AU656810B2 - A method of recovering the overspray from aqueous coating agents used in spray booths - Google Patents

Abstract

A process for the recovery of oversprays from water-coatings during spray coating in spray booths, in which the overspray is collected in an aqueous washing liquid which is continuously returned in a circulation (A) to the spray booth, a part of the washing liquid which contains the overspray being fed from the circulation (A) to an ultrafiltration circulation (B), and permeate being withdrawn in the ultrafiltration circulation (B) and returned to supplement the washing liquid into the circulation (A), in which the washing liquid used is the water-coating, which is used in the spray booth for spray coating, in a form diluted with water, the liquid circulating in the ultrafiltration circulation (B) is the water-coating, which is used in the spray booth for spray coating, if required, in a form diluted with water, and the process is carried out continuously, the washing liquid having in the circulation (A) an approximately constant solids content which is in the range from 4 - 20 % by weight, and the liquid circulating in the ultrafiltration circulation (B), by withdrawal of permeate, being kept at an approximately constant solids content in the range from 15 % by weight to the solids content which results from the spraying viscosity of the water-coating and is higher than the solids content in the circulation (A), and a part of the liquid circulating in the ultrafiltration circulation (B) being used as a water-coating for spray coating in the spraying booth or for the treatment thereof.

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT P/00/0011 Regulation 3.2 6568

ORIGINAL

Name of Applicant: HERBERTS GESELLSCHAFT MIT BESCHRANKTER

HAFTUNG

o o

D

Actual Inventor(s): Dietrich SAATWEBER, Bernhard RICHTER, KRUMM and Dirk HOLFTER Waltraud Address for service in Australia: r CARTER SMITH BEADLE Qantas House 2 Railway Parade Camberwell Victoria 3124 Australia Attorney Code CD Invention Title: A METHOD OF RECOVERING THE OVERSPRAY FROM AQUEOUS COATING AGENTS USED IN SPRAY BOOTHS The following statement is a full description of this invention, including the best method of performing it known to us: Our Ref: #11786 GN:WB 09-29her -1- I I

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The invention relates to a method of recovering the overspray from aqueous coating agents used in spray booths, the overspray being recycled in the coating agent used in the spray booth.

It is becoming increasingly important, for known environmental reasons, to use water-dilutable systems in place of lacquers diluted with organic solvents. When water-dilutable systems are used, the air is less contaminated, if at all, by emission of solvents. One io general weakness, however, as regards processing of coating agents such as water-dilutable lacquers by spraying, is that there are problems in disposing of the overspray (the lacquer mist). These problems occur particularly in largea ~scale series lacquering operations, as in the car industry.

/5 it is known to purify the exhaust air from spray cabins by washing with water in order to dispose of the overspray.

For example, DE-Al-29 45 523 describes a process in which the overspray from conventional solvent-dilutable lacquers is washed with water, and the water is subjected to 2o ultrafiltration.

S DE-Al-32 27 227 describes a similar system for disposing of the overspray occurring when using water lacquers. The overspray is absorbed on the walls of the spray booth, which are sprinkled with water, and the sprinkling water is conveyed in a circuit and processed in an ultrafilter. It has been shown, however, that the overspray collected in the sprinkling water cannot be re-used in coating agents and therefore has to be dumped. Admittedly, ultrafiltration is I I r 2 applied to waste-water purification and particularly to the removal of undesired low-molecular substances during electrodeposition coating, thus improving the efficiency by recycling (DE-C2-21 58 668, DE-B2- 22 29 677, EP-A1-0 127 685, EP-A1-0 137 877, US-A-3 663 405 and US-A-3 663 406), but problems occur when using ultrafiltration for recycling of overspray. Ultrafiltration of electrodeposition coating lacquers is a method of recycling the permeate and the retentate to the bath material, whereas when overspray from water lacquers is recycled, the different form of application (spraying instead of electrodeposition) and the resulting greater differences in viscosity lead to stability problems, such as the risk of coagulation, sedimentation, phase separation or formation of deposits. In DE-Al-34 28 300, to avoid the stability problems resulting from the method e.cco::ding to DE-A1-32 27 227, it is recommended to use demineralised water for sprinkling the spray booth. It has been shown, however, that even the use of demineralised water as a sprinkling liquid is not sufficient to improve the stability of the absorbed overspray until it is suitable for re-use in the lacquer material. In WO 91/09666 an effort is made to solve the stability problems in the method in DE-A1-32 27 227 by sprinkling the spray booth with water mixed with an anti-coalescence agent, i.e. amines. This involves addition oZ environmentally polluting substances and also the composition of the lacquer is altered by the ~added chemicals.

