A low-temperature circulation dyeing method for polylactic acid fibers

Abstract

This invention relates to the field of dyeing and finishing technology, and discloses a low-temperature cyclic dyeing method for polylactic acid (PLA) fibers, comprising: 1) modifying curcumin with 12-bromo-1-dodecanool to obtain modified curcumin dye; 2) preparing a pre-emulsion using a eutectic solvent composed of coumarin and thymol and an emulsifier, and then modifying the PLA fibers; 3) low-temperature cyclic dyeing. This invention uses long-alkyl-linked curcumin for modification, which improves its hydrophobic compatibility and fixation stability, enhancing its diffusion and fixation ability in PLA fibers; and uses coumarin / thymol for surface modification of PLA fibers, which improves its surface affinity and dyeability without damaging the fiber's structure and mechanical properties. The combination of these two methods enables PLA fibers to achieve efficient dyeing under low-temperature and normal-pressure conditions, and can be combined with a low-temperature cyclic dyeing process to obtain high dye depth and excellent color fastness, thereby achieving the goals of energy saving, emission reduction, and recyclability in green dyeing and finishing.

Description

Technical Field

[0001] This invention relates to the field of dyeing and finishing technology, and in particular to a low-temperature cyclic dyeing method for polylactic acid fibers. Background Technology

[0002] Under the existing dyeing and finishing technology, low-temperature cyclic dyeing is an energy-saving process further developed from cyclic dyeing. It achieves multiple stable dyeing cycles at lower dyeing temperatures, based on the premise of reusable dyeing liquor. Compared to traditional high-temperature dyeing and single-cycle dyeing processes, low-temperature cyclic dyeing significantly reduces heat consumption and minimizes problems such as dye hydrolysis, dispersant failure, and fiber damage under high-temperature conditions, thus contributing to the long-term stability and reproducibility of the dye liquor system. Simultaneously, the chemical oxygen demand (COD) load of the dyeing waste liquor is lower under low-temperature conditions, making it more suitable for multiple cycles, thereby further reducing water consumption and wastewater discharge. This technology effectively alleviates the environmental pressure of high energy consumption and high emissions in the printing and dyeing industry while balancing dyeing quality and resource utilization efficiency. It meets the practical needs of green manufacturing and sustainable development, possessing clear technological advantages and significant social and industrial application value.

[0003] Polylactic acid (PLA) fiber is an eco-friendly polyester fiber obtained through fermentation and polymerization of plant starches such as corn and sugar beets. It possesses excellent properties such as biodegradability and good biocompatibility, and is widely used in clothing, home textiles, and medical fields. However, PLA fiber has two significant drawbacks: First, it has poor dyeability. Lacking polar hydrophilic groups in its molecular chain, it is difficult to dye, resulting in poor dye penetration, low colorfastness, and requiring high-temperature, high-pressure dyeing. To ensure dyeing results, the dye bath for dark-colored PLA fabrics needs to be raised to 110–120°C and held for a corresponding time. Therefore, it is difficult to use low-temperature cyclic dyeing processes for PLA fiber. Second, PLA fiber has poor heat resistance. High-temperature dyeing significantly reduces fiber strength and elongation at break, severely impacting subsequent processing. Therefore, high temperatures not only lead to a substantial increase in energy consumption but also increase the risk of thermal degradation of PLA fiber.

[0004] On the other hand, curcumin is a natural yellow polyphenol pigment extracted from the rhizome of turmeric. It is a renewable and environmentally friendly source and has been used in green dyeing and functional finishing in recent years, enabling the coloring of fibers such as cotton, silk, and wool. However, curcumin molecules are highly polar and have insufficient compatibility with hydrophobic polyester fibers (such as polylactic acid). Furthermore, it is prone to migration / desorption during the dyeing process, resulting in low dye uptake and unsatisfactory color fastness in terms of wash and rubbing resistance.

[0005] In summary, existing technologies urgently need to address issues such as poor dyeability and insufficient heat resistance of polylactic acid fibers, as well as the low dyeing rate and color fastness of natural dye curcumin on hydrophobic polyester fibers. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a low-temperature cyclic dyeing method for polylactic acid (PLA) fibers. This invention modifies PLA fibers with long-alkyl-linked curcumin, improving its hydrophobic compatibility and fixation stability, and enhancing its diffusion and fixation capabilities within PLA fibers. Simultaneously, it modifies the surface of PLA fibers with coumarin / thymol, significantly improving its surface affinity and dyeability without damaging the fiber's structure and mechanical properties. The combination of these two methods enables efficient dyeing of PLA fibers under low-temperature and normal-pressure conditions, and, when combined with a low-temperature cyclic dyeing process, achieves high dye depth and excellent color fastness, thereby realizing the goals of energy conservation, emission reduction, and recyclability in green dyeing and finishing.

[0007] The specific technical solution of this invention is as follows: This invention provides a low-temperature cyclic dyeing method for polylactic acid fibers, which includes the following steps:

[0008] 1) Synthesis of modified curcumin dye: Curcumin was dissolved in a solvent, potassium carbonate and potassium iodide catalysts were added, and a solvent containing the bromine reagent 12-bromo-1-dodecanool was added stepwise to carry out the reaction. The product was separated to obtain the modified curcumin dye.

[0009] 2) Preparation of modified polylactic acid fiber: Coumarin and thymol are mixed in a molar ratio of 1:0.75-2.5 (preferably 1:1.75-2.5), heated and stirred until dissolved to a transparent state, to obtain a eutectic solvent; the eutectic solvent and emulsifier are added to water for pre-emulsification to obtain a pre-emulsion; polylactic acid fiber is placed in the pre-emulsion for modification treatment, washed, and modified polylactic acid fiber is obtained.

