Polyvinylidene fluoride resin with wide-bimodal molecular weight distribution as well as emulsion synthesis method and application of polyvinylidene fluoride resin

Abstract

The invention provides polyvinylidene fluoride resin with wide-bimodal molecular weight distribution as well as a preparation method and application of the polyvinylidene fluoride resin. The method comprises the following steps: carrying out polymerization reaction on polyvinylidene fluoride under the condition of using a compound initiator, at least one chain transfer agent and at least one emulsifier, the compound initiator is a combination of two or more medium and low temperature initiators, or a combination of at least one high temperature initiator and at least one medium and low temperature initiator, the decomposition temperature of the high temperature initiator is higher than that of the medium and low temperature initiators, and the half-life period t1 / 2 of the high temperature initiator at the same temperature is longer. The polyvinylidene fluoride resin disclosed by the invention has wide / bimodal distribution, relatively high molecular weight and relatively wide molecular weight distribution, not only has excellent mechanical properties, but also has good processability, relatively wide use temperature range, good corrosion resistance and permeation resistance, and excellent heat resistance and ageing resistance; the method is suitable for preparing the lining layer of the marine flexible riser.

Description

Technical Field

[0001] The present application relates to the field of polymers, and more specifically to a method for synthesizing polyvinylidene fluoride resin with a broad-bimodal molecular weight distribution, polyvinylidene fluoride with excellent performance synthesized by the method, and its application in flexible risers for use in marine environments. Background Art

[0002] The oil and gas reserves in the ocean are extremely rich worldwide, and the production of marine oil and gas is steadily increasing year by year, accounting for an increasing proportion. As the main structural form connecting the underwater production system and deepwater floating facilities, compared with steel sea pipes, marine flexible risers are laid in actual complex and harsh marine environments, with short construction periods, recyclability, and low engineering costs. They are one of the important facilities for deepwater oil and gas field development projects and play a vital role in deep-sea oil and gas exploitation. With the increase in demand for marine oil and gas resources, deep-sea oil and gas development, exploration and production activities are increasing, and the industry's demand for flexible risers suitable for use in marine environments is also continuing to grow. As of 2017, the global use of marine oil and gas hoses has accumulated 15,341 km, of which 57.4% is used for deepwater oil and gas field development with a water depth of more than 300 meters. At present, with the improvement of domestic deep-sea oil and gas exploitation technology, the annual demand for marine flexible risers is about 2,000 kilometers or more. Among them, there are strict requirements on the selection of materials for the inner lining of marine flexible risers. It is not only required to have a wide operating temperature, good corrosion resistance and permeability resistance, but also high heat resistance and anti-aging performance.

[0003] Among many engineering plastics, polyvinylidene fluoride (PVDF) has good chemical stability and aging resistance, and is more suitable for such flexible pipe linings. For example, for a 12-inch offshore flexible riser, an average of 20 kg of PVDF is required per meter, 40,000 tons of PVDF are required each year, and the market value exceeds 20 billion. However, there are still many serious problems with the PVDF materials in the prior art, such as poor heat resistance, poor environmental stress cracking resistance, and processing performance and mechanical properties that cannot simultaneously meet the use requirements of the lining materials for offshore flexible risers. People are eager to develop PVDF resins with further improved performance. One idea is to prepare PDVF resins with a wide-bimodal molecular weight distribution. The so-called "wide-bimodal molecular weight distribution" means that the PVDF resin has a wide molecular weight distribution (PDI), and has both an ultra-high molecular weight peak and a medium molecular weight peak, wherein the ultra-high molecular weight wide PVDF resin peak provides it with excellent mechanical properties, and the medium molecular weight wide PVDF resin provides excellent processing properties.

[0004] The existing technical processes for achieving such molecular weight distribution mainly include physical methods and chemical methods. The physical method refers to obtaining PVDF resin with the above molecular weight distribution by physically blending several batches of products with different molecular weights and molecular weight distributions. However, this method is prone to uneven mixing, obvious differences between different batches, and poor stability.

[0005] The chemical method is to specially design the reagents, steps and processes used in the polymerization reaction to make the PVDF product have the desired molecular weight distribution. However, so far, there are very few reports on such chemical methods, and none of them can achieve the expected molecular weight distribution and expected purpose. The common problems of the chemical methods reported in the prior art are: low molecular weight, which cannot meet the molecular weight requirements for marine use; poor mechanical properties, weather resistance or processing properties, and poor durability when used in marine environments.

[0006] The art particularly hopes to develop a novel polymerization reaction method that can effectively synthesize an ultra-high molecular weight polyvinylidene fluoride resin that has both a wide distribution and a double peak. This PVDF resin has excellent mechanical properties, processing properties, chemical corrosion resistance, heat resistance and weather resistance, and meets the requirements for flexible risers used in harsh marine environments in marine oil and gas exploration and transportation applications.

[0007] In view of the problems existing in the above-mentioned prior art, the inventors of the present application, through extensive and in-depth research, unexpectedly developed a method for synthesizing ultra-high molecular weight polyvinylidene fluoride with a wide-bimodal molecular weight distribution, thus solving a long-standing problem in the prior art that has been urgently needed to be solved. Summary of the invention

[0008] The first aspect of the present application provides a method for preparing polyvinylidene fluoride resin by emulsion polymerization, the method comprising: polymerizing vinylidene fluoride under the conditions of using a composite initiator, at least one chain transfer agent, and at least one emulsifier;

[0009] The composite initiator is a combination of two or more medium and low temperature initiators, or a combination of at least one high temperature initiator and at least one medium and low temperature initiator, wherein the decomposition temperature of the high temperature initiator is higher than the decomposition temperature of the medium and low temperature initiator.

