Abstract:

Objective Supercritical CO₂ fluid dyeing is attracting attentions as a green technology that is promising to root out the printing and dyeing wastewater problem. There are still deficiencies in understanding the dyeing mechanisms, among which the understanding of diffusion performance of dyestuffs would play a key role in exploring the dyeing process with supercritical CO₂ fluids. This research aims to study the diffusion behaviour of disperse dyes in supercritical CO₂ fluid dyed polyester fibers.

Method The diffusion behaviour of disperse dyes (Disperse Red 167, Disperse Orange 30, Disperse Blue 79) in polyester fibers under different dying conditions of supercritical CO₂ fluid dyeing was investigated using confocal Raman microscopy. Based on the Raman spectra of the dyes, fibers before and after dyeing and the Raman characteristic peaks of the dyes in the dyed fibers were analyzed. The distribution of the dyes in the fibers was also studied by selecting the corresponding depth imaging map according to the characteristic peaks of the dyes. The accuracy of the data was later verified by comparing the Raman data with the exfoliated colour data. The diffusion coefficient was also evaluated.

Results Firstly, the position of the main Raman peak (1 616 cm^-1) of the polyester fiber and the Raman characteristic peak of disperse dyes in the fiber was obtained by comparing the Raman spectra of the samples. The depth imaging function of confocal Raman microscope was used to analyze the fibers after dyeing under different conditions, revealing that the dye in the fibers increased significantly from 5 to 30 min of dyeing. The adsorption capacity of three dyes in the fibers increased with pressure, the adsorption capacity of Disperse Red 167 and Disperse Orange 30 in the fiber increased with the increase of temperature, and the adsorption capacity of disperse Blue 79 in the fiber showed an increase and then a decrease. The dye was found to be evenly distributed in the fibers at the early stage of dyeing (dyeing time 5 min). The Raman data were compared with the stripping data to verify the validity of the Raman data, and the diffusion coefficients of the three dyes were calculated under the dyeing condition of 120 ℃ temperature and 27 MPa pressure. The results show that Raman spectroscopy is able to facilitate quantitatively analyses of dyeing polyester fibers with disperse dyes.

Conclusion The diffusion of disperse dyes in polyester fibers after supercritical CO₂ dyeing was studied by confocal Raman microscopy without damaging the polyester fibers. The content of Disperse Red 167 and Disperse Orange 30 in polyester fibers increased with increasing dyeing time and pressure, while the content of Disperse blue 79 increased and then decreased with increasing dyeing temperature. The diffusion process of the dye was analyzed by studying the I_Dyes/I_Fibers at different fibre depths under different dyeing conditions. It was found that in the early stages of dyeing (5 min into dyeing) the dye already showed a uniform distribution in the fibers, which was related to the dissolution of the polyester fibers in the supercritical CO₂ fluid and the high diffusivity of the supercritical CO₂ fluid itself. The Raman data was compared with conventional stripping data which demonstrated the accuracy of the Raman data and the suitability of Raman spectroscopy to quantify the disperse dye staining in polyester fibers.

Key words: disperse dye, polyester fiber, supercritical CO₂, confocal Raman microscope, diffusion