Note: Descriptions are shown in the official language in which they were submitted.
CA 022~611 1998-11-18
PAT96637 FILE~
05.24.1996 TE', .'.~
BASF Lacke + Farben Aktiengesellschaft, Munster
Powder coating comprising at least one hydroxyl-
containing binder and at least one crosslinking agent
having free isocyanate groups, and its use for
producing a multicoat finish
the present invention relates to a powder coating com-
prising
A) at least one hydroxyl-containing binder (A) and
B) at least one crosslinking agent having free iso-
cyanate groups which is dispersed in the binder in
the form of discrete particles with diameters of
between 0.1 to [sic] 150 micrometers.
The present invention also relates to the use of the
powder coatings for producing a multicoat finish.
W093/25599 discloses compositions based on hydroxyl-
containing binders and on crosslinking agents whichcontain isocyanate groups and are dispersed in the
binder in the form of discrete particles with diameters
of between 0.1 to [sic] 150 micrometers. These com-
poslt1ons are employed, in particular, as hot-melt
adhesive compositions, but also as powder coatings.
CA 022~611 1998-11-18
-- 2
Transparent powder coatings and the use of the
transparent powder coatings for producing a multicoat
finish, however, are not described in W093/25599.
5 Furthermore, EP-A-431 413 discloses a process for pro-
ducing powder coatings based on hydroxyl-containing
binders and on crosslinking agents which contain iso-
cyanate groups, where the binder/crosslinking agent
mixture is added in the form of a melt to an inert,
emulsifier-containing solvent and then the powder coat-
ing is separated off. Transparent powder coatings and
the use of the transparent powder coatings to produce a
multicoat finish, however, are likewise not described
in EP-A-431 413.
DE-A-32 30 757 discloses a process for producing solid
polyisocyanates of reduced reactivity which are
stabilized by being enveloped in a polymer, and the use
of these polyisocyanates, together with hydroxyl-
20 containing binders which are solid or liquid at roomtemperature, in reactive compositions for the produc-
tion of polyurethanes. Powder coatings and the use of
the reactive compositions for producing multicoat
finishes are, again, not described in DE-A-32 30 757.
Moreover, DE-A-35 17 333 discloses a process for pro-
ducing dispersions of finely divided polyisocyanates,
in which the polyisocyanates are treated with a
stabilizer, then dispersed and subsequently again
CA 022~611 1998-11-18
-- 3
dispersed, under the action of high shear forces,
and/or subjected to milling.
Furthermore, WO 94/09913 discloses a process ~or
producing powder coatings, where the binder and the
crosslinking agent are dissolved using supercritical
gas, especially supercritical carbon dioxide, and the
powder coating particles are produced by subsequent
spraying. The production of transparent powder coatings
based on hydroxyl-containing binders and on cross-
linking agents containing isocyanate groups, however,
is not described in WO 94/09913.
Furthermore, WO 94/11120 discloses a process for
producing powder coatings, where the binder and the
crosslinking agent are dissolved using pressure-
liquefied gas, especially liquid nitrogen, and the
powder coating particles are produced by subsequent
spraying.
Finally, DE-A-44 06 157 discloses a powder coating
containing a hydroxyl-containing binder and a polyiso-
cyanate which has free isocyanate groups as crosslink-
ing agent. Said binder consists of a polyol component
at least 50 ~ of whose hydroxyl groups consist of
secondary- and/or tertiary-bonded OH groups. The powder
coatings described therein, however, have the dis-
advantage that the coating material exhibits incomplete
crosslinking and, as a result, deficient mechano-
CA 022~611 1998-11-18
~, -- 4
technological properties.
Furthermore, EP-B-100 507, EP-B-100 508, EP-A-62 780
and the review article by Blum, R. and Schupp, H. in
Prog. Org. Coat. 1990, 18(3), pages 275 - 288, disclose
compositions of polyisocyanate containing free iso-
cyanate groups and polyol, in which the polyisocyanate
is dispersed in the polyol in the form of discrete
solid particles. These polyisocyanate particles are
deactivated on their surface. The compositions des-
cribed therein have a liquid to pastelike consistency,
whereas powder coatings are not described in these
documents.
Finally, EP-A-148 462, EP-A-153 579 and DE-A-34 03 500
also disclose one-component polyurethane systems in
which the polyisocyanate is dispersed in the polyol in
the form of discrete solid particles. These documents
too, again, do not describe powder coatings.
The object of the present invention therefore, is to
provide powder coatings which can be cured at low
temperatures of, for example, < 160~C, preferably <
140~C, and which even at these low baking temperatures
lead to transparent coatings having the good properties
required. For instance, the powder coatings should lead
to coatings having a good chemical resistance and good
weather stability, and a degree of yellowing stability
which is as good as possible. Furthermore, when used as
CA 022~611 1998-11-18
-- 5
transparent powder coatir.g for producing a multicoa~
finish the powder coatings should exhibit good
compatibility with the base coat film and good color
neutrality.
