CA2138154C - Simultaneous production of dicarboxylic acids and diamines from polyamides - Google Patents

Abstract

Disclosed is a method for the simultaneous preparation of dicarboxylic acids and diamines from (a) polymers based on polyamides made by reacting dicarboxylic acids or derivatives thereof with diamines or (b) compounds containing essentially such polymers by decomposing these polymers into their constituent monomers by treating the polymers or compounds with a base and subsequently converting electrochemically the dicarboxylic-acid salts thus produced into the corresponding dicarboxylic acids and bases.

Description

O.Z. 0050/43334 Simultaneous production of dicarboxylic acids and diamines from polyamides The present invention relates to a process for the simultaneous production of dicarboxylic acids and diamines from a) polymers based on polyamides of dicarboxylic acids or their derivatives with diamines, or b) compositions containing essentially such polymers, by splitting these polymers into their monomeric con-stituents.
The splitting of polyamides such as nylon 66 (PA
66) into their monomeric constituents can be carried out in a neutral or acid medium but in general it is prefer ably carried out in a basic medium, inter alia because of the shorter reaction time.
FR-A-926 873 describes the splitting of poly-amides such as PA 66 and PA 610 with inorganic bases, for example with a from 10 to 15% strength by weight alkali metal hydroxide solution such as sodium hydroxide solu-tion, at 200°C and about 15 bar. The resulting diamine is then extracted or distilled out of the reaction mixture and further purified by vacuum distillation. According to this reference, the free dicarboxylic acid is obtained by addition of a strong acid such as hydrochloric acid to the diamine-free reaction mixture and subsequent preci-pitation.
In IT-A-553 182 an excess of 20~ strength by weight of sodium hydroxide solution at 220°C and 25 bar reduces the reaction time compared with the process of FR-A-926 873. The diamine is extracted from the aqueous solution with n-butanol. One example concerns the removal of insoluble titanium dioxide, previously present in the polymer in the form of fibers, by filtration after the splitting. The dicarboxylic acid is likewise freed by addition of a strong mineral acid.
FR-A-1 070 841 describes the splitting of PA 66 - ~~~~ O.Z. 0050/43334 with alkali metal or alkaline earth metal hydroxide solutions. According to this reference, the reaction mixture is initially worked up by acidifying with sul-furic acid and then the precipitated adipic acid is separated off. Thereafter the filtrate is admixed with potassium hydroxide solution, which brings down hexa-methylenediamine as an oily layer which can be separated off and purified. This reference also describes the splitting and workup of polymers and copolymers that contain polycaprolactam (PA 6).
DE-A-1 088 063 describes the splitting of PA 66 in a 10% strength by weight methanol NaOH solution. The disodium adipate obtained is converted into the free acid by acidification, while hexamethylenediamine (H1~) can be obtained in pure form by distillation.
US-A-2 840 606 describes the splitting of PA 66 into disodium adipate and HIS in an isopropanol/water mixture. According to this teaching, the HIS is isolated from the alcohol phase by distillation. The adipic acid is obtained by acidifying the aqueous phase with sulfuric acid and may be purified by crystallization.
DE-A 39 26 642 describes a process and an apparatus based on a four-compartment electrolysis cell for obtaining an acid from its salt. However, no mention is made of reaction parameters and examples in DE-A 39 26 642.
A feature common to all these processes is the isolation of adipic acid through acidification of the respective alkali metal or alkaline earth metal salt solutions. The inevitable inorganic salt coproduct, usually sodium chloride or sodium sulfate, not only interferes with the attempt to purify the dicarboxylic acid by crystallization, since it inhibits the latter, but also constitutes a considerable disposal problem.
A further disadvantage is that the processes described cannot be suitably employed for working up technical, for example fiber-reinforced, mineral-filled AMENDED SHEET

and/or impact-modified, PA 66 molding compositions, since the various additives would disrupt the smooth running of the processes described.
It is an object of the present invention to provide a process for the simultaneous production of dicarboxylic acids and diamines that shall be free of the abovementioned disadvantages.
We have found that this object is achieved by a process for coproduction of dicarboxylic acids and diamines from (a) polymers based on polyamides of dicarboxylic acids or their derivatives with diamines, or (b) materials comprising essentially such polymers, by cracking these polymers into their monomeric constituents using a base in an aqueous medium and removing the resulting diamines, characterized in that the base is dissolved in a mixture of from 100 to 75% by weight of water and from 0 to 25% by weight of a C1-C4-alkanol and the resulting dicarboxylic acid salts of adipic acid or sebacic acid are converted into the corresponding dicarboxylic acids and bases by electrochemical means.
Suitable polymers based on polyamides of dicarboxylic acids or their derivatives, for example the corresponding acid halides, preferably the acid chlorides, with diamines are from observations to date poly-hexamethyleneadipamide, polyhexamethylenesebacamide and polytetramethyleneadipamide, preferably polyhexamethylene-adipamide.
Suitable compositions containing essentially, ie.
to at least 50% by weight, such polymers also include for example copolyamides with PA 66 and also PA 66 or copoly amides with PA 66 containing fibers and/or additives.
The bases used for splitting the polymers are in general alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, preferably sodium hydroxide, or mixtures thereof, preferably a mixture of sodium hydroxide and potassium hydroxide.

