A chlorine-free bleaching agent for cotton linter pulp and a bleaching method

Abstract

The present application relates to a kind of cotton linter pulp chlorine-free bleaching agent and bleaching method, by mass fraction, including persulfate 1.4~7.9 parts, pyrophosphate 2.7~9.2 parts, polyphosphate 1.4~7.9 parts, anionic surfactant 1.4~7.9 parts, 1000 parts of water;The pH of the bleaching agent is 8.5~11.0.The present application is mainly used as persulfate, and low pollution bleaching agent is formed by being matched with pyrophosphate, polyphosphate, anionic surfactant, with good bleaching effect, can effectively remove the colored material attached to cellulose, the degree of damage to cellulose is small, the obtained refined cotton whiteness is above 70, the degree of polymerization is above 1000, the relative unbleached cotton linter pulp is reduced by 15%~64%, without bleaching terminator;The bleaching method of the present application is mild, does not contain chlorine element, reduces pollution load, the COD content in waste liquid after bleaching is below 280 mg / L, green environmental protection.

Description

Technical Field

[0001] This invention relates to the field of bleaching, specifically to a chlorine-free bleaching agent and bleaching method for cotton linter pulp. Background Technology

[0002] Cotton linter pulp is generally produced through the caustic soda process. After bleaching, the resulting refined cotton product can be used in the production of cellulose esters, nitrocellulose, and cellulose acetate, and has a wide range of applications. To strengthen environmental protection and promote green development, the bleaching industry's shift towards elemental chlorine-free (ECF) bleaching and total chlorine-free (TCF) bleaching is an inevitable trend. Currently, domestic refined cotton production mainly involves high-temperature alkaline cooking of cotton linters followed by ECF or TCF bleaching.

[0003] In ECF (Electro-Cured Fluorescent Cotton), chlorine dioxide and sodium hypochlorite are commonly used bleaching agents. Patent CN202110932036.3 discloses a method for preparing a stable chlorine dioxide bleaching solution, which directly introduces chlorine gas and chlorine dioxide into a buffer absorbent, causing the chlorine gas to react and generate hypochlorite with bleaching properties. This method effectively treats the chlorine gas while improving the bleaching effect of the stable chlorine dioxide bleaching solution. Chlorine dioxide can selectively oxidize lignin and colored substances, resulting in refined cotton products with higher whiteness and less fiber damage. However, organic chlorides will still be generated in the bleaching wastewater.

[0004] In TCF (Total Fluidized Cotton), hydrogen peroxide is a widely used bleaching agent. Patent CN201510453418.2 discloses an apparatus and method for bleaching cotton linters with high concentrations of hydrogen peroxide to prepare refined cotton. This method uses a mixture of H2O2, an activator, and an alkaline solution as the bleaching agent to treat the cotton linters. Although H2O2 can effectively reduce or eliminate the colored groups of lignin under alkaline conditions and can break down lignin to dissolve it, hydrogen peroxide is unstable and easily decomposes, especially in the presence of transition metal ions, which accelerates the decomposition. Furthermore, hydrogen peroxide itself is a weak oxidizing agent, and its effect is not ideal, especially when treating lignin that has already been passivated after cooking. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a chlorine-free bleaching agent and bleaching method for cotton linter pulp, thereby solving the technical problems of easy generation of organic chlorides or poor bleaching effect in the bleaching of cotton linter pulp in the prior art.

[0006] To achieve the above-mentioned technical objectives, the technical solution provided by the present invention is as follows: In a first aspect, the present invention provides a chlorine-free bleaching agent for cotton linter pulp, comprising, by weight, 1.4 to 7.9 parts of persulfate, 2.7 to 9.2 parts of pyrophosphate, 1.4 to 7.9 parts of polyphosphate, 1.4 to 7.9 parts of anionic surfactant, and 1000 parts of water.

[0007] Secondly, the present invention provides a bleaching method for cotton linter pulp, comprising the following steps: soaking cotton linter pulp in a bleaching agent, mixing evenly to obtain a mixture; adjusting the pH of the mixture to alkaline, carrying out a bleaching reaction, and washing and drying after the reaction to complete the bleaching of cotton linter pulp.

[0008] Compared with the prior art, the beneficial effects of the present invention include: This invention uses persulfate as the main bleaching agent, combined with pyrophosphate, polyphosphate, and anionic surfactant to form a low-pollution, chlorine-free bleaching agent for bleaching cotton lint pulp. The pyrophosphate and polyphosphate form a polyphosphate system, which effectively enhances the activation ability of persulfate by forming superoxide radicals and singlet oxygen. The anionic surfactant has good surface activity and strong hydrophilicity, effectively reducing the tension at the oil-water interface, improving bleaching efficiency, and also playing a certain role in detergency. The bleaching agent of this invention has excellent bleaching effect, effectively removing colored substances attached to cellulose with minimal damage to the cellulose. The bleaching method of this invention is mild, chlorine-free, and requires no bleaching terminator, thus reducing the pollution load. The COD content in the waste liquid after bleaching is below 280 mg / L, making it green and environmentally friendly. Bleaching cotton linter pulp can produce refined cotton with high whiteness and high degree of polymerization. The refined cotton has a whiteness of over 70 and a degree of polymerization of over 1000, with a relative degree of depolymerization of 15% to 64% compared to unbleached cotton linter pulp. Attached Figure Description

