Preparation and application of hyperbranched polyvinylidene fluoride resin by suspension polymerization with chain transfer agent

Abstract

The present application relates to a kind of hyperbranched polyvinyl fluoride resin prepared by suspension polymerization with chain transfer agent and its preparation and application, the preparation method of the resin includes: under the condition of using at least one initiator, at least one chain transfer agent, at least one dispersing agent, make polyvinyl fluoride occur polymerization reaction;The chain transfer agent is at least one chain transfer agent of multiple chain transfer sites (≥2) or the combination of liquid phase transfer agent and gas phase transfer agent forms multiple chain transfer sites.Compared with prior art, the polyvinyl fluoride resin of the present application has hyperbranched configuration, and high mechanical strength and good processing performance are considered, the resin is especially suitable for being used as the inner lining material of marine non-adhesive flexible riser, submarine cable packaging layer and other fields, its excellent chemical corrosion resistance, gas permeation resistance and heat aging performance can effectively block the erosion of external corrosive medium to internal tensile armor layer.

Description

Technical Field

[0001] This invention relates to the field of polymers, and in particular to a hyperbranched polyvinylidene fluoride resin prepared by suspension polymerization using a chain transfer agent, its preparation method, and its application. Background Technology

[0002] Traditional linear polyvinylidene fluoride (PVDF) holds a significant position in petrochemical, new energy, and specialty functional materials fields due to its superior comprehensive properties. However, there is an irreconcilable contradiction between its performance advantages and processing performance. PVDF's high performance relies on its high molecular weight (>600,000 g / mol). While increasing the molecular weight can enhance its mechanical properties, it leads to an exponential increase in melt viscosity, with the melt index (MFI) dropping as low as 0.2-3 g / 10 min, creating a serious processing bottleneck. Extrusion / injection molding requires high temperatures (250~265 ℃) and high pressure, and is prone to surface defects such as "sharkskin." During blow molding, insufficient melt strength can lead to significant deviations in preform wall thickness. Rapid cooling generates internal stress, making the processed products prone to cracking, requiring complex annealing processes for repair, which extends the production cycle and reduces production efficiency.

[0003] To address the high viscosity processing challenges of traditional linear PVDF, the industry primarily employs methods such as additive-assisted processing, blending modification, and molecular weight reduction. Additive-assisted processing involves incorporating plasticizers or lubricants to improve melt flow properties. However, small-molecule plasticizers are prone to migration and precipitation, leading to reduced toughness, embrittlement, media contamination, and decreased chemical resistance in long-term use. This results in dimensional changes in the molded product, poor joint sealing, and failures such as oil leakage from risers, failing to meet the requirements of dynamic flexible risers in complex operating conditions. Blending modification involves blending organic polymers or inorganic nanoparticles with PVDF to improve processing flowability. For example, when PMMA is blended with PVDF, the carbonyl groups in PMMA's molecular chain form hydrogen bonds with the fluorine atoms of PVDF, disrupting the crystalline structure of PVDF and reducing melt viscosity by 30%–40%, increasing the melt viscosity index (MFI) to 10–15 g / 10 min. However, this method suffers from poor compatibility and is prone to phase separation. When inorganic nanoparticles such as Al2O3 are blended with PVDF, the nano-Al2O3 acts as a heterogeneous nucleation point to refine the grains and reduce the size of the crystalline region, which can reduce the melt viscosity by 30%. However, inorganic nanoparticles are prone to agglomeration, which leads to an increase in viscosity and can clog processing equipment. Adding a large amount of chain transfer agent can reduce the molecular weight of PVDF to improve the molecular weight fraction (MFI) and significantly improve processing performance, but this will severely sacrifice the mechanical properties of the material and produce a large amount of low molecular weight PVDF, which will lead to changes in the material's heat resistance and accelerated aging at high temperatures.

