CA1059062A - Method for diaphragm electrolysis of alkali metal halides - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for diaphragm electrolysis of an alkali metal halide which comprises electrolyzing an alkali metal halide solution by passing an electric current through an anode compart-ment and a cathode compartment of an electrolytic cell with an ion-exchange membrane of a graft copolymer of a polyolefin main chain and a side chain composed mainly of a hydroxystyrene compound having the formula (I)

Description

~OS9~16Z

BACKGROUND OF THE INVENTION
___________________________ 1. Field of the Invention This invention relates to a method for electrolyzing halides of monovalent alkali metals using an electrolytic dia-phragm, and more specifically, to a method for electrolyzing halides of monovalent alkali metals using a cation exchange membrane composed of a graft copolymer of a polyolefin main chain to which a side chain composed mainly of a hydroxystyrene compound having the following formula is grafted:
Cl~= CH 2 l~
(OH)n wherein n is an integer of 1 or 2.

2. Description of the Prior Art .
In the method of electrolyzing an aqueous solution of an alkali metal halide in an electrolytic cell including an anode, a cathode, a diaphragm disposed between them for separating the electrolytic cell into an anode compartment and a cathode compart-ment, and means provided outside of the cell for passing an electric current between the anode and the cathode, the feeding of the aqueous solution of the alkali metal halide into the anode compartment and the subsequent passing of a current between both electrodes result in the conversion of the halogen ion to halogen at the anode. The alkali metal ion moves to the cathode compart-ment vla the diaphragm, and an alkali metal hydroxide and hydrogen gas are generated on the cathode. Previously, a porous diaphragm made of asbestos has frequently been used for this purpose. How-ever, since the asbestos porous diaphragm does not possess selec-tive permeability between positive and negative ions and between ~059~6Z
monovalent and polyvalent ions, a part of the hydroxide ion formed in the cathode compartment diffuses into the anode compartment through the diaphragm to cause a reduction in current efficiency.
At the same time, very small amounts of divalent or higher cations such as iron, magnesium or calcium contained as impurities also move to the cathode compartment together with the alkali metal ion, and thus cannot be removed. Thus, in order to prevent diffusion of hydroxide ions into the anode compartment, a technique is employed of flowing a part of the anodic solution into the cathode compartment through the diaphragm. However, an enormous cost in the subsequent concentrating and purifying steps is required because the aqueous solution of the alkali metal hydroxide obtained in the cathode compartment contains a large quantity of the alkali metal halide and traces of polyvalent metal ions, and the concentration of the resulting alkali metal hydroxide cannot be increased.
It is known that the use of a cationic exchange membrane as the diaphragm obviates the above defect. If a cation exchange membrane having an ideal selective permeability to monovalent cations is used as a diaphragm, alkali metal hydroxides in high concentrations can be obtained from the cathode compartment with-out involving the above difficulties because the cation exchange membrane does not permit the permeation of hydroxide ions, halogen ions and polyvalent metal ions such as iron or magnesium.
However, it is very difficult in practice to produce membranes having an ideal permselectivity to monovalent cations. Conven-tional cation exchange membranes have proved to be not entirely feasible for one or more reasons. For example, these membranes cannot ensure sufficient current efficiency or sufficient purity or concentration of the product. Moreover, since the membranes 1059~62 are exposed to severe conditions, they do not have sufficient endurance for use for prolonged periods of time.

SUMMARY OF THE INVENTION
____________ ___________ An object of this invention is to provide a method for electrolyzing alkali metal halides using a diaphragm having excèllent performance.
Another object of this invention is to provide an electrolytic diaphragm having high permselectivity to monovalent cations, mechanical strength and durability.
The present invention provides a method for electro-lyzing alkali metal halides using a cation exchange membrane as a diaphragm, the cation exchange membrane being composed of a graft copolymer of a polyolefin main chain to which a side chain composed mainly of a hydroxystyrene compound of the following formula tI) is grafted:

f~H=CH2 ~OH)n (I) wherein n is an integer of 1 or 2.

DETAILED DESCRIPTION OF THE INVENTION

_____________________________________ The graft copolymer used in this invention can be cross-linked or sulfonated, or both cross-linked and sulfonated.
The diaphragm used in this invention can be produced, for example, by the following method.

The polyolefin used in this invention can be an aliphatic hydEocarbon polymer, especially those polymers of monomer units 1 having 2 to 10 carbon atoms, such as polyethylene, polypropylene or polybutene; aromatic hydrocarbon polymers, especially polymers of compounds represented by~the general formula (II) Rl~ / R2 / C = C \ (II) wherein Rl, R2 and R3 each represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and A is an aryl group having the formula R6 ~ R4 or ~ in which R4, R5, R~4 R~5 R6, R'4, R'5 and R'6 each represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, such as polystyrene, poly(a-methylstyrene) or poly(tert-butylstyrene); alicyclic hydrocarbon polymers, especially polymers of the compound represented by the general formula (III) R7CH = CHR8 (III) in which R7 and R8 each represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and at least one of R7 and R8 is a cycloalkyl group, such as polyvinyl cyclohexane; or copolymers derived from two or more aliphatic, alicyclic or aromatic monomers that constitute the above polymers. The polyolefin that makes up the main chain of the graft copolymer can have branched chains.
The range of the degree of polymerization of these polymers is such that the polymers are solid at normal temperatures (e.g., 20-30C). The polymers can be used in various desired forms such as powders, granules, fibers or films. If a polymer in a film form is used, the product can be used directly as a diaphragm.

~)59~162 The hydroxystyrene compound that constitutes the hydroxystryene side chain of the graft copolymer can be any isomer, or a mixture of these isomers. A suitable proportion of the hydroxystyrene compound side chain is about 5 to 500~
by weight, preferably 20 to 200% by weight, based on the poly-olefin main chain. Instead of the hydroxystyrene compound, an acyloxystyrene can be grafted to the polyolefin main chain with subsequent hydrolysis of the acyloxy group. Examples of suitable acyloxystyrenes are mono-, or 1,2-, 1,3- or 3,4-diacetoxystyrene, mono-, or 1,2-, 1,3- or 3,4-dipropionyloxystyrene, mono-, or 1,2-, 1,3- or 3,4-dibutyryloxystyrene, and mono-, or 1,2-, 1,3- or 3,4-dibenzoyloxystyrene. MoSt generally, para-acetoxystyrene or 3,4-diacetoxystyrene is used.
In order to reduce the water content of the graft co-polymer containing the hydroxystyrene compound side chain and to increase the current efficiency thereof, the graft copolymer can, if desired, be cross-linked using a difunctional compound reactive with phenolic hydroxy groups, a polyene compound having at least 2 polymerizable double bonds in the molecule, or an organic sulfonic acid compound.
Examples of suitable difunctional compounds are di-epoxides, for example, aliphatic, aromatic and alicyclic di-epoxides, such as ethylene glycol diglycidy] ether, diethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, cyclo-hexane diol diglycidyl ether, and epoxy resins; diisocyanates such as hexamethylene diisocyanate, tolylene diisocyanate, xylylene diisocyanate, diphenylmethane-4,4'-diisocyanate, or hexahydrotolylenediisocyanate; and acid dihalides such as adipoyl dichloride, terephthaloyl dichloride and hexahydro-terephthaloyl dichloride. A suitable amount of the difunctionalcompound is about 0.01 to 0.5 equivalent per equivalent of the phenolic hydroxy group.

