Regenerated fiber decoloring method based on controllable phase separation in response polymer fiber

Abstract

The invention relates to the field of recycling of renewable resources, and discloses a regenerated fiber decolorization method based on controllable phase separation in response polymer fibers, which comprises the following steps: providing a response polymer with reversible hydrophilic / hydrophobic transition characteristics to specific external stimuli; under a first specific external stimulation condition, enabling the response polymer to permeate into the regenerated fiber to be decolored, and forming a polymer-dye compound with dye molecules in the fiber; changing the external stimulation condition to a second specific external stimulation condition, triggering the response polymer permeating into the fiber to generate controllable phase separation, and forming a polymer condensed phase, so as to extract the dye from the fiber matrix; and separating the formed polymer condensed phase from the decolored regenerated fiber. According to the method, efficient and low-damage physical extraction of the dye in the fiber is achieved, the decoloring effect is good, the fiber performance retention rate is high, the response polymer can be recycled, and remarkable economic and environmental benefits are achieved.

Description

Technical Field

[0001] This invention relates to the field of renewable resource recycling technology, and in particular to a method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers. Background Technology

[0002] Regenerated fibers, as an important resource derived from waste textiles or industrial textile waste, are crucial for building a sustainable material cycle and reducing dependence on virgin resources. However, these recycled fibers often retain colors imparted by various dyes, which greatly limits their high-value reuse, especially in applications with specific color requirements or those requiring a light-colored base. Therefore, effective decolorization is a key preliminary step to improve the quality of recycled fibers and broaden their application range.

[0003] Currently, industry and academia have explored and applied various decolorization technologies for recycled fibers. Traditional chemical methods, mainly including oxidative bleaching (such as using hypochlorite, hydrogen peroxide, etc.) and reductive bleaching (such as using sodium hydrosulfite), are among the most widely used technologies. Although these methods can remove color to some extent, their strong chemical action often leads to significant degradation of the fiber macromolecular structure, resulting in a substantial decrease in fiber mechanical strength, reduced degree of polymerization, and poorer spinnability. Furthermore, these processes typically involve large amounts of chemical consumption and the discharge of high-concentration, difficult-to-treat wastewater. The residual chemicals, salts, and dye degradation products in the wastewater pose a serious threat to the environment and increase the complexity and cost of subsequent wastewater treatment. At the same time, for some dark-colored or specific types of dyes, the uniformity and thoroughness of chemical decolorization are often difficult to guarantee.

[0004] As an alternative, solvent extraction attempts to dissolve and remove dyes using organic solvents. However, this method faces challenges such as volatile organic compound (VOC) emissions, potential health risks from solvent handling, and high energy consumption during solvent recovery. Residual solvents in the treated fibers may also affect subsequent processing and end-use. Physical adsorption techniques, such as using adsorbents like activated carbon, can remove dyes from solution, but they often struggle to effectively penetrate the fiber matrix to remove deeply embedded dyes, resulting in decolorization primarily affecting the surface. The regeneration process of saturated adsorbents can also be complex or costly, and may generate secondary pollution. Enzymatic decolorization, as a relatively mild method, is often limited by factors such as enzyme cost, stability, selectivity for specific dye-fiber systems, and slow reaction kinetics, limiting its versatility, especially for synthetic dyes.

[0005] In summary, existing decolorization technologies for regenerated fibers generally face a difficult balance between decolorization efficiency and fiber performance preservation, accompanied by significant environmental and economic cost pressures. Therefore, developing an innovative decolorization method that can completely remove dyes from the interior and surface of regenerated fibers while maximizing the protection of the fiber's inherent properties, and which is also environmentally friendly and economically feasible, has become an urgent technical challenge to be solved in this field. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a new method for decolorizing regenerated fibers in an efficient, environmentally friendly and low-damage manner, in particular to solve the problems of incomplete removal of dyes inside the fibers, easy fiber damage and high environmental impact of the existing technology.

[0007] To address the aforementioned technical problems, this invention provides a method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers.

[0008] Specifically, the method provided by this invention includes the following steps: Step 1 involves providing one or more responsive polymers that exhibit reversible hydrophilic / hydrophobic transition properties in response to specific external stimuli. These responsive polymers are specially molecularly designed, with optimized molecular parameters (e.g., molecular weight, molecular weight distribution, hydrophilic / hydrophobic segment ratio, responsive group density, etc.) to ensure that, under the first specific external stimulus conditions, they not only dissolve or swell significantly in the treatment medium, but more importantly, effectively penetrate the inherent, complex internal pore structure of the regenerated fiber material, including micron- and submicron-sized channels. This deep penetration into the fiber interior is a crucial prerequisite for achieving subsequent efficient decolorization.

[0009] Step 2 involves bringing the regenerated fiber to be decolorized into full contact with the aforementioned responsive polymer under specific external stimuli. Under these conditions, the responsive polymer molecular chains that have penetrated into the fiber will interact non-covalently with the dye molecules present in the fiber pores, such as through hydrophobic interactions, π-π stacking, hydrogen bonding, or electrostatic attraction, thereby forming a polymer-dye complex. This process can be viewed as an "in-situ encapsulation" of the dye molecules inside the fiber.

[0010] Step 3, the core step of this invention, involves changing the external stimulus to a second specific external stimulus. This change in external stimulus triggers a significant phase transition in the responsive polymer that previously penetrated into the pores within the regenerated fiber. Because the polymer's molecular design ensures its phase separation behavior within the confined space of the fiber is controllable, the polymer transforms from a dissolved or swollen state to a contracted or condensed state, forming independent polymer condensates rich in encapsulated dye molecules. These condensates typically manifest as dispersed micro-regions or nano / micro-scale aggregates. Importantly, they effectively "extract" dye molecules from the fiber matrix, while their controllable formation avoids clogging of the fiber channels and facilitates subsequent migration and separation.

