Vinylidene fluoride emulsion polymerization using poly(vinyl alcohol) as the only stabilizer.

Abstract

The present invention relates to a method for producing a stable PVDF latex by emulsion polymerization in the absence of a fluorinated surfactant, said latex being stabilized by poly(vinyl alcohol) (PVOH).

Description

[Technical Field]

[0001] The present invention relates to a method for producing a stable PVDF latex by emulsion polymerization in the absence of a fluorinated surfactant, said latex being stabilized by poly(vinyl alcohol) (PVOH). [Background technology]

[0002] Polyvinylidene fluoride (PVDF) is a fluoropolymer valued for its excellent mechanical and thermal properties as well as its chemical inertness. It is used in high-performance applications such as lithium-ion batteries, coatings, and water treatment. PVDF is synthesized by aqueous free-radical emulsion or suspension polymerization of vinylidene fluoride (VDF). The emulsion polymerization mechanism of VDF is more complex than other emulsion polymerization processes because VDF can become a gas or a supercritical fluid depending on the polymerization conditions. In VDF emulsion polymerization, the formulation contains water, VDF, surfactants, a water-soluble initiator, and optionally several additives (chain transfer agents, buffers, and antifouling agents). Polymerization is carried out in a high-pressure reactor, and VDF initially exists in both the gas and aqueous phases, as well as in the PVDF particles that form during polymerization.

[0003] To obtain a stable dispersion of PVDF particles and good polymer properties, an appropriate surfactant must be used. Therefore, various types of surfactant / stabilization techniques are used industrially and / or described in the literature for the emulsion polymerization of PVDF, such as molecular fluorinated surfactants, molecular non-fluorinated surfactants, or "surfactant-free" techniques.

[0004] Water-soluble polymers (WSPs) can be used industrially as stabilizers in emulsion polymerization. These polymers can provide good colloidal stability through a combination of adsorption onto hydrophobic surfaces and grafting onto the polymer (a chemical bond connecting the WSP to the particle).

[0005] Commonly used WSPs are hydroxyethyl cellulose (paint industry) and poly(vinyl alcohol) (PVOH) (adhesives industry), the latter of which is used in particular as a stabilizer for the emulsion (co)polymerization of vinyl acetate to produce poly(vinyl acetate)-based latexes, and which has the advantage of being readily biodegradable.

[0006] PVOH polymers are synthesized by partial hydrolysis of polyvinyl acetate (PVAc). Their colloidal and interfacial properties are influenced by their degree of hydrolysis (DH) and molecular characteristics, such as molar mass or blockiness (characterized by the arrangement of vinyl acetate units within the hydrolyzed polymer chain). PVOH with a degree of hydrolysis between 70 and 90 mol% is known to aggregate in water due to intramolecular and intermolecular hydrophobic interactions between vinyl acetate units. Its amphiphilic structure allows it to stabilize hydrophobic particles in water by adsorption and grafting. The hydrophobic vinyl acetate units provide strong adsorption to hydrophobic particles, while the hydrophilic hydroxyl units act as stabilizing sites. In addition to adsorption, grafting of PVOH onto particles also occurs, relying on chain transfer to PVOH during the polymerization of hydrophobic monomers.

[0007] The use of PVOH as a suspending agent in VDF suspension polymerization is known. In the absence of a suspending agent, reactor fouling can be observed.

[0008] US Patent Application Publication No. 2010 / 298487 teaches the preparation of PVDF by polymerization in a dispersed aqueous medium. The aqueous suspension polymerization can be carried out in the presence of PVOH as a dispersing agent. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] US Patent Application Publication No. 2010 / 298487 Summary of the Invention [Problem to be solved by the invention]

[0010] It has now been found that stable PVDF particles can be obtained by emulsion polymerization of VDF using PVOH as the sole stabilizer. [Means for solving the problem]