[0004] Conventional methods for synthesizing PVDF include emulsion polymerization and suspension polymerization. While emulsion polymerization can efficiently synthesize polymers, it requires thorough removal of emulsifiers in post-processing. This process is complex, energy-intensive, and difficult to completely remove, and residual emulsifiers can potentially lead to degradation of the material under harsh environments. In contrast, suspension polymerization exhibits significant advantages in this regard. Its post-processing is simple, eliminating the need for complex demulsification and emulsifier removal steps. This not only simplifies the production process but also eliminates the potential harm of emulsifier residues to the long-term performance of the product (such as resistance to media, aging, and heat resistance). Furthermore, suspension polymerization is typically carried out at lower temperatures, which helps to form more regular molecular chains with fewer defects, resulting in polyvinylidene fluoride resin products with better overall properties.

[0005] Chinese patent CN119080979A discloses a suspension polymerization method for preparing polyvinylidene fluoride (PVDF) and its applications. The method includes: polymerizing PVDF in a polymerization reactor containing a dispersant, an initiator system, at least one chain transfer agent, and water to form a PVDF suspension. PVDF prepared by this suspension polymerization method has a high molecular weight, a higher melting point, and a wide molecular weight distribution. It possesses excellent mechanical properties, good processability, a wide operating temperature range, good corrosion and permeability resistance, and excellent heat resistance and aging resistance, making it suitable for preparing the inner lining of marine dynamic flexible risers. However, the use of a complex initiator system to control the molecular weight distribution does not change the linear topology of the polymer. Essentially, it still improves processability by introducing low molecular weight components, which may result in a sacrifice of mechanical properties and long-term stability.

[0006] In summary, while existing technologies improve the processing conditions of PVDF resin, they often come at the cost of mechanical property loss and insufficient long-term stability. Therefore, a solution based on molecular structure innovation is urgently needed. Summary of the Invention

[0007] The purpose of this invention is to address at least one of the aforementioned problems by providing a method for preparing hyperbranched polyvinylidene fluoride (PVDF) resin using suspension polymerization with a chain transfer agent, and its application. This method utilizes a chain transfer agent with multiple chain transfer sites to controllably synthesize PVDF resin with a hyperbranched three-dimensional structure. While maintaining high molecular weight and good mechanical properties, this resin, due to its unique topological structure, can effectively reduce melt viscosity, thereby achieving excellent processing performance.

[0008] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides a method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent. In a polymerization reaction system containing an initiator, a chain transfer agent, and a dispersant, polyvinylidene fluoride is polymerized by suspension polymerization. The chain transfer agent is a chain transfer agent containing multiple chain transfer sites or a compound chain transfer agent formed by combining it with at least one of a liquid-phase transfer agent and a gas-phase transfer agent.

[0009] Furthermore, the amount of the initiator is 0.01~1% of the mass of vinylidene fluoride; The amount of the chain transfer agent used is 0.01~1% of the mass of vinylidene fluoride; The amount of the dispersant used is 0.01~3% of the mass of vinylidene fluoride.

[0010] Further, the initiator is selected from one or more of the following: tert-butyl peroxycarbonate-2-ethylethyl ester, cyclohexanone peroxide, tert-pentyl peroxycarbonate, tert-butyl peroxyacetate, tert-butyl peroxybenzoate, n-butyl 4,4-bis(tert-butyl peroxycarbonate), di-tert-butyl peroxide, di-tert-pentyl peroxide, cumene hydroperoxide, diisopropyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyisopropyl peroxide, tert-butanol peroxide, tert-butyl peroxylaurate, isobutyl peroxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, tert-butyl peroxyneodecanate, benzoyl peroxide, tert-butyl peroxyisobutyrate, tert-butyl peroxyneodecanate, succinic acid peroxide, acetyl peroxide, octanoyl peroxide, decanoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisoheptanenitrile, ammonium persulfate, and potassium persulfate.

