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Description  |
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This invention relates to the application of protective and decorative
coatings to surfaces, particularly the surfaces of automobile bodies.
It is well known to employ, especially in the automobile industry, coating
compositions which contain metallic pigments; these are the so-called
"glamour metallic" finishes whereby a differential light reflection
effect, depending on the viewing angle, is achieved. To maximise this
"flip" tone effect, careful formulation of the coating composition in
regard both to the film-forming resin and to the liquid medium is
required. Difficulties may be encountered in formulating a single
composition which both meets this objective and at the same time achieves
a high degree of gloss in the final finish such as is usually desired in
the automobile field. For this reason, one of the procedures which has
been proposed for producing metallic finishes is a two-coat procedure, in
which there is first applied by spraying, to the surface of the substrate,
a basecoat containing the metallic pigment and formulated so as to give
the optimum "flip" effect, and there is then applied over the basecoat,
again by spraying, an unpigmented topcoat which will yield the desired
degree of gloss without in any way modifying the characteristics of the
basecoat.
For a successful two-coat metallic finish system of this
"basecoat/clearcoat" type, an essential criterion is that the basecoat
film must be able to resist attack by the solvents in the clearcoat
composition when the latter is subsequently applied, in order to avoid
disturbance of the metallic pigment and hence impairment of the "flip"
effect; it is, moreover, very desirable that the basecoat film should
possess this property without the need for an extended intermediate drying
or curing operation.
In the known basecoat/clearcoat systems, in which both the basecoat and the
clearcoat compositions are organic solvent-borne, this requirement has in
most cases been met by the use of an additive capable of imparting a
gel-like character to the freshly formed basecoat film; the additive which
has predominantly been used is cellulose acetate butyrate. The transition
between the relatively low viscosity, which the basecoat composition is
required to have at the spray gun, and this gel-like character is assisted
by arranging that the liquid diluent of the composition contains volatile
components which are preferentially lost by evaporation during the passage
from the spray gun to the substrate.
For reasons of avoiding atmospheric pollution, considerable interest has
developed in recent years in coating compositions which employ water as
the diluent rather than organic solvents. A number of such compositions
has been proposed for use in the automobile industry. It has not, however,
hitherto been possible to employ water-borne compositions satisfactorily
as the basecoat component of a basecoat/clearcoat system. One of the
factors tending to inhibit the successful achievement of such an objective
is the extreme difficulty of arranging for controlled selective loss of
diluent from the basecoat composition by evaporation between spray gun and
substrate, except by means of very expensive regulation of the ambient
humidity in the spray area. We have, however, now found that a
satisfactory water-borne basecoat composition can be based upon an aqueous
dispersion of a crosslinked polymer microgel.
Thus according to the present invention there is provided a process for the
production of a multi-layer protective and/or decorative coating upon a
substrate surface, which comprises the steps of:
(1) applying to the surface a basecoat composition comprising (a)
film-forming material (b) a volatile liquid medium for the said material
and (c) pigment particles dispersed in the said liquid medium;
(2) forming a polymer film upon the surface from the composition applied in
step (1);
(3) applying to the basecoat film so obtained a transparent topcoat
composition comprising (d) a film-forming polymer and (e) a volatile
carrier liquid for the said polymer; and
(4) forming a second polymer film upon the basecoat film from the
composition applied in step (3),
characterised in that the constituents (a) and (b) of the basecoat
composition are provided by a dispersion in an aqueous medium of
crosslinked polymer microparticles which have a diameter in the range 0.01
to 10 microns, are insoluble in the said aqueous medium and are stable
towards gross flocculation, the dispersion having a pseudoplastic or
thixotropic character.
The crosslinked polymer microparticles may be composed of various types of
polymer. Of particular interest for this purpose are the acrylic addition
polymers, derived from one or more alkyl esters of acrylic acid or
methacrylic acid, optionally together with other ethylenically unsaturated
monomers. Suitable acrylic or methacrylic esters include methyl
methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate,
ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. Suitable other,
copolymerisable monomers include vinyl acetate, vinyl propionate,
acrylonitrile, styrene and vinyl toluene. Since the polymer is required to
be crosslinked, there may be included in the monomers from which the
polymer is derived a minor proportion of a monomer which is polyfunctional
with respect to the polymerisation reaction, e.g. ethylene glycol
dimethacrylate, allyl methacrylate or divinylbenzene; alternatively there
may be included in those monomers minor proportions of two other monomers
carrying pairs of chemical groups which can be caused to react with one
another either during or after the polymerisation reaction, such as epoxy
and carboxyl (as for example, in glycidyl methacrylate and methacrylic
acid), anhydride and hydroxyl or isocyanate and hydroxyl.
