RU2398789C1 - Method of producing granular heat-resistant copolymers of tetrafluoroethylene with ethylene and copolymers obtained using said method - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method of producing granular heat-resistant copolymers of tetrafluoroethylene with ethylene used as strong insulating material in electrical engineering, in electronics, nuclear, oil-extraction and chemical industries. The copolymers are obtained through copolymerisation of monomers in an aqueous medium in the presence of a persulphate initiator, an aliphatic alcohol and modifying additives. A mixture of the said monomers is loaded into the reaction zone in molar ratio 3:1. The mixture is replenished by a replenishing mixture which contains 50-53 mol % tetrafluoroethylene and 47-50 mol % ethylene. The aliphatic alcohol used is tert-butanol taken in amount of 0.5-25 wt % of aqueous phase which is added to the reaction mass before loading the monomers. An extra modifying comonomer of hexafluoropropylene or perfluoropropylvinyl ether is used in amount of 5-50 wt % of the weight of the loaded mixture of monomers.

EFFECT: obtaining copolymers with high packed density 0,50-0,60 g/cm³ and wide processing range 100-150°C.

4 cl, 1 tbl, 5 dwg, 14 ex

Description

The invention relates to the production of bulk heat-resistant copolymers of tetrafluoroethylene (TFE) with ethylene (E), which are valuable structural materials and have improved processability. They have high strength and dielectric characteristics combined with high chemical resistance, heat and radiation resistance. These copolymers are used as durable insulating material in electrical engineering, in the electronic, nuclear, oil and chemical industries. In particular, they are used to insulate cables operating at a temperature of 180-200 ° C, retain flexibility and do not crack under load.

It is known that TFE-E copolymers are made in the form of granules or powders, which are then processed at high temperatures: by casting, extrusion, or powder spraying; therefore, in addition to high heat resistance, they have good flowability and particles of uniform size and shape. The fractional composition and particle shape of the powders depend on the method of their preparation. Powders obtained in the form of aqueous suspensions or isolated from dispersions by coagulation contain particles ranging in size from fractions of a micron to several micrometers. Such polydisperse powders, being in the bins of the processed equipment, undergo partial separation due to constant shaking: larger particles seem to float up, and small ones sink down. The same thing happens in vibrating batchers and dryers. Therefore, a large number of fine dust particles can accumulate in the loading opening of the injection or tablet equipment, as a result of which the flow of powder to the processing parts of the machine will stop. With poor flowability, the powder freezes in the hopper and its flow into the machine spontaneously stops. Dramatically decreases equipment performance.

The uniformity of the raw materials also has a noticeable effect on the thermal regime of processing in an injection or extrusion machine: individual large particles warm up more slowly than small ones, which can cause the formation of small swelling and roughness on the surface of the finished product.

Particularly high demands are made on the flowability and particle size distribution of the powder when producing polymer coatings by sputtering, since when using polydisperse powders it is not possible to obtain a uniform suspended layer. Coatings have an uneven surface and vary in thickness. In addition, polydisperse powders, due to the different thermal conductivity of small and large particles, fuse poorly and form weak coatings.

A known suspension method for producing TFE copolymers with E with a processing interval of more than 100 ° C (US Pat. NRB No. 40561, class C08F 214/26, 214/00, publ. 15.01.87). The copolymer is obtained in an aqueous-organic medium containing 30-80 wt.% Tert-butyl alcohol using organic peroxides at a temperature of 50-70 ° C and a pressure of 3.5-5.0 MPa. The product isolated from a 20-25% suspension is washed with water, ethanol and dried at 67-72 ° C. The resulting powder has a melting point of 232-250 ° C, melt flow in the range of 2.5-60 g / 10 min, strength 28-32 MPa and elongation of 300-400%. The disadvantage of this method is that the synthesis is carried out in the absence of emulsifiers. The product is obtained in the form of a suspension from which a powder is emitted, which is very heterogeneous in particle size, which further complicates its processing. The resulting polymer has too high values of melt flow and mass loss upon heating. With its industrial implementation, there is a need for the regeneration of large amounts of solvent, which reduces the productivity of the process and increases the cost of the target product. In addition, the synthesis is carried out at high pressures of 3.5-5.0 MPa, which is unsafe, since the mixture of TFE monomers with E is explosive.

