Process for reconcentrating overspray from one-component coating compositions

The present invention relates to a multi-stage membrane filtration process for reconcentrating the overspray from a one-component, aqueous coating composition containing high molecular weight components having a weight average molecular weight of at least 2000 and at least 5% by weight, based on the total weight of the organic components of the coating composition, of a low molecular weight component having a weight average molecular weight of less than 2000, that has been diluted with spray booth water from spray booths having wet flushing to form a booth/water overspray mixture by PA1 a) preconcentrating the booth water/overspray mixture in an ultrafiltration unit to obtain a first retentate containing high molecular weight components and a first permeant containing water and at least a portion of the low molecular weight components, PA1 b) treating said first permeant and the third permeant obtained in step c) below in a reverse osmosis unit to obtain a second permeant containing essentially pure water for recycle as booth water and a retentate containing low molecular weight components, PA1 c) treating said first and second retentates in a nanofiltration unit to obtain a third permeant containing water and a minor portion of low molecular weight components and a third retentate which largely corresponds in composition and concentration to said one-component, aqueous coating composition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for reconcentrating the 
overspray of one-component, aqueous coating compositions from spray booths 
having wet extraction by a multi-stage membrane process using the 
resulting permeant as booth water and using the resulting retentate as an 
aqueous coating composition or as a constituent of an aqueous coating 
composition. 
2. Description of the Prior Art 
The term "overspray" means those coating components that miss the target 
substrate during spray application of the coating and in the absence of 
particular precautions are lost. 
Increasing environmental problems have prompted the recent development of a 
wide variety of processes aimed at reducing the volume of special waste 
arising from coatings overspray. Conventionally, the overspray diluted by 
the spray booth water is coagulated in collecting basins for disposal. The 
underlying concept in some of the newer processes for water-thinnable 
coatings dispenses with coagulation and, instead, reconcentrates the 
overspray with care for reuse as a coating composition. 
DE-OS 2,353,469 describes reconcentrating the overspray by ultrafiltration. 
The diluted overspray flows past a semipermeable membrane such that the 
low molecular weight substances, in particular water but also low 
molecular weight dissolved binder components and auxiliary substances, 
pass through the membrane (i.e., the permeant or penetrant), while the 
principal components of the coating composition are retained by the 
membrane (i.e., the retentate). 
DE-OS 3,428,300 describes the desirability of exclusively using 
demineralized water as the spray booth water in order to avoid overspray 
coagulation. The same objective is achieved by the process described in 
DE-OS 2,945,523 by the addition of emulsifying agents. 
EP-A-0,141,171 discloses the possibility of continuous ultrafiltration. A 
portion of the mixture of booth water and overspray circulated in the 
spray booth circuit undergoes continuous separation and ultrafiltration. 
The permeate is returned to the booth water. The physical and chemical 
properties of the retentate is checked to determine its suitability for 
reuse as a coating composition. The reconcentrated overspray may be reused 
after, for example, dilution or concentration. 
WO 91/09666 describes the reworking of heat-curable, aqueous alkyd or 
acrylic resins by ultrafiltration in the presence of an aliphatic amine to 
prevent the coating compositions from coalescing and a glycol derivative 
to adjust the ultrafiltration throughput. These additives, however, have 
the disadvantages of affecting the quality of the worked-up coating 
composition and also polluting the exhaust air from the booth. 
The new literature (for example JOT 10 (1992) 32 to 38, JOT 3 (1992) 28 to 
33) discloses that the prior art ultrafiltration leads to serious problems 
with many coating systems, thus making it impractical. These problems may 
be due, for example, to a marked foaming tendency due to low molecular 
weight emulsifying agents or possibly low molecular weight binder 
components which pass into the permeant and accumulate. The lost 
components may be crucial to the coating quality of the recycled material, 
making direct reuse of the retentate as coating composition impossible. 
An object of the present invention is to develop a process which enables 
overspray from water-thinnable coating compositions to be reworked to form 
new coating compositions having substantially the original composition 
and, thus, the original quality. 