[0010] According to one embodiment of the first aspect of the present application, the high-temperature initiator is selected from one or more of the following: cyclohexanone peroxide, tert-butyl peracetate, tert-butyl perbenzoate, 4,4-bis(tert-butyl peroxy) valerate, di-tert-butyl diperoxyisophthalate, methyl ethyl ketone peroxide, di-tert-butyl peroxide, di-tert-amyl peroxide, isopropylbenzene hydroperoxide, diisopropylbenzene peroxide, tert-butyl isopropylbenzene peroxide, 1,1,3,3-tetramethyl hydroperoxide, tert-butyl peroxide, tert-butyl peroxide, tert-butyl peroxide, tert-butyl peroxide, 1,1,3,3-tetramethyl hydroperoxide, tert-butyl alcohol peroxide, tert-butyl peroxylaurate, and tert-butyl peroxy-3,5,5-trimethylhexanoate.

[0011] According to another embodiment of the first aspect of the present application, the medium and low temperature initiator is selected from one or more of the following: ammonium persulfate, potassium persulfate, sodium persulfate, isobutyl peroxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, tert-butyl peroxyneodecanoate, benzoyl peroxide, tert-butyl peroxyisobutyrate, tert-butyl peroxypivalate, succinic acid peroxide, acetyl peroxide, octanoyl peroxide, decanoyl peroxide, and lauroyl peroxide.

[0012] According to another embodiment of the first aspect of the present application, the weight ratio of the high temperature initiator to the medium and low temperature initiator is 50:1 to 1:10.

[0013] According to another embodiment of the first aspect of the present application, based on 100 wt % of the weight of vinylidene fluoride, the amount of the composite initiator is 0.01-1 wt %.

[0014] According to another embodiment of the first aspect of the present application, the high-temperature initiator is added to the polymerization reaction system at one time at the beginning of the reaction; or a portion of the high-temperature initiator is added at the beginning of the reaction, and the remaining high-temperature initiator is added to the polymerization reaction system in an intermittent or continuous manner during the polymerization reaction.

[0015] According to another embodiment of the first aspect of the present application, a portion of the medium and low temperature initiator is added to the polymerization reaction system at the beginning of the reaction, and the remaining medium and low temperature initiator is added to the polymerization reaction system intermittently or continuously during the polymerization reaction.

[0016] According to another embodiment of the first aspect of the present application, the chain transfer agent is a liquid phase chain transfer agent. According to another embodiment of the first aspect of the present application, the chain transfer agent is a combination of a liquid phase chain transfer agent and a gas phase chain transfer agent.

[0017] According to another embodiment of the first aspect of the present application, the liquid phase chain transfer is selected from one or more of the following: methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, diethyl carbonate, dibutyl carbonate, n-butyl mercaptan, tert-butyl mercaptan, isopropanol, methanol, ethanol, n-butanol, acetone, chloroform, and cyclohexane.

[0018] According to another embodiment of the first aspect of the present application, the gas phase chain transfer agent is selected from one or more of the following: methane, ethane, propane, butane, and hydrogen.

[0019] According to another embodiment of the first aspect of the present application, when the chain transfer agent is a combination of a liquid phase chain transfer agent and a gas phase chain transfer agent, a weight ratio of the liquid phase chain transfer agent to the gas phase chain transfer agent is 100:1 to 1:1.

[0020] According to another embodiment of the first aspect of the present application, based on 100 wt % of the weight of vinylidene fluoride, the amount of the chain transfer agent is 0.01-2 wt %.

[0021] According to another embodiment of the first aspect of the present application, the emulsifier is selected from one or more of the following: perfluorohexyl acetic acid or its salt, perfluorohexylphenyl acetic acid or its salt, 4,4,5,5,6,6,6-heptafluoro-3,3-bis(trifluoromethyl)hexanoic acid or its salt, 4,4,5,5,6,6,6-heptafluoro-3,3-bis(trifluoromethyl)phenylhexanoic acid or its salt, 4-(2,2,3,3,5,5,5-heptafluoro-4,4-bis(trifluoromethyl)pentyl)benzoic acid Or its salt, perfluorodioxaheptacarbonic acid or its salt, perfluorotrioxaheptacarbonic acid or its salt, perfluorodioxaoctacarbonic acid or its salt, branched perfluoropolyether, linear perfluoropolyether, sodium dodecyl sulfate, sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium dodecyl diphenyl ether disulfonic acid salt, sodium hexadecyl sulfonate, sodium hexadecyl sulfate, sodium hexadecyl sulfate, sodium hexadecyl sulfate, sodium octyl sulfonate, sodium octyl sulfate, polypropylene glycol (Mn400-2000), polyethylene glycol (Mn400-2000), isomeric tridecyl polyoxyethylene ether, octyl polyoxyethylene ether, dodecyl polyoxyethylene ether.

[0022] According to another embodiment of the first aspect of the present application, based on 100 weight % of the weight of vinylidene fluoride, the amount of the emulsifier used is 0.1-3 weight %.

[0023] According to another embodiment of the first aspect of the present application, the polymerization reaction further uses a stabilizer, and the stabilizer is paraffin or a long-chain alkane. According to another embodiment of the first aspect of the present application, based on the weight of the vinylidene fluoride as 100 weight%, the amount of the stabilizer is 0-5 weight%.

[0024] According to another embodiment of the first aspect of the present application, the polymerization reaction temperature is 50-130° C., and the polymerization reaction pressure is 2.7-5.0 MPa.

[0025] The second aspect of the present application provides a polyvinylidene fluoride having a weight average molecular weight of 200,000 to 3 million, a PDI of 3-15, and a molecular weight distribution including a first peak of 800,000 to 3 million and a second peak of 200,000 to 800,000. According to an embodiment of the second aspect of the present application, the polyvinylidene fluoride has a melting point of 155-168°C and a melt index of 1-5 g / 10 minutes.

[0026] According to another embodiment of the second aspect of the present application, the polyvinylidene fluoride is prepared by the method of the present invention.

[0027] A third aspect of the present application provides a dynamic flexible riser for use in a marine environment, at least a portion of the riser is made of the polyvinylidene fluoride resin of the present application.