[0009] Figure 1 This invention relates to the effect of PA concentration on the bleaching effect of cotton linter pulp. Figure 2 This invention relates to the effect of PB concentration on the bleaching effect of cotton linter pulp. Figure 3 This invention relates to the effect of SDBS concentration on the bleaching effect of cotton linter pulp. Figure 4 This invention relates to the effect of bleaching temperature on the bleaching effect of cotton linter pulp. Figure 5 This invention relates to the effect of bleaching time on the bleaching effect of cotton linter pulp. Figure 6 This invention relates to the effect of pH on the bleaching effect of cotton linter pulp. Figure 7 This invention relates to the effect of PMS concentration on the bleaching effect of cotton linter pulp. Figure 8 This is the FTIR diffraction pattern of cellulose cotton linter pulp before and after bleaching in Example 8 of this invention; Figure 9 This is the XRD pattern of cotton linter pulp and refined cotton in Example 8 of this invention; Figure 10 These are SEM images of cotton linter pulp and refined cotton in Example 8 of this invention, wherein (a) is cotton linter pulp, magnified 200 times; (b) is cotton linter pulp, magnified 500 times; (c) is refined cotton, magnified 200 times; and (d) is refined cotton, magnified 500 times. Figure 11 This is a diagram showing the quenching experiment results in this invention; Figure 12 The sulfate radical (SO4·) in different reaction systems in this invention - EPR spectra of hydroxyl radicals (·OH) and hydroxyl radicals (·OH); Figure 13 Different reaction systems in this invention 1 EPR spectrum of O2; Figure 14 These are the EPR spectra of O2·- in different reaction systems in this invention. Detailed Implementation

[0010] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0011] Given that existing elemental chlorine-free bleaching processes easily generate pollutants, such as organochlorides, and that total chlorine-free (TCF) bleaching processes have low bleaching efficiency, this invention provides a chlorine-free bleaching agent and method for cotton linter pulp. Using potassium persulfate as the main bleaching agent, combined with sodium pyrophosphate, sodium tripolyphosphate, and sodium dodecylbenzene sulfonate to form a low-pollution bleaching agent, this method bleaches cotton linter pulp, resulting in refined cotton with high whiteness and high degree of polymerization. This invention reduces pollutant formation at the source, is environmentally friendly, and achieves high bleaching efficiency.

[0012] Sodium pyrophosphate is used for bleaching paper and plant fibers, while sodium tripolyphosphate has chelating, suspending, dispersing, colloidal, emulsifying, and pH buffering effects, and can be used as a major additive in synthetic detergents. This invention utilizes a polyphosphate system composed of sodium pyrophosphate and sodium tripolyphosphate, which synergistically promotes the activation of potassium persulfate, effectively activating it to produce active substances and significantly improving its oxidative degradation efficiency for organic matter, thereby enhancing the bleaching effect. Sodium dodecylbenzenesulfonate, as a common anionic surfactant, is widely available, low in cost, and possesses good surface activity and strong hydrophilicity, effectively reducing the tension at the oil-water interface and improving bleaching efficiency. Furthermore, sodium dodecylbenzenesulfonate has a good detergency effect on particulate dirt on natural fibers and can also absorb nitrogen, phosphorus, and certain metal ions from water.

[0013] In a first aspect, the present invention provides a chlorine-free bleaching agent for cotton linter pulp, comprising, by weight, 1.4 to 7.9 parts of persulfate, 2.7 to 9.2 parts of pyrophosphate, 1.4 to 7.9 parts of polyphosphate, 1.4 to 7.9 parts of anionic surfactant, and 1000 parts of water; the pH value of the chlorine-free bleaching agent for cotton linter pulp is 8.5 to 11.0.

[0014] Preferably, by weight, it includes 2.7–7.9 parts persulfate, 4–7.9 parts pyrophosphate, 2.7–6.6 parts polyphosphate, 1.4–6.6 parts anionic surfactant, and 1000 parts water; the pH value of the chlorine-free bleaching agent for cotton linter pulp is 9.0–10.5.

[0015] Preferably, by weight, it includes 4-6.6 parts persulfate, 5.3-6.6 parts pyrophosphate, 4-5.3 parts polyphosphate, 2.7-6.6 parts anionic surfactant, and 1000 parts water; the pH value of the chlorine-free bleaching agent for cotton linter pulp is 9.0-10.0.

