Polyester features high tensile strength, excellent wear resistance and superior dimensional stability, which makes it widely adopted in various textile products. Nevertheless, its low surface energy and limited polar sites lead to poor wettability and moisture absorption & transmission capacity, impairing the efficiency of dyeing and finishing processes. From the perspective of eco-friendly processing, developing a mild, controllable surface modification method for polyester with guaranteed performance bears practical significance for advancing the green sustainable development and industrial transformation & upgrading of the textile industry.

Traditional water-bath dyeing of polyester is plagued by excessive water consumption, massive wastewater discharge and severe pollution. Statistics show that roughly 4 tons of water are consumed for dyeing each ton of polyester fabric. The discharged printing and dyeing wastewater contains abundant surfactants, alkalis, dyes, especially heavy metal salts that are hard to biodegrade, thereby causing heavy metal contamination to water and soil resources. Current anhydrous dyeing technologies for polyester mainly include solvent dyeing, supercritical CO₂ fluid dyeing and vacuum sublimation dyeing. However, solvent dyeing generally delivers low dye uptake owing to limited dye solubility in solvents; meanwhile, stricter requirements are imposed on fire safety management and applicable dye categories during operation. Supercritical CO₂ fluid dyeing still faces drawbacks such as difficult equipment cleaning, high processing costs and insufficient varieties of dedicated dyes. Vacuum sublimation dyeing causes severe dye contamination on equipment together with tough cleaning work and stringent requirements for dyeing facilities.

Plasma treatment is a dry finishing technology that saves water and energy while mitigating environmental pollution. Plasma treatment triggers surface etching and changes in the microscopic structure of polyester fibers, enabling easier penetration of dye molecules into fiber interiors and accordingly improving the dyeability of fabrics. Disperse dyes bind with the amorphous regions of polyester via van der Waals forces, hydrogen bonds and dipole forces, among which hydrogen bonds play a dominant role. The hydrogen atoms contained in disperse dye molecules can form hydrogen bonds with oxygen and nitrogen atoms inside fibers. During plasma modification, oxygen and nitrogen from the process gas are grafted onto the fiber surface. The generated carboxyl groups endow the material with hydrophilicity, optimizing the interfacial interaction between dye liquor and fiber surface and facilitating the wetting and spreading of dye solutions.

Analysis of Plasma Treatment Processes

Analysis of Fabric Surface Morphology

Aiming at the inherent drawbacks of polyester fibers including smooth surface and weak dye affinity, low-temperature plasma treatment was applied to modify the outer-layer polyester fibers of woven yarns. Scanning Electron Microscopy (SEM) was adopted to characterize fiber surface morphologies under identical power with different treatment durations.

As displayed in Figure 1(a), untreated polyester fibers exhibit continuous, flat and dense surfaces, which hinders the adhesion of functional dyes onto fiber surfaces. As shown in Figure 1(b)–(d), plasma treatment at a power of 200 W modifies fiber surface topography. After 30 s of treatment, slight superficial etching emerges on fiber surfaces while the overall structure remains smooth with marginal improvement in surface roughness. When the treatment time is extended to 60 s, the etching effect is intensified, forming relatively continuous micro-grooves along the axial direction of fibers. The elevated surface roughness effectively increases the specific surface area of fibers without damaging their bulk morphology, providing abundant potential physical anchoring sites for subsequent thermochromic dye molecules. Further prolonging the treatment duration to 90 s results in uneven surface etching with locally deepened grooves and disrupted surface continuity, indicating that overlong plasma treatment induces excessive evolution of surface morphology and obstructs the controllable regulation of interfacial structures.

Figure 1(e) presents the morphology of fibers loaded with thermochromic dyes after plasma treatment at 200 W for 60 s. Continuously distributed adhered phases can be observed on fiber surfaces, demonstrating that moderate plasma etching optimizes the microscopic surface structure and creates favorable physical interfacial conditions for stable immobilization of thermochromic dyes on polyester fibers.

Figure 1 SEM images of polyester samples subjected to plasma treatment with varied power and treatment durations

Analysis of Fabric Contact Angle

The water contact angle of untreated polyester fabric is 125.0°, whereas water droplets are instantly absorbed by plasma-treated fabrics (Figure 2). This phenomenon originates from cracks generated on fiber surfaces via etching, which enlarges the contact area between water droplets and fibers and enhances the water transport capacity of fibers. Similarly, dye molecules can permeate into fiber interiors more efficiently.

Figure 2 Comparison of hydrophilicity of polyester fabrics before and after plasma treatment

Analysis of Fabric Elemental Composition

Figure 3 illustrates the X-ray Photoelectron Spectroscopy (XPS) spectra of polyester samples before and after plasma treatment. After nitrogen plasma treatment at 200 W for 60 s, obvious variations are observed in the surface elemental proportions of samples: the atomic content of oxygen rises from 17.06% to 33.89%, while the carbon content declines correspondingly; the O/C atomic ratio increases from 0.21 to 0.55. These data prove that plasma treatment elevates the oxygen content and polarity of fiber surfaces. According to the high-resolution C1s spectra, untreated samples are dominated by the C–C/C–H peak located at 284.8 eV. After plasma treatment, the peak intensity assigned to C–O/C–OH near 286.5 eV is significantly strengthened, accompanied by an increment of ester carbonyl (O–C=O) related components at 288.9 eV. This outcome verifies the increased proportion of oxygen-containing functional groups on fiber surfaces, which indicates that plasma action mainly induces oxygen enrichment and functional group rearrangement on the fiber superficial layer without destroying the bulk chemical framework of polyester. The O1s spectra further validate the above conclusions. Following plasma treatment, the peak intensity corresponding to hydroxyl groups and other polar oxygen-containing structures near 533.0 eV is enhanced, and the growth of high-binding-energy components further confirms the introduction of polar oxygen-containing structures onto surfaces. Such structures support the stable attachment of thermochromic dye molecules through hydrogen bonding and other intermolecular interactions.

Figure 3 XPS spectra of polyester samples before and after plasma treatment