Process for preparing a vinylidene fluoride copolymer, vinylidene fluoride copolymer and ultrafiltration membrane

Abstract

The present application belongs to the technical field of fluorine chemical industry, and particularly relates to a preparation method of vinylidene fluoride copolymer, vinylidene fluoride copolymer and ultra-micro filtration membrane. The preparation method of vinylidene fluoride copolymer takes vinylidene fluoride as a first monomer and 1-chloro-1-fluoroethylene or trifluorochloroethylene as a second monomer; the first monomer and the second monomer are added into a polymerization reactor for emulsion polymerization; the emulsion polymerization adopts a composite dispersant which is compounded by at least two of dispersant a and hydroxyl phosphate or polycarboxylate, wherein the dispersant a is composed of xanthan gum and / or cellulose derivative. The preparation method adopts one-step emulsion polymerization, and the process is simple, and the polymerization reaction pressure is below 5 MPa, and the reaction condition is mild.

Description

Technical Field

[0001] This invention belongs to the technical field of fluorochemicals, specifically relating to a method for preparing vinylidene fluoride copolymer, the vinylidene fluoride copolymer, and an ultrafiltration membrane. Background Technology

[0002] Currently, polyvinylidene fluoride (PVDF) has become the mainstream material for ultrafiltration membranes. The preparation methods for PVDF ultrafiltration membranes can all adopt the general preparation methods for microfiltration membranes, with phase inversion being a commonly used method. In order to make membrane separation applications more widespread, it is generally possible to modify the membrane material in various ways. PVDF as a membrane material has the following disadvantages: (1) poor strength and low porosity; (2) after wet membrane formation, the membrane shrinkage is severe during the drying process. Therefore, it is necessary to modify it. The modification method generally involves modifying the surface of the ultrafiltration membrane, that is, improving the adhesion, hydrophilicity and biocompatibility of the microfiltration membrane surface without changing the material's bulk properties.

[0003] Currently, PVDF membrane modification methods include surface chemical modification. Existing technologies generally employ the method of introducing C-Cl chemical bond active sites into PVDF. The C-Cl chemical bonds introduced into PVDF serve as active sites for the reaction, allowing for in-situ grafting and substitution reactions with low-molecular-weight polyamine compounds. Under mild conditions, active polyamine groups can be introduced onto the surface of the ultrafiltration membrane, which can then undergo condensation reactions with low-molecular-weight polycarboxyl compounds.

[0004] The copolymerization of vinylidene fluoride (PVDF) with a chlorine-containing secondary monomer can introduce C-Cl chemical bond active sites into PVDF. Current technologies generally use trifluorochloroethylene (TCC) as the chlorine-containing secondary monomer to form PVDF-TCC copolymers. However, the types of chlorine-containing secondary monomers are currently limited, and TCC is easily liquefied. To ensure complete vaporization and homogeneous mixing of the PVDF and TCC monomers, the monomer storage tank must first be heated to completely vaporize each monomer; secondly, a diaphragm compressor must be used for circulation to ensure complete and homogeneous mixing, while simultaneously insulating all necessary pipelines to prevent liquefaction during circulation. Furthermore, the polymerization process is complex and involves high polymerization pressure.

[0005] US Patent 4946900A discloses a process for preparing a heterogeneous copolymer of vinylidene fluoride and trifluorochloroethylene. This copolymer is obtained through two-stage copolymerization in a suspension or emulsion. The first stage involves the copolymerization of vinylidene fluoride and trifluorochloroethylene; the second stage involves the polymerization of vinylidene fluoride after the introduction of trifluorochloroethylene is stopped, with a polymerization pressure of 5-10 MPa. Compared to normal emulsion polymerization processes, this preparation process has relatively complex staged polymerization control and higher pressure conditions, which is not conducive to industrial production.

[0006] Furthermore, when the molecular weight of polyvinylidene fluoride (PVDF) is low, the high content of small molecules makes PVDF highly susceptible to decomposition, leading to poor mechanical properties, such as low tensile strength and / or low elongation at break. Therefore, effectively increasing the molecular weight of PVDF can, to some extent, improve the mechanical properties of the copolymer.

[0007] Patent CN110078856A discloses a tetrafluoroethylene-modified polyvinylidene fluoride resin and its preparation method, which uses tetrafluoroethylene and vinylidene fluoride as monomer raw materials to carry out a polymerization reaction, wherein the molar ratio of tetrafluoroethylene to vinylidene fluoride is (1-60):(40-99). Although the elongation at break of polyvinylidene fluoride is improved to a certain extent, the molecular weight of the resulting polymer is still relatively small, and the tensile strength of the polyvinylidene fluoride is low, reaching a maximum of only 40 MPa.

