Aqueous can coatings of improved impermeability

There is disclosed a metal container containing a beverage or food having its integral surface coated with an adherent sanitary liner, the coating being a cured layer of a water-based coating composition consisting essentially of from about 5 percent to about 60 percent by weight of an amine-neutralized or partially neutralized interpolymer formed in the presence of a vinyl polymerization catalyst and in the absence of mercaptan, the interpolymer being comprised of from about 10 percent to about 30 percent of acrylamide or methacrylamide in units of the structure: ##STR1## where R' is methyl or hydrogen and R is hydrogen or lower alkyl, from about 5 percent to about 15 percent of units formed by vinyl polymerization of monoethylenically unsaturated carboxylic acid; from about 10 percent to about 50 percent of units derived by vinyl polymerization of isobutyl methacrylate; and from about 20 percent to about 45 percent of nitrile, such as acrylonitrile; wherein the interpolymer is solubilized by neutralizing acid groups of the interpolymer with a sufficient amount of monomeric amine to produce at least 0.200 milliequivalents of salt per gram of resin solids; and from about 40 percent to about 95 percent by weight of a liquid medium consisting of a mixture of water-soluble or water-miscible organic solvents and water, wherein at least 60 percent by weight of the mixture is water. These cured coatings are characterized by improved impermeability to lubricants and flavor components and also better resist water at elevated temperature so as to also be useful as an exterior coating.

The present invention relates to aqueous thermosetting coatings 
particularly adapted for coating the interior of sanitary cans. 
The development of water-based coating compositions for use as internal 
sanitary liners for metal containers has received considerable recent 
attention, and one system for providing such coating compositions is set 
forth in U.S. Pat. No. 3,991,216 issued Nov. 9, 1976. However, typical 
systems in accordance with the teachings of said patent lack desired film 
impermeability which is a characteristic which is particularly difficult 
to obtain when the cured coatings are adequately flexible. This invention 
is directed to overcoming the inadequacies which characterize these 
typical systems. 
The water-based coating compositions of said U.S. Pat. No. 3,991,216 
consist essentially of aqueous dispersions of amine-solubilized 
carboxy-functional interpolymers which are formed by the 
interpolymerization of monoethylenically unsaturated methylolated 
carboxylic acid amide units, monoethylenically unsaturated carboxylic acid 
units and a mixture of selected monoethylenically unsaturated monomers 
providing hardening units and flexibilizing units. The interpolymer is 
formed by polymerization in the presence of a vinyl polymerization 
catalyst and in the absence of mercaptan and the acid groups of the 
interpolymer are neutralized or partially neutralized with enough 
monomeric amine to produce at least 0.200 milliequivalents of salt per 
gram of resin solids so that the liquid medium of the coating composition 
can consist of a mixture of water-soluble or water-miscible organic 
solvents and water, wherein at least 60 percent by weight of the mixture 
is water. All proportions herein are by weight unless otherwise specified 
and the disclosure of said U.S. Pat. No. 3,991,216 is hereby incorporated 
by reference. 
Referring more particularly to the mixture of selected monoethylenically 
unsaturated monomers providing hardening units and flexibilizing units 
used in said patent, a typical preferred mixture is provided by styrene 
and ethyl acrylate. To provide desired cure and water-dispersibility it 
has been found that about 20% of isobutoxymethyl acrylamide together with 
about 8.5% acrylic acid (or 10% of methacrylic acid) is preferred. The 
balance of styrene and ethyl acrylate is then adjusted to contain enough 
ethyl acrylate to provide approximate minimum acceptability which is 
indicated by a wedge bend rating of about 6.5 (on a scale of 1-10 where 10 
is perfect and 6 is the lower limit of acceptability). This provides a 
typical composition containing 45.0% styrene, 25.0% ethyl acrylate, 20.0% 
isobutoxymethyl acrylamide and 10.0% methacrylic acid. When this 
interpolymer is prepared as described in U.S. Pat. No. 3,991,216 and 
applied from aqueous medium and cured, the film is inadequately 
impermeable, and the measurement of impermeability will now be described. 
An impermeable film is needed from two standpoints. First, foods and 
beverages contain components which provide flavor, and it is desired that 
these and similar components not be extracted by contact with the film. 
