Note: Descriptions are shown in the official language in which they were submitted.
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A method for environmentally benign paint spraying with an
air-drying lacquer dissolved, emulgated ox dispersed in water
The invention relates to a method for environmentally
benign paint spraying with an air-drying lacquer dissolved,
emulgated or dispersed in water in a paint spraying station
that is provided with a cubicle with water-rinsed rear wall
for collecting and washing out of overspray'in cubicle waste
water, the entire cubicle waste water being separated by ul-
trafiltration into permeate and lacquer-containing residue,
the permeate being returned to the paint spraying station,
the lacquer-containing residue being enriched with lacquer by
removing water until its lacquer content is essentially equal
to that of spraying composition that is freshly fed to the
spray painting station, the residue that is enriched with
lacquer in this way being admixed to the freshly fed spraying
composition for reuse in spray painting, and with a component
for preventing coalescence of the lacquer being admixed to
the spraying composition fed to the paint spraying station.
In prior art the overspray usually will be disposed of,
which constitutes a heavy burden for the environment and is
also very costly. For instance, a binding agent for oven-dry-
ing lacquer, which can be dispersed in water, is known from
EP-0032554: the disposal of such a binding agent constitutes
a heavy burden to the environment, because it contains phos-
phor. Besides, special measures have to be taken so as to
avoid conglutination of the appliances used, fox separating
the over~pray from the cubicle waste water, in this context
cf. for example DE-3704683.
The separation of cubicle waste water into permeate and
lacquer-containing residue by means of ultrafiltr~tion is
known,.for example, from EP-0307047, EP-0245863, EP-0137877
or EP-0127685. On the other hand, it is known from EP-0318827
or EP-0271015 that the permeate has to be deacidified before
it is returned to the paint spraying residue. Both of these
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aspects of the state of the art are taken into account in EP-
0217212.
Experiments for the recuperation of the lacquer from the
overspray in cubicle waste water are made reference to in
"Oberflache + JoT" 5/1987, page 36, however, in this publica-
tion they are considered as "successful only in certain
cases'~. In "Oberflache + JOT" 2/1988, pages 24-25, lacquer
and binding agent circuits in the spraying cubicle are quali-
fied as ~'future", while the recuperation of the lacquer by
means of special reprocessing as mentioned as known. Also in
DE-3800980, the recuperation of the laccluer by means of re-
processing is known.
In "Oberflache + JOT" 5/1988, pages 61-63, the use of
amines for preventing coalescence of air-drying lacquer and
the additional cleaning of the cubicle waste water by reverse
osmosis follawing its separation into permeate and lacquer
residue are mentioned. In this publication, the recuperation
of the lacquer from the overspray is considered as being "in
its initial stages of development~'.
Therefore it is in accordance with prior art that the
lacquer-containing residue has to be disposed of.
From EP-0141171 or US-4607592 a method for paint spray-
ing with a dispersion of lacquer in water is known. The paint
spraying station is equipped with a cubicle with water-rinsed
rear wall for collecting and washing out of overspray in cu-
bicle waste water. If the concentration of the overspray in
the cubicle waste water reaches about 20% a partial current
of the cubicle waste water circuit is conveyed through a fil-
t~ation unit so as to be separated into filtrate and lacquer-
containing residue. The filtrate consists of cleaned water
and is returned to the paint spraying station for water rins-
ing of the cubicle rear wall. At the lacquer-containing resi-
due several parameters are measured such as for example con-
ductibility, in order to regulate filtration in a way that
the measured parameters will be approximately equal to those
in the freshly fed dispersion. The residue, which is enriched
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with lacquer in this way, will then be admixed to freshly fed
dispersion and thus reused for paint spraying. The overspray
in the cubicle waste water is not stable, and it is specified
in EP-0141171 or US-4607592 that the mixture must pass
through dispergers so as to temporarily stabilize it. Be-
sides, it is a drawback of this method according to EP-
0141171 or US-4607592 that only a partial current of the cu-
bicle waste water is treated and as a consequence the unit
needs a costly pipe and contral system. It is probably for
this reason that this method failed to gain acceptance (cf.
the above quoted article in "Oberflache ~ JOT").
It is known from JP-49-51324 to enrich the lacquer-con-
taining residue with lacquer by removing water from the total
cubicle waste water and to reuse the residue enriched with
lacquer in this way far paint spraying. Though it is recom-
mended to use deionized water in order to avoid contamination
of the spraying composition, this only serves the purpose of
avoiding to soil the spraying composition and it is expressly
pointed out that also regular tap water may be used.
Thus, it corresponds to the state of the art that the
lacquer-containing residue is not stable and no appropriate
countermeasures are known.
As a consequence, the prior art methods of the kind men-
tioned above are unreliable, which is not acceptable in view
of the present demands for the protection of environment.
zn contrast, it is the object of the invention to speci-
fy a method of the kind mentioned at the outset which is
reliable and by means of which no quantities of cubicle waste
water, permeate and lacquer-containing residue must be dis-
posed of which would constitute a significant burden for the
environment.