This object is surprisingly achieved by the powder
coatings of the type specified at the outset, which are
characterized in that the binder (A) employed comprises
A1) at least one hydroxyl-containing acrylate copolymer
(A1) having a glass transition temperature of from 30
to 80~C, a number-average molecular weight of from 2000
to 15,000 daltons and an OH number of from 60 to 180 mg
of KOH/g, more than 50 ~ of the hydroxyl groups of the
binder (A1) being primary-bonded OH groups, and/or
A2) the reaction product of a carboxyl-containing
acrylate copolymer and a compound containing epoxide
groups, or the reaction product of an acrylate
copolymer containing epoxide groups and a carboxyl-
containing compound, the carboxy/epoxy reaction being
able in each case to be carried out prior to or during
the reaction of the binder (A) with the crosslinking
agent (B).
The present invention also relates to the use of the
powder coatings for producing a multicoat finish.
It is surprising and was not foreseeable that the
CA 022~611 1998-11-18
-- 5
powder coatings are curable at low temperatures of, for
example, < 160~C, preferably < 140~C, and that even at
these low baking temperatures they lead to transparent
coatings having the good properties desired, for
example good chemical resistance, good weather
stability and a degree of yellowing resistance which is
as good as possible. Furthermore, when used as trans-
parent powder coatings to produce a multicoat finish,
the powder coatings show good compatibility with the
base coat film and good color neutrality.
In the text which follows, then, the individual con-
stituents of the novel powder coatings will be illus-
trated in more detail.
It is essential to the invention that the binder and
the crosslinking agent, containing free isocyanate
groups, form a heterogeneous system in which the
polyisocyanate is dispersed in the binder in the form
of discrete particles. The polyisocyanate is preferably
in crystalline form. Owing to the heterogeneous
distribution of the polyisocyanate, storage-stable
pulverulent compositions are obtained. In the course of
thermal curing of the powder coatings, phase separation
disappears and the crosslinking reaction between the
isocyanate groups and the hydroxyl groups is able to
take place.
The novel powder coatings contain the polyisocyanates
CA 022~611 1998-11-18
-- 7
in the form of discrete particles with diameters of
between 0.1 to [sic] 150 micrometers, preferably from
0.1 to 50 micrometers, particularly preferably from 1
to 20 micrometers and, with very particular preference,
from 1 to 10 micrometers.
In order to prepare the novel powder coatings it is
possible to employ polyisocyanates which are deactiva-
ted on their surface by reaction with a deactivating
agent. The deactivating agent is expediently chosen so
that it is bonded to the surface of the polyisocyanate
particles by chemical or physical forces and thus
brings about a phase separation between polyisocyanate
particles and binder particles.
For deactivation, the usually from 0.5 to 25 percent by
equivalents, preferably from 0.2 to 12 percent equi-
valents, of the total isocyanate groups present are
reacted with the deactivating agent.
Examples of suitable deactivating agents are: water,
primary and secondary mono-, di- and polyamines,
hydrazine derivatives, amidines, guanidines, mono- and
polyalcohols and also compounds containing carboxyl
groups, phenolic hydroxyl groups, amide groups or
hydrazide groups. Other examples of suitable deactivat-
ing agents are the compounds mentioned in EP-B-100508
in column 6, lines 10 to 37, and column 6, line 53, to
column 7, line 19, and in EP-B-100507, column 5, lines
CA 022~611 1998-11-18
20 to 58, and also the deactivating agents mentioned in
EP-A-431 413, DE-A-35 17 333, DE-A-32 30 757,
DE-A-35 29 530, DE-A-34 03 500 and EP-A-153 579.
For further details of the deactivation, reference is
made, for example, to EP-A-62 780, pages 6 and 7, to
EP-B-100508, columns 6 and 7, and to EP-B-100507,
column 5.
The preparation of the novel powder coatings using the
deactivated polyisocyanates is carried out in accor-
dance with the customary methods of preparing such
powder coatings. For example, they can be prepared by
finely premilling the polyisocyanate, preferably to a
particle size of between 0.1 and 50 micrometers,
particularly preferably from 1 to 20 micrometers and,
with very particular preference, from 1 to 10 micro-
meters. The polyisocyanate is then, together with the
binder and with further customary auxiliaries and
additives, mixed first of all in dry form and then
mixed and extruded using a twin-screw extruder or in a
compounder at temperatures of between 60 and
120 degrees C, preferably between 80 and 120 degrees C.
After comminution beforehand, the extrudate is milled
in a mill to mean particle sizes of between 10 and
60 micrometers, preferably between 20 and 40 micro-
meters. Any coarse fract~ions present are removed by
sieving or classifying.
CA 022~611 1998-11-18
g
The function of the deactivating agent can also be
taken over by specific additives or binder con-
stituents.
The novel powder coatings can also be prepared by the
method described in WO 94/09913. For details, there-
fore, reference is made to WO 94/09913, pages 5 to 12.
