EP0262419A2 - Process for removing acids from cataphoretic painting baths - Google Patents

Abstract

The present invention relates to a novel process for removing acids from cathodic electrocoating baths, in which electrically conductive substrates are coated with cationic resins present in the form of aqueous dispersions, part or all of the dip-coating bath being subjected to an ultrafiltration in which the ultrafiltration membrane retains the cationic resin and an ultrafiltrate is formed which contains water, solvent, low molecular weight substances and ions, and part or all of the ultrafiltrate being recycled to the coating bath, in which process (a) part or all of the ultrafiltrate, before recycling to the electrocoating bath and (b) an aqueous solution of an organic or inorganic base, which may or may not contain salts, are each introduced into one or more chambers, separated from one another by an anion exchange membrane, of an exchange cell, solutions (a) and (b) being led over the surface of the membrane.

Description

The present invention relates to a new process for removing acid from cathodic electrocoating baths in which electrically conductive substrates are coated with cationic resins present in the form of aqueous dispersions, at least part of the dipcoating bath being subjected to ultrafiltration, in which the ultrafiltration membrane does this retains cationic resin and an ultrafiltrate is formed, which contains water, solvents, low molecular weight substances and ions, and at least a part of the ultrafiltrate is returned to the immersion bath.

Cathodic electrocoating is known and is e.g. described in detail in F. Loop, "Cathodic electrodeposition for automotive coatings" World Surface Coatings Abstracts (1978), para. 3929.

In this process, electrically conductive substrates are coated with cationic resins in the form of aqueous dispersions. Resins that can be deposited by cathode usually contain amino groups. In order to convert them into a stable aqueous dispersion, they are protonated with acids (also called solubilizing agents in some publications), such as formic acid, acetic acid, lactic acid or phosphoric acid. During electrodeposition coating, the protonation in the immediate vicinity of the metallic object to be coated is reversed by neutralization with the hydroxyl ions formed by electrolytic water decomposition, so that the binder precipitates on the substrate ("coagulates"). The acid is not co-precipitated, so that the acid accumulates in the bath with increasing painting time. This causes the pH to drop, which leads to destabilization of the electrocoat. The excess acid must therefore be neutralized or removed from the bath.

US Pat. No. 3,663,405 describes the ultrafiltration of electrocoat materials. In ultrafiltration, the electrocoat is passed under a certain pressure along a membrane that retains the higher molecular weight components of the lacquer, which allows low molecular weight components such as organic impurities, decomposition products, resin solubilizing agents (acids) and solvents to pass through. To remove these low molecular weight components, part of the ultrafiltrate is discarded and thus removed from the system. A other part of the ultrafiltrate is led into the rinsing zone of the painting line and is used there to rinse off the paint dispersions still adhering to the painted objects ("drag-out"). Ultrafiltrate and rinsed paint dispersions are fed back into the electrocoating tank to recover the discharge. Since the solubilizing agent is used in large quantities, it is not possible to remove it from the bath in sufficient quantities by discarding ultrafiltrate.

US Pat. No. 3,663,406 describes the combined use of ultrafiltration and electrodialysis for working up and controlling the solubilizing agent budget of electrocoating materials. Electrodialysis is installed in the electrodeposition basin so that the counter electrode to the coated object is separated from the actual varnish by a semipermeable membrane and an electrolyte that contains the solubilizing agent. When an electric field is applied, the ions which are oppositely charged to the ionic resin groups migrate through the ion exchange membrane into the electrolyte and can be discharged from there via a separate circuit. These electrodialysis units installed in the electrocoat require a lot of space and are very maintenance-intensive. The membranes can clog with paint particles or can be mechanically damaged by the objects to be painted, so that an exchange of the membranes is necessary. This is time and cost intensive and can put the painting process out of operation for a certain time.

For this reason, there are processes that make it possible to move the electrodialysis from the electrocoating tank to the system periphery. Such processes are described in DE 32 43 770 and EP 01 56 341, in which the part of the ultrafiltrate which is returned to the rinsing zone and then to the electrodeposition basin is subjected to an electrodialysis treatment before it enters the rinsing zone. This allows the solubilizing agents (acids) enriched in the ultrafiltrate to be removed from the painting process. The major disadvantage of this electrodialysis process is that lead, which originates from a corrosion protection pigment, is also deposited on the cathode in addition to other cations. The cathode was therefore designed to be movable and thus regenerable, which is very complex.