This object is achieved, according to the invention, by
means of a method of the kind mentioned in the preamble and
which is characterized in that a component for retardation of
the drying is admixed to the spraying composition that is fed
to the paint spraying station, that a component for adjusting
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the throughput in the ultrafiltration is admixed to the cubi-
cle waste water, and that the conductibility of the water
returned to the paint spraying station is monitored and kept
below a predetermined limiting value.
Preferably, the component for adjusting the throughput
in the ultrafiltration is a derivative of glycol or of a
higher alcohol.
Preferably, the lacquer is selected from an alkyd resin,
an acrylic resin, a polyvinyl acetate resin, a silicone
w--- resin, a copolymer thereof, a mixed polymer thereof or a mix-
ture thereof.
Preferably, only such water is freshly fed to the paint
spraying station which has a conductibility of less than 10-4
n lcm'1, which is preferably achieved by reverse osmosis.
Preferably, the component for preventing coalescence of
the lacquer is selected from one or several aliphatic amines
having an average molecular weight of 75 to 133, mixtures
thereof or derivatives thereof.
Preferably, the spraying composition and/or the cubicle
waste water additionally contains a biocidal component.
Preferably, the lacquer being used is capable of being
emulgated or dispersed in water, and the spraying composition
and/or the cubicle waste water further comprise a component
for stabilizing the emulsion or dispersion, respectively, of
the lacquer in the water)
Preferably, the enrichment of the lacquer-containing
residue substantially up to the lacquer content of the fresh-
ly fed spraying composition either proceeds in essentially
continuous manner, the residue enriched with lacquer being
reused for spray painting in an essentially continuous way,
or in essentially discontinuous manner, i.e. in batches
(batch method), the lacquer-enriched residue being stored in
an intermediate reservoir prior to being reused for spray
painting.
Besides the surmounting the above mentioned drawbacks of
the prior art, an additional advantage of the method of the
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invention is contemplated in that its reliability (particu-
larly in respect of the danger of lacquer being deposited in
the pipes and tanks) allows an optimum design of the appara-
tus from an economical point of view and a reuse of the per-
meate and lacquer-containing residue.
The invention will be set forth hereinafter with refer-
ence being made to the drawing in which:
Fig. 1 is a diagram of a closed circuit passing the paint
spraying station and designed for the continuous
implementation of the method of the invention, and
Fig. 2 is a diagram of a closed circuit passing the paint
spraying station and designed for the discontinuous
implementation of the method of the invention in
batches (batch method).
Figs. 1 and 2 snow a per se well-known paint spraying
station with a cubicle 1, a water-rinsed .rear wall 2 and a
tank 3 as well as a rinsing conduit 4 with associated pump 5.
The rinsing system of the rear wall is designed to collect
and wash out the overspray in water in a known way, which is
collected as cubicle waste water in the tank 3.
Fresh spraying composition based on air-drying lacquer
dissolved, emulgated or dispersed in water is fed to the
paint spraying station by way of a conduit 6 and a union 7
and sprayed through the nozzle D.
In Fig. 1 the cubicle waste water is conveyed to an in-
termediate reservoir 10 by way of the discharge 8 with asso-
ciated pump 9, and from there it is conveyed to an ultrafil-
tration unit Z3 by way of a conduit 11 with associated pump
12. In the ultrafiltration unit 13 the cubicle waste water is
separated into permeate and lacquer-containing residue. The
permeate is returned to the paint spraying station by way of
the conduit 14, the intermediate reservoir 15 and the conduit
16.
In Fig. 2 the cubicle waste water is conveyed to an in-
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termediate reservoir 20, which is equipped with a stirrer 21,
by way of the discharge 8 with associated pump 9, and from
there it is conveyed to an ultrafiltration unit 13 by way of
a conduit 21 with associated pump 12. In the ultrafiltration
unit 13 the cubicle waste water is separated into permeate
and lacquer-containing residue. The permeate is returned to
the paint spraying station by way of the conduit 14, the in-
termediate reservoir 15 and the conduit 16.
In Fig. 1 the lacquer containing residue from ultrafil-
tration is conveyed from the ultrafiltration unit 13 via the
conduit 17 and to an intermediate reservoir 18 and from there
to the union 7 by means of the pump 19.
In Fig. 2 the lacquer containing residue from ultrafil-
tration is conveyed from the ultrafiltration unit 13 back to
the intermediate reservoir 20 by way of the conduit 22. From
the intermediate reservoir 20 a discharge conduit 26 leads to
an intermediate reservoir 23 by way of a stop valve 27 and
from there to the union 7 via a conduit 24 with associated
pump 25.
Hence, in both cases of Figs. 1 and 2 the residue from
ultrafiltration enriched with lacquer in the ultrafiltration
unit 13 is reused for paint spraying in the nozzle D. In or-
der to make this reuse possible, also the measures described
hereinafter will be taken.