Second, the metal surface which is coated frequently contains lubricants, 
and it is desired to prevent these lubricants from passing through the 
film to contaminate the food or beverage and spoiling the flavor. It is 
the capacity of agents to move into and through the film which must be 
measured. Since beverages are primarily contemplated, especially beer, it 
is convenient to measure the impermeability of the film by its capacity to 
pick up and remove the n-heptyl ester of p-hydroxybenzoic acid which is an 
oily material commonly used to inhibit microbial growth in packaged beer. 
This agent which is commercially available as Staypro WS-7 is frequently 
extracted from beer when an off taste is encountered and it is concluded 
that coatings which extract "Staypro" will also extract flavor-producing 
constituents and allow oily lubricants to migrate. 
This Staypro is easily tracked using the following procedure. 
Water containing 8% of ethyl alcohol has 12 ppm (parts per million) of 
Staypro added thereto and 200 cc of this solution is used for test. 25 
square inches of aluminum foil coated with the coating under test on one 
side thereof is immersed in the solution and held therein at 75.degree. F. 
for 7 days. The amount of Staypro in the solution is ascertained by 
preparing a series of standard solutions containing various amounts of 
Staypro to see how much each of these absorb at 255 m.mu.. The absorbance 
of the test solution at 255 m.mu. at the end of 7 days is then measured 
and compared with the standards. 
As will be evident, if none of the Staypro is absorbed, the final Staypro 
concentration will be 12 ppm, the same as at the start, so a Staypro 
rating of 12 indicates total impermeability. In contrast, complete 
absorption is indicated by a final Staypro concentration of 0 ppm, so a 
Staypro rating of 0 indicates total permeability. 
The test is a difficult one as can be judged from the fact that aqueous 
systems containing typical acrylic copolymers in admixture with aminoplast 
resins provide a Staypro rating of less than 1 (0.7-0.8 is usual). These 
same aqueous systems exhibit poor resistance to water at the elevated 
temperatures which are encountered in the processing of sanitary cans. The 
aqueous coatings of this invention exhibit excellent processing resistance 
and it is concluded that processing resistance also requires a high degree 
of film impermeability which is the factor measured by the Staypro rating. 
The styrene-ethyl acrylate-isobutoxymethyl acrylamide-methacrylic acid 
interpolymer described previously and which typifies performance in 
accordance with said U.S. Pat. No. 3,991,216 has a Staypro rating of 3.7. 
In this invention a Staypro rating of at least about 6 is desired, 
preferably at least about 9. 
In accordance with this invention, the monomer mixtures contemplated by 
said U.S. Pat. No. 3,991,216 are modified by the addition of from about 
20% to about 45% of a monoethylenic nitrile selected from acrylonitrile 
and methacrylonitrile. Acrylonitrile is preferred. Comparative data which 
shows the new result is set forth in Table I, the wedge bend rating being 
of significance as previously noted and being included in the table. 
Table I 
__________________________________________________________________________ 
Wedge 
(*=AA) 
Staypro 
Bend 
AN STY 
EA NBMA 
IBMA 
IBMAM 
MAA Rating 
Rating 
__________________________________________________________________________ 
1- 
0 45 25 0 0 20 10 3.7 6.5 
2- 
35 0 0 0 41.5 
15 8.5* 6.1 8.3 
3- 
40 0 35 0 0 20 5.0* 4.8 6.3 
4- 
30 20 0 23 0 20 7.0* 4.1 5.0 
__________________________________________________________________________ 
In the above table, AN=acrylonitrile, STY=styrene, EA=ethyl acrylate, 
NBMA=n-butyl methacrylate, IBMA=isobutyl methacrylate, 
IBMAM=isobutoxymethyl acrylamide, MAA=methacrylic acid, and AA=acrylic 
acid. 
Runs 1 and 2 show a contrast between a typical system in accordance with 
the teachings of U.S. Pat. No. 3,991,216, as previously noted (see run 1) 
and a typical system in accordance with the teachings of this invention 
(run 2). By using styrene and ethyl acrylate, a mixture of hardening and 
flexibilizing monomers as taught in the patent, the Staypro rating is poor 
and the wedge bend rating is marginally satisfactory, these normally being 
antagonistic properties. When we use acrylonitrile which tends to form a 
relatively hard homopolymer with isobutyl methacrylate which is classified 
in the patent as a hardening monomer, we simultaneously improve both the 
Staypro rating and the wedge bend rating. This is surprising. 
