*3.3. Supporting Evidence: SEM, Extraction Test, and Tensile Strength*

Scanning electron micrographs (SEM) reveal the smooth surface of original paraaramid fibers, and a 4 min NTP treatment did not show any visible changes on the surface (Figure 11), explaining why the NTP treatment alone would not significantly improve dyeing. The samples dyed with AESO or AA/Soy pretreatment have a resin-like coating on the para-aramid fibers, in which dye particles are embedded. It is notable that the coating survived two washes with detergent, indicating a polymerized network. Agglomeration of dye particles can also be observed, especially in the AA/Soy pretreated sample, indicating that our pretreatment procedure needs further improvement in order to evenly distribute the AESO or AA/Soy and deliver the NTP treatment.

After a 15 h extraction with three solvents of different polarity, the color strength of all the tested samples decreased. Because the K/S values were measured at different wavelengths for the blue cationic dye vs. the red disperse dye, we report here the percentage decrease in K/S value after extraction for comparison. For the samples dyed with AA/Soy pretreatment followed by NTP treatment, the K/S value decreased by 20%, 64%, and 63% after the extraction using hexane, ethanol, or 2:1 (*v*/*v*) chloroform:methanol, respectively. For the sample dyed with Disperse Red 1 acrylate, the K/S value decreased by 6%, 54%, and 84% after the extraction using hexane, ethanol, or 2:1 (*v*/*v*) chloroform:methanol, respectively. This indicates that not all the AA/Soy were polymerized (and the unpolymerized soybean oil could be easily extracted by hexane), or not all the AA/Soy were covalently bonded to the fabric. However, there was evidence that almost all the dye molecules were covalently bonded to the NTP-induced polymerized network. When the solvents were evaporated from the extractable matters and the dry lipid-dye matrices were re-dissolved in hexane, then water was added to study the partition of the dye between the hexane and water, no blue color was observed in the aqueous layer, indicating that this water-soluble dye was chemically linked to the soybean oil network.

In addition, significant color strength remained after extraction with the three solvents of different polarities, indicating a covalently bonded network formed on the surface of para-aramid fabrics, with one exception of the 84% reduction in K/S value for the sample dyed with Disperse Red 1 acrylate and extracted with chloroform:methanol, which indicates the lack of bonding between the polymerized dye and the para-aramids; the extracted sample exhibited a yellow color close to that of the undyed fabric. In this respect, the method developed in this study performed better in binding with the inert para-aramids. Furthermore, the samples dyed with Disperse Red 1 acrylate became more rigid because of the short chain of the starting monomers, while the samples dyed with AA/Soy remained flexible due to the preserved long chains of soybean oil.

It is worthwhile to mention that in dyeing textiles, dyes are usually fixed on textiles through physical entrapment or the relatively weak hydrogen bonds and van der Waals force of attraction, or stronger ionic bonds in the case of acid dye or basic dye; few dye molecules binded to textiles via covalent bonds, except for reactive dyes with cotton, wool, or polyamides such as nylon [31,32].

Tensile test results show that our developed method of dyeing para-aramids did not reduce the strength of para-aramid fibers measured as breakpoint peak load.

#### **4. Conclusions**

We demonstrated that deriving more functionalities of soybean oil on the surface of para-aramids further improved the dyeing color strength and colorfastness. Compared with the pretreatment using soybean oil that obtained the highest K/S value up to 3.89, pretreatment with either AESO or AA/Soy achieved K/S values higher than nine in this study. Importantly, we demonstrated through sequential experimentation that a simple mixture of AA/Soy could replace the pricy AESO to render comparable dyeing performance. NTP treatment after the AESO or AA/Soy pretreatment was essential for our developed method, because the NTP not only induced the formation of a polymerized

network on the surface of para-aramids that bonded with dye molecules, but also generated covalent bonds anchoring the polymerized network to the para-aramids, which is difficult given the high crystallinity and chemical inertness of the para-aramids.

Among the auxiliary additives tested in this study, Polysorbate 80 (TWEEN) as a surfactant negatively affected the dyeing, and the effect of benzyl alcohol as a swelling agent was minimal. However, NaCl as an electrolyte showed positive effect. Therefore, we recommend an optimal formulation and condition as NTP time = 360 s (specific for the NTP source used in this study), NaCl = 5% wt.%, and AA/Soy = 3.25 for dyeing para-aramids without other chemical additives. The dyeing method developed in this study did not sacrifice the strength of para-aramids, showing the potential for this method to replace hazardous chemicals currently used in dyeing practices with renewable materials and environmentally friendly technologies, while achieving improved dyeing of para-aramid textiles for new applications.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/polym14030628/s1, Table S1: Characteristic FTIR bands of materials and the references; Figure S1: Scanned images of dyed samples in contrast with original undyed fabric (labels correspond to the sample numbers in Tables 3–5 and Figure 8; G4 (W2) indicates sample G4 after 2nd detergent wash); Figure S2: FTIR spectra of (A) para-aramid sample after soaking in AA/Soy and subsequent NTP treatment; (B) dyed para-aramid sample pretreated with AA/Soy with subsequent NTP treatment; (C) dyed para-aramid sample pretreated with AESO with subsequent NTP treatment.

**Author Contributions:** Conceptualization, C.J.D. and X.P.Y.; methodology, X.P.Y. and C.J.D.; validation, X.P.Y., T.W. and C.M.; formal analysis, C.J.D., C.M. and X.P.Y.; investigation, C.M., T.W. and X.P.Y.; resources, C.J.D. and X.P.Y.; data curation, C.M. and X.P.Y.; writing—original draft preparation, X.P.Y.; writing—review and editing, X.P.Y., T.W. and C.J.D.; visualization, C.M. and X.P.Y.; supervision, X.P.Y. and T.W.; 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 of the U.S. Department of Agriculture HATCH project No. TEN00521.

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

#### **References**

