High-bulk high-strength bamboo chemimechanical pulp and preparation method thereof

Abstract

The invention discloses high-bulk high-strength bamboo chemimechanical pulp and a preparation method thereof, and belongs to the field of pulping processes. The method comprises the following steps: carrying out polycondensation treatment on bamboo chips by adopting a sodium sulfite solution, and controlling polycondensation reaction conditions, wherein the yield of slurry is 70-80%, the content of lignin in extruded slurry is 20-30%, the content of a beta-O-4 structure in a lignin structure is reduced by less than 20%, the content of a beta-beta structure is reduced by less than 2%, and the content of a beta-5 structure is reduced by less than 0.5%. Extruding the treated material to discharge waste liquid to obtain extruded slurry; the extruded slurry is placed in an alkaline hydrogen peroxide solution for directional oxidation treatment, and the carboxyl value of ground wood lignin in the obtained oxidized slurry is controlled to be higher than 0.12 mol / kg; and finally, carrying out disc milling treatment to obtain the chemimechanical pulp. According to the preparation method disclosed by the invention, through a synergistic process of moderate polycondensation and directional oxidation, stable carbon-carbon bonds are constructed by polycondensation to support the bulk, and carboxyl is introduced by virtue of oxidation ring opening to enhance hydrogen bond combination between fibers, so that the bulk and the strength of the slurry are synchronously improved, and the technical bottleneck that the two are difficult to obtain simultaneously in a traditional process is broken through.

Description

Technical Field

[0001] This invention belongs to the field of pulping technology, specifically relating to a high bulk thickness and high strength bamboo chemimechanical pulp and its preparation method. Background Technology

[0002] In recent years, the production and consumption of packaging paper, linerboard, and corrugated base paper have continued to expand. As a core indicator determining the performance of packaging paper materials, bulk (i.e., the volume per unit mass of paper or paperboard) is mainly contributed by chemimechanical pulp (hereinafter referred to as "chemimechanical pulp"). The industry generally believes that the lignin content in the pulp is positively correlated with bulk; that is, the higher the lignin retention, the more outstanding the bulk performance. Meanwhile, mechanical strength, as a basic requirement for the use of paper products, is the primary parameter for evaluating the suitability of pulp.

[0003] From a fiber structure perspective, plant cell walls are mainly composed of three components: lignin, hemicellulose, and cellulose. In natural fibers, lignin and hemicellulose are chemically linked through ether bonds to form a complex structure that coats the exterior of cellulose microfibrils. carbon bond (β) β, β 5) Crosslinking forms a three-dimensional network polymer, exhibiting overall hydrophobic properties. In chemimechanical pulp, the hydroxyl groups on cellulose and hemicellulose chains form hydrogen bonds, constituting the main source of paper strength; while lignin, with its hydrophobicity and rigid structure, effectively prevents excessive compaction between fibers, thereby maintaining the bulk of the paper.

[0004] It is generally believed that the bulk of chemimechanical pulp mainly depends on the lignin content—the more lignin retained, the stronger the hydrophobic effect, and the higher the bulk; while the strength properties depend on the degree of exposure of hydroxyl groups on the surface of microfibrils—the higher the accessibility of hydroxyl groups, the more hydrogen bonding sites, and the better the strength properties. However, due to the natural structure of lignin coating the outside of microfibrils, retaining lignin will limit the exposure of microfibrils, leading to a decrease in strength; conversely, removing a large amount of lignin to improve strength will inevitably result in a loss of bulk. This inherent "strength" factor... The contradiction between "bulkness" and "volume" severely restricts the application of chemimechanical pulp in the field of high-value-added packaging materials, becoming a technical bottleneck that urgently needs to be overcome in this field.

[0005] Currently, mainstream pulping processes mainly include two categories: chemimechanical pulping (CMP) and chemical pulping. CMP typically uses sodium hydroxide to soften the raw materials, with the core objective of preserving lignin and other components as much as possible. This is achieved primarily through the mechanical action of a disc mill to exfoliate lignin and expose microfibrils. This method usually yields pulp with a yield exceeding 85% and a high degree of lignin retention, resulting in excellent bulk properties. However, disc milling has limited effect on fiber fibrillation; excessive mechanical treatment can lead to over-cutting of fibers and a decrease in average length, which weakens strength properties. Therefore, CMP pulps generally exhibit high bulk but low strength.

