CA2079693A1

CA2079693A1 - A method of recovering the oversray from aqueous coating agents used in spray booths

Info

Publication number: CA2079693A1
Application number: CA002079693A
Authority: CA
Inventors: Dietrich Saatweber; Bernhard Richter; Waltraud Krumm; Dirk Holfter
Original assignee: Individual
Current assignee: Axalta Coating Systems Germany GmbH and Co KG
Priority date: 1991-10-05
Filing date: 1992-10-02
Publication date: 1993-04-06
Also published as: CZ302192A3; AU656810B2; CZ284487B6; DE4133130A1; SK279689B6; ATE134535T1; KR930007516A; SK302192A3; JP3378596B2; DE59205463D1; AU2612492A; EP0536648A1; ES2086605T3; EP0536648B1; JPH05228422A

Abstract

Herberts Gesellschaft mit beschrankter Haftung A b s t r a c t 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containing the overspray being supplied from the circuit (A) to an ultrafiltration circuit (B), and permeate being withdrawn from the ultrafiltration circuit (B) and returned to the circuit (A) to make up the washing liquid, wherein the washing liquid is the aqueous coating agent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 (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 ultra-filtration circuit (B) is used as the aqueous coating agent for spraying in the booth, or for preparation thereof.

TOTAL P. 21

Description

` 3 2~79~3 Herber~s Gesallschaft mit beschrankter Kaftung A ~t~od o~ rec~v~ri~ the ovQrspr~y rom aqueous coati~
~e3~ u~ed_in s~ray booth~

The invention relates to a method of xecovering the overspray ~rom aqueous coating agents used in s~ra~ booths, the o~erspray being recycle~ in the coating agent used in the s~ray booth.

It is ~ecoming increa~ingly important, or known environmental reasons, to use water~dilUtable systems in place ~f lac~uers diluted with organic solvents. When ~ater-dilutable sys~ems are used, the aiX is less contaminated, if at all, by emission o~ sol-~ents. One general weakness, howe~er, as ~egaxds processing of coating agents such ~s water-dilu~able lacquers by spraying, is that there are pro~lems in disposing of the overspray (the lac~uer mist). These problems occUr pa~ticularly in large-scale seri~s lacquering operations, as in the car industry.

It is known to purify the exhaust air ~rom spray c~bins by washing wi~h water in order ~o dispose of the overspra~.
For example, DE-A~-29 45 523 desc~ibes a process in which ~he o~exsp~ay fxom conventional solvent-dilutable lacquers is washe~ with water, and the ~ater is subjected to ultrafiltration.

~E-A1-32 27 227 describes a similar system for disposing of the ove~spray occurring when using wa~er lacquers. The ove~sp~ay is absorbed on ~he walls o~ t~e spray booth, which are sprin~led with water, and the sp~inkling water is conveyed in a circuit and processed in an ultra~ilter. It has been shown, ho~e~er, that t~e ove~pray collected in ~he sprinkling water cannot be re-used in co~ing agents and therefore has to be dumped. ~dmittedly, ultrafi}~ratio~ is 2~79693 applied to waste-water puriCication and particularly to the removal of ~ndesired lo~-molecular substances during elec~rodeposi~ion coating, thus improving the e~icienc~ by recycling (D~-C2-21 58 668, D~-~2- ~2 2~ 6?7, EP-Al-o 127 685, EP-Al-0 137 877, US-A-3 663 405 and US-A-3 663 405), but problems occur when usi~g ultra~iltration for recycling o overspray. Ultra~iltratlon of electro~eposition coa~ing lacquers is a me~hod of recycling the pe~meate and the retentate to the bath material, whereas when overspra~ from water l~cquers is re~ycled, the di~er~nt form o~
~ppli~ation ~spraying instead o~ electrodeposition) and the re~ulting greater differQnces in ~iscosity lead ~o stability pro~lems, such as the risX of coagulation, sedimentation, phase separation ox formation of deposits. In 3E-Al-34 28 300, to a~oid the stability proble~s resulting from the method accordin5 to DE~ 32 27 227, it is reco~mended to use demineralised water for sprinXling the spra~ booth, I~
has been shown, however, that even the use o~ demineralised water as a sprin~ g li~uid is not su~ficient to improve the stability o~ the absorbed overspray until it is suitable for re-use in the lacquer material. In W0 91/09666 an efort is made to solve the stability pro~lems in the methcd in DE-Al-32 27 2Z7 by sprinkling ~he spray booth with ~a~er mixed with an anti-coalescence agent, i.e. amin~s. Thi5 involves addition of environmentally polluting substances and also the composition o the lac~uer is altered by t~e a~ded c~emicals.

