Epoxy phosphate-carboxyl copolymers and aqueous coatings containing the same

Epoxy phosphate-carboxyl copolymers substantially free of epoxy functionality and aqueous coatings containing the same are disclosed. These copolymers are formed by copolymerizing monoethylenically unsaturated monomers, including carboxyl-functional monomer, in an organic solvent solution containing an epoxy-functional epoxy phosphate, there being at least 5% of carboxyl-functional monomer and at least 10% of total monomers present in the copolymer. The copolymer is converted into a water soluble salt with volatile amine and phenoplast resins are preferably used to provide at least part of the curing agent.

DESCRIPTION 
1. Field of Invention 
This invention relates to aqueous coatings containing epoxy phosphates 
possessing improved stability in aqueous medium, especially in combination 
with phenoplast resins. 
2. Background Art 
Aqueous coatings containing epoxy phosphates are known. While these 
contribute good coating properties when cured with aminoplast or 
phenoplast curing agents, they have been difficult to use effectively 
because the aqueous dispersions containing these epoxy phosphates have 
lacked stability. In some instances the aqueous dispersions settle out and 
become useless. In other instances, the coating properties change with 
time, rendering the coatings unreliable and unpredictable. Application 
problems, such as the presence of large particles which damage the final 
films, frequently characterize the difficulties which are encountered. 
This stability problem has been particularly evident when phenoplast 
curing agents are used in an effort to provide coatings possessing 
superior corrosion resistance, as is desired in coatings for sanitary can 
use. 
DISCLOSURE OF INVENTION 
In this invention, an epoxy-functional epoxy phosphate is copolymerized in 
organic solvent solution with monoethylenically unsaturated monomers, 
including at least about 5% of a carboxyl-functional monomer and at least 
about 10% of total monomers (based on the copolymer) to provide a 
copolymer having carboxyl functionality and which is dispersible in water 
with the aid of an amine to provide a dispersion of uniformly fine 
particle size which is stable, even in the presence of large amounts of 
phenoplast curing agent. 
During the copolymerization reaction substantially all of the epoxy 
functionality is consumed by esterification with the carboxyl-functional 
monomer and the product is carboxyl-functional so that it can be dispersed 
in water by salt formation with an amine. The volatile amines used for 
this purpose are common knowledge and are illustrated in the examples. 
The epoxy phosphate is conveniently formed by reacting a resinous 
polyepoxide with ortho phosphoric acid (pyrophosphoric acid is regarded to 
be an equivalent because it generates ortho phosphoric acid) in an organic 
solvent solution. Only one of the phosphoric acid OH groups appears to 
react, and based on such stoichiometry, excess 1,2-oxirane functionality 
is present in the polyepoxide to provide an epoxy-functional epoxy 
phosphate. The organic solvent is preferably water miscible, alcoholic 
solvents like 2-ethoxy ethanol and 2-butoxy ethanol being preferred. 
Monoethylenically unsaturated monomers, including carboxyl-functional 
monomers, are then dissolved therein, and these monomers are then 
copolymerized in the presence of a free radical-generating catalyst to 
provide a copolymer in which the epoxy phosphate is part of the copolymer 
molecule. While some grafting may occur, the primary mechanism for 
combining the addition polymeric structure with the epoxy phosphate is 
believed to involve esterification of the epoxy groups in the epoxy 
phosphate by the carboxy groups in the carboxyl-functional monomer. At 
least about 5% of carboxyl-functional monomer, based on the weight of the 
copolymer, is employed to consume epoxy functionality and to provide 
excess carboxyl functionality for water dispersibility with the aid of a 
volatile amine which forms salt groups therewith. The unreacted acidity of 
the orthophosphoric acid also forms salt groups with the volatile amine 
and this aids water dispersibility. Some water miscible solvent is also 
present, and this further assists dispersibility in water. 
The carboxyl-functional monomers which are copolymerizable and which form 
salts to aid water dispersibility are well known and are preferably 
constituted by acrylic and methacrylic acids. The proportion of total 
monomer in the copolymer is preferably from 10% to 200% of the weight of 
the epoxy phosphate, most preferably not in excess of 100%, and the 
carboxy-functional monomer is preferably used in an amount of from 10% to 
35% of the epoxy ester. The copolymer acid value is desirably 30-120, most 
preferably 50-90.