[0006] In contrast, chemical pulping involves cooking the pulp at high temperatures using appropriate chemical agents to achieve deep lignin removal while preserving cellulose to the maximum extent. This method fully exposes the microfibrils, giving the pulp excellent strength properties, but the significant removal of lignin also leads to a substantial reduction in pulp bulk.

[0007] Existing chemical mechanical and chemical pulping processes struggle to achieve a balance between high bulk and high strength in pulp. Summary of the Invention

[0008] To address the inherent "strength" of pulp in the pulping process The invention addresses the contradictory relationship between "bulk thickness" and "high strength" bamboo chemimechanical pulp, aiming to provide a method for its preparation by employing "moderate polycondensation." The synergistic process of "directional oxidation" enables the directional modification of the lignin structure in chemimechanical pulp. By constructing structurally stable carbon-carbon bonds through moderate polycondensation, its contribution to bulk is strengthened. At the same time, the oxidation ring-opening reaction provides carboxyl groups as potential hydrogen bonding sites, effectively improving the bonding strength between fibers. This successfully produces high-performance chemimechanical pulp products with both high bulk and high strength, breaking through the technical bottleneck of traditional processes where these two aspects are difficult to achieve simultaneously.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing high bulk thickness and high strength bamboo chemimechanical pulp includes the following steps: (1) Preparation of bamboo raw materials (including but not limited to moso bamboo, ci bamboo, powder bamboo, dragon bamboo, hard-headed yellow bamboo, Mian bamboo and other chemical mechanical pulp fiber raw materials): The bamboo raw materials are processed by a chipper and passed through a wood chip sieve to obtain qualified bamboo chips with a length of 20 mm-50 mm, a width of 5 mm-20 mm, and a thickness of 3 mm-8 mm.

[0010] (2) After the qualified bamboo chips are washed with hot water and dehydrated, they are added to a sodium sulfite solution for polycondensation treatment (first stage chemical treatment); the solute in the sodium sulfite solution is sodium sulfite or a mixture of sodium sulfite and sodium hydroxide; relative to the amount of oven-dried bamboo chips, the amount of sodium sulfite is 100-300 kg / (t oven-dried wood chips) (hereinafter abbreviated to kg / t), and the amount of sodium hydroxide is 100-300 kg / t; preferably, the temperature of the polycondensation treatment is 110-165℃, preferably 150-165℃, the time is 30-80 min, and the system concentration is not less than 30% (the mass of all solid substances in the system divided by the total mass). The reaction conditions in this step are adjusted according to the differences in the raw material structure. The ultimate goal is to control the pulp yield after treatment to 70%-80%; control the lignin content in the pulp to 20-30%, the hemicellulose content to 23-28%, and the β-O-4 structure content in the lignin structure to decrease by less than 20% (i.e., the proportion of β-O-4 structure in lignin relative to C9 groups measured by two-dimensional NMR; "decreased" refers to the difference between the β-O-4 structure content in the pulped lignin and the β-O-4 structure content in the raw material lignin, the same below), the β-β structure content to decrease by less than 2%, and the β-5 structure content to decrease by less than 0.5%. After the polycondensation treatment, the pulp is processed using a twin-screw extruder to extrude the sodium sulfite waste liquid, obtaining extruded pulp.

[0011] (3) The extruded slurry is added to an alkaline hydrogen peroxide solution for oxidation treatment; relative to the extruded slurry, the amount of hydrogen peroxide in the alkaline hydrogen peroxide solution is 30-80 kg / t, and the amount of sodium hydroxide is 20-40 kg / t; the oxidation treatment temperature is 90-120℃, the system concentration is 20%, and the time is 10-20 min. The reaction conditions in this step are adjusted according to the differences in the raw material structure. The ultimate goal is to control the carboxyl value measured by the NMR phosphorus spectrum of the ground wood lignin in the treated slurry to be higher than 0.12 mol / kg. The slurry after the oxidation treatment (second stage chemical treatment) is processed by a twin-screw extruder to extrude the alkaline hydrogen peroxide waste liquid. The lignin in bamboo does not contain hydrophilic groups and cannot provide strength; this invention transforms the lignin in bamboo into a substance with a certain carboxyl content through the oxidation treatment of alkaline hydrogen peroxide, so that it can form hydrogen bonds with the hydroxyl groups of substances such as cellulose, thereby improving the strength of the material.

