Powder coating compositions

A powder coating composition comprises a component (A) containing carboxylic acid or anhydride functionality and a component (B) containing hydroxy functionality. The components (A) and (B) are both solids at temperatures up to 50.degree. C. and at least one of them is a synthetic resin having a glass transition temperature in the range 0.degree. to 120.degree. C. The carboxylic acid or anhydride-functional component (A) is a compound or polymer containing at least two cyclic carboxylic anhydride groups per molecule or is a compound containing a moiety of the formula: ##STR1## where X is a hydrogen atom or an alkyl group, or a cyclic anhydride thereof. The hydroxy-functional component (B) contains at least two hydroxy groups per molecule and contains at least one amine group in its molecule to catalyze the reaction of the carboxylic acid or anhydride groups of component (A) with the hydroxy groups of component (B). The composition is prepared by mixing the ingredients, extruding them at a temperature above the glass transition temperature of at least one of the components but below the fusion temperature, and comminuting the extrudate. The composition is applied to substrates and heated to cause fusion and reaction of the ingredients to cure the coating layer formed.

This invention relates to thermosetting powder coating compositions. Such a 
coating composition comprises powdered solid materials containing groups 
capable of reacting with each other when heated. The coating composition 
is applied to a substrate, particularly a metallic substrate, in powder 
form, usually by electrostatic spraying. The coating composition is heated 
on the substrate to fuse at least the major component of the coating to 
cause it to flow and to react with another component to cure the coating. 
Examples of powder coating compositions comprising a component containing 
hydroxyl groups and a component containing anhydride groups are described 
in U.S. Pat. Nos. 4069275 and 4101606, British Pat. Nos. 1366081 and 
1561828 and European Patent Application No. 73022. Such powder coatings 
are generally heat-cured at a temperature in the range from 130.degree. to 
200.degree. C. The use of an amine catalyst for the curing reaction 
between the hydroxyl and anhydride groups is described in U.S. Pat. No. 
4069275. The amine catalyst is generally a solid of melting point 
50.degree. to 200.degree. C. which is mixed into the powder coating 
composition. 
Examples of liquid coating compositions comprising a component containing 
hydroxyl and amine groups and a component containing anhydride groups are 
described in U.S. Pat. No. 4452948, which relates to a two-pack 
solvent-based coating composition, and U.S. Pat. No. 4308188, which 
relates to water-borne coatings comprising the half-ester zwitterion 
reaction product of a polymer containing cyclic acid anhydride groups with 
a hydroxy-functional tertiary carbinamine. 
A powder coating composition according to the invention comprises a 
component (A) containing carboxylic acid or anhydride functionality and a 
component (B) containing hydroxy functionality, the components (A) and (B) 
both being solids at temperatures up to 50.degree. C. and at least one of 
(A) and (B) being a synthetic resin having a glass transition temperature 
(Tg) in the range 0.degree.-120.degree. C., and the composition is 
characterised in that: the carboxylic acid or anhydride-functional 
component (A) is a compound or polymer containing at least two cyclic 
carboxylic anhydride groups per molecule or is a compound containing a 
moiety of the formula: 
##STR2## 
where X is a hydrogen atom or an alkyl group, or a cyclic anhydride 
thereof, 
provided that the carboxylic acid or anhydride-functional component (A) 
does not contain an olefinic double bond in the alpha, beta- or beta, 
gamma-position with respect to any carboxylic acid or anhydride group and 
provided that if the component (A) is an addition polymer the carboxylic 
acid or anhydride groups are separated from the addition polymer chain by 
at least one intervening carbon atom, and 
the hydroxy-functional component (B) contains at least two hydroxy groups 
per molecule and contains at least one amine group in its molecule to 
catalyse the reaction of the carboxylic acid or anhydride groups of 
component (A) with the hydroxy groups of component (B). 
