A method for the cyclic dyeing of flame-retardant viscose fibers with isomeric hexadecanes as medium

Abstract

The present application belongs to the field of textile chemistry and dyeing and finishing engineering, and particularly relates to a kind of flame-retardant viscose fiber and its alkane medium circulation dyeing method.The method comprises the following steps: S1, alkali pretreatment step: the flame-retardant viscose fiber is immersed in the alkali solution with a concentration of 10-20 g / L for treatment, S2, dyeing step: the flame-retardant viscose fiber treated in S1 is placed in a dyeing system containing isohexadecane and reactive dyes for dyeing, the amount of isohexadecane is 40-60 mL / g fiber, S3, soaping step;S4, medium circulation step.The present application solves the technical problems of the existing reactive dye water bath dyeing process applied to flame-retardant viscose fiber, such as the decline of fiber flame-retardant performance due to strong alkali conditions, the serious pollution of a large amount of high-salinity wastewater, and the difficulty in recycling water medium.

Description

Technical Field

[0001] This invention belongs to the field of textile chemistry and dyeing and finishing engineering, and specifically relates to a method for cyclic dyeing of flame-retardant viscose fibers and their alkane media. Background Technology

[0002] Flame-retardant viscose fiber is a high-performance, environmentally friendly fiber that combines the excellent moisture absorption, soft hand feel, and comfort of viscose fiber with special flame-retardant properties. It has broad application prospects in protective clothing for fire protection, metallurgy, and power industries, as well as in home textiles. Reactive dyes, due to their complete color spectrum, bright colors, and excellent wet fastness, are the preferred dyes for dyeing cellulose fibers. Therefore, achieving efficient and high-quality dyeing of flame-retardant viscose fiber with reactive dyes is key to expanding its application areas and meeting diverse market demands. However, the traditional water bath dyeing process relied upon by reactive dyes is somewhat incompatible with the characteristics of flame-retardant viscose fiber, posing a significant challenge to achieving this goal.

[0003] Traditional water bath dyeing requires high temperature, high electrolyte (such as sodium sulfate), and strong alkali (such as sodium carbonate) conditions. This severely damages the flame retardants (usually phosphorus or nitrogen-containing compounds) added to the fiber, leading to a significant decrease or even complete loss of the fiber's flame retardant properties, directly endangering the user's safety. Furthermore, the introduction of flame retardants often increases the fiber's hydrophobicity or alters the physical structure of the fiber surface, further hindering the uniform penetration of the water-based dye into the fiber, easily causing defects such as uneven dyeing and color spots. After dyeing, the wastewater, containing large amounts of dissolved salts, alkalis, and residual dyes, has a complex composition. Purification and reuse are extremely energy-intensive and costly, usually requiring direct discharge or simple treatment before disposal, resulting in a significant waste of water resources.

[0004] To address the aforementioned issues, organic solvent-based non-aqueous dyeing technology has become a research hotspot. Among these, isohexadecane, due to its non-polarity, chemical stability, low toxicity, and ease of recycling, has been attempted for application in the dyeing of cellulose fibers. However, existing technologies largely focus on verifying the dyeing effect in a single application. How to achieve efficient and repeated recycling of the dyeing medium while ensuring the dyeing quality of flame-retardant viscose fibers remains a technological gap. The main challenges are: 1) Before dyeing, the fibers need alkali treatment and carry a certain amount of alkali solution to promote dye fixation, but the residue of alkali solution complicates the composition of the recycling medium, increasing the difficulty of purification; 2) After multiple cycles, the accumulation of trace impurities and pigments in the medium may affect the stability of the dyeing effect.

[0005] Therefore, there is an urgent need in this field to develop a new process that can maintain the flame retardant properties of fibers while achieving efficient and recyclable dyeing, so as to promote the widespread application of flame retardant viscose fibers. Summary of the Invention

[0006] This invention aims to solve the technical problems of reduced flame retardant properties of flame-retardant viscose fibers due to strong alkaline conditions and severe pollution from large amounts of high-salt wastewater when the existing reactive dye water bath dyeing process is applied to flame-retardant viscose fibers. It provides a cyclic dyeing method for flame-retardant viscose fibers using isohexadecane to achieve efficient, uniform, and environmentally friendly dyeing of flame-retardant viscose fibers, while also achieving the purpose of media recycling.

[0007] The technical solution adopted by this invention to solve its technical problem is:

[0008] A method for cyclic dyeing of flame-retardant viscose fibers using isohexadecane as a medium, the method comprising the following steps: S1, alkali pretreatment step: immersing the flame-retardant viscose fibers in an alkali solution with a concentration of 10~20 g / L, controlling the alkali content of the fibers to be 100% (owf)~500% (owf), that is, the mass of alkali solution carried by the fibers is 100%~500% of their own oven-dry mass.

