A precision liquid filter and a production process thereof

Abstract

This invention relates to a precision liquid filter media and its manufacturing process, belonging to the field of liquid filtration technology. The precision liquid filter media of this application comprises the following raw materials in weight percentages: 70%-75% polyethylene resin and 25%-30% modified calcium sulfate whiskers. This application involves the addition of 2,6-di-tert-butylphenol to ethylene glycol, followed by modification of calcium sulfate whiskers with a phosphorus oxychloride phosphate antioxidant modifier using the phosphorus oxychloride method. The modified calcium sulfate whiskers are then blended with polyethylene resin and the modified calcium sulfate whiskers and processed through a molding process to obtain the precision liquid filter media. This process significantly enhances mechanical properties, improves heat resistance, and significantly strengthens oxidation resistance.

Description

Technical Field

[0001] This invention belongs to the field of liquid filtration technology, specifically relating to a precision liquid filter material and its manufacturing process. Background Technology

[0002] Liquids are indispensable in chemical production, and heterogeneous separation of liquids plays a crucial role in industrial processes. Common methods for heterogeneous liquid separation include sedimentation and filtration. Liquid-solid filtration is categorized into coarse filtration (>100μm), sub-precision filtration (100-10μm), precision filtration (10-1μm), and ultra-precision filtration (1-0.1μm). Precision filtration is widely used due to its superior filtration efficiency, maximizing the removal of solid particles from liquids and improving their clarity.

[0003] The most practical precision filter material currently available is polymer microporous tube, which has high filtration efficiency: (1) The minimum filtration pore size is 0.5μm, and the filtration efficiency is greater than 99.9%; (2) It is easy to remove the filter cake. It can be easily and quickly removed by simply backflushing the filter cake; (3) It has high regeneration efficiency. The microporous tube can be efficiently and simply regenerated by backflushing, and the microporous tube can be used for a long time; (4) It has good corrosion resistance. The microporous tube is resistant to acids, alkalis, salts and most organic solvents; (5) It is easy to maintain and repair. The filter structure is relatively simple, the microporous tube has a low relative density, and maintenance and repair are easy.

[0004] Polyethylene (PE) materials are favored for their non-toxicity, odorlessness, good water resistance, and excellent chemical stability, and are widely used in polymer microporous tube materials. Among them, high-density polyethylene (HDPE) is more suitable for filter media due to its hardness, abrasion resistance, and chemical stability. However, HDPE materials have poor oxidation resistance, as well as poor mechanical strength, brittleness, and poor resistance to environmental stress cracking, which affects the service life of the filter media. Furthermore, polyethylene materials have poor heat resistance, which limits their applicability to filtrates. Summary of the Invention

[0005] The purpose of this invention is to provide a precision liquid filter material and its manufacturing process.

[0006] The technical problem this invention aims to solve is that existing precision liquid filter media fail to simultaneously address the issues of poor oxidation resistance, poor mechanical strength, and poor heat resistance of high-density ethylene.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A precision liquid filter media comprises the following raw materials in weight percentages:

[0009] High-density polyethylene 70%-75%

[0010] Modified calcium sulfate whiskers: 25%-30%.

[0011] As a further embodiment of the present invention: the preparation method of the modified calcium sulfate whiskers is as follows: anhydrous calcium sulfate whiskers (purchased from Zhengzhou Bokaili Ecological Engineering Co., Ltd.) are added to anhydrous ethanol and heated in a water bath at 20-30°C. The ratio of calcium sulfate whiskers to anhydrous ethanol is 2g:50mL. Potassium hydroxide (purchased from Sinopharm Chemical Reagent Co., Ltd.) is added to anhydrous ethanol solution of phosphate ester antioxidant modifier (purchased from Sinopharm Chemical Reagent Co., Ltd.) to adjust the pH to neutral. After the reaction is completed, the mixture is filtered, washed three times with anhydrous ethanol, and dried at 100°C to obtain modified calcium sulfate whiskers.

