Hydroxyl-terminated fluorine-containing crystalline prepolymer and preparation method thereof

Abstract

The invention relates to a hydroxyl-terminated fluorine-containing crystal prepolymer and a preparation method thereof, and belongs to the field of fluorine-containing materials and adhesives. The problems of poor mechanical property and poor solvent resistance formed by a common technology are solved. Comprising the following steps: adding a fluorine-containing crystalline polymer into an organic solvent to prepare a fluorine polymer solution, adding an oxidizing agent and an alkaline inorganic substance, carrying out a heating reaction to obtain a mixed solution, removing impurities, and drying to obtain the fluorine-containing carboxyl-terminated prepolymer. Dissolving the hydroxyl-terminated fluorine-containing crystal prepolymer in tetrahydrofuran, adding sodium borohydride and an iodine simple substance solution for reduction, adding hydrochloric acid for reaction to generate a white precipitate, filtering, removing a solvent, dissolving, washing, and drying to obtain the hydroxyl-terminated fluorine-containing crystal prepolymer. The molecular weight of the hydroxyl-terminated fluorine-containing crystalline prepolymer is 800-9000 Daltons, a middle molecular chain is a crystalline fluorine-containing copolymer, and the two ends of the molecular chain are hydroxyl-terminated. The preparation method disclosed by the invention is simple, few in side reaction and low in cost, and the cured product has relatively high mechanical property and solvent resistance.

Description

Technical Field

[0001] The invention relates to a fluorine-containing crystalline prepolymer with terminal hydroxyl groups and a preparation method thereof, and belongs to the field of fluorine-containing materials and adhesives. Background Art

[0002] Fluorinated materials are a type of polymer that contains fluorine atoms on carbon atoms in the main chain or side chain. They can be mainly divided into two categories: fluorinated resins and fluorinated rubbers. Fluorinated materials have excellent physical and chemical properties, including excellent chemical stability, good acid and alkali resistance, and potential for application in extreme environments; low surface energy, good hydrophobicity, self-cleaning properties, and anti-adhesion capabilities; excellent insulation properties and good biocompatibility. They are widely used in scenarios with very harsh operating environments such as high temperature, high humidity, oil resistance, and solvent resistance.

[0003] According to the state of the material during use, fluorine-containing materials can be divided into crystalline fluorine-containing materials, rubber fluorine-containing materials and liquid fluorine-containing materials. Among them, crystalline fluorine-containing materials are mainly represented by polyvinylidene fluoride (PVDF). PVDF has a high degree of crystallinity and fluorine content (59%), a melting point of 160°C, and good physical and chemical properties. However, PVDF, as an adhesive material and engineering parts, often needs to be processed at a temperature above the melting point, and when used as an adhesive material, due to its large molecular weight, it has a high viscosity in the molten state and poor fluidity, making it difficult to achieve bonding between complex parts, and often requires high pressure during use. The bonding process is complicated and cumbersome, and the energy consumption is high. For rubber fluorine-containing materials, due to the low glass transition temperature and low crystallinity, it is in a rubber state at room temperature and has good flexibility. However, since it is a polymer itself, it also encounters problems such as high temperature, high pressure, and poor fluidity during processing and bonding, which are similar to traditional fluoroplastics. And because of its non-crosslinked and amorphous characteristics, its mechanical properties, solvent resistance, and corrosion resistance are not as good as fluoroplastics.

[0004] CN 111471186 B ​​and US6329471 use a similar preparation method to this patent to prepare a low molecular weight, carboxyl-terminated or hydroxyl-terminated liquid fluorine-containing material that is liquid at room temperature. The advantages of this material are that it has a low molecular weight and has fluidity far exceeding that of high molecular weight fluorine-containing materials at medium and low temperatures (below 100°C), and can be cured at this temperature. The cured material has excellent heat resistance, oxidation resistance, and chemical resistance, so it can be cured into any desired shape at a relatively low temperature and used as a high temperature resistant and solvent resistant adhesive material. However, the fluorine-containing polymers used in these two patents are all low crystallinity fluorine-containing rubber materials, which have poor solvent resistance compared to crystalline fluoroplastics. The prepared low molecular weight liquid fluorine-containing material is an amorphous polymer after curing, and its mechanical strength, especially solvent resistance, is still slightly insufficient in some fields with higher requirements.

