithionite eliminated the need to back-titrate with acetic acid to achieve the acid leuco form of the vat dye [32].

The correlation between dye exhaustion and the solubility parameter for PLA fiber and disperse dyes has been reported by Karst et al. [34, 35] , who showed that the closer the solubility parameter of the dye was to the fiber, the better the exhaustion of the dye. Solubility parameter (SP) is the square root of the cohesive energy density. SP of the PP and commercially available vat dyes were calculated using the Fedors group contribution method [36]. Dyes having SP values closest to that of PP were given priority in this research. Calibration curves for each dye were prepared using UV/Vis NIR Spectrophotometry. The single stage, acid leuco vat dyeing process was utilized to dye the PP flat fabric. Various mixing parameters (Flory-Huggins interaction parameter and free energy of mixing) were predicted using Accelrys' Materials Studio® software. Dyes having the least free energy of mixing with PP were given priority in the experiments.

Exhaust batch dyeing experiments were conducted on a single bath Roaches Colortec® High Temperature Dyeing Machine at Georgia Tech, as well as on a Roaches Pyrotec Multi-Beaker Dyeing Machine at the University of Georgia, utilizing the optimized, single stage acid leuco dyeing procedure. Percent exhaustion was measured to evaluate the efficacy of each dye.

Dyed fabrics were evaluated for various fastness properties (washing, crocking and drycleaning) in order to certify the viable vat dye candidates. The impact of the dyeing process on the solid state structure of PP was investigated using X-ray diffraction and tensile testing. Rate of dyeing plots were prepared for the single dyeings as well as for combination dyeings. Adsorption isotherms were prepared to calculate the thermodynamic parameters of dyeing for single colorants. The K/S values of the dyed samples were determined.

In order to perform the continuous steam-pad dyeing, the PP fabric was dipped and squeezed into the solutions 2-3 times to perform the padding operations using dye pad and chempad formulations. The developed, optimized pad-steam process sequence for simulated continuous pad/steam coloration of unmodified PP fabric with acid leuco vat dyes was based upon the method first described by Ulrich et al. [27]. The fabric was padded into the dye pad formulation, dried in a convection oven, padded with the chempad formulation, steam-fixed in the lab box steamer and finally cold-rinsed, soaped, washed and air-dried.

The simulated continuous pad-dry heat process was based on the diffusion of acid leuco vat dye molecules into the swollen amorphous regions of the PP fiber at a temperature of 130oC. The PP fabric was padded into the vat acid formulation, the excess squeezed out by hand, and the saturated fabric placed into a forced convection oven at 130oC for 10 minutes.

#### **5. Results and discussion**

#### **5.1 Solubility parameter calculations**

The chemical structures of oxidized vat dyes were obtained from the Colour Index, 1971 Edition [37]. The chemical structures of the corresponding reduced acid leuco vat dyes were

Recently, researchers [32, 33] investigated the process of dyeing polyester with indigo vat dye utilizing the non-ionic, acid leuco form of indigo (sparingly water soluble) to dye polyester fabrics and obtained good fastness results by optimizing the concentration of sodium hydroxide and sodium hydrosulphite. The authors noted that using excess sodium dithionite eliminated the need to back-titrate with acetic acid to achieve the acid leuco form

The correlation between dye exhaustion and the solubility parameter for PLA fiber and disperse dyes has been reported by Karst et al. [34, 35] , who showed that the closer the solubility parameter of the dye was to the fiber, the better the exhaustion of the dye. Solubility parameter (SP) is the square root of the cohesive energy density. SP of the PP and commercially available vat dyes were calculated using the Fedors group contribution method [36]. Dyes having SP values closest to that of PP were given priority in this research. Calibration curves for each dye were prepared using UV/Vis NIR Spectrophotometry. The single stage, acid leuco vat dyeing process was utilized to dye the PP flat fabric. Various mixing parameters (Flory-Huggins interaction parameter and free energy of mixing) were predicted using Accelrys' Materials Studio® software. Dyes having the least free energy of

Exhaust batch dyeing experiments were conducted on a single bath Roaches Colortec® High Temperature Dyeing Machine at Georgia Tech, as well as on a Roaches Pyrotec Multi-Beaker Dyeing Machine at the University of Georgia, utilizing the optimized, single stage acid leuco dyeing procedure. Percent exhaustion was measured to evaluate the efficacy of

Dyed fabrics were evaluated for various fastness properties (washing, crocking and drycleaning) in order to certify the viable vat dye candidates. The impact of the dyeing process on the solid state structure of PP was investigated using X-ray diffraction and tensile testing. Rate of dyeing plots were prepared for the single dyeings as well as for combination dyeings. Adsorption isotherms were prepared to calculate the thermodynamic parameters of dyeing for single colorants. The K/S values of the dyed

In order to perform the continuous steam-pad dyeing, the PP fabric was dipped and squeezed into the solutions 2-3 times to perform the padding operations using dye pad and chempad formulations. The developed, optimized pad-steam process sequence for simulated continuous pad/steam coloration of unmodified PP fabric with acid leuco vat dyes was based upon the method first described by Ulrich et al. [27]. The fabric was padded into the dye pad formulation, dried in a convection oven, padded with the chempad formulation, steam-fixed

The simulated continuous pad-dry heat process was based on the diffusion of acid leuco vat dye molecules into the swollen amorphous regions of the PP fiber at a temperature of 130oC. The PP fabric was padded into the vat acid formulation, the excess squeezed out by hand, and the saturated fabric placed into a forced convection oven at 130oC for 10

The chemical structures of oxidized vat dyes were obtained from the Colour Index, 1971 Edition [37]. The chemical structures of the corresponding reduced acid leuco vat dyes were

in the lab box steamer and finally cold-rinsed, soaped, washed and air-dried.

mixing with PP were given priority in the experiments.

of the vat dye [32].

each dye.

minutes.

samples were determined.

**5. Results and discussion** 

**5.1 Solubility parameter calculations** 

determined using the general hydrosulfite/hydroxide reaction. The Fedors group contribution method was applied after structural analysis of the acid leuco vat dyes to compute the solubility parameter. The table of atomic and group contributions to energy of vaporization and molar volumes from Fedors was used in this computation [36].

Table 5.1 shows the calculated solubility parameters (SPs) for oxidized and reduced vat dyes in which Vat Red 15, Vat Orange 7, Vat Yellow 2, Vat Blue 18 and Vat Orange 1 have the lowest values. The solubility parameter of PP was calculated to be 8.1 (cal/cm3)1/2. Since Vat Red 15 and Vat Orange 7 are cis- and trans- isomers, respectively, the two molecules yielded equal solubility parameters.


Table 5.1. Calculated Solubility Parameters of Vat Dyes Using Fedors' Method [36]
