Hydrotalcite capable of improving thermal stability and initial whiteness of PVC (polyvinyl chloride) and preparation method of hydrotalcite

Abstract

The invention provides hydrotalcite for improving thermal stability and initial whiteness of PVC (polyvinyl chloride) and a preparation method of the hydrotalcite. The chemical composition of the hydrotalcite comprises M < 2 + > xM < 3 + > y (OH) 2x + 3y-x / 2 (CO3) x / 4. MH2O, wherein M < 2 + > is a divalent metal ion or a mixture of divalent metal ions and is selected from one or two of Mg < 2 + >, Ca < 2 + >, Ni < 2 + >, Zn < 2 + >, Mn < 2 + >, Cu < 2 + >, Co < 2 + >, Pd < 2 + > and Fe < 2 + >; m < 3 + > is a trivalent metal ion and is selected from one of Al < 3 + >, Ga < 3 + > and In < 3 + >; x and y are positive numbers, and the molar ratio of x to y satisfies x / y = 2.0-3.0; m is the content of crystal water, and m is 3.5-4.0. By regulating and controlling the specific surface area to be less than 20 m < 2 > / g, D50 to be less than 0.150 mu m and the grain size to be greater than or equal to 800 and combining a surface modification process, the compatibility and dispersity of the hydrotalcite and PVC are improved. The preparation method comprises the steps of raw material mixing and sanding, hydro-thermal synthesis, solid-liquid separation and washing, surface modification, drying and post-treatment, the hydrotalcite capable of improving PVC thermal stability and initial whiteness is obtained, the hydrotalcite can be uniformly dispersed in a PVC matrix, thermal decomposition is inhibited by absorbing HCl, the static aging time is prolonged, and meanwhile light scattering is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of polyvinyl chloride (PVC) heat stabilizers, and in particular to a hydrotalcite having excellent thermal stability and an effect of improving initial whiteness and a preparation method thereof, which is suitable for the production of PVC pipes, profiles, films and other products. Background Art

[0002] Polyvinyl chloride (PVC), an important general-purpose plastic, is widely used in building materials, wire and cable, packaging materials, and medical devices due to its excellent mechanical properties, chemical resistance, flame retardancy, and low cost. However, the presence of unstable chlorine atoms in the PVC molecular chain makes it susceptible to dehydrochlorination reactions under external conditions such as heat, light, and oxygen, leading to molecular chain breakage or cross-linking, which in turn causes thermal stability issues such as discoloration and decreased mechanical properties. Furthermore, the addition of heat stabilizers or improper control of process conditions during PVC processing often leads to a decrease in initial whiteness, which particularly significantly affects the appearance quality of light-colored or white products.

[0003] While traditional thermal stabilizers such as lead salts, metal soaps, and organotin compounds can alleviate the thermal decomposition of PVC to some extent, they suffer from drawbacks such as high toxicity, severe environmental pollution, and high cost. In recent years, hydrotalcite (LDH), a layered double hydroxide, has become a research hotspot as an alternative to traditional thermal stabilizers due to its unique layered structure, ion exchange properties, and excellent thermal stability. By absorbing the HCl produced by PVC decomposition, LDH effectively inhibits the dehydrochlorination reaction. Its white powder properties also help improve the initial whiteness of PVC. However, existing hydrotalcite technology still faces numerous challenges. The high polarity of LDH leads to poor compatibility with the PVC matrix, making it prone to agglomeration, resulting in uneven dispersion and reduced thermal stabilization efficiency. Traditional preparation methods (such as co-precipitation and hydrothermal methods) are complex, require harsh reaction conditions, and produce products with wide particle size distributions, low purity, and high costs, limiting their industrial application. Furthermore, the thermal stabilization efficiency of LDH alone is insufficient to meet the requirements of high-performance PVC products. While composite modification techniques (such as with rare earth elements or organotin compounds) can improve performance, they further increase costs and environmental risks. Summary of the Invention

[0004] The present invention aims to provide a hydrotalcite that improves the thermal stability and initial whiteness of PVC and a preparation method thereof. By optimizing the chemical composition, particle size distribution, and surface modification process of the hydrotalcite, the compatibility and dispersibility of the hydrotalcite with polyvinyl chloride (PVC) are effectively improved, overcoming the agglomeration problem caused by the polar surface of conventional hydrotalcite. This allows for more efficient suppression of the dehydrochlorination reaction during the thermal decomposition of PVC and prolonged thermal stability of the material.

[0005] The present invention reduces production costs while simplifying the preparation process by regulating the crystal size and specific surface area of hydrotalcite and combining it with a low-cost, environmentally friendly modifier. It avoids the use of toxic heavy metals or complex composite systems, ultimately obtaining a hydrotalcite product that has excellent thermal stability, improves initial whiteness, and is easy to industrialize. This product meets the application needs of high-performance PVC products in the fields of construction, packaging, and wire and cable.

[0006] The present invention provides a hydrotalcite for improving the thermal stability and initial whiteness of PVC, the chemical composition of which comprises: 2+ x M 3+ y (OH) 2x+3y-x / 2 (CO3) x / 4 mH2O; where M 2+ is a divalent metal ion or a mixture of divalent metal ions selected from Mg 2+ , Ca 2+ , Ni 2+ , Zn 2+ , Mn 2+ , Cu 2+ ,Co 2+ , Pd 2+ and Fe 2+ One or two of M 3+ is a trivalent metal ion selected from Al 3+ , Ga 3+ ,In 3+ x and y are positive numbers, and the molar ratio of x to y satisfies x / y=2.0-3.0; m is the crystal water content, m=3.5-4.0.

[0007] By precisely controlling the types of metal ions, interlayer anions, and crystalline water content, the hydrotalcite achieves efficient inhibition of PVC thermal decomposition and whiteness maintenance at the molecular level, while taking into account the feasibility and environmental friendliness of industrial production.

[0008] Among them, Mg 2+ , Ca 2+ 、Zn 2+ Divalent metal ions and Al 3+ Layered double hydroxides (LDHs) formed by trivalent metal ions such as CO3 2- ) Adsorb Cl generated by PVC decomposition through ion exchange - , reducing the HCl concentration in the system, thereby inhibiting the chain process of dehydrochlorination reaction. In addition, the layered structure of LDH can provide a physical barrier, reduce heat conduction, and further delay the decomposition of PVC. Zn 2+ The introduction of can further improve the initial whiteness and reduce yellowing during PVC processing.

[0009] By M 2+ / M 3+ The charge difference between the two layers forms a positive charge, which attracts anions between the layers and achieves charge balance. 3+ , Ga 3+ ,In 3+ The high charge density of the layers determines the positive charge of the layers, affecting the adsorption capacity of interlayer anions; the strong coordination ability forms a stable octahedral structure, inhibiting the peeling of the layers; and the synergistic optimization of the interlayer ion exchange capacity with divalent metal ions. The combination of the two can improve the HCl absorption rate.

[0010] Hydroxyl (OH - ) and carbonate (CO3 2- ) is located between the hydrotalcite layers, stabilizing the structure through electrostatic interaction and enhancing the HCl adsorption capacity; CO3 2- The intercalation can regulate the interlayer spacing and optimize the thermal stability. The hydroxyl group is located at the center of the octahedron of the layer, and 2+ / M 3+ The rigidity of the laminate is maintained by hydrogen bonding, and the surface hydroxyl groups provide HCl adsorption sites through the reaction OH−+HCl→H2O+Cl−OH - +HCl→H2O+Cl - Inhibit PVC decomposition. Carbonate is embedded in the interlayer to balance the positive charge. Its size and charge density affect the interlayer spacing. It reacts with the Cl generated by PVC decomposition. - An exchange reaction occurs, which slows down the HCl removal process.

