H2O2 concentration
In the one-bath scouring and bleaching of flax roving, the perhydroxyl anions produced by the decomposition of hydrogen peroxide can interact with some components of cellulose to achieve the purpose of discoloration and removal of impurities. If the concentration of hydrogen peroxide is too low, it can not reach the bleaching effect, can not meet the production requirements. If the concentration of hydrogen peroxide is too high, although it can achieve a very high whiteness, but because of its oxidation, the flax fiber will be seriously damaged. Therefore, strict control of the concentration of hydrogen peroxide is one of the key conditions in scouring and bleaching.
Figure 1 shows the effect of H2O2 concentration on whiteness, breaking tenacity, weight loss ratio and capillary effect of flax roving bleached using 2, 4, 6, 8, 10, 12 g/L H2O2, 4 g/L NaOH,4 g/L synthesized non-silicon OBS PAA-Mg༌3 g/L Na2CO3༌and 3 g/L NaHSO3 under 90 ℃ for 60 min. Apparently, with the increase of hydrogen peroxide concentration, the whiteness of flax roving gradually improved and the breaking tenacity gradually decreased (Fig. 1a), the weight loss ratio increased gradually and the capillary effect fluctuated but not much (Fig. 1b). Such a relationship between bleaching properties and H2O2 concentration may be ascribed to increase in the amount of active oxidation species when hydrogen peroxide concentration increased (Yu, et al. 2018). However, it was also pointed out that the whiteness and weight loss ratio of the flax roving remained almost constant when the concentration of H2O2 was over 8 g/L, but the breaking tenacity decreased greatly. This may be associated with the excessive decomposition of H2O2, which contributed little to the bleaching result, on the contrary, the flax fiber was severely damaged. Therefore, the concentration of hydrogen peroxide should be selected about 8 g/L.
NaOH concentration
In one-bath scouring and bleaching process of flax roving, alkali is another important factor, which helps to the removal of impurities such as pectin, nitrogen containing and waxes. And the lignin attached to the surface of the fibers will be partially sulfonated to remove by sodium bisulfite under the presence of strong alkali. At the same time, the addition of alkali can ensure that the working liquid remains within a certain pH value range, promoting H2O2 decomposition to produce more active components, in order to remove impurities and improve the whiteness.
Figure 2 shows the effect of NaOH concentration on whiteness, breaking tenacity, weight loss ratio and capillary effect of flax roving bleached using 8 g/L H2O2, 3, 4, 5, 6, 7, 8 g/L NaOH, 4 g/L PAA-Mg, 3 g/L Na2CO3, and 3 g/L NaHSO3 under 90 ℃ for 60 min. As can be seen from Fig. 2, with the increase of sodium hydroxide concentration, the two indexes of whiteness and breaking tenacity of the bleached flax roving showed opposite trends, the whiteness increased gradually while the breaking tenacity decreased gradually (Fig. 2a), the weight loss ratio and capillary effect of flax roving increased with the increase of sodium hydroxide concentration (Fig. 2b). The reason was that with the increase of concentration of sodium hydroxide, the impurities which influenced whiteness and absorption of flax were removed more thoroughly, fibrous tissue became uniform, hygroscopicity enhanced, so capillary effect and whiteness gradually improved. But when pH value increased to a certain degree, the decomposition of hydrogen peroxide was intense, and the interaction with the fiber was enhanced, caused the fiber damage, so led to the fiber strength decreased obviously. Therefore, considering the influence of alkali concentration on the whiteness and breaking tenacity, as well as the requirements of the later spinning process, the sodium hydroxide concentration should be controlled at about 5 g/L.
NaHSO3 concentration
Treatment time can affect bleaching effect of the flax roving, if the bleaching time is too short, the impurities will be not removed thoroughly, and if bleaching time is too long, although can achieve excellent whiteness, but the extension of time will increase the reaction of hydrogen peroxide and cellulose in the fiber, and even cause fiber damage. In addition, it will cause a waste of energy. Figure 4 shows the effect of time on whiteness, breaking tenacity, weight loss ratio and capillary effect of flax roving bleached using 8 g/L H2O2, 5 g/L NaOH,4 g/L PAA-Mg༌3 g/L Na2CO3༌and 3 g/L NaHSO3 under 90 ℃ for 15, 30, 45, 60, 75, 90 min.