In addition, when overspray absorbed in water is concentrated by ultrafiltration, there are considerable changes in the aqueous coating agent, because ultrafiltration preferentially results in withdrawal of water-soluble and low-molecular constituents such as solvents or neutralising agents, which are important in stabilising the aqueous lacquer dispersions and their properties as lacquers.

Dirk Saarbach and Georg Schlumpf, in Oberflache JOT 1991, Part 3, pages 18 to 20, "Die Nasslackierung ohne Emissionen oder Sondermll" "Wet lacquering without emission or special refuse"] describe lacquering of office furniture and recycling the water lacquer. A continuous circuit is also described. It is mentioned that the components of the system, i.e. "water-dilutable stoving lacquer", "ultrafiltration technique" and, "spray system", have to be adapted to one another. The addition of substances to the recycling process is also described. However, there is no suggestion as to how the process can be free from stability problems, and also the composition of the original lacquer is altered by the additives.

This process is suitable only for lacquers which do not coagulate or become unstable in other ways which affect their properties as lacquer, when concentrated by ultrafiltration.

The aim of the invention therefore is to provide a method of recovering the overspray from aqueous coating agents so as to enable the overspray to be re-used in the aqueous coating agent and so as to avoid problems of stability and enable high-quality coating agents (which are usually sensitive), such as aqueous car lacquers, to be used on an industrial scale.

It has been found that this problem can be solved by a method of recovering the overspray from aqueous coating agents applied in spray booths, the overspray being collected in an aqueous washing liquid which is continuously supplied to the spray booth in a circuit A, a part of the washing liquid containing the overspray being supplied from the circuit A to an ultrafiltration circuit B, and permeate being withdrawn from the ultrafiltration circuit B and returned to the circuit A to make up the washing liquid, -4characterised in that the washing liquid is the aqueous coating agent used for spraying in the booth, after dilution with water, the liquid in the ultrafiltration circuit B is the aqueous coating agent used for spraying in the booth, optionally diluted with water, and the process is performed continuously, the washing liquid in circuit A having an approximately constant content of solids in the range from 4 20 wt.%, and the liquid in the ultrafiltration circuit B being kept by withdrawal of permeate at an approximately' constant content of solids in the range from 15 wt.% to the content of solids occurring at the spray viscosity of the aqueous coating agent and higher than the content of solids in circuit A, and a part of the liquid in the ultrafiltration circuit B is used as the aqueous coating agent for spraying in the booth, or for preparation thereof.

Preferably, the volumes of liquid in circuit A and in circuit B are kept constant.

The liquid in the ultrafiltration circuit B is the aqueous coating agent used for spraying, either in the state ready for spraying or in a form diluted with water. The i liquid in the ultrafiltration circuit B, therefore, can be supplied to the spray units in the booth either directly or after processing. For the purpose of processing, in a preferred embodiment, a part of the liquid in the ultrafiltration circuit B can be mixed with fresh topping-up concentrate from the aqueous coating agent and 20 optionally with water and/or optionally other coating-agent constituents. A separate 'mixer can optionally be used for mixing. In another preferred embodiment, the topping-up concentrate, optionally with water and/or other coating-agent constituents, can be added in metered manner, optionally via a mixer, to the liquid in the ultrafiltration circuit B, so that the spray units can be directly supplied with the liquid in the ultrafiltration circuit B.

