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Metallic paint coatings have in recent years become very important in the
automobile industry. Whilst in the past a high proportion of automobile
bodies with such coatings were provided with single-coat metallic paints,
automobiles are nowadays increasingly painted by a twocoat metallic-finish
process, namely the so-called "wet-on-wet process". In this process, a
base paint, which is essentially physically drying, and which is pigmented
with aluminium, optionally with the addition of other colouring pigments,
is first applied, and, after brief air-drying, a clear lacquer is applied
as the top coat. The base paint consists in the main of a solvent, a
physically drying binder, such as cellulose acetobutyrate, an oil free
polyester, melamine resin and pigments. The pigments used include, in
addition to aluminium, inorganic pigments such as titanium dioxide, lead
chromate and iron oxides, as well as organic pigments, such as
phthalocyanines, quinacridones or halogenated thioindigo pigments. The
solids content of the base paints is about 15% and hence very low. The
remaining 85% consist of volatile solvent, which is in the main composed
of esters, such as ethylglycol acetate, butyl acetate and ethyl acetate,
and aromatics, such as toluene or xylene.
The second coat, which is applied after an air-drying time of only a few
minutes, is a clear lacquer which may be an air-drying lacquer, as is used
in the case of, for example body repair work, or a baking lacquer, such as
is used when an automobile is painted in conveyor-belt production. In the
latter case, the two coats are hardened conjointly under conventional
baking conditions (e.g. 10-30 minutes at 120.degree.-150.degree. C.).
The high proportion of solvent in the base paint may cause considerable
pollution of the environment and in addition results in correspondingly
high costs: that is to say costs due to the heavier coatings required as a
result of the use of the solvent, and costs due to the removal of solvent
under conditions which do not pollute the environment. There has therefore
been no lack of attempts to increase the solids content or reduce the
proportion of volatile solvents. In the course of these, it has been found
that if the solids content is increased the application characteristics of
the paint deteriorate and the quality of the total system suffers, so that
this method of producing a metallic base paint which does not pollute the
environment and is economical is not viable. Furthermore, attempts have
been made to reformulate the metallic base paint so as to use
water-soluble binders, in order thereby to achieve a non-polluting
composition and good economics. The most serious disadvantages of these
attempts has proved to be insufficient shelf life, difficulties in
application, nonuniformity of the metallic effect, and a tendency of the
aluminium constituents to turn grey.
According to the present invention there is provided a water-dilutable
binder mixture for preparation of a metallic base paint which comprises
(A) a water-soluble condensation product of (a) saturated or unsaturated,
oil-free polyesters containing OH groups and having been prepared from at
least one polycarboxylic acid and at least one polyhydric alcohol, (b) a
polycarboxylic acid mixture containing trimellitic acid, (c) at least one
epoxidised oil and (d) a basic compound;
(B) at least one metal powder,
(C) at least one water-miscible solvent; and, optionally,
(D) other pigments and/or dyestuffs; and, optionally,
(E) other conventional additives,
which mixture can be used for the production of a two-coat metallic paint
coating, and does not suffer from the above disadvantages in respect of
shelf life, ease of application, greying and pollution of the environment.
Aqueous paints based on the binder mixtures according to the invention
surprisingly require, when applied to a suitable substrate, a very short
drying time at room temperature, in spite of a high water content (for
example from 40 to 70% by weight, based on the sum of water and solvent),
to achieve a dust-dry or tack-free coating; in general, this drying time
is less than 10 minutes, mostly less than six minutes and in particular
from three to five minutes. The reasons for this short drying time are not
known. Possibly it is the result of the ease with which the resin releases
the solvent, and, where relevant, also the result of the formation of
readily volatile azeotropic mixtures. Drying at an elevated temperature is
also feasible, and this substantially reduces the drying time, which is
particularly advantageous where the time available within a predetermined
process of operation is short and inadequate for physical drying. The
composition furthermore exhibits good stability when stored at room
temperature. The binder mixture has excellent intermediate adhesion.