[0008] Patent CN106336476A discloses a method for preparing a vinylidene fluoride copolymer with high elongation at break. It adopts a suspension polymerization process and uses hexafluoropropylene as the second monomer to prepare the vinylidene fluoride copolymer, which effectively improves the elongation at break of the polymer. However, the polymerization reaction of this preparation method requires a large pressure, which is 9.0-12.0 MPa.

[0009] In summary, there is an urgent need for a method for preparing vinylidene fluoride copolymers with mild reaction conditions and simple process. This method can effectively solve the problem of introducing active sites of C-Cl chemical bonds and obtain high molecular weight vinylidene fluoride copolymers, thereby giving the copolymers superior mechanical properties. Summary of the Invention

[0010] The purpose of this invention is to provide a method for preparing vinylidene fluoride copolymers. This method employs a one-step emulsion polymerization process, which is simple and operates under mild reaction conditions with a polymerization pressure below 5 MPa. Furthermore, this method increases the molecular weight of the vinylidene fluoride copolymer, thereby improving its mechanical properties to some extent, particularly tensile strength and elongation at break.

[0011] The specific technical solution is as follows:

[0012] A method for preparing a vinylidene fluoride copolymer, wherein vinylidene fluoride is used as the first monomer and 1-chloro-1-fluoroethylene or trifluorochloroethylene is used as the second monomer.

[0013] Emulsion polymerization is carried out by adding a first monomer and a second monomer to a polymerization reactor; wherein the temperature of emulsion polymerization is 55-95℃ and the pressure of emulsion polymerization is 1-5MPa.

[0014] The mass ratio of the first monomer to the second monomer is 1:0.05-0.25. When the monomer mass ratio is below this range, the copolymer performance is not significantly improved compared to the homopolymer; when the monomer mass ratio is above this range, the copolymer is prone to forming a rubbery state, which is not conducive to the production in the polymerization reactor, nor is it conducive to the dissolution of the copolymer.

[0015] The emulsion polymerization employs a composite dispersant, which is composed of at least two of dispersant a, hydroxyphosphate, or polycarboxylate, wherein dispersant a is composed of xanthan gum and / or cellulose derivatives.

[0016] The copolymerization reaction of the first monomer, vinylidene fluoride, and the second monomer, 1-chloro-1-fluoroethylene, is as follows:

[0017] In the formula, m is 4000-8000 and n is 4000-8000.

[0018] The copolymerization reaction of the first monomer, vinylidene fluoride, and the second monomer, trifluorochloroethylene, is as follows:

[0019] In the formula, m is 3000-6000 and n is 3000-6000.

[0020] The emulsion polymerization mentioned above refers to a one-step polymerization of the first monomer and the second monomer in a reaction system consisting of deionized water, pH adjuster, composite dispersant, initiator and molecular weight adjuster, under the polymerization temperature and polymerization pressure conditions described above.

[0021] In the understanding of those skilled in the art, compared to the trifluorochloroethylene monomer conventionally used for introducing C-Cl chemical bonds in existing technologies, 1-chloro-1-fluoroethylene has fewer fluorine atoms in its molecular structure, making it difficult to combine with vinylidene fluoride to form copolymers; that is, copolymerizing 1-chloro-1-fluoroethylene with vinylidene fluoride is more difficult. Furthermore, 1-chloro-1-fluoroethylene is extremely unstable and easily decomposes into hydrogen fluoride, hydrogen chloride, and carbon monoxide under the high temperature and pressure of traditional preparation processes. Therefore, existing technologies generally do not use 1-chloro-1-fluoroethylene as a comonomer.

[0022] In the preparation method described in this invention, the dispersant used in the emulsion polymerization is a composite dispersant composed of at least two of dispersant a, hydroxyphosphate or polycarboxylate. This composite dispersant can make the reaction relatively stable, and at the same time, the polymerization temperature is low and the polymerization rate is easier to control.

[0023] The preparation method utilizes the composite dispersant of the aforementioned compound system, completing the polymerization reaction in a one-step process. This method not only simplifies the process steps but also maintains the polymerization pressure below 5 MPa. The low polymerization pressure and mild reaction conditions result in a stable and easily controlled reaction. The polymerization process exhibits relatively even distribution across different time points, maintaining a steady overall reaction rate, which is beneficial for increasing the polymer's molecular weight. The resulting vinylidene fluoride copolymer has a weight-average molecular weight of 800,000-1,000,000, a tensile strength of 50-60 MPa, and an elongation at break exceeding 350%.

