RU2168519C1 - Method of preparing polytetrafluoroethylene - Google Patents

Abstract

FIELD: preparation of polytetrafluoroethylene by polymerization of tetrafluoroethylene. SUBSTANCE: method comprises purging reactor with inert gas to oxygen content of 0.01-0.05 %, purifying water for polymerization to remove organic and inorganic impurities by filtration through two successively disposed inverted osmotic membranes located so that 25-50% of stream with concentrated admixtures being admitted from space above first membrane and 20- 50 of stream with concentrated impurities being admitted from space above second membrane with said stream recycled to first membrane. Process of admitting tetrafluoroethylene for polymerization is carried out at rate which maintains temperature in gas phase of reactors at 40-60 C. Tetrafluoroethylene to be polymerized contains 0.05-0.4 mg of alkylamine per liter of gaseous tetrafluoroethylene. Alkylamine includes tertiary alkylamines from group of triethyl- tripropyl-or tributylamines. Method makes it possible to increase safety of process and yield of polymer and reduced consumption of initiating system. EFFECT: more efficient preparation method. 5 cl, 1 dwg, 4 tbl

Description

 The invention relates to the field of chemistry and technology of polymers, in particular to the production of polytetrafluoroethylene (PTFE) by polymerization of tetrafluoroethylene (TFE).

A known method of entrusting PTFE with the polymerization of TFE in a medium of demineralized water in the presence of a polymerization initiator at 2.17 MPa and a temperature of 20-40 ^o C [Pat. US 5405923, CL C 08 F 2/18, C 08 F 14/26]. The disadvantage of this method is the high polymerization pressure, which leads to an increase in the probability of explosive decomposition of TFE, which increases the risk of the process, increased adhesion of the resulting PTFE to the walls of the reactor, and therefore, to the complexity of the technology due to the need for periodic cleaning of the inner surface of the reactor [Yu.A. Panshin, S.G. Malkevich, U.S. Dunaevskaya. Ftoroplasty. L., Chemistry, 1978, from 19-35].

 The closest in technical essence to the proposed invention is the method of obtaining described in the patent of the Russian Federation [Patent of Russia 2056437, cl. C 08 F 114/26]. It is taken as a prototype. In accordance with this method, the process for producing PTFE is carried out by polymerizing TFE in a liquid medium (in particular, water subjected to demineralization) in the presence of an initiator. Before TFE is fed into the reactor, it is purged with an inert gas until the oxygen content in the exhaust inert gas is less than 0.05% in order to prevent explosive decomposition of TFE (to increase process safety). The PTFE powder obtained by polymerization and isolation from the suspension is washed continuously with demineralized water. Washing is combined with grinding.

The disadvantages of the prototype method include the following:
- the safety of the polymerization process is not fully ensured, since the removal of oxygen from the reactor by purging with an inert gas to a residual oxygen content of less than 0.05% is a necessary but not sufficient safety condition. In addition, the upper limit of the residual oxygen content in the reactor is such that the process of explosive decomposition of TFE remains potentially possible;
- use for the polymerization of demineralized water, i.e. water purified only from mineral impurities. It is known [Yu.A. Panshin, S. G. Malkevich, W. S. Dunaevskaya. Ftoroplasty. L., Chemistry, 1978, p. 29-31] that the quality of the obtained PTFE, the stability of the quality from the polymerization process to the process, the yield of PTFE on the spent TFE, depend on the level of presence of organic impurities in water, quantified by titration of water with a solution of potassium permanganate. In the case of the standard procedure for obtaining demineralized water, these organic impurities enter the purified water according to the law of the case and in significant quantities [L.A. Kulsky. Fundamentals of chemistry and water technology. Kiev, Naukova Dumka, 1991, p. 407], which leads to the following negative points in the production of PTFE using demineralized water: 1) an increase in the flow rate of the initiator due to its interaction with organic impurities; 2) a decrease in the yield of PTFE; 3) a decrease in the molecular weight of PTFE and a change in the molecular weight distribution, and therefore, a deterioration in the quality of PTFE; 4) the ingress of an additional amount of organic impurities during the washing and grinding of the obtained PTFE, which negatively affects the production of products from such PTFE, especially at the stage of sintering at 370-390 ^o C (may crack products) and changes the color of the products from white to yellowish with a splash of dark inclusions. The latter does not meet the requirements of the international standard ASTMD 4894 "Specifications for materials based on PTFE with a granular structure, obtained by molding and plunger extrusion." In this regard, the aim of the present invention is to eliminate the disadvantages inherent in the prototype.