[0016] Preferably, by weight, it includes 5.3 parts persulfate, 6.6 parts pyrophosphate, 5.3 parts polyphosphate, 6.6 parts anionic surfactant, and 1000 parts water; the pH value of the chlorine-free bleaching agent for cotton linter pulp is 9.5.

[0017] Preferably, the persulfate includes potassium peroxymonosulfate, the pyrophosphate includes sodium pyrophosphate, the polyphosphate includes sodium tripolyphosphate, and the anionic surfactant includes sodium dodecylbenzenesulfonate.

[0018] Secondly, the present invention provides a method for bleaching cotton linter pulp, comprising the following steps: The cotton linter pulp was soaked in bleaching agent and mixed evenly to obtain a mixture; The pH of the mixture is adjusted to alkaline, and a bleaching reaction is carried out. After the reaction is completed, the mixture is directly washed and dried to complete the bleaching of cotton lint pulp and obtain refined cotton.

[0019] Preferably, the liquor ratio of cotton linter pulp to bleach is 1:(10-60). Here, the liquor ratio refers to the mass ratio of cotton linter pulp to bleach.

[0020] In the bleaching reaction of this invention, the product is directly washed and dried after the reaction is completed, eliminating the need for a bleaching terminator and saving costs.

[0021] Preferably, the pH of the mixture is adjusted to 8.5–11. Pyrophosphates (e.g., PA) and polyphosphates (e.g., PB) can effectively enhance the activation ability of persulfate under weakly alkaline conditions by forming superoxide radicals and singlet oxygen; under alkaline conditions, hydroxyl radicals (·OH) promote persulfate activation, which is beneficial to the generation of active substances. Simultaneously, cellulose undergoes hydrolysis under excessively alkaline conditions, leading to a decrease in the degree of polymerization; therefore, controlling the appropriate pH in the reaction system is a key factor in ensuring the production of refined cotton with high whiteness and a high degree of polymerization.

[0022] Preferably, the bleaching reaction is carried out at 30–80°C for 10–60 minutes. The bleaching temperature can affect the reaction rate of the bleaching system. Increasing the bleaching time allows the bleaching agent and cotton linter pulp to react fully, effectively removing colored substances, thereby affecting the whiteness and degree of polymerization of refined cotton.

[0023] Preferably, the bleaching reaction is stirred once every 3 to 8 minutes.

[0024] The present invention will be further described in detail below through specific embodiments; to avoid redundancy, the main raw materials, instruments and related testing standards / methods will be described here: 1. Main raw material: Cotton linter pulp: from Hubei Jinhanjiang Refined Cotton Co., Ltd., unbleached, with a whiteness of 42-45 and a degree of polymerization of 2600-2800; Potassium persulfate (PMS), sodium pyrophosphate (PA), sodium tripolyphosphate (PB), tert-butanol (TBA), histidine (L-his), and p-benzoquinone (BQ) were all purchased from Shanghai Maclean Biochemical Technology Co., Ltd.; sodium dodecylbenzenesulfonate (SDBS) was purchased from Tianjin Tianli Chemical Reagent Co., Ltd.; methanol (MeOH) and potassium dichromate were purchased from Sinopharm Group. All the above reagents were of analytical grade.

[0025] 2. Main instruments: Fourier Transform Infrared Spectrometer (FTIR), PE Corporation, USA; X-ray Diffractometer (XRD), SmartLab, Rigaku Corporation, Japan; Scanning Electron Microscope (SEM), JSM-7500F, NEC Corporation, Japan; Paramagnetic Resonance Spectrometer (EPR), Bruker EMXplus, Bruker Corporation, USA.

[0026] 3. Test standards or method descriptions (1) Whiteness determination: The whiteness of refined cotton was determined by a digital whiteness meter in accordance with GBT-9107-2023 "Whiteness Determination Method". Whiteness is one of the important physical indicators reflecting the quality of refined cotton.

[0027] (2) Determination of degree of polymerization (DP): According to GBT-9107-2023 "Method for Determination of Degree of Polymerization", the degree of polymerization of refined cotton degradation is calculated by measuring the time required for cellulose copper ammonia solution and blank copper ammonia solution to flow through the two marks of the viscometer. A high degree of polymerization means that cellulose is more robust and durable, with higher strength and toughness.

[0028] (3) Fourier Transform Infrared Spectroscopy (FTIR) Test: Cotton lint pulp and refined cotton were each ground and processed into thin slices. The samples were then measured using an infrared spectrometer. The scanning range was 500~4000 cm⁻¹. -1 .

[0029] (4) X-ray diffraction (XRD) test The fibrous crystal structure was examined using X-ray diffraction. A copper target was used to generate X-rays (0.154 nm), and the scanning range was 5–80°. The crystallinity was then calculated. The formula for calculating crystallinity (Xc) is: Xc=( Ι 200 - Ι am ) / Ι 200 ×100% In the above formula Ι 200 The value represents the intensity of the 200-plane diffraction peak, which is the maximum diffraction intensity at 2θ = 22.8°. The value of 1am represents the intensity of the peak at 2θ = 18°, which is the diffraction intensity in the amorphous region.