It has now surprisingly been found that this object may be achieved with 
the specific multi-stage membrane process described in greater detail 
hereinafter. 
Multi-stage membrane processes are known and are applied, inter alia, in 
the whey, sea water, oil emulsions or latex waste water reworking sectors 
(see, for example, M. Mulder "Basic principles of membrane technology" 
Kluwer Academic Publishers (1991), R. Rautenbach, R. Albrecht 
"Membran-trennverfahren: Ultrafiltration und Umkehrosmose" [Membrane 
Separation Processes: Ultrafiltration and Reverse Osmosis], Otto Salle 
Verlag, Frankfurt am Main (1981) and DE-OS 4,126,483). These references 
describe the many possibilities, including combining ultrafiltration with 
reverse osmosis. The permeant from ultrafiltration, which contains low 
molecular weight components, is reconcentrated by reverse osmosis. The 
retentate from reverse osmosis is fed again into the ultrafiltration feed 
stream. 
EP-A-0,553,684 describes a multi-stage membrane process for reconcentrating 
the overspray from water-dilutable coating compositions in spray booths 
having wet flushing, in which the booth circulation water is 
preconcentrated in a continuous manner in an ultrafiltration unit and the 
permeant is returned as circulation water. Final concentration is 
performed batch-wise in a further ultrafiltration unit. The permeant from 
the final concentration stage may be reconcentrated by a reverse osmosis 
stage downstream. Because there is no provision for returning the 
retentate from reverse osmosis into the ultrafiltration feed, it is not 
possible with this arrangement to recover the overspray as a coating 
having essentially the original composition. 
EP-A 0,567,915 describes the possibility of returning the retentate from 
the reverse osmosis stage either into the preconcentration stage or into 
the final concentration stage. Reconcentration of low molecular weight 
components by reverse osmosis is only practical up to relatively low 
concentrations of at the most 5%, due to osmotic pressure build-up, so 
that redilution occurs as a result of returning the retentate from reverse 
osmosis into the ultrafiltration stages. Because of the poor retention 
performance of ultrafiltration in the case of soluble low molecular weight 
coating components, the coating composition recovered in this process does 
not have virtually the original composition. 
SUMMARY OF THE INVENTION 
The present invention relates to a multi-stage membrane filtration process 
for reconcentrating the overspray from a one-component, aqueous coating 
composition containing high molecular weight components having a weight 
average molecular weight of at least 2000 and at least 5% by weight, based 
on the total weight of the organic components of the coating composition, 
of a low molecular weight component having a weight average molecular 
weight of less than 2000 , that has been diluted with spray booth water 
from spray booths having wet flushing, to form a booth/water overspray 
mixture by 
a) preconcentrating the booth water/overspray mixture in an ultrafiltration 
unit to obtain a first retentate containing high molecular weight 
components and a first permeant containing water and at least a portion of 
the low molecular weight components, 
b) treating said first permeant and the third permeant obtained in step c) 
below in a reverse osmosis unit to obtain a second permeant containing 
essentially pure water for recycle as booth water and a retentate 
containing low molecular weight components, 
c) treating said first and second retentates in a nanofiltration unit to 
obtain a third permeant containing water and a minor portion of low 
molecular weight components and a third retentate which largely 
corresponds in composition and concentration to said one-component, 
aqueous coating composition.

DETAILED DESCRIPTION OF THE INVENTION 
In the context of the present invention "ultrafiltration" means a known 
membrane separation process performed using membranes having an exclusion 
limit of 1,000 to 100,000, preferably from 10,000 to 100,000 g/mol, at 
differential pressures of 1 to 10, preferably 2 to 7 bar. 