[0012] (4) The pulp sample that has completed two stages of chemical treatment enters the disc mill for further processing. It is milled to the required degree of freeness according to the needs of the downstream paper machine. Then, it is prepared into a finished chemimechanical pulp through conventional processes such as anti-latenting, screening, and washing.

[0013] The core innovation of this invention lies in the fundamental optimization of the traditional pulping and cooking process. It abandons the traditional approach of aiming for massive lignin dissolution and instead adopts a novel treatment strategy centered on "controlled condensation." Specifically, by precisely controlling the chemical treatment conditions, it moderately breaks the low-energy ether bonds (mainly β-O-4) in the lignin structure, causing them to form lignin fragment free radicals. Under the premise of controlling the amount of lignin dissolution, it promotes the recombination of these free radicals to form more stable carbon. Carbon bonds (β-β, β-5) significantly enhance the structural rigidity of the lignin network. Although some lignin dissolution loss is inevitable during this process, the substantial enhancement of the lignin skeleton's rigidity allows it to effectively support the fiber network and maintain excellent bulk properties during subsequent pulping and papermaking processes.

[0014] In terms of strength enhancement, this invention introduces "directional oxidation" as a supplementary step. The slurry modified by polycondensation is further treated with alkaline hydrogen peroxide. This treatment is not for the purpose of traditional bleaching, but rather to directionally oxidize the benzene rings in the lignin structure on the slurry surface, converting the hydrophobic benzene rings into hydrophilic carboxyl groups through a ring-opening reaction. The carboxyl group structure contains both hydrogen bond donors (hydroxyl groups) and acceptors (carbonyl oxygen), and its abundant introduction significantly increases the hydrogen bond sites between fibers, thereby effectively improving the strength properties of the slurry.

[0015] This invention achieves this through "appropriate condensation" The synergistic process of "directional oxidation" enables the directional modification of the lignin structure in chemimechanical pulp. By constructing carbon-carbon bonds to enhance its contribution to bulk, and by modifying the hydrophobic benzene ring to a hydrophilic carboxyl group, the bonding strength between pulp fibers is effectively improved, and a high-performance chemimechanical pulp product with both high bulk and high strength is successfully prepared.

[0016] This invention, through precise control of the preparation process conditions, achieves a functional transformation of pulp properties based on the use of traditional pulping chemical reagents. Compared with the chemical treatment of sodium sulfite and alkaline hydrogen peroxide in traditional pulping, this invention has the following essential differences: This invention functionally restructures the traditional sulfite treatment process, transforming it from a lignin-removing pretreatment method to a targeted strengthening of the lignin's internal structure. This process is placed at the beginning of the pulping process, selectively utilizing the activation function of sulfite ions to induce controlled free radicalization of the lignin structure through "structural strengthening" of wood chips. By precisely controlling key parameters such as temperature, concentration, and reaction time, the integrity of the lignin backbone structure is effectively protected while controlling the dissolution behavior of lignin fragments, guiding them to construct a more stable carbon-carbon crosslinked structure than ether bonds through condensation polymerization. This structural remodeling process significantly improves the rigidity of the lignin network, with technical effects manifested on two levels: firstly, effectively inhibiting excessive lignin dissolution during subsequent pulping processes; and secondly, enhancing the fiber skeleton's resistance to deformation during the papermaking stage, providing excellent bulk properties for paper products.

