Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02209320 1997-06-30
PROCESS FOR PRODOCTION OF A POWDER COATING
BACRGROOND OF THE INVENTION
Field of the Invention
The present invention relates to a process for
production of a powder coating. More particularly, the
present invention relates to a process for production
of a powder coating using tert-butanol as a solvent,
wherein resin, a crosslinking agent, additives and the
like are dissolved, then frozen, and lyophilized.
Description of Related Art
A powder coating is suitable, in particular, for
use as a clear top coat for automobiles.
Conventional powder coatings have been produced by
grinding and mixing resin, crosslinking agents,
additives and the like, blending them in a melt
kneader, followed by grinding. In such processes,
however, since resin and crosslinking agents are melted
with heat and kneaded, the resulting reaction between
the resin and the crosslinking agent proceeds to
polymerization. As a result, melt viscosity of the
coating increases and the finish of a coated product
may be deteriorated. Sometimes, gelated materials may
be produced, which may cause deathblow or disastrous
defects for coating appearance, particularly for thin
clear films. Dust, lumps and the like mixed during the
production process can not be removed in the
conventional processes because it is difficult to
filter the coating composition. The inclusion of dust,
lumps, or the like in the composition creates
significant problems, particularly when the composition
is used as a clear top coat for automobiles which
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requires high-grade films. Moreover, the heat
generated during production partly facilitates the
progression of crosslinking reactions and the formation
of dust, gelated materials or the like. Therefore, it
is substantially impossible to recover and reuse the
coating for applications which require a high-grade
coating appearance.
A process different from the above-mentioned ones,
has been proposed in Japanese Laid-Open Patent No.
92318/1975 which includes lyophilization. However,
this reference includes no particular limitations for
solvent, resin or the like. Further, there are no
other publications which would enable those skilled in
the art to carry out a suitable lyophilization.
Therefore, it is not easy to find the optimum balance
of resin solubility, crosslinking agent, solvent,
additives, etc. and lyophilization technique from the
description in this document. In addition, it also is
difficult and/or not optimal to use benzene, which is
often used-for lyophilization, from the viewpoint of
toxicity. Disadvantageously, a large amount of dioxane
which has good solubility should not be used for
sanitary reasons.
SUMMARY OF THE INVENTION
The present invention, for example,
provides a solvent composition
or resin composition which completely dissolves resin,
a crosslinking~agent, additives, etc. and that can be
readily lyophilized without toxicity, and produces a
thermosetting powder coating which forms a good film
with a high quality finish and without substantial dust
and/or gelated materials mixed therein.
In accordance with these and other aspects, there
is provided a process for production of a powder
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coating comprising: copolymerizing a monomer mixture
comprising a (meth)acrylate monomer having branched or
cyclic substituents of four or more carbon atoms in its
side chain and a (meth)acrylate monomer having
functional groups to form a thermosetting vinyl
copolymer (a) having a glass transition temperature of
approximately 40 to 100°C and a number average
molecular weight of approximately 1,000 to 10,000;
dissolving or dispersing said copolymer (a) and a
crosslinking agent (b) in a solvent comprising
approximately 50% by weight or more of tent-butanol, to
form a resulting coating solution having a melting
point of riot less than approximately -30°C; and
lyophilizing said coating solution under a pressure not
higher than approximately 50 mmHG.
In further accordance with these and other
aspects, there is provided a resin solution having a
melting point of not less than approximately -30°C
suitable for use in preparing a powder coating, said
solution comprising: a thermosetting vinyl copolymer
(a) having a glass transition temperature of
approximately 40 to 100°C and a number average
molecular weight of approximately 1,000 to 10,000
prepared from a monomer mixture comprising a
(meth)acrylate monomer having branched or cyclic
substituents of four or more carbon atoms in its side
chain and a (meth)acrylate monomer having functional
groups; a crosslinking agent (b); and a solvent
comprising approximately at least 50% by weight based
on the weight of the solvent, of tert-butanol. .