[0011] According to a first aspect, the present invention relates to a process for producing a stable PVDF latex by emulsion polymerization of VDF in the absence of fluorinated or low molar mass surfactants, said process comprising: i. charging a polymerization reactor with deionized water and a stabilizer; ii. optionally adding a chain transfer agent; iii. optionally adding a buffer; iv. optionally adding a paraffin antifouling agent; v. Raising the temperature to 50-120°C; vi. feeding vinylidene fluoride into the reactor until a pressure of 100 kPa to 10 MPa is reached; vii adding an initiator to initiate the polymerization; viii. continuously supplying vinylidene fluoride, and optionally additional initiator, to maintain said pressure; ix. stopping the vinylidene fluoride feed when the desired polymer loading is reached; x optionally continuing the supply of initiator to consume residual vinylidene fluoride; wherein the stabilizer is PVOH having a degree of hydrolysis in the range of 70-99 mol%, preferably 80-98 mol%.

[0012] This polymeric stabilizer results in a stable PDVF latex while avoiding the need to use any fluorinated surfactants or surfactants of low molar mass (especially having a molar mass below 500 g / mol).

[0013] The present invention makes it possible to overcome the drawbacks of the current state of the art. It particularly provides a method for emulsion polymerization of VDF, in which PVDF particles can be stabilized using PVOH (a non-fluorinated surfactant) as the sole stabilizer. The method according to the invention also makes it possible to prepare high-solids PVDF latexes (up to 60% by weight) with thermally stable non-fluorinated surfactants (decomposition temperature above 250°C). Advantageously, PVOH is used as a biodegradable non-fluorinated surfactant for the preparation of high-solids PVDF latexes. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will now be described in detail.

[0015] According to a first aspect, the present invention provides a process for emulsion polymerization of VDF to produce stable PVDF latexes using PVOH as the sole stabilizer and in the absence of fluorinated surfactants or low molar mass surfactants.

[0016] PVOH is prepared by hydrolysis of PVAc and has the following formula (I):

[0017] [ka]

[0018] The degree of hydrolysis is defined as DH (%) = (n / (n+m))*100, where n is the number of repeating vinyl alcohol units in the PVOH chain and m is the number of repeating vinyl acetate units in the PVOH chain. The ratio n / (n+m)*100 varies from 70 to 99 mol%, preferably from 80 to 98 mol%.

[0019] The Mw of the PVOH used in the present invention is greater than 500 g / mol, preferably greater than 1000 g / mol, more preferably greater than 5000 g / mol.

[0020] The term "vinylidene fluoride polymer" as used herein includes within its meaning both general solids, homopolymers, and copolymers. Such copolymers include those containing at least 50 mol% vinylidene fluoride copolymerized with at least one comonomer selected from the group consisting of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, pentafluoropropene, perfluoromethyl vinyl ether, perfluoropropyl vinyl ether, and other monomers that readily copolymerize with vinylidene fluoride. Terpolymers of vinylidene fluoride, hexafluoropropene, and tetrafluoroethylene, as well as terpolymers of vinylidene fluoride, trifluoroethylene, and tetrafluoroethylene, also represent types of vinylidene fluoride copolymers that can be prepared by the methods embodied herein.

[0021] Although the methods of the present invention are generally illustrated with reference to the polymerization of vinylidene fluoride homopolymers, one skilled in the art will recognize that similar polymerization techniques can be applied to the preparation of copolymers of vinylidene fluoride with fluorinated or non-fluorinated reactive comonomers.

[0022] These polymers are made by the process of emulsion polymerization.

[0023] Emulsion polymerization is generally systems in which emulsifiers (e.g. low molar mass surfactants) are used below their critical micelle concentration (hence polymerization in the absence of micelles), or - surfactant-free systems (surfactant-free polymerization), or - Systems in which low molar mass surfactants are replaced by either reactive or non-reactive polymers Includes.

[0024] The latter is the case for the emulsion systems used in this invention, which use PVOH as a stabilizer for the resulting polymer particles.