[0017] The traditional alkaline hydrogen peroxide treatment process has been functionally restructured, shifting its application objective from bleaching by destroying chromophores to the targeted modification of lignin surface properties. By removing stabilizers that inhibit the rapid reaction of hydrogen peroxide and synergistically increasing process parameters such as reaction temperature, the highly reactive characteristics of hydrogen peroxide are effectively activated, prompting selective oxidative ring-opening reactions in the aromatic ring structure of lignin on the pulp surface. This results in the construction of a hydrophilic interface rich in carboxyl groups. These introduced carboxyl groups serve as highly efficient hydrogen bonding sites, significantly enhancing the bonding strength between fibers and providing excellent strength properties to the finished paper during subsequent paper forming processes.

[0018] This invention breaks through the traditional thinking in chemimechanical pulping theory, which relies on high lignin content to maintain bulk and on microfibril exposure to improve strength. It creatively proposes a synergistic modification strategy of "moderate polycondensation-directional oxidation." This technology, through precise control of the lignin structure in the fiber raw material, achieves the following dual effects while allowing for moderate lignin dissolution: 1. Constructing a rigid network through moderate polycondensation: Under controllable conditions, lignin fragment free radicals are induced to recombine and form a stable carbon-carbon bond structure. This rigid framework provides excellent bulk support for the pulp. II. Introducing binding sites through directional oxidation: By using selective ring-opening reactions to convert the lignin benzene ring structure into carboxyl groups, the hydrogen bonding sites on the fiber surface are significantly increased, thereby effectively improving the pulp strength. Attached Figure Description

[0019] Figure 1 Flowcharts for three different processing techniques: TAT, LSAPT, and HST; Figure 2 The bulk-tensile relationship of pulp obtained from Sichuan Cizhu bamboo through three different treatment processes: TAT, LSAPT and HST. Figure 3 The bulk-tensile relationship of pulp obtained from Mianzhu, Sichuan through three different treatment processes: TAT, LSAPT and HST. Figure 4 The bulk-tensile relationship of Sichuan hard yellow pulp obtained by three different treatment processes: TAT, LSAPT and HST. Figure 5 The bulk-tensile relationship of pulp obtained from Zhejiang green bamboo through three different treatment processes: TAT, LSAPT and HST.

[0020] Figure 6 Comparative experiment on the bulk-tensile strength relationship of pulp obtained from Sichuan Cizhu under different treatment processes. Detailed Implementation

[0021] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments are not intended to limit the present invention. In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail. The raw materials used in the present invention are all commercially available products.

[0022] The following section illustrates the unique innovation of the proposed "moderate polycondensation-directional oxidation" strategy in improving pulp properties by designing and comparing three pulping processes with different raw materials and different modification pathways. The three process designs are as follows: Comparison Process 1: Two-Stage Alkali Treatment Process (TAT) As a conventional process for traditional unbleached chemimechanical pulping, TAT employs a staged alkali treatment method, primarily aimed at enhancing solvent penetration and achieving gradual fiber softening. This process retains a high lignin content, resulting in a high bulk of the pulp. However, due to the lack of directional regulation of lignin structure and the introduction of effective fiber binding sites, its tensile strength development is limited. The experimental procedure is as follows: Figure 1 As shown in Figure A.

[0023] The specific experimental procedure is as follows: Bamboo chips are first screened through a TMI standard sieve, and qualified bamboo chips are collected, sealed in plastic bags, and their moisture content is balanced. Qualified bamboo chips are first washed with hot water, dehydrated, and then subjected to the first stage of chemical pre-impregnation (impregnation conditions are shown in Table 1): impregnation temperature 95 ℃, time 120 min, concentration 25%. After impregnation, the bamboo chips are extruded using a twin-screw extruder (TSPI), and the second stage of impregnation chemicals are added and thoroughly mixed. The mixture is then sent to a reaction chamber for chemical impregnation (time 90 min, temperature 90-95 ℃). The impregnated material undergoes the first stage of high-concentration disc milling (concentration 25-30%). The milled material undergoes high-concentration extraction (collecting the extruded concentrated waste liquid and measuring its pH, residual alkali, and solids content), followed by subsequent disc milling. By inputting different electrical energies, four materials with different degrees of free carbon dioxide are milled. After desensitization and washing, the degree of free carbon dioxide in the material is measured, and the material is formed into a 60 g / m³ sheet in a forming machine according to relevant standards. 2 After drying the sample at constant temperature and humidity, the physical and strength properties of the sample were measured.