In the case of this method, the binder is first of all
dissolved in supercritical gas, preferably super-
critical carbon dioxide, and then the crosslinking
agent and, if desired, further customary auxiliaries
and additives are added. In this procedure, the cross-
linking agent is preferably milled prior to its
incorporation into the binder solution to a particle
size of between 0.1 and 50 micrometers, preferably from
1 to 20 micrometers and, with very particular prefer-
ence, between 1 and 10 micrometers. The temperature and
pressure which are to be established in each case in
the course of preparing the powder coating solution
depend in this procedure on the supercritical gas used.
Normally, the temperature is between -85 and +200~C. If
working with supercritical carbon dioxide, the tempera-
ture is normally between 15 and 150~C, preferably
between 31 and 100~C. Moreover, the temperature is
preferably adjusted so that it is below the melting
point of the pulverulent isocyanate. The pressure to be
established is usually between 0 and 345 bar, prefer-
ably between 7 and 150 bar, and with particular
CA 022~611 1998-11-18
_ O
preference between more than 20 and 70 bar.
The powder particles are subsequently produced by
spraying (spray drying). In this procedure the polyiso-
cyanate is obtained in the form of discrete particlesdispersed in the binder.
Furthermore, it is also possible to dissolve only the
binder and any further auxiliaries and additives in the
supercritical gas, especially carbon dioxide, and to
supply this binder solution and, separately therefrom,
the pulverulent, premilled crosslinking agent to a
dual-substance nozzle, to mix both components in the
nozzle and to produce the powder coating by spraying.
Examples of suitable supercritical gases are, in
addition to carbon dioxide, water, methane, ethane,
ethylene, propane, pentane, methanol, ethanol, fluoro-
carbons and so on. For details, reference may again be
made to WO94/09913, pages 10 to 12.
In addition to the use of supercritical gases it is
also possible to prepare the novel powder coatings by
using liquefied gases, as is described in detail in
WO94/11120 and [sic] to which reference is made
regarding further details.
Suitable liquefied gases for preparing the novel powder
coatings in this context are all those which have been
CA 022~611 1998-11-18
- 1 1 -
liquefied at a pressure of not more than 20 bar,
preferably not more than 10 bar and, with particular
preference, at normal ambient pressure by having been
cooled to the appropriately low temperatures in accor-
dance with methods known to the person skilled in theart. Examples of suitable liquefied gases which may be
mentioned are liquid nitrogen, liquid air and liquid
helium. Owing to its ready availability and low price,
liquid nitrogen is employed in particular, although
other liquefied gases are also employed depending on
the intended application of the powder coating
suspensions.
The use of gas which has been liquefied at a pressure
of not more than 20 bar insures that the suspension can
be handled without complex apparatus for maintaining
high pressures, whereas the use of supercritical gases,
for example of liquefied C02 [sic], requires the main-
tenance of higher pressures in the course of produc-
tion, storage, transportation and processing of thesuspensions. The use of gases which have been liquefied
at a pressure of not more than 20 bar does not of
course, however, rule out the exposure of the suspen-
sions employed in accordance with the invention to an
elevated pressure, for example in the course of trans-
portation in a pipe circuit, but just means that such a
pressure is not necessary in order to maintain the
suspension.
CA 022~6ll l998-ll-l8
- 12
Subsequently, the powder coating particles are in turn
produced by spraying (spray drying). In this procedure,
the polyisocyanate is produced in the form of discrete
particles dispersed in the binder.
Finally, the novel powder coatings can also be obtained
by first of all melting the binder and the crosslinking
agent separately and mixing them in the liquid melt
state (in this context the temperature of the melt
should in each case be chosen so as to achieve an
appropriately low viscosity of the melt which insures
good mixing of the components under the stated
conditions) for a time which is as short as possible,
preferably for a time of less than 1 minute, parti-
cularly preferably for a time of from 0.01 to10 seconds, and then rapidly cooled, preferably
quenched. Afterwards, the heterogeneous composition
obtained is milled to the desired particle size. With
this method too the novel heterogeneous system is
formed.
With this method, however, care must be taken to insure
that the mixing of the liquid-melt binder and cross-
linking agent at the elevated temperatures is carried
out in so short a time that the polyisocyanates react
only on their surface with the hydroxyl groups of the
binder and thus produce the novel heterogeneous system.
Therefore, the liquid-melt binder and liquid-melt
crosslinking agent are preferably mixed using an
CA 022~6ll l998-ll-l8
- 13 -
intensive mixer or the like, while extruders are in
general unsuitable owing to the long residence time.
Regarding the structural components of the novel powder
coatings, the following individual comments should be
made.