Water-emulsion methods for the preparation of TFE-E copolymers are more convenient and safe. Known water-emulsion method of obtaining resistant to cracking modified TFE copolymers with E with improved color and heat resistance (US Pat. RF No. 2156776, IPC 7 C08F 214/26, 4/40, publ. 09/27/2000). A mixture of TFE monomers with E in a molar ratio of 4: 1 with a content of 0-15 mol% of a modifying comonomer: hexafluoropropylene (HFP), perfluoroalkyl vinyl ether (PFAVE), vinylidene fluoride (VDF), etc. is loaded into the reactor. The synthesis is carried out in the presence of a fluorinated emulsifier - mixtures of perfluoroenanthate and perfluoropelargonic acids or their salts, in an amount of 0.2 wt.% from the aqueous phase. An organic redox system (β-hydroxyethyl tert-butyl peroxide - ascorbic acid) in a molar ratio of (20-25): 1 is used as an initiator. The synthesis is carried out at a temperature of 23-27 ° C and a pressure of 2.1 MPa. The speed of the process is maintained by constant batch dosing of ascorbic acid (AK), and the pressure by constant feeding into the reaction zone of a feed mixture of TFE monomers with E in a 1: 1 molar ratio. The result is an aqueous dispersion with a concentration of 18-23 wt.%, Which is used for coating. From this dispersion, powder is isolated by freezing or mechanical coagulation with the addition of electrolytes to increase the efficiency of polymer isolation. The copolymers obtained by the known method are resistant to cracking at 200 ° C and have high heat resistance (weight loss during heating of 0.30-0.35 wt.%). They practically do not change color at processing temperatures of 275-300 ° C and operation up to 200 ° C. The disadvantage of this method is that in order to obtain large, free-flowing powders by freezing, special equipment is required, while at the coagulation stage, productivity is reduced. The powder isolated from the dispersion by mechanical coagulation is very fine, with a particle size of less than 30 microns. It has poor flowability and low bulk density of 0.2-0.3 g / cm ³ , which complicates its processing and reduces the productivity of processing equipment. In the case of mechanical coagulation with the addition of electrolytes used to increase the efficiency of polymer separation, the heat resistance of the product sharply worsens, since the polymer separated from the dispersion in the form of a floating powder is difficult to wash from the added electrolyte and organic solvents, for example ethyl alcohol, have to be additionally used, which complicates process in its industrial implementation. In addition, the known method provides for the use of specific peroxide, which is not produced by industry, this increases the cost of the polymer and complicates its production, since it requires the organization of additional production of explosive reagent. The use of AK creates a high acidity of the medium, which during the polymerization process causes corrosion of the equipment used, pollutes the product, worsening its properties.

A known method of producing thermoprocessable TFE copolymers in the form of microspheres with high bulk density, suitable for powder spraying (US Pat. EP No. 1172380 A2, class C08F 6/22, publ. 16.01.2002). The dispersion of a copolymer with a concentration of 25-30 wt.%, Obtained by the water-emulsion method, is mechanically coagulated in a special apparatus equipped with reflective partitions. The apparatus is filled with dispersion, a coagulant (for example, HNO ₃ or NaOH) is added, and the contents are mixed with a multi-tiered blade mixer, rotating at a speed ranging from 5 rpm to 25 rpm at a temperature of 15-70 ° C. The time spent on coagulation varies widely from 25 minutes to 10 hours. A copolymer powder with large particles of 230 μm is obtained, which has a high bulk density of 0.82 g / cm ³ and a specific surface area of 8 m ² / g. The disadvantage of this method is that the copolymer has very poor heat resistance. Floating powder isolated from the dispersion with the addition of electrolytes is practically impossible to wash from the added coagulant. Products made from such a product have a high mass loss upon heating, and upon heat treatment acquire an extraneous dark color.