[0024] In this invention, the emulsion polymerization temperature in the method for preparing vinylidene fluoride copolymer is 70-90℃; the emulsion polymerization pressure is 2-3MPa.

[0025] In this invention, the mass ratio of the first monomer to the second monomer in the method for preparing the vinylidene fluoride copolymer is 1:0.1-0.25.

[0026] In this invention, the amount of composite dispersant added in the method for preparing vinylidene fluoride copolymer is 0.01%-1% of the mass of the first monomer. In the prior art, the amount of dispersant generally used is 0.1%-2% of the mass of the first monomer, while the amount of composite dispersant used in this invention is reduced by 0.09%-1.99% compared with the prior art.

[0027] In this invention, when the dispersant a is composed of xanthan gum and cellulose derivative in the method for preparing vinylidene fluoride copolymer, the mass ratio of xanthan gum to cellulose derivative is 2.5-4:1.

[0028] In this invention, when the composite dispersant contains dispersant a in the preparation method of the vinylidene fluoride copolymer, dispersant a is added when the reaction inlet flow rate of vinylidene fluoride fluctuates within 2% (i.e. after the polymerization reaction is stable), while the other components of the composite dispersant are added before the polymerization reaction occurs.

[0029] In this invention, the compounding method of the composite dispersant in the method for preparing the vinylidene fluoride copolymer is selected from one of the following A, B, C, D, or E:

[0030] A. The composite dispersant is composed of dispersant a, hydroxyphosphate and polycarboxylate, wherein dispersant a is composed of xanthan gum and cellulose derivative; the mass ratio of xanthan gum: cellulose derivative: hydroxyphosphate: polycarboxylate is 0.5-0.8:0.2:1:0.5-0.8;

[0031] B. The composite dispersant is composed of dispersant a and hydroxyphosphate, wherein dispersant a is composed of xanthan gum and cellulose derivative; the mass ratio of xanthan gum: cellulose derivative: hydroxyphosphate is 5-8:2:15-18.

[0032] C. The composite dispersant is composed of hydroxyphosphate and polycarboxylate; the mass ratio of hydroxyphosphate to polycarboxylate is 2:1-3.

[0033] D. The composite dispersant is composed of dispersant a, hydroxyphosphate and polycarboxylate, wherein dispersant a is xanthan gum; the mass ratio of xanthan gum:hydroxyphosphate:polycarboxylate is 5-40:10:5-8.

[0034] E. The composite dispersant is composed of dispersant a and hydroxyphosphate, wherein dispersant a is xanthan gum; the mass ratio of xanthan gum to hydroxyphosphate is 8:15.

[0035] In this invention, the dispersant a in the method for preparing vinylidene fluoride copolymer is composed of cellulose derivatives such as cellulose acetate or methylcellulose; the hydroxyphosphate is calcium hydroxyphosphate or sodium hydroxyphosphate; and the polycarboxylate is sodium polyaspartate or potassium polyaspartate.

[0036] The specific steps of the method for preparing the vinylidene fluoride copolymer in this invention are as follows:

[0037] (1) Add deionized water, pH adjuster and the composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0038] (2) Add the first and second monomers, and start heating and pressurizing until the temperature is 55-95℃ and the pressure is 1-5MPa;

[0039] After maintaining constant temperature and pressure for 10-15 minutes, add initiator and molecular weight regulator to carry out polymerization reaction. During the reaction, control the temperature inside the reactor at 55-95℃ and the pressure at 1-5MPa.

[0040] After the polymerization reaction has proceeded for 1-5 hours, the resulting product is washed and dried to obtain the vinylidene fluoride copolymer. The reaction ends when the amount of monomers reaches the required copolymerization ratio after 1-5 hours. The copolymerization ratio referred to here is the mass ratio of the first monomer to the second monomer in the polymerization reaction.

[0041] In the present invention, in the method for preparing vinylidene fluoride copolymer, the pH adjuster in step (1) is one or more of sodium dihydrogen phosphate, potassium dihydrogen phosphate or sodium acetate; the amount of pH adjuster is 0.02-5% of the mass of the first monomer.

[0042] The initiator in step (2) is persulfate; the amount of initiator is 0.002-0.5% of the mass of the first monomer.