~059C~6Z

1 Examples of suitable organic sulfonic acid compounds are aliphatic or aromatic sulfonic acids having 1 to 20 carbon atoms or their alkyl esters, such as benzensulfonic acid, methyl benzenesulfonate, para-toluenesulfonic acid, methyl para-toluene-sulfonate, ethanesulfonic acid, and methyl ethanesulfonate.
When the graft copolymer is cross-linked with an organic sulfonic acid compound, the graft copolymer is immersed in a solution of the organic sulfonic acid compound, and react~d by heating, or the copolymer is impregnated with this solution and reacted by heating. A suitable reaction temperature is generally about 100 to 150C. The reaction time differs according to the end use of the copolymer, or the degree of cross-linking, but generally, a suitable reaction time is about 5 minutes to 5 hours or longer.
The polyene compound having at least two polymerizable double bonds in the molecule can, for example, be an aliphatic compound, an alicyclic compound containing double bonds in the ring or in a substituent, and an aromatic compound containing unsaturated substituents. Suitable aliphatic compounds are aliphatic hydrocarbons, and aliphatic esters, such as diesters formed between unsaturated acids and dihydric alcohols, diesters formed between diacids and unsaturated alcohols, diesters formed between unsaturated acids and unsaturated alcohols, diesters formed between unsaturated acids and unsaturated diols, and diesters formed between unsaturated dicarboxylic acids and un-saturated alcohols, and the like. These compounds have 4 to 20 carbon atoms. Specific examples of the polyene compounds are divinylbenzenes (o-, m- or p- isomers, or mixtures of these isomers), isoprene, butadiene, cyclopentadiene, ethylidene norbornene, diol esters of acrylic acid or methacrylic acid, or 1059~6Z t divinyl esters of adipic acid. Of these, divinylbenzenes and isoprene are used preferably. All of the o-, m- and p-isomers of divinylbenzene can be used in this invention. Generally, a mixture of these isomers is used. Generally, commercially available divinylbenzene sometimes contains about 45~ by weight of ethylvinylbenzene, but this mixture can be used as such in the present invention.
A membrane composed of a graft copolymer of a polyolefin as a main chain and a side chain composed of the hydroxystyrene compound and the polyene compound grafted to the main chain, in which the polyolefin is cross-linked by the polyene compound of the side chain, can also be used as the diaphragm in accordance with this invention. In this copolymer, too, the above-described various polyolefins and polyene compounds, and the hydroxystyrene compounds can be used as constituents of the copolymer. Irre-spective of the method of introducing the polyene compound, the amount of the hydroxystyrene compound and polyene compound is preferably about 5 to 500~ by weight based on the polyolefin main chain. If the amount is less than about 5%, the resulting copolymer has insufficient properties as an electrolytic diaphragm. On the other hand, if the amount is larger than about 500% by weight, the resulting graft copolymer has insufficient strength and softness, and becomes difficult to use.
The grafting ratio, or the rate of introduction (based on the polyolefin), of the polyene compound is generally about 0.5 to 100%. If the grafting ratio is less than about 0.5%, the degree of cross-linking is low, and no outstanding effect is obtained by incorporating the polyene compound. On the other hand, if the grafting ratio is more than about 100~, cross-linking becomes excessive, and the polymer generally tends to be hard, 1 brittle and tearable, and tends to have a high electric resist-ance that makes the passage of electricity through the polymer difficult. In view of the ion transport number, electric resistance and strength of the membrane, an especially preferred grafting ratio is about 20 to 200% for the hydroxystyrene compound, and about 2 to 50~ for the polyene compound.
Preferably, the hydroxystyrene : polyene weight ratio is about 200 : 1 to 1 : 1, especially 50 : 1 to 2 : 1.

Furthermore, an ion-exchange membrane of a sulfonated product of the graft copolymer, either cross-linked or uncross-linked, can also be used as a diaphragm in this invention. Inthese graft copolymers, sulfonic acid groups are introduced mainly into the hydroxystyrene portion, but can be introduced into the polyene compound portion grafted for cross-linking purposes.
The rate of introduction of sulfonic acid groups is not particularly limited, but generally, about 0.5 to 2 sulfonic acid groups can be introduced per unit of the hydroxystyrene compound. A suitable thickness of the membrane composed of the ~ graft copolymer or the sulfonated product thereof is about 0.05 to 0.5 mm (in a wet condition).
~e ~ O~rO ~C r ; The mcmbranc composed of the above graft copolymer or its sulfonated product can be those prepared by any desired method. For example, a membrane composed of the graft copolymer containing a hydroxystyrene compound side chain can be prepared by subjecting a polyolefin film to ionizing radiation in vacuo, in air or in an inert gas such as nitrogen, and then immersed in a solution of a hydroxystyrene compound monomer or an acyloxy-styrene compound monomer or a mixture of these (when the acyloxy-styrene compound monomer is used, the grafted acyloxystyrene is 1059~62 hydrolyzed to convert the acyloxystyrene to the hydroxystyrene compound). The membrane can also be obtained by immersing the polyolefin film in a solution of the styrene compound monomer, and applying ionizing radiation (when an acyloxystyrene compound monomer is used, hydrolysis is carried out subsequently as described above). Alternatively, a powdery or granular poly-olefin is used in the above process instead of the film-form polyolefin to obtain a powdery or granular graft copolymer, and such a copolymer can be fabricated into film form using any desired film-forming techniques such as press-forming or melt-extrusion. If desired, the grafted copolymer obtained in a film form is subjected to the above-described cross-linking treatment.
The cross-linking treatment using the polyene compound can be performed easily by utilizing ionizing radiation as in the case of the graft copolymerization.
In order to obtain a graft copolymer in which a side chain composed of the hydroxystyrene compound and the polyene compound is grafted to a polyolefin main chain and which is cross-linked with the polyene compound of the side chain, a solution containing both the styrene compound monomer and the polyene compound monomer is used in the above-described method for graft copolymexization using ionizing radiation, whereby the styrene compounds and polyene compounds are both grafted to the polyolefin, and cross-linking occurs by the polyene com-pound so grafted.
The styrene compound and the polyene compound are used for the graft copolymerization reaction as solutions in organic solvents which uniformly dissolve the styrene compound and the polyene compound, but do not dissolve the polyolefin. Examples of suitable organic solvents are ketones such as acetone or methyl ethyl ketone, esters such as ethyl acetate or butyl ~OS906Z
acetate, alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol or butyl alcohol, ethers such as tetrahydrofuran, aromatic hydrocarbons such as benzene or toluene, aliphatic or alicyclic hydrocarbons such as n~heptane or cyclohexane, or a mixture thereof. Because these aliphatic or alicyclic hydro-carbons have high affinity for the hydrocarbon polymers, they swell the polymers and permit easy introduction of the monomer.
Thus, the grafting reaction is accelerated, and the grafting becomes uniform. The amount of these hydrocarbons should be such ~ that they do not dissolve the polymer at the reaction temperatures, and is determined according to the type of the polymer.
The concentration of the monomer in the reaction solu-tion is not critical, but generally, a suitable concentration of the monomer is about 0.1 to 80% by weight, preferably 5 to 50 by weight, based on the solution.
When the monomeric mixture to be grafted contains an unsaturated compound such as ethyl vinylbenzene, for example, such an unsaturated compound is also graft copolymerized and con-tained in the side chain. The presence of unsaturated compounds other than acyloxystyrene compounds, hydroxystyrene compounds and polyene compounds, especially monounsaturated compounds other than those having a polymerization inhibiting action, for example, styrene, l-hexene and acrylic acid esters in addition to ethyl vinylbenzene, does not adversely affect the reaction.
However, if the amount of such an unsaturated compound is too large, the effect of the present invention is reduced. For practical purposes, therefore, the amount of such a compound can be about 30~ by weight or less based on the total monomeric mixture.