[0011] Further, the chain transfer agent for the multi-chain transfer site is selected from one or a combination of several of the following: pentaerythritol tetrakis(3-mercaptopropionic acid), trimethylolpropane tris(3-mercaptopropionate), mercaptoethanol, 1,4-butanedithiol, 1,6-hexanedithiol, tris(2-mercaptoethyl)amine, glycerol trimercaptopropionate, 1,3,5-benzenetrithiol, 4,4'-thiodiphenylthiol, pentaerythritol tetrakis(3-mercaptobutyrate), pentaerythritol tetrakis(2-(dithiobenzoyl)thiopropionate), glucose pentathiopropionate, and tetrakis(dimethylsiloxy)silane glucose thiopropionate.

[0012] Furthermore, when the chain transfer agent is a compound transfer agent, the mass ratio of the chain transfer agent with multiple chain transfer sites to the liquid-phase chain transfer agent or the gas-phase transfer agent is 1:100 to 1:10.

[0013] The liquid phase transfer agent is selected from one or a combination of several of the following: methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, diethyl carbonate, n-butanethiol, tert-butanethiol, n-octanethiol, dodecyl mercaptan, tert-dodecyl mercaptan, chloroform, and methanol. The gas phase transfer agent is selected from one or a combination of several of methane, ethane, propane, butane, and hydrogen.

[0014] Furthermore, the dispersant is selected from one or a combination of several of the following: methylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, cellulose acetate, cellulose phosphate, polyvinyl alcohol, polyethylene glycol, polypropylene glycol, polyethylene glycol-propylene glycol copolymer, fatty acid polyethylene glycol ester, polyacrylamide, sodium hexametaphosphate, sodium metasilicate, and sodium disilicate.

[0015] Furthermore, the suspension polymerization method is carried out according to the following steps: S1. Add deionized water and dispersant to the polymerization reactor, evacuate the vacuum, and remove oxygen until the oxygen content is qualified. S2. Start stirring to raise the temperature of the reactor; S3. Adding VDF monomer and continuously replenishing VDF monomer will maintain the polymerization reaction pressure; S4. Add initiator and chain transfer agent; S5. After the reaction is complete, the obtained polyvinylidene fluoride suspension is washed and dried to obtain hyperbranched polyvinylidene fluoride resin.

[0016] Furthermore, the reaction temperature in step S2 is 30~70℃; The polymerization reaction pressure in step S3 is 5.0~9.0 MPa.

[0017] Furthermore, the initiator is added to the polymerization reaction system all at the beginning of the reaction; or a portion of the initiator is added at the beginning of the reaction, and the remaining initiator is added to the polymerization reaction system intermittently or continuously during the polymerization reaction.

[0018] Furthermore, one of the chain transfer agents is added to the polymerization reaction system all at the beginning of the reaction; or a portion of the chain transfer agent is added at the beginning of the reaction, and the remaining chain transfer agent is added to the polymerization reaction system intermittently or continuously during the polymerization reaction; or no chain transfer agent is added at the beginning of the reaction, but is added intermittently or continuously only during the polymerization reaction.

[0019] In a second aspect, the present invention provides a hyperbranched polyvinylidene fluoride resin, prepared by the method described above, having a weight-average molecular weight of 800,000 to 4,000,000, a branching factor of 0.3 to 0.8, a molecular weight distribution index of 1 to 10, a melt index of 0.1 to 5 g / 10 min, a melting point of 165 to 175 °C, and a tensile strength of 50.0 to 80.0 MPa.

[0020] In a third aspect, the present invention provides the application of hyperbranched polyvinylidene fluoride resin in the preparation of an inner lining material for marine non-adhesive flexible risers under high temperature and high pressure conditions.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses a multi-chain point transfer agent to controllably synthesize PVDF resin with a hyperbranched three-dimensional structure. This synthesis method effectively solves the contradiction between performance and processing in traditional PVDF synthesis processes, reducing processing difficulty while effectively improving the mechanical strength of PVDF products. Compared with PVDF prepared by traditional suspension polymerization, the hyperbranched three-dimensional structure prepared by the multi-chain point transfer agent brings more stable chemical and physical properties to PVDF, enabling it to be applied in the harsh marine environment of marine flexible pipes, maintaining its structural function under harsh conditions.

