Electrolysis diaphragm and method for its manufacture

0001The present invention relates to a diaphragm for electrolysis, based on fluorinated resins, having a marked hydrophilic character, as well as the process for preparing this diaphragm.

0002For a number of years, conventional asbestos diaphragms, for electrolysis, deposited on the cathodes of cells intended in particular for obtaining chlorine and soda, have been gradually replaced by diaphragms based on fluorinated resins possibly containing fibers. reinforcement. Such diaphragms have many advantages due in particular to the chemical properties of fluorinated resins but also a notable drawback, also inherent in these resins, which is due to their low wettability. This defect is mitigated when fibers such as asbestos are incorporated in significant proportions in the diaphragms, but we know the danger that this material presents for its users. Many solutions have been proposed to overcome this drawback: in addition to the use of particular fillers such as oxides or hydroxides of titanium, zirconium or aluminum or asbestos, it has also been suggested the introduction of groups containing sulfur. sulfonic groups in particular, either by in situ treatment of the resin used, as described in US Pat. No. 4,153,520, or by addition of previously sulfonated resin, described in particular in the French patent published under No. 2 152 988 .

0003It has now been found that hydrophilic diaphragms, that is to say easily wetted by an electrolyte, can be obtained by a simple process which gives them properties favorable for electrolysis, particularly in contact with concentrated detergents.

0004One of the objects of the invention is a microporous diaphragm based on fluorinated resin, intended in particular for the electrolysis of alkali metal halide, coated on at least part of the internal surface of the pores with a carboxylic acid copolymer unsaturated and non-ionic unsaturated monomer.

0005Another object of the invention is the process for obtaining this diaphragm comprising the formation of a porous sheet based on fluorinated resin, the impregnation of this sheet with a mixture containing at least one unsaturated carboxylic acid, at least one nonionic monomer and at least one polymerization initiator, this mixture having a low viscosity, the copolymerization of this mixture, draining the sheet after impregnation and copolymerization of the comonomers in said sheet.

0006The microporous sheet can be prepared by a wide variety of methods, many of these methods being well known today.

0007The fluorinated resins which can be used are in particular polytetrafluoroethylene, polytrifluoroethylene, polyhexafluoropropylene, polyvinyl fluoride, polyvinylidene fluoride, polyperfluoroalkoxyethylene, polyhaloethylenes comprising one or two chlorine atoms and three or two fluorine atoms on each ethylene unit and in particular polychlorotrifluoroethylene, the corresponding polyhalopropylenes, copolymers of ethylene and / or of propylene and halogenated, at least partially fluorinated unsaturated hydrocarbons having 2 or 3 carbon atoms. Among these compounds, mention may be made in particular of the products known under the brands "TEFLON by Du Pont de Nemours," SOREFLON by the company Produits Chimiques Ugine Kuhlmann, "HALAR by Allied Chemicals Co.

0008These resins can be reinforced with different fibers, either mineral, such as asbestos, glass, zirconia or carbon quartz, or organic, such as polypropylene or optionally halogenated and, in particular fluorinated, polyhalovinylidene fibers. etc ...

0009The proportion of reinforcing fibers can be from 0 to 200% of the weight of the resin. As already mentioned above when a relatively high proportion of asbestos is present, greater than 30% of the weight of resin, the diaphragm generally has satisfactory wettability without additional treatment.

0010The overall porosity must preferably be 50 to 95% and the equivalent average pore diameter is between 0.1 and 12 micrometers and preferably between 0.2 and 6 micrometers, this equivalent diameter being the diameter of a cylindrical pore theoretical which allows the same speed of passage of a slightly viscous liquid, under a determined pressure, as the real pore.

0011The carboxylic acid monomers used carry one or two carboxylic groups. It can be acrylic, methacrylic acids and their halogenated derivatives, phenylacrylic, ethylacrylic, maleic, itaconic, butyl-acrylic, vinylbenzoic etc ... Acrylic and methacrylic acids are preferred.

0012The nonionic monomers can carry a single ethylenic bond, such as styrene, methylstyrene, ethylvinylbenzene, chloro- or fluorostyrenes, or -methylstyrenes, as well as vinylpyridine or pyrrolidone. They can have several unsaturations and also promote crosslinking of the polymer layer formed. By way of examples, mention may be made of divinylbenzenes and in particular the para isomer which is preferred, trivinylbenzene, divinylnaphthalenes, divinylethyl or methylbenzenes. trivinyl 1-3-4 cyclohexane etc ...

0013It is possible and preferred to use both at least one non-ionic monounsaturated monomer and one pluri-unsaturated monomer. The numerical proportion of the molecules or units of these two types of monomers is then between 0.1 and 10 and preferably between 0.4 and 2.5. The commercially available divinylbenzene ethylvinylbenzene mixture is advantageously used.