As can be seen from Fig. 4, with the prolonging of bleaching time, the weight loss ratio and capillary effect of the flax roving increased continuously, the reason was that the impurities in the flax roving were gradually removed, the crystallinity of the fiber became uniform, and the moisture absorption was enhanced. It was also possible that the fibers have been excessively damaged and the weight loss ratio continued to increase (Fig. 4b). The whiteness of flax roving improved obviously with prolonging of bleaching time, but after bleaching 60 min, and the whiteness increased not obviously, on the contrary, the breaking tenacity of the fiber decreased significantly (Fig. 4a). The main reason was that the effective bleaching components in the working solution reacted more thoroughly with the impurities in the fiber with the bleaching time increase, at the same time, the damage degree of the fibers increased either. However, after reached a degree similar to saturation, even if the bleaching time was extended, the bleaching effect would not be significantly improved, but only the consumption increased. Therefore, considering the bleaching effect and production cost, the bleaching time should be 60 min.
Bleaching temperature
It is very important to control the bleaching temperature of flax roving to achieve good bleaching effect. Figure 5 shows the effect of time on whiteness, breaking tenacity, weight loss ratio and capillary effect of flax roving bleached using 8 g/L H2O2, 5 g/L NaOH, 4 g/L PAA-Mg,3 g/L Na2CO3༌and 3 g/L NaHSO3 under 60, 70, 80, 90, 95, 100 ℃ for 60 min.
As can be seen from Fig. 5, when the bleaching temperature varies in the range of 60 - 100 ℃, with the increase of the bleaching temperature, the whiteness (Fig. 5a), weight loss ratio and capillary effect (Fig. 5b) of flax roving increased continuously, but its breaking tenacity decreased obviously (Fig. 5a). This was mainly because higher temperature helped the decomposition of hydrogen peroxide, and the produced effective bleaching components reacted with the impurities in the flax roving, so that the impurities were removed thoroughly. But above 90℃, the hydrogen peroxide decomposed so fast that its active component HOO- was released into the air before it could interact with the impurities, making the increase in whiteness less noticeable. The decrease in breaking tenacity indicated that the fibers had been damaged. Considering the bleaching effect and production cost, the bleaching temperature should be at about 90 ℃.
PAA-Mg concentration
The synthesized non-silicon OBS PAA-Mg is a product of oligomer acrylic acid compounding with magnesium salt, which can effectively inhibit the catalysis of metal ions, thereby inhibiting the ineffective decomposition of hydrogen peroxide. The purpose of using this stabilizer is to solve the silicon scale problem caused by traditional stabilizer sodium silicate. Fig. 6 shows the effect of time on whiteness, breaking tenacity, weight loss ratio and capillary effect of flax roving bleached using 8 g/L H2O2, 5 g/L NaOH, 1, 2, 3, 4, 5, 6 g/L PAA-Mg, 3 g/L Na2CO3, and 3 g/L NaHSO3 under 90 ℃ for 60 min.
As can be seen from Fig. 6, with the increase of PAA-Mg concentration, the whiteness increased gradually, the breaking tenacity decreased first and then increased slightly (Fig. 6a). The weight loss ratio of the bleached flax roving decreased gradually, and the decomposition rate of H2O2 decreased gradually (Fig. 6b). The reason was that when the concentration of PAA-Mg was lower, it also had less stabilizing effect on hydrogen peroxide, and the violent decomposition of H2O2 were not controlled, so that some effective component escaped into air without time to interact with the impurities, and the good bleaching effect was not achieved. With the increase of PAA-Mg concentration, it also increased the stability of H2O2 decomposition, the active component reacted with the impurities in the fiber, and achieved the best bleaching effect. When the concentration of PAA-Mg was too high, which over-inhibited the decomposition of H2O2, thus the whiteness reduced and the breaking tenacity increased slightly. Therefore, the concentration of PAA-Mg should be about 5 g/L.
