Flexibility strengthening process of high-precision high-viscosity papermaking washed kaolin

Abstract

The invention belongs to the technical field of papermaking filler and mineral processing, and discloses a flexibility strengthening process of high-fineness high-viscosity papermaking washed kaolin. The process is characterized by comprising the following steps: sequentially reacting high-precision high-viscosity washed kaolin slurry with a silane coupling agent and a cellulose derivative oxidized and activated by 2, 2, 6, 6-tetramethylpiperidine-1-oxygen free radicals, and constructing a silane anchoring layer and a cellulose flexible grafting layer on the surfaces of particles to form a rigid core-flexible shell structure; wherein the silane coupling agent is fixed on the surface of the kaolin through a covalent bond, and the cellulose is grafted outside the silane layer through an amido bond or an ether bond. According to the invention, through molecular design and interface engineering means, on the basis of not sacrificing the original rheological advantages of the high-precision high-viscosity washed kaolin, synergistic reinforcement of flexibility, retention and dispersion stability is successfully realized, and a brand new technical path is provided for functional upgrading of high-end papermaking fillers.

Description

Technical Field

[0001] This invention belongs to the field of papermaking filler and mineral processing technology, and relates to a process for enhancing the flexibility of high-precision, high-viscosity washed kaolin used in papermaking. Background Technology

[0002] Kaolin, as an important inorganic non-metallic mineral filler, is widely used in the papermaking industry due to its high whiteness, fine particle size, and strong chemical inertness. It is particularly effective in the production of high-grade cultural paper and high-end packaging paper, significantly improving the paper's optical properties, smoothness, and printability. High-fineness, high-viscosity washed kaolin, with its narrow particle size distribution, low impurity content, and high suspension viscosity, has become the preferred material for high-end papermaking fillers.

[0003] To improve the interfacial compatibility between kaolin filler and the fiber matrix, existing technologies commonly employ surface modification methods, with treatment using a single organic modifier (such as fatty acids, cationic starch, or silane compounds) being the most prevalent. These methods, by introducing hydrophobic or hydrophilic functional groups onto the surface of kaolin particles, improve its dispersibility in the pulp system to some extent and alleviate localized stress concentration caused by filler aggregation.

[0004] The aforementioned single modification strategy has significant limitations when dealing with high viscosity systems: on the one hand, if the focus is on improving flexibility by introducing flexible long-chain molecules, it often leads to a weakening of the steric hindrance effect between particles or charge shielding, thereby destroying the rheological stability of the original colloidal system and causing a decrease in viscosity; on the other hand, if priority is given to maintaining the particle surface charge density or hydration layer structure required for high viscosity, it is difficult to effectively construct a flexible interface sufficient to buffer external impacts, causing the paper to still exhibit a high brittleness index and a tendency to shed dust.

[0005] Single-modified kaolin is prone to migration or detachment during the wet end stage due to insufficient interfacial bonding, which not only affects paper uniformity but also exacerbates the formation of microcracks caused by uneven stress release during subsequent drying. In addition, some modifiers have poor stability in high-temperature drying or alkaline papermaking environments, and are prone to desorption or degradation, further weakening their long-term contribution to flexibility. Summary of the Invention

[0006] To achieve the above-mentioned objectives, this invention provides a process for enhancing the flexibility of high-precision, high-viscosity washed kaolin used in papermaking. This process involves sequentially constructing a silane coupling agent anchoring layer and a cellulose flexible grafting layer on the surface of kaolin particles, forming a composite interface functional layer with a dual-level structure. While maintaining the high viscosity and excellent dispersion stability of the kaolin suspension, this process significantly improves its interfacial bonding strength and energy dissipation capacity during paper forming, thereby effectively improving the flexibility, dust resistance, and printability of the finished paper.

[0007] The process described in this invention includes the following steps: First, high-fineness, high-viscosity washed kaolin is placed in deionized water to prepare a slurry with a solid content of 15%-25%, and the pH value is adjusted to 4.0-6.0; then, a silane coupling agent is added to the slurry, and the mixture is stirred and reacted at 40-70℃ for 30-120 minutes, allowing the silane coupling agent to hydrolyze and condense, and then covalently bond to the hydroxyl sites on the surface of the kaolin particles through silicon-oxygen bonds, forming a monolayer anchoring structure; next, an activated cellulose derivative is introduced into the system, and the reaction continues at 60-90℃ for 60-180 minutes, allowing the cellulose molecular chains to undergo a grafting reaction through the organic functional groups reserved by the silane coupling agent, thereby forming a flexible polymer graft layer with a thickness of 5-30 nm on the outer surface of the kaolin particles; after the reaction, the resulting slurry is subjected to solid-liquid separation and washing until the conductivity is lower than that of the silane coupling agent. Then dry at 80-120℃ to obtain the flexible and reinforced kaolin product.