[0010] 3) Low-temperature cyclic dyeing: Prepare a dye mixture by mixing modified curcumin dye, dispersant and water; dye the modified polylactic acid fiber at low temperature, wash and dry; by referring to the absorbance standard curve, use the residual liquid after dyeing as a medium to continue to add dye, and cycle multiple times to obtain dyed polylactic acid fiber.

[0011] To address the problems of poor dyeing depth and low color fastness of polylactic acid (PLA) fibers, which hinder the use of low-temperature cyclic dyeing processes, and the insufficient compatibility of curcumin with PLA and its tendency to migrate / desorb during dyeing, resulting in low dye uptake and unsatisfactory color fastness in washing and rubbing, this invention addresses these issues through a two-pronged approach: dye structure modification and fiber surface modification. On one hand, long alkyl-linked branch modification of curcumin enhances its hydrophobic compatibility and fixation stability, strengthening its diffusion and fixation capabilities in PLA fibers. On the other hand, the introduction of coumarin / thymol allows for gentle surface etching and reconstruction of PLA fibers, significantly improving surface affinity and dyeability without damaging the fiber's structure and mechanical properties. This combination enables efficient dyeing of PLA fibers under low-temperature, ambient-pressure conditions, and allows for high dyeing depth and excellent color fastness through low-temperature cyclic dyeing processes, thereby achieving energy-saving, emission-reducing, and recyclable green dyeing and finishing goals. Specifically:

[0012] (1) In this invention, 12-bromo-1-dodecanool is used to modify curcumin, and the resulting modified curcumin dye has the best dyeing effect on polylactic acid. Compared with bromododecane, 1,12-dibromododecane, and other bromools with different chain lengths, 12-bromo-1-dodecanool has both a suitable C12 hydrophobic segment and a terminal hydroxyl structure. The C12 segment can better enhance the hydrophobic compatibility and van der Waals interaction between the modified curcumin and polylactic acid fibers, and increase the driving force for the transfer of dye from the dye bath to the fiber phase; the terminal hydroxyl can improve the dispersion stability of the modified dye in the liquid dye system, and enhance the interfacial interaction between it and the polylactic acid ester group and the surface active sites of the eutectic solvent modified fiber through hydrogen bonding or dipole interaction. Experimental results show that the dyeing uptake rate of the 12-bromo-1-dodecanool modified system reaches approximately 96%, significantly higher than that of bromoalcohol reagent systems with other chain lengths. Under the same C12 chain length, the dyeing uptake rate of the 12-bromo-1-dodecanool modified sample is approximately 98%, higher than that of bromododecane and 1,12-dibromododecane modified samples. This indicates a synergistic effect between the C12 chain length and the terminal hydroxyl group, which can achieve higher dye utilization and lighter dyeing residue at low temperatures. This characteristic is beneficial for reducing the dyeing temperature of polylactic acid fibers, mitigating fiber thermal and hydrolytic damage, and providing a stable basis for subsequent residual solution recycling dyeing. Therefore, this invention preferentially uses 12-bromo-1-dodecanool as a brominating agent for curcumin modification.

[0013] (2) This invention uses a specific coumarin / thymol eutectic solvent to modify the surface of PLA fibers. This allows for surface etching of PLA fibers without damaging their internal structure and mechanical properties, achieving surface reconstruction. After this process, PLA fibers with strong hydrophilicity and biocompatibility can be prepared, enabling dyeing at low temperature and normal pressure with dyeing efficiency comparable to that under high temperature and high pressure. The invention chooses thymol / coumarin because this natural hydrophobic eutectic solvent has better compatibility with the hydrophobic polyester structure of PLA. Simultaneously, it can disturb the chain segments near the ester groups of PLA through hydrogen bonding, achieving controllable surface etching and a loose microstructure. Example 2 showed the highest dyeing rate and K / S value, and electron microscopy also demonstrated the most significant etching effect on the PLA surface, indicating that this system can effectively increase dye adsorption sites and low-temperature diffusion channels. Compared to oxalic acid / choline chloride and urea / choline chloride, the thymol / coumarin system avoids the problem of insufficient compatibility between hydrophilic eutectic solvents and PLA, and also has the advantages of being natural, low-volatility, emulsifiable, recyclable, and energy-saving at low temperatures. Therefore, it is most suitable as the PLA fiber pretreatment modification system of this invention.

[0014] Furthermore, the ratio of coumarin to thymol is also crucial. If the ratio is too low, the thymol content is insufficient, limiting the available phenolic hydroxyl hydrogen bond donors in the system. This makes it difficult for coumarin and thymol to form a sufficiently continuous hydrogen bond network, weakening the eutectic effect and reducing the uniformity and stability of the eutectic solvent. Simultaneously, insufficient hydrophobicity reduces compatibility and penetration with the hydrophobic polyester surface of PLA, resulting in weaker swelling, etching, and reconstruction effects on the PLA surface segments. This also limits the increase in dye adsorption sites and diffusion channels during subsequent dyeing. Conversely, if the thymol ratio is too high, exceeding the effective binding range of coumarin, free thymol may exist in the system, reducing the stability of the hydrogen bond network and leading to excessive hydrophobicity. Since this invention uses an emulsion to treat PLA, excessive hydrophobicity will reduce the emulsion dispersion stability, causing uneven treatment; it may also cause excessive swelling or etching of the PLA surface, affecting fiber mechanical properties and cyclic dyeing stability. Therefore, controlling the coumarin / thymol molar ratio at 1:0.75–2.5, especially 1:1.75–2.5, can achieve a good balance between stable DES formation, effective PLA surface reconstruction, and minimally damaging staining.

[0015] Preferably, in step 1), the molar ratio of curcumin to bromine reagent is 1:10-10:1, the addition time is 30-180 min (more preferably 1-3 h), the reaction temperature is 50-100℃ (more preferably 60-80℃), and the reaction time is 8-14 h (more preferably 10-13 h).