3 a A
It is preferable to use from 1.8 to 4.0, preferably from 2.0 to 3.0, equivalents of alkali metal - 4~~~~~~ ~ O. Z. 0050/43334 hydroxide per repeat unit of polymer, for example - (- (CH2),,-CO-NH- (CHz) 6-NH-CO-] - in the case of PA 66. If less than 1.8 equivalents of base are used, the result is in general an undesirably high proportion of oligomer. If more than 4.0 equivalents of base are used per repeat unit, this leads in general, in particular in the case of glass fiber-reinforced and/or mineral-filled polyamide molding compositions, to a high degree of degradation of the glass fibers or of the mineral filler.
In general, the alkali metal hydroxide is used in the form of a from 5 to 25, preferably from 10 to 15, strength by weight solution in water. If desired, instead of water it is possible to use a water-C1-C4-alkanol mixture which contains from 0 to 50, preferably from 0 to 25, % by weight of a C1-C4-alkanol or a mixture thereof.
The reaction is in general carried out at a temperature within the range from 100 to 300°C, prefer ably from 150 to 250°C. The pressure is in general within the range from 0.1 to 10 MPa, although it is also possible to employ a pressure outside this range. Prefer-ence is given to working under the autogenous pressure.
Owing to the alkali metal hydroxide, the reaction mixture pH is in general alkaline, preferably within the range from 8 to 14.
The duration of the reaction depends essentially on the concentrations of the starting materials, on the temperature and on the pressure and will in general be within the range from 0.5 to 15, preferably from 3 to 10, h.
The splitting with a base can be carried out continuously or batchwise.
It can be carried out in customary apparatus with or without stirrer, preference being given to using a pressure vessel equipped with a stirrer system that is particularly suitable for solids dispersion, for example a propeller stirrer or a cross-bar stirrer.
In a preferred embodiment, the starting polymer or polyamide-containing compositions are mechanically comminuted to an average particle size of 0.1 to 50, preferably from 1 to 10, mm before splitting. The comminution can be carried out in a commercial mill, for example in a cutting mill, or, preferably, in particular when the compositions used contain hard materials such as metal inserts, for example bolts, in a hammer mill.
Metal parts present in the material thus comminuted can be removed in a drying separation process using an air table, preferably with subsequent induction separation, using for example a free-fall tube separator, for complete removal of the metal parts, or in a wet separation process, for example by means of a hydro-cyclone.
Brief description of the drawings Fig. 1 is a diagram of a three compartment electrolysis all with three liquid cycles (KL1 to KL3).
Fig. 2 is a diagram of a four compartment electrolysis all with four liquid cycles (KLl to KL4).
This invention will be better understood upon reading the following non restrictive description and examples.
Detailed description of the invention In a particularly preferred embodiment, the polymer or composition feedstock is comminuted in a hammer mill to a size of not more than 50 mm in length, any metal parts present are separated off, and the millbase freed of metal parts is then comminuted to a size within the range from 5 to 12 mm in a cutting mill. If desired, the polymer or polymer-containing composition thus pretreated can then be additionally washed and dried before it is subjected to splitting with a base.

5a The reaction mixture obtained on splitting the polyamides consists in general of a liquid phase, which contains the diamine, the dicarboxylate salt, and insoluble constituents.
According to the invention, the reaction mixture obtained on splitting the polyamides is then subjected to an electrochemical treatment to convert the dicarboxylate salt into the corresponding dicarboxylic acid, for which it is advantageous to remove troublesome impurities beforehand.
For instance, in a further preferred embodiment, constituents of the reaction mixture that are insoluble after the polyamides have been split into their monomeric constituents are removed.