[0011] Controlling the content of crystal water (mH2O) influences the thermal stability and dispersibility of hydrotalcite. Excessive or insufficient amounts can lead to performance degradation. Crystal water connects the interlaminar anions through hydrogen bonds, preventing interlaminar collapse. It is gradually removed at PVC processing temperatures (160-200°C), absorbing some heat and reducing the risk of PVC thermal decomposition. When m=3.5-4.0, the water molecules are evenly distributed. Excessive amounts (m>4.0) can cause interlaminar expansion, while insufficient amounts (m<3.5) can reduce structural stability.

[0012] The present invention uses this chemical composition in combination with other necessary physical properties. The specific surface area of the hydrotalcite of the present invention is less than 20m 2 / g, reducing surface active sites and inhibiting agglomeration; the particle size D50 is less than 0.150 μm, improving the dispersion uniformity in the PVC matrix; the hydrotalcite of the present invention has a <003> The crystallite size in the direction of the hydrotalcite is at least 800Å, enhancing the stability of the crystal structure and prolonging the thermal stability period. By precisely controlling the chemical composition and structural parameters, the hydrotalcite of the present invention combines high compatibility, efficient HCl adsorption capacity, and low cost advantages, making it suitable for PVC products with stringent requirements on thermal stability and appearance.

[0013] Specific surface area is the total surface area per unit mass of the material. A smaller specific surface area indicates a denser surface of hydrotalcite particles with fewer active sites, effectively reducing interparticle agglomeration and thus achieving more uniform dispersion within the PVC matrix. A low specific surface area reduces particle surface energy, weakens van der Waals forces, inhibits particle agglomeration, reduces agglomeration, and improves dispersibility. The dense surface reduces physical friction with the PVC molecular chains, avoiding processing defects caused by localized stress concentration and stabilizing the interface. While ensuring sufficient HCl adsorption sites, the adsorption capacity is balanced to avoid side reactions caused by excessive surface defects. Evenly dispersed hydrotalcite can more efficiently absorb HCl produced by PVC decomposition, prolonging static aging time while reducing localized discoloration of the material caused by agglomeration.

[0014] D50 indicates the size below which 50% of the particles are smaller. Controlling D50 to less than 0.150 μm means that the hydrotalcite particles reach submicron size, approaching the nanometer scale. Ultrafine particles are more evenly distributed in the PVC melt, reducing island structures, avoiding stress concentration, and enhancing dispersibility. Small particle size increases specific surface area, improving interfacial contact and allowing for more complete contact with PVC molecular chains, thus inhibiting HCl removal reactions. Submicron particles reduce light scattering, improving optical properties, and enhancing the initial whiteness of PVC products. Experimental results show that hydrotalcite with a D50 of less than 0.150 μm can achieve a PVC film transmittance of greater than 88%, a yellowing index of less than 2.0, and a 40%-60% increase in thermal stability.

[0015] <003> The crystal face is the characteristic diffraction peak of the hydrotalcite layered structure, and its grain size reflects the order of the layers and the stability of the interlayer structure. Large grain size (≥800Å) indicates that the layers are tightly arranged and the carbonate groups are evenly distributed between the layers, which enhances the interlayer stability and can adsorb HCl stably for a long time. The complete layered structure reduces the fracture of the layers caused by shear force during processing, inhibits the peeling of the layers, and maintains long-term thermal stability. The ordered structure provides more effective ion channels, optimizes the ion exchange capacity, and accelerates the adsorption of Cl. - With CO3 2- The hydrotalcite with a grain size of ≥800Å has a higher thermal stability efficiency and a higher whiteness retention rate than traditional hydrotalcite.

[0016] The low specific surface area inhibits agglomeration, and the small particle size improves dispersibility, jointly ensuring the uniform distribution of hydrotalcite in PVC. The large grain size and stable interlayer structure synergistically enhance the HCl adsorption capacity and extend the thermal stability time. By optimizing the surface properties, particle size and crystal structure, the comprehensive performance of PVC products with high thermal stability, high whiteness and low yellowing is improved.

[0017] The present invention prepares a high-performance hydrotalcite through four core steps: raw material optimization, structural regulation, impurity removal, and functional modification, effectively improving the thermal stability and initial whiteness of PVC. Through raw material refinement, directional reaction regulation, deep impurity removal, and surface functionalization design, the hydrotalcite of the present invention improves the thermal stability and initial whiteness of PVC and its preparation method. Through a full-chain innovation chain of molecular structure design, process optimization, and performance verification, it addresses the PVC industry's urgent need for high-efficiency, environmentally friendly heat stabilizers.

[0018] The present invention provides a method for preparing hydrotalcite for improving the thermal stability and initial whiteness of PVC, which specifically comprises the following steps: S1 Raw material mixing and sand milling: Weigh divalent metal salts or hydroxides, trivalent metal hydroxides and sodium salts (such as carbonates, bicarbonates) in proportion and add them to deionized water. Grind them thoroughly with a sand mill until D50 is less than 0.6µm. Mechanical sand milling breaks up the agglomerates of metal hydroxides to ensure uniform particle size and achieve raw material homogenization, providing highly active raw materials for subsequent hydrothermal reactions. During the sand milling process, shear force and collision effects refine the particles, increase the specific surface area, and promote sufficient contact between sodium salts and metal ions. The local high temperature generated by grinding can trigger a preliminary hydrolysis reaction and accelerate the formation of a layered structure.

[0019] S2 Hydrothermal Synthesis: The ground slurry is transferred to a reactor, heated to 75-100°C, and stirred at 60-120 rpm for 3-8 hours to obtain a reaction mother liquor; at 75-100°C, the hydrothermal reaction promotes the directional growth of the hydrotalcite layered structure through the Ostwald ripening mechanism, ensuring <003> Crystal plane size ≥800Å; CO3 2- In alkaline environment, it is embedded in the interlayer to form a stable Mg6Al2(OH) 16 CO3·4H2O structure; the reaction time is 3-8 hours, which can adjust the grain size and crystallinity. Small grains are generated in a short time, while the grain size increases in a long time.

[0020] S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered or centrifuged to obtain a filter cake. A multi-stage countercurrent washing system is used to repeatedly wash the filter cake with deionized water 3-5 times the weight of the filter cake until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salts and by-products. Multi-stage countercurrent washing can efficiently remove residual sodium salts and by-products to avoid impurities causing PVC discoloration. The filtrate is washed until it is neutral to prevent alkaline residues from affecting the processing stability of PVC. The final stage wastewater is reused for the previous stage rough washing, reducing fresh water consumption by more than 70%.

[0021] Filtrate refers to the liquid produced after the deionized water used to wash the filter cake contacts the filter cake during the multi-stage countercurrent washing process. It comes from the countercurrent contact of the washing water (deionized water) with the filter cake (hydrotalcite crude product) in the washing tank, dissolving or flushing out the impurities remaining in the filter cake; these impurities include unreacted sodium salts (such as NaOH, Na2CO3), by-products (such as Na2SO4, NaCl) and a small amount of free metal ions (such as Al 3+ Mg 2+ The filtrate acts as an impurity carrier, removing residues from the filter cake through step-by-step washing. The pH and conductivity of the filtrate are monitored to determine the washing endpoint and ensure that the hydrotalcite purity meets the required standards. A neutral filtrate pH indicates that alkaline or acidic substances such as sodium salts have been effectively removed, while a decrease in conductivity indicates a reduction in residual ions, thereby ensuring the purity of the final hydrotalcite product.