[0016] Preferably, in step 1), the molar ratio of curcumin to potassium carbonate is 2-200:1, and the molar ratio of curcumin to potassium iodide catalyst is 2-2000:1.

[0017] Preferably, in step 1), the solvent is one or a mixture of several of acetone, ethyl acetate, and N,N-dimethylformamide.

[0018] Preferably, in step 2), the heating and stirring temperature is 30-80℃ (more preferably 40-80℃) and the time is 1-4h (more preferably 1-3h); the pre-emulsification temperature is 40-80℃ (more preferably 50-70℃) and the time is 10-60min (more preferably 20-50min).

[0019] Preferably, in step 2), the emulsifier is polyethylene glycol mono(4-tert-octylphenyl) ether (X-100).

[0020] Different types of emulsifiers affect the stability of coumarin / thymol emulsions and their modification effect on PLA fibers. Emulsifiers influence the wetting, penetration, and etching uniformity of coumarin / thymol on the PLA surface by regulating the dispersion state, particle size, and interfacial contact behavior of coumarin / thymol droplets in the aqueous phase. Inappropriate emulsifier selection may lead to emulsion stratification, droplet coalescence, or insufficient contact between coumarin / thymol and PLA, thereby reducing the surface reconstruction effect; it may also affect subsequent dyeing due to competitive adsorption of the emulsifier on the fiber surface. Therefore, this invention selects nonionic X-100 as the emulsifier. Its hydrophilic polyoxyethylene segments and hydrophobic aromatic structure are beneficial for the stable dispersion of hydrophobic coumarin / thymol, allowing it to act uniformly on the PLA fiber surface, thereby achieving more stable and controllable surface etching and improved low-temperature dyeing performance.

[0021] Preferably, in step 2), the volume fraction of the eutectic solvent in the pre-emulsion is 3-5% (more preferably 3.5-4.5%), and the volume fraction of the emulsifier is 0.5-2% (more preferably 0.5-1.5%).

[0022] The volume fractions of the eutectic solvent and emulsifier are limited to the above range primarily to ensure stable dispersion of coumarin / thymol in the aqueous phase and to produce moderate and uniform surface etching on PLA. Example 2 uses 4% coumarin / thymol and 1% X-100, which falls within this preferred range, achieving significant improvement in PLA surface reconstruction and dyeing performance. If the coumarin / thymol concentration is too low, the effective coumarin / thymol content on PLA is insufficient, making it difficult to fully swell and disturb the fiber surface segments, resulting in insignificant surface etching, limited increase in dye adsorption sites and diffusion channels, and insufficient improvement in subsequent dyeing rate and K / S ratio. If the coumarin / thymol concentration is too high, the etching effect is too strong, potentially causing excessive swelling, localized erosion, or even mechanical damage to the PLA surface, while also increasing the emulsion dispersion burden and residual organic load, which is detrimental to low-temperature cyclic dyeing stability. If the emulsifier content is too low, it is difficult to stably disperse the hydrophobic coumarin / thymol, which can easily lead to droplet aggregation, layering, and uneven treatment. If the emulsifier content is too high, it may over-coat the coumarin / thymol droplets or compete for adsorption on the PLA surface, thereby weakening the effective contact between the coumarin / thymol and the fiber and affecting subsequent dye adsorption. Therefore, the above range is a reasonable range that balances emulsion stability, effective PLA surface reconstruction, and minimal fiber damage.

[0023] Preferably, in step 2), the mass ratio of polylactic acid fiber to pre-emulsion is 1:80-120 (more preferably 1:90-110).

[0024] Limiting the mass ratio of PLA fiber to pre-emulsion within the aforementioned range is essentially to control the effective contact and uniformity of the pre-emulsion on the fiber surface. This ratio ensures that the fiber is fully wetted and spread in the treatment solution, allowing coumarin / thymol droplets to uniformly contact the PLA surface, while maintaining a sufficient effective concentration of coumarin / thymol to prevent it from being adsorbed or consumed too quickly by the fiber surface during treatment. Example 2 uses a 1:100 ratio, which falls within this range, corresponding to a significant improvement in subsequent surface etching and dyeing performance. If the amount of pre-emulsion is too low, i.e., the bath ratio is too small, the fiber will not be sufficiently dispersed and wetted in the system, making it difficult for the coumarin / thymol droplets to act uniformly on the entire fiber surface, easily leading to uneven treatment; at the same time, if the fiber load per unit volume of emulsion is too high, the effective amount of coumarin / thymol in some areas will be insufficient, the degree of surface reconstruction will be insufficient, and the subsequent increase in dye adsorption sites and diffusion channels will be limited. If the amount of pre-emulsifying solution is too high, i.e., the bath ratio is too large, although the wetting is sufficient, the effective contact efficiency between coumarin / thymol and the fiber decreases. Some coumarin / thymol remains in the liquid phase and cannot fully participate in surface etching, resulting in waste of reagents and an increase in residual liquid load. Excessive treatment liquid volume also increases heating and heat preservation energy consumption, which does not meet the process objectives of low temperature, energy saving, and cyclic dyeing and finishing. Therefore, the above range is a reasonable range that balances sufficient fiber wetting, effective utilization of coumarin / thymol, uniform surface reconstruction, and process economy.

[0025] Preferably, in step 2), the temperature of the modification treatment is 20-80℃ (more preferably 30-70℃), and the time is 80-200min (more preferably 100-140min).

[0026] Preferably, in step 2), the cleaning process uses ethanol and deionized water.

[0027] Preferably, in step 3), the procedure for each staining is as follows: increase the temperature to 90-98°C at room temperature at 0.8-1.2°C / min, hold for 50-70 min, and then decrease to room temperature at 1.5-2.5°C / min.

[0028] Preferably, in step 3), the amount of dye used is 1-5% of the polylactic acid fiber (more preferably 1.5-3%); the pH of the water is 7.