- 6 ~ 1381 ~ 4 p. Z. 0050/43334 Examples of insoluble constituents are glass fibers, carbon fibers, carbon black, minerals and rubber and any metals not removed or not completely removed beforehand, unless dissolved by the base.
Suitable processes for separating off the insoluble constituents are known processes such as filtration, sedimentation or centrifuging.
The removed insolubles may if desired be washed in a further operation with water and/or an organic sol vent, preferably with a C1-C4-alcohol or with the solvent component used in the splitting with the base. This operation can be carried out with the apparatus used for the separation, for example a belt filter, and/or in a further separating operation, in which case the insolubles are in general first intimately mixed with the water or solvent (mixture) used. The solids obtained at this stage can if desired be further used as filler when dry.
The filtrates from the washing operations can if desired be combined with the filtrate from the reaction mixture.
In a further preferred embodiment, the diamines obtained in the base splitting are preferably separated off after the insolubles have been separated off and before the electrochemical treatment.
The removal of the diamines can be carried out by known methods such as distillation, in particular recti-fication, or extraction, an extraction with organic solvents being preferable on energy grounds.
Prior to the extraction it can be advantageous to remove volatile organic components, if present, for example by rectification. The rectification can be carried out in conventional apparatus (for example tray columns or packed columns with arranged or dumped packing).
To extract the diamines it is possible to employ the customary known solvents (see DE-A-1,163,334) such as Z13815~
- 7 - O.Z. 0050/43334 hal:ogenated hydrocarbons, for example chloroform or methylene chloride, C,,-C8-alcohols such as n-butanol, isobutanol, sec-butanol, 1-pentanol, 2-pentanol, 3-pentanol, neopentyl alcohol, 1-hexanol, 2-hexanol, 3-hexanol, 1-heptanol, 2-heptanol, 3-heptanol, 4-heptanol, 1-octanol, 2-octanol, 3-octanol, 4-octanol, preferably n-butanol, isobutanol, C5-C8-cycloalkanes such as cyclopentane, cyclohexane, cycloheptane, cyclooctane, preferably cyclohexane, or aromatic hydrocarbons, prefer-ably benzene and its C1-C4-alkyl derivatives such as toluene and xylenes, or mixtures thereof. Particular preference is given to using mixtures of benzene, toluene, n-butanol and isobutanol. Very particular preference is given to mixtures of from 25 to 60~ by weight of benzene and/or toluene and from 40 to 75~ by weight of a mixture of n-butanol and isobutanol with the total isobutanol content of the mixture generally not being higher than 40~ by weight.
The extraction can be carried out batchwise or continuously, in general in conventional extraction apparatus such as mixer-settlers or in pulsed or unpulsed extraction columns (for example tray or packed columns).
The isolation of the diamines from the extract is in general effected by rectification in a conventional apparatus in which the diamines are advantageously obtained in vapor form as a side takeoff from the stripping portion of the rectification column. The rectification is in general carried out at from 10 to 100 kPa, preferably at from 50 to 80 kPa.
Any diamine still present in the rectification residue can if desired be separated therefrom in a further distillation step, for example using a thin-film evaporator, at from 0.5 to 50, preferably at from 2 to 30, kPa.
The extractant obtained at the top of the recti-fication column is advantageously directly recirculated into the extraction or freed in a further distillation -~138154o,Z. 0050/43334 step of lower and/or higher boiling impurities.
The raffinate can if desired be subjected to a further rectification in order to separate off dissolved extractant and/or other volatile organic compounds.
Impurities that interfere with the electrochemical treatment such as alkaline earth metal cations, silica and polyphosphate anions or high mole-cular weight organic amine compounds can advantageously be removed from the aqueous solutions freed of insolubles and diamines by treating these solutions for example with adsorbents and/or suitable precipitants.
The adsorbents used are preferably activated carbon or selective ion exchangers, if desired based on chelates. Suitable precipitants include for example car-bonates of alkali metals and/or ammonium carbonates.
From observations to date the manner of the electrochemical treatment has in principle no bearing on the success of the process of the invention.
The electrochemical treatment may for example take one of the following forms (a) to (f):
(a) In this version the splitting of the dicarboxy-late salt into the corresponding dicarboxylic acid and the corresponding base can be carried out in a two-part electrodialysis cell using bipolar membranes. In general, the electrodialysis cell has between the anode and the cathode from 1 to 200, preferably from 20 to 70, electrodialysis units separated from one another by bipolar membranes . The bipolar membranes are separated from one another by cation exchange membranes, so that an electrodialy-sis unit has the following structure: bipolar membrane (anode side) - anolyte compartment - cation exchange membrane - catolyte compartment - bipolar membrane (cathode side). The individual electrodialysis units are preferably electrically connected in series.
In this version it is advantageous to feed the ~13815~ ~.Z. 0050/43334 aqueous dicarboxylate salt solution into the anolyte compartment. In the electric field of an applied direct voltage the alkali metal cations generally migrate through the cation exchange membrane into the catolyte compartment. The hydroxyl anions required for compensating the separated charges are formed by the dissociation of the water in the bipolar membranes on the cathode side. In this way the corresponding alkali metal hydroxide solution collects in the catolyte compartment. In the anolyte compartment the dicarboxylate anion can combine with the hydrogen ions from the bipolar membrane on the anode side to form the free dicarboxylic acid.
It is advantageous to feed the dicarboxylate salt solution into the anolyte compartments in parallel.
The product streams from the anolyte compartments, containing the free acid and unconverted dicarboxyl ate salt, and the product streams from the catolyte compartments are advantageously combined with one another. The free dicarboxylic acid is in general obtained by crystallization from the combined product streams from the anolyte compartment without coprecipitation of the corresponding dicarboxylate salt, which is preferably subjected again to the electrodialysis process.
The electrodialysis process can be carried out not only continuously but also batchwise. A prefer-red form of the continuous process involving a plurality of electrodialysis cells comprises dividing the total conversion between from 2 to 20, preferably from 4 to 6, electrodialysis cells and effecting only partial conversion in each electro-dialysis cell.
It is particularly advantageous here to guide the 3 5 f lows in countercurrent . The outf low from an anolyte compartment forms the inflow into the next anolyte compartment, etc., so that the outflow from the last - 10 - O.Z. 0050/43334 anolyte compartment is rich in dicarboxylic acid and lean in dicarboxylate salt. The outflow from the last catolyte compartment, containing a low concentration of alkali metal hydroxide, forms the inflow into the last but one catolyte compartment, etc., so that the first unit has a high concentration of dicarboxylate salt in the anolyte compartment and a high concentration of alkali metal hydroxide in the catolyte compartment. The result is that the alkali metal hydroxide concentration differences in the anolyte and catolyte compartments are small within a unit. This ultimately leads in general to an energy saving due to a higher current yield and on average to lower cell voltages.
The current densities are in general within the range from 0.1 to 2, preferably from 0.5 to 1.0, kA/m~. The cell voltage is in general from 3 to 8, preferably from 4 to 6, V per electrodialysis unit.
The pH is in general within the range from 2 to 10 in the anolyte compartments and within the range greater than 13 in the catolyte compartments.
The compartment width is in general from 0.2 to 5, preferably from 0.5 to l, mm.
The electrodialysis temperature is in general within the range from 40 to 110°C, preferably from 65 to 90°C.
The inflow and outflow velocities are in general within the range from 0.05 to 0.2 m/sec.
The concentration of dicarboxylate salt used is in general from 5 to 40~ by weight, preferably from 10 to 20~ by weight.
If desired, the conductivity in the anolyte system can be increased by adding salts or acids such as sodium sulfate or sulfuric acid. Substances of this type are in general added within the range from 0.1 to 10~ by weight, preferably from 1 to 60 by weight, based on the total weight of the solution - 11 - O.Z. 0050/43334 present in the anolyte compartment.
To the catolyte compartment it is advantageous to add the substances which are obtained in the course of the operation, preferably the corresponding alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, preferably sodium hydroxide.
The inflow into the catolyte compartment general-ly comprises fully demineralized water, but at the beginning it is preferable to employ the from 1 to 25, preferably from 5 to 10, o strength by weight alkali metal hydroxide solution formed in the course of the electrodialysis.
(b) A three-part electrodialysis cell with bipolar membranes has the advantage over the procedure described under (a) that the feed materials need not be very pure. Furthermore, generally significantly lower salt contents are obtained not only in the dicarboxylic acid solution obtained but also in the corresponding alkali metal hydroxide solution.
The three-compartment system contains not only a cation exchange membrane but also an anion exchange membrane, so that the structure of an electrodialy-sis unit is as follows: bipolar membrane (anode side) - anolyte compartment - anion exchange membrane - center compartment - cation exchange membrane - catolyte compartment - bipolar membrane (cathode side) .
The dicarboxylate salt solution is advantageously introduced into the center compartment. Under the influence of a direct current electric field the dicarboxylate anions generally migrate through the anion exchange membrane into the anolyte compart-ment, where they can combine with the hydrogen ions present there to form the free acid. Apart from selectivity losses of the anion exchange membrane the free acid can be withdrawn from the anolyte compartment devoid of salt. As in (a) the catolyte - 12 - O.Z. 0050/43334 compartment yields the alkali metal hydroxide solution. The outflow from the center compartment, still containing residual quantities of dicarboxylate salt, can be disposed of or advantageously added again to the feed for the electrodialysis process. Again as in (a) the flows can be guided countercurrently in order to increase the current yield.
To increase the conductivity the anolyte compart went can have added to it for example an oxo acid such as sulfuric acid, phosphoric acid or nitric acid.
The catolyte compartment can advantageously have added to it the substances which are obtained in the course of the operation, preferably the correspond ing alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, preferably sodium hydroxide.
As for the rest, the process of (b) can be carried out under the same conditions as described 2 0 under ( a ) .
(c) In principle it is also possible to use electro-dialysis cells having four compartments. The layout generally resembles that of an electrodialysis cell with three compartments except that, to protect the bipolar membranes from possible fouling, a further ion exchange membrane, preferably a cation exchange membrane, is included. In general, an electrodialy-sis unit will have the following structure: bipolar membrane (anode side) - anolyte compartment - cation exchange membrane - anode-near center compartment -anion exchange membrane - cathode-near center compartment - cation exchange membrane - catolyte compartment - bipolar membrane (cathode side).
The dicarboxylate salt solution is advantageously introduced into the cathode-near center compartment with the dicarboxylic acid solution being withdrawn from the anode-near center compartment and the - 13 - O.Z. 0050/43334 alkali metal hydroxide solution from the cathode compartment.
