Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
w CA 02379219 2002-O1-14
Henkel KGaA - H 4051 _ 1 _
Dr. Endres / KK
13.07.1999
"Process for the treatment of the circulating water in painting booths"
This invention relates to a process for the treatment of the circulating water
in
painting booths, particularly for vehicle painting. The paint particles washed
out of
the waste air and introduced into the circulating water are kept in suspension
in finely
dispersed form and removed from the circulating water continuously or
discontinuously. This is achieved by addition of suitable additives to the
circulating
water, the concentration of which in the circulating water is set to the
relatively low
values of between 0.01 and 0.5 wt. % based on the circulating water.
In painting booths in which water-based or solvent-based paints are applied to
the
surfaces to be painted, the entire quantity of paint used is not applied to
the intended
surface. Rather, a considerable proportion of the paint particles becomes
mixed with
the air flowing through the painting booth or settles on the walls of the
painting
booth. The paint particles, the so-called paint overspray, are washed out of
the air
stream with water by means of suitable washing devices, such as Venturi
scrubbers.
The water mixed with paint is collected in circulating water tanks and
circulated. As
the paint particles in the circulating water stick together and to equipment
parts,
however, they are released and coagulated by traditional methods by the
addition of
release and coagulation agents. The coagulated paint particles settle or float
on the
circulating water according to the method employed. The thus-produced paint
sludge
may be removed from the bottom or the surface of the circulating water tank by
means of suitable removal devices. This traditional method requires the use of
inorganic or organic release and coagulation agents which may be selected, for
example, from sheet silicates, waxes and organic polymers. The quantity used
of
these products is generally from 10 to 50 wt. % based on the paint overspray
ready
for spraying. The release of the paint particles and removal of the paint
sludge is
not, however, usually complete so that paint sludge is deposited and has to be
removed by hand from time to time, involving a great deal of work. In
addition, an
additional quantity of sludge is produced where inorganic coagulants are used,
and
this leads to additional disposal costs.
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There is therefore a need for improved processes for the treatment of the
circulating
water in painting booths, which may involve a reduced use of chemicals, with
which
fewer problems caused by sticky paint particles arise and/or in which there is
less
manual outlay for cleaning and maintaining the painting booths and the
disposal costs
are distinctly minimized. ' _
In a first embodiment, the present invention provides a process for the
treatment of
the circulating water in painting booths, wherein the paint particles are
dispersed by
addition of dispersants which are selected from:
(a) homo- and co-polymers based on malefic acid, acrylic acid and/or
methacrylic
acid having molecular weights of between 2,500 and 500;000;
(b) non-ionic surfactants;
(c) anionic surfactants, no polyaspartic acid being additionally added to the
circulating water in this case;
(d) inorganic or non-polymeric organic complexing agents and mixtures thereof;
the total concentration thereof being between 0.01 and 2.0 wt. % , preferably
between
0.02 and 0. 5 wt. % , based on the circulating water.
The success of the process according to the present invention is based on the
fact that
the paint particles remain dispersed in the circulating water and are not
deposited as
paint sludge which is difficult to remove. The circulating water may therefore
be
circulated together with the paint particles dispersed therein without
equipment parts
becoming sticky.
Homo- and co-polymers based on malefic acid, acrylic acid and/or methacrylic
acid
are intended to mean those polymers wherein at least part of the monomers
consists
of the above-mentioned polymerizable carboxylic acids. Other polymerizable
monomers may be co-incorporated into the polymers. Polymers which exclusively
contain acrylic acid and/or methacrylic acid are used, for example. The
molecular
weight of the polymers is preferably from 15,000 to 250,000, particularly up
to
~ ~ 50,000. The polymers may be added to the circulating water as they are or
in salt
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form. Depending on the pH of the circulating water, the acid/base equilibrium
between acid and salt form is automatically adjusted.
In the process according to the present invention, non-ionic surfactants,
which are
preferably selected from alkoxylates of fatty acids, fatty alcohols or fatty
amines
having 7 to 36, preferably 10 to 22, carbon atoms in the fatty alkyl group and
having
5 to 100, preferably 10 to 80, alkylene oxide units, may be used as
dispersants.
These alkoxylates may have the terminal groups thereof closed, i.e. represent
so-
called mixed ethers.
A butyl group may be present, for example, as a terminal group. Ethylene oxide
groups or combinations of ethylene oxide groups and propylene oxide groups are
considered in particular as alkylene oxide groups. An alkoxylate of a Clma-
fatty
alcohol mixture with an average of five ethylene oxide and four propylene
oxide units
in the molecule may be used for example.