[0029] Preferably, in step 3), the absorbance standard curve is prepared by using a mixed solution of water and acetone with a volume ratio of 1:1-1:10 (more preferably 3:2-3:10) as the solvent to prepare a modified curcumin dye solution with a mass concentration of 0-2 g / L. The absorbance of the dye at the maximum absorption wavelength under different mass concentrations is measured, and the relationship curve between dye mass concentration and absorbance is plotted.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] (1) The present invention modifies curcumin with a long alkyl chain without destroying the original color-developing structure of curcumin to enhance its high color fastness and high color depth dyeing of PLA fibers.

[0032] (2) This invention modifies the PLA fiber by using a coumarin / thymol system as a modifier at low temperature to activate the molecular chain segments on the PLA fiber surface without changing the aggregated structure of the PLA fiber or affecting its mechanical properties. The process is simple and controllable.

[0033] (3) The modified curcumin and modified PLA fiber of the present invention can achieve low-temperature cyclic dyeing, eliminate the conditions of high temperature and high pressure, maintain a high dyeing rate, effectively utilize dyeing wastewater, optimize the process, protect the environment, and improve the dyeing effect.

[0034] (4) The processing technology of the present invention is mild, pollution-free, low-cost, and energy-saving, and the process is controllable, non-toxic and pollution-free. Attached Figure Description

[0035] Figure 1 SEM images of PLA fibers before and after modification are shown; original PLA fibers (a, b), modified PLA fibers of Example 2 (c, d), modified PLA fibers of Example 1 (e), and modified PLA fibers of Example 3 (f).

[0036] Figure 2 The K / S curve and dyeing rate curve of the modified PLA fiber are shown.

[0037] Figure 3 Infrared spectra of curcumin dyes modified by brominated reagents with different alkyl chain lengths;

[0038] Figure 4 NMR spectra of curcumin dyes modified with brominated reagents of different alkyl chain lengths;

[0039] Figure 5 K / S curves and dye uptake curves for curcumin dyes modified with brominated reagents of different alkyl chain lengths;

[0040] Figure 6 Infrared spectra of curcumin dyes modified with bromine reagents with different tail groups;

[0041] Figure 7 NMR spectra of curcumin dyes modified with bromine reagents with different tail groups;

[0042] Figure 8 K / S curves and dyeing rate curves for curcumin dyes modified with brominated reagents with different tail groups;

[0043] Figure 9 The staining K / S curves and staining rate curves for Examples 7-9 and Comparative Example 9 are shown.

[0044] Figure 10 Photographs of the waste liquid from the cyclic staining process (1-10 indicate the number of cycles). Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. It should be noted that, unless otherwise defined, the technical or scientific terms used in the embodiments of this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this disclosure pertains.

[0046] (I) Effects of modification with different types of eutectic solvents on PLA fibers and their dyeing effects

[0047] Example 1

[0048] Preparation of (oxalic acid / choline chloride) eutectic solvent: Accurately weigh oxalic acid and choline chloride at a molar ratio of 1:1, and place them into an Erlenmeyer flask in the order of choline chloride first and then oxalic acid. Add a magnetic rotor and place the flask on a constant temperature magnetic stirrer. Use an oil bath and stir at 80°C until the mixture becomes transparent. Continue stirring for 2 hours and then remove the flask for later use.

[0049] Treatment of PLA fibers and fabrics with (oxalic acid / choline chloride) eutectic solvent: PLA fibers were pre-washed with ultrapure water for 20 min and dried in a 60°C oven for later use. Eutectic solvent and emulsifier were placed in deionized water at volume fractions of 3.5% (v / v) and 0.5% (v / v), respectively, and pre-emulsified at 50°C for 20 min. Then, PLA fibers were placed in the pre-emulsion solution (the mass ratio of PLA fibers to pre-emulsion solution was 1:90) and treated at 40°C for 100 min. After washing repeatedly with ethanol and deionized water for 20 min, the fibers were dried in a 60°C oven for later use and testing.

[0050] Example 2

[0051] Preparation of (coumarin / thymol) eutectic solvent: Accurately weigh coumarin and thymol according to a molar ratio of 1:2, and put them into an Erlenmeyer flask in the order of coumarin first and then thymol. Add a magnetic rotor and place the flask on a constant temperature magnetic stirrer. Use an oil bath and stir at 70°C until the mixture becomes transparent. Continue stirring for 2 hours and then remove the flask for later use.

[0052] Treatment of PLA fibers and fabrics with a eutectic solvent (coumarin / thymol): PLA fibers were pre-washed with ultrapure water for 20 min and dried in a 60°C oven for later use. Eutectic solvent and emulsifier were added to deionized water at volume fractions of 4% (v / v) and 1% (v / v), respectively, and pre-emulsified at 60°C for 30 min. Then, PLA fibers were placed in the pre-emulsion (the mass ratio of PLA fibers to pre-emulsion was 1:100) and treated at 60°C for 120 min. The fibers were then repeatedly washed with ethanol and deionized water for 20 min and dried in a 60°C oven for later use, ready for testing.

[0053] Example 3

[0054] Preparation of (urea / choline chloride) eutectic solvent: Weigh urea and choline chloride precisely at a molar ratio of 2:1, and place them in an Erlenmeyer flask in the order of choline chloride first and then urea. Add a magnetic rotor and place the flask on a constant temperature magnetic stirrer. Use an oil bath and stir at 80°C until the mixture becomes transparent. Continue stirring for 3 hours and then remove the flask for later use.

[0055] Treatment of PLA fibers and fabrics with a eutectic solvent (urea / choline chloride): PLA fibers were pre-washed with ultrapure water for 30 min and dried in a 70°C oven for later use. Eutectic solvent and emulsifier were added to deionized water at volume fractions of 4.5% (v / v) and 1.5% (v / v), respectively, and pre-emulsified at 70°C for 50 min. Then, PLA fibers were placed in the pre-emulsion (the mass ratio of PLA fibers to pre-emulsion was 1:110) and treated at 70°C for 140 min. After washing repeatedly with ethanol and deionized water for 30 min, the fibers were dried in a 70°C oven for later use and testing.