In other respects, the process of (c) can be carried out under the same conditions as described under (b).
(d) The electrochemical cleavage of the dicarboxylate salt into the dicarboxylic acid and the correspond-ing base can be carried out in a further embodiment in a two-part membrane electrolysis cell known per se from chlor-alkali electrolysis. The membrane electrolysis cell comprises in general from 1 to 100, preferably from 20 to 70, electrolysis units grouped together in a block. In this block, the individual electrolysis units can be electrically connected in series by electrically connecting the cathode of one unit to the anode of the next unit or by using internally connected bipolar electrodes.
The products generally flow in and out via separate collector lines for each compartment type. The two-part membrane electrolysis unit generally has the following structure going from the anode to the cathode:
anode - anolyte compartment - cation exchange membrane - catolyte compartment - cathode.
The aqueous dicarboxylate salt solution is advan-tageously introduced into the anolyte compartment.
Under the electric field of the applied direct voltage the alkali metal cations generally migrate through the cation exchange membrane into the catolyte compartment, where they are converted into alkali. The hydroxyl anions required for compensat-ing the separated charges are released in the cathode reaction. The cathode reaction can be for example the cathodic evolution of hydrogen or a cathodic reduction of oxygen. The anolyte compart-went generally retains the organic acid radical which combines with the hydrogen ions or their - 14 - O.Z. 0050/43334 hydrated forms released in the course of the anode reaction to form the corresponding free acid. An example of an anode reaction is the anodic evolution of oxygen or the anodic oxidation of hydrogen. The anode compartment will thus have in general become leaner in the salt and richer in the free dicarboxylic acid.
The membrane electrolysis process can be carried out not only batchwise but also continuously. If it is carried out over the continuous process, one option is to divide the conversion between from 2 to 20, preferably from 4 to 6, cells and to guide the flows countercurrently (see (a)).
The dicarboxylate salt solution used, which may contain a plurality of such salts, has in general a concentration of from 1% by weight up to the satura tion limit of the salt(s), preferably from 5 to 35, particularly preferably from 15 to 30, % by weight.
The current densities are in general within the range from 0.5 to 10, preferably from 1 to 4, kA/m2.
The cell voltage is in general from 3 to 10 V, preferably from 4 to 6 V, per membrane electrolysis unit.
The pH is in general within the range from 2 to 10 in the anolyte compartment and within the range greater than 13 in the catolyte compartment.
The compartment width is in general from 0.5 to 10, preferably from 1 to 5, mm.
The temperature selected for carrying out the membrane electrolysis process is in general within the range from 50 to 110°C, preferably from 65 to 90°C.
To ensure mass transport, the compartment con tents are in general recirculated either by means of 3 5 pumps or through natural convection, ie . through the mammoth pump effect due to gas evolution at elec-trodes. The flow velocities in the compartments are - 15 - O.Z. 0050/43334 in general within the range from 0.05 to 0.5, preferably from 0.1 to 0.2, m/sec.
(e) A particularly preferred embodiment is the electrochemical splitting of the dicarboxylate salts into the corresponding dicarboxylic acids and bases in a three-part membrane electrolysis cell.
The three-part membrane electrolysis unit has in general the following structure:
anode - anolyte compartment - cation exchange mem brane - center compartment - cation exchange mem brane - catolyte compartment - cathode.
The aqueous dicarboxylate salt solution is in general introduced into the center compartment. To increase the electric conductivity in the center compartment, a mineral acid or a salt can be added to the center compartment electrolyte. Examples are sulfuric acid, nitric acid, sodium sulfate and sodium nitrate.
The center compartment generally retains the organic acid radical, which can react with the hydrogen ions liberated in the course of the anode reaction and which have migrated into the center compartment through the anode-side cation exchange membrane to form the free acid. The acid is in general removed from the center compartment system together with unconverted salt. The anolyte used can be an aqueous mineral acid such as sulfuric acid, nitric acid or hydrochloric acid, preferably sulfuric acid. The anolyte's essential function is, together with the anode-side cation exchange mem-brane, to protect the organic dicarboxylic acid from anodic oxidation.
As for the rest, the process of (e) can be carried out under the conditions described at (d).
(f) The electrochemical cleavage of the dicarboxylate salts into the corresponding dicarboxylic acids and bases can also be carried out in a four-part - 16 - O.Z. 0050/43334 membrane electrolysis cell.
The four-part membrane electrolysis unit general-ly has the following structure:
anode - anolyte compartment - cation exchange mem brane - anode-near center compartment - anion exchange membrane - cathode-near center compartment - cation exchange membrane - catolyte compartment cathode.
The aqueous dicarboxylate salt solution is advan tageously introduced into the cathode-near center compartment.
To increase the electric conductivity in the center compartment, a mineral acid or a salt such as sulfuric acid, nitric acid, sodium sulfate or sodium nitrate can be added to the center compartment electrolyte.
The acid anion generally passes from the cathode-near center compartment into the anode-near center compartment, where it reacts with hydrogen ions, which are evolved in the course of the anode reac-tion and pass into the anode-near center compartment through the anode-side cation exchange membrane, to form the free acid. The acid is in general withdrawn from the center compartment system in high purity.
The remaining salt solution is in general withdrawn from the cathode-near center compartment and recir-culated into the adipate dissolution stage as a part-stream or disposed of. The anolyte used is in general an aqueous mineral acid, preferably sulfuric acid. The anolyte's essential function, together with the anode-side cation exchange membrane, is to protect the organic acid from anodic oxidation.
As for the rest, the process of (f) can be carried out under the conditions mentioned at (d).
In the above-described alternatives the cation exchange membranes used are particularly preferably polymers based on perfluorinated olefins or copolymers of - 17 - O.Z. 0050/43334 styrene and divinylbenzene containing sulfonic acid and if desired carboxyl groups as charge carriers. Very particular preference is given to using membranes that contain sulfonic acid groups only, since in general they are more resistant to fouling by multivalent cations than other membranes. Membranes of this type are known (for example Nafion~ membranes of type 324). They consist of a copolymer of tetrafluoroethylene with a perfluorinated monomer that contains sulfone groups. In general they have a high chemical and thermal stability. The ion exchange membrane can be reinforced with a Teflon support fabric. It is also possible to use copolymers based on styrene and divinylbenzene.
Suitable anion exchange membranes are for example the membranes described in detail in EP-A-449,071 so no details will be given here.
The electrode materials used can be in general perforated materials, constructed for example in the form of nets, lamellae, oval profile webs or round profile webs.
The oxygen overvoltage at the anodes is in general set at less than 400 mV within the current density range according to the invention in order that the formation of ozone or per-compounds may be prevented.
Suitable anode materials of low oxygen overvol-tage are for example titanium supports with electrically conducting interlayers of borides and/or carbides and/or silicides of subgroups IV to VI such has tantalum borides, titanium borides or titanium suboxide, doped or undoped tin oxides, or tantalum and/or niobium with or without platinum metal doping, whose surface has in general been doped with electrically conducting, non-stoichiometric mixed oxides of subgroups IV to VI and metals or metal oxides of the platinum group or platinum metal compounds such as platinates. Atop these inter-layers is in general the active electrode material, which preferably consists of mixed oxides of tantalum with - 18 - O.Z. 0050/43334 iridium, platinum or rhodium and platinates of the type Lio,3Pt3O,,. To enlarge the surface area it is customary to use superficially roughened or macroporous titanium supports.
The cathodes are in general made of electrode materials having a low hydrogen overvoltage in order to avoid additional voltage losses in the membrane electrolysis or electrodialysis cell. Suitable cathodes are for example iron or nickel supports which have been surface coated with finely divided cobalt, nickel, molybdenum, tungsten, manganese, Raney metal compounds of nickel or of cobalt, nickel- or cobalt-aluminum alloys, or nickel-iron alloys or cobalt-iron alloys containing from 65 to 90~ by weight of iron.
To improve selectivity and membrane life the cathode side can be equipped with cation exchange mem branes containing hydroxyl ion blockers. The selectivity can be further improved by keeping the level of calcium, magnesium and aluminum ions and also the silica content in each case below 5 ppm.
The dicarboxylic acid obtained by the electro-chemical treatment is in general present as an aqueous solution having a concentration within the range from 1 to 30, preferably from 4 to 30, o by weight. This solu-tion can contain the conductivity salt, if present, in a concentration within the range from 0.05 to 15, prefer-ably from 0.06 to 6, ~ by weight and the mineral acid, if present, in a concentration within the range from 0.05 to 15, preferably from 0 to 6, ~ by weight.
The alkali obtained according to the invention generally contains an alkali metal hydroxide in a con-centration within the range from 5 to 35, preferably from 15 to 25, ~ by weight.
Particularly preferably, the alkali metal hydroxide solution obtained according to the invention can be recirculated or otherwise used, in which case if desired it can be concentrated beforehand in a - 19 - O.Z. 0050/43334 conventional manner, for example by evaporation.
To obtain the dicarboxylic acid in pure form, it is in general crystallized out of the solution obtained according to the invention, then separated off, for example by filtration, and dried.
The dicarboxylic acid is preferably obtained from the electrodialysis or membrane electrolysis solutions by cooling or evaporation crystallization. Then the dicar-boxylic acids are in general separated from the resulting suspensions, for example by filtration, decanting or centrifuging.
The cooling crystallization is customarily carried out at from 0 to 50°C, preferably at from 10 to 40°C, advantageously at pressures within the range from 1 to 100 kPa, preferably from 4 to 20 kPa.
The dicarboxylic acids separated off can be preferably obtained in a pure form by washing, for example with water or Cl-C4-alkanols, and if desired by recrystallization. If a plurality of dicarboxylic acids are present at the same time, the individual dicarboxylic acids can be isolated in pure form by utilizing the solubility differences in a conventional manner such as fractional crystallization.
The aqueous solutions obtained by crystallization and washing can be concentrated in a conventional manner and resubjected to a crystallization, for example by adding them to as-electrodialyzed or as-electrolyzed solutions that have still to be crystallized. They can also be for example added to the reaction mixture obtained from the base treatment of the polymers or compounds used.
One advantage of the process of the invention over known processes is that it obviates the formation and disposal of salts which are customarily obtained When the dicarboxylic acids are freed from their salts by acidification. A further advantage is that even fiber-reinforced, mineral-filled and/or impact-modified molding 20 - O.Z. 0050/43334 compositions can be processed. Furthermore, the sub-stances produced by the process of the invention, such as dicarboxylic acids, diamines and bases and also, as the case may be, glass fibers and mineral fillers, can be used for making new products.

