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Claims  |
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We claim:
1. A process for preparing a multilayer coating on a substrate consisting
of at least one base coating containing pigments on said substrate and at
least one transparent top coating on said base coating with improved
pigment settling on storage of said base coating material, and excellent
pigment orientation in said multilayer coating, comprising:
(1) applying a coating composition as said base coating consisting
essentially of
(a) 10 to 60% by weight of a film-forming synthetic resin;
(b) 40 to 80% by weight of an organic solvent in which said synthetic resin
is soluble;
(c) 2.5 to 25% by weight of polymer microparticles having ionic or a
mixture of polar and ionic groups on their surfaces and having a diameter
of 0.01 to 5 microns prepared by emulsion-polymerizing
ethylene-unsaturated monomers in an aqueous phase, at least one of said
monomers selected from the group consisting of acrylic acid, methacrylic
acid, and substituted alkyl esters thereof and containing a polar
functional group and at least another of said monomers selected from the
group consisting of an alkyl ester of acrylic acid, an alkyl ester of
methacrylic acid, styrene and methyl styrene, said another of said
monomers free of polar functional groups and a multi-functional
crosslinking agent present in an amount of 5 to 70% by weight of the total
weight of said ethylenically unsaturated monomers and said crosslinking
agent, said crosslinking agent crosslinking said microparticles to a
degree that said polar and said ionic groups are fixed on the surface of
said microparticles, and subsequently removing water from said aqueous
phase, said microparticles being insoluble in the solution of said
synthetic resin in said organic solvent, and having said polar and ionic
groups fixed on the surfaces, rearrangement of said polar and ionic groups
into the particle interior being prevented by the crosslinking from said
crosslinking agent; and
(d) 2 to 40 weight percent pigments
(2) applying said transparent top coating on said base coating; and
(3) curing (1) and (2) together to form a crosslinked transparent top coat
film upon a base coat film.
2. The process of claim 1, wherein said microparticles (c) are prepared in
the form of an aqueous dispersion, and as such are added to a solution of
said synthetic resin (a) in said organic solvent (b) and thereupon water
from said aqueous dispersion is removed by azeotropic distillation.
3. The process of claim 1, wherein said microparticles (c) are prepared by
spray drying, freeze drying, centrifuging, salting out or freezing out of
the aqueous phase and thereupon are added to a solution of said synthetic
resin (a) in said solvent (b).
4. The process of claim 1, wherein said microparticles (c) are obtained in
the form of an aqueous dispersion and as such are added to the components
for the preparation of said synthetic resin (a), said synthetic resin (a)
is prepared in the presence of said microparticles (c) and water is
removed from said aqueous dispersion by azeotropic distillation during the
preparation of said synthetic resin (a).
5. The process of claim 1, wherein said synthetic resin (a) is a homo- or
co-polymer prepared by polymerizing ethylene-unsaturated monomers having
at least a portion of said synthetic resin as alkylesters of acrylic or
methacrylic acid.
6. The process of claim 1, wherein said synthetic resin (a) is an alkyd
resin or a polyester resin.
7. The process of claim 1, wherein said synthetic resin (a) is an
aminoplastic resin.
8. The process of claim 1, wherein said microparticles (c) are prepared by
homo- or co-polymerizing ethylene-unsaturated monomers having at least a
portion of alkylesters of acrylic or methacrylic acid.
9. The process of claim 1, wherein said top coating is applied as a
film-forming synthetic resin in an organic solvent.
10. The process of claim 1, wherein said top coating is deposited in the
form of a coating composition containing:
(a) a film-forming synthetic resin;
(b) an organic solvent in which said synthetic resin is soluble; and
(c) polymer microparticles having polar, ionic or a mixture of polar and
ionic groups on their surfaces and having a diameter from about 0.01 to 5
microns prepared by emulsion-polymerizing ethylene-unsaturated monomers,
said monomers having one portion comprising 2 or more double bonds per
molecule, in an aqueous phase and by subsequent removal of water from said
aqueous phase, and said microparticles being insoluble in the solution of
said synthetic resin in said organic solvent.
11. The process of claim 1, wherein said top coating is a pulverulent
coating composition.
12. The process of claim 1, wherein said pigments (d) are metallic
pigments.
13. The process of claim 1, wherein the concentration of said
microparticles (c) in said coating composition amounts to about 2-60% by
weight referred to the total weight of said synthetic resin (a) and said
microparticles (c).
14. A coated substrate having a multilayer coating thereon prepared by the
process of claim 12.
15. The process of claim 1, wherein said polymer microparticles have a
diameter of 0.05 to 0.5 microns.
16. The process of claim 1, wherein said polar functional groups are
selected from hydroxy groups, tertiary amine, and sulfonic acid groups.