[0009] S2. Dyeing Step: The flame-retardant viscose fiber treated in S1 is placed in a dyeing system containing isohexadecane and reactive dye for dyeing. Under the mechanical force of the dyeing machine, the reactive dye is promoted to diffuse into the flame-retardant viscose fiber and fix it. The amount of isohexadecane is 40~60 mL / g of flame-retardant viscose fiber, and the amount of reactive dye is 1% (owf)~5% (owf) of the mass of the flame-retardant viscose fiber. S3, Soaping Step: After completing the dyeing step, the dyed flame-retardant viscose fiber is soaped to remove excess dye; S4, Media Circulation Step: The residual liquid after dyeing in step S2 is collected and allowed to settle to recover the upper layer of isohexadecane; an adsorbent is added to the recovered upper layer of isohexadecane to remove residual impurities and pigments, and then finely filtered through an organic nanofiltration membrane resistant to organic solvents under the transmembrane pressure difference provided by a vacuum pump to obtain isohexadecane; the isohexadecane is used as the medium for the dyeing system in the next dyeing process of step S2.

[0010] In this invention, the reactive dye is dispersed in hydrophobic isohexadecane, while the flame-retardant viscose fiber contains moisture and alkali introduced in the pretreatment step (S1). Due to the high affinity between the aqueous environment inside the fiber and the dye, under external mechanical force, the reactive dye molecules dispersed in the isohexadecane migrate from the nonpolar medium phase to the polar fiber surface driven by the concentration gradient and affinity, and further diffuse into the water-rich amorphous region inside the fiber. Under the pre-existing alkaline conditions inside the fiber, the reactive dye covalently bonds with the cellulose hydroxyl groups, completing the dyeing process.

[0011] The core of this invention lies in its unique media recycling process design: First, the flame-retardant viscose fiber is pretreated to carry a certain amount of alkali. Second, the alkali-containing fiber is placed in a non-aqueous isohexadecane system containing reactive dyes for dyeing. Since the reactive dyes have extremely low solubility in isohexadecane, and the pre-existing water and alkali inside the fiber provide a good environment for the dye to dissolve and react, the dye is transferred from the medium to the fiber surface under the drive of the concentration gradient and diffuses and fixes inward. Then, the dyeing residue is allowed to stand to achieve oil-water separation, and the upper isohexadecane medium is recovered. Activated carbon and other adsorbent materials are added to the recovered medium to remove residual impurities and pigments. Subsequently, under the condition of a transmembrane pressure difference provided by a vacuum pump, it is finely filtered through an organic nanofiltration membrane, and the obtained isohexadecane is used for the next cycle of dyeing.

[0012] In this invention, the isohexadecane dyeing system comprises reactive dyes and isohexadecane. Before dyeing, the flame-retardant fibers are impregnated in an alkaline solution of 10-20 g / L, controlling the alkali content of the fibers to be 100% (owf) to 500% (owf). During dyeing, the amount of reactive dye used is 0.5% (owf) to 5% (owf), and the amount of isohexadecane used is 40-60 mL / g of flame-retardant viscose fiber. Ideally, the amount of isohexadecane used is 40-50 mL / g of flame-retardant viscose fiber, and the amount of reactive dye used is 1% (owf) to 3% (owf).

[0013] Preferably, in step S1, the alkali in the alkaline solution is sodium carbonate, sodium bicarbonate, trisodium phosphate, sodium hydroxide, or a combination thereof, and the concentration of the alkaline solution is 14-16 g / L. More preferably, it is 300% (owf) - 500% (owf). This concentration and dosage achieve good dyeing performance without damaging the fiber structure or its flame-retardant properties.

[0014] Preferably, in step S2, the reactive dye is selected from one or more combinations of Reactive Yellow S-3R, Reactive Red 3BS, and Reactive Red 195.

[0015] Preferably, the soaping solution consists of 3 g / L standard soap flakes and 3 g / L sodium carbonate.

[0016] Preferably, the dyeing temperature in step S2 is 30-100℃ and the time is 10-200 min; the soaping temperature in step S3 is 80-110℃ and the time is 5-60 min. The preferred dyeing parameters are 30-70℃ and 10-50 min (the optimal embodiment is 60℃ and 40 min). In the dyeing process, good dyeing performance is observed within the range of 50℃-70℃, 40-60 min, and 90-100℃ for 10-20 min.

[0017] Preferably, the adsorbent material in step S4 is activated carbon, modified activated carbon, zeolite, or a combination thereof, with activated carbon being the preferred choice. The amount of adsorbent material used is 0.1% to 1% of the isohexadecane after oil-water separation, preferably 0.6% to 0.8%. This amount of activated carbon adsorbent material can effectively remove residual impurities and pigments from the isohexadecane medium.