300 g of a nylon 66 having a viscosity number (VN) - 149 (unit: 1 cm'/g (measured on a 0.5o strength by weight solution of the nylon in 96~ strength by weight sulfuric acid at 25°C in accordance with DIN 53727) and comminuted to about 8 mm (average particle diameter) were heated together with 780 g of 15% strength by weight aqueous sodium hydroxide solution with stirring in a pressure vessel at 220°C for 6 hours.
On cooling there was obtained a slightly yel-lowish, homogeneous aqueous solution containing the reaction products hexamethylenediamine and sodium adi-pate.
To separate off the hexamethylenediamine, the solution was repeatedly extracted with a total of 800 g of a mixture of 50~ by volume of toluene and 50~ by volume of n-butanol. The combined organic phases were subjected to a fractional distillation. Toluene, n-butanol and water were separated off first under atmospheric pressure.
141 g of hexamethylenediamine were obtained in the form of a colorless melt at 128 to 131°C/100 mbar.
The aqueous sodium adipate solution remaining following the extraction step was evaporatively con centrated under atmospheric pressure to a 27 o strength by weight sodium adipate solution, extractant residues being removed as well.
The concentrated sodium adipate solution was then admixed with 0.5 g of pulverized activated carbon per 100 ml of solution and heated to 50°C. After 1 h the activated carbon was filtered off and 80 mg of sodium carbonate per 100 g of solution were added with stirring.

