Process for the preparation of an ethylene-tetrafluoroethylene copolymer having a low oligomer content

Abstract

The application provides a preparation method of an ethylene-tetrafluoroethylene copolymer with low oligomer content, and belongs to the technical field of fluorine-containing polymers. A solvent is injected into a polymerization kettle, the material in the polymerization kettle is adjusted to a certain solid content, and then monomers, an initiator, a chain transfer agent and a modified monomer are continuously added, so that a polymerization reaction is carried out in the solvent system to prepare the ethylene-tetrafluoroethylene copolymer with low oligomer content. The resin with low ETFE oligomer content can be prepared by using conventional separation equipment without removing the ETFE oligomer by using hot water granulation, and the prepared ETFE resin has an ETFE oligomer content of less than 0.2 wt%.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fluoropolymer technology, and particularly to a method for preparing ethylene-tetrafluoroethylene copolymer with low oligomer content. BACKGROUND

[0002] The existing ETFE polymerization technology is usually a semi-continuous polymerization process with organic solvent as the medium. In this process, there is no ETFE resin in the polymerization kettle before the polymerization starts, only solvent, pre-added monomer and chain transfer agent and other raw materials. After the polymerization starts, initiation, growth, chain transfer and termination reactions occur in the form of solution polymerization in the solvent. At the same time, when the growing chain grows to a certain degree of polymerization, it will precipitate from the solvent phase to form a gel-like solid particle swelled by the solvent. The growing chain in the solid particle can continue to grow, but because of the gel effect, the growth rate is significantly accelerated and the termination rate is slowed down, often resulting in high molecular weight resin, which is beneficial to the performance of the product.

[0003] The disadvantage of the above process is that the ETFE resin has a wide molecular weight distribution. Because, in the early stage of polymerization, the resin solid content is low, the gel effect contributes less, and the solution polymerization form contributes more dead chains, resulting in smaller molecular weight resin, while in the later stage of polymerization, the gel effect contributes more, producing more high molecular weight resin, some of which is even too high. Too much low molecular weight resin will bring more raw material loss and energy consumption and higher equipment design requirements for later volatile removal. When the volatile removal effect is not good, it will bring problems such as die flow, discoloration, product blistering and mold contamination to ETFE processing. And too high molecular weight will bring shrinkage, distortion and other dimensional stability problems to ETFE products caused by residual stress.

[0004] Patent document US8292203B2 discloses a method for preparing ETFE resin with low oligomer content, the main feature of which is that the ETFE slurry prepared by polymerization in a solvent medium is granulated in hot water, which can simultaneously remove residual monomer, solvent and part of oligomers, and the oligomer content in the obtained ETFE resin is less than 0.20wt%.

[0005] Although the method in US8292203B2 can effectively control the content of ETFE oligomers, it will also produce a large amount of wastewater, and additional water removal process is needed for the recovery of solvent and monomer, resulting in the increase of site and equipment cost, energy consumption and material consumption.

[0006] Therefore, the present application provides a method for preparing ethylene-tetrafluoroethylene copolymer with low oligomer content. SUMMARY

[0007] The present application aims to provide a method for preparing ethylene-tetrafluoroethylene copolymer with low oligomer content, which can prepare ETFE resin with low ETFE oligomer content by using conventional separation equipment without using hot water granulation to remove ETFE oligomer, and the prepared ETFE resin has ETFE oligomer content lower than 0.2 wt%.

[0008] To achieve the above object, the present application adopts the following technical solutions:

[0009] The present application provides a method for preparing ethylene-tetrafluoroethylene copolymer with low oligomer content, characterized in that the method comprises the following steps:

[0010] Injecting solvent into a polymerization kettle, adjusting the solid content of the materials in the polymerization kettle, and then continuously adding monomers, initiators, chain transfer agents and modified monomers to carry out polymerization reaction in the solvent system to prepare ethylene-tetrafluoroethylene copolymer with low oligomer content.