The invention was therefore based on the object of removing excess acid from the ultrafiltrate of cathodic electrocoating baths without the disadvantages described above.

Surprisingly, it was found that the acid can be removed from the ultrafiltrate via an exchange cell, ie by dialysis, without electrodialysis.

Furthermore, it was found that all cations and solvents remain in the ultrafiltrate after the dialysis treatment.

• a) mindestens einen Teil des Ultrafiltrates vor Rückführung in das Elektrotauchbad und
• b) eine wäßrige Lösung einer organischen oder anorganischen Base, die gegebenenfalls noch Salze enthält,

jeweils in mindestens eine, durch eine Anionenaustauschermembran voneinander getrennte Kammer einer Austauschzelle einleitet und die Lösungen a) und b) an der Oberfläche der Membran vorbeiführt.Accordingly, a process has been found for removing acids from cathodic electrocoating baths in which electrically conductive substrates are coated with cationic resins present in the form of aqueous dispersions, at least part of the immersion painting bath being subjected to ultrafiltration, in which the ultrafiltration membrane contains the cationic resin holds back, and an ultrafiltrate is formed, which contains water, solvents, low molecular weight substances and ions and at least a portion of the ultrafiltrate is returned to the immersion bath by

• a) at least part of the ultrafiltrate before being returned to the electro-immersion bath and
• b) an aqueous solution of an organic or inorganic base, which may also contain salts,

introduces in each case into at least one chamber of an exchange cell which is separated from one another by an anion exchange membrane and solutions a) and b) pass the surface of the membrane.

A large number of paints can be used for cathodic electrocoating. The paints obtain their ionic character from cationic resins, which usually contain amino groups, which are mixed with conventional acids, e.g. Formic acid, acetic acid, lactic acid or phosphoric acid are neutralized, forming cationic salt groups. Such cationically separable compositions are described, for example, in US Pat. No. 4,031,050, US Pat. No. 4,190,567, DE-OS 27 52 555 and EP-OS 12 463.

These cationic resin dispersions are combined with pigments, soluble dyes, solvents, flow improvers, stabilizers, antifoams, crosslinking agents, curing catalysts, lead and other metal salts and other auxiliaries and additives to give the electrocoating materials.

For cathodic electrocoating, a solids content of the electrocoating bath of 5 to 30, preferably 10 to 20,% by weight is generally established by dilution with deionized water. The deposition is generally carried out at temperatures of 15 to 40 ° C. for a period of 1 to 3 minutes and at pH bath values of 5.0 to 8.5, preferably pH 6.0 to 7.5, with deposition voltages between 50 and 500 volts. After rinsing off the film deposited on the electrically conductive body, it is cured at about 140 ° C. to 200 ° C. for 10 to 30 minutes, preferably at 150 to 180 ° C. for about 20 minutes.

Electro dip painting baths are operated continuously, i.e. the objects to be coated are constantly introduced into the bath, coated and then removed again. That is why it is also necessary to constantly supply the bathroom with paint.

After a few months of operation, undesirable contaminants and solubilizing agents accumulate in the bathroom. Examples of such contaminants are oils, phosphates and chromates which are introduced into the bath from the substrates to be coated, carbonates, excess solubilizers, solvents, oligomers which accumulate in the bath because they are not deposited with the resin. Such undesirable components negatively affect the coating process, so that the chemical and physical properties of the deposited film become unsatisfactory.

In order to remove these impurities and to keep the composition of the electrocoating bath relatively uniform, part of the bath is drawn off and subjected to ultrafiltration.

The solutions to be ultrafiltered are brought under pressure in a cell, for example either by compressed gas or a liquid pump, into contact with a filtration membrane which is arranged on a porous support. Any membrane or filter that is chemically compatible with the system and has the desired separation properties can be used. The contents of the ultrafiltration cell are preferably stirred in order to prevent accumulation of the retained material on the membrane surface and to prevent these substances from being firmly deposited on the membrane. Ultrafiltrate is continuously produced, which is collected until the retained solution in the cell has reached the desired concentration or the desired proportion of solvents or solvents with dissolved low-molecular substances is removed. Suitable devices for ultrafiltration are described, for example, in US Pat. No. 3,495,465.