- 21 - O.Z. 0050/43334 After 1 h the stirrer was switched off and, after a further 4 h, the solution was filtered. This prepurified sodium adipate solution was then subjected to a treatment with a selective ion exchange resin (Lewatit TP*208 (from BAYER) ) .

This experiment was carried out using a pigmented (with carbon black), (thermally stabilized) and glass fiber-reinforced nylon 66 having a viscosity number (VN) - 140 (measured in accordance with DIN 53727), see Example 1) and a glass fiber content of 36 o by weight (determination of the calcination loss of glass fiber-reinforced plastics in accordance with DIN 53 395) which had been comminuted to about 8 mm (average particle diameter). In a pressure vessel 490 g of this composite material were heated together with 1180 g of a 100 strength by weight sodium hydroxide solution with stirring at 220°C for 8 hours. After the reaction mixture had cooled down, the insoluble constituents such as glass fibers were filtered off and repeatedly washed with water.
The mother filtrate and the combined wash fil-trates were repeatedly extracted with a total of 1100 g of a mixture of 50% by volume of toluene and 50% by volume of n-butanol to separate off the hexamethylenedi-amine . The combined organic phases were subj ected to a fractional distillation. First low boilers such as toluene, n-butanol and water were separated off under atmospheric pressure. 145 g of hexamethylenediamine were obtained at 128 to 131°C/100 mbar. The aqueous sodium adipate solution remaining following the extraction step was evaporated to a concentration of 27.20 by weight of sodium adipate, extractant residues being removed as well. The rest of the workup was carried out similarly to Example 1.