Examples of synthetic resins of Tg 0.degree.-120.degree. C. suitable for 
use as anhydride component (A) include anhydride-functional polymers 
formed by the reaction of a hydroxy-functional polymer with a 
tricarboxylic compound capable of introducing anhydride groups. Preferred 
tricarboxylic compounds are those containing a moiety of the formula: 
##STR3## 
where X is an alkyl group of 1 to 4 carbon atoms or especially a hydrogen 
atom, or a cyclic anhydride thereof. Anhydrides are generally preferred to 
the corresponding acids since the reaction may require less heating and 
produce higher yields. A particularly preferred anhydride is 
tricarballylic anhydride of the formula 
##STR4## 
Tricarballylic acid, 
##STR5## 
is also effective to introduce anhydride groups on heating, as is citric 
acid. Hemimellitic anhydride (benzene 1,2,3-tricarboxylic acid anhydride) 
or acid can also be used, although the aliphatic compounds are preferred. 
Alicyclic compounds can also be used, for example anhydrides of 
cyclohexane-1,2,3-tricarboxylic acid and derivatives thereof substituted 
by an alkyl group. An alicyclic tricarboxylic acid anhydride can be formed 
by the Diels-Alder reaction of a 2,4-dienoic acid such as sorbic acid with 
maleic anhydride according to the formula: 
##STR6## 
where R is hydrogen or an alkyl group (R is methyl when the dienoic acid 
is sorbic acid). The cyclohexene tricarboxylic acid anhydride produced can 
be hydrogenated to form cyclohexane-1,2,3-tricarboxylic acid anhydride 
which is reacted with a hydroxy-functional polymer to produce an 
anhydride-functional component (A) for use in a powder coating composition 
according to the invention. 
Anhydride-functional polymers of Tg 0.degree.-120.degree. C. can also be 
produced by the reaction of a hydroxy-functional polymer with trimellitic 
anhydride acid chloride in the presence of a tertiary base; by 
transacidolysis of the acetate of a hydroxy-functional polymer with 
trimellitic anhydride; or by thermal esterification of the 
hydroxy-functional polymer with trimellitic anhydride. These three 
reactions are described by Puskas and Fields in Ind. Eng. Chem. Prod. Res. 
Develop., Volume 9, No. 3 (1970) at page 403-407. The thermal 
esterification of hydroxy-functional polymers with trimellitic anhydride 
is also described in European Patent Application No. 73022. These 
anhydride-functional polymers can be used as component (A) in the powder 
coating compositions of the present invention. The reaction products of 
hydroxy-functional polymers with aliphatic or alicyclic tricarboxylic 
compounds such as tricarballylic anhydride or acid are however preferred 
since the reaction with such a compound converts a substantially greater 
proportion of the hydroxy groups to anhydride groups compared to reaction 
with trimellitic anhydride. Moreover, reaction with an aliphatic or 
alicyclic tricarboxylic anhydride can be carried out at lower temperatures 
and can form an anhydride-functional polymer from a hydroxy-functional 
polymer with a smaller increase in molecular weight and less color in the 
product than is the case with reaction with trimellitic anhydride. 
The anhydride component (A) may be an anhydride-tipped polymer of Tg 
0.degree.-120.degree. C. formed by the reaction of a tricarboxylic 
compound capable of introducing anhydride groups with a hydroxy-tipped 
polymer, for example a hydroxy-tipped polyester. Such polyesters can be 
prepared by the reaction of an excess of one or more glycols with one or 
more polycarboxylic acids or anhydrides. The glycol and polycarboxylic 
acid or anhydride reagents used in preparing the polyester can be those 
known for preparing polyesters for powder coatings. The glycol component 
preferably consists at least partly of a branched-chain glycol such as 
neopentyl glycol or trimethylpentane diol, optionally with ethylene glycol 
or propylene glycol. The polycarboxylic acid or anhydride component 
preferably consists at least partially of an aromatic acid or anhydride 
such as isophthalic acid, terephthalic acid or phthalic acid or anhydride, 
or an alicyclic acid or anhydride such as cyclohexane dicarboxylic acid, 
optionally with an aliphatic dicarboxylic acid such as adipic acid. The 
polyester-forming reagents preferably include a minor amount of a 
trifunctional reagent such as trimethylol-propane or trimellitic anhydride 
to increase the average functionality of the polyester and to introduce 
branching. Such anhydride-tipped polyesters can be used in the coating 
composition with synthetic resins containing hydroxy and amine groups or 
with lower molecular weight crystalline compounds containing at least two 
hydroxy groups and at least one amine group. 