[0024] The process of this invention: a two-stage treatment process using low-dosage sodium sulfite / oxidation treatment (LSAPT). This process, the core innovation of this invention, is designed based on the principle of "moderate polycondensation-directional oxidation." In the first stage, the sulfonation of sulfites moderately breaks the ether bonds (such as β-O-4) in lignin and guides the lignin fragments to undergo condensation reactions, constructing a stable carbon-carbon bond network (β-β, β-5), thus maintaining excellent bulk even with a slight reduction in lignin content. In the second stage, alkaline hydrogen peroxide is used to selectively oxidize and open the benzene ring structure of lignin, generating a large number of carboxyl groups. These carboxyl groups act as efficient hydrogen bond donors and acceptors, significantly enhancing the bonding force between fibers, thereby greatly improving the tensile strength of the pulp. The LSAPT process achieves synergistic optimization of bulk and tensile strength. The experimental procedures are as follows: Figure 1 As shown in Figure B.

[0025] The specific process is as follows: Bamboo strips are first screened using a TMI standard sieve. Qualified bamboo strips are collected, sealed in plastic bags, and their moisture content is balanced. The qualified bamboo strips are then washed with hot water, dehydrated, and then subjected to high-temperature cooking in a cooking pot with sodium sulfite solution to complete the polycondensation treatment (first-stage chemical treatment). After cooking, TSPI treatment is performed. The extruded material is washed and then undergoes oxidation treatment (second-stage chemical treatment). After bleaching, the material undergoes high-concentration extraction, washing, and subsequent disc milling. By inputting different electrical energies, four materials with different degrees of free Canadian carbonate are milled. The materials are then desensitized, washed, and their free Canadian carbonate content is measured. Finally, the material is formed into a 60 g / m³ sheet in a forming machine according to relevant standards. 2 After drying the sample at constant temperature and humidity, the physical and strength properties of the sample were measured.

[0026] Comparison Process 2: High-Dosage Sodium Sulfite Single-Stage Treatment Process (HST) This process falls under the category of conventional semi-chemical pulping, employing high-concentration sulfite treatment at high temperatures for intensification. Its core mechanism involves the significant removal of lignin to promote microfibril exposure, thereby substantially improving pulp strength. However, due to excessive lignin dissolution, its ability to support the fiber network and provide pore structure is severely weakened, leading to a significant decrease in pulp bulk. The experimental procedures are as follows: Figure 1 As shown in Figure C.

[0027] The specific process is as follows: Bamboo strips are first screened using a TMI standard sieve. Qualified bamboo strips are collected, sealed in plastic bags, and allowed to reach equilibrium moisture content. The qualified bamboo strips are then washed with hot water, dehydrated, and then subjected to high-temperature cooking in a cooking pot with added chemicals. After cooking, TSPI treatment is performed. The extruded material undergoes subsequent disc milling. By inputting different electrical energies, four different Canadian free radical fractions are obtained. The material is then desensitized, washed, and its Canadian free radical fraction is measured. Finally, it is formed into a 60 g / m³ material in a forming machine according to relevant standards. 2 After drying the sample at constant temperature and humidity, the physical and strength properties of the sample were measured.

[0028] Papermaking and performance testing: Papermaking: carried out in accordance with GB7981-1999 Laboratory papermaking of pulp - conventional paper forming equipment method; Basis weight: Determination of basis weight of paper and paperboard in accordance with GB / T 451.2-2002; Thickness: Determined in accordance with GB / T 451.3-2002: Determination of thickness of paper and paperboard; Tensile strength: Tested in accordance with GB / T 12914-2002: Determination of tensile strength of paper and paperboard (constant rate tensile test).

