*2.6. Determination of Water Absorption and Moisture Content* 2.6.1. Water Absorption

Water absorption was evaluated by immersing samples into distilled water. Each thread sample was immersed for 0.5, 1, 3, 5, 7, and 30 min. The samples were weighed before and after immersion, and then the water absorption was calculated using Equation (1).

$$\mathbf{W} \left( \% \right) \;= \left[ \frac{m\_d}{m\_o} \right] \times 100 \tag{1}$$

where W is the water absorption of the samples in (%), *m<sup>a</sup>* is the sample mass (mg) after water absorption, and *m<sup>o</sup>* is the initial mass of the samples before water absorption [25].

#### 2.6.2. Moisture Content

Moisture content was determined by recording the sample mass at similar time intervals, and moisture content was calculated as shown in Equation (2).

$$\text{MCC } (\%) = \frac{Mcws - Mcs}{Mcs - Mc} \times 100\tag{2}$$

where *Mcws* is the container plus wet sample mass (mg), *Mcs* is the mass of the container plus dried sample (mg), and *Mc* is the container mass (mg) [26].

#### *2.7. Surface Analysis*

#### 2.7.1. Surface Morphology

The surface topography of coated threads was observed under a Dino-Lite Optical Microscope and analyzed with the DinoCapture V2 program. The samples were observed under 440× magnification.

#### 2.7.2. Attenuated Total Reflection–Fourier-Transform Infrared Spectroscopy (ATR–FTIR)

FTIR was carried out using Alpha Platinum-ATR (Bruker, Massachusetts, United States). FTIR spectra of the samples were recorded at 32 scans with a resolution of 4 cm−<sup>1</sup> and a wavenumber range between 400 and 4000 cm−<sup>1</sup> .

#### *2.8. Antibacterial Analysis*

Fabricated cotton threads with different concentrations of CA and different coating layers were investigated based on a previous published study [27], which was conducted using agar plate diffusion method (GB/T 20944.1-2007). Gram-negative (*Escherichia coli*) and Gram-positive (*Staphylococcus aureus*) bacteria were diluted into a suspension (1 <sup>×</sup> <sup>10</sup><sup>6</sup> CFU/mL) and spread on the sterile agar plate. The fabricated cotton threads were placed transversely onto the agar surface to ensure contact, and the plates were incubated at 37 ◦C for 24 h. The zone of inhibition was evaluated by the inhibition zone diameter values of the samples and calculated based on the formula shown in Equation (3).

$$\text{H (mm)} = \frac{D - d}{d} \tag{3}$$

where H refers to the zone of inhibition (mm), *D* refers to the total diameter of the sample and inhibition zone (mm), and *d* refers to the diameter of the cotton thread (mm).

### *2.9. Statistical Analysis*

The selected data were given as mean values with standard deviations. The number of replicates was constant, where *n* = 3 replicates for each observation. Analysis of the data obtained from the experiments was performed using the ANOVA function in Microsoft Excel with a confidence level of *p* < 0.05.

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

*3.1. ATR–FTIR Characterization of Uncoated CT and CT Coated with CMC Cross-Linked with CA (CT/CMC + CA)*

The presence of CMC and CMC + CA coating on CT was confirmed by FTIR analysis. The ATR–FTIR measured in this study is within the mid-IR spectrum of 400–4000 cm−<sup>1</sup> . Figure 1 shows the spectrum of the pristine CT and CT coated with CMC cross-linked with 2M CA samples at different coating layers with different drying regimes. CA spectra showed characteristic peaks at 3290 cm−<sup>1</sup> corresponding to O–H stretching for H2O and 1745 and 1698 cm−<sup>1</sup> , which matched with C=O stretching of carboxylic acid [28]. As regards CMC, FTIR peaks were observed at 3356, 1587, and 1051 cm−<sup>1</sup> corresponding to O–H stretching, carboxylate C=O stretching, and C–O–C stretching, respectively [29]. Based on Figure 1, there is no significant difference between coating layers at *p* > 0.05, which

proves that coating layers have no adverse effect on the chemical functionalities of the samples. Additionally, similarities among the spectrum prove that CA was homogeneously cross-linked with CMC. However, as regards different drying regimes, there are slightly different transmittance intensities to the CMC + CA spectra. *Polymers* **2022**, *14*, x 7 of 23

**Figure 1.** FTIR spectra of (**a**) single-coated and (**b**) double-coated CMC cross-linked with 2M CA in different drying regimes. **Figure 1.** FTIR spectra of (**a**) single-coated and (**b**) double-coated CMC cross-linked with 2M CA in different drying regimes.

