*2.6. FTIR Analysis*

In order to monitor some of the chemical changes occurring during the pretreatment and dyeing processes, Attenuated Total Reflection—Fourier Transform Infrared (ATR-FTIR) spectroscopy was used to analyze the fabric samples at different stages of the pretreatment and dyeing processes with the Victoria Blue R dye. In summary, fabric samples were placed on a potassium bromide (KBr) plate, pressed under the germanium crystal of ATR (UMA 400, Varian Inc., Palo Alto, CA, USA) and scanned in the mid-IR region (500–4000 cm−<sup>1</sup> with a 4 cm−<sup>1</sup> resolution) with an FTIR spectrometer (Excalibur 3100, Varian Inc., Palo Alto, CA, USA) equipped with an Attenuated Total Reflection (ATR) accessory. The ATR-FTIR spectra were displayed in absorbance units with each spectrum representing an average of 128 scans and taking into account the background spectrum acquired using a blank KBr plate.

#### **3. Results and Discussion**

#### *3.1. Experiments A–F: Analysis*

Experiment A was a 4-factor, 2-level, full factorial experiment designed to probe the effects of dye solvent (water or glycerol), pretreatment with acetic acid and/or NTP treatment for 60 sec and the addition of the swelling agent benzyl alcohol. The dyeing results are also shown in Table 1.


**Table 1.** Experiment A: experimental design and results @.

@ Experiment conducted at soaking time in 20 wt.% acetic acid solution = 15 h, dye concentration = 0.1 wt.%, Benzyl alcohol concentration = 2 wt.%, T = 140 ◦C, dyeing time = 1 h. \* Values represent mean and standard error of 3 replicates.

Overall, the highest K/S value achieved in Experiment A is only 1.74 (sample A7) through the combination of water as solvent, pretreatments with acetic acid and NTP (60 s) and the addition of swelling agent benzyl alcohol. The Analysis of Variance (ANOVA) factorial model terms were selected based on the half-normal probability plot, which indicated that all the factors and some of their interactions were significant (*t*-tests for coefficients, *p* < 0.02). Since all of the factors are involved in interactions, statistical interpretations of only the significant interaction terms are shown in Figure 2.

Evidently in Figure 2, the dye solvent had the most significant influence on the color strength (K/S value), with water performing significantly better than glycerol. Adding benzyl alcohol to the dye bath increased the color strength, and the effect of benzyl alcohol was slightly enhanced by the acetic acid pretreatment with water as the dye solvent. The effects of pretreatment with acetic acid and NTP were not obvious, slightly improving the color strength with their synergy.

A significant loss of water was observed in the dye bath during Experiment A due to the evaporation of water at the dyeing temperature T = 140 ◦C. The use of glycerol as the dye solvent effectuated dyeing at T = 140 ◦C without the concomitant evaporative loss of solvent from the dye bath but reached significantly lower values of color strengths. The cationic dye might not be well-dissociated in glycerol as the solvent, and the high viscosity of pure glycerol compared to water and the occurrence of hydrogen bonding between the

dye and glycerol might hinder the mobility of the cationic dye and retard its diffusion onto the para-aramids.

**Figure 2.** Statistical inference of Experiment A: significant factorial interactions.

To overcome the drawbacks of using glycerol as the dye solvent, full factorial Experiment B was designed using 50% or 80% aqueous glycerol solutions as the dye solvent. Additional factors were investigated and included a pretreatment of soaking the paraaramid fabrics in soybean oil and/or applying further treatment with NTP and adding benzyl alcohol as the swelling agent. Table 2 lists the detailed experimental design and resultant K/S values. Even though using 50% or 80% aqueous glycerol solutions as the dye solvent suppressed the loss of the dye bath solution, the highest K/S value achieved in this experiment was only 0.94 (sample B11) with 50% aqueous glycerol solution as the solvent and soybean oil and NTP (60 s) as the pretreatment.



@ Experiment carried out with soaking in soybean oil time = 15 h, dye concentration = 0.1 wt.%, Benzyl alcohol concentration = 2 wt.%, T = 140 ◦C and dyeing time = 1 h. \* Mean and standard error of 3 replicates.

