**5.2 Chemistry of acid leuco dyeing**

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 nonionic (acid leuco) form (IV):

Commercially Adaptable Coloration Processes for Generic Polypropylene Fiber 165

market, thus precluding its experimental confirmation as a viable vat blue colorant for

Fig. 5.2. Conversion of Oxidized Vat Dye Keto Structures into Reduced Acid Leuco Structures: (a) Vat Red 1; (b) Vat Yellow 2; (c) Vat Blue 1 (Indigo); (d) Vat Blue 6; (e) Vat

Mixing energies of different acid leuco vat dyes with isotactic PP chain of DP=10 were predicted using Accelrys' Materials Studio® (MS) software. The Blends Analysis Feature of the MS software was used to perform dye-PP mixing simulations. The results in Figure 5.3 demonstrated that the dyes C. I. Vat Blue 1, Vat Blue 8, Vat Red 1, and Vat Blue 6 had the lowest free energy / interaction parameters of mixing with PP. Vat Orange 1 and Vat Yellow 2 were intermediate in the parameters, while Vat Brown 1 had the highest free energy / interaction parameter of mixing. Vat Brown 1 was thus predicted to have poorer interactions with isotactic PP and resulting poorer exhaustion and color strength (K/S value) properties than the other dye candidates, predictions that were later confirmed by the exhaust batch

**C using accelrys'** 

**5.4 Predicted free energy of mixing of acid leuco vat dyes at 90o**

 **software** 

generic PP.

Blue 8; and (f) Vat Brown 1

dyeing experimental data.

**materials studio®**

Fig. 5.1. Various Stages of Conversion of Vat Orange 1 from the Keto to the Acid Leuco Structure: (I) Original Keto Structure; (II) Alkaline Leuco; (III) Monoionic; and (IV) Acid Leuco

The neutral acid leuco form has been reported as having affinity with hydrophobic fibers such as polyester and PP [27, 32, 33].

#### **5.3 Chemical structures of vat dye candidates for the PP single-stage, acid leuco vat dyeing process**

Figure 5.2 shows the conversion of a vat dye into its acid leuco form with the action of sodium hydroxide and excess sodium hydrosulphite. The acid leuco forms were utilized to calculate the solubility parameters of the vat dyes using Fedors' group contribution method (Table 5.1).

From the Colour Index search, the chemical structure of C. I. Vat Blue 8 deemed it a candidate for the dye trichromatic series (Figure 5.2 e). The SP for Vat Blue 8 was calculated in its acid leuco form to be 15.4 (cal/cc)1/2, which was lower than those of Vat Blue 6 (18.6 (cal/cc)1/2 ) and Vat Blue 1 (16.7 (cal/cc)1/2 ). The predicted mixing energy was also much lower for Vat Blue 8 (10.2 kcal/mole) than Vat Blue 6 (22.1 kcal/mole), further strengthening the theory that Vat Blue 8 could be a more viable performer than Vat Blue 6 in the trichromatic series. However, an exhaustive search of dye vendor sources revealed that Vat Blue 8 is currently not commercially available on the world

II

IV

Fig. 5.1. Various Stages of Conversion of Vat Orange 1 from the Keto to the Acid Leuco Structure: (I) Original Keto Structure; (II) Alkaline Leuco; (III) Monoionic; and (IV) Acid

The neutral acid leuco form has been reported as having affinity with hydrophobic fibers

**5.3 Chemical structures of vat dye candidates for the PP single-stage, acid leuco vat** 

Figure 5.2 shows the conversion of a vat dye into its acid leuco form with the action of sodium hydroxide and excess sodium hydrosulphite. The acid leuco forms were utilized to calculate the solubility parameters of the vat dyes using Fedors' group contribution method

From the Colour Index search, the chemical structure of C. I. Vat Blue 8 deemed it a candidate for the dye trichromatic series (Figure 5.2 e). The SP for Vat Blue 8 was calculated in its acid leuco form to be 15.4 (cal/cc)1/2, which was lower than those of Vat Blue 6 (18.6 (cal/cc)1/2 ) and Vat Blue 1 (16.7 (cal/cc)1/2 ). The predicted mixing energy was also much lower for Vat Blue 8 (10.2 kcal/mole) than Vat Blue 6 (22.1 kcal/mole), further strengthening the theory that Vat Blue 8 could be a more viable performer than Vat Blue 6 in the trichromatic series. However, an exhaustive search of dye vendor sources revealed that Vat Blue 8 is currently not commercially available on the world

Leuco

III

**dyeing process** 

(Table 5.1).

such as polyester and PP [27, 32, 33].