Throughout this specification and claims, all proportions are by weight, 
unless otherwise specified. 
Any organic solvent-soluble resinous polyepoxide may be used herein. By a 
polyepoxide is meant a 1,2-epoxy equivalency of at least about 1.2. 
Diepoxides are preferred, especially diglycidyl ethers of bisphenols 
having a 1,2-epoxy equivalency in the range of 1.3-2.0. The class of 
bisphenols is well known, and bisphenol A is usually used in commerce. 
Diglycidyl ethers of bisphenol A are commonly available in commerce and 
the commercial materials are fully useful herein. It is preferred to 
employ those having an average molecular weight (by calculation) of from 
about 500, more preferably at least about 1000, up to about 5000. Epon 
1007 from Shell Chemical Company will be used as illustrative. Epon 1004 
and Epon 1001 are also useful herein and will further illustrate preferred 
polyepoxides. 
The proportion of phosphoric acid is not critical herein so long as epoxy 
functionality is left to permit the carboxyl-functional monomer to couple 
with the polyepoxide by esterification which takes place in the presence 
of the phosphate groups (an esterification catalyst may be added if 
desired). It is appropriate to use the phsophoric acid in an amount such 
that the epoxy ester contains from 0.05 mol to 1.0 mol of phosphoric acid 
per epoxide equivalent in the polyepoxide, which is preferably a 
diglycidyl either as noted above. It is preferred to employ from 0.1 to 
0.3 mol of ortho phosphoric acid per epoxide equivalent in the 
polyepoxide. 
It is desired to point out that it is preferred to minimize the proportion 
of phosphoric acid in order to provide the best resistance to chemical 
attack in the final cured coatings. Of course, enough phosphoric acid must 
be used to provide the desired curing catalysis. This means that the epoxy 
phosphate which is formed will include unreacted epoxy groups. It is 
believed that only about one P--OH group reacts in the ortho phosphoric 
acid, so some P--OH groups remain in the epoxy phosphate. It is these 
unreacted epoxy groups which led to instability in the prior art aqueous 
dispersions which were formed, especially when phenoplast curing agents 
were used. In this invention, it appears that the copolymerization 
reaction with carboxyl-functional monomer forces the consumption of the 
unreacted epoxy groups and it also enhances dispersibility in water with 
the aid of an amine so that nonwater soluble curing agents are more stably 
suspended in the aqueous coating compositions which are formed. 
While some grafting is thought to occur, it is thought that graft coupling 
is very limited and that esterification is the prime coupling mechanism. 
Catalysis of the copolymerization reaction is conventional, and will be 
illustrated herein using cumene hydroperoxide. A copolymerizatin 
temperature over 100.degree. C. is preferred to force consumption of the 
unreacted epoxy groups, and a temperature of 125.degree. C. will be used 
as illustrative. 
Mercaptan chain terminators are desirably avoided when sanitary can use is 
intended. The alcoholic solvents which are preferred likely have some 
limited chain terminating function. 2-butoxy ethanol is the preferred 
solvent and will be used in the Example. 
The other monomers which may be copolymerized with the epoxy phosphates in 
accordance with this invention are those commonly used in the production 
of film-forming addition copolymers from monoethylenically unsaturated 
monomers. These are illustrated by styrene, vinyl toluene, vinyl acetate 
and acrylate and methacrylate esters, like methyl methacrylate, methyl 
acrylate, ethyl acrylate and n-butyl and isobutyl acrylate and 
methacrylate. 
The copolymers of this invention are cured with curing agents, especially 
aminoplast and/or phenoplast resins. These may be added either to the 
copolymer solution or to the aqueous dispersion containing the dispersed 
copolymer to provide a stable and curable aqueous thermosetting coating 
composition. When the curing agent is added to the copolymer solution, it 
is preferred to heat the mixture in order to heat-compatilize the mixture. 
This better enables the copolymer to colloidally disperse the curing agent 
in the aqueous dispersion. This is particularly advantageous when some of 
the preferred curing agents which are not water soluble are employed. The 
preferred curing agents are phenoplast resins which are not soluble in 
water directly. These provide greater resistance to chemical attack and 
may be used as the sole curing agent, or together with aminoplast resins. 
