The effects of 4,6-Dichloro-2-Sodiooxy-1,3,5-Triazine on the Fibrillation Propensity of Lyocell Fibers

ABSTRACT Dichlorotriazine reactive crosslinker has been used to reduce the fibrillation of Lyocell. The fibrillation propensity and mechanism of 4, 6-dichloro-2-sodium dioxy-1, 3, 5-triazine treatment on Lyocell fibers were studied, and the mechanical properties and dyeing properties of the treated Lyocell fibers were evaluated. The results showed that the three-step reaction process can make lyocell have more uniform behavior of decreased fibrillation propensity, in which the first step is physical mixing of triazine and cellulose, the second and third steps are the stepwise reactions of cellulose with two chlorines of Dichlorotriazine under different conditions, respectively. At the end of the reaction, non-fibrillating Lyocell fibers were produced. In addition, the structure, mechanical properties and dyeing properties of the Lyocell fibers were not affected by the cross-linking chemical reaction process. In conclusion, this study provides a better uniform effect and economically reasonable route for the preparation of commercial non-fibrillating Lyocell fibers.


Introduction
Lyocell fibers are claimed as green fiber with a good application prospect in the 21 st century.It is spun from cellulose solution in N-methylmorpholine-N-oxide system with a dry-jet wet spinning.(Jiang et al. 2020;Poongodi et al. 2021).
However, the main problem that restricts the further promotion of Lyocell fibers is its obvious fibrillation properties.Lyocell fibers crystal units form a highly oriented arrangement by dense stacking manner (Gerhard et al. 2013;Okugawa, Yuguchi, and Yamane 2021;Sawada et al. 2021), which led to the crystal units have a strong binding force in the longitudinal direction, but weak transverse force (Zhang, Okubayashi, and Bechtold 2005).When the fiber is influenced by frictional force, the crystallites or microfibrils slide past each other in the longitudinal direction and produce fibrillation tendency (Schurz and Lenz 1994).In addition, small molecules could promote this process such as water and ethanol (Okugawa, Yuguchi, andYamane 2020, 2021).
Process control is hard to solve the fibrillation problem of Lyocell fibers effectively (Mi et al. 2015;Mortimer and Péguy 1996).Cellulase and alkali pre-treatment were used to decrease the fibrillation of Lyocell fibers (Natarajan, Rajan, and Subrata 2022;Periyasamy 2020).Nevertheless, chemicals are used to discard microfibrils of Lyocell in this process (Natarajan, Rajan, and Subrata 2022).Therefore, it will not change the fibrillation propensity of Lyocell fibers permanently.
The most mature way to solve this problem is to form covalent bonds between cellulose molecules through cross-linking (Bates et al. 2006;Bates, Phillips, and Renfrew 2007;Natarajan, Rajan, and Subrata 2022;Perepelkin 2007).For example, Tencel-A100 and Tencel-LF are produced by this technology (Taylor 2016).
However, there are a few problems with all kinds of cross-linking fibers.The fibers produced by acrylamide have defects such as excessive formaldehyde content (Faizan et al. 2018;Jaturapiree et al. 2011;Petersen 1987), and this kind of reactive crosslinker has potential toxicity (Bates, Phillips, and Renfrew 2007), which has been gradually eliminated.Resin reactive crosslinker will cause problems such as the degradation of fabric performance (Faizan et al. 2018;Petersen 1987).Dichlorotriazine reactive crosslinker also had problems such as nonuniform non-fibrillating propensity.
In this paper, the fibrillation propensity and mechanism of Lyocell fibers by 4,6-Dichloro-2-sodiooxy-1,3,5-triazine was studied systematically to mitigate its production cost and enhance the uniformity of decreased fibrillation propensity.Firstly, the effect of different technologies on the fibrillation propensity was studied.Then, a new cross-linking process was suggested by studying the reaction mechanism and optimizing the process parameters.Finally, non-fibrillating Lyocell fibers were continuously prepared on the beltline, which has the characteristics of low cost, good uniformity, excellent mechanical properties, etc.It provides important reference value for the industrialization of non-fibrillating Lyocell fibers.