[0018] Preferably, the fine filtration organic nanofiltration membrane described in step S4 is a polyimide membrane, a polyether membrane, a sulfonated polysulfone membrane, or a combination thereof, preferably a polyimide membrane, and the transmembrane pressure difference provided by the vacuum pump is 0.5~2.0 MPa.

[0019] Preferably, the isohexadecane dyeing system and flame-retardant viscose fiber are placed in the dye cup of an adjustable sampler; the adjustable sampler raises the temperature to the dyeing temperature at a constant rate and holds it during the dyeing and fixing process, while stirring at a constant rate to ensure that the dye is evenly applied to the flame-retardant viscose fiber.

[0020] Preferably, in step S4, oil-water separation and adsorption are carried out at room temperature and pressure, and nanofiltration membrane filtration is carried out at room temperature and under the transmembrane pressure difference provided by a vacuum pump.

[0021] A flame-retardant viscose fiber prepared by the isohexadecane cyclic dyeing method described in this invention has a dyeing uniformity value of less than 2.0 and a limiting oxygen index retention rate of not less than 90% after dyeing. Furthermore, after at least 5 or 10 cycles of use, the K / S value change rate is less than 5%, the uniformity value is less than 2.0, and the color fastness to soap washing is grade 4 or above.

[0022] The beneficial effects of this invention are:

[0023] 1. This invention is carried out in isohexadecane medium, which avoids the dissolution and destruction of flame retardants by large amounts of water, salt and alkali in traditional water bath dyeing, and the flame retardant properties of the dyed fibers are largely preserved.

[0024] 2. Utilizing the property that reactive dyes are readily soluble in water but sparingly soluble in isohexadecane, and the polar hydrophilic surface of flame-retardant viscose fibers, a dye transfer system of "non-aqueous medium-aqueous fiber" was constructed. Driven by concentration gradient and affinity, the dye is efficiently and uniformly transferred from the non-polar medium to the fiber surface, and diffuses and fixes in the presence of trace amounts of alkali solution, realizing the efficient dyeing of flame-retardant viscose fibers by reactive dyes in a non-aqueous system.

[0025] 3. To address the issue of trace amounts of moisture, impurities, and pigments in the residual dyeing solution, a multi-stage purification process of "oil-water separation + adsorption purification + membrane fine filtration" was established. Residual pigments and impurities are removed by activated carbon adsorption, and then small molecular residues are retained by an organic nanofiltration membrane to obtain high-purity isohexadecane, which can be completely recycled for the next dyeing process. This recycling process allows the medium to be reused at least 10 times while maintaining essentially the same dyeing effect, eliminating the generation of dyeing wastewater at the source and truly achieving green and recyclable dyeing of flame-retardant viscose fibers.

[0026] Furthermore, the process principles and methods proposed in this invention are not only applicable to flame-retardant viscose fibers, but also provide a new technical route and cyclic dyeing method for reactive dyeing of other functional cellulose fibers that are sensitive to water and alkali (such as flame-retardant Lyocell fibers, antibacterial fibers, and UV-resistant fibers). Attached Figure Description

[0027] Figure 1 This is a sample image of flame-retardant viscose fiber after being dyed with reactive dyes in isohexadecane medium, as shown in Example 1 of the present invention.

[0028] Figure 2 This is a sample image of flame-retardant viscose fiber after being cyclically dyed with reactive dyes five times in isohexadecane medium, according to Example 1 of the present invention.

[0029] Figure 3 This is a sample image of flame-retardant viscose fiber after being dyed with reactive dyes in an aqueous medium, as shown in Comparative Example 1 of this invention.

[0030] Figure 4 These are SEM images of flame-retardant viscose fibers from Example 1 and Comparative Example 1 after being dyed with reactive dyes in aqueous and isohexadecane media; (a) original fiber; (b) after water dyeing; (c) after isohexadecane dyeing; (d) after 5 cycles of isohexadecane dyeing.

[0031] Figure 5 This is a sample image of Comparative Example 8 of the present invention, which shows flame-retardant viscose fibers dyed five times in isohexadecan medium using reactive dyes.

[0032] Figure 6This is a sample image of Comparative Example 9 of the present invention, which is a pair of flame-retardant viscose fibers dyed 5 times in isohexadecane medium using reactive dyes.

[0033] Figure 7 This is a sample image of Comparative Example 10 of the present invention, after being cyclically dyed 10 times with reactive dyes in isohexadecane medium. Detailed Implementation

[0034] The technical solution of the present invention will be further described in detail below through specific embodiments. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0035] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art.

[0036] Unless otherwise specified, the reagents used in the following examples can be purchased from a regular biochemical reagent store.

[0037] Reactive Yellow S-3R dye was purchased from Huntsman Chemical Trading (Shanghai) Co., Ltd.

