Abstract:

Objective The carbene-type dye that emerged in recent years could achieve strong covalent bonding between dyes and fibers, which is expected to solve the thermal migration problem of conventional disperse dyes. To date, the carbene dyes are still not widely available, and the reaction mechanism between dyes and fibers is yet to be fully understood. This research developed a novel carbene-type dye based on the structure of α-(trifluoromethyl)phenyl diazo ester, which was used for dyeing and fixing various synthetic fibers. This paper article aims to reveal the reaction mechanism between dyes and fibers by further simulation experiments and theoretical calculations.

Method Dye D1 was synthesized through a two-step post-modification of an azo dye containing one hydroxyl group. The absorption and thermal properties of dye D1 were studied. Then, dye D1 was applied to dyeing polyester, polypropylene, polyurethane and polyamide fibers. The color firmness was confirmed by testing fixation, dye migration and fastness properties. Simulation reactions between D1 and small molecule analogues, such as benzene, dioxane, cyclohexane, and piperidine, were used to clarify the reaction site on fibers. Theoretical calculations of reactions between 4-(trifluoromethyl)phenyldiazoacetic acid methyl ester and methane or methylamine were performed. Based on the above, the reaction mechanism between the dye and fiber was speculated.

Results Dye D1 with an overall yield of 79% was obtained through the two-step post-modification of azo dye. The color of dye D1 was red and its maximum absorption wavelength in UV-vis absorption spectrum located at 492 nm, with a molar extinction coefficient of 35 300 L/(mol·cm). The thermogravimetric curve of dye D1 showed a weight loss of 5.6% at 100-200 ℃, with a quickest weight loss temperature of 152 ℃. The weight loss at this stage was related to the dissociation of the diazo group by releasing nitrogen gas. The DSC curve indicated one endothermic peak with the temperature of 141 ℃, and one exothermic peak corresponding to a temperature of 160 ℃. During the heating process, dye D1 converted into carbene intermediates, accompanied with the removal of nitrogen gas. Highly active carbenes would undergo insertion reactions with C—H or N—H bonds on fibers. Four types of synthetic fibers dyed with dye D1 showed good fixation values of 73% for polyester, 48% for polypropylene, 85% for polyurethane and 53% for polyamide. Dye migrations of dyed fibers were only 12%-14%, and the color fastnesses to soaping, rubbing and sublimation reached level 4 or above. Simulation reaction results demonstrated that dye D1 and benzene underwent an additional reaction with an optimal yield of 54% at the temperature of 140 ℃. Dye D1 and dioxane underwent ring expansion reaction, and the optimal yield is 62% when the reaction temperature was 110 ℃. A C—H bond insertion reaction between D1 and cyclohexane occurred at an optimal reaction temperature of 150 ℃ in a 78% yield, and an N—H bond insertion reaction between D1 and pyridine took place at an optimal reaction temperature of 130 ℃ in a 50% yield. The reaction of dyes in a mixed solvent of benzene and dioxane produced cycloaddition product with a yield of 12% and ring expansion reaction product with a yield of 51%. The reaction of dyes in a mixed solvent of cyclohexane and piperidine produced cyclohexane C—H insertion product with a yield of only 6%, and piperidine N—H insertion product with a yield of 35%. Theoretical calculations showed a potential barrier of 31.3 kJ/mol for the conversion of 4-(trifluoromethyl)phenyldiazoacetic acid methyl ester to carbenes. N—H bond insertion reactions was mainly dominated by singlet—state carbenes. C—H bond insertion reactions was initiated from triplet carbenes with a potential barrier of 65 kJ/mol.

Conclusion The carbene-type dye based on α-(trifluoromethyl)phenyl diazo ester structure could react with synthetic fibers. The dyed synthetic fibers exhibited remarkable color firmness with the fixation values between 48%-85%, good resistance to dye migration and excellent color fastnesses to soaping, rubbing and sublimation. For polyester fiber, ethylene glycol ester units could react with carbene intermediates through C—H bond insertion reaction, and benzene ring could react with carbene intermediates through addition reaction. For polypropylene fiber, C—H bond insertion reaction was the main reaction type between dyes and fibers. For polyurethane and polyamide fibers, the insertion reaction between carbene intermediates and N—H bonds on fiber macromolecules was the main reaction type.

Key words: diazo compound, disperse dye, synthetic fiber, dyeing property, reaction mechanism