[0012] As a further embodiment of the present invention: the preparation method of the phosphate ester antioxidant modifier is as follows: phosphorus oxychloride (purchased from Jinan Rongguang Chemical Co., Ltd.) is added to a reaction vessel, and modified antioxidant phenol is added while stirring. The temperature is 10-18℃ and stirred for 20 minutes. Then the temperature is raised to 40-50℃, distilled water is slowly added to the system, and then the temperature is adjusted to 50-60℃. The reaction is carried out for a period of time to obtain the phosphate ester antioxidant modifier.

[0013] As a further aspect of the present invention: the molar ratio of phosphorus oxychloride and modified antioxidant phenol is 1:1-1.2:1, and the molar ratio of modified antioxidant phenol and distilled water is 10:1.

[0014] As a further embodiment of the present invention: the preparation method of the modified antioxidant phenol is as follows: 2,6-di-tert-butylphenol (purchased from Shanghai Kaisai Chemical Co., Ltd.) and potassium hydroxide catalyst are added to a reaction vessel, nitrogen gas is introduced, and the temperature is raised to 50°C until the material is completely melted; while stirring, the temperature is raised to 100°C, and a vacuum is drawn to remove water from the material; the temperature is further raised to 110°C, and ethylene glycol (purchased from Chaoyang Guangda Chemical Co., Ltd.) is slowly added. After a period of time, the temperature is raised to 130°C and reacted for 3-4 hours; after the reaction is completed, the temperature is lowered to 90°C, acetic acid (purchased from Sinopharm Chemical Reagent Co., Ltd.) is added for neutralization, and the mixture is dissolved, crystallized, filtered, washed, and dried with methanol (purchased from Sinopharm Chemical Reagent Co., Ltd.) to obtain solid modified antioxidant phenol.

[0015] If too little potassium hydroxide catalyst is added, the reaction time needs to be extended, the degree of side reactions increases, and the product color is affected; if too much is added, it will affect the subsequent neutralization, filtration, and washing processes, and the amount of reagent used will be large. The optimal amount of potassium hydroxide is about 1% of the mass of 2,6-di-tert-butylphenol.

[0016] As a further aspect of the present invention: the molar ratio of 2,6-di-tert-butylphenol to ethylene glycol is 1:1.1; the mass ratio of 2,6-di-tert-butylphenol to potassium hydroxide is 1:0.01.

[0017] To improve the conversion rate of 2,6-di-tert-butylphenol, ethylene glycol needs to be in excess. However, too much excess not only increases the cost, but also makes ethylene glycol prone to self-polymerization, reducing the yield. A molar ratio of 1:1.1 for 2,6-di-tert-butylphenol and ethylene glycol is preferable.

[0018] The dropping time should not be too short, otherwise ethylene glycol will undergo self-polymerization, thus reducing the yield. The dropping time should also not be too long. The dropping time has little effect on the reaction. It is advisable to add 0.25 mol of ethylene glycol and require 1 hour to add 0.5 mol and require 1 hour.

[0019] As a further aspect of the present invention, the preparation process of the precision liquid filter material includes the following steps:

[0020] S1: Modified calcium sulfate whiskers and high-density polyethylene (purchased from Lanzhou Petrochemical Branch of China National Petroleum Corporation) are placed together in a high-speed mixer and mixed for 5 minutes. The uniformly mixed material is then melt-blended using a twin-screw extruder to obtain a precision liquid filter material raw material.

[0021] S2: The obtained precision liquid filter material raw material is loaded into the molding mold, and the powder is pressed into shape using a DSB-30A four-column hydraulic press. The molding pressure range is 2.5MPa-22.5MPa. When the pressure reaches the set target value, the pressure is held. After completion, the pressure is slowly released and the blank is removed from the mold.

[0022] S3: Place the demolded blank in a precision oven (HH-132) for sintering. During the sintering process, temperature is controlled: first, heat to 120-130℃ at a rate of 5℃ / min and hold for 10 minutes to release the internal stress of the blank. During the second heating, heat to the target sintering temperature at a rate of 0.5℃ / min and hold for 45 minutes. After the holding time is completed, slowly cool to room temperature.