[0011] In a specific embodiment, the specific operation steps of the ethylene-tetrafluoroethylene polymerization are as follows:

[0012] (1) After the polymerization system including but not limited to polymerization kettle, monomer tank, solvent tank and other equipment pipelines are vacuumed and deoxygenated and qualified for nitrogen, the polymerization kettle is pumped with solvent, the solid content of the materials in the polymerization kettle is adjusted to a certain value, and the solvent loading coefficient is controlled at 0.5-0.8;

[0013] (2) The mixture of ethylene and tetrafluoroethylene is introduced into the polymerization kettle until the pressure of the polymerization kettle is 1.1-2.2 MPa, the modified monomer and the chain transfer agent are pumped by the metering pump, the polymerization kettle is heated to 35-85℃, and the initiator is pumped by the metering pump to initiate the polymerization reaction;

[0014] (3) The mixture of ethylene and tetrafluoroethylene is continuously introduced to maintain the specified pressure of the reaction, the modified monomer is continuously pumped by the metering pump to participate in the polymerization reaction, and when the solid content of the polymerization system reaches 5-50%, the monomer is stopped from being introduced, the polymerization kettle is rapidly depressurized, and the reaction is terminated;

[0015] (4) The polymerization kettle is discharged, and the monomer and the solvent are recovered to obtain ethylene-tetrafluoroethylene copolymer with low oligomer content.

[0016] In a specific embodiment, the method for adjusting the solid content in the kettle before the polymerization starts includes but is not limited to:

[0017] (1) A certain amount of ETFE slurry of the previous batch is reserved in the kettle;

[0018] (2) ETFE powder resin and solvent are mixed in the kettle at a certain ratio.

[0019] In a specific embodiment, the solid content before the polymerization starts is 0.5-50%.

[0020] In one embodiment, the nitrogen is not more than 2% and the oxygen is not more than 15 ppm in the polymerization system.

[0021] In one embodiment, the solvent is one of chlorofluorocarbon, hydrofluorocarbon, hydrofluoroether, perfluoroether.

[0022] In one embodiment, the monomer is ethylene, tetrafluoroethylene, and the mole ratio of ethylene to tetrafluoroethylene is 60:40 to 5:95.

[0023] In one embodiment, the initiator is an organic peroxide, azo compound or other compound capable of generating free radicals or energy source, and the amount of initiator added is 0.2-1.0 g per liter of solvent.

[0024] In one embodiment, the chain transfer agent is one or mixture of alkane, alcohol, ketone, ester, aldehyde, hydrogen, and the amount of chain transfer agent added is 0.4-2.2 g per liter of fluorine-containing solvent.

[0025] In one embodiment, the modified monomer is one or mixture of hexafluoropropylene, hexafluoroisobutene, perfluoroalkyl vinyl ether, perfluoroalkyl ethylene.

[0026] In one embodiment, the initiator is an organic peroxide, azo compound or other compound capable of generating free radicals or energy source, and the amount of initiator added is 0.2-1.0 g per liter of solvent.

[0027] In one embodiment, the initiator is an organic peroxide, azo compound or other compound capable of generating free radicals or energy source, and the amount of initiator added is 0.2-1.0 g per liter of solvent.

[0028] S1: 5-10 parts of 4-vinyl-1,2-benzenedicarboxylic acid and 10-20 parts of Cr(NO3)3·9H2O were weighed by mass fraction, placed in a sealed reaction kettle, and nitrogen was introduced to replace the air. Then 200-300 parts of deionized water was added and mixed and stirred. The mixed solution was poured into a hydrothermal reaction kettle with a polytetrafluoroethylene liner and sealed, and then placed in an oven with the temperature adjusted to 200-230°C. After crystallization, it was taken out and naturally cooled to room temperature. The precursor was obtained.

[0029] S2: 10-15 parts of the precursor, 100-140 parts of toluene, 3-7 parts of (ferrocenyl) hexanethiol, and 2-10 parts of triethylamine were weighed by mass fraction, placed in a sealed reaction kettle, and nitrogen was introduced to replace the air. The temperature was adjusted to 50-70°C and refluxed. Toluene was removed by distillation and dried to obtain the initiator synergist.

[0030] Compared with the prior art, the powder spraying ethylene-tetrafluoroethylene copolymer prepared by the embodiment of the present application has the following beneficial effects:

[0031] The present application only needs to prepare a resin with low ETFE oligomer content by using a conventional separation device, without using hot water granulation to remove ETFE oligomer. According to the polymerization process in the present application, the obtained ETFE resin has an ETFE oligomer content of less than 0.2 wt%. There is no die-swell phenomenon, and the size stability of the molded, extruded and injection molded products is good. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The polymerization process flow chart of the present application is shown in the figure. DETAILED DESCRIPTION

[0033] The technical solutions of the present application will be further described by specific embodiments. Those skilled in the art should understand that the specific embodiments are only used to help understand the present application, and should not be regarded as specific limitations of the present application. If not specifically indicated, the technical means used in the following examples are conventional means known to those skilled in the art, and the raw materials used are conventional commercial goods.