The source of ionizing radiation can be ~-rays, X-rays, electron beams, a-rays, or mixtures thereof. A suitable intensity, ~059~6Z
1 that is, dose, of the ionizing radiation is about 103 to 1011 rads per hour. With electron beams, doses of as high as 109 to 1011 rads per hour can be used. Although lower doses can be used, a long time is required to obtain the desired amount of irradia-tion. Furthermore, higher doses can also be used, but are not feasible because higher doses may result in the structural change of the polyolefin, for example, excessive cross-linking, cleavage of the main chain, and deformation and breakage of the polymer by heat.
tO The use of electron beams generated from an electron beam accelerator is especially effective since high dose irradiation can be obtained within short periods of time. The total dose of ionizing radiation required for graft copolymeriza-tion is usually from about 105 rads to 101 rads.
The temperature employed for ionizing radiation must be one at which the polyolefin is not dissolved and deformed.
In view of the life of the generated radicals (which is short at high temperatures), a feasible temperature generally ranges from about -100C to 40C. There is no particular lower limit to this temperature except that arising due to economical and technical problems.
The graft copolymerization reaction temperature generally ranges from the temperature at which the reaction mixture is a liquid, to about lbOC. If the reaction temperature is too low, the time required for the reacti,on increases, and if .i 9e~ 7~,0 ~
the reaction temperature is too high, gcll~e~ or homopolymeriza-tion under heat tends to occur. A suitable temperature can be selected so that such difficulties do not occur. For practical purposes, temperatures of about 0 to 70C are suitable. Where the ionizing radiation is applied in air, the graft copolymeriza-tion is preferably carried out at a temperature of about 60C

~059062 1 or more because the peroxide generated must be decomposed.
Ionizing radiation in advance in air or in a stream of nitrogen is commercially advantageous.
The resulting graft copolymer, if desired, is washed with an organic solvent, for example, an alcohol such as methanol, ethanol or propanol, a ketone such as acetone or methyl ethyl ketone, or an aromatic hydrocarbon such as benzene or toluene, or mixtures of these solvents. Hydrolysis of graft copolymers containing a side chain comprising the acyloxystyrene, similar to an ordinary hydrolysis of phenol esters, is much easier to perform than the hydrolysis of esters of primary alcohols, and can be carried out easily under mild conditions. Specifically, the graft copolymer is placed in a solution of an acid such as hydrochloric acid, sulfuric acid or organic sulfonic acid or a base such as sodium hydroxide or ammonia as a catalyst in water or in a mixture of water and an organic water soluble solvent to hydrolyze the acyloxy group of the side chain. Since the hydrolysis is primarily carried out in a heterogeneous system, it is preferably performed in a mixture of water and a water-soluble organic solvent such as an alcohol or ketone in order to increase the affinity between the substrate and the catalyst and also to dissolve the organic acid that is split off in the case of using an acidic catalyst. A suitable hydrolysis tempera-ture is about 50 to 100C.
In order to obtain membranes composed of a sulfonated product of the graft copolymer, either cross-linked or uncross-linked, any known method for sulfonating phenols can be used.
For example, the sulfonation can be effected by sulfonating the graft copolymer with concentrated sulfuric acid, sulfuric anhydride, or chlorosulfonic acid, etc. in the presence lOS9~6Z
1 or absence of a solvent. Examples of suitable solvents which can be used in this process are halogenated hydrocarbons such as chloroform or carbon tetrachloride, polar solvents such as pyridine or dimethylformamide, or solvents such as ether or dioxane. Catalysts such as silver sulfate can also be used in this process.
When concentrated sulfuric acid is used, the film is immersed in concentrated sulfuric acid, and allowed to react for about 1 hour to about 10 days at about 0 to 40C. If heating to a temperature of about 60C is carried out, the treating time can be shortened. If the temperature is too high, the base polymer is attacked with the properties of the polymer being degraded. In order to achieve a mild reaction, up to about 80 by weight of a solvent such as acetic acid or dioxane can be used. When fuming sulfuric acid containing about 5 to 60% by weight of sulfuric anhydride is used, the film is suitably treated at room temperature for about 2 to 10 hours. If the reaction proceeds excessively, the base polymer is also attacked. Where chlorosulfonic acid is used, the graft copolymer is dissolved in a solvent such as chloroform, dioxane, carbon tetrachloride or a mixture of these solvents in a concentration of about 1 to 60% by weight, and reacted at about 0 to 60C for about 1 hour to 10 days. Then, the reaction product is washed with water.
The reaction conditions such as the temperature, the type of reagent, the concentration, or the reaction time are controlled as required so that the proportion of sulfonic acid groups introduced becomes the desired value.
If the graft copolymer is treated with concentrated sulfuric acid at room temperature for about 10 hours, about one sulfonic acid group is introduced per hydroxystyrene group unit.