In the method according to the invention, in contrast to the prior art, the washing liquid in circuit A is not water, 29 Novcmber, 1994 I I f which in the prior art must either be completely demineralised or contain chemical additives. According to the invention, the circuit A is started and continued with the aqueous coating agent used for spraying, after dilution with water.

in the method according to the invention, continuous recycling occurs in a number of circuits. In circuit A, which constitutes the booth circulating system, the process is started by using a washing liquid in the form of the aqueous coating agent diluted with water. The coating agent absorbs overspray and is continuously supplied to circuit B, an ultrafiltration circuit. At the same time, circuit A continuously absorbs aqueous permeate from circuit B. By varying the proportion of overspray absorbed to the proportion delivered to circuit B, the content of solids in circuit A can be kept approximately constant at a value of at least 4 preferably at least 7 wt.% and up to 20 wt%.

The ultrafiltration circuit B, in which the aqueous coating agent is diluted or circulated at the spray consistency, is used for an ultrafiltration process in which aqueous permeate is supplied to circuit A, and the overspray concentrated in the ultrafiltration circuit B is discharged for mixing with a topping-up concentrate for the aqueous coating agent. In circuit B, the content of solid in the circulated dilute aqueous coating agent is kept at a constant value between 15 wt.% and the content of solids at the spray viscosity, the concentration in circuit B being always higher than in circuit A. in the method according to the invention, circuits A and B are adapted to one another, by means of the amount of washing fluid withdrawn from A and the amount of retentate and permeate taken from circuit B, so as to obtain the previously-explained approximate constant content of solids in the two circuits.

The percentages by weight given hereinbef ore for circ xit A and circuit B relate in each case to the total weight of the aqueous circulated system.

The topping-up concentrate for the aqueous coating agent is mixed with the circuit liquid B until it has the viscosity for spraying, and is supplied for spraying.

The method according to -the invention, the matching of circuit A and B and the use of dilute aqueous coating agent as the sprinkling liquid in the spray booth eliminates the stability problems arising in the prior art. The method according to the invention, therefore, enables direct re-use to be made of the overspray from aqueous coating agents even when they tend to become unstable or to lose their.

properties as lacquer a change in viscosity) when concentrated by ultrafiltration, as is observed particularly in the case of high-quality lacquers for series lacquering of cars.

The method according to the invention avoids use of an 0* additional quantity of tap water or completely demineralised water for sprinkling the spray-booth walls, as was necessary in tbie prior art and adversely affected the stability of the overzpray when collected. For optimum absorption of the overspray, the booth walls are sprinkled with dilute aqueous coating agent. Demineralised water is used only as a replacement for water withdrawn in the process, e.g. by evaporation.

The advantage of the method according to the invention is that the composition and the equilibrium in the circuits are retained, thus avoiding major changes in the composition of the aqueous coating agent. This prevents interfering factors leading to instability, and practically 1004% of the overspray can be recycled without expensive adjustments and with inexpensive analysis.

in contrast to the known methods, the recycling circuits can be operated practically without additives. This ensures that no subsequently-introduced additives change and possibly impair the properties of the aqueous coating agents. All necessary additives, e.g. anti-f oaminq agents, additives for avoiding surface disturbances, or levelling additives can be incorporated e.g. in the topping-up lacquer concentrate.

The equilibrium can be adjusted in simple manner with conventional apparatus. It can be controlled e.g. via the ratio of the amount discharged to the amount collected in the respective circuits, the area and capacity of the ultrafiltration membranes, the concentration of the toppingup concentrate for aqueous coating agent, and the throughput of the aqueous coating agent in the spray booth. These parameters can easily be determined by the skilled man, by 2 trial and calculation.

The method according to the invention can be 6arried out on normal apparatus, and therefore does not need any special adaptation of existing equipment.

The spray booths used according to the invention are conventional spray booths. They can operate with a conventional air-supply and exhaust-air circuit. The spray booths can e.g. be equipped with at least one wet sprinkling i wall or with a Venturi washing system, as is usual particularly in the motor-vehicle lacquering industry. in the case of wet sprinkling walls, all the walls can be used feor wet sprinkling. If wet sprinkling walls are present, the washing liquid serves as a sprinkling liquid. The washing liquid can e.g. be collected at the bottom of the spray booth and returned to the circuit for repeated sprinkling of the walls or for repeated Venturi washing.

spray booths of this kind are well-known to the skilled man.