[0023] The beneficial effects of this invention are:

[0024] Calcium sulfate whiskers possess excellent properties such as high strength, wear resistance, high temperature resistance, acid and alkali resistance, and corrosion resistance. Their manufacturing cost is lower than other materials, and they are non-toxic. Due to their high surface energy, calcium sulfate whiskers are widely used. When mixed with high-density polyethylene resin, they can improve the mechanical strength, impact resistance, and high-temperature resistance of the material. However, calcium sulfate whisker particles are prone to severe agglomeration, and this agglomeration is hard. Even after grinding and sieving, the particles still exist as agglomerates, forming larger particles, thus reducing the specific surface area of ​​the calcium sulfate whisker powder. When dispersed in high-density polyethylene resin, this agglomeration affects the chemical properties of the blend. Modified calcium sulfate whiskers, however, have an organic coating on their surface, reducing surface energy and keeping the particles in a stable state. The particles are more dispersed, and even the agglomerates are soft agglomerates that are easily broken. This greatly improves the dispersibility of the calcium sulfate whisker powder, resulting in uniform dispersion of the modified calcium sulfate whiskers and enhanced stability of the resulting blend.

[0025] 2,6-Di-tert-butylphenol is readily soluble in organic solvents such as alcohols, esters, alkanes, and aromatic hydrocarbons. It is one of the main components of antioxidants 1010, 1076, 702, 3114, and 4426. Grafting 2,6-di-tert-butylphenol onto the surface of calcium sulfate whiskers increases the compatibility and chemical stability of the calcium sulfate whiskers with high-density polyethylene. When 2,6-di-tert-butylphenol is used as a modifier, it needs to undergo a chemical bonding reaction with a surface containing hydroxyl groups to achieve surface modification. However, anhydrous calcium sulfate whiskers have a smooth surface without hydroxyl groups, making it difficult to directly modify them with 2,6-di-tert-butylphenol. Therefore, a special process is needed to solve the bonding problem between 2,6-di-tert-butylphenol and the surface of anhydrous calcium sulfate whiskers.

[0026] Phosphate ions can undergo chemical adsorption on the surface of anhydrous calcium sulfate whiskers. Phosphorus oxychloride has high reactivity and can be used as a phosphorylation reagent to ensure a more complete reaction. The aromatic phosphate ester groups in the material give it better resistance to extraction. First, 2,6-di-tert-butylphenol is added to ethylene glycol to generate a modified antioxidant phenol. Then, the modified antioxidant phenol is used to prepare a phosphate ester antioxidant modifier via the phosphorus oxychloride method.

[0027] Phosphate ester antioxidant modifiers, with their polar phosphate groups and phenols, can combine the effects of inorganic-organic surface modification, and have significant effects on increasing the stability of powder materials, reducing surface energy, and increasing hydrophobic properties. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] Preparation method of modified antioxidant phenols:

[0031] 515g of 2,6-di-tert-butylphenol and potassium hydroxide catalyst were added to a reaction vessel, nitrogen gas was introduced, and the temperature was raised to 50°C until the material was completely melted. While stirring, the temperature was raised to 100°C, and a vacuum was drawn to remove water from the material. The temperature was then raised to 110°C, and 313.5g of ethylene glycol was slowly added. After 5.5 minutes of addition, the temperature was raised to 130°C and the reaction was carried out for 3 hours. After the reaction was completed, the temperature was lowered to 90°C, and acetic acid was added to neutralize the mixture. The mixture was then dissolved in methanol, crystallized, filtered, washed, and dried to obtain solid modified antioxidant phenol.

[0032] Example 2

[0033] Preparation method of modified antioxidant phenols:

[0034] 515g of 2,6-di-tert-butylphenol and potassium hydroxide catalyst were added to a reaction vessel, nitrogen gas was introduced, and the temperature was raised to 50°C until the material was completely melted. While stirring, the temperature was raised to 100°C, and a vacuum was drawn to remove water from the material. The temperature was then raised to 110°C, and 313.5g of ethylene glycol was slowly added. After 8.5 minutes of addition, the temperature was raised to 130°C and the reaction was carried out for 3.5 hours. After the reaction was completed, the temperature was lowered to 90°C, and acetic acid was added to neutralize the mixture. The mixture was then dissolved in methanol, crystallized, filtered, washed, and dried to obtain solid modified antioxidant phenol.