The anhydride component (A) can alternatively be an anhydride-functional 
polymer of Tg 0.degree.-120.degree. C. formed by the reaction of a 
tricarboxylic compound capable of introducing anhydride groups with an 
addition copolymer having pendent hydroxyl groups, for example a copolymer 
of a hydroxyalkyl ester of an olefinically unsaturated acid such as 
hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl acrylate 
or hydroxypropyl methacrylate. The addition copolymer preferably contains 
10 to 50 percent by weight hydroxyalkyl acrylate or methacrylate units 
together with comonomer units of one or more esters of methacrylic or 
acrylic acid such as methyl methacrylate, ethyl methacrylate or ethyl 
acrylate and/or styrene. An alternative type of addition copolymer having 
pendent hydroxyl groups is a styrene/allyl alcohol copolymer. 
Alternative hydroxy-functional polymers which can be reacted with a 
tricarboxylic compound such as tricarballylic anhydride to form an 
anhydride-functional component (A) are hydrolysed epoxy resins, 
hydroxy-functional polyurethanes and hydroxy-functional polycarbonates. 
A hydroxy-functional polymer which is reacted with a tricarboxylic compound 
capable of introducing anhydride groups to form component (A) preferably 
contains no amine groups so that anhydride groups can be introduced into 
the polymer without premature curing. 
We have found that addition polymers in which a carboxylic acid anhydride 
group is immediately adjacent the addition polymer chain, for example 
polymers of an alpha, beta-unsaturated carboxylic acid anhydride such as 
the maleic anhydride copolymers described in British Patent No. 1366081 
and U.S. Pat. Nos. 4069275 and 4101606 and the itaconic anhydride 
copolymers described in U.S. Pat. No. 4452948, as well as materials in 
which an olefinic double bond is present in the alpha, beta- or beta, 
gamma-position with respect to any carboxylic acid or anhydride group, are 
not suitable for forming the powder coating compositions of the invention. 
They lead to a rough, brittle and sometimes foamed product on heat curing. 
We believe that this may be caused by decarboxylation of the polymers, 
giving off gas during curing. 
The anhydride component (A) may be an addition polymer of Tg 
0.degree.-120.degree. C. of a cyclic carboxylic anhydride in which the 
anhydride groups are separated from the addition polymer chain by at least 
one intervening carbon atom. Examples of suitable anhydride monomers are 
vinyl trimellitate anhydride whose preparation is described in U.S. Pat. 
No. 4308188, a vinyl ester of an acylated citric acid anhydride as 
described in European Patent Application No. 85884, or a compound of the 
formula: 
##STR7## 
where R' represents hydrogen or methyl and R" represents an alkylene group 
having 2 to 4 carbon atoms which can be prepared by the reaction of a 
hydroxyalkyl acrylate or methacrylate with tricarballylic anhydride. 
The anhydride component (A) can alternatively be a polymer of Tg 
0.degree.-120.degree. C. containing cyclic carboxylic anhydride groups 
introduced by the ene reaction. Such polymers are the reaction product of 
a polymer containing residual unsaturation, for example an unsaturated 
polyester, with an olefinically unsaturated cyclic carboxylic anhydride, 
for example maleic anhydride. 
When the hydroxy-functional component (B) is a synthetic resin of Tg 
0.degree.-120.degree. C. the component (A) can be a tricarboxylic compound 
containing a moiety of the formula: 
##STR8## 
or a cyclic anhydride thereof. The tricarboxylic compound is preferably an 
anhydride. Tricarballylic anhydride is the preferred anhydride and is 
capable of producing particularly smooth hard glossy cured films when used 
in a powder coating with a hydroxy-functional polymer (B), for example an 
acrylic polymer containing hydroxy and amine groups. Tricarballylic acid 
can also be used, as can tricarballylic anhydride in which the free 
carboxyl group is esterified with an alkanol, e.g. of up to 4 carbon 
atoms. 
Alternative low molecular weight anhydride components (A) which can be used 
in the powder coating composition with a synthetic resin (B) of Tg 
0.degree.-120.degree. C. containing hydroxy and amine groups are oligomers 
of trimellitic anhydride containing at least two anhydride groups such as 
those described in British Patent No. 1561828 and aromatic dianhydrides 
such as benzophenone tetracarboxylic dianhydride. 