They are used for the booth circuit, circuit A.

A conventional ultrafiltration unit is used in circuit B.

The aqueous coating agent in circuit B is circulated by the ultrafiltration unit, e.g. via a heatable or coolable container for the ultrafiltration retentate (also called concentrate in this specification). The ultrafiltration retentate is returned from the container to the ultrafiltration unit. A pre-filter for removing coarse impurities can be connected upstream of the ultrafiltration unit. Aqueous coating agent enriched with overipray and diverted from the circuit A is supplied to the circuit B, e.g. to the container for ultrafiltration concentrate. The aqueous permeate can be returned from the filtration unit directly to circuit A, e.g. to the bottom of the spray booth, which can be trough-shaped. Optionally all or part eo of the aqueous permeate can be conveyed through a hyperfiltration unit, where reverse osmosis occurs, as a result of which low-molecular constituents are withdrawn from the aqueous permeate, and can be returned to circuit B, e.g. to the container for the ultrafiltration concentrate.

The water coming out of the hyperfiltration unit is returned o: to circuit A, e.g. to the base of the spray booth.

Ultrafiltration, reverse osmosis and hyperfiltration are o: known to the skilled man. They can be carried out in conventional units. These filter processes are described in 7 the opening parts of the aforementioned patent applications and patent specifications and also e.g. by Wilhelm R. A.

Auck and Herrmann A. MGller in "Grundoperationen chemischer Verfahrenstechnik" "Basic operations in chemical processing technology"], Verlag Chemie, 1982, pages 153 155, or by Robert Rautenbach and Rainer Albrecht in "Grundlagen det chemischen Technik, Membrantrennverfahren, Ultrafiltration und Umkehrosmose" "Principles of chemical engineering, membrane separation processes, ultrafiltration and reverse osmosis"], Otto Salle Verlag and Verlag Sauerlnder, 1981, and by Thomas D. Brock in "Membrane Filtration", Springer Verlag, 1983.

No high pressures are required for ultrafiltration in the method according to the invention. The pressure must be at least sufficient to drive the water and low-molecular substaice at a measurable speed through the membrane. These pressures are e.g. of the order of 0.7 to 11 bar, preferably about 5 bar.

The pressure for reverse osmosis is, by definition, greater than the osmotic pressure. There is no sharp boundary between ultrafiltration and reverse osmosis or hyperfiltration. In the method according to the invention, "reverse osmosis" or "hyperfiltration" means e.g. filtration at which low-molecular components of the permeate are substantially completely separated from water.

Since hyperfiltration units can be used in the method according to the invention, there is the additional advantage of being able to recycle water-soluble components which partly or completely enter the permeate during ultr-,ltration, e.g. water-soluble synthetic resins such as polyv.nyl alcohol, special water-soluble melamine resins, e.g. Cymel 327, or the water-soluble component of organic solvents or low-molecular compounds as described e.g. in the report by Dietrich Saatweber in "Untersuchungen zum Einfluss der Ultrafiltration auf die Eigenschaften von Elektrotauchlacken" "The effect of ultrafiltration on the properties of electrodeposition coating lacquers"] VII FATIPEC KONGRESS, Kongress-Buch pages 467 474. Any changes in the proportions by weight of components which do not enter the permeate and components which enter the permeate, e.g. resulting from accidental loss of ultrafiltrate, can optionally be made up by suitably formulating the toppingconcentrate for the aqueous coating agents. Alternatively, as previously described, the components can be withdrawn from the permeate by reverse osmosis or hyperfiltration and can flow directly back into circuit B. If desired, electrolytes which may have been brought in during the process, can be removed from the permeate with ion exchangers by usual methods.

The accompanying drawing is a flow chart illustrating the process according to the invention, or the device used according to the invention in the case of an example, the example constituting a preferred embodiment of the method atcording to the invention.