[0056] (a) Microscopic morphology test of PLA fiber surface

[0057] The surface microstructure of the PLA fibers treated in Examples 1-3 was tested: Figure 1As shown, a systematic analysis of the SEM images (ad) of the original PLA fibers reveals that the untreated PLA fibers (a and b) have a dense and uniform surface structure, exhibiting high overall smoothness. No obvious roughening features, expansion, thinning, breakage, or fiber splitting were observed, indicating that their macroscopic morphology is well maintained and their structural integrity is high. After modification with the coumarin / thymol system of Example 2 (c and d), the fiber surface underwent significant morphological reconstruction, exhibiting typical etching characteristics, including crack generation, attachment of granular deposits, and the formation of multi-scale hierarchical pore structures. This phenomenon demonstrates that the coumarin / thymol system has a strong surface regulation capability for PLA fibers, effectively disrupting the orderly stacking of the original molecular chains and inducing local swelling and even dissolution behavior, thereby constructing a surface morphology with multi-level structural features. This structural evolution not only significantly increases the specific surface area of ​​the material but also provides richer and more active interaction sites for subsequent interfacial reactions and functional modifications. The other two systems (oxalic acid / choline chloride in Example 1 and urea / choline chloride in Example 3) did not significantly affect the modification of PLA fibers, as shown in Figures e and f. The fibers remained unchanged and did not significantly help with subsequent dyeing.

[0058] (b) Dyeing effect of PLA fibers

[0059] The dyeing effect of PLA fibers treated in Examples 1-3 was tested:

[0060] Weigh 2g of PLA fibers treated in Examples 1-3. 2% (based on fabric weight) of modified curcumin dye (Example 4) is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min at room temperature, maintain this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0061] like Figure 2 As shown, by comparing the dyeing effects of PLA fibers modified with eutectic solvent systems in different embodiments, it is clear that compared with Examples 1 and 3, the PLA treated with the coumarin / thymol system in Example 2 showed the most significant improvement in both the K / S value and the dyeing rate. Combined with the analysis results of the PLA fiber surface microstructure test, it was determined that the coumarin / thymol system was the best choice for modifying PLA fibers.

[0062] (II) Effects of different alkyl chain lengths and tail-end group grafting modification of curcumin on its dyed PLA fibers

[0063] Example 4 (The brominating reagent is 12-bromo-1-dodecanool)

[0064] Weigh 3.68 g of curcumin and dissolve it in 25 ml of acetone and 75 ml of N,N-dimethylformamide, then add 0.350 g of potassium carbonate and 0.01 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 75 °C for 1 h. Then, dissolve 9.29 g of 12-bromo-1-dodecanool in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 h. After injection, react for another 10 h, then wash, purify, and column chromatography to obtain modified curcumin (H12).

[0065] Example 5 (The brominating reagent is 12-bromo-1-dodecanool (H12))

[0066] Weigh 7.36 g of curcumin and dissolve it in 50 ml of acetone and 50 ml of N,N-dimethylformamide, then add 0.750 g of potassium carbonate and 0.021 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 80 °C for a period of time. Then, dissolve 18.58 g of 12-bromo-1-dodecanool in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 hours. After injection, react for another 8 hours, then wash, purify, and pass through a column to obtain modified curcumin (H12).

[0067] Example 6 (The brominating reagent is 12-bromo-1-dodecanool (H12))

[0068] Weigh 0.737 g of curcumin and dissolve it in 25 ml of acetone and 25 ml of N,N-dimethylformamide, then add 0.351 g of potassium carbonate and 0.015 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 70 °C for a period of time. Then, dissolve 0.93 g of 12-bromo-1-dodecanool in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 hours. After injection, react for another 12 hours, then wash, purify, and pass through a column to obtain modified curcumin (H12).

[0069] Comparative Example 1 (The brominating reagent was 8-bromo-1-octanol (H8))

[0070] Weigh 3.68 g of curcumin and dissolve it in 25 ml of acetone and 75 ml of N,N-dimethylformamide, then add 0.350 g of potassium carbonate and 0.01 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 80 °C for 1 h. Then, dissolve 7.31 g of 8-bromo-1-octanol in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump over 2 h. After injection, react for another 10 h, then wash, purify, and column chromatography to obtain modified curcumin (H8).

[0071] Comparative Example 2 (The brominating reagent is 10-bromo-1-decyl alcohol (H10))

[0072] Weigh 3.68 g of curcumin and dissolve it in 25 ml of acetone and 75 ml of N,N-dimethylformamide, then add 0.350 g of potassium carbonate and 0.01 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 70 °C for 1 h. Then, dissolve 8.29 g of 10-bromo-1-decyl alcohol in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 h. After injection, react for another 10 h, then wash, purify, and pass through a column to obtain modified curcumin (H10).

[0073] Comparative Example 3 (The brominating reagent was 14-bromo-1-tetradecanool (H14))

[0074] Weigh 3.68 g of curcumin and dissolve it in 25 ml of acetone and 75 ml of N,N-dimethylformamide, then add 0.350 g of potassium carbonate and 0.01 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 75 °C for 1 h. Then, dissolve 10.25 g of 14-bromo-1-tetradecanool in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 h. After injection, react for another 10 h, then wash, purify, and column chromatography to obtain modified curcumin (H14).