[0043] The molecular weight regulator in step (2) is one or more of acetone, ethyl propionate, ethyl acetate, butyl acetate or methyl acetate; the amount of molecular weight regulator is 0.01-0.5% of the mass of the first monomer.

[0044] The drying temperature in step (3) is 60-100℃ and the drying time is 8-12h.

[0045] A polyvinylidene fluoride prepared by the above preparation method has a weight-average molecular weight of 800,000 to 1,000,000, a tensile strength of 45 to 60 MPa, and an elongation at break of 260 to 360%.

[0046] The polyvinylidene fluoride (PVDF) described herein has a particle size D50 (D50: also known as median diameter or median particle size, refers to the particle size value corresponding to a cumulative distribution percentage of 50%) of 20-30 μm; the PVDF dissolves in N,N-dimethylacetamide (DMAc) solvent in 50-60 min, and the viscosity of a 10 wt% solution obtained at 30 °C is 1000-3000 mPa·s; this viscosity range meets the requirements for membrane processing. The PVDF copolymer obtained by this invention exhibits excellent solubility; when the PVDF copolymer is stirred and dissolved in an organic solvent, no bubbles are generated, and the resulting solution is clear and transparent.

[0047] The chlorine content of the vinylidene fluoride copolymer is 10wt%-30wt%.

[0048] An ultrafiltration membrane is a hollow fiber homogeneous membrane made from the above-mentioned vinylidene fluoride copolymer by a non-solvent phase separation method (NIPS method); the membrane thickness of the ultrafiltration membrane is 0.5-1 mm; and the tensile strength is 4-10 MPa.

[0049] When the above-mentioned polyvinylidene fluoride copolymer is applied to a water treatment membrane, the tensile strength of the polyvinylidene fluoride described in this invention can reach 50-60 MPa. Therefore, the water treatment membrane prepared using this polyvinylidene fluoride has high mechanical strength and will not easily break under high pressure back shock during actual use, which greatly extends the service life of the water treatment membrane.

[0050] In existing technologies, to improve the mechanical strength of water treatment membranes, the polyvinylidene fluoride (PVDF) used in the preparation of the membranes is generally modified. This modification method requires multiple steps, a long reaction time, and high polymerization pressure, typically 7.5-9.5 MPa. In contrast, this invention requires only a one-step process, achieving high-mechanical-performance PVDF at a polymerization pressure below 5 MPa.

[0051] The beneficial effects of this invention are as follows: Using vinylidene fluoride as the first monomer and 1-chloro-1-fluoroethylene or trifluorochloroethylene as the second monomer, this invention employs a one-step polymerization method. Combined with the used composite dispersant system, this significantly increases the molecular weight of the polyvinylidene fluoride copolymer. Compared to existing technologies, the polyvinylidene fluoride copolymer prepared by the method described in this invention achieves a weight-average molecular weight of 800,000-1,000,000, an increase of 300,000-500,000. Simultaneously, the mild introduction of C-Cl chemical bond active sites makes the prepared copolymer easier to modify.

[0052] The prepared polyvinylidene fluoride exhibits excellent mechanical properties. In existing technologies, the tensile strength of polyvinylidene fluoride is generally between 30-40 MPa. Compared to existing technologies, the tensile strength of the polyvinylidene fluoride prepared in this invention is increased by 10-20%. Furthermore, the prepared polyvinylidene fluoride exhibits excellent solubility in organic solvents, with a short dissolution time, and the resulting adhesive solution has low viscosity and is clear and transparent. Attached Figure Description

[0053] Figure 1 The NMR fluorine spectrum of the vinylidene fluoride-chlorotrifluoroethylene copolymer is shown. Detailed Implementation

[0054] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0055] 1. Method for determining weight-average molecular weight: The weight-average molecular weight (Mw) of the copolymer was determined by GPC method.

[0056] 2. Methods for determining tensile strength and elongation at break: Determined according to GB / T 1040-2006 standard.

[0057] 3. The preparation method of the ultrafiltration membrane described in each embodiment and comparative example is as follows: The obtained vinylidene fluoride copolymer is mixed with polyvinylpyrrolidone (porogen) and N,N-dimethylacetamide (solvent), and stirred to obtain a casting solution. After the casting solution reaches a homogeneous and stable state, degassing treatment is performed until there are no bubbles in the casting solution. Then, a hollow fiber membrane spinning machine is used for spinning to obtain a hollow fiber ultrafiltration membrane with a thickness of 400 μm.