Preparation of non-fibrillating Lyocell fibers
The non-fibrillating Lyocell fibers were prepared by using the self-made production test line (Figure 2).The fibers pass through the hot roll and the running speed of the fibers is 40 m/min.Steam is introduced to prevent the moisture on the fibers from evaporating.Setting temperature sensors to control the fibers temperature through the hot rolling temperature.
Depending on the pH value of the experiment, sodium hydroxide, sodium carbonate or sodium bicarbonate was mixed with 1% 4,6-Dichloro-2-sodiooxy-1,3,5-triazine solution.And then it is added to the Lyocell fibers in a soaking ratio of 1:5.
An accurately control regarding the parameter of pH value, reaction time and reaction temperature, carry out of the cross-linking reaction is compulsory: The pH value is controlled by the content of sodium hydroxide and other reagents in the added solution.
The reaction time is controlled by the running speed and the winding distance of the fibers on the roller.
The reaction temperature is controlled by hot rolling temperature.

Wet abrasion resistance measurement
The propensity to fibrillate was determined by the time of wet abrasion resistance (Bates et al. 2006;Bates, Phillips, and Renfrew 2007).The basic principle of this method is that rub a single fiber constantly under wet conditions, and the wear of time is different for different fibrillation propensity of Lyocell fibers.A higher time indicates a lower fibrillation propensity.As shown in Figure 3.The fibers are stretched under a certain weight and are constantly rubbed with a roller.A layer of cotton cloth is uniformly wound around the surface of the roller to provide friction.The time needed to break the fibers, that is the time of wet abrasion resistance.

FTIR analysis
Use Nicolet-10 Fourier transform infrared spectrometer to identify the infrared spectrum of the fibers.The IR spectra was scanned over the wave number range of 4000-500 cm −1 .

XPS analysis
XPS spectra were measured by X-Ray Photoelectron Spectroscopy (ESCALAB 250, USA).The samples were irradiated with monochromatic Al K Alpha (100 eV) using a spot size of 500 µm × 500 µm.In addition, high-resolution scan XPS spectra of N 1s and Cl 2p were recorded with pass energy of 30 eV, and the Energy Step Size was 0.100 eV, from which the surface chemical compositions were obtained.To ensure reproducibility, the samples were analyzed in duplicate or triplicate and data analysis was performed using software for the equipment.

XRD analysis
X-Ray diffraction patterns of Lyocell fibers were measured by a reflection method and recorded on X-ray diffraction apparatus (Siemens-Bruker D5000, Germany) using a Cu Kα radiation.Scattered radiation was detected in the range of 2θ = 5-60° at a scan step size of 0.05°.

Tensile mechanical properties
The mechanical properties of the fibers were evaluated by XQ-1 tensile tester (Donghua University, China).The test length was 20 mm, tensile speed is 5 mm/min.To ensure reproducibility, mean test value results of 50 samples were taken.

Analysis of fibers' morphology by SEM analysis
The apparent morphology of the Lyocell fibers were verified by SEM (S-4700, Japan).The fiber surfaces were coated with approximately 20 nm of copper to make samples more conductive and suitable for SEM analysis.The SEM was operated using 10 kV.

Dying
Lyocell fibers were dyed in a Dyeing Machine (HBC-24, China) with 0.5% on a mass of fibers (o.m.f.) of the specified dye and at a liquor-to-goods ratio of 30:1 following commercial conditions recommended by the dye manufacturers.Fibers were initially immersed in liquor containing the dye and 20 g/L sodium sulfate before raising the temperature to 30°C and further running the machine at this temperature for 15 min.Ten grams per liter sodium carbonate was then added, and raise the temperature to 60°C at the rate of 1°C/min.And the dyeing continued for another 60 min.The fibers were then removed from the dye bath and rinsed thoroughly in deionized water prior to after-soaping, final rinsing and air drying.
For all application methods, at the end of the dying and soaping, the fibers were removed from the bath and the absorbance of the liquid measured at the wavelength of the maximum absorption.The substantivity (S) and fixation (F) of the compound for dried substrate was then calculated by the reference method (Phillips, Reisel, and Renfrew 2008).