When component (B) is a synthetic resin of Tg 0.degree.-120.degree. C. the 
component (A) can be a mixture of a polymer containing cyclic anhydride 
groups and a tricarboxylic compound containing a moiety of the formula: 
##STR9## 
or a cyclic anhydride thereof. Such a mixture can be formed by reacting a 
hydroxy-functional polymer with a stoichiometric excess of the 
tricarboxylic compound, which is preferably an anhydride such as 
tricarballylic anhydride. The ratio of anhydride groups in the 
tricarboxylic compound to hydroxy groups in the hydroxyl-functional 
polymer can for example be 1.5:1 to 4:1. 
A preferred type of hydroxy component (B) is an addition copolymer of Tg 
0.degree.-120.degree. C. having pendent hydroxyl and amine groups. The 
amine groups are preferably tertiary amine groups. The addition polymer 
can for example be a copolymer containing 5 to 80 percent by weight, 
preferably 10 to 50 percent by weight, of units of a hydroxyalkyl ester of 
an olefinically unsaturated carboxylic acid such as hydroxyethyl 
methacrylate, hydroxyethyl acrylate, or hydroxypropyl methacrylate and 2 
to 50 percent by weight, preferably 5 to 20 percent by weight, of units of 
a dialkylaminoalkyl acrylate or methacrylate, for example 
diethylaminoethyl methacrylate or dimethylaminoethyl methacrylate or of a 
dialkylaminoalkyl-substituted amide such as dimethylaminopropyl 
methacrylamide. The amine groups can alternatively be secondary amine 
groups particularly if these are linked to a tertiary alkyl group; for 
example the addition copolymer may be a copolymer of a hydroxyalkyl 
acrylate or methacrylate and t-butylaminoethyl methacrylate. 
Alternatively, tertiary or secondary amine groups can be introduced into 
an acrylic resin by copolymerising glycidyl acrylate or methacrylate with 
the unsaturated hydroxyalkyl ester and subsequently reacting all or part 
of the glycidyl groups with a secondary or primary amine. The hydroxy 
component (B) suitably contains 2 to 50, preferably 5 to 20 percent by 
weight of the resulting modified units. The addition polymer generally 
contains one or more comonomers which do not have reactive functional 
groups, for example an acrylic ester such as methyl methacrylate, ethyl 
methacrylate or ethyl acrylate, or styrene. Such addition polymers can be 
used in the powder coating composition with any of the anhydride or 
acid-functional components (A) described above. 
The addition polymer can be a grafted copolymer of acrylic monomers, 
including a hydroxy-functional monomer and an amine-functional monomer, 
onto an unsaturated polyester. The polyester segment is preferably of low 
molecular weight formed from the glycol and polycarboxylic acid and/or 
anhydride reagents described above and including an unsaturated acid or 
anhydride such as maleic anhydride or itaconic acid. 
An alternative type of hydroxy- and amine-functional resin of Tg 
0.degree.-120.degree. C. suitable for use as component (B) can be prepared 
by the reaction of an epxoy resin and an amine. The epoxy resin can for 
example be a condensed glycidyl ether of bisphenol A such as that sold 
under the trade mark "Epikote 1004", an epoxy novolac resin or a 
cycloaliphatic diepoxide. The product of the epoxy resin and amine 
contains hydroxy and amine groups according to the following reaction: 
##STR10## 
where Z is the residue of the epoxy resin, R.sup.IV is an organic group 
linked to the nitrogen atom through a carbon-nitrogen bond, preferably an 
alkyl or hydroxyalkyl group, and R.sup.V is hydrogen or an organic group 
linked to the nitrogen atom through a carbon-nitrogen bond or R.sup.IV and 
R.sup.V are such organic groups joined to form a heterocyclic ring. The 
amine is preferably a secondary amine and preferably contains at least one 
hydroxyalkyl group, for example N-methylethanolamine, N-ethylethanolamine 
or diethanolamine, to introduce further hydroxyl groups. Such epoxy 
resin/amine adducts are preferably used in the powder coating composition 
with a synthetic resin containing anhydride groups, for example an 
anhydride-tipped polyester. 