[0058] Example 1

[0059] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0060] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant consists of the following: 0.10 g of dispersant a, 0.10 g of hydroxycalcium phosphate and 0.05 g of sodium polyaspartate; dispersant a is composed of 0.08 g of xanthan gum and 0.02 g of cellulose acetate.

[0061] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of initiator, ammonium persulfate, and 0.25g of molecular weight regulator, ethyl acetate, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0062] Figure 1 In the spectrum, the peaks at 93 and 118-121 represent copolymer peaks. The presence of peaks at both locations in this spectrum proves that the product is a copolymer of vinylidene fluoride and trifluorochloroethylene.

[0063] Example 2

[0064] First, the composite dispersant in Example 2 has the same composition as the composite dispersant in Example 1.

[0065] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0066] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate, and 0.10g of hydroxycalcium phosphate and 0.05g of sodium polyaspartate from the composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0067] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure for 10min, add dispersant a (composed of 0.08g xanthan gum and 0.02g cellulose acetate), 0.06g of initiator potassium persulfate, and 0.25g of molecular weight regulator ethyl acetate to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has been going on for 2h, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9h to obtain the vinylidene fluoride copolymer.

[0068] Example 3

[0069] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0070] (1) Add deionized water, 0.40g of pH adjuster sodium dihydrogen phosphate and 0.25g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm. The composite dispersant consists of 0.10g of dispersant a and 0.15g of hydroxyapatite; dispersant a is composed of 0.08g of xanthan gum and 0.02g of methylcellulose.

[0071] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 70℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.10g of the initiator, ammonium persulfate, and 0.15g of the molecular weight regulator, acetone, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 70±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 3 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 80℃ for 8 hours to obtain the vinylidene fluoride copolymer.

[0072] Example 4

[0073] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0074] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant consists of the following components: 0.10 g of hydroxycalcium phosphate and 0.15 g of sodium polyaspartate.

[0075] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of initiator, ammonium persulfate, and 0.20g of molecular weight regulator, methyl acetate, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 60℃ for 12 hours to obtain the vinylidene fluoride copolymer.

[0076] Example 5

[0077] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0078] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant is composed of the following: 0.07 g of dispersant a, 0.10 g of hydroxycalcium phosphate and 0.08 g of sodium polyaspartate; dispersant a is composed of 0.05 g of xanthan gum and 0.02 g of cellulose acetate.

[0079] (2) Add 85g of the first monomer, vinylidene fluoride, and 15g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 80℃ and the pressure reaches 3.0MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of potassium persulfate initiator and 0.15g of ethyl acetate molecular weight regulator to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 80±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 3 hours, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0080] Example 6

[0081] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0082] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant is composed of the following: 0.07 g of dispersant a, 0.10 g of hydroxycalcium phosphate and 0.08 g of sodium polyaspartate; dispersant a is composed of 0.05 g of xanthan gum and 0.02 g of cellulose acetate.

[0083] (2) Add 88g of the first monomer, vinylidene fluoride, and 12g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 80℃ and the pressure reaches 2.0MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.15g of potassium persulfate initiator and 0.25g of ethyl acetate molecular weight regulator to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 80±0.5℃ and the pressure at 2.0±0.05MPa. After the polymerization reaction has proceeded for 4 hours, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0084] Example 7

[0085] First, the composite dispersant in this embodiment has the same composition as the composite dispersant in Example 1.

[0086] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0087] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate, and 0.10g of hydroxycalcium phosphate and 0.05g of sodium polyaspartate from the composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0088] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, tri-1-chloro-1-fluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at constant levels for 10min, add dispersant a (composed of 0.08g xanthan gum and 0.02g cellulose acetate), 0.042g of initiator ammonium persulfate, and 0.25g of molecular weight regulator ethyl acetate from the composite dispersant to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2h, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9h to obtain the vinylidene fluoride copolymer.

[0089] Example 8

[0090] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0091] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant is composed of the following: 0.07 g of dispersant a, 0.10 g of hydroxycalcium phosphate and 0.08 g of sodium polyaspartate; dispersant a is composed of 0.05 g of xanthan gum and 0.02 g of cellulose acetate.

[0092] (2) Add 85g of the first monomer, vinylidene fluoride, and 15g of the second monomer, 1-chloro-1-fluoroethylene. Start heating and pressurizing until the temperature reaches 80℃ and the pressure reaches 3.0MPa. After maintaining the temperature and pressure at constant levels for 10min, add 0.042g of potassium persulfate initiator and 0.15g of ethyl acetate molecular weight regulator to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 80±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 3h, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9h to obtain the vinylidene fluoride copolymer.

