Straw fiber composite material and preparation method thereof

Abstract

The present application belongs to the technical field of material preparation, and relates to a straw fiber composite material and a preparation method thereof, in particular to a preparation process of a biochar filled rice straw fiber composite material. The present application investigates the law and mechanism of the influence of CS and CPAM as retention aids on the performance of the straw fiber composite material, on the basis of which, a filling process of asymmetric bridging flocculation biochar after CS coating and CPAM is proposed, and the influence of different filling processes on the particle size distribution, Zeta potential, filler retention rate, sizing efficiency and material performance of the straw fiber composite material is studied.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation technology and relates to a straw fiber composite material and its preparation method. Specifically, it discloses a filling process for CS-coated CPAM asymmetric bridging flocculent biochar and a method for preparing the composite material. Background Technology

[0002] Currently, the application of biochar as a filler in fiber composites has attracted widespread attention. Biochar, due to its abundant pore structure and large specific surface area, can significantly improve the adsorption performance and mechanical strength of composites. However, because biochar particles carry a negative charge on their surface, their binding force with fibers is weak, resulting in a low retention rate of the filler in the fibers, affecting the overall performance of the composite. To address this issue, researchers have attempted to improve the retention rate and sizing effect of biochar in fibers through different retention aids and sizing agents. Cationic starch (CS) and cationic polyacrylamide (CPAM) have become research hotspots due to their excellent flocculation and bonding properties. CS carries a positive charge and can adsorb negatively charged biochar onto the fibers through electrostatic attraction, improving the filler retention rate. CS-coated fillers not only reduce the adsorption amount of AKD sizing agent and improve sizing efficiency, but also enable the filler to form a stronger bond with the fiber, enhancing the strength of the material.

[0003] The amide groups on CPAM can combine with the hydroxyl groups on the fiber to form hydrogen bonds, improving the mechanical properties of straw fiber composites; however, this improvement is still limited. Some biochar particles not covered by CPAM fail to fully bond with the fiber, affecting the overall strength of the material. Furthermore, CPAM can also act as a flocculant, flocculating negatively charged biochar particles through electrostatic attraction and bridging. However, when CPAM connects with the biochar filler through bridging, most of the biochar surface remains uncovered. This uncovered biochar surface adsorbs AKD sizing agent, reducing AKD retention on the fiber and affecting the sizing effect. Moreover, the bonding force between the incompletely covered biochar and the fiber is relatively weak, resulting in limited improvement in the material's mechanical properties. Summary of the Invention

[0004] In view of this, the present invention discloses a straw fiber composite material and its preparation method, and simultaneously proposes a filling process for coated biochar via asymmetric bridging. First, CS coats biochar through electrostatic attraction, then CPAM is added and deposited on the fiber. Subsequently, CPAM connects the fiber and the CS-coated biochar through asymmetric bridging to improve the retention of biochar on the fiber.

[0005] It should be noted that the problems existing in the above-mentioned technologies can all be improved by optimizing the molecular structure of CPAM, increasing the adsorption time, or introducing other auxiliary treatment methods, such as using composite retention aids. Currently, the relevant technologies for improving filler retention and reducing the impact of fillers on material properties mainly include changing the filling method, fiber / filler composites, filler modification, and filler pre-flocculation. The concept of post-coating flocculation refers to first coating and modifying the filler, and then performing pre-flocculation. By effectively combining the two technologies, high-performance filling materials can be prepared. Chen Anan used APAM, CS, and CPAM for a pretreatment process of post-coating flocculation of calcium silicate filler. First, negatively charged APAM was added to the calcium silicate suspension. Electrostatic repulsion dispersed the calcium silicate particles in the suspension, increasing the contact area with the subsequently added CS. CS can be fixed on the filler surface or used to coat the filler particles, promoting the formation of primary flocs in the calcium silicate filler.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] The first technical objective of this invention is to provide a process for adding CS-coated CPAM asymmetric bridged flocculent biochar, the process being as follows:

[0008] Using biochar as a filler, AKD as a sizing agent, and cationic starch (CS) and cationic polyacrylamide (CPAM) as retention aids, a high-load porous filler-straw fiber composite material was prepared by a coating-fed asymmetric bridging flocculation process. This ensured the filler content while maintaining the material's mechanical properties and sizing efficiency. Biochar was suspended in water to prepare a biochar suspension; CS and CPAM were dissolved separately in water to prepare solutions; CS solution was added to the obtained biochar suspension and stirred, followed by the addition of CPAM solution and stirring again to obtain biochar@CS+CPAM; following GB / T24325-2009 standard, KP pulp boards and rice straw fibers were soaked separately for 8 hours, then pulped to 45±5°SR using a PL4-00 Wali pulper and mixed. The mixed slurry consists of 70% rice straw fiber and 30% KP fiber, diluted with water to a concentration of 0.2%. AKD emulsion is diluted with water to a concentration of 2%, and AKD is added to the slurry to obtain a fiber + AKD mixed solution. Finally, the mixed solution is mixed with the previously prepared biochar@CS + CPAM solution. This biochar addition process is called "fiber + AKD + ​​[biochar@CS + CPAM]" (@ indicates CS coating of biochar, [] indicates preferential mixing, and + indicates asymmetric bridging of CPAM).