[0011] Step 4 involves physically separating the dye-rich polymer condensate from the decolorized regenerated fibers. This can be achieved, for example, through conventional solid-liquid separation methods such as filtration and centrifugation.

[0012] The innovations of this invention are mainly reflected in the following aspects: Firstly, by responding to the "gated" penetration and in-situ encapsulation of polymers in the pores inside the fiber, it can effectively act on dye molecules deep within the fiber, rather than just surface dyes, thereby achieving more thorough decolorization.

[0013] Secondly, by utilizing the controllable phase separation mechanism that occurs within the confined space of the fiber in response polymers, dyes can be physically "extracted" from the fiber matrix. This process mainly relies on the phase change behavior of the polymer, avoiding the use of strong chemical oxidants, reducing agents, or large amounts of organic solvents, thereby significantly reducing chemical damage to fiber materials (such as cellulose, polyester, etc.) and better maintaining the original mechanical properties and physical morphology of the regenerated fibers.

[0014] Third, since the phase transition of the polymer is reversible, the separated dye-rich polymer condensate can be redissolved and release the encapsulated dye by changing the external stimulus conditions again (e.g., returning to the first specific external stimulus condition or selecting another condition conducive to polymer dissolution). In this way, the responding polymer can be recycled and reused in subsequent decolorization batches, and the dye can be enriched for further processing or recycling, significantly reducing material consumption and environmental pollution, and meeting the requirements of green chemistry and sustainable development.

[0015] Fourth, the entire decolorization process is usually carried out under relatively mild conditions, such as a temperature range close to room temperature or pH adjustment near neutral, which reduces energy consumption and simplifies the process.

[0016] To achieve the above method, the external stimulus can be a temperature change, for example, using a temperature-sensitive polymer with a minimum critical dissolution temperature (LCST) or a maximum critical dissolution temperature (UCST). At temperatures below the LCST (or above the UCST), the polymer dissolves and permeates the fiber; when the temperature rises above the LCST (or falls below the UCST), phase separation occurs in the polymer.

[0017] The external stimulus can also be a change in pH, for example, using a pH-sensitive polymer containing acidic or basic groups. Within a certain pH range, the polymer dissolves or swells; changing the pH to another range causes phase separation.

[0018] In addition, external stimuli can also be switching between CO2 and inert gas atmospheres (for CO2-responsive polymers) or changes in specific ion concentrations, or combinations of these stimuli.

[0019] The specific chemical composition of the responsive polymer may include, for example, a homopolymer of N-isopropylacrylamide, or a random copolymer, block copolymer, or graft copolymer formed by N-isopropylacrylamide and one or more other copolymerizable monomers (such as acrylic acid, acrylamide, dimethylaminoethyl methacrylate, etc.). Its permeability and phase change behavior within the fiber can be optimized by controlling the type and ratio of comonomers.

[0020] To promote the penetration of the responsive polymer into the interior of the regenerated fiber, mechanical stirring, ultrasonic-assisted treatment, or dynamic impregnation can be used in the contact step.

[0021] After separating the dye-rich polymer condensate from the decolorized fibers, the decolorized fibers can be properly washed to remove any possible residual polymer.

[0022] Before decolorization, the regenerated fibers can be pretreated, such as by washing to remove impurities and some of the floating color, or by moderate wetting and swelling treatment to improve the polymer's permeability.

[0023] This invention achieves efficient and low-damage physical extraction of dyes by precisely controlling the permeation and phase change behavior of responsive polymers within the complex porous medium of regenerated fibers. This provides an innovative and practically promising technical approach for the high-value utilization of regenerated fibers.

[0024] In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention employs a specially designed responsive polymer and regulates its penetration under a first specific external stimulus to achieve "gated" penetration into the micron- or submicron-sized pore structure within regenerated fibers. This allows the polymer to deeply penetrate the fiber and form complexes with dye molecules in situ. Subsequently, under a second specific external stimulus, these polymers that have penetrated into the fiber undergo controlled, non-clogging phase separation, forming a dye-rich condensed phase that is extracted from the fiber matrix. This mechanism of deep penetration and efficient extraction, compared to traditional surface treatments or methods with insufficient penetration, can more thoroughly remove stubborn dyes from within the fiber, thereby significantly improving the decolorization depth and overall color uniformity of the regenerated fibers.

[0025] 2. This invention primarily relies on the reversible phase transition behavior of the responsive polymer under mild physical conditions (such as suitable temperature and pH range) to achieve the physical encapsulation and extraction of dyes, rather than through strong chemical oxidation, reduction reactions, or the use of large amounts of organic solvents that would damage the dye structure or dissolve the dye. This avoids the degradation and damage to the fiber macromolecular chains (such as cellulose and polyester) under harsh chemical conditions, and can maximize the preservation of the original mechanical strength, degree of polymerization, spinnability, and other important physical properties of the regenerated fibers, achieving low-damage treatment of the fibers and meeting the requirements of green chemistry and sustainable development.

[0026] 3. The responsive polymer used in this invention, after dye extraction, exhibits a dye-rich condensed phase that is easily separated from the fiber. More importantly, by altering the external stimulus conditions, the polymer can be redissolved and the encapsulated dye released, thereby achieving efficient recovery and recycling of the responsive polymer. This not only significantly reduces the single-use consumption and waste of chemical decolorizing agents, lowering material costs, but also significantly alleviates the pressure on subsequent wastewater treatment and the environmental burden, making the entire decolorization process more economical and environmentally friendly.