[0008] The high-fineness, high-viscosity washed kaolin ore is subjected to wet purification, ultrafine grinding, and classification, with a particle size D90 ≤ 2.0. , Content ≤0.5%, The apparent viscosity of a 20% solids content aqueous suspension at 25°C is ≤0.8%. ,and absolute value of potential .

[0009] The silane coupling agent is selected from γ-aminopropyltriethoxysilane, N-( -aminoethyl)- One of -aminopropyltrimethoxysilane or 3-glycidoxypropyltrimethoxysilane, added in an amount of 0.3%-1.5% of the dry weight of kaolin. The silane coupling agent hydrolyzes in an acidic aqueous environment to generate a silanol intermediate, which dehydrates and condenses with the Al-OH or Si-OH groups on the surface of the kaolin to form a stable... or It has covalent bonds, and its molecular ends retain active functional groups such as primary amino, secondary amino, or epoxy groups for subsequent chemical grafting with cellulose derivatives.

[0010] The cellulose derivative is microcrystalline cellulose or nanocellulose activated by oxidative oxidation with 2,2,6,6-tetramethylpiperidine-1-oxy radicals, and its carboxyl content is [missing information]. The degree of polymerization is 100-500, the average diameter is 5-50 nm, and the length is 100-2000 nm. The activation treatment specifically includes: dispersing microcrystalline cellulose in a buffer solution with a pH of 10.0, adding a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxo radical and... Sodium hypochlorite solution was added dropwise at 0-5℃, and the reaction temperature was controlled to be less than 10℃. The reaction was carried out for 2-4 hours until the carboxyl content reached the target value. Then, the solution was purified by dialysis and freeze-dried for later use.

[0011] The cellulose grafting reaction was carried out under weakly alkaline conditions, with the system pH controlled between 8.0 and 9.5. When the silane coupling agent was terminally amino, the carboxyl groups on the cellulose surface underwent an amidation condensation reaction with the amino group under the synergistic activation of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, forming a stable amide bond. When the silane coupling agent was terminally epoxy, the hydroxyl groups on the cellulose surface underwent nucleophilic ring-opening addition to the epoxy ring under alkaline conditions, forming an ether bond. Both grafting pathways ensured that the cellulose segments were firmly anchored to the kaolin surface, preventing desorption during the subsequent wet end of papermaking.

[0012] In the grafting reaction, the amount of cellulose derivative added is 1.0%-4.0% of the dry weight of kaolin. Within this range, the grafted layer can provide sufficient flexible buffering effect to absorb external stress without shielding the surface charge of kaolin or damaging the original hydration layer structure due to excessive coating, thereby maintaining the high viscosity characteristics of the suspension system.

[0013] In a preferred embodiment of the present invention, the silane coupling agent is The amount added was 0.8% of the dry weight of kaolin; the cellulose derivative was 2,2,6,6-tetramethylpiperidine-1-oxy radical oxidized nanocellulose with a carboxyl content of 1.8 mmol / g, and the amount added was 2.5% of the dry weight of kaolin; the grafting reaction was carried out at pH=8.8 and temperature 80℃ for 120 min.

[0014] In another preferred embodiment of the present invention, the silane coupling agent is 3-glycidoxypropyltrimethoxysilane, and the amount added is 1.2%; the cellulose derivative is 2,2,6,6-tetramethylpiperidine-1-oxy radical oxidized microcrystalline cellulose, the carboxyl content is 1.2 mmol / g, and the amount added is 3.0%; the grafting reaction is carried out at pH=9.2 and temperature 70°C for 150 min.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] 1. The key to the process described in this invention lies in constructing a rigid core-flexible shell composite particle structure. The kaolin matrix serves as the rigid core, maintaining its original high refractive index, high whiteness, and high filling efficiency; the silane coupling agent layer acts as a molecular-level bridge, achieving strong chemical coupling between the inorganic and organic phases; the cellulose grafted layer serves as the flexible shell, its multi-hydroxyl structure not only forming a large network of hydrogen bonds with pulp fibers, enhancing interfacial bonding, but also dissipating energy through conformational rearrangement under stress, effectively inhibiting microcrack propagation. The synergistic effect of these three components overcomes the performance paradox of traditional single-modification techniques where increased flexibility inevitably leads to decreased viscosity.

[0017] 2. The cellulose grafted layer is hydrophilic and biocompatible, and does not produce hydrophobic aggregation in the wet end environment of papermaking. Moreover, its molecular chain segments can deform synchronously with fiber shrinkage during paper drying, avoiding interfacial delamination caused by thermal stress differences. In addition, cellulose itself is a natural polymer, which is structurally stable under alkaline papermaking conditions and drying temperatures, with no risk of degradation, ensuring long-term flexibility.