The goal is achieved through the use of the following technical solutions:
- purge the reactor with inert gas until the oxygen content in the inert gas of 0.01-0.005%. When the oxygen content is above 0.01%, the probability of explosive decomposition of TFE upon contact with oxygen remains quite high. Ensuring that the oxygen content is below the level of 0.005% leads to a significant complication of the scheme and more appropriate ways to ensure almost complete process safety are described below;
- TFE is fed into the polymerisation reactor at a speed that ensures the temperature of the gas phase in the reactor 40-60 ^o C. At temperatures below 40 ^o C there is a decrease in process performance, at temperatures above 60 ^o C increases the likelihood of explosive decomposition of TFE;
- TFE supplied to the polymerization reactor contains 0.05-0.4 mg per liter of gaseous TFE alkylamine, which ensures the stability of the polymerization rate over time and thereby reduces the risk of the polymerization process. As the alkylamine, triethylamine, tributylamine or other similar compounds are used;
- as the source water, as a result of the purification of which pure water was obtained for the polymerization of TFE, river water was used, which was subjected to standard preliminary treatment:
- coagulation of impurities suspended in river water by adding coagulants such as aluminum sulfate, aluminum hydroxochloride;
- sludge to precipitate the precipitate;
- filtration of settled water through successive bulk filters with quartz sand and activated carbon in order to more deeply remove residual suspended impurities and chlorine;
- the addition of scale inhibitors (decarbonizers) to the water such as trisodium phosphate, sodium hexametaphosphate and others, which transfer hardness salts to the precipitate.

 After standard pretreatment procedures, water containing a series of soluble organic and inorganic impurities is passed through two sequentially located reverse osmosis membrane elements. Moreover, part of the water (25-50%) supplied to the first membrane along the way is constantly diverted from the supmembrane space along with organic and inorganic impurities that have not passed through the membrane, including hardness salts. The water stream passing through the first membrane is fed to the second membrane along the way in such a way that 20-30% of the indicated stream is constantly diverted from the supmembrane space along with impurities that have not passed through the second membrane and is fed into the flow line to the first membrane along the way. Water passing through both membrane elements and purified from organic and inorganic impurities is used to conduct TFE polymerization (Fig. 1). As the material of the membranes used for water purification, polyamide-polysulfone, cellulose acetate, and others can be used. In the case of special high-quality PTFE grades, water purified by reverse osmosis can be subjected to additional desalination on ion-exchange resins.

 The upper limit of the value of water flows, which contain concentrated organic and inorganic impurities and are discharged without passing through the first (50% of the total flow) and second (30% of the flow to the second membrane element) membrane, is due to the fact that a further increase in the part of the discharged flows does not reduce impurities in the filtrate after the second membrane.

 The lower limit of the selection amount of flows with concentrated impurities that did not pass through the first (25% of the total water flow to the 1st membrane) and the second (20-30% of the water flow to the 2nd membrane) is due to the fact that, at flows less than indicated, there is a decrease in the level of water purification, especially from organic impurities, which leads to a decrease in the yield of PTFE and the deterioration of its quality when using water of this quality in the polymerization.

 The invention is illustrated by the following examples.

EXAMPLE 1 (control)
Deep desalted (demineralized water) for TFE polymerization was prepared using the standard demineralization procedure described in [L. A.Kulsky. Fundamentals of chemistry and water technology. Kiev, Naukova Dumka, 1991] and which consists in passing the initial (subjected to purification) water through a three-stage ion exchange unit. Each stage consisted of series-connected H-cationocytes (filled with Ky-2 ion-exchange resin) and anionitic (filled with AB-17-8 ion-exchange resin) filters. The height of the resin layer in the apparatus is 1.5 m. The feed rate is 10 m ³ / h.

 The characteristics of the source and purified water are given in table. 1.