The effect of production process on the fibrillation propensity of Lyocell fibers
The chemical reaction between dichlorotriazine and cellulose is a nucleophilic substitution reaction.The first step of the reaction is the nucleophilic addition of the hydroxyl groups of the cellulose to the carbon atoms with the lowest electron cloud density.Due to the high electronegativity of the chlorine atom, the negative charge is concentrated on the carbon adjacent to the chlorine atom.The first step of the reaction will generate a negatively charged intermediate product.The second step of the reaction is that the chlorine atoms leave the intermediate and forms a covalent bond product.Therefore, when one of the chlorine atoms undergoes nucleophilic reaction and is eliminated, the density of the electron cloud of the carbon atom increases.This may make the reaction between another chlorine and cellulose more difficult (Ibbett et al. 2010;Phillips, Reisel, and Renfrew 2008).In addition, the reactivity between dichlorotriazine and cellulose increases with increasing temperature and did not react at low-temperature (Ibbett et al. 2010;Phillips, Reisel, and Renfrew 2008).Therefore, the technological process has an essential effect on fibrillation propensity of Lyocell fibers.There are four common processes as shown in Figure 4(a).
After the first step reaction of process 3, the Lyocell fibers don't produce non-fibrillating propensity.After the second heating reaction, the fibers have decreased fibrillation Figure 4(b), and the uniformity of process 3 is higher than that of process 2 Figure 4(c).This is because the cross-linking reaction does not occur at lower temperature, which can promote uniform distribution of 4,6-Dichloro-2-sodiooxy-1,3,5-triazine on fibers, thus enhance the non-fibrillating propensity.
Process 4 did not exert influences on the fibrillation propensity in the first and second reactions.It produces decreased fibrillation propensity during the final heating stage Figure 4(b).And the decreased fibrillation propensity and uniformity are significantly improved compared with other processes Figure 4(c).The decreased fibrillation propensity of process 4 is better than that process 3, and the degree of confidence is greater than 98% by the One way ANOVA.This is because the gradient heating process avoids the hydrolysis of 4,6-Dichloro-2-sodiooxy-1,3,5-triazine, which helps to improve the definitive decreased fibrillation propensity and uniformity.
Therefore, the most reasonable process is the three-step reaction (Process 4).The first step envisages a tantamount distribution of the 4,6-Dichloro-2-sodiooxy-1,3,5-triazine at lowtemperature to improve the non-fibrillating uniformity of Lyocell.The second step is to generate the first chemical bond between one chlorine in 4,6-Dichloro-2-sodiooxy-1,3,5-triazine with cellulose at medium temperature.The third step is to generate the second chemical bond between another chlorine in 4,6-Dichloro-2-sodiooxy-1,3,5-triazine with cellulose at highly temperature.At the end of the reaction, non-fibrillating Lyocell fibers were produced.This process provides a better uniform effect and economic route for the preparation of commercial non-fibrillating Lyocell fibers.Next, the process steps will be optimized.

Effect of different conditions on the Lyocell fibrillation propensity
For the reaction process of process 4 in the preceding text, the reaction conditions were optimized in this section.

Effect of reaction temperature on the Lyocell fibrillation propensity
The reaction temperature will affect the reactivity between Dichlorotriazine and cellulose and determine the ultimate non-fibrillating propensity and uniformity.
The influence of temperature in the first step on the non-fibrillating propensity and uniformity as shown in Figure 5(a).In the range of 20-50°C, there is no significant difference in the final nonfibrillating propensity at different temperatures.However, the excessively high temperature will reduce the uniformity of non-fibrillating considerably.This is because the increase in temperature will lead to the early chemical reaction when the 4,6-Dichloro-2-sodiooxy-1,3,5-triazine has not been evenly distributed, resulting in a significant decline in non-fibrillating uniformity.
Following consideration of the energy consumption, the optimal reaction temperatures of the first step, the second reaction and the third step are 30°C, 50°C and 90°C, respectively.

Effect of reaction pH value and reaction time e on the Lyocell fibrillation propensity
In general, the increase of pH value will increase the amount of ionized cellulose and the reactivity of cross-linking reaction, but if the pH value is too high, that will promote fibrillation of Lyocell fibers.The first step reaction mainly takes place in the process of uniform distribution of 4,6-Dichloro-2-sodiooxy-1,3,5-triazine, and the pH value of the system has no significant effect on this process.Therefore, the pH values of the second and third reactions were investigated.
Figure 6(a) shows the effect of the pH value of second reaction on non-fibrillating propensity.Under the condition of pH 12.0-13.5, the cross-linking reaction can be completed within 2 min.The non-fibrillating propensity of Lyocell fibers increases firstly and then decreases when pH value increases.Because an increase of pH value will increase the reactivity between Dichlorotriazine and cellulose, but high pH could promote fibrillation of Lyocell.
Figure 6(b) indicates the effect of pH value on the non-fibrillating propensity in the third step reaction.It can be seen that the non-fibrillating propensity increases significantly with the increase of pH value at 12.0-13.5.In addition, the pH value will influence the reaction time.When the pH value is 12, the reaction is quite slow.When the pH value is 12.5, the reaction takes a long time to complete, which is difficult to achieve in practical production.When the pH value is 13 or 13.5, the reaction can be completed in a short time.And there was no significant difference between them.
Following consideration of the energy consumption, the best pH value of the second step reaction is 13, and the best pH value of the third step reaction is 13.

