CA2003645A1 - Process for cathodic electrodip lacquering and the use of nitric acid and/or nitrous acid for the neutralization of coating baths - Google Patents

Abstract

Abstract 1. The use of nitric acid and/or nitrous acid for the neutralization of coating baths for cathodic electro dip lacquering.
2.1. The anode plates of cathodic electro dip lacquering baths are severely attacked by halide ions. This has hitherto been prevented by the addition of a salt, e.g. copper nitrate. A process is now to be provided for protecting the anode continuously without the addition of undesirable accompanying substances.
2.2. Cathodic electro dip lacquering is carried out in an aqueous coating bath containing a basic or amphoteric film former capable of being converted into an aqueous solution or dispersion by neutral-ization with acid, free nitric acid and/or nitrous acid being used for neutralization, and/or the pro-cess being carried out in the presence of an electro dialysis membrane which has been charged with nitric acid and/or nitrous acid.
2.3. Cathodic electro dip lacquering of surfaces of electrically conductive substrates which are connected as cathode.

Description

ZC~645 Process for cathodlc electrodip lacquerlng i nd the use of nitric acid and/or nitrous acid for the neutralization of coating baths.
The basic principle of cathodic electro dip'lacquering, which has become widely established in practice, consists of immersing a workpiece having an electrically conductive surface in a lacquer which is diluted with water and connecting the workpiece to the negative terminal of a source of direct current. ~ ' A DC voltage of from 50 to 500 Volt is applied between the anode and the cathode and deposition of the lacquer particles on the cathodically connected workpiece takes place under the action of this voltage. The coating bath contains an aqueous solution or aqueous dispersion of salts of cationic film formers with orqanic and/or inorganic acids and optionally other lacquer raw materials such as, for example, binders, plasticizers, cross-linking agents, pigments, fillers and additives. One important component for the process is the anode plate. Two measure 20 are conventionally employed to deal with the possibility '~
of anodic solution of the plate in the coating bath. ' Firstly, the anode plates are made of special steels te.g. chromium-molybdenum steel) to improve their service life. Secondly, coating is carried out by the electro- -' ;'' 25 dialysis process to~avoid acid accumulation and reversible ' '~coating, e.g. the coating of lead compounds. In the electro-`dialysis process, the anode plate is separated from the --' ;'~-immersion bath (anolyte space) by special membranes.
The rate at which the anode plate dissolves may be ~-~
drasticaily'increasbd biy even sligh't concentrations o halide ions in the elec~ro dip baths. It is virtually ''~
impossible to prevent the presence of small concentrations ' of~haiide ions, especially chloride ions, and these may cause trouble when acting in combination with other ' -' ~' 35 influence~ such as heat, flow velocity, and surface defects on the anode plate. The freshly installed membranes ~ .
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. , ~C~3645 of a cathodic dip lacquering installation normally contain chlorides which are released into the anolyte space in the course of the process and may cause a considerable amount of the unprotected anode plate to dissolve. It S is known to employ passivating methods as protective measures. In DE-A-27 51 498, for example, a certain propor-tion of dissolved chloride ions together with nitrate ions and/or nitrite ions is described as being particularly advantaqeous for preven~ing perforation corrosion and 10 the resulting further breakdown of the anode. The nitrates ;
and nitrites are added to the coating bath in the form of salts, preferably copper nitrate. The disadvantage of this procedure lies in its lack of continuity and in the presence of impurities which may cause trouble, e.g. for production ~echnical or ecological reasons.
The process has not become established in the art of cathodic electro dip lacquering.
It is an object of the present invention to provide a process for cathodic electro dip lacquering which does not have the disadvantage of severe corrosion of the - unprotected anode plate and in which the disadvantages of the methods ~nown in the art do not exist.
This problem is solved by using aqueous solutions of free nitric acid and/or nitrous acid for at least partial neutralization of the basic or amphoteric film-formers present in the coating baths used for cathodic eIectro dip lacquering.
This invention therefore relates to a process for the cathodic electro dip lacquering cf surfaces of conductive subs~trates which are electrically connected as cathode in an aqueous coating bath containing a basic or amphoteric film former capable of being converted into an aqueous solution or dispersion by neutralization with acid, one or more organic and/or inorganic acids and optionally cross-linking agents, plasticizers, pigments, fillers, organic solvents and/or other conventional additives used in lacquer technology, characterised ~: ' '- ' ' ' '.:'" ' ' -,: '",- ':' .