Application of the mixture presents no problems; because of the rapid
drying, it does not run off vertical surfaces and shows no clouding;
instead, the metallic effect develops very well. Furthermore, it lends
itself excellently to overcoating, i.e. the dried coat is not attacked by
the subsequently applied top coat. If aluminium pigments are used, the
so-called "greying" effect is not observed.
Suitable oil-free polyesters possessing free OH groups (component (a)) may
be prepared from known polycarboxylic acids, such as for example phthalic
acid, isophthalic acid, terephthalic acid, trimellitic acid, fumaric acid,
maleic acid, endomethylene-tetrahydrophthalic acid, succinic acid, adipic
acid, suberic acid, azelaic acid, sebacic acid, alkyl-substituted
compounds of these acids and their possible anhydrides. Examples of
alcohol components of these polyesters include known polyhydric alcohols,
such as ethylene glycol, the propanediols, butanediols and pentanediols,
neopentyl glycol, hexanediols, diethylene glycol, glycerol,
trimethylolethane, trimethylolpropane, pentaerythritol and
dipentaerythritol. The components may be employed individually or as
mixtures. The polyesters in general have OH numbers ranging from 60 to
150, especially from 80 to 120.
Polycarboxylic acid mixtures containing trimellitic acid (component (b))
include those which contain compounds of the formulae (I) to (III) (see
accompanying sheet of formulae) and which are disclosed in German Patent
Application No. P 27 07 018.4. In formulae (I) to (III),
R, R' and R", which may be the same or different, represent divalent to
hexavalent, and preferably divalent, straight-chain or branched
hydrocarbon radicals having a total of from 1 to 12, preferably of 2 to 6,
carbon atoms, which radicals optionally possess up to two olefinic double
bonds or are optionally interrupted by up to three ether bridges,
x and y are integers from 2 to 6,
t is an integer from 1 to 5 and
z and u are zero or an integer from 1 to 5,
and in the formulae shown only as linear structures, that is to say shown
for the case of divalent radicals R, R' and R", branches may be present,
not only in the radicals R' and R" but also in the radical R. Such
branched polycarboxylic acids cannot readily be represented by one formula
alone. If, due to the possibility of R, R' and R" having a higher valency
than 2, the compounds (I) to (III) are branched, the indices x, y and z in
the various branches may be the same or different. Examples of compounds
of formula (I) include esters of trimellitic acid containing several ester
groups and derived from polyhydric, for example dihydric to hexahydric,
saturated or unsaturated alcohols having 2 to 12 C atoms, including those
alcohols which contain up to 3 ether groups. Examples of suitable alcohols
for the preparation of these compounds are the diols already mentioned
above, as well as their higher homologues and their unsaturated analogues,
for example butenediols, as well as oligomers of ethylene glycol, and the
polyhydric alcohols mentioned above, though the diols are preferred.
Examples of suitable compounds of formulae (I), (II) and (III) are those in
which R, R' and R" are preferably divalent but can also be branched,
depending on the structure and number of hydroxyl groups of the starting
alcohol. In formulae (II) and (III), R, R' and R" may be the same or
different and have the meaning given above for R.
Examples of typical radicals R, R' and R" include --(CH.sub.2).sub.n --
(wherein n is 2 to 10),
##STR1##
--C.sub.2 --CH.dbd.CH--CH.sub.2 --, --CH.sub.2 --CH.sub.2 --O--CH.sub.2
--CH.sub.2 -- and --CH.sub.2 --CH--CH.sub.2 --, as well as radicals of
formulae (IV) (see accompanying sheet of formulae).
Accordingly, if x is 2, the compounds of formula (II) are
diester-tetracarboxylic acids and if x is 3, they are
triester-hexacarboxylic acids. The compounds of formula (III) are, for
example, oligomeric polycarboxylic acids possessing several ester groups.