Nanofiltration is also a pressure permeation process and, in terms of 
separation performance, is classified between ultrafiltration and reverse 
osmosis (see, for example, R. Rautenbach, G. Schneider, Final Report on 
the DFG [German Research Association] research project on 
"Nanofiltration", RWTH [Technical University of Rhineland-Westphalia] in 
(Aachen (1993). In the context of the present invention "nanofiltration" 
means a known membrane separation process performed using membranes having 
an exclusion limit of 200 to 2,000, preferably from 500 to 1,000 g/mol, at 
differential pressures of 12 to 40, preferably 12 to 30 bar. 
In the context of the present invention "reverse osmosis" means a known 
membrane separation process in which membranes are utilized that are 
capable of 95 wt-%, preferably at least 98 wt-%, retention of common salt. 
The trans-membrane pressure difference is about 15 to 100, preferably 25 to 
75 bar. 
The permeant flow, i.e., the speed of reconcentration, in all of the 
membrane separation processes previously described is influenced 
predominantly by the trans-membrane pressure difference. When the process 
according to the invention is carried out, the permeant flows are about 5 
to 200, preferably 10 to 100 I/m.sup.2 /h in the ultrafiltration stage, 
about 5 to 100, preferably 10 to 75 I/m.sup.2 /h in reverse osmosis, and 
about 5 to 100, preferably 10 to 50 I/m.sup.2 /h in nanofiltration. 
All conventional commercial membrane modules are suitable for the 
ultrafiltration unit utilized in the process according to the invention, 
such as cushion, plate, spirally wound, tubular, capillary or hollow fiber 
modules. Examples of materials used to manufacture the membranes include 
polysulphone, polyacrylonitrile, polyethylene, Teflon resin, porous 
carbon, ceramic, cellulose acetate, polyurea, aromatic or aliphatic 
polyamides, sulphonated polyaryl ethers, polyfuran, polybenzimidazole, 
various fluoropolymers and polyether aromatics such as polyimide or 
polyimidazopyrrolidone. Polysulphone or polyacrylonitrile plate or tubular 
modules are preferably used. 
All conventional commercial membrane modules, such as spirally wound, 
tubular, capillary or hollow fiber modules, are suitable for the reverse 
osmosis unit. Cushion or plate modules may be used, but are less 
preferred. The membranes may be manufactured from the same materials as 
the ultrafiltration membranes. Polysulphone or polyacrylonitrile spirally 
wound modules are preferably used. 
The same conventional commercial module types and membrane materials may be 
used for the nanofiltration unit as described for ultrafiltration. Cushion, 
plate or tubular modules of polypiperazinamide, polysulphone or 
polyacrylonitrile are preferably used. 
The process according to the invention is suitable for reconcentrating 
booth water/overspray mixtures formed during the processing of 
one-component, aqueous coating compositions. Examples of these 
compositions include physically drying coating compositions based on 
polyurethane or polyacrylate dispersions, coating compositions based on 
fatty acid-modified, polyurethane or polyacrylate dispersions, aqueous 
coating compositions based on polybutadiene, unsaturated polyesters or 
polyacrylates or coating compositions containing based on 
hydroxy-functional polyesters, polyacrylates or polyurethanes combined 
with amino resins or blocked polyisocyanates as cross-linking resins. 
Pigments, fillers and other additives for coating compositions, such as 
levelling agents, gloss improvers, anti-sedimentation agents, thickeners, 
thixotropic agents, antioxidants and thermal stabilizers, which may 
optionally be present in the coating compositions, may also be 
reconcentrated. 
The binders of the coating compositions are either dissolved or dispersed 
by the use of internal or external emulsifying agents. The transitions 
between these states are fluid. The binders generally have average 
molecular weights of 2,000 to 100,000 g/mol, and frequently have broad 
molecular weight distributions so that low molecular weight components 
having molecular weights of less than 2,000 g/mol may also be present. The 
low molecular weight components exert a decisive influence on important 
product properties such as flow and gloss. 
The cross-linking resins in stoving systems, such as amino cross-linking 
resins or blocked polyisocyanates, conventionally have average molecular 
weights of 500 to 2,000 g/mol. Therefore, they make a substantial 
contribution to the proportion of low molecular weight components in 
stoving coating compositions. 