As binder (component (A)) the novel powder coatings
comprise
A1) at least one hydroxyl-containing acrylate copolymer
(A1) having a glass transition temperature of from 30
to 80~C, preferably from 40 to 60~C (measured in each
case with the aid of differential scanning calometrie
[sic] (DSC)), a number-average molecular weight of from
2000 to 15,000 daltons, preferably from 2500 to 10,000
(determined in each case by gel permeation chromatog-
raphy using a polystyrene standard), and an OH number
of from 60 to 180 mg of KOH/g, preferably from 100 to
170 mg of KOH/g, more than 50 ~, preferably at least
60 ~, of the hydroxyl groups of the binder (A1) being
primary-bonded OH groups, and/or
A2) the reaction product of a carboxyl-containing
acrylate copolymer and a compound containing epoxide
groups, or the reaction product of an acrylate
copolymer containing epoxide groups and a carboxyl-
containing compound, the carboxy/epoxy reaction being
able in each case to be carried out prior to or during
CA 022~6ll l998-ll-l8
- 14
the reaction of the binder (A) with the crosslinking
agent (B).
Suitable binders for the powder coating are, for
5 example, hydroxyl-containing polyacrylate resins (Al)
which can be prepared by copolymerization of at least
one ethylenically unsaturated monomer containing at
least one hydroxyl group in the molecule with at least
one further ethylenically unsaturated monomer contain-
ing no hydroxyl group in the molecule, at least one ofthe monomers being an ester of acrylic acid or meth-
acrylic acid. Suitable hydroxyl-containing polyacrylate
resins (Al ) are also, for example, known from
DE-A-43 37 480, DE-A-41 18 052 and EP-A-591 862.
Examples of hydroxyl-containing monomers are hydroxy-
ethyl (meth)acrylate, hydroxypropyl (meth)acrylate and
hydroxybutyl (meth)acrylate, preference being given to
the use of hydroxyethyl methacrylate together, if
20 desired, with other hydroxyl-containing monomers.
Examples of ethylenically unsaturated monomers contain-
ing no hydroxyl group in the molecule are alkyl esters
of acrylic and methacrylic acid containing 1 to
25 20 carbon atoms in the alkyl radical, especially methyl
acrylate, methyl methacrylate, ethyl acrylate, ethyl
methacrylate, butyl acryl.ate, butyl methacrylate, 2-
ethylhexyl acrylate and 2-ethylhexyl methacrylate.
Other examples of ethylenically unsaturated monomers
CA 022~6ll l998-ll-l8
- 15
containing no hydroxyl groups in the molecule are acid
amides, for example acrylamide and methac~ylamide,
aromatic vinyl compounds, such as styrene,
methylstyrene and vinyltoluene, nitriles, such as
acrylonitrile and methacrylonitrile, vinyl halides and
vinylidene halides, such as vinyl chloride and
vinylidene fluoride, vinyl esters, such as vinyl
acetate, and carboxyl-containing monomers, such as
acrylic acid and methacrylic acid, for example.
Furthermore, as binder or as further binder in the
novel powder coating, it is also possible to employ the
reaction product of a carboxyl-containing acrylate
copolymer and a compound which contains epoxide groups,
or the reaction product of an acrylate copolymer which
contains epoxide groups and a carboxyl-containing
compound.
In this context the carboxy/epoxy reaction can be
carried out prior to mixing with the crosslinking agent
containing isocyanate groups; in other words, the
reaction product is employed as binder. However, it is
also possible to employ a mixture of the component
containing epoxide groups and the component containing
carboxyl groups in the novel powder coating.
In this case, the carboxy/epoxy reaction does not take
place until during the curing of the powder coating,
the hydroxyl groups formed in the carboxy/epoxy
CA 022~611 1998-11-18
- 16
reaction then reacting in situ with the isocyanate
crosslinking agent.
Examples of suitable polyacrylate resins containing
epoxide groups for use in the novel powder coating are
those which can be prepared by copolymerization of at
least one ethylenically unsaturated monomer containing
at least one epoxide group in the molecule with at
least one further ethylenically unsaturated monomer
containing no epoxide group in the molecule, at least
one of the monomers being an ester of acrylic acid or
methacrylic acid. Polyacrylate resins of this kind,
containing epoxide groups, are known, for example, from
EP-A-299 420, DE-B-22 14 650, DE-B-27 49 576,
US-A-4,091,048 and US-A-3,781,379).
Examples of ethylenically unsaturated monomers contain-
ing no epoxide group in the molecule are the compounds
already mentioned in the case of the description of the
hydroxyl-containing acrylate copolymers.
The polyacrylate resin containing epoxide groups
usually has an epoxide equivalent weight of from 400 to
2500, preferably from 420 to 700, a number-average
molecular weight (determined by gel permeation chroma-
tography using a polystyrene standard) of from 2000 to
20,000, preferably from 3000 to 10,000, and a glass
transition temperature (TG) of from 30 to 80~C,
preferably from 40 to 70~C, particularly preferably
CA 022~611 1998-11-18
- i7
from 40 to 60~C (measured using Differential Scanning
Calorimetrie [sic] (DSC)). About 50~C is very parti-
cularly preferred. It is also possible ~o employ
mixtures of two or more acrylate resins.