[0025] In the emulsion polymerization process, a reactor is charged with deionized water, PVOH and optionally a paraffin antifoulant and / or a chain transfer agent.

[0026] The mixture is stirred and deoxygenated. A predetermined amount of chain transfer agent (CTA) is then introduced into the reactor; however, a CTA may not be used in this process. The reactor temperature is raised to the desired level, and VDF is fed into the reactor. An initial charge of VDF is introduced, and once the pressure in the reactor reaches the desired level, at least one radical initiator is added to initiate and maintain the polymerization reaction. VDF, and optionally additional initiator, are continuously fed to maintain the desired pressure. The reaction temperature can be varied depending on the characteristics of the initiator used, and those skilled in the art will know how to do so. Typically, the reactor temperature is between 30°C and 120°C, preferably between 50°C and 120°C. The polymerization pressure may vary, typically within the range of 100 to 10,000 kPa. Once the desired polymer conversion is reached in the reactor, the monomer feed is stopped, but initiator is optionally added to consume any residual monomer. The residual gases (including unreacted monomers) are then vented and the latex is recovered from the reactor, after which the polymer may be separated from the latex by standard techniques such as freeze-thaw, spray drying, lyophilization, high-shear coagulation separation, etc.

[0027] The reaction can be initiated and maintained by the addition of any suitable initiator known for the polymerization of fluorinated monomers, including inorganic peroxides, "redox" combinations of oxidizing and reducing agents, and organic peroxides.

[0028] The radical initiator is added to the reaction mixture in an amount sufficient to initiate and maintain the polymerization reaction at the desired reaction rate, and the order of addition can vary depending on the desired process and latex emulsion properties.

[0029] According to one embodiment, the radical initiator is water-soluble and comprises a persulfate, such as sodium persulfate, potassium persulfate, or ammonium persulfate, and the amount of persulfate added to the reaction mixture (based on the total weight of the monomers added to the reaction mixture) is 0.005 to 1.0 wt %.

[0030] According to one embodiment, the radical initiator comprises an organic peroxide, such as alkyl, dialkyl, or diacyl peroxides, peroxydicarbonates, and peroxyesters, in an amount of 0.5 to 2.5% by weight based on the total monomers.

[0031] Chain transfer agents are optionally added to the polymerization to control the molecular weight of the product. They can be added in one portion at the beginning of the polymerization, or they can be added stepwise or continuously during the reaction. The amount and mode of addition of the chain transfer agent depend on the activity of the particular chain transfer agent employed and the desired molecular weight of the polymer product. If added, the amount of chain transfer agent is 0.05 to 5 wt. %, preferably 0.1 to 2 wt. %, based on the total weight of monomers added to the reaction mixture.

[0032] Examples of chain transfer agents useful in the present invention include, but are not limited to, oxygenated compounds such as alcohols, carbonates, ketones, esters, ethers, etc.; hydrohalocarbons such as halocarbons, chlorocarbons, hydrochlorocarbons, chlorofluorocarbons, and hydrochlorofluorocarbons; ethane, propane, etc.

[0033] The optional addition of paraffin wax or hydrocarbon oil to the reaction acts as an antifouling agent to minimize or prevent polymer buildup on reactor components. Any long-chain saturated hydrocarbon wax or oil can perform this function. The amount of oil or wax added to the reactor is an amount that serves to minimize the formation of polymer buildup on the reactor components. The amount is generally proportional to the internal surface area of ​​the reactor and can vary from 1 to 40 mg per square centimeter of internal reactor surface area. The amount of paraffin wax or hydrocarbon oil is preferably 5 mg / cm of internal reactor surface area.2 is.

[0034] Optionally, a buffer such as sodium acetate can be added to the reaction.

[0035] Advantageously, no catalysts are used in the polymerization of VDF according to the invention.

[0036] The polymerization generally results in a latex having a solids level of 10-60% by weight, preferably 10-45% by weight, and an average particle size of the latex of less than 500 nm, preferably less than 400 nm, more preferably less than 300 nm. The average particle size is generally at least 20 nm, preferably at least 50 nm.