~OS9C3 62 When a strong sulfonating agent such as chlorosulfonic acid is used, the treatment of the polymer in a solution of chloroform or dioxane, etc. results in the introduction of about 2 sulfonic acid groups per hydroxystyrene group unit, and a membrane having a high ion-exchange capacity can be obtainedO
Membranes composed of the above graft copolymers or their sulfonated products have reduced permeability to halogen ions, low electric resistance, high permselectivity to monovalent cations, high mechanical strength and high durability.

In the practice of the method of this invention, any type of electrolytic cell including an anode, a cathode and a cationic exchange membrane as hereinabove described and provided between the electrodes can be used. For example, a two-compart-ment electrolytic cell divided into an anode compartment and a cathode compartment by the cationic exchange membrane, a three-compartment electrolytic cell in which an anode compartment and an intermediate compartment are separated from each other by a non-selective diaphragm and the intermediate compartment and a cathode are partitioned by the above cationic exchange membrane, or a modified three-compartment or four-compartment electrolytic cell built up by providing another non-selective diaphragm in the cathode compartment of the above three-compartment electro-lytic cell. The arrangement of the electrodes is also optional.
For example, they are arranged perpendicularly, or obliquely in parallel to each other, or not in parallel to each other. E~owever, unless there is a special reason to do otherwise, electrodes provided in parallel to each other are generally used. The mode of providing the cationic exchange membrane can also be varied just as in the case of the electrodes, but usually, the membrane is provided perpendicularly. Generally, an iron or iron-type electrode is used as the cathode, and a graphite or dimensionally ~59062 stable electrode is used as the anode. The scope o this inven-tion, however, is not limited and any appropriate selection of these embodiments can be made.
An aqueous solution of an alkali metal halide to be electrolyzed is fed into the anode compartment or the intermediate compartment, and an electrolytic current is passed between the electrodes whereby an alkali metal hydroxide is f~rmed in the cathode compartment. At the surface of the cathode, hydrogen-gas is formed, and at the surface of the anode, halogen is 10generated.
The alkali metal halides that can be electrolyzed by the method of this invention are halides of monovalent alkali metals. A suitable concentration of the solution electrolyzed generally ranges from about 1~ by weight up to a saturated solu-tion. A suitable current density is about 1 to 40 A~dm2.
When sodium chloride is electrolyzed using the cationic exchange membrane in accordance with this invention, the tempera-ture of the electrolytic cell can range from room temperature to about 90C, and a suitable current density ranges from about S to 30 A/dm . If, for example, sodium hydroxide in a concen-tration of 15g by weight is to be produced at a current density of 10 A/dm2 using a cationic exchange membrane having a thickness of 0.15 mm, a current efficiency o~ 70 to 95% can be obtained, and the resulting sodium hydroxide solution contains sodium chloride in an amount of less than about 0.02~ by weight.
The following Examples are given to further illustrate the present invention in detail but the invention is not to be construed as being limited thereby. Vnless otherwise indicated, all parts, percents, ratios and the like are by weight.

An electrolytic cell made of poly(methyl methacrylate) was used which was partitioned by a diaphragm having an effective area of 10 cm2 into an anode compartment having a volume of 7 cm3 and a cathode compartment having a volume of 3.5 cm3 and in which a graphite plate was used as an anode and a stainless steel plate was used as a cathode. A solution feed opening and discharge openings for the solution and gas were provided in each of the anode compartment and the cathode compartment. Brine saturated at room temperature with sodium chloride was fed into the anode compartment at a rate of 15 ml/min., and a prescribed amount of a O.lN aqueous solution of sodium hydroxide was circulated into the cathode at a rate of 5 ml/min. Both of these solutions were pre-heated by a coil immersed in a constant-temperature water tank, and the temperature inside the tank was maintained at 70C.
Current of a specific den~ ty wa,s passed for a prescribed period e o O/,~e ~- of time, and then the cathoritc was analyzed to examine the con-centrations of sodium hydroxide and sodium chloride. On the basis of the concentrations, the current efficiency was calculated.
Separately, a polyethylene film having a thickness of 0.1 mm was inserted into a glass ampoule, and subsequently, a tetrahydrofuran-n-heptane mixed solution (1:2 by volume) contain-ing 20 wt% of p-acetoxystyrene monomer was added thereto, followed by heat-sealing the glass ampoule in vacuum. After irradiation of ~-rays of 105 rad/hr at a temperature of 25C for 10 hours, the resulting film was taken out of the glass ampoule and sufficiently washed with acetone to remove the p-acetoxystyrene homopolymer. The thus treated film was then refluxed in a con-centrated hydrochloric acid-methanol mixed solution (1:4 by volume) under heating for a period of 30 minutes, followed by 1 hydrolysis to obtain a film in which a p-hydroxystyrene side chain was grafted at a grafting ratio of 52 wt~ (based on the poly-ethylene). This film was further immersed in concentrated sul-furic acid at a temperature of 25C for 72 hours and then sul-fonated to obtain a cationic exchange membrane. The thus obtained cationic exchange membrane was used as a diaphragm. This cationic exchange membrane had a cation transport number of 0.97, an electric resistance, in 0.5N sodium chloride, of 1.7~-cm2, and an acidic ion-exchange capacity of 2.2 meq/g. A potential was applied to the cell and after about 10 minutes a current of 1.0 A (lO A/dm2) was passed for 5 hours. It was found that the concentration of sodium hydroxide in the catholyte was 16.8% by weight, that the concentration of sodium chloride was 0.02~ by weight, and that the current efficiency was 83%.

A polyethylene film having a thickness of 0.1 mm was inserted in one leg of an H-type glass cell and a benzene-acetone solution (2:1 by volume) containing 20 wt% of a p-acetoxystyrene monomer was added to the other leg, followed by heat-sealing the H-type glass cell in vacuum. The portion containing the monomer solution was cooled and frozen, followed by covering sufficiently with lead plates. Then the H-type cell was cooled to -30C and electron beams of 20 Mrad were irradiated onto the polyethylene film using an electron beam accelerator under the conditions, i.e., of an acceleration voltage of 2 MeV, and an acceleration current of lmA, at this temperature.
After irradiation of the polyethylene film, the monomer solution was melted and transferred to the film side, and reacted at room temperature (i.e., about 20 to 30C) for 24 hours. After completion of the reaction, the cell was opened and the resulting film was then taken out, followed by washing thoroughly with acetone. The 1C~59062 thus treated film was then hydrolyzed in the same manner as described in Example 1 to obtain a film in which a p-hydroxy-styrene side chain was graft-copolymerized at a grafting ratio of 92 wt% (based on the polyethylene).
Electrolysis was then carried out in the same manner as described in Example 1 using the film thus obtained as a diaphragm. It was fou~d th,at the concentration of sodium c~f~io~
hydroxide in the aathori-tc was 14.2% by weight and the concen-tration of sodium chloride was 0.01% by weight. The current efficiency was 74~.