[0038] Isohexadecane medium, purchased from Guangzhou Chengyi Chemical Co., Ltd., China;

[0039] Flame-retardant viscose fiber, purchased from Lenzing Fibre Company;

[0040] The standard soap flakes meet the requirements of GB / T 3921 Textiles - Test for color fastness to washing with soap.

[0041] Example 1 (Best Practice):

[0042] This embodiment provides a method for cyclic dyeing of flame-retardant viscose fibers using isohexadecane as a medium. The steps are as follows:

[0043] (1) Alkali pretreatment: Prepare a 14 g / L sodium carbonate solution. Take 1.0 g of flame-retardant viscose fiber and immerse it in 50 mL of the alkali solution for 20 min at 40℃. Take out the fiber and roll it with a small rolling mill, controlling the roll residue (alkali residue rate) to be 300%, that is, the mass of the fiber with alkali residue is 3.0 g.

[0044] (2) Preparation of dyeing working solution: Add 40 mL of isohexadecane and 0.02 g of reactive yellow S-3R dye to the dye cup of the adjustable sampler, turn on the stirrer to make the dye uniformly dispersed in the isohexadecane (the dye exists in the form of fine solid particles) to obtain the dyeing working solution.

[0045] (3) Dyeing: Immerse 1.0 g of flame-retardant viscose fiber in the dyeing working solution in step (2) and dye for 40 min at a temperature of 60 ℃;

[0046] (4) Washing: The flame-retardant viscose fiber dyed in step (3) is placed in 30 mL of soaping solution (3 g / L standard soap flakes and 3 g / L sodium carbonate) and washed at 95 °C for 15 min.

[0047] (5) Purification: The dyeing residue from step (3) was allowed to stand for 30 minutes and then subjected to oil-water separation to recover the upper layer of isohexadecane; 0.05 g of activated carbon was added to the recovered isohexadecane and stirred at room temperature for 30 minutes to remove the pigment and other impurities of the dye; the isohexadecane medium containing activated carbon was subjected to fine filtration through a polyimide membrane under a transmembrane pressure difference of 1.0 MPa provided by a vacuum pump, and the isohexadecane on the permeate side was collected.

[0048] (6) Cyclic staining: The isohexadecane obtained after purification in step (5) is stained again according to steps (1) to (4), and the process is repeated for 5 cycles.

[0049] (6) Testing: Color depth testing, i.e., K / S value, is performed on fibers after the first dyeing and the cycle dyeing. It is used as the main indicator to measure the dyeing effect. The testing method is as follows:

[0050] After dyeing and drying, the maximum K / S value of each sample was measured at 12 points using a Datacolor SF600X colorimeter, and the average value was taken as the final color yield of the sample according to formula (1). The standard deviation of the dyed fabric was calculated based on the measured K / S values, and the unevenness of the dyed fabric was calculated according to formula (2).

[0051] (1)

[0052] (2)

[0053] In the formula: K and S are the absorption coefficient and backscattering coefficient, respectively; R is the reflectivity; λ is the maximum absorption wavelength of the corresponding dye, in nm; n is the number of sampling points; The average color depth of the fiber measured n times; S λ The standard deviation is the smaller the non-uniformity, the better the leveling performance; the larger the non-uniformity, the worse the leveling performance.

[0054] The relevant test results for Example 1 are shown below. Figures 1-3 And Table 1.

[0055] Comparative Example 1

[0056] Unlike Example 1:

[0057] This embodiment uses a traditional water bath staining process, and its staining method is basically the same as that described in Example 1. The difference is that in step (2) of this embodiment, water is used instead of isohexadecane, and the amount used is 40 mL; 1.2 g of Na2SO4 is added after staining at 60 ℃ for 30 min, followed by 1.4 g of Na2CO3 after 30 min. The test results of this embodiment are shown in Table 1.

[0058] Table 1. Dyeing effect of flame-retardant viscose fiber under different systems

[0059]

[0060] Figure 1 , Figure 2 and Figure 3 These are: samples after initial staining, samples after five cycles of a complete purification process, and samples stained using a traditional water bath; from... Figure 1 and Figure 3 As shown in Table 1, compared with traditional aqueous dyeing (Comparative Example 1), the isohexadecane-based dyeing method (Example 1) exhibits superior leveling effect while achieving comparable dyeing depth. The K / S value of Example 1 is 11.416, which is basically consistent with 11.503 in Comparative Example 1, indicating that the non-aqueous system can achieve a dyeing level similar to that of the aqueous medium; its non-uniformity value is 1.652, lower than 1.817 in Comparative Example 1, indicating that the dye is more evenly distributed on the fiber surface and inside. The color fastness to washing for both systems is grade 4-5, indicating that isohexadecane-based dyeing does not affect the dye fixation performance. The limiting oxygen index of Example 1 is 30.54%, which shows a smaller change compared to Comparative Example 1, indicating that this non-aqueous dyeing process has a limited impact on the flame retardant properties of the material. Furthermore, from... Figure 1 and Figure 2 As can be seen, unlike Comparative Example 1, after being recycled 5 times, the K / S value of Example 1 remained at 11.064, and the leveling property was further optimized to 1.534, showing good system stability and recyclability.

