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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 formulate coating compositions, especially those for
use in the automobile industry, upon acrylic resins, that is to say,
polymers or copolymers of one or more alkyl esters of acrylic acid or
methacrylic acid. Such compositions, however, give rise to certain
difficulties, especially under the conditions of spray application as
normally used in the painting of car bodies. Good flow-out of the coating
after application, which is necessary in order to maximise gloss, may be
hard to achieve simultaneously with the avoidance of excessive flow of the
composition during the spraying operation, the latter commonly leading to
the defect known as "sagging" or "running", particularly at sharp edges or
corners of a substrate of complicated shape.
We have now found that these difficulties can be minimised or eliminated if
the coating composition contains polymer microparticles of a specified
type.
According to the present invention there is provided a process for the
production of a surface coating upon a substrate which comprises (i)
applying by spray to the substrate surface a composition comprising (A) a
film-forming acrylic resin as herein defined; (B) a volatile organic
liquid diluent in which the acrylic resin is carried; (C) polymer
microparticles as hereinafter defined, in an amount of at least 3% of the
aggregate weight of the film-forming resin (A) and the microparticles,
which are insoluble in and stably dispersed in the combination of the
acrylic resin (A) and the diluent (B); (D) a pigment, other than a
metallic flake pigment, which is also dispersed in the combination of the
acrylic resin and the diluent; and (ii) subsequently evaporating the
volatile diluent to form a polymer film upon the surface.
The acrylic resin used in the process of the invention may be any
film-forming polymer or copolymer of one or more alkyl esters of acrylic
acid or methacrylic acid, optionally together with other ethylenically
unsaturated monomers such as vinyl acetate, vinyl propionate,
acrylonitrile, styrene or vinyl toluene. These polymers may be of either
the thermoplastic type or the thermosetting, cross-linking type. Suitable
acrylic or methacrylic esters for either type of polymer include methyl
methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate,
ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. Where the
polymer is required to be of the cross-linking type, suitable functional
monomers to be used in addition to the latter include acrylic acid,
methacrylic acid, 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
composition may in such a case contain also a cross-linking agent such as
a dissocyanate, 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 cross-linking agents are melamine-formaldehyde
condensates in which a substantial proportion of the methylol groups have
been etherified by reaction with butanol.
For the purposes of the foregoing general definition of the invention, the
cross-linking agent, where present, is considered as being a part of the
film-forming polymer (A).
The composition used according to the invention may incorporate a suitable
catalyst for the cross-linking reaction between the acrylic polymer and
the cross-linking agent, for example an acid-reacting compound such as
acid butyl phosphate, acid butyl maleate 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.
According to one embodiment of the invention, the acrylic resin (A) is in a
state of stable dispersion in the diluent liquid (B), which in such a case
will be a non-solvent for the resin. Methods of preparing such polymer
dispersions are well known in the art and are further referred to below in
connection with the production of the polymer microparticles (C).
In another embodiment, the acrylic resin (A) is dissolved in the diluent
(B); the polymer may then be prepared by solution polymerisation of the
constituent monomer or monomers, in the presence of suitable catalysts or
initiators where necessary. Conveniently the polymerisation may be carried
out in the same organic liquid that is to provide the diluent (B), or in a
liquid which is to form part of that diluent. Alternatively the resin (A)
may be prepared in a separate previous operation (e.g. by aqueous emulsion
polymerisation of monomer) and then dissolved in a suitable organic
liquid.
In yet another embodiment, the acrylic resin (A) may be partly in
dispersion and partly in solution in the diluent (B).
The volatile organic liquid constituent (B) of the composition may be any
of the liquids, or mixtures of liquids, which are conventionally used as
polymer solvents in coating compositions, for example aromatic
hydrocarbons such as toluene and xylene and petroleum fractions of various
boiling point ranges having 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 acrylic resin (A),
according to principles which are well known in the coatings art, in order
that the resin shall be soluble or insoluble in the diluent as required.