[0075] (a) Structural characterization of the modified curcumin dye:

[0076] The molecular structures of H8, H10, H12 (Example 4) and H14 were determined by 1H nuclear magnetic resonance spectroscopy (1H NMR). Figure 4 ) and Fourier transform infrared spectroscopy (FT-IR) Figure 3Synergistic characterization was performed. For curcumin, the multiple signals in its 1H NMR spectrum in the range of 6.95-7.82 ppm can be attributed to aromatic ring protons (Ha-He), while the characteristic peak at 3.80-3.88 ppm corresponds to methoxy protons (Hg). The proton signal related to the β-diketone structure (Hf, 3.38 ppm) may partially overlap with the peak of water in the solvent (3.33 ppm). Taking the representative product H12 as an example, the continuous multiple peaks appearing in the δ 1.03-1.85 ppm range are attributed to the introduced aliphatic chain methylene group (H3-H12), while the characteristic signals observed at 4.01, 3.40, and 3.16 ppm correspond to H1, H2, and H13, respectively, indicating that the target molecular structure has been successfully constructed and the chemical environment has been significantly modulated.

[0077] FT-IR analysis further corroborated the structural evolution at the vibrational spectral level. For example... Figure 3 As shown, 3435 cm -1 The broad and strong absorption bands at 2927 and 2853 cm⁻¹ are attributed to the stretching vibration of the hydroxyl group (-OH), indicating significant hydrogen bonding interactions in the system; -1 The absorption peaks at these locations correspond to the symmetric and antisymmetric stretching vibrations of the methylene group (-CH2-), reflecting the successful introduction of the aliphatic chain structure. Furthermore, the absorption peaks at 1625, 1511, and 1465 cm⁻¹ also indicate this. -1 The characteristic peaks at 1260 and 1139 cm⁻¹ are attributed to the C=C skeletal vibration of the aromatic ring, while the peaks at 1260 and 1139 cm⁻¹ are attributed to the C=C skeletal vibration of the aromatic ring. -1 The absorption peak at that point corresponds to the stretching vibration of the aromatic ether bond (COC). These spectroscopic features indicate that the hydroxyl functional group is effectively grafted onto the curcumin skeleton through an etherification reaction, achieving molecular-level structural reconstruction and functional modification.

[0078] (b) Testing the dyeing effect of grafted curcumin dye on PLA fibers.

[0079] Weigh 2g of modified PLA fiber (Example 2). 2% (based on fabric weight) of modified curcumin dye is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fiber in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min at room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0080] like Figure 5As shown, the K / S values ​​of curcumin dyes modified with different alkyl chain lengths on fabrics generally showed a trend of first increasing and then decreasing. Samples H8 and H10 had relatively high K / S values, while H12 and H14 showed slightly lower values. Fabrics dyed with samples H8 and H10 exhibited an orange hue, while those dyed with H12 and H14 gradually shifted towards a bright yellow hue. Since orange has a stronger absorption capacity for blue light in the visible light region, its overall color depth usually reflects a higher K / S value; therefore, samples H8 and H10 exhibited higher color intensity. However, as the overall dyeing color gradually shifted towards bright yellow, the overall color depth relatively decreased, resulting in a slight decrease in the K / S values ​​of samples H12 and H14. But the K / S value is not only affected by the amount of dye adsorbed but also closely related to the overall hue of the dye. When the dyeing color changes from orange to bright yellow, even if the amount of dye adsorbed on the fiber remains high, the overall color depth may still decrease to some extent. Therefore, the slightly lower K / S value of sample H12 compared to H8 does not necessarily mean that its dye adsorption capacity is significantly reduced, but rather reflects the overall color depth difference caused by the change in dyeing hue.

[0081] Combination Figure 5 The dye uptake results further show that sample H12 has the highest dye uptake (approximately 96%), indicating that dye molecules can be more effectively adsorbed and fixed on the fiber surface under this chain length condition. Meanwhile, the fabric dyed with sample H12 exhibits a purer yellow hue, with uniform overall color and good overall color stability. The H12 chain length ensures high dye utilization while achieving a more ideal and stable yellow dyeing effect, demonstrating the best overall dyeing performance.

[0082] Comparative Example 4 (The brominating reagent was bromododecane)

[0083] Weigh 3.68 g of curcumin and dissolve it in 50 ml of acetone and 50 ml of N,N-dimethylformamide, then add 0.350 g of potassium carbonate and 0.01 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 80 °C for 1 h. Then, dissolve 8.72 g of bromododecane in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 h. After injection, react for another 8 h, then wash, purify, and pass through a column to obtain modified curcumin.

[0084] Comparative Example 5 (bromination reagent: 1,12-dibromododecane)

[0085] Weigh 3.68 g of curcumin and dissolve it in 25 ml of acetone and 75 ml of N,N-dimethylformamide, then add 0.351 g of potassium carbonate and 0.015 g of potassium iodide. Transfer the solution to a three-necked flask, place it in an oil bath equipped with a stirrer and condenser, and heat at 70 °C for 1 h. Then, dissolve 11.48 g of 1,12-dibromododecane in 25 ml of N,N-dimethylformamide. After complete dissolution, inject the solution into the three-necked flask using a micro-injection pump, completing the injection within 2 h. After injection, react for another 12 h, then wash, purify, and pass through a column to obtain modified curcumin.

[0086] (a) Structural characterization of the modified curcumin dye

[0087] like Figure 6 As shown and Figure 7 As shown, the infrared and nuclear magnetic resonance analyses are the same as in the previous section, and the infrared and nuclear magnetic resonance analyses can clearly determine the successful synthesis of the product and that the original curcumin chromogenic group was not destroyed.

[0088] (b) Testing the dyeing effect of grafted curcumin dye on PLA fibers.

[0089] Weigh 2g of modified PLA fiber (Example 2). 2% (based on fabric weight) of modified curcumin dye is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fiber in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min at room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0090] like Figure 8 As shown, the hydroxyl group at the tail (Example 4) has a significant advantage in dyeing rate compared to the absence of a group (Comparative Example 4) and bromine atoms (Comparative Example 5), which is in line with the green, environmentally friendly, and low-temperature cycling concept of this invention. Although the other two bromine reagents have a slight advantage in K / S value, they do not meet the dyeing rate requirements and are therefore difficult to achieve the purpose of cyclic dyeing. Therefore, the bromine reagent with a hydroxyl group at the tail was selected to modify curcumin.