[0093] Example 9

[0094] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0095] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant is composed of the following: 0.07 g of dispersant a, 0.10 g of hydroxycalcium phosphate and 0.08 g of sodium polyaspartate; dispersant a is composed of 0.05 g of xanthan gum and 0.02 g of cellulose acetate.

[0096] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of initiator, ammonium persulfate, and 0.25g of molecular weight regulator, ethyl acetate, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0097] Example 10

[0098] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0099] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and 0.25g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm. The composite dispersant consists of the following components: 0.10g of dispersant a xanthan gum, 0.10g of hydroxycalcium phosphate and 0.05g of sodium polyaspartate.

[0100] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of initiator, ammonium persulfate, and 0.25g of molecular weight regulator, ethyl acetate, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0101] Example 11

[0102] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0103] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and 0.23g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm. The composite dispersant consists of the following: 0.08g of dispersant a xanthan gum and 0.15g of hydroxyapatite.

[0104] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of initiator, ammonium persulfate, and 0.25g of molecular weight regulator, ethyl acetate, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0105] Comparative Example 1

[0106] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0107] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and 0.15g of dispersant xanthan gum to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0108] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at constant for 10min, add 0.10g of dispersant xanthan gum, 0.042g of initiator ammonium persulfate, and 0.25g of molecular weight regulator ethyl acetate to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has been carried out for 4h, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9h to obtain the vinylidene fluoride copolymer.

[0109] Comparative Example 2

[0110] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0111] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and 0.25g of dispersant calcium hydroxyphosphate to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0112] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.06g of potassium persulfate initiator and 0.25g of ethyl acetate molecular weight regulator to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2 hours, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0113] Comparative Example 3

[0114] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0115] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and 0.25g of dispersant sodium polyaspartate to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0116] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.042g of the initiator, ammonium persulfate, and 0.25g of the molecular weight regulator, methyl acetate, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 3 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0117] Comparative Example 4

[0118] First, the composite dispersant in this comparative example has the same composition as the composite dispersant in Example 1.

[0119] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0120] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and dispersant a composed of 0.08g xanthan gum and 0.02g cellulose acetate in the composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm.

[0121] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at constant levels for 10min, add 0.10g of calcium hydroxyphosphate, 0.05g of sodium polyaspartate, 0.06g of potassium persulfate initiator, and 0.25g of ethyl acetate molecular weight regulator from the composite dispersant to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 2h, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9h to obtain the vinylidene fluoride copolymer.

[0122] Comparative Example 5

[0123] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0124] (1) Add deionized water, 0.40g of pH adjuster sodium dihydrogen phosphate and 0.25g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm. The composite dispersant consists of 0.10g of dispersant a and 0.15g of hydroxyapatite; dispersant a is composed of 0.08g of xanthan gum and 0.02g of methylcellulose ether.

[0125] (2) Add 80g of the first monomer, vinylidene fluoride, and 20g of the second monomer, chlorotrifluoroethylene. Start heating and pressurizing until the temperature reaches 70℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at this level for 10 minutes, add 0.10g of the initiator, ammonium persulfate, and 0.15g of the molecular weight regulator, acetone, to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 70±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has proceeded for 3 hours, stop the reaction, release the pressure, wash the resulting product, and dry it at 80℃ for 8 hours to obtain the vinylidene fluoride copolymer.

[0126] Comparative Example 6

[0127] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0128] (1) Add deionized water, 0.40g of pH adjuster potassium dihydrogen phosphate and 0.25g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm. The composite dispersant consists of 0.10g methylcellulose ether and 0.15g calcium phosphate.

[0129] (2) Add 85g of the first monomer, vinylidene fluoride, and 15g of the second monomer, 1-chloro-1-fluoroethylene. Start heating and pressurizing until the temperature reaches 95℃ and the pressure reaches 8.0MPa. After maintaining the temperature and pressure at constant levels for 10min, add 0.042g of potassium persulfate initiator and 0.15g of ethyl acetate molecular weight regulator to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 950±0.5℃ and the pressure at 8±0.05MPa. After the polymerization reaction has proceeded for 3h, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9h to obtain the vinylidene fluoride copolymer.

[0130] Comparative Example 7

[0131] The specific steps of the method for preparing the vinylidene fluoride copolymer are as follows:

[0132] (1) Add deionized water, 0.40 g of pH adjuster potassium dihydrogen phosphate and 0.25 g of composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20 ppm. The composite dispersant consists of the following: 0.10 g of dispersant a, 0.10 g of hydroxycalcium phosphate and 0.05 g of sodium polyaspartate; dispersant a is composed of 0.08 g of xanthan gum and 0.02 g of cellulose acetate.