17. The process of claim 16, wherein the amount of ethylenically
unsaturated monomer containing said hydroxy, tertiary amine or sulfonic
acid groups used in the preparation of the polymeric microparticles ranges
from 10 to 50 weight percent and the amount of said ethylenically
unsaturated monomer free from said hydroxy, tertiary amine or sulfonic
acid groups ranges from 70 to 10 percent by weight of the total weight of
said ethylenically unsaturated monomers and crosslinking agent.
18. The process of claim 1, wherein the amount of the multifunctional
crosslinking agent used for the preparation of the polymer microparticles
amounts to 10 percent to 50 percent by weight of the total weight of
ethylenically unsaturated monomers and crosslinking agent.
19. The process of claim 1, wherein the concentration of said
microparticles (c) in said base coat composition amounts to about 5 to 50
percent and preferably 10 to 35 percent by weight referred to the total
weight of said synthetic resins (a) and said microparticles (c).
20. The process of claim 1, wherein said film-forming synthetic resins (a)
are a combination of an aminoplast resin and a copolymer of a mixture of
ethylenically unsaturated monomers, at least one of said monomers selected
from the group consisting of acrylic acid, methacrylic acid, a substituted
alkyl ester of acrylic acid containing a free hydroxy group, a substituted
alkyl ester of methacrylic acid containing a free hydroxy group, a
substituted alkyl ester of acrylic acid containing a free epoxy group and
a substituted alkyl ester of methacrylic acid containing a free epoxy
group and at least another of said monomers selected from the group
consisting of unsubstituted alkyl ester of acrylic acid, unsubstituted
alkyl ester of methacrylic acid, styrene and methyl styrene aminoplast
resin to copolymer being from 50:50 to 16:84 weight percent.
21. The process of claim 1, wherein said film-forming synthetic resins (a)
are a combination of an aminoplast resin and a second resin selected from
the group consisting of alkyd resins and polyester resins and the weight
ratio of aminoplast resin to said second resin is from 50:50 to 16:84.
22. The process of claim 1, wherein said transparent coating is deposited
in the form of a coating composition containing:
(a) film-forming synthetic resins; and
(b) organic solvents.
23. The process of claim 22, wherein said film-forming synthetic resins are
a combination of a thermosetting acrylic copolymer and an aminoplast
resin.
24. The process of claim 22, where said film-forming resins are a
combination of an alkyd resin and a second resin selected from the group
consisting of polyester resins and aminoplast resins.
25. The process of claim 22, where said film-forming resins are a
combination of an alkyd resin and a second resin selected from the group
consisting of polyester resins, thermosetting acrylic copolymers, and
aminoplast resins.
26. The process of claim 22, wherein said transparent coating is deposited
in the form of a coating composition consisting essentially of
(a) film-forming synthetic resins;
(b) organic solvents; and
(c) polymeric microparticles having a diameter of 0.01 to 5 microns,
prepared by emulsion polymerization in water of a mixture of ethylenically
unsaturated monomers, at least one of said monomers selected from the
group consisting of acrylic acid, methacrylic acid, and substituted alkyl
esters thereof and containing polar functional groups and at least another
of said monomers being selected from the group consisting of alkyl esters
of acrylic acid, alkyl esters of methacrylic acid, styrene and methyl
styrene and being free of polar functional groups, and a multifunctional
crosslinking agent present in an amount of at least 5 and up to 50 percent
by weight of the total weight of said ethylenically unsaturated monomers
and crosslinking agent, and by subsequent removal of water from said
aqueous phase, said microparticles being insoluble in the solution of said
synthetic resin in said organic solvent.
27. The process of claim 26, wherein the concentration of said
microparticles in said transparent top coating composition amounts to from
2 to 15 percent by weight referred to the total weight of said synthetic
resins and microparticles. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The field of the invention is combined coating processes and the invention
is particularly concerned with preparing a multilayer coating consisting
of at least one base coating containing pigments and at least one
transparent top coating, the multilayer coating being applied to a
substrate.
It is known to provide substrates with multilayer coatings as disclosed in
U.S. Pat. Nos. 3,639,147 and 4,220,679, the disclosures of which are
incorporated herein. Metal and plastic substrates are suitable for these
multilayer processes. In particular, the present invention relates to a
process for the deposition of a multilayer coating on automobile bodies or
parts thereof which are made of metal or plastic and which as a rule are
pretreated with a primer system. Until now, automobile bodies have been
predominantly made of metal, however, bodies or parts thereof made of
plastic are explicitly included herein as substrates.