[0037] According to one embodiment, the vinylidene fluoride polymer latex particles have an average particle size comprised between 20 and 300 nm, as measured by dynamic light scattering (DLS).

[0038] One or more other water-miscible solvents, such as ethylene glycol, may be mixed into the latex in small amounts to improve freeze-thaw stability.

[0039] The VDF-containing polymer latex may be dried to a powder by means known in the art, including but not limited to spray drying, freeze drying, coagulation drying, drum drying, etc. The dried VDF-containing polymer powder may have an average particle size of 0.5 to 200 microns, or 1 to 100 microns, or 2 to 50 microns, or 3 to 20 microns.

[0040] Latices of VDF-containing polymers are suitable for the preparation of coatings, films, membranes and binders for battery applications. [Example]

[0041] The following examples illustrate the invention without limiting it.

[0042] Examples 1-3: Emulsion polymerization of VDF in the presence of PVOH Using various experimental conditions, commercially available PVOH (Mw ≒ 31,000 gmol -1 , 86.7-88.7 mol% hydrolyzed) was used as a stabilizer. Example 1: 0.20 g potassium persulfate (KPS), 0.12 g sodium acetate, 0.06 g PVOH, 100 mL water Example 2: 0.20 g KPS, 0.12 g sodium acetate, 0.08 g PVOH, 100 mL water Example 3: 0.20 g KPS, 0.12 g sodium acetate, 0.13 g PVOH, 100 mL water

[0043] The operating conditions and the properties of these experiments and the resulting latexes are summarized in Table 1. The reactor was equipped with a stainless steel four-blade agitator with an agitation speed of 500 rpm.

[0044] [Table 1]

[0045] In all cases, stable PVDF latexes with solid contents ranging from 14.0 to 23.0% and average particle sizes ranging from 149 to 235 nm were obtained after 1 hour of reaction.

[0046] The solids content decreases with the weight ratio KPS / PVOH, i.e., as the amount of PVOH increases, which can be attributed to the chain transfer reactions that occur along the PVOH chain during polymerization causing inhibition, thus resulting in lower solids content with increasing amounts of PVOH.

[0047] Example 4: Emulsion polymerization of VDF in the presence of PVOH A commercially available PVOH (M w ≒ 130,000 gmol -1 , 86.7-88.7 mol% hydrolyzed) was used as a stabilizer.

[0048] The operating conditions and properties of these experiments and the resulting latexes are summarized in Table 2. The reactor was equipped with a stainless steel four-blade agitator with an agitation speed of 500 rpm.

[0049] [Table 2]

[0050] After 1 hour of reaction, a stable PVDF latex was obtained with equiaxed particles having a diameter of 197 nm and a solid content of 19%. The particle size and solid content after 1 hour of reaction were very similar to those in Example 2 (with the same KPS / PVOH weight ratio), indicating that the molar mass of PVOH does not significantly affect the solid content and particle size under these reaction conditions. However, the polydispersity, molar mass, and dispersity obtained in Example 4 were different from those obtained in Example 2.

[0051] Examples 5-7: Emulsion polymerization of VDF in the presence of PVOH Using various experimental conditions, commercially available PVOH (M w ≒ 13,000-23,000 gmol -1 , 86.7-88.7 mol% hydrolyzed) was used as a stabilizer. Example 5: 0.20 g KPS, 0.12 g sodium acetate, 0.13 g PVOH, 100 mL water Example 6: 0.20 g KPS, 0.12 g sodium acetate, 0.08 g PVOH, 100 mL water Example 7: 0.20 g KPS, 0.12 g sodium acetate, 0.06 g PVOH, 100 mL water

[0052] The operating conditions and properties of these experiments and the resulting latexes are summarized in Table 3. The reactor was equipped with a stainless steel four-blade agitator with an agitation speed of 500 rpm.

