RU2156776C1 - Modified tetrafluoroethylene/ethylene copolymer production process - Google Patents

Abstract

FIELD: polymer production. SUBSTANCE: invention relates to production of copolymers exhibiting high strength and dielectric characteristics along with chemical stability, heat (up to 200 C) and radiation resistance, which are, in particular, useful as structural materials in a variety of mechanical engineering areas and as strong electric- insulating materials in electrical industry. Copolymers of invention are prepared by aqueous-emulsion copolymerization of tetrafluoroethylene with ethylene and modifying comonomer in presence of initiator and fluorinated emulsifier, which process is carried out by feeding, into reaction zone, replacing amounts of equimolar mixture tetrafluoroethylene and ethylene at constant copolymerization rate. Process is distinguished by that above-said initiator is represented by system of beta-hydroxyethyl-tert-butyl peroxide/ascorbic acid at molar ratio (20-25): 1, copolymerization rate is maintained constant by portionwise feed of ascorbic acid after 18-20% of the total weight of replacing monomer mixture is consumed, and molar ratio of peroxide to ascorbic acid in reaction zone is maintained (7-10): 1 during polymerization process. EFFECT: simplified process and improved working characteristics of copolymers. 1 tbl, 10 ex

Description

 The present invention relates to the production of modified copolymers of tetrafluoroethylene / TFE / and ethylene / E /, in particular copolymers of TFE / E, modified with a comonomer introduced in the copolymerization step to create materials with an improved range of performance properties.

Such copolymers are valuable industrial materials with high strength and dielectric characteristics in combination with chemical resistance, heat resistance / up to 200 ^o C / and radiation resistance. They find application, in particular, as structural materials in various fields of mechanical engineering and as durable electrical insulation materials in the electrical industry.

 Typically, TFE-E copolymers are prepared by copolymerizing monomers under the action of free radical initiators in an organic, aqueous-organic or aqueous medium under pressure and at temperatures determined by the decomposition temperature of the initiator used.

 For the synthesis of copolymers based on TFE with E, it is especially important to create conditions for safe process management, since the monomeric mixture of TFE with E is an explosive mixture. Safe conditions for the process of copolymerization of TFE with E are of particular importance when it is carried out in an industrial environment, which is ensured by the application of synthesis methods in a water-emulsion medium.

 Known [Japanese application N 39-22586, NKI 26 B 11, publ. 12.10.64] a method of producing TFE copolymers with E copolymerization of these monomers in an aqueous emulsion medium under the action of free radical initiators, preferably ammonium persulfate, in the presence of a fluorinated emulsifier, mono- or dihydric alcohols / butanol, methanol, t-butanol, etc. / and molecular weight regulator / MM /, for example mercaptan. Using this method, TFE-E copolymers are obtained with a TFE content in the copolymer of 10 to 90 mol%. %

The temperature of the copolymerization reaction is determined depending on the decomposition temperature of the initiator / 10-80 ^o C, preferably 70 ^o C /, the pressure is 20-100 ATM / preferably 30 ATM /. The process is conducted with periodic feeding of a feed mixture of monomers. The addition of alcohol provides a fairly high speed of the process / 58-167 g / l • hour /. The polymer latex is coagulated by freezing, the resulting copolymer is washed with water and alcohol and dried under vacuum. Get a copolymer of TFE with E with a softening temperature of 225-283 ^o C and high physical and mechanical characteristics / tensile strength - up to 500 kgf / cm ² ; elongation at break up to 400% /.

The disadvantages of the method are:
1. The lack of thermal stability of the copolymer, due to the use of initiators forming in the copolymer end-resistant to thermal effects. Thus, during persulfate initiation, -OH, -SO ₃ H end groups are formed in the copolymer, which are subject to thermal decomposition at processing and operating temperatures, which is accompanied by coloring of the copolymer and foaming of the polymer melt during processing as a result of evolution of volatile decomposition products.

2. Low resistance to cracking of the resulting copolymer, since the TFE and E copolymer, which does not contain a modifier, has a high degree of crystallinity, which determines the rigidity of the macromolecular chain, which leads to cracking of materials made from it at 200 ^o C in a stressed state [Panshin Yu. A ., Loginova N.N., Podlesskaya N.K. and other Plastics, 1978, N 3, S. 70-71].