[0030] (5) Scanning electron microscopy (SEM) test Small pieces were selected from both cotton linter pulp and refined cotton for sample preparation. Scanning electron microscopy (SEM) was used for magnified observation and image capture to obtain images of the surface morphology and structure of the cotton fibers. Before testing, the samples underwent gold sputtering. The SEM testing conditions were: accelerating voltage: 0.1KV-20KV, beam current intensity: 10... -13 ~2×10 -9 .

[0031] (6) Chemical oxygen demand (COD) cr ) Measurement COD crCOD (hereinafter referred to as COD) is Chemical Oxygen Demand, also known as Chemical Oxygen Consumption. It primarily uses potassium dichromate as an oxide to oxidize and decompose oxidizable substances in water (such as organic matter, nitrites, ferrous salts, sulfides, etc.), and then calculates the oxygen consumption based on the amount of oxidant consumed. The unit is expressed in mg / L.

[0032] Example 1 In this embodiment, the bleaching agent has a PMS concentration of 6.6 g / L, a PA concentration of 6.6 g / L, a PB concentration of 5.3 g / L, and an SBDS concentration of 6.6 g / L. It is obtained by mixing PMS, PA, PB, and SBDS with water (without adjusting the pH value).

[0033] A bleaching method for cotton linter pulp includes the following steps: Weigh 5 g of cotton linter pulp and place it in 150 g of the above-mentioned bleaching agent (liquor ratio of 1:30), mixing thoroughly. Then place it in a controllable constant temperature water bath and bleach at 50 ℃ for 30 min, stirring every 5 min during the bleaching process to ensure uniform reaction. After the reaction is complete, rinse with tap water and then dry in an oven at 50 ℃ to obtain refined cotton.

[0034] Example 2 The only difference from Example 1 is that the PA concentration was adjusted to 2.7 g / L, 4 g / L, 5.3 g / L, 7.9 g / L, and 9.2 g / L, respectively. All other steps and conditions were the same as in Example 1.

[0035] The whiteness and degree of polymerization of the refined cotton obtained in Examples 1 and 2 were tested to investigate the effect of different PA concentrations on the bleaching effect of cotton linter pulp. The results are as follows: Figure 1 As shown.

[0036] Depend on Figure 1 It can be seen that, under constant conditions, the whiteness of refined cotton initially decreases, then increases, and then decreases again with increasing PA concentration. The highest whiteness (85.1) is achieved when the PA concentration reaches 6.6 g / L. This indicates that the present invention uses potassium persulfate as the main bleaching agent to bleach cotton pulp after cooking, and an appropriate amount of PA can activate PMS, thus improving the whiteness of the treated refined cotton. When the PA concentration continues to increase, the whiteness remains essentially unchanged, possibly because the active sites of the polyphosphate system have reached their maximum, so further increases in PA have little bleaching effect. The degree of polymerization of refined cotton initially increases and then decreases slightly, reaching 1552 when PA is 6.6 g / L.

[0037] Example 3 The only difference from Example 1 is that the PB concentration was adjusted to 1.4 g / L, 2.7 g / L, 4 g / L, 6.6 g / L, and 7.9 g / L, respectively. All other steps and conditions were the same as in Example 1.

[0038] The whiteness and degree of polymerization of the refined cotton obtained in Example 3 were tested to investigate the effect of different PB concentrations on the bleaching effect of cotton linter pulp. The results are as follows: Figure 2 As shown.

[0039] Depend on Figure 2 It can be seen that the whiteness of refined cotton increases with the increase of PB concentration. When the PB concentration increases to 5.3 g / L, the whiteness is 85. When the PB concentration exceeds 5.3 g / L, the whiteness increases slowly and the degree of polymerization decreases slightly. This indicates that further increasing the PB concentration will cause some damage to the cellulose of refined cotton.

[0040] Example 4 The only difference from Example 1 is that the SDBS concentration was adjusted to 1.4 g / L, 2.7 g / L, 4 g / L, 5.3 g / L, and 7.9 g / L, while the other steps and conditions were the same as in Example 1.

[0041] The whiteness and degree of polymerization of the refined cotton obtained in Example 4 were tested to investigate the effect of different SDBS concentrations on the bleaching effect of cotton linter pulp. The results are as follows: Figure 3 As shown.

[0042] Depend on Figure 3 It is observed that the whiteness of refined cotton increases with increasing SDBS concentration, reaching 85 when the SDBS concentration is 6.6 g / L. When the SDBS concentration exceeds 6.6 g / L, the increase in whiteness slows down. With increasing SDBS concentration, the degree of polymerization generally decreases, possibly because sodium dodecylbenzene sulfonate improves the wettability of cotton linter pulp, facilitating the diffusion of bleaching agents into the fiber interior, resulting in increased whiteness and decreased degree of polymerization. Considering all factors, an SDBS concentration below 6.6 g / L is preferred.

