Highly acid-absorbing hydrotalcite, preparation method and PVC material

Abstract

The application provides a hydrotalcite and a preparation method thereof. The hydrotalcite is prepared by a method combining low-temperature water bath and high-temperature hydrothermal reaction, and a regulator is introduced in the hydrothermal reaction to adjust the interlayer spacing of the hydrotalcite and improve the acid absorption capacity of the hydrotalcite. The obtained hydrotalcite is used as a thermal stabilizer of PVC general plastic, and can significantly improve the thermal stability of the plastic. The application also relates to the manufacturing of PVC plastic.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of inorganic material preparation, and particularly relates to a production method of hydrotalcite. BACKGROUND

[0002] Hydrotalcite is a layered double hydroxide compound, which has been applied in flame retardants, heat stabilizers, nucleating agents, ultraviolet inhibitors and infrared absorbers, and has very good use effect. At present, it is mainly used as an acid absorbent, a PVC heat stabilizer and a composite heat stabilizer, a PO film heat stabilizer, a PVDC heat stabilizer, a PVC smoke suppressant and a flame retardant, and is applied in the process of polymer material processing. Among them, the PVC heat stabilizer has the largest amount.

[0003] PVC, also known as polyvinyl chloride, is the world's largest general-purpose plastic with a very wide range of applications, involving pipe materials, profiles, films, wire and cable, interior decoration, toys, medical treatment, building materials and many other industries. Since 2020, the apparent consumption of PVC resin industry in China has maintained a growth trend, with annual sales of more than 21 million tons. Research shows that PVC begins to remove HCl when the temperature exceeds 100℃, and the removed HCl automatically catalyzes the degradation of PVC molecules, resulting in a rapid zippered elimination reaction to generate colored polyene sequences. Therefore, many PVC products will appear yellowing and reddening during processing and use, which brings a lot of trouble to the application of PVC.

[0004] Due to the defects of PVC molecules, the birth of heat stabilizers has solved this problem. Heat stabilizers can prevent or delay the degradation of PVC. Heat stabilizer treatment can replace the active atoms in the PVC molecular chain, can add to the unsaturated double bonds of PVC, can inhibit the oxidation of polyene structure, and can absorb infrared light in addition. Most importantly, PVC heat stabilizers can absorb HCl to inhibit its self-catalysis. Therefore, a substance with strong acid absorption capacity and without affecting the performance of PVC is needed as an acid absorbent component in heat stabilizers. Hydrotalcite is a layered double hydroxide compound, which has very strong acid absorption performance due to its layered structure and carrying a large amount of hydroxyl groups. Due to its strong acid absorption capacity, it is widely used in PVC heat stabilizers, especially in the current mainstream calcium-zinc heat stabilizer, and hydrotalcite is an indispensable component in it.

[0005] Currently, the preparation methods of hydrotalcite include hydrothermal method, coprecipitation method, calcination reduction method, urea method, etc. The method disclosed in CN111825111A is to use magnesium hydroxide as a magnesium source to prepare hydrotalcite, and the amount of the acidic substance used for activating the magnesium source in the early stage is 50% of the magnesium hydroxide, which is large in amount and leads to complex and multiple components of the sewage in the later stage, thereby causing troubles for sewage treatment in the later stage. CN115872429A discloses a production method, which uses magnesium oxide and sodium aluminate to prepare ultra-fine hydrotalcite. The acid absorption performance of the hydrotalcite obtained by the disclosed technology still needs to be further improved. SUMMARY

[0006] The purpose of the present application is to provide a preparation method of high-acid-absorption hydrotalcite.

[0007] Another purpose of the present application is an environmentally friendly process for producing high-acid-absorption hydrotalcite.