[0029] Example 1 Using Sichuan bamboo as raw material, the experimental parameters for each process are shown in Table 1: Table 1. Process parameters for each process

[0030] The yields of the three chemical treatment processes and the main chemical composition of the pulp samples after chemical treatment are shown in Table 2. The chemically treated pulp was then subjected to disc milling to prepare four pulp samples with free volume distributions ranging from 100 to 500 mL. The relationship between the bulk and strength of each group of pulp samples is shown in Table 3. Figure 2 .

[0031] Table 2 shows the pulp yield and chemical composition of the obtained pulp samples.

[0032] Table 3 shows the bulk and strength of the pulp obtained from each process.

[0033] Table 4. Structural information of lignin in pulp samples.

[0034] Note: In the table, H, S, and G represent the three basic structural units of lignin, and A (β-O-4), B (β-β), and C (β-5) represent the main bonds connecting these basic structural units. The structural characteristics of the ground wood lignin in the pulp were measured using two-dimensional nuclear magnetic resonance (2D HSQC NMR). The content of each structure in the ground wood lignin was quantitatively analyzed using the internal standard (aromatic ring) method. The content of 0.5 I in the lignin structure was used as an example. S2，6 +I G2 +0.5I H2，6 The total aromatic rings of lignin are calculated as the sum of the integral areas of the characteristic signals of the three structural units. The ratio of the two-dimensional NMR signal of each structure to the total aromatic rings is its corresponding relative content A, which is calculated as shown in equation (1).

[0035] (1) In the formula, A x The percentages for each structure are relative to the C9 unit content in lignin, %; I x I represents the integral area of ​​the strongest signal in the spectrum for each structural unit of lignin; S2, 6 The signal integration area of ​​carbons at positions 2 and 6 of the S structure in lignin in the spectrum; I G2 The signal integral area of ​​the carbon at position 2 of the G structure in lignin in the spectrum; I H2, 6 The signal integration area of ​​carbons at positions 2 and 6 of the H structure in lignin in the spectrum.

[0036] As shown in Tables 2 and 4, the chemical treatment yield of the LSPAT process is greater than 70% (75.21%), and the lignin content of the resulting pulp is not less than 20% (23.21%). Compared with the raw material, the content of β-O-4 structure in the lignin structure decreases by less than 20% (6.47%), the content of β-β structure decreases by less than 2% (0.7%), the content of β-5 structure decreases by less than 0.5% (0.39%), and the carboxyl content is greater than 0.12 mol / kg (0.15 mol / kg), meeting all the technical indicators mentioned in this invention. Figure 2 It can be seen that P LSAPT The balance between bulk density and tensile strength is significantly improved.

[0037] Example 2 Using Sichuan Mianzhu bamboo as raw material, the experimental parameters for each process are shown in Table 5.

[0038] Table 5 Process parameters for each process

[0039] The yields of the three chemical treatment processes and the main chemical composition of the pulp samples after chemical treatment are shown in Table 6. The chemically treated pulp was then subjected to disc milling to prepare four pulp samples with free volume distributions ranging from 100 to 400 mL. The relationship between the bulk and strength of each group of pulp samples is shown in Tables 7 and 8. Figure 3 .

[0040] Table 6 shows the pulp yield and chemical composition of the obtained pulp samples.

[0041] Table 7 shows the bulk and strength of the pulp obtained from each process.

[0042] Table 8. Structural information of lignin in the pulp samples.

[0043] As shown in Tables 6 and 8, the chemical treatment yield of the LSPAT process is greater than 70% (73.87%), and the lignin content of the resulting pulp is not less than 20% (20.71%). Compared with the raw material, the content of β-O-4 structure in the lignin structure decreases by less than 20% (7.17%), the content of β-β structure decreases by less than 2% (0.28%), the content of β-5 structure decreases by less than 0.5% (0.23%), and the carboxyl content is greater than 0.12 mol / kg (0.12 mol / kg), meeting all the technical indicators mentioned in this invention. Figure 3 It can be seen that P LSAPT The balance between bulk density and tensile strength is significantly improved.

[0044] Example 3 Using Sichuan hard-head yellow as raw material, the experimental parameters of each process are shown in Table 9.

