Powder coatings with catalyzed transesterification cure

Improved powder coating compositions comprise a hydroxyl functional binder having carboxylic ester functionality and a transesterification cure catalyst comprising an epoxide and a non-acidic nucleophile, preferably onium salts. Resins such as hydroxyl functional acrylic, hydroxyl functional polyester, ester functional polyesters, and methyl succinate esters of bisphenol epoxides transesterify in the presence of an epoxide and phosphonium salts such as tetrabutylphosphonium acetate and yield coatings having exceptional hardness, impact resistance and solvent resistance at bake temperatures of about 300.degree. F.

BACKGROUND OF THE INVENTION 
Transesterification of an ester moiety with an alcohol component is known 
to proceed according to the scheme: 
##STR1## 
The reaction is an equilibrium reaction, that can be driven to completion 
by removal of the evolved alcohol especially if it is a lower molecular 
weight alcohol such as methanol or ethanol. Transesterification is an 
especially suitable reaction for producing thermoset coatings because the 
lower alcohols evolved during the cure easily pass out of the coating and 
allow the reaction to go to completion. Highly crosslinked films result. 
Several catalyst types are known for transesterification. These include 
acids, bases, and metal salts of organic acids. A number of patents, 
including U.S. Pat. Nos. 4,362,847; 4,376,848; 4,332,711; and 4,459,393 
describe metal ion complexes and/or metal salts used for promoting 
transesterification. These are incorporated herein by reference for 
general exemplification of resinous binder types that can be crosslinked 
by transesterification and to illustrate the prior art catalysts such as 
octoates or naphthenates of lead, zinc, calcium, barium, and iron. 
In U.S. Pat. No. 4,559,180 Green teaches a process for the 
transesterification of a carboxylic or carbonic acid ester under 
transesterification conditions with an alcohol in the presence of either a 
Group V element containing Lewis base or a cyclic amidine and an epoxide. 
Kooijmans et al (U.S. Pat. No. 4,362,847 and U.S. Pat. No. 4,332,711) teach 
thermosetting binders for paints comprising a non-acidic 
hydroxyl-containing resin and a non-acidic polyester having a 
beta-hydroxyl ester group. 
Dante and Parry have shown that phosphonium halides, such as ethyltriphenyl 
phosphonium iodide, are efficient catalyts for (a) 1,2-epoxide reactions 
with phenols to yield hydroxy ethers (U.S. Pat. No. 3,477,990), and (b) 
polyepoxide reactions with carboxylic acids or acid anhydrides (U.S. Pat. 
No. 3,547,885). Parry has shown that polyepoxides and phenols can be 
reacted to form phenolic hydroxy ethers with phosphonium salts as 
catalysts. The counterion of the phosphonium moiety is the anion portion 
of a carboxylic acid, or acid ester, such as in ethyltriphenyl phosphonium 
acetate (U.S. Pat. No. 3,948,855). 
Barnhoorn et al (U.S. Pat. No. 4,459,393) teach self-crosslinking 
thermosetting resin compositions obtained from the reaction of a 
beta-hydroxyalkyl ester or an alpha,beta-carboxylic acid with a primary 
mono- or polyamine to give a product having 1 to 2 amino hydrogens and 
further reacted with a polyglycidyl ether of a polyhydric phenol so that 
the final resin adduct has more than one beta-hydroxyalkyl ester group and 
amino groups having 1 to 2 amino hydrogen atoms per molecule. 
Subramanyam et al (U.S. Pat. No. 4,376,848) teach the preparation of water 
dilutable electrocoating compositions having tertiary amino-containing 
basic binders by reacting a secondary amino group compound with an 
olefinically double-bonded epoxy and the copolymerization of this product 
with at least one ethylenically bonded polymerizable monomer wherein said 
binders can self-cure and be cured in combination with amine resins and/or 
phenolic resins. 
In the powder coatings field there is an ongoing need for improved cure 
chemistry. The transesterification cure patents noted above suffer from 
low cure efficiency, high temperature baking schedules, require expensive 
activated esters, and give poor hydrolytic film stability. Although U.S. 
Pat. Nos. 3,477,990, 3,547,885 and 3,948,855 teach phosphonium salts as 
catalysts for epoxy/phenol, epoxy/acid and epoxy/epoxy reactions they did 
not recognize that the combination catalyst of phosphonium salts with free 
epoxides are effective transesterification catalysts.

