Cotton fabric has attracted much attention in the development of functional textiles due to its excellent moisture absorption, breathability, and biodegradability. However, its surface inertness and lack of active functional groups limit the loading efficiency and binding fastness of functional substances such as dyes and nanoparticles. Although traditional chemical modification methods such as alkali treatment or graft modification can improve surface properties, they have problems such as wastewater pollution, high energy consumption, and fiber damage. In recent years, low-temperature plasma technology, as a green, efficient, and solvent-free surface modification method, has provided a new approach for functionalizing cotton fabrics through the synergistic effect of physical etching and chemical activation.

Plasma Surface Treatment Technology

Plasma surface treatment can achieve the following effects at the nanoscale by interacting with high-energy particles (such as electrons and ions) and active free radicals on the fiber surface (Figure 1-1): (1) Clean etching: removing wax and pectin from the surface of cotton fibers, forming rough structures, increasing specific surface area and binding sites; (2) Chemical activation: Introducing polar groups such as hydroxyl (- OH) and carboxyl (- COOH) on the surface of fibers to enhance surface wettability and chemical bonding ability; (3) Synergistic Enhancement: Surface etching and activation synergistically optimize the penetration and adsorption of functional substances.

Figure 1-1 Schematic diagram of oxygen plasma pretreatment of cotton fabric

The Effect of Plasma Surface Treatment on Cotton Fabric

We used a low-temperature plasma treatment device to pretreat the surface of cotton fabric. Firstly, cut the cotton fabric into a suitable size, then place it horizontally in the cylindrical chamber of the processing device, and then turn on the vacuum pump to pump the pressure inside the chamber to below 10 Pa. Next, open the gas cylinder and introduce oxygen. After setting the flow rate, processing power, and processing time, start processing the surface of the cotton fabric.

Quality loss and morphological changes of cotton fabric after plasma surface treatment

Plasma surface treatment significantly changes the surface composition and microstructure of cotton fabrics through high-energy particle bombardment and reactive free radical oxidation. As shown in Figure 1-2, the quality loss rate of cotton fabric increases with the increase of processing power (100-300W) and time (10-30min), and the highest quality loss rate reaches 8.59% (100W, 30min), indicating the synergistic effect of surface etching effect and impurity removal. On the one hand, due to the removal of impurities, the wax and pectin on the fiber surface are decomposed into volatile small molecules such as CO ₂ and H ₂ O by plasma, resulting in mass loss; On the other hand, it is due to surface etching: high-energy particle bombardment causes cellulose molecular chains to break, forming micrometer sized grooves and nanopores (SEM images 1-3), directly leading to a decrease in fiber mass.

Figure 1-2 Quality loss rate of cotton fabric after plasma surface  treatment

By comparing and analyzing the SEM images in Figures 1-3, it can be clearly revealed that oxygen plasma treatment etches the surface of cotton fibers. The surface of untreated cotton fibers presents a typical smooth layered structure (Figure 1-3a), while after plasma treatment, the fiber surface forms varying degrees of dense grooves and microporous structures (Figure 1-3b-f). The reason for the change in morphology is that in oxygen plasma, active substances can cause chemical erosion (atomic) and physical erosion (ion).

Figure 1-3 Scanning electron microscopy image of cotton fabric after plasma treatment (a) untreated, (b)100W-10M,(c)100W- 20M,(d)100W-30M,(e)200W-10M,(f)300W-10M

The Effect of Plasma Surface Treatment on the Wettability of Cotton Fabric

Evaluate the improvement of wetting and hydrophilicity of cotton fabrics by plasma treatment technology using a contact angle measuring instrument. Due to the high porosity of cotton fabric itself and the good hydrophilicity of the treated cotton fabric, it is difficult to measure the contact angle of water droplets instantly. In this case, the wettability of cotton fabric can be evaluated by the absorption rate of water droplets by cotton fabric. As shown in Figure 1-4, the static contact angle of untreated cotton fabric is 154.70 °, which decreases to 136.62 ° after 9.72 seconds, indicating a certain degree of hydrophobicity. This is attributed to the formation of a continuous film on the surface of cotton fibers after the slurry dries, which hinders the rapid penetration of moisture. After plasma treatment of the sample (100W-30min), the water droplets were completely absorbed within 0.18s, attributed to the multi-level capillary channels formed by the etched micro grooves and nano pores, which strengthened the capillary effect and drove the rapid spreading of water.

Figure 1-5 shows the contact angle test results of different cotton fabrics (a) without treatment, (b)100W-10M,(c)100W-20M,(d) 100W-30M,(e)200W-10M,(f)300W-10M

Infrared Spectral Analysis of Cotton Fabric after Plasma Surface Treatment

The infrared spectra of different cotton fabrics shown in Figure 1-5 are helpful for studying the effect of plasma treatment on the chemical composition of cotton fabrics. The peaks at 2915cm-1 and 2849cm-1 correspond to asymmetric and symmetric stretching of methylene (- CH2-) in the long alkyl chain, indicating the presence of a wax layer on the surface of cotton fabrics. After plasma treatment, different treatment schemes showed similar results, with peak intensities significantly reduced or even disappeared at 2915cm-1 and 2849cm-1, attributed to the decomposition and removal of wax on the surface of cotton fabrics by plasma treatment. In addition, after plasma treatment, the peak intensity in the 1657-1605 cm-1 region (C=O) increased, and a small peak appeared at 1655 cm-1, confirming the oxidation effect on the surface of cotton fabric after oxygen plasma treatment. That is, after the wax layer was removed, the cotton fabric was oxidized by plasma active substances from oxygen, thereby generating a relatively large number of C=O bonds. In addition, this confirms that oxygen plasma induces the generation of additional polar groups on the fiber surface to enhance the hydrophilicity of the fabric, which is consistent with the analysis of contact angle testing.

Figure 1-5 Infrared spectra of different cotton fabrics