[0018] 3. The process described in this invention utilizes an aqueous reaction system throughout, eliminating the need for organic solvents and meeting green manufacturing requirements. Each reaction step can be completed in a conventional stirred tank, and process parameters are easily controlled, making it suitable for industrial-scale production. The resulting flexible and reinforced kaolin can directly replace existing washed kaolin in the production of high-grade cultural paper, coated paper base coating, food-grade packaging paper, and other high-end paper products, without requiring adjustments to existing papermaking processes.

[0019] 4. The high-precision, high-viscosity papermaking washed kaolin provided by this invention strengthens the flexibility of the material by sequentially chemically modifying it with silane coupling agent and cellulose to construct a dual-functional interface layer on the surface of kaolin particles that combines chemical anchoring strength and mechanical buffering capacity, thus solving the technical problem of difficulty in balancing flexibility and dispersion stability in the prior art. Detailed Implementation

[0020] A high-precision, high-viscosity papermaking process for enhancing the flexibility of washed kaolin lies in its molecular-level interface engineering. This involves sequentially constructing a silane coupling agent anchoring layer and a flexible cellulose grafting layer on the surface of kaolin particles, forming a rigid core-flexible shell composite particle structure. This structure, while maintaining the original high viscosity, high dispersion stability, and high whiteness—key rheological and optical properties of kaolin—significantly enhances the interfacial bonding strength between kaolin and pulp fibers, and imparts energy dissipation capabilities to the filler particles. This effectively solves the technical challenges of high paper brittleness and severe dusting.

[0021] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0022] Example 1: The silane coupling agent is (Added amount 0.8%); Cellulose derivative is 2,2,6,6-tetramethylpiperidine-1-oxy radical oxidized nanocellulose (added amount 2.5%, carboxyl content 1.8 mmol / g); Reaction temperature 55℃ in the silane stage and 80℃ in the grafting stage; pH 5.0 in the silane stage and 8.8 in the grafting stage;

[0023] Preparation process: Kaolin slurry preparation → pH adjustment → silane coupling agent reaction → cellulose grafting reaction → solid-liquid separation → washing → programmed temperature drying → finished product.

[0024] Example 2: The amount of silane coupling agent added was 0.3%, and the rest of the formulation and process were the same as in Example 1;

[0025] Preparation process: Same as in Example 1.

[0026] Example 3: The amount of silane coupling agent added was 1.5%, and the rest of the formulation and process were the same as in Example 1;

[0027] Preparation process: Same as in Example 1.

[0028] Example 4: The amount of cellulose derivative added was 1.0%, and the rest of the formulation and process were the same as in Example 1;

[0029] Preparation process: Same as in Example 1.

[0030] Example 5: The amount of cellulose derivative added was 4.0%, and the rest of the formulation and process were the same as in Example 1;

[0031] Preparation process: Same as in Example 1.

[0032] Example 6: The silane coupling agent is 3-glycidoxypropyltrimethoxysilane (addition amount 1.2%), and the rest of the formulation and process are the same as in Example 1;

[0033] Preparation process: Same as in Example 1 (the pH of the grafting reaction was adjusted to 9.2).

[0034] Example 7: Grafting reaction temperature 60℃, other formulations and processes are the same as in Example 1;

[0035] Preparation process: Same as in Example 1.

[0036] Example 8: Grafting reaction temperature 90℃, other formulations and processes are the same as in Example 1;

[0037] Preparation process: Same as in Example 1.

[0038] Comparative Example 1: No cellulose derivative grafting, only silane coupling agent (addition amount 0.8%) modification; the rest of the formulation and process are the same as in Example 1;

[0039] Preparation process: Kaolin slurry preparation → pH adjustment → silane coupling agent reaction → solid-liquid separation → washing → drying → finished product.

[0040] Comparative Example 2: Without silane coupling agent, cellulose derivative (2.5%) was directly added for grafting; the rest of the formulation and process were the same as in Example 1;

[0041] Preparation process: Kaolin slurry preparation → pH adjustment → cellulose grafting reaction → solid-liquid separation → washing → drying → finished product.

[0042] Test method:

[0043] Kaolin performance testing: apparent viscosity at 20% solid content was measured using a rotational viscometer; absolute potential was measured using a Zeta potentiometer; dispersion stability index was evaluated after standing for 72 hours.

[0044] Paper performance test: basis weight 80 g / m³ 2 For paper, measure the increase in breaking length; test filler retention rate; evaluate dust loss using standard friction tests.

[0045] Structural testing: Observe the integrity of the rigid core-flexible shell structure; verify the chemical bonding between silane and cellulose.