I

market, thus precluding its experimental confirmation as a viable vat blue colorant for generic PP.

Fig. 5.2. Conversion of Oxidized Vat Dye Keto Structures into Reduced Acid Leuco Structures: (a) Vat Red 1; (b) Vat Yellow 2; (c) Vat Blue 1 (Indigo); (d) Vat Blue 6; (e) Vat Blue 8; and (f) Vat Brown 1

#### **5.4 Predicted free energy of mixing of acid leuco vat dyes at 90o C using accelrys' materials studio® software**

Mixing energies of different acid leuco vat dyes with isotactic PP chain of DP=10 were predicted using Accelrys' Materials Studio® (MS) software. The Blends Analysis Feature of the MS software was used to perform dye-PP mixing simulations. The results in Figure 5.3 demonstrated that the dyes C. I. Vat Blue 1, Vat Blue 8, Vat Red 1, and Vat Blue 6 had the lowest free energy / interaction parameters of mixing with PP. Vat Orange 1 and Vat Yellow 2 were intermediate in the parameters, while Vat Brown 1 had the highest free energy / interaction parameter of mixing. Vat Brown 1 was thus predicted to have poorer interactions with isotactic PP and resulting poorer exhaustion and color strength (K/S value) properties than the other dye candidates, predictions that were later confirmed by the exhaust batch dyeing experimental data.

Commercially Adaptable Coloration Processes for Generic Polypropylene Fiber 167

**K/S of Trichromatic Series Plus Orange Dyes at 90C** 

Red 1 Orange 1 Yellow 2 Blue 6 Blue 1

0 2 4 6 8 10 12 14 16 18 20 22 24 **% owf**

Fig. 5.4. K/S Values at Wavelengths of Minimum Reflectance versus % owf for Acid Leuco

**5.6 Correlation of experimental K/S values with calculated acid leuco dye solubility** 

Figure 5.5 showed the correlation between K/S values of dyed PP fabrics and calculated SP/predicted mixing energies of six acid leuco vat dyes. Upon increasing solubility parameter and mixing energy from Vat Blue 1 to Vat Brown 1, the K/S value decreased. This correlation demonstrated the viability of the theoretical approaches (calculated SP and predicted mixing energy) to screening viable vat dye candidates for the coloration of

The predicted mixing energy for Vat Orange 1 was high, but the corresponding high experimental fabric K/S value and low acid leuco vat dye SP value created an anomaly (Figure 5.5). The discrepancy was explained by the more complicated, high molecular weight chemical structure of C. I. Vat Orange 1 compared to the other certified dyes (MW = 468 g/mole, six fused aromatic rings in a benzenoid structure with a plane of symmetry running through the molecule, a dibromine salt, etc., Figure 5.1). Figure 5.3(a) models the difficulty in placing the Vat Orange 1 molecule within the polymeric chains of PP in the amorphous regions. In addition, utilizing the "like dissolves like" rule of organic chemistry and designating PP as the solvent and the Vat Orange 1 as the solute, the highly aromatic nature of the dye dictates poor compatibility with the aliphatic PP chains. Utilizing these two factors, the Materials Studio® Software predicted a comparatively high mixing energy between PP and Vat Orange 1 (Figure 5.3(b)). With the experimentally-observed high K/S value for the dyed fabric, however, the low SP of Vat Orange 1 (14.6 (cal/cc) 1/2) was a more accurate predictor of the good compatibility of the colorant with PP than was the mixing

Vat Dyed PP Fabrics

generic PP.

energy (Figure 5.5).

**parameters and predicted mixing energies** 

**K/S**

Fig. 5.3. (a) 3-D space Configuration of Vat Orange 1 Interacting with Isotactic PP in Materials Studio Workspace; (b) Free Energy vs. Mole Fraction of Different Dyes with PP at 363oK Predicted Using Materials Studio® Software

#### **5.5 K/S value determinations**

The Ultrascan XE sensor was standardized using a light trap and the standard white tile. The sensor was tested for accuracy before PP fabric measurements by scanning the diagnostic green tile, and then comparing the X, Y, Z tristimulus values obtained with the values printed on the green tile. Ten readings taken at different places were recorded for each of the dyed fabric samples, and the average of the ten readings was computed to derive the K/S value for each sample at the wavelength of minimum fabric reflectance. The K/S results in Table 5.2 revealed a correlation with the blend miscibility approach for the candidate six vat dyes, e.g., C. I. Vat Brown 1 gave the least color transfer onto the PP fabric due to its very high solubility parameter/mixing energy.