Coatings deposited on substrates are both for protective and decorative
purposes. The coating compositions used in the preparation of such
coatings frequently contain pigments which improve the protective and
decorative effect of the coatings. The term pigment herein means colored
and hueless, organic and inorganic pigments, filler or dyestuffs which are
soluble or insoluble in solvents or vehicles. Especially as regards
automobile enameling, metallic pigments have for some time been widely
used. They offer a varying reflection of incident light as a function of
the angle of observation. This effect is frequently called the "flip-flop"
effect and depends on the orientation of the flake-like metallic pigments
in the finished coating.
Great efforts are exerted when making metallic-effect coatings that an
optimal orientation of the metallic pigments be achieved. Until now,
coating compositions based on acrylate polymers or polyesters containing
hydroxyl groups have been used for such purposes in combination with the
conventional crosslinking agents and in organic solvents containing
cellulose esters, for instance cellulose acetobutyrate and other enameling
accessories. These known coating compositions suffer from the drawback
that their solid content is low and hence their organic solvent content is
high. This is uneconomical and results in appreciable stress on the
ecology. Another drawback is that the metallic pigments settle markedly
and are difficult to stir again into the orginial state.
It has been proposed to solve the above cited problems by the use of
microparticles in the coating composition. The term microparticles means
polymer particles insoluble in a coating composition. U.S. Pat. No.
4,220,679 discloses a process for the preparation of a protective and/or
decorative multilayer coating on a substrate, wherein the base coat
deposited is a coating composition containing polymeric microparticles
having a diameter of 0.01 to 10 microns. The microparticles are stably
dispersed by a steric stabilization (steric barrier). The steric
stabilization is implemented by polymer chains arranged around the
particles and these polymer chains are solvated by the solvent of a
film-forming polymer system. The microparticles therefore are provided
with a shell or a boundary zone of polymer material anchored by primary
and/or secondary valences to the particle core. Again, these coating
compositions which are known from U.S. Pat. No. 4,220,679, tend to have
strong settling of the metallic pigments, thereby degrading the metallic
effect of the finished coatings.
SUMMARY OF THE INVENTION
Having in mind the limitations of the prior art, applicants have disclosed
that coatings of improved decorative effectiveness are obtained,
especially where metallic pigments are used, provided the coating
composition contains special microparticles which are highly crosslinked
and comprise polar groups on their surface. These special particles
strongly delay or reduce settling, and even in the event of settling, the
sediments which result are loose and easily stirred up.
An object of the present invention therefore is a process for preparing a
multilayer coating wherein first the base coating is deposited in the form
of a coating composition which contains:
(A) a film-forming synthetic resin;
(B) an organic solvent in which the synthetic resin is dissolved;
(C) microparticles having polar groups at their surfaces and having a
diameter from about 0.01 to 5 microns prepared by emulsion-polymerizing
ethylene-unsaturated monomers, at least a portion of which contains two or
more double bonds per molecule, in an aqueous phase and with ensuing
removal of the water from the aqueous phase and which are insoluble in the
synthetic resin solution in the organic solvent and
(D) pigments. Next a coating composition is deposited as a transparent top
coating of the multilayer coating and both compositions are hardened or
cured together.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The film-forming polymer constituent (A) of the basecoat composition used
in the first step of the process may be any of the polymers known to be
useful in coating compositions. One suitable class of polymer consists of
those which are derived from one or more ethylene unsaturated monomers.
Particularly useful members of this class are the acrylic polymers which
are well established for the production of coatings in the automobile
industry, that is to say polymers or copolymers of one or more alkyl
esters of acrylic acid or methacrylic acid, optionally together with other
ethylene unsaturated monomers. These polymers may be of either the
thermoplastic type or the thermosetting, crosslinking type. Suitable
acrylic esters for either type of polymer include methyl methacrylate,
ethyl methacrylate, propyl methacrylate, butyl methacrylate, ethyl
acrylate, butyl acrylate and 2-ethylhexyl acrylate. Suitable other,
copolymerizable monomers include vinyl acetate, vinyl propionate,
acrylonitrile, styrene and vinyl toluene. Where the polymer is required to
be of the crosslinking type, suitable functional monomers to be used in
addition to the latter include acrylic acid, methacrylic acid,
hydroxyethyl acrylate, hydroxyethyl methacrylate, 2-hydroxypropyl
acrylate, 2-hydroxypropyl methacrylate, N-(alkoxymethyl) acrylamides and
N-(alkoxymethyl) methacrylamides, where the alkoxy group may be, for
example, a butoxy group, glycidyl acrylate and glycidyl methacrylate. The
base-coat composition may in such a case contain also a crosslinking agent
such as a diisocyanate, a diepoxide or, especially, a nitrogen resin, that
is to say a condensate of formaldehyde with a nitrogeneous compound such
as urea, thiourea, melamine or benzoguanamine, or a lower alkyl ether of
such a condensate in which the alkyl group contains from 1 to 4 carbon
atoms. Particularly suitable crosslinking agents are melamine-formaldehyde
condensates in which a substantial proportion of the methylol groups have
been etherified by reaction with butanol or alcohols like ethanol or
methanol.