[0005] Therefore, it is urgent to propose a terminal hydroxyl group fluorine-containing crystalline prepolymer and a preparation method to solve the above technical problems. Summary of the invention

[0006] The purpose of the present invention is to solve the problems of high cost, complicated lamination process, poor mechanical properties and solvent resistance formed by conventional technologies. A brief overview of the present invention is given below to provide a basic understanding of certain aspects of the present invention. It should be understood that this overview is not an exhaustive overview of the present invention. It is not intended to identify the key or important parts of the present invention, nor is it intended to limit the scope of the present invention.

[0007] The technical solution of the present invention:

[0008] A method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer comprises the following steps:

[0009] Step 1: adding a fluorine-containing crystalline polymer to an organic solvent, and heating and stirring for 3 to 10 hours to prepare a fluorine polymer solution;

[0010] Step 2: Add an oxidant and an alkaline inorganic substance to the fluorine-containing polymer solution in sequence, control the temperature between 20 and 60°C, and continue stirring for 12 to 24 hours in an inert gas atmosphere to prepare a mixed solution:

[0011] Step 3: The mixed solution is eluted in deionized water to remove impurities, and the lower layer of the mixed solution is placed in a vacuum drying oven and dried to a constant weight to obtain a fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer;

[0012] Step 4: dissolve the fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer in tetrahydrofuran solution, then add tetrahydrofuran solution containing sodium borohydride and iodine, control the temperature to 0°C for reaction for 0.5 hours, then react at 80-100°C for 6 hours, after the reaction is completed, cool to room temperature, add hydrochloric acid and stir for reaction for 0.5 hours to generate a white precipitate.

[0013] Step 5: Filter the white precipitate, remove the solvent from the filtrate by vacuum distillation, then dissolve it with tetrahydrofuran, wash it with a saturated aqueous solution of anhydrous sodium sulfite after dissolution, and then wash it with deionized water, collect the lower layer of liquid and put it into a vacuum oven for drying. After drying, cool it to room temperature to prepare a terminal hydroxyl fluorine-containing crystalline prepolymer.

[0014] Preferably, the fluorine-containing crystalline polymer is a crystalline copolymer formed by copolymerization of one or more monomers selected from vinylidene fluoride, chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, ethylene, propylene and other double bond-containing monomers.

[0015] Preferably, the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, methyl vinyl ketone, dichloromethane, chloroform, tetrahydrofuran and N-methylpyrrolidone.

[0016] Preferably, the oxidant is one or more of hydrogen peroxide, peracetic acid, persulfate, organic peroxide, potassium permanganate, ammonium perchlorate, metal oxide and transition metal ion.

[0017] Preferably, the alkaline inorganic substance is one or more of tertiary amine, tertiary phosphorus, hydroxide or carbonate of alkali metal.

[0018] Preferably, the concentration of the fluoropolymer solution is 10-50%.

[0019] Preferably, the order of adding the oxidant and the alkaline inorganic substance is to add the alkaline inorganic substance first, and then add the oxidant after the alkaline inorganic substance is completely dissolved.

[0020] Preferably, the amount of the alkaline inorganic substance is 0.001 to 0.01 mole per gram of the crystalline fluorine-containing polymer, and the amount of the oxidant is 1 to 3 times the amount of the alkaline inorganic substance.

[0021] A fluorine-containing crystalline prepolymer with terminal hydroxyl groups is prepared by any of the above methods.

[0022] Preferably, the above-mentioned terminal hydroxyl fluorine-containing crystalline prepolymer has a molecular weight of 800-9000 Daltons, the middle molecular chain is a crystalline fluorine-containing copolymer, and both ends of the molecular chain are terminal hydroxyl groups.

[0023] The present invention has the following beneficial effects:

[0024] The preparation method of the present invention is simple, has few side reactions and low cost, and the product has high mechanical properties and solvent resistance after curing;

[0025] The terminal hydroxyl fluorine-containing prepolymer prepared by the present invention has the characteristic of low viscosity at a temperature above its own melting point;

[0026] The terminal hydroxyl crystalline fluorine-containing prepolymer prepared by the present invention has the characteristics of crystal units. As a result, after curing, the crystal units in the system can act as additional physical cross-linking points, thereby showing more excellent mechanical properties and solvent resistance. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is described below by specific embodiments. However, it should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0028] A terminal hydroxyl fluorine-containing crystalline prepolymer, characterized in that the molecular weight is 800-9000 Daltons, and there is a hydroxyl group at each end of the molecule. The crystalline fluorine-containing polymer is degraded in a solvent by the action of an alkali and an oxidant to generate a terminal carboxyl fluorine-containing crystalline prepolymer, and then a terminal hydroxyl fluorine-containing crystalline prepolymer is prepared by reduction.