[0028] In the following specific implementation section, the method, fluorocarbon resin and application of the present application are further introduced in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Shows a gel permeation chromatography graph of PVDF prepared in one embodiment of the present application;

[0030] Figure 2 A polymerization reactor used to prepare PVDF in one embodiment of the present application is shown. DETAILED DESCRIPTION

[0031] "Range" disclosed herein is in the form of lower limit and upper limit. It can be one or more lower limits, and one or more upper limits respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower limit and upper limit define the boundaries of a particular range. All ranges that can be defined in this way are inclusive and combinable, i.e. any lower limit can be combined with any upper limit to form a range. For example, a range of 60-120 and 80-110 is listed for a particular parameter, and it is understood that a range of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.

[0032] In this application, unless otherwise specified, the numerical range "ab" represents an abbreviation of any real number combination between a and b, where a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations.

[0033] In this application, unless otherwise specified, all embodiments and preferred embodiments mentioned herein can be combined with each other to form a new technical solution.

[0034] In this application, unless otherwise specified, all technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.

[0035] In this application, if there is no special explanation, the "including" mentioned herein means open or closed. For example, the "including" may mean that other components not listed may also be included, or only the listed components may be included.

[0036] One of the inventive points of the present invention is that, by designing the initiator and the chain transfer agent during the polymerization reaction of the vinylidene fluoride monomer, the polyvinylidene fluoride obtained by the polymerization reaction has the desired molecular weight, molecular weight distribution and various properties.

[0037] The method of the present invention comprises the steps of making vinylidene fluoride undergo emulsion polymerization reaction under the conditions of using a composite initiator, at least one chain transfer agent and at least one emulsifier.

[0038] According to one embodiment of the present application, the composite initiator includes at least one high-temperature initiator and at least one medium- and low-temperature initiator.

[0039] According to another embodiment of the present application, the composite initiator is a combination of two or more medium and low temperature initiators.

[0040] Under the same half-life, the decomposition temperature of the high temperature initiator is higher than the decomposition temperature of the medium and low temperature initiator. According to an exemplary embodiment of the present application, the high temperature initiator usually needs to be decomposed at a higher temperature (for example, at a temperature higher than 100°C) to produce enough active groups (activation), while the low temperature initiator usually decomposes at a relatively low temperature (for example, below 80°C, or below 60°C) to produce enough active groups (activation). For example, potassium persulfate will decompose at a temperature of 50-60°C to produce active free radicals, and more active free radicals will be produced at a temperature of 70-80°C. As the temperature rises, the more free radicals the initiator produces, but the temperature is too high, and the concentration of the active free radicals produced is too high, which will be quenched instantly, but it will not be able to trigger the monomer to polymerize. Therefore, for different high temperature initiators or medium temperature initiators, the corresponding suitable initiation temperatures are different, and the speed of free radical generation and the speed of quenching need to be comprehensively considered.

[0041] According to an exemplary embodiment of the present application, the high temperature initiator is selected from one or more of the following: cyclohexanone peroxide, tert-butyl peracetate, tert-butyl perbenzoate, 4,4-bis(tert-butyl peroxy) valerate, di-tert-butyl diperoxyisophthalate, methyl ethyl ketone peroxide, di-tert-butyl peroxide, di-tert-amyl peroxide, isopropylbenzene hydroperoxide, diisopropylbenzene peroxide, tert-butyl isopropylbenzene peroxide, 1,1,3,3-tetramethyl hydroperoxide, tert-butyl peroxide, tert-butyl peroxide, tert-butyl peroxide, tert-butyl peroxide, tert-butyl peroxide, tert-butyl peroxide, 1,1,3,3-tetramethyl hydroperoxide, tert-butyl alcohol peroxide, tert-butyl peroxylaurate, tert-butyl peroxy-3,5,5-trimethylhexanoate; preferably di-tert-butyl peroxide (also known as di-tert-butyl peroxide).

[0042] According to another exemplary embodiment of the present application, the medium and low temperature initiator is selected from at least one of the following: ammonium persulfate, potassium persulfate, sodium persulfate, isobutyl peroxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, tert-butyl peroxyneodecanoate, benzoyl peroxide, tert-butyl peroxyisobutyrate, tert-butyl peroxypivalate, succinic acid peroxide, acetyl peroxide, octanoyl peroxide, decanoyl peroxide, lauroyl peroxide; preferably potassium persulfate, diisopropyl peroxydicarbonate, or a combination thereof.

[0043] According to a specific embodiment of the present application, the composite initiator is a combination of two or more medium and low temperature initiators, preferably a combination of two or more medium and low temperature initiators, more preferably a combination of potassium persulfate and diisopropyl peroxydicarbonate, wherein the weight ratio of potassium persulfate to diisopropyl peroxydicarbonate is 1:20 to 1:1, preferably 1:15 to 1:5, and more preferably 1:10 to 3:20.

[0044] According to one embodiment of the present application, when the composite initiator is a combination of two or more medium and low temperature initiators, one of the two or more medium and low temperature initiators (e.g., potassium persulfate) is added once at the start of the reaction (e.g., before or at the start of the copolymerization reaction), and a portion of the other medium and low temperature initiators (e.g., diisopropyl peroxydicarbonate) (e.g., 5%-50%, preferably 10-30%) is added at the start of the reaction, and the remaining part of the material is added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0045] According to a specific embodiment of the present application, the composite initiator is a combination of at least one high temperature initiator and at least one medium and low temperature initiator, and the weight ratio of the high temperature initiator to the medium and low temperature initiator is 50:1 to 1:10, preferably 50:3 to 1:1.

[0046] According to one embodiment of the present application, when the composite initiator is a combination of at least one high-temperature initiator and at least one medium- and low-temperature initiator, the high-temperature initiator is added all at once at the start of the reaction (for example, before or at the start of the copolymerization reaction), and a portion of the medium- and low-temperature initiator (for example, 5%-60%, preferably 10-50%) is added at the start of the reaction, and the remaining portion of the material is added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0047] According to another embodiment of the present application, when the composite initiator is a combination of at least one high-temperature initiator and at least one medium-low-temperature initiator, a portion of the high-temperature initiator and the medium-low-temperature initiator (for example, 5%-60%, preferably 10-50%) is added at the beginning of the reaction, and the remaining portion of the material is added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0048] According to one embodiment of the present application, the ratio of the total weight of the composite initiator added to the reaction system during the entire polymerization reaction to the total weight of the vinylidene fluoride monomer added to the reaction system during the entire polymerization reaction is 0.01:100 to 1:100, preferably 0.1:100 to 0.8:100, and more preferably 0.25:100 to 0.7:100.