Additional aspects and advantages of the invention
will be set forth in the description which follows, and
in part will be obvious from the description, or may be
learned by practice of the invention. The objects. and
advantages of the invention may be realized and
obtained by means of the instrumentalities and
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combinations particularly pointed out in the appended
claims.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present inventors have studied intensively to
solve the problems described above. As a result, the
inventors have found a process to conveniently produce
a powder coating with excellent finish by
lyophilization using a specific solvent composition and
resin composition, and have attained the present
invention.
That is, the present invention relates to a
process for producing a powder coating, wherein
thermosetting vinyl copolymer (a) having a glass
transition temperature of approximately 40 to 100°C and
a number average molecular weight of approximately
1,000 to 10,000, which can be obtained by
copolymerization of a monomer mixture containing
(meth)acrylate monomer having branched or cyclic
substituents of four or more carbon atoms in its side
chain and (meth)acrylate monomer having functional
groups as essential ingredients, and a crosslinking
agent (b) can be dissolved or dispersed in a solvent
consisting of approximately 50% by weight or more tert-
butanol, and the resulting coating solution having
melting point of not less than approximately -30°C can
be lyophilized under pressure of not higher than
approximately 50 mmHg.
Powder coatings according to the present invention
preferably can form a film with a high quality finish
without reaction due to heat generated during
production. Coatings prepared with the present powder
coatings are also able to be recovered and reused.
A suitable vinyl copolymer (a) can be obtained by
copolymerization of a monomer mixture containing
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approximately 20 to 65% by weight based on the weight
of the monomer mixture of a (meth)acrylate monomer
having branched or cyclic substituents of four or more
carbon atoms in its side chain. Approximately 20 to
70% by weight based on the weight of the monomer
mixture of the (meth)acrylate monomer having functional
groups can be used. A suitable (meth)acrylate monomer
having functional groups can be selected from epoxy-
containing unsaturated monomers, hydroxyl-containing
unsaturated monomers, and carboxyl-containing
unsaturated monomers. Further, the vinyl copolymer (a)
can have glycidyl functionality. The crosslinking
agent (b) can be an aliphatic polycarboxylic acid or an
acid anhydride of an aliphatic polycarboxylic acid.
Approximately 10 to 60% by weight based on the weight
of the monomer mixture of a non-functional unsaturated
monomer can optionally be used simultaneously. A
suitable crosslinking agent (b) can be dodecanoic
diacid, and the solvent composition may be composed of
approximately 5o to 100% by weight of tert-butanol, 0
to 50% by weight of dioxane, and 0 to 20% by weight of
other solvents. The solvent composition in one
embodiment, may comprise not less than 80% by weight of
tert-butanol.
The thermosetting vinyl copolymer (a) in the
powder coating composition used for the process for
production of the a powder coating according to the
present invention can be obtained by copolymerization
of at least one (meth)acrylate monomer having branched
or cyclic substituents of four or more carbon atoms in
its side chain and at least one (meth)acrylate monomer
having functional groups. Other polymerizable
unsaturated monomers may optionally be added as needed.
(Meth)acrylate monomers having branched or cyclic
substituents of four or more carbon atoms in their side
chains may include, for example, a methacrylic acid
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ester having branched substituents of four or more
carbon atoms such as iso-butyl (meth)acrylate, tert-
butyl (meth)acrylate; a (meth)acrylate ester having
alicyclic rings in substituents such as cyclohexyl
(meth)acrylate, isobornyl (meth)acrylate,
tricyclodecanyl (meth)acrylate, etc.
Polymerizable functional group-containing
(meth)acrylate monomers may include, for example, epoxy
containing unsaturated monomers such as glycidyl
(meth)acrylate, glycidyl allyl ether, 3,4epoxycyclo-
hexylmethyl (meth)acrylate, ~-methylglycidyl (met-
h)acrylate; hydroxyl-containing unsaturated monomer
such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl
(meth)acrylate, 3-hydroxypropyl (meth)acrylate,
hydroxybutyl (meth)acrylate, (poly)ethylene glycol
mono(meth)acrylate, hydroxyethylvinyl ethers; carboxyl-
containing unsaturated monomers such as acrylic acid,
methacrylic acid, crotonic acid, itaconic acid, malefic
acid, fumaric acid.