[0008] The hydrotalcite preparation method of the present application comprises the following steps:

[0009] 1) mixing magnesium hydroxide, an acidic activator and water to prepare a magnesium hydroxide suspension;

[0010] 2) adding sodium bicarbonate to the magnesium hydroxide suspension, and placing it in a water bath kettle for water bath treatment to prepare a first mixed slurry;

[0011] 3) mixing the first mixed slurry with a sodium aluminate solution, stirring for a period of time to obtain a second mixed slurry;

[0012] 4) filtering the second mixed slurry, mixing the filter cake with water and an interval adjusting agent to obtain a third mixed slurry;

[0013] 5) placing the third mixed slurry in an autoclave for hydrothermal reaction.

[0014] 6) filtering the mixed slurry after reaction, washing, modifying and drying the filter cake to obtain hydrotalcite.

[0015] In the present application, the magnesium hydroxide can be industrial-grade magnesium hydroxide raw powder, and the purity is preferably not less than 99%. There is no special requirement for the particle size, and D50 in the range of 20-200 μm is acceptable. The smaller the particle size, the easier the slurry is, so the particle size D50 of the raw powder is preferably in the range of 20-80 μm, for example, 40-50 μm.

[0016] In the present invention, the acidic activator used in step 1) can be a strong acid and a weak acid, the strong acid is for example hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, the weak acid is usually an organic carboxylic acid, for example acetic acid, fumaric acid, maleic acid, acetic acid, oxalic acid, itaconic acid and citric acid. The weak acid is preferred in the present invention. The amount of the acidic activator can be 0.5-1.5% of the molar amount of magnesium hydroxide, preferably 0.8-1%. The purpose of the acidic activator is to etch the surface of magnesium hydroxide, causing surface defects. Considering the availability of the material and the solubility, the weak acid activator is preferably not more than 100 molar equivalent of carboxylate.

[0017] In step 1), the particle size of magnesium hydroxide can be reduced by heating, stirring or grinding, among which ball milling is preferred, preferably the particle size of magnesium hydroxide in the suspension is reduced to D100 not more than 10 μm, more preferably D50 not more than 1.5 μm, more preferably not more than 1.2 μm. In the preferred embodiment, the acidic activator is added after the particle size is reduced to the desired value.

[0018] In step 2), in order to prepare the first mixed slurry, the amount of sodium bicarbonate is 0.25-0.5 times of the molar amount of magnesium hydroxide, which is compatible with the molar amount of sodium aluminate added in the next step. The reaction is stirred under water bath conditions for a suitable time, during which the high-activity intermediate [Mg x (OH) y (H2O) z ] n+ A n- , A n- is carbonate, bicarbonate, organic weak acid radical. The water bath reaction temperature is 60-90°C, preferably 70-80°C, too low temperature, the formation of the active intermediate is slow, which will prolong the stirring time; too high temperature, then the formed high-activity intermediate may be partially decomposed to form magnesium hydroxide crystals. The water bath reaction time is 2-30 min, typically 5-15 min. If necessary, the pH value of the slurry can be adjusted with a base or an acid, so that the pH value is maintained between 10-12, preferably 10-11. The available base can be sodium aluminate, ammonia, sodium hydroxide or potassium hydroxide, the available acid is preferably small molecular weight acid such as hydrochloric acid, acetic acid.

[0019] In step 3), the sodium aluminate solution is added to the obtained first mixed slurry, and stirred until the pH value is no longer changed. It is maintained at the water bath temperature for 4-8 h, typically 5-6 h. In this process, the hydrotalcite crystal nucleus is formed in the second mixed slurry. The pH value in this process is usually between 10-12, if there is deviation, the pH value can be adjusted to this interval by a base or the aforementioned weak organic acid, preferably 10-11.