Batchwise electrolysis in a three-compartment * trademark A

- 22 - O.Z. 0050/43334 electrolysis cell as per variant e) The three-compartment electrolysis cell used was that diagrammatically depicted in Figure 1 with three liquid cycles (RL1 to RL3). All product-contacting parts with the exception of the electrodes consisted of polypropylene, glass or quartz. The anode (El) (in compartment (A)) was a titanium expanded-mesh anode having an area of 100 cm2 and a coating suitable for oxygen evolution. The cathode (E2) (in compartment (C)) likewise had an area of 100 cmz. It consisted of a chromium-nickel stainless steel (1.4571) which had been coated with a nickel network activated for hydrogen evolution. The two membranes (M1 and M2) of the type Nafion~ 324 were positioned directly on the electrodes (El and E2) and were separated from each other by a 1 mm wide center compartment (B) with a polypropylene spacer.
The anode (RL1) and cathode (RL2) cycles were kept in natural circulation owing to the gas evolutions at the electrodes. The cycle of the center compartment (B), (RL3), was recirculated using a cycle pump (P). The flow velocity in the center compartment (B) was 0.1 m/sec.
The anolyte used comprised 1131 g of 5o strength by weight sulfuric acid introduced at location (1), the catolyte comprised 1219 g of loo strength by weight sodium hydroxide solution introduced at location (2), and the center compartment electrolyte comprised 911 g of the 27% strength by weight sodium adipate solution of Example 1 to which 19 g of 96o strength by weight of sulfuric acid were added, so that 930 g of a solution containing 21.7 by weight of sodium adipate, 2.9~ by weight of adipic acid and 2.8~ by weight of sodium sulfate were introduced at location (3).
A temperature of 80°C, atmospheric pressure, a current density of 3.0 kA/m~, a cell voltage of 4.0 V (at the beginning) and 5.1 V (at the end of the run) produced in a current yield of 78o and after a reaction time of ~1~815~
- 23 - O.Z. 0050/43334 2 h 13 min the following electrolytes:
anolyte (removed at location (4)): 771 g of 6.6%
strength by weight sulfuric acid, catolyte (removed at location (5)): 1348 g of l4.lo strength by weight sodium hydroxide solution, center compartment electrolyte (removed at location (6)): 838 g of a solution containing 18.7%
strength by weight of adipic acid, 2.3o by weight of sodium adipate and 3.2~ by weight of sodium sulfate.