The optionally copolymerizable non-functional
unsaturated monomers if employed, may include, for
example, methacrylic acid ester such as methyl
(meth)acrylate, ethyl (meth)acrylate, n-butyl
(meth)acrylate, octyl (meth)acrylate, lauryl
(meth)acrylate, dodecyl (meth)acrylate, stearyl
(meth)acrylate, dodecyl (meth)acrylate; vinyl aromatic
monomer such as styrene, vinyltoluene, etc.
(Meth)acrylate monomers having branched or cyclic
substituents of four or more carbon atoms which can be
used for production of vinyl copolymer (a) may be
suitably employed in an amount from approximately from
20 to 65% by weight, preferably 30 to 60% by weight,
based on the weight of the monomer mixture. However,
it should be understood that the copolymerized amount
is not limited to the numerals above. However, when
the copolymerized amount of (meth)acrylate monomers
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having branched or cyclic substituents of four or more
carbons is less than 20% by weight, solubility in the
solvent described below, particularly in tert-butanol
may be affected, and often is reduced. A reduced
solubility may result in a heterogeneous coating
solution and a deteriorated finish of the film formed
from such a powder coating.
Functional group-containing (meth)acrylate
monomers can be used in the range between approximately
20 and 70% by weight, preferably 30 and 45% by weight
based on the weight of the monomer mixture. However,
the range is not strictly limited.
Further, other non-functional unsaturated monomers
can optionally be included in the range between
approximately 10 and 60% by weight (not strictly
limited). When styrene is used, it is suitably not
more than 35% by weight, preferably not more than 25%
by weight. The amount of styrene is not strictly
limited to the range; however, when 36% by weight or
more styrene is copolymerized, solubility in tert-
butanol can be significantly reduced, resulting in a
heterogeneous coating solution.
The vinyl copolymer (a) used in the present
invention should preferably have a glass transition
temperature of approximately 40 to 100°C, most
preferably 50 to 80°C. The glass transition
temperature should not be strictly limited to the
range. However, when the glass transition temperature
is below about 40°C, blocking resistance of the
resulting powder coating can be deteriorated, and when
the glass transition temperature is above about 100°C,
viscosity upon heat flow can increase, which may spoil
finish properties, and at the same time, decrease
solubility in a solvent for lyophilization, and further
deteriorate finish properties.
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The above-described glass transition temperature
(Tg,°C) is obtained by converting a value (°K),
calculated from the following Fox's equation, into
(°c):
100/Tg=W1/Tgl+W2/Tg2+W3/Tg3+W4/Tg4
(wherein W1, W2, W3, W4 respectively represent the % by
weight of the monomer used for each copolymer, and Tgl,
Tg2, Tg3, Tg4 represent glass transition temperature
(°K) of each polymer).
The crosslinking agent (b) is not particularly
limited, but the conventionally known crosslinking
agents can be used as long as they react with the
functional group of the vinyl copolymer (a) to harden.
Examples of suitable crosslinking agents (b) include,
aliphatic polycarboxylic acids such as adipic acid,
sebacic acid, suberic acid, succinic acid, glutaric
acid, malefic acid, fumaric acid, dodecanoic diacid,
piperic acid, azelaic acid, itaconic acid, citraconic
acid and (poly)acid anhydride thereof; aromatic
polycarboxylic acids such as terephthalic acid,
isophthalic acid, phthalic trimellitic acid,
pyromellitic acid and (poly)acid anhydride thereof;
alicyclic polycarboxylic acids such as
hexahydrophthalic acid, hexahydroisophthalic acid,
methylhexahydrophthalic acid; and (anhydrous)
polycarboxylic acids such as anhydrides thereof. Other
examples include, a block isocyanate compound obtained
by blocking aliphatic, alicyclic or aromatic polyiso-
cyanate such as isophorone diisocyanate, hexamethylene
diisocyanate, hydrogenated xylene diisocyanate,
hydrogenated tolylene diisocyanate, with a blocking
agent such as phenols, caprolactons, alcohols;
polyepoxy compounds such as trisepoxypropyl
isocyanurate, hydrogenated bisphenol A, Celoxide 2021
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CA 02209320 1997-06-30
(manufactured by Dicel Chemical Industry, Ltd.), EHPE-
3150 (manufactured by Dicel Chemical Industry, Ltd.).