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s in that aqueous solutions of free nitric acid and/or nitrous acid are used as at least part of the acid for neutralization and/or the process is carried out in the presence of an electrodialysis membrane which has been charged with nitric acid and/or nitrous acid. The latter method is particularly advantageous since it virtually eliminates the influence:of halide ions from the membrane substance. The nitric acid and~or nitrous acid may be added to the first filling material or to the refilling material for a cathbdic coating bath. Similarly, the electro dialysis membrane charged with nitric acid and/or nitrous acid may be used in the starting phase and/or when membrane change is carried out.
The proportion of nitric acid and/or nitrous acid in the neutralizing agent is advantageously about 1 to 10 mol-%, based on the total quantity of neutralizing agent required. The acids may be used as aqueous solutions, e.g. at a concentration of up to 20~ by weight.
The quantity of nitric acid and/or nitrous acid is preferably so chosen that its molar ratio to the quantity of halide ions present in the coating bath is from 0.5 to 10 :~1, preferably from 0.5 : 1 to 5 : 1, most prefer-ably from 0.~ : 1 to 1.5 Although it has long been known to use mineral acids as neutralizing agent, neither nitric acid nor nitrous acid are mentioned in the Patent literature and techhical literature, e.g. in DE-A-2 751 498. The use of these two acias as the only neutralizing agent is less preferred as it deleteriously affects the properties required for technical application.
The film formers which afe to be cathodically deposited and require to be neutralized are conventional basic or amphoteric polyaddition products and/or homopolymers and/or copolymers and/or polycondensates, i.e. any binders - 35 which can be cathodically deposited either alone or in combination with other binders and cross-linking agents.
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2C~)3645 The binders or film formers are those components of the lacquer which remain in the lacquer after removal of the pigments and fillers and volatile constituents (organic solvents and water). Cross-linking agents, plasti-cizers, additives, etc. are therefore to be regardedas binders, provided they are non-volatile, but as these components have special functional properties, they will be referred to in the description by their specific names mentioned above.
The preparation of aqueous solutions, aqueous emulsions or aqueous dispersions is known. They may be prepared, for example, from solutions of the binders or binders and cross-linking agents in organic solvents by dissolving or dispersing these solutions in water together wlth suitable aaids and -a proportion of nitric acid and/or nitrous acid. The nitric acid and/or nitrous acid may, of course, also be added at a later stage of the preparation, for example during or after preparation of the first filling or refilling material which optionally contains pigments and fillers,or it may be added during or after preparation of the coating bath, for example, as in every method of procedure. Another example of the known possible methods of preparation is the prepara-tion of a solvent-free or low solvent aqueous dispersion in which the addition according to the invention of the acids is carried out before or after preparation of the dispersion or only after other lacquer raw materials have been added. ~ -The neutralizing agents used in addition to nitric acid and/or nitrous acid may be any inorganic and/or organic acids or acid derivatives known for'this purpose. -As a general rule, however, it is only suitable to use acids which when used in combination with film formers capable of cathodic deposition ensure complete solubility ~ -~ 35 or dispersibility in water at as low a molar ratio as ` .
- possible (i.e. the quantity of acid groups should be kept as low as possible) and have no damaging side effects . ' ' - ':: ; . ~ ~ :-.
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2C~)~645 such as corrosiveness or physiological toxicity.
The neutralizing agents mainly used for this purpose in practice are formic acid, acetic acid, propionic acid, lactic acid and alkylphosphoric acids.The use of hydrochloric acid as neutralizing agent can only be recommended for special, exceptional cases in view of their introduction of chloride ions into the coating bath and the in most cases poor dispersibility in water of cathodically depositable film formers which have been 1~ neutralized with this acid.
The aqueous coating bath, which may be at a pH of from 1 to 9, preferably from 5 to 8, contains cathodic-ally depositable binders and optionally other lacquer raw materials which need not necessarily be cationic, e.g. ot'ner binders, cross-linking agents, plasticizers, pigments, fillers, lacquer powders, reisin powders, addi-tives ~e.g. catalysts, anti-foamants, anti-pitting agents, bonding agents and substances for improving levelling) and organic solvents.
The conditions unde~ which cathodic deposition is to be carried out depend on the requirements of the user and the nature and quantity of the raw materials used.
For example, DC voltageis of from 50 to 500 Volt, tempera-tures of the coating bath of from 20C to 40C and coating times from 0.3 to 5 minutes are generally employed. The coated metal parts may be stoved, for example, at tempera-tures from 80C to 200C, e.g. for 15 to 40 minutes.
Perfect, smooth coatings are obtained in thicknesses ` ;
of ,up to 60 ~m or more, depending on the nature of the cathodic dip lacquering system employed and the conditions of deposition.
The cathodically depositable binders are basic synthet-ic resins or natural resins which have been basically modified and/or amphoteric synthetic resins. Synthetic ~ :
resins containing primary and/or secondary and/or tertiary amino groups and/or quaternary ammonium groups and/or sulphonium groups and/or phosphonium groups are preferred. ~