[0091] (III) Effects of Eutectic Solvent Modification and Modified Curcumin on Low-Temperature Cyclic Dyeing of PLA Fibers

[0092] Example 7

[0093] Weigh 2g of modified PLA fiber (Example 2), and add 1.5% (based on fabric weight) of modified curcumin dye (Example 4). Calculate the required amount of liquid dye using solid content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the product in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min at room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0094] The dyeing waste liquid is collected, and its absorbance is measured by ultraviolet spectrophotometer. The amount of residual dye in the dyeing liquid and the amount of dye to be added are compared and analyzed by the absorbance standard curve. The dyeing cycle is repeated multiple times.

[0095] Example 8

[0096] Weigh 2g of the PLA fiber modified in Example 2. 2% (based on fabric weight) of the curcumin dye modified in Example 4 is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min at room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0097] The dyeing waste liquid is collected, and its absorbance is measured by ultraviolet spectrophotometer. The amount of residual dye in the dyeing liquid and the amount of dye to be added are compared and analyzed by the absorbance standard curve. The dyeing cycle is repeated multiple times.

[0098] Example 9

[0099] Weigh 2g of the PLA fiber modified in Example 2. 3% (based on fabric weight) of the curcumin dye modified in Example 4 is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min at room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0100] The dyeing waste liquid is collected, and its absorbance is measured by ultraviolet spectrophotometer. The amount of residual dye in the dyeing liquid and the amount of dye to be added are compared and analyzed by the absorbance standard curve. The dyeing cycle is repeated multiple times.

[0101] Comparative Example 6 (Unmodified ordinary PLA fiber + Unmodified ordinary curcumin dye)

[0102] Weigh 2g of ordinary PLA fiber. 2% (based on fabric weight) of ordinary curcumin dye is required. Calculate the required amount of liquid dye using solid content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: at room temperature, heat to 95℃ at a rate of 1℃ / min, maintain this temperature for 60min, and then cool to room temperature at a rate of 2℃ / min. After completion, wash with clean water. Dry at 60℃ after washing, ready for testing.

[0103] Comparative Example 7 (Unmodified common curcumin dye)

[0104] Weigh 2g of the modified PLA fiber from Example 2. 2% (based on fabric weight) of common curcumin dye is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min from room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0105] Comparative Example 8 (Unmodified ordinary PLA fiber)

[0106] Weigh 2g of ordinary PLA fiber. 2% (based on fabric weight) of the modified curcumin dye from Example 4 is required. Calculate the required amount of liquid dye using solids content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: heat to 95°C at a rate of 1°C / min from room temperature, hold at this temperature for 60min, and then cool to room temperature at a rate of 2°C / min. After completion, wash with clean water. Dry at 60°C after washing, ready for testing.

[0107] Comparative Example 9 (Unmodified ordinary PLA fiber + unmodified ordinary curcumin dye, dyed at 110℃)

[0108] Weigh 2g of ordinary PLA fiber. 2% (based on fabric weight) of curcumin dye is required. Calculate the required amount of liquid dye using solid content conversion. Add an appropriate amount of water (pH=7) at a bath fabric ratio of 10:1, and then place the fabric in an infrared dyeing machine. The dyeing temperature profile is as follows: at room temperature, heat to 110℃ at a rate of 1℃ / min, maintain this temperature for 60min, and then cool to room temperature at a rate of 2℃ / min. After dyeing, wash with clean water. Dry at 60℃ after washing, ready for testing.

[0109] (a) Single-sample comprehensive staining performance test:

[0110] Depend on Figure 9 As can be seen, although Example 7 maintained a high dyeing rate under conditions of 1.5% owf and 95 ℃, the K / S value was low, indicating insufficient dye supply and that the fiber had not yet reached a fully dyed state. In Example 9, after increasing the dye dosage to 2.5% owf, the K / S value only increased slightly, while the dyeing rate decreased significantly, indicating that the fiber dye adsorption / fixation tended to be saturated, and excess dye could not be effectively converted into color depth, but instead increased the residual liquor load.

[0111] In contrast, Example 8 exhibits both a high K / S value and dye uptake rate at 2.0% owf and 95 °C, indicating that this dosage is optimally matched with the dye adsorption, diffusion, and fixation capabilities of modified PLA, enabling high dye utilization and ideal color depth.

[0112] Compared with Comparative Example 9, which dyes ordinary PLA with ordinary curcumin at 2.0% owf and 110 ℃, Example 8 can achieve a similar or even better dyeing effect under the condition of reducing the temperature by 15 ℃. This shows that the "modified curcumin-modified PLA" system significantly reduces the heat driving force required for PLA dyeing, realizes low-temperature and high-efficiency dyeing, and demonstrates the process advantages of less damage, low energy consumption and high dyeing uptake.

[0113] (b) Cyclic staining performance test:

[0114] Tables 1-3 show the cyclic staining data for Example 8 and Comparative Example 6. Detailed analysis is as follows:

[0115] As shown in Table 1, during the cyclic dyeing process, the dyeing rate of modified curcumin was consistently higher than that of ordinary curcumin, and remained at a higher level even after 10 cycles, while the K / S value did not change significantly. In contrast, the dyeing rate and K / S value of ordinary curcumin decreased after multiple cycles. This indicates that modified curcumin exhibits better dyeing performance and stability in the cyclic system. This may be due to the improved dispersibility of modified curcumin, leading to enhanced interaction with PLA fibers, making it easier for the dye to diffuse into the fiber interior and be adsorbed.