The analysis of variance (ANOVA) of this factorial design indicates that the main effect of all the factors and some of their interactions are significant (*p* < 0.005). The half-normal probability plot was used to select the ANOVA model terms, and only the significant interaction terms of the model are presented in Figure 3.

**Figure 3.** Statistical inference of Experiment B: significant factorial interactions.

The 50% aqueous glycerol solution performed better than the 80% aqueous glycerol solution as the dye solvent (Figure 3). Benzyl alcohol appeared to be incompatible with aqueous glycerol or soybean oil, which might hinder its diffusion and swelling functions and consequently limited the benefits of soybean oil on dyeing. Soaking the para-aramids in soybean oil slightly improved the dyeing color strength, while soaking the para-aramids in soybean oil with subsequent NTP treatment for 60 s increased the color strength significantly. Without a pretreatment of soybean oil pretreatment, NTP had negligible effect on the dyeing strength.

The full factorial design for Experiment C used a lower dyeing temperature (T = 90 ◦C), compared 50% aqueous glycerol solution with water as the dye solvent and used soybean oil pretreatments and NTP treatment times of 90 s. The experimental design and dyeing results are compiled in Table 3. The highest K/S value achieved in this experiment is 1.72 (sample C11), which is higher than the highest value obtained in Experiment B (0.94). Experiment B was carried out at a higher temperature (T = 140 ◦C vs. T = 90 ◦C for Experiment C), used soybean oil with a shorter NTP treatment time (60 s vs. 90 s for Experiment C) and used 50% or 80% aqueous glycerol solutions as the dye solvent (vs. water or 50% aqueous glycerol in Experiment C).

The half-normal probability plot was used to select the ANOVA model terms and resulted in a significant model (F-test, *p* < 0.0001). All of the terms included in the model were significant (*t*-tests, *p* < 0.007), except for the main effect of swelling agent (*t*-test, *p* = 0.59). Since all of the factors were involved in interactions, only the significant interaction terms are interpreted in Figure 4.


**Table 3.** Experiment C: experimental design and the results @.

@ Experiment was carried out with soaking in soybean oil for time = 15 h, dye concentration = 0.1 wt.%, Benzyl alcohol concentration = 2 wt.%, T = 90 ◦C and dyeing time = 1 h. \* Mean and standard error of 3 replicates.

**Figure 4.** Statistical inference of Experiment C: significant factorial interactions.

According to the half-normal probability plot, the solvent had the most influence on color strength among all of the factors, with water as the solvent performing significantly better than the 50% aqueous glycerol solution. Soybean oil pretreatment followed by NTP significantly improved the color strength, although NTP treatment without the soybean oil pretreatment also improved the color strength, but to a lesser degree. Benzyl alcohol performed better in water than in 50% aqueous glycerol solution as the solvent, another indication that the benzyl alcohol was not compatible with glycerol in Experiment B. Further, the effect of benzyl alcohol was minimal on the samples pretreated with soybean oil, but benzyl alcohol significantly improved dyeing without the soybean oil pretreatment, also indicating that benzyl alcohol was not compatible with soybean oil. Without a subsequent NTP treatment, soybean oil soaking resulted in slightly lower color strength, which seems contradictory to the results in Experiment B. However, Experiment C was carried out at a much lower temperature (T = 90 ◦C vs. T = 140 ◦C in experiment B with water or 50% aqueous glycerol solution (vs. 50% or 80% aqueous glycerol in Experiment B in the dye bath, and these factors may have limited the beneficial effects of the soybean oil in achieving dyeing without an NTP treatment. The longer NTP treatment time in Experiment C (90 s vs. 60 s in Experiment B also significantly improved dyeing even without the soaking in soybean oil pretreatment.

From the results of the previous Experiments A–C, it is clear that the use of glycerol in the dye solvent does not help with dyeing, and there is a potential for improving dyeing using a pretreatment consisting of soaking in soybean oil followed by NTP. Accordingly, Experiment D used a lower dyeing temperature (T = 90 ◦C), water as the dye solvent, a wider range of NTP treatment times (*t* = 30–150 s), eliminated the swelling agent benzyl alcohol as a factor and added surfactant as a new factor using TWEEN 80, because of its reported benefit in dyeing aramids [25,26]. Results of the 3-factor 2-level full factorial design are presented in Table 4.