In addi~ion, when vverspray absorbed in ~ater is concentrate~ by ultra~ ration, there are considerable ~hanges in the aqueous coating agent, because ult~afiltration pre~erentially results in withdraWal o wa er-soluble and low-molecular constituen~s such as solvents or neutralising agents, which are impo~tant in s~abilising the a~ueous lao~uer dis~ersions and their properties as lacquers.

$ ~ ~

~irk Saarbach and Georg Schlumpf, in Ober~lache + JO~ 19gl, Part 3, pages 18 ~o 20, "~ie NasslackierUng ohne ~missiQnen oder Sondermull" ~ Wet lacquering without emission or special ~e~use"] describe lac~uering of office ~urni~ure and recycling the water lacquer. A continuous circui~ is ~lso described. It is mentioned that the compo~ents of the sy~tem, i.e~ "wa~er-dilutable stoving lacquer", ~'ultra~iltration techni~ue" and "spra~ s~stem", ha~e to be adapted to one another. The addition o~ substances to the recycling process is also de~cribed. However, therP i~ no suggestion as to how the process ca~ be free from stabillty problems, and also the composition of ~he original lacquer is altered by the addi~i~es.

This process is suitable onl~ ~o~ lacquers which do not coagulate or become unstable in other ~ays which a fe~t theix properties as lac~uer, when concentrated by ultrafiltratio~.

The aim of ~he invention there~o~e is to provide a method o~
recovexing the overspray ~rom aqueous coating agents so as to ena~le the oversp~a~ to be re-used in the aqueous coating agent and so as to avoid problems of stability and e~able high-qualit~ coa~ing age~t~ (w~ are usua~ly sensitive), such as a~ueou~ ca~ lac~uers, to be used on an industrial scale.

It has bee~ ound that this problem can be solved by ~
method of recovering the overspray ~rom aqueous coa~ing agents applied in sp~ay booths, the overspray being collected in an ~queous washing liquid which is continuously supplied to the spray booth in a circuit A, a part of the w~shing liquid containing the overspray being supplied fro~
the ci~uit A to an ultra~iltration cixcuit B, and permeate being withdrawn from the ult~afiltxation circuit ~ and returned ~o the cirouit A to make up the washing liquid, 2~7~93 characterised in that the washing liquid is the aqueous coatin~ agent used or spraying in the booth, after dilution with water, the li~uid in the ultra~iltration circuit B is the aqueous coating agent used for spraying in the booth, optionally diluted with water, and the process is performed conti~uously, the washing liquid in circ~it A having an approximately constant co~ten~ o~ solids i~ the range f~om 4 - 20 Wt.~, and the liquid in ~he ultrafiltration circuit B
being kept by withdxawal of pe~meate a~ an approximately constant content o~ solids in the range ~rom 15 wt.~ to ~he contént of ~olids oc~urring at the spray viscosit~ of the aqueous coating agent and higher than the content of solids in circuit A, and a pa~t of the li~uid in the ultra-filtration circuit B is used as the aqueous coating agent for 5praying in the boo~h, or for preparation thereof~