[0009] Furthermore, the amount of AKD added in the fiber + AKD solution is 1.5% (relative to the dry weight of the fiber in the composite membrane), and the amount of biochar added in the biochar suspension is 20% of the dry weight of the fiber raw material in each composite membrane material. 1.2wt% CS and 0.12wt% CPAM are added to the biochar suspension respectively (since water will be lost during the preparation of the membrane material, the above percentages of additives are all relative to the dry weight of the fiber in the composite membrane), and the mixture is stirred again to obtain a biochar@CS+CPAM solution.

[0010] The second technical objective of this invention is to provide a method for preparing straw fiber composite materials, the method specifically comprising the following steps:

[0011] KP pulp board and rice straw fiber were soaked separately and then pulped to 45±5°SR, and then mixed to obtain mixed pulp.

[0012] Water was added to the mixed slurry to dilute the concentration to 0.2%, and then AKD was added sequentially to obtain a "fiber + AKD" solution. Biochar was suspended in water to prepare a biochar suspension. CS and CPAM were dissolved in water to prepare solutions. CS solution was added to the obtained biochar suspension and stirred. Then, CPAM solution was added to the suspension and stirred again to obtain biochar@CS + CPAM. Finally, the straw fiber composite material was prepared using a ZCX-A paper forming machine.

[0013] Furthermore, the mixed slurry consists of 70% rice straw fiber and 30% KP fiber.

[0014] Furthermore, the process for preparing straw fiber composite materials using the ZCX-A paper forming machine is as follows:

[0015] The mixed slurry with different additives was poured into the paper forming machine for sheet forming. During the paper forming process, water was first added to 100ml, then the slurry was poured in, and water was added to 350ml. After uniform stirring, the mixture was taken out. During the paper forming process, the water was vacuumed to bond the fiber and cotton cloth together. After extrusion, compaction, and drying, the straw fiber composite material was obtained.

[0016] Furthermore, the drying temperature is 105°C and the drying time is 15 minutes.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] (1) The effects of CS and CPAM on the zeta potential, filler retention, sizing degree, and mechanical properties of the slurry were compared. When 1.2 wt% CS or 0.12 wt% CPAM were added alone, the zeta potential of the slurry changed from negative to positive, the filler retention increased by 73.91% and 79.77%, respectively, the sizing degree increased by 247.10% and 253.20%, respectively, the dry tensile index increased by 25.95% and 28.85%, respectively, and the tear index increased by 23.79% and 25.97%, respectively.

[0019] Positively charged CS is added to a negatively charged biochar suspension, causing CS to deposit on the biochar surface, thus coating the biochar filler; this is called "biochar@CS". Due to electrostatic repulsion, the second additive, CPAM, cannot adsorb onto the "biochar@CS," and CPAM and "biochar@CS" remain dispersed in the suspension. The suspension containing CPAM and "biochar@CS" is then added to a slurry containing AKD, where CPAM adsorbs onto the negatively charged fibers. CPAM does not bridge with the coated biochar in the biochar suspension itself; instead, it connects the biochar and fibers through an asymmetric bridging mechanism after addition to the slurry. CPAM still needs to be added to the biochar suspension to increase the contact opportunities between the fibers and AKD, thereby improving AKD retention on the fibers.

[0020] When CS is added to the slurry as a stabilizer, it adsorbs onto AKD colloidal particles, imparting a positive charge to them. This increases the zeta potential of the emulsion, enhances the electrostatic repulsion between AKD particles, effectively improves the emulsion's stability, reduces the number of AKD particles that fail due to hydrolysis, and prolongs the hydrolysis time of AKD. From a molecular structure perspective, CS, as a high-molecular-weight polymer containing multiple hydroxyl groups, can bind with cellulose molecules through hydrogen bonds when added to the slurry and adhering to the fibers. This increases the number of hydrogen bonds between fibers, improves the internal binding force of the fibers, forms a cohesive network, and improves material strength. When adsorbed onto fillers, it can mitigate the strength loss caused by filler addition through flocculation.

[0021] (2) Compared with "fiber + AKD + ​​biochar", the "fiber + AKD + ​​[biochar@CS + CPAM]" filling process increased the average particle size of biochar flocs by 1.94 times, increased the surface charge by 52.83 mV, decreased air permeability by 19.16%, increased sizing degree by 35.02%, increased contact angle by 64.13%, increased filler retention rate by 84.77%, increased total retention rate by 25.70%, increased dry tensile index by 35.14%, increased tear index by 53.62%, and increased filler adhesion coefficient by 35.30%. CS adsorbs biochar particles through electrostatic attraction, and the biochar particles coated by CS are connected to the fibers through the asymmetric bridging effect of CPAM, increasing the size of the biochar flocs. The flocs are beneficial for filling the gaps between fibers, reducing the air permeability of the material. The coating reduces the contact area between biochar and AKD, thus reducing the adsorption of AKD by biochar and improving sizing efficiency. The addition process of asymmetric bridging flocculation after coating can improve the biochar retention rate and the binding between biochar and fiber. CS and CPAM form a network structure through chemical bonds, which improves the mechanical properties of straw fiber composite materials.