Examples of compounds suitable for reaction with the
acrylate copolymers containing epoxide groups are
carboxylic acids, especially saturated, straight-chain,
aliphatic dicarboxylic acids having 3 to 20 carbon
atoms in the molecule. Very particular preference is
given to the use of a dicarboxylic acid having
12 carbon atoms in the molecule, for example dodecane-
1,12-dicarboxylic acid. For modifying the properties in
the finished powder coatings it is possible, if
desired, also to employ other carboxyl-containing
compounds. Examples thereof which may be mentioned are
saturated branched or unsaturated straight-chain di-
and polycarboxylic acids and also polymers containing
carboxyl groups.
Also suitable furthermore, for the preparation of the
novel powder coatings are carboxyl-containing poly-
acrylate resins which can be prepared by copolymeri-
zation of at least one ethylenically unsaturated
monomer containing at least one acid group in the
molecule with at least one further ethylenically
unsaturated monomer containing no acid group in the
molecule.
CA 022~611 1998-11-18
-- ~ 8
Preference is given to the use of carboxyl-containing
acrylate copolymers having a glass transition tempera-
ture of from 30 to 80~C, preferably from 40 to 60~C
(measured in each case with the aid of differential
scanning calometrie [sic] (DSC)), a number-average
molecular weight of from 2000 to 15,000 daltons,
preferably from 2500 to 10,000 (determined in each case
by gel permeation chromatography using a polystyrene
standard), and an acid number of from 60 to 180 mg of
KOH/g, preferably from 100 to 170 mg of KOH/g.
The compound containing epoxide groups and the
carboxyl-containing compound are normally employed in a
quantity such that there are from 0.5 to 1.5, prefer-
ably from 0.75 to 1.25, equivalents of carboxyl groupsper equivalent of epoxide groups. The quantity of
carboxyl groups present can be determined by titration
with an alcoholic KOH solution.
In the novel powder coatings it is preferred as binders
to employ acrylate copolymers containing hydroxyl
and/or carboxyl and/or epoxide groups, including a
content of aromatic vinyl compounds of less than 50 ~
by weight, particularly preferably of less than 30 ~ by
weight, with very particular preference of from 10 to
25 ~ by weight, based in each case on the overall
weight of the monomers employed.
The abovementioned polyacrylate resins containing
CA 022~611 1998-11-18
hydroxyl and/or epoxide and/or carboxyl groups can be
prepared by polymerization in accordance with generally
well-known methods.
It is of course also possible to employ any desired
mixtures of the binders mentioned.
In the novel powder coating it is also possible, to
give if desired with the hydroxyl-containing acrylate-
based binder (component A1 and/or A2), to employhydroxyl-containing polyester resins which have been
prepared in a customary manner from aromatic or
aliphatic or cycloaliphatic di- and/or polyols,
combined if desired with monools, and aromatic or
aliphatic or cycloaliphatic di- and/or polycarboxylic
acids, combined if desired with monocarboxylic acids.
Owing to the better chalking resistance and weather
resistance, it is preferred to employ aliphatic and/or
cycloaliphatic structural components for the prepara-
tion of the polyester resins.
For example, the polyester resins can have been pre-
pared using ethylene glycol, propanediol, butanediol,
neopentyl glycol, hexanediol, cyclohexanediol, 4,4'-
dihydroxydicyclohexyl-2,2-propane, trimethylolpropane,
hexanetriol, pentaerythritol. Other suitable diols are
esters of hydroxycarboxylic acids with diols, in which
case the abovementioned diols can be employed as diol.
Examples of hydroxycarboxylic acids are hydroxypivalic
CA 022~611 1998-11-18
- 20
acid or dimethylolpropanoic acid. Examples of acids
which can be employed are adipic acid, glutaric acid,
acelaic [sic] acid, sebacic acid, fumaric acid, maleic
acid, tetrahydrophthalic acid, hexahydrophthalic acid,
cyclohexanedicarboxylic acid, phthalic acid, terephtha-
lic acid, isophthalic acid and trimellitic acid. Also
suitable are the esterifiable derivatives of the above-
mentioned polycarboxylic acids, for example their mono-
or polyesters with aliphatic alcohols having 1 to
4 carbon atoms or hydroxy alcohols having 1 to 4 carbon
atoms. Furthermore, it is also possible, where they
exist, to employ the anhydrides of the abovementioned
acids.
Suitable hydroxyl-containing polyester resins are also
known, for example, from DE-A-25 47 124 and
EP-A-408 465.
As crosslinking agents the novel powder coatings con-
tain at least one isocyanate. Suitable isocyanates are
those which are customary and solid at room tempera-
ture, preferably crystalline isocyanates. They can be
obtained by addition of diisocyanates onto di-, tri- or
polyols or di-, tri- or polyamines, or from diisocyan-
ates by dimerization to form uretdiones, trimerizationto form isocyanurates and with amines or water to form
biurets. Di- and polyiso~yanates containing allophan-
ate, carbodiimide and ester groups are also suitable as
crosslinking agents for the novel powder coatings.