A further example of a polymer of Tg 0.degree.-120.degree. C. suitable for 
use as hydroxy component (B) is a hydroxy-functional polyester modified to 
contain amine groups, preferably tertiary amine groups. Such a polyester 
may be prepared by reacting an excess of one or more glycols with a 
polycarboxylic acid and/or anhydride component. The glycols used in 
preparing the polyester are preferably of the same type as described above 
in connection with anhydride-functional polyesters. The polycarboxylic 
acid and/or anhydride component preferably consists at least partly of 
aromatic dicarboxylic acid or anhydride as described above and also 
includes an olefinically unsaturated dicarboxylic acid. The 
hydroxy-functional polyester thereby formed contains double bonds from the 
unsaturated dicarboxylic acid. Amine groups can be introduced by a 
Michael-type addition to the double bond. For example, a polyester 
containing itaconic acid residues can be reacted with a secondary amine to 
introduce tertiary amine groups according to the following reaction: 
##STR11## 
where Z' and Z" represent polymer residues and R.sup.IV and R.sup.V are 
defined as above. The amine is preferably a secondary amine to introduce 
tertiary amine groups into the polymers, for example diethylamine, 
di-n-propylamine, di-n-butylamine, N-methylethanolamine, 
N-ethylethanolamine or diethanolamine. The use of an aminoalcohol such as 
N-methylethanolamine or diethanolamine introduces further hydroxy groups 
into the polyester as well as tertiary amine groups. 
In an alternative method of forming a hydroxy- and amine-functional 
polyester an unsaturated carboxy-functional polyester is formed by 
reacting a minor proportion of an unsaturated monocarboxylic acid such as 
acrylic or methacrylic acid with the glycol and polycarboxylic acid and/or 
anhydride reagents. The acrylic or methacrylic residue is incorporated at 
the end of the polymer chain and can be reacted with an aminoalcohol by 
the Michael-type addition reaction described above, preferably with a 
di(hydroxyalkyl)amine such as diethanolamine, to introduce hydroxy and 
amine groups. 
The hydroxy-functional polyesters of Tg 0.degree.-120.degree. C. containing 
amine groups are preferably used in powder coating compositions with a low 
molecular weight anhydride component (A) such as tricarballylic anhydride 
or with an addition copolymer containing anhydride groups along its chain. 
Examples of solid, low molecular weight compounds containing at least two 
hydroxy groups and at least one amino group which can be used as hydroxy 
component (B) when the anhydride component (A) is a resin of Tg 
0.degree.-120.degree. C., for example an anhydride-tipped polymer, include 
tertiary carbinamines of the formula: 
##STR12## 
where R.sup.VI represents a methyl group or a hydroxyalkyl group having 1 
to 4 carbon atoms, for example tris-(hydroxymethyl)-methylamine or 
2-amino-2-methyl-propane-1,3-diol. The corresponding secondary and 
tertiary amines in which the amine nitrogen atom is substituted, for 
example with a methyl group or a hydroxyethyl group, can also be used. 
Alternative hydroxy amines are the reaction products of glycidol with a 
diamine, particularly a heterocyclic diamine such as piperazine. The 
reaction product of glycidol and piperazine is a bis(dihydroxyalkyl) amine 
of the formula: 
##STR13## 
The amine groups in the hydroxy component (B) are most preferably tertiary 
amine groups, although primary or secondary amine groups adjacent to a 
tertiary carbon atom are also preferred particularly when lower molecular 
weight amino alcohols such as the tertiary carbinamines described above 
are used. 
The powder coating compositions can be formulated using known methods. The 
components (A) and (B) are generally mixed in the dry state together with 
any other ingredients of the coating composition such as pigments or 
fillers. The dry mixture is then extruded at a temperature above the glass 
transition temperature of at least one synthetic resin component but below 
the temperature at which substantial fusion takes place, for example at a 
temperature in the range 80.degree. to 120.degree. C. The extrudate is 
comminuted to form the powder coating composition, which is then applied 
to the substrate to be coated, which may or may not be preheated, 
preferably by electrostatic spraying. The coated substrate is then heated 
to a temperature, usually in the range 120.degree. to 220.degree. C., at 
which the synthetic resin component of the powder coating flows and the 
coating cures. 
The components (A) and (B) are preferably used in relative proportons to 
provide from about one hydroxy group per carboxylic acid group to about 
one hydroxy group per carboxylic anhydride group .+-.10%. 