The drawing shows a conventional spray booth 1 supplied from above, through a duct 2 with air which can be introduced into an exhaust air duct 5 with baffles 4 at the bottom 3 of the spray cabin.

The overspray 7 (spray mist) formed during spraying with the spray unit 6 is collected on a wet sprinkling wall 8. The sprinkling liquid flowing down the wall 8 is collecte. iiA the trough-like bottom 3 of the spray booth and returned thence through a line 9, a pump 10 and an overflow 11 to the top end of the wet sprinkling wall a.

The booth circuit, circuit A, is thus formed b y the wet sprinkling wall 8, the bottom 3 of the spray booth, the line 9 and pump 10, and the overflow 11. The sprinlkling liquid is the aqueous coating agent used in the spray unit 6 in the booth and diluted with water and kept at a solids 'content having an approximately constant value in the range from 4 to 20 wt.%.

A line 12 branches off the line 9 shown in the drawing and leads via a control device 13 to a container 14 for SI r t

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receiving the ultr.filtration retentate. The container 14 can have cooling devices 15 and heatin ''evices 16 for adjusting the desired operating temper,..ures. The sprinkling liquid from line 12, enriched with overspray, is mixed with the circulating liquid in container 14.

The ultrafiltration retentate from container 14, together with the liquid introduced from circuit A, is conveyed through line 17, a pump 18 and an optionally connected prefilter 19 to an ultrafiltration unit 20, from which permeate is continuously withdrawn. The retentate is returned through line 21 to the container 14.

Usually the output of permeate during ultrafiltration is e.g. below 1% of the volume flowing through the filter, so that the retentate has practically the same composition as the circulating liquid flowing into the system, particularly when the volume of the container 14 is very large compared with the volume flowing through the ultrafiltration unit.

By means of this effect, the overspray from circuit A can be concentrated by the method according to the invention without substantially altering the composition and the content of solids in circuit B.

4 According to the invention, an ultrafiltration circuit, circuit B, is made up of the container 14, the line 17 with S the pump 18 and the optionally connected pre-filter 19, the ultrafiltration unit 20 and the line 21. In the method according to the invention, the circuit B in combination with the control system 13, which regulates the supply of overspray-enriched sprinkling liquid from circuit A, is adjusted so that the liquid in circuit B is kept at a solids content at an approximately constant value in the range between 15 wt.% and the content at which the coating agent has the viscosity for spraying.

12 Permeate obtained from the ultrafiltration unit 20 is withdrawn through line 22 and returned to the circuit A via the base 3 of the spray booth 1.

A line 23 can be connected to and branch off from the line 22 and leads to a hyperfiltration unit 24, in which dissolved or low-molecular components are separated by reverse osmosis from the aqueous permeate and returned through a line 25 to container 14. After being separated from dissolved or low-molecular constituents, the permeate from the hyper-filtration unit 24 can be returned through line 26 to the base 3 of the spray booth 1 and thence to the circuit A.

A line 27 branches from and can be connected to the line 21, which returns ultrafiltration retentate from the ultra- S" filtration unit 20 to the container 14. Line 27 conveys a part of the concentrate from the ultrafiltration unit to a topping-up device 28. Device 28 is supplied through a line 29 with topping-up concentrate which is diluted by mixina with the ultrafiltration retentate to obtain the aqueous coating agent, which can be used for spraying in the spray unit 6 in booth 1. The topping-up device 28 can thus be connected to the spray unit 6 via a line 30, optionally via a storage container 31. Other constituents can be added to the mixture in the topping-up device 28, e.g. water or volatile constituents which were withdrawn from circuit A or B during the process.

The method according to the invention can be worked with water-dilutable coating agents which are commercially available and familiar to the s!:illed man, e.g. waterdilutable lacquers as used for series lacquering of cars or motor vehicles. The method can also be applied to waterdilutable coating agents having a high content of solids, e.g. water-dilutable fillers, likewise used in series 13 lacquering of cars.

The method according to the invention is preferably applied to aqueous coating agents having a solids content of 15 to preferably equal or greater to the solids content of the liquid in the ultrafiltration circuit B.