[0034] <Example evaluation method>

[0035] 1. Melting point test (Tm, ℃)

[0036] 5-10 mg of ETFE sample was taken into the sample cell, and the temperature was raised from 100℃ to 300℃ at a rate of 10℃ / min, and the change of the curve was recorded. The DSC instrument was DCS702 from Japan Precision Instrument Co., Ltd.

[0037] 2. Melt index (MFI, g / 10min)

[0038] ETFE was tested according to international standard ASTM-3159. The load for testing was 5 kg, the temperature was 297℃, and the instrument used was a melt index tester of model MF30 from Italy Ceast Company.

[0039] 3. Mechanical property test

[0040] A sample rotating type Geer Oven (manufactured by ESPEC CORP., GPHH-101) was used as the measuring device, and a sample obtained by cutting the obtained ETFE film to a size of 7 cm x 15 cm was put in, and dumbbell-shaped pieces were punched out according to the shape specified in ASTM D638 TYPE V, and the breaking strength (MPa) and breaking elongation (%) in the longitudinal direction (MD direction) and the transverse direction (TD direction) were measured. Moreover, the average values in the longitudinal direction and the transverse direction were taken as the breaking strength and breaking elongation of the ETFE film.

[0041] 4. ETFE oligomer content (mass %)

[0042] Into a pressure vessel having a PTFE (polytetrafluoroethylene) inner cylinder, 30 g of the ETFE prepared in the example and 300 g of 1,3-dichloro-1,1,2,2,3-pentafluoropropane were charged, and the pressure vessel was heated with a heating furnace at 150°C for 12 hours. The pressure vessel taken out from the heating furnace was cooled to room temperature, and the mixture of ETFE and 1,3-dichloro-1,1,2,2,3-pentafluoropropane was filtered, and 1,3-dichloro-1,1,2,2,3-pentafluoropropane contained in the filtrate was completely evaporated by a rotary evaporator, and the mass of the extracted ETFE oligomer was measured.

[0043] <Example 1>

[0044] A method for producing an ethylene-tetrafluoroethylene copolymer having a low oligomer content, characterized by comprising the steps of:

[0045] (1) After the polymerization system including but not limited to a polymerization kettle, a monomer tank, a solvent tank, and the like are vacuumed and deoxygenated, and nitrogen is qualified, the polymerization kettle is pumped with a solvent, perfluoro(methylcyclohexane), the solid content of the material in the polymerization kettle is adjusted to 1%, and the solvent loading coefficient is controlled to 0.5;

[0046] (2) A mixture of ethylene and tetrafluoroethylene is introduced into the polymerization kettle until the pressure of the polymerization kettle is 1.35 MPa, wherein the molar ratio of ethylene to tetrafluoroethylene is 75:25, a modified monomer, hexafluoropropene, and a chain transfer agent, methanol, are pumped by a metering pump, the chain transfer agent is added in an amount of 0.45 g per liter of fluorine-containing solvent, the polymerization kettle is heated to 55°C, and an initiator, tert-butyl peroxyisobutyrate, is pumped by a metering pump to initiate the polymerization reaction, and the initiator is added in an amount of 0.35 g per liter of solvent;

[0047] (3) The mixture of ethylene and tetrafluoroethylene is continuously introduced to maintain the reaction pressure, wherein the molar ratio of ethylene to tetrafluoroethylene is 49:51, and a modified monomer is continuously pumped by a metering pump to participate in the polymerization reaction, and when the solid content of the polymerization system reaches 5%, the introduction of the monomer is stopped, the polymerization kettle is rapidly depressurized, and the reaction is terminated.

[0048] (4) The polymerization kettle is discharged, and the monomer and the solvent are recovered to obtain an ethylene-tetrafluoroethylene copolymer having a low oligomer content.

[0049] In addition, an initiation synergist is also added to participate in the polymerization before the initiator in step (2) is added, and the initiation synergist is added in an amount of 2% of the initiator.