The polymer microparticles (C) present in the composition of the invention
are particles of colloidal dimensions, having a diameter of from 0.01-10
microns, composed of a polymer which has a glass-liquid transition
temperature greater than 0.degree. C. and is insoluble in the combination
of the acrylic resin (A) and the diluent (B), the microparticles being
stably dispersed in that combination (in the sense that they do not
undergo flocculation or aggregation) as a consequence of the method of
their preparation. This method involves two essential stages: (i) the
polymerisation of monomer, from which the microparticles are to be
derived, in an inert liquid in which the resulting polymer is insoluble
and under conditions such that the polymer is obtained in a state of
stable dispersion, and (ii) the subsequent polymerisation, in the same
inert liquid and in the presence of the microparticles formed in the first
stage, of ethylenically unsaturated monomer giving rise to a second
acrylic polymer which is compatible with the film-forming acrylic resin
(A) and is soluble in the combination of polymer (A) and diluent (B). This
second acrylic polymer, which for convenience will be referred to here as
the "auxiliary" polymer, may be either soluble or insoluble in the inert
liquid in question, depending on the monomer or monomers from which it is
derived.
The insolubility of the microparticles in the combination of the acrylic
resin (A) and the diluent (B) may be achieved by suitable selection of the
composition of the microparticulate polymer, that is to say, the polymer
may be one which is inherently insoluble in that combination, but
preferably it is achieved by introducing a sufficient degree of
cross-linking into a polymer which, if not cross-linked, would actually be
soluble in the combination of the resin (A) and diluent (B). Where
insolubility of the microparticles is achieved through cross-linking, it
is preferred that the degree of cross-linking should not be greater than
that necessary to render the polymer insoluble. Insolubility of the
microparticles in the combination of the resin (A) and diluent (B) may be
checked by means of the following test. The microparticles (1 part by
weight) are shaken for 30 minutes with the diluent (B) (100 parts by
weight); the suspension is then centrifuged at 17,000 r.p.m. for 30
minutes. The supernatant liquid is decanted off and the residual polymer
then dried for 30 minutes at 150.degree. C., after which its weight is
compared with that of the microparticles originally taken. This test may
be difficult to apply where the specific gravity of the diluent is close
to or greater than that of the microparticles, but such diluents (e.g.
chlorinated solvents) would not normally be used in the compositions under
consideration. Where the result of this test indicates that the
microparticles are acceptably insoluble in the diluent (B) alone, it can
be assumed that the particles will be at least equally insoluble when the
resin (A) is also present in solution in the diluent; there would be
practical difficulties in carrying out the test actually in the solution
of the resin (A) in diluent (B). Where the resin (A) is in a state of
dispersion in the diluent (B), its presence will not normally have any
influence on the degree of insolubility of the microparticles.
The microparticulate polymer may be of various types. It may, for example,
be an acrylic addition polymer, derived from one or more of the same
monomers as have been described above in connection with the acrylic resin
constituent (A). Where it is desired that such a polymer should be
cross-linked, this may be achieved by either of two general methods;
firstly, by including 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 or
divinylbenzene; or secondly, by including in those monomers minor
proportions of two other monomers carrying pairs of chemical groupings
which can be caused to react with one another either during or after the
polymerisation reaction, such as epoxy and carboxyl (e.g. glycidyl
methacrylate and methacrylic acid), anhydride and hydroxyl or isocyanate
and hydroxyl. Alternatively, the microparticles may 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, hexane triol, 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, muconic 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 that the polyester should be
cross-linked, this may again be achieved by incorporating in the starting
composition materials of functionality greater than two, although in this
case, because of the characteristically broad distribution of molecular
species formed in a condensation polymerisation, it may be difficult to
ensure that all those species are in fact cross-linked.
The chemical compositions and degree of cross-linking of the
microparticulate polymer are so chosen as to ensure that the polymer per
se has a glass-liquid transition temperature (Tg) greater than 0.degree.
C., that is to say, under normal conditions of use of the composition
which is spray-applied according to the invention, the microparticles are
hard and glassy.
As already stated, it is necessary that the polymer microparticles be
stably dispersed in the combination of the acrylic resin (A) and the
liquid diluent. By "stably dispersed" is meant that the particles are
prevented from flocculating or aggregating by means of a steric barrier
around the particles, of polymer chains which are solvated by the said
combination and hence are 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
combination in question; 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 particles. It will be
understood that the stabilising polymer chains to be used in any
particular instance will be selected with reference to the nature of the
liquid diluent and film-forming acrylic resin concerned. In general terms
this means that the chains will be of a degree of polarity similar to that
of the diluent and film-forming resin, so that the combination of the
latter will be inherently a solvent for the polymer of which the chains
are composed. Since, in the automobile finishes to which the present
invention is primarily directed, the liquid diluent will conventionally be
of a relatively high degree of polarity (containing, for example, a
substantial proportion of "strong" ester and ketone solvents) it follows
that the stabilising chains on the microparticles will usually require to
be of a composition such that they are inherently soluble in that type of
liquid.