The solids content of the aqueous coating agent can be determined in the individual circuits, e.g. to DIN 53216.

Preferably, a weighed portion of the sample to be determined is placed on a flat-bottomed metal dish and first heated to about 950C for half an hour. This evaporates part of the water, so that the results cannot be distorted by spraying.

The water-dilutable coating agents, e.g. water-dilutable lacquers and fillers, can optionally contain pigments and/or extender pigments, or can be formulated as clear lacquers.

They can contain organic solvents or be free from organic solvents.

As mentioned, the water-dilutable coating agents used in the method according to the invention can be transparent or pigmented. They can optionally contain extender pigments, additives, coalescence agents, volatile organic liquids or other conventional raw materials for lacquers. They can dry by physical or chemical means. The systems can be self-crosslinking or they can contain external crosslinking agents e.g.

for stoving lacquers.

The water-dilutable lacquers suitable for the method according to the invention are e.g. aqueous dispersions, e.g. systems comprising finely divided polymers or synthetic resins in water and based on homopolymers or copolymers of styrenevinyl chloride, vinyl acetate, maleic anhydride, maleic acid ester, maleic acid semi-ester, vinyl propionate, (meth)acrylic acid and esters thereof, amides and nitriles. Examples of these coating agents can be found in the Lehrbuch der Lacke und Beschichtungen Manual of I j lacquers and coatings], published by Dr. Hans Kittel, Volume 1, Part 3, Verlag W. A. Colomb in der H. Heenemann GmbH, 1974, pages 920 to 1001 and in Lackkunstharze Synthetic lacquer resins] by Hans Wagner and Hans Friedrich Sarx, Carl Hanser Verlag, Munich, 1971, pages 207 to 242, and in A Manual of Resins for Surface Coatings, SITA Technology, London, 1987, Volume II, pages 249 to 296.

Water-dilutable lacquers constituting emulsions of polymers or plastics are other examples. They can e.g. contain binders, dissolved in solvent or solvent-free, optionally with cross-linking agents and emulsified in water.

Emulsions with particularly finely distributed binder droplets are called micro-emulsions. The large group of water-dilutable lacquers produced from "water-soluble" polymers or synthetic resins, which are further examples of the method according to the invention, contain binders which S" bear acid or basic groups, e.g. carboxylic acid groups, carboxylic anhydride groups, sulphonic acid groups, primary, secondary or tertiary amino groups, sulphonium groups or phosphonium groups. The polymers or synthetic resins are converted into a water-dilutable form by neutralisation or partial neutralisation of the groups with basic compounds, e.g. with amine, alkanolamine, ammonia, sodium hydroxide or potassium hydroxide, or with acid compounds, e.g. formic acid, acetic acid, lactic acid, alkyl phosphoric acid or carbon dioxide. The binder basis consists e.g. of one or more binders in the group consisting of polyurethane resins, polyester resins, poly(meth)acrylate resins, epoxy resins, epoxy resin esters, fatty oils (e.g.

linseed oil) and synthetic oils polybutadiene oil).

Examples of "water-soluble" lacquers are given in Lehrbuch der Lacke und Beschichtungen, published by Dr. Hans Kittel, Volume I, Part 3, Verlag W. A. Colomb in der H. Heenemann GmbH, 1974, pages 879 919 and A Manual of Resins for Surface Coatings, SITA Technology London, 1987, Volume III, pages 169 280 and in EP-A-0 032 554, EP-A-0 270 795 and EP-A-0 309 901.

Mixtures of aqueous dispersions with "water-soluble" polymers or synthetic resins, known as hybrid systems, are also suitable for the method according to the invention.

The cross-linking agents for the aqueous coating agents can e.g. be urea resins, triazine resins (such as melamine resin or benzoguanamine resins), phenolic resins, blocked polyisocyanates (such as blocked diisocyanates, triisocyanates, isocyanurates, diurets or prepolymers of isocyanates) and mixtures of various cross-linking agents.