**Table 4.** Experiment D: experimental design and the results @.

@ Experiment carried out with soaking in soybean oil for time = 15 h, dye solvent = water, dye concentration = 0.1 wt.%, T = 90 ◦C and dyeing time = 1 h. \* Mean and standard error of 3 replicates.

ANOVA for the factorial design indicated a significant model (*F*-test, *p* < 0.0001). All three of the factors and their two-way interactions were significant (*t*-test, *p* < 0.0002). Since all of the factors are involved in interactions, only the interaction plots are shown in Figure 5.

According to the half-normal probability plot, the surfactant TWEEN 80 had the highest impact on the dyeing color strength, followed by NTP treatment time, then soybean oil pretreatment in descending order. The use of TWEEN 80 resulted in weaker color strength (Figure 5). The surfactant, although helping to form a dye dispersion, might also hinder the diffusion of the dye onto the para-aramid fiber fabrics, even in the presence of soybean oil. The soybean oil pretreatment improved dyeing, which was further improved by longer subsequent NTP treatment times. Evidently, the longer NTP treatment time of 150 s, comparing to the treatment time of 30 s, resulted in higher color strength in the presence or the absence of TWEEN 80 surfactant.

**Figure 5.** Statistical inference of Experiment D: significant factorial interactions.

It appeared that the NTP treatment time is crucial to the dyed color strength, especially for the soybean oil pretreated samples. Therefore, Experiment E was designed to optimize the NTP treatment time. In Experiment E, samples were soaked in soybean oil for 15 h, water was used as the dye solvent with a 0.1 wt.% dye concentration and with no other additives and the dyeing temperature was T = 90 ◦C for 1 h.

Results of the one-factor factorial design with NTP treatment time varied from 0–150 s are presented in Figure 6. The effects of NTP treatment at these experimental conditions show (Figure 6) that an NTP treatment time of 120 s provided the optimal result in dyeing the soybean oil pretreated samples in terms of color strength as represented by K/S values. NTP treatment times shorter than 120 s achieved dyeing, but to a lesser extent and NTP treatment times longer than 120 s decreased the K/S value, probably due to the degradation of the soybean oil network by the increased exposure to the NTP treatment.

**Figure 6.** Effect of NTP time on K/S value of soybean oil-soaked para-aramids.

Based on the results of Experiments A–E, Experiment F was designed to optimize dyeing performance by developing a suitable recipe that included auxiliary additives (swelling agent and acetic acid) in the dye bath and a suitable dyeing process that in terms of the dyeing temperature (T = 70 ◦C or 100 ◦C) and the effects of a 120 s NTP treatment prior to versus post soaking in soybean oil. The addition of acetic acid into the dye bath brought down the pH to about 3, which should make the conditions favorable for cationic dyes [14]. Results of the 4-factor full factorial design are shown in Table 5.


**Table 5.** Experiment F: experimental design and the results @.

@ Experiment carried out with soaking in soybean oil for time = 15 h, dye solvent = water, dye concentration = 0.1 wt.%, dyeing time = 1 h, Additive concentrations = 1 wt.% Acetic acid and/or 2 wt.% Benzyl alcohol and the NTP treatment time = 120 s prior to or post soaking in soybean oil for 15 h. \* Mean and standard error of 3 replicates. # The change in K/S value was calculated by subtracting the K/S after the 1st detergent wash from the K/S value measured after the 2nd detergent wash.

The half-normal probability plot indicated that impact of the factors is in the order of Dye temperature > NTP > Acetic acid. Even though the overall ANOVA model was significant (*p* < 0.0001), the coefficient for benzyl alcohol was not significant (*t*-test, *p* = 0.63). The only significant interaction term is Temperature × NTP (*t*-test, *p* = 0.0003), while the Temperature × Benzyl–OH (Benzyl alcohol) interaction was marginally significant (*t*-test, *p* = 0.085). Figure 7 plots out the main effects of all the factors along with the two interactions.

Acetic acid was the only factor not involved in any significant interactions (Figure 7A). The addition of 1 wt.% acetic acid in the dye bath slightly decreased the dyeing color strength. All of the other three factors were involved in the two interactions, so their effects are discussed in the context of their interactions (Figure 7B).