The crosslinking agent (b) can be used alone or as
a combination of two or more crosslinking agents.
Among the above groups, aliphatic polycarboxylic acid
and acid anhydrides thereof are preferred. Among them,
dodecanoic diacid is excellent in blocking resistance
and finish appearance of the coating.
As optional additives (c), those conventionally
included in a coating can be used and include, for
example, anti-foaming agent, surface controlling agent,
antioxidant, W absorber, W stabilizer, anti-blocking
agent, fluidization controlling agent, static
controlling agent, coloring pigment, filler, and
hardening accelerator.
In the present invention, as a solvent which can
dissolve or disperse vinyl copolymer (a) and cross-
linking agent (b), tert-butanol alone or a mixed
solvent of tert-butanol and dioxane may be used.
These solvents having a high solubility of resin,
crosslinking agents, additives, etc. and a high melting
point, and a high vapor pressure are particularly
suitable because solvents having these properties save
energy required for freezing, can afford without highly
reduced pressure and require a shorter time under
reduced pressure for lyophilization. In addition, so
as not to lower the melting point below approximately -
40°C, other solvents, for example, methyl ethyl ketone
or toluene may be used simultaneously with tert-
butanol. The lower limitation of melting point of -
40°C is not strict, even though, when the melting point
is lower than -40°C, the solvent may require
substantial energy for freezing, and further the resin
and a crosslinking agent may separate upon cooling,
thus deteriorating the finish properties of the
coating.
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When a mixed solvent is used as a solvent, tert-
butanol should preferably be used in an amount of not
less than approximately 50% by weight, more preferably
not less than 70% by weight, most preferably not less
than 80% by weight, all weights being based on the
total weight of the solvent. On the other hand,
dioxane should be preferably decreased to less than
approximately 50% by weight, preferably less than 30%
by weight, more preferably less than 20% by weight
based on the total weight of the solvent. When other
solvents are used, they should be used in the amount of
not more than approximately 20% by weight, again based
on the total weight of the solvent.
The amount of tert-butanol should not be strictly
limited to the amount described above. However, when
the amount of tert-butanol is less than 50% by weight,
the vapor pressure of the solvent may decrease,
requiring highly reduced pressure and a longer time
periods under reduced pressure, sometimes reducing
solubility of the crosslinking agent. The amount of
dioxane should not be strictly limited to the amount
above. However, the use of more than 50% by weight of
dioxane can possibly be accompanied with sanitary
problems. Large amounts of dioxane may also decrease
the vapor pressure of the solvent, requiring highly
reduced pressure and prolonged time under reduced
pressure.
The process for producing a powder coating of the
present invention preferably includes dissolving the
aforementioned vinyl copolymer (a), a crosslinking
agent (b), additives and the like, filtering using a
suitable filtration device, freezing generally at 10 to
-30°C, reducing pressure below 50 mmHg, and collecting
using a cold trap. After lyophilization, fine
particles having particle diameter of about 10 ~m can
be readily obtained by simple grinding and filtration
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using a screen. Accordingly, a powder coating can be
produced without requiring a grinding step, etc.
The present invention will be illustrated in
detail in the following examples.
Examples
Production of resin solution CAL
100 parts by weight of tert-butanol was charged in
a reactor equipped with a thermometer, a thermostat, a
stirrer, a reflux condenser and a dropping device, and
heated to 82°C while blowing nitrogen gas, to which was
added dropwise a mixture of cyclohexyl (meth)acrylate
(50 parts by weight), tert-butyl-(meth)acrylate (15
parts by weight), glycidyl (meth)acrylate (35 parts by
weight), azobisdimethyl valeronitrile (7 parts by
weight) over about 3 hours. After completion of
addition, the mixture was left at 82°C for 2 hours to
complete the reaction to produce resin solution (1).