[0029] Specific implementation method 1: A method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer in this implementation method comprises the following steps:

[0030] Step 1: adding a fluorine-containing crystalline polymer to an organic solvent and heating and stirring for 3 to 10 hours to ensure that the fluorine-containing crystalline polymer is completely dissolved to prepare a fluorine-containing polymer solution;

[0031] Step 2: Add an oxidant and an alkaline inorganic substance to the fluorine-containing polymer solution in sequence, control the temperature between 20 and 60°C, and continue stirring for 12 to 24 hours in an inert gas atmosphere to prepare a mixed solution:

[0032] Step 3: The mixed solution is eluted in deionized water to remove impurities, and the lower layer of the mixed solution is placed in a vacuum drying oven and dried to a constant weight to obtain a fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer;

[0033] Step 4: dissolving the fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer in a tetrahydrofuran solution, then adding a tetrahydrofuran solution containing sodium borohydride and elemental iodine, controlling the temperature to react for 0.5 hours at 0°C, then reacting at 80-100°C for 6 hours, and after the reaction is completed, cooling to room temperature, adding hydrochloric acid, stirring and reacting for 0.5 hours to generate a white precipitate;

[0034] Step 5: Filter the white precipitate, remove the solvent from the filtrate by vacuum distillation, then dissolve it with tetrahydrofuran, wash it with a saturated aqueous solution of anhydrous sodium sulfite after dissolution, and then wash it with deionized water, collect the lower layer of liquid and put it into a vacuum oven for drying. After drying, cool it to room temperature to prepare a terminal hydroxyl fluorine-containing crystalline prepolymer.

[0035] Through the above steps, the terminal hydroxyl fluorine-containing crystalline prepolymer finally obtained has a high purity and a terminal hydroxyl (-OH) group, which is convenient for subsequent chemical reactions or cross-linking. Due to its crystallinity, it usually has good physical stability and is suitable for the application requirements of various high-performance materials.

[0036] Specific implementation method 2: This implementation method is a method for preparing a terminal hydroxyl fluorine-containing crystalline prepolymer, wherein the fluorine-containing crystalline polymer is a crystalline copolymer formed by copolymerizing one or more of vinylidene fluoride, trifluorochloroethylene and tetrafluoroethylene, hexafluoropropylene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, ethylene, propylene and other double bond-containing monomers.

[0037] The preferred fluorine-containing crystal units account for 60-90% or 10-40%, and the melting point is between 120-150°C.

[0038] The preparation method obtains a crystalline polymer by copolymerizing a fluorine-containing monomer with other double-bond-containing monomers. By introducing terminal hydroxyl groups, the polymer is endowed with good reactivity and functionality. The terminal hydroxyl-containing fluorine-containing crystalline prepolymer finally obtained has excellent crystallinity, high temperature resistance, and chemical corrosion resistance, and is suitable for a variety of applications such as high-performance sealing, coatings, and electronic materials.

[0039] Specific implementation method three: In this implementation method, a method for preparing a terminal hydroxyl-containing fluorine-containing crystalline prepolymer is provided, wherein the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, methyl vinyl ketone, dichloromethane, chloroform, tetrahydrofuran and N-methylpyrrolidone.

[0040] By selecting appropriate organic solvents and reaction conditions, terminal hydroxyl-containing fluorine-containing crystalline prepolymers can be effectively prepared.

[0041] 0-30% of acetone and butanone can also be added to the solvent system to adjust the polarity, volatility and solubility of the solvent, thereby optimizing the polymerization reaction of the terminal hydroxyl fluorine-containing crystalline prepolymer. The appropriate amount of acetone and butanone helps to increase the polymerization rate, control the reaction temperature and optimize the crystallinity of the final polymer. According to the needs of the specific reaction, the reasonable selection of the ratio of acetone and butanone can achieve precise control of the polymer properties.

[0042] Specific implementation method 4: In this implementation method, a method for preparing a terminal hydroxyl-containing fluorine-containing crystalline prepolymer is provided, wherein the oxidant is one or more of hydrogen peroxide, peracetic acid, persulfate, organic peroxide, potassium permanganate, ammonium perchlorate, metal oxides and transition metal ions.