EXAMPLE 2
Purification of the source water (see table. 1) was carried out using a reverse osmosis unit, a diagram of which is shown in the drawing. As the membrane elements, elements of the Dow Chemical company of two brands BW 30LE-440 and SWHR 30-4040 were used. The process parameters and the quality of purified water are given in table. 2 and 3, respectively.

 As can be seen from the table. 1 and 3, the salinity level in water (p. 3) purified on ion-exchange resins and reverse osmosis membranes is approximately the same, but the content of organic impurities in water purified by ion exchange is 1.7-2.3 times higher than in water purified by the proposed method (table. 3, op. 1, 2, 5, 6).

EXAMPLE 3
The polymerization of TFE was carried out in a reactor with a volume of 3.2 m ³ , equipped with a stirrer, a jacket, a device for emergency pressure relief. As the initiation system, a system of 3 components was used: salts of supra sulfuric acid, salts of ferrous iron and inorganic acid (sulfuric or hydrochloric). 1800 liters of purified water from impurities were loaded into the reactor (Example 2), two of the three components of the initiation system (aqueous solution of ammonium or potassium persulfate with a concentration of 3%, hydrochloric or sulfuric acid with a concentration of 4%). The reactor was closed, tested for leaks, purged with an inert gas to remove oxygen. Liquid tetrafluoroethylene containing alkylamine from a TFE collector was piped to a polymerization reactor. As it passed through the pipeline, TFE with an alkylamine evaporated and, entering the reactor, created the necessary pressure. Then, up to 2 L of the third component of the iron (II) salt initiation system with a concentration of 0.5 g / L was added to the reactor. As the pressure in the reactor decreased (due to the polymerization process), TFE was periodically fed into the reactor from the liquid TFE collection. Since the polymerization process proceeded in the gas phase, the analysis of the alkylamine content in TFE was carried out from the gas phase. The resulting PTFE powder was separated from the mother liquor, washed with water purified according to example 2, milled in the presence of this water, dried in a stream of hot air to a moisture content of 0.02%.

 The parameters and characteristics of the process are given in table. 4.

Thus from the table. 4 it follows that the maximum safety of the process is achieved when the temperature of the gas phase of the reactor is in the range of 40-70 ^o C, the alkylamine content is 0.05-0.4 mg / l of the gas phase of the reactor, the oxygen content in the gas phase of the reactor is 0, 03-0.005%. The use of water purified from organic and inorganic impurities by passing the feed water through two sequentially located reverse osmosis membranes (Example 4-16 of Table 4) makes it possible to increase the yield of PTFE by about 3-10% compared with the prototype and to reduce the consumption of the system of initiation of the salt of sulfuric acid + inorganic acid + salt of iron (II) about 1.9-6.8 times (comparison was carried out with the experiments of the prototype, providing the maximum possible yield of PTFE).

Claims (5)

 1. A method of producing polytetrafluoroethylene by purging the reactor with an inert gas to deoxygenate the reactor volume, polymerizing tetrafluoroethylene in the reactor in a medium purified from organic and inorganic impurities, containing an initiation system, isolating polymer powder from the resulting suspension, followed by washing, grinding and drying, characterized in that, as purified water, water is used that is passed through standard procedures for purification from suspended impurities, chlorine and decarbonization, through two subsequent properly located reverse osmosis membranes, moreover, 25-50% of the mass of the stream entering it with concentrated organic and inorganic impurities is constantly diverted from the space above the first membrane, and 20-30% of the mass of the stream entering it from the space above the second membrane with the return of this part to the first membrane.  2. The method according to claim 1, characterized in that the reactor is purged to an oxygen content of 0.005-0.01% in the gas leaving the reactor. 3. The method according to PP.1 and 2, characterized in that the supply of tetrafluoroethylene is carried out at a speed that ensures the temperature in the gas phase of the reactor 40-60 ^o C.  4. The method according to claim 3, characterized in that tetrafluoroethylene containing 0.05-0.4 kg of alkylamine per liter of gaseous tetrafluoroethylene is used.  5. The method according to claim 4, characterized in that tetrafluoroethylene containing tertiary alkyl amines selected from the group of triethyltropyl or tributyl amines is used.

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