The mode of anchoring of the stabilising chains to the microparticles is
conveniently discussed in connection with methods of making the particles,
as follows.
The polymer microparticles may be produced in various ways. Preferably they
are produced by a process of dispersion polymerisation of monomers, in an
organic liquid in which the resulting polymer is insoluble, in the
presence of an amphipathic steric stabilising agent. Suitable processes of
dispersion polymerisation are well known and extensively described in the
literature. Thus, so far as the dispersion polymerisation of ethylenically
unsaturated monomers such as acrylic or methacrylic acid esters, vinyl
esters and styrene or its derivatives is concerned, the procedure is
basically one of polymerising the monomers in an inert liquid in which the
monomers are soluble but the resulting polymer is not soluble, in the
presence dissolved in the liquid of an amphipahtic stabilising agent or of
a polymeric precursor which, by copolymerisation or grafting with a
portion of the monomers, can give rise in situ to such a stabilising
agent. Reference may be made, for example, to British Pat. Specifications
Nos. 941,305; 1,052,241; 1,122,397 and 1,231,614 for a general description
of the principles involved, as well as to "Dispersion Polymerisation in
Organic Media", ed. K. E. J. Barrett (John Wiley and Sons, 1975).
The choice of suitable unsaturated monomers for the production of
microparticles having the required Tg and other characteristics will
present no difficulty to those skilled in the polymer art. Amongst
suitable monomers, there may be mentioned methyl methacrylate, styrene and
vinyl acetate, the homopolymers of which have Tg values of about
105.degree., 100.degree. and 32.degree. C. respectively. Where a lower Tg
than that of the homopolymer is desired, these "hard" monomers may be
copolymerised with a suitably chosen proportion of a "soft" monomer, such
as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate or butyl
methacrylate. In the case where the microparticle polymer is produced by a
process of dispersion polymerisation of the monomer or monomers in an
organic medium, there may be a limit imposed upon the proportion of "soft"
monomers which can be included by the need to ensure that the resulting
copolymer is not too soluble, even in organic liquids of low polarity such
as hydrocarbons, for the formation of a stable dispersion of the
microparticles to take place. The skilled person will, however, readily be
able to find, if necessary with the aid of simple experimentation,
suitable monomer compositions to satisfy the given requirements.
The production specifically of dispersion of cross-linked addition polymer
particles can be achieved by including, in the monomers selected, pairs of
monomers containing (in addition to the polymerisable unsaturated groups)
groups capable of entering into chemical reaction with each other; for
example, the epoxide and carboxyl groups contained in glycidyl
methacrylate and methacrylic acid. By following the procedure particularly
described in British Pat. Specifications Nos. 1,095,288 and 1,156,012, for
example, particles are obtained in which there are present such
complementary groups which, although not at that stage co-reacted, can be
caused to co-react and so form crosslinks by subsequently heating the
dispersion to a suitably elevated temperature. Cross-linked addition
polymers may also be prepared in dispersion by including in the monomers
undergoing dispersion polymerisation a minor proportion of a monomer which
is difunctional with respect to the polymerisation reaction, such as
ethyleneglycol dimethacrylate or divinylbenzene. Small proportions of
comonomers incorporating carboxyl groups, e.g. acrylic acid or methacrylic
acid may be included (where the microparticles are to be cross-linked,
such proportions would be in excess of those used in order to achieve
cross-linking by reaction with a co-reactive monomer such as glycidyl
methacrylate). Conversely, small (additional) proportions of an epoxide
monomer, e.g. glycidyl methacrylate, may be included. Other functional
monomers, such as hydroxyethyl acrylate or acrylamide, may also be
included in minor proportions in the monomers from which the
microparticles are to be derived.
The production of dispersions of condensation polymers is described, for
example, in British Pat. Specifications Nos. 1,373,531; 1,403,794 and
1,419,199, and methods of obtaining cross-linked polymer particles are
included in these descriptions. The general principles involved here are
the same as those referred to above in connection with addition polymer
dispersions, but there is a difference of detail arising from the commonly
more highly polar nature of the monomers or starting materials from which
condensation polymers are derived. This is, namely, that the monomers in
question are usually insoluble in the inert liquid in which the
polymerisation is to be carried out. Accordingly the first step in the
dispersion polymerisation of the monomers is to bring them into a state of
colloidal dispersion in the inert liquid, either as liquid or as solid
particles. In the second step, polymerisation of the monomers takes place
within those same particles. An amphipathic stabilising agent is required
in each stage, firstly in order to stabilise the particles of monomer and
secondly in order to stabilise the particles of polymer formed, but in
suitable cases a single stabilising agent can be found which will perform
both these functions. In place of using a preformed amphipathic
stabilising agent in this process, there may be employed instead a
suitable polymeric precursor which, by copolymerisation or grafting with a
portion of the monomers being polymerised, can give rise to such a
stabilising agent in situ. Reference may be made in this connection to
British Pat. Application No. 19487/76.