[0022] S4 surface modification: The washed hydrotalcite solid is re-slurried, heated to 75-85°C, and an anionic modifier is added at 1-5% by weight of the hydrotalcite, and stirred for 30-60 minutes. The anionic modifier covers the surface of the hydrotalcite through physical adsorption or chemical bonding, reducing its polarity, improving its compatibility with the PVC matrix, and lowering the dispersion index. The hydrophobic groups of the modifier (such as alkyl chains) reduce the van der Waals forces between particles and inhibit secondary agglomeration during drying and storage. Some modifiers can synergistically inhibit PVC oxidation, further improving whiteness.

[0023] S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 50-150°C to constant weight. The powder after crushing has a loose density of 0.4-0.6 g / cm 3 , verifying the final hydrotalcite product; gradient drying at 50-150℃ to a moisture content of <1% to prevent residual moisture from triggering hydrolysis side reactions during PVC processing; air flow milling (pressure 0.6-0.8MPa) to refine the particles to D50 < 0.150μm and a bulk density of 0.4-0.6g / cm³ to ensure powder fluidity and processing dispersibility; and by testing specific surface area, grain size and thermal stability time, ensure that the product meets PVC application standards.

[0024] Raw material mixing and sand milling are key pretreatment steps in the preparation of high-performance hydrotalcite. Through precise proportioning and physical refinement, they provide a uniform, highly active precursor for subsequent hydrothermal synthesis. These two steps directly determine the chemical composition, crystal structure, and particle morphology of the hydrotalcite, thus influencing its thermal stability and dispersibility in PVC.

[0025] In the preparation method of the hydrotalcite of the present invention, the raw material mixing and sand grinding steps specifically include the following operations: S1.1 Raw material mixing: according to chemical formula M 2+ x M3+ y (OH) 2x+3y-x / 2 (CO3) x / 4 ·mH2O molar ratio, weigh divalent metal salts or hydroxides, trivalent metal hydroxides and sodium salts, ensuring divalent (such as Mg 2+ 、Zn 2+ ) and trivalent metal ions (such as Al 3 + ) in accordance with the requirements of the layered structure, maintaining the charge balance of the layer (such as Mg6Al2 structure), avoiding the collapse of the layer or the decrease of ion exchange capacity due to the imbalance of the ratio; mixed with deionized water at a liquid-solid ratio of 3:1-5:1, controlling the ratio of deionized water to solid raw materials, balancing the fluidity and reaction activity of the slurry, facilitating mixing and subsequent sand milling operations, and appropriate moisture promotes the dissolution of metal hydroxides and ion migration; the introduction of sodium salts (such as NaHCO3) provides interlayer anions (CO3 2- ) source, adjust the interlayer distance, CO3 2- Embedded between layers, it stabilizes the structure and enhances HCl adsorption capacity.

[0026] S1.2 Sand milling dispersion: Grind the mixed slurry in a sand mill at 20-40℃ for 2-4 hours, and cyclically grind until the particle size D50 is less than 0.6μm. Through mechanical shear force and collision, the raw material particles are broken into submicron level, increasing the specific surface area, improving the reaction activity, and accelerating the crystal growth in hydrothermal synthesis; homogenize the slurry to avoid local uneven reaction caused by large particles (such as unreacted Al(OH)3 residue); temperature control prevents side reactions caused by frictional heat during sand milling (such as decomposition of metal hydroxides), maintains the chemical stability of the raw materials, and ensures the purity of subsequent reactions; cyclic grinding, through multiple grinding cycles, ensures that all particles reach the target particle size, reduces the proportion of coarse particles, avoids energy waste caused by excessive grinding, and ensures that the raw materials are fully dispersed and refined.

[0027] The raw material mixing and sand grinding steps lay the foundation for the efficient synthesis of hydrotalcite through precise chemical proportioning and physical refinement. The precise proportioning and fine particle size make the layer structure complete and improve the HCl absorption efficiency. The homogeneous slurry ensures uniform grain growth during the hydrothermal reaction and the specific surface area is stable at <20m 2 / g, reducing agglomeration during PVC processing.

[0028] Solid-liquid separation and washing in this invention are key steps in the hydrotalcite preparation process to ensure product purity, remove impurities, and optimize subsequent process performance. Multi-stage separation and countercurrent washing efficiently remove residual sodium salts, byproducts, and unreacted materials, minimizing the negative impact of impurities on the thermal stability and whiteness of PVC. Controlling the filter cake moisture content reduces drying energy consumption and improves product quality.

[0029] In the preparation method of the hydrotalcite of the present invention, the solid-liquid separation and washing steps specifically include the following operations: S3.1 Preliminary solid-liquid separation: Pump the slurry after the hydrothermal reaction into the separation equipment, filter at 0.4-0.6MPa or centrifuge at 3000-5000rpm to initially remove most of the liquid mother liquor (containing unreacted sodium salts and by-products such as Na2SO4) to reduce the burden of subsequent washing; collect filter cakes with a thickness of 3-5cm to ensure the uniformity of the filter cake to avoid excessive thickness resulting in reduced washing efficiency or excessive thinness increasing the equipment load; 0.4-0.6MPa filtration pressure can efficiently process high-solid content slurries, while 3000-5000rpm centrifugal force is suitable for the rapid sedimentation of submicron particles (Stokes' law).

[0030] S3.2 Multi-stage countercurrent washing: Use 3-4 stages of washing tanks in series, and gradually reduce the residual sodium salt and by-products (such as Cl - 、SO4 2- ), improving the purity of hydrotalcite and efficiently removing impurities. Fresh deionized water is injected from the final stage and flows countercurrently to the preceding stage, where it comes into countercurrent contact with the filter cake. The total amount of wash water is 3-5 times the weight of the filter cake. The final stage wastewater is reused for the preceding stage rough wash, resulting in a total water consumption of only 3-5 times the weight of the filter cake (compared to 8-10 times required for traditional single-stage washing), with a water saving rate of over 70%. The filter cake is dehydrated by high-speed centrifugation to a moisture content of 10-30%, balancing washing efficiency and energy consumption. Countercurrent contact (fresh water is added from the final stage) maximizes the concentration gradient and improves mass transfer efficiency (according to Fick's diffusion law). The temperature is controlled at 40-60°C to increase the solubility of sodium salts and accelerate impurity elution. After three stages of countercurrent washing, the conductivity drops from an initial 2000μS / cm to <100μS / cm, and the pH stabilizes at 6.5-7.5.

[0031] S3.3 Final solid-liquid separation: Use a high-speed centrifuge to perform secondary dehydration on the washed product to obtain a hydrotalcite solid with a moisture content of 5-15%; use high-speed centrifugal secondary dehydration to further reduce the moisture content of the filter cake to 5-15%, reducing subsequent drying energy consumption; moderate dehydration avoids excessive compression of the filter cake that may cause particle agglomeration or damage to the layer structure; according to thermodynamic calculations, for every 10% reduction in moisture content, the drying energy consumption is reduced by approximately 15%.

[0032] The solid-liquid separation and washing steps ensure the high purity and structural stability of hydrotalcite through efficient impurity removal, water conservation and energy reduction, and precise control of moisture content. They are key process steps for optimizing the performance of PVC heat stabilizers.