[0022] (3) This invention investigated the laws and mechanisms of the influence of CS and CPAM as retention aids on the properties of straw fiber composites. Based on this, a filling process of CS-coated CPAM asymmetric bridging flocculent biochar was proposed, and the effects of different filling processes on the filler particle size distribution, Zeta potential, filler retention rate, sizing efficiency, and material properties of straw fiber composites were studied. The results showed that CS adsorbs biochar particles through electrostatic attraction, and the CS-coated biochar particles are connected to the fibers through the asymmetric bridging effect of CPAM, increasing the size of the biochar flocs. The flocs are beneficial for filling the gaps between fibers, reducing the material's air permeability. The coated biochar reduces the contact area with AKD, reduces the adsorption of AKD by biochar, and improves the sizing efficiency. The filling process of coating with asymmetric bridging flocculents can improve the biochar retention rate and improve the bonding between biochar and fibers. CS and CPAM form a network structure through chemical bonds, improving the mechanical properties of straw fiber composites. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1The effects of CS content on Zeta potential (a), filler retention (b), sizing degree (c), dry tensile index (d), and tear index (e).

[0025] Figure 2 The effects of CPAM content on Zeta potential (a), filler retention (b), sizing degree (c), dry tensile index (d), and tear index (e).

[0026] Figure 3 These are the size distribution curves of fillers added using different filling processes.

[0027] Figure 4 It is the average particle size (d50) of fillers added using different filling processes.

[0028] Figure 5 The diagrams show the mechanisms of the fiber + AKD + ​​[biochar@CS + CPAM] (a), fiber + AKD + ​​[biochar@CS] (b), and fiber + AKD + ​​[biochar / CPAM] (c) addition processes.

[0029] Figure 6 The effects of the filling process on the dry tensile index (a), tear index (b), filler adhesion coefficient (c), sizing degree (d), contact angle (e), and air permeability (f) of straw fiber composite materials are investigated. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0031] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in the embodiments of this application, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in this application is merely for describing particular implementations and is not intended to limit the scope of this disclosure.

[0032] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and technical means not specifically mentioned herein refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0033] To better illustrate the content of this application, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well-known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0034] Without conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of this application.

[0035] This invention discloses a straw fiber composite material and its preparation method, specifically a preparation process for a biochar-filled rice straw fiber composite material. This invention investigates the influence and mechanism of CS and CPAM as retention aids on the properties of straw fiber composite materials. Based on this, a filling process for CS-coated CPAM asymmetric bridging flocculated biochar is proposed, and the effects of different filling processes on the filler particle size distribution, Zeta potential, filler retention rate, sizing efficiency, and material properties of the straw fiber composite material are studied.

[0036] To better understand the present invention, the following embodiments are provided for further detailed description of the present invention, but they should not be construed as limiting the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the protection scope of the present invention.

[0037] Example 1

[0038] A process for preparing a biochar-filled rice straw fiber composite material, the process comprising the following steps:

[0039] Parameter range and steps:

[0040]

[0041] step:

[0042] Preparation of biochar suspension: Biochar was suspended in water to obtain a 10 wt% biochar suspension.

[0043] Preparation of CS and CPAM solutions: Dissolve CS in water to prepare a 0.5% solution, and dissolve CPAM in water to prepare a 0.1% solution.

[0044] Preparation of biochar@CS+CPAM: Add CS solution to biochar suspension and stir until homogeneous; then add CPAM solution and stir until homogeneous to obtain biochar@CS+CPAM suspension.

[0045] Fiber treatment: In accordance with GB / T 24325-2009 standard, KP pulp board and rice straw fiber were soaked for 8 hours, then pulped to 45±5°SR using a PL4-00 Wali pulper and mixed to obtain mixed pulp.

[0046] Slurry dilution: Dilute the mixed slurry with water to a concentration of 0.2%.

[0047] AKD treatment: Add 2% AKD emulsion to the mixed slurry and stir evenly to obtain a fiber + AKD mixed solution.

[0048] Mixing: The fiber + AKD mixed solution was mixed with the biochar@CS + CPAM solution and stirred evenly to prepare a high-load porous filler straw fiber composite material.

[0049] Example 2

[0050] Parameter range and steps:

[0051]

[0052] step:

[0053] Preparation of biochar suspension: Biochar was suspended in water to obtain a 20 wt% biochar suspension.

[0054] Preparation of CS and CPAM solutions: Dissolve CS in water to prepare a 1% solution, and dissolve CPAM in water to prepare a 0.2% solution.

[0055] Preparation of biochar@CS+CPAM: Add CS solution to biochar suspension and stir until homogeneous; then add CPAM solution and stir until homogeneous to obtain biochar@CS+CPAM suspension.