The chemical composition and degree of crosslinking of the microparticle
polymer may be such that it has a Tg (glass-rubber transition temperature)
below room temperature, in which case the microparticles will be rubbery
in nature; alternatively, it may be such that the Tg is above room
temperature, that is to say the particles will be hard and glassy.
As already specified, it is necessary that the polymer microparticles be
dispersed in the basecoat composition in a state which is stable towards
gross flocculation, that is to say, a state in which, even at low solids
contents, the dispersion contains few if any multi-particle aggregates;
this does not, however, preclude the possibility of a slight degree of
particle flocculation especially at higher solids contents. This state may
be achieved, for example, by steric stabilisation, that is to say by the
provision around the particles of a barrier of chains of a different
polymer which are solvated by the aqueous medium of the composition and
hence exist in a chain-extended configuration. In this context, the term
"solvated" implies that the polymer chains in question, if they were
independent molecules, would be actually soluble in the said aqueous
medium; however, because the chains are in fact attached to the
microparticles at one or more points along their length, the steric
barrier remains permanently attached to the microparticles. Polymer
microparticles which are sterically stabilised in this way may
conveniently be produced by a process of dispersion polymerisation of the
appropriate monomers in the aqueous medium, in the presence of a steric
stabiliser. The stabiliser is amphipathic in nature, that is to say it
contains in the molecule two essential polymeric components of differing
characteristics: one component is a polymer chain which is solvated by the
aqueous medium, and the other component is a polymer chain which is not
solvated by that medium and in consequence anchors itself to the polymer
microparticles which are by definition insoluble in the aqueous medium.
Suitable dispersion polymerisation processes are described in our British
Patent Application No. 7,940,088 (now published as Application No.
2,039,497 A). The aqueous medium in which the polymerisation is carried
out consists of water admixed with a volatile organic co-solvent, the
mixture as a whole being capable of dissolving the monomers, most or all
of which would be substantially insoluble in water alone. These procedures
involve the additional requirement that the polymerisation be conducted at
a temperature which is at least 10.degree. C. higher than the glass
transition temperature of the polymer which is to be formed, and in such a
manner that at no time is there present a separate monomer phase. The
amphipathic steric stabiliser may be added to the polymerisation mixture
as a pre-formed substance, or it may be formed in situ during the
polymerisation from a polymer which is soluble in the aqueous medium and
is able to copolymerise with, or, through hydrogen abstraction, to undergo
grafting by, some of the monomer being polymerised. The sterically
stabilised microparticle dispersions which are obtained by these
procedures are very suitable for the formulation of basecoat compositions
to be used in accordance with the present invention, since it is possible
to remove the organic co-solvent from them by distillation without
impairing the stability of the disperse phase, yielding a product in which
the continuous phase consists solely of water.
Alternatively, the dispersion of polymer microparticles may be obtained by
the aqueous emulsion polymerisation of the appropriate monomers, in which
case the stability towards flocculation is conferred by the presence on
the particles of electrically charged species derived from a
water-soluble, ionised surfactant and/or a water-soluble, ionisable
polymerisation initiator. Such polymerisation processes are extensively
described in the literature.
Yet again, the polymer microparticles may be made by a process of
non-aqueous dispersion polymerisation of monomers, followed by transfer of
the resulting polymer to the aqueous medium. Such a procedure is described
in our British Patent Application No. 42457/77. (now published as
Application No. 2,006,229 A). It involves in a first step forming a
sterically stabilised dispersion in a non-aqueous liquid of a polymer
which is insoluble both in the non-aqueous liquid and in water, using any
of the procedures which are well known in the art for making such
dispersions; then in a second step polymerising in the dispersion thus
obtained, in the presence of a steric stabiliser, one or more monomers
which can give rise to a second polymer which is inherently soluble in the
desired aqueous medium at an appropriate pH; and finally transferring the
resulting composite polymer microparticles from the non-aqueous medium to
the aqueous medium.