[0043] Specific implementation mode 5: In the preparation method of a fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to the present implementation mode, the alkaline inorganic substance is one or more of tertiary amine, tertiary phosphorus, hydroxide or carbonate of an alkali metal.

[0044] Different types of alkaline inorganic substances can adapt to different reaction environments and reaction systems. For example, some fluorinated monomers may be unstable in acidic environments, while the addition of alkaline inorganic substances can provide a milder environment, which helps to achieve highly selective and efficient polymerization reactions.

[0045] Specific implementation method 6: In the preparation method of a fluorine-containing crystalline prepolymer with terminal hydroxyl groups in this implementation method, the concentration of the fluorine polymer solution is 10-50%.

[0046] Lower concentration (10% to 20%): At lower concentrations, the solubility of fluoropolymers is higher, the reaction system is more diluted, and polymerization can be carried out under milder conditions. Lower concentrations can reduce the occurrence of side reactions, control the speed of the polymerization reaction, and make the polymerization process more controllable.

[0047] Higher concentration (30% to 50%): As the concentration increases, the viscosity of the fluoropolymer solution increases, which may lead to an increase in the reaction rate. At high concentrations, the frequency of collisions between reactant molecules increases, and the polymerization reaction may be faster, but it is also necessary to pay attention to controlling the temperature and reaction time to avoid overreaction or uneven polymer production.

[0048] Specific implementation method seven: In the preparation method of a hydroxy-terminated fluorine-containing crystalline prepolymer of this implementation method, the oxidant and the alkaline inorganic substance are added in the order of first adding the alkaline inorganic substance, and then adding the oxidant after the alkaline inorganic substance is completely dissolved.

[0049] The method effectively controls the reaction rate and stability through a reasonable reaction sequence, reduces the occurrence of side reactions, and thus improves the yield and quality of the terminal hydroxyl-containing fluorine-containing crystalline prepolymer.

[0050] Specific embodiment eight: In the method for preparing a hydroxy-terminated fluorine-containing crystalline prepolymer of this embodiment, the amount of the alkaline inorganic substance used is 0.001 to 0.01 mole per gram of the crystalline fluorine-containing polymer, and the amount of the oxidant used is 1 to 3 times the content of the alkaline inorganic substance.

[0051] Accurately control the dosage ratio of alkaline inorganic substances and oxidants. By using appropriate amounts of alkaline inorganic substances (0.001 to 0.01 mol / g of crystalline fluoropolymer) and oxidants (1 to 3 times the amount of alkaline inorganic substances), the reaction rate can be effectively controlled and the quality and purity of the product can be improved.

[0052] Specific implementation method 9: A fluorine-containing crystalline prepolymer with terminal hydroxyl groups in this implementation method is prepared by any of the specific implementation methods mentioned above.

[0053] Specific implementation method ten: a terminal hydroxyl group fluorine-containing crystalline prepolymer of this implementation method has a molecular weight of 800-9000 Daltons, a middle molecular chain is a crystalline fluorine-containing copolymer, and both ends of the molecular chain are terminal hydroxyl groups.

[0054] The present invention also discloses a curing method: a terminal hydroxyl group-containing fluorine-containing crystalline prepolymer is cured by reacting with an organic compound containing two or more isocyanate groups, or with an organic compound containing two or more epoxy groups, and the molar ratio of the terminal hydroxyl group-containing fluorine-containing crystalline prepolymer to the curing substance containing the isocyanate group or epoxy group is 1:1.02.

[0055] The specific curing process is: the terminal hydroxyl crystalline prepolymer and the curing agent are fully mixed at 30-70°C, then poured into a mold, placed in a vacuum oven at 50-80°C for 10 minutes for vacuum treatment to remove internal bubbles, and then maintained at normal pressure for 1-7 days to complete the curing.