[0056] Fiber treatment: In accordance with GB / T 24325-2009 standard, KP pulp board and rice straw fiber were soaked for 8 hours, then pulped to 45±5°SR using a PL4-00 Wali pulper and mixed to obtain mixed pulp.

[0057] Slurry dilution: Dilute the mixed slurry with water to a concentration of 0.2%.

[0058] AKD treatment: Add 2% AKD emulsion to the mixed slurry and stir evenly to obtain a fiber + AKD mixed solution.

[0059] Mixing: The fiber + AKD mixed solution was mixed with the biochar@CS + CPAM solution and stirred evenly to prepare a high-load porous filler straw fiber composite material.

[0060] Example 3

[0061]

[0062]

[0063] step:

[0064] Preparation of biochar suspension: Biochar was suspended in water to obtain a 20 wt% biochar suspension.

[0065] Preparation of CS and CPAM solutions: Dissolve CS in water to prepare a 1.5% solution, and dissolve CPAM in water to prepare a 0.2% solution.

[0066] Preparation of biochar@CS+CPAM: Add CS solution to biochar suspension and stir until homogeneous; then add CPAM solution and stir until homogeneous to obtain biochar@CS+CPAM suspension.

[0067] Fiber treatment: In accordance with GB / T 24325-2009 standard, KP pulp board and rice straw fiber were soaked for 8 hours, then pulped to 45±5°SR using a PL4-00 Wali pulper and mixed to obtain mixed pulp.

[0068] Slurry dilution: Dilute the mixed slurry with water to a concentration of 0.2%.

[0069] AKD treatment: Add 2% AKD emulsion to the mixed slurry and stir evenly to obtain a fiber + AKD mixed solution.

[0070] Mixing: The fiber + AKD mixed solution was mixed with the biochar@CS + CPAM solution and stirred evenly to prepare a high-load porous filler straw fiber composite material.

[0071] To further demonstrate the beneficial effects of the present invention and to better understand it, the following experiments are conducted to further clarify the technical features disclosed in the present invention, but these should not be construed as limiting the invention. Any other improvements made by those skilled in the art based on the above description of the invention, without inventive effort, are also considered to fall within the protection scope of the present invention.

[0072] (1) Preparation and characterization of straw fiber composite materials

[0073] Biochar filler pretreatment: Taking the addition process of coated biochar flocculated by asymmetric bridging as an example. Biochar was suspended in water to prepare a suspension with a solid content of 2%; 50 mL of the prepared biochar suspension was transferred to a beaker, and CS and CPAM were dissolved in water to prepare solutions with concentrations of 10.0 g / L and 1.0 g / L, respectively; CS was added to the biochar suspension, and the mixture was stirred at 400 rpm / min for 5 min, followed by the addition of CPAM, and stirring again for 5 min. This biochar addition process is called “fiber + AKD + ​​[biochar@CS + CPAM]” (@ indicates CS coating of biochar, [] indicates preferential mixing, and + indicates CPAM bridging).

[0074] Preparation of straw fiber composite materials: Following GB / T 24325-2009 standard, KP pulp board and rice straw fiber were soaked separately for 8 hours, then pulped to 45±5°SR using a PL4-00 Wali pulper and mixed. The mixed pulp consisted of 70% rice straw fiber and 30% KP fiber, diluted with water to a concentration of 0.2%, followed by the sequential addition of CS, AKD, CPAM, and finally biochar. The mixture was stirred at 800 rpm for 10 minutes and then prepared using a ZCX-A paper forming device to achieve a basis weight of 90 g / m³. 2 The samples were dried at 105℃ for 15 min and then left to stand for 24 h at room temperature (23±1℃) and relative humidity (30±2%) before testing.

[0075] Zeta potential: Add 20wt% biochar to the slurry and add the corresponding proportion of wet end additives to control the total slurry concentration to 0.2% and the total volume to 500mL. Measure the slurry zeta potential using a zeta potential meter.

[0076] (2) Performance Testing

[0077] The dry tensile strength, sizing degree, and air permeability of the prepared straw fiber composite material were measured.

[0078] Total fiber retention (R) T ) and filler retention rate (R F Calculate using formulas 1, 2, and 3 respectively:

[0079]

[0080]

[0081]

[0082] In the formula, A1(g) represents the mass of the straw fiber composite material, A2(g) represents the mass of the slurry used to prepare the straw fiber composite material, m(g) represents the ash mass of the straw fiber composite material after calcination at 575℃, α(%) represents the mass loss ratio of biochar after calcination, m1(g) represents the mass of biochar before calcination, and m2(g) represents the mass of biochar after calcination.

[0083] The filler bondability factor (FBF) is used to analyze the bond strength between the filler and the fiber, and is expressed by Formula 4:

[0084]

[0085] In the formula, X1 (N·m / g) represents the dry tensile index of the straw fiber composite material, X2 (N·m / g) represents the dry tensile index of the unfilled straw fiber material, and F (%) represents the filler content of the straw fiber composite material.