CA 022~611 1998-11-18
- 21
Owing to their good stability to ultraviolet light,
(cyclo)aliphatic diisocyanates give rise to products
with a low tendency to yellow. Preferably, therefore,
use is made of compounds containing isocyanate groups,
based for example on isophorone and isophorone deriva-
tives.
Examples of suitable isocyanates for the preparation of
the solid di- and polyisocyanates employed in accor-
dance with the invention are the following: phenylenediisocyanate, tolylene diisocyanate, xylylene diiso-
cyanate, bisphenylene diisocyanate, naphthalene diiso-
cyanate, diphenylmethane diisocyanate, isophorone
diisocyanate, cyclobutane diisocyanate, cyclopentylene
diisocyanate, cyclohexylene diisocyanate, methylcyclo-
hexylene diisocyanate, dicyclohexylmethane diisocyan-
ate, ethylene diisocyanate, trimethylene diisocyanate,
tetramethylene diisocyanate, pentamethylene diisocyan-
ate, hexamethylene diisocyanate, propylene diiso-
cyanate, ethylethylene diisocyanate and trimethylhexanediisocyanate. Also suitable are diisocyanates of the
general formula (III')
CA 022~611 1998-11-18
- 22
IRl IRl
OCN - C - X - C - NCO (III')
R2 R2
in which X is a divalent, aromatic hydrocarbon radical,
preferably an unsubstituted, halo-, methyl- or methoxy-
substituted naphthalene, diphenylene or 1,2-, 1,3- or
1,4-phenylene radical, particularly preferably a 1,3-
phenylene radical, and Rl and R2 are an alkyl radical
having 1 - 4 carbon atoms, preferably a methyl radical.
Diisocyanates of the formula (III') are known (their
preparation is described, for example, in EP-A-101 832,
US-A-3,290,350, US-A-4,130,577 and US-A-4,439,616) and
in some cases are commercially available (1,3-bis-(2-
isocyanatoprop-2-yl)-benzene, for example, is sold by
American Cyanamid Company under the tradename TMXDI
(META) ).
Other suitable diisocyanates are those of the formula
(IV'):
H NCO
~\ ¦ / R'- NCO (IV')
in which R is a divalent alkyl or aralkyl radical
having 3 to 20 carbon atoms and R~ is a divalent alkyl
or aralkyl radical having 1 to 20 carbon atoms.
.
CA 022~611 1998-11-18
- 23
Examples of preferred crosslinking agents which may be
mentioned are the following isocyanates: the addition
product of tolylene diisocyanate and trimethylolpro-
pane, the cyanurate of 3 moles of tolylene diisocyan-
ate, dimerized tolylene diisocyanate containinguretdione groups, and solid isocyanates based on the
uretdione of isophorone diisocyanate, such as, for
example, the product commercially available under the
designation IPDI-BF 1540 from Huls.
If the novel powder coatings are employed as
transparent powder coatings, they additionally contain
at least one light stabilizer, preferably in a quantity
of from 0.1 to 5 ~ by weight, based on the overall
weight of the transparent powder coating.
Examples which may be mentioned of suitable light
stabilizers are W absorbers based on benzotriazole
derivatives and triazine derivatives, such as, for
example, the commercially available products Tinuvin~
900 and Tinuvin~ 1130 from Ciba Geigy or Cyoguard~
1161 from Clariant, and light stabilizers based on
sterically hindered amines (HALS), such as, for
example, the commercially available products Tinuvin~
25144, Tinuvin 242 and Tinuvin 123 from Ciba Geigy.
In addition to the components already described, the
powder coating can contain the customary fillers and
pigments in proportions of from about 1 to 50 ~ by
CA 022~611 1998-11-18
2-- --
weight, based on the overall weight of the powder
coating. Examples are: both inorganic and organic color
pigments and/or fillers, for example titanium dioxide,
heavy spar, silicates and iron oxide in various color
modifications.
In addition the powder coatings can if desired contain
one or more catalysts which catalyze the urethane
curing. These catalysts are usually employed in a
quantity of from 0.001 to 3 ~ by weight, based on the
overall weight of the powder coating. Examples of suit-
able catalysts which may be mentioned are, in particu-
lar, organic metal catalysts, for example organic
tin(II), tin(IV), iron, lead, cobalt, bismuth,
antimony, zinc and magnesium compounds. Also suitable
are amine catalysts, for example diazabicyclooctane and
diazabicycloundecane.
If desired, the powder coatings can also contain one or
more catalysts suitable for the epoxy resin curing, if
they contain as binder a mixture of compounds contain-
ing carboxyl groups and epoxide groups and if the
carboxy/epoxy reaction takes place simultaneously with
the curing of the powder coating. Suitable catalysts
are phosphonium salts of organic or inorganic acids,
quaternary ammonium compounds amines, imidazole and
imidazole derivatives. The catalysts are generally
employed in proportions of from 0.001 ~ by weight to
about 2 ~ by weight, based on the overall weight of the
CA 022~611 1998-11-18
- 25 -
epoxy resin and of the crosslinking agent.