Electrolysis was carried out in the same way as inExample 1 using the same diaphragm as in Example 1 except that it was not sulfonated. It wa~ fpu~d that the concentration of ~ 70/~'e sodium hydroxide in the c~ was 16.9% by weight, that the concentration of sodium chloride was 0.01% by weight, and that the current e~ficiency was 77%.

The same graft copolymer film as used in Example 1 except that the film was not sulfonated but was immersed in a 10 acetone solution of bisphenol A diglycidyl ether (containing 1%
of triethylenetetramine as a curing agent) for 2 hours, and then taken out, followed by air-drying. The resulting film was further heated to 100C for 30 minutes in an air thermostat to obtain a cross-linked film.

Electrolysis was then carried out in the same manner as described in Example 1 using the resulting cross-linked film as a diaphragm. It wa~ f~und that the concentration of sodium hydroxide in the oathor-~c was 16.8% by weight, and that the concentration of sodium chloride was 0.005% by weight. The current efficiency was 85%.

P~

l~S9(~62 Electrolysis was carried out in the same way as in Example 1 using a diaphragm obtained by graft-copolymerizing a 0.1 mm thick polypropylene film with p-hydroxystyrene at a grafting ratio of 98~ by weight in the same manner as described in Example 2.
It w~s fo~und that the concentration of sodium hydroxide in the ~athor~e was 16.0% by weight, that the concentration of sodium chloride was 0.02% by weight, and that the current efficiency was 76%.

Electrolysis was carried out in the same way as in Example 1 using a diaphragm obtained by sulfonating the graft-copolymer membrane obtained in Example 5. It ~as fQund that thec~o /~e concentration of sodium hydroxide in the e*~e~itr-was 16.2% by weight, that the concentration of sodium chloride was 0.02%, and that the current efficiency was 78%.

Electrolysis was carried out continuously for 1000 hours using the same apparatus as used in Example 1 and a diaphragm obtained by graft-copolymerizing a 0.2 mm thick polyethylene film with p-hydroxystyrene at a grafting ratio of 105% by weight using ionizing radiation from electron beams. The current density used was 15 A/dm2. In order to completely prevent C12 from attacking the ion-exchange membrane, a neutral porous diaphragm (made of a fluorine resin) was provided to separate the ion-exchange membrane in the anode compartment from the anode, and a suitable amount of water was added to the cathode compartment ~o m 'ntain the con-c~o ~o/,~
centration of sodium hydroxide in the c~tho~ite at 11% by weight, and brine comprising saturated sodium chloride at room tempera-ture ti.e., about 20-30C) was supplied between the ion-exchange membrane and the neutral porous diaphragm. The total current efficiency of the lOS9C3 6Z
1 continuous electrolysis for 1000 hours was 75~. The average concentration of sodium chloride in the catholyte was 0.002%
by weight, and no reduction in current efficiency was observed even at the end of the electrolysis.

Electrolysis was rarried out continuously for 1000 hours in the same way as in Example 7 using a diaphragm obtained by sulfonating the graft copolymer membrane used in Example 7.
When the concentration of sodium hydroxide in the catholyte was 8~ by weight, the total current efficiency of the continuous electrolysis for 1000 hours was 85%, and the concentrati~n of sodium chloride in the catholyte was less than 0.001% by weight.
No reduction in current efficiency was observed even at the end of the electrolysis.

A polyethylene film having a thickness of 0.2 mm was cooled to -20C, and electron beams of 20 Mrad were irradiated thereon under a nitrogen atmosphere. Subsequently, the resulting polyethylene film was inserted into a glass ampoule and a solu-tion in which a monomer mixture of p-hydroxystyrene and divinyl-benzene (divinylbenzene content of 55 wt%, an m- to p- weight ratio of about 2 : 1, and the remainder being mainly ethylvinyl benzene) at a mixing ratio of 3 : 1 by weight was dissolved in the same weight of benzene was added thereto, followed by thorough degassing in vacuum by repeating a freezing-melting procedure five times and heat-sealing. This glass ampoule was then heated to 60C and the contents were reacted for one hour. After completion of the reaction, the seal of the glass ampoule was broken and the resulting film was 1C~59~62 1 then taken out, followed by washing thoroughly with acetone.
The thus treated film was then hydrolyzed in the same manner as described in Example 1 to obtain a film at a total grafting ratio of 75 wt% (based on the polyethylene). As a result of analysis, the p-hydroxystyrene content was 69 wt% in the total of the graft copolymer.
Electrolysis was then carried out in the same manner as described in Example 1 using the film thus obtained as a diaphragm. It was found that the concentration of sodium hydroxide in the catholyte was 17.2% by weight, that the concen-tration of sodium chloride was 0.02~ by weight, and that the current efficiency was 87%.

EXAMPLE lO
Electrolysis was carried out in the same way as in Example 1 using a diaphragm obtained by sulfonating the same graft copolymer membrane as used in Example 9 in a 50% by weight dioxane solution of chlorosulfonic acid at 70C for 5 hours.
It was found that the concentration of sodium hydroxide in the cath~lyte was 17.3% by weight that the concentration of sodium chloride was 0.03% by weight, and that the current efficiency was 88%.

Electron beams of 30 Mrad were irradiated on a poly-propylene film having a thickness of 0.1 mm in air.The resultin~
film was subjected to grafting and hydrolysis in the same manner as described in Example 9 to obtain a film at a total grafting ratio of 115 wt% (based on the polypropylene), the p-hydroxystyrene content being 91 wt~ in the total of the graft copolymer.
Electrolysis was then carried out in the same manner as described in Example 1 using the polypropylene film thus 1059~)6Z

obtained as a diphragm. It wa~f o d that the concentration ;~ C O ~
of sodium hydroxide in the c~t-h~rit~ was 17.0~ by weight, that the concentration of sodium chloride was 0.02~ by weight, and that the current efficiency was 85%.

On a polyethylene film having a thickness of 0.2 mm was grafted p-acetoxystyrene in the same manner as described in Example 5, and subsequently, divinylbenzene (in which a benzene-acetone mixed solution containing 3 wt% of divinylbenzene usedas a monomer solution) was further grafted thereon in the same manner as described in Example 5. The thus treated film was then hydrolyzed to obtain a film at a total grafting ratio of 90 wt% (based on the polyethylene). The p-hydroxystyrene content was 94 wt~ and the divinylbenzene content was 6 wt~ in the total of the graft copolymer, respectively.
Electrolysis was then carried out in the same manner as described in Example 1 using the polyethylene film thus obtained as a diaphragm. It was~ fou~d that the concentration ~Dof~o /~/7'C~
of sodium hydroxide in the aathoritc was 18.4~ by weight, that the concentration of sodium chloride was 0.01% by weight, and that the current efficiency was 89.5%.