The dyeing temperature and NTP after soaking in soybean oil significantly impacted the dyeing results, whereas the use of benzyl alcohol did not significantly influence the dyeing color strength. In Experiment F, the largest K/S value achieved was K/S = 2.28 (sample F15) in dyeing conditions with T = 100 ◦C in a water-based dye bath, with a pretreatment of soaking in soybean oil followed by a 120 s NTP treatment and no other additives of benzyl alcohol or acetic acid in the dye bath. It is noteworthy that T = 100 ◦C is the highest temperature that can be used for the water-based dye bath at atmospheric pressure without significant loss of water due to evaporation. In general, dyeing at T = 100 ◦C was better than dyeing at T = 70 ◦C. While NTP treatment (120 s) prior to soaking in soybean oil did not show a significant effect on dyeing, NTP treatment after soaking in soybean oil significantly improved the color strength, especially when the dyeing was carried out in the higher temperature regime of T = 100 ◦C. This clearly points out that the improved dyeing effect should be attributed to the chemical reactions between soybean oil and NTP.

**Figure 7.** Analysis of Experiment F. (**A**) Main effects of all of the factors and (**B**) the significant interactions.

All samples were laundered with a detergent wash for a second time according to the same protocol described in Section 2.5, to determine colorfastness in terms of the change in K/S values (denoted as ΔK/S in Table 5). A negative sign of the ΔK/S value indicates a decrease in K/S value after the second wash with detergent. Generally, higher temperature and NTP treatment after soaking in soybean oil resulted in better dyeing strength and more durable dyeing (colorfastness) after laundering.

For the purposes of providing a visual demonstration comparing the dyeing strengths of different treatments, select samples were imaged using a flatbed scanner (Figure 8). Compared with the previous investigation of dyeing para-aramids with this method and using a disperse dye [27], it is demonstrated that the pretreatment of soybean oil followed by NTP treatment method is compatible with dyeing with both disperse dye and cationic dye. Further, this method enables dyeing to a significantly high color strength, although the disperse dye requires glycerol as a dispersant, but water as the solvent is better suited for the use of cationic dye. Taken together, these results demonstrate the potential for this method to replace hazardous chemicals currently used in dyeing practices with renewable materials and environmentally friendly ("green") technologies, to improve the dyeing of para-aramid textiles.

#### *3.2. FTIR Analysis*

As shown in Figure 9, the following characteristic peaks of soybean oil (line I) are well represented: 3009 cm−<sup>1</sup> (=C–H stretching of aliphatic alkenes) in unsaturated fatty acids, 2922 cm−<sup>1</sup> (–CH2 asymmetric stretching), 2852 cm−<sup>1</sup> (–CH3 symmetric stretching), 1746 cm−<sup>1</sup> (–C=O triglycerides carbonyl stretching), 1462 cm−<sup>1</sup> (–CH2 antisymmetric deformation) and 1160 cm−<sup>1</sup> (C–O stretching in the esters) [30–33].

**Figure 8.** Scanned images comparing dyed samples with the undyed fabric (labels correspond to the sample numbers in Tables 1–5; F15 (W2) denotes sample F15 after second detergent wash).

**Figure 9.** FTIR spectra of the para-aramid fabrics, materials and after certain treatments as follows: (**I**) Soybean oil; (**II**) Paraaramid fabric (untreated); (**III**) Para-aramid fabric treated with NTP; (**IV**) Para-aramid fabric after soaking in soybean oil; (**V**) Para-aramid fabric after soaking in soybean oil followed by a subsequent NTP treatment; (**VI**) Para-aramid fabric dyed with NTP as the only pretreatment; (**VII**) Para-aramid fabric dyed after soaking in soybean oil (without an NTP treatment); (**VIII**) Para-aramid fabric dyed after soaking in soybean oil and an NTP treatment; and (**IX**) Victoria Blue R cationic dye.