The powder coating can for example be applied to steel pipelines, office 
furniture, motor car components, reinforcing bars for concrete or 
aluminium window frames. 
Whilst any synthetic resin used as a component in the powder coating 
composition can generally have a Tg in the range 0.degree.-120.degree. C., 
special precautions such as low-temperature grinding and storage are 
required if the Tg is near the lower end of this range. The Tg of any 
synthetic resin is used in the powder coating composition is preferably 
25.degree.-110.degree. C. and most preferably 40.degree.-90.degree. C. 
The powder coatings of the invention are capable of forming cured films 
with particularly good gloss and weathering properties such as resistance 
to sunlight, particularly when the anhydride component (A) and hydroxy 
component (B) are acrylic polymers or when a hydroxy-functional acrylic 
polymer (B) is used with a tricarboxylic compound such as tricarballylic 
anhydride as component (A). The presence of amine groups in the 
hydroxy-functional components (B) allows curing at lower temperature. For 
example, a powder coating containing an acrylic polymer containing 
hydroxyl and amine groups and an anhydride-functional component (A) will 
cure to a hard solvent-resistant coating at 170.degree. C., whereas if an 
acrylic polymer containing hydroxyl groups but no amine groups is used 
there is insufficient curing at 170.degree. C. 
The invention is illustrated by the following Examples.

EXAMPLE 1 
(a) Preparation of Tricarballylic Anhydride 
Tricarballylic acid was prepared by the method of Clarke and Murray (Org. 
Synth. Coll. Vol. 1, P.523). Tricarballylic acid (580 g) was mixed with 
acetic anhydride (679 g), acetic acid (1779 g) and chloroform (2834 g) and 
the mixture stirred at eflux for 4.5 hours and allowed to cool. The 
precipitate which formed after 32 hours was collected, dried in vacuo 
below 110.degree. C., and dissolved in 10 times its weight of a 
crystallisation mixture comprising chloroform (1000 parts), glacial acetic 
acid (900 parts) and acetic anhydride (100 parts), premixed at least 12 
hours before use. The solution was filtered and allowed to cool. The white 
precipitate which formed within 36 hours was collected and dried in vacuo 
below 100.degree. C. 
210 g of material containing 90 percent of tricarballylic anhydride, the 
residue being principally tricarballylic acid, was obtained. Analysis was 
by gas chromatography of the silanised product. 
(b) Preparation of Hydroxy-and Amine-Functional Acrylic Polymer 
Methyl methacrylate (2650 g), ethyl acrylate (600 g), hydroxyethyl acrylate 
(315 g), diethylaminoethyl methacrylate (500 g), methyl isobutyl ketone 
(MIBK) (783 g) and azobisisobutyronitrile (AZBN) (160 g) were mixed, 
filtered and pumped over 4 hours into a further 3000 g of MIBK refluxing 
at about 112.degree. C. in a reaction vessel equipped with stirrer, 
thermometer and reflux condenser. One hour after addition was complete, a 
further 4 g of AZBN dissolved in 225 g of MIBK was added, the mixture held 
at reflux for one hour, and a further "booster" of the same composition 
added. 
After a further hour, solvent was distilled off under vacuum to yield a 
glassy resin of cone and plate viscosity 3.5 poise (0.35 Pas) at 
200.degree. C., Tg 40.degree. C., molecular weight 6000 by g.p.c. and 
hydroxyl equivalent weight 1560. 
(c) Powder Coating Compositions 
The acrylic polymer prepared in Example 1(b) ground to a coarse powder 
(606.1 g), tricarballylic anhydride (31.9 g), rutile titanium dioxide (350 
g), Modaflow 3 (Trade Mark) acrylic polymer flow promoting agent (10 g), 
and benzoin (2 g) were premixed and co-extruded at 80.degree. C. The 
extrudate was micronised to a powder of average particle size 50 microns, 
electrostatically sprayed onto steel panels and stoved for 15 minutes at 
170.degree. C. The films obtained were glossy, hard and crosslinked, 
resisting rubbing with an acetone-soaked cloth. 