The compounds of formulae (I) to (III) may be prepared, for example, by
hydrolysis of a mixture containing anhydride groups, such as is obtained
on reacting trimellitic anhydride with the corresponding dihydric and/or
polyhydric alcohols in a manner known per se. This hydrolysis can be
carried out in the presence or absence of a solvent. If it is carried out
in the presence of a solvent, solvent-free polycarboxylic acid mixtures
can also be obtained by cautious distillation of the solvent, if necessary
under reduced pressure. However, in many cases it is possible to select
such a solvent mixture that a polycarboxylic acid solution suitable for
further processing is obtained. The hydrolysis is particularly simple if
the amount of water required for the hydrolysis is added to a melt or a
solution of an anhydride mixture at an elevated temperature. To complete
the hydrolysis, the mixture is stirred for some time at the same
temperature.
Examples of solvents for the preparation of component (b) include, ethylene
glycol monobutyl ether acetate, butyl acetate, methyl isobutyl ketone and
mixtures containing them, for example a mixture of ethylene glycol
monobutyl ether acetate and xylene, for example in a ratio of 1:1 to 1:3.
A particularly suitable method of characterising the mixtures containing
anhydride groups is gel permeation chromatography (GPC) using a gel of
polystyrene, cross linked with divinylbenzene, as the adsorbent and
tetrahydrofuran as the eluent. By this method, the individual components
of the anhydride mixtures may be determined accurately, so that the
particular proportion of the individual components can readily and
accurately be adjusted. In this way, the structure and amount of the
compounds of formulae (I) to (III) can be accurately determined.
Advantageously, the content of compounds of formulae (I) to (III) in
component (b) is 70 to 99.8, preferably 80 to 95, % by weight, whilst the
content of trimellitic acid is 0.1 to 30, preferably 5 to 20, % by weight.
The content of oligomeric polycarboxylic acids (formula (III)) in the
mixture of the compounds of formulae (I) to (III) can generally be between
0.1 and 99.8%, preferably between 30 and 70%. In addition, the
polycarboxylic acid hardeners can consist of mixtures of individual
components with different radicals R, R' and R" and different indices x,
y, z, t and u.
The ratio in which the polyester (a) and the polycarboxylic acid mixture
(b) are mixed is generally very important. Suitable values of the ratio of
the solids in component (a) to the solids in component (b) are in general
from 50:50 to 90:10, preferably from 60:40 to 85:15, and more particularly
from 65:35 to 75:25.
Epoxidised oils, for example those based on soya bean oil, linseed oil,
tall oil and castor oil, and generally having, in each case, an epoxy
equivalent of 100 to 360, especially of 150 to 200, are employed as
component (c). The condensation product of components (a) to (c) is
rendered water-soluble by neutralisation with a basic compound (d), such
as, for example, ammonia and aliphatic di- and tertiary amines, such as
diisopropanolamine, dimethylaminoethanol, diethylaminoethanol,
trimethylamine, triethylamine and tripropylamine.
The component (B) used in the mixtures of the invention consists of at
least one metal powder such as copper, copper alloys, aluminium and steel,
used individually or as mixtures; aluminium powder is preferred, at least
as the predominant constituent; the amount of metal powder is in general
0.8 to 50, preferably 0.8 to 15, in particular 0.8 to 5, % by weight,
relative to the binder.
Examples of optional dyestuffs or pigments (component (D)), which may be
inorganic or organic, are titanium dioxide, graphite, carbon black, zinc
chromate, strontium chromate, barium chromate, lead chromate, lead
cyanamide, lead silicochromate, calcium molybdate, manganese phosphate,
zinc oxide, cadmium sulphide, chromium oxide, zinc sulphide,
nickel-titanium yellow, chromium-titanium yellow, red iron oxide, black
iron oxide, ultramarine blue, phtalocyanine complexes, naphthol red,
quinacridones, halogenated thioindigo pigments or the like. Their
proportion is chosen to be such that it does not supress the desired
metallic effect.