[0049] According to one embodiment of the present application, the chain transfer agent is a liquid phase chain transfer agent, or a combination of a liquid phase chain transfer agent and a gas phase chain transfer agent.

[0050] The liquid phase chain transfer is selected from one or more of the following: methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, diethyl carbonate, dibutyl carbonate, n-butyl mercaptan, tert-butyl mercaptan, isopropanol, methanol, ethanol, n-butanol, acetone, chloroform, cyclohexane; preferably diethyl malonate, ethyl acetate, or a combination thereof.

[0051] The gas phase chain transfer agent is selected from one or more of the following: methane, ethane, propane, butane, hydrogen; preferably hydrogen, propane, or a combination thereof.

[0052] According to one embodiment of the present application, when the chain transfer agent is a combination of a liquid phase chain transfer agent and a gas phase chain transfer agent, the weight ratio of the liquid phase chain transfer agent to the gas phase chain transfer agent is 100:1 to 1:1, preferably 100:3 to 22:3.

[0053] According to one embodiment of the present application, the ratio of the total weight of all chain transfer agents added to the reaction system during the entire polymerization reaction to the total weight of the vinylidene fluoride monomer added to the reaction system during the entire polymerization reaction is 0.01:100 to 2:100, preferably 0.02:100 to 1:100, more preferably 0.05:100 to 0.8:100, and further preferably 0.1:100 to 0.5:100.

[0054] According to one embodiment of the present application, the chain transfer agent is a liquid phase chain transfer agent, and a portion of the chain transfer agent (e.g., 1%-50%, preferably 5-30%) is added at the start of the reaction, and the remaining portion is added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0055] According to another embodiment of the present application, the chain transfer agent is a liquid phase chain transfer agent, and no chain transfer agent is added at the beginning of the polymerization reaction, and all chain transfer agents are added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0056] According to another embodiment of the present application, the chain transfer agent is a combination of a liquid-phase chain transfer agent and a gas-phase chain transfer agent, all of the gas-phase chain transfer agents are added to the polymerization reaction system at one time at the start of the polymerization reaction, while the liquid-phase chain transfer agent is not added at the start of the polymerization reaction, and all of the liquid-phase chain transfer agents are added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0057] According to another embodiment of the present application, the chain transfer agent is a combination of a liquid-phase chain transfer agent and a gas-phase chain transfer agent, all of the gas-phase chain transfer agent is added to the polymerization reaction system at the start of the polymerization reaction, and a portion of the liquid-phase chain transfer agent (for example, 1%-50%, preferably 5-30%) is added at the start of the reaction, and the remaining portion of the liquid-phase chain transfer agent material is added to the reaction system in a continuous or intermittent manner during the polymerization reaction.

[0058] According to one embodiment of the present application, the emulsifier used in the polymerization reaction is selected from one or more of the following: perfluorohexyl acetic acid or its salt, perfluorohexylphenyl acetic acid or its salt, 4,4,5,5,6,6,6-heptafluoro-3,3-bis(trifluoromethyl)hexanoic acid or its salt, 4,4,5,5,6,6,6-heptafluoro-3,3-bis(trifluoromethyl)phenylhexanoic acid or its salt, 4-(2,2,3,3,5,5,5-heptafluoro-4,4-bis(trifluoromethyl)pentyl)benzoic acid Or its salt, perfluorodioxaheptacarbonic acid or its salt, perfluorotrioxaheptacarbonic acid or its salt, perfluorodioxaoctacarbonic acid or its salt, branched perfluoropolyether, linear perfluoropolyether, sodium dodecyl sulfate, sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium dodecyl diphenyl ether disulfonic acid salt, sodium hexadecyl sulfonate, sodium hexadecyl sulfate, sodium hexadecyl sulfate, sodium hexadecyl sulfate, sodium octyl sulfonate, sodium octyl sulfate, polypropylene glycol (Mn400-2000), polyethylene glycol (Mn 400-2000), isomeric tridecyl polyoxyethylene ether, octyl polyoxyethylene ether, dodecyl polyoxyethylene ether; preferably, the emulsifier is perfluorohexyl acetate, more preferably perfluorohexyl ammonium acetate.

[0059] According to one embodiment of the present application, the ratio of the total weight of all emulsifiers added to the reaction system during the entire polymerization reaction to the total weight of the vinylidene fluoride monomer added to the reaction system during the entire polymerization reaction is 0.1:100 to 3:100, for example, 0.5:100 to 2:100, or 0.8:100 to 1:100, or within the numerical range obtained by combining any two of the above end values.

[0060] According to one embodiment of the present application, the emulsifier is added into the polymerization reaction system at one time at the start of the polymerization reaction.

[0061] According to another embodiment of the present application, a stabilizer is further added to the polymerization reaction system, and the stabilizer is preferably paraffin or a long-chain alkane. According to a preferred embodiment of the present application, the paraffin is selected from one or more of the following: paraffin No. 50, paraffin No. 58, paraffin No. 60, hexadecane, heptadecane, octadecane, eicosane, docosane, tetracosane.

[0062] According to one embodiment of the present application, when a stabilizer is used, the ratio of the total weight of all stabilizers added to the reaction system during the entire polymerization reaction to the total weight of the vinylidene fluoride monomer added to the reaction system during the entire polymerization reaction is 0:100 to 5:100, for example, 0.1:100 to 4:100, or 1:100 to 3:100, or within the numerical range obtained by combining any two of the above end values.