Suitable monomeric starting materials for preparing condensation polymer
microparticles are those which are well known for use in making such
polymers by melt or solution polymerisation techniques. For example, in
the case of polyester microparticles, suitable materials in general are
the polyhydric alcohols and polycarboxylic acids, and here, as with the
polymers of the addition type discussed above, it will be evident to the
skilled person which particular reactants are to be selected in order to
produce a microparticulate polymer having the requisite Tg and other
characteristics.
It will, of course, be understood that, in the case of both polyester and
polyamide microparticles, the mixture to be polymerised must incorporate
some proportion of a starting monomer which has a functionality greater
than two, where it is desired that the microparticles should be
cross-linked.
In all the above-described dispersion polymerisation processes, the
amphipathic steric stabilising agent is a substance the molecule of which
contains a polymeric component which is solvatable by the liquid in which
the dispersion is made and another component which is relatively
non-solvatable by that liquid and is capable of associating with the
polymer particles produced. Such a stabilising agent will be soluble as a
whole in the dispersion liquid, but the resulting solution will usually
contain both individual molecules and micellar aggregates of molecules, in
equilibrium with each other. The type of stabilising agent preferred for
use in the invention is a block or graft copolymer containing two types of
polymeric component: one type consists, as stated above, of polymer chains
which are solvatable by the dispersion liquid and the other type consists
of polymer chains of different polarity from the first type which
accordingly are not solvatable by that liquid and are capable of becoming
anchored to the polymer microparticles. A particularly useful form of such
a stabilising agent is a graft copolymer comprising a polymer backbone,
which is the non-solvatable or "anchor" component, and a plurality of
solvatable polymer chains pendant from the backbone. Specific examples of
such graft copolymers include those in which the backbone is an acrylic
polymer chain, derived predominantly from methyl methacrylate, and the
pendant chains are residues of poly-(12-hydroxystearic acid) which are
readily solvatable by an aliphatic hydrocarbon medium. These copolymers
may be made, for example, by first reacting poly-(12-hydroxystearic acid)
with glycidyl acrylate or glycidyl methacrylate, whereby the terminal -
COOH group in the polymeric acid is converted to an ester derivative
containing a polymerisable unsaturated grouping, and then copolymerising
that derivative with methyl methacrylate, optionally together with minor
proportions of other copolymeric monomers. By employing acrylic acid or
methacrylic acid as such minor comonomers, it is possible to introduce
carboxyl groups into the backbone chain of the graft copolymer with
beneficial results inasmuch as the backbone is thereby rendered more polar
than it is if composed of methyl methacrylate units alone. This increased
polarity causes the backbone to be even less solvatable by a non-polar
diluent such as an aliphatic hydrocarbon, and in consequence enhances the
force whereby it becomes anchored to the microparticle.
The foregoing patent and literature references, and the foregoing
description, relating to dispersion polymerisation processes suitable for
the production of the polymer microparticles are equally applicable to the
production of the film-forming acrylic polymer (A), where this is to be in
a state of stable dispersion in the diluent (B), except that the polymer
(A) will not normally be cross-linked at this stage even though it may
become cross-linked as the result of a curing operation following
application of the composition to a substrate.
After formation of the polymer microparticles in dispersion in the manner
just described, the acrylic auxiliary polymer is then formed by
polymerisation of ethylenically unsaturated monomer in the presence of the
microparticles. This further polymerisation is accomplished by feeding in
a charge of suitable monomer to the same reaction mixture in which the
microparticles have been produced under essentially the same conditions as
were operated in the first stage, but with the differences that the
monomer or monomers involved will overall be of a different composition to
those used in the first stage and that the acrylic auxiliary polymer,
unlike the microparticles, will not necessarily be wholly insoluble in the
inert liquid in which the polymerisation is carried out.