The foregoing discussion has centred on the case where the crosslinked
polymer microparticles are composed of an addition polymer, this being the
most convenient type of polymer for the present purpose. It is possible,
however, for the microparticles alternatively to be composed of a
condensation polymer, for example a polyester prepared from a polyhydric
alcohol and a polycarboxylic acid. Suitable polyhydric alcohols include
ethylene glycol, propylene glycol, butylene glycol, 1:6-hexylene glycol,
neopentyl glycol, diethylene glycol, triethylene glycol, tetraethylene
glycol, glycerol, trimethylolpropane, trimethylolethane, pentaerythritol,
dipentaerythritol, tripentaerythritol, hexanetriol, oligomers of styrene
and allyl alcohol (for example that sold by Monsanto Chemical Company
under the designation RJ 100), the condensation products of
trimethylolpropane with ethylene oxide or propylene oxide (such as the
products known commercially as "Niax" triols: "Niax" is a Registered Trade
Mark). Suitable polycarboxylic acids include succinic acid (or its
anhydride), adipic acid, azelaic acid, sebacic acid, maleic acid (or its
anhydride), fumaric acid, muconic acid, itaconic acid, phthalic acid, (or
its anhydride), isophthalic acid, terephthalic acid, trimellitic acid (or
its anhydride) and pyromellitic acid (or its anhydride). Such polymers are
caused to be crosslinked by the incorporation of materials of
functionality greater than two in the starting composition, although in
this case, because of the characteristically broad distribution of
molecular species formed in a condensation polymerisation as compared with
that of an addition polymerisation, it may be difficult to ensure that all
these species are in fact crosslinked.
It will be appreciated that the methods which have been referred to above
for making polymer microparticles by polymerisation of addition-type
monomers in an aqueous medium are not, in general, applicable to
condensation-type monomers because of the inhibiting effect of water on
the condensation reaction. Condensation polymer microparticles may,
however, readily be made by dispersion polymerisation in non-aqueous media
according to the procedures described in our British Patent Specification
Nos. 1,373,531; 1,403,794 and 1,419,199 and methods of obtaining
crosslinked microparticles are included in these descriptions. Such
microparticles may then be subjected, in dispersion in the non-aqueous
medium, to the second polymerisation step of the procedure of British
Patent Application No. 42457/77 (published Application No. 2,006,229A)
referred to above, followed by their transfer to the chosen aqueous
medium.
By "aqueous medium" is meant herein either water alone or water in
admixture with a water-miscible organic liquid such as methanol; the
aqueous medium may also contain water-soluble substances introduced for
the purpose of adjusting the pH of the basecoat composition, as discussed
in more detail below.
The pigment particles which, as already defined, are dispersed in the
aqueous medium of the basecoat 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 of particular relevance to the case of
basecoat 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, suitable
metallic pigments including in particular aluminium flake and copper
bronze flake. However, the invention offers advantages also in the
production of "solid colour" finishes, as discussed below. In general,
pigments of any kind may be incorporated in the basecoat compositions in
an amount of from 2% to 100% of the total weight of the composition. Where
metallic pigmentation is employed, this is preferably in an amount of from
5% to 30% by weight of the aforesaid total weight.
Such pigments, whether metallic or otherwise, may be incorporated into the
basecoat compositions with the aid of known pigment dispersants suitable
for use in aqueous systems.
The presence of the crosslinked polymer microparticles in the basecoat
composition confers upon the film derived from the latter the desired
ability to withstand subsequent application of the topcoat composition
without disturbance of the film or of the pigmentation, in particular
metallic pigmentation, which it contains and without which, therefore, a
successful basecoat/clearcoat system cannot be achieved.
In addition to this essential feature, it is also required, as stated
above, that the dispersion of insoluble microparticles should possess a
pseudoplastic or thixotropic character. By this is meant that the apparent
viscosity of the dispersion will differ according to the degree of shear
to which the dispersion is subjected and, more particularly, that the
apparent viscosity under low shear is greater than it is under high shear.