The mixtures employed may also optionally contain other conventional
additives (E), such as, for example, fillers, stabilisers, wetting agents,
dispersing assistants, levelling agents, anti-foam agents and catalysts,
individually or as mixtures, in the conventional amounts as well as
inferior amounts of solvents being water-immiscible. These substances may
be added to the individual components and/or to the total mixture. Other
resins may also be employed as conventional additives, for example to
increase the reactivity, for instance amine resins, especially melamine
resins, for example in amounts of from 1 to 20% by weight, relative to the
other binders.
Examples of suitable fillers include talc, mica, kaolin, chalk, quartz
powder, asbestos powder, slate powder, barium sulphate, various silicas,
silicates, glass fibres, organic fibres and the like.
Examples of suitable water-immiscible solvents include higher alcohols, for
example with 4 to 8 carbon atoms, such as n-butanol and the various
pentanols, hexanols and octanols, as well as aliphatic and aromatic
hydrocarbons such as petrol fractions, toluene, the various xylenes, and
esters of glycol ethers, for example the acetic acid esters of ethylglycol
or butylglycol (i.e. of ethylene glycol monoethyl ether or ethylene glycol
monobutyl ether).
To prepare the metallic base paints, water is added to the binders
according to the invention. The paints contain conventional water-miscible
solvents, for example monohydric or polyhydric alcohols, esters, glycol
ethers or ketones. Specific examples of such solvents which may be
mentioned are tert.-butanol, ethylglycol, butylglycol, butyldiglycol (i.e.
ethylene glycol dibutyl ether), ethylene glycol dimethyl ether, diethylene
glycol dimethyl ether, cyclohexanone, methyl ethyl ketone, acetone,
isophorone and mixtures of these.
In producing metallic paint coatings the aqueous binder mixture is applied
to a prepared surface, for example sprayed onto an automobile body which
has been treated with a filler, and after air-drying is covered with a top
lacquer coat. Both coats may then be baked in a single operation, for
example for 40 to 10 minutes at 100.degree. to 180.degree. C., the
temperature being inter-related with the baking time.
The thicknesses of the individual coats to be applied to the substrate are
in general from 10 to 40, preferably from 15 to 25, .mu.m for the base
paint, and in general from 20 to 70, preferably from 30 to 50, .mu.m for
the top lacquer coat.
Examples of articles and surfaces suitable for coating with the binder
mixtures according to the invention are those of metal, plastic, wood and
the like.
The invention is illustrated by the following Examples in which parts and %
are always by weight.
EXAMPLE 1
53 Parts of a 40% aqueous solution of a water-soluble commercial
condensation product based on a polyester which contains OH groups (the
polyester having an OH number of about 100, and being derived from
phthalic anhydride, trimethylolpropane and 2,2-dimethylpropanediol), a
bis-anhydride, obtained by reacting 2 mols of trimellitic anhydride with 1
mol of propanediol and subsequent rearrangement of the product, and an
epoxidized soya bean oil having an epoxy equivalent weight of 170-180, the
aqueous solution being neutralised with dimethylaminoethanol to pH 6.5,
are compounded with 0.8 parts of a highly disperse capillary-active
silica, 9.8 parts of aluminium paste (metal content 65%; solvent: white
spirit and benzene; average particle size determined according to DIN
4190, 20 .mu.m; specific gravity 1.5), 34 parts of ethanol and 2.3 parts
of water to give a metallic base paint. This base paint is diluted by
adding about 15 parts of a mixture of ethanol and water (in the ratio of
60:40) so as to have a cup flow time appropriate for spraying, namely 16
seconds at 20.degree. C. (measured in a DIN cup with 4 mm nozzle,
according to DIN 53 211), and is sprayed pneumatically onto an automobile
sealer to give a dry film thickness of 16 .mu.m. After air-drying for 5
minutes, a lacquer having the composition shown below is sprayed, as a top
coat, onto the film, and the two coats are baked conjointly, in one
operation, for 20 minutes at 140.degree. C.