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Examples of these include the conventional binders containing amlno groups and/or quaternary ammonium groups and based on modified epoxide resins, (meth)acrylate resins, polyamino amide resins, polybutadiene resins, modified Mannich bases of bisphenol-A, modified epoxide/car-bon dioxide/amine reaction products, polyesters, poly-urethanes and polyureas.
Examples of cross-linking agents suitable for use in combination with the film formers present in the coating bath include those which react with reactive hydroxyl groups and/or reactive amino groups, such as melamine resins, phenol resins, masked isocyanates, cross-linking agents capable of trans-esterification and trans-amidation, acetal-functional cross-linking agents and urea condensation 15 products according to DE-A-3 325 061, page 11, cross-linking agent 3.
The cathodically depositable binder may also be self^
cross-linking, i.e. the groups which react with one another are present in one and the same molecule. The same possi-bilities of cross-linking apply to such a binder as to - the combinations of film formers and cross-linking agents.
According to one embodiment of the invention,,the ~ `
process for cathodic electro dip lacquering is carried out in the presence of an electrodialysis membrane which has been charged with nitric acid and/or nitrous acid.
The membranes used for this purpose are conventional membranes of the type used for electrodialysis. They generally aonsist of anion exchanger resins applied to supporting fabrics by means of film forming agents.
! Membranes of this type frequently contain chloride ions bound in the form of salts. The negative effect ~-of such chloride ions as of other halide ions and of those present in the solutions can be eliminated by ;
charging the membranes with nitric acid and/or nitrous acid in accordance with this invention.
For charging the membranes with nitric acid and~or .~ -~
~ nitrous acid, the membranes may, for example, first be -~ ~

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treated with a solution of an alkali metal liquor (e.g. ~-an aqueous liquor such as sodium hydroxide solution) to replace the anions or chloride ions present by a hydroxyl functionality. The membranes now charged ~lth hydroxyl lons may be rinsed with water and then treated with an aqueous solution of nitric acid and/or an aqueous solution of nitrous acid to prepare them for their use according to the invention.
All percentages given in the following Examples of Preparation and Examples are percentages by weight unless otherwise indicated.
Example of Preparation:
Cationic binder 210 g of Diethanoiamine, 102 g of 3-(N,N-dimethylamino)-propylamine and 616 g of an adduct of 1 mol of 1,6-diamino-hexane and 2 mol. of the glycidyl ester of a branched C10-monocarboxylic acid (Cardura(R) E 10 having a molar mass of 250 g/mol) are added to 2892 g of an epoxide resin based on bisphenol-A (epoxide equivalent weight 482, Epikote 1001(R)) in 1637 g of 2-butoxyethanol.
The mixture is maintained at 85 to 90C for 4 hours and 120C for one hour with stirring.
Cross-linkinq aqent 768 g of Trimellitic acid anhydride are dissolved -in 1187 g of 2-butoxyethanol at 100C and mixed with 2000 g of the glycidyl ester of a branched C10-monocarboxyl- ;~
ic acid (Cardura(R) E 10) and with 2.75 g of N,N-dimethyl-benzylamine. ~ ;~
The mixture is heated to 120C with stirring and -30 kept at this telmperatujrq until the acid number has fallen ~; -below 3 mg of KOH/g of solid resin. -:.'. ': ~.,:

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.;:;; . , o Coatinq bath A (Comparison Example~
Preparation of the first filling material:
440 g of cationic binder, 163 g`of cross-linking agent, S16 g oE carbon black, 56 g of kaolin, 4 g of zinc oxide and 27 g of 8-hydroxyquinoline are thoroughly mixed by vigorous stirring in a dissolver and then ground in a pear~ mill. 18 g of formic acid at a concentration of 50% in water and 2100 g of completely salt-free water are added with stirring after the mixture has been ground.
Coatin~ ath 3 The method of preparation is analogous to that employed for coating bath A excepjt that in the first filling material, only 17.4 g of a 50~ soIution of formic acid in water -are used instead of 18 g and that 0.7 g of 25% nitric acid are added after the addition of 2100 g of salt-free ~ ;
20 water. .
PreParation of the bath ;
The first filling material A is adjusted to a solids - ;
content of 13% with completely salt-free water so that the total volume o the coating bath is about 4 litres.
25 Example 1 i '`
The coating bath~A is introduced after 2 days' stirring;;
` at room temperature into an electro dialysis cell designed for circulation of the electrodialysis liquid. The conduct- , ivity of the anolyte space is adjusted to 500 ~S.cm 1 `~
by means of completely salt-free water.
The conductivity in the anolyte liquid is increased to 1000 yS.cm~l by the coating of steel sheets under continuous circulation, and the chloride content is then measured. ; -~
35~ ~ This liquid is then used for carrying out a continuous ele~ctrolysis experiment over a period of 4 hours at 10 `~
mA.cm and 22C~ The weight loss of the anode plate i , ~
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2C~3645 is then measured. The values are shown in the ~able.
ExamPle 2 The same procedure is employed with coating bath B
as with coating bath A. Marked protection against corrosion is again obtained.
Example 3 1 g of 10% Hydrochloric acid is added to coating bath A to simulate unintentional contamination with a relatively large amount of chloride. The procedure described in Example 1 is then carried out. The addition of nitric acid provides marked protection of the anode against corrosion.
ExamPle 4 1 g of 10~i Hydrochloric acid and 0.7 g of 25% nitric acid are added to coating bath B and the same procedure as in Example 1 is employed. The results show that the - addition of nitric acid has protected the anode against corrosion.
The results obtained in Examples 1 to 4 are entered -~
20 in the following Table: l Chloride Nitric acid Weight loss of - -content content anode plate ppm mmol/l mmol/l % by weight Example ~-25 ~1 ~ 24 0.68 O 1.38 2 24 0.68 0.69 0.01 4 49 1.4 1.39 0.01 ~ It may beiseen that,~the addition of free nitric acid ~
30 ~ to the bath provides considerable protection against ~ --corrosion. . - -Example 5 An ion exchanger membrane conventionally used for electrophQret~ic deposition at the cathode is extracted with~sodium hydroxide solution for 48 hours so that the ~ ..

ZC~3645 chloride ions which are bound in the form of salt are replaced by an OH functionality. The membrane is then rinsed with water;and immersed in an aqueous nitric acid solution.
S The membrane now charged with nitric acid i5 introduced between the anode chamber and cathode chamber of an electro dip bath. It is found that the corrosion of the anode can be greatly reduced by the use of the charged membrane.

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Claims (7)

1. Process for the cathodic electro dip lacquering of surfaces of electrically conductive substrates connected as cathode in an aqueous coating bath containing a basic or amphoteric film former capable of being converted into an aqueous solution or dispersion by neutralization with acid, one or more organic and/or inorganic acids and optionally cross-linking agents, plasticizers, pigments, fillers, organic solvents and/or other conventional addi-tives of lacquer technology, characterised in that free nitric acid and/or nitrous acid is used as at least part of the acid used for neutralization and/or the process is carried out in the presence of an electrodialysis membrane which has been charged with nitric acid and/or nitrous acid.

2. Process according to Claim 1, characterised in that the free nitric acid and/or nitrous acid is added to a first filling material or a refilling material for the coating bath.

3. Process according to Claim 1 or 2, characterised in that the membrane which is charged with nitric acid and/or nitrous acid is used in the starting phase of the electrodialysis process.

4. Process according to one of the Claims 1 to 3, charac-terised in that when a membrane change is carried out, the membrane is freshly charged with nitric acid and/or nitrous acid.

5. Process according to one of the preceding Claims, characterised in that the free nitric acid and/or nitrous acid is used in such a quantity that the molar ratio to the halide ions present in the coating bath is from 0.5 : 1 to 10 1.

6. Process according to one of the preceding Claims, characterised in that the free nitric acid and/or nitrous acid is added in the form of an aqueous solution at a concentration of up to 20% by weight.

7. The use of aqueous solutions of free nitric acid and/or nitrous for at least partial neutralization of coating baths for cathodic electro dip lacquering.