[0027] 4. The core decolorization process of this invention is typically carried out at temperatures close to room temperature or at pH levels near neutral, eliminating the need for extreme conditions such as high temperature and high pressure, thus reducing energy consumption and equipment requirements. By rationally selecting and designing polymers or combinations thereof with different response types (temperature-sensitive, pH-sensitive, CO2-sensitive, etc.) and adjusting their molecular parameters, it can adapt to the decolorization needs of different types of recycled fibers (such as cotton, polyester, viscose, etc.) and different types of dyes, demonstrating good process versatility. Its relatively simple operation steps and easily controllable process parameters also facilitate industrial-scale production. Detailed Implementation

[0028] Unless otherwise specified, all raw materials and reagents used in the following examples, comparative examples, and test cases are commercially available analytical grade or industrial grade products, which can be used directly or purified using conventional methods. The deionized water used was prepared in the laboratory and had a conductivity of less than 2 μS / cm.

[0029] Main experimental materials: 1. Regenerated fibers: Regenerated cotton fiber (RCF-1): derived from waste pure cotton T-shirts, mechanically opened and impurity removed, with an average fiber length of 22±3mm, a fineness of 1.8±0.3dtex, and an initial color of mixed light colors.

[0030] Regenerated cotton fiber (RCF-2): It comes from factory cutting scraps (mainly denim), and is mechanically opened and impurity removed. The average fiber length is 18±4mm, the fineness is 2.0±0.4dtex, and the initial color is dark blue.

[0031] Recycled polyester staple fiber (RPET-1): It is derived from recycled polyester bottle flakes and is produced through melt spinning and cutting processes. The average fiber length is 38±2mm, the fineness is 1.5±0.2dtex, and the initial color is off-white but slightly yellowish.

[0032] Recycled polyester staple fiber (RPET-2): It is derived from colored waste polyester clothing and is produced by chemical depolymerization and repolymerization, melt spinning and cutting processes. The average fiber length is 45±3mm, the fineness is 1.7±0.2dtex, and the initial color is light gray.

[0033] 2. Main chemical reagents: N-Isopropylacrylamide (NIPAM): Purity ≥98.5%, purchased from Aladdin Reagent (Shanghai) Co., Ltd. Purified by recrystallization in a toluene / n-hexane mixed solvent before use.

[0034] Acrylic acid (AA): purity ≥99.0%, purchased from Sinopharm Chemical Reagent Co., Ltd. Purified by vacuum distillation before use.

[0035] Dimethylaminoethyl methacrylate (DMAEMA): purity ≥99.0%, containing stabilizer MEHQ, purchased from TCI (Shanghai) Chemical Industry Development Co., Ltd. The stabilizer was removed by passing it through an alkaline alumina column before use.

[0036] Azobisisobutyronitrile (AIBN): Purity ≥98.0%, purchased from Maclean Biotechnology Co., Ltd. Purified by recrystallization from ethanol before use.

[0037] Potassium persulfate (KPS): purity ≥99.0%, purchased from Tianjin Kemeo Chemical Reagent Co., Ltd.

[0038] N,N-Dimethylformamide (DMF): Analytical grade, purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Dryed using 4A molecular sieve before use.

[0039] Tetrahydrofuran (THF): chromatographic grade, purchased from Thermo Fisher Scientific (China) Co., Ltd.

[0040] Hydrochloric acid (HCl): analytical grade, 36.0-38.0 wt% aqueous solution, purchased from Beijing Chemical Plant.

[0041] Sodium hydroxide (NaOH): analytical grade, flakes, content ≥96.0%, purchased from Tianjin Damao Chemical Reagent Factory.

[0042] Nonionic surfactant (JFC): fatty alcohol polyoxyethylene ether, industrial grade, purchased from BASF (China) Co., Ltd.

[0043] Sodium hypochlorite (NaClO): analytical grade, available chlorine content ≥5.0%, purchased from Sinopharm Chemical Reagent Co., Ltd.

[0044] Sodium dithionite (Na2S2O4): Industrial grade, content ≥85.0%, purchased from Shandong Jinhe Industrial Group Co., Ltd.

[0045] Examples of preparation of responsive polymers: Example P1: Preparation of the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM-1) 10.0 g (88.4 mmol) of purified N-isopropylacrylamide (NIPAM) monomer was weighed and dissolved in 80 mL of deionized water that had been deoxygenated under nitrogen for 30 minutes. 0.10 g (0.37 mmol) of potassium persulfate (KPS) was added as an initiator. The mixture was magnetically stirred in a water bath at 70 ± 1 °C for 8 hours under nitrogen protection. After the reaction was complete, the reaction solution was cooled to room temperature and then heated to above 40 °C to precipitate the polymer. The precipitate was collected by centrifugation, washed three times with deionized water at 40-50 °C, and then dried to constant weight in a vacuum drying oven at 45 °C to obtain a white powder, PNIPAM-1.

[0046] According to GPC analysis (THF as mobile phase, polystyrene as standard), the number-average molecular weight (Mn) of PNIPAM-1 was 18,500 g / mol, and the polydispersity index (PDI) was 1.65. Its lowest critical temperature of dissolution (LCST) in a 0.5 wt% aqueous solution was determined by UV-Vis spectrophotometry to be 32.5 °C.

[0047] Example P2: Preparation of thermosensitive / pH dual-responsive copolymer P(NIPAM-co-AA)-1 Weigh 8.50 g (75.1 mmol) of purified NIPAM monomer and 0.54 g (7.5 mmol) of purified acrylic acid (AA) monomer (NIPAM to AA molar ratio 90:10), and dissolve them together in 100 mL of N,N-dimethylformamide (DMF) after deoxygenation under nitrogen for 30 minutes. Add 0.15 g (0.91 mmol) of azobisisobutyronitrile (AIBN) as an initiator. Under nitrogen protection, the mixture is magnetically stirred in an oil bath at 70 ± 1 °C for 12 hours. After the reaction is complete, the reaction solution is cooled to room temperature and then added dropwise to excess diethyl ether to precipitate. Filter and collect the precipitate, wash 2-3 times with diethyl ether, and then dry to constant weight in a vacuum drying oven at 45 °C to obtain a white powder, P(NIPAM-co-AA)-1.