The clear lacquer for the two-coat metallic paint finishing treatment
consists of 56.6 parts of a commercial acrylic resin which can be
crosslinked with an extraneous hardening agent (a 65% solution in a 3:1
mixture of xylene and n-butanol; dynamic viscosity of a 50% solution in
xylene: 1 140 mPa.s/20.degree. C.; acid number 13, based on solid resin),
28.7 parts of a partially isobutanol-etherified melamine resin (about 55%
solution in a 44:1 isobutanol/xylene mixture, molar ratio of
melamine:formaldehyde=1:4, etherified with 3 mols of isobutanol; dynamic
viscosity of a 55% solution in butanol: 500 mPa.s/20.degree. C. as a
hardener, 1.5 parts of an aromatic hydrocarbon mixture of boiling range
180.degree. to 210.degree. C., 5.3 parts of isobutanol and 9.7 parts of
xylene. This clear lacquer is brought to a spraying consistency (a 26
second flow time at 23.degree. C., measured in a DIN cup with 4 mm nozzle,
according to DIN 53 211) with a hydrocarbon mixture of boiling range
150.degree. to 180.degree. C., and is applied to give a dry film thickness
of 40 .mu.m.
EXAMPLE 2
Example 1 is repeated, with the difference that in place of the 9.8 parts
of aluminium paste only 6.6 parts of aluminium paste and 3.2 parts of a
copper phthalocyanine pigment in the pure .beta.-modification (density 1.6
g/cm.sup.3, specific surface area 57 m.sup.2 /g, mean particle size 0.066
.mu.m) are employed.
EXAMPLE 3
Example 1 is repeated, with the difference that in place of the 9.8 parts
of aluminium paste only 6.6 parts of aluminium paste and 3.2 parts of a
benzimidazolone pigment (density 1.5 g/cm.sup.3, specific surface area
17.5 m.sup.2 /g, mean particle size 0.23 .mu.m) are employed.
Examples 2 and 3 give coatings with good properties similar to those
obtained in Example 1.
COMPARATIVE EXAMPLE 1
45 Parts of a partially condensed commercial alkyd resin consisting of a
castor oil alkyd and a maleate oil (70% solution in a 10:1
water/diacetone-alcohol mixture; dynamic viscosity (diluted 100:20 with
water) 90 mPa.s/20.degree. C.; density 1.07) are compounded with 4 parts
of a partially methanol-etherified melamine resin (about 70% in
isobutanol, molar ratio of melamine:formaldehyde=1:4, etherified with 3
mols of methanol; dynamic viscosity (70% solution in isobutanol) 2 400
mPa.s/20.degree. C.; acid number <1), 0.8 part of pyrogenic silica, 9.8
parts of the aluminium paste mentioned in Example 1 and 5.8 parts of
water, to give a metallic paint, which is diluted, sprayed and baked, all
as in Example 1.
COMPARATIVE EXAMPLE 2
Comparative Example 1 was repeated, but 22.5 parts of an oil-free saturated
commercial polyester resin (80% solution in a 12:11:7 mixture of
butylglycol/butyldiglycol/ethylglycol; dynamic viscosity (60% solution in
ethylglycol) at 20.degree. C.: 500 mPa.s; density 1.11; acid number (based
on solid resin, determined according to DIN 53 402): 60) were used as the
resin component.
The results of various tests on the products of Example 1 and Comparative
Examples 1 and 2 are shown in the following Table.
TABLE
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Comparative Comparative
Example 1 Experiment 1
Experiment 2
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Shelf life of the
No change Severe greying of the
Has gelled
base paint: examined
aluminum particles
after 90 days storage
at 20.degree. C.
Application by
No run-off
Run-off, "sags" and
Run-off, "sags" and
spraying blisters blisters
Dryness of the base
Dust-dry, non-tacky
Slightly tacky
Very tacky
paint after 5 min-
utes' air-drying,
before overcoating
Assessment of the
Flawless metallic-
Cloudy (flop) effect;
Severe dissolving of
baked two-coat
effect finish with
gloss distinctly
the base paint sur-
finish high gloss
poorer than Example 1
face; aluminum
particles have
floated up into the
top coat
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##STR2##
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