Batchwise electrolysis in a three-compartment electrolysis cell as per variant f) The four-compartment electrolysis cell used is diagrammatically depicted in Figure 2 with four liquid cycles (RL1 to RL4). All product-contacting parts with the exception of the electrodes consisted of polypropy-lene, glass or quartz. Anode (E1) (in compartment (A)) was a titanium expanded-mesh anode having an area of 100 cm2 and a coating suitable for oxygen evolution.
Cathode (E2) (in compartment (D)) likewise had an area of 100 cm2. It consisted of a chromium-nickel stainless steel (1.4571) which had been coated with a nickel network activated for hydrogen evolution. The two elec-trode-near cation exchange membranes (Ml and M3) of the type Nafion~ 324 were positioned directly on the electrodes (El and E2 respectively) and were separated by two center compartments, (B) and (C), each 1 mm in width, with a centrally disposed anion exchange membrane (M2) of the type Tokuyama Soda° AMH. The center compartments, (B) and (C), were provided with two polypropylene spacers which served to keep the flow channel free and to prevent direct contact between the membranes.
The anode (RL1) and cathode (RL4) cycles were kept in natural circulation owing to the gas evolutions at the electrodes. The cycles of the center compartments (B) and (C), (RL2) and (RL3), were recirculated using the cycle pumps (P1) and (P2). The flow velocities in the X13 815 4 O. Z . 0050/43334 center compartments (B) and (C) were in each case 0.1 m/sec.
The anolyte used comprised 1099 g of 5.10 strength by weight sulfuric acid introduced at location (1) , the catolyte comprised 1103 g of 4 .1 o strength by weight sodium hydroxide solution introduced at location (2) , the electrolyte of the anode-near center compartment (B) comprised 1095 g of 2.Oo strength by weight sulfuric acid introduced at location (3), and the electrolyte of the cathode-near center compartment (C) comprised 1499 g of the 27.2 strength by weight sodium adipate solution of Example 2, introduced at location (4).
During the reaction a total of 915 g of water was additionally introduced into the cathode-near center compartment (C).
A temperature of 80°C, atmospheric pressure, a current density of 3.0 kA/m~, a cell voltage of 7.0 V (at the beginning) and 8.7 V (at the end of the run) produced with a current yield of 72~, and after a reaction time of 5 h, during which the pH in the cathode-near center compartment (C) was within the range from 10 to 12, the following electrolytes.:
anolyte (removed at location (5)): 784 g of 7.1~
strength by weight sulfuric acid, catolyte (removed at location (6)): 1579 g of l3.lo strength by weight sodium hydroxide solution, product of the anode-near center compartment (B) (removed at location (7)): 2020 g of a solution containing 14.6 strength by weight of adipic acid, 1.1%
by weight of sulfuric acid, product of the cathode-near center compartment (C) (removed at location (8)): 1086 g of a 2.3~ strength by weight sodium adipate solution.

Claims (5)

CLAIMS~

1. A process for coproduction of dicarboxylic acids and diamines from (a) polymers based on polyamides of dicarboxylic acids or their derivatives with diamines, or (b) materials comprising essentially such polymers, by cracking these polymers into their monomeric constituents using a base in an aqueous medium and removing the resulting diamines, characterized in that the base is dissolved in a mixture of from 100 to 75% by weight of water and from 0 to 25% by weight of a C1-C4-alkanol and the resulting dicarboxylic acid salts of adipic acid or sebacic acid are converted into the corresponding dicarboxylic acids and bases by electrochemical means.

2. A process as claimed in claim 1, characterized in that the electrochemical conversion is carried out in a three- or four-part membrane electrolysis cell at a temperature within the range from 65 to 90°C.

3. A process as claimed in claim 1, characterized in that the electrochemical conversion is carried out in a three-compartment electrodialysis unit at a temperature within the range from 40 to 110°C.

4. A process as claimed in any one of claims 1 to 3, characterized in that the adipic acid or sebacic acid is crystallized out from its solutions resulting from the electrochemical treatment.

5. A process as claimed in any one of claims 1 to 4, characterized in that the base obtained from the electrochemical treatment is used for cracking the polymers into their monomeric constituents.