[0116] With increasing cycle count, the unevenness of fabrics dyed with ordinary curcumin gradually increased, while the unevenness of modified curcumin remained relatively low, indicating that the modified system maintained good dispersion stability and dyeing uniformity during cyclic dyeing. Regarding color fastness, the dry and wet rubbing fastness and soaping fastness of fabrics dyed with ordinary curcumin decreased after multiple cycles, while the various fastness values ​​of fabrics dyed with modified curcumin showed little change and remained at a good overall level, indicating that the modified dye was more stable in fixation within the fiber.

[0117] As can be seen from Table 2, the L value of ordinary curcumin-dyed fabrics increases with the number of cycles, indicating that the color gradually becomes lighter. At the same time, the a and b values ​​change to some extent, indicating that there is a shift in the overall color. However, the L, a*, b, and C values ​​of modified curcumin-dyed fabrics change less, indicating that the fabric color remains basically stable during the cyclic dyeing process.

[0118] It is worth noting that after 10 cycles of staining, the staining effect of the ordinary curcumin system decreased significantly, while the modified curcumin system maintained a good staining effect. This is mainly because the unstained components of ordinary curcumin gradually accumulate during the cycle, affecting subsequent staining; while the modified curcumin has a higher staining rate and less residue, thus maintaining relatively stable staining performance after multiple cycles.

[0119] As can be seen from the COD results in Table 3, the residual COD of the modified curcumin system is significantly lower than that of the ordinary curcumin system, and it remains at a low level even after 10 cycles, indicating that its dye utilization rate is higher and its environmental impact is smaller.

[0120] Table 1

[0121]

[0122] Table 2

[0123]

[0124] Table 3

[0125]

[0126] like Figure 10 As shown in the residual liquid image, the residual liquid of the ordinary curcumin system (Comparative Example 6) after cyclic dyeing remains a deep orange-yellow with low transparency, indicating that a significant amount of undyed or unfixed curcumin components remain in the dye bath. This is related to the insufficient compatibility between ordinary curcumin and the hydrophobic polyester structure of PLA, making it difficult for the dye to fully transfer to the fiber phase. In contrast, the residual liquid of the curcumin-12 alcohol modified dye (Example 8) is significantly lighter, appearing as a light yellow and clearer overall. This indicates that the hydrophobic compatibility between the dye and PLA is enhanced after long alkyl chain modification, allowing more dye to enter and fix in the fiber, reducing the amount of residual dye in the residual liquid. Therefore, this figure visually demonstrates that the curcumin-12 alcohol system has a lower color intensity and less dye residue in the residual liquid, which is more conducive to subsequent replenishment control and cyclic dyeing stability.

[0127] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0128] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A low-temperature cyclic dyeing method for polylactic acid fibers, characterized in that... Includes the following steps: 1) Synthesis of modified curcumin dye: Curcumin was dissolved in a solvent, potassium carbonate and potassium iodide catalysts were added, and a solvent containing the bromine reagent 12-bromo-1-dodecanool was added stepwise to carry out the reaction. The product was separated to obtain the modified curcumin dye. 2) Preparation of modified polylactic acid fiber: Coumarin and thymol were mixed at a molar ratio of 1:0.75-2.5 and heated and stirred until dissolved to a transparent state to obtain a eutectic solvent; the eutectic solvent and emulsifier were added to water for pre-emulsification to obtain a pre-emulsion; polylactic acid fiber was placed in the pre-emulsion for modification treatment, and then washed to obtain modified polylactic acid fiber; 3) Low-temperature cyclic dyeing: Prepare a dye mixture by mixing modified curcumin dye, dispersant and water; dye the modified polylactic acid fiber at low temperature, wash with water and dry; use the residual dyeing liquid as a medium to continue adding dye, and cycle multiple times to obtain dyed polylactic acid fiber.

2. The low-temperature cyclic staining method as described in claim 1, characterized in that: In step 1), The molar ratio of curcumin to bromine reagent is 1:10-10:1, the addition time is 30-180 min, the reaction temperature is 50-100℃, and the reaction time is 8-14 h.

3. The low-temperature cyclic staining method as described in claim 1 or 2, characterized in that: In step 1), The solvent is one or a mixture of several of acetone, ethyl acetate, and N,N-dimethylformamide; The molar ratio of curcumin to potassium carbonate is 2-200:1, and the molar ratio of curcumin to potassium iodide catalyst is 2-2000:

1.

4. The low-temperature cyclic staining method as described in claim 1, characterized in that: In step 2), The emulsifier is polyethylene glycol mono(4-tert-octylphenyl) ether; In the pre-emulsion, the volume fraction of the eutectic solvent is 3-5%, and the volume fraction of the emulsifier is 0.5-2%.

5. The low-temperature cyclic staining method as described in claim 1 or 4, characterized in that: In step 2), The heating and stirring temperature is 30-80℃, and the time is 1-4 hours; The pre-emulsification temperature is 40-80℃ and the time is 10-60 min.

6. The low-temperature cyclic staining method as described in claim 1, characterized in that: In step 2), the mass ratio of polylactic acid fiber to pre-emulsion is 1:80-120.

7. The low-temperature cyclic staining method as described in claim 1 or 6, characterized in that: In step 2), the temperature of the modification treatment is 20-80℃ and the time is 80-200min.

8. The low-temperature cyclic staining method as described in claim 1 or 6, characterized in that: In step 2), the cleaning process uses ethanol and deionized water.

9. The low-temperature cyclic staining method as described in claim 1, characterized in that: In step 3), the procedure for each staining is as follows: raise the temperature to 90-98℃ at room temperature at 0.8-1.2℃ / min, hold for 50-70min, and then lower it to room temperature at 1.5-2.5℃ / min.

10. The low-temperature cyclic staining method as described in claim 1 or 9, characterized in that: In step 3), The amount of dye used is 1-5% of the polylactic acid fiber; The pH of the water is 7.

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