0014The proportion by weight of unsaturated acid on all of the carboxylic and nonionic comonomers is between 40 and 98% by weight and preferably between 70 and 95% and it is important that this mixture of monomers optionally and preferably added with diluent, present a low viscosity preferably less than 2 cp so as to be able to penetrate under a slight depression (from 1 to 100 mmHG below atmospheric pressure) in the pores of the microporous substrate. In order to control the amount of monomers introduced and the dispersion in the porosity, an inert diluent is added to the mixture of monomers, in particular methanol, ethanol, isopropanol, butanols, acetone, methyl isobutyl ketone, dioxane , chloro or dibromomethane, optionally halogenated aliphatic hydrocarbons having from 2 to 10 carbon atoms, dimethylformamide, dimethylacetamide, dimethyl sulfoxide etc ... Ethanol is the preferred diluent; in general, the diluents must have a relatively low voltage at room temperature and be miscible with the comonomers and possibly with water. Per 100 parts by weight of comonomers, preferably 1,600 to 30 parts of diluent are used. The copolymer formed from the comonomers thus diluted will be present in an at least monomolecular layer on at least part of the internal surface of the pores.

0015A radical polymerization initiator is added to the mixture of comonomers; it must not cause significant polymerization at room temperature in the absence of activating radiation (ultraviolet), but cause polymerization of the comonomers in a time preferably less than 12 h, at a temperature below that of softening of the fluorinated polymer put in ceuvre, and preferably less than 100 ° C. Mention may be made, among the polymerization initiators, of benzoyl, lauroyl, t-butyl, cumyl peroxides, t-butyl peracetate or perbenzoate and also azobisisobutyronitrile.

0016The temperature conditions of the polymerization can be adapted to the choice of diluent so as to avoid its too rapid departure during the polymerization in situ. Activators can be used for this, for example dimethylaniline which, combined with benzoyl peroxide, makes it possible to obtain a polymerization around 40 ° C.

0017The process for preparing these wettable microporous diaphragms therefore comprises in its first phase the preparation of a microporous sheet. Among the preferred methods for this, mention may be made of those employing porophoric fillers as described in the French patents published under the numbers 2 229 739; 2,280,435; 2280609 and 2,280,435, the descriptions of which are incorporated herein by reference. It is also possible to introduce a porophore filler into a latex of fluorinated resin and in particular of polytetrafluoroethylene containing a plasticizing agent, 900 to 1,200 and preferably 400 to 900 parts by weight of porophores, 0.5 to 2 parts of agent plasticizer and 1 to 20 parts of water being added to 100 parts of a latex resin containing 40 to 60% by weight of dry matter, mix the whole in a moderately agitated mixer, that is to say the rotor rotates at less than 100 revolutions / min, preform by rolling a sheet using the paste obtained, dry it and then sinter at a temperature of the order of the melting point of the polymer used. The porophore agent which is preferably calcium carbonate is then removed by immersion in acid which is preferably acetic acid in aqueous solution at 15-20% by weight.

0018One can also obtain porous sheets, in particular in the case where the fluoropolymer used is a copolymer of ethylene and chlorotrifluoroethylene, or a PTFE latex, associated with mineral or organic fibers (asbestos, zirconia, fibrillated polyolefins) by dispersing the copolymer in an amount of 5 to 50% of the weight of fibers in the electrolyte, that is to say containing about 15% of sodium hydroxide and 15% of sodium chloride to which a surfactant is added.

0019This suspension is deposited on a surface allowing filtration; this surface can in particular be a perforated cathode. After spinning and drying, the sheet formed during filtration is heated to 260 ° C temperature which is maintained from 30 mm to 1 hour.

0020The porous sheet thus formed is then impregnated with a mixture of comonomers and of polymerization initiator and, in general of inert diluent. The proportion of diluent mentioned above must be chosen according to various other parameters and in particular, the proportion of the crosslinking agent comonomer, in particular divinylbenzene, relative to the unsaturated carboxylic acids and the proportion of polymerization initiator in particular. benzoyl peroxide. The overall condition which must be met, and which involves a choice in the combination of the various other parameters, is that 0.1 to 6% of the total pore volume, before the in situ copolymerization, of the microporous-support sheet, are occupied by carboxylic copolymer. The proportion by weight of divinylbenzene can be between 2.5 and 25 parts per 100 parts of unsaturated carboxylic acid. It is also good to use only small amounts of polymerization initiator, for example, less than 5 parts by weight of benzoyl peroxide per 100 parts of comonomers and little or no copolymerization accelerator such as dimethylaniline (less than 2 parts).

0021This impregnation can be done for example by immersion in a tank containing this liquid mixture and filtration under vacuum from 10 to 100 mmHg.

0022The sheet optionally on its support, and in particular on a cathode, is then introduced into an enclosure where the temperature, or actinic rays, in particular ultraviolet rays, allow the action of the polymerization initiators. It can be immersed in a liquid, water for example. It is important that the temperature is not too high, generally less than 150 ° C. and does not substantially modify the structure of the microporous sheet by too rapid departure of the diluent or destruction of the deposited copolymer. The polymerization time (which corresponds approximately to the half-life of the initiator used) is preferably less than 12 hours.