[0086] (3) Results Analysis

[0087] Table 1 shows that the retention rates of biochar with different particle sizes range from 16.45% to 27.94%. As the particle size decreases, the retention rate also decreases. Biochar particle sizes below 50 μm are much smaller than the size of a single sieve in a paper forming machine (70–300 μm). Relying solely on mechanical retention results in very low biochar retention rates. Therefore, improving the retention rate of the filler is crucial for preparing high-performance straw fiber composite materials.

[0088] Table 1. Filler retention rate for particle sizes C3 to C18

[0089]

[0090]

[0091] (4) The effect of unit retention system on straw fiber composite materials

[0092] 1. The effect of CS retention system on straw fiber composite materials

[0093] The experiment compared the effects of CS content on the slurry's Zeta potential, filler retention, sizing degree, and material mechanical properties. The results are as follows: Figure 1 As shown in ae: From Figure 1As can be seen from diagram a, without the addition of CS, the zeta potential of the slurry is negative. With increasing CS content, the zeta potential gradually approaches the zero charge potential, and when the CS content reaches 1.2 wt%, the zeta potential changes from negative to positive. CS is a positively charged cationic polymer that can neutralize the negative charge in the slurry. The carboxyl groups are the source of the negative charge in the fibers and also the adsorption sites for CS. The zeta potential reflects the surface charge density of the fibers and fillers in the slurry and can serve as an important indicator for evaluating colloidal stability. Near the zero charge potential, the slurry stability is usually best, resulting in optimal filler retention, better material properties, and higher sizing efficiency.

[0094] Depend on Figure 1 As shown in section b, when the CS content is below 1.2 wt%, the filler retention rate gradually increases with increasing CS content, reaching a maximum increase of 73.91% compared to no CS addition. This is because CS carries a positive charge and has a strong affinity for negatively charged substances in the slurry, enabling it to flocculate biochar filler through electrostatic attraction, causing biochar to aggregate into large flocs and subsequently fix them in the fibers, thereby improving the filler retention rate. However, when the CS content exceeds 1.2 wt%, the biochar retention rate decreases. This is because excessive CS suspended in the slurry generates electrostatic repulsion between it and the flocs.

[0095] from Figure 1 As shown in Figure c, the sizing degree of the straw fiber composite material gradually increases with the increase of CS content. When the CS content is 1.2 wt%, the sizing degree of the material reaches 169.37 s, which is 247.10% higher than that without CS. CS reduces the adsorption of AKD by the filler by coating biochar particles, thereby increasing the retention rate of AKD on long fibers. After adding CS to the slurry, the biochar filler adsorbs CS, and the resulting electrostatic repulsion reduces the adsorption of AKD, improving the sizing efficiency of AKD. CS can also improve the sizing efficiency by increasing the stability of the AKD sizing agent in the slurry.

[0096] Figure 1 The study demonstrated the effect of CS content on improving the mechanical properties of straw fiber composites. When the CS content was 1.2 wt%, the dry tensile index increased by 25.95% and the tear index increased by 23.79% compared to no CS added. CS contains hydroxyl groups, which can bind with fibers to form hydrogen bonds, enhancing the bonding force between fibers and thus improving the mechanical properties of the material. However, excessive CS leads to a decrease in mechanical properties because the excess CS introduces a large amount of positive charge, causing the fibers to adhere together due to charge neutralization, resulting in uneven fiber distribution after molding.

[0097] 2. The effect of CPAM retention system on straw fiber composite materials

[0098] The effects of CPAM content on slurry zeta potential, filler retention, sizing degree, and material mechanical properties were compared, and the results are as follows: Figure 2 As shown in ac: From Figure 2 As can be seen, without the addition of CPAM, the zeta potential of the slurry is negative. With increasing CPAM content, the zeta potential gradually approaches zero charge. When the CPAM content is 0.12 wt%, the zeta potential becomes positive. This is because CPAM is also a positively charged cationic polymer, possessing a charge neutralization effect, which can reduce the surface charge of the slurry.

[0099] from Figure 2 As can be seen from b, the filler retention rate gradually increases with the increase of CPAM content. When the CPAM content is 0.12 wt%, the filler retention rate increases by 79.77% compared with no CPAM added. CPAM interacts with various components in the slurry (including fibers, fine fibers, and biochar fillers) through charge neutralization and bridging, forming larger flocs. Initially, CPAM is adsorbed together with biochar by electrostatic attraction, and then forms positively charged biochar flocs. The mutual attraction between the flocs and the negatively charged biochar particles free in the slurry causes them to adhere together and settle on the fibers. The increase in the size of the biochar flocs improves their retention rate. In this case, CPAM can be considered as a "binder" or "adhesive" for straw fiber composites. The optimal biochar content plays a key role in improving the biochar retention rate. When the CPAM content exceeds 0.12 wt%, the filler retention rate decreases slightly. These phenomena are consistent with both charge neutralization and bridging mechanisms. [8] Excessive CPAM can cause electrostatic repulsion between flocs, thus affecting the restabilization of the flocs.