[0020] In step 4), the interlayer spacing regulator is a sodium salt or potassium salt of an organic weak acid, and the diameter of the acid radical ion is preferably 1.5 times or more than the diameter of the carbonate radical ion. Examples of the interlayer spacing regulator that can be used include sodium citrate, sodium acetate, sodium oxalate, sodium malate, and the like. In the present application, the use of the interlayer spacing regulator can partially destroy the hydrogen bonds formed between the layers of the hydrotalcite, thereby partially expanding the spacing between the layers of the hydrotalcite crystal, resulting in a partial expansion of the interlayer spacing. This partial destruction of the interlayer spacing can effectively improve the adsorption rate and amount of hydrogen chloride, thereby improving the acid absorption performance. In typical embodiments of the present application, a carboxylate salt is used as the interlayer spacing regulator, but other active groups are not excluded, such as substances containing exposed ammonium groups.

[0021] In the present application, the amount of the interlayer spacing regulator can be 3-10%, preferably 5-10%, and most preferably 7-9% of the molar amount of the magnesium hydroxide. If the amount is too low, the improvement in acid absorption capacity is not obvious; if the amount is too high, the double-layer structure of the hydrotalcite can be substantially damaged.

[0022] In the present application, the hydrothermal reaction of step 5) can be carried out under conventional conditions, and the hydrothermal reaction temperature can be 150-200°C, preferably 160-180°C, and the reaction time can be 2-6h, preferably 3-4h. The pH value of the reaction slurry is maintained at 8-12, preferably 10-11.

[0023] After the hydrothermal reaction, the slurry is filtered, washed, and dried to obtain the hydrotalcite. As a further preferred, the hydrotalcite is modified with a modifier to reduce the hydrophilicity of the hydrotalcite. Commonly used modifiers in the art are silane coupling agents KH560, K silane coupling agent H550, stearic acid, oleic acid, and castor oil, and the like hydrophobic substances.

[0024] The hydrotalcite prepared according to the method of the present application is rectangular or approximately rectangular, with a D50 of 0.2-1.0μm and a D90<1.0μm. The modified hydrotalcite is particularly suitable for use as an acid absorber in PVC heat stabilizers and is applied in PVC.

[0025] The present application prepares a hydrotalcite with higher acid absorption capacity by adding an activator to magnesium hydroxide and ball-milling and activating to form a suspension slurry, then adding baking soda and sodium aluminate, then forming a hydrotalcite crystal nucleus after a low-temperature water bath reaction, and then controlling the crystal growth under high-temperature and high-pressure hydrothermal conditions. The prepared hydrotalcite has higher acid absorption capacity and can provide higher thermal stability for PVC materials, and the crystal morphology is rectangular or approximately rectangular.

[0026] The hydrotalcite prepared by the method of the present application has a relatively low particle size, and the particle size is not more than 1μm. Moreover, all the by-products are converted into sodium carbonate, which can be causticized to generate calcium carbonate and sodium hydroxide. The sodium hydroxide can be used to prepare sodium aluminate, achieving zero emissions in production. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 are XRD patterns of the hydrotalcites of Examples 1-3 and Comparative Example 1-4, respectively;

[0028] Figures 2 to 8 are electron micrographs of the hydrotalcites of Examples 1-3 and Comparative Example 1-4, respectively;

[0029] Figure 9 is a device for detecting the acid absorption performance of hydrotalcite. DETAILED DESCRIPTION

[0030] The advantages of the technical solutions of the present application are illustrated below by way of examples and comparative examples.

[0031] Example 1

[0032] (1) 104.62 g (1.804 mol) of magnesium hydroxide, 2.31 g (0.012 mol, carboxylate 0.036 mol, 1% of the amount of hydroxide) of citric acid, and 200 ml of water were mixed, ball-milled into a magnesium hydroxide slurry, and configured into a suspension. The particle size D50 of the magnesium hydroxide suspension was 1.25 μm, and the particle size D100 was 7.33 μm;

[0033] (2) 90.9 g (1.082 mol) of baking soda was added to the magnesium hydroxide suspension, and the mixture was treated in a 75°C water bath for 5 min to prepare a first mixed slurry;

[0034] (3) The first mixed slurry was mixed with 285 ml of a sodium aluminate solution (containing Al2O3 42.75 g, 0.42 mol), and stirred in a water bath for 6 h while maintaining the pH value at 11 to prepare a second mixed slurry;

[0035] (4) The second mixed slurry was suction-filtered, the filter cake was mixed with water, 27.93 g of sodium citrate (anion number 0.108 mol, anion charge number 0.325 mol, 9% of the amount of hydroxide of magnesium hydroxide), and the pH value was adjusted to 11 with sodium hydroxide to obtain a third mixed slurry;

[0036] (5) The third mixed slurry was placed in an autoclave and heated to 170°C for 3 h.