The li~uid i~ the ul~rafiltration circuit B is the a~ueous coating agent used or spraying, either in t~e state ready for spraying or in a form diluted wi~h water~ ~he liquid in ~he ultra~iltration circuit B, therefore, can be su~plied to the spray units in the booth either directly or after processing. For the purpose of processing, in a preferred embodiment, a ~a~ o~ ~he liquid in the ultraCiltration circuit B can be mixed with fresh topping-Up ~oncentrate fro~ the aqueous coating agent and optionall~
with w~ter and/ar cptionally other coating agen~
constituen~s. A separate mixer can optionally ~e used for mixing. In another preferred embodiment, the topping-up concen~rate, optionally ~ith wa~er and/o~ o~her coating-agent constituents, can be added in metered manner, optionally via a mixer, ~o the liquid in the ultrafiltration circui~ B, s~ that the spray units can be directly supplied with t~e liquid in the ultrafiltration circuit B.

In the method according to the invention, in cont~aS~ to the prior art, the washing liquid in circuit A is not water, ~7~93 which in the prior art must either be completely demineralised or contain chemical additives. According to the inve~ior~ e ci~cUi~ .~ is sta~ted and continued with the aqueous coatins agent used for spraying, aftex dilution with water.

In the method according to the in~ention, contin~ous recycling occurs in a number o~ ci~cuits. In ~i~cuit A, which constitutes the booth circulatin~ sy~tem, the process i~ startéd b~ using a washing li~uid in t~e form o~ the aqueous ~oating agent diluted with water. The coatin~ a~ent ab~or~s overspra~ and is con~inuously supplied to circuit B, an ultraf1ltration circuit~ ~t the same time, circuit A
continuo~sly a~sor~s aqueous permeate from circuit B. B~
varyin~ the pxopor~ion of overspxa~ a~so~ed to the propo~tion delivered to circuit 3, thç content of ~olid~ in circult A can be ~ept approximately co~tant at a value of at least 4 wt.%, pxeferably at least 7 wt.% and up to 20 wt.~.

The ~ltraf iltration circuit B, in which the aqueous coating agent is dilu~ed o~ circulated at the spxay consistency, is used for an ultrafiltration process in which a~ueaus perm~ate is ~upplied to circuit A~ and the ovexsp~ay concentrated in the ul~rafiltration ~ixcuit B is discha~ged for mixing with a topping-up concentra~e for the a~ueous coatin~ agent. In circuit ~, the co~tent o~ solid in ~he circulated dilute aqueous coating ayent is kept at a constant value between lS wt. % and the content o~ solids at the spray viscosity, the concentration in circuit B bein~
always higher ~han in circuit A. In the method according to the invention, circuits ~ an~ ~ are adapted to on another, by means of the amount of washing fluid withdrawn from A and the amount of retenta~e and per~eate taken ~om CirC'lit B, so as to o~tain the pre~iously~explained approximate constant content of solids in the two circui's.

8 2~7~93 The ~ercen~a~es b~ weight given hereinbefore for circuit A
and circuik ~ relate in each case to the ~otal weight of the a~ueous circulated s~stem.

The topping-up concentrate ~or ~he aqueous coatins age~ is mixed with the circuit liquid B until it has the viscosity ~or spraying, and is supplie~ for spra~ving.

The method according to the invention, the matching of circuit A and 3 and the use o~ dilu~e aqueous coating agent as ~he sp~inkling li~id in the spray booth eliminates the stability problems arising in the prior art. The methad according to the invention, ~herefore, ena~les direct re-use to be made of the overspra~ ~rom a~ueous coating agents even when they tend to become unstable or to lose ~heir .
pro~ertie~ as lac~uex (e.g. a chan~e in viscosity) when concentrated by ultra~iltration, as is o~se~ved particularly in the case of high-~uality lac~uexs for series lac~uering of cars.

The method ac~ordin~ to the invention a~oids use of an additional quantity of tap water or completel~ demineralised water for s~rin~ling ~e sp~ay-booth walls, as was necessar~
i~ the p~ior art and adversely a~fected the stability of the overspray when collected. ~o~ optimum absorption of the oversp~ay, the booth walls are sprinkled wi~h dilu~e aqueous coating agent. Demineralised wa~er is used only as a replacement for ~ater wi~hdrawn in the proc~ss, e.g. by eva~ora~ion.

