Note: Descriptions are shown in the official language in which they were submitted.
2150525
AQUEOUS TWO-COMPONENT POLYURETHANE COATING
COMPOSITIONS AND A METHOD FOR THEIR PREPARATION
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to aqueous two-component
polyurethane coating compositions having a bimodal particle size
distribution and a method for their preparation using special mixing
equipment.
Background of the Invention
Two-component polyurethane coating compositions are not mixed
until shortly before application due to their limited pot life.
While in the past such two-component systems were used in the
form of solutions in organic solvents, in recent years a large number of
water-dispersible two-component systems have been developed. The
water-dispersible two-component systems usually contain as binder a
resin component containing hydroxyl groups (polyol) and a polyiso-
cyanate component (curing agent, cross-linking agent). Such systems,
which can also be used in accordance with the present invention, are
disclosed, for example, in EP-A 358,979, 496,210, 469,389, 520,266,
540,985, 548,669, 562,282, 562,436 and 583,728. A disadvantage of
these two-component polyurethane coating systems is that they still
require, in addition to water, a relatively high proportion of organic
solvents, i.e., 15 to 25%. Another frequent disadvantage of such
aqueous coating systems is that they are not capable of producing the
coating quality obtained with two-component systems based on purely
organic solvents.
Mo4221
Le A 30 399-US/CA/FC
2150525
-2-
It is known that coating dispersions having as small a particle size
as possible are used to obtain high quality coating surfaces. Whereas the
binder is generally capable of being dispersed in such two-component
polyurethane coating systems if its particle size is sufficiently small, i.e.
lower than 200 nm, the dispersion of the inherently hydrophobic
isocyanate component causes considerable problems. These problems
can only be partially reduced by modifying the isocyanate component to
render it more hydrophilic. The reason for this is that during emulsification
the isocyanate component is already stabilised on the surface of the
emulsion particles as they are formed, and the stabilising layer on the
surface of the particles prevents any further comminution thereof.
Therefore, aqueous polyurethane coating emulsions usually have a
bimodal particle size distribution with a first distribution maximum of a
particle size of less than 100 nm (binder/polyol component) and a second
distribution maximum of a particle size of above 10,000 nm (isocyanate
component), a considerable portion of the isocyanate component having
particle sizes above 20,000 nm.
Hydrophilic polyisocyanates and hydrophilic polyols have already
been developed (by chemical modification), but these give rise to cured
coating films having insufficient moisture resistance. Coating films having
improved moisture resistance are obtained only by using hydrophobic or
at most slightly hydrophilic isocyanate components.
Due to the fact that the dispersibility of the isocyanate component
is restricted by the kinetics which cause the isocyanate particles to
become coated with stabilising polyol, it is an object of the present
invention to obtain as fine a dispersion as possible within sufficiently short
times, within which no noticeable surface stabilization takes place. In
Mo4221
Le A 30 399-US/CA/FC
2150525
-3-
particular any increase in temperature, which accelerates the reaction,
must be avoided during the dispersion process.
It has now been found that this object can be achieved if, after
prior mixing of the binder/polyol, the isocyanate, water and optionally
solvent, emulsifiers and additives, the resulting composition is forced
under high pressure through a nozzle having a small size in at least one
dimension.
SUMMARY OF THE INVENTION
The present invention relates to a process for the preparation of
an aqueous coating composition containing an isocyanate-reactive
component and a polyisocyanate component by mixing the components
with water and forcing the mixture at a pressure of 1 to 30 MPa through
a nozzle of small size in at least one dimension.
The present invention also relates to a bimodal aqueous coating
composition which contains in dispersed form an isocyanate-reactive
component as binder and a polyisocyanate component and has a particle
size distribution with a first distribution maximum at a particle size of 40
to 200 nm and a second distribution maximum at a particle size of 200 to
2,000 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an embodiment of an emulsifying device according to
the invention.
Figure 2 is an additional embodiment of an emulsifying device
according to the invention.