Closest to the claimed combination of essential features is a method for producing crack-resistant TFE-E copolymers in an aqueous medium (Pat. SU No. 1840101, class C08F 214/26, 210/02, 2/44, publ. 06.27.2006). The copolymerization of equimolar amounts of TFE and E is carried out in the presence of radical initiators and modifying additives - alkaline or ammonium salts of monobasic perfluoro- or omega-monohydroperfluorocarboxylic acids, which are introduced into the reaction mass in the amount of 0.2-0.8 wt.% By weight monomers. In order to increase the economy and simplify the technology, the synthesis is carried out in the presence of potassium or ammonium persulfate, and C ₁ -C ₂ aliphatic alcohols are used to control the molecular weight of the copolymer. A mixture of these monomers is loaded into the reaction zone, which contains 50-60 mol.% TFE and 50-40 mol.% E. The copolymerization is carried out at a temperature of 60-70 ° C and a pressure of 2.6-2.8 MPa, supported by the constant addition of feed a mixture of monomers containing 45-50 mol.% TFE and 50-55 mol.% E. The product is obtained in the form of a floating powder or its mixture with a dispersion having a concentration of 8-10 wt.%. The copolymer obtained after isolation and drying has a temperature loss of strength (CCI) of 280-288 ° C, a tensile strength of 31.5-34.0 MPa and an elongation of 290-330%.

The disadvantage of this method is that the product is obtained in the form of a heterogeneous powder, which is a mixture of very small particles (less than 30 microns), isolated from the dispersion, and large - from coagulate. It has a low bulk density (0.2-0.3 g / cm ³ ) and poor flowability, which makes it difficult to process. In addition, the copolymers obtained by the known method have insufficient heat resistance, they have practically no processing temperature range (20-25 ° C) and they darken when manufacturing products.

The authors of the present invention unexpectedly found that the use of tert-butanol as an aliphatic alcohol, which is introduced into the reaction mass before loading monomers in an amount of 0.5-5.0 wt.% From the aqueous phase, allows to obtain the product in the form of a dispersion with larger particles . The dispersion is coagulated without the addition of special coagulants. After drying of the coagulate, the target product is obtained in the form of a powder of uniform bulk size with a high bulk density. The resulting copolymer is heat-resistant, with a wide processing interval.

The technical result, the achievement of which the present invention provides, is to obtain a free-flowing heat-resistant copolymer with a high bulk density and a processing interval in the range of 100-150 ° C.

The specified technical result is achieved by the fact that in the method for producing copolymers of tetrafluoroethylene and ethylene, copolymerization of monomers in an aqueous medium in the presence of a persulfate initiator, aliphatic alcohols and modifying additives, comprising loading a mixture of these monomers into the reaction zone, followed by feeding a mixture of these monomers, according to the invention, with In order to obtain a loose heat-resistant copolymer with a high bulk density, tert-butanol in an amount of 0.5- is used as an aliphatic alcohol. 25 wt.% From the aqueous phase (preferably 0.7-10 wt.%), Which is introduced into the reaction mass before loading the monomers.

An additional modifying comonomer, hexafluoropropylene or perfluoropropyl vinyl ether, is used in an amount of 5-50 wt.% By weight of the loading mixture of monomers, which is introduced at a time at the beginning of polymerization.

A loading mixture of tetrafluoroethylene monomers with ethylene in a molar ratio of 3: 1 was used, and feeding was carried out with a make-up mixture containing 50-53 mol% of tetrafluoroethylene and 47-50 mol% of ethylene.

The copolymers of tetrafluoroethylene with ethylene, obtained by the proposed method, having a flowability within 7-10 seconds, bulk density 0.50-0.60 g / cm ³ , average particle size 50-80 microns and a temperature range of processing from 100 ° C to 150 ° FROM.

The following examples illustrate the invention.