DETAILED DESCRIPTION OF THE INVENTION 
The instant invention relates to new coatings which comprise a binder 
having hydroxyl functional groups and lower alkyl ester carboxylic ester 
functional groups adapted to cure after application to a substrate by 
transesterification; and an in-situ formed transesterification catalyst 
comprising a non-acidic nucleophile and an epoxide wherein the said 
epoxide is a monomeric or polymeric epoxide selected from the group 
consisting of C.sub.2-18 alkylene oxides, arylalkylene oxides, 
cycloaliphatic oxides, and a polymeric or oligomeric epoxide having at 
least one epoxide group per molecule. The nucleophile is selected from the 
group consisting of non-acidic nucleophile or onium salts. 
For the instant powder coating compositions, the hydroxyl and/or ester 
functionality may be derived from individual compounds, dimers, oligomers 
or polymers and mixtures or both ester precursor functionality may be part 
of the same oligomer or polymer including polyesters, polyepoxides, 
polyacrylates and methacrylates, polyamides, polyamines, polycarbonates 
and mixtures thereof. The catalyst is preferably formed in-situ in the 
coating by the addition of about 0.001 to 1.0 milliequivalents non-acidic 
nucleophile and about 0.001 to 1.0 milliequivalents epoxide per gram of 
coating. Preferred catalysts are those derived from the reaction of an 
epoxide including polymeric epoxides and onium salts. Especially preferred 
are phosphonium salt catalysts. 
The instant in-situ catalysts effect cure for various polyfunctional alkyl 
esters admixed with various polyols. For example, a coating can contain: 
(a) a blend of polyalkyl esters (R--(CO.sub.2 R').sub.m) and polyols 
(R"--(OH).sub.n); or 
(b) a multifunctional compound containing both ester and hydroxyl 
functionality, e.e., R'"(CO.sub.2 R').sub.x (OH).sub.y ; or 
(c) a blend of (A) and (B); and 
(d) an effective amount of a transesterification catalysts comprising an 
epoxide and a non-acidic nucleophile. 
In the above coatings the further addition of a monoalcohol and/or 
monoester is useful for limiting crosslink density of the coating and act 
as film softeners. 
The catalyst for the transesterification is formed in situ by the reaction 
of a nucleophilic compound X with an oxirane: 
##STR2## 
wherein X is a non-acidic nucleophile or non-acidic nucleophilic onium 
salt. Most preferred are terminal oxiranes wherein R.sup.VI and R.sup.VII 
both are H as they are most reactive with nucleophiles. R.sup.V can be H 
or simple alkyl, simple aryl, or more complex moieties. R.sup.IV 
represents simple or complex alkyl or aryl radicals. 
Examples of epoxides useful for the in-situ preparation of the cure 
catalyst include C.sub.2-18 alkylene oxides and oligomers and/or polymers 
having epoxide functionality including multiple epoxy functionality. 
Particularly suitable alkylene oxides include propylene oxide, 
1,2-butylene oxide, 1,2-hexylene oxide, tert-butyl glycidyl ether, phenyl 
glycidyl ether, glycidyl acetate, and glycidyl benzoate. Useful 
multifunctional oxiranes include bisphenol A diglycidyl ether, diglycidyl 
adipate, 1,4-diglycidyl butyl ether, Novalac resins and other commercial 
epoxy resins. Bisphenol A diglycidyl ether epoxides which are solids at 
20.degree. C. are preferred epoxides. Also useful are acrylic polymers 
having epoxide functionality such as acrylic copolymers derived from 
glycidyl methacrylate. Oxirane compounds wherein only R.sup.VI and 
R.sup.VII are H include isobutylene oxide (2-methyl-1,2-propene oxide), 
2-methyl-1,2-hexene oxide, 2-phenyl-1,2-propene oxide (alphamethyl styrene 
oxide), 2-phenoxy methyl-1,2-propene oxide, and the like. Other oxiranes 
include 2,3-dimethyl-2-butene oxide, 2-methyl-2-butene oxide, oleic acid 
oxide, and 1-phenyl propene oxide. 