Inorganic or non-polymeric organic complexing agents may further be used as
dispersants. These are preferably selected from:
(i) organic carboxylic acids having two to ten heteroatoms, which may
coordinate on
metal ions, particularly from citric acid, tartaric acid, malic acid, gluconic
acid,
nitrilotriacetic acid, ethylenediamine tetraacetic acid, methylglycine
diacetic acid;
(ii) organic phosphoric acids, particularly from 1-hydroxyethane-1,1-
diphosphonic
acid, aminotrimethylene phosphoric acid and phosphonobutane tricarboxylic
acid;
(iii) oligomeric or polymeric inorganic phosphates, particularly Na
triphosphate, Na
pyrophosphate and Na hexametaphosphate.
Oxygen atoms (alcohols, carboxylates) or nitrogen atoms are considered in
particular
as heteroatoms of the organic complexing agents which may coordinate on metal
ions.
It is also applicable in each case that the complexing agents may be used in
the form
of the acids or in salt form. Depending on the pH of the circulating water, a
mixture
. -~ comprising acid and salt form will be reached according to the acid/base
equilibrium.
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The organic complexing agents should not be polymers and should differ in this
way
from the polymeric carboxylic acids of (a), which also have complexing
properties.
The inorganic complexing agents may, however, be entirely polymeric, such as
Na
pyrophosphate.
Mixtures of representatives of the individual groups of dispersants listed may
also be
used. Mixtures comprising polyacrylates and phosphoric acids and mixtures
comprising niosurfactants and phosphoric acids are particularly suitable.
Where dispersants selected from (a), (b) and (d) are used, 0.2 to 2 wt. % ,
based on
the circulating water, of polyaspartic acid may be additionally added to the
circulating
water. The use of polyaspartic acid as such is known from patent application
PCT/EP99/00350, which is not a prior publication. According to this document,
the
polyaspartic acid may be used together with anionic surfactants. In the
process
according to the present invention, however, anionic surfactants may also be
used
without the co-use of polyaspartic acid. The anionic surfactants to be used as
dispersants are preferably selected from soaps, alkyl sulfates, alkyl
sulfonates,
alkylbenzene sulfonates, alkylether sulfates each having 7 to 44, preferably 8
to 22,
carbon atoms in the alkyl group, and from sulfonated malefic acid esters.
In the process according to the present invention, the pH of the circulating
water is
preferably adjusted to a range between about 5 and about 10.5. A weakly
alkaline
pH may be preferred. Particularly where phosphoric acids are used as
dispersants,
particularly advantageous results are obtained when the pH in the circulating
water
is between about 8 and about 10. At such a pH, dispersants capable of
protolysis will
be mainly present in salt form, irrespective of the form in which they are
introduced
into the circulating water. Depending on the form in which the dispersants are
used,
it may be necessary additionally to use pH adjusters, such as alkali metal
hydroxides,
to give the preferred pH range.
Depending on the type of paint used, it may further be advisable additionally
to add
. ~ defoaming agents to the circulating water. The use of defoaming agents is
generally
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known in the technical field in question as foam problems may arise in the
circulating
water particularly where water-based paints are used.
The process according to the present invention is particularly suitable for
painting
equipment in which the circulating water exhibits high turbulence. The
dispersing
effect of the dispersants to be used according to the present invention is
increased by
the flowing of the circulating water.
When the dispersants according to the present invention are used, the
circulating
water may be circulated for a certain period of time without the need to
remove the
entrained paint particles. If the equipment is operated for a fairly long
time,
however, as time progresses a concentration of paint particles in the
circulating water
is reached which should not be further exceeded in order to prevent
breakdowns. The
range in which this concentration limit lies depends on the actual design of
the
equipment in question. For this reason, the process according to the present
invention preferably provides for the removal of a proportion of the paint
particles
from the circulating water continuously or discontinuously. The preferred
procedure
in this case is to separate some of the paint particles from the circulating
water by
means of a membrane filtration, preferably an ultra filtration. The permeate
of the
membrane filtration is returned to the circulating water and the retentate
disposed of.
For this purpose, a portion of the circulating water may be passed over the
membrane
via a by-pass discontinuously or continuously.
The process according to the present invention functions particularly reliably
when
it may be ensured, by addition of the dispersants to the circulating water in
a
concentration between 0.01 and 2.0 wt. % based on the circulating water, that
the
paint particles in the circulating water have an average particle size, which
may be
determined by means of laser diffraction, of below about 20 Vim. To achieve
this,
an addition of suitable dispersants is required for circulating water of a
hardness of
at least 2 °dH (German hardness) (corresponds to 0.714 mval/1 of
alkaline earth metal
ions).
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Accordingly, in a generalized aspect, the present invention relates to a
process for the
treatment of the circulating water in painting booths wherein the circulating
water has
a hardness of at least 2 ° German hardness and wherein the paint
particles are
dispersed by addition of dispersants in a concentration between 0.01 and 2.0
wt.
based on~the circulating water iwsuch a way that they exhibit an average
particle size,
which may be determined by means of laser diffraction, of below 20 win.