In the case of anionically modified binders the coating composition 
additionally contains amines, such as ammonia, triethylamine or 
dimethylethanolamine, as neutralizing agents. The degree of retention of 
these amines in the reverse osmosis stage is also high. 
The majority of water-reducible coating compositions contain 0.1 to 15%, 
preferably 0.5 to 10%, of low molecular weight solvents such as 
glycolether, N-methylpyrrolidone or methoxypropylacetate. These solvents 
are also have an important influence on the properties of the resulting 
coatings. 
In the coating compositions used in the process according to the invention, 
the proportion of components having a weight average molecular weight of 
less than 2,000 g/mol is at least 5 wt-%, preferably 10 to 60 wt-%; the 
proportion of components having a molecular weight of less than 1,000 
g/tool is preferably at least 5 wt-%; and the proportion of coating 
components having a molecular weight of less than 500 g/mol, preferably 
less than 200 g/mol, is at least 0.5 wt-%, preferably at least 1.0%. The 
preceding percentages are based on the total weight of the organic 
components of the coating compositions. 
The solids content of these coating compositions is generally about 20 to 
70, preferably 30 to 70 wt-%, while that of the overspray diluted with the 
spray booth water is generally about 0.05 to 20, preferably 0.5 to 10 wt-%. 
In the context of the present invention "reconcentration" means the 
recovery from the overspray/booth water of an aqueous coating composition 
having virtually the original composition such that it may be reused as a 
coating composition or as a constituent of a coating composition. 
The process according to the invention is suitable for all one-component, 
aqueous coating compositions which have previously been worked up solely 
by ultrafiltration. Preferably, the process according to the invention is 
used with coating compositions which either cannot be worked up by 
ultrafiltration or which are obtained in an unsatisfactory form due to the 
loss of components essential to the coating composition. 
FIG. 1 set forth the process according to the invention in which: 
(1) represents the booth water circuit; 
(2) represents the part of the booth water circuit containing a booth 
water/overspray mixture to be reconcentrated; 
(3) represents the ultrafiltration stage; 
(4) represents the retentate from ultrafiltration; 
(5) represents the permeant from ultrafiltration; 
(6) represents the reverse osmosis stage; 
(7) represents the retentate from reverse osmosis; 
(8) represents the permeant from reverse osmosis; 
(9) represents the nanofiltration stage; 
(10) represents the retentate from nanofiltration and 
(11) represents the permeant from nanofiltration. 
FIGS. 2 and 3 set forth other embodiments of the invention, in which the 
process according to the invention is carried out in batch-wise manner. In 
these FIGS. (1) to (11) are defined as previously set forth and 
additionally 
(12) represents an intermediate tank for the booth water/overspray mixture 
which is to be supplied to reconcentration; 
(13) represents an intermediate tank for the permeant from reverse osmosis 
which is virtually pure water for reuse; 
(14) represents an intermediate tank for the permeant from the reverse 
osmosis stage and 
(15) represents an intermediate tank for permeant from the nanofiltration 
stage. 
The mixture of booth water and overspray (2), which is to be reconcentrated 
according to the invention, represents either the total quantity of mixture 
from wet extraction of the overspray or a portion of the mixture leaving 
the booth. In order to carry out the process according to the invention, 
the mixture (2) is guided into an ultrafiltration stage (3). In the 
ultrafiltration stage (3) the high molecular weight coating components are 
pre-concentrated in the retentate (4) to 10 to 90 wt-%, preferably from 25 
to 75 wt-%, of the original coating concentration. The major part of the 
low molecular weight coating components are in the permeant (5) from the 
ultrafiltration stage. They are reconcentrated in a reverse osmosis stage 
(6) to form the reverse osmosis retentate (7). The permeant (8) from the 
reverse osmosis stage contains virtually pure water and is returned into 
the booth circulation water circuit (1). The reverse osmosis retentate (7) 
is mixed with the ultrafiltration retentate (4) and reconcentrated in a 
nanofiltration stage (9) until the nanofiltration retentate (10) has 
attained the original coating concentration and, thus, virtually the 
original coating composition and quality. The nanofiltration retentate may 
therefore be used again without further modification as a ready-to-spray 
coating composition for the same purpose, or may be admixed with fresh 
coating composition. The nanofiltration permeant (11), which contains 
small proportions of low molecular weight compounds that do not remain in 
the retentate (10), is mixed with the ultrafiltration permeant (5) and 
supplied to the reverse osmosis stage (6). 