[0043] Example 5 The only difference from Example 1 is that the bleaching temperatures are adjusted to 30°C, 40°C, 60°C, 70°C, and 80°C respectively, while the other steps and conditions are the same as in Example 1.

[0044] The whiteness and degree of polymerization of the refined cotton obtained in Example 5 were tested to investigate the effect of different bleaching temperatures on the bleaching effect of cotton linter pulp. The results are as follows: Figure 4 As shown.

[0045] like Figure 4As shown, the whiteness of refined cotton increases with increasing bleaching temperature. Higher bleaching temperatures accelerate the reaction rate of PMS, leading to a more vigorous reaction and increased whiteness. However, this also causes fiber damage and a decrease in the degree of polymerization. Considering all factors, a bleaching temperature of 30–50 °C is preferred.

[0046] Example 6 The only difference from Example 1 is that the bleaching time is adjusted to 10 min, 20 min, 40 min, 50 min, and 60 min respectively, while the other steps and conditions are the same as in Example 1.

[0047] The whiteness and degree of polymerization of the refined cotton obtained in Example 6 were tested to investigate the effect of different bleaching times on the bleaching effect of cotton linter pulp. The results are as follows: Figure 5 As shown.

[0048] Depend on Figure 5 It can be seen that the whiteness of refined cotton gradually increases with increasing bleaching time. The increase is rapid within the first 30 minutes, but slows down after 30 minutes. Within the first 10 minutes, a large amount of the active substances in the bleaching agent oxidize and degrade the colored substances attached to the cellulose; as time continues, these colored substances gradually decrease. Due to the extended bleaching time, while increasing whiteness, the active substances in the bleaching agent react with the cellulose, causing cellulose damage and a decrease in the degree of polymerization. Considering all factors, the optimal bleaching time is 10–30 minutes.

[0049] Example 7 The only difference from Example 1 is that the pH values ​​of the bleaching agent were adjusted to 8.5, 9.0, 9.5, 10.0, 10.5, and 11 with sodium hydroxide solution and sulfuric acid, respectively. The other steps and conditions are the same as in Example 4.

[0050] The whiteness and degree of polymerization of the refined cotton obtained in Example 7 were tested to investigate the effect of different pH values ​​on the bleaching effect of cotton linter pulp. The results are as follows: Figure 6 As shown.

[0051] Depend on Figure 6 It is known that when the pH is less than 9.5, the whiteness of refined cotton increases with increasing pH. However, when the pH exceeds 9.5, both whiteness and degree of polymerization tend to decrease. Above 9.5, the activation ability of PMS weakens, and under excessively alkaline conditions, it may induce the conversion of PMS to sulfate free radicals. Sulfate free radicals play a relatively minor role in the bleaching process. Therefore, excessively alkaline conditions lead to a decrease in the whiteness of refined cotton, and cellulose also undergoes hydrolysis, resulting in a decrease in the degree of polymerization. Considering all factors, a pH of 9.0–10.0 is preferred.

[0052] Example 8 The only difference from Example 1 is the bleach used; specifically: In this embodiment, the bleaching agent has a PMS concentration of 5.3 g / L, a PA concentration of 6.6 g / L, a PB concentration of 5.3 g / L, an SBDS concentration of 6.6 g / L, and a pH of 9.5; the specific conditions for the bleaching method of cotton lint pulp are the same as in Example 1.

[0053] The tested refined cotton had a whiteness of 83.6 and a degree of polymerization of 2210.

[0054] Example 9 The only difference from Example 8 is that the PMS concentration was adjusted to 1.4 g / L, 2.7 g / L, 4 g / L, 6.6 g / L, and 7.9 g / L, and the pH was 9.5. All other steps and conditions were the same as in Example 8.

[0055] The whiteness and degree of polymerization of the refined cotton obtained in Examples 8 and 9 were tested to investigate the effect of different PMS concentrations on the bleaching effect of cotton linter pulp. The results are as follows: Figure 7 As shown.

[0056] Depend on Figure 7 It is known that the whiteness of refined cotton increases with increasing PMS concentration, but the increase slows down when the PMS concentration reaches 5.3 g / L. Increasing the PMS dosage increases the amount of active substances in the bleaching agent, enhancing its oxidizing power and increasing whiteness, but it also causes cellulose degradation and a decrease in the degree of polymerization. Considering all factors, the preferred PMS concentration is 2.7–5.3 g / L.

[0057] The cotton lint pulp used and the refined cotton obtained in Example 8 are characterized and analyzed below.

[0058] 1. FTIR analysis The changes in cellulose structure in cotton linter pulp before and after bleaching were understood by analyzing the characteristic absorption peaks of various functional groups in the infrared spectrum. The results are as follows: Figure 8 As shown.