The ~d~antage o~ the me~ho~ accordin~ to the invention is that the composition and the equilibrium in the cixcuits are retained, ~hus avoiding m~jor cha~es in the composition o~
the ~ueous coating agent~ Thi~ prevent5 interfering ~actors leading to instability, and practi~ally 100~ o~ the overspray can be ~ecycled withouk expensive adj ustments and 2~7~693 wi~h inexpensive analysis.

I~ contrast to the known methods, ~e recycling circuits can be operated practically without additives. This ensures that no subsequently-introduced additives change and possi~l~ impair the prope~ies of the a~ueous coating agents. All necessary additives, e.g. anti-foaming agents, additi~e~ ~or a~oiding sur~ace disturbances, o~ levelling addi~ives can be incorporated e.g. in the topping-up lacquer concentrate.

The e~uilibriu~ can be adjus~ed in simple manner with conventional apparatus. It can be controlled e.g. via the ratio of the amount discharged to the amoun~ collected in the respective circuits, the area and capacity of the ultraf~ltration membranes, t~e concentration o the ~opping-up concentrate ~or aqueous coating agent, and the throughput of the aqueous coating a~ent in the spray boot.h. These parameters can easil~ be determlned by the skilled man, by txial and calculation.

The method accoxdin~ to the invention can be carried out on normal apparatus, and ther~or~ does not need any specia~
adaptation o~ existin~ eguipment.

The spray boo~h~ use~ acco~ding to the i~ention are conventional spray booths. They can oper~te with ~
conventio~al air-supply and exhaus~-air circuit. The spray booths can e.g. be e~uipped with at least one wet sprinkling wall or with a ~entu~i washing system, a5 is usual particularly in the motor-vehicle lacquering lndustry. In the case of wet sprin~ling walls, all the walls can be used ~or wet sp~inkling. If wet sprin~ling walls are present, thè washing liquid ser~es as a sprin~ling liquid. The was~ing liquid ca~ e~g~ ~e c~llected at ~he bottom o~ the spra~ booth and ~eturned to the circuit fox repeated 207~3 sprinkling of the walls or for repeated Venturi ~ashing.
spray booths of this ~ind are well-~noWn to the skilled man.
They are used fox the booth circuit, circuit ~.

A conventional ultrafiltration u~it is used in ci~cuit B.
The aqueous coating agent in circuit B is circulated by the ultrafiltration unit, e.~. via a heatable ox coolable container ~ox t~e ultxafiltration xetentate (also called concentrate in this s~eci~ication)~ ~he ultrafiltration retentate is ~eturned from the container to the ~ltra~iltr~tion unit. ~ pre-filter for removlng coaxse impuxi~ies can be connected upstream of the ultrafiltration unit. Aque~us coatins agent enriched wi~h o~erspray and diverted from the circuit A i5 supplied to the cixcuit ~, e.g. to the container for ultrafiltration concentrate. The a~ueous pexmeate can be returned fro~ the filtration unit directly to circuit ~, e.g. to ~he ~ottom o~ the spray booth, which can be trough-shaped. optionally all or part ~ he aqueous perme~te can be con~eyed ~-hrough a hypex~iltxation unit, whexe reverse osmosis occurs, a~ a result of which low-molecular constituents are withdrawn from the aqueous permeate, and can be returned to circuit B, e.~. to the container for the ultra~iltration concent~ate.
The water coming out o~ the hyperfil~ration unit is xeturned to cixcuit ~, e.g. to the base of t~e spra~ booth.