[0063] According to one embodiment of the present application, when a stabilizer is used, the stabilizer is added into the polymerization reaction system at one time at the start of the polymerization reaction.

[0064] According to one embodiment of the present application, the polymerization reaction temperature is 50-130°C, preferably 80-120°C, and more preferably 100-110°C.

[0065] According to another embodiment of the present application, the polymerization reaction is carried out under anaerobic conditions, for example, the oxygen content in the reactor for carrying out the polymerization reaction is less than 50 ppm, more preferably less than 20 ppm.

[0066] According to another embodiment of the present application, the vinylidene fluoride monomer in gaseous form is charged into the reactor until the pressure in the reactor reaches 2.7-5.0 MPa, preferably 2.8-4.8 MPa, and more preferably 3.5-4.5 MPa.

[0067] According to an exemplary embodiment of the present application, the method of the present invention is carried out in the following manner:

[0068] 1. Add water (e.g., deionized water), stabilizer and emulsifier into a polymerization reactor, seal the polymerization reactor and then evacuate it until the oxygen content in the reactor is lower than a preset standard value, e.g., the oxygen content is less than 50 ppm;

[0069] 2. Raise the temperature in the reaction vessel to the reaction temperature mentioned above under stirring, and charge the VDF monomer in gaseous form into the reaction vessel until the pressure mentioned above is reached;

[0070] 3. Add initiator to the reactor in the manner described above (a portion of the composite initiator can be added at the start of the polymerization reaction, or all of the high-temperature initiator can be added at once), and a portion of the chain transfer agent or no chain transfer agent is added as described above;

[0071] 4. During the polymerization reaction, VDF monomer is continuously added to keep the pressure in the reactor at the pressure level described above; and the remaining initiator and chain transfer agent (or all of the chain transfer agent) are added to the reactor in an intermittent or continuous manner as described above;

[0072] 5. After the reaction is completed, the obtained polyvinylidene fluoride emulsion is subjected to coagulation and demulsification, washing, drying and granulation to obtain the polyvinylidene fluoride product of the present invention.

[0073] The polyvinylidene fluoride of the present invention has a wide molecular weight distribution, for example, its polydispersity index (PDI, which is the ratio of weight average molecular weight Mw to Mn, the larger the PDI, the wider the molecular weight distribution) is 3-15, preferably 6-12. The molecular weight distribution of the polyvinylidene fluoride of the present invention includes two peaks, wherein the first peak of higher molecular weight is 800,000 to 3 million, preferably 1 million to 2.8 million; the second peak of lower molecular weight is 200,000 to 800,000, preferably 250,000 to 700,000.

[0074] The polyvinylidene fluoride of the present invention has a relatively high molecular weight, and its Mw is 200,000 to 3,000,000.

[0075] According to one embodiment of the present application, the melting point of the polyvinylidene fluoride of the present invention is 155-168°C, preferably 159-165°C; the melt index of the polyvinylidene fluoride measured at 230°C and 10 kg load is 1-5 g / 10 min, preferably 1.5-3 g / 10 min.

[0076] For the polyvinylidene fluoride for marine applications reported in the prior art, any one of these properties is very difficult to achieve, and polyvinylidene fluoride having an excellent combination of the above properties has never been reported.

[0077] The advantages of the present invention are:

[0078] 1. The present invention has specially designed the synthesis process and successfully synthesized polyvinylidene fluoride with a wide-bimodal molecular weight distribution by emulsion polymerization process. The polyvinylidene fluoride has excellent processing performance, mechanical properties, thermal stability, aging resistance, corrosion resistance, permeability resistance and environmental stress cracking resistance, and has a wide operating temperature, which can perfectly meet the requirements of the harsh marine environment for dynamic flexible risers (liners);

[0079] 2. The process technology of the present invention is simple and efficient, and all the reagents used are conventional reagents widely used in the art, which can be conveniently used to improve and upgrade existing production lines and processes on an industrial scale.

[0080] The method of the present application is specifically described in the following examples, the purpose of which is to better understand the content of the present application. It should be understood that these examples are merely illustrative and not restrictive. The water used in the examples is deionized water, and the reagents used are all purchased from the market and used directly without further purification.

[0081] Example

[0082] Example 1

[0083] This example uses Figure 2 The polymerization reaction was carried out in a stainless steel horizontal reactor with a capacity of 50 L and equipped with a stirring device, a vacuum device, a heating and temperature control device, a gas / liquid / solid feeding device.

[0084] 32 kg of deionized water, 120 g of No. 58 solid paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started so that the temperature in the reactor was raised to 110° C. and maintained at this temperature throughout the polymerization reaction.

[0085] Fill the reactor with vinylidene fluoride (VDF) gas until the pressure in the reactor reaches 4.5 MPa. Add 50 grams of di-tert-butyl peroxide and 10 grams of diisopropyl peroxydicarbonate into the reactor at one time, and add 3 grams of diethyl malonate to start the polymerization reaction.

[0086] During the polymerization reaction, VDF gas was continuously added into the reactor to keep the pressure in the polymerization reactor at 4.5 MPa.

[0087] When the cumulative amount of VDF charged into the reactor has reached 2 kg, 6 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 4 kg, 8 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 6 kg, 8 g of diethyl malonate is added to the reactor. At the same time, after the polymerization reaction starts, 2 g of diisopropyl peroxydicarbonate is added to the reactor every 15 minutes until the reaction ends. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0088] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0089] Add the same weight of water to the polyvinylidene fluoride emulsion obtained by the polymerization reaction, and stir the mixture at a high speed to break the emulsion and coagulate. Then separate the solid powder from it, mix the obtained powder with deionized water in a weight ratio of 1:3, stir and wash, and then perform solid-liquid separation. The mixed washing step is repeated at least three times, and the conductivity of the separated deionized water is detected after each washing until the measured conductivity is less than 1μs / m and the washing is stopped. The washed solid material is dried in an oven at 110°C for 4 hours, and then a twin-screw extruder and a pelletizer are used to extrude the dried powder into a pellet product. The molecular weight and distribution of the polyvinylidene fluoride resin product are measured using gel permeation chromatography (GPC), and the GPC chromatogram curve is shown as follows: Figure 1 As shown; the melt index of the polyvinylidene fluoride resin under a load of 12.5 kg and a temperature of 230°C was measured using an automatic melt indexer, and its melting point was tested using differential scanning calorimetry (DSC); its tensile strength was measured using a universal tensile testing machine; the analysis results are shown in Table 1 below.