Electrolysis was carried out in the same way as in Example 1 except that a suitable amount of water was added to the cathode compartme~t to~ maintain the concentration of sodium hydroxide in the cathor-tc constant at 12~ by weight, and the current density was adjusted to 15 A/dm2.

When a membrane prepared by graft copolymerizing a 0.2 mm thick polyethylene film with p-hydroxystyrene and divinyl-benzene at a total grafting ratio of 102~ by weight (based on the ~059062 polyethylene), the p-hydroxystyrene content being 73% by weight in the total of the graft copolymer, using ionizing radiation from electron beams, was used as a diaphragm, the concentration of sodium chloride in the catholyte was 0.002% by weight, and the current efficiency was 92%.
When a membrane prepared by graft copolymerizing the same polyethylene film as used above with p-hydroxystyrene alone at a grafting ratio of 74% by weight was used as a diaphragm, the concentration of sodium chloride in the catholyte was 0.008%
by weight, and the current efficiency was 77%.

ExAMæLE 14 _ Electrolysis was carried out in the same way as in Example 13 using a diaphragm prepared by sulfonating the same graft copolymer membrane as used in Example 13 having both p-hydroxystyrene and divinylbenzene grafted thereto, in a 50% by weight dioxane solution of chlorosulfonic acid at 70C for 5 hours. It was found that the concentration of sodium chloride in the catholyte was 0.002% by weight, and the current efficiency was 92%.

Electrolysis was.carried out in the same way as in Example 13 using a diaphragm prepared by graft copolymerizing a 0.1 mm thick polypropylene film with p-hydroxystyrene and di-vinylbenzene using ionizing radiation from electron beams at a total grafting ratio of 125~ by weight, the p-hydroxystyrene content being 97% by weight in the total of the graft copolymer.
It was found that the concentration of sodium chloride in the catholyte was 0.002% by weight, and the current efficiency was 88%.

Electrolysis was carried out in the same way as in Example 13 using a diaphragm obtained by sulfonating the same graft copolymer membrane as used in Example 15 in a 50% by weight dioxane solution of chlorosulfonic acid at 70C for 5 hours.
It w~s~fo~nd that the concentration of sodium chloride in the cathoritc was 0.001% by weight, and the current efficiency was 90% .

_~MP~E 17 The grafting reaction and the hydrolysis were carried out on a polystyrene film having a thickness of 0.1 mm in the same manner as described in Example 9 except that a mixed solution of methanol and benzene (2 : 1 by volume) was used. Thus, a film having a total graft copolymer content of 85 wt% (based on the polystyrene) and a p-hydroxystyrene content of 90 wt% in the total of the graft copolymer was obtained.
Electrolysis was then carried out in the same manner as described in Example 13 using the resulting polystyrene film as a diaphragm. It ~a~ ~ound that the concentration of sodium chloride in the cathorit~ was 0.002% by weight, and the current efficiency was 85%.

ExAMæLE 18 The graft copolymerization was carried out on a poly-ethylene film having a thickness of 0.2 mm in the same manner as described in Example 9 (but the divinylbenzene content was 90%
by weight, with the remainder being ethylvinyl benzene). Thus, a film having a total grafting ratio of 82% by weight (based on the polyethylene) and a p-hydroxystyrene content of 72% by weight in the total of the graft copolymer was obtained.

~05906Z
1 Electrolysis was then carried out in the same manner as described in Example 13 using the resulting film as a diaphragm. It was found that the concentration of sodium chloride in the catholyte was 0.002~ by weight and the current efficiency was 94%.
EXAMPLE l9 ~ lectrolysis of a saturated potassium chloride solution was carried out in the same manner as described in Example 18 using the same apparatus and the same diaphragm as described in Example 18. The concentration of potassium hydroxide was 12%

by weight, the concentration of potassium chloride was 0.02~ by weight and the current efficiency was 98% by weight.

An electrolytic cell made of poly(methyl methacrylate) was used which was partitioned by a diaphragm having an effective area of lO cm2 into an anode compartment having a volume of 7 cm3 and a cathode compartment having a volume of 3.5 cm3 and in which a graphite plate was used as an anode and a stainless steel plate was used as a cathode. A solution feed opening and discharge open-ings for the solution and gas were provided in each of the anode compartment and the cathode compartment. Brine which was saturated with sodium chloride at room temperature (i.e., about 20 to 30C) and in which magnesium chloride was dissolved to the concentration of 20 ppm was fed into the anode compartment at a rate of 15 ml/min, and a prescribed amount of a O.lN aqueous solution of sodium hydroxide was circulated into the cathode compartment at a rate of 5 ml/min. Both of these solutions were preheated by a coil immersed in a constant-temperature water tank, and the temperature inside the tank was maintained at 70C. Current of a specific density was passed for a prescribed period of time, 1~591~62 1 and then the catholyte was analyzed to examine the concentrations of sodium hydroxide, sodium chloride and magnesium hydroxide~ On the basis of the concentrations, the current efficiency was calculated.
Separately, one leg of a glass H-tYpe cell (diameter 10 mm, thickness 0.5 mm) was charged with a 0.1 mm thick poly-ethylene film washed thoroughly with acetone and the other leg was charged with a solution of a mixture of 3,4-diacetoxystyrene and divinylbenzene (containing 55% by weight of divinylbenzene with an m- to p-weight ratio of about 2 : 1, and the remainder being mainly ethylvinylbenzene)in a weight ratio (3,4-diacetoxy-styrene : divinylbenzene) of 9 : 1 in two times its weight of a mixture of benzene and acetone (in a benzene : acetone volume ratio of 3 : 1). By repeating a freezing-melting procedure five times, the cell was thoroughly degassed in vacuo, and then heat-sealed. The monomer solution part was frozen, and sufficiently covered with a lead plate. While the entire H-type cell was being cooled at -30C, electron beams in a dose of 30 Mrads were applied to the polyethylene film at an acceleration voltage of 1.5 MeV using an electron beam accelerator.
After the irradiation, the monomer solution was transerred to the film-containing portion to dip the film in the solution and allow it to react for 24 hours at 25C. After the reaction, the cell was opened. The film was withdrawn, and thoroughly washed with benzene and acetone.
The thus obtained film was then placed in a 100 ml of flask equipped with a cooling tube. Subsequently, 50 ml of a mixture of concentrated hydrochloric acid and methanol in a mixing ratio by weight of 1 : 4 was added, and the flask was heated for 30 minutes over a hot water bath. As a result of the 1 calculation from the difference in the weight of this film before and after the hydrolysis treatment, a grafting ratio of 3,4-dihydroxystyrene per se was 73% by weight, and a total grafting ratio was 98% by weight.
Electrolysis was then carried out using the resulting polyethylene film as a diaphragm. A potential was applied to the cell and after about 10 minutes a current of 1.0 A (10 A/dm2) was ~-~ passed for 5 hours. ~t w~s found that the concentration of sodium O ~f e hydroxide in the cathoritc was 17.2~ by weight, that the con-centrations of sodium chloride and magnesium hydroxide were 0.02%by weight and 6 ppm, respectively, and that the current efficiency was 84.5%.