While a precise comparison with run 2 using a typical softening monomer has 
not yet been carried out, run 3 provides an approximate comparison, and 
the result is a reduction in both Staypro rating and wedge bend rating. If 
we bring a proportion of styrene into the interpolymer and use n-butyl 
methacrylate instead of isobutyl methacrylate, the results are 
unsatisfactory in both of the critical areas with which the invention is 
concerned. 
Further preferred interpolymers are presented in Table II: 
Table II 
______________________________________ 
AN IBMAM IBMA MAA 
______________________________________ 
1- 35 15 40 10 
2- 35 17.5 37.5 10 
3- 37.5 15 37.5 10 
______________________________________ 
The headings in Table II are explained in the text following Table I. 
These three interpolymers are similar to run 2 of Table I, but proportions 
are balanced and methacrylic acid is selected in order to maximize Staypro 
Rating while maintaining the wedge bend rating at a minimum acceptable 
rating in the range of about 6-7. 
With the data of Tables I and II in mind, this invention uses an 
interpolymer of monomer components as tabulated in Table III: 
Table III 
______________________________________ 
Preferred 
Monoethylenic Propor- Propor- 
Monomer Type 
Illustration tions % tions % 
______________________________________ 
1- nitrile acrylonitrile* 20-45 25-40 
methacrylonitrile 
2- isobutyl 10-50 20-45 
methacrylate 
3- N-methylol amide 
N-methylol acryl- 
10-30 12-25 
amide, acrylamide 
or ether thereof 
such as the propyl 
butyl, or isobutyl* 
ether 
4- carboxy- acrylic acid, 5-15 7-12 
functional methacrylic acid*, 
crotonic acid, 
fumaric acid, 
itaconic acid 
5- other non- ethyl acrylate 0-10 0 
reactive methyl methacrylate, 
styrene 
______________________________________ 
The asterisks in the above table indicate materials which are presently 
preferred. The most preferred interpolymer involves the use of from 30-40% 
acrylonitrile and from 35-45% of isobutyl methacrylate. 
These monomers are formed into an interpolymer in the presence of a vinyl 
polymerization catalyst and in the absence of mercaptan, as described in 
U.S. Pat. No. 3,991,216, polymerization in organic solvent solution being 
preferred. From about 5% to about 60% of the aqueous coating composition 
is constituted by the interpolymer either partially or completely 
neutralized by amine to provide a salt content as noted previously. 
Correspondingly, from about 40% to about 95% of the coating composition is 
provided by a liquid medium consisting of water-soluble or water-miscible 
solvents and water (at least 60% water). 
Appropriate solvents and amines are disclosed in said U.S. Pat. No. 
3,991,216. 
While ethers of N-methylol acrylamide or methacrylamide with lower alcohols 
as in U.S. Pat. No. 3,991,216 are preferred, it is also possible to use 
the same monomers without etherification since the ether groups are 
removed on baking to regenerate the N-methylol group for cure.

A typical interpolymer in accordance with this invention is prepared and 
applied as follows: 
EXAMPLE A 
Into a 3 liter roundbottom flask equipped with reflux condenser, heating 
means, stirrer, thermometer and inert gas blanket are charged 200 g of 
2-butoxyethanol and 350 g of n-butanol. After heating to reflux, a mixture 
of 150 g isobutoxy methylcrylamide, 415 g isobutyl methacrylate, 350 g 
acrylonitrile, 85 g acrylic acid and 25 g t-butylperbenzoate is added over 
3 hours. After the addition was complete, the reaction is held at reflux 
for one hour then an additional 5 g of t-butyl perbenzoate and 100 g 
n-butanol are added and reflux is continued for two hours. A dispersion is 
obtained by adding 40 g dimethylaminoethanol followed by 2130 g water 
under rapid agitation. 
To 2500 g of the resulting dispersion is added 443 g water. Then 680 g of 
distillate is stripped off. The solvent composition is then adjusted by 
adding 100 g isopropanol and 60 g n-butanol. 
The product is applied to aluminum substrate and cured 75 sec. at 
360.degree. F., to provide very good film properties. The Staypro rating 
of 6.1 and the wedge bend rating of 8.3 for this example were reported in 
Table I. This aqueous coating also provides excellent resistance to 
exposure to high temperature water (liquid and vapor) as is encountered in 
the processing of sanitary cans which is normally carried out at a 
temperature in the range of 240.degree.-250.degree. F. For this reason, 
these impenetrable coatings of this invention are of value in the coating 
of the exterior of sanitary cans.