Laser diffraction is an example of a method of measurement of particle sizes
in the
micrometer range, in which the light scatter brought about by the particles is
measured. A particle size measuring instrument made by Sympatec may be used
for
this purpose, for example. The average particle size is defined as that
particle size
which establishes when the particles are kept in suspension by a slight
movement of
the suspension, for example, by stirring. Ultrasound, which mechanically
destroys the
particles, is not used in this case.
The above statements apply to the dispersants which may be used under this
generalized aspect of the present invention. This also applies to the
preferred
procedure described above.
In the two embodiments expressed by the independent claims, the present
invention
is suitable for dispersing water-based and/or solvent-based paints.
CA 02379219 2002-O1-14
Examples
Herberts' water-based metallic paint Aqua Pearl Base, Sierra red II was used
for the
trials. In each case, a quantity of test paint was introduced into water so
that the
paint solids content was 0.5 wt. % based on the amount of water.
Dusseldn,~/RRnrat~,
tap water, which has a hardness of 18 °dH (corresponding to 6.426
mval/1 of alkaline
earth metal ions) was used as the water.
In a first set of trials, Sympatec's particle size measuring instrument based
on laser
diffraction was used to measure the average particle size (defined as 50 %
value)
which was established under gentle stirring using the various dispersants. The
results
are contained in Table 1.
Table 1: Average particle size (50 % value)
Example Dispersant/quantity with respect 50 % value
to water (~.m)
Ref. 1 none ' 255
Example 1 Coconut amine x 12 EO~I~, 0.25 11. 8
%
Example 2 Coconut amine x 12 EO~I~, 0.25 11. 3
%
+ polyaspartic acid, 0.76
~l~ EO = ethylene oxide
In the above set of trials, it was shown that the particle size correlates to
the stability
of the paint suspension: at a particle size below about 20 ~.m, no
sedimentation or
flotation may be observed, even 30 minutes after the dispersion was prepared.
Paint
does not settle on the glass wall of the test vessel. Further dispersants were
therefore
only then tested to see whether they exhibit this behavior. Table 2 contains
examples.
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Table 2: Dispersibility: dispersion stability after 30 minutes
Ex. No. Di ersant uanti
sp q ty (wt. % based Dispersion Paint on vessel
on water) , stable? ~ wall?
Ref. 2 none no (flotation)yes
Ref. 3 polyaspartic acid, 0. no (flotation)no
76 %
Ex. 3 modified polyacrylic yes no
acid
molecular weight 4000,
0.23
Ex. 4 coconut amine x 12 EO yes no
Ex. 5 sulfosuccinic acid-bis-2-yes no
ethylhexylester, Na salt,
0.38%
Ex. 6 Na oleate, 0.5 % yes no
The sediment volume was quantitatively determined in a further set of trials.
For this
purpose, the dispersant was placed in a liter of water and stirred for 10
minutes at
300 rpm. The test paint described above was then added in a quantity such that
the
paint solids content was 0.5 wt. % based on the water. Stirring was repeated
for 10
minutes at 300 rpm. The dispersion was then transferred into a measuring
funnel
(Imhoff cone) and the sediment volume, in ml, measured after a residence time
of 10
minutes. Good dispersion is evident from the fact that no sediment appears.
Table
3 contains the results.
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Table 3: Sediment volume
Ex. No. Dispersant quantity (wt. % based on Sediment
water) volume (ml)
Ref.4 none 100
Ref. S polyaspartic acid, 0.2 % - 2.5
Ex. 7 polyacrylate (molecular weight 0
10, 000), 0.18
Ex. 8 malefic acid/olefm copolymer (molecular0
weight
12,000), 0.13
Ex. 9 polyacrylate (molecular weight 30,000),
0.2%
Ex. 10 malefic acid/acrylic acid copolymer 0
(molecular weight '
70,000), 0.2%
Ex. 11 coconut amine x 12 EO, 0.1 %
Ex. 12 Na tripolyphosphate, 0.5 % p
Ex. 13 Na pyrophosphate, 0. 5 % 0
Ex. 14 Na hexametaphosphate, 0.5
Ex. 15 citric acid, 0.5 % 0
Ex. 16 1-hydroxyethane-1,1-diphosphonic acid, 0
0.3 %
Ex. 17 aminotrimethylene phosphoric acid, 0.250
% '
Ex. 18 phosphonobutane tricarboxylic acid,
0.25 % 0
Ex. 19 nitrilotriacedc acid, 0.2 % 0
Ex. 20 Cl~"8-fatty alcohol x 25 EO, 0:5 % 0
Ex. 21 Cmla-fatty alcohol x 80 EO, 0.5 % 0
Ex. 22 coconut amine x 15 EO, 0. 5 % 0
Ex. 23 Clma-fatty alcohol sulfate, 0.18% 0
Ex. 24 ring-opened Soya polyol x 10 EO, 0.5 0
%