Although ultrafiltration can be used to remove numerous impurities from the immersion bath, it is not possible to remove solubilizing agents from the bath satisfactorily. One of the reasons for this is that in industrial use, the ultrafiltrate is used to wash and rinse freshly coated objects to rinse off loose paint particles. This washing solution is returned to the immersion bath. Although part of the ultrafiltrate is usually discarded, this is usually not sufficient to remove the excess acid. It is therefore necessary to feed at least part of the ultrafiltrate to an exchange cell.

The dialysis process is carried out in an exchange cell which contains at least two chambers separated by an anion exchange membrane, so that two separate liquid flows are possible. Exchange cells of this type are e.g. used for the known methods of electrodialysis, but in the present case the electrode chambers are omitted since no electrical field is required. Suitable equipment is e.g. described in EP-PS 126 830. Suitable as exchange cells are e.g. apparatus equipped with membrane stacks, which a large number, e.g. Contain 2 to 800 chambers arranged parallel to each other. Since no electric field has to be applied, one is not bound to these so-called plate membrane modules. All other exchange cells can also be used, such as hollow fiber, tube or winding modules. The chambers of the exchange cells can alternatively be charged with aqueous solutions a) and b), an aqueous solution of an organic or inorganic base, which may also contain salts, being used as solution b). The hydroxides or carbonates of the alkali or alkaline earth metals or of ammonium are used as inorganic bases. Sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, calcium hydroxide, barium hydroxide, ammonia or ammonium carbonate are preferred. Amines such as the trialkylamines, e.g. Trimethylamine and triethylamine, or diazabicyclooctane and dicyclohexylethylamine or polyamines, such as polyethyleneimines and polyvinylamines, or quaternary ammonium hydroxides are used. Solution b) generally has a pH of 7 to 14, preferably 11 to 13.

Optionally, in addition to the bases mentioned, at least one salt, preferably consisting of a cation of the above-mentioned bases and an anion of the above-mentioned conventional acids, is also present in solution b) a concentration of 0.001 to 10 equivalents per liter, preferably 0.001 to 1 equivalent per liter. Sodium and potassium acetate and sodium and potassium lactate are preferred.

The process can be carried out continuously or batchwise. In the case of the batch process, the solutions pass through the exchange cell a number of times and during continuous operation once. The two solutions can be passed through the exchange cell in parallel, cross or countercurrent. The exchange cells can be arranged in the form of a multi-stage cascade, in particular during continuous operation.

Known anion exchange membranes, which e.g. have a thickness of 0.1 to 1 mm and a pore diameter of 1 to 30 µm or a gel-like structure. Since it is a diffusion process, particularly thin membranes, e.g. those with a thickness of less than 0.2 mm are preferred.

The anion exchange membranes are constructed according to a generally known principle from a matrix polymer which is functionalized with cationic groups.

Examples of matrix polymers are polystyrene, which is e.g. Divinylbenzene or butadiene has been crosslinked, high or low density polyethylene, polysulfone or polytetrafluoroethylene.

The matrix polymers are functionalized e.g. by copolymerization, grafting or condensation reaction with monomers containing cationic groups. Examples of such monomers are vinylbenzylammonium, vinylpyridinium or vinylimidazolidinium salts. Amines which still have quaternary ammonium groups are introduced into the matrix polymer via an amide or sulfonamide condensation reaction.

Polystyrene-based membranes are e.g. commercially available under the names Selemion® (Asahi Glas), Neosepta® (Tokoyama Soda) or Aciplex® (Asahi Chem.).

Membranes based on polyethylene grafted with quaternized vinylbenzylamine are available under the name Raipore® R-5035 (from RAI Research Corp.), with grafted polytetrafluoroethylene under the name Raipore R-1035.

EP-A-166 015 describes membranes based on polytetrafluoroethylene with a quaternary ammonium group bonded via a sulfonamide group.

The anion exchange membranes have good stability towards the alkaline medium.

Although the process is characterized by high exchange rates, depending on the process conditions and the electrocoating bath compositions used, the exchange rates may drop after some operating time. In these cases, an intermediate rinsing of the membranes with e.g. diluted acids.

The flow rate at which solutions a) and b) are passed through the exchange cell is generally 0.001 m / s to 2.0 m / s, preferably 0.01 to 0.10 m / s.