[0100] from Figure 2 As shown in Figure c, the sizing degree of the straw fiber composite material increases with increasing CPAM content. The sizing degree reaches its highest value of 163.54 s after adding 0.12% CPAM, representing a 253.20% increase compared to the absence of CPAM. However, when the CPAM content exceeds 0.12 wt%, the promoting effect on sizing becomes insignificant. With increasing CPAM content, the excess cations carried by CPAM significantly increase the repulsion effect on biochar particles, leading to a substantial loss of filler and fine fibers adsorbing AKD. This increases the porosity between fibers, making the liquid more easily permeable. Furthermore, the large amount of CPAM added also causes strong flocculation in the slurry, causing long fibers in the slurry to curl and flocculate together, reducing the contact area between long fibers and AKD particles, and lowering the sizing efficiency of AKD.

[0101] from Figure 2 As can be seen from the results, the mechanical properties of straw fiber composites improve with increasing CPAM content. When the CPAM content reaches 0.12 wt%, the dry tensile index increases by 28.85%, and the tear index increases by 25.97%. CPAM can improve material strength by forming hydrogen bonds with fibers. Furthermore, fine fiber components that were originally trapped by fibers and screens are retained more effectively through the bridging effect of CPAM, which is beneficial for fiber network construction. However, when the CPAM content exceeds 0.12 wt%, the mechanical properties of the material begin to decrease. Excessive addition of CPAM leads to excessive flocculation, causing fibers to aggregate and fail to disperse evenly, thus affecting the mechanical properties of the material.

[0102] (5) Filler filling process

[0103] 1. Particle size distribution and average size of biochar filler treated by different filling processes, as shown in the figure. Figure 4 and Figure 5 As shown.

[0104] Figure 4 The results show that the particle size distribution of biochar treated with the "fiber + AKD + ​​biochar" addition process ranges from 13 to 25 μm. The particle size distribution is significantly wider with the "fiber + AKD + ​​[biochar@CS + CPAM]" process, ranging from 20 to 50 μm. For biochar packings flocculated using CS or CPAM, i.e., the "fiber + AKD + ​​[biochar@CS]" and "fiber + AKD + ​​[biochar / CPAM]" treatments, the particle size increase is relatively small, ranging from 17 to 35 μm and 17 to 39 μm, respectively. For binary pre-flocculated biochar packings, i.e., the "fiber + AKD + ​​[biochar@CS / APAM]" treatment, the particle size distribution ranges from 23 to 43 μm.

[0105] from Figure 5The effects of different filler processes on the particle size distribution of biochar flocs can be more clearly observed. Compared with the "fiber + AKD + ​​biochar" treatment, the "fiber + AKD + ​​[biochar@CS + CPAM]" process increased the average particle size of biochar by 1.94 times, which is 1.54 times and 1.32 times larger than the "fiber + AKD + ​​[biochar@CS]" and "fiber + AKD + ​​[biochar / CPAM]" processes, respectively, and 1.11 times larger than the "fiber + AKD + ​​[biochar@CS / CPAM]" process. All four filler processes can increase the size of biochar flocs, with the "fiber + AKD + ​​[biochar@CS + CPAM]" process more likely to produce larger filler flocs. This can be attributed to the electrostatic interaction between biochar particles and CS, which effectively coats the biochar surface. The CS-coated biochar particles are then connected to the fibers through the asymmetric bridging effect of CPAM, increasing the size of the biochar flocs.

[0106] To further understand the changes in the charge properties of the biochar filler, the surface charge of the biochar was measured. Table 2 shows that the initial Zeta potential of the biochar was -17.49 mV. After treatment with "fiber + AKD + ​​[biochar@CS + CPAM]", the Zeta potential of the biochar turned positive to +35.34 mV, indicating that the wet-end additive deposition on the biochar surface was very effective. However, in the processes of "fiber + AKD + ​​[biochar@CS]" and "fiber + AKD + ​​[biochar / CPAM]", the Zeta potential of the biochar only reached +13.17 mV and +15.49 mV, respectively. In the process of "fiber + AKD + ​​[biochar@CS / APAM]", the Zeta potential of the biochar was 3.09 mV. Therefore, it can be predicted that biochar particles treated by the "fiber + AKD + ​​[biochar@CS + CPAM]" process are more likely to remain on the fiber.

[0107] Table 2. Zeta potentials of solutions with different addition processes

[0108]

[0109] Based on the preparation process and existing conclusions, the mechanism of CPAM action through asymmetric bridging flocculation after CS coating is deduced, such as... Figure 5 As shown, the specific steps are as follows:

[0110] (1) Positively charged CS is added to a negatively charged biochar suspension. CS is deposited on the surface of biochar, thus coating the biochar filler, and is called "Biochar@CS". Due to electrostatic repulsion, the second additive CPAM added subsequently cannot be adsorbed on "Biochar@CS", and CPAM and "Biochar@CS" are dispersed in the suspension.