[0022] 2. The polyPVDF resin prepared by this invention has a hyperbranched configuration, and also has a high molecular weight and a wide molecular weight distribution. This structure endows the resin with high mechanical strength and excellent processing performance, and also gives it excellent chemical corrosion resistance, gas permeability resistance and heat aging resistance, which can effectively block the external corrosive media from eroding the internal tensile armor layer. Detailed Implementation

[0023] The present invention will now be described in detail with reference to specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0024] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, each point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value, or combined with other lower or upper limits, to form a range not explicitly stated. In the description of this application, it should be noted that, unless otherwise stated, "above" includes the stated number, and "multiple" in "one or more" means two or more.

[0025] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0026] Numerous details are explored in the following description to provide a more thorough explanation of embodiments of this application; however, it will be apparent to those skilled in the art that embodiments of this application may be practiced without these specific details.

[0027] The purpose of this invention is to provide a controllable synthesis method for hyperbranched polyvinylidene fluoride (PVDF) resin. This method is based on a suspension polymerization system and precisely controls the chain transfer activity and branching point growth path during the free radical reaction process by using a single chain transfer agent with multiple chain transfer sites or a combination of single-site and multi-site chain transfer agents, thereby preparing a hyperbranched PVDF resin with a highly branched topology. This resin possesses both high mechanical strength and excellent processing performance, making it particularly suitable for manufacturing the inner lining of marine dynamic flexible risers, meeting the stringent requirements for weather resistance, permeability resistance, and long-term reliability in deep-sea oil and gas extraction.

[0028] To further understand the present invention, the following embodiments are provided. It is worth noting that, unless otherwise specified, all raw materials used in the present invention are commercially available; and all methods and equipment employed are common in the art.

[0029] The PVDF resin product synthesized in this invention was tested and characterized using the following methods: 1. Molecular weight and molecular weight distribution Molecular weight and molecular weight distribution were determined using the methods described in ISO 16014-1, -2, -4 via high-temperature gel permeation chromatography (GPC, Waters, Styragel HR5, Waters Technologies, Milford, Massachusetts). Specific details are as follows: a PVDF sample solution with a concentration of 5 mg / mL was prepared, using DMF (containing 50 mmol / L lithium bromide) as the mobile phase at a flow rate of 1.0 mL / min and a column temperature of 50 °C. Detection was performed using a GPC system calibrated with narrow-distribution polystyrene standards via a differential refractive index detector. The relative weight-average molecular weight (Mb) of the polymer was calculated based on the elution volume versus calibration curve. w Number-average molecular weight (M) n ) and molecular weight distribution index (PDI).

[0030] 2. Branching factor (g') The radius of gyration of each fraction eluted from gel permeation chromatography (GPC, as described above) was determined by analyzing light scattering at different angles using multi-angle laser light scattering (MALLS, Waytt, Down 8 detector, Wyatt Technologies, Santa Barbara, California). The laser source used had a wavelength of 658 nm and a power of 120 mW. A specific refractive index was taken as 0.104 ml / g. Data were evaluated using WyattASTRA 4.7.3 and CORONA 1.4 software. The branching factor was determined as described below.

[0031] The parameter g' is the ratio of the mean square radius of gyration of the measured sample to that of a linear polymer with the same molecular weight. A linear molecule shows a g' of 1, while a value less than 1 indicates the presence of branched structures. The value of g' as a function of molecular weight M is calculated from the equation: g'(M) = <Rg 2 > 样品，M / <Rg 2 > 线性参考，M in, <Rg 2 > is the mean square radius of gyration of the fraction with molecular weight M. <Rg 2 > 线性参考，M The linear PVDF reference can be measured using the same apparatus and method described above to confirm this.