The change in viscosity consequent upon a change in the shear applied may
be instantaneous or it may require a finite time interval which is
nevertheless within the time scale of the viscosity measurement. The
reason for requiring this character in the dispersion on which the
basecoat composition is based is most apparent in the case where that
composition contains a metallic flake pigment. It is then desirable that
the total concentration of non-volatile solids present should be
relatively low, in order to achieve a substantial shrinkage of the
basecoat film after application to the substrate and during the drying
operation, and thereby to ensure correct orientation of the metallic flake
and hence optimum "flip" effect. It is, however, necessary, where the
basecoat composition is to be applied to a substrate by spraying, that the
composition should have a low enough viscosity for efficient atomisation
at the spray gun, and yet, once it has reached the substrate, that its
viscosity should be high enough to prevent "sagging" or "running" of the
film, or the development of "sheariness" (uneven distribution and
orientation) of the metallic flake, even if the loss of water and other
solvents by evaporation between the spray gun and the substrate is only
slight (owing to a high ambient humidity).
The possession of such pseudoplastic properties is often expressed by
quoting values of .eta..sub.a (apparent viscosity in poise) at selected
values of D (shear rate in sec.sup.-1). In the case of metallic pigmented
basecoat compositions to be used according to the invention, the value of
.eta..sub.a, at a solids content of less than 30% by weight non-volatiles,
should preferably be less than 0.5 poise at a shear rate D of 10,000
sec.sup.-1, and more than 20 poise at a value of D of 1.0 sec.sup.-1. Even
more preferably, the basecoat composition should have a value of
.eta..sub.a of less than 0.25 poise at a value of D of 10,000 sec.sup.-1
and of more than 40 poise at a value of D of 1.0 sec.sup.-1. In the case
of "solid colour" basecoat compositions incorporating pigments other than
metallic flake, it is preferred that .eta..sub.a, again at solids contents
below 30%, should be less than 1 poise at D=10,000 sec.sup.-1 and more
than 5 poise at D=1.0 sec.sup.-1 ; even more preferably, .eta..sub.a is
less than 0.7 poise at D=10,000 sec.sup.-1 and more than 10 poise at D=1.0
sec.sup.-1. It is, however, in the nature of pseudoplastic or thixotropic
behaviour that it cannot be fully and accurately defined by a few selected
viscosity/shear data; much depends upon the actual method of viscosity
measurement employed. It is, therefore, not intended that the values
quoted above should be considered as rigid limits which must be adhered to
in order to achieve the benefits of the invention; they are, rather, given
as an approximate guide only and the skilled person can readily determine,
by simple practical tests, whether a particular dispersion, or the
basecoat composition derived from it, possesses the necessary degree of
pseudoplasticity or thixotropy.
There are various ways in which a pseudoplastic or thixotropic character
can be conferred upon the basecoat dispersion. In certain cases, no
special measures are required. This may be so, for example, where the
microparticles have been prepared by the above-mentioned procedure
described in British Patent Application No. 42457/77 (published
Application No. 2,006,229A). This procedure involves first of all the
production of the polymer microparticles proper by a process of
non-aqueous dispersion polymerisation of the appropriate monomers,
followed by polymerisation of further monomers in order to produce a
second polymer which is essentially noncrosslinked and is of a hydrophilic
character such that it is inherently capable of dissolving in the aqueous
medium in which the final dispersion is to be formed, at an appropriate
pH. Not all this second polymer does, however, in fact dissolve in the
aqueous medium on the transferring to it of the product of these two
non-aqueous polymerisation steps. A substantial part of the second polymer
remains associated with the polymer microparticles, and the microparticles
are thereby stabilised in dispersion in the aqueous medium; at the same
time, however, this associated polymer may result in the aqueous
dispersion exhibiting pseudoplastic or thixotropic properties. Further
monomers which are suitable for use in the second polymerisation step are,
for example, a hydroxyalkyl ester of acrylic acid or methacrylic acid, a
monoacrylic or monomethacrylic ester of a polyglycol such as polyethylene
glycol, a monovinyl ether of such a polyglycol, or vinyl pyrrolidone,
optionally in admixture with smaller proportions of non-hydrophilic
monomers such as methyl methacrylate, butyl acrylate, vinyl acetate or
styrene. Alternatively, or additionally, the required solubility in the
aqueous medium can be achieved by using as a major monomer constituent an
acrylic ester containing basic groups, for example dimethylaminoethyl
methacrylate or diethylaminoethyl methacrylate, these groups being
subsequently converted to salt groups by reaction with a suitable acid,
for example lactic acid, dissolved in the aqueous medium. Yet again, the
second polymer may be derived from comonomers containing a substantial
proportion of a polymerisable carboxylic acid, such as acrylic acid or
methacrylic acid, and is then capable of dissolving in an aqueous medium
containing a dissolved base, such as dimethylaminoethanol. In general,
therefore, the second polymer may be non-ionic, anionic or cationic in
character.