In general, the auxiliary polymer will be required to have a composition
such that it is compatible with the film-forming acrylic polymer (A);
indeed, it may be identical with that polymer and, in certain
circumstances as described below, even wholly replace it. The monomer or
monomers from which the auxiliary polymer is to be derived will be chosen
with this requirement in mind, from the range of ethylenically unsaturated
monomers already described in connection with the production of the
polymer (A), or other suitable monomers, as will be apparent to those
skilled in the art.
On introducing the microparticles, associated with auxiliary polymer in the
way described above, into the dispersion or solution of the acrylic
polymer (A) in the diluent (B), part of the auxiliary polymer may be
dissolved away by that more polar medium, but it is believed that a
substantial proportion of the auxiliary polymer chains remain attached to
the microparticles (albeit now solvated by the medium), for example by
virtue of their having become entangled with the chains of the
microparticle polymer during their formation, or as a result of actual
grafting on to those chains. Whatever the mechanism may be, the effect of
the presence of the auxiliary polymer is to stabilise the microparticles
in the new, more polar environment. If desired, this stability may be
enhanced by ensuring that covalent linkages are developed between the
chains of the auxiliary polymer and those of the microparticles. This may
be done, for example, by including an unsaturated carboxylic acid in the
monomers from which the auxiliary polymer is derived. The carboxyl groups
so introduced are able to react with epoxide groups, present in the
microparticle polymer as the result of the use of a slight excess of the
latter groups for the purpose of cross-linking that polymer by reaction
with carboxyl groups in the manner described above. The incorporation of
the microparticles, associated with auxiliary polymer, into the
composition of the invention may be achieved either by blending the
dispersion obtained after formation of the auxiliary polymer directly with
the dispersion or solution of the film-forming acrylic polymer (A) in the
diluent (B), or, in the extreme case, by simply adding to the
microparticle dispersion sufficient strong solvent to dissolve away enough
of the acrylic auxiliary polymer from the microparticles to provide itself
the whole of the film-forming polymer constituent (A), whilst still
leaving a residue of the auxiliary polymer associated with the
microparticles which will ensure their continued stabilisation. Another
possibility is to separate the microparticles from the dispersion in which
they are made, for example by centrifuging, filtration or spray-drying,
and then to blend the microparticles with the dispersion or solution of
the film-forming acrylic polymer (A). It will be understood from the
foregoing description that, for the purposes of the definition of the
invention hereinbefore given, the film-forming resin (A) is considered to
comprise that portion of the auxiliary polymer which is dissolved away
from the microparticles when the latter are incorporated into the coating
composition.
As an alternative to the use in organic media of dispersion polymerisation
methods, the polymer microparticles may, for example, be produced by
aqueous emulsion polymerisation of suitable unsaturated monomers, using
procedures well known in the art. The microparticles are then obtained in
the form of a charge-stabilised dispersion, from which the particles
themselves can be separated, e.g. by spray drying. For incorporation into
the coating composition, the microparticles are then re-dispersed in the
solution or dispersion in the diluent of the film-forming polymer,
preferably by methods imparting high shear to the mixture such as pug
milling or triple roll milling, in an analagous fashion to the dispersion
of a pigment. By further analogy to pigment dispersion, the requisite
steric stability of the microparticles may then be achieved simply as a
result of an innate tendency of the film-forming polymer (especially where
it is soluble in, and therefore solvated by, the diluent) to associate
with the particles, for example through the interaction of polar groups
present in the film-forming polymer and in the microparticle respectively.
In producing the microparticles by aqueous emulsion polymerisation, some
difunctional unsaturated compounds may be included in the polymerising
monomers in order to give rise to a cross-linked polymer which will be
insoluble in the solution or dispersion of the film-forming acrylic
polymer (A) in the diluent (B), whatever the nature of the latter. Here
again, as in the case of microparticles made by dispersion polymerisation
in inorganic media, it is essential to continue the emulsion
polymerisation with a second feed of monomers which does not include any
difunctional (i.e. cross-linking) material and which gives rise to an
acrylic polymer which is compatible with the polymer (A) and is soluble in
the solution or dispersion of polymer (A) in diluent (B), in other words
to associate with the microparticles an auxiliary polymer having the same
function as that previously described.