[0090] Example 2

[0091] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0092] 32 kg of deionized water, 120 g of No. 58 solid paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started so that the temperature in the reactor was raised to 110° C. and maintained at this temperature throughout the polymerization reaction.

[0093] Fill the reactor with vinylidene fluoride (VDF) gas until the pressure in the reactor reaches 4.5 MPa. Add 50 grams of di-tert-butyl peroxide and 10 grams of diisopropyl peroxydicarbonate into the reactor at one time to start the polymerization reaction.

[0094] During the polymerization reaction, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 6 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 4 kg, 8 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 6 kg, 8 g of diethyl malonate is added to the reactor. At the same time, after the polymerization reaction starts, 2 g of diisopropyl peroxydicarbonate is added to the reactor every 15 minutes until the reaction ends. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0095] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0096] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0097] Example 3

[0098] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0099] 32 kg of deionized water, 120 g of No. 58 solid paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started so that the temperature in the reactor was raised to 110° C. and maintained at this temperature throughout the polymerization reaction.

[0100] Fill the reactor with vinylidene fluoride (VDF) gas until the pressure in the reactor reaches 4.5 MPa. Add 50 grams of di-tert-butyl peroxide and 10 grams of diisopropyl peroxydicarbonate into the reactor at one time, and fill the reactor with 3 grams of propane to start the polymerization reaction.

[0101] During the polymerization reaction, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 6 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 4 kg, 8 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 6 kg, 8 g of diethyl malonate is added to the reactor. At the same time, after the polymerization reaction starts, 2 g of diisopropyl peroxydicarbonate is added to the reactor every 15 minutes until the reaction ends. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0102] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0103] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0104] Example 4

[0105] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0106] 32 kg of deionized water, 120 g of No. 58 solid paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started so that the temperature in the reactor was raised to 110° C. and maintained at this temperature throughout the polymerization reaction.

[0107] Fill the reactor with vinylidene fluoride (VDF) gas until the pressure in the reactor reaches 4.5 MPa. Add 50 grams of di-tert-butyl peroxide and 10 grams of diisopropyl peroxydicarbonate into the reactor at one time, and fill the reactor with 3 grams of hydrogen to start the polymerization reaction.

[0108] During the polymerization reaction, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 6 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 4 kg, 8 g of diethyl malonate is added to the reactor. When the cumulative amount of VDF charged into the reactor has reached 6 kg, 8 g of diethyl malonate is added to the reactor. At the same time, after the polymerization reaction starts, 2 g of diisopropyl peroxydicarbonate is added to the reactor every 15 minutes until the reaction ends. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0109] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0110] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0111] Example 5

[0112] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0113] 32 kg of deionized water, 120 g of octadecane and 200 g of a 30 wt % aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 70° C., and this temperature was maintained throughout the polymerization reaction.

[0114] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 3 g of potassium persulfate and 10 g of diisopropyl peroxydicarbonate were added into the reactor at one time to start the polymerization reaction.

[0115] During the polymerization reaction, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 10 grams of ethyl acetate is added to the reactor. Then ethyl acetate is added every 15 minutes, 10 grams each time, for a total of 10 times. At the same time, after the polymerization reaction starts, 2 grams of diisopropyl peroxydicarbonate are added to the reactor every 15 minutes, for a total of 10 times. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0116] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0117] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0118] Example 6

[0119] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0120] 32 kg of deionized water, 120 g of octadecane and 200 g of a 30 wt % aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 70° C., and this temperature was maintained throughout the polymerization reaction.

[0121] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 3 g of potassium persulfate and 3 g of propane were added into the reactor at one time to start the polymerization reaction.

[0122] During the polymerization reaction, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 10 grams of ethyl acetate is added to the reactor. Then ethyl acetate is added every 15 minutes, 10 grams each time, for a total of 10 times. At the same time, after the polymerization reaction starts, 2 grams of diisopropyl peroxydicarbonate are added to the reactor every 15 minutes, for a total of 10 times. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0123] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0124] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0125] Example 7

[0126] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0127] 32 kg of deionized water, 120 g of octadecane and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 110° C., and this temperature was maintained throughout the polymerization reaction.

[0128] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 50 g of di-tert-butyl peroxide and 3 g of propane were added into the reactor at one time to start the polymerization reaction.

[0129] During the polymerization reaction, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5 MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 10 grams of ethyl acetate are added to the reactor. Subsequently, ethyl acetate is added every 15 minutes, 10 grams each time, for a total of 10 times. At the same time, after the polymerization reaction begins, potassium persulfate is added to the reactor in the form of an aqueous solution at a uniform rate, and a total of 3 grams of potassium persulfate is added during the entire polymerization reaction. A total of 8 kg of VDF monomer is input into the reactor during the entire reaction process.

[0130] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0131] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0132] Example 8

[0133] In this example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0134] 32 kg of deionized water, 120 g of eicosane and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, the reactor was sealed and evacuated to remove oxygen until the oxygen content was less than 50 ppm. At this time, the stirring device was started and the heating device was started to raise the temperature in the reactor to 110°C and maintained at 246902 during the entire polymerization process.

[0135] At this temperature.

[0136] A mixed gas of vinylidene fluoride (VDF) gas and propane (wherein the propane content is 0.0375 volume %) is charged into the reactor until the pressure in the reactor reaches 4.5 MPa. 50 g of di-tert-butyl peroxide and 100 g of ethyl acetate are added into the reactor at one time to start the polymerization reaction.