[0033] The multi-stage countercurrent washing method of this invention effectively improves the quality and process economics of hydrotalcite by removing impurities in stages, optimizing resource utilization, and precisely controlling purity, achieving efficient impurity removal, water conservation, and environmental protection. This operation gradually removes sodium salts, byproducts, and free ions, preventing residual impurities from affecting the thermal stability and appearance of PVC. The reuse of final-stage wastewater reduces fresh water consumption by over 70%, meeting green production requirements. The staged washing process reduces energy consumption and time costs, making it suitable for industrial continuous production.

[0034] In the preparation method of the hydrotalcite of the present invention, the multi-stage countercurrent washing operation specifically includes: using 3-4 washing tanks in series, fresh deionized water is injected from the last stage and flows countercurrently to the previous stage step by step, and contacts the filter cake in countercurrent; the first stage rough washing uses 50% of the water to remove most of the sodium salt and by-products, the secondary fine washing uses 30% of the water to reduce the conductivity to below 500 μS / cm, and the final washing uses 20% of the water to adjust the filtrate pH to a neutral 6.5-7.5. The total amount of washing water is 3-5 times the weight of the filter cake, and the water temperature is controlled at 40-60°C to improve dissolution efficiency. At the same time, the wastewater from the last stage is reused for the previous rough washing, and the filter cake with a moisture content of 10-30% is obtained by high-speed centrifugal dehydration.

[0035] Among them, the first stage rough washing quickly removes the main impurities, uses high water volume (1.5-2.5 times the weight of the filter cake) to flush the filter cake, and removes more than 80% of the sodium salts (such as Na + 、CO3 2- ) and by-products (such as Na2SO4); initially reduce the conductivity, reducing the filtrate conductivity from the initial 2000-3000μS / cm to 800-1000μS / cm; high water volume provides sufficient mass transfer driving force, in accordance with Fick's diffusion law, accelerating the elution of impurities. Secondary fine washing deep purification further reduces the conductivity to below 500μS / cm, removes residual ions (such as Cl - 、Al 3+); Protect the layered structure, gentle washing to prevent high-speed water flow from damaging the hydrotalcite layers; countercurrent concentration gradient enhances elution efficiency (based on countercurrent mass transfer theory). The final wash precisely controls the pH, adjusting the filtrate to neutral (pH 6.5-7.5) to eliminate the negative impact of alkaline or acidic residues on PVC processing; final purity is guaranteed, ensuring that the final hydrotalcite sodium residue is less than 50ppm and the conductivity is less than 100μS / cm; neutral pH avoids protonation or deprotonation of hydroxyl groups on the hydrotalcite surface, maintaining HCl adsorption activity. Controlling the water temperature between 40-60°C improves dissolution efficiency, while increasing the temperature increases the solubility of sodium salts (for example, the solubility of Na2CO3 increases with temperature), accelerating impurity elution; energy-saving balance, below 80°C to avoid heat waste and prevent the removal of water between the hydrotalcite layers. Final-stage wastewater reuse saves water and reduces energy consumption. Cleaner final-stage wastewater is reused for the first-stage rough wash, resulting in a total water consumption of only 3-5 times the filter cake weight (compared to 8-10 times for traditional processes). This reduces wastewater discharge by 70%, complying with environmental management system requirements. High-speed centrifugal dehydration reduces drying load, controls filter cake moisture content, and reduces subsequent drying energy consumption. Moderate dehydration prevents particle agglomeration and plate compression deformation. A centrifugal force of 5000-8000 rpm effectively removes surface bound water while retaining crystallized water (m=3.5-4.0).

[0036] Multi-stage countercurrent washing achieves the production goals of high-purity and low-energy hydrotalcite through staged impurity removal, countercurrent efficiency enhancement, and resource recycling, providing the PVC industry with a solution that combines performance and sustainability.

[0037] The modifier of this invention is compatible with the hydrotalcite surface and the PVC matrix, enhancing dispersibility through physical adsorption or chemical bonding. Hydrotalcite has a high surface polarity, while PVC is a non-polar or low-polar material. The modifier of this invention possesses appropriate polarity to reduce the surface polarity of the hydrotalcite and improve compatibility with the non-polar PVC matrix. Furthermore, the modifier is stable at PVC processing temperatures, tolerating them (160-200°C), preventing the decomposition or volatilization of certain modifiers at high temperatures, which could affect the thermal stability of the hydrotalcite. Furthermore, the color of the modifier itself does not affect the whiteness, fluidity, or processing temperature of the PVC, avoiding the misconception that all surfactants are suitable.

[0038] In the preparation method of the hydrotalcite of the present invention, the anionic surfactant in the surface modification step is selected from one or two of alkali metal salts of higher fatty acids, alkyl sulfates, alkyl sulfonates, alkyl aryl sulfonates and sulfosuccinates.

[0039] Among them, the alkali metal salts of higher fatty acids (such as sodium stearate, sodium laurate), long-chain alkyl (such as C 17 H 35 -) covers the surface of hydrotalcite by physical adsorption, reduces polarity, reduces the van der Waals force between particles, inhibits agglomeration and performs hydrophobic modification; alkali metal ions (such as Na + , K + ) and the surface hydroxyl groups (OH - ) electrostatic bonding enhances adsorption stability and anchors metal ions; in PVC processing, it reduces melt friction, improves fluidity and processing efficiency, and acts as a lubricant. Calcium stearate, as a metal soap, offers excellent thermal stability and lubricity at a reasonable price, making it widely used in PVC processing. Its long-chain alkyl group helps reduce the surface polarity of hydrotalcite. Lauric acid, a medium-chain fatty acid, is similar to calcium stearate, but with a shorter chain length, differing in dispersibility and compatibility.

[0040] Surfactants such as alkyl sulfates / sulfonates have strong hydrophilicity and form a more stable dispersion layer on the surface of hydrotalcite. Alkyl sulfates (such as sodium lauryl sulfate), sulfate groups (-SO4 - ) forms hydrogen bonds or electrostatic interactions with the hydroxyl groups on the surface of hydrotalcite, tightly adsorbing to achieve strong polar adsorption; it forms micelles in the solution, wraps the particles to assist dispersion, and improves dispersion uniformity; it has a high decomposition temperature (>200°C), making it suitable for high-temperature extrusion processes.

[0041] Alkyl sulfonates (such as sodium dodecylbenzenesulfonate), sulfonic acid groups (-SO3 - ) is more stable than sulfate groups, maintains adsorption capacity in a wide pH range, and stabilizes adsorption; the benzene ring structure provides a physical barrier, and steric hindrance reduces particle collision and agglomeration; it has good compatibility with PVC plasticizers, synergistically increases capacity, and reduces interface defects.

[0042] Alkyl aryl sulfonates (such as sodium xylene sulfonate) have aromatic ring structures that enhance the rigidity of surfactant molecules, forming a dense coating layer and achieving rigid support for the aromatic ring. They are more stable than linear alkyl sulfonates in acidic or oxidative environments and are resistant to chemical corrosion. They are also resistant to high temperatures and are suitable for PVC products that require multiple processing or high-temperature post-treatment.

[0043] Sulfosuccinate salt modifiers are more environmentally friendly, but the cost is higher. Sulfosuccinate salts (such as sodium dioctyl sulfosuccinate) have a synergistic effect of the sulfonic acid group (strong polarity) and the long alkyl chain (hydrophobicity) amphiphilic structure to achieve efficient coating; reduce interfacial tension, promote the uniform dispersion of hydrotalcite in the PVC melt, form a microemulsion; promote interlayer CO3 2- With Cl - The exchange of ions is enhanced, and the HCl absorption efficiency is improved.