The structure and reaction mechanism of the cross-linking Lyocell in preparation
Non-fibrillating Lyocell fibers were prepared by using the optimum process described previously (Process 4).The optimal reaction conditions of the first second and third steps are 30°C, 50°C (pH = 13, 2 min) and 90°C (pH = 13, 2 min), respectively.In this section, the reaction mechanism was discussed.

FTIR and XPS test
In order to deeply understand the process, the Lyocell fibers state at different stages of the "three-step process" was characterized by FTIR spectroscopy, XPS spectroscopy and XRD test.
As shown in Figure 7(a), the characteristic peaks at 1690 cm −1 are the stretching vibration peak of triazine ring skeleton and the stretching vibration peak of double bond.And there was no new characteristic peak in the FTIR spectra at the first reaction step.It is shown that 4,6-Dichloro- 2-sodiooxy-1,3,5-triazine did not react with cellulose at the first reaction step.In addition, the infrared absorption frequency and the vibration strength of an organic molecule depend on the force constant of its specific chemical bond, while the force constant concerns the way in which the electrons are distributed in the molecule.The induced effect of the chlorine atom changes the force constant of the double bond and increases the vibration frequency.Therefore, the wavelength of the stretching vibration peak of triazine ring and the double bond is higher.
There was a new characteristic peak at 1560 cm −1 during the second step of the reaction.It shows that the cellulose reacts with the first chlorine atom of the 4,6-Dichloro-2-sodiooxy-1,3,5-triazine.At the end of this reaction, the 4,6-Dichloro-2-sodiooxy-1,3,5-triazine eliminates one chlorine atom, leading to a decrease in wavelength.Therefore, the wavelength of stretching vibration peak of triazine ring and the double bond reduced from 1690 cm −1 to 1560 cm −1 .
The characteristic peak of 1560 cm −1 disappeared when the third step reaction was carried out.It shows that the chemical reaction occurs between cellulose with the second chlorine atom of 4,6-Dichloro-2-sodiooxy-1,3,5-triazine.At the end of this reaction, 4,6-dichloro-2-sodium oxy- 1,3,5-triazine eliminated two chlorine atoms, leading to a decrease in the force constant of the double bond, while the wavelength decreases.Therefore, there is no obvious characteristic peak on the FTIR spectrum.
High-resolution scan XPS spectra of N 1s and Cl 2p are shown in Figure 7(b).The phenomenon in Figure 7(b) verifies the conclusion of Figure 7(a).There are chlorine atoms on the Lyocell fibers after the second step reaction.And chlorine atoms on the Lyocell fibers disappear after the third step reaction.Nitrogen is visible on the Lyocell fibers when the second step reaction because of the reaction between 4,6-Dichloro-2-sodiooxy-1,3,5-triazine with cellulose.And the nitrogen content does not vary after the third step reaction.It is worth noting that the signal intensity of the spectrum is weak due to the minimal element content.

XRD test
Figure 8 shows the XRD test consequences of Lyocell fibers at different stages.After calculation, the Crystallinity Index (Crl) did not change significantly in the whole process.This shows that the reaction process occurs on the amorphous region or surface of the crystalline region, which does not change the crystalline structure of Lyocell.

SEM test
The reference method promoted fibrillation of fibers (Mi et al. 2015).SEM test results showed that non-fibrillating Lyocell were obtained after the third step reaction (Figure 9).Before that, Lyocell fibers did not have the capability of decreased fibrillation.

Reaction mechanism
In summary, the first step of the "three-step process" is to distribute the 4,6-Dichloro-2-sodiooxy-1,3,5-triazine at low-temperature, where no chemical reaction has occurred.The second step is to generate the first chemical bond between cellulose and 4,6-Dichloro-2-sodiooxy-1,3,5-triazine at medium temperature.The third step is that cellulose and 4,6-Dichloro-2-sodiooxy-1,3,5-triazine generate a second chemical bond at highly temperature and finally produce non-fibrillating propensity.This process is illustrated in Figure 10.