EXAMPLE 2 
(a) Preparation of Anhydride-Functional Polyester 
Isophthalic acid (1992 g), adipic acid (584 g), pentaerythritol (136 g), 
neopentyl glycol (1664 g) and xylene (75 g) were charged to a reaction 
vessel fitted with stirrer, condenser and thermometer, and heated to 
reflux. After about 12 hours, the temperature had risen to 240.degree. C., 
500 g of water had been removed and the acid value fallen to 10. Xylene 
was removed by distillation in vacuo, and replaced by MIBK (150 g). 
Tricarballylic anhydride (894 g) was charged at 120.degree. C., and the 
temperature maintained for 12 hours, ensuring only gentle reflux. A Dean 
and Starke trap was then fitted and vigorous refluxing to remove water 
continued for 4 hours. Solvent was removed in vacuo to yield a glassy 
polymer of anhydride equivalent weight approx. 1000, Tg 50.degree. C. and 
viscosity 40 poise (4 Pas) at 200.degree. C. 
(b) Preparation of Powder Coatings 
The anhydride-functional polyester prepared in (a) above (1000 g), 
tris(hydroxymethyl)methylamine (40 g), TiO.sub.2 (1350 g), Modaflow 3 (10 
g) and benzoin (2 g) were mixed, extruded at 100.degree. C., and 
micronised, sprayed and stoved as described in Example 1 to yield hard, 
tough coatings. 
EXAMPLE 3 
(A) Preparation of Hydroxy-Functional Acrylic Polymer 
The procedure of Example 1(b) was repeated exactly, except that the monomer 
mixture comprised only methyl methacrylate (2850 g), ethyl acrylate (900 
g) and hydroxyethyl acrylate (315 g) to prepare an amine-free polymer. The 
product had a viscosity of 4 poise (0.4 Pas) at 200.degree. C., Tg 
45.degree. C., moleculr weight 8000 by g.p.c., and hydroxyl equivalent 
weight of 1560. 
(b) Preparation of Anhydride-Functional Acrylic Polymer 
The amine-free hydroxyacrylic polymer (1560 g), tricarballylic anhydride 
(182 g) and MIBK (87 g) were stirred together at 120.degree. C. with 
reflux for 16 hours. A Dean and Starke trap was then fitted, and vigorous 
reflux continued for 2 hours. The product was then dried in vacuo to a 
glassy brittle anhydride-functional resin. 
(c) Preparation of Powder Coating 
The anhydride-functional resin of Example 3(b) (875 g), 
tris(hydroxymethyl)-methylamine (20 g) TiO.sub.2 (350 g), Modaflow 3 (10 
g) and Benzoin (2 g) were mixed, coextruded, micronised, sprayed and 
stoved as described in Example 1(c) to yield hard, cured coatings. 
EXAMPLE 4 
The anhydride-functional polymer of Example 3(b) (1750 g) was mixed with 
the hydroxy- and amine-functional acrylic polymer of Example 1(b) (1560 
g), TiO.sub.2 (1815 g), Modaflow 3 (52 g) and benzoin (10 g) and 
co-extruded, micronised, sprayed, and stoved as described in Example 1(c) 
to form a hard, solvent-resistant coating. 
EXAMPLE 5 
(a) Preparation of Anhydride Component 
The amine-free hydroxyacrylic polymer of Example 3(a) (1560 g), 
tricarballylic anhydride (33.0 g) and MIBK (95 g) were refluxed together 
for 8 hours, followed by in vacuo drying below 120.degree. C. The product 
was a mixture of anhydride-functional acrylic polymer and tricarballylic 
anhydride, both components active in curing. 
(b) Preparation of Powder Coating 
The anhydride component prepared in Example 5(a) (950 g) was mixed with the 
hydroxy- and amine-functional acrylic polymer of Example 1(b) (1560 g), 
TiO.sub.2 (1375 g), Modaflow 3 (40 g) and benzoin (8 g) and extruded, 
micronised, sprayed and stoved as described in Example 1. A hard, smooth 
cured film was obtained. 
EXAMPLE 6 
The hydroxy and amine-functional polymer of Example 1(b) (606.1 g) was 
mixed with tricarballylic acid (34.2 g) and pigments and additives and 
formed into a powder coating and applied as described in Example 1(c). 
Comparative Example 
The procedure of Example 1(c) was repeated using the amine-free 
hydroxy-functional acrylic polymer of Example 3(a) in place of the polymer 
of Example 1(b). The films produced were uncured and of poor appearance 
(low gloss).