[0133] (2) Add 50.5g of the first monomer, vinylidene fluoride, and 49.5g of the second monomer, chlorotrifluoroethylene (the mass ratio of the first monomer to the second monomer is 1:0.98). Start heating and pressurizing until the temperature reaches 90℃ and the pressure reaches 2.55MPa. After maintaining the temperature and pressure at constant for 10 minutes, add 0.042g of initiator ammonium persulfate and 0.25g of molecular weight regulator ethyl acetate to carry out the polymerization reaction. During the reaction, control the temperature inside the reactor at 90±0.5℃ and the pressure at 2.55±0.05MPa. After the polymerization reaction has been carried out for 2 hours, stop the reaction, release the pressure, wash the obtained product, and dry it at 70℃ for 9 hours to obtain the vinylidene fluoride copolymer.

[0134] The relevant performance tests were performed on the vinylidene fluoride copolymers obtained in Examples 1-11 and Comparative Examples 1-7, as detailed in Table 1.

[0135] Table 1. Performance comparison of the vinylidene fluoride copolymers obtained in each embodiment and the comparative example.

[0136]

[0137] Table 2 Comparison of the performance of ultrafiltration membranes obtained from each embodiment and comparative example.

[0138]

[0139] As can be seen from the above data, compared with the prior art, the tensile strength of the polyvinylidene fluoride of the present invention is increased by 10-20%.

Claims

1. A method for preparing a vinylidene fluoride copolymer, characterized in that, Vinylidene fluoride is used as the first monomer, and 1-chloro-1-fluoroethylene or trifluorochloroethylene is used as the second monomer; Emulsion polymerization was carried out by adding a first monomer and a second monomer to a polymerization reactor. The emulsion polymerization temperature is 55-95℃, and the emulsion polymerization pressure is 1-5MPa; The mass ratio of the first monomer to the second monomer is 1:0.05-0.25; The emulsion polymerization uses a composite dispersant; The compounding method of the composite dispersant is selected from one of the following A, B, C, D or E: A. The composite dispersant is composed of dispersant a, hydroxyphosphate and polycarboxylate, wherein dispersant a is composed of xanthan gum and cellulose derivative; The mass ratio of xanthan gum, cellulose derivative, hydroxyphosphate, and polycarboxylate is 0.5-0.8:0.2:1:0.5-0.

8. B. The composite dispersant is composed of dispersant a and hydroxyphosphate, wherein dispersant a is composed of xanthan gum and cellulose derivatives; The mass ratio of xanthan gum, cellulose derivative, and hydroxyphosphate is 5-8:2:15-18. C. The composite dispersant is composed of hydroxyphosphate and polycarboxylate; The mass ratio of the hydroxyphosphate to the polycarboxylate is 2:1-3; D. The composite dispersant is composed of dispersant a, hydroxyphosphate and polycarboxylate, wherein dispersant a is xanthan gum; The mass ratio of xanthan gum, hydroxyphosphate, and polycarboxylate is 5-40:10:5-8. E. The composite dispersant is composed of dispersant a and hydroxyphosphate, wherein dispersant a is xanthan gum; The mass ratio of xanthan gum to hydroxyphosphate is 8:15; The component of dispersant a is a cellulose derivative, which is either cellulose acetate or methylcellulose. The hydroxyphosphate is calcium hydroxyphosphate or sodium hydroxyphosphate; The polycarboxylate is sodium polyaspartate or potassium polyaspartate. When the composite dispersant contains dispersant a, dispersant a is added when the reaction inlet flow rate of vinylidene fluoride fluctuates within 2%; the other components of the composite dispersant are added before the polymerization reaction occurs.

2. A method for preparing a vinylidene fluoride copolymer, characterized in that, Vinylidene fluoride is used as the first monomer, and 1-chloro-1-fluoroethylene or trifluorochloroethylene is used as the second monomer; Emulsion polymerization was carried out by adding a first monomer and a second monomer to a polymerization reactor. The emulsion polymerization temperature is 55-95℃, and the emulsion polymerization pressure is 1-5MPa; The mass ratio of the first monomer to the second monomer is 1:0.05-0.25; The emulsion polymerization uses a composite dispersant; The compounding method of the composite dispersant is selected from one of the following A, B, C, D or E: A. The composite dispersant is composed of dispersant a, hydroxyphosphate and polycarboxylate, wherein dispersant a is composed of xanthan gum and cellulose derivative; The mass ratio of xanthan gum, cellulose derivative, hydroxyphosphate, and polycarboxylate is 0.5-0.8:0.2:1:0.5-0.