Ultrafiltration, xevexse osmosis a~d hyperfiltration are known to the s~illed man. They can be carried out in conventional units. These filter processes are descxibed in the opening parts of the a~orementioned pat~nt applications and paten~ s~e~ifications and also e.~. by ~ilhelm R. A.
Auck and ~errmann A. M~ller in "Grundoperationen chemischer Verfahre~stechnik" ~= "Basic operations in chemical processing t2chnology"J, Verlag Chemie, 1982, pages 153 -155, or by Robext Rau~enbach and Rainer Albrecht in "Grundlagen de~ chemischen Technik, ~embrantrennverfahren, 2~79693 Ul~rafiltra~ion und U~kehrosmose" [= "Principles o~ chemicai engineering, membrane separatio~ processes, ultraflltration and re~erse osmo~is"~, Otto Salle Verlag and Verlag S~uerl~nder, t 981, and by Thomas D. ~xock in ~'Membrane ~iltration", Springer Verlag, lg~3.

No high pressures axe ~e~uired for ultra~iltration in the method according to the invention. The pressure must be a~
le~st suf~i~ien~ ~o drive the water and low-molecular substance at a measurable speed through the membrane. These pressures are e.g. of th~ orde~ of 0.7 to 11 bar, prefexably about 5 bar.

The pressure for xevexse osmosis is, by defini~ion, greater than the osmotic pressure. There ls no sharp boundaxy ~etween ultra~iltration and reverse osmosis or hyper~iltration. Irl the me~od a~coxdin~ to the invention, "revexse osmosisl' or "hyper~iltration" means e.g. filtration at which low-molecular components of the permeate are ~ubstantially completely separated ~rom wa~e~.

Since hyperfiltration units can be used in the method acc~rding to the in~ention, there is the additional advantage o~ being a~le to rec~cle water-soluble components which partly or completely enter the p~rmeate during ul~afiltration, e.g. water-soluble synthetic resins ~uc~ as polyvin~l alcohol, special water-soluble melamine xesins, e.g~ Cymel 327, ~r the water-soluble component o~ org~ni~
solvents or low-molecular compounds as de~c~ibed e.g. in the report by Die~rich Saa~weber in "Untersuchungen zum Einfl~ss der Ultrailtration auf die ~igenscha~ten von ~lektrotauchlac~en" ~= "The e~ect of ultrafiltration on ~he pro~erties of elect~odeposi~ion coatinq lacquers"~ VII
FATIP~C KONGR~SS, Kongress-3uch pages 467 - 474. ~ny changes in the proportions by weigh~ of co~ponents which do not enter the permeate and components which enter the ~7~3 permeate, e.g. resulting from accidental loss of ultrafiltrate, can optionally be made up by suitably formulating the topping-up concentrate for the aqueous coating agents. Alternatively, as previously described, the components can be withdrawn from the permeate by reverse osmosis or hype~filtration 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 according 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 collected in 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 8.

The booth circuit, circuit A, is thus formed by the wet sprinkling wall 8, the bottom 3 of the spray booth, th~ line 9 and pump 10, and the overflow 11. The sprinkling 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 evice 13 to a container 14 for recei~ing the u7tra~iltration reten~ate. The container '4 c~n have cooling devices 15 and heating devices 16 for adjusting the desired operating temperatures. The sprinkli~g liquid ~rom line 12, enriched with overspray, is mixed with the cixculating liquid i~ container 14.

The ul~xafiltration retentate f~om container 1~, together ~ith the liquid introduced from circuit A, is conveyed through line 17, a pump 18 and an optionally connected ~re-filtex 19 to an ultra~iltration unit 20, ~rom which permeate is ~ontinuously withdrawn. The rete~tate is returned t~rough line 21 to the container 14.

Usually the outpu~ of permeate during ult~afiltration is e.g. below 1~ of the volume flowing ~hrough the filter, so that the ~etentate has practicall~ ~he same composition as the circulating li~uid flowing into the system, pa~ticularly when the volume of the containex 14 is very lar~e compared with the volu~e flowing thxough the ultrafiltration uni~.
By means ~f this ee~l the overspray from circuit A can be concentrated b~ t~e method according ~o the invention without substantially altering the composition and the content of solids in circui~ 3.