[0037] (7) After the final discharge, the slurry was suction-filtered, washed, modified with stearic acid in a 70°C water bath, and finally suction-filtered and dried to obtain the hydrotalcite LDH-1, the XRD pattern of which is shown in Figure 1 , and the electron micrograph is shown in Figure 2 .

[0038] Example 2

[0039] (1) A mixture of 104.62 g (1.804 mol) of magnesium hydroxide, 2.16 g (0.036 mol, 1% of carboxylate is hydroxyl) of acetic acid and 200 ml of water was ball-milled into a magnesium hydroxide slurry and configured into a suspension. The magnesium hydroxide suspension was tested for particle size D50 = 1.13 μm, D100 = 7.05 μm

[0040] (2) 90.9 g (1.082 mol) of baking soda was added to the magnesium hydroxide suspension, and the mixture was placed in a 75°C water bath for 5 min to prepare a first mixed slurry;

[0041] (3) The first mixed slurry was mixed with 285 ml of sodium aluminate solution (containing Al203 42.75 g, 0.42 mol), and stirred in a water bath for 6 h while maintaining pH value at 11 to prepare a second mixed slurry;

[0042] (4) The second mixed slurry was suction filtered, and the filter cake was mixed with water, 26.65 g (0.325 mol, 9% of anion charge number is the number of hydroxyl) of sodium acetate, and adjusted to pH value 11 with sodium hydroxide to obtain a third mixed slurry;

[0043] (5) The third mixed slurry was placed in an autoclave and heated to 170°C for 3 h.

[0044] (7) After the final discharge, the slurry was suction filtered, washed, modified with stearic acid in a 70°C water bath, and finally suction filtered and dried to obtain the hydrotalcite LDH-2, whose XRD pattern is shown in Figure 1 , and the electron microscope photograph is shown in Figure 3 .

[0045] Example 3

[0046] (1) A mixture of 104.62 g (1.804 mol) of magnesium hydroxide, 2.16 g (0.036 mol, 1% of carboxylate is hydroxyl) of acetic acid and 200 ml of water was ball-milled into a magnesium hydroxide slurry and configured into a suspension. The magnesium hydroxide suspension was tested for particle size D50 = 1.13 μm, D100 = 7.05 μm

[0047] (2) 90.9 g (1.082 mol) of baking soda was added to the magnesium hydroxide suspension, and the mixture was placed in a 75°C water bath for 5 min to prepare a first mixed slurry;

[0048] (3) The first mixed slurry was mixed with 285 ml of sodium aluminate solution (Ao = 150 g / L, Nk = 145 g / L), and stirred in a water bath for 6 h while maintaining pH value at 11 to prepare a second mixed slurry;

[0049] (4) The second mixed slurry was filtered, the filter cake was mixed with water, 21.79 g (anion number 0.1625 mol, anion charge number 0.325 mol, 9% of the amount of magnesium hydroxide hydroxyl group) sodium malate, and the pH value was adjusted to 11 with sodium hydroxide to obtain a third mixed slurry;

[0050] (5) The third mixed slurry was placed in an autoclave and heated to 170°C for 3 h.

[0051] (7) After the final discharge, the slurry was filtered, washed, modified with stearic acid in a water bath at 70°C, and finally filtered and dried to obtain the hydrotalcite LDH-3, the XRD pattern of which is shown in Figure 1 , and the electron micrograph is shown in Figure 4 .