[0048] According to GPC analysis (DMF containing 0.05M LiBr as the mobile phase and polystyrene as the standard), the number-average molecular weight (Mn) of P(NIPAM-co-AA)-1 was 25,200 g / mol, and the polydispersity index (PDI) was 1.82. The LCST (lower limit of dissolved oxygen) in a 0.5 wt% aqueous solution at pH 7.0 was determined to be 38.8 °C using UV-Vis spectrophotometry. At pH 4.0, due to the protonation of the AA units, no obvious LCST behavior was observed in the polymer within the 5-60 °C range, but a pH-induced dissolution-precipitation transition was observed, with the pH response point (turbidity abrupt change) approximately between pH 4.8 and 5.2.

[0049] Example P3: Preparation of copolymer P(NIPAM-co-DMAEMA)-1 optimized for specific permeation and phase separation. 7.92 g (70.0 mmol) of purified NIPAM monomer and 1.57 g (10.0 mmol) of purified dimethylaminoethyl methacrylate (DMAEMA) monomer (NIPAM to DMAEMA molar ratio 87.5:12.5) were weighed and dissolved together in 90 mL of deionized water deoxygenated under nitrogen for 30 minutes. 0.12 g (0.44 mmol) of KPS was added as an initiator. The mixture was magnetically stirred in a constant temperature water bath at 65 ± 1 °C for 10 hours under nitrogen protection. After the reaction was complete, the pH of the solution was adjusted to 9.0, and then the temperature was raised to 50 °C to precipitate the polymer. The precipitate was collected by centrifugation, washed three times with deionized water at pH 9.0 and 40-50℃, and then dried to constant weight in a vacuum drying oven at 45℃ to obtain a light yellow powder P(NIPAM-co-DMAEMA)-1.

[0050] According to GPC analysis (with 0.1 M NaNO3 in the aqueous mobile phase and polyethylene glycol as the standard), the number-average molecular weight (Mn) of P(NIPAM-co-DMAEMA)-1 is 22,100 g / mol, and the polydispersity index (PDI) is 1.75. The LCST in a 0.5 wt% aqueous solution at pH 7.4 was 34.2 °C, determined by UV-Vis spectrophotometry; at pH 5.0, the LCST increased to 41.5 °C due to partial protonation of the DMAEMA units. Because this polymer contains cationic DMAEMA units, it is expected to have better interactions with negatively charged fiber surfaces and dyes at specific pH levels, and its phase transition behavior is dually regulated by pH and temperature.

[0051] Example of decolorization process for regenerated fibers: Example 1: Decolorization of dyed regenerated cotton fibers (RCF-2) using PNIPAM-1 1.1 Fiber pretreatment: Take 10.0g of oven-dry weight of dyed recycled cotton fiber (RCF-2), add it to 200mL of deionized water containing 1.0g / L nonionic surfactant (JFC), stir and wash in a 50℃ water bath for 40 minutes, then wash with deionized water until neutral, squeeze dry and set aside.

[0052] 1.2 Preparation of Response Polymer Solution: Weigh 2.0 g of PNIPAM-1 prepared in Example P1 and dissolve it in 198 mL of deionized water to prepare a 1.0 wt% polymer solution.

[0053] 1.3 “Gated” Infiltration and In-situ Dye Encapsulation: Pretreated moistened RCF-2 fibers (containing approximately 10 g of water) were added to the above PNIPAM-1 solution (total liquid bath ratio approximately 1:20). The solution was mechanically stirred at 80 rpm for 90 minutes at 20°C (lower than the LCST of PNIPAM-1, 32.5°C).

[0054] 1.4 Controllable phase separation of polymer and dye extraction: The above system was rapidly heated to 45°C (higher than the LCST of PNIPAM-1), and stirred at low speed (30 rpm) for 45 minutes at this temperature to allow PNIPAM-1 to undergo phase separation inside the fiber and in the solution, encapsulating the dye to form a condensed phase.

[0055] 1.5 Separation and Post-treatment: While still hot (45℃), filter through a 200-mesh stainless steel filter to separate the fibers from the treatment solution. Collect the fibers and rinse them once quickly with 100mL of deionized water at 45℃. Dry the decolorized fibers in a 70℃ oven for 6 hours. Collect the filtrate and rinsing solution (a polymer condensed phase suspension rich in dye).

[0056] Example 2: Decolorization of dyed recycled polyester fiber (RPET-2) using P(NIPAM-co-AA)-1 1.1 Fiber pretreatment: Take 10.0g of dyed recycled polyester fiber (RPET-2) by oven dry weight and perform cleaning pretreatment as in step 1.1 of Example 1.

[0057] 1.2 Preparation of Response Polymer Solution: Weigh 2.5g of P(NIPAM-co-AA)-1 prepared in Example P2, dissolve it in 247.5mL of deionized water, adjust the pH to 7.0 with 0.1M NaOH solution, and prepare a 1.0wt% polymer solution.

[0058] 1.3 “Gated” Infiltration and In-situ Dye Encapsulation: Pretreated, moistened RPET-2 fibers were added to the above P(NIPAM-co-AA)-1 solution. The solution was mechanically stirred at 100 rpm for 120 minutes at 25°C (below the polymer's LCST of 38.8°C at pH 7.0) and pH 7.0.