Example 1. In a stainless steel reactor with a capacity of 1.7 dm ³ , equipped with a propeller stirrer (1400 rpm) and a syringe device for introducing additives, 600 cm ^{3 of} distilled water cooled to 10 ° C are loaded, 1 g of initiator is ammonium persulfate and 1.5 g of a mixture of ammonium salts of perfluoroenanthate and perfluoropelargonic acid in a mass ratio of 1: 1. 0.7 wt.% Of the aqueous phase of tert-butyl alcohol is added to the reactor, it is closed, it is checked for leaks and evacuated to a residual pressure of 0.001 MPa. After that, a loading mixture containing 75 mol of TFE and 25 mol of E (3: 1) is fed to the reactor to a pressure of 0.05 MPa. The mixer is turned on, the contents of the reactor are heated to 70 ° C, and the loading mixture of monomers is added to a pressure of 2.1 MPa. At each pressure drop of 0.1 MPa, a make-up mixture of monomers containing 53 mol of TFE and 47 mol of E is added to the reactor to an initial pressure of 2.1 MPa. Make-up of the reaction mass is carried out until 180 g of the make-up mixture are consumed. Then the reaction is stopped and the remaining gas mixture is blown into an evacuated container for reuse. The reactor is cooled to room temperature, purged with nitrogen from the remaining monomer, open and unload 785 g of the reaction mixture in the form of an aqueous dispersion with a concentration of 20 wt.%, Which is coagulated by freezing. The powder is washed with water at a temperature of 40-60 ° C until there is no emulsifier, and then dried for 24 hours at a temperature of 150 ° C and its properties are determined by the methods of TU 2213-036-7623164-2003. 180 g of a white powder of TFE-E copolymer are obtained, which has a melting point of 260 ° C., a tensile strength of 39 MPa and an elongation of 350%.

The bulk density of the copolymer is characterized by the ratio of the weight of the powder to its volume in accordance with GOST 11035-64.

The flowability of the copolymer is determined by the time of expiration of 50 g of powder through a funnel of a certain size in accordance with GOST 25139-82.

The particle size of the powder is determined on a laser diffraction device Analizette-22 company "Fritch". The differential particle size distribution curve for the resulting copolymer powder is shown in FIG. 1 (curve 1).

The melting temperature (T _PL ) and the temperature of the onset of decomposition are determined by differential scanning calorimetry on a DSC-141 instrument from Setaram according to ASTM 4591-99. The copolymer has a melting point of 260 ° C and a decomposition onset temperature of 380 ° C.

The processing temperature range is estimated by the difference between the melting point and the decomposition start temperature.

The copolymerization conditions and properties of the copolymers obtained in example 1, as well as in all subsequent examples, are shown in the table.

Figure 1-5 shows the differential particle size distribution curves for the obtained copolymer powders.

Examples 2-4. The copolymerization process is carried out as described in example 1, but vary the amount of tert-butyl alcohol in the range from 0.5 to 10 wt.% From the aqueous phase. 5-7 minutes after heating and supply of monomers, a pressure drop begins. The reaction is carried out until 180 g of the make-up mixture of monomers are used up. The product is discharged from the reactor in the form of an aqueous dispersion having a concentration of 20 wt.%. After coagulation, washing and drying, a copolymer powder with a melting point of 260 ° C, a tensile strength of 36-39 MPa and a relative elongation of 330-350% is obtained. The differential particle size distribution curves of the obtained copolymer powders are shown in FIG. 1 (curves I-IV).

Examples 5 and 6. The copolymerization process is carried out similarly to that described in example 3, but modifying comonomer — hexafluoropropylene (HFP) or perfluoropropyl vinyl ether (PFPVE) in the amount of 5-50 wt.% Of the monomer feed mixture is added to the vacuum in the reactor. After heating, after 5-7 minutes, a pressure drop begins. The reaction is carried out until 180 g of the make-up mixture of monomers are used up. The product is discharged from the reactor in the form of an aqueous dispersion with a concentration of 20-25 wt.%. After coagulation, washing and drying, a copolymer powder is obtained which has a melting point of 255-260 ° C, a tensile strength of 38-40 MPa and an elongation of 350%. The differential particle size distribution curves for the obtained copolymer powders are shown in FIG. 2 (curves I-II).

Examples 7 and 8. The copolymerization process is carried out similarly to that described in example 3, but tert-butyl alcohol is added in an amount of 15 wt.% Or 25 wt.% From the aqueous phase. After heating and feeding the monomers, a pressure drop begins. The reaction is carried out until 180 g of the make-up mixture of monomers are used up. The product is discharged from the reactor in the form of a thick "porridge" with a copolymer concentration of 20 wt.%. After coagulation, washing and drying, a highly flowing copolymer powder is obtained which has a melting point of 260 ° C. The differential particle size distribution curves for the resulting copolymer powder are shown in FIG. 3 (curves I and II).