For the purposes of the foregoing general definition of the invention, the
crosslinking agent, where present, is considered as being a part of the
film-forming polymer (A).
The base-coat composition may incorporate a suitable catalyst for the
crosslinking reaction between the film-forming polymer (A) and the
crosslinking agent, for example an acid-catalyst compound such as acid
butyl maleate, maleic acid, acid butyl phosphate or p-toluene sulphonic
acid. Alternatively the catalytic action may be supplied by the
incorporation of free acid groups in the film-forming polymer, for example
by the use of acrylic acid or methacrylic acid as comonomer in the
preparation of an acrylic polymer. Metal salts or combinations of metal
salts and acids are suitable as catalysts. Again blocked catalysts, for
instance acid amides, are also suitable.
The film-forming polymer may be prepared by solution polymerization of the
monomer(s), in the presence of suitable catalysts or initiators such as
organic peroxides or azo compounds, e.g. benzoyl peroxide or
azodiisobutyronitrile. Conveniently the polymerization may be carried out
in the same organic liquid that is to form the diluent constituent (B) of
the base-coat composition, or in a liquid which is to form a part of that
diluent. Alternatively the acrylic polymer may be prepared in a separate
previous operation (e.g. by aqueous emulsion polymerization) and then
dissolved in a suitable organic liquid.
Other suitable members of the class of polymer derived from ethylene
unsaturated monomers are vinyl copolymers, that is to say copolymers of
vinyl esters of inorganic or organic acids, for example vinyl chloride,
vinyl acetate and vinyl propionate; the copolymers may optionally be
partially hydrolyzed so as to introduce vinyl alcohol units. An example of
such a copolymer is that containing 91% vinyl chloride, 6% vinyl alcohol
and 3% vinyl acetate by weight.
Instead of being a polymer derived from ethylene unsaturated monomers, the
polymer constituent (A) of the base-coat composition may be an alkyd resin
or a polyester.
Such polymers may be prepared by condensation of polyhydric alcohols and
polycarboxylic acids, with or without the inclusion of natural or
synthetic drying oil fatty acids. Suitable polyhydric alcohols include
ethylene glycol, propylene glycol, butylene glycol, 1,6-hexylene glycol,
neopentyl glycol, diethylene glycol, triethylene glycol, tetraethylene
glycol, glycerol, trimethylolpropane, trimethyolethane, pentaerythritol,
dipentaerythritol, tripentaerythritol, hexanetriol, oligomers of styrene
and allyl alcohol (for example that sold by Monsanto Chemical Company
under the designation RJ 100) and the condensation products of
trimethylolpropane with ethylene oxide or propylene oxide (such as the
products known commercially as "Niax" triols). Suitable polycarboxylic
acids include succinic acid (or its anhydride), adipic acid, azelaic acid,
sebacic acid, maleic acid (or its anhydride), fumaric acid, malonic acid,
itaconic acid, phthalic acid (or its anhydride), isophthalic acid,
terephthalic acid, trimellitic acid (or its anhydride) and pyromellitic
acid (or its anhydride). Where it is desired to produce air-drying alkyd
resins, suitable drying oil fatty acids which may be used include those
derived from linseed oil, soya bean oil, tall oil, dehydrated castor oil,
fish oils or tung oil. Normally it is preferred that the oil length of
such an alkyd resin should not exceed 50%. Alkyd resins with synthetic,
non-drying fatty acids, for instance those prepared using the
glycidylester of versatic acid (Cardura E-trade name of Shell Corp.) are
also suitable. All these polyester and alkyd resins contain a proportion
of free hydroxyl and/or carboxyl groups which are available for reaction,
if desired, with suitable crosslinking agents as discussed above.
The polymer constituent (A) of the base coat composition may contain minor
amounts of a cellulose ester, in particular cellulose acetate butyrate,
depending on the requirements concerning the allowed amount of solvent in
the base coat formulation.