The permeant (8) leaving the reverse osmosis stage (6) generally contains 
"virtually pure" water. This means that the maximum organics of this 
permeant is generally less than about 0.5, preferably less than 0.1 wt-%. 
It is possible in accordance with the present invention to ultimately 
obtain as the nanofiltration retentate (10), an aqueous coating 
composition which largely corresponds in composition and concentration to 
the original coating composition used. This is attributable principally to 
the step in the process in which the retentate (7) from reverse osmosis 
undergoes nanofiltration (9) together with the retentate (4) from 
ultrafiltration. Despite the limited extent to which low molecular weight 
components retained in the reverse osmosis are retained by nanofiltration, 
the reconcentration of such low molecular weight components from the 
combined permeants (5) and (11) has the effect of adjusting the balance 
and results in the presence of such low molecular weight components in the 
retentate (10), specifically in a percentage which corresponds to their 
percentage in mixture (2) or in the original coating composition. 
The continuous process according to the invention enables the solids 
content in the booth circulation water circuit (1) to be maintained 
permanently at a constant value of 0.05 to 20 wt-%, preferably 0.5 to 10 
wt-%. 
The process according to the invention may also be carried out batchwise 
(FIGS. 2 and 3). In this case the overspray/booth water mixture (2) is 
first pumped from the booth circulation water circuit (1) into an 
intermediate tank (12). Thereafter, as in the continuous process, the high 
molecular weight components are pre-concentrated in the ultrafiltration 
stage (3), the low molecular weight components are reconcentrated in the 
reverse osmosis stage (6) and the final concentration takes place in the 
nanofiltration stage (9). The reverse permeant (8) is collected in a tank 
(13) for subsequent reuse as booth water. 
In a second embodiment of the batchwise process, which is less preferred, 
the ultrafiltration, reverse osmosis and nanofiltration stages may be 
carried out independently of one another (FIG. 3). In this case the 
overspray/booth water mixture from the intermediate tank (12) is first 
preconcentrated in the ultrafiltration stage (3). The ultrafiltration 
retentate (4) is first collected in a further intermediate tank (14). The 
ultrafiltration permeant (5) is reconcentrated in the reverse osmosis 
stage (6). The reverse osmosis permeant (8) is collected in the tank (13) 
for reuse as booth water. The reverse osmosis retentate (7) is mixed with 
the ultrafiltration retentate (4)in the intermediate tank (14) and is 
supplied to the nanofiltration stage (9), where re-concentration takes 
place until the original coating composition concentration is reached. The 
nanofiltration permeant (11) is collected in a tank (15). It is mixed into 
the ultrafiltration permeant (5) upstream of the reverse osmosis stage (6) 
during the next reconcentration operation. 
In the process according to the invention all of the auxiliary substances 
and additives present in the overspray are also present practically 
without loss in the retentate (10) which is ultimately obtained. 
Accordingly, the only losses which must be made up in the process 
according to the invention are for the volatile coating components which 
may result from evaporation. Demineralized water is used as the booth 
water. 
In the process according to the invention known materials and pumps are 
used in the individual separation stages, provided that they enable the 
process conditions according to the invention to be maintained. Pumps 
which are preferably used are those which subject the material to the 
lowest possible shear stress, e.g., diaphragm-actuated pumps. 
The process according to the invention is generally carried out at room 
temperature (for example, 15.degree.to 25.degree. C.). It may be necessary 
to cool the mixtures because of heat generated by friction during the 
process. 