The graft copolymerization reaction was carried out on a polyethylene film having a thickness of 0.2 mm using a mixed solution of 3,4-dihydroxystyrene and divinylbenzene (containing 90% by weight of divinylbenzene) by means of ionizing radiation from electron beams to obtain a film having a total grafting ratio of 105~ by weight (based on the polyethylene) and a 3,4-dihydroxystyrene content of 82% by weight.
Electrolysis was then carried out in the same manneras described in Example 20 using the thus treated polyethylene film as a diaphragm. In the above experiment, a neutral porous diaphragm was provided to separate the diaphragm of the graft copolymer as described above in the anode compartment from the anode, and brine was supplied between the diaphragm of the graft copolymer and the neutral porous diaphragm. It~ s,f nd that c~G
the concentration of sodium hydroxide in the cathorite was 19.2%

by weight, that the concentration of sodium chloride was 0.005%

by weight, and that the concentration of magnesium hydroxide was 2 ppm. Further, the current efficiency was found to be gl%.

1059~6Z

Electrolysis was carried out in the same manner as described in Example 20 using a diaphragm obtained by graft-copolymerizing a polyethylene film having a thickness of 0.2 mm with a 3,4-dihydroxystyrene side chain at a grafting ratio of 95% by weight using ionizing radiation from electron beams. It ' was ~o,un~ that the concentration of sodium hydroxide in the o~thor-i-t~ was 15.5~ by weight, that the concentration of sodium chloride was 0.01% by weight, and the concentration of magnesium hydroxide was 4 ppm. Further, the current efficiency was found to be 79%.

Electrolysis was carried out in the same manner as described in Example 20 using a diaphragm obtained by graft-copolymerizing a polypropylene film having a thickness of 0.2 mrn with a 3,4-dihydroxystyrene side chain at a grafting ratio of 92% by weight using ionizing radiation from electron beams. It was f~u~d that the concentration of sodium hydroxide in the c"~f~o /~
oathorito was 16.2~ by weight, that the concentration of sodium chloride was 0.02~ by weight, and that the concentration of magnesium hydroxide was 5 ppm. Further, the current efficiency was found to be 75%.

A 0.2 mm thick polyethylene film was placed in a glass ampoule, and a solution of a mixture of 3,4-diacetoxystyrene and divinylbenzene in a weight ratio (3,4-diacetoxystyrene : divinyl-benzene) of 9 : 1 in 9 times its weight of a mixture of benzene and acetone in a benzene : acetone volume ratio of 3 : 1 was placed in the ampoule. The ampoule was sufficiently degassed in vacuo by repeating a freezing-melting procedure five times and ~OS906Z
1 then heat-sealed. Using a cobalt 60 source, ~-rays were applied to the ampoule at a dose of 1.1 x 105 rads/hour for 24 hours.
Then, the film was taken out of the ampoule, washed sufficiently with benzene and acetone to remove a by-product copolymer composed of 3,4-diacetoxystyrene and divinylbenzene. The grafted film was hydrolyzed in the same manner as described in Example 20. The grafting ratios of 3,4-diacetoxystyrene and divinylbenzene cal-; culated from the difference in weight of the grafted film before and after the hydrolysis were 50~ and 19%, respectively.
Electrolysis was then carried out in the same manner as described in Example 20 using the thus obtained polyethylene film as a diaphragm. It was/fo~nd that the concentration of ~O/Jf ~!s sodium hydroxide in the c~thoritc was 17.5% by weight, that the concentration of sodium chloride was 0.02~ by weight, and that the concentration of magnesium hydroxide was 4 ppm. Further, the current efficiency was found to be 82%.

The grafting reaction and the hydrolysis were carried out on a polyethylene film having a thickness of 0.2 mm in the same manner as described in Example 20 except that using a cobalt 60 source, ~-rays were applied at a dose of 1.1 x 105 rads/hour at room temperature (i.e., about 20 to 30C) for 24 hours instead of the electron beams, and that a monomeric solu-tion of 3,4-dihydroxystyrene in 2 times its weight of a mixture of benzene and acetone (3:1 by volume) was used, to obtain a film in which a 3,4-dihydroxystyrene side chain was graft-copolymerized at a grafting ratio of 64% by weight (based on the polyethylene). The resulting film was further sulfonated with concentrated sulfuric acid at room temperature for 72 hours to obtain a cationic exchange membrane.

1~9~16Z

1 Electrolysis was then carried out in the same manner as described in Example 20 using the resulting cationic exchange membrane as a diaphragm. It was found that the concentration of cr~ f~o/,~7~e sodium hydroxide in the ea~ ~e was 17.2% by weight, that ~he concentration of sodium chloride was 0.02% by weight, and that the concentration of magnesium hydroxide was 6 ppm. The current efficiency was also found to be 84.5~.

SUPPLEMENTARY DISCLOSURE

As noted in the above examples, when an electrical potential is applied to the electrolytic cell there is an initial period of approximately 10 minutes before the desired operating current is obtained. This may be explained by the fact that when using the membranes composed of the graft co-polymers or the sulfonated products thereof of the present invention, the hydrogen ion of the hydroxy group of the hydroxy-styrene side chain is inevitably replaced with an alkali metal ion. Thus the electrolysis does not proceed smoothly until the hydrogen atoms are replaced. Once replacement occurs the desired current level can be maintained.
To obviate this problem it is possible to control the voltage applied across the cell during this initial period.
The preferred technique, however, is to affirm~tivcly replace the hydrogen ions with alkali metal ions prior to electrolysis.
This is done by contacting the membrane with an aqueous solution of an alkali metal hydroxide such as sodium hydroxide, potassium hydroxide and lithium hydroxide having a concentration of about 0.1 to 12N, an aqueous solution of an alkali metal hydroxide at the concentration as described above, additionally containing an organic solvent which can be uniformly mixed with `" 1059~6Z