Yet another type of polymer which may be employed as the constituent (A)
comprises the nitrogen resins, which have already been described in the
role of crosslinking agents for acrylic polymers of the thermosetting
type. These same resins can be employed as film-forming materials in their
own right and, for this purpose; the preferred resins are again melamine-
or urea-formaldehyde condensates in which a substantial proportion of the
methylol groups are etherified by reaction with butanol, methanol etc. In
order to assist curing of the resin, there is preferably also incorporated
into the base-coat composition a suitable catalyst, such as one of those
already described. From what has been said above, it will be clear that
there may also be employed as the film-forming constituent (A) a mixture
of a thermosetting acrylic polymer and a nitrogen resin in such
proportions that part of the latter functions as crosslinking agent and
part as a supplementary film-former in its own right.
The volatile organic liquid constituent (B) of the base-coat composition
may be any of the liquids, or mixtures of liquids, which are
conventionally used as polymer solvents in coating compositions, for
example aliphatic hydrocarbons such as hexane and heptane, aromatic
hydrocarbons such as toluene and xylene, and petroleum fractions of
various boiling point ranges which are predominantly aliphatic but have a
significant aromatic content, esters such as butyl acetate, ethylene
glycol diacetate and 2-ethoxyethyl acetate, ketones such as acetone and
methyl isobutyl ketone, and alcohols such as butyl alcohol. The actual
liquid or mixture of liquids selected as the diluent (B) will depend upon
the nature of the film-forming polymer (A), according to principles which
are well known in the coatings art, in order that the polymer shall be
soluble in the diluent.
The microparticles (C) of the present invention are crosslinked particles
and evince polar groups on their surfaces. These polar groups can be
achieved for instance by polymerizing acrylate or methacrylate monomers
having polar groups in an aqueous medium. Alternatively the polar groups
of the microparticles (C) also may arise as polymer terminal groups from
the dissociation of the polymerization initiator, for instance from the
decay of potassium persulfate. They may also be obtained from the
emulsifier added. This emulsifier for instance can be an unsaturated,
water-soluble polyester which is copolymerized with the monomers and of
which the carboxyl groups induce the electrostatic stabilization in the
aqueous phase. It can also be apposed by means of secondary valences. The
polar groups furthermore can be introduced by hydrolysis, for instance
hydrolysis of ester groups, or they can be produced by subsequent
reactions of addition to and at excess double bonds. Due to the high
polarity of the aqueous medium, the polar groups arrange themselves in
preferred manner on the particle surface, whereas non-polar groups
preferredly are arranged in the particle core.
The microparticles so prepared then are transferred into the non-polar
organic medium of the coating substance, as is explained more
comprehensively further below. Care must be taken to prevent the polar
groups from re-orienting themselves in the microparticle inner part, as in
such an eventuality the microparticles become ineffective in the sense of
the invention described. A high degree of crosslinking of the particles by
means of polyfunctional monomers prevents this re-orientation of the polar
groups when transferring the microparticles into an organic, non-polar
medium, whereby polar groups are also present in the coating composition
on the surface of the microparticles (C).
Because of the polar groups, there are interactions between the
microparticles and a spatial lattice is formed. The interactions for
instance may be due to dipole-dipole forces or hydrogen bridge bonds. Due
to the interactions, a loose spatial lattice is formed between the
particles in the coating substance, which can be reversibly dismantled by
shearing forces. The loose lattice, i.e., gel structure renders the
coating composition structurally viscous or thixotropic.
Controlling pigment settling in top coat enamels by the use of thixotropic
agents is in fact known. However as regards metallic enamels, the
conventionally used thixotropic agents such as Bentone pastes are
inapplicable because in contrast to the microparticles described in the
present invention, they do not improve the settling behavior of metallic
pigments to the desired degree, rather they degrade the metallic effect
and delay the physical drying of the base enamel.
In addition to the interactions between the microparticles, there are also
interactions between the microparticles and the pigments. Frequently
pigments themselves have polar groups on their surfaces, or are
correspondingly pretreated. On account of the interactions between the
microparticles and the pigments, settling of these pigments is prevented,
and an improved effect is obtained in the finished coating. This applies
especially to metallic enamels.
The preparation of the microparticles (C) is carried out by
emulsion-polymerizing two or more ethylene-unsaturated monomers of which
one preferably comprises a hydroxyl, carboxyl amide, epoxide or other such
polar group and at least one is free from such groups. A further required
constitutent is a polyfunctional crosslinking agent. Typically the monomer
containing the --OH or --COOH group is acrylic acid, methacrylic acid,
hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate
or hydroxypropylmethacrylate. The ethylene unsaturated monomers which are
free from --OH and --COOH groups may be, for example, the alkyl esters of
acrylic or methacrylic acid, particularly those having from 1 to 4 carbons
in the acrylate; and methyl, ethyl, propyl or butyl acrylate; and methyl,
ethyl, propyl or butyl methacrylate. Other suitable monomers include
styrene or alpha-methyl styrene.