[0032] 3. Melt Flow Index The test was conducted according to ISO 1133 standard using a fully automated plastic melt index tester (GT-7200-MIA, High-Speed ​​Rail Testing Instruments (Dongguan) Co., Ltd.). Test conditions were: temperature 230 ℃, load 10.0 kg, preheating time 5 min, followed by automatic cutting and weighing of the polymer extruded within the specified time. Results are expressed in g / 10 min.

[0033] 4. Melting point The test was performed using a differential scanning calorimeter (DSC 250, TA Instruments, USA). Approximately 5-8 mg of sample was weighed and placed in a DSC crucible. Under a nitrogen atmosphere, the temperature was increased from room temperature to 220°C at a rate of 5°C / min and held at this temperature for 3 minutes to eliminate thermal history. The temperature was then decreased to room temperature at a rate of 10°C / min, and finally increased again to 220°C at a rate of 10°C / min. The second heating curve was recorded, and the peak temperature of the endothermic peak was determined as the melting point of the polymer. T m ) 5. Mechanical properties Tests were performed using a universal testing machine (68TM-5, Instron, Norwood, Massachusetts) according to ISO 527-1 standard. Polymer samples were prepared into Type 1A standard tensile specimens by compression molding or injection molding and tested at a tensile rate of 50 mm / min at room temperature until the specimen broke. Stress-strain curves were recorded, and the tensile strength (in MPa) was reported.

[0034] Example 1 A hyperbranched polyvinylidene fluoride resin is prepared according to the following steps: Diisopropyl peroxide dicarbonate was initially added, followed by continuous replenishment; pentaerythritol tetra(3-mercaptobutyrate) and diethyl carbonate were initially added, followed by replenishment in stages, and the reaction was carried out at 50 °C.

[0035] Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 50 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 3.5 g of diisopropyl peroxide, 4 g of diethyl carbonate, and 0.3 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added to initiate the polymerization reaction. VDF monomer is continuously added, maintaining the polymerization pressure at 7.2 MPa. During polymerization, 6 g of diethyl carbonate, 2 g of diisopropyl peroxide, and 0.2 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added every 20 min until the total VDF feed reaches 9 kg. The reaction time is approximately 4 hours. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0036] Example 2 A hyperbranched polyvinylidene fluoride resin is prepared according to the following steps: Diisopropyl peroxide dicarbonate was initially added, followed by continuous replenishment; pentaerythritol tetra(3-mercaptobutyrate) and ethyl acetate were initially added, followed by staged replenishment, and the reaction was carried out at 50 °C.

[0037] Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 50 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 3.5 g of diisopropyl peroxide, 4 g of ethyl acetate, and 0.3 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added to initiate the polymerization reaction. VDF monomer is continuously added, maintaining the polymerization pressure at 7.2 MPa. During polymerization, 8 g of diethyl carbonate, 2 g of diisopropyl peroxide, and 0.2 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added every 20 min until the total VDF feed reaches 9 kg. The reaction time is approximately 4 h. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0038] Example 3 A hyperbranched polyvinylidene fluoride resin is prepared according to the following steps: Diisopropyl peroxide dicarbonate was initially added, followed by continuous replenishment; methane and pentaerythritol tetra(3-mercaptobutyrate) were initially added, followed by replenishment in stages, and the reaction was carried out at 50 °C.

[0039] Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 50 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 12 g of diisopropyl peroxide, 2 g of methane, and 0.3 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added to initiate the polymerization reaction. VDF monomer is continuously added, maintaining the polymerization pressure at 7.2 MPa. During polymerization, 1 g of methane, 3 g of diisopropyl peroxide, and 0.2 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added every 20 min until the total VDF feed reaches 9 kg. The reaction time is approximately 4 hours. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0040] Example 4 A hyperbranched polyvinylidene fluoride resin is prepared according to the following steps: Potassium persulfate was initially added, followed by supplementary additions in stages; pentaerythritol tetra(3-mercaptobutyrate) and diethyl carbonate were initially added, followed by supplementary additions in stages, and the reaction was carried out at 55 °C.