[0059] 1.4 Controlled Phase Separation of Polymer and Dye Extraction: Phase transition was triggered in two steps: First, the system temperature was rapidly increased to 45°C (above the LCST of the polymer at pH 7.0). Then, 0.5M HCl solution was slowly added dropwise to adjust the pH of the system to 4.0 (the pH range in which the polymer precipitates). Under these conditions, low-speed stirring (30 rpm) was continued for 60 minutes.

[0060] 1.5 Separation and Post-treatment: Fibers and treatment solution were separated by filtration through a 200-mesh stainless steel filter at pH 4.0 and 45℃. The fibers were collected and quickly rinsed once with 50 mL of deionized water at pH 4.0 and 45℃. The decolorized fibers were then dried in a 70℃ oven for 6 hours. The filtrate and rinsing solution were collected.

[0061] Example 3: Decolorization of dyed regenerated cotton fibers (RCF-1) and polymer recovery using P(NIPAM-co-DMAEMA)-1. 1.1 Fiber pretreatment: Take 10.0g of oven-dry weight dyed recycled cotton fiber (RCF-1) and perform cleaning pretreatment as in step 1.1 of Example 1.

[0062] 1.2 Preparation of Response Polymer Solution: Weigh 3.0 g of P(NIPAM-co-DMAEMA)-1 prepared in Example P3, dissolve it in 297 mL of deionized water, adjust the pH to 6.0 with 0.1 M HCl solution, and prepare a 1.0 wt% polymer solution.

[0063] 1.3 “Gated” Infiltration and In-situ Dye Encapsulation: Pretreated, moistened RCF-1 fibers were added to the above P(NIPAM-co-DMAEMA)-1 solution. The solution was mechanically stirred at 80 rpm for 100 minutes at 20°C (below the polymer's LCST at pH 6.0) and pH 6.0.

[0064] 1.4 Controllable phase separation of polymer and dye extraction: The above system was rapidly heated to 50°C (higher than the LCST of the polymer at pH 6.0) and stirred at low speed (30 rpm) for 50 minutes at this temperature.

[0065] 1.5 Separation and Post-treatment: While still hot (50℃), filter through a 200-mesh stainless steel filter to separate the fibers from the treatment solution. Collect the fibers and rinse them once quickly with 80mL of 50℃ deionized water. Dry the decolorized fibers in a 70℃ oven for 6 hours.

[0066] 1.6 Polymer Regeneration and Recovery: Combine the filtrate and rinsing solution collected in step 1.5, cool to 20°C, and then adjust the pH to 9.0 with 0.5M NaOH solution. At this point, P(NIPAM-co-DMAEMA)-1 redissolves, while some dye may still exist as insoluble matter or have reduced affinity for the polymer. Separate the dye precipitate or insoluble matter by centrifugation (5000 rpm, 15 minutes). Take the supernatant (recovered polymer solution) and determine its solid content to calculate the polymer recovery rate. Add a small amount of fresh P(NIPAM-co-DMAEMA)-1 to the initial concentration and volume of the recovered polymer solution, adjust the pH to 6.0, and use it for the next decolorization cycle (repeat the decolorization cycle twice).

[0067] Comparative Examples 1-4: Comparative Example 1: Decolorization using conventional non-responsive adsorbents Compared with Example 1, the differences are as follows: PNIPAM-1 is not used in step 1.2, but an equal mass (2.0 g) of activated carbon powder (200 mesh) is dispersed in 198 mL of deionized water as the treatment solution; in step 1.3, the mixture is stirred at 20°C for 90 minutes; in step 1.4, the temperature is raised to 45°C and stirred for 45 minutes (at which time the activated carbon does not undergo a phase change); the other conditions and operations are the same.

[0068] Comparative Example 2: Decolorization using traditional chemical oxidation method The difference compared to Example 1 is that the polymer treatment in steps 1.2-1.4 is omitted. 10.0 g of pretreated RCF-2 fiber was added to a sodium hypochlorite solution containing 5.0 g / L available chlorine (pH adjusted to 10.5, bath ratio 1:20) and treated at 40°C for 60 minutes. After treatment, it was thoroughly washed with deionized water until neutral, and a small amount of sodium thiosulfate solution was added to remove residual chlorine. After washing again, it was dried according to step 1.5 of Example 1.

[0069] Comparative Example 3: Using the PNIPAM-1 of this invention, but failing to achieve effective "gated permeation" i.e., phase separation. Compared with Example 1, the difference is that in step 1.3, after the permeation time at 20°C is shortened from 90 minutes to 5 minutes, the system is immediately heated to 45°C to carry out the phase separation and extraction operation in step 1.4; the other conditions and operations are the same.

[0070] Comparative Example 4: Using P(NIPAM-co-AA)-1 as described in this invention, but improper control of phase separation conditions led to differences in polymer behavior. Compared with Example 2, the difference is that in step 1.4, after the system temperature is raised to 45°C, a sufficient amount of 1M HCl solution is quickly added at once to drastically adjust the pH of the system to 2.0 (far below its precipitation pH range), and the system is stirred for 60 minutes under these conditions; the other conditions and operations are the same.

[0071] Test Examples 1-3: Test Example 1: Comparison of Decolorization Effects Experimental instructions and procedures: Whiteness, color difference, and apparent color depth (K / S value) were measured for the original regenerated fiber samples to be decolorized (RCF-1, RCF-2, RPET-1, RPET-2, selected according to the specific fiber type used in the examples and comparative examples) and the decolorized regenerated fiber samples obtained after treatment in Examples 1-3 and Comparative Examples 1-4, in order to evaluate the decolorization effect of different treatment methods.

[0072] 1. Sample preparation: Take about 2.0g of dried fiber sample from each treatment group, comb it and press it into a circular test piece with uniform thickness and a flat surface (about 30mm in diameter and 2-3mm in thickness). Prepare 3 parallel test pieces for each sample.