[0137] During the polymerization reaction, a mixed gas of VDF gas and propane (wherein the propane content is 0.0375 volume %) is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 10 grams of ethyl acetate are added to the reactor. Subsequently, ethyl acetate is added every 15 minutes, 10 grams each time, and a total of 10 times. At the same time, after the polymerization reaction starts, potassium persulfate is added to the reactor in the form of an aqueous solution at a uniform rate, and a total of 3 grams of potassium persulfate are added during the entire polymerization reaction. A total of 8 kilograms of VDF monomer and 3 grams of propane are input into the reactor during the entire reaction process.

[0138] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0139] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0140] Comparative Example 1

[0141] In this comparative example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0142] 32 kg of deionized water, 120 g of No. 58 solid paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 120° C., and this temperature was maintained throughout the polymerization reaction.

[0143] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 50 g of di-tert-butyl peroxide and 24 g of diethyl malonate were added into the reactor at one time to start the polymerization reaction.

[0144] During the polymerization reaction, VDF gas was continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5 MPa. A total of 8 kg of VDF monomer was input into the reactor during the entire reaction.

[0145] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0146] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0147] Comparative Example 2

[0148] In this comparative example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0149] 32 kg of deionized water, 120 g of No. 58 solid paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 120° C., and this temperature was maintained throughout the polymerization reaction.

[0150] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 50 g of di-tert-butyl peroxide and 3 g of diethyl malonate were added into the reactor at one time to start the polymerization reaction.

[0151] In the process of polymerization, VDF gas is continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5MPa. When the cumulative amount of VDF charged into the reactor has reached 2 kg, 6 grams of diethyl malonate are added into the reactor. When the cumulative amount of VDF charged into the reactor has reached 4 kg, 8 grams of diethyl malonate are added into the reactor. When the cumulative amount of VDF charged into the reactor has reached 6 kg, 8 grams of diethyl malonate are added into the reactor. A total of 8 kg of VDF monomers are input into the reactor during the whole reaction process.

[0152] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0153] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0154] Comparative Example 3

[0155] In this comparative example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0156] 32 kg of deionized water, 120 g of octadecane and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 80° C., and this temperature was maintained throughout the polymerization reaction.

[0157] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 3 g of potassium persulfate and 100 g of ethyl acetate were added into the reactor at one time to start the polymerization reaction.

[0158] During the polymerization reaction, VDF gas was continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5 MPa. A total of 8 kg of VDF monomer was input into the reactor during the entire reaction.

[0159] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0160] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0161] Comparative Example 4

[0162] In this comparative example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0163] 32 kg of deionized water, 120 g of octadecane and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 80° C., and this temperature was maintained throughout the polymerization reaction.

[0164] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 3 g of potassium persulfate and 50 g of ethyl acetate were added into the reactor at one time to start the polymerization reaction.

[0165] During the polymerization reaction, VDF gas was continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5 MPa. When the cumulative amount of VDF added to the reactor reached 2 kg, 10 g of ethyl acetate was added to the reactor. Subsequently, ethyl acetate was added every 15 minutes, 5 g each time, for a total of 10 times. A total of 8 kg of VDF monomer was input into the reactor during the entire reaction process.

[0166] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0167] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0168] Comparative Example 5

[0169] In this comparative example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0170] 32 kg of deionized water, 120 g of octadecane and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen therein until the oxygen content therein was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 80° C., and this temperature was maintained throughout the polymerization reaction.

[0171] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure inside the reactor reached 4.5 MPa. 3 grams of potassium persulfate and 3 grams of hydrogen were added into the reactor at one time to start the polymerization reaction.

[0172] During the polymerization reaction, VDF gas was continuously added to the reactor to keep the pressure in the polymerization reactor at 4.5 MPa. A total of 8 kg of VDF monomer was input into the reactor during the entire reaction.

[0173] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0174] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0175] Comparative Example 6

[0176] In this comparative example, the same reactor as in Example 1 was used to carry out the polymerization reaction.

[0177] 32 kg of deionized water, 120 g of No. 58 paraffin and 200 g of a 30 wt% aqueous solution of ammonium perfluorohexylacetate were added to a 50 L horizontal reactor, and the reactor was sealed and evacuated to remove oxygen until the oxygen content was less than 50 ppm. At this time, the stirring device was started, and the heating device was started to raise the temperature in the reactor to 75° C., and maintained at this temperature throughout the polymerization reaction.

[0178] The reactor was charged with vinylidene fluoride (VDF) gas until the pressure in the reactor reached 4.5 MPa. 10 g of diisopropyl peroxydicarbonate and 50 g of ethyl acetate were added into the reactor at one time to start the polymerization reaction.

[0179] During the polymerization reaction, VDF gas was continuously added into the reactor to keep the pressure in the polymerization reactor at 4.5 MPa.

[0180] After the polymerization reaction started, 3 grams of diisopropyl peroxydicarbonate was added every 15 minutes for a total of 10 times, and 5 grams of ethyl acetate was added every 15 minutes for a total of 10 times. During the entire reaction process, a total of 8 kilograms of VDF monomer was input into the reactor.

[0181] After the reaction is completed, the reactor is cooled to room temperature, and the pressure is released to discharge the materials.

[0182] The polyvinylidene fluoride emulsion obtained by the polymerization reaction was subjected to coagulation and demulsification, washing, drying and granulation in the same manner as in Example 1 to obtain a granular product. The various properties thereof were characterized as described in Example 1, and the analysis results are shown in Table 1 below.

[0183] Table 1 Performance test results of polyvinylidene fluoride resins prepared in Examples 1-8 and Comparative Examples 1-6

[0184]

[0185] It can be seen from Table 1 above that the PVDF resins prepared by the emulsion method in Examples 1-8 of the present application all have a very wide molecular weight distribution, and the molecular weight distribution includes two peaks. These PVDF resin products all show excellent melt index and tensile strength, and the pellets can be melt-extruded very smoothly into uniform and high-quality pipes.