Examples of suitable phosphonium catalysts are
ethyltriphenylphosphonium iodide, ethyltriphenylphos-
phonium chloride, ethyltriphenylphosphonium thiocyan-
ate, ethyltriphenylphosphonium acetate-acetic acid
complex, tetrabutylphosphonium iodide, tetrabutylphos-
phonium bromide and tetrabutylphosphonium acetate-
acetic acid complex. These and other suitable
phosphonium catalysts are described, for example, in
US-A 3,477,990 and US-A 3,341, 580.
Examples of suitable imidazole catalysts are 2-
styrylimidazole, l-benzyl-2-methylimidazole~ 2-methyl-
imidazole and 2-butylimidazole. These and other
imidazole catalysts are described, for example, in
Belgian Patent No. 756,693.
It is advantageous, furthermore, to use devolatilizing
agents, for example benzoin in proportions of from 0.1
to 3 ~ by weight, based on the overall weight of the
powder coating, such agents facilitating the evapora-
tion of volatile constituents through the film which
forms in the course of the coating operation (compare,
for example, US-A 5,055,524, column 5, lines 12-21 and
EP-A 0 509 393, page 3, lines 38-40).
Furthermore, leveling assistants are generally added to
the powder coating in proportions of from 0. 05 to 5 ~
_ _ .
CA 022~611 1998-11-18
- 25
by weight, based on the overall weight of the powder
coating. Examples which may be mentioned are oligomeric
poly(meth)acrylates, for example polylauryl
(meth)acrylate, polybutyl (meth)acrylate, poly-2-
ethylhexyl (meth)acrylate, fluorinated polymers orpolysiloxanes.
The novel powder coating can additionally be employed
in the form of an aqueous dispersion. In the text which
follows, the term powder coating dispersion is used as
a synonym for powder coating slurry. The aqueous powder
coating dispersion, especially transparent powder
coating dispersion, consists in this case of a solid,
pulverulent component I and an aqueous component II,
the component I being the above-described powder coat-
ing and the component II being an aqueous dispersion
containing at least one nonionic or anionic thickener
and, if desired, catalysts, auxiliaries, antifoams,
dispersion auxiliaries, wetting agents, preferably
carboxy-functional dispersants, antioxidants, W
absorbers, free-radical scavengers, small quantities of
solvents, leveling agents, biocides and/or water
retention agents.
The aqueous powder coating dispersion can be prepared
by first of all - as described above - preparing the
powder coating. The powd~er coating can then be wet-
milled, or have stirred into it dry-milled powder
coating, in order to prepare the aqueous powder coating
CA 022~611 1998-11-18
- 27 -
dispersion. Wet milling is particularly preferred.
The present invention relates accordingly also to a
process for preparing an aqueous powder coating disper-
sion based on the component I described which inaccordance with the invention is dispersed in a
component II. The latter consists of an aqueous
dispersion of catalysts, auxiliaries, antifoams, anti-
oxidants, wetting agents, W absorbers, free-radical
scavengers, biocides, water retention agents, small
quantities of solvents and/or dispersion auxiliaries,
preferably carboxy-functional dispersion auxiliaries.
Following the dispersion of component I in component II
the mixture is, if desired, milled, the pH is adjusted
to 4.0 to 10.0, preferably 5.5 to 9.5, and the dis-
persion is filtered.
The mean particle size is between 1 and 25 ~m,
preferably below 20 ~m, particularly preferably from 3
to 10 ~m. The solids content of the aqueous powder
coating dispersion is between 15 and 50 ~.
Prior to or following wet milling and/or introduction
of the dry powder coating into the water it is possible
to add to the dispersion from 0 to 5 ~ by weight of a
defoamer mixture, an ammonium and/or alkali metal salt,
a carboxy-functional or nonionic dispersion auxiliary,
wetting agent and/or thickener mixture and the other
.. . . _ ,
CA 022~611 1998-11-18
- 28 -
additives. Preferably, in accordance with the inven-
tion, defoamers, dispersion auxiliaries, wetting agents
and/or thickeners are first of all dispersed in wate~.
Then small portions of the powder coating are stirred
in. Subsequently, defoamers, dispersion auxiliaries,
thickeners and wetting agents are again incorporated by
dispersion. Finally, powder coatings are stirred in in
small portions again.
In accordance with the invention the pH is preferably
established using ammonia or amines. In this context
the pH may first of all rise so that a strongly basic
dispersion is formed. However, the pH falls back to the
values indicated above within a number of hours or
days.
The powder coating and the aqueous powder coating
dispersion can be applied to a very wide variety of
substrates, for example metal, wood, glass or plastic.