[0073] 2. Whiteness Test: A digital whiteness meter conforming to GB / T 7974 standard was used to test the whiteness of each sample. Whiteness values ​​were recorded at five points at different locations for each sample, and the average value was taken as the whiteness value of that sample. Finally, the average whiteness value of three parallel samples was calculated. The CIE whiteness value was recorded.

[0074] 3. Color Difference Test: Color parameters of each sample were tested using a colorimeter. The light source was D65, and the observer's viewing angle was 10°. The instrument was calibrated using a standard white board and black tube before testing. L values ​​were taken from 5 points at different locations on each sample. * ,a * ,b * The values ​​are averaged. Using the untreated original dyed fibers as a reference, the total color difference ΔE of each treated sample is calculated. * The calculation formula is: Where ΔL * ,Δa * ,Δb * The treated sample and the original sample are L respectively. * ,a * ,b * The difference in values.

[0075] 4. Apparent Color Depth (K / S Value) Test: Using a colorimetric spectrophotometer, scan the spectral reflectance curves of each sample within the visible light range (400-700nm). Select the wavelength of maximum absorption (λ). max The reflectance R at a wavelength (or, for samples without a distinct absorption peak, the wavelength most sensitive to color changes, such as blue typically at 600-650 nm, and gray evaluated across the entire visible light range) is used to calculate the apparent color depth K / S value according to the Kubelka-Munk equation: K / S = (1-R). 2 / (2R).

[0076] The experimental data are shown in Table 1: Table 1 Comparison of the decolorization effects of different treatment methods on regenerated fibers As can be seen from the data in Table 1, the regenerated fibers treated using the methods proposed in this invention (Examples 1, 2, and 3) have higher CIE whiteness and L... * The K / S values ​​were significantly higher than those of the original dyed fibers and the samples of Comparative Example 1 (treated with conventional adsorbent) and Comparative Example 2 (treated with traditional chemical oxidation method). Simultaneously, their K / S values ​​were also significantly reduced, indicating that the dye was effectively removed and the decolorization effect was excellent. This is mainly attributed to the responsive polymer used in this invention, whose molecular structure is designed to achieve effective "gated" penetration into the microporous structure inside the fiber under a first specific external stimulus. This ability to penetrate deep into the fiber allows the polymer molecular chain segments to fully interact in situ with dye molecules deeper within the fiber pores and form complexes, rather than merely interacting on the fiber surface or in large pores, thus laying the foundation for subsequent efficient extraction.

[0077] Compared to Comparative Examples 3 and 4, which used the same or similar responsive polymers but had improperly controlled process conditions, the decolorization effect of the embodiments of the present invention is also superior. For example, in Comparative Example 3, due to insufficient penetration time, the responsive polymer failed to fully penetrate into the fiber interior. Even if phase separation was subsequently triggered, it could not effectively capture the dye inside the fiber, resulting in poor decolorization. In Comparative Example 4, although the polymer may have penetrated, improper control of phase separation conditions (such as drastic pH changes) may have caused uncontrollable, rapid, or large-scale aggregation of the polymer inside the fiber, which may have partially blocked the pores or reduced the encapsulation efficiency, affecting the effective extraction and migration of the dye. This further confirms the importance of the core technical features of the present invention: not only does the polymer need to be responsive, but more importantly, it needs to achieve its "gated" penetration behavior within the confined space of the fiber and its subsequent "controllable phase separation" behavior. These two synergistic effects are necessary to ensure that the polymer forms dispersed, dye-rich, and easily separable micro-regions inside the fiber, thereby efficiently extracting the dye from the fiber matrix.

[0078] In summary, this invention overcomes the limitations of traditional methods in deep decolorization and fiber protection by designing a responsive polymer system and regulating its unique behavior within the fiber—namely, effective internal penetration and in-situ dye complexation—combined with subsequent controllable phase separation within a confined space to achieve physical extraction of the dye. This decolorization mechanism, based on the specific physicochemical behavior of polymers in the fiber microenvironment, provides an innovative solution for the efficient, low-damage, and environmentally friendly treatment of regenerated fibers, significantly outperforming traditional decolorization techniques that rely on simple physical adsorption or strong chemical reactions.

[0079] Test Example 2: Comparison of Damage to Fiber Physical Properties Experimental instructions and procedures: The original regenerated fiber samples to be decolorized (RCF-2, RPET-2, selected according to the specific fiber type used in the examples and comparative examples) and the decolorized regenerated fiber samples obtained after treatment in Examples 1-3 and Comparative Examples 1-4 were tested for single fiber breaking strength and breaking elongation to evaluate the degree of damage to the physical properties of the fibers by different treatment methods.

[0080] 1. Sample conditioning: Condition all fiber samples to be tested (raw fibers and fibers after each treatment) under standard atmospheric conditions (temperature 20±2℃, relative humidity 65±5%) for at least 24 hours.

[0081] 2. Single fiber sample preparation: Randomly select a single fiber from each fiber sample after conditioned, avoiding the selection of fibers with obvious damage or nodules.

[0082] 3. Single fiber tensile and elongation test: The test shall be conducted in accordance with GB / T 14337 (Test Method for Tensile Properties of Short Chemical Fibers). The spacing length shall be set to 10 mm, and the tensile speed shall be 10 mm / min. At least 50 single fibers shall be tested in each sample group. The breaking strength (cN) and breaking elongation (%) of each fiber shall be recorded.