[0041] Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 55 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 2.5 g of potassium persulfate, 4 g of diethyl carbonate, and 0.3 g of pentaerythritol tetrakis(3-mercaptobutyrate) are added, with continuous addition of VDF monomer to maintain the polymerization pressure at 7.2 MPa. During polymerization, 0.2 g of potassium persulfate, 8 g of diethyl carbonate, and 0.2 g of pentaerythritol tetrakis(3-mercaptobutyrate) are added every 20 min until the total VDF feed reaches 9 kg. The reaction time is approximately 3 hours. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0042] Example 5 A hyperbranched polyvinylidene fluoride resin is prepared according to the following steps: Potassium persulfate was initially added, followed by additional additions in stages; pentaerythritol tetra(3-mercaptobutyrate) and ethyl acetate were initially added, followed by additional additions in stages, and the reaction was carried out at 55 °C.

[0043] Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 55 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 2.5 g of potassium persulfate, 3 g of ethyl acetate, and 0.3 g of pentaerythritol tetrakis(3-mercaptobutyrate) are added, with continuous addition of VDF monomer to maintain the polymerization pressure at 7.2 MPa. During polymerization, 0.2 g of potassium persulfate, 6 g of ethyl acetate, and 0.2 g of pentaerythritol tetrakis(3-mercaptobutyrate) are added every 20 min until the total VDF feed reaches 9 kg. The reaction time is approximately 4 h. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0044] Example 6 A hyperbranched polyvinylidene fluoride resin is prepared according to the following steps: Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 50 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 3.5 g of diisopropyl peroxide and 0.3 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added to initiate the polymerization reaction. VDF monomer is continuously added, maintaining the polymerization pressure at 7.2 MPa. During polymerization, 2 g of diisopropyl peroxide and 0.2 g of pentaerythritol tetrakis (3-mercaptobutyrate) are added every 20 min until the total VDF feed reaches 9 kg. The reaction time is approximately 4 h. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0045] Comparative Example 1 A polyvinylidene fluoride resin is prepared according to the following steps: Diisopropyl peroxide was added initially and then added in stages; diethyl carbonate was added initially and then added in stages; the reaction was carried out at 50°C. 35 kg of deionized water and 5 g of hydroxymethyl cellulose were added to a 50 L vertical reactor. The reaction system was then subjected to vacuum deoxygenation until the oxygen content was below 30 ppm. Stirring was started, and the reactor temperature was raised to 50 °C. Vinylidene fluoride monomer was added, and the reactor pressure was increased to 7.2 MPa. 3.5 g of diisopropyl peroxide and 4.3 g of diethyl carbonate were added to initiate the polymerization reaction. VDF monomer was continuously added, maintaining the polymerization pressure at 7.2 MPa. During polymerization, 6.2 g of diethyl carbonate and 2 g of diisopropyl peroxide were added every 20 minutes until the total VDF feed reached 9 kg. The reaction time was approximately 4 hours. After the reaction, the reactor was cooled to room temperature, and the pressure was released. The resulting polyvinylidene fluoride suspension was washed by adding twice its weight of deionized water to the washing device, heating to 70 °C, and stirring was started to wash the PVDF material. The washed material was then filtered to obtain a solid powder. The washing / separation step was repeated three times. The washed PVDF powder was dried and then granulated by a twin-screw extruder and pelletizer to obtain granular products. Relevant analysis and testing were performed, and the results are shown in Table 1.