[0050] The preparation method of the initiation synergist is as follows:

[0051] S1: take 5g of 4-vinyl-1,2-benzenedicarboxylic acid, 10g of Cr(NO3)3·9H2O, place in a sealed reaction kettle, replace air with nitrogen, then add 200g of deionized water, mix and stir; pour the mixed solution into a hydrothermal reaction kettle with a polytetrafluoroethylene lining, seal, then put into an oven, adjust the oven temperature to 200℃, after crystallization, take out, and let it cool to room temperature naturally; obtain a precursor;

[0052] S2: take 10g of the precursor, 100g of toluene, 3g of (ferrocenyl)hexanethiol, and 2g of triethylamine, place in a sealed reaction kettle, replace air with nitrogen, adjust the temperature to 50℃, and heat under reflux; remove toluene by distillation, dry, and obtain an initiation synergist.

[0053] [Example 2]

[0054] A method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content, characterized in that it comprises the following steps:

[0055] (1) After the polymerization system, including but not limited to polymerization kettle, monomer tank, solvent tank, and other equipment pipelines, is vacuumed and deoxygenated, and qualified with nitrogen, pump 1,1,2,2-tetrafluoroethyl ethyl ether into the polymerization kettle, adjust the solid content in the polymerization kettle to 2%, and control the solvent loading coefficient to 0.6;

[0056] (2) Introduce a mixture of ethylene and tetrafluoroethylene into the polymerization kettle until the pressure of the polymerization kettle is 1.45 MPa, wherein the molar ratio of ethylene to tetrafluoroethylene is 70:30, meter in modified monomer perfluoroalkyl vinyl ether and chain transfer agent 2-mercaptoethanol, the chain transfer agent is added at a dosage of 0.85 g per liter of fluorine-containing solvent, the polymerization kettle is heated to 55℃, and meter in initiator tert-butyl peroxyneodecanoate to initiate polymerization, the initiator is added at a dosage of 0.45 g per liter of solvent;

[0057] (3) Continuously introduce a mixture of ethylene and tetrafluoroethylene to maintain the reaction pressure, wherein the molar ratio of ethylene to tetrafluoroethylene is 54:46, continuously pump in the modified monomer to participate in the polymerization reaction, and when the solid content of the polymerization system reaches 8.5%, stop introducing the monomer, quickly release the pressure of the polymerization kettle, and terminate the reaction.

[0058] (4) Discharge the polymerization kettle, and recover the monomer and solvent to obtain an ethylene-tetrafluoroethylene copolymer with low oligomer content.

[0059] In addition, an initiation synergist is also added to participate in the polymerization before the initiator is added in step (2), and the addition amount of the initiation synergist is 4% of the initiator.

[0060] The preparation method of the initiation synergist is as follows:

[0061] S1: Weigh 6.5 g of 4-vinyl-1,2-phthalic acid and 13 g of Cr(NO3)3·9H2O, place them in a closed reactor, introduce nitrogen to replace the air, add 225 g of deionized water, and mix and stir; pour the mixture into a hydrothermal reactor with a polytetrafluoroethylene liner and seal it, then place it in an oven, adjust the oven temperature to 210°C, take it out after crystallization, and let it stand and cool naturally to room temperature to obtain a precursor;

[0062] S2: Weigh 12 g of the precursor, 110 g of toluene, 4 g of (ferrocenyl)hexanethiol, and 4 g of triethylamine, place them in a sealed reaction kettle, introduce nitrogen to replace the air, adjust the temperature to 55°C, and heat under reflux; distill off the toluene, and dry to obtain an initiator synergist.

[0063] <Example 3>

[0064] A method for preparing an ethylene-tetrafluoroethylene copolymer with a low oligomer content, characterized by comprising the following steps:

[0065] (1) After the polymerization system, including but not limited to the polymerization kettle, monomer storage tank, solvent tank and other equipment pipelines, is vacuumed and deoxygenated and nitrogen-free, the solvent 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether is pumped into the polymerization kettle, and the solid content of the materials in the polymerization kettle is adjusted to 4.5%, and the solvent loading coefficient is controlled at 0.7;

[0066] (2) A mixture of ethylene and tetrafluoroethylene is introduced into the polymerization kettle until the pressure of the polymerization kettle reaches 1.65 MPa, wherein the molar ratio of ethylene to tetrafluoroethylene is 65:35, and a metering pump is used to pump in the modified monomer hexafluoroisobutylene and the chain transfer agent cyclohexane, and the amount of the chain transfer agent added is 1.35 g per liter of fluorinated solvent. The polymerization kettle is heated to 65° C. and an initiator ammonium persulfate is pumped in by a metering pump to initiate the polymerization reaction, and the amount of the initiator added is 0.65 g per liter of solvent;