The dialysis process is generally carried out at temperatures from 0 to 100 ° C., preferably 20 to 50 ° C. and at pressures from 1 to 10 bar, preferably at atmospheric pressure. The pressure drop across the membranes used is up to 5 bar, in particular up to 0.2 bar.

The process of cathodic electrocoating is used to coat electrically conductive surfaces, e.g. Automotive bodies, metal parts, sheets, etc. made of brass, copper, aluminum, metallized plastics or materials coated with conductive carbon, as well as iron and steel, which may have been chemically pretreated, e.g. are phosphated.

The process of removing acid from the electrocoating bath is characterized by high exchange rates.

example 1

Through the middle chamber of a round three-chamber exchange cell with two anion exchange membranes of the type Selahion DMV from Asahi Glas with a membrane distance of 1 cm and an area per membrane of 3.14 cm², 150 g of ultrafiltrate (solution a )) pumped in a circle from pH 5.74 through a storage vessel until a pH of 6.5 was reached. 150 g of a 0.02 normal aqueous sodium hydroxide solution (solution b)) having a pH value of 12.2 were pumped through the two outer chambers at 25 ° C. in a circle over the same period of time. To At the end of the experiment, no change in weight of the two solutions was found.

Changes in the composition of the solutions and the measurement data are listed in the table.

Example 2

This example was carried out analogously to Example 1, with the difference that a mixture of 0.02 equivalents / l sodium hydroxide and 0.17 equivalents / l sodium acetate was used as solution b).

Example 3

Through the middle chamber of a laboratory plate stack cell with two anion exchange membranes of the type Selemion DMV with a membrane spacing of 0.3 cm and an area per membrane of 37.8 cm², 900 g of ultrafiltrate with a pH of 5.74 as at Example 1 were at 25 ° C. pumped in a circle until a pH of 6.5 was reached.

Example 4

Through the middle chamber of a laboratory plate stack cell (as described in Example 3), 1000 g of ultrafiltrate of another electrocoat material with a pH of 5.88 was pumped in a circle as in Example 1 until a pH of 6.5 was reached. A 0.01 N sodium hydroxide solution with a pH of 11.8 was used as solution b).

Example 5

The same experimental setup and an ultrafiltrate of the same electrocoat material as in Example 4 were used. A 0.001 N sodium hydroxide solution with a pH of 10.4 was used as solution b). The pH of solution b) was kept between 9.4 and 10.6 by regular addition of 0.01N NaOH solution.

Claims (7)

1. A process for removing acid from cathodic electrocoating baths, in which electrically conductive substrates are coated with cationic resins present in the form of aqueous dispersions, at least part of the dipcoating bath being subjected to ultrafiltration, in which the ultrafiltration membrane retains the cationic resin and an ultrafiltrate is formed which contains water, solvents, low molecular weight substances and ions and at least part of the ultrafiltrate is returned to the immersion bath, characterized in that a) at least part of the ultrafiltrate before being returned to the electro-immersion bath and b) an aqueous solution of an organic or inorganic base, which may also contain salts introduces in each case into at least one chamber of an exchange cell which is separated from one another by an anion exchange membrane and solutions a) and b) pass the surface of the membrane. 2. The method according to claim 1, characterized in that the solutions a) and b) at a flow rate of 0.001 to 2.0 m / s, preferably 0.01 to 0.10 m / s and at a temperature of 0 to 100 ° C, preferably 20 to 50 ° C along the anion exchange membrane. 3. The method according to any one of claims 1 to 2, characterized in that a solution a) is used which contains at least one of the acids formic acid, acetic acid, lactic acid or phosphoric acid. 4. The method according to any one of claims 1 to 3, characterized in that dilute inorganic bases such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, calcium hydroxide, barium hydroxide, ammonia and ammonium carbonate with pH values of 7 to 14, preferably as aqueous solution b) 11 to 13 used. 5. The method according to any one of claims 1 to 3, characterized in that solutions of amines or quaternary ammonium compounds with pH values of 7 to 14, preferably 11 to 13, are used as aqueous solutions b). 6. The method according to any one of claims 1 to 5, characterized in that a solution b) is used, which optionally in addition to the bases mentioned in claim 4, at least one salt consisting of a cation of the bases mentioned in claim 4 and an anion of acids mentioned in claim 3, in a concentration of 0.001 to 10 equivalents per liter, preferably 0.001 to 1 equivalent per liter dissolved.