The spectrum of untreated para-aramids (line II) has main peaks at 3313 cm−<sup>1</sup> (–N–H stretching), 1638 cm−<sup>1</sup> (amide I C=O stretching) and 1538 cm−<sup>1</sup> (–N–H deformation) [34]. The absorption bands at 1513 cm−<sup>1</sup> (amide II), at 1017 cm−<sup>1</sup> and at 820 cm−<sup>1</sup> deriving from the C–H bonds on the para-aromatic rings and the absorption band at 1305 cm−<sup>1</sup> deriving

from the C–N bond stretch of amide III of the para-aramid fiber fabrics can be considered as internal standards for para-aramid [35]. NTP treatment of the para-aramid sample (line III) did not induce discernible changes in the observed FTIR peaks. A similar result was observed in Kašparová et al. [36], which also reported some changes in meta-aramids after NTP treatment and may reflect the different internal molecular structures between meta-aramids and para-aramids that make para-aramids more difficult to dye even with an NTP treatment.

The sample in line IV for the para-aramid fabric soaked in soybean oil shows peaks characteristic of soybean oil and of the para-aramids. Subsequent NTP treatment induces some important changes in the observed spectrum (line V). First, the unsaturated fatty acids peak at 3009 cm−<sup>1</sup> decreases, but the other major characteristic peaks of soybean oil were still visible. This observation suggests that the double bonds in the unsaturated fatty acids were rapidly consumed by the NTP treatment, presumably by reaction with the plasma's reactive oxygen and nitrogen species (RONS). Second, there were changes in the 1380–1250 cm−<sup>1</sup> region, including the formation of a small peak appearing at 1275 cm−1, which is assigned to the C–C stretching vibration of the C–C(=O)–C groups in aliphatic ketone molecule [33]. These results suggest that the plasma's RONS attack the C=C double bonds, concomitant to the decrease in the peak at 3009 cm−<sup>1</sup> (=C–H stretching of aliphatic alkenes). However, the ketone molecule is unstable. A likely explanation is that high-energy reactive oxygen species in the NTP (generated with ambient air as the feedgas) attack the electron rich double bonds in the unsaturated fatty acids and form epoxides, although the characteristic oxirane absorption peak typically observed at ~822 cm−<sup>1</sup> would be obscured by the strong para-aramid absorption peak at 820 cm−1. The –C=O triglycerides carbonyl stretching and C–O stretching in the esters at 1746 cm−<sup>1</sup> and 1160 cm<sup>−</sup>1, respectively, remain strong in line V, indicating that the NTP did not break the ester bonds in soybean oil.

Strong acids such as phosphoric acid and sulfuric acid have been used to pretreat para-aramids for the improvement of dyeing or interfacial bonding; a new absorption peak at ~3440 cm−<sup>1</sup> (which is ascribed to the hydroxyl group O–H) or the shift and broadening of the 3313 cm−<sup>1</sup> (–N–H stretching) band was reported, along with some decrease in mechanical strength. This distinct change was explained as a consequence of the increased number of –OH groups in the modified para-aramids due to hydrolysis [2,3]. However, this effect is not observed for the para-aramid sample pretreated with acetic acid in this study, and the acetic acid pretreatment did not significantly improve the color strength of dyeing.

FTIR spectra for dyed para-aramid fabric samples (Figure 9) are shown in line VI (with only NTP pretreatment), line VII (with soybean oil soaking but no NTP) and line VIII (with soybean oil soaking and NTP). The three spectra all have a new peak emerging at 786 cm−1, which is typically the region of C–H bending for 1,3-disubstituted or 1,2,3 trisubstituted aromatic rings, possibly indicating a new bond forming on the para-aromatic rings. Otherwise, the spectrum of the sample dyed only with NTP pretreatment (line VI) is not much different from that of undyed para-aramids (line II or III). The para-aramid fabric sample in line VII (pretreated with soybean oil soaking and without subsequent NTP treatment, showed peaks characteristic of soybean oil that were significantly diminished. NTP treatment after oil soaking induced further decreases in the signal intensity of soybean oil (line VIII). In addition to reactions with NTP, there are two possible explanations for the diminished oil signal: (1) unfixed oil could be washed off after dyeing and subsequent washing with detergent; and (2) the triacylglycerol could be hydrolyzed at the dyeing conditions resulting in the destruction of ester bonds, as indicated by the diminished peaks at 1746 cm−<sup>1</sup> and 1160 cm<sup>−</sup>1.