[0045] Table 9 Process parameters for each process

[0046] The yields of the three chemical treatment processes and the main chemical composition of the pulp samples after chemical treatment are shown in Table 10. The chemically treated pulp was then subjected to disc milling to prepare four pulp samples with free volume distributions ranging from 100 to 400 mL. The relationship between bulk and strength for each group of pulp samples is shown in Table 11. Figure 4 .

[0047] Table 10 shows the pulp yield and chemical composition of the obtained pulp samples.

[0048] Table 11 shows the bulk and strength of the pulp obtained from each process.

[0049] Table 12 Structural information of lignin in ground wood pulp samples

[0050] As shown in Tables 10 and 12, the chemical treatment yield of the LSPAT process is greater than 70% (71.11%), and the lignin content of the resulting pulp is not less than 20% (24.58%). Compared with the raw material, the content of β-O-4 structure in the lignin structure decreases by less than 20% (15.59%), the content of β-β structure decreases by less than 2% (0.66%), the content of β-5 structure decreases by less than 0.5% (0.04%), and the carboxyl content is greater than 0.12 mol / kg (0.13 mol / kg), meeting all the technical indicators mentioned in this invention. Figure 4 It can be seen that P LSAPT The balance between bulk density and tensile strength is significantly improved.

[0051] Example 4 Using Zhejiang green bamboo as raw material, the experimental parameters for each process are shown in Table 13.

[0052] Table 13 Process parameters for each process

[0053] The yields of the three chemical treatment processes and the main chemical composition of the pulp samples after chemical treatment are shown in Table 14. The chemically treated pulp was then subjected to disc milling to prepare four pulp samples with free volume distributions ranging from 100 to 400 mL. The relationship between the bulk and strength of each group of pulp samples is shown in Table 14. Figure 5 .

[0054] Table 14 shows the pulp yield and chemical composition of the obtained pulp samples.

[0055] Table 15. Looseness and strength of pulp obtained from each process

[0056] Table 16 Structural information of lignin in ground wood pulp samples

[0057] As shown in Tables 14 and 16, the chemical treatment yield of the LSPAT process is greater than 70% (78.42%), and the lignin content of the resulting pulp is not less than 20% (21.23%). Compared with the raw material, the content of β-O-4 structure in the lignin structure decreases by less than 20% (11.42%), the content of β-β structure decreases by less than 2% (0.67%), the content of β-5 structure decreases by less than 0.5% (0.09%), and the carboxyl content is greater than 0.12 mol / kg (0.17 mol / kg), meeting all the technical indicators mentioned in this invention. Figure 5 It can be seen that P LSAPT The balance between bulk density and tensile strength is significantly improved.

[0058] Comparative experiment Using Sichuan bamboo as raw material, the experimental parameters for each process are shown in Table 17.

[0059] Table 17 Process parameters for each process

[0060] The LSPAPT process is the "moderate polycondensation, directional oxidation" process of this invention; the LSAPT-A process is the "insufficient polycondensation, directional oxidation" process; and the LSAPT-B process is "moderate polycondensation," but it uses a traditional bleaching process in the oxidation step, adding hydrogen peroxide stabilizers "sodium silicate and DTPA." The difference between the "directional oxidation" process and the bleaching process is that the bleaching process needs to maintain the "oxidizing property" of the system in order to destroy the secondary chromophores that continuously appear in the pulp, and needs to control the reaction between hydrogen peroxide and the conjugated chromophores in lignin; while the "directional oxidation" reaction does not need to control the reaction rate of hydrogen peroxide, but instead requires hydrogen peroxide to react rapidly with the lignin on the pulp surface, destroying the benzene ring structure of lignin to generate more active carboxyl groups.

[0061] The yields of the three chemical treatment processes and the main chemical composition of the pulp samples after chemical treatment are shown in Table 18. The chemically treated pulp was then subjected to disc milling to prepare four pulp samples with free volume distributions ranging from 100 to 400 mL. The relationship between the bulk and strength of each group of pulp samples is shown in Table 19. Figure 6 .

[0062] Table 18 shows the pulp yield and chemical composition of the obtained pulp samples.