Mechanical and fibrillation properties
Mechanical properties are one of the main factors restricting the application of Lyocell fibers.Table 1 presents the results of fibers tests of Lyocell treated by 4,6-Dichloro-2-sodiooxy-1,3,5-triazine.As a result of cross-linking the tenacity and elongation of the Lyocell fibers are reduced, but in an acceptable range.In addition, there was no significant differences in the non-fibrillating propensity between this study and the products sold in the market (Lyocell-LF and Lyocell-A100).

Dyeing capacity of non-fibrillating Lyocell fibers
Dyeing performance is part of the main factors restricting the application of Lyocell fibers.Therefore, this section explores the influence of cross-linking process of the dyeing performance of Lyocell fibers.The photos of dyed fibers are presented in Figure 11 Dyeing properties of different Lyocell fibers were estimated by measuring dye substantivity (S) and fixation (F) (Table 2).In addition, fibrillation properties after dyeing were also analyzed (Table 2).
The non-fibrillating ability of the crosslinked fibers will not change significantly no matter what dye is used.The substantivity (S) and fixation (F) (Table 2) indicate that the dyeing capacity of Lyocell and non-fibrillating Lyocell fibers could be an effective commercial treatment.The visual analysis of the dyed samples presented in Figure 11, no differences are observed.This aspect is supported by the values for substantivity (S) and fixation (F).It is shown the crosslinking reaction process does not affect the dyeing performance of the Lyocell fibers.In addition, there were no significant differences in the dyeing capacity between the fibers treated according to the procedure presented within the study and the products sold in the market (Lenzing Lyocell LF).

Conclusion
In order to improve the non-fibrillating propensity and uniformity of non-fibrillating Lyocell fibers, the influence of cross-linking process, reaction mechanism and parameter optimization was studied in this article.The cross-linking process has a significant effect on the non-fibrillating propensity and uniformity of Lyocell fibers.The direct heating process will result in a significant decline in the nonfibrillating propensity of Lyocell fibers.The low temperature process can promote uniform distribution of 4,6-Dichloro-2-sodiooxy-1,3,5-triazine to improve the uniformity of nonfibrillating Lyocell fibers.
Uniform and excellent non-fibrillating Lyocell fibers can be prepared by a three-step reaction process.The FTIR and XPS test results showed that the 4,6-Dichloro-2-sodiooxy-1,3,5-triazine was evenly distributed on the fibers at low temperature.The 4,6-Dichloro-2-sodiooxy-1,3,5-triazine and cellulose form the first and second chemical bonds at medium and highly temperatures, respectively.And non-fibrillating Lyocell fibers are produced after the formation of the second chemical bond.In addition, the chemical reactions occur on the surface of the crystalline region or amorphous region of the cellulose.The chemical reaction process does not change the structure of Lyocell fiber and it has little effect on the mechanical properties and dyeing capacity of the Lyocell fibers.
Overall, this study provides a better process for the preparation of non-fibrillating Lyocell fibers.

Figure 3 .
Figure 3. Test method of wet abrasion resistance.
Figure 4(b) shows the change curve of fibrillation propensity of Lyocell fibers under different processes.And Figure 4(c) shows the final fibrillation propensity of fibers by different processes.

Figure 4 .
Figure 4. Comparison of different process routes: (a) Temperature rise curve; (b) Change curve of fibrillation propensity; (c) Final fibrillation propensity.

Figure 5 .
Figure5.Effect of reaction temperature on non-fibrillating propensity: (a) First step reaction; the temperature of the first step is variable; the temperature of the second step is 50°C, the temperature of the third step is 90°C; (b) Second step reaction; the temperature of the first step is 20; the temperature of the second step is variable, the temperature of the third step is 90°C; (c) Third step reaction; the temperature of the first step is 20; the temperature of the second step is 50°C, the temperature of the third step is variable.

Figure 6 .
Figure 6.Time varying curve of non-fibrillating propensity under different pH: (a) the second step reaction; (b) Third step reaction.(The temperature of the different step is 30, 50 and 90°C, respectively).

Figure 9 .
Figure 9. Microscopic photographs of Lyocell after fibril induction for 30 min in 1 g/l NaOH solution; (a) after the first step; (b) after the second step, and (c) after the third step.

Table 1 .
Mechanical properties and fibrillation properties for different Lyocell.