The nucleophilic compound X can include covalent materials such as tertiary 
amines, tertiary phosphines, sulfides and the like as described below. The 
compound X can be ionic wherein the anion component possesses the 
nucleophilic moiety. These include various "onium" halides and 
carboxylates as detailed below. Various other heterocyclic compounds are 
nucleophiles and can be used, such as imidazoles, imidazolines, thiazoles 
and the like. Compounds such as secondary amines or mercaptans can also be 
used though they are less preferred as they must react twice to form the 
active catalyst. 
By non-acidic nucleophile is meant a nucleophile not bearing an active 
hydrogen, which becomes acidic upon reaction with an epoxy. Secondary 
amines have an active hydrogen and, hence, must be reacted twice to 
generate the effective catalyst. These secondary amines, although useful, 
are not preferred. 
Nucleophiles of the ionic type include: 
(1) quaternary ammonium compounds such as for example tetraethyl ammonium 
chloride, tetrapropyl ammonium acetate, and hexyl trimethyl ammonium 
bromide; 
(2) quaternary phosphonium compounds such as tetrabutyl phosphonium bromide 
and chloride, tetraphenyl phosphonium iodide and the like. Ethyl triphenyl 
phosphonium acetate is a preferred nucleophile because it is commercially 
available at low cost. 
(3) "pseudo" halides; 
(4) an N-alkylated pyridinium salt such as hexadecyl pyridinium bromide, 
chloride, and acetate. 
Other onium catalyst components include arsonium compounds such as 
tetraphenyl arsonium chloride and bromide and the like. Various sulfonium 
compounds are useful; for example, tributyl sulfonium chloride, dibutyl 
phenyl sulfonium acetate, S-butyl 1,4-butyl sulfonium benzoate and the 
like. Useful pseudo halides include cyanides, azides, cyanates and the 
like. 
The onium catalyst component of the present invention and particularly 
phosphonium salt catalysts are advantageous in that they are not only 
excellent catalysts but function as latent catalysts for the 
transesterification. Formulated powder paints (before application) have 
superior storage stability and the catalysis occurs only on baking 
following application of the coating to a substrate. Substantially no 
reaction occurs on storage at ambient temperature and the catalyst becomes 
active at cure temperatures of 250.degree.-400.degree. F. 
Coatings catalyzed by the epoxide/nucleophile transesterification catalysts 
are conveniently referred to as ENCAT coatings. ENCAT coatings comprise a 
wide variety of monomers, oligomers, and resins having the requisite 
hydroxyl and/or ester functionality include polyesters, polyacrylates, 
polyepoxides, polyamides, polyamines, monoalcohols, monoesters, polyols 
and mixtures thereof. 
This invention can be used to form films which have exceptional physical 
properties and are derived from ester-terminated epoxy compounds. Films 
prepared from blends of polyesters containing hydroxyls and esters of low 
boiling alcohols have good physical properties and low raw materials 
costs. 
Cure conditions vary with the concentrations of free epoxy and nucleophile 
as well as the type of alcohol which leaves during the transesterification 
reaction. Lower boiling alchols allow faster, lower temperature cures than 
higher boiling alcohols. Thus, the adduct of GT-7074 (Ciba-Geigy) and 
monomethyl succinate will cure well at 250.degree. F. in 20 to 30 minutes, 
while the mono-n-butyl succinate adduct only partially cures at 
300.degree. F. in 20 minutes. 
ENCAT catalysts are effective curing agents for epoxy esters of bisphenol A 
type epoxies and dicarboxylic acid monoesters. After cure, these coatings 
have a composition which is similar to that of epoxy/anhydride coatings, 
but our formulations do not contain irritating acid anydrides. Epoxy 
esters prepared with methyl succinate can be cured at about 300.degree. F. 
to form films with 3H and 5H pencil hardness, impact resistance of 140 
inch/pounds forward and reverse, excellent solvent resistance, no cracking 
or tape pull off in mandrel bending. 
Coating compositions are formulated from one or more of the various 
ester-containing components, one or more of the hydroxyl-containing 
components, and various blends of catalyst components. Generally, the 
coating will be formulated with about equal quantities of the co-reactive 
esters and alcohols although other ratios are useful for specific 
purposes. The coatings may include other less reactive esters or alcohols 
that are not considered in determining this reactive ratio. Likewise 
reactive diluents containing only a single reactive ester of reactive 
alcohol can be included in the coatings formulation. 
