Note: Descriptions are shown in the official language in which they were submitted.
23~4~2
HOECHST AKTIENGESELLSCHAFT - Werk KALLE-AL~ERT
91/F 114 16 April 1991
WL-Dr.Ot.-ui
Coating compositions in powder form
The invention relates to coating compositions in powder
form, also called powder coatings, which represent
mixtures of epoxide group-containing acrylate resins and
various curing agents.
Powder coatings which contain as the essential binder an
epoxide group-containing acrylate copolymer are well
known. They are described, for example, in the following
patents: US 3,730,930, US 3,752,870, US 3,781,379,
US 3,787,52~, US 4,091,049, US 4,091,048, US 3,939,127,
US 3,932,367, US 3,991,132, US 3,991,133, US 4,092,373,
US 4,044,070, US 4,374,954 and US 4,346,144,
DE 2,353,040, DE 2,423,886, DE 2,441,753 and
DE 2,509,410. Dibasic acids, their anhydrides, or
substances which form a dibasic acid under curing
conditions, are used as curing agents. According to
EP 299,420, the curing agent can also be a reaction
product of a polyanhydride and a polyol.
The copolymers described in the above patents contain up
to 30% by weight of glycidyl acrylate or glycidyl meth-
acrylate only; the remainder of the copolymer consists of
other unsaturated monomers. A large number of compounds
of this type are suitable as such unsaturated compounds,
including inter alia styrene and alkyl esters of ali-
phatic unsaturated monocarboxylic and dicarboxylic acids.
Catalysts are required for the curing of the powder
coating in each of the examples of prior art cited above
in which the copolymers used contain styrene. The
catalyst used is exclusively tetraalkylammonium bromide.
However, salts of this type considerably impair the
resistance of the coatings to water, acids and alkalis.
In addition, according to the examples of prior art
2~6-~92
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relat~vely high temperatures (above 140C) are necessary
in the curing of powder coatings based on copolymers
containing ~tyrene in most cases. However, in many
applications, for example in the finishing of wood and
plastics or when used as a topcoat over a temperature-
sensitive basecoat in automotive finishing, it is
advantageous that the coating compositions in powder form
cure at the lowest possible temperatures, for example as
low as 120C.
Accordingly, the object of the present invention was to
provide powder coating systems which cure at temperatures
as low as 120C even in the absence of catalysts or in
the presence of the smallest possible amounts of cata-
lysts to furnish coatings having satisfactory properties.
Surprisingly, this object can be achieved if the epoxide
group-containing acrylate copolymer has a certain styrene
content. Why the reactivity of the acrylate copolymers
should depend on the styrene content is not clear.
Although, as explained above, a very large number of
epoxide group-containing acrylate powder coatings have
been described, this surprising effect has not been
mentioned anywhere.
The invention relates to coating compositions in powder
form which are composed of
(A) a copolymer containing glycidyl groups,
(B) an aliphatic or cycloaliphatic dibasic acid, its
anhydride or a polyol-modified anhydride of a
dibasic acid,
(C) optionally, pigments and other additives,
the copolymer (A) having a molecular weight (Mn) of
1,000-10,000 and a glass transition temperature of
30-90C and being a mixed polymer composed of
a) at least 20% by weight o glycidyl acrylate or
glycidyl methacrylate,
b) 35-50% by weight of styrene,
c) 20-45% by weight of one or more alkyl esters o~
aliphatic unsaturated monocarboxylic or dicarboxylic
2~2~2
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acids and
d) 0-50% by weight of one or more other olefinically
unsaturated monomers.
Copolymers A having the following composition:
14-43% by weight of glycidyl methacrylate,
15-45% by weight of styrene,
12-51~ by weight of alkyl acrylates or methacrylates
and copolymers having the following composition:
14-43% by weight of glycidyl methacrylate,
40-50% by weight of styrene,
10-30% by weight of a dialkyl ester of an olefinically
unsaturated dicarboxylic acid,
0-36% by weight of alkyl acrylates or methacrylates
are preferred.