Example 9k (control). The copolymerization process is carried out similarly to that described in example 3, but using a loading mixture containing 60 mol of TFE and 30 mol of E (2: 1). After heating and feeding the monomers, a pressure drop begins after 17 minutes. The reaction is carried out until 180 g of the make-up mixture of monomers are used up. The product is discharged from the reactor in the form of an aqueous dispersion with a concentration of 20 wt.%. After coagulation, washing and drying, a copolymer powder is obtained which has a melting point of 260 ° C., a tensile strength of 47 MPa, and an elongation of 330%. The differential particle size distribution curve for the resulting copolymer powder is shown in FIG. 4 (curve I).

Examples 10k and 11k (control). The copolymerization process is carried out similarly to that described in example 2, but isopropyl or ethyl alcohol, respectively, is added to the reactor. After heating and feeding the monomers, a pressure drop begins. The reaction is carried out until 180 g of the make-up mixture of monomers are used up. The product is discharged from the reactor in the form of an aqueous dispersion with a concentration of 20 wt.%. After coagulation, washing and drying, a highly flowing copolymer powder is obtained which has a melting point of 260 ° C. The differential particle size distribution curves for the obtained copolymer powders are shown in FIG. 4 (II) (curves 1 and 2).

Example 12k (control of the prototype). The copolymerization process is carried out in the reactor described in example 1, but 1 g of ammonium persulfate, 0.15 g of ammonium salt of perfluoroenanthic acid are loaded into the reactor, 0.2 wt.% Of the aqueous phase of ethyl alcohol is added and a loading mixture containing 67 mol of TFE is introduced. and 33 mol E (2: 1). After the supply of monomers and heating, a pressure drop begins. The reaction is carried out until 180 g of the make-up mixture are used up. The product is discharged from the reactor in the form of a mixture of a floating powder and an aqueous dispersion with a concentration of 8.5 wt.%. The powder after coagulation, washing and drying has a melting point of 250 ° C, a tensile strength of 42 MPa and an elongation of 330%. The differential particle size distribution curve for the resulting copolymer powder is shown in FIG. 5 (curve I).

Examples 13k and 14k (control). The copolymerization process is carried out similarly to that described in example 1, but the conditions for the selection of the polymer vary: the obtained 20% dispersion is coagulated by mechanical stirring or with the addition of an electrolyte. After coagulation, washing and drying, a copolymer powder is obtained which has a melting point of 260 ° C., a tensile strength of 47 MPa, and an elongation of 330%. The differential particle size distribution curves for the obtained copolymer powders are shown in FIG. 5 (II) (curves 1 and 2), respectively.

From the presented data it is seen that the proposed method provides a heat-resistant copolymer of tetrafluoroethylene with ethylene with a sufficiently wide processing interval in the form of a loose powder with a high bulk density.

From the experiments (examples 1-6 in the table) it is seen that the use of tert-butanol as an aliphatic alcohol in an amount of 0.5-10.0 wt.% From the aqueous phase allows to obtain copolymers that have heat resistance in the required range of 0.25- 0.65 wt.% And a wide range of processing from 100 ° C to 150 ° C. The obtained bulk powders have large particles with a size of 55-80 microns. Products from them are characterized by high tensile strength, sufficient elongation at break and meet the requirements of consumers.

The use of tert-butyl alcohol in the above ranges allows, in contrast to isopropyl and ethyl alcohols (examples 10k and 11k in the table), to gently control the molecular weight of the copolymer and provides copolymers with different melt flow rates ranging from 0.3 to 54 g / 10 min.

It can be seen (example 2 in the table) that with a decrease in the amount of added tert-butanol less than declared (less than 0.5 wt.% Of the aqueous phase), the flowability and bulk density of the copolymer worsen, its processing temperature interval decreases. With an increase in the amount of added tert-butanol more than the claimed (optimally 10 wt.%) From the aqueous phase) (examples 4, 7 and 8 in the table) it is seen that the melt flow rate increases sharply, the heat resistance worsens and the temperature range of the copolymer processing decreases. The mass loss during heating of the product increased from 0.65 wt.% To 0.96-1.25 wt.%, MFI - up to 150 g / 10 min, and the temperature range of processing decreased to 100-105 ° C.