Orthogonal optimization of application process of non-silicon OBS PAA-Mg
Based on the influences of single factor on bleaching effects of the flax roving, the optimal scouring and bleaching process of the synthesized non-silicon OBS PAA-Mg was determined by orthogonal experiment of L9 (34). Table 1 shows the orthogonal experiment and range analysis.
Table 1 Orthogonal experiment and results
No.
|
A (Temperature)
℃
|
B (H2O2 Con.)
%
|
C (NaOH Con.)
%
|
D (OBS Con.)
g/L
|
Whiteness
%
|
Breaking tenacity
cN/tex
|
1
|
85 (1)
|
7.5 (1)
|
4.5 (1)
|
4.5 (1)
|
64.29
|
60.65
|
2
|
85 (1)
|
8.0 (2)
|
5.0 (2)
|
5.0 (2)
|
66.35
|
54.10
|
3
|
85 (1)
|
8.5 (3)
|
5.5 (3)
|
5.5 (3)
|
66.81
|
62.60
|
4
|
90 (2)
|
7.5 (1)
|
5.0 (2)
|
5.5 (3)
|
65.54
|
57.65
|
5
|
90 (2)
|
8.0 (2)
|
5.5 (3)
|
4.5 (1)
|
65.93
|
60.95
|
6
|
90 (2)
|
8.5 (3)
|
4.5 (1)
|
5.0 (2)
|
66.75
|
55.15
|
7
|
95 (3)
|
7.5 (1)
|
5.5 (3)
|
5.0 (2)
|
63.70
|
59.70
|
8
|
95 (3)
|
8.0 (2)
|
4.5 (1)
|
5.5 (3)
|
66.49
|
63.40
|
9
|
95 (3)
|
8.5 (3)
|
5.0 (2)
|
4.5 (1)
|
67.11
|
56.00
|
k1
|
65.82
|
64.51
|
65.84
|
65.77
|
|
|
k2
|
66.07
|
66.26
|
66.33
|
65.60
|
|
|
k3
|
65.77
|
66.89
|
65.48
|
66.28
|
|
|
R
|
0.30
|
2.38
|
0.85
|
0.68
|
|
|
k1′
|
59.12
|
59.33
|
59.73
|
59.20
|
|
|
k2′
|
57.92
|
59.48
|
55.92
|
56.32
|
|
|
k3′
|
59.70
|
57.92
|
61.08
|
61.22
|
|
|
R′
|
1.78
|
1.56
|
5.16
|
4.90
|
|
|
It can be seen from Table 1, for the whiteness, the order of primary and secondary factors was B﹥C﹥D﹥A, the optimization scheme was A2 B3 C2 D3. For the breaking tenacity, the order was C﹥D﹥A﹥B, the optimization scheme was A3 B2 C3 D3. Although the analysis of the two indexes appeared different optimization process, but considering the influence of the evenness of flax roving on the test results, as well as the specific requirements of breaking tenacity in production practice, namely, the strength of the bleached flax roving should be within a reasonable range of proper spinning, and the requirement of whiteness should be appropriately reduced in order to prevent the flax fiber from being seriously damaged. On the basis of the above analysis, combined with the performance of flax fiber and the stability of hydrogen peroxide, the optimal technological conditions for the application of the synthesized non-silicon OBS PAA-Mg to the one-bath scouring and bleaching process of the flax roving were obtained as follows: 30% H2O2 8.5 g/L, NaOH 5 g/L, Na2CO3 3 g/L, NaHSO3 3 g/L, PAA-Mg 5.5 g/L, scoured and bleached at 90 ℃ for 60 min.
The optimal process determined by orthogonal experiment was used for verification experiments, and the properties of the samples were determined. The results showed that the whiteness was 68.75%, and the breaking tenacity was as high as 113.4 cN/tex, which was much higher than any one of the results on single factor and orthogonal experiments, so the optimized process was feasible.