Ac~ording to the invention, an ultra~iltration circuit, circuit B, is made up o~ the container 14, the line ~7 ~ith the pump 18 and the optionally connected pre-filter 19, the ultrafiltration unit 20 and the line 21. In the method acco~din~ to the in~ention, ~he ~ircuit ~ in combina~ion with the control system 13, which xegulates the suppl~ of oYerspxa~-en~iched sprinkling li~uid from circuit A, is adjusted ~o that the li~uid in circuit B is kept at a solids content a~ an approxi~ately cons~ant ~alue in ~he range between lS wt.~ and th~ cont~nt a~ which t~e coating agent has the viscosity for spraying.

2~7~3 Permeate obtained fxom the ultra il~ration unit 20 is withdrawn through line 2~ and returned to the circui~ A via the base 3 of the spray booth 1.

line 23 can be connected to and bxan~ of~ from the line 22 and leads ~o a hyperfiltration unit 24, in which di~solved or lo~r-molecular components are separated by xeverse osmosis ~rom the aqueous permeate and returned through a line 25 to container 14. After ~eing separated from dissolved or low-molecula~ consti~uents, the pexmeate ~rom the hyper-~iltration unit 24 can be returned through lin~ 26 to the base 3 of the spray booth 1 and thence ~o ~e c~rcuit ~.

line 27 branches ~rom and can be connected to t~e line 21, which returns ultra~iltration retent2te ~rom the ult~a-iltration unit 20 to the container 14. Line 2/ conveys a part of the concentrate ~rom the ultrafiltration unit to a topping-up de~ice 28. Device 28 is supplied through a line 29 with topping up con~2~t~ate which is diluted by mixin~
wi~h the ultrafilt~a~ion retentate to obtain the a~ueous coa~in~ agent, which can be used ~or spra~ing in the spray unit 6 i~ ~ooth 1. The topping-up device 2~ can thus be connected to the spray unit 6 via a line 30, optio~ally via a st~rage ~ontai~er 31, Other constituents can be added ~o the mixtu~e in the topping-up device 2~, e.g. water or volatile constituen.s which were withdrawn from circuit ~ or B during ~he process.

The method acco~ing to the invention can be worked ~ith water-dilutable coating agents which are comme~cially available and ~amiliar to the skilled man, e.g. wa~er-diluta~le lac~uers as used for series lacquering o~ cars or motor vehicles. The method can also be applied to w~ter-dilutable coating agent~ having a high conten~ of solids, e.g~ water-dilutable ~illers, likewise used in series ~07~9~

lacquering of cars.

The method according to the invention is preferably applied to aqueous coating agents having a solids content of 15 to 80 wt.%, 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 95C 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-dilutabl2 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-cross-linking 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 ~ater and based on homopolymers or copolymers of styrene, vinyl chloride, vinyl acetate, maleic anhydride, maleic acid ester, maleic acid semi-ester, vinyl propionate, (meth)acrylic acid and esters thereof, amid~s and nitriles. Examples of these coating agents can be found in the Lehrbuch der Lacke und Beschichtungen [= Manual of ' ' ' . ~ ' ' . ~ ' ' ~9~

lac~uers and coatings], ~ublished by Dr. Hans ~itte~, Volume 1, Part 3, Ve~lag W. A. Colomb in der ~. Heenemann Gmb~, 1974, pages ~20 to 1001 and in Lackkunstharze ~= Synthetic lacquer re~ins~ by Hans Wagner and Hans Friedrich Sarx, Carl H~nser Vex~ag, ~unich, 1971, pages 207 to 242, and in A
Manual o~ ~esins ~or Surface Coatings, SIT~ ~echnology, London, 1987, Volume II, ~ages 24g to 29~. .