A component for preventing coalescence of lacquer is
admixed to the spraying composition so as to avoid that the
lacquer that is circulating in the system will settle and
clog up the system. This component is admixed to the cubicle
waste water and/or to the spraying composition fed to the
paint spraying station at the nozzle D. Hecause of the large
evaporation area and the long sojourn time of the lacquer in
the cubicle waste water in connection with the described re-
circulation, preferably a component with low vapor pressure
at room temperature is selected. such a component is selected
from the group of amines in a known manner, however, accord-
ing to the invention, this component is selected from one or
,
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several aliphatic amines of an average molecular weight of 75
to 133, their mixtures or derivatives. As examples of such
substances there may be quoted isopropanolamine, dimethyle-
thanolamine, diisopropanolamine and their mixtures.
Furthermore, in accordance with the invention, a compo-
nent fox retardation of the drying of the lacquer is admixed
to the spraying composition which is fed to the paint spray-
ing station, so as to prevent that the lacquer circulating in
the pump will cure (polymerize) or be oxidized and/or conglu-
tinate the system in an insoluble manner. As examples of such
substances there may be quoted ketoximes, butoximes, etc. and
their mixtures.
Besides, a component for adjusting the throughput in the
ultrafiltration is admixed to the cubicle waste water, which
for instance may be a glycol derivative. As examples of such
substances there may be quoted butylglycol, butyldiglycol,
propyleneglycols and the like. However, for this purpose de-
rivatives of higher alcohols may also be used, for instance a
glycerine ester.
Also the conductibility of the water that is freshly fed
to the paint spraying station is monitored and kept below a
limit in order to avoid that ions and especially calcium ions
coagulate the lacquer that is circulating in the system and
that the system will be clogged up in this way. Preferably,
only such water will be freshly fed to the paint spraying
station, which has a conductibility of less than
4 ft-1cm 1. This conductibility is preferably achieved by
reverse osmosis. The fresh water will be needed when the sys-
tem is filled up for the first time and also for compensating
evaporation loss.
When the lacquer being used is capable of being emulgat-
ed or dispersed in water, there is further admixed to the
spraying composition and/or the cubicle waste water a compo-
vent for stabilizing the emulsion or dispersion, respective-
ly, of the, lacquer in the water. As examples of such sub-
stances there may be quoted derivatives of linear primary
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fatty alcohols such as sulfates, ethoxylates and ethoxysul-
fates of synthetic linear primary fatty alcohols.
Finally, for accomplishing direct reuse, the lacquer-
containing residue from the ultrafiltration is enriched with
lacquer by removal of water from the cubicle waste water,
until its lacquer content is essentially similar to that of
the freshly fed spraying composition so that the recuperated
composition is equally sprayable as the freshly fed composi-
tion. This may be done in two ways.
---- In the-embodiment according to Fig. 1 the enrichment of
the lacc~.ier-containing residue from ultrafiltration is con-
tinuous. The residue from ultrafiltration that has been en-
riched with lacquer until becoming sprayable is essentially
reused continuously for paint spraying, even 'though it may
occasionally be held up in the intermediate reservoir 18. for
this purpose, the capacity of the ultrafiltration unit 13 is
dimensioned in a way that it removes water from the cubicle
waste water at a speed essentially equal to the speed at
which permeate is fed to the paint spraying station.
In the embodiment according to Fig. 2 the enrichment of
the lacquer-containing residue is done in an essentially dis-
continuous way, i.e. in batches {batch methodj. With the help
of a pump, the intermediate reservoir 20, the ultrafiltration
unit 13 and the conduits 11 and 22 form a circuit in which
the above increase in cancentration of the lacquer content
takes place. The residue that is enriched with lacquer up to
sprayability will occasionally be held up in the intermediate
reservoir 23 before it is reused. In the intermediate reser-
voir 23 the color of the spraying composition may be adjusted
at one's discretion.
Tn this context, the capacity of the ultrafiltration
unit 13 can be dimensioned in a way that it removes water
from the cubicle waste water at a speed lower than the speed
at which permeate is fed to the paint spraying station. This
embodiment of the method of the invention is typically suited
for operating the paint spraying station during the normal
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day-time work hours, the capacity of the ultrafiltration unit
13 being dimensioned in a way that the residue of the ultra-
filtration will be enriched with lacquer up to sprayability
at times other than the work hours and especially duxing
night-time.
To perform the method of the invention the lacquer used
will preferably be an alkyd resin, an acrylic resin, a poly-
vinyl acetate resin, a silicone resin, a copolymer thereof, a
mixed polymer thereof or a mixture thereof, because in combi-
----------- nation with the other measures these substances are best
suited for achieving the object of this invention mentioned
hereinabove.
It is another aim to further limit the burden to the
environment by increasing the period of use of the cubicle
waste water between water changes which might be compelling
due to the multiplication of microorganisms. For this pur-
pose, a biocidal component will preferably be admixed to the
spraying composition and/or the cubicle waste water. ~1s exam-
ples of such substances there may be quoted triazine deriva-
tives such as especially hexahydro-1,3,5,-triazine and benzyl
alcohol as well as its derivatives in concentrations of 0.1-
0.3 ~ by volume (rela'~2d to the volume of the cubicle waste
water) .