[0061] Depend on Figure 4 It can be seen that the SEM images of the fibers after dyeing in Example 1 and Comparative Example 1, after five cycles of medium dyeing, did not have a significant impact on the fiber surface morphology, indicating that isohexadecane has little impact on the structure of flame-retardant fibers. Overall, isohexadecane medium dyeing significantly improves leveling properties and has good potential for recycling, while ensuring dyeing depth, color fastness, and stable fiber surface morphology.

[0062] Comparative Example 2:

[0063] Referring to Example 1, this study discusses the effect of alkali concentration on the cyclic staining of isohexadecane in step (1) alkali pretreatment. Other staining methods and process conditions are the same as in Example 1. The test results of this comparative example are shown in Table 2.

[0064] Table 2. Effect of alkali concentration on cyclic dyeing of flame-retardant viscose fibers

[0065]

[0066] As shown in Table 2, different alkali concentrations significantly affect the dyeing effect of flame-retardant viscose fibers. Within the range of 10–20 g / L, the K / S value initially increases and then fluctuates, with the highest value (15.229) at 12 g / L, indicating that an appropriate amount of alkali is beneficial for improving the reactivity between the dye and the fiber and the dyeing efficiency. At 14–16 g / L, the leveling performance is significantly improved, with the unevenness decreasing to 1.678 and 1.636, indicating a more uniform dye distribution in this range. After five cycles, the K / S values ​​under each concentration condition showed little difference from the initial values, with limited variation, and the leveling performance remained largely consistent, indicating that the system exhibited stable dyeing performance and good repeatability during multiple reuses. Simultaneously, the color fastness to soaping for all samples remained at grade 4–5, indicating that changes in alkali concentration and repeated use did not significantly affect the dye fixation performance. Overall, this system demonstrates good cyclic dyeing stability, and an alkali concentration of 14 g / L is recommended.

[0067] Comparative Example 3:

[0068] Referring to Example 1, this study discusses the effect of the amount of alkali solution in the flame-retardant viscose fiber on the cyclic dyeing of isochetane during step (1) alkali pretreatment. Other dyeing methods and process conditions are the same as in Example 1. The test results of this comparative example are shown in Table 3.

[0069] Table 3. Effect of Alkali Content in Flame-Retardant Viscose Fibers on Cyclic Dyeing with Isohexadecane

[0070]

[0071] Table 3 shows that as the amount of alkali solution on the fiber increased from 100% (owf) to 5% (owf), the K / S value of the flame-retardant viscose fiber significantly increased from 6.909 to 13.068, indicating that appropriately increasing the amount of alkali solution is beneficial to promoting the reaction between the dye and the fiber, improving the dyeing rate and overall color depth. Simultaneously, the levelness of dyeing improved significantly, with the unevenness decreasing from 3.668 to 1.107, indicating that increasing the amount of alkali solution helps the dye to be evenly distributed on the fiber surface and inside. When the amount of alkali solution on the fiber was between 300% (owf) and 500% (owf), both the K / S value and levelness tended to stabilize, indicating that the system reached an optimal dyeing state. After 5 cycles, the K / S values ​​of each group were very close to the initial values, and the change in levelness was also low, with the overall trend consistent with the first dyeing. This indicates that the dyeing performance of this system fluctuated little during multiple reuses, demonstrating good repeatability and stability.

[0072] Comparative Example 4

[0073] Referring to Example 1, the effect of different dye amounts on the cyclic staining of isohexadecane in step (2) of preparing the staining working solution is discussed. Other staining methods and process conditions are the same as in Example 1. The test results of this comparative example are shown in Table 4.

[0074] Table 4. Effect of dye dosage on cyclic dyeing with isohexadecane

[0075]

[0076] Table 4 shows that as the dye concentration increased from 1% (owf) to 5% (owf), the K / S value of the flame-retardant viscose fiber significantly increased from 9.385 to 25.134, indicating that the dyeing depth increased significantly with increasing dye concentration. The K / S value increased most significantly in the 1% (owf) to 3% (owf) range, indicating that the increased dye concentration gradient effectively promoted the adsorption and diffusion of dye on the fiber. When the concentration increased to 4% (owf) to 5% (owf), the increase in K / S value slowed down, reflecting that the binding sites inside the fiber gradually became saturated. Regarding evenness, the unevenness was lowest at 2% (owf) (1.622), exhibiting excellent dyeing uniformity. When the concentration exceeded 3% (owf), the unevenness increased significantly (reaching 3.815 at 5% (owf), indicating that high-concentration dyes are prone to local aggregation or excessively rapid surface adsorption, thus affecting the uniformity of dye distribution. The color fastness to soap washing for all samples was grade 4-5, indicating strong dye fixation. Furthermore, after 5 cycles, the K / S values ​​at each dosage were basically close to the initial values, with minimal variation, and the leveling trend remained consistent, indicating good cyclic stability of the system. Overall, 2% (owf) to 3% (owf) is the optimal dosage range for balancing dyeing depth and leveling.