 3. The need to regenerate a relatively large number of alcohols introduced to increase the speed of the copolymerization process.

4. Relatively high temperatures / 70 ^o C / and pressure / 30-100 atm / of the copolymerization process, which does not allow to provide sufficiently reliable safe conditions for the process of synthesis of the copolymer.

где R, R' - радикалы алифатического алкила, в присутствии эмульгатора /перфтороктаноата аммония/, ускорителя сополимеризации /т-бутанол, н-бутанол и др. / и регулятора ММ /метанол, ацетон и др./. Предпочтительными пероксидами в данном процессе являются трет-бутил-пероксибутират, трет-бутил-пероксипевалат. При этом процесс сополимеризации проводят при температуре 50-80^oC и давлении 1,0-80 кг/см², предпочтительно 30 кгс/см², которое поддерживают постоянным подачей подпиточной смеси мономеров. Время сополимеризации 6,5 час, скорость процесса /в соответствии с примером 1/ составляет 6,7 г/л•час. Полимерный латекс коагулируют методом вымораживания и получают термостабильный полимер, который при переработке из расплава не образует в нем пузырей и не изменяет свой цвет. Введение третьего сомономера обеспечивает стойкость сополимера к растрескиванию.Improved heat and color fastness of the copolymer can be achieved by using compounds that decompose into radicals containing heat-resistant groups as a copolymerization initiator. So in the application of Japan N 55-23846 [MKI C 08 F 210/02, publ. 06/25.80] describes a method for the copolymerization of TFE with E, including with a modifying comonomer, for example, hexafluoropropylene / HFP /, isobutylene, vinylidene fluoride / VDF /, chlorotrifluoroethylene / CTFE /, perfluoroalkylvinyl ether / PFAVE /, etc., in an aqueous emulsion or suspension medium using peroxy as the initiator formula

where R, R 'are radicals of aliphatic alkyl, in the presence of an emulsifier / ammonium perfluorooctanoate /, a copolymerization accelerator / t-butanol, n-butanol, etc. / and a MM regulator / methanol, acetone, etc. /. Preferred peroxides in this process are tert-butyl peroxybutyrate, tert-butyl peroxypevalate. The copolymerization process is carried out at a temperature of 50-80 ^o C and a pressure of 1.0-80 kg / cm ² , preferably 30 kgf / cm ² , which is supported by a constant supply of a feed mixture of monomers. The copolymerization time of 6.5 hours, the speed of the process / in accordance with example 1 / is 6.7 g / l • hour. Polymer latex is coagulated by freezing and a thermostable polymer is obtained which, when processed from a melt, does not form bubbles in it and does not change its color. The introduction of the third comonomer ensures the resistance of the copolymer to cracking.

The disadvantages of the method are:
1. The relatively high temperature / 50-70 ^o C / copolymerization, which complicates the creation of conditions for the safe conduct of the process.

 2. The need for regeneration of alcohols introduced to increase the speed of the copolymerization process.

 3. Low process speed.

Known [US patent 3847881, MKI C 08 F 15/40, publ. 12.11.74] a method for producing cracking-resistant copolymers at elevated temperatures, consisting of 40-60 mol% of ethylene and 60-40 mol% of tetrafluoroethylene or perhalogenated olefin and small amounts / 0.1 to 10 mol% of the mass of tetrafluoroethylene / modifying comonomer - 3,3,3-trifluoro-2-trifluoromethylpropene. The copolymerization is carried out in an aqueous-organic medium / methanol addition - 10% of the aqueous phase / under the action of the initiator trichloroacetyl peroxide dissolved in 1,1,2-trifluoro-1,2,2-trichloroethane / freon-113 / chloroform or heptane. The copolymerization temperature is 0-5 ^o C, the pressure is 125 psiq / 8.75 atm /. The speed of the process is 50-60 g / l • hour. Get the copolymers resistant to cracking at a temperature of 200 ^o C. The process proceeds relatively safely.