[0059] Depend on Figure 8 It can be known that 3400~3200cm -1 A broad and moderately strong absorption peak appears, attributed to the OH stretching vibration. At this point, hydrogen atoms readily attract lone electrons from other oxygen atoms with higher electronegativity to form hydrogen bonds. (2988~2890 cm⁻¹) -1 The absorption peaks are attributed to the CH stretching vibration peaks, which are characteristic absorption peaks of cellulose. The wavenumber is around 1400 cm⁻¹. -1 The left and right sides represent C-H2 bending vibration and CH bending vibration, respectively. The wavenumber is around 1100 cm⁻¹. -1The bands on the left and right represent the stretching vibration of cellulose COC and the absorption bands of OH association. (892cm) -1 The peaks are attributed to the glycosidic vibration and the anodic carbon (C1) stretching vibration. After bleaching, the absorption intensity of the OH stretching vibration did not change significantly, while the absorption intensity of the cellulose COC stretching vibration increased slightly. This may be because PMS has a certain oxidizing ability, increasing the oxygen-containing groups in the cotton linter pulp. The absorption peak intensities of the glycosidic vibration and the anodic carbon (C1) stretching vibration peaks increased, possibly because a small amount of cellulose was degraded during the bleaching process, exposing more glycosidic bonds and anodic carbons. This also indicates that bleaching slightly damages the cellulose structure, leading to a decrease in the degree of polymerization of cellulose.

[0060] Infrared spectral analysis revealed the absence of absorption peaks for benzene ring functional groups, indicating that the unbleached cotton linter pulp contains virtually no lignin. The low whiteness of the unbleached cotton linter pulp is likely due to colored impurities remaining on the cellulose during the cooking process. Furthermore, the characteristic absorption peaks of the cotton linter cellulose did not change significantly in position or intensity before and after bleaching, and no new absorption peaks appeared. This demonstrates that the bleaching method of this invention only removes colored substances from the cellulose, causing minimal damage to the fiber. This aligns with the high whiteness and high degree of polymerization of refined cotton measured in this invention, indicating that the bleaching method of this invention is an effective and feasible bleaching process.

[0061] 2. XRD Analysis Cellulose crystallinity refers to the percentage of crystalline regions in the total cellulose volume. It is an important parameter describing the crystal structure of cellulose and is closely related to lignin, cellulose, and hemicellulose. X-ray diffraction was used to analyze the changes in cellulose crystallinity in unbleached cotton linter pulp and refined cotton. The results are as follows: Figure 9 As shown in Table 1.

[0062] Table 1. Effect of the bleaching method of the present invention on the crystallinity of cotton linter pulp.

[0063] Depend on Figure 9 It is known that the main component of unbleached cotton linter pulp is cellulose, and diffraction peaks appear at 2θ = 14.98°, 22.8°, and 34.49°. The refined cotton after bleaching also exhibits these three characteristic peaks. The cellulose crystal structure is consistent before and after bleaching, with no significant difference. This is because the PMS bleaching method only removes the colored substances adhering to the surface of the cotton linter pulp fibers and does not change the crystal structure of cellulose. Table 1 shows that the crystallinity of cellulose does not change much before and after bleaching, which also indicates that the bleaching method of this invention can selectively oxidize and remove colored substances with minimal impact on the crystal structure of cellulose. This is consistent with the results of infrared spectroscopy analysis.

[0064] 3. Scanning electron microscopy (SEM) analysis To further investigate the effects of PMS bleaching on the surface morphology and damage level of cellulose, scanning electron microscopy was used to observe the surface morphology of unbleached cotton linter pulp and refined cotton. The results are as follows: Figure 10 As shown.

[0065] Depend on Figure 10 As can be seen, the unbleached cotton linter pulp shown in Figures (a) and (b) has an intact cotton fiber morphology and structure, but the surface contains some fine fibers and a few fragments. After treatment by the bleaching method of this invention, the fiber surfaces shown in Figures (c) and (d) become smooth and flat, and the fragments on the fiber surface are also removed. Moreover, the fibers do not exhibit surface disorder or cracks, and the surface etching is minimal. Therefore, the integrity of the fibers is not compromised, which is consistent with the result that the degree of polymerization does not decrease rapidly. This indicates that the bleaching method of this invention has a relatively small impact on the morphology and structure of cellulose, and is an excellent process for obtaining high-whiteness, high-polymerization-degree refined cotton.

[0066] 4. COD Analysis Traditional cotton bleaching processes use chlorine-containing bleaching agents, resulting in bleaching wastewater containing toxic, teratogenic, and mutagenic organic compounds such as chloroform, chlorophenols, dioxins, and furans. Direct discharge of this wastewater causes significant environmental pollution and is extremely difficult to treat. This invention, however, uses PMS as the main bleaching agent, which does not produce toxic byproducts such as chlorine compounds. This reduces the formation of industrial pollutants at the source, and the wastewater produced by this system has a very low COD, thus reducing subsequent wastewater treatment costs. The COD of the bleaching wastewater generated after bleaching in Example 8 was tested three times, and the results are shown in Table 2. The average COD of the bleaching wastewater was 270.9 mg / L, far lower than that of traditional chlorine-containing bleaching processes, indicating that the bleaching method of this invention is a green bleaching technology with safe and reliable characteristics, effectively eliminating the generation of organochlorine compounds.