[0063] As shown in Table 18, the LSPAT-A process has a weaker polycondensation reaction intensity and a higher yield compared to the LSPAT process, resulting in a higher lignin content in the pulp. In contrast, the LSPAT-B process differs from the LSPAT process only in the mechanism of the oxidation reaction, so the three lignin components of the pulp obtained by the two processes are not significantly different.

[0064] Table 19 shows the bulk and strength of the pulp obtained from each process.

[0065] The relationship between bulk thickness and tensile strength of the slurry obtained from the three processes is as follows: Figure 6 As shown, compared to the LSAPT process of "moderate polycondensation and directional oxidation," the LSAPT-A process has a lower polycondensation reaction intensity, resulting in only dissociation and no reconstruction of the lignin structure during pulping. Therefore, the resulting pulp has a lower bulk at the same strength. The LSAPT-B process, having undergone moderate polycondensation, produces a pulp with a bulk similar to that of the LSAPT process, but its oxidation process has a relatively weaker ring-opening reaction, resulting in slightly lower strength. This difference is verified in Table 20. The LSAPT-A process significantly reduces the rigid carbon-carbon bond structure in the lignin, leading to a higher lignin content but lower bulk. The LSAPT-B process produces pulp with a lignin structure similar to that of the LSAPT process, but with a lower carboxyl value, resulting in slightly lower strength.

[0066] Table 20 Structural information of lignin in ground wood pulp samples

[0067] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing high-bulk, high-strength bamboo chemimechanical pulp, characterized in that, Includes the following steps: Bamboo strips are added to a sodium sulfite solution and subjected to polycondensation treatment at 110-165℃. The resulting pulp is then extruded to remove waste liquid, yielding extruded pulp. The yield of extruded pulp is 70%-80%. The lignin content in the extruded pulp is controlled at 20-30%. The content of β-O-4 structure in the lignin structure decreases by less than 20%, the content of β-β structure decreases by less than 2%, and the content of β-5 structure decreases by less than 0.5%. The extruded slurry was added to an alkaline hydrogen peroxide solution for directional oxidation treatment. The resulting slurry was then extruded to expel the waste liquid, yielding an oxidized slurry. The carboxyl value of the groundwood lignin in the oxidized slurry was controlled to be higher than 0.12 mol / kg. The oxidized pulp is processed through disc milling, descaling, screening and washing to obtain the finished chemimechanical pulp.

2. The preparation method according to claim 1, characterized in that, The bamboo strips are obtained from bamboo raw materials through slicing, screening, washing and dehydration; the length of the bamboo strips is 20 mm-50 mm, the width is 5 mm-20 mm, and the thickness is 3 mm-8 mm.

3. The preparation method according to claim 2, characterized in that, The bamboo material is at least one of the following: moso bamboo, ci bamboo, pink bamboo, dragon bamboo, hard-headed yellow bamboo, and mian bamboo.

4. The preparation method according to claim 1, characterized in that, The solute in the sodium sulfite solution is sodium sulfite; the amount of sodium sulfite used is 100-300 kg / t relative to the amount of oven-dried bamboo chips.

5. The preparation method according to claim 1, characterized in that, The solute in the sodium sulfite solution is a mixture of sodium sulfite and sodium hydroxide; relative to the amount of oven-dried bamboo chips used, the amount of sodium sulfite used is 100-300 kg / t, and the amount of sodium hydroxide used is 100-300 kg / t.

6. The preparation method according to any one of claims 1 to 5, characterized in that, The polycondensation treatment is performed at a temperature of 150-165℃ for 30-80 minutes.

7. The preparation method according to claim 1, characterized in that, Compared to oven-dry extruded slurry, the amount of hydrogen peroxide used in the alkaline hydrogen peroxide solution is 30-80 kg / t, and the alkaline environment is provided by sodium hydroxide, with the amount of sodium hydroxide being 20-40 kg / t.

8. The preparation method according to claim 1, characterized in that, The oxidation treatment is performed at a temperature of 90-120℃ for 10-20 minutes.

9. A high bulk thickness and high strength bamboo chemimechanical pulp, which is prepared by the preparation method described in any one of claims 1 to 8.

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