[0035] Example 3

[0036] Preparation method of modified antioxidant phenols:

[0037] 515g of 2,6-di-tert-butylphenol and potassium hydroxide catalyst were added to a reaction vessel, nitrogen gas was introduced, and the temperature was raised to 50°C until the material was completely melted. While stirring, the temperature was raised to 100°C, and a vacuum was drawn to remove water from the material. The temperature was then raised to 110°C, and 313.5g of ethylene glycol was slowly added. After 11 minutes of addition, the temperature was raised to 130°C and the reaction was carried out for 4 hours. After the reaction was completed, the temperature was lowered to 90°C, and acetic acid was added to neutralize the mixture. The mixture was then dissolved in methanol, crystallized, filtered, washed, and dried to obtain solid modified antioxidant phenol.

[0038] Example 4

[0039] Preparation method of phosphate ester antioxidant modifiers:

[0040] Add 306g of phosphorus oxychloride to the reaction vessel, and add 532g of the modified antioxidant phenol prepared in Example 1 while stirring. Keep the temperature below 18°C ​​and stir for 20 minutes. Then raise the temperature to 40°C, slowly add 36g of distilled water to the system, and then adjust to 50°C. React for 30 minutes to obtain the phosphate ester antioxidant modifier.

[0041] Example 5

[0042] Preparation method of phosphate ester antioxidant modifiers:

[0043] Add 336.6g of phosphorus oxychloride to the reaction vessel, and add 532g of the modified antioxidant phenol prepared in Example 2 while stirring. Keep the temperature below 18°C ​​and stir for 20 minutes. Then raise the temperature to 45°C, slowly add 39.6g of distilled water to the system, and then adjust to 55°C. React for 30 minutes to obtain the phosphate ester antioxidant modifier.

[0044] Example 6

[0045] Preparation method of phosphate ester antioxidant modifiers:

[0046] Add 367.2g of phosphorus oxychloride to the reaction vessel, and add 532g of the modified antioxidant phenol prepared in Example 3 while stirring. Keep the temperature below 18°C ​​and stir for 20 minutes. Then raise the temperature to 50°C, slowly add 43.2g of distilled water to the system, and then adjust to 60°C. React for 30 minutes to obtain the phosphate ester antioxidant modifier.

[0047] Example 7

[0048] Preparation method of modified calcium sulfate whiskers:

[0049] 784g of anhydrous calcium sulfate whiskers were added to 19.6L of anhydrous ethanol and heated in a water bath at 20°C. Then, an anhydrous ethanol solution containing potassium hydroxide-neutralized phosphate ester antioxidant modifier prepared in Example 4 was added. After reacting for 5 minutes, the mixture was filtered, washed three times with anhydrous ethanol, and dried at 100°C to obtain modified calcium sulfate whiskers.

[0050] Example 8

[0051] Preparation method of modified calcium sulfate whiskers:

[0052] 784g of anhydrous calcium sulfate whiskers were added to 19.6L of anhydrous ethanol and heated in a water bath at 25°C. Then, an anhydrous ethanol solution containing potassium hydroxide-neutralized phosphate ester antioxidant modifier prepared in Example 5 was added. After reacting for 5 minutes, the mixture was filtered, washed three times with anhydrous ethanol, and dried at 100°C to obtain modified calcium sulfate whiskers.

[0053] Example 9

[0054] Preparation method of modified calcium sulfate whiskers:

[0055] 784g of anhydrous calcium sulfate whiskers were added to 19.6L of anhydrous ethanol and heated in a water bath at 30°C. Then, an anhydrous ethanol solution containing potassium hydroxide-neutralized phosphate ester antioxidant modifier prepared in Example 6 was added. After reacting for 5 minutes, the mixture was filtered, washed three times with anhydrous ethanol, and dried at 100°C to obtain modified calcium sulfate whiskers.

[0056] Example 10

[0057] High-density polyethylene 70%

[0058] 30% modified calcium sulfate whiskers

[0059] The preparation process of precision liquid filter media includes the following steps:

[0060] S1: 70g of polyethylene and 30g of modified calcium sulfate whiskers prepared in Example 7 were placed together in a high-speed mixer and mixed for 5 minutes. The uniformly mixed material was then melt-blended using a twin-screw extruder to obtain a precision liquid filter material raw material.