[0186] In sharp contrast, the PDI of Comparative Examples 1-6 is much narrower, without a characteristic bimodal distribution, and the melt index and tensile strength are far inferior to those of the examples of the present invention. When the pellets are melt-extruded into a pipe, significant defects and discontinuities appear in the pipe.

Claims

1. A method for preparing polyvinylidene fluoride by emulsion polymerization, the method comprising: Allowing vinylidene fluoride to polymerize under the conditions of using a composite initiator, at least one chain transfer agent, and at least one emulsifier; The composite initiator is a combination of two or more medium and low temperature initiators, or a combination of at least one high temperature initiator and at least one medium and low temperature initiator, wherein the decomposition temperature of the high temperature initiator is higher than the decomposition temperature of the medium and low temperature initiator.

2. The method according to claim 1, characterized in that The high temperature initiator is selected from one or more of the following: cyclohexanone peroxide, tert-butyl peracetate, tert-butyl perbenzoate, 4,4-bis(tert-butyl peroxy) valerate, di-tert-butyl diperoxyisophthalate, methyl ethyl ketone peroxide, di-tert-butyl peroxide, di-tert-amyl peroxide, isopropylbenzene hydroperoxide, diisopropylbenzene peroxide, tert-butyl isopropylbenzene peroxide, 1,1,3,3-tetramethyl hydroperoxide, tert-butyl alcohol peroxide, tert-butyl peroxylaurate, and tert-butyl peroxy-3,5,5-trimethylhexanoate; The medium and low temperature initiator is selected from one or more of the following: ammonium persulfate, potassium persulfate, sodium persulfate, isobutyl peroxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, tert-butyl peroxyneodecanoate, benzoyl peroxide, tert-butyl peroxyisobutyrate, tert-butyl peroxypivalate, succinic acid peroxide, acetyl peroxide, octanoyl peroxide, decanoyl peroxide, and lauroyl peroxide.

3. The method according to claim 1 or 2, characterized in that The weight ratio of the high temperature initiator to the medium and low temperature initiator is 50:1 to 1:10; Based on the weight of vinylidene fluoride being 100 weight percent, the amount of the composite initiator used is 0.01-1 weight percent.

4. The method according to claim 1 or 2, characterized in that: The high temperature initiator is added to the polymerization reaction system at the beginning of the reaction; or a part of the high temperature initiator is added to the polymerization reaction system at the beginning of the reaction, and the remaining high temperature initiator is added to the polymerization reaction system in an intermittent or continuous manner during the polymerization reaction; A portion of the medium and low temperature initiator is added to the polymerization reaction system at the beginning of the reaction, and the remaining medium and low temperature initiator is added to the polymerization reaction system in an intermittent or continuous manner during the polymerization reaction.

5. The method according to claim 1, characterized in that The chain transfer agent is a liquid phase chain transfer agent, or a combination of a liquid phase chain transfer agent and a gas phase chain transfer agent; The liquid phase chain transfer is selected from one or more of the following: methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, diethyl carbonate, dibutyl carbonate, n-butyl mercaptan, tert-butyl mercaptan, isopropanol, methanol, ethanol, n-butanol, acetone, chloroform, cyclohexane; The gas phase chain transfer agent is selected from one or more of the following: methane, ethane, propane, butane, hydrogen; When the chain transfer agent is a combination of a liquid phase chain transfer agent and a gas phase chain transfer agent, the weight ratio of the liquid phase chain transfer agent to the gas phase chain transfer agent is 100:1 to 1:1; Based on 100% by weight of vinylidene fluoride, the amount of the chain transfer agent is 0.01-2% by weight.

6. The method according to claim 1, characterized in that The emulsifier is selected from one or more of the following: perfluorohexyl acetic acid or its salt, perfluorohexylphenyl acetic acid or its salt, 4,4,5,5,6,6,6-heptafluoro-3,3-bis(trifluoromethyl)hexanoic acid or its salt, 4,4,5,5,6,6,6-heptafluoro-3,3-bis(trifluoromethyl)phenylhexanoic acid or its salt, 4-(2,2,3,3,5,5,5-heptafluoro-4,4-bis(trifluoromethyl)pentyl)benzoic acid or its salt, perfluorodioxafluoroheptafluoroacetic acid or its salt. Carbonic acid or its salt, perfluorotrioxaheptacarbonic acid or its salt, perfluorodioxaoctacarbonic acid or its salt, branched perfluoropolyether, linear perfluoropolyether, sodium dodecyl sulfate, sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium dodecyl diphenyl ether disulfonic acid salt, sodium hexadecyl sulfonate, sodium hexadecyl sulfate, sodium hexadecyl sulfate, sodium hexadecyl sulfate, sodium octyl sulfonate, sodium octyl sulfate, polypropylene glycol (Mn400-2000), polyethylene glycol (Mn400-2000), isomeric tridecyl polyoxyethylene ether, octyl polyoxyethylene ether, dodecyl polyoxyethylene ether; based on the weight of vinylidene fluoride as 100% by weight, the amount of the emulsifier is 0.1-3% by weight; The polymerization reaction also uses a stabilizer, which is paraffin or long-chain alkane, preferably dodecane, hexadecane, octadecane or eicosane. The amount of the stabilizer is 0-5% by weight based on 100% by weight of the vinylidene fluoride.

7. The method according to claim 1, characterized in that The polymerization reaction temperature is 50-130° C., and the polymerization reaction pressure is 2.7-5.0 MPa.

8. A polyvinylidene fluoride having a weight average molecular weight of 200,000 to 3,000,000, a PDI of 3-15, and a molecular weight distribution including a first peak of 800,000 to 3,000,000 and a second peak of 200,000 to 800,000; The melting point is 155-168° C. and the melt index is 1-5 g / 10 min. The melt index is measured under the conditions of a temperature of 230° C. and a load of 12.5 kg.

9. The polyvinylidene fluoride according to claim 8, which is prepared by the method according to any one of claims 1 to 7.

10. A flexible riser for use in a marine environment, at least a portion of the riser being made of the polyvinylidene fluoride according to claim 8 or 9.

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