The disadvantages of the method are:
1. The complexity of the process associated with the need to separate and regenerate relatively large amounts of methanol from water, which also contains chloroform, and freon-113.

 2. The use for modification of a specific expensive monomer 3,3,3-trifluoro-2-trifluoromethylpropene, which is not produced in industry.

Closest to the claimed method for the combination of essential features is a method for producing modified TFE copolymers with E according to US patent N 4338237 [MKI C 08 K 5/32, publ. 6.07.82]. According to this method, a water-emulsion copolymerization of 40-60 mol.% TFE, 60-40 mol. % ethylene and 0-15 mol.% modifying α-olefin comonomer (HFP, VDF, CTFE or PFAVE) in the presence of a fluorinated emulsifier and initiator taken from the group of manganese acids, their salts or manganese compounds capable of converting to these acids or their salts under copolymerization conditions. The copolymerization temperature is 26-27 ^o C, the pressure is 17 bar, the process speed is maintained constant / within 20-100 g / l • hour / by continuous dosing of the initiator - potassium permanganate, the pressure is maintained by constant feeding into the reaction zone of a feed mixture of TFE with E in molar 1: 1 ratio / equimolar composition /. As a molecular weight regulator, dimethyl, methylethyl or diethyl malonic acid esters are used. Along with the emulsifier, a dispersion stabilizer — mono- or diammonium oxalate, is also introduced into the reaction medium. Buffer compounds such as chloride, fluoride, borate, bisulfate or ammonium nitrate are added to maintain the pH of the reaction medium. The copolymerization process is carried out until the formation of 22-27 wt.% Copolymer in the dispersion. Latex is coagulated mechanically using a high-speed mixer. The polymer is separated from water, washed and dried in a nitrogen atmosphere at a temperature of 200 ^o C for 15 hours.

The copolymer synthesized by the described method has good processability, physicomechanical properties at high temperatures, and cracking resistance at 180 ^o C. Carrying out the copolymerization process at room temperature reduces the likelihood of explosive decomposition of the monomer mixture of TFE with E, increases the safety of processes proceeding with a relatively high speed / 40-100 g / l • hour /.

The disadvantages of the prototype method are:
1. The complexity of the copolymerization process associated with the need to strictly adhere to the pH of the reaction medium, and therefore the introduction of buffer additives, as well as the need to use a dispersion stabilizer along with a fluorinated emulsifier to obtain a sufficiently high latex concentration / 22-27% /.

 2. Low color and thermal stability of the copolymer at processing temperatures due to the formation of unstable end groups / OH groups or salt residues of manganese acids / in the polymer. To avoid significant color changes, the polymer is dried under a nitrogen atmosphere.

 The technical result, the achievement of which the claimed method provides, is to simplify the process and improve the color and thermal stability of the copolymer.

 To achieve the technical result, the authors propose in a method for producing modified TFE copolymers with E water-emulsion copolymerization of TFE with E and a modifying comonomer in the presence of an initiator, a fluorinated emulsifier with an equimolar composition fed into the reaction zone of a feed mixture of TFE with E at a constant speed of the copolymerization process, in use β-hydroxyethyl-tert-butyl peroxide-ascorbic acid / β-OEP-AK / initiator system before copolymerization to the reaction zone load β-OEP and AA in a molar ratio of / 20-25 /: 1, and the copolymerization process rate should be maintained by constant batch feed of AK into the reaction zone after 18-20% of the total weight of the feed mixture of monomers is used up, while the molar ratio between β-OEP and AK in the reaction zone in the copolymerization process to maintain equal to / 7-10 /: 1.

It is known to use β-OEP in the copolymerization of VDF with TFE and other fluoromonomers in an aqueous medium in the presence of a fluorinated emulsifier [UK patent N 1312036, MKI C 08 F 1/60, 3/22, publ. 04/04/73]. In this case, the process proceeds under rather severe conditions: at a temperature of 80-110 ^o C and pressure from atmospheric to 1400 kg / cm ² . Such conditions are unacceptable for the copolymerization of TFE with E, especially under industrial conditions, since, as mentioned earlier, a mixture of TFE with E is explosive.