[0077] Comparative Example 5:

[0078] Referring to Example 1, this study discusses the effect of the amount of isohexadecane used in step (2) of preparing the dyeing working solution on the cyclic dyeing of flame-retardant viscose fibers. Other dyeing methods and process conditions are the same as in Example 1. The test results of this comparative example are shown in Table 5.

[0079] Table 5. Effect of hexadecane dosage on the cyclic dyeing of flame-retardant viscose fibers

[0080]

[0081] Table 5 shows that as the amount of isohexadecane increased from 10 mL to 40 mL, the K / S value of the flame-retardant viscose fiber gradually increased from 7.899 to 11.181, indicating that appropriately increasing the amount of medium is beneficial to improving the dispersion state and mass transfer conditions of the dye in the system, thereby enhancing the dyeing effect. When the amount was further increased to 50 mL, the K / S value decreased to 10.014, indicating that excessive medium may dilute the effective concentration of dye, reduce the concentration gradient between dye and fiber, and is not conducive to the continuous improvement of dyeing depth. In terms of leveling, the unevenness decreased significantly from 3.258 at 10 mL to 1.251 at 50 mL, showing that with the increase of medium amount, the system fluidity is enhanced, the dye distribution is more uniform, and the dyeing uniformity is significantly improved. After 5 cycles, the K / S values ​​of each group were basically close to the initial values, and the trend of leveling was consistent. In particular, the unevenness further decreased after cycling under the conditions of 40-50 mL, indicating that the system has good cycling stability and reuse potential. The color fastness to soap washing for all samples was grade 4-5, indicating that the dye fixation performance was stable. Based on comprehensive analysis, 40 mL is the optimal dosage that balances dyeing depth and levelness.

[0082] Comparative Example 6:

[0083] Referring to Example 1, this study discusses the effect of dyeing temperature on the cyclic dyeing of isohexadecane in step (3). Other dyeing methods and process conditions are the same as in Example 1. The test results of this comparative example are shown in Table 6.

[0084] Table 6. Effect of staining temperature on cyclic staining with isohexadecane

[0085]

[0086] Table 6 shows that dyeing temperature significantly affects the dyeing effect of flame-retardant viscose fibers. As the temperature increases from 30 ℃ to 40 ℃, the K / S value increases significantly to 14.952, but the leveling performance is poor. At 60 ℃, the unevenness decreases to 1.564, exhibiting the best leveling effect. After 5 cycles, the K / S value changes little under each temperature condition, and the leveling performance index remains basically consistent, indicating that the system has good cyclic stability and reliable repeated dyeing under different temperature conditions. All samples showed a washing fastness of 4-5, indicating that cyclic use did not affect the dye fixation performance; the recommended dyeing temperature is 60 ℃.

[0087] Comparative Example 7:

[0088] Referring to Example 1, this study discusses the effect of dyeing time on the cyclic dyeing of isohexadecane in step (3). Other dyeing methods and process conditions are the same as in Example 1. The test results of this comparative example are shown in Table 7.

[0089] Table 7 Effect of staining time on cyclic staining with isohexadecane

[0090]

[0091] Table 7 shows that as the dyeing time increased from 10 min to 50 min, the K / S value generally increased from 9.959 to 13.004, indicating that extending the dyeing time is beneficial for the dye to diffuse fully into the fiber and increase the dyeing depth. Regarding levelness, the unevenness generally ranged from 1.0 to 1.9, with relatively small variations, indicating that the dyeing uniformity was relatively stable under different time conditions. After 5 cycles, the K / S values ​​of each group were similar to the initial values, and the levelness indicators remained basically consistent, showing good repeatability and system stability. Meanwhile, all samples showed a washing fastness of 4-5, indicating that cyclic dyeing did not affect the dye fixation performance.

[0092] Comparative Example 8:

[0093] Referring to Example 1, in the purification process of step (5), the dyeing residue was only separated into oil and water; activated carbon adsorption and fine filtration with a polyimide membrane were not used. Other dyeing methods and process conditions were the same as in Example 1. The comparative test results are as follows: Figure 5 (Sample after 5 cycles of oil-water separation only) and Table 8.