The crosslinking agent may be any such agent which contains at least two
ethylene unsaturated double bonds and will give a crosslinked polymer in
aqueous emulsion polymerization that is insoluble in the organic solvent
which is ultimately used to make up the acrylic resin compositions. As
examples of suitable crosslinking agents there may be mentioned the
following although it is noted that the invention is not limited thereto:
ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate,
methylene bisacrylamide, methylene bismethacrylamide, divinyl benzene,
vinyl methacrylate, vinyl crotonate, vinyl acrylate, divinyl acetylene,
trivinyl benzene, glycerine trimethacrylate, pentaerythritol
tetramethacrylate, triallyl cyanurate, divinyl ethane, divinyl sulfide,
divinyl sulfone, hexatriene, triethylene glycol dimethacrylate, diallyl
cyanamide, glycol diacrylate, ethylene glycol divinyl ether, diallyl
phthalate, trimethylolpropane diallyl ether, divinyl dimethyl silane
glycerol trivinyl ether, trimethylolpropane triacrylate, trimethylolethane
triacrylate, trimethylolpropane trimethacrylate, trimethylolethane
trimethacrylate, tetramethylene glycol dimethacrylate, triethylene glycol
dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycol
dimethacrylate, pentaerythritol diacrylate, pentaerythritol triacrylate,
pentaerythritol tetraacrylate, dipentaerythritol diacrylate,
dipentaerythritol triacrylate, dipentaerythritol tetraacrylate,
dipentaerythritol pentaacrylate, dipentaerythritol hexacrylate,
tripentaerythritol octoacrylate, pentaerythritol dimethacrylate,
pentaerythritol trimethacrylate, dipentaerythritol dimethacrylate,
dipentaerythritol tetramethacrylate, tripentaerythritol octamethacrylate,
pentaerythritol diitaconate, dipentaerythritol trisitaconate,
dipentaerythritol pentaitaconate, dipentaerythritol hexaitaconate,
1,3-butanediol diacrylate, 1,3-butanediol dimethacrylate, 1,4-butanediol
diitaconate, sorbitol triacrylate, sorbitol tetraacrylate, sorbitol
tetramethacrylate, sorbitol pentaacrylate, sorbitol hexacrylate, modified
1,4-butylene diacrylate, 1,6 hexanediol diacrylate, modified
trimethylolpropane triacrylate, modified pentaerythritol triacrylate, poly
functional isocyanates with hydroxy monomers (isophorone diisocyanate plus
hydroxy ethyl acrylate), methacrylated epoxy resin, and the like, and
mixtures and prepolymers thereof.
The amount of ethylene unsaturated monomers used to make up the microgel
can be widely varied. Typical amounts will fall in the range of 10-90% by
weight of monomer or monomers containing --OH or --COOH groups and 90-10%
by weight of monomer or monomers which are free from such groups.
The amount of crosslinking monomer that is used is important although this
can be varied over a relatively large range. Desirably the amount of
crosslinking monomer constitutes at least about 2% and up to 20% by weight
of the total amount of all materials used for the emulsion polymerization.
(The lower limit of 2% applies only to very effective crosslinking agents,
for instance divinylbenzene.) In the normal situation this is roughly
equivalent to a range of 5-70%, preferably 8-50%, by weight of the total
weight of ethylene unsaturated monomers and crosslinking monomer.
Conventional emulsifiers or surfactants and initiators for emulsion
polymerization are used to prepare the microgel emulsion. Typical
emulsifiers (or surfactants) include such anionic alkali metal salts of
organic carboxylic acids or soaps, e.g., potassium oleate or stearate.
Alkyl sulfates and alkyl- or alkylarylsulfonates may also be used.
Preferred for use are the sodium alkyl sulfosuccinamates or
alkylarylsulfonates e.g., disodium N-octadecyl sulfosuccinamate, sodium
diamyl octadecyl sulfosuccinamate, sodium diamyl sulfosuccinamate and
sodium dodecylbenzene sulfonate. Advantageously, two or more such
surfactants are used together since this seems to given an emulsion of
better stability. Ionic and non-ionic emulsifiers may also be combined.
Suitable initiators are the free radical yielding peroxides and persalts
e.g., benzoyl peroxide, lauroyl peroxide, ammonium, potassium or sodium
persulfate, etc.
Advantageously the emulsion polymerization is carried out by adding the
monomers gradually to heated water containing the emulsifier and
initiator. Preferably the system is heated to, for example,
75.degree.-90.degree. C. during the monomer addition and for a short time
afterwards although it will be appreciated that the polymerization
conditions can be widely varied and will depend on other factors, e.g.,
the monomers involved. It is, in any case, important to obtain a stable
emulsion in which the polymer particles have sizes of the order of about
0.01 to 5 microns, preferably 0.01 to 0.5 microns.