Where the polymer microparticles have been made by a process of aqueous
emulsion polymerisation as is well known in the art, a second, inherently
water-soluble, polymer may be produced by further polymerisation, in the
same aqueous medium and in the presence of the microparticles, of monomers
giving rise to a polymer which contains acidic salt-forming groups that
can confer water-solubility. Thus suitable monomers are polymerisable
carboxylic acids such as acrylic acid or methacrylic acid, if desired
together with minor proportions of non-hydrophilic monomers such as methyl
methacrylate and also of hydrophilic monomers which give rise to
water-insoluble homopolymers, e.g hydroxyethyl methacrylate and
hydroxypropyl methacrylate.
Where the polymer microparticles have been made by a process of dispersion
polymerisation in an aqueous medium as described above with reference to
our British Patent Application No. 7,940,088 (published Application No.
2,039,497A), the second, water-soluble polymer can conveniently be
generated by further polymerisation in the same medium of suitable
monomers such as those mentioned in the preceding paragraph and also basic
monomers such as dimethylaminoethyl methacrylate, from which water-soluble
salt derivatives can be generated.
Not all water-soluble polymers generated in situ in the presence of the
microparticles by any of the abovedescribed methods will be capable of
conferring the desired pseudoplastic properties upon the basecoat
composition, but suitable polymer compositions can be arrived at by a
process of simple trial experimentation involving, for example,
measurements of viscosity at selected different shear rates as described
above, or actual application of the compositions to a substrate.
Instead of generating a suitable water-soluble polymer in situ, or in
addition thereto, such a polymer may be added, as a pre-formed separate
ingredient, to the aqueous dispersion of the microparticles. Suitable
polymers are those which, when dissolved in the aqueous medium even at low
concentrations, bring about a considerable enhancement of the viscosity of
the composition. For example, there may be added one or more of the
thickeners which are well known for use in coating compositions based on
aqueous polymer latices. Not all such thickeners are, however, suitable
for the present purpose, since some thickeners are not capable of
conferring the necessary pseudoplastic properties upon the composition to
which they are added. On the other hand, certain thickeners which when
alone dissolved in the aqueous medium do not exhibit such properties may
confer those properties upon the dispersion of the microparticles through
interactions between it and the microparticles (e.g. through hydrogen
bonding or interaction of polar groups). One commercially available
thickener which has been found to be very suitable is "Acrysol" ASE60,
made by Rohm & Haas ("Acrysol" is a Registered Trade Mark).
Whilst it follows that any inherently water-soluble polymer which is
associated with the polymer microparticles or is added to the
microparticle dispersion, for the purpose of imparting pseudoplastic or
thixotropic properties to the dispersion, must of its nature be
noncrosslinked, such polymer may nevertheless if desired, be of the
crosslinkable type. That is to say, it may contain chemically reactive
groups whereby it may be caused, optionally with the aid of an added
crosslinking agent, to become crosslinked after application of the
basecoat composition, and preferably also of the topcoat composition, to
the substrate. Thus the polymer may contain, as already indicated,
hydroxyl or carboxyl groups derived from monomers bearing those groups,
and may be subsequently crosslinked with the aid of an amino resin, e.g. a
methylated melamine-formaldehyde condensate, which is soluble in the
aqueous medium.
From the foregoing description, it will be seen that the basecoat
composition can consist exclusively of the polymer microparticles, the
pigment particles, the aqueous medium in which both groups of particles
are dispersed, and inherently water-soluble polymer which imparts
pseudoplastic properties to it. However, it is much preferred that the
composition should also incorporate a film-forming polymer which is
soluble in the aqueous medium, in order to ensure that, subsequent to the
application of the basecoat to a substrate and evaporation of the aqueous
medium, there is material present which can coalesce so as to fill the
voids between the microparticles and thus produce a coherent, adequately
integrated film in step (2) of the process. This function may indeed be
fulfilled by a portion of an inherently water-soluble polymer which is
present in the composition, as described above, for the purpose of
conferring pseudoplastic or thixotropic properties upon it, but, in view
of the generally low proportions of such polymer which is required for
that purpose, it may be desirable to supplement it by one or more other
water-soluble film-forming materials introduced into the composition,
which may optionally be chemically reactive with constituents already
present. Thus the composition may contain oligomeric substances which can
be converted to high molecular weight products subsequent to application
of the composition but which do not in themselves contribute significantly
to the viscosity of the composition before application.