[0061] S2: The obtained precision liquid filter material raw material is loaded into the molding mold, and the powder is pressed into shape using a DSB-30A four-column hydraulic press. The molding pressure range is 2.5MPa. When the pressure reaches the set target value, the pressure is held. After completion, the pressure is slowly released and the blank is removed from the mold.

[0062] S3: Place the demolded blank in a precision oven (HH-132) for sintering. During the sintering process, temperature is controlled: first, heat to 120℃ at a rate of 5℃ / min and hold for 10 minutes. When heating for the second time, heat to the target sintering temperature at a rate of 0.5℃ / min and hold for 45 minutes. After holding at the temperature, slowly cool to room temperature.

[0063] Example 11

[0064] High-density polyethylene 72.5%

[0065] Modified calcium sulfate whiskers 27.5%

[0066] The preparation process of precision liquid filter media includes the following steps:

[0067] S1: 72.5g of polyethylene and 27.5g of modified calcium sulfate whiskers prepared in Example 8 were placed together in a high-speed mixer and mixed for 5 minutes. The uniformly mixed material was melt-blended using a twin-screw extruder to obtain a precision liquid filter material raw material.

[0068] S2: The obtained precision liquid filter material raw material is loaded into the molding mold, and the powder is pressed into shape using a DSB-30A four-column hydraulic press. The molding pressure range is 2.5MPa. When the pressure reaches the set target value, the pressure is held. After completion, the pressure is slowly released and the blank is removed from the mold.

[0069] S3: Place the demolded blank in a precision oven (HH-132) for sintering. During the sintering process, temperature is controlled: first, heat to 120℃ at a rate of 5℃ / min and hold for 10 minutes. When heating for the second time, heat to the target sintering temperature at a rate of 0.5℃ / min and hold for 45 minutes. After holding at the temperature, slowly cool to room temperature.

[0070] Example 12

[0071] High-density polyethylene 75%

[0072] 25% modified calcium sulfate whiskers

[0073] The preparation process of precision liquid filter media includes the following steps:

[0074] S1: 75g of polyethylene and 25g of modified calcium sulfate whiskers prepared in Example 9 were placed together in a high-speed mixer and mixed for 5 minutes. The uniformly mixed material was then melt-blended using a twin-screw extruder to obtain a precision liquid filter material raw material.

[0075] S2: The obtained precision liquid filter material raw material is loaded into the molding mold, and the powder is pressed into shape using a DSB-30A four-column hydraulic press. The molding pressure range is 2.5MPa. When the pressure reaches the set target value, the pressure is held. After completion, the pressure is slowly released and the blank is removed from the mold.

[0076] S3: Place the demolded blank in a precision oven (HH-132) for sintering. During the sintering process, temperature is controlled: first, heat to 120℃ at a rate of 5℃ / min and hold for 10 minutes. When heating for the second time, heat to the target sintering temperature at a rate of 0.5℃ / min and hold for 45 minutes. After holding at the temperature, slowly cool to room temperature.

[0077] Comparative Example 1

[0078] The 25g modified calcium sulfate whiskers in Example 12 were replaced with 24.3g of calcium sulfate whiskers and 0.6g of 2,6-di-tert-butylphenol, and the rest were the same as in Example 12.

[0079] Comparative Example 2

[0080] The 25g modified calcium sulfate whiskers in Example 12 were replaced with 25g calcium sulfate whiskers, and all other aspects were the same as in Example 12.

[0081] Performance testing

[0082] (1) Mechanical property test: Impact test was carried out on the XJJ-5 impact testing machine of Hebei Chengde Experimental Machine Co., Ltd. in accordance with GB1043-1993. The results are shown in Table 1.

[0083] (2) Tensile strength test: Tensile tests were conducted on the RGD-5 electronic tensile testing machine of Shenzhen Ruigeer Co., Ltd. in accordance with GB / T1040-1992. The results are shown in Table 1.