The method according to the invention is particularly suitable for water-dilutable lacquers described e.g. in DE-A-36 28 124, DE-A-36 28 125, DE-A-37 39 332, DE-A-38 05 629, DE-A-38 38 179, EP-A-0 038 127, S"EP-A-0 089 497, EP-A-0 123 939, EP-A-0 158 099, EP-A-0 226 171, EP-A-0 238 037, EP-A-0 234 361.

EP-A-0 298 148, EP-A-0 287 144, EP-A-0 300 612, EP-A-0 315 702, EP-A-0 346 886, EP-A-0 399 427, US-A-4 822 685, US-A-4 794 147 and WO 87/05305, and used in the form of sprays (manual, automatic or electrostatic) for series lacquering of cars, and the method is particularly preferred for water-dilutable fillers (hydrofillers) described in e.g. EP-A-0 015 035, EP-A-0 269 828, EP-A-0 272 524 or WO 89/10387 and water-clear lacquers for series lacquering of cars, described e.g. DE-A-0 266 152.

The method according to the invention can also be preferably applied to water-dilutable anti-gravel lacquers, as described e.g. in DE-A-38 05 629. These anti-gravel lacquers can be applied directly to metal substrates, or to metal substrates initially primed with usual primer media, e.g. electrodeposition coating lacquer, optionally covered with a filler. They improve the gravel resistance of multi-layer lacquers.

The invention provides a method of substantially complete re-use, without difficulty, of overspray produced during spraying of aqueous coating agents. The method according to the invention is particularly suitable in connection with manual, automatic or electrostatic spraying, as carried out particularly in series lacquering of cars. It can therefore be applied e.g. to water-dilutable lacquers for series lacquering of cars, e.g. pigment-containing single-colour lacquers or lacquers containing decorative pigments, e.g.

metallic lacquers, and is preferably applied to waterdilutable fillers (hydrofillers) and to water-dilutable clear lacquers for series lacquering of cars. It enables S. practically 100% of the overspray to be re-used, thus protecting the environment. It is surprising that stable re-usable lacquer materials are obtained without the need for subsequent additives.

The following example illustrates the invention.

Example A hydrofiller (aqueous filler) with a solids content of I 50 wt.% was used for spraying in a booth 1 in a device as described in the preferred embodiment in the accompanying drawing. The same hydrofiller was used for wet sprinkling S of the booth, i.e. in the booth circuit A, and was diluted with demineralised water to a solids content of 10 wt.%.

During each working day, the booth circuit absorbed about 1% of its volume of overspray of the sprayed hydrofiller, i.e.

about 20% of the volume of sprayed hydrofiller.

The ultrafiltration circuit B was filled with 40 wt.% dilute hydrofiller. Its volume was about 25% of the volume of circuit A.

During each working day, about 6.5% of the volume of circuit A was continuously transferred to circuit B. The liquid from circuit B was continuously conveyed through an ultrafiltration plant 20 suitable for the lacquered material. During each working day, about 20% of the volume of ultrafiltrate was withdrawn from circuit B and supplied to circuit A through line 22.

In the topping-up device 28, three parts of an approximately 53% hydrofiller topping-up concentrate were continuously mixed with 1 part of the material from circuit B and supplied through a filter to the spray unit 6 in booth 1, either directly or via a storage container 31.

The hydrofiller topping-up concentrate was adjusted so that when mixed with the material from circuit B, the hydrofiller was ready for spraying. To this end, the topping-up .9 concentrate also contained the volatile organic constituents which had been lost in circuit A during the separation of booth air and circulating liquid. The amounts to be added were calculated by analysis of the retentate obtained in circuit B.

0soo Water losses resulting e.g. from evaporation can be compensated by direct addition of demineralised water to circuit A.

*9*o In the preceding example, the solids content was determined to DIN 53216. Time/temperature combination: half an hour at plus 1 hour at 1250C.

The claims form part of the disclosure of this specification.