8. B. The composite dispersant is composed of dispersant a and hydroxyphosphate, wherein dispersant a is composed of xanthan gum and cellulose derivatives; The mass ratio of xanthan gum, cellulose derivative, and hydroxyphosphate is 5-8:2:15-18. C. The composite dispersant is composed of hydroxyphosphate and polycarboxylate; The mass ratio of the hydroxyphosphate to the polycarboxylate is 2:1-3; D. The composite dispersant is composed of dispersant a, hydroxyphosphate and polycarboxylate, wherein dispersant a is xanthan gum; The mass ratio of xanthan gum, hydroxyphosphate, and polycarboxylate is 5-40:10:5-8. E. The composite dispersant is composed of dispersant a and hydroxyphosphate, wherein dispersant a is xanthan gum; The mass ratio of xanthan gum to hydroxyphosphate is 8:15; The component of dispersant a is a cellulose derivative, which is either cellulose acetate or methylcellulose. The hydroxyphosphate is calcium hydroxyphosphate or sodium hydroxyphosphate; The polycarboxylate is sodium polyaspartate or potassium polyaspartate. The specific steps of the preparation method are as follows: (1) Add deionized water, pH adjuster and the composite dispersant to the polymerization reactor, and evacuate until the oxygen content is ≤20ppm; (2) Add the first and second monomers, and start heating and pressurizing until the temperature inside the reactor is 55-95℃ and the pressure is 1-5MPa; After maintaining constant temperature and pressure for 10-15 minutes, add initiator and molecular weight regulator to carry out polymerization reaction. During the reaction, control the temperature inside the reactor at 55-95℃ and the pressure at 1-5MPa. After the polymerization reaction has proceeded for 1-5 hours, the resulting product is washed and dried to obtain the vinylidene fluoride copolymer.

3. The method for preparing vinylidene fluoride copolymer according to claim 1 or 2, characterized in that, The emulsion polymerization temperature is 70-90℃; the emulsion polymerization pressure is 2-3MPa.

4. The method for preparing vinylidene fluoride copolymer according to claim 1 or 2, characterized in that, The mass ratio of the first monomer to the second monomer is 1:0.1-0.

25.

5. The method for preparing vinylidene fluoride copolymer according to claim 1 or 2, characterized in that, The amount of the composite dispersant added is 0.01%-1% of the mass of the first monomer.

6. The method for preparing vinylidene fluoride copolymer according to claim 1 or 2, characterized in that, When the dispersant a is composed of xanthan gum and cellulose derivative, the mass ratio of xanthan gum to cellulose derivative is 2.5-4:

1.

7. The method for preparing vinylidene fluoride copolymer according to claim 2, characterized in that, In step (1), the pH adjuster is one or more of sodium dihydrogen phosphate, potassium dihydrogen phosphate, or sodium acetate; the amount of pH adjuster used is 0.02-5% of the mass of the first monomer. The initiator in step (2) is persulfate; the amount of initiator used is 0.002-0.5% of the mass of the first monomer; The molecular weight regulator in step (2) is one or more of acetone, ethyl propionate, ethyl acetate, butyl acetate, or methyl acetate; the amount of molecular weight regulator used is 0.01-0.5% of the mass of the first monomer. The drying temperature in step (3) is 60-100℃ and the drying time is 8-12h.

8. The vinylidene fluoride copolymer prepared by the method described in claim 1 or 2, characterized in that, The copolymer has a weight-average molecular weight of 800,000-1,000,000, a tensile strength of 45-60 MPa, and an elongation at break of 260-360%. The particle size D50 of the vinylidene fluoride copolymer is 20-30 μm; the dissolution time of the vinylidene fluoride copolymer in N,N-dimethylacetamide solvent is 50-60 min, and the viscosity of the 10 wt% adhesive solution obtained by dissolution at 30 °C is 1000-3000 mPa·s. The chlorine content of the vinylidene fluoride copolymer is 10wt%-30wt%.

9. An ultrafiltration membrane, characterized in that, The membrane is a hollow fiber homogeneous membrane prepared by a non-solvent phase separation method using a vinylidene fluoride copolymer obtained by the preparation method described in claim 1 or 2; the membrane thickness of the ultrafiltration membrane is 0.5-1 mm; and the tensile strength is 4-10 MPa.

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