The use of the β-OEP-AK system in the synthesis of polymers is not described in the scientific and technical literature, although AK is known to be used as a component of the redox system with peroxides, diazo compounds during polymerization of butadiene and isoprene at a temperature of 15-47 ^o [Dolgoplosk B.A., Tinyakova E.I. Free radical generation. M .: Nauka, 1982, p. 140-150]. In this case, the presence of a variable valence metal / in the promoter system / metal salt is mandatory, since in its absence the free radical generation rate is so low that it practically does not allow the use of such a redox system to initiate the copolymerization of vinyl monomers.

The authors of the present invention found that when using β-OEP in combination with AK, the process of copolymerization of TFE with E and the third modifying comonomer proceeds stably with a fairly high speed at a temperature of 15-35 ^o C without the use of any promoters. At the same time, maintaining in the reaction zone a molar ratio between β-OEP and AK equal to / 7-10 /: 1 by batch feed of AK allows not only to maintain a constant process rate, but, as it turned out unexpectedly, to ensure the process of controlling the MM of the resulting copolymer, which eliminates the need for introducing a chain growth regulator into the reaction mass. Simultaneous administration of AK does not give the effect of regulating the MM polymer. In addition, the process is accompanied by heating the reaction mass to 50-60 ^o C / cm our test case 6 /. The inventive method allows you to maintain a stable temperature, while the process proceeds practically at room temperature, which reduces the possibility of explosive decomposition of the monomer mixture and, therefore, creates the conditions for safe process. The resulting copolymer has high thermal and color stability, since upon decomposition of β-OEP under these conditions, end groups / -CH ₃ , -CF ₂ H / are formed that are resistant to thermal decomposition.

 As indicated above by the claimed method before starting the copolymerization is loaded β-OEP and AK in a molar ratio / 20-25 /: 1. With a decrease in AK content, for example, with a molar ratio of β-OEP: AK = 27: 1, a significant induction period is observed. The consumption of the monomer mixture begins only after 2-3 hours, the speed of the polymerization process is 7-10 g / l • hour / cm. our test case 7 /. With an increase in AA content, for example, with a ratio of β-OEP: AK = 18: 1, a rapid consumption of β-OEP is observed, the reaction mass is heated, the copolymerization process decreases / cm. our test case 8 /.

 The molar ratio of β-OEP and AK during the copolymerization process according to the claimed method is maintained equal to / 7-10 /: 1. Deviation from the specified limits of the ratio of β-OEP and AK does not allow to maintain a constant process speed, in addition, an increase in the AA content in the reaction mass to a deterioration in the color fastness of the synthesized polymer at processing temperatures / cm. our control example 9 /, and a decrease in AK - to slow down and stop the process / cm. control example 10 /. A batch feed of AK should be done after 18-20% of the total weight of the feed monomer mixture has been consumed. After consuming 80% of the monomer mixture, there is no need to introduce an additional portion of AK, because the process proceeds with sufficient speed until the full consumption of a given amount of make-up monomer mixture. Preferred to maintain a stable temperature regime is batch dosing of AA after 18-20%, 36-40%, 54-60% and 72-80% of the total weight of the feed mixture of monomers, i.e. after the consumption of each subsequent 18-20% make-up monomer mixture. β-OEP is introduced into the reaction zone in amounts traditional for the initiator / 0.05-0.25% of the total mass of the aqueous phase /, adopted for water-emulsion systems.

 As modifying comonomers in the present method can be used HFP, PFAVE, VDF, propylene / PP /, etc., or mixtures thereof. The modifying comonomer is introduced into the reaction zone before loading TFE with E or it can be dosed during the copolymerization process.

 The copolymer is obtained in the form of latex with a copolymer concentration of 18-23%, which can be used directly to obtain coatings, or the copolymer is isolated from the latex by freezing or mechanical coagulation, including using electrolytes to increase the efficiency of the mechanical coagulation process.

 As a fluorinated emulsifier, perfluoroenanthic acid, perfluoropelargonic acid or their salts, as well as mixtures thereof, are used.