[0046] Comparative Example 2 A polyvinylidene fluoride resin is prepared according to the following steps: Potassium persulfate was added initially, then gradually added in stages; diethyl carbonate was added initially, then gradually added in stages, and the reaction was carried out at 55 °C. 35 kg of deionized water and 5 g of hydroxymethyl cellulose were added to a 50 L vertical reactor. The reaction system was then subjected to vacuum deoxygenation until the oxygen content was below 30 ppm. Stirring was started, and the reactor temperature was raised to 55 °C. Vinylidene fluoride monomer was added, and the reactor pressure was increased to 7.2 MPa. 2.5 g of potassium persulfate and 4.3 g of diethyl carbonate were added, and VDF monomer was continuously added to maintain the polymerization pressure at 7.2 MPa. During polymerization, 0.2 g of potassium persulfate and 8.2 g of diethyl carbonate were added every 20 min until the total VDF feed reached 9 kg. The reaction time was approximately 4 h. After the reaction, the reactor was cooled to room temperature, and the pressure was released. The resulting polyvinylidene fluoride suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring was started to wash the PVDF material. The washed material was then filtered to obtain a solid powder. The washing / separation step was repeated three times. The washed PVDF powder was dried and then granulated by a twin-screw extruder and pelletizer to obtain granular products. Relevant analysis and testing were performed, and the results are shown in Table 1.

[0047] Comparative Example 3 The majority of the contents were the same as in Example 1, except that pentaerythritol tetra(3-mercaptobutyrate) was replaced with an equal mass of other conventional chain transfer agent containing multiple chain transfer sites, carbon tetrachloride.

[0048] Add 35 kg of deionized water and 5 g of hydroxymethyl cellulose to a 50 L vertical reactor. Vacuum deoxygenation of the reaction system is performed until the oxygen content is below 30 ppm. Stirring is started, and the reactor temperature is raised to 50 °C. Vinylidene fluoride monomer is added, and the reactor pressure is increased to 7.2 MPa. 3.5 g of diisopropyl peroxide, 4 g of diethyl carbonate, and 0.3 g of carbon tetrachloride are added to initiate the polymerization reaction. VDF monomer is continuously added, maintaining the polymerization pressure at 7.2 MPa. During polymerization, 6 g of diethyl carbonate, 2 g of diisopropyl peroxide, and 0.2 g of carbon tetrachloride are added every 20 minutes until the total VDF feed reaches 9 kg. The reaction time is approximately 4 hours. After the reaction is complete, the reactor is cooled to room temperature, and the pressure is released. The obtained polyvinylidene fluoride (PVDF) suspension was washed by adding twice the weight of deionized water to the washing device, heating to 70 °C, and stirring to wash the PVDF material. The resulting solid powder was then filtered. This washing / separation step was repeated three times. The washed PVDF powder was dried and finally granulated using a twin-screw extruder and pelletizer to obtain granular products. Relevant analytical tests were performed, and the results are shown in Table 1.

[0049] Table 1 Performance test data of polyvinylidene fluoride resin Table 1 shows that, compared to the comparative examples without added chain transfer agents or using only conventional chain transfer agents, the examples with added chain transfer agents containing multiple chain transfer sites exhibited a significantly reduced branching factor g' while maintaining a high weight-average molecular weight, demonstrating the successful construction of a hyperbranched structure. Furthermore, this structure significantly improved the melt flow index and processing fluidity, but due to the molecular chain entanglement of the hyperbranched PVDF resin, its tensile strength remained significantly higher than the comparative examples, achieving the preparation of PVDF resins that combine easy processing and high strength at high molecular weights.

[0050] The foregoing has shown and described the basic process, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent, characterized in that, In a polymerization reaction system containing an initiator, a chain transfer agent, and a dispersant, vinylidene fluoride is polymerized by suspension polymerization. The chain transfer agent is a chain transfer agent containing multiple chain transfer sites, or a compound chain transfer agent formed by combining it with at least one of a liquid-phase transfer agent and a gas-phase transfer agent.

2. The method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent according to claim 1, characterized in that, The amount of the initiator used is 0.01~1% of the mass of vinylidene fluoride; The amount of the chain transfer agent used is 0.01~1% of the mass of vinylidene fluoride; The amount of the dispersant used is 0.01~3% of the mass of vinylidene fluoride.