In most cases it is possible to reuse the resulting compositions without 
further modification. However, in order to avoid fluctuations in quality 
it is also possible before reuse to mix with the concentrate a quantity of 
fresh coating composition corresponding to the quantity arising through 
overspray in this case the retentate is used as a constituent of a new 
coating composition. 
The examples which follow aim to explain the invention in greater detail 
without, however, restricting it. The advantages of the process according 
to the present invention may be seen by comparing the example according to 
the invention with the comparison example. All parts and percentages are by 
weight unless otherwise specified. 
EXAMPLE 1 
The following coating composition was applied by spraying: 
49.5% polyester-polyurethane dispersion (at a concentration of 42% in a 
52.3:4.6 blend of water/N-methylpyrrolidone, neutralized with 1.1% 
dimethylethanolamine, weight average molecular weight =11,000, 
non-uniformity U=3.0) 
29.7% white pigment (Bayertitan R-KB-4, available from Bayer AG) 
9.9% amino cross-linking resin (Luwipal LR 8839, available from BASF AG, 
90% in isobutanol) 
8.5% water 
1.2% dimethylethanolamine 
1.0% flow aid (Tegopren 100, Tego Chemie Service GmbH, 10% in water) 
0.2% cross-linking agent (Fluorad FC 129.3 M) 
The coating was diluted with water to a spraying viscosity of 35 s 
(DIN-4-beaker/23.degree. C.) before spray application. The solids content 
was about 59% and the pH was about 8.8. 
Demineralized water was used as the booth water. 
The solids content of the booth water/overspray mixture was 5% when the 
spraying operation was complete. 
A polyacrylonitrile membrane having an exclusion limit of 50,000 g/mol was 
used in the ultrafiltration stage for preconcentration. Ultrafiltration 
was continued at a pressure of 4 bar until the solids content of the 
retentate was 30%. 
Reconcentration of the combined permeants from the ultrafiltration and 
nanofiltration stages took place in the reverse osmosis stage using a 
modified polyamide membrane capable of over 98% retention of common salt 
at a pressure of 30 bar. The permeant, containing less than 0.1% organics, 
was reused as the booth water in the next spraying operation. 
The combined retentates from ultrafiltration and reverse osmosis were 
reconcentrated to the original solids content of 59% in the nanofiltration 
stage which took place in parallel with ultrafiltration and reverse 
osmosis. A polypiperazinamide membrane having an exclusion limit of 1,000 
g/mol was used at a pressure of 20 bar. The nanofiltration permeant was 
mixed with the ultrafiltration permeant and supplied to the reverse 
osmosis unit. 
The resulting coating composition was identical to the original coating 
composition in all properties relating to coating technology, including 
hardness, drying rate, gloss and resistance to condensation or solvent, 
thus enabling it to be reused as a coating composition for the same 
purpose without modification. 
EXAMPLE 2 
(not according to the invention) 
The same coating composition as in Example 1 was applied by spraying. 
In this example reconcentration was performed solely by ultrafiltration. 
The membrane was a polyacrylonitrile having an exclusion limit of 50,000 
g/tool, as in the ultrafiltration stage of Example 1. 
The working pressure was 4 bar. The pH was held constant at 8.8 by the 
addition of dimethylethanolamine. Reconcentration had to be terminated 
when the solids content reached only 48% because the permeant flow had 
dropped to 0.6 I/m.sup.2 /h. 
At this time 40% of the amino resin used had passed through the membrane. 
Even after the addition of amino resin to make for its loss in the 
permeant, it was not possible to prepare coatings having the gloss, 
solvent resistance and hardness of the original coating. Thus, it was not 
possible to use the retentate for the preparation of coating compositions 
without loss of properties. 
Although the invention has been described in detail in the foregoing for 
the purpose of illustration, it is to be understood that such detail is 
solely for that purpose and that variations can be made therein by those 
skilled in the art without departing from the spirit and scope of the 
invention except as it may be limited by the claims.