The quantity of catalyst components needed in the instant compositions can 
vary widely. Either catalyst component concentration can range from about 
0.001 to 1.0, preferably 0.05 to 0.5, milliequivalents per gram of binder. 
The ratio of equivalents nucleophile to epoxide can vary from about 10:1 
to 1:20. Preferably, an equal equivalent of nucleophilic component and of 
oxirane component will be used. However, the equivalent ration can vary 
especially toward an excess of oxirane. The concentration of catalyst 
components needed will depend upon the reactivity of the individual 
components with one another as well as upon curing temperature and time. 
One of the powder coatings of the instant invention adapted to cure by 
transesterification comprise a major portion of an ester derivative 
produced by an epoxy-acid reaction of an epoxy with a monoalkyl ester of a 
diacid or acid anhydride and transesterification catalyst comprising an 
epoxide and a nucleophilic compound, especially a phosphonium salt. 
Any bisphenol A, epichlorohydrin type epoxy that is of sufficient molecular 
weight to be a solid at room temperature is suitable to make the ester 
derivative. It is expected that Novalac type epoxy resins will also be 
suitable. Generally the monoalkyl ester of a diacid is mixed with the 
epoxy at a 1:1 mole ratio of free carboxylic acid to epoxide, but a 10 to 
20% excess of either component can be tolerated. Excess epoxy will remain 
in the derivative and provide the free, unreacted epoxy, as catalyst 
component. Reaction of the epoxy with the free acid can be carried out at 
any temperature above about 80.degree. C. Higher temperatures, such as 
about 100.degree. to 150.degree. C. may be desirable to melt the epoxy and 
keep it free flowing during the reaction. Small quantities of a catalyst, 
such as a phosphonium salt, a tertiary amine or another nucleophile can be 
added at levels up to about 1% to shorten the reaction time. 
Monoalkyl esters of various diacids or anhydrides can be used. It is 
convenient to mix an equal molar quantity of an anhydride, such as 
succinic anhydride, with a lower alkyl alcohol, such as methanol, and 
simply allow the alcohol to open the anhydride ring at about 50.degree. to 
100.degree. C. Monoesters of other diacids such as monomethyl adipate or 
monoethyl glutarate can be used. Lower alkyl, primary alcohols, such as 
methanol, ethanol, n-butanol and the like are preferred, because these low 
molecular weight alcohols will easily evaporate and leave the film during 
cure. Secondary or higher primary alcohols, however, can be used, alone or 
in combination with lower primary alcohols. 
The free epoxy in this formulation can be any bisphenol A or glycidyl type 
epoxy that is of sufficient molecular weight or tack so as not to cause 
the power paint to block or sinter during storage. 
The phosphonium salt catalysts can be represented generally by the formula: 
EQU R.sub.1 R.sub.2 R.sub.3 R.sub.4 P.sup.+ X.sup.- 
where R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are independently selected 
from the group consisting of aromatic, aliphatic, alkenyl, cycloaliphatic 
and cycloalkenyl radicals; and X is an anion. Suitable anions include 
halide, preferably bromide or chloride, and carboxylic acid residue, 
preferably acetate anion. 
Useful phosphonium salts include such compounds as tetrabutylphosphonium 
acetate, ethyltriphenylphosphonium acetate, various phosphonium halides 
and the like. Although other nucleophiles will provide some cure, 
phosphonium salts have been found to be particularly useful for powder 
formulations. Phosphonium salt catalysts are particularly advantageous for 
use with epoxy resin binders, with acrylate resin binders and with 
polyester resin binders. 
Various hydroxyl functional acrylic resins can be used in ENCAT powder 
coatings. Powder coating resins typically have Tg's above 40.degree. C. 
and sufficiently high molecular weights so that the formulated powder 
paint remains a free flowing powder at room temperature, but lower Tg 
polymers can be used preferably in a minor proportion. Hydroxyl 
functionality can be provided by monomers such as hydroxypropyl 
methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate and the 
like at levels of about 5 to 40 weight percent basis total monomer. 