After different concentrations of CA were added to the CT/CMC samples, peaks can be observed at 3284, 1747, and 1698 cm−1, and the cross-linking mechanism involves the attachment of carboxylate groups of CA to the hydroxyl group of CMC, which could be attributed to the esterification between citric acid and CMC, demonstrating chemical linkage formation among them [30]. In addition, the peaks at 1747and 1698 cm−1 became more intense, with the increase in CA concentrations, as shown in Figure 2a,b, stipulating a higher cross-linking reaction. The ATR–FTIR results suggested that there was an occurrence of cross-linking interaction between CT/CMC and CA. Figure 2a,b demonstrates a similar observation at 3284 cm−1, where peaks at lower concentration showed less intense peaks due to the esterification reaction during cross-linking [31]. Thus, the results obtained are in tune with those of previous studies [28,32]. After different concentrations of CA were added to the CT/CMC samples, peaks can be observed at 3284, 1747, and 1698 cm−<sup>1</sup> , and the cross-linking mechanism involves the attachment of carboxylate groups of CA to the hydroxyl group of CMC, which could be attributed to the esterification between citric acid and CMC, demonstrating chemical linkage formation among them [30]. In addition, the peaks at 1747 and 1698 cm−<sup>1</sup> became more intense, with the increase in CA concentrations, as shown in Figure 2a,b, stipulating a higher cross-linking reaction. The ATR–FTIR results suggested that there was an occurrence of cross-linking interaction between CT/CMC and CA. Figure 2a,b demonstrates a similar observation at 3284 cm−<sup>1</sup> , where peaks at lower concentration showed less intense peaks due to the esterification reaction during cross-linking [31]. Thus, the results obtained are in tune with those of previous studies [28,32].

**Figure 2.** FTIR spectrum comparison between (**a**) single-coated CMC cross-linked with 2M–4M CA, (**b**) double-coated CMC cross-linked with 2M–4M CA. **Figure 2.** FTIR spectrum comparison between (**a**) single-coated CMC cross-linked with 2M–4M CA, (**b**) double-coated CMC cross-linked with 2M–4M CA.

The slight differences in transmittance intensity among different drying regimes are due to the transmittance value among the drying regimes. IR has the highest transmittance values, followed by OD and OIR drying regimes. A high transmittance value indicates a large population of specific functional groups that emits vibrational energies, which corresponds to the reflected light [33]. The large population of specific bonds suggests that IR is an effective drying regime since CMC+CA is more concentrated due to due to the efficient moisture removal. As the moisture is efficiently removed from the coated CT, macromolecular compaction occurs, and the population of specific bonds per mm2 increases. The concentrated amount of CMC+CA coating in IR-dried samples causes them to emit a high transmission value to a specific functional group population bond. More apparent differences are shown in Figure 1 of OD and OIR samples, regardless of layers. The slight differences in transmittance intensity among different drying regimes are due to the transmittance value among the drying regimes. IR has the highest transmittance values, followed by OD and OIR drying regimes. A high transmittance value indicates a large population of specific functional groups that emits vibrational energies, which corresponds to the reflected light [33]. The large population of specific bonds suggests that IR is an effective drying regime since CMC + CA is more concentrated due to due to the efficient moisture removal. As the moisture is efficiently removed from the coated CT, macromolecular compaction occurs, and the population of specific bonds per mm<sup>2</sup> increases. The concentrated amount of CMC + CA coating in IR-dried samples causes them to emit a high transmission value to a specific functional group population bond. More apparent differences are shown in Figure 1 of OD and OIR samples, regardless of layers. Even though the amounts of CMC and CA used were controlled, distinct transmittance values among different drying regimes indicated different efficacies of moisture removal. The efficiency of moisture removal will be highlighted in Section 3.2.

In Figure 3, four enlargements of Figure 1, which exhibit the FTIR spectra of dried samples by different drying regimes on a specific range of wavelengths, are shown. The stretching bands of the functional groups of the IR-dried samples were similar to those of the OD and OIR samples. According to Figure 2, the OH group appeared within the broad adsorption peak in all drying methods. A slight decrease in the wavenumber was observed by comparing the CT/CMC + CA samples and uncoated CT samples. The carbonyl peaks of CMC + CA emerged at 1747 and 1698 cm−<sup>1</sup> in the spectral region between 1750 and 1600 cm−<sup>1</sup> for all samples, both in single and double coats. There was a shift towards a smaller wavenumber within this wavenumber, and a similar trend was observed at peaks of 1378 and 1137 cm−<sup>1</sup> . When the peaks shift to a smaller wavenumber, this indicates that the molecule within this wavenumber has increased in its mass. The vibration frequency is inversely proportional to the mass of the vibrating molecule. Therefore, the heavier the molecule, the lower the vibration frequency, thus the smaller the wavenumbers [34]. At a wavenumber of 770 cm−<sup>1</sup> , no changes were observed between the drying regimes. This result helps confirm that different drying regimes did not affect the sample compositions. *Polymers* **2022**, *14*, x 10 of 23

**Figure 3.** Enlargement of the FTIR spectra of (**a**) single-coated and (**b**) double-coated CMC crosslinked with 2M CA in different drying regimes. **Figure 3.** Enlargement of the FTIR spectra of (**a**) single-coated and (**b**) double-coated CMC crosslinked with 2M CA in different drying regimes.

tional to the coating layer and concentration of solids in the coating formulation. Coated samples gained the basis weight and thickness as expected compared with uncoated samples, with double-coated samples accumulating the highest weight gain and thickness. CT coated with CMC increased its weight and thickness but not as much as CT coated with CMC+CA. The substantial increase in CMC+CA samples' weight proved that cross-linked CMC+CA has better coating efficiency due to the accumulation of CA attached to the -OH group of CMC after the esterification process. It was also found that the CT/CMC weight and average thickness increased significantly with CA concentration. This is because the higher the CA concentration, the higher the probability of CA to cross-link with CMC. At

3.2.1. Basis Weight and Thickness

*3.2. Physical Properties of Uncoated CT and Coated CT/CMC+CA* 