From control experiments (examples 10k and 11k in the table) it is seen that the use of additives of isopropyl and ethyl alcohols is impractical, since in their presence the fluidity of the copolymer melt sharply increases, its friability and bulk density deteriorate, and the average particle size decreases. It can be seen that the heat resistance of the copolymer worsened, its weight loss during heating increased to 1.38-1.57 wt.%, And the temperature range of processing decreased to 50-55 ° C.

From the experiments (examples 3, 5 and 6 in the table) it is seen that when using modifying comonomers (hexafluoropropylene or perfluoropropyl vinyl ether), the heat resistance of the TFE-E copolymer improves and its processing temperature range increases.

Experiments (see examples 3 and 9k in the table) show that the use of loading mixtures of monomers enriched in ethylene in the ratio of TFE: E equal to 2: 1, as in the prototype, is impractical, since the heat resistance of the copolymer worsens (mass loss when heated increased to 0 , 87 wt.%), And the temperature range of processing narrows to 80 ° C.

The experience of the prototype (example 12k in the table) shows that although the copolymer obtained in the form of a floating powder has an average large particles of 60 microns in size, it has poor flowability and low bulk density. It is also seen that the heat resistance of the copolymer is deteriorating (its weight loss during heating increased to 0.95 wt.%), And the temperature range of processing sharply decreases to 20 ° C.

Control experiments (examples 13k and 14k in the table) show that when the powder is isolated from the dispersion simply by mechanical coagulation, a product with small particles of 20 μm is obtained, with poor flowability and low bulk density (example 13k). When coagulants are added, as proposed in analogues and (Example 14k), the particle size of the powder increases, its flowability and bulk density improve, but its heat resistance deteriorates sharply (mass loss upon heating increased to 1.63 wt.%).

Examples (1-6 in the table) show that the product obtained by the proposed method is obtained in the form of a sufficiently concentrated dispersion with a polymer concentration of 20 wt.%, Which can be used directly or after concentration for coating and impregnating various substrates.

From the graphs of the distribution of particle sizes (examples 1-7) it is seen that the powders of the copolymers obtained by the proposed method are more homogeneous. They are more convenient to process (in the case of airborne dryers, as well as in granulation) and are suitable for powder coating.

Thus, all copolymers of tetrafluoroethylene with ethylene obtained by the proposed method have a high flowability of 5-10 seconds, bulk density of 0.40-0.55 kg, processing temperature range from 100 ° C to 150 ° C, high thermal stability of 0.25 0.65 wt.% And physico-mechanical properties that meet the requirements for this product.

Claims (4)

1. A method of producing copolymers of tetrafluoroethylene with ethylene by copolymerization of monomers in an aqueous medium in the presence of a persulfate initiator, aliphatic alcohols and modifying additives, comprising loading a mixture of these monomers into the reaction zone, followed by feeding with a mixture of the same monomers, characterized in that, in order to obtain a loose heat-resistant a copolymer with a high bulk density, tert-butanol is used as an aliphatic alcohol in an amount of 0.5-25 wt.% of the aqueous phase, preferably 0.7-10 wt.%, which the second is introduced into the reaction mass before loading the monomers. 2. The method according to claim 1, characterized in that they use additional modifying comonomer hexafluoropropylene or perfluoropropyl vinyl ether in an amount of 5-50 wt.% By weight of the loading mixture of monomers, which is introduced at a time at the beginning of polymerization. 3. The method according to claim 1, characterized in that they use a loading mixture of tetrafluoroethylene monomers with ethylene in a molar ratio of 3: 1, and make-up is fed with a make-up mixture containing 50-53 mol.% Tetrafluoroethylene and 47-50 mol.% Ethylene. 4. The copolymers of tetrafluoroethylene with ethylene, obtained by the method according to claims 1-3, having a flowability in the range of 7-10 s, bulk density 0.50-0.60 g / cm ³ , average particle size 50-80 microns and temperature range for processing from 100 ° C to 150 ° C.

Images