[0067] Table 2 COD content in waste liquid from the bleaching method of the present invention

[0068] 5. Mechanism of the bleaching method of the present invention Under normal circumstances, sulfate radicals (SO4·) are mainly generated in the PMS oxidation system. - ), hydroxyl radicals (·OH), superoxide radicals (O2· - Free radicals and singlet oxygen ( ) 1O2) Non-free radical reactive substances. These reactive substances play an important role in removing colored substances attached to the surface of cellulose. In order to investigate the main reactive substances in the bleaching system of this invention, different quenchers were added in this experiment to identify the main free radicals in the PMS bleaching system. Specifically, methanol (MeOH)[k(SO4· - = 2.5 × 10 7 M -1 s -1 k(·OH) = 1.0 × 10 9 M -1 s -1 [Regarding sulfate free radicals (SO4·] - Both tert-butanol (TBA) and hydroxyl radicals (·OH) have high quenching effects. - = (4.0 - 9.1) × 10 5 M -1 s -1 k(·OH) = (3.8-7.6)×10 9 M -1 s -1 [Regarding sulfate free radicals (SO4·] - L-histidine (L-his) has a weak quenching effect, but a better quenching effect on hydroxyl radicals (·OH), and is very stable. 1 O2) = 3.2 × 10⁷ M -1 s -1 It can effectively quench singlet oxygen ( 1 O2), p-benzoquinone (BQ) [k(·O 2 - ) = (0.9~1.0) × 10 9 M -1 S -1 It is then used to quench superoxide radicals (O2·). - The experimental conditions for each group were as follows: the bleaching agent of Example 8 was added to the reactor, the pH was adjusted to 9.5, and the quenching agent was added immediately. The reaction was carried out in a controlled water bath at 50°C for half an hour before the reaction was terminated.

[0069] Control group: the bleach from Example 8; Methanol group: bleach from Example 8 + 500 mmol / L methanol; tert-butanol group: bleach from Example 8 + 500 mmol / L tert-butanol; L-histidine group: bleach from Example 8 + 10 mmol / L L-histidine; p-Benzoquinone group: bleaching agent from Example 8 + 3 mmol / L p-Benzoquinone.

[0070] Quenching experiment results are as follows Figure 11 As shown. After adding methanol and tert-butanol to the bleaching agent in Example 8, the whiteness of the refined cotton obtained was not significantly different from that obtained in Example 8, indicating that the bleaching effect was not significantly inhibited. This indicates that sulfate free radicals (SO4·) - Singlet oxygen (·OH) and hydroxyl radicals (·OH) have a relatively small effect on removing colored substances in the bleaching method of this invention. It was found that the whiteness of refined cotton decreased significantly upon the addition of histidine and p-benzoquinone to the bleaching agent. This indicates that singlet oxygen (·OH) has a relatively small effect on improving the whiteness of refined cotton. 1 O2) and superoxide radicals (O2· - ) plays a major role. Furthermore, after adding four quenchers to the bleaching agent in Example 8, the degree of polymerization increased, approaching that of unbleached cotton linter pulp, indicating that all four active substances may cause some damage to the cellulose structure, leading to a slight decrease in the degree of polymerization. Among them, singlet oxygen ( 1 O2 has the greatest destructive effect on the structure of cellulose.

[0071] To further determine the generation of various active substances in the PMS bleaching system, an EPR spectrometer was used, with DMPO or TEMP as free radical scavengers. DMPO was used to capture sulfate free radicals (SO4· ... - ), hydroxyl radicals (·OH) or superoxide anion radicals (O2· - TEMP is used to capture singlet oxygen. Free radical scavengers were added to different reaction systems, the pH was adjusted to 9.5, and the reaction was carried out at 50°C for 15 minutes before testing. The results were as follows: PMS group: PMS concentration of 5.3 g / L, pH value of 9.5, with added free radical scavenger of 50 mmol / L.

[0072] PMS+SDBS group: PMS concentration was 5.3 g / L, SBDS concentration was 6.6 g / L, pH value was 9.5; 50 mmol / L of free radical scavenger was added.

[0073] The PMS+PA+PB group consisted of PMS at a concentration of 5.3 g / L, PA at a concentration of 6.6 g / L, PB at a concentration of 5.3 g / L, and a pH of 9.5, with an added free radical scavenger at a concentration of 50 mmol / L.

[0074] The PMS+PA+PB+SDBS group consisted of PMS at a concentration of 5.3 g / L, PA at 6.6 g / L, PB at 5.3 g / L, SBDS at 6.6 g / L, and a pH of 9.5, with an added free radical scavenger at 50 mmol / L.