In this connection there may be mentioned diols of low volatility such as
2-ethyl-1,3-hexanediol, low molecular weight polypropylene glycols, low
molecular weight adducts of ethylene oxide with dihydric or trihydric
alcohols such as neopentyl glycol, bisphenol A, cyclohexanedimethanol,
glycerol and trimethylolpropane, .beta.-hydroxyalkylamides such as
N,N,N.sup.1,N.sup.1 -tetrakis-(.beta.-hydroxyethyl)adipamide and cyclic
amides and esters such as .epsilon.-caprolactam and
.epsilon.-caprolactone. Where such materials are not significantly soluble
in pure water, they should be soluble in the aqueous medium consisting of
water together with the water-miscible organic liquid as previously
described. Any of these oligomeric substances can be converted to a high
molecular weight polymer, after application of the basecoat composition to
the substrate, by linking them through their hydroxyl or other reactive
groups by means of a polyfunctional reactant also present in the
composition. Particularly useful for this purpose are amino resins soluble
in the aqueous medium of the composition, in particular
melamine-formaldehyde condensates such as hexa(alkoxymethyl)melamines and
their low molecular weight condensates.
As an alternative to its containing constituents which produce a
film-forming polymer subsequently to its application to the substrate, the
basecoat composition may contain a pre-formed water-soluble acrylic
polymer which does not confer pseudoplastic properties upon it, or it may
contain in dispersion particles of non-crosslinked polymer which are
stabilised in a similar fashion to the crosslinked microparticles present.
Either of such alternative constituents may, if desired, contain
functional groups such as hydroxyl groups whereby they can become
crosslinked, after application of the composition to the substrate, by
means of a crosslinking agent, e.g. an amino resin.
The relative proportions of the various constituents of the basecoat
composition may vary widely and the optimum proportions in any individual
system are often best determined by experiment, but some generally guiding
principles can be stated. In particular, if the proportion of the polymer
microparticles is too high in relation to the other film-forming material
present in the composition, as described above, there will not be
sufficient of the latter material to fill the voids between the
microparticles; in consequence, on subsequent application of the clearcoat
composition there will be a tendency for that composition to sink into the
basecoat film, with resultant loss of gloss. If, on the other hand, the
proportion of microparticles is too low, it may not confer on the basecoat
composition the desired degree of protection against attack by the solvent
present in the clearcoat composition; to some extent, a lower level of
microparticles can be compensated for in this respect by allowing a longer
period for the basecoat film to flash off or dry off before the clearcoat
is applied, but this diminishes one of the chief advantages to be gained
from the present invention. In general, a satisfactory level of the
microparticles will lie in the range 5-80% by weight of the total
non-volatile content of the basecoat composition. The optimum level
depends, however, in some degree upon whether the pigment present in the
basecoat composition is metallic or non-metallic. For "metallic"
compositions, the preferred range of microparticle content is 40-75% by
weight on the foregoing basis. For "solid colour" compositions, because of
the generally higher proportions of pigment required in order to achieve
adequate opacity at fairly low film thicknesses, the preferred
microparticle content range is rather lower, namely 10-50% by weight on
the same basis as before. The reduced proportion of microparticles avoids
an excessively high total volume fraction of dispersed material which
could result in a porous basecoat film and hence sinkage into it, and poor
gloss, of the topcoat film.
Again in general terms, it may be stated that the proportions used in the
composition of a thickener, or of a second polymer conferring
pseudoplastic properties, may range from 0.3% to 50% by weight of the
total non-volatile content; the amount present of other film-forming
material may be in the range 0-30% by weight and, where a cross-linking
agent such as an amino-resin is present, this also may amount to up to 30%
by weight of the total non-volatile content of the basecoat composition.
The basecoat composition may, if desired, additionally contain a catalyst
for any crosslinking reaction which is required to take place after
application of the composition to the substrate. This may be a
water-soluble acidic compound, such as p-toluenesulphonic acid,
orthophosphoric acid, maleic acid or other strong carboxylic acid such as
tetrachlorophthalic acid; alternatively, it may be a heat-labile salt of
such an acid with a volatile amine.