Therefore the aqueous microgel must be transferred into the organic medium
of the coating composition. Basically two different procedures are
suitable to this end. Either the microparticles can be separated from the
aqueous emulsion, and this can be done by methods such as spray drying,
centrifuging, salting out, freezing out, freeze-drying, and the separated
microparticles are then added to a mixture of the remaining constituents
of the coating composition, or else first the microgel is added to
individual constituents of the coating composition.
In the second option for transferring the microparticles into the organic
phase, the aqueous emulsion is added to a solution of the synthetic resin
(A) in the organic solvent (B) or to individual constituents of the
coating composition and thereupon the water is removed in part or in
whole. It is possible also to add the aqueous emulsion to the initial
materials for the preparation of the synthetic resin (A) and to prepare
this synthetic resin in the presence of the microparticles.
In one illustrative procedure, the organic solvent (B) or an ingredient of
the mixture of solvents used is added to the aqueous emulsion. The water
than can be removed by azeotropic distillation, freezing-out or shake-out.
In the latter case the solvent may not be miscible with water, or only
partly, and the microgel must migrate preferredly into the organic phase.
A suitable organic solvent is n-butanol or a mixture of n-butanol and
n-propanol.
If the aqueous microgel emulsion is azeotropically distilled any convenient
organic solvent which forms an azeotrope with water may be used.
Typically suitable solvents include CELLOSOLVE ACETATE, butanol, naphtha,
mineral spirits and the like. The amount of solvent used can be varied but
should be enough to facilitate azeotropic removal of the water. All of the
water of the microgel emulsion may be removed or only a portion thereof.
Usually at least 50% by weight of the water should be removed.
Alternatively the microparticles (C) can be precipitated from the aqueous
emulsion using the solvent. The coagulate can be separated by filtering
from the main quantity of water and can be further dried by azeotropic
distillation, however, the coagulate may also float out of the aqueous
phase, so that the main quantity of water can be merely drained off.
U.S. Pat. No. 4,220,679 does indeed disclose the possibility of preparing
the microparticles by emulsion-polymerization in the aqueous phase;
however, these microparticles are only weakly crosslinked, or not
crosslinked at all, whereby any polar groups present when fed into a
non-polar medium would reorient themselves and no longer be located on the
particle surface. Moreover an accessory polymer is used as regards these
known microparticles and in association with any polar groups that would
have remained on the surfaces would build up a non-polar solvating
envelope. This envelope provides then the steric stabilization considered
to be necessary.
U.S. Pat. No. 4,220,679 therefore does not make it obvious to use
microparticles with polar groups also fixed to their surfaces when in the
organic medium of the coating substance and causing attractive
interactions, rather it leads one skilled in the art away from the present
invention because of its feature of steric stabilization which was
explicitly held essential and mandatory.
The proportion of the microparticles (C) of the coating composition for the
base coat advantageously amounts to 3 to 60% by weight referred to the sum
of the synthetic resin (A) and the microparticles (C).
The pigment particles (D) included in the base-coat composition may range
in size from 1 to 50 microns and may be of any of the pigments
conventionally used in surface coating compositions, including inorganic
pigments such as titanium dioxide, iron oxide, chromium oxide, lead
chromate and carbon black, and organic pigments such as phthalocyanine
blue and phthalocyanine green, carbazole violet, anthrapyrimidine yellow,
flavanthrone yellow, isoindoline yellow, indanthrone blue, quinacridone
violet and perylene reds. For the present purposes, the term "pigment" is
here meant to embrace also conventional fillers and extenders, such as
talc or kaolin.
The process of the invention is, however, of particular value in the case
of base-coat compositions containing metallic flake pigmentation which are
intended for the production of "glamour metallic" finishes chiefly upon
the surfaces of automobile bodies as previously discussed. The presence of
the polymer microparticles (C) in base-coats containing metallic
pigmentation gives a valuable degree of improvement in metal control
during the application of the base-coat and the subsequent application of
the transparent top-coat. Suitable metallic pigments include in particular
aluminum flake and copper bronze flake. In general, pigments of any kind
may be incorporated in the base-coat composition in an amount of from 2%
to 100% of the aggregate weight of the film-forming polyer (A) and the
microparticles (C). Where metallic pigmentation is employed, this is
preferably in an amount of from 5% to 20% by weight of the aforesaid
aggregate weight.
Such pigments, whether metallic or otherwise, may be incorporated into the
base-coat compositions with the aid of known dispersants. Thus, in the
case where the main film-forming polymer is of the acrylic type, an
acrylic polymer of similar composition may be employed as pigment
dispersant. Any such polymeric dispersant is also considered to be part of
the film-forming constituent (A).