The polymer microparticles (C) used in the process of the invention are
preferably present in proportions of from 2% to 30% of the aggregate
weight of the film-forming acrylic resin (A) and the microparticles. More
preferably, the proportion of microparticles is from 5% to 20% of that
aggregate weight. The precise proportion which is used will depend upon
the particular application properties which it is desired that the coating
composition should possess. For the purposes of this definition, the term
"polymer microparticles" is to be understood as referring to the
microparticles proper together with that part of the auxiliary polymer
associated therewith which cannot be dissolved away from the particles by
the diluent (B), under the conditions of the insolubility test described
above.
The compositions used in the process of the invention incorporate, in
addition to the acrylic resin (A), the diluent (B) and the polymer
microparticles (C), pigments (D) as conventionally used in the coatings
art, other than metallic flake pigments. Such pigments may range in
particle size from 1 to 50 microns and may be inorganic in nature, for
example titanium dioxide, iron oxide, chromium oxide, lead chromate or
carbon black, or organic in nature, for example phthalocyanine blue,
phthalocyanine green, carbazole violet, anthrapyrimidine yellow,
flavanthrone yellow, isoindoline yellow, indanthrone blue, quinacridone
violet and perylene red. Any of these pigments may be present in the
compositions in a proportion of from 2% to 50% of the aggregate weight of
the acrylic resin (A) and the microparticles (C). The term "pigment" is
here meant to embrace also conventional fillers and extenders, such as
talc or kaolin. Such pigments may be incorporated into the compositions
with the aid of known dispersants, for example with the aid of an acrylic
polymer which is compatible with the acrylic film-forming polymer (A). Any
such polymeric dispersant is also considered to be part of the
film-forming constituent (A).
If desired, the compositions may additionally incorporate other known
additives, for example viscosity modifiers such as bentone or cellulose
acetate butyrate.
In the case where the film-forming acrylic polymer (A) is of the
thermosetting or cross-linking type, there may, as already stated, be
incorporated in the composition used according to the invention a
cross-linking agent. The proportion of cross-linking agent to
cross-linkable acrylic polymer in the composition may vary widely, but in
general a ratio of from 50:50 to 90:10 by weight of polymer to
cross-linking agent is satisfactory. The precise proportion to be employed
depends upon the properties required in the final film, but a preferred
range affording a good balance of properties is from 60:40 to 85:15 by
weight of polymer to cross-linking agent. Where it is of particular
importance that the film should exhibit good resistance towards acid
corrosion induced by severe atmospheric pollution, an especially preferred
range of ratios of polymer to cross-linking agent is from 70:30 to 85:15
by weight.
As already indicated, the composition may also incorporate a suitable
catalyst for the cross-linking reaction, for example an acid-reacting
compound, or the film-forming acrylic polymer (A) of the composition may
be arranged to contain free acid groups.
The coating compositions may be applied to a substrate according to the
invention by any of the known spraying procedures, such as compressed air
spraying, electrostatic spraying, hot spraying and airless spraying, and
either manual or automatic methods are suitable. By these procedures there
can be achieved an improvement of 15-20% in the thickness of paint which
can be applied before sagging occurs, without the loss of gloss, poor
appearance or lower spray solids which result from attempts to improve sag
resistance by other methods (for example, by increasing the average
molecular weight of the film-forming polymer or by including in the
composition inorganic sag-control aids).
EXAMPLE
(a) Preparation of Polymer Microparticles
To a vessel fitted with stirrer, thermometer, reflux condenser and
provision for adding a liquid feed to the returning condensate was
charged:
Aliphatic hydrocarbon (boiling range 170.degree.-210.degree. C.; aromatic
content 5%): 12.081 parts
Hexane: 2.959 parts
Heptane: 15.821 parts
The vessel and contents were purged with inert gas; the temperature of the
reactants was raised to 100.degree. C.
The following premixed ingredients were added as a single rapid addition to
the refluxing solvents:
Methyl methacrylate: 0.972 parts
Methacrylic acid: 0.020 parts
Azodiisobutyronitrile: 0.077 parts
Graft copolymer stabiliser (33% solution; as described below): 0.362 parts
The contents of the vessel were then held at reflux for 30 minutes to form
a seed-polymer dispersion. The following ingredients were premixed and fed
into the hydrocarbon returning from the condenser at a uniform rate over a
period of 3 hours:
Methyl methacrylate: 18.460 parts
Methacrylic acid: 0.188 parts
Glycidyl methacrylate: 0.188 parts
Azodiisobutyronitrile: 0.247 parts
Graft copolymer stabiliser solution (as described below): 3.885 parts
Dimethylamino ethanol: 0.036 parts
After completion of the feed, the reaction mixture was maintained und | | |