[0044] During the preparation of the hydrotalcite of the present invention, surfactants are selected singly or compounded according to the performance requirements of the PVC product, such as whiteness, heat resistance, and cost, to achieve the best modification effect.

[0045] The surface modification step of this invention effectively improves the compatibility and dispersibility of hydrotalcite with the PVC matrix through particle redispersion, modifier coating, and post-processing optimization. This solves the agglomeration problem of traditional hydrotalcite caused by surface polarity, ultimately improving the thermal stability and initial whiteness of PVC products. This step breaks up filter cake agglomerations through repulping, creating a uniformly dispersed particle surface. Modifier addition reduces surface polarity through physical adsorption or chemical bonding, inhibiting secondary agglomeration. Post-processing terminates the reaction and removes residual modifier to ensure product stability.

[0046] In the preparation method of the hydrotalcite of the present invention, the surface modification step specifically includes the following operations: S4.1 Filter Cake Reslurry: Mix the washed filter cake with deionized water at a solid-to-liquid ratio of 1:2-1:4 (w / w). Stir at 300-500 rpm for 10-15 minutes to form a uniform suspension with a solid content of 20-30%. Use shear force (stirring at 300-500 rpm) to disperse particle aggregates in the filter cake, breaking up agglomerations and forming a uniform suspension. Balance slurry fluidity (for ease of subsequent operations) with reaction efficiency (sufficient particle concentration) to achieve solid content control. Surface activation: Deionized water wets the hydroxyl groups on the particle surface, providing active sites for modifier adsorption. The solid-to-liquid ratio should be 1:2-1:4. Too dilute (>1:4) reduces reaction efficiency, while too concentrated (<1:2) results in uneven dispersion.

[0047] S4.2 Modifier Addition and Dispersion: Pre-dissolve the modifier in a small amount of ethanol or hot water and slowly add it dropwise to the slurry. Simultaneously, raise the temperature to 75-85°C and increase the stirring speed to 1500-2000 rpm for 30-60 minutes to ensure that the modifier evenly coats the particle surface. Dissolve and disperse the modifier (such as calcium stearate) in ethanol or hot water (40-60°C) to promote its uniform diffusion to the particle surface. Raise the temperature to enhance the thermal motion of the modifier molecules and accelerate adsorption kinetics. High-speed shearing achieves uniform coating of the modifier through turbulent flow, avoiding excessive local concentrations. The adsorption mechanism is that the hydrophobic alkyl chains of calcium stearate are adsorbed via van der Waals forces. The coverage of some modifiers, such as calcium stearate, varies with the duration of exposure.

[0048] S4.3 Post-modification treatment: Add a small amount of deionized water to quench or adjust the pH to neutral to stop the modification reaction, centrifuge or filter to separate the slurry, and collect the modified slurry; quench (rapid cooling) to terminate the adsorption process and prevent over-reaction; adjust the pH to neutral (6.5-7.5) to avoid residual acid and alkali corrosion of the hydrotalcite layer; solid-liquid separation by centrifugation or filtration to remove free modifiers and solvents (such as ethanol) to ensure product purity (modifier residue <0.5%); maintain the interlayer structure of the hydrotalcite through post-treatment (XRD <003> Crystal plane size ≥800Å) to avoid agglomeration during drying and ensure stability. The quenching effect is a rapid cooling to lock in the modifier distribution state and prevent desorption at high temperatures.

[0049] The surface modification step gives hydrotalcite excellent interfacial properties by precisely controlling the dispersion, adsorption and post-treatment conditions. It is the core process link for PVC products to achieve high thermal stability, high whiteness and environmental protection.

[0050] Drying and post-processing are the final, critical steps in the preparation of hydrotalcite. Efficient dehydration, particle optimization, and rigorous quality control ensure the product possesses low moisture content, high dispersibility, and stable physical and chemical properties, thus meeting the stringent requirements for thermal stability and initial whiteness in PVC processing. This method removes residual liquid through solid-liquid separation, reducing drying energy consumption. Drying and pulverization control moisture content and particle size, optimizing processing fluidity. Quality testing verifies product parameters to ensure performance meets standards.

[0051] In the preparation method of the hydrotalcite of the present invention, the drying and post-treatment steps specifically include the following operations: S5.1 Solid-Liquid Separation: The modified slurry is subjected to solid-liquid separation by centrifugation at 3000-5000 rpm or filter pressure at 0.4-0.6 MPa. This separates residual liquid (such as unabsorbed modifier, solvent, or water) from the modified slurry and removes free liquid, reducing the subsequent drying load. The resulting solid filter cake is collected and the moisture content is controlled to 15-25% to avoid over-drying that may cause particle sintering or structural damage. The centrifugal force and pressure are selected based on Stokes' sedimentation law to ensure efficient separation of submicron particles.

[0052] S5.2 Drying: Use spray drying or vacuum drying at 50-150°C to constant weight. Dehydration is achieved by spray drying (180-200°C at the inlet) or vacuum drying (50-150°C) to remove bound and free water, keeping the final moisture content below 1% to prevent hydrolysis during PVC processing. Structural protection is achieved by increasing the temperature gradually to avoid high temperatures (>150°C) that can lead to the removal of crystal water (m=3.5-4.0) or laminate collapse. Rapid evaporation during spray drying reduces particle agglomeration, while vacuum drying is suitable for heat-sensitive materials.

[0053] S5.3 Crushing: Use 0.6-0.8MPa air flow or ball mill for 30-60 minutes to crush to D50 < 0.150μm, and then control the bulk density to 0.4-0.6g / cm after sieving. 3 , a hydrotalcite product with improved thermal stability and initial whiteness of PVC is obtained; particle size control is achieved by air flow crushing or ball milling to refine the particles to D50 < 0.150 μm, reducing light scattering and improving PVC whiteness; bulk density is optimized by controlling the density to 0.4-0.6 g / cm 3 , improve powder fluidity and facilitate uniform dispersion of PVC ingredients. Air flow milling achieves ultra-fine grinding through high-speed collision, and ball milling refines particles through shear force and grinding.

[0054] S5.4 Quality Inspection: Through the inspection data, we ensure that each batch of products meets the technical indicators and is suitable for industrial production. The specific surface area of the hydrotalcite product obtained is less than 20m 2 / g, inhibiting particle agglomeration and ensuring uniform dispersion in PVC; <003> The crystallite size is at least 800Å, maintaining the stability of the layered structure and improving HCl absorption efficiency. Its D50 is less than 0.150μm, reducing light scattering and ensuring initial whiteness. BET surface area testing verifies the distribution of surface active sites, XRD analysis ensures the integrity of the crystal structure, and a laser particle size analyzer monitors the uniformity of the particle size distribution.

[0055] Precise process control and rigorous quality inspection during the drying and post-processing steps ensure that the hydrotalcite product possesses high purity, excellent properties, and industrial stability, providing a high-performance, low-cost, green heat stabilizer solution for the PVC industry. Low moisture content prevents bubble formation during processing, improving the surface finish of PVC products. Ultrafine particle size and low specific surface area synergistically inhibit agglomeration, extending static aging time. HCl absorption efficiency is improved, reducing scrap rates. Initial whiteness and yellowing index are optimized, making it suitable for high-end transparent products. Furthermore, drying energy consumption is reduced, pulverization efficiency is improved, and overall costs are lowered. Quality inspection reduces defective product rates, ensuring compliance with green manufacturing standards.