They can also be employed in the field of container
coating, coil coating and film coating. The powder
coatings and the aqueous powder coating dispersions are
preferably employed to produce a multicoat finish,
particularly preferably to produce a clear coat film on
a base coat film, preferably in the automotive
industry. The powder coating and the aqueous powder
coating dispersion is particularly suitable for
application to waterborne coating materials based on a
polyester, polyurethane resin and an amino resin.
.. .. . . ..
CA 022~611 1998-11-18
- 29
The present invention therefore also relates to a
process for producing a multicoat protective and/or
decorative coating, in which first of all a base coat
is applied, a polymer film is formed from the coat, a
top coat is applied to the base coat thus obtained, and
then the base coat is dried together with the top coat,
characterized in that the top coat employed is the
novel powder coating or the aqueous powder coating
dispersion.
The powder coating is applied by means of customary
methods, for example electrostatic spraying or tribo-
electric spraying. The novel powder coating dispersions
can be applied by the methods known from liquid-
coatings technology. In particular, they can be applied
by means of spraying techniques. Also suitable is
electrostatically assisted high-speed rotation or
pneumatic application.
The powder coating dispersions applied to the base coat
film are in generally flashed off prior to baking. This
is expediently effected first of all at room tempera-
ture and then at slightly elevated temperature. In
general the elevated temperature is from 40 to 70~C,
preferably from 50 to 65~C. Flashing off at room
temperature is carried out for 2 to 10 minutes,
preferably for 4 to 8 minutes. At elevated temperature,
flashing off is carried out again for the same period
CA 022~611 1998-11-18
- 30
of time.
Following the application of the powder coatings or of
the powder coating dispersion, curing is usually
carried out at temperatures of between 120 and
220 degrees C, preferably between 120 and 200 degrees C
and, with particular preference between 140 and 180~C.
The baking period is between 5 and 60 minutes, prefer-
ably between 20 and 40 minutes.
With the novel process using the powder coating dis-
persions it is possible to obtain film thicknesses of
from 30 to 50 ~m, preferably from 35 to 45 ~m. Clear-
coats of comparable quality could only be obtained to
date in accordance with the prior art, using trans-
parent powder coatings, by applying film thicknesses of
from 65 to 80 ~m.
In the following text the invention is illustrated in
more detail with reference to working examples. All
parts or percentages in the examples are by weight
unless expressly stated otherwise.
1. Preparation of a hydroxyl-containina binder
260 parts of xylene are charged to a laboratory reactor
and heated to 120~C. Over the course of 4 hours, from
two separate feeds, a mixture 1 comprising 30.15 parts
of methyl methacrylate, 13.40 parts of styrene,
CA 022~6ll l998-ll-l8
- 31
16.75 parts of hydroxyethyl methacrylate and 6.7 parts
of n-butyl acrylate and a mixture 2 comprising
3.00 parts of tert-butyl perethylhexanoate and
4.0 parts of xylene are metered in. During the addition
of the mixtures 1 and 2 and for one hour thereafter,
the temperature is held at 120~C. Then the xylene is
distilled off by gradually raising the temperature to
180~C. Finally, a vacuum of 100 mbar is applied for one
hour and the resin is discharged. The resin obtained
has a number-average molecular weight of 3500 daltons,
a glass transition temperature of 55~C and an OH number
of 117 mg of KOH/g.
2. Preparation and a~plication of a powder coatinq
0.07 parts of a commercial cycloaliphatic diamine
(commercial product Laromin C260 from BASF AG,
Ludwigshafen) are dissolved under an inert gas atmo-
sphere in a melt, at 140~C, of 81.72 parts of the
acrylate resin described above. The melt is then cooled
to below the solidification point and coarsely com-
minuted. 14.83 parts of a commercial, crystalline iso-
cyanate which has been premilled to a mean particle
size of 15 ~m and is based on a dimerized 2,4-tolylene
diisocyanate (commercial product Desmodur~ TT from
Bayer AG, Leverkusen), 1.93 parts of a commercial light
stabilizer based on benzotriazole (Tinuvin~ 900 from
Ciba Geigy), 0.87 parts of a commercial light
stabilizer based on HALS (Tinuvin 144 from Ciba
CA 022~611 1998-11-18
- 32 -
Geigy), 0.19 parts of a commercial leveling agent based
on acrylate (commercial product Perenol F40) and
0.39 parts of benzoin are then added. The mixture is
subsequently subjected to intense mixing in a fluid
mixer from Henschel. The mixture thus prepared is
extruded using a Buss PLK 46 single-screw compounder.
The extrudate is cooled directly with a cooling roll
and broken into chips. The chips of extrudate are
milled to a mean particle size distribution of from 20
to 30 ~m using a Hosokawa ACM 2 classifier mill and
subjected to protective sieving at 125 ~m.
The powder coating thus obtained is sprayed electro-
statically using a standard equipment incorporating
corona guns. The films obtained, with a film thickness
of from 60 to 100 ~m, are baked at 160~C for
30 minutes. A glossy coating is obtained which is
resistant to MEK.