Figure 3 is a flow chart of a first embodiment of a continuous
process according to the invention.
Figure 4 is an embodiment of a jet dispersing machine used as an
emulsifying device according to the invention.
Mo4221
Le A 30 399-US/CA/FC
CA 02150525 2002-12-12
-4-
Figure 5 is a flow chart of a second embodiment of a continuous
process according to the invention.
Figure 6 is a preferred embodiment of a jet dispersing machine used
as an emulsifying device according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Suitable nozzles for use in accordance with the process according to
the invention are slot nozzles, annular nozzles or hole-type nozzles. The
small
size (slot width, annular width, hole diameter) can be from approximately 0.2
to 1 mm. The nozzle sizes in the direction of the flow can be one to three
times, preferably one and a half times to twice, the nozzle width or diameter.
Devices of this kind are known as jet dispersing machines or high-pressure
homogenizers. The jet dispersing machine in accordance with EP-A
101,007 (U.S. Patents 4,996,004 and 5,116,536) is especially preferred
since it can be used to form very fine dispersions even at relatively low
pressures.
The pressure to be applied is typically from 1 to 30 MPa (10 to 300
atmospheres), preferably 1 to 8 MPa and more preferably 2 to 6 MPa.
These latter pressures are especially preferred for the jet dispersing
machines.
Optionally it may be useful to employ several jet dispersing machines
in which several nozzles are arranged in succession so that the emulsion is
pressed through a nozzle several times within a short period. With jet
dispersing machines of this kind having several nozzle passages, an
increased pressure corresponding to the number of nozzles must be applied.
In general, however, passage through a nozzle more than three times does
not lead to any significant improvement in the emulsion.
By means of the emulsification method according to the invention,
polyurethane coating emulsions can be produced which are stable for
Mo4221
Le A 30 399-US/CA/FC
_ 2150525
-5-
several hours and which display considerably improved surface quality
following application and curing.
It is also possible to considerably reduce the content of solvent
and/or hydrophiling agent in the dispersion. In particular dispersions
having a solvent content of less than 15% can easily be prepared
according to the invention. Depending on the pressure applied during
dispersion, the number of nozzle passages and the two-component
system used, it is also possible to prepare emulsions which are
completely free of solvent and hydrophiling agents.
The high surface quality of the coatings which is attainable by the
method according to the invention can be directly attributed to the particle
size distribution in the emulsions according to the invention.
The present invention also relates to bimodal aqueous coating
compositions, based on resins containing isocyanate-reactive hydrogen
atoms and polyisocyanates, which have a particle size distribution with a
first distribution maximum at a particle size of 40 to 200 nm and a second
distribution maximum at a particle size of 200 to 2,000 nm, preferably
300 to 1,000 nm. The particle sizes of the distribution maxima differ by a
factor of at least 2.
In particular 99% by weight of the particles of the emulsion
according to the invention have a particle size of less than 5,000 nm.
According to the invention all isocyanate-reactive components
(binders), preferably those containing hydroxyl groups, and
polyisocyanate cross-linking components hitherto used in two-component
polyurethane coating systems can be used. Suitable isocyanate-reactive
(binder) resins are for example polyurethane resins (which can be cross-
linked by polyisocyanates via the active hydrogen atoms present in the
urethane groups), such as those disclosed in U.S. Patent 4,711,918,
Mo4221
Le A 30 399-US/CA/FC
i I ' I i! I
CA 02150525 2002-12-12
-6-
polyacrylates having hydroxyl groups, preferably those having a molecular
weight of 1,000 to 10,000, such as those disclosed in U.S. Patent 5,075,370,
and the polyester resins having hydroxy groups, which may optionally be
urethane-modified, that are known from polyester and alkyd resin chemistry,
such as those disclosed in U.S. Patent 5,387,642. Preferably, hydrophilic
polyols are used.