[0067] (3) Continuously introduce a mixture of ethylene and tetrafluoroethylene to maintain the reaction pressure, wherein the molar ratio of ethylene to tetrafluoroethylene is 55:45, and continuously pump the modified monomer into the polymerization reaction by a metering pump. When the solid content of the polymerization system reaches 12%, the monomer introduction is stopped, the polymerization kettle is quickly depressurized, and the reaction is terminated.

[0068] (4) Discharging the polymerization reactor, recovering the monomers and solvent to obtain an ethylene-tetrafluoroethylene copolymer with a low oligomer content.

[0069] In addition, before adding the initiator in step (2), an initiation synergist is added to participate in the polymerization, and the amount of the initiation synergist added is 7.5% of the initiator.

[0070] In a specific embodiment, the initiation synergist is prepared by:

[0071] S1: 8 g of 4-vinyl-1,2-benzenedicarboxylic acid and 17.5 g of Cr(NO3)3·9H2O were weighed and placed in a sealed reaction kettle, nitrogen was introduced to replace the air, then 265 g of deionized water was added, and the mixture was stirred; the mixed solution was poured into a hydrothermal reaction kettle with a polytetrafluoroethylene lining, and sealed, then placed in an oven, the temperature of the oven was adjusted to 220°C, and after crystallization, it was taken out and naturally cooled to room temperature; a precursor was obtained;

[0072] S2: 14 g of 130 g of toluene, 6 g of (ferrocenyl)hexanethiol, and 8 g of triethylamine were weighed and placed in a sealed reaction kettle, nitrogen was introduced to replace the air, the temperature was adjusted to 60°C, and reflux heating was performed; toluene was removed by distillation, and the initiator synergist was obtained after drying.

[0073] <Example 4>

[0074] A method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content, characterized in that it comprises the following steps:

[0075] (1) The polymerization system, including but not limited to polymerization kettle, monomer tank, solvent tank, and other equipment pipelines, was vacuumed and deoxygenated, and then nitrogen was introduced to replace the air, the polymerization kettle was pumped into solvent perfluorocyclohexane, the solid content in the polymerization kettle was adjusted to 9%, and the solvent loading coefficient was controlled at 0.8;

[0076] (2) A mixture of ethylene and tetrafluoroethylene was introduced into the polymerization kettle until the pressure of the polymerization kettle reached 1.85 MPa, wherein the molar ratio of ethylene to tetrafluoroethylene was 51:49, the modified monomer perfluorobutyl ethylene and the chain transfer agent acetone were pumped in by a metering pump, the chain transfer agent was added at an amount of 2.05 g per liter of fluorine-containing solvent, the polymerization kettle was heated to 75°C, and the initiator dicumyl peroxide was pumped in by a metering pump to initiate the polymerization reaction, and the initiator was added at an amount of 0.95 g per liter of solvent;

[0077] (3) The mixture of ethylene and tetrafluoroethylene was continuously introduced to maintain the reaction pressure, wherein the molar ratio of ethylene to tetrafluoroethylene was 56:44, and the modified monomer was continuously pumped in by a metering pump to participate in the polymerization reaction, until the solid content of the polymerization system reached 16.5%, then the monomer was stopped, the polymerization kettle was rapidly depressurized, and the reaction was terminated.

[0078] (4) The polymerization kettle was discharged, and the monomer and solvent were recovered to obtain an ethylene-tetrafluoroethylene copolymer with low oligomer content.

[0079] In addition, an initiation synergist was also added to participate in the polymerization before the initiator was added in step (2), and the amount of the initiation synergist added was 9% of the initiator.