[0084] (3) Bending strength test: Bending test was carried out on the RGD-5 electronic tensile testing machine of Shenzhen Ruigeer Co., Ltd. in accordance with GB / T9341-1988. The results are shown in Table 1.

[0085] (4) Oxygen barrier performance test: The oxygen barrier performance of the membrane before and after modification was determined by the differential pressure gas permeation apparatus VAC-V2 Jinan Langguang Electromechanical Technology Co., Ltd. according to GB / T1038-2000. The results are shown in Table 1.

[0086] (5) Thermal performance test: According to GBT27761-2011, thermogravimetric analysis was performed using a STA409PC synchronous thermal analyzer with a heating rate of 20℃ / min, N2 atmosphere, nitrogen flow rate of 40ml / min, and temperature test range of 50-600℃. The results are shown in Table 2.

[0087] Table 1:

[0088]

[0089] Table 2:

[0090]

[0091] As shown in Tables 1-2, the precision liquid filter media prepared in Examples 10-12 have good antioxidant properties, improve the mechanical properties of the original composite material, extend the service life of the filter media, and are stable in nature.

[0092] In the description of this specification, the references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0093] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A precision liquid filter media, characterized in that, The raw materials include the following weight percentages: High-density polyethylene 70%-75% Modified calcium sulfate whiskers 25%-30%; The method for preparing the modified calcium sulfate whiskers: Add 2g of anhydrous calcium sulfate whiskers to 50mL of anhydrous ethanol and heat in a water bath at 20-30℃. Then add anhydrous ethanol solution containing potassium hydroxide-neutralized phosphate antioxidant modifier. After the reaction is complete, filter, wash three times with anhydrous ethanol, and dry at 100℃ to obtain modified calcium sulfate whiskers. The preparation method of the phosphate ester antioxidant modifier: Phosphorus oxychloride was added to the reaction vessel, and modified antioxidant phenol was added while stirring. The temperature was 10-18℃ and stirred for 20 minutes. Then the temperature was raised to 40-50℃, and distilled water was slowly added to the system. The temperature was then adjusted to 50-60℃ and the reaction was carried out for a period of time to obtain phosphate ester antioxidant modifier. The molar ratio of phosphorus oxychloride to modified antioxidant phenol is 1:1-1.2:1, and the molar ratio of modified antioxidant phenol to distilled water is 10:

1. The preparation method of the modified antioxidant phenol: 2,6-Di-tert-butylphenol and potassium hydroxide catalyst were added to a reaction vessel, nitrogen gas was introduced, and the temperature was raised to 50°C until the material was completely melted. While stirring, the temperature was raised to 100°C, and a vacuum was drawn to remove water from the material. The temperature was then raised to 110°C, and ethylene glycol was slowly added. After a period of time, the temperature was raised to 130°C and the reaction was carried out for 3-4 hours. After the reaction was completed, the temperature was lowered to 90°C, acetic acid was added to neutralize, and the mixture was dissolved in methanol, crystallized, filtered, washed, and dried to obtain solid modified antioxidant phenol. The molar ratio of 2,6-di-tert-butylphenol to ethylene glycol is 1:1.1; the mass ratio of 2,6-di-tert-butylphenol to potassium hydroxide is 1:0.

01.

2. A manufacturing process for producing the precision liquid filter media as described in claim 1, characterized in that, Includes the following steps: (1) Modified calcium sulfate whiskers and high-density polyethylene are placed together in a high-speed mixer and mixed for 5 minutes. The uniformly mixed material is then melt-blended using a twin-screw extruder to obtain a precision liquid filter material raw material. (2) The obtained precision liquid filter material raw material is loaded into the molding mold, and the powder is pressed into shape by a four-column hydraulic press. When the pressure reaches the set target value, the pressure is held. After completion, the pressure is released and the blank is removed from the mold. (3) Place the demolded blank in a precision oven for sintering. During the sintering process, temperature control is carried out: first heat to 120-130℃ at a rate of 5℃ / min and keep the temperature constant for 10min. When heating for the second time, heat to the target sintering temperature at a rate of 0.5℃ / min and keep the temperature constant for 45min. After the temperature is maintained, slowly lower it to room temperature.

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