[0094] Table 8. Effect of oil-water separation alone on cyclic staining with isochetane.

[0095]

[0096] Table 8 shows that as the number of dyeing cycles increased from 1 to 5, the K / S value fluctuated between 11.354 and 13.427, without showing a monotonically decreasing trend. However, the levelness of dyeing increased continuously from 1.542 to 3.451. Regarding the color fastness to washing, it gradually decreased from grade 4-5 in the first cycle to grade 3 in the fifth cycle, indicating that without fine purification, cyclic dyeing significantly damages the dye fixation performance. Simple oil-water separation is mainly used to remove the macroscopic aqueous phase, residual alkali, and inorganic salts dissolved in the aqueous phase from the dyeing residue. However, it is difficult to remove trace amounts of pigments, aggregated dyes, dye hydrolysis products, microemulsion droplets, and colloidal impurities remaining in the isohexadecane phase. With increasing cycles, these impurities gradually accumulate in the medium, altering the dye dispersion state and mass transfer process in the subsequent dyeing system, leading to uneven dyeing of fibers and a decrease in the fixation stability of some dyes. Therefore, although the K / S value remained at a certain level after 5 cycles, the levelness of dyeing and the color fastness to soaping deteriorated significantly. This result indicates that oil-water separation can only achieve coarse separation of the recovered medium and cannot meet the requirements for medium purity and compositional stability in multiple cycles of dyeing with isohexadecane media. Further adsorption purification and fine membrane filtration are necessary to suppress the accumulation of trace impurities.

[0097] Comparative Example 9:

[0098] Referring to Example 1, this discussion focuses on the purification process in step (5). In this step, the dyeing residue undergoes only oil-water separation and activated carbon adsorption; other dyeing methods and process conditions remain the same as in Example 1. The comparative test results are as follows: Figure 6 (Sample after 5 cycles of oil-water separation + activated carbon adsorption but without membrane filtration) and Table 9.

[0099] Table 9. Effect of residual liquid without polyimide membrane filtration on isohexadecane cyclic staining.

[0100]

[0101] As shown in Table 9, with the increase in the number of dyeing cycles from 1 to 5, the K / S value fluctuated between 11.226 and 12.417, without showing a significant downward trend, indicating that activated carbon adsorption could remove most of the residual dye. The levelness increased from 1.625 to 2.283, with fluctuations but an overall deteriorating trend, indicating that a small amount of minute impurities remained in the medium and accumulated with each cycle. The color fastness to washing decreased from grade 4-5 in the first and second cycles to grade 4 in the third to fifth cycles, indicating that the dye fixation performance decreased slightly in the later stages of the cycle. Compared with Comparative Example 8, the levelness and color fastness to washing were improved after 5 cycles with the addition of activated carbon adsorption, indicating that activated carbon can adsorb residual pigments, aggregated dyes, and some low-molecular-weight organic impurities in the isohexadecane phase, thereby mitigating the adverse effects of impurity accumulation on dyeing stability. However, activated carbon adsorption is selective and cannot completely remove colloidal dye hydrolysates, microemulsion droplets, inorganic salt particles, and other fine suspended matter. At the same time, a small amount of activated carbon particles may remain during the adsorption process. If these fine particles are not further retained, they will continue to affect the dispersion state of the subsequent dyeing system and the mass transfer process on the fiber surface.

[0102] Therefore, while using only "oil-water separation + activated carbon adsorption" can improve the cyclic dyeing effect, it is still insufficient to guarantee the uniformity and color fastness stability after multiple cycles. Compared with Example 1, omitting organic nanofiltration membrane filtration still results in the gradual accumulation of fine impurities in the system, leading to a decrease in cyclic dyeing stability. This result proves that organic nanofiltration membrane filtration is not a dispensable conventional filtration step in this invention, but a crucial fine purification step used to retain activated carbon particles, colloidal dye hydrolysates, microemulsion droplets, and fine suspended matter.

[0103] As can be seen from Examples 1, 8, and 9, the "oil-water separation + adsorption + organic nanofiltration membrane filtration" of the present invention forms a hierarchical synergistic purification mechanism targeting impurities of different forms: oil-water separation removes macroscopic aqueous phase, alkaline solution, and soluble salts; activated carbon adsorption removes residual pigments and low-molecular-weight organic impurities from the oil phase; and the organic nanofiltration membrane further traps microemulsion droplets, colloidal impurities, and adsorbent particles. These three components work synergistically in sequence, and none can be omitted, in order to ensure that the recovered isohexadecane medium maintains stable dyeing depth, levelness, and wash fastness during multiple dyeing cycles.

[0104] Comparative Example 10:

[0105] Referring to Example 1, this discussion focuses on the cyclic staining process in step (6), where the number of cyclic staining cycles is increased to 10, while other staining methods and process conditions remain the same as in Example 1. The comparative test results are as follows: Figure 7 As shown in Table 10.