The nature of the film-forming polymer constituent of the topcoat
composition employed in step (3) of the process of the invention is in no
way critical. There may in general be used any suitable film-forming
polymer, which may be of either the thermosetting or the thermoplastic
type. One suitable class of polymer consists of those which are derived
from one or more ethylenically unsaturated monomers. Particularly useful
members of this class are the acrylic addition 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
ethylenically unsaturated monomers. Suitable acrylic esters include methyl
methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate,
ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. Suitable other,
copolymerisable monomers include vinyl acetate, vinyl propionate,
acrylonitrile, styrene and vinyl toluene. Where the acrylic polymer is
required to be of the thermosetting crosslinking type, suitable functional
monomers to be used in addition to the latter include acrylic acid,
hydroxyethyl acrylate, hydroxyethyl methacrylate, 2-hydroxypropyl
acrylate, 2-hydroxypropyl methacrylate, N-(alkoxymethyl)acrylamides and
N-(alkoxymethyl)methacrylamides, where the alkoxy groups may be, for
example, a butoxy group, glycidyl acrylate and glycidyl methacrylate. The
topcoat 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 nitrogenous 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.
The topcoat composition may incorporate a suitable catalyst for the
crosslinking reaction between the acrylic polymer and the crosslinking
agent, for example an acid-reacting compound such as acid butyl maleate,
acid butyl phosphate or p-toluene sulphonic acid. Alternatively the
catalytic action may be supplied by the incorporation of free acid groups
in the acrylic polymer, for example by the use of acrylic acid or
methacrylic acid as comonomer in the preparation of the polymer.
The topcoat polymer may be either in solution or in stable dispersion in
the volatile carrier liquid of the topcoat composition, in other words,
the carrier liquid may be either a solvent or a non-solvent for the
topcoat polymer. Where the liquid is to be a solvent, it may be any of the
organic liquids, or mixtures of organic liquids, which are conveniently
used as polymer solvents in coating composition, 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 carrier liquid will depend upon the nature of the
topcoat polymer, according to principles which are well-known in the
coatings art, in order that the polymer shall be soluble in liquid.
Where the carrier liquid is to be an organic non-solvent, it will tend to
be of rather lower polarity than those just mentioned and may consist of
one or more aliphatic hydrocarbons such as hexane, heptane or petroleum
fractions of low aromatic content, optionally in admixture with liquids of
high polarity such as have already been referred to, provided that the
total mixture is a non-solvent for the topcoat polymer. In such a case,
the topcoat composition will be a non-aqueous polymer dispersion, and this
will in general be a sterically stabilised dispersion in which the polymer
particles are stabilised by means of a block or graft copolymer, one
polymeric constituent of which is non-solvatable by that liquid and is
associated with the disperse polymer. The well-known principles according
to which such dispersions may be prepared have been extensively described
in the patent and other literature, for example in British Patent
Specification Nos. 941,305; 1,052,241; 1,122,397; and 1,231,614 and in
"Dispersion Polymerisation in Organic Media" ed. K. E. J. Barrett (John
Wiley and Sons, 1975).
Alternatively the topcoat composition may, like the basecoat composition,
be water-borne and in this case also the topcoat polymer may be either in
solution or in a state of stable dispersion in an aqueous medium. In the
case of a dispersion, this may be sterically stabilised, as for example
when it has been prepared by an aqueous dispersion polymerisation
procedure such as that of British patent application No. 7,940,088
(published Application No. 2,039,497A) already referred to, or
charge-stabilised, as for example when it has been prepared by the well
known aqueous emulsion polymerisation techniques. Unlike the polymer
microparticles of the basecoat composition, the topcoat polymer will
always be of the thermosetting type and hence capable of becoming
crosslinked after application to the substrate, optionally with the aid of
a crosslinking agent.
Usually, the topcoat composition will be substantially colourless so that
the pigmentation effect due to the basecoat is not significantly modified,
but it may be desirable in some cases, more usually where the basecoat
contains a metallic pigment, to provide a transparent tinting of the
topcoat composition.
In the first operational step of the process of the invention, the basecoat
composition is applied to the surface of the substrate, which may be
previously primed or otherwise treated as conventional in the art. The
substrates which are of principal interest in the context of the invention
are metals such as steel or aluminium which are commonly used for the
fabrication of automobile bodies, but other materials such as glass,
ceramics, wood and even plastics can be used provided they are capable of
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