Acrylate and methacrylate esters of polyols with two or more hydroxyl 
groups also can be utilized to develop specific properties. Other monomers 
that can be copolymerized with the hydroxy functional monomers to yield 
hydroxy functional acrylate resins are the vinylic monomers such as 
styrene and various other acrylate and methacrylate monomers. Various 
alkyl carboxylic diesters or polyester can be added to provide 
crosslinking. Dimethylterephthalate is particularly used in this regard. 
Of the onium catalysts those comprising phosphonium salts and epoxides are 
preferred for use with acrylic resins. 
Hydroxyl functional polyesters commonly used for powder paints can be cured 
by the ENCAT catalysts of the present invention. As noted above, diesters 
such as dimethyl terephthalate or ester functional polyesters are suitable 
components for crosslinking purposes. Polyesters with terminal ester 
functionality, such as those obtained by transesterification of dimethyl 
isophthalate and a diol such as neopentyl glycol (with a molar excess of 
diester) can be crosslinked with diols and polyols including for example 
neopentyl glycol, pentaerythritol, trimethylol propane and/or hydroxy 
functional polyester oligomers. As with the acrylic resins, phosphonium 
salt/epoxide catalysts are the preferred catalysts because they (1) are 
effective at lower levels (2) do not discolor on baking as do tertiary 
amines and phosphines and (3) maintain their catalytic activity during 
storage of the powder paints. 
The presence of carboxylic acid functionality in the coating will retard 
the onset of transesterification. Indeed, if excessive amounts of 
carboxylic acid are present, the oxirane component will be entirely 
consumed without the formation of the catalytic intermediate necessary for 
transesterification. This restriction must be kept in mind while 
formulating coatings based on the instant invention. The retarding effect 
of small quantities of carboxylic acid can be used advantageously to 
improve the package stability of the instant coating compositions. 
Catalysts prepared with phosphonium salts and epoxides are preferred for 
good storage stability in coatings formulations. Powder paints with 
phosphonium salts do not lose epoxide functionality or lose their ability 
to cure well with time. Catalysts containing tertiary amines lack the 
stability of those prepared with phosphonium salts. Catalysts prepared 
with onium salts are much preferred for powder coatings. 
Evaluation of Film Properties 
Coatings were applied at 1-2 mil thickness to phosphate treated cold-rolled 
steel panels and baked for 20 minutes at 300.degree. F. to 350.degree. F. 
Film properties were evaluated using the following evaluation: 
(1) marring after 100 MEK double rubs; 
(2) pencil hardness; 
(3) cracking after forward and reverse impact with a 5/8-inch ball up to 
160-inch pounds impact; and 
(4) cracking or loss of adhesion after conical mandrel bend down to 
1/8-inch diameter 180.degree. bend. 
MEK Double Rubs 
A cotton rag is wrapped around the index finger, soaked with methylethyl 
ketone and then wiped with a 2-inch stroke across the coated surface. Up 
and back motions with moderate pressure are counted as one rub. Resoak rag 
with MEK after each 20 rubs. Record number of rubs to the point where the 
coating is just removed, or after 100 rubs record percentage of mar if the 
coating has not been removed. 
The following illustrative Examples should not be narrowly construed. 
Unless otherwise indicated, parts and percentages are by weight and 
temperature is given in degrees Centigrade. 
EXAMPLE 1 
Preparation of Ester Resin Vehicle from Epoxy 
An epoxy derivative was prepared from the following coponents: 
______________________________________ 
Ingredients Grams 
______________________________________ 
Methanol 173 
Succinic Anhydride 494 
Bisphenol A type Epoxy (1,000 equivalent weight) 
5,300 
______________________________________ 
The anhydride and methanol were gradually heated under reflux to 
100.degree. C. and maintained two hours at 100.degree. to 110.degree. C. 
The epoxy and 15 grams benzyldimethylamine were added and the mixture 
heated to about 120.degree. C., held at that temperature for about two 
hours and then cooled. The product had a residual acid of 0.08 MEQ/g. and 
a residual epoxy of 0.06 MEQ/g. 
EXAMPLE 2-7 
Powder coatings were formulated from the vehicle of Example 1 by extruding 
the mixes shown in Table I at 80.degree. C. (Mixes were preblended in a 
Welex blender for one minute to assure uniformity. Phosphonium salts in 
methanol were adsorbed onto the TiO.sub.2 to aid in dispersion). The hot 
blend was passed from the extruder through a chilled two roll mill to form 
a flake. The flake was ground in a Bantam mill and sieved using a 140-mesh 
screen. 