Production of resin solutions (B to I~
In the same manner as described for resin solution
(A), each solution was produced using the formulation
given in Table 1.
Example 1
Resin solution (A) (200 parts by weight),
dodecanoic diacid (25 parts by weight), tert-butanol
(190 parts by weight) were added and, after
dissolution, cooled to -10°C and frozen. Subsequently,
pressure was reduced under 1.0 mmHg, and the
temperature was gradually brought back to ambient
temperature to prevent melting. The resulting solid
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was filtered through 150 mesh screen and lightly
ground to produce a powder coating.
Examples 2 to 7
Using resin solutions (A) to (F), a coating was
produced in the same manner as described in Example 1.
Comparative Examples 1 to 4
Using resin solutions (A), (G), (H), (I) and
solvents given in Table 2, coatings were produced in
the same manner as described in Example 1.
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~
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M .-i M ~ ~D
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CA 02209320 1997-06-30
a a O
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w 8 ~ ~ ~ o
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CA 02209320 1997-06-30
Comparative Example 5
Solvent was removed from the resin solution (A) by
distillation under reduced pressure to obtain solid
resin. This solid resin (100 parts by weight) and
dodecanoic diacid (25 parts by weight) was dry blended
in a Henschel mixer at room temperature, then melt
kneaded in an extruder. After cooling, pulverization
by a pin disk, and filtration through 150 mesh screen,
a powder coating was obtained.
Film properties of the prepared powder coatings
are given in Table 2. Testing in Table 2 was carried
out as follows:
Evaluation of solubility in a solution state
Solubility of solution before lyophilization was
evaluated in the state of a coating solution at 25°C.
As for resin, evaluation was made according to the
following standard: O: completely dissolved; o:
blueing without precipitation; D: clouded and slightly
precipitated. Solubility of a crosslinking agent,
dodecanoic diacid, was also evaluated according to the
following standard: O: completely dissolved; o:
crystalline found to separate out.
On a dull steel plate of 0.8 mm thick which was
subjected to chemical conversion treatment with zinc
phosphate, epoxy cation electrodeposition coating was
applied to give a film having a dry thickness of 20
microns and baked. On the resulting electrodeposition
coating, a surface for intermediate coat of an
automobile was baked to give a film of 20 microns of
dry thickness. This was followed by water sanding with
#400 sand paper, then dewatered and dried.
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Subsequently, Magicron base coat HM-22 (manufactured by
Kansai Paint Co., Ltd., metallic coating, trade name)
was coated to give a hardened film of about 15 microns
thick, baked and hardened in a dryer at 140°C for about
30 minutes to give a material to be tested.
Subsequently, electrostatic coating was conducted
on the surface of said material to give a powder
coating of about 70 microns thick, then heated in a
dryer at 160°C for 30 minutes to harden. The resulting
coated plate was tested by the following test:
Film Appearance
Film appearance was evaluated from the viewpoint
of gloss and smoothness according to the following
standard: O: good; O: slightly bad smoothness but good
gloss; D: slightly bad; X: bad.
Foaming was evaluated according to the following
standard: O: good; e: a small amount of foaming
generated.
60° gloss
Specular reflectivity at 60° was measured
according to JISK-5400.
The present invention can provide a powder coating
with good and/or high quality finish properties without
increasing melting viscosity of coating because
reaction due to heat during production does not take
place.
Additional advantages and modifications will
readily occur to those skilled in the art. Therefore,
the invention in its broader aspects is not limited to
the specific details, and representative devices, shown
and described herein. Accordingly, various
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modifications may be made without departing from the
spirit or scope of the general inventive concept as
defined by the appended claims and their equivalents.
Although only a few exemplary embodiments of this
invention have been described in detail above, those
IQ skilled in the art will readily appreciate that many
modifications are possible in the exemplary embodiments
without materially departing from the novel teachings
and advantages of this invention. Accordingly, all
such modifications are intended to be included within
15 the scope of this invention.
_1~~.