Water-dilutable lacquers consti~uting emulsions of polymers o~ plastics are other e~amples. The~ can e.g. contain bi~ders, ~issolved in sol~ent or solven~ ee, optionally wlth cross linki~q agents and emulsified in watex.
~mulsions with particularly finely dis~ributed binder droplets are ealled micro-emulsions. The large grou~ of water-dllutable lacquers produced Cro~ "water-soluble"
pol~me~s o~ synthetic resins, which are further examples o~
the method according to the invention, contain binders which bear acid or basic groups, e.g. car~oxyli~ acid groups, carboxylic anhydride groups, sulphonic aci~ ~roups, primary, secondary or ~er~iary amino groups, sulphonium groups or phosphonium groups. The polymers or synt~etic resins are con~erted in~o a water-diluta~le form by neutralis~tion or partial neutralisation o the groups with basic compounds, e.g. wi~h amine, alk~nolamine, ammonia, sodium hydroxide or potassium hydroxide, or with acid compou~ds, e.g. formic acid, acetic acid, lactic acid, al~l phosphoric actd or carbon dioxide. The binder basis consists e.g. o one or more binders in the group consisting of polyurethane resins, polyes~er resins, poly(me~h~acr~late ~esins, epoxy Xesins, epo~y ~esin esters, fatty oils (e.g.
li~seed oil) and synthetic oils (e.g. polybutadiene oil).

~amples ~f "wa~er-soluble~ lacquers are given in ~eh~buch der Lac~e und Beschicht~ngen, published by Dr. Hans Xittel, Volume I, Pax~ 3, Verlag W~ ~. Colomb in de~ ~. Heenemann Gmb~, 1974, pages ~7~ - 919 and A Man~al of Resins or 9 ~

Surface Coating5, SIT~ Technology London, 1987, Volume III, pages 169 - 280 and in ~P-A-0 032 S54, EP-A-0 270 7g5 and ~P-~-0 3~g ~01.

~ixtures o~ aqueous dispersions with "wate~-soluble"
S polymers or s~nthetic ~esins, known as hyb~id systems, are also suitable ~o~ the method according to ~he invention.

~he cross~lin~ing ag~nts ~or the aqueous coating agents can e.g. be urea resins, triazine ~esins (such as mela~ine resin or benzoguanamine resins~, phenolic resins, bloc~ed polyisocyanates (such as blocked diisocyanates, triisocyanates, isocyanurates, diurets or prepolymers of isocyanates) and mix~ures of va~ious cross~ king age~ts.

The method according to the inve~tion is particularly ~u~table fo~ wate~-dilutable lacquers described e.g. in DE-A-36 2~ 124, PE-A-36 28 125, DE~A-37 39 332, D~ 38 05 629, DE-A-3% 38 179, EP-A-0 038 1~7, ~P-~-0 089 497, E~-A-0 123 939, EP-~-0 15~3 Q99, EP-~-0 ~6 171, EP-A-0 238 037, EP-A-0 23~ 361.
EP-A 0 298 148, EP-A` 0 287 1~4, :EP~-0 300 612, ~P-~-0 315 7Q2, EP-A-0 346 886, EP-A-~ 39g 4~7, US~ 22 685, US-A-4 794 147 and W0 87/05305, and used in the form o~ sp~ays ~manual, aU~OmatiC o~ electrostatic~ for series l~c~uering of cars, and thQ method is par~iculaxl~
prefe~ed ~o~ wate~-dilutable fillers (hydxofille~s) describ~d in e.g~ E~-A-0 015 035, ~P-~-0 269 32Z, ~P-~-0 272 524 or W0 89/103~7 and water-clear lac~uers for series lacquering of cars, described e.g. ~E-A-0 266 15~

The method accordi~g ~o the in~ention can also be preferably applied to water-dilutable anti-gravel lacquers, a~
described e.g. in DE-A-38 05 629. These anti-gravel lac~uers can be applied di~ec~ly to metal substrates t or to metal substrates initially primed with usual primer ~7g~3 media, e.g. electrodeposition coating lac~uer, op~ionally covered with ~ filier. They impro~e the gravel resis~ance of multi-layer lac~uers.