[0053] [Table 3]

[0054] In all cases, stable PVDF latexes were obtained after 1 hour of reaction. For the same amount of KPS, the solids content decreases with the weight ratio KPS / PVOH for the reasons explained above for Examples 1-3.

[0055] With the same mass of PVOH and the same weight ratio of KPS / PVOH, the solids content and particle size after 1 hour of reaction are very similar in Examples 2, 4, and 6, indicating that the molar mass of PVOH does not have a significant effect on the stabilization of PVDF under these reaction conditions.

[0056] Example 8: Emulsion polymerization of VDF in the presence of PVOH A commercially available PVOH (0.08 g, M) was dissolved in 0.20 g of KPS, 0.12 g of sodium acetate, and 100 mL of water. w ≒ 9000~10000gmol -1 , 80 mol % hydrolyzed) was used as a stabilizer.

[0057] The operating conditions and properties of these experiments and the resulting latexes are summarized in Table 4. The reactor was equipped with a stainless steel four-blade agitator with an agitation speed of 500 rpm.

[0058] [Table 4]

[0059] After 3 hours and 30 minutes of reaction, a stable PVDF latex was obtained with equiaxed particles of 202 nm diameter and 31.2% solids content. The particle size is very similar to that of Example 4, but the solids content is significantly higher, indicating good PVDF stabilization by Example 8.

[0060] Example 9: Emulsion polymerization of VDF in the presence of PVOH A commercially available PVOH (0.08 g, M) was dissolved in 0.20 g of KPS, 0.12 g of sodium acetate, and 100 mL of water. w ≒ 31,000~51,000gmol -1 , 98-98.8 mol% hydrolyzed) was used as a stabilizer.

[0061] The operating conditions and properties of these experiments and the resulting latexes are summarized in Table 5. The reactor was equipped with a stainless steel four-blade agitator with an agitation speed of 500 rpm.

[0062] [Table 5]

[0063] After 1 hour of reaction, the resulting product consisted mainly of coagulated material, but a liquid, homogeneous fraction of the sample could be collected and the solids content determined gravimetrically and the particle size determined by DLS.

[0064] With the same PVOH mass and weight ratio of KPS / PVOH as in Examples 2, 4, and 6, no stable latex was obtained using this PVOH, which may indicate that the stabilization of PVDF particles depends more on the degree of hydrolysis than on the molar mass of PVOH.

[0065] Comparative Example Examples 10-14: Emulsion polymerization of VDF in the presence of different stabilizers The PVDF latex obtained in the presence of PVOH was compared with that obtained using other known stabilizers (Table 6). The reactor was equipped with a stainless steel four-blade agitator at a stirring speed of 500 rpm.

[0066] Example 4, which gives particularly good results in terms of latex stabilization, was selected for comparison.

[0067] [Table 6]

[0068] (a) Reaction conditions: 35 bar, 80°C, 1 hour reaction, 0.20 g KPS, 0.12 g sodium acetate, 0.08 g stabilizer, 100 mL water (b) Reaction conditions: 35 bar, 80°C, 1 hour 30 minutes of reaction, 0.88 g of KPS, 0.0056 g of sodium acetate, 0.15 g of molecular surfactant Capstone®, 100 mL of water (c) Reaction conditions: PEG-OH(M n ≒2000gmol -1 ). The stabilizing properties of PVDF, as described in WO 2019 / 063445. (d) Reaction conditions: 35 bar, 80°C, 2 hours 30 minutes of reaction, 0.20 g of KPS, 0.12 g of sodium acetate, 0.08 g of stabilizer, 100 mL of water (e) Reaction conditions: 35 bar, 80°C, 1 hour 30 minutes of reaction, 0.20 g of KPS, 0.12 g of sodium acetate, 0.08 g of stabilizer, 100 mL of water