[0111] (2) A suspension containing CPAM and "biochar@CS" was added to a slurry containing AKD. CPAM adsorbed onto the negatively charged fibers. Although CPAM cannot directly adsorb "biochar@CS", it can adsorb onto the fiber surface through electrostatic attraction, causing changes in the chemical bonds and molecular configuration of CPAM. This leads to a greater loss of CPAM molecular entropy and a decrease in resistance to the adsorption of "biochar@CS". This allows CPAM, which initially could only adsorb onto the fibers, to also connect the fibers and "biochar@CS" through asymmetric bridging, thus better achieving the flocculation of the biochar filler. In contrast, as... Figure 6 bc. Traditional filler processes will directly form loose filler flocs after adding CS and CPAM.

[0112] Agatha Poraj-Kozminski demonstrated through AKD flocculation experiments and adsorption kinetic measurements that intermolecular electrostatic repulsion prevents the connection between CPAM and AKD. CPAM does not bridge the coated biochar in the biochar suspension; instead, it connects the biochar and fibers through an asymmetric bridging mechanism after the slurry is added. CPAM still needs to be added to the biochar suspension to increase the contact opportunities between the fibers and AKD, thereby improving AKD retention on the fibers. Since CPAM does not adsorb onto positively charged AKD particles but rather onto negatively charged fibers, competing with AKD for the negative charge on the fibers, AKD needs to be added to the slurry first to provide sufficient contact sites. Furthermore, CPAM flocculates fibers, causing fiber crimping and reducing the contact area with AKD.

[0113] 2. The effect of the filling process on the properties of straw fiber composite materials

[0114] Table 3 Comparison of packing retention rates

[0115]

[0116] Table 3 shows the total retention rate and filler retention rate of straw fiber composites under different filling processes. Compared with the "fiber + AKD + ​​biochar" process, the "fiber + AKD + ​​[biochar@CS + CPAM]" process increased the filler retention rate and total retention rate of straw fiber composites by 84.77% and 25.70%, respectively. Compared with the "fiber + AKD + ​​[biochar@CS]" filling process, the filler retention rate and total retention rate increased by 27.99% and 19.02%, respectively. Compared with the "fiber + AKD + ​​[biochar / CPAM]" process, the filler retention rate and total retention rate increased by 9.07% and 9.52%, respectively. Compared with the "fiber + AKD + ​​[biochar@CS / APAM]" process, the filler retention rate and total retention rate increased by 7.73% and 6.58%, respectively. The advantages of using the "fiber + AKD + ​​[biochar@CS + CPAM]" process are very obvious. Due to the coating of CS and the asymmetric bridging effect of CPAM, the coated biochar particles can be combined with fine fibers to form larger flocs, thereby improving the retention rate of fillers and fibers.

[0117] The effect of different filling processes on the properties of straw fiber composite materials, such as Figure 6 As shown in Figure 4, the "fiber + AKD + ​​[biochar@CS + CPAM]" process significantly improved the material's performance. Compared to the "fiber + AKD + ​​biochar" treatment, the dry tensile index increased by 35.14%, the tear index increased by 53.62%, the filler adhesion coefficient increased by 35.30%, the sizing degree increased by 35.02%, the contact angle increased by 64.13%, and the air permeability decreased by 19.16%. CS and CPAM can improve the material's mechanical properties through ionic bonds and asymmetric bridging. CS coats the biochar surface through electrostatic attraction, reducing the adsorption of AKD and improving the sizing efficiency of AKD. CPAM acts as a bridge, first adsorbing onto the fiber and then combining with the CS-coated biochar surface, forming a network structure between the filler and fiber through chemical bonds. This transforms the originally loose physical and mechanical mixture into a strong chemical bond, improving the material's mechanical properties. The increase in sizing degree is not due to the direct adsorption of added wet-end additives by AKD granules, but rather to the adsorption of these additives onto fillers and fibers. The sizing efficiency is improved through the co-flocculation of fillers, fibers, and AKD granules.

[0118] Compared to the "fiber + AKD + ​​[biochar@CS]" process, the "fiber + AKD + ​​[biochar@CS + CPAM]" filling process resulted in a 16.48% increase in the dry tensile index, a 21.49% increase in the tear index, a 16.44% increase in the filler adhesion coefficient, a 13.55% increase in sizing degree, a 21.42% increase in the contact angle, and a 12.10% decrease in air permeability. CPAM significantly improves the bonding between biochar and fiber through asymmetric bridging. Positively charged CPAM initially adsorbs onto the negatively charged fiber surface through electrostatic attraction, causing changes in CPAM chemical bonds and molecular configuration, leading to increased entropy loss. This reduces the resistance to adsorbing CS-coated biochar, allowing CPAM, which initially could only adsorb onto the fiber, to also connect the fiber and coated biochar through asymmetric bridging, thus improving the material's mechanical properties and adhesive strength. After adsorbing anionic waste in the slurry, AKD particles may become amphoteric substances. The added CPAM can play a bridging role in the negatively charged regions of AKD, thereby improving the retention rate of AKD.