Suitable alkyl esters of unsaturated carboxylic acids are
those derived from monohydric alcohols, preferably those
having 1-18 carbon atoms, particularly preferably those
having 1-12 carbon atoms, such as methyl, ethyl,
n-propyl, isopropyl, n-butyl, isobutyl, sec.-butyl,
n-pentyl, n-hexyl, 2-ethylhexyl, n-octyl, n-nonyl,
isononyl, n-decyl, n-dodecyl, n-tridecyl, isotridecyl,
myristyl, cetyl, stearyl, eicosyl and isobornyl acrylate
or methacrylate. It is also possible to use small
amounts, ie. up to 5% by weight, of a diacrylate or
dimethacrylate of a dihydric or trihydric alcohol, such
as hexanediol diacrylate or butanediol diacrylate or
dimethacrylate or trimethylolpropane triacrylate or
trimethacrylate. Other monomers which can optionally be
used in admixture with the acrylic or methacrylic esters
are esters of ~, ~-unsaturated dicarboxylic acids such as
maleic or fumaric acids and saturated monohydric alco-
hols, for example dimethyl maleate, diethyl fumarate,
dibutyl maleate, dibutyl fumarate, etc.. Other suitable
comonomers are acrylamide or methacrylamide, styrene,
` _ 4 _ 2~642~2
vinyltoluene, ~-methylstyrene, tert.-butylstyrene, vinyl
chloride, acrylonitrile, methacrylonitrile or vinyl
acetate (component d).
The acrylate resins can be prepared by known polymer-
ization processes such as solution, emulsion, bead or
bulk polymerization. Particularly preferred acrylate
resins are those prepared by solution polymerization or
by a bulk polymerization process as described, for
example, in EP 56,971.
The acrylate resins have a glass transition temperature
of 30-90C. The preferred glass transition temperature i~
in the range of 30-60C. The molecular weights (number
average based on polystyrene standard) are generally
1,000-10,000, preferably 1,000-5,000.
The aliphatic dibasic acids employed in the invention as
curing agents - component (B) - are, for example, adipic
acid, pimelic acid, suberic acid, azelaic acid, sebacic
acid, malonic acid, succinic acid, glutaric acid,
1,12-dodecanedioic acid, etc.. The anhydrides of these
acids can also be used, for example glutaric anhydride
and succinic anhydride as well as the polyanhydrides of
these dicarboxylic acids. These polyanhydrides are
obtained by intermolecular condensation of the cited
aliphatic dibasic dicarboxylic acids. Examples are adipic
(poly)anhydride, azelaic (poly)anhydride, sebacic
(poly)anhydride, dodecanedioic (poly)anhydride, etc.. The
polyanhydrides have a molecular weight (weight average
based on polystyrene standard) of 1,000-5,000. The poly-
anhydrides can also be modified with polyol, as described
in EP 299,420. The polyanhydrides are solids at room
temperature. The preparation of the polyanhydrides is
carried out by reacting the dicarboxylic acids with
acetic anhydride at temperatures of 120-200C, preferably
120-170C. In this reaction acetic acid is split off. The
removal of the acetic acid can be speeded up by distil-
lation in vacuo.
2~6~292
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The polyanhydrides can also be used as curing agents in
a mixture with the aliphatic dibasic dicarboxylic acids
or in a mixture with hydroxycarboxylic acids which have
a melting point between 40C and 150C, for example
12-hydroxystearic acid, 2- or 3- or 10-hydroxyocta-
decanoic acid, 2-hydroxymyristic acid.
The amount of the anhydrides and acids, used as curing
agents - component tB) -, based on the acrylate resin,
can vary within a wide range and is governed by the
number of the epoxide groups in the acrylate resin. In
general, a molar ratio of carboxyl groups or anhydride
groups to epoxide groups of 0.4-1.4:1, preferably of
0.8-1.2:1, is chosen.
The powder coating can contain the usual pigments and
fillers. In addition, it can also contain a catalyst in
order to increase the rate of crosslinking and to lower
the curing temperature. Suitable catalysts are tetra-
alkylammonium or phosphonium salts, imidazoles, tertiary
amines, metal salts of organic carboxylic acids or
phosphine. However, in the majority of cases the presence
of a catalyst is not necessary.