Comparison of application effects of different stabilizers
The performance of the treated flax roving using various stabilizers was tested, and the results were shown in Table 2.
Table 2 Comparison of application effects of different stabilizers
Stabilizer
|
Whiteness (%)
|
Capillary effect (cm/30 min)
|
Breaking tenacity (cN/tex)
|
Sodium silicate
|
68.53 ± 0.20
|
14.4 ± 0.1
|
101.2 ± 6.7
|
DM-1403
|
67.39 ± 0.18
|
14.3 ± 0.2
|
109.5 ± 7.4
|
PAA-Mg
|
68.75 ± 0.25
|
15.4 ± 0.2
|
113.4 ± 7.8
|
As can be seen from Table 2, the application performance comparison of the three stabilizers in scouring and bleaching one-bath process to flax roving shows that the synthesized non-silicon OBS PAA-Mg is the best no matter the whiteness, the capillary effect or the breaking tenacity. DM-1403 is one of the substitutes to sodium silicate, which is a high molecular polymer containing hydroxyl, carboxyl and thiamine group. It is a non-silicon oxygen bleaching stabilizer that integrates micelle adsorption, inter-molecular and intra-molecular chelation. PAA-Mg belongs to the macromolecule anion stabilizer, the main component carboxylate radical of the poly(acrylic acid) molecule can chelate to the metal cation, and the component magnesium salt can easily form magnesium hydroxide colloid in the alkaline medium, the metal ions in solution mainly exist in the form of complex ions with negative charge, such as hydroxide. The complex ions are also easily adsorbed by magnesium hydroxide micelles and lose their catalytic activity. Most of the complex ions will be removed in the washing process and will not adhere on the fiber surface, so the treated flax roving has high capillary effect and good feel.
XRD of unbleached and bleached flax roving
The X-ray diffraction pattern was used to characterize the difference in crystal structure of flax roving before and after bleaching with different stabilizers. It can be seen from Fig. 7 that the X-ray diffraction curves and diffraction peaks of flax roving treated with different stabilizers are very similar to the sample untreated. The crystalline structure of flax cellulose is cellulose Ⅰ, the diffraction peaks of (100) plane and (010) plane appear at 14.83 and 16.04, respectively, and the diffraction peak of (110) plane emerge at 22.68 (French 2014, 2020). This is the basis for conclusion that the bleached flax roving has not changed the crystal structure of the original cellulose, and the skeleton structure of the fibers was not damaged. The differences between the samples are that the intensity of all diffraction peaks in bleached flax roving is slightly weakened. The possible reason is that the flax fiber has been damaged to different degrees by the scouring and bleaching with various stabilizers, which leads to the slightly decrease of the crystallinity of flax fiber. It is concluded that the bleached flax fibers basically preserve natural crystal regions distribution.
SEM of unbleached and bleached flax roving
SEM examination of the flax roving was made with S-3400 Scanning Electron Microscope by Hitachi Co. (Japan). SEM images of the samples unbleached and bleached with different stabilizers were shown in Fig. 8. The conclusion is that the surface of flax fibers has changed significantly, the surface of the flax fiber became relatively clean and smooth, and the impurities were mostly removed, but the smoothness by using different stabilizers was inconsistency. There were lots of granular solids on the fiber surface (Fig. 8a), which may be the colloidal impurities existed in the flax fiber itself. The surface of the flax fiber bleached using traditional stabilizer sodium silicate was almost no granular solids (Fig. 8b), it implies that the impurities have been removed completely. Stabilizer DM - 1403 was an alternative product of sodium silicate, the surface of flax fiber was smooth and the impurities were removed thoroughly (Fig. 8c). The surface of flax fiber bleached using this synthesized PAA-Mg as stabilizer was smooth and the impurities were also removed completely (Fig. 8d). It showed that the synthesized non-silicon OBS PAA-Mg can replace sodium silicate and achieve the application effect of other substitutes.