[0083] 4. Data Processing: Calculate the average breaking strength and average breaking elongation of 50 single fibers in each sample group. Calculate the strength retention rate of each treated fiber relative to the original fiber, using the formula: Strength Retention Rate (%) = (Average Breaking Strength of Treated Fiber / Average Breaking Strength of Original Fiber) × 100%; The experimental data are shown in Table 2: Table 2. Effects of different treatment methods on the physical properties of regenerated fibers Note: Strength retention rate is calculated relative to the corresponding type of raw fiber.

[0084] Table 2 clearly shows that the regenerated fibers treated with the decolorization method proposed in this invention (Examples 1 and 2) have significantly higher average breaking strength and strength retention rate than fibers treated with the traditional chemical oxidation method (Comparative Example 2). For example, the RCF-2 fiber treated in Example 1 achieved a strength retention rate of 91.4%, while the chemically treated Comparative Example 2 only achieved 55.8%. This indicates that the method of this invention can better protect the mechanical properties of the fibers and reduce damage during the treatment process. The fundamental reason is that the decolorization mechanism of this invention mainly relies on the phase change behavior of the responding polymer under relatively mild physical conditions. It achieves "physical extraction" through polymer penetration into the fiber, physical encapsulation of the dye, and subsequent controllable phase separation, rather than using strong chemical reagents to chemically destroy the dye molecules. This avoids the significant degradation effect of strong oxidants or reducing agents on the fiber macromolecular chains (such as cellulose chains or polyester chains).

[0085] Compared to comparative examples (such as Comparative Examples 3 and 4) that used the same responsive polymer but had improper process control, the embodiments of the present invention also demonstrate advantages in maintaining fiber strength. For example, in Comparative Example 3, although the final decolorization effect was not as good as the examples, its fiber damage was also relatively small, indicating the gentleness of the interaction between the polymer and the fiber. In contrast, in Comparative Example 4, if the phase separation process was too violent or local conditions (such as pH) were not properly controlled, leading to unexpected polymer aggregation or adverse interactions with the fiber, it could also cause some physical damage to the fiber, although usually much less than the damage caused by chemical methods. This further emphasizes the importance of the "controllable phase separation" technical feature in the present invention, ensuring that the phase change behavior of the polymer inside the fiber is gentle and orderly, thereby minimizing the negative impact on the fiber structural integrity while efficiently extracting dye.

[0086] Therefore, this invention, through the core mechanism of "gated" penetration and "controlled phase separation" of polymers within the confined space inside the fiber, not only achieves highly efficient deep decolorization, but more importantly, its gentle physical process effectively avoids the severe fiber degradation problems often associated with traditional chemical decolorization methods. This low-damage characteristic allows the treated recycled fibers to better maintain their original physical and mechanical properties, which is crucial for subsequent high-value reuse (such as re-spinning and manufacturing high-quality recycled products), fully demonstrating the technological advantages of this invention in environmental protection and sustainable resource utilization.

[0087] Test Example 3: Performance Evaluation of Response Polymer Recycling and Reuse Experimental instructions and procedures: This test case aims to evaluate the recycling efficiency of the responsive polymer P(NIPAM-co-DMAEMA)-1 used in Example 3 and its retention of decolorization properties on recycled cotton fibers (RCF-1) after multiple cycles.

[0088] 1. Initial decolorization and polymer recovery: The first decolorization treatment of RCF-1 was carried out according to steps 1.1 to 1.5 of Example 3.

[0089] Polymer regeneration and recovery were performed according to step 1.6 of Example 3: The filtrate and rinsing solution collected from the initial decolorization were combined, cooled, and the pH was adjusted to 9.0 to redissolve the polymer. The dye precipitate was separated by centrifugation. The supernatant (recovered polymer solution) was taken, and its volume was accurately measured. A certain volume of the recovered polymer solution was taken and dried in an oven at 105°C to constant weight. The mass of the remaining solid was weighed to calculate the concentration of the recovered polymer solution. Based on the volume and concentration of the recovered solution, the mass of the polymer recovered initially was calculated and compared with the initial mass of polymer added (3.0 g) to calculate the initial polymer recovery rate.

[0090] Recovery rate (%) = (dry weight of recovered polymer / dry weight of initially added polymer) × 100%; 2. Evaluation of polymer recycling and decolorization effect: The initially recovered polymer solution was replenished with a small amount of fresh P(NIPAM-co-DMAEMA)-1 solid, based on its measured concentration, to restore its concentration to 1.0 wt%. Simultaneously, deionized water was added to restore the total volume to approximately the same as the initial volume (about 300 mL) of the polymer solution used for the initial decolorization. The pH was adjusted to 6.0 using 0.1 M HCl solution.

[0091] Using the adjusted recycled polymer solution described above, a second decolorization treatment was performed on a new batch of 10.0 g of pretreated RCF-1 fibers, following the same procedures as steps 1.3 to 1.5 of Example 3. The whiteness value of the fibers after decolorization was recorded.

[0092] Repeat the above steps of polymer recovery (the second half of step 1, i.e., centrifuging to separate the dye, measuring the concentration and mass of the recovered polymer, and calculating the recovery rate for this cycle) and recycling (adjusting the concentration, volume, and pH before using it for the next round of decolorization), for a total of 3 cycles of decolorization (i.e., a total of 4 batches of fibers decolorized, including the first decolorization).

[0093] Record the CIE whiteness value of the decolorized fibers after each cycle of use, as well as the recovery rate of each polymer recycling.

[0094] The experimental data are shown in Table 3: Table 3 Recovery rate and cyclic decolorization performance of the responsive polymer P(NIPAM-co-DMAEMA)-1 Loop count Polymer recovery rate (%) CIE whiteness of RCF-1 after decolorization First decolorization --- 78.2 (Data from Example 3) First recycling 93.5 --- Second bleaching --- 76.8 Second recycling 91.8 --- 3rd bleaching --- 75.1 3rd recycling 90.3 --- 4th bleaching --- 73.9 Note: Whiteness data for the first decolorization is cited from Example 3. Polymer recovery rate refers to the percentage of polymer recovered from the treatment solution in the current operation relative to the polymer previously added to the system.