0023A preferred means of polymerization is immersion in water between 40 ° C and 100 ° C.

0024Table I given with the examples below clearly illustrates the influence of various factors such as the porosity of the diaphragm or, which is directly the cause, the proportion of porophore agent, the weight ratio between the carboxylic acids and the nonionic monomers and the amount of diluent added on the pressure drop of the electrolyte through the diaphragm or in other words on the hydrostatic pressure, due to the anolyte, necessary to ensure satisfactory percolation and on the electrical voltage in the cell. We will also see that the factors mentioned can be chosen to achieve a specific goal.

0025Examples of implementation of the invention are given below for the sole purpose of illustrating it.

Example 1

0026Is introduced into 167 g of polytetrafluoroethylene latex at 60% dry extract, brand "SOREFLON", from the company "Produits Chimiques Ugine Kuhlmann", 700 g of powdered calcium carbonate, trade name "CALIBRITE 1400" from the company OMYA and 42 g of PEROLENE (PEROLENE SPZ) in aqueous solution at 62 g / I.

0027The mixture is homogenized for 5 minutes in a WERNER type mixer whose Z-shaped rotors rotate at a speed of 45 revolutions / min.

0028The dough obtained is put into sheets using a LESCUYER type roller mixer. The thickness is reduced to 1.2 mm. The initial cylinder rotation speed of 15 rpm is gradually reduced to 5 rpm in 2 to 4 minutes.

0029The sheet thus formed is dried for 15 hours at 90 ° C then 2 hours at 120 ° C and then sintered in a hot air circulation oven whose temperature has risen, at a rate of 100 ° C / h, to 360 ° C where it is kept for 15 minutes.

0030The calcium carbonate is removed by immersion for 72 hours in an aqueous solution of acetic acid at 25% by weight added with 2 g / I of fluorinated surfactant brand “ZONYL FSN from EI Du Pont De Nemours Co. •.

0031The diaphragm is then rinsed with water and then immersed for 12 hours in ethanol.

0032The solution below is then filtered through the microporous diaphragm formed under a vacuum of 50 mm of mercury:<ul id="ul0001" list-style="none"><li>- ethanol 330 parts</li><li>- 100 parts methacrylic acid</li><li>- commercial divinylbenzene 10 parts</li><li>- benzoyl peroxide 2 parts</li></ul>

0033The parts mentioned are expressed by weight.

0034Commercial divinylbenzene contains 45% by weight of ethylvinylbenzene and 55% of divinylbenzene.

0035The copolymerization is brought about by immersion for 2 hours in water at 80 ° C.

0036This diaphragm, which has been given remarkable wettability, is kept in water until it is used. It is then placed, in contact with a cathode, in braided laminated iron from GANTOIS •, an electrolysis cell.

0037The anode is made of expanded titanium coated with Pt-Ir alloy.

0038The distance between the electrodes is 5.5 mm: it is maintained by a rubber seal.

0039The electrolyte introduced into the anode compartment is a brine containing 300 g / l of sodium chloride.

0040After 200 hours of operation, the operating conditions then being stable, the temperature is 85 ° C, the current density is 25 A / dm², the electrical voltage is 3.35 V, the electrolyte charge is 40 cm. The sodium hydroxide of the catholyte has a concentration of 123 g / I, the faradic yield (OH ion) is 94_%.

Comparative example

0041A microporous diaphragm prepared as above, with the exception of the treatment with the carboxylic acid comonomers, nonionic monomers which is not applied to it, is used under the same conditions as those of Example 1.

0042After 15 hours of operation, the voltage rises to 4.0 V and the charge rises to 60 cm. It then increases very quickly and electrolysis must be stopped.

Example 2

0043The test of Example 1 is repeated by varying the quantity of calcium carbonate and the proportion of the diluent and peroxide comonomers of the impregnation mixture.

0044The various data are mentioned in Table 1 in which:<ul id="ul0002" list-style="none"><li>-AM = methacrylic acid</li><li>- DVB = commercial mixture at 55% by weight of divinylbenzene and 45% of ethylvinylbenzene</li><li>- PB = benzoyl peroxide.</li></ul>

0045The results indicated are recorded after 200 hours of walking, unless otherwise indicated.

0046The first two control tests (1 and 2) had to be stopped after 25 hours, which is the time when the measurements of charge h and voltage U were made. The same is true of test 235.

0047The figures concerning the materials used are parts by weight, those of calcium carbonates are related to 100 parts of fluoropolymer (dry).

0048The electrolyte charge R is the hydrostatic pressure on the diaphragm expressed in cm or the electrolyte height of density 1.2 approximately, multiplied by this last figure.

0049The amount of NaOH is expressed in grams / liter.

0050The yield R (OH)% is a faradaic yield calculated from the sodium hydroxide formed.

0051T% is the percentage of the pore volume occupied by the dry polymer. (See Table on page 6)<tables id="tabl0001" num="0001"><img file="EP0033262B1_D0001.tif" /></tables>