[0056] In summary, the present invention has the following beneficial effects: 1. Different from traditional hydrotalcite, the hydrotalcite of the present invention optimizes the chemical composition. By accurately selecting the combination of divalent metal ions and trivalent metal ions, combined with the molar ratio of x / y=2.0-3.0 and the crystal water content, a stable layered double hydroxide structure is formed, which not only significantly enhances the absorption capacity of HCl, but also utilizes Zn 2+ The synergistic effect of the two agents reduces the yellowing phenomenon during PVC processing. At the same time, the hydrophobic modification of the anionic modifier (such as alkyl sulfonate) greatly reduces the surface polarity of the hydrotalcite, solves the agglomeration problem of traditional hydrotalcite caused by polarity, and improves the compatibility with the PVC matrix. 2. The preparation method of the hydrotalcite of the present invention includes the steps of raw material mixing and sand milling, hydrothermal synthesis, solid-liquid separation and washing, surface modification, drying, and post-treatment. The sand milling technology refines the raw materials to the submicron level (D50 < 0.6 μm), laying the foundation for the directional grain growth of the hydrothermal reaction. The multi-stage countercurrent washing system is combined with efficient removal of sodium salts and by-products, saving more than 70% of water. Surface modification processes such as gradient heating and high-speed shear dispersion achieve uniform coating of the modifier, reducing the dispersion index of the hydrotalcite and significantly improving its dispersion uniformity in PVC. 3. Different from the conventional production process of hydrotalcite, the present invention adopts green production design. The multi-stage countercurrent washing system uses the final stage wastewater to reuse the previous stage rough washing, which controls the total water consumption to 3-5 times the weight of the filter cake. At the same time, the low-temperature hydrothermal reaction and gradient drying reduce energy consumption. The air flow milling technology is combined to accurately control the final product particle size (D50 < 0.150μm) and bulk density (0.4-0.6g / cm 3 ), taking into account both powder fluidity and processing performance, and adapting to the needs of industrial continuous production; 4. In the preparation process of hydrotalcite of the present invention, the present invention adjusts the specific surface area to less than 20m 2 / g inhibits particle agglomeration, and combined with a large grain size ≥800Å, it enhances the stability of the laminate and improves the HCl adsorption efficiency. At the same time, it utilizes anion modification technology to form a dense hydrophobic layer, reducing secondary agglomeration caused by van der Waals forces, and synergistically inhibits PVC oxidation and yellowing, achieving a double breakthrough in technical parameters and functional properties. At the same time, it extends the static thermal stability time of PVC, increases the film transmittance, and reduces the yellowing index, meeting the appearance requirements of high-end light-colored products. In addition, the process avoids toxic additives such as lead salts and organic tin throughout, reducing the overall cost and achieving both environmental and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a schematic flow chart of a method for preparing hydrotalcite for improving the thermal stability and initial whiteness of PVC; Figure 2 Schematic diagram of X-ray diffraction characterization of hydrotalcite samples of Example and Comparative Example; Figure 3 1 is a schematic diagram summarizing the test results of the initial whiteness of PVC of hydrotalcite samples 1 to 7 in the examples; Figure 4 1 is a schematic diagram summarizing the PVC static aging test results of hydrotalcite samples 1 to 7 in the examples. DETAILED DESCRIPTION

[0058] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art may make non-creative modifications to this embodiment as needed. However, as long as such modifications are within the scope of the claims of the present invention, they are protected by patent law.

[0059] Example 1

[0060] S1 Raw material mixing and sand milling: Weigh 1.2 mol Mg(OH)2, 0.4 mol Al(OH)3 and 0.2 mol NaHCO3, add them into 1 L deionized water, and grind them thoroughly with a sand mill until D50 < 0.6 µm; S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 95°C, and stirred at 100 rpm for 3 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered and separated to obtain a filter cake, which is then washed repeatedly with deionized water 4 times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and modified by adding 3% calcium stearate by weight of the hydrotalcite, and stirred for 40 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 70°C to constant weight. The powder after crushing has a loose density of 0.5 g / cm 3 , and the final hydrotalcite sample 1 was verified to have a specific surface area of <20m 2 / g, D50<0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 6Mg(OH)2+2Al(OH)3+NaHCO3+3H2O→Mg6Al2(OH) 16 CO3·4H2O+NaOH.

[0061] Example 2

[0062] S1 Raw material mixing and sand grinding: Weigh 1.2 mol Mg(OH)2, 0.4 mol Al(OH)3 and 0.24 mol Na2CO3, add them into 1 L deionized water, and grind them thoroughly with a sand mill until D50 < 0.6 µm; S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 95°C, and stirred at 60 rpm for 3 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered and separated to obtain a filter cake, which is then washed repeatedly with deionized water three times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and modified by adding 3% calcium stearate by weight of the hydrotalcite, and stirred for 30 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 70°C to constant weight. The powder after crushing has a loose density of 0.45 g / cm 3 , and the final hydrotalcite sample 2 was verified to have a specific surface area of <20m 2 / g, D50<0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 6Mg(OH)2+2Al(OH)3+1.2Na2CO3+3H2O→Mg6Al2(OH) 16 CO3·4H2O+2Na2CO3+NaOH.

[0063] Example 3

[0064] S1 Raw material mixing and sand milling: Weigh 1.8 mol Mg(OH)2, 0.4 mol Al(OH)3 and 0.2 mol NaHCO3, add them into 1 L deionized water, and grind them thoroughly with a sand mill until D50 < 0.6 µm; S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 95°C, and stirred at 120 rpm for 3 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered and separated to obtain a filter cake, which is then washed repeatedly with deionized water 5 times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and modified by adding 3% calcium stearate by weight of the hydrotalcite, and stirred for 60 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 70°C to constant weight. The powder after crushing has a loose density of 0.4 g / cm 3 , the final hydrotalcite sample 3 was verified to have a specific surface area of <20m² / g, D50 <0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 9Mg(OH)2+2Al(OH)3+NaHCO3+3H2O→Mg9Al2(OH) 22 CO3·4H2O+NaOH.

[0065] Example 4

[0066] S1 Raw material mixing and sand milling: Weigh 0.8 mol Mg(OH)2, 0.4 mol Al(OH)3 and 0.2 mol NaHCO3, add them into 1 L deionized water, and grind them thoroughly with a sand mill until D50 < 0.6 µm; S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 95°C, and stirred at 70 rpm for 3 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered and separated to obtain a filter cake, which is then washed repeatedly with deionized water 4 times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and modified by adding 3% lauric acid by weight of the hydrotalcite, and stirred for 50 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 70°C to constant weight. The powder after crushing has a loose density of 0.55 g / cm 3 , and the final hydrotalcite sample 4 was verified to have a specific surface area of <20m 2 / g, D50<0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 4Mg(OH)2+2Al(OH)3+NaHCO3+3H2O→Mg4Al2(OH) 12 CO3·4H2O+NaOH.

[0067] Example 5

[0068] S1 Raw material mixing and sand milling: Weigh 1.2 mol Zn(OH)2, 0.4 mol Al(OH)3 and 0.2 mol NaHCO3, add them into 1 L deionized water, and grind them thoroughly with a sand mill until D50 < 0.6 µm; S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 95°C, and stirred at 110 rpm for 3 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered and separated to obtain a filter cake, which is then washed repeatedly with deionized water 5 times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and modified by adding 3% calcium stearate by weight of the hydrotalcite, and stirred for 45 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 70°C to constant weight. The powder after crushing has a loose density of 0.5 g / cm 3 , and the final hydrotalcite sample 5 was verified to have a specific surface area of <20m 2 / g, D50<0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 6Zn(OH)2+2Al(OH)3+NaHCO3+3H2O→Zn6Al2(OH) 16 CO3·4H2O+NaOH.