The resulting powder was sprayed on to Parker Bonderite 1000 24 ga. panels, 
and baked at 300.degree. F. for 20 minutes. Film thicknesses ranged from 
1.2 to 1.8 mil. 
TABLE I 
__________________________________________________________________________ 
Epoxy 
Resin of 
Example 1 
GT7013 
Nucleophile* 
TiO.sub.2 
Benzoin 
PL200*** 
Example 
(g) (g) (g) (g) (g) (g) 
__________________________________________________________________________ 
2 1,500 225 113 TPP.sup.(1) 
1,268 
9 18 
3 1,000 100 15 DABCO.sup.(2) 
760 6 10 
4 1,000 100 30 DABCO.sup.(2) 
760 6 10 
5 1,000 100 30 TBPAA.sup.(3) 
760 6 10 
6 1,000 100 15 TBPAA.sup.(3) 
760 6 10 
7 1,500 150 20 ETPPAA.sup.(4) 
0 9 15 
__________________________________________________________________________ 
*GT7013 = 400 equivalent weight bisphenol A type epoxy (CibaGeigy) 
.sup.(1) TPP = triphenylphosphine 
.sup.(2) DABCO = 1,4diazabiscyclo(2.2.2)octane 
.sup.(3) TBPAA = tetrabutylphosphonium acid acetate (70% in 
methanol)(Morton Thiocol) 
.sup.(4) ETPAA = ethyltriphenylphosphonium acid acetate (70% in 
methanol)(Morton Thiocol) 
***PL200 = flow and leveling aid for powder formulations (SBS Chemical) 
EXAMPLE 8 
Film Properties 
The film properties of the powder coatings of Examples 2-7 are shown in 
Table II. 
TABLE II 
______________________________________ 
1/8" 5/8" Impact 
Pencil 
Example 
MEK DR* Mandrel Bend 
(in. lb.) 
Hardness 
______________________________________ 
2 10 Not tested; 
assume no cure 
3 90 -- 30 3H 
4 40 -- 30 H 
5 100 no cracks 160 5H 
(no mar) 
6 100 no cracks 160 4H 
(slight mar) 
7 100 no cracks 100 3H 
(full mar) 
9 100 no cracks 50 4H 
(full mar) 
10 100 fine cracks 10 H 
(full mar) 
______________________________________ 
*Solvent rubs with a methylethyl ketone soaked rag to the point of first 
film removal. #5 had no evidence of marring after 100 rubs. 
EXAMPLE 9 
Preparation of Acrylic (ENCAT) Powder Coating 
Powder coatings were formulated according to the method of Example 2 using 
commercial hydroxyl functional acrylic as follows: 
______________________________________ 
Ingredients Grams 
______________________________________ 
Poly-Tex 1000 Acrylic Resin* (hydroxyl value = 93, 
663 
AN = 16) 
GT 7013, 400 eq. wt. Epoxy** 
133 
PL-200 9 
Benzoin 9 
TiO.sub.2 550 
Tetrabutyl phosphonium acetate, 70% in Methanol 
30 
Dimethyl terephthalate 117 
______________________________________ 
*Celanese 
**CibaGeigy 
The coating was applied to cold roll steel test panels using an 
electrostatic spray. The coating was baked at 325.degree. F. for 20 
minutes. 
The results in Table II show that tetrabutyl phosphonium acetate is an 
effective transesterification catalyst for hydroxy functional acrylic 
binders. 
EXAMPLE 10 
Preparation of Hydroxyl Functional Polyester Powder Coating 
The powder coating was formulated as follows: 
______________________________________ 
Ingredients Grams 
______________________________________ 
Cargil 3008 Polyester Resin* (hydroxyl value = 295, 
500 
AN = 3.5) 
TiO.sub.2 760 
Dimethyl terephthalate 500 
GT 7013** 200 
Ethyltriphenyl phosphonium acetate, 70% in Methanol 
22 
Benzoin 6 
PL-200 10 
______________________________________ 
*Cargill, Incorporated 
**CibaGeigy 
As shown in Table II, ethyl triphenyl phosphonium acetate is an effective 
catalyst component using hydroxyl functional polyester powder coatings.