Claims (4)

1. A method of recovering the overspray from aqueous coating agents applied in spray booths, the overspray being collected in an aqueous washing liquid which is continuously supplied to the spray booth in a circuit a part of the washing liquid containing the overspray being supplied from the circuit to an ultrafiltration circuit and permeate being withdrawn from the ultrafiltration circuit and returned to the circuit to make up the washing liquid, characterised in that the washing liquid is the aqueous coating agent used for spraying in the booth, after dilution with water, the liquid in the ultrafiltration circuit is the aqueous coating agent used for spraying in the booth, optionally diluted with water, and the process is performed "continuously, the washing liquid in circuit having an approximately constant content of solids in the range from 4 20 and the liquid in the ultrafiltration circuit being kept by withdrawal of permeate at an approximately constant content of solids in the range from 15 to the content of solids occurring at the spray viscosity of the aqueous coating agent and higher than the content of solids in circuit and a part of the liquid in the ultiafiltration o*o* circuit is used as the aqueous coating agent for spraying in the booth, or for preparation thereof.

2. A method according to claim 1, characterised in that the volumes of liquid in circuit and in circuit (B) are kept constant.

3. A method according to claim I or 2, characterised by use of an aqueous coating agent having a content of solids of 15 to 80 the content being equal to or greater than the content of solids in the liquid in the ultrafiltration circuit

19- 4. A method according to claim 1, 2 or 3, characterised in that a part of the liquid in the ultrafiltration circuit B is mixed with fresh topping-up concentrate from the aqueous coating agent to form the aqueous coating agent which is supplied to the booth for spraying. 5. A method according to claim 1, 2 or 3, characterised in that the circulating liquid used in the ultrafiltration circuit B, is the coating agent ready for spraying. 6. A method according to claim 5, characterised in that the topping-up concentrate from the aqueous coating agent, optionally with water and/or other coating-agent constituents, optionally using mixing devices, is supplied in metered amounts to the ultrafiltration circuit B and the coating agent required for spraying is withdrawn from the ultrafiltration circuit B. 0 7. A method according to any of the preceding claims, characterised in that iaqueous pigmented or transparent lacquers are used therein. 8. A method according to any of claims 1 to 6, characterised in that aqueous 00: 15 fillers are used therein. 9000 **0000 00 9. A method according to any of the preceding claims, characterised by use of 0 0o aqueous coating agents as used for series lacquering of cars or motor vehicles. A method according to claim 4 or claim 6 wherein the part of the liquid in the ultrafiltration circuit B is mixed with fresh topping-up concentrate from the aqueous coating agent and with water rnd/or other coating-agent constituents. 11. A method according to any one of claims 4, 6 or 10 wherein the part of the liquid in the ultrafiltration circuit B mixed with the fresh topping-up concentrate or the fresh topping-up concentrated and water and/or the other coating agent 29 November, 1994 constituents is mixed using mixing devices. 12. A method of recovering the overspray from aqueous coating agents applied in spray booths substantially as hereinbefore described. DATED this 29 November, 1994 CARTER SMITH BEADLE Fellows Institute of Patent Attorneys of Australia Patent Attorneys for the Applicant: HERBERTS GESELLSCHAFT MBH *5S*0 S S *o *o**oo *o 29 November, 1994 S21 ABSTRACT A method of recovering the overspray from aqueous coating agents applied in spray booths, the overspray being collected in an aqueous washing liquid which is continuously supplied to the spray booth in a circuit, a part (12) of the washing liquid containing the overspray being supplied from the circuit to an ultrafiltration circuit, and permeate (22) being withdrawn from the ultrafiltration circuit and returned to the circuit to make up the washing liquid, wherein the washing liquid is the aqueous coating agent used for spraying in the booth, after dilution with water, the liquid in the ultrafiltration circuit is the aqueous coating agent used for spraying in the booth, optionally diluted with water, and the process is performed continuously, the washing liquid in circuit having an approximately constant content of solids in the range from 4 20 and the liquid in the ultrafiltration circuit being kept by withdrawal of permeate at an approximately constant content of solids in the range from wc.% to the content of solids occurring at the spray viscosity of the aqueous coating agent and higher than the content of solids in circuit, and a part of the liquid in the ultra-filtration circuit is used as the aqueous coating agent for spraying in the booth, or for preparation thereof. 4 6 e* *00 QBO