The coating composition for the transparent top coat can be film-forming
synthetic resin in an organic solvent. The synthetic resins described as
component (A) are illustratively suitable for such application. The
synthetic resin can be present in the solvent in dissolved or dispersed
form.
The top coating also may be a coating composition corresponding to that of
the base coat but contrary to latter being free of pigments. The top coat
furthermore may be a pulverulent coating composition. These so-called
power enamels can be applied by spraying electrostatically.
The process of the present invention is carried out so that first the
coating composition is deposited on the base layer. Advantageously this
coating composition is allowed to dry for a short time, whereby the
organic solvent escapes at least in part and a film is formed which will
not be degraded when subsequently the top coating is deposited. The drying
may take place at room temperature, or higher temperatures may be applied.
Thereupon the coating composition of the top layer is deposited and
thereupon both coatings are hardened by baking. The baking temperature
depends on the kind of coating composition used and is for instance about
100.degree. to 140.degree. C. The layer thickness of the base coating
ordinarily is about 10 to 15 microns, that of the hardened top coating
about 20 to 60 microns.
In a variation of the process described, two or more layers of the coating
composition of the base or top coatings are deposited also in order to
obtain a thicker layer and a better decorative effect and, better
protection.
The present invention also applies to a coating composition for preparing
coatings on substrates and containing:
(A) a film-forming synthetic resin;
(B) an organic solvent in which the synthetic resin is dissolved;
(C) microparticles comprising polar groups on their surfaces and prepared
by emulsion-polymerizing ethylene-unsaturated monomers of which at least a
portion comprises two or more double bonds per molecule in the aqueous
phase and by subsequently removing the water, and which are insoluble in
the solution of the synthetic resin in the organic solvent; and
(D) pigments.
The present invention furthermore is concerned with a substrate coated by a
multilayer coating prepared by the process of the present invention.
The present invention is explained below in more detail in relation to
specific examples.
EXAMPLE 1: PREPARING A MICROGEL EMULSION
(A)
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Initial mixture 1:
water 1,568 g
sodium dodecylbenzene sulfonate
10 g
Initial mixture 2:
water 792 g
sodium dodecylbenzene sulfonate
18 g
hexanediol diacrylate 347 g
methylmethacrylate 277 g
hydroxypropylmethacrylate
69 g
Initial mixture 3:
water 413 g
ammonium persulfate 8.25 g
The initial mixture 1 is filled into a reactor and heated to 80.degree. C.
After reaching this temperature, 15% of the initial mixture 2 is added
with stirring and the temperature is raised again to 80.degree. C.
Thereupon 10% of the initial mixture 3 is added over 5 minutes. An
exothermal reaction takes place. The temperature is kept at 80.degree. C.
for 15 minutes by cooling. Then the inital mixture 2 is uniformly added
over 2 hours and the initial mixture 3 over 3 hours. Then the temperature
is kept another hour at 80.degree. C.
Care is taken during this preparation that the initial mixture 2 is
steadily stirred within its supply vessel.
(B)
The microgel emulsion was then converted to a microgel dispersion in the
following manner using:
66.6 parts of the microgel emulsion
16.7 parts of n-butanol
16.7 parts of methyl amyl ketone
The microgel emulsion is added to a five liter, three neck reaction flask
equipped with a thermometer, an agitator and a Dean Stark water trap.
Under agitation the n-butanol is added to the microgel emulsion in the
flask. Agitation is continued for ten minutes and then stopped. The
n-butanol causes the acrylic microgel emulsion to coagulate and float to
the surface. The bottom layer is removed from the reaction flask by
suction. The methyl amyl ketone is then added to the flask to further
dilute the acrylic microgel dispersion. The resulting mixture is heated to
reflux and the residual water is removed through the Dean-Stark water
trap. When the contents of the reaction flask reach 105.degree. C., the
contents are cooled and filtered. The resulting acrylic microgel
dispersion exhibits the following characteristics:
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Nonvolatiles 35.1%
Acid Number 8.5
Viscosity 12 seconds #4 Ford Cup
Color White
______________________________________
EXAMPLES 2-3
(A)
Microgel emulsions II and III are prepared with materials as shown on Table
I. The procedure for the preparation of these emulsions is as follows:
To a five liter, three neck reaction flask equipped with a condenser,
thermometer and agitator, Aerosol 18, Aerosol AY-65, sodium bicarbonate
and first deionized water are charged. The ammonium persulfate and second
deionized water are premixed and added to a small addition funnel. The
monomers are premixed and added to a separate addition funnel. The
surfactants and water are heated to 87.degr | | |