[0075] like Figure 12 As shown, sulfate radicals (SO4·) were observed in different reaction systems.- Characteristic peaks for hydroxyl radicals (·OH) and sulfate radicals (SO4·OH), with four strong peaks corresponding to hydroxyl radicals (·OH) and interspersed in the middle. - Characteristic peaks of ). For example, Figure 13 As shown, the 1:1:1 triplet represents singlet oxygen (…). 1 Characteristic peaks of O2). For example Figure 14 As shown, the four large and two small sextet peaks are superoxide radical (O2·) - Characteristic peaks of ) are observed. These active substances endow the bleaching system of this invention with strong oxidizing properties.

[0076] Depend on Figures 12-14 It can be seen that when only PMS is in the reaction system, the strength of the generated free radicals is not high, resulting in poor bleaching effect. When SDBS is added to the reaction system, the strength of the free radicals does not change much. However, when PA and PB are added to the reaction system, the strength of the free radicals is significantly enhanced, and the bleaching effect is significantly enhanced. Polyphosphate can act as a nucleophile (Nu) to attack the asymmetric OO covalent bond of PMS, promoting heterolytic cleavage of the OO bond, generating sulfate free radicals and hydroxyl free radicals as shown in equation (1), and then further generating superoxide free radicals and singlet oxygen as shown in equations (2) and (3). Therefore, the PMS / polyphosphate bleaching system of the present invention has higher oxidation performance, the colored substances attached to cotton lint pulp are removed more efficiently, and the whiteness of refined cotton is improved.

[0077] (1) (2) (3) This invention uses PMS as the main bleaching agent to formulate a bleaching agent for bleaching unbleached cotton linter pulp. The resulting refined cotton has a whiteness of 70 or higher (70-86) and a degree of polymerization of 1000 or higher (1000-2210), with a relative reduction in polymerization of 15%-64% compared to the unbleached cotton linter pulp. Test results show that the process of this invention not only has a good bleaching effect and can effectively remove colored substances attached to cellulose, but also causes minimal damage to cellulose. The bleaching method of this invention obtains refined cotton with high whiteness and high degree of polymerization in a shorter time and under mild process conditions. Compared with traditional bleaching methods, this process does not contain chlorine and does not produce organochlorides or other teratogenic or mutagenic substances, thus reducing the pollution load. The COD content in the wastewater after bleaching is below 280 mg / L, making it environmentally friendly.

[0078] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A chlorine-free bleaching agent for cotton linter pulp, characterized in that, By weight, it comprises 2.7–7.9 parts persulfate, 4–7.9 parts pyrophosphate, 2.7–6.6 parts polyphosphate, 2.7–6.6 parts anionic surfactant, and 1000 parts water; the pH value of the chlorine-free bleaching agent for cotton linter pulp is 9.0–10.

5. Persulfates include potassium peroxymonosulfate, pyrophosphates include sodium pyrophosphate, polyphosphates include sodium tripolyphosphate, and anionic surfactants include sodium dodecylbenzenesulfonate.

2. The chlorine-free bleaching agent for cotton linter pulp according to claim 1, characterized in that, By weight, it includes 4-6.6 parts persulfate, 5.3-6.6 parts pyrophosphate, 4-5.3 parts polyphosphate, 2.7-6.6 parts anionic surfactant, and 1000 parts water; the pH value of the chlorine-free bleaching agent for cotton linter pulp is 9.0-10.

0.

3. The chlorine-free bleaching agent for cotton linter pulp according to claim 1, characterized in that, By weight, it includes 5.3 parts persulfate, 6.6 parts pyrophosphate, 5.3 parts polyphosphate, 6.6 parts anionic surfactant, and 1000 parts water; the pH value of the chlorine-free bleaching agent for the cotton linter pulp is 9.

5.

4. A bleaching method for cotton linter pulp, characterized in that, Includes the following steps: Cotton lint pulp is soaked in bleaching agent and mixed evenly to obtain a mixture; the bleaching agent is the bleaching agent according to any one of claims 1-3; The pH of the mixture is adjusted to alkaline, and a bleaching reaction is carried out. After the reaction is completed, the mixture is washed and dried to complete the bleaching of the cotton linter pulp.

5. The bleaching method for cotton linter pulp according to claim 4, characterized in that, The bath ratio of the cotton lint pulp to the bleaching agent is 1:(10-60).

6. The bleaching method for cotton linter pulp according to claim 4, characterized in that, Adjust the pH of the mixture to 8.5–11.

7. The bleaching method for cotton linter pulp according to claim 4, characterized in that, The bleaching reaction is carried out at 30–80°C for 10–60 minutes.

8. The bleaching method for cotton linter pulp according to claim 4, characterized in that, During the bleaching reaction, the mixture is stirred every 3 to 8 minutes.

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