Table 5.2. Final K/S Values of PP Fabrics Colored by Single Stage Vat Acid Leuco Dyeing Method (8% owf)

The K/S versus % owf plot detailed that for all dyes, increasing the amount of colorant in the dyebath resulted in a gradual color buildup on the dyed fabric, followed by saturation (Figure 5.4). K/S values of the colored fabrics with the colorants of the trichromatic series plus orange exhibited similar K/S plots, whereas those dyed with Vat Blue 1 exhibited much higher K/S values, reinforcing the compatibility of the component colorants of the trichromatic series plus orange colorants in PP fabric dyeing, along with the incompatibility of Vat Blue 1 with the group.

Fig. 5.3. (a) 3-D space Configuration of Vat Orange 1 Interacting with Isotactic PP in

363oK Predicted Using Materials Studio® Software

due to its very high solubility parameter/mixing energy.

Wavelength of Minimum Reflectance (nm)

**5.5 K/S value determinations** 

C.I. Name of Dye

Method (8% owf)

of Vat Blue 1 with the group.

Materials Studio Workspace; (b) Free Energy vs. Mole Fraction of Different Dyes with PP at

The Ultrascan XE sensor was standardized using a light trap and the standard white tile. The sensor was tested for accuracy before PP fabric measurements by scanning the diagnostic green tile, and then comparing the X, Y, Z tristimulus values obtained with the values printed on the green tile. Ten readings taken at different places were recorded for each of the dyed fabric samples, and the average of the ten readings was computed to derive the K/S value for each sample at the wavelength of minimum fabric reflectance. The K/S results in Table 5.2 revealed a correlation with the blend miscibility approach for the candidate six vat dyes, e.g., C. I. Vat Brown 1 gave the least color transfer onto the PP fabric

Vat Red 1 510 4.4 16.0 Vat Blue 6 600 3.9 18.6 Vat Yellow 2 420 2.1 15.0 Vat Orange 1 440 5.0 14.6 Vat Blue 1 640 6.1 16.7 Vat Brown 1 400 1.7 19.3 Table 5.2. Final K/S Values of PP Fabrics Colored by Single Stage Vat Acid Leuco Dyeing

The K/S versus % owf plot detailed that for all dyes, increasing the amount of colorant in the dyebath resulted in a gradual color buildup on the dyed fabric, followed by saturation (Figure 5.4). K/S values of the colored fabrics with the colorants of the trichromatic series plus orange exhibited similar K/S plots, whereas those dyed with Vat Blue 1 exhibited much higher K/S values, reinforcing the compatibility of the component colorants of the trichromatic series plus orange colorants in PP fabric dyeing, along with the incompatibility

K/S at Wavelength of Minimum Reflectance

SP (Acid Leuco Form) (cal/cm3)1/2

Fig. 5.4. K/S Values at Wavelengths of Minimum Reflectance versus % owf for Acid Leuco Vat Dyed PP Fabrics

### **5.6 Correlation of experimental K/S values with calculated acid leuco dye solubility parameters and predicted mixing energies**

Figure 5.5 showed the correlation between K/S values of dyed PP fabrics and calculated SP/predicted mixing energies of six acid leuco vat dyes. Upon increasing solubility parameter and mixing energy from Vat Blue 1 to Vat Brown 1, the K/S value decreased. This correlation demonstrated the viability of the theoretical approaches (calculated SP and predicted mixing energy) to screening viable vat dye candidates for the coloration of generic PP.

The predicted mixing energy for Vat Orange 1 was high, but the corresponding high experimental fabric K/S value and low acid leuco vat dye SP value created an anomaly (Figure 5.5). The discrepancy was explained by the more complicated, high molecular weight chemical structure of C. I. Vat Orange 1 compared to the other certified dyes (MW = 468 g/mole, six fused aromatic rings in a benzenoid structure with a plane of symmetry running through the molecule, a dibromine salt, etc., Figure 5.1). Figure 5.3(a) models the difficulty in placing the Vat Orange 1 molecule within the polymeric chains of PP in the amorphous regions. In addition, utilizing the "like dissolves like" rule of organic chemistry and designating PP as the solvent and the Vat Orange 1 as the solute, the highly aromatic nature of the dye dictates poor compatibility with the aliphatic PP chains. Utilizing these two factors, the Materials Studio® Software predicted a comparatively high mixing energy between PP and Vat Orange 1 (Figure 5.3(b)). With the experimentally-observed high K/S value for the dyed fabric, however, the low SP of Vat Orange 1 (14.6 (cal/cc) 1/2) was a more accurate predictor of the good compatibility of the colorant with PP than was the mixing energy (Figure 5.5).