[0119] Compared to "fiber + AKD + ​​[biochar / CPAM]", "fiber + AKD + ​​[biochar@CS + CPAM]" exhibits a 20.43% increase in dry tensile index, a 37.22% increase in tear index, a 20.38% increase in filler adhesion coefficient, an 8.40% increase in sizing degree, a 16.01% increase in contact angle, and a 4.81% decrease in air permeability. CPAM asymmetrically bridges the CS-coated biochar, allowing CS to escape from the biochar interstices and spread onto the fiber, forming hydrogen bonds and improving the material's mechanical properties and adhesive strength. The addition of CS deposits on the biochar surface, coating it and reducing both AKD adsorption and CS adsorption from the AKD emulsion, preventing a decrease in sizing degree due to AKD hydrolysis.

[0120] Compared with "fiber + AKD + ​​[biochar @ CS / APAM]", "fiber + AKD + ​​[biochar @ CS + CPAM]" has a 20.21% lower dry tensile index, a 6.81% lower tear index, a 17.24% lower filler adhesion coefficient, a 17.53% higher sizing degree, a 15.73% higher contact angle, and a 9.42% higher air permeability.

[0121] The reason for the reduced air permeability of straw fiber composites is that the flocs obtained by the "fiber + AKD + ​​[biochar@CS + CPAM]" process have the widest distribution range (20-50μm), which is more conducive to filling the gaps between fibers and has a stronger obstruction effect on air.

[0122] Based on the above analysis, this invention investigated the influence and mechanism of CS and CPAM as retention aids on the properties of straw fiber composites. Based on this, a CS-coated CPAM asymmetric bridging flocculent biochar addition process was proposed, and the effects of different addition processes on the filler particle size distribution, Zeta potential, filler retention rate, sizing efficiency, and material properties of straw fiber composites were studied. The following conclusions were drawn:

[0123] (1) The effects of CS and CPAM on the zeta potential, filler retention, sizing degree, and mechanical properties of the slurry were compared. When 1.2 wt% CS or 0.12 wt% CPAM were added alone, the zeta potential of the slurry changed from negative to positive, the filler retention increased by 73.91% and 79.77%, respectively, the sizing degree increased by 247.10% and 253.20%, respectively, the dry tensile index increased by 25.95% and 28.85%, respectively, and the tear index increased by 23.79% and 25.97%, respectively.

[0124] (2) Compared with "fiber + AKD + ​​biochar", the "fiber + AKD + ​​[biochar@CS + CPAM]" filling process increased the average particle size of biochar flocs by 1.94 times, increased the surface charge by 52.83 mV, decreased air permeability by 19.16%, increased sizing degree by 35.02%, increased contact angle by 64.13%, increased filler retention rate by 84.77%, increased total retention rate by 25.70%, increased dry tensile index by 35.14%, increased tear index by 53.62%, and increased filler adhesion coefficient by 35.30%. CS adsorbs biochar particles through electrostatic attraction, and the biochar particles coated by CS are connected to the fibers through the asymmetric bridging effect of CPAM, increasing the size of the biochar flocs. The flocs are beneficial for filling the gaps between fibers, reducing the air permeability of the material. The coating reduces the contact area between biochar and AKD, thus reducing the adsorption of AKD by biochar and improving sizing efficiency. The addition process of asymmetric bridging flocculation after coating can improve the biochar retention rate and the binding between biochar and fiber. CS and CPAM form a network structure through chemical bonds, which improves the mechanical properties of straw fiber composite materials.

[0125] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A process for adding CS-coated CPAM asymmetric bridged flocculent biochar, characterized in that, The process is as follows: Using biochar as filler, AKD as sizing agent, and cationic starch CS and cationic polyacrylamide CPAM as retention aids, a high-load porous filler straw fiber composite material was prepared by a coating-fed asymmetric bridging flocculation process. The specific operation is as follows: 1) Prepare a biochar suspension by suspending biochar in water; prepare solutions by dissolving CS and CPAM in water respectively; add CS solution to the obtained biochar suspension and stir, then add CPAM solution to the suspension and stir again to obtain a biochar@CS+CPAM solution; 2) Soak the KP pulp board and rice straw fiber separately, then pulp them to 45±5°SR, and mix them together; The concentration of the mixed slurry is 0.2%; AKD is added to the mixed slurry to obtain a fiber + AKD mixed solution; 3) Mix the fiber + AKD mixed solution with the biochar @CS + CPAM solution to obtain "fiber + AKD + ​​[biochar @CS + CPAM]", where @ represents the coating of biochar by CS, [] represents preferential mixing, and + represents the asymmetric bridging of CPAM; Since water is lost during the preparation of membrane materials, the percentages of additives are all relative to the dry weight of fibers in the composite membrane; The AKD addition amount in the fiber + AKD solution is 1.5%; the biochar addition amount in the biochar suspension is 20% of the dry weight of the fiber raw material in each composite membrane material. 1.2wt% CS and 0.12wt% CPAM are added to the biochar suspension respectively, and the mixture is stirred again to obtain the biochar@CS+CPAM solution.

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