[0069] Example 6

[0070] S1 Raw material mixing and sand milling: Weigh 1.2 mol Ca(OH)2, 0.4 mol Al(OH)3 and 0.2 mol NaHCO3, add them into 1 L deionized water, and grind them thoroughly with a sand mill until D50 < 0.6 µm; S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 95°C, and stirred at 90 rpm for 3 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered and separated to obtain a filter cake, which is then washed repeatedly with deionized water three times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and modified by adding 3% calcium stearate by weight of the hydrotalcite, and stirred for 40 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 70°C to constant weight. The powder after crushing has a loose density of 0.5 g / cm 3 , and the final hydrotalcite sample 6 was verified to have a specific surface area of <20m 2 / g, D50<0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 6Ca(OH)2+2Al(OH)3+NaHCO3+3H2O→Ca6Al2(OH) 16 CO3·4H2O+NaOH.

[0071] Example 7

[0072] S1 Raw material mixing and sand milling: Weigh 1.2 mol MgCl2 and 0.2 mol Al2(SO4)3·18H2O and dissolve them in 1 L of deionized water. At the same time, weigh 2.88 mol NaOH and 0.2 mol Na2CO3 and dissolve them in another 1 L of deionized water. The two solutions are continuously added dropwise to a reactor containing 500 ml of deionized water at flow rates of 12.5 ml / min and 20 ml / min, respectively, and stirred thoroughly to mix. The mixed slurry is then transferred to a sand mill and circulated and milled at 30°C for 3 hours until the particle size D50 is less than 0.6 μm.

[0073] S2 hydrothermal synthesis: The ground slurry was transferred to a reactor, heated to 80°C, stirred at 90 rpm, and reacted for 3 hours to obtain a reaction mother liquor.

[0074] S3 Solid-Liquid Separation and Washing: After the reaction is complete, the mother liquor is filtered to separate the filter cake. A multi-stage countercurrent washing system is used, with deionized water four times the weight of the filter cake, to wash the filtrate step by step until the pH of the filtrate is neutral, removing residual sodium salts and by-products. The final stage wastewater is reused for the previous rough wash, resulting in a total water consumption of only four times the weight of the filter cake, with a water conservation rate exceeding 70%.

[0075] S4 Surface modification: The washed hydrotalcite solid was re-slurried, heated to 80°C, and calcium stearate was added as a modifier at a concentration of 3% by weight of the hydrotalcite. The mixture was stirred at high speed for 45 minutes to ensure that the modifier evenly coated the particle surface. S5 Drying and Post-treatment: The modified slurry was centrifuged at 4000 rpm to achieve solid-liquid separation. The obtained solid was vacuum dried at 70°C to constant weight and then pulverized using 0.7 MPa air flow to control the final powder bulk density to 0.5 g / cm 3 , and the final hydrotalcite sample 6 was verified to have a specific surface area of <20m 2 / g, D50<0.150μm, <003> The grain size in the direction is >800Å; The reaction formula is as follows: 6MgCl2+Al2(SO4)3·18H2O+NaOH+Na2CO3+3H2O→Mg6Al2(OH) 16 CO3·4H2O+3Na2SO4.

[0076] Comparative Example 1 Commercially available hydrotalcite products, chemical composition: Mg6Al2(OH) 16 CO3·4H2O, including: 1.2molMg(OH)2, 0.4molAl(OH)3, 0.2molNaHCO3 Preparation method: Mg(OH)2, Al(OH)3 and NaHCO3 are dissolved in deionized water, and the metal ions are co-precipitated to form hydrotalcite by controlling the pH value and temperature.

[0077] 1. Performance testing The Example samples 1 to 7 and the comparative example hydrotalcite samples were subjected to X-ray diffraction (XRD) characterization, initial whiteness test, static aging test, specific surface area (BET) test and <003> Crystal size test.

[0078] 1.1 X-ray diffraction (XRD) characterization X-ray diffraction (XRD) characterization is used to analyze the crystal structure and phase composition of the sample and determine the crystal size and crystallinity of the hydrotalcite. Figure 2 The XRD patterns of the hydrotalcite samples of the embodiments and comparative examples are shown.

[0079] As can be seen from the figure, all samples exhibit typical diffraction peaks characteristic of hydrotalcite, indicating successful synthesis. Specifically, the diffraction peaks of the example samples are sharper, indicating higher crystallinity and a more complete crystalline structure; the diffraction peaks of the comparative example samples are relatively broad, indicating lower crystallinity and the presence of more defects or impurities.

[0080] 1.2 Initial whiteness test Initial whiteness testing was conducted using experimental formulas and conditions to assess the initial whiteness of PVC products and evaluate the impact of hydrotalcite on PVC whiteness. Initial whiteness testing was conducted on hydrotalcite samples from Examples and Comparative Examples using the experimental formula (50g PVC, 50g calcium powder, 15g DOTP, 0.5g zinc stearate, 1g hydrotalcite) and conditions (open mill temperature 203°C, material temperature 185-195°C, sampling every 15 minutes). Figure 3 A summary of the PVC initial whiteness test results of Example hydrotalcite samples 1 to 7 is shown.

[0081] It can be seen from the figure that after the example sample is added to PVC, the initial whiteness of the PVC product is significantly improved, indicating that the hydrotalcite of the present invention can effectively improve the whiteness of PVC.

[0082] 1.3 Static aging test Static aging tests simulate the stability of PVC at high temperatures and test the thermal stability of PVC products. Static aging tests were conducted on hydrotalcite samples 1 through 7 from Examples using the test formula (100g PVC, 50g DOTP, 0.5g zinc stearate, 1g hydrotalcite) and oven temperature (190°C). Figure 4 A summary of the PVC static aging test results of Example hydrotalcite samples 1 to 7 is shown.

[0083] As can be seen from the figure, the example samples exhibit excellent thermal stability under high-temperature conditions, significantly extending the thermal stability time of PVC products. This is attributed to the uniform particle size distribution of the hydrotalcite of the present invention, its good compatibility with PVC, and its ability to more effectively absorb HCl generated by PVC decomposition, thereby inhibiting the dehydrochlorination reaction.

[0084] 1.4 Specific surface area (BET) test and <003> Crystal size test Specific surface area (BET) test, measuring the specific surface area of the sample and evaluating its adsorption performance; <003> Crystal size test, calculation of hydrotalcite through XRD data <003> Table 1 lists the BET specific surface areas and <003> Summary of crystal size test results.

[0085] Table 1 BET and <003> Summary of crystal size test results

[0086] It can be concluded from the table that the specific surface areas of the samples in the examples are all less than 20m² / g, indicating that the particle size distribution is uniform and the powder agglomerates less, which is conducive to dispersion in PVC. <003> The crystal plane sizes are all larger than 800Å, indicating that the crystal structure is complete and the crystallinity is high, which helps to improve the thermal stability efficiency.