The properties of the obtained copolymers were determined as follows:
- tensile strength and elongation at break according to GOST 11262-80;
- the melt flow rate was determined at 300 ^o C and a load of 10 kg according to GOST 11645-73;
- thermal stability was determined by the loss of mass of the copolymer powder when heated in air at a temperature of 275 ^o C for 5 hours;
- the color fastness of the copolymer was evaluated visually by the color of the plate with a thickness of 2 mm, obtained by pressing at a temperature of 300 ^o C and a specific pressure of 29.4 MPa;
- resistance to cracking was determined by the following method: a 2 mm diameter bundle obtained by extruding the copolymer powder on an IIRT-2M device was wound onto metal rods of 2 mm and 10 mm diameter, the ends of the bundles were clamped, and the entire assembly was lowered into a silicone bath with a temperature of 200 ^o C for 10 minutes If no fractures or cracks were formed on the wound samples, the material was considered resistant to cracking.

 Example 1

650 ml of deoxygenated distilled water, 1.3 g of ammonium salt of perfluoropelargonic acid / emulsifier concentration of 0.2% are charged into a 1.3 liter stainless steel reactor equipped with a paddle stirrer with a speed of 1200 rpm and a jacket for heating and cooling. by weight of the aqueous phase /, 1 g β-OEP / peroxide concentration of 0.15% by weight of the aqueous phase /. The reactor is sealed, checked for leaks by excessive nitrogen pressure, vacuum to a residual pressure of 5-7 mm RT.article and introduced at a temperature of 23 ^o C 10 g of HFP, then 50 g of a mixture of TFE with E in a molar ratio of 4: 1. The pressure is set to 20 atm. The stirrer is then turned on and 0.065 g AK / 0.01% by weight of the aqueous phase / is injected with a syringe, molar ratio β-OEP: AK = 20: 1. After the introduction of AK, the temperature of the reaction mass rises to 27 ^o C. The copolymerization process is carried out at 27 ^o C and a pressure of 21 atm, maintaining the indicated pressure in the reactor by feeding a feed mixture of TFE-E monomers in an equimolar ratio. After the consumption of 32 g / 20% / of the monomer feed mixture, AK in the amount of 0.045 g is injected using a syringe to provide a soap ratio in the β-OEP reaction zone: AK = 7: 1. Next, AK is introduced in batches in the amount of 0.045 g after the consumption of 40, 60, 80% of the total weight of the feed monomer mixture, maintaining the ratio between β-OEP and AK 7: 1. After the consumption of 160 g of the feed mixture of monomers, the process is stopped, closing the supply line of the feed mixture of monomers and cooling the reactor to room temperature. The total copolymerization process time is 2.5 hours.

Get polymer milk latex with a copolymer concentration of 24 wt. %, which is coagulated by freezing. The polymer is washed until the washings are neutral to the presence of an emulsifier (using the Nessler reagent), filtered and dried to constant weight at 50 ^° C. under vacuum. 160 g of finely dispersed copolymer powder are obtained.

 The main process parameters and properties of the obtained copolymer according to example 1 and all subsequent examples are shown in the table.

 Example 2

 The process is carried out analogously to example 1, but after loading the reactor, AK is introduced in an amount of 0.053 g, which corresponds to a molar ratio of β-OEP: AK = 25: 1. The portioned amount of AA introduced during the process is 0.033 g, which ensures the maintenance of a molar ratio between β-OEP and AA in the reaction zone of 10: 1. The copolymerization time is 3 hours. The yield of copolymer is 150 g.

 Example 3

The process is carried out analogously to example 1, but after loading the reactor, 0.06 g of AA is introduced, which corresponds to a molar ratio of β-OEP: AK = 22: 1. The process is carried out at a temperature of 25 ^o C and a pressure of 18 amt. In the process, after spending 18% of the total weight of the make-up monomer mixture, AK is added in portions in an amount of 0.04 g each, while the molar ratio between β-OEP and AK is maintained at 8: 1. The copolymerization time is 2.5 hours. The yield of copolymer 160 g.

 Example 4

The process is carried out analogously to example 1, but as the third modifying comonomer use PFPVE in an amount of 15 g. The copolymerization is carried out at a temperature of 27 ^o C and a pressure of 18 atm. The copolymerization time is 2 hours. Product yield 160 g.