[0056] Example 1

[0057] A method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer:

[0058] Step 1: Place 10 g of vinylidene fluoride-ethylene copolymer (molar ratio 90:10, number average molecular weight Mn=3*104) in N,N-dimethylformamide to prepare a polymer solution with a mass fraction of 12%, increase the temperature to 50°C, and continue to stir slowly for 3 hours to ensure that the vinylidene fluoride-ethylene copolymer is completely dissolved;

[0059] Step 2: Then raise the temperature to 80°C, introduce nitrogen, weigh 2.5g of peracetic acid with a mass concentration of 30%, and directly add it to the ethylene-trifluoroethylene copolymer. Then, after 5 hours of continuous stirring, slowly add 3g of sodium carbonate to the mixed system, slowly stir until the sodium carbonate is completely dissolved, and continue stirring at the same temperature for 12 hours;

[0060] Step 3: After the reaction system is cooled to room temperature, a large amount of deionized water is added for washing, the lower layer of solution is taken out and repeatedly washed three times with deionized water, and finally the lower clear liquid is taken out and placed in a vacuum oven, set at 100°C and -0.1Mpa to dry to constant weight, to obtain a carboxyl-terminated fluorine-containing crystalline prepolymer;

[0061] Step 4: dissolving the above fluorinated carboxyl-terminated fluorinated crystalline prepolymer in a tetrahydrofuran solution, slowly adding it into a tetrahydrofuran solution containing sodium borohydride and iodine, controlling the temperature to react for 0.5 hours at 0°C, then reacting at 80-100°C for 6 hours, then cooling to room temperature, adding hydrochloric acid, stirring and reacting for 0.5 hours, and reacting to generate a white precipitate;

[0062] Step 5: After filtering the white precipitate, the filtrate is subjected to reduced pressure distillation to remove the solvent, then dissolved with tetrahydrofuran, washed with a saturated aqueous solution of anhydrous sodium sulfite, and then repeatedly washed with deionized water, the lower layer of liquid is collected, and then placed in a vacuum oven to dry at a temperature above the melting point of the product. The obtained liquid fluorine-containing terminal hydroxyl fluorine-containing crystalline prepolymer is cooled to room temperature, and the product crystallizes as a waxy solid.

[0063] Example 2

[0064] A method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer:

[0065] Step 1: 20 g of vinylidene fluoride-chlorotrifluoroethylene copolymer (molar ratio 90:10, number average molecular weight Mn=3*104) was placed in a solvent prepared by dimethyl sulfoxide and acetone at a ratio of 9:1, so that the concentration of the polymer solution was 8%, the temperature was raised to 50°C, and the mixture was stirred slowly for 10 hours to ensure that the vinylidene fluoride-chlorotrifluoroethylene copolymer was completely dissolved;

[0066] Step 2: Then raise the temperature to 80°C, keep the system in a continuous nitrogen flow, weigh 7.2g of 30% hydrogen peroxide, and slowly add it dropwise to the ethylene-tetrafluoroethylene copolymer solution using a constant pressure dropping funnel, control the dropping rate to be completed in about 2 hours, then react for 3 hours with constant stirring, and then add 60g of 50% sodium hydroxide aqueous solution dropwise, control the dropping rate to be completed within 1 hour;

[0067] Step 3: Maintain the temperature and continue to stir to continue the reaction for 20 hours. After the reaction system cools to room temperature, add a large amount of deionized water for washing. Take the lower layer of solution and wash it repeatedly three times with deionized water. Finally, take out the lower clear liquid, place it in a vacuum oven, set at 100°C, and dry it to constant weight under -0.1Mpa to obtain a terminal carboxyl fluorine-containing crystalline prepolymer.

[0068] Step 4: dissolving the above fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer in a tetrahydrofuran solution, slowly adding it into a tetrahydrofuran solution containing sodium borohydride and elemental iodine, controlling the temperature to react for 0.5 hours at 0°C, then reacting at 80°C for 6 hours, then cooling to room temperature, adding hydrochloric acid, stirring and reacting for 0.5 hours, and reacting to generate a white precipitate;

[0069] Step 5: After filtering the white precipitate, the filtrate is subjected to reduced pressure distillation to remove the solvent, then dissolved with tetrahydrofuran, washed with a saturated aqueous solution of anhydrous sodium sulfite, and then repeatedly washed with deionized water, the lower layer of liquid is collected, and then placed in a vacuum oven to dry at a temperature above the melting point of the product. The obtained liquid fluorine-containing terminal hydroxyl fluorine-containing crystalline prepolymer is cooled to room temperature, and the product crystallizes as a waxy solid.