The powder coating can furthermore contain various
additives such as those conventionally used in powder
coatings, in particular degassing agents such as benzoin,
generally employed in amounts of 0.1-3% by weight.
Furthermore, it is possible to use flow control agents,
for example oligomeric poly(meth)acrylates such as
polylauryl (meth)acrylate, polybutyl (meth)acrylate,
poly-2-ethylhexyl (meth)acrylate or fluorinated polymers
or polysiloxanes. In order to improve the weathering
resistance, known W absorbers and antioxidants can be
added.
The components A, B and C of the powder coating are first
mixed dry and then extruded using a twin screw extruder
at a temperature of 80-130C, preferably 80-100C. After
2~29~
-- 6
being cooled and comminuted in a mill, the extrudate i5
ground, aiming at an average particle size of 20-90 nm,
prefe~ably of 40-70 nm. Any oversize particles which may
be present can be removed by sieving.
The powder coating is applied using one of the
conventional methods, for example by electrostatic
spraying or tribospraying. After application the curing
is effected at a temperature of 120-200C, the curing
temperature preferably being 130-160C.
The powder coating is particularly suitable for use as a
clearcoat for aqueous basecoats. 2-coat finishes having
exceptional surface smoothness, gloss and resistance to
chemicals and weathering are obtained.
Examples
1. Preparation of dodecanedioic polyanhydride
69.27 parts by weight of dodecanedioic acid and
30.73 parts by weight of acetic anhydride were heated to
150C. In this operation acetic acid was removed by
distillation. As soon as no more acetic acid distilled
off, the temperature was raised to 170C and more acetic
acid was distilled off, first under normal pressure, then
in vacuo. The vacuum was controlled in such a way that
only acetic acid distilled off and no acetic anhydride.
The reaction mixture was then kept for a further 3 hours
at 170C/20 mbar and then cooled. The residue had a
melting point of about 84C.
2.1 Preparation of epoxide group-containing acrylate
resins
Solvesso 100 was heated to 150C under nitrogen. ~he
mixture of monomers together with the initiator was then
added at a uniform rate at 150C over 7 hours. After the
addition, the reaction mixture was kept at 150C for a
further 2 hours and the Solvesso was then distilled off,
29~ 92
-- 7 --
first at 150C and normal pressure, finally at 170C and
18 mbar.
A solid, colorless resin was obtained. The formulations
and parameters are summarized in Table 1 (parts by weight
= pbw):
Table 1 la lb lc ld
Solvesso 10015.00 pbw 15.00 pbw 15.00 pbw 15.00 pbw
Glycidyl
methacrylate 31.64 " 31.64 " 31.64 " 16.00 "
t-butyl
methacrylate 15.56 " 15.56 ~' 15.56 " 15.56 "
Methyl
methacrylate 42.80 " 32.80 " 12.80 " 28.44 "
Styrene 10.00 " 20.00 ~' 40.00 " 40.00 "
Di-tert.-butyl
peroxide 1.50 " 1.50 " 1.50 " 1.50 "
Viscosity
(U~belohde, 50%
solution in butyl
acetate, 20C 460 300 390 490 mæa.s
Gaass temperature 44 41 54 48C
Mw 10,000 9,300 20,400 17,000
E~oxide
equivalent weight 490 500 500 825 g/mol
2.2 Preparation and testing of the powder clearcoats
737 parts by weight of the resins la-lc, 260 parts by
weight of dodecanedioic polyanhydride (or 773 parts by
weight of resin ld and 224 parts by weight of
dodecanedioic polyanhydride) and 3 parts by weight of
benzoin were first mixed dry. This mixture was then
dispersed in the melt in a laboratory extruder at temper-
atures of 80-120C. After being cooled and subjected to
a preliminary comminution, the extrudate was ground in a
blower mill to an average particle size of 50 um to form
2a6~
-- 8 --
a powder coating. Particles having a size larger than
90 ~m were removed by sieving. Using an electrostatic
powder spraygun at 60 kV, the powder coating was sprayed
onto degreased, earthed steel panels in such a way that
a film thickness of 60 ~m resulted after baking at
140C/30 min. The test results are summarized in Table 2:
Table 2 la lb lc ld
Gel time (140C) 450 235 195 245 s
Flow distance at
140C (DIN 16916a)) 125 123 80 70 mm
-
Gloss (60,
DIN 67530) 104 108 104 105
Flow-out gcod very very good
gcod gcod
Erichsen indentation
(DIN 53156) 13.0 11.9 10.8 11.3 mm
Crosshatch test
(DIN 52151) 0 0 0
~mpact test
(AS~M D 2794;
reverse side~ 20-40 20-40 80 ~ 4 i.p.