 Example 5

 The process is carried out analogously to example 1, but propylene in the amount of 20 g is used as the third modifying comonomer. The copolymerization time is 3.5 hours. The yield of copolymer is 150 g.

Example 6. / control /
The process is carried out analogously to example 1, but AK is loaded at the beginning of the process at a time in an amount of 0.18 g / 0.027 wt.% From the aqueous phase /, which corresponds to a molar ratio of β-OEP: AK = 7: 1. After 3 hours of copolymerization time, 63 g of a feed mixture of monomers was consumed. In the next 2 hours - 15 g. The total time of the copolymerization process is 5 hours. A polymer milky latex is obtained with a copolymer concentration of 11.5 wt.%. The copolymer yield of 75 g.

Example 7. / control /
The process is carried out analogously to example 1, but before the copolymerization, AK is introduced in an amount of 0.048 g / molar ratio β-OEP: AK = 27: 1 /. The consumption of the monomer mixture begins after 2 hours / induction period is observed /. Subsequent administration of AA during the process is carried out in batches in an amount of 0.045 g after using 20, 40, 60 and 80% of the total weight of the feed monomer mixture. During the process, the molar ratio between β-OEP and AK is 7: 1. The copolymerization time is 6 hours. The copolymer yield of 80 g.

Example 8. / control /
The process is carried out analogously to example 1, but the amount of AK introduced at the beginning of the process is 0.073 g, which corresponds to a molar ratio of β-OEP: AK = 18: 1. The copolymerization time is 5 hours. The yield of copolymer 72 g.

Example 9. / control /
The process is carried out analogously to example 1, but AK is introduced during the process after spending 20, 40, 60 and 80% of the total weight of the feed monomer mixture in an amount of 0.065 g, which corresponds to the ratio β-OEP: AK = 5: 1. The copolymerization time is 2 hours. The yield of copolymer 160 g.

Example 10. / control /
The process is carried out analogously to example 1, but AK is introduced during the process in an amount of 0.027 g, which corresponds to a molar ratio of β-OEP: AK in the process of copolymerization of 12: 1. The copolymerization time is 8 hours. Yield 105 g.

 The inventive method in comparison with the prototype method is simpler, since its synthesis technology does not require the introduction of special additives to regulate the MM, maintain the pH of the reaction medium, process accelerator, dispersion stabilizer.

According to the claimed method receive copolymers that are resistant to cracking, with high thermal and color stability, because the copolymer does not contain thermally unstable end groups, which are potential centers of the beginning processes of thermo-oxidative degradation. As can be seen from the table, the copolymers obtained by the present method have a thermal stability of 0.3-0.35%, practically do not change color at processing temperatures / 275-300 ^o C / and operation up to 200 ^o C; have resistance to cracking at 200 ^o C in tension. The copolymers obtained by the prototype method using manganese salts as an initiator or by the similar method in persulfate initiation have low color and thermal stability due to the formation of unstable end groups in the copolymer. The inventive method provides safe conditions for the process, because it is carried out almost at room temperature and allows you to maintain a stable temperature.

Claims (1)

 A method of producing modified copolymers of tetrafluoroethylene with ethylene by water-emulsion copolymerization of tetrafluoroethylene with ethylene and a modifying comonomer in the presence of an initiator and a fluorinated emulsifier with an equimolar composition fed into the reaction zone with an equimolar composition at a constant speed of the copolymerization process, characterized in that the system is used as an initiator β-hydroxyethyl-tert-butyl peroxide - ascorbic acid, before copolymerization to the reaction zone β-hydroxyethyl-tert-butyl peroxide and ascorbic acid are charged in a molar ratio (20 - 25): 1, and the copolymerization process is supported by a constant portion supply of ascorbic acid to the reaction zone after 18 - 20% of the total weight of the monomer feed mixture is used up, while the molar the ratio between β-hydroxyethyl-tert-butyl peroxide and ascorbic acid in the reaction zone during the copolymerization is maintained equal to (7 - 10): 1.

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