[0052] Comparative Example 1

[0053] The operation of Example 2 was repeated, except that step (4) was omitted and no crystal growth regulator was added. The hydrotalcite LDH-4 was obtained, the XRD pattern of which is shown in Figure 1 , and the electron micrograph is shown in Figure 5 .

[0054] Comparative Example 2

[0055] 1) 72.15 g of magnesium oxide and 0.4 g of acetic acid were weighed into water, stirred for 3 min, and then 75.76 g of sodium bicarbonate was added;

[0056] 2) The obtained slurry was placed in a 75°C water bath and stirred for 5 min, during which high-activity intermediates [Mg x (OH) y (H2O) z ] n+ were generated in the slurry;

[0057] 3) 307 mL of a sodium aluminate solution with a concentration of 2.94 mol / L was added to the slurry of the reaction product of step 2, stirred for 6 h, and the pH value was adjusted to 11 with acetic acid and sodium hydroxide;

[0058] 4) After the pH value was stable, the slurry was placed in an autoclave and reacted at 180°C for 6 h.

[0059] 5) After the reaction was completed, the slurry was cooled, discharged, filtered, dried, and pulverized, and the finished product was the hydrotalcite LDH-5, the XRD pattern of which is shown in Figure 1 , and the electron micrograph is shown in Figure 6 .

[0060] Comparative Example 3

[0061] Take magnesium hydroxide 29 g, aluminum hydroxide 15.6 g, n (Mg / Al) =2.1, mixed and ground to a medium particle size of 3-4 μm, add 145 g of water and stir evenly, heat to 65℃, add 60% nitric acid 26.25 g to the solution, continue to stir for 30 min. The material is transferred to a ball mill and ground at 400 r / min for 80 min. The ball-milled material is transferred to a reaction bottle, and a mixed alkali solution of liquid alkali and sodium carbonate is added dropwise to the reaction bottle until the pH is 12.2, and stirring is continued for 30 min. The material is transferred to a hydrothermal synthesis kettle and crystallized at 230℃ for 24 h. After crystallization, the sample is filtered, washed and dried to obtain a magnesium-aluminum hydrotalcite, LDH-6, and the electron micrograph is shown in Figure 7

[0062] In addition, a magnesium-aluminum hydrotalcite with good sales and strong acid absorption capacity (HT3 hydrotalcite from Kaisima (Dandong) High-tech Materials Science and Technology Co., Ltd.) is purchased from the market, numbered LDH-7, as Comparative Example 4, and the electron micrograph is shown in Figure 8 .

[0063] The specific surface area of the samples obtained in the examples and comparative examples is detected by a specific surface area instrument, and the results are shown in Table 1.

[0064] Table 1 Comparison of specific surface areas of different hydrotalcites

[0065] Sample Specific surface area m 2 / g]] Example 1 42.12 Example 2 39.38 Example 3 40.66 Comparative Example 1 12.05 Comparative Example 2 11.22 Comparative Example 3 11.32 Comparative Example 4 10.15

[0066] The acid absorption performance of the hydrotalcite is detected by the device shown in Figure 9 . A flask is filled with concentrated sulfuric acid, a separatory funnel is filled with concentrated hydrochloric acid, a gas collection bottle is filled with 1 g of hydrotalcite powder, and a beaker is filled with a certain concentration of sodium hydroxide solution. When concentrated hydrochloric acid is added to concentrated sulfuric acid, HCl acid gas is generated, which is introduced into the gas collection bottle through a conduit and contacts the hydrotalcite. The hydrotalcite absorbs the HCl gas, and the excess tail gas is absorbed by sodium hydroxide through a conduit. The entire reaction time is 10 min, 20 min, and 30 min, respectively, and the dropwise addition rate of concentrated hydrochloric acid is constant.

[0067] The hydrotalcite after the above reaction is dissolved with 10 mL of 3.5 mol / L nitric acid to make 100 mL of solution, 50 mL of which is taken out with a pipette into a conical flask, then potassium dichromate indicator is added, and 0.0141 mol / L silver nitrate is titrated. When the solution turns brick red, stop titrating, and record the amount of silver nitrate consumed. The greater the amount of silver nitrate consumed, the stronger the acid absorption capacity of the surface material.