[0095] Table 3 shows that the responsive polymer used in this invention has excellent recovery performance and reusability. In terms of polymer recovery rate, even after multiple cycles, the recovery rate of P(NIPAM-co-DMAEMA)-1 remains above 90% (93.5% for the first recovery and 90.3% for the third), indicating that the polymer experiences minimal loss during multiple dissolution-phase separation-redissolution cycles. This efficient recovery is attributed to the reversible response of the responsive polymer to external stimuli (such as temperature and pH). After dye extraction, by simply changing the external conditions (e.g., in Example 3, changing from low temperature and high pH to high temperature and low pH to dissolve the polymer, and then cooling and increasing the pH to redissolve the polymer and release the dye), the polymer can effectively transition from a dye-rich condensed phase to a dissolved state, facilitating separation and recovery from the dye precipitate, thus laying the material foundation for recycling.

[0096] From the perspective of cyclic decolorization effect, although the CIE whiteness of the decolorized recycled cotton fiber decreased slightly with the increase of the number of cycles (from 78.2 in the first cycle to 73.9 in the fourth cycle), it remained at a high level overall, indicating that the recovered responsive polymer maintained good decolorization ability after multiple uses. The slight decrease in whiteness may be due to the gradual accumulation of a small amount of irreversibly adsorbed dye or impurities on the polymer chain, or the occurrence of minor structural changes or losses in the polymer during repeated treatment. However, the maintenance of this performance further verifies the effectiveness of the polymer's "controllable phase separation" mechanism in this invention, that is, the phase transition process of the polymer is relatively mild and reversible, and does not lead to serious damage to its key functional groups or significant loss of responsive activity due to multiple cycles.

[0097] Therefore, the decolorization method based on controllable phase separation within responsive polymer fibers proposed in this invention not only demonstrates excellent single-batch decolorization efficiency and fiber protection, but also possesses significant economic and environmental benefits due to the highly efficient recyclability and reusability of the responsive polymer in its core material. By enabling the polymer's sensitive and reversible response to external stimuli, the decolorizing agent can be reused multiple times, significantly reducing chemical consumption and waste emissions, and lowering processing costs. This aligns perfectly with the current urgent need for green and sustainable manufacturing technologies, highlighting the enormous potential of this invention in industrial applications.

[0098] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers, characterized in that, Includes the following steps: (a) Provides a responsive polymer with reversible hydrophilic / hydrophobic switching properties in response to external stimuli, said responsive polymer being able to penetrate into the internal pore structure of regenerated fibers under a first specific external stimulus condition; (b) The regenerated fiber is contacted with the responsive polymer under the first specific external stimulus condition, such that the responsive polymer penetrates into the interior of the regenerated fiber and interacts with dye molecules in the fiber to form a polymer-dye complex; (c) Change the external stimulus conditions to a second specific external stimulus condition, so that the responsive polymer that has penetrated into the internal pores of the regenerated fiber undergoes controllable phase separation to form a dye-rich, independent polymer condensate phase, which encapsulates the dye molecules and separates from the fiber matrix. (d) Separating the dye-rich polymer condensate phase from the decolorized regenerated fiber; The molecular parameters of the responsive polymer are designed to achieve micron- to submicron-scale pore structures that enter the regenerated fiber under the first specific external stimulus condition, and to cause controllable, non-clogging phase separation in the pores inside the fiber under the second specific external stimulus condition. The polymer condensate formed in step (c) is a dispersed micro-region or nano / micro-scale aggregate.

2. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 1, characterized in that, The external stimulus is selected from one or more combinations of temperature, pH value, CO2 / inert gas atmosphere, and ion concentration.

3. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 2, characterized in that, The responsive polymer is a thermosensitive polymer, the first specific external stimulus condition is a temperature below its minimum critical dissolution temperature or above its maximum critical dissolution temperature, and the second specific external stimulus condition is a temperature above its minimum critical dissolution temperature or below its maximum critical dissolution temperature.

4. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 2, characterized in that, The responsive polymer is a pH-sensitive polymer, the first specific external stimulus condition is a pH value that causes the polymer to be in a dissolved or highly swollen state, and the second specific external stimulus condition is a pH value that causes the polymer to undergo phase separation.

5. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 1, characterized in that, The responsive polymer comprises a homopolymer of N-isopropylacrylamide, or a copolymer of N-isopropylacrylamide with at least one other copolymerizable monomer.

6. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 5, characterized in that, The copolymer is a copolymer of N-isopropylacrylamide and acrylic acid, wherein the molar ratio of N-isopropylacrylamide to acrylic acid is 85:15 to 95:

5.

7. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 5, characterized in that, The copolymer is a copolymer of N-isopropylacrylamide and dimethylaminoethyl methacrylate, wherein the molar ratio of N-isopropylacrylamide to dimethylaminoethyl methacrylate is 85:15 to 90:

10.

8. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 1, characterized in that, The process of the responsive polymer penetrating into the interior of the regenerated fiber in step (b) is assisted by at least one of mechanical stirring, ultrasonic assistance, or dynamic impregnation.

9. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 1, characterized in that, The method further includes step (e): The external conditions of the separated dye-rich polymer condensate are adjusted back to the first specific external stimulus condition or another condition that is conducive to polymer dissolution, so that the polymer is redissolved and the dye is released, and the polymer is recovered for recycling.

10. The method for decolorizing regenerated fibers based on controllable phase separation within responsive polymer fibers according to claim 1, characterized in that, Prior to step (b), a pretreatment step is included to clean and / or wet and swell the regenerated fibers.

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