Suitable polyisocyanate components include organic polyisocyanates
which have aliphatically, cycloaliphatically, araliphatically and/or
aromatically
bonded, free isocyanate groups, and are liquid at room temperature. The
polyisocyanate component should in general have a viscosity of 20 to 2,000
mPa.s, preferably less than 1000 mPa.s and more preferably less than 500
mPas. Polyisocyanates having a higher viscosity or solid polyisocyanates
can also be used if the viscosity of the polyisocyanate component is lowered
by a corresponding content of solvent. Particularly preferred polyisocyanates
are those exclusively containing aliphatically and/or cycloaliphatically bound
isocyanate groups and having an average NCO functionality of 2.2 to 5.0 and
a viscosity of 50 to 500 mPa.s at 23 C. If the viscosity of the polyisocyanate
is sufficiently low a dispersion of sufficiently small particle size can be
successfully obtained according to the invention without the addition of
solvent.
Conventional additives and modifying agents which are known for use
in surface coatings may also be present in the systems according to the
invention.
The present invention is not confined to the use of component
systems developed specially for water-dispersible coating systems as
described, for example, in the European patent applications listed
Mo4221
Le A 30 399-US/CA/FC
2150525
-7-
above. On the contrary it is possible according to the invention to use a
large number of two-component systems previously not dispersible in
water. However, in general when two-component systems developed
especially for dispersion in water are used according to the invention, the
energy expended to disperse these compositions, i.e., the pressure
applied, is particularly favorable.
Further details and preferred embodiments of the invention are
shown in the attached Figures 1 to 6, without limiting the general
disclosure.
Figure 1 represents a preferred emulsifying device according to
the invention consisting of a tube 1 with an insert 2 which has an
attachment 3 closed in the direction of flow. Attachment 3 has distributed
over its circumference, radial holes 4 which act as emulsifying nozzles.
The pre-emulsion is introduced under high pressure in the direction of
arrow 6 and leaves the emulsifying device in the form of a fine emulsion
in the direction of the arrow 5. Figure 2 shows a device corresponding to
that of Figure 1, but with two dispersing inserts I and II arranged in
succession. The holes 4 in the second emulsifying insert in the direction
of the flow may optionally be holes of a smaller diameter than those in
the first emulsifying attachment.
Figure 3 illustrates an embodiment of the method according to the
invention for continuous operation. A polyol/water dispersion is fed from
tank 20 by means of pump 21 via duct 22 to jet dispersing machine 1.
The isocyanate component is fed from tank 10 by means of pump 11 via
duct 12. Following the dispersion in jet dispersing machine 1, the coating
dispersion enters coating applicator 30, for example, a spray gun. Pumps
11 and 21 transport the components at a rate calibrated against the
admission pressure of the dispersing machine, so that a constant ratio of
Mo4221
Le A 30 399-US/CA/FC
- - ------- -- - - - ------ ----
2150525
-8-
the polyol dispersion to the isocyanate component enters the jet
dispersing machine 1.
Figure 4 shows an enlarged diagram of the jet dispersing machine
of Figure 3. The polyol dispersion is introduced in the direction of arrow
22 into the pre-emulsifying chamber 15, the droplets of polyol being
represented as small, open circles. The isocyanate component is
introduced in the direction of the arrow 12 through the pre-dispersing
nozzle 13 into the pre-dispersing chamber 15. The isocyanate
component is indicated by large drops 14. After passing through the jet
dispersing machine, the bimodal aqueous emulsion leaves the jet
dispersing machine in the direction of arrow 5.
Figure 5 illustrates an embodiment of the method according to the
invention for an intermittent coating operation. The pre-dispersion 15 is
produced in a storage tank 30 by means of a stirrer 31. The pre-emulsion
is fed under pressure via pump 40 to the jet dispersing machine 1 and
enters the applicator (not shown), for example, a spray gun via valve 44.