The inve~tion provides a method of substantially ~omplete re-use, without di~ficulty, of o~exspray prod~ced during spraying of aqueous coa~ing agents. The method according to the invention is pa~ticularly suitable in connection with manual, automatic or electrostatic spraying, as car~ied out particularl~ in series lacqueri~g o~ cars. It can the~efore be applied e.g. L~o wa~er-dilutable lacquers for se~ies lacquering OL cars, e~g. pigment-containing single-colou~
lacquexs o~ lacquer~ containing deoo~ative pigments, ~.g.
metallic lacquers, and is p~eferably applied to water-dilutable ~illers (h~drofille~s) and to water-dilutable clear lacquers for series lacquering of cars. I~ e~ables practicall~ 100~ of the o~erspray to be re-u~ed, thus protecting ~he environment. It is surprising that stable re-usable lacquer ma~e~ials are o~tained without the need f or subsequent additives.

The ~ollowing example illustrates the invention.

~xample A h~d~o~illex (aqueous ~iller) with a solids conten~ of 50 wt.96 was used for spraying in a booth 1 in a device as de~cribed in the pre~erred embodimen~ in the accomp~nying dxawing. The same hydro~iller w~s used for wet sprinkling of the booth, i.e. in the booth cir~uit A, and was dilu~ed with demineralised water to a sol~ds co~tent of lo w..~.
~ux~ng each working day, the booth circuit absorbed about 1%
o its volume of overspray of the sprayed hydro~iller, i.e.
about ~0~ of ~he volume of sprayed hydrofiller.

The ult~afiltration circui~ 3 was fllled with 40 wt~% dilute . 2~ 93 hydrofille~. Its Yolume was about 25% of the volume of ~ircuit A.

During each wo~king day, abou~ 6.5% o~ the ~olume of circuit ~ was continuo~sly trans~erred to ci~cuit B. The liquid from ci~cuit B was continuo~sl~ conveyed through an ul~rafiltration plant 20 suitable fo~ the lacquered material. During each working day, a~out 20% o~ the volume of ult~a~iltrate was wi~hdrawn f~om circuit B a~d supplied to ci~cuit ~ through line ~2.

In the tvpping-u~ device 28, three par~s o~ an approximately 53~ hydrofiller topping-up concentrate were con~inuousl~
mixed with 1 paxt of the material from circuit ~ and supplied thro~h a ~ilter to the spray unit 6 in booth 1, ei~her directly or via a storage container 3~.

The hydrofiller topping-up co~centrate was adjus'ed so th~t when mixed with the material from circuit B, the h~dro~iller was ready for spraying. To this end, t~e topping-up concentrate also contained the volatile organic constituents which had been lost in circuit A during t~e separation of 20 booth air and cixculatin~ liquid. The amounts to be added we~e calculated b~ analysis o~ the retentate obtained in circuit B.

Water loss~s resulting e.g. from e~aporation can be compensated by direct addition OL demi~exal~sed water to circuit A~

In the pre~eding example, the solids content was determined to DIN 53216. Time/tempe~a~ure combination: half an hour at 95C plus 1 hour at 125C.

Claims

C L A I M S

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), 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, 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 (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 us d 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 (A) and in circuit (B) are kept constant.

3. A method according to claim 1 or 2, characterised by use of an aqueous coating agent having a content of solids of 15 to 80 wt.%, the content being equal to or greater than the content of solids in the liquid in the ultrafiltration circuit (B).

4. A method according to claim 1, 2 or 3, characterised in what a part of the liquid in the ultrafiltration circuit (B) is mixed with fresh topping-up concentrate from the aqueous coating agent and optionally with water and/or other coating-agent constituents, optionally using mixing devices to form the aqueous coating agent which is supplied to one 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).

7. A method according to any of the preceding claims, characterised in that aqueous pigmented or transparent lacquers are used therein.

8. A method according to any of claims 1 to 6, characterised in that aqueous fillers are used therein.

9. A method according to any of the preceding claims, characterised by use of aqueous coating agents as used for series lacquering of cars or motor vehicles.