[0087] The present invention provides a hydrotalcite and its preparation method that combine high compatibility, high thermal stability, simplified processing, and environmental friendliness. By optimizing the hydrotalcite's chemical composition, particle size distribution, and surface modification process, the hydrotalcite's compatibility and dispersibility with polyvinyl chloride (PVC) are effectively improved, overcoming the agglomeration problem of conventional hydrotalcite caused by its polar surface. This method more effectively inhibits the dehydrochlorination reaction during the thermal decomposition of PVC, thereby extending the material's thermal stability. Furthermore, by regulating the hydrotalcite's grain size and specific surface area and combining it with a low-cost, environmentally friendly modifier, the present invention simplifies the preparation process, reduces production costs, and avoids the use of toxic heavy metals or complex composite systems. The result is a hydrotalcite product that combines excellent thermal stability, enhances initial whiteness, and is easily industrially produced. This product meets the application needs of high-performance PVC products in fields such as construction, packaging, and wire and cable.

Claims

1. A hydrotalcite for improving the thermal stability and initial whiteness of PVC, characterized in that: Its chemical composition includes: M 2+ x M 3+ y (OH) 2x+3y-x / 2 (CO3) x / 4 mH2O; where M 2+ is a divalent metal ion or a mixture of divalent metal ions selected from Mg 2+ , Ca 2+ , Ni 2+ , Zn 2+ , Mn 2+ , Cu 2+ ,Co 2+ , Pd 2+ and Fe 2+ One or two of M 3+ is a trivalent metal ion selected from Al 3+ , Ga 3+ ,In 3+ x and y are positive numbers, and the molar ratio of x to y satisfies x / y=2.0-3.0; m is the crystal water content, m=3.5-4.

0.

2. The hydrotalcite for improving the thermal stability and initial whiteness of PVC according to claim 1, characterized in that: The specific surface area of the hydrotalcite is less than 20m 2 / g, D50 is less than 0.150 μm, and the hydrotalcite has a <003> The grain size in the direction is at least 800Å.

3. The method for preparing hydrotalcite for improving the thermal stability and initial whiteness of PVC according to claim 1, characterized in that: The specific steps include: S1. Raw material mixing and sand milling: Weigh divalent metal salt or hydroxide, trivalent metal hydroxide and sodium salt in proportion, add to deionized water, and grind thoroughly with a sand mill until D50 < 0.6µm; S2 hydrothermal synthesis: transfer the ground slurry into a reactor, heat to 75-100°C, and stir at 60-120 rpm for 3-8 hours to obtain a reaction mother liquor; S3 solid-liquid separation and washing: After the reaction is completed, the mother liquor is filtered or centrifuged to obtain a filter cake, which is repeatedly washed with deionized water 3-5 times the weight of the filter cake using a multi-stage countercurrent washing system until the pH of the filtrate is neutral to obtain hydrotalcite solid and remove residual sodium salt and by-products; S4 surface modification: re-slurry the washed hydrotalcite solid, heat to 75-85°C, add 1-5% of the weight of the hydrotalcite anionic modifier, and stir for 30-60 minutes; S5 Drying and Post-treatment: The modified slurry is subjected to solid-liquid separation, and the obtained solid is dried at 50-150°C to constant weight. The powder after crushing has a loose density of 0.4-0.6 g / cm 3 , verifying that the final hydrotalcite product is obtained.

4. The method for preparing hydrotalcite according to claim 3, wherein: The raw material mixing and sand grinding steps specifically include the following operations: S1.1 Raw material mixing: according to chemical formula M 2+ x M 3+ y (OH) 2x+3y-x / 2 (CO3) x / 4 ·mH2O molar ratio, weigh divalent metal salts or hydroxides, trivalent metal hydroxides and sodium salts, and mix with deionized water at a liquid-to-solid ratio of 3:1-5:1; S1.2 Sand grinding dispersion: Grind the mixed slurry in a sand mill at 20-40℃ for 2-4 hours, and grind it repeatedly until the particle size D50 is less than 0.6μm to ensure that the raw materials are fully dispersed and refined.

5. The method for preparing hydrotalcite according to claim 3, characterized in that: The solid-liquid separation and washing steps specifically include the following operations: S3.1 Preliminary solid-liquid separation: Pump the slurry after the hydrothermal reaction into the separation equipment, filter at 0.4-0.6 MPa or centrifuge at 3000-5000 rpm to separate the product, and collect the filter cake with a thickness of 3-5 cm; S3.2 Multi-stage countercurrent washing: Use 3-4 stages of washing tanks in series. Fresh deionized water is injected from the last stage and flows countercurrently to the previous stage step by step, contacting the filter cake in countercurrent. The total amount of washing water is 3-5 times the weight of the filter cake. The filter cake is dehydrated by high-speed centrifugation to a moisture content of 10-30%; S3.3 Final solid-liquid separation: Use a high-speed centrifuge to perform secondary dehydration on the washed product to obtain hydrotalcite solid with a water content of 5-15%.

6. The method for preparing hydrotalcite according to claim 5, characterized in that: The multi-stage countercurrent washing operation specifically includes: using 3-4 stages of washing tanks in series, fresh deionized water is injected from the last stage and flows countercurrently to the previous stage step by step, and contacts the filter cake in countercurrent; 50% of the water is used for the first stage rough washing to remove most of the sodium salts and by-products, 30% of the water is used for the secondary fine washing to reduce the conductivity to below 500 μS / cm, and 20% of the water is used for the final washing to adjust the pH of the filtrate to a neutral pH of 6.5-7.

5. The total amount of washing water is 3-5 times the weight of the filter cake, and the water temperature is controlled at 40-60°C to improve the dissolution efficiency. At the same time, the wastewater from the last stage is reused for the previous stage rough washing, and the filter cake with a moisture content of 10-30% is obtained through high-speed centrifugal dehydration.

7. The method for preparing hydrotalcite according to claim 3, characterized in that: In the surface modification step, the anionic surfactant is selected from one or two of alkali metal salts of higher fatty acids, alkyl sulfates, alkyl sulfonates, alkyl aryl sulfonates and sulfosuccinates.

8. The method for preparing hydrotalcite according to claim 3, characterized in that: The surface modification step specifically includes the following operations: S4.1 Filter Cake Reslurry: Mix the washed filter cake with deionized water at a solid-to-liquid ratio of 1:2-1:4 (w / w), stir at 300-500 rpm, and slurry for 10-15 minutes to form a uniform suspension with a solid content of 20-30%; S4.2 Modifier addition and dispersion: Pre-dissolve the modifier in a small amount of ethanol or hot water and slowly add it dropwise to the slurry. Simultaneously, raise the temperature to 75-85°C and increase the stirring speed to 1500-2000 rpm for 30-60 minutes to ensure that the modifier evenly coats the particle surface. S4.3 Post-modification treatment: Add a small amount of deionized water to quench or adjust the pH to neutral to stop the modification reaction, centrifuge or filter to separate the slurry, and collect the modified slurry.

9. The method for preparing hydrotalcite according to claim 3, characterized in that: The drying and post-processing steps specifically include the following operations: S5.1 Solid-liquid separation: The modified slurry is subjected to solid-liquid separation by centrifugation at 3000-5000 rpm or filter pressure at 0.4-0.6 MPa, and the resulting solid filter cake is collected; S5.2 Drying: Dry by spray drying or vacuum drying at 50-150°C to constant weight; S5.3 Crushing: Use 0.6-0.8MPa air flow or ball mill for 30-60 minutes to crush to D50 < 0.150μm, and then control the bulk density to 0.4-0.6g / cm after sieving. 3 , obtaining a hydrotalcite product that improves the thermal stability and initial whiteness of PVC; S5.4 Quality Inspection: Verify that the specific surface area of the hydrotalcite product is less than 20m 2 / g, <003> The grain size in the direction is at least 800Å and the D50 is less than 0.150μm.

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