1 the above described aqueous solution and which is also capable of swelling the membrane, or a solution of an alkali metal hydroxide, at the concentration as described above, dissolved in a lower aliphatic alcohol having 1 to 4 carbon atoms at a temperature of room temperature (e.g., about 20-30C) to about 90C for a period about 30 minutes to about 5 hours. Suitable examples of organic solvents which can be employed include methanol, ethanol, dioxane, tetrahydrofuran, acetone, methyl- -ethylketone, etc. Further, suitable examples of lower aliphatic alcohols which can be employed include methanol, ethanol, etc.
When the membranes used are composed of sulphonated products containing sulphonic acid groups, the hydrogen atom of the sulphonic acid group similarly is also inevitably replaced by an alkali metal ion at the beginning of the electrolysis or in the replacement treatment as set forth above.
When!the~membr~nes are employed which are composed of graft copolymers which are produced by grafting side chains composed mainly of an acyloxy styrene onto a polyolefin and hydrolyzing the resulting graft copolymer in the presence of an alkali metal hydroxide as a catalyst, the hydrogen atom of the hydroxy group o~ the hydroxystyrene side chain thus produced in the membranes is already replaced by an alkali metal ion.
Therefore, in such a case, no particular replacement treatment is needed and the use of such copolymers as membranes results in an electrolysis which proceeds smoothly at the desired current level as soon as the potantial is applied to the cell.
The following examples are given to further illustrate in detail the above embodiments of the present invention but the invention is not to be construed as being limited thereby.
Unless otherwise indicated, all parts, percents, ratios and the like are by weight.

B

i~S9~62 ` 1 EXAMPLE 26 . . _ The method of manufacturing a membrane and the electrolysis were carried ou~ in the same way as in Example 2 e,,~7,Yge except that the cationic ~ membrane to be used as a diaphragm was subjected to a pretreatment in which the membrane was immersed in 0.5N sodium hydroxide solution at a temperature of 8`0C for a period of 2 hours prior to the electrolysis. The membrane had a cation transport number of 0.97, an electrical resistance in 0~5N sodium chloride, of 1.7~ -cm2, and an acidic ion-exchange capacity of 2.2 meq/g. The current of l.0 A(lOA/dm2) was passed for 5 hours with a constant current level throughout tha~ t~m,e. The concentration of sodium ~ /,~f~
hydroxide in the c~h~te, the concentration of sodium chloride and the current efficiency were the same as found in Example 2.

The method of manufacturing a membrane and the electrolysis were carried out in the same way as in Example 7 except that the membrane which was to be used as a diaphragm was subjected to a pretreatment, before electrolysis, comprising immersing the diaphragm in a mixture of a lN sodium hydroxide aqueous solution and dioxane (l:l by volume). The results were the same as found in Example 7 except that there was no initial period where the current level was less than the desired rate.

The method of manufacutring a membrane and the electro-lysis were carried out in the same way as in Example 21 above except that the polyethylene film was subjected to pretreatment, before electrolysis, in which the film was immersed in a mixture of a lN sodium hydroxide solution comprising water and dioxane (1:1 by volume) at a temperature of 80C for a period of 2 hours.

1059~6Z
1 There was no initial period where the current level was not at the desired rate. The results were the same as found in Example 21.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

i~

Claims (24)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for diaphragm electrolysis of an alkali metal halide which comprises electrolyzing an alkali metal halide solution by passing an electric current through an anode com-partment and a cathode compartment of an electrolytic cell with an ion-exchange membrane of a graft copolymer of a polyolefin main chain and a side chain composed mainly of a hydroxystyrene compound having the formula wherein n is 1 or 2 and grafted to said polyolefin main chain separating the anode compartment and the cathode compartment, the hydroxystyrene content being 5 to 500% by weight based on the polyolefin main chain.

2. The method of claim 1, wherein said polyolefin is an aliphatic polymer, an aromatic polymer or an alicyclic polymer.

3. The method of claim 2, wherein said polyolefin is polyethylene.

4. The method of claim 2, wherein said polyolefin is polypropylene.

5. The method of claim 2, wherein said polyolefin is polystyrene.

6. The method of claim 1, wherein said hydroxystyrene compound is hydroxystyrene.

7. The method of claim 1, wherein said hydroxystyrene compound is dihydroxystyrene.

8. The method of claim 1, wherein said graft copolymer is cross-linked.

9. The method of claim 8, wherein said graft copolymer is cross-linked with a difunctional compound reactive with phenolic hydroxyl groups.

10. The method of claim 9, wherein said difunctional com-pound is used in an amount of about 0.01 to 0.5 equivalent per equivalent of phenolic hydroxyl group.

11. The method of claim 9, wherein said difunctional compound is a diepoxide, a diisocyanate or an acid dihalide.

12. The method of claim 9, wherein said difunctional compound is ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, cyclohexane diol diglycidyl ether, an epoxy resin, hexamethylene diisocyanate, tolylene diisocyanate, xylylene diisocyanate, diphenylmethane-4,4'-diisocyanate, hexahydrotolylene diisocyanate, adipoyl dichloride, terephthaloyl dichloride or hexahydroterephthaloyl dichloride.

13. The method of claim 8, wherein said graft copolymer is cross-linked with an organic sulfonic acid compound.

14. The method of claim 8, wherein said graft copolymer is cross-linked with a polyene compound containing at least two polymerizable double bonds in the molecule.

15. The method of claim 14, wherein the amount of said polyene compound is about 0.5 to 100% by weight based on the polyolefin main chain.

16. The method of claim 13, wherein said compound is divinylbenzene.

17. The method of claim 13, wherein said compound is iso-prene, butadiene, cyclopentadiene, ethylidene norbornene, a diol ester of acrylic acid or methacrylic acid, or a divinyl-ester of adipic acid.

18. The method of claim 1, wherein said side chain comprises a hydroxystyrene compound and a polyene compound containing at least two polymerizable double bonds.

19. The method of claim 18, wherein the weight ratio of the hydroxystyrene compound to the polyene compound is about 200 : 1 to 1 : 1.

20. The method of claim 1, wherein said graft copolymer is sulfonated.

21. The method of claim 20, wherein said graft copolymer con-tains about 0.5 to 2 sulfone groups introduced per hydroxystyrene group unit.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE

22. The method of claim 1 further including the step of pretreating the ion-exchange membrane, prior to electrolysis, comprising contacting the membrane with a solution of an alkali metal hydroxide.

23. The method of claim 22 wherein the solution of an alkali metal hydroxide is selected from the group consisting of:
i) aqueous solution of an alkali metal hydroxide;
ii) an aqueous solution of an alkali metal hydroxide additionally containing an organic solvent capable of being uniformly mixed with said aqueous solution and capable of swelling said ion-exchange membrane;
iii) a solution of an alkali metal hydroxide in a lower aliphatic alcohol.

24. The method of claim 23 wherein the concentration of said alkali metal hydroxide in said solution of metal hydroxide is about 0.1 to 12N and the contacting of said ion-exchange membrane with said alkali metal hydroxide solution is conducted at a temperature of about room temperature to about 90°C for a period of about 30 minutes to about 5 hours.