On interrupting the coating process by shutting valve 44, valve 45 is
opened and the coating emulsion is fed back through return duct 46, so
that no build-up of pressure occurs on the exit side of jet dispersing
machine 1. In the embodiment illustrated in Figure 5, the pump 40 is a
simple diaphragm piston metering pump, downstream of which a piston-
type accumulator 41 is arranged, through which a continuous pulsation-
free supply of the pre-emulsion to jet dispersing machine 1 is ensured.
The piston-type accumulator consists of a housing and a piston 42
located therein, onto the rear side 43 of which impinges a gas under
constant pressure. The piston-type accumulator 41 thus compensates for
the periodic flow rates of the diaphragm piston pump. Preferably the
valve 45 is designed in the form of a pressure-maintaining valve which
Mo4221
Le A 30 399-US/CA/FC
- 2150525
-9-
opens or shuts according to the changes in the discharge via valve 44,
so that the jet dispersing machine 1 can be operated under constant
conditions.
Figure 6 shows another preferred embodiment of the jet dispersing
machine which can be employed according to the invention. The jet
dispersing machine according to Figure 6 has several holes, 4a to 4f,
staggered along the axis of insert 3. In addition on the low-pressure side
5 of the jet dispersing machine there is an inlet tube 50 which is movable
by means of drive 53 in an axial direction, i.e., in the direction of arrow
52. By means of inlet tube 50 the flow rate of the emulsion can be
varied by the staggered orifices 4a to 4f being successively opened or
covered. In this way it is possible to establish a variable coating
operation. For example, it is possible to incorporate the dispersing
machine according to Figure 6 into a spray gun and, by an appropriate
mechanism, to move the inlet tube 50 directly by means of the hand lever
of the spray gun. Accordingly, the dispersing machine according to Figure
6 can be inserted into the head of the automatic coating machine on a
coating production line, the metering of the coating emulsion being
controlled electronically by drive 53.
25
Mo4221
Le A 30 399-US/CA/FC
uI I. I li i
CA 02150525 2002-12-12
-10-
EXAMPLES
I) Polyol Systems
Polyol 1: The polyol according to Example 3 of EP-A 578,940 (which
corresponds to Example 3 of U.S. Pat. No. 5,349,041).
Polyol 2: The polyol according to Example 2 of EP-A 496,210 (which
corresponds to Example 2 of U.S. Pat. No. 5,304,400).
II) Polyol Components
The polyol component is produced by mixing the polyol type with water.
Polyol component 1 2 3
Polyol type 1 1 2
Quantity of polyol type (g) 888 801 1908
Quantity of water (g) 900 1050 342
Quantity of polyol
component (g) 1788 1851 2250
III) Polyisocyanate Systems
Polyisocyanate 1: Polyisocyanate 3 according to the examples of EP-A
358,979 (which corresponds to Example 3 of U.S. Pat.
No. 5,075,370) but with a viscosity of the
Mo4221
Le A 30 399-US/CA/FC
CA 02150525 2002-12-12
-11-
100% product of about 1200 mPa.s/23 C and an
average NCO content of 22.5%.
Polyisocyanate 2: according to Example 3 of EP-A 540,985 (which
corresponds to Example 3 of U.S. Pat. No.
5,252,696).
Polyisocyanate 3: Polyisocyanate with an isocyanurate structure based
on isophorone diisocyanate, obtainable from Bayer
AG, Leverkusen, under the name Desmodur Z 4370.
IV) Polyisocyanate Components
The polyisocyanate type is mixed with Baysilone OL 44, obtainable from
Bayer AG, Leverkusen and optionally with butyl diglycol acetate.
Polyisocyanate component 1 2 3
Polyisocyanate type 1 2 3
Quantity of polyisocyanate
type (g) 900 936 739
Quantity of Baysilone OL 44 12 12 11
Quantity of butyl diglycol
acetate (g) 300 201 --
Quantity of polyisocyanate
component (g) 1212 1149 750
Mo4221
Le A 30 399-US/CA/FC
_ 2150525
-12-
V) Preparation of the pre-emulsion
The polyol component is initially introduced into the machine. The
polyisocyanate component is added manually over a period of 3
minutes with intensive stirring. The pre-emulsion thus formed can
be used directly for the fine dispersion according to the invention.