[0070] Example 3

[0071] A method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer:

[0072] Step 1: 10 g of vinylidene fluoride-hexafluoropropylene copolymer (molar ratio 90:10, number average molecular weight Mn=2*104) is placed in a solvent of N,N-dimethylacetamide to prepare a polymer solution with a mass fraction of 12%, the temperature is raised to 50°C, and slowly stirred for 2 hours until the ethylene-hexafluoropropylene copolymer is completely dissolved;

[0073] Step 2: Then weigh 5g of sodium persulfate and add it to the reaction system, then add 17g of potassium hydroxide, keep the system temperature at 80°C, nitrogen atmosphere, slowly stir for 24 hours, wait until the reaction system is cooled to room temperature, add a large amount of deionized water for washing, take the lower layer of solution and wash it repeatedly with deionized water three times, finally take out the lower clear liquid, place it in a vacuum oven, set it at 100°C, -0.1Mpa and dry it to constant weight to obtain a carboxyl-terminated fluorine-containing crystalline prepolymer;

[0074] Step 3: dissolving the above fluorinated carboxyl-terminated fluorinated crystalline prepolymer in a tetrahydrofuran solution, slowly adding it into a tetrahydrofuran solution containing sodium borohydride and elemental iodine, controlling the temperature to react for 0.5 hours at 0°C, then reacting at 90°C for 6 hours, then cooling to room temperature, adding hydrochloric acid, stirring and reacting for 0.5 hours, and reacting to generate a white precipitate;

[0075] Step 4: After filtering the white precipitate, the filtrate is subjected to reduced pressure distillation to remove the solvent, then dissolved with tetrahydrofuran, washed with a saturated aqueous solution of anhydrous sodium sulfite, and then repeatedly washed with deionized water, the lower layer of liquid is collected, and then placed in a vacuum oven to dry at a temperature above the melting point of the product, and the obtained liquid fluorine-containing terminal hydroxyl fluorine-containing crystalline prepolymer is cooled to room temperature, and the product crystallizes as a waxy solid.

[0076] Comparative Example 1

[0077] Step 1: Place 10 g of vinylidene fluoride-hexafluoropropylene copolymer (molar ratio 80:20, number average molecular weight Mn=6*104) in a solvent of N,N-dimethylacetamide to prepare a polymer solution with a mass fraction of 12%, raise the temperature to 50°C, and stir slowly for 2 hours until the ethylene-hexafluoropropylene copolymer is completely dissolved;

[0078] Step 2: Then weigh 5g of sodium persulfate and add it to the reaction system, then add 17g of potassium hydroxide, keep the system temperature at 80°C, nitrogen atmosphere, slowly stir for 24 hours, wait until the reaction system is cooled to room temperature, add a large amount of deionized water for washing, take the lower layer of solution and wash it repeatedly with deionized water three times, finally take out the lower clear liquid, place it in a vacuum oven, set it at 100°C, -0.1Mpa and dry it to constant weight to obtain a carboxyl-terminated fluorine-containing crystalline prepolymer;

[0079] Step 3: dissolving the above fluorinated carboxyl-terminated fluorinated crystalline prepolymer in a tetrahydrofuran solution, slowly adding it into a tetrahydrofuran solution containing sodium borohydride and elemental iodine, controlling the temperature to react for 0.5 hours at 0°C, then reacting at 90°C for 6 hours, then cooling to room temperature, adding hydrochloric acid, stirring and reacting for 0.5 hours, and reacting to generate a white precipitate;

[0080] Step 4: After filtering the white precipitate, the filtrate is distilled under reduced pressure to remove the solvent, then dissolved with tetrahydrofuran, washed with a saturated aqueous solution of anhydrous sodium sulfite, and then repeatedly washed with deionized water, the lower layer of liquid is collected, and then placed in a vacuum oven to dry. The resulting yellow liquid is hydroxyl-terminated liquid fluororubber.

[0081] 100℃ viscosity (Pa*s) Number average molecular weight Melting point(℃) Example 1 1.6 3200 73 Example 2 2.8 4400 81 Example 3 1.4 2800 66 Comparative Example 1 1.7 3500 none

[0082] Mix Examples 1-3 and Comparative Example 1 at 100°C with HDI trimer polyisocyanate in an isocyanate / hydroxyl molar ratio of 1.2, add 0.5 parts per million of dibutyltin dilaurate catalyst, stir evenly, and place the vacuum degassing casting mold in a 90°C oven for curing for 2 days. The cured product was tested for tensile strength at a tensile rate of 100 mm / min. The cured product was soaked in different solvents at 40°C for 7 days and then taken out, and the weight gain rate of the sample after soaking (weight gain / original weight) was tested. By comparison, it can be seen that the tensile strength and solvent resistance of the cured product of the terminal hydroxyl fluorine-containing crystalline prepolymer are much higher than those of the terminal hydroxyl liquid fluororubber with similar molecular weight. This is mainly because the physical cross-linking effect formed by its internal crystallization plays a role in enhancing and reducing the solvent permeability.