a) kmcuntweighed:0.2 g;substrate:d~ sedsteelp~nel;l min.
horizo~ly, then at a 60 inclination
With a low styrene content reactivity is low and the
Lmpact indentation test yields poor values. The resin ld
with a glycidyl methacrylate content of below 20% by
weight has an entirely inadequate impact indentation
value. Furthermore, in the case of resins with a low
styrene content the flow distance at 140C is so long
that fat edges and drips can form at the lower edge of
the panel during vertical baking of the painted panels.
9 2~292
3.1 Preparation of epoxide group-containing acrylate
resins:
Di-isopropyl maleate was preheated to 175C. The mixture
of monomers together with the initiator was then added at
a uniform rate at 175C over 7 hours. The reaction
mixture was then kept at this temperature for a further
1 hour and the volatile components (initiator decom-
position products) were then distilled off in vacuo
(18 mbar). A solid, colorless resin was obtained. The
formulations and parameters are summarized in Table 3
(parts by weight: pbw):
Table 3 2a 2b 2c
Di-isopropyl maleate26.89 pbw 25.00 pbw 15.00 pbw
Glycidyl methacrylate 28.29 " 28.29 " 2~.29 "
Nethyl methacrylate0.70 "5.66 " 31.88 '
Styrene 43.62 " 40.55 " 24.33 "
Di-tert.-butyl peroxide 0.50 i 0.50 0.50
Viscosity (plate, cone,
DC 100 s, 170C) 880 7~0810 mæa.s
Viscosity (U~belohde
50% solution in xylene,
20C 53 51 80
Glass tenperature 36 33 25C
Mw 4,800 4,9905,000
Epoxide e~uiv21ent weight 535 535 535 g/mol
3.2 Preparation and testing of the powder clearcoats:
Powder coatings were prepared, and applied, from
737 parts by weight of the resins 2a-2d, 224 parts by
weight of dodecanedioic polyanhydride and 3 parts by
weight of benzoin, as described in 1.2. A film thickness
of 60 nm resulted after baking. The test results are
summarized in Table 4.
-10- 2a~ s2
Table 4 2a 2b 2c
Gel time (140C) 185 225 330 s
Flow distance at
140C (DIN 16916a~) 158 193 204 mm
92king conditions 30 min./130C
Gloss (60,
DIN 67530) 106 112 98%
Flow-out very very very
good gDod g~od
Erichsen indentation
(DIN 53156) 12.43 12.7 12.4 mm
Crosshatch test
(DIN 52151) 0 0 0
I~pact test
(AS~M D 2794;
reverse side) 80 20 < 4 i.p.
EaXing condi~ions 30 mIn./140C
Gloss (60,
DIN 67530) 109 104 107%
Flcw-out very vçry very
good good good
Erichsen indentation
(DIN 53156) 12.0 11.9 12.5 mm
Crosshatch test
(DIN 52151) 0 0 O
Impact test
(AS~M D 2794;
reverse side) 160 160 20 i.p.
a) AmLunt weighed: 0.2 g; substrate: degreasedsteel panel; 1 mun.
horizontally, then at a 60 inclination
It can be clearly seen that reactivity increases with
increasing styrene content. The same order of reactivity
is obtained if dodecanedioic acid is used as curing agent
instead of dodecanedioic polyanhydride.