[0046] The test data comparisons are shown in Table 1 and Table 2.

[0047]

[0048]

[0049] Examples 1-8 showed a ≥12% increase in paper breaking length and a ≥0.95 dispersion stability index, which were far superior to the comparative examples. Comparative example 1, lacking flexible grafting, showed only a slight increase in flexibility, while comparative example 2, lacking silane anchoring, experienced a significant decrease in viscosity and stability. This demonstrates that the synergistic effect of silane anchoring and cellulose grafting is key to achieving a balance between performance and efficiency.

[0050] Increasing the proportion of silane coupling agent (Examples 2→1→3) resulted in simultaneous improvement in retention and flexibility; increasing the amount of cellulose (Examples 4→1→5) significantly optimized the paper's flexibility but slightly reduced its viscosity; and maintaining stable performance at reaction temperatures in the range of 60-90℃.

[0051] All examples maintained high viscosity It is suitable for high-solids-content papermaking processes; it has excellent dispersion stability and no sedimentation when left to stand; it significantly improves the flexibility and retention rate of the finished paper, and reduces dust and filler loss.

[0052] Compared to single silane modification (Comparative Example 1), the paper produced in this example showed a 260% increase in paper flexibility and a 21% increase in retention rate; compared to direct grafting (Comparative Example 2), the viscosity increased by 29% and the dispersion stability increased by 9%, solving the industry problem of traditional modification where viscosity and flexibility cannot be achieved simultaneously.

[0053] The process described in this invention achieves a synergistic effect of high viscosity and high dispersibility of kaolin and high flexibility of paper by modifying the interface at two levels and using different parameter combinations. It is suitable for high-grade cultural paper, high-end packaging paper and other applications.

[0054] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin, characterized in that, Includes the following steps: High-precision, high-viscosity washed kaolin is placed in deionized water to prepare a slurry with a solid content of 15%-25%, and the pH value is adjusted to 4.0-6.

0. A silane coupling agent is added to the slurry, and the mixture is stirred to allow the silane coupling agent to hydrolyze and condense, and then covalently bond to the hydroxyl sites on the surface of the kaolin particles through silicon-oxygen bonds, forming a monolayer anchoring structure. Then, a cellulose derivative activated by oxidative oxidation of 2,2,6,6-tetramethylpiperidine-1-oxygen free radical is introduced into the system, and the reaction continues to cause the cellulose molecular chain to undergo a grafting reaction through the organic functional groups reserved by the silane coupling agent, forming a flexible polymer grafting layer on the outer surface of the kaolin particles. After the reaction is complete, the resulting slurry is subjected to solid-liquid separation and washing until the conductivity is lower than that of the solid-liquid mixture. After drying, a flexible and reinforced kaolin product is obtained.

2. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 1, characterized in that, The particle size of the high-fineness, high-viscosity washed kaolin , Content ≤0.5%, Content ≤0.8%.

3. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 1, characterized in that, The silane coupling agent is selected from... Triethoxysilane, N-( -aminoethyl)- The amount of 3-aminopropyltrimethoxysilane or 3-glycidoxypropyltrimethoxysilane added is 0.3%-1.5% of the dry basis mass of kaolin.

4. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 3, characterized in that, When the terminal of the silane coupling agent is an amino group, the grafting reaction is carried out under the synergistic activation of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, causing the carboxyl and amino groups on the surface of the cellulose derivative to undergo an amidation condensation reaction to form amide bonds.

5. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 3, characterized in that, When the terminal of the silane coupling agent is an epoxy group, the grafting reaction is carried out under weakly alkaline conditions, causing the hydroxyl groups on the surface of the cellulose derivative to undergo nucleophilic ring-opening addition to the epoxy ring, forming an ether bond.

6. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 1, characterized in that, The cellulose derivative is microcrystalline cellulose or nanocellulose activated by 2,2,6,6-tetramethylpiperidine-1-oxy radical oxidation, and its carboxyl content is 0.8-2.5%. The degree of polymerization is 100-500, the average diameter is 5-50nm, and the length is 100-2000nm.

7. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 6, characterized in that, The amount of the cellulose derivative added is 1.0%-4.0% of the dry weight of the kaolin.

8. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin according to claim 1, characterized in that, During the grafting reaction, the pH value of the system is controlled between 8.0 and 9.

5.

9. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin as described in claim 1, characterized in that, The silane coupling agent is premixed by diluting it with five times its mass of anhydrous ethanol and then added dropwise to the kaolin slurry at a rate of 2 mL / min.

10. The process for enhancing the flexibility of high-precision, high-viscosity papermaking washed kaolin according to claim 1, characterized in that, The washing step involves multiple centrifugation-dispersion cycles until the conductivity of the washing liquid is lower than [a certain value]. .

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