[0069] As shown in Table 4, after 1 hour of reaction, the solids content obtained in the presence of PVOH was significantly higher than that achievable without stabilizer (Example 10) or in the presence of PEG-OH (Example 12), and the particles were smaller in the presence of PVOH. This indicates better stabilization of PVDF particles in the presence of PVOH. Furthermore, in the absence of surfactant, no stable latex with a solids content higher than 15.3% was obtained. In the presence of PEG-OH, a stable latex with a solids content of 20.2% and an average particle size of 220 nm was obtained (Example 13), but only after 2.5 hours of reaction, and higher solids contents could not be reached without coagulation in the presence of PEG-OH. In the presence of PVOH, a latex with a solids content of 26.2% and an average particle size of 230 nm was obtained without coagulation after 1.5 hours of reaction (Example 14).

[0070] Finally, the latex obtained in the presence of PVOH (Example 4) was compared with that obtained in the presence of a molecular surfactant (FS) using a standard and reference formulation (Example 11). In the presence of the molecular surfactant, a stable latex with a solids content of 15.6% and an average particle size of 180 nm (PdI = 0.02) was obtained after 1 hour and 30 minutes of reaction. Therefore, under these reaction conditions, the use of PVOH allows a higher solids concentration to be reached in a shorter reaction time than in the presence of FS.

Claims

1. 1. A method for producing a stable poly(vinylidene fluoride) (PVDF) latex by emulsion polymerization of vinylidene fluoride (VDF) in the absence of any fluorinated or low molar mass surfactant, said method comprising: i. charging a polymerization reactor with deionized water and stabilizers; ii. optionally adding a chain transfer agent; iii. optionally adding a buffer; iv. optionally adding a paraffin antifouling agent; v. increasing the temperature to 50-120°C; vii. feeding VDF into the reactor until a pressure of 100 kPa to 10 MPa is reached; vii. adding an initiator to initiate polymerization; viii. continuously feeding VDF and optionally additional initiator to maintain said pressure; ix. stopping the VDF feed when the desired polymer loading is reached; x optionally continuing the supply of initiator to consume residual vinylidene fluoride; wherein the stabilizer is poly(vinyl alcohol) (PVOH) having a degree of hydrolysis ranging from 70 to 99 mol%, preferably from 80 to 98 mol%.

2. The poly(vinyl alcohol) stabilizer has the following formula (I): 【Chemical 1】 2. The method of claim 1, wherein n is the number of repeating vinyl alcohol units and m is the number of repeating vinyl acrylate units in the PVOH chain.

3. 3. The method of claim 1, wherein the initiator is a persulfate selected from sodium persulfate, potassium persulfate, or ammonium persulfate, and the amount of persulfate added to the reaction mixture is 0.005 to 1.0 wt. %.

4. 4. The method of claim 1, wherein the initiator is an organic peroxide selected from alkyl, dialkyl, or diacyl peroxides, peroxydicarbonates, and peroxyesters, and is used in an amount of about 0.5 to about 2.5 weight percent, based on the total weight of monomers added to the reaction mixture.

5. 5. The process according to claim 1, wherein the chain transfer agent is selected from oxygenated compounds such as alcohols, carbonates, ketones, esters and ethers; halocarbons and hydrohalocarbons such as chlorocarbons, hydrochlorocarbons, chlorofluorocarbons and hydrochlorofluorocarbons; ethane and propane, and is used in an amount of 0.05 to 5% by weight, preferably 0.1 to 2% by weight, relative to the total weight of monomers added to the reaction mixture.

6. 6. The method according to claim 1, wherein the paraffin-based antifouling agent is selected from long-chain saturated hydrocarbon waxes and oils and is used in an amount of 1 to 40 mg per square centimeter of reactor surface area.

7. The method of any one of claims 1 to 6, wherein the buffer is sodium acetate.

8. 8. The method according to any one of claims 1 to 7, wherein said latex particles of vinylidene fluoride polymer have an average particle size comprised between 20 and 300 nm.

9. A method according to any one of the preceding claims, wherein said latex particles of vinylidene fluoride polymer have a solids content of 10 to 60, preferably 15 to 45% by weight.

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