VI) Preparation of the emulsions:
A according to the invention
Immediately after mixing the components the pre-emulsions
are forced at a pressure of 50 bars through a tube with an
internal diameter of 10 mm, which contains an emulsifying
insert according to Fig. 1, of which the inner tube 3 has 2
radial holes of a diameter of 0.5 mm and the thickness of
the wall of the tube is 1.3 mm.
B according to the prior art
The pre-emulsions are continuously fed into a rotor mixing
chamber with a free volume of the mixing chamber of 20
cm3 and are continuously passed to a coating supply
container downstream of the mixing chamber. The
components are mixed by a rotor mixer with a diameter of
about 20 mm at a speed of rotation of 6000 r.p.m.
Mo4221
Le A 30 399-US/CA/FC
_ 2150525
-13-
C according to the invention
The same procedure was followed as under A, with.the
exception that the total pressure was 20 bars.
D according to the invention
The same procedure was followed as under A, except that
the emulsifying device according to Fig. 2 was used, but
with 3 identical emulsifying inserts as under A. The total
pressure was 100 bars.
VII) Assessment of the emulsions
The following tests were carried out on the resulting emulsions:
- determination of the particle size distribution by measuring
the Fraunhofer diffraction (light wavelength: 632 nm)
- determination of the particle size distribution maxima using
an ultracentrifuge, as described by H. G. Muller in Colloid
Polym. Sci. 267 (1989), pp. 1113 - 1116.
The results are shown in the following table.
Mo4221
Le A 30 399-US/CA/FC
2150525
-14-
Assessment of the emulsions prepared by various emulsifying
methods
emulsion example particle size (pm) particle size
preemulsion emulsifying <10% <50% <90% <99% maxima (pm)
example no. method
1 A 0.22 0.47 1.28 3.5 0.06 0.42
1 B 0.71 7.6 16.2 25 0.06 >10
2 A 0.24 0.47 1 3 0.06 0.3
2 B 0.66 4.2 11.4 25 0.06 >10
3 A 0.33 0.83 2.4 5.8 0.04 0.7
3 B 0.87 7.41 14 16.3 0.04 >10
4 C 0.36 0.95 2.3 4.5 0.06 0.9
4 D 0.24 0.48 0.97 1.3 0.06 0.3
VIII) Production of coatings and methods of assessment
The emulsions were applied to glass plates in horizontal and vertical
positions using a flow cup Airmix spray gun commonly used for
spray-coating. After a ventilation time of 10 mins. at room
temperature the coating is dried for 10 mins. at 80 C and then for 30
minutes at 130 C in the same position (either horizontal or vertical).
Separate visual assessments of the coatings from the point of view of
microstructure and pinholing were carried out (+ = good;
- = unsatisfactory). DOI (distinctness of image) values were
determined in a special matching cabinet by grading the mirror image
of a sample card. The DOI sample card shows a number of
incompletely closed circles of various diameters to which values of 10
Mo4221
Le A 30 399-US/CA/FC
2150525
-15-
(largest circle) to 100 (smallest circle) are assigned in grades each
consisting of 10 units. The DOI value represents the value of that
circle whose mirrored image can still just be identified as being
incomplete.
The results are shown in the following table:
Assessment of the coatings visually and by the DOI method
coating of visual assessment DOI
example:
preemulsion emulsifying pinholing microstructure
of example no. method
horiz. vert. horiz. vert. horiz. vert.
1 A + + + + 90 90
1 B - - - - 70 60
2 A + + + + 90 80
2 B - - - - 80 60
3 A + + + + 80 70
3 B - - - - 50 40
4 C + + + + 90 80
4 D + + + + 90 80
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.
Mo4221
Le A 30 399-US/CA/FC