[0080] The preparation method of the initiation synergist is as follows:

[0081] S1: 10 g of 4-vinyl-1,2-benzenedicarboxylic acid and 20 g of Cr(NO3)3·9H2O were weighed and placed in a sealed reaction kettle, nitrogen was introduced to replace the air, 300 g of deionized water was then added, and the mixture was stirred; the mixed solution was poured into a hydrothermal reaction kettle with a polytetrafluoroethylene lining, and sealed, then placed in an oven, the temperature of the oven was adjusted to 230°C, and after crystallization, it was taken out and naturally cooled to room temperature; a precursor was obtained;

[0082] S2: 15 g of the precursor, 140 g of toluene, 7 g of (ferrocenyl)hexanethiol, and 10 g of triethylamine were weighed and placed in a sealed reaction kettle, nitrogen was introduced to replace the air, the temperature was adjusted to 70°C, and reflux heating was performed; toluene was removed by distillation, and the initiator synergist was obtained after drying.

[0083] <Comparative Example 1>

[0084] A method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content, characterized in that it comprises the following steps:

[0085] (1) The polymerization system, including but not limited to polymerization kettle, monomer tank, solvent tank, etc., was vacuumed and deoxygenated, and then nitrogen was introduced to replace the air, the polymerization kettle was pumped into the solvent perfluoro(methylcyclohexane), the solid content in the polymerization kettle was adjusted to 1%, and the solvent loading coefficient was controlled at 0.5;

[0086] (2) The mixture of ethylene and tetrafluoroethylene was introduced into the polymerization kettle until the pressure of the polymerization kettle reached 1.35 MPa, wherein the molar ratio of ethylene to tetrafluoroethylene was 75:25, the modified monomer hexafluoropropene and the chain transfer agent methanol were pumped by a metering pump, the chain transfer agent was added at an amount of 0.45 g per liter of fluorine-containing solvent, the polymerization kettle was heated to 55°C, and the initiator tert-butyl peroxyisobutyrate was pumped by a metering pump to initiate the polymerization reaction, and the initiator was added at an amount of 0.35 g per liter of solvent;

[0087] (3) The mixture of ethylene and tetrafluoroethylene was continuously introduced to maintain the reaction pressure, wherein the molar ratio of ethylene to tetrafluoroethylene was 49:51, the modified monomer was continuously pumped by a metering pump to participate in the polymerization reaction, and when the solid content of the polymerization system reached 5%, the introduction of the monomer was stopped, the polymerization kettle was rapidly depressurized, and the reaction was terminated.

[0088] (4) The polymerization kettle was discharged, and the monomer and solvent were recovered to obtain an ethylene-tetrafluoroethylene copolymer with low oligomer content.

[0089] <Comparative Example 2>

[0090] A method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content, characterized in that it comprises the following steps:

[0091] (1) The polymerization system, including but not limited to polymerization kettle, monomer tank, solvent tank and other equipment pipelines, is vacuumed and deoxygenated, and qualified with nitrogen, and then the polymerization kettle is pumped with solvent perfluoro(methylcyclohexane), and the solvent loading factor is controlled at 0.5;

[0092] (2) The polymerization kettle is fed with a mixture of ethylene and tetrafluoroethylene, and the pressure of the polymerization kettle is 1.35 MPa, wherein the molar ratio of ethylene to tetrafluoroethylene is 75:25, and the metering pump is pumped with modified monomer hexafluoropropene and chain transfer agent methanol, and the chain transfer agent is added at an amount of 0.45 g per liter of fluorine-containing solvent, and the polymerization kettle is heated to 55°C, and the metering pump is pumped with initiator tert-butyl peroxyisobutyrate to initiate the polymerization reaction, and the initiator is added at an amount of 0.35 g per liter of solvent;

[0093] (3) The mixture of ethylene and tetrafluoroethylene is continuously fed to maintain the reaction pressure, wherein the molar ratio of ethylene to tetrafluoroethylene is 49:51, and the metering pump is continuously pumped with modified monomer to participate in the polymerization reaction, and when the solid content of the polymerization system reaches 5%, the feeding of the monomer is stopped, the polymerization kettle is rapidly depressurized, and the reaction is terminated.

[0094] (4) The polymerization kettle is discharged, and the monomer and solvent are recovered to obtain an ethylene-tetrafluoroethylene copolymer with low oligomer content.

[0095] The test results of the ethylene-tetrafluoroethylene copolymers prepared in the above examples and comparative examples are shown in the following table:

[0096]

[0097] The above examples can find that the oligomer content prepared by the method is significantly reduced, and the ETFE resin prepared has an ETFE oligomer content of not higher than 0.2 wt%, while maintaining the excellent mechanical properties and high melting point of the ethylene tetrafluoroethylene copolymer.