[0106] Table 10 Effect of the number of cycles on cyclic staining with isohexadecane

[0107]

[0108] according to Figure 1 and Figure 6 As shown in Table 10 (sample after 10 cycles of complete purification process), the purified isohexadecane exhibits good stability in the cyclic dyeing of flame-retardant viscose fibers. With the number of dyeing cycles increasing from 1 to 10, the K / S value of the fiber remained relatively stable between 12.2 and 12.7, with minimal fluctuation, indicating stable dye adsorption on the fiber and no significant attenuation trend. The evenness index fluctuated between 1.341 and 1.639, also showing that the fiber maintained a uniform coloring effect during multiple dyeing cycles, without significant streaks or color spots. The wash fastness remained at grade 4-5, indicating a tight bond between the dye and the fiber, and that the fiber's color fastness was not affected during the cyclic dyeing process. In summary, the purified isohexadecane medium can support multiple rounds of cyclic dyeing of flame-retardant viscose fibers while ensuring the stability of dyeing depth, uniformity, and color fastness, demonstrating its good applicability and sustainability in recyclable dyeing processes.

[0109] Compared with traditional water bath dyeing, this invention achieves excellent dyeing results while ensuring a salt-free dyeing process, low water consumption, and recycling of the dyeing medium, truly achieving the dual goals of environmentally friendly dyeing and functional preservation.

Claims

1. A method for cyclic dyeing flame-retardant viscose fibers using isohexadecane as a medium, characterized in that, The method includes the following steps: S1, Alkali pretreatment step: The flame-retardant viscose fiber is immersed in an alkaline solution with a concentration of 10~20 g / L, and the alkali content of the fiber is controlled to be 100% (owf)~500% (owf); S2. Dyeing Step: The flame-retardant viscose fiber treated in S1 is placed in a dyeing system containing isohexadecane and reactive dye for dyeing. Under the mechanical force of the dyeing machine, the reactive dye diffuses into and fixes into the flame-retardant viscose fiber. The amount of isohexadecane is 40-60 mL / g fiber, and the amount of reactive dye is 1% (owf)-5% (owf) of the mass of the flame-retardant viscose fiber. S3. Soaping Step: After completing the dyeing step, the dyed flame-retardant viscose fiber is soaped to remove floating dye. S4. Media Circulation Step: The residual liquid after dyeing in step S2 is collected and allowed to settle. The upper layer of isohexadecane is recovered. Adsorbent material is added to the recovered upper layer of isohexadecane to remove residual impurities and pigments. Then, under the transmembrane pressure difference provided by the vacuum pump, it is finely filtered through an organic nanofiltration membrane resistant to organic solvents to obtain isohexadecane. The isohexadecane is used as the medium for the dyeing system in the next dyeing process of step S2.

2. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S1, the alkali in the alkaline solution is selected from one or more combinations of sodium carbonate, sodium bicarbonate, trisodium phosphate, and sodium hydroxide; the concentration of the alkaline solution is 14~16 g / L, and the alkali content of the fiber is controlled to be 300% (owf)~500% (owf).

3. The isohexadecane cyclic staining method according to claim 1, characterized in that, In step S2, the reactive dye is selected from one or more combinations of Reactive Yellow S-3R, Reactive Red 3BS, and Reactive Red 195.

4. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S2, the amount of the reactive dye used is 1% (owf) to 3% (owf) of the mass of the flame-retardant viscose fiber.

5. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S2, the temperature of the dyeing step is 30~100℃ and the time is 10~200min; in step S3, the temperature of the soaping step is 80~110℃ and the time is 5~60min.

6. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S3, the soaping solution used in the soaping treatment contains 3 g / L of standard soap flakes and 3 g / L of sodium carbonate.

7. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S4, the adsorbent material is selected from one or more combinations of activated carbon, modified activated carbon, and zeolite; the amount of adsorbent material added is 0.1% to 1% of the mass of the upper isohexadecane medium recovered after oil-water separation.

8. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S4, the organic nanofiltration membrane is selected from one or more combinations of polyimide membranes, polyether membranes, and sulfonated polysulfone membranes.

9. The method for cyclic staining with isohexadecane according to claim 1, characterized in that, In step S4, oil-water separation and adsorption are carried out at room temperature and pressure, and nanofiltration is carried out at room temperature by providing a transmembrane pressure difference through a vacuum pump. The transmembrane pressure difference provided by the vacuum pump is 0.5~2.0 MPa.

10. A flame-retardant viscose fiber prepared by the isohexadecane cyclic dyeing method according to any one of claims 1-9, characterized in that, The uniformity of the dyeing of the flame-retardant viscose fiber is less than 2.0.

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