The FTIR spectrum of the Victoria Blue R is also shown in Figure 9 (line IX). The para-aramid fabric samples showed a low uptake of dye, and most of the dye peaks were obscured by the characteristic peaks of the para-aramids and the oil. Accordingly, no

significant dye peaks were detected in the dyed samples, except for a small shoulder at 1355 cm−<sup>1</sup> in the spectra of the dyed samples (lines VII and VIII).

It is likely that the soybean oil well diffused throughout the tightly woven fabric and adsorbed onto the surfaces of yarns and fibers, and the NTP treatment induced the formation of a polymerized network in situ, enabling dyeing to a higher color strength. The unsaturated fatty acids in soybean oil play important roles in the dyeing process. Cross-linking/polymerization by the action of reactive plasma species are possible, because hydrogenation, nitration and epoxidation reactions have been observed along with polymerization under atmospheric NTP [37]. In a study on the tribological properties of air plasma polymerized soybean oil, Zhao et al. [33] concluded that the free radicals in the long-chain oil molecules, which were formed by the opening of the double bonds under the plasma conditions, could capture the reactive O and N species to produce and incorporate carbonyl, organic amine and nitrogen heterocyclic groups into the polymerized oil network. Moreover, depending on the duration of the NTP treatment in the dyeing procedure, the soybean oil might undergo processes such as complex oxidations, decomposition or fragmentation, to form reactive intermediate sites (e.g., ethers, furans, peroxides, carboxylic acids or 1,2,4-trioxolane [38,39]) on the para-aramid fiber fabrics that serve as catalysts or ligands to bind dye molecules.

#### **4. Conclusions**

Using a cationic dye and an environmentally friendly alternative dyeing method, we found that the best formulation for dyeing para-aramid textiles involved (i) pre-treating the para-aramid fiber fabrics with soybean oil followed by a brief NTP treatment, and (ii) using water at T = 100 ◦C as the dye solvent and omitting auxiliary chemical additives. Dyeing temperature and NTP treatment time were the most important factors, with the dyeing temperature of T = 100 ◦C resulting in higher color strength (larger K/S values), and the optimized NTP treatment time of 120 s for soybean oil pretreated para-aramids. These conditions achieved a K/S value up to 2.28, which is significantly larger than the K/S ~1 for untreated samples. For comparison, NTP alone or the use of the auxiliary additives of acetic acid and benzyl alcohol only slightly improved dyeing with the cationic dye. As a dye solvent, glycerol was inferior to water for dyeing with the cationic dye, and the addition of the surfactant TWEEN 80 in the dye bath negatively impacted the dyeing performance.

In the present study, FTIR analysis revealed that NTP induced chemical reactions in soybean oil on the surface of para-aramid fabrics were responsible for significantly improving the color strength. These chemical reactions likely involved high-energy, shortlived reactive plasma species that first attacked the double bonds in the unsaturated fatty acids. While the results of this study provide ample evidence that the pretreatment of soaking in soybean oil and ensuing NTP treatment improved dyeing color strength, determining the chemical pathways that led to better dyeing warrant further research, to control these processes with other dyestuffs. Future research along this line of sustainable dyeing method will aim at deriving more functionalities of soybean oil on the surface of para-aramids to further improve the dyeing strength and colorfastness.

**Author Contributions:** Conceptualization, C.J.D. and X.P.Y.; methodology, X.P.Y. and C.J.D.; validation, X.P.Y. and M.M.; formal analysis, C.J.D. and X.P.Y.; investigation, M.M. and X.P.Y.; resources, C.J.D. and X.P.Y.; data curation, M.M.; writing—original draft preparation, X.P.Y.; writing—review and editing, X.P.Y. and C.J.D.; visualization, M.M. and X.P.Y.; supervision, X.P.Y.; project administration, C.J.D. and X.P.Y.; funding acquisition, C.J.D. and X.P.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partially supported by the Department of Agriculture HATCH project No. TEN00521.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We thank the support by the U.S. Department of Agriculture HATCH project No. TEN00521.

**Conflicts of Interest:** The authors declare no conflict of interest.