It is desired to point out that these nonwater-soluble phenoplast resin 
curing agents were previously characterized by poor stability in aqueous 
medium, but good stability is provided herein. 
Water dispersible aminoplast resins and phenoplast resins are themselves 
well known and are boardly useful herein. These are illustrated by 
hexamethoxymethyl melamine and A-stage phenol-formaldehyde resols. The 
water insoluble curing agents which may be used are heat-hardening 
formaldehyde condensates which are dispersed in the water medium by the 
copolymer salt. Heat-hardening products employ at least about 1 mol of 
formaldehyde per mol of the phenol. Polymethylol phenols produced by the 
reaction with formaldehyde in alkaline medium may be used, but it is 
preferred to employ a cresol-formaldehyde reaction product containing 
about 1 mol of formaldehyde per mol of cresol. Ortho cresol is used in the 
cresol-formaldehyde resin available in commerce. These cresol-formaldehyde 
resins are preferably used in conjunction with aminoplast resins of higher 
functionality, like hexamethoxymethyl melamine. These mixtures cure to 
provide outstanding properties. 
The curing agent should be used in an amount of from 5% to 50% of total 
nonvolatile resin solids, and up to about 30% of total nonvolatile resin 
solids is phenoplast resin. 
The amines used for salt formation to provide dispersibility in water are 
well known, and may even be constituted by ammonia which is regarded to be 
an amine in this art. Dimethyl ethanol amine is well known to solubilize 
carboxyl-functional resins and will be used herein as illustrative. 
The coatings herein can be applied in any desired fashion and are cured by 
baking. This removes volatile components, such as water, organic solvent, 
and the volatile amine which provides the salt groups in the copolymer. 
The epoxy phosphate component of the copolymer serves as the catalyst for 
the cure, but extraneous catalysts, like p-toluene sulphonic acid may be 
used, though this is not desirable since it impairs water resistance. 
EXAMPLE 
600 grams of 2-butoxy ethanol are mixed with 24 grams of 85% ortho 
phosphoric acid in a reactor equipped with a reflux condenser and a trap 
to remove water of condensation. The mixture is then heated to 120.degree. 
C. with agitation. 875 grams of the Shell product Epon 1007 are then added 
slowly to the reactor forming a hot melt, and the mixture is held for 2 
hours at 125.degree. C. A premix of 160 grams of styrene, 8 grams of ethyl 
acrylate and 135 grams of methacrylic acid with 38 grams of cumene 
hydroperoxide are then added to the epoxy phosphate formed by the 
previously described reaction. Addition of the premix is made over a 21/2 
hour period with the temperature at 125.degree. C. After addition, the 
mixture is held at 125.degree. C. for 1 hour to complete the 
polymerization. The acid value of the copolymer solids is 74. 
Another premix is now made to include 10 grams of cumene hydroperoxide (to 
insure completion of polymerization) 130 grams of 2-butoxy ethanol, 260 
grams of an ortho-cresol-formaldehyde heat-hardening resin and 230 grams 
of hexamethoxymethyl melamine. This premix is then added to the reactor 
and the contents held at 125.degree. C. for 1 hour. Then the reaction is 
completed by adding 10 grams of cumene hydroperoxide and holding for 1 
hour at 125.degree. C. 
155 grams of dimethyl ethanol amine are than added in admixture with 145 
grams of deionized water to the cooled reaction product over a 15 mixture 
period, and then the product is thinned to desired solids content by the 
addition of deionized water (first by the addition of 2610 grams over 1 
hour) and then by the addition of 950 grams. This provides a milky 
dispersion. 
The mixture of phenoplast and aminoplast resin in this example can be 
replaced with 260 grams of an alkaline-condensed polymethylol phenol 
resin. 
The dispersion of this example is coated upon a flat aluminum substrate and 
cured in a 425.degree. F. oven for 30 seconds to provide a cured film 
having a thickness of 0.3 mil. The cured film resisted 100 double rubs 
with a methyl ethyl ketone-saturated cloth and was excellently flexible. 
As a result the cured coated aluminum substrate can withstand fabrication 
to produce a can end for a two-piece sanitary can intended for the 
packaging of beer.