[0068] Table 2 Comparison of acid absorption performance of different hydrotalcites

[0069]

[0070] Hydrotalcite was used as an acid absorber in calcium-zinc heat stabilizer in PVC and the heat stabilization time of PVC was tested. The longer the time, the better the heat stabilization effect, which indicates that the acid absorption capacity of hydrotalcite in application is stronger. The specific formula is shown in Table 3:

[0071] Table 3 Formulas used in Congo red experiment

[0072] Ingredient Ratio PVC 100 Plasticizer 45 Calcium carbonate 30 Hydrotalcite 70 Calcium zinc heat stabilizer without hydrotalcite 30

[0073] Table 4. Congo red test results

[0074] Sample Heat stabilization time / min LDH-1 272 LDH-2 245 LDH-3 260 LDH-5 191 LDH-6 210 LDH-7 203 LDH-8 215

[0075] Through the experiment, it is shown that the hydrotalcite prepared by the method of the present application has stronger acid absorption capacity than the hydrotalcite prepared by the traditional method.

Claims

1. A method for preparing highly acid-absorbing hydrotalcite, characterized in that, Includes the following steps: 1) Magnesium hydroxide, an acidic activator, and water are mixed to prepare a magnesium hydroxide suspension. The particle size of magnesium hydroxide in the suspension is D50 < 1.5 μm and D100 < 10 μm. The acidic activator is hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, or an organic carboxylic acid with a molar equivalent of not more than 100 based on the carboxylate ion. The amount of acidic activator added is 0.5-1.5% of the molar amount of magnesium hydroxide. 2) Add sodium bicarbonate to the magnesium hydroxide suspension, place it in a water bath for water bath treatment, and prepare the first mixed slurry. The amount of sodium bicarbonate is 0.25-0.5 times the molar amount of magnesium hydroxide. 3) Mix the first mixed slurry with sodium aluminate solution, stir in a water bath for a period of time to obtain the second mixed slurry. The amount of sodium aluminate solution used is based on a magnesium-aluminum ratio of 2-2.

5. 4) Filter the second mixed slurry, and mix the filter cake with water and a gap conditioner to obtain a third mixed slurry. The gap conditioner is a sodium or potassium salt of an organic weak acid, whose anion diameter is larger than that of a carbonate ion. The amount of the gap conditioner added is 5-10% of the molar amount of hydroxide ions in magnesium hydroxide, based on the anion charge. 5) The third mixed slurry is placed in an autoclave for hydrothermal reaction; 6) After filtering the mixed slurry from the reaction, the filter cake is washed, modified, and dried to obtain hydrotalcite; In step 6), the hydrotalcite is modified with a modifier to reduce its hydrophilicity. The modifier is silane coupling agent KH560, silane coupling agent KH550, stearic acid, oleic acid, or castor oil. In steps 2) and 3), the water bath reaction temperature is 60-90℃ and the water bath reaction time is 4-8h.

2. The method according to claim 1, wherein, In step 1), the particle size of magnesium hydroxide in the suspension is D50 < 1.2 μm and D100 < 8 μm.

3. The method according to claim 1, wherein, In step 2), the pH value of the first mixed slurry is 8-12.

4. The method according to claim 1, wherein, In step 4), the radius of the anion of the gap regulator is 1.5 times or more that of the carbonate ion.

5. The method according to claim 1, wherein, In step 5), the pH value is 8-12.

6. A PVC heat stabilizer, comprising hydrotalcite prepared by the method described in any one of claims 1-5, characterized in that, The shape is rectangular or near-rectangular, with a particle size D50 of 0.2-1.0 μm and D90 < 1.0 μm.

7. A PVC composite material comprising the PVC heat stabilizer of claim 6.

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