[0083]

[0084] It should be noted that in the above embodiments, as long as the technical solutions are not contradictory, they can be arranged and combined, and those skilled in the art can exhaust all possibilities based on the mathematical knowledge of arrangement and combination. Therefore, the present invention will no longer describe the technical solutions after arrangement and combination one by one, but it should be understood that the technical solutions after arrangement and combination have been disclosed by the present invention.

[0085] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a hydroxy-terminated fluorine-containing crystalline prepolymer, characterized in that: The steps include: Step 1: adding a fluorine-containing crystalline polymer to an organic solvent, and heating and stirring for 3 to 10 hours to obtain a fluorine polymer solution; Step 2: Add an oxidant and an alkaline inorganic substance to the fluorine-containing polymer solution in sequence, control the temperature between 20 and 60°C, and continue stirring for 12 to 24 hours in an inert gas atmosphere to obtain a mixed solution: Step 3: The mixed solution is eluted in deionized water to remove impurities, and the lower layer of the mixed solution is placed in a vacuum drying oven and dried to a constant weight to obtain a fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer; Step 4: dissolving the fluorine-containing carboxyl-terminated fluorine-containing crystalline prepolymer in a tetrahydrofuran solution, then adding a tetrahydrofuran solution containing sodium borohydride and elemental iodine, controlling the temperature to react for 0.5 hours at 0°C, then reacting at 80-100°C for 6 hours, and after the reaction is completed, cooling to room temperature, adding hydrochloric acid, stirring and reacting for 0.5 hours to generate a white precipitate; Step 5: Filter the white precipitate, remove the solvent from the filtrate by vacuum distillation, then dissolve it with tetrahydrofuran, wash it with a saturated aqueous solution of anhydrous sodium sulfite after dissolution, and then wash it with deionized water, collect the lower layer of liquid and put it into a vacuum oven for drying. After drying, cool it to room temperature to obtain a terminal hydroxyl fluorine-containing crystalline prepolymer.

2. The method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer according to claim 1, characterized in that: The fluorine-containing crystalline polymer is a crystalline copolymer formed by copolymerizing one or more of vinylidene fluoride, chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, ethylene, propylene and other double-bond-containing monomers.

3. The method for preparing a fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to claim 1, characterized in that: The organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, methyl vinyl ketone, dichloromethane, chloroform, tetrahydrofuran and N-methylpyrrolidone.

4. The method for preparing a fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to claim 1, characterized in that: The oxidant is one or more of hydrogen peroxide, peracetic acid, persulfate, organic peroxide, potassium permanganate, ammonium perchlorate, metal oxide and transition metal ion.

5. The method for preparing a fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to claim 1, characterized in that: The alkaline inorganic substance is one or more of tertiary amine, tertiary phosphorus, hydroxide or carbonate of alkali metal.

6. The method for preparing a fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to claim 1, characterized in that: The concentration of the fluoropolymer solution is 10 to 50%.

7. The method for preparing a fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to claim 1, characterized in that: The order of the oxidant and the alkaline inorganic substance is to add the alkaline inorganic substance first, and then add the oxidant after the alkaline inorganic substance is completely dissolved.

8. The method for preparing a hydroxyl-terminated fluorine-containing crystalline prepolymer according to claim 1, characterized in that: The amount of the alkaline inorganic substance used is 0.001 to 0.01 mole per gram of the crystalline fluorine-containing polymer, and the amount of the oxidant used is 1 to 3 times the content of the alkaline inorganic substance.

9. A fluorine-containing crystalline prepolymer having terminal hydroxyl groups, characterized in that: The method is prepared by any one of claims 1 to 8.

10. The fluorine-containing crystalline prepolymer having terminal hydroxyl groups according to claim 9, characterized in that: The molecular weight of the terminal hydroxyl fluorine-containing crystalline prepolymer is 800-9000 Daltons, the middle molecular chain is a crystalline fluorine-containing copolymer, and both ends of the molecular chain are terminal hydroxyl groups.

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