Commercially Adaptable Coloration Processes for Generic Polypropylene Fiber 169

Vat Orange 1 5 5 5 5 5 5 5 Vat Yellow 2 5 4-5 4-5 5 4-5 5 4-5 Vat Blue 6 5 4-5 4-5 4-5 4-5 4-5 4-5 Vat Red 1 5 4-5 4-5 4 4-5 4-5 4-5 Vat Blue 1 5 4-5 5 4 4-5 5 4-5

Dry-cleaning fastness of the dyed materials was determined by AATCC Test Method 132- 2004. The fastness ratings confirmed that all of the certified vat dyes imparted good,

Vat Orange 1 5 5 4-5 4-5 4-5 4-5 4-5 Vat Yellow 2 5 4-5 4-5 4-5 4 4-5 4-5 Vat Blue 6 5 4-5 4 4 4 4 4-5 Vat Red 1 4-5 5 4 4-5 4-5 4-5 4-5 Vat Blue 1 4-5 4-5 4 4-5 4-5 4-5 4

Table 5.5. Dry-Cleaning Fastness Ratings of PP Fabrics Dyed with Certified Vat Dyes

e.g., high color yields, in dyeing PP fabrics by the optimized acid leuco vat process.

C.I. Name of Dye Change in Color Staining on the Various Components of Multifiber

Solubility parameter and molecular dynamics simulation approaches were developed to screen viable vat dye candidates for generic PP aqueous dyeing, and the dyes targeted as viable candidates by the theoretical techniques provided excellent experimental correlations,

A viable method to commercially aqueous batch dye generic, unmodified PP fiber textiles in a conventional process has been developed for a certified trichromatic series (red, yellow and blue) plus orange of vat dyes with adequate fastness properties to washing, crocking and dry-cleaning in their acid leuco forms: C. I. Vats Red 1, Yellow 2 and Blue 6 plus Orange 1. The same method was shown to adequately color PP textiles with C. I. Vat Blue 1 (Indigo) as a stand-alone colorant with adequate fastness properties to washing, crocking and dry-cleaning to produce the popular "denim" shade. The developed single-stage acid leuco method for dyeing generic PP fabrics at pH 7 provided good fastness properties and

Of the vat dyes currently available on the commercial market, C. I. Vat Dyes Orange 1, Yellow 2, Red 1 and Blue 1, all possessing low solubility parameters closest to that of generic PP's 8.1 (cal/cc)1/2, were demonstrated to be viable candidates for generic PP fiber coloration, while C. I. Vat Blue 6 was deemed a marginal candidate. However, Vat Blue 6 was the best-performing blue vat dye available outside of Vat Blue 1, and since the latter

Table 5.4. Wash Fastness Ratings of PP Fabrics Dyed with Certified Vat Dyes

acceptable dry-cleaning fastness properties to dyed PP fabric (Table 5.5).

Staining on the Various Components of Multifiber Fabric Style # 10 Acetate Cotton Nylon 66 Polyester Acrylic Wool

Staining on the Various Components of Multifiber Fabric Style # 10 Acetate Cotton Nylon 66 Polyester Acrylic Wool

C.I. Name of Dye

C.I. Name of Dye

Fabric Style # 10

**6. Conclusions** 

Change in Color

Change in Color

good color yields without fiber "ring-dyeing."

**5.7.3 Fastness to dry-cleaning (Perchloroethylene)** 

Fig. 5.5. Correlation of Dyed PP Fabric K/S Values with Calculated Acid Leuco Dye Solubility Parameters and Predicted Mixing Energies

#### **5.7 Evaluation of fastness properties 5.7.1 Fastness to crocking**

Crock fastness was determined using the electronic crock meter with 10 complete cycles in both dry as well as wet conditions according to AATCC Standard Test Method 8-2004. Vat Blue 6 exhibited less resistance to crocking as it stained the cotton cloth square to a rating of 3-4, whereas Vat Orange 1, Vat Red 1, Vat Blue 1 and Vat Yellow 2 all showed good to excellent crocking resistance (Table 5.3). The higher crock fastness ratings in the wet condition to that of the dry condition was attributed to the reduction in the frictional force between the low surface energy PP fabric and the rubbing finger of the crock meter caused by the lubricating effect of water at the interface.



#### **5.7.2 Fastness to washing**

Wash fastness of the dyed materials was determined by AATCC Standard Test Method 61- 2003, no. 2A. The fastness ratings (Table 5.4) revealed that Vat Orange 1 exhibited excellent wash fastness on the PP fabric, whereas the other vat dyes yielded good, acceptable wash fastness properties to the PP fabric.


Table 5.4. Wash Fastness Ratings of PP Fabrics Dyed with Certified Vat Dyes