[0098] Although the present application has been described in detail in the foregoing general description, specific embodiments and experiments, modifications or improvements can be made on the basis of the present application, which is obvious to those skilled in the art. Therefore, these modifications or improvements made on the basis of not deviating from the spirit of the present application, all belong to the scope of protection claimed by the present application.

Claims

1. A method for preparing an ethylene-tetrafluoroethylene copolymer having a low oligomer content, characterized in that: The steps include: (1) The equipment pipelines of the polymerization system including the polymerization kettle, monomer storage tank and solvent tank are vacuum-evacuated to remove oxygen and nitrogen until the nitrogen is no more than 2% and the oxygen is no more than 15 ppm. The solvent is pumped into the polymerization kettle, and the solid content of the ETFE material in the polymerization kettle is adjusted to 0.5-9%, and the solvent loading coefficient is controlled at 0.5-0.8; (2) A mixture of ethylene and tetrafluoroethylene is introduced into the polymerization kettle until the pressure of the polymerization kettle reaches 1.1-2.2 MPa, and a modified monomer and a chain transfer agent are pumped into the polymerization kettle by a metering pump. The polymerization kettle is heated to 35-85° C., and an initiator synergist is added to participate in the polymerization. The initiator is pumped into the polymerization reaction by a metering pump to initiate the polymerization reaction. The amount of the initiator synergist added is 1-10% of the initiator; (3) continuously introducing a mixture of ethylene and tetrafluoroethylene to maintain the specified reaction pressure, and continuously pumping the modified monomer into the polymerization reaction with a metering pump until the solid content of the polymerization system reaches 5-50%, then stopping the introduction of monomers, and rapidly depressurizing the polymerization kettle to terminate the reaction; (4) discharging the polymerization reactor, recovering the monomers and solvent to obtain an ethylene-tetrafluoroethylene copolymer with a low oligomer content; The method for adjusting the solid content in the kettle in step (1) comprises: (1) Keep a certain amount of the previous batch of ETFE slurry in the kettle; (2) Mixing ETFE powder resin and solvent in a kettle according to a certain ratio; The preparation method of the initiator synergist is: S1: Weigh 5-10 parts by mass of 4-vinyl-1,2-phthalic acid and 10-20 parts of Cr(NO3)3·9H2O, place them in a closed reactor, introduce nitrogen to replace the air, then add 200-300 parts of deionized water, and mix and stir; pour the mixture into a hydrothermal reactor with a polytetrafluoroethylene liner and seal it, then place it in an oven, adjust the oven temperature to 200-230°C, take it out after crystallization, and let it stand and cool naturally to room temperature to obtain a precursor; S2: Weigh 10-15 parts of the precursor, 100-140 parts of toluene, 3-7 parts of (ferrocenyl)hexanethiol, and 2-10 parts of triethylamine by mass, place them in a closed reaction vessel, introduce nitrogen to replace the air, adjust the temperature to 50-70°C, and reflux; distill off the toluene, and dry to obtain an initiator synergist.

2. The method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content according to claim 1, characterized in that: The solvent is one of chlorofluorocarbon, hydrofluorocarbon, hydrofluoroether and perfluoroether.

3. The method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content according to claim 1, characterized in that: The monomers are ethylene and tetrafluoroethylene, and the molar ratio of ethylene to tetrafluoroethylene is 60:40 to 5:

95.

4. The method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content according to claim 1, characterized in that: The initiator is an organic peroxide, an azo compound or other compound capable of generating free radicals or an energy source, and the amount of the initiator added is 0.2-1.0 g per liter of solvent.

5. The method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content according to claim 1, characterized in that: The chain transfer agent is one or a mixture of alkanes, alcohols, ketones, esters, aldehydes, and hydrogen. The amount of the chain transfer agent added is 0.4-2.2 g per liter of fluorine-containing solvent.

6. The method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content according to claim 1, characterized in that: The modified monomer is one or a mixture of hexafluoropropylene, hexafluoroisobutylene, perfluoroalkyl vinyl ether, and perfluoroalkyl ethylene, and the added amount of the modified monomer accounts for 0.1-5 mol% of the total amount of ethylene and tetrafluoroethylene.

7. The method for preparing an ethylene-tetrafluoroethylene copolymer with low oligomer content according to claim 1, characterized in that: The amount of the initiator synergist added in step (2) is 2-5% of the initiator.

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