Abstract:

Objective Flax should be degummed before spinning using cotton spinning system. The degumming process is to remove pectin, hemicellulose, lignin, and other non-cellulosic substances from the raw flax. The whiteness of flax fiber obtained by the conventional alkali degumming process is low, and bleaching is a necessary step. The oxidative degumming process is easy to cause cellulose degradation, which leads to the decrease of fiber breaking strength. Meanwhile, residual oxidized lignin after degumming tends to reduce fiber elongation because of its high lignin rigidity.

Method N-Hydroxy-3,4,5,6-tetraphenylphthalimide (NHTPPI)catalytic oxidation with alkali boiling in one bath was adopeed to degum flax. The NHTPPI catalytic oxidation system selectively oxidizes primary hydroxyl groups at the C6 position of cellulose. This feature was utilized to achieve effective inhibition of the oxidative degradation of cellulose macromolecular chains while removing non-cellulosic material, bleaching fibers, and oxidizing lignin. Finally, the benzene ring and side chain of oxidized lignin were dissolved in sodium hydroxide solution.

Results Single-factor tests were carried out on the concentration of NHTPPI, 9,10-anthraquinone and hydrogen peroxide, pH value, reaction temperature and sodium hydroxide. The results of the single-factor tests showed that the optimum concentrations of hydrogen peroxide was 9 g/L, the reaction temperature was 80 ℃, the pH value was 10.5, the amount of catalyst NHTPPI was 0.6 g/L, the concentrations of co-catalyst 9,10-anthraquinone was 0.5 g/L and the concentrations of sodium hydroxide was 5 g/L. Influences of the factors on the flax fiber breaking strength and whiteness after degumming were then investigated using SPSS statistical analysis software. The results showed that the concentrations of hydrogen peroxide had an extremely significant effect on the flax fiber breaking strength and whiteness. The reaction temperature had a highly significant effect on flax fiber breaking strength and whiteness, and the sodium hydroxide concentration had a significant effect on flax fiber breaking strength and whiteness. It was found that the pH had no significant effect on flax fiber breaking strength and whiteness, the NHTPPI concentration had no significant effect on flax fiber breaking strength and whiteness, and the 9,10-anthraquinone concentration had no significant effect on flax fiber breaking strength and whiteness. A preliminary single-factor test was conducted to determine the ranges of these significant variables, that hydrogen peroxide concentration was in the range of 6-12 g/L, reaction temperature was in the range of 60-100 ℃ and sodium hydroxide concentration was in the range of 2-8 g/L. Their interaction effects and optimization of the reaction conditions were investigated using a central combined design CCD. Response surface analysis was carried out using Design-Expert to obtain the optimum process for flax degumming by NHTPPI catalytic oxidation and alkali boiling in one bath. The results showed that the optimum process for the catalytic oxidation of flax and the degumming in one bath by alkali boiling was as follows: NHTPPI concentration of 0.6 g/L, 9,10-anthraquinone concentration of 0.5 g/L, pH value of 10.5, hydrogen peroxide concentration of 10.35 g/L, reaction temperature of 83.6 ℃, sodium hydroxide concentration of 5.67 g/L. SEM, FT-IR and ¹³C NMR characterizations showed that after catalytic oxidation with alkali boiling in one bath, most of the non-cellulose components in the flax fiber were removed, the surface was smooth, carboxyl groups were generated on the fiber and only primary hydroxyl groups at the C6 position of flax were selectively catalytically oxidized, which could effectively inhibit the oxidative degradation of the cellulose macromolecular chain and enhance the fiber breaking strength. Compared with the three processes of catalytic oxidation with alkali boiling in one bath degumming, oxidative degumming with sodium periodate and conventional alkali degumming with hydrogen peroxide bleaching, catalytic oxidation with alkali boiling in one bath degumming of flax fiber had the highest breaking strength and the shortest reaction time. The length of fibers obtained by the three processes was about 28 mm, and the whiteness was all over 70%.

Conclusion The average breaking strength of the refined flax fiber prepared in repeat validation tests using the optimum process was 4.39 cN/dtex and the whiteness was 70.53%, which are very close to the predicted values of the CCD model (4.38 cN/dtex, 71.25%). The results show that the CCD model has good predictability for NHTPPI catalytic oxidation and alkali boiling one bath degumming process, and the fine of flax obtained conforms to the national standard. The process has a broad application prospect in the field of flax degumming.

Key words: flax, pretreatment, catalytic oxidation, alkali boiling, degumming