3. The method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent according to claim 1, characterized in that, The initiator is selected from one or more of the following: tert-butyl peroxycarbonate-2-ethylethyl ester, cyclohexanone peroxide, tert-pentyl peroxypentanoate, tert-butyl peroxyacetate, tert-butyl peroxybenzoate, n-butyl 4,4-bis(tert-butyl peroxypentanoate), di-tert-butyl peroxide, di-tert-pentyl peroxide, cumene hydroperoxide, diisopropyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-butyl cumene peroxide, tert-butanol peroxide, tert-butyl peroxylaurate, isobutyl peroxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, tert-butyl peroxyneodecanate, benzoyl peroxide, tert-butyl peroxyisobutyrate, tert-butyl peroxyneodecanate, succinic acid peroxide, acetyl peroxide, octyl peroxide, decyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisoheptanenitrile, ammonium persulfate, and potassium persulfate.

4. The method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent according to claim 1, characterized in that, The chain transfer agent for the multi-chain transfer site is selected from one or a combination of several of the following: pentaerythritol tetrakis(3-mercaptopropionic acid), trimethylolpropane tri(3-mercaptopropionate), mercaptoethanol, 1,4-butanedithiol, 1,6-hexanedithiol, tri(2-mercaptoethyl)amine, glycerol trimercaptopropionate, 1,3,5-benzenetrithiol, 4,4'-thiodiphenylthiol, pentaerythritol tetra(3-mercaptobutyrate), pentaerythritol tetra(2-(dithiobenzoyl)thiopropionate), glucose pentathiopropionate, and tetra(dimethylsiloxy)silane glucose thiopropionate.

5. The method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent according to claim 1, characterized in that, When the chain transfer agent is a compound transfer agent, the mass ratio of the chain transfer agent with multiple chain transfer sites to the liquid-phase chain transfer agent or the gas-phase transfer agent is 1:100 to 1:

10.

6. The liquid phase transfer agent is selected from one or a combination of several of the following: methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, diethyl carbonate, n-butanethiol, tert-butanethiol, n-octanethiol, dodecyl mercaptan, tert-dodecyl mercaptan, chloroform, and methanol. The gas phase transfer agent is selected from one or a combination of several of methane, ethane, propane, butane, and hydrogen.

7. The method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent according to claim 1, characterized in that, The dispersant is selected from one or a combination of several of the following: methylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, cellulose acetate, cellulose phosphate, polyvinyl alcohol, polyethylene glycol, polypropylene glycol, polyethylene glycol-propylene glycol copolymer, fatty acid polyethylene glycol ester, polyacrylamide, sodium hexametaphosphate, sodium metasilicate, and sodium disilicate.

8. The method for preparing hyperbranched polyvinylidene fluoride resin by suspension polymerization using a chain transfer agent according to claim 1, characterized in that, Includes the following steps: S1. Add deionized water and dispersant to the polymerization reactor, evacuate the vacuum, and remove oxygen until the oxygen content is qualified. S2. Start stirring to raise the temperature of the reactor; S3. Adding VDF monomer and continuously replenishing VDF monomer will maintain the polymerization reaction pressure; S4. Add initiator and chain transfer agent; S5. After the reaction is complete, the obtained polyvinylidene fluoride suspension is washed and dried to obtain hyperbranched polyvinylidene fluoride resin. In S2, the reaction temperature is 30~70℃; In S3, the polymerization reaction pressure is 5.0~9.0 MPa.

9. A hyperbranched polyvinylidene fluoride resin, characterized in that, The sample is prepared by any one of the methods described in claims 1 to 8, and has a weight-average molecular weight of 800,000 to 4,000,000, a branching factor of 0.3 to 0.8, a molecular weight distribution index of 1 to 10, a melt index of 0.1 to 5 g / 10 min, a melting point of 165 to 175 °C, and a tensile strength of 50.0 to 80.0 MPa.

10. The application of the hyperbranched polyvinylidene fluoride resin as described in claim 9 in the preparation of the inner lining material for marine non-adhesive flexible risers under high temperature and high pressure conditions.

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