CA1065988A - Water-based liners for beverage containers - Google Patents

Abstract

WATER-BASED LINERS FOR BEVERAGE CONTAINERS

Abstract of the Disclosure Water-based coating compositions for use as internal sanitary liners for metal containers containing beer, carbonated and non- I
carbonated soft drinks, and fruit juices consist essentially of aqueous dispersions of amine-solubilized interpolymers, said interpolymers being formed from substituted carboxylic acid amide units; ethylenically unsaturated acid units; hardening units from monomers such as styrene, vinyl toluene or alkyl methacrylates having from 1 to 4 carbon atoms and flexibilizing units from certain alkyl acrylates or methacrylates.
The interpolymers are solubilized by neutralizing the acid units of the interpolymer with monomeric amines. In order to produce a stable composition, the amount of amine utilized in neutralizing the acid groups of the interpolymer should be sufficient to produce at least 0.200 milliequivalents of salt per gram of resin solids. These compositions provide containers with cured liners which do not impart undesirable turbidity or taste characteristics to the beer, soft drink, or fruit juices packaged therein.

Description

~065'~

This invention relates to a metal container for containlng a beverage and having its internal surface coated with an adherent sanitary liner.
Background of the Invention Beer, carbonated and non-c~rbonated soft drinks, and fruit ~uices (hereinafter referred to generically as beverages) are often packed in _I~ ~

' ~06~

containers made from aluminum, tin-free steel, blackplate or tinplate, which is cold rolled steel to which a thin layer of tin is applied. Many of these beverages exert corrosive action upon the metal and in order to adequately protect the container and to prevent contamination of the packaged material, a sanitary liner must be applied to the internal surface of the container. Ilowever, the use of such liners also presents several problems, one of the most troublesome being the residual turb~dity and taste which tends to result from some liner materials.
Because of their relatively taste-free characteristics, vinyl polymers based on vinyl chloride have been extensi~ely employed in sanitary liners in contact with beverages. Wllile sucl- vinyl polymers have been useful in the past, they possess a serious disadvantage whicll diminishes their usefulness as sanitary liners at the present time. Thus, these vinyl polymers are generally applied from volatile organic solvent solutions at relatively low solids contents and these solvent rich solu-tions either add to llydrocarbon air pollution or require expensive control equipment.
In recent times, the increased emphasis on safety and environ-mental pollution problems have resultcd ln n nccd for water-bused composi-tions for sucll liners. ~y "water-bascd" it is meallt compositions in solvents comprised predominantly of water, tllus greatly reducing tlle handling and emissions oE organic solvent vapors. Ilowever, the types of solvent-based sanitary liners known and used heretofore are not obtainable as satisfactory water-based systems; indeed, it has been found that water-based materials as a class generally provide liners whlch impart undesirable turbidity and taste characteristics to beverages, even when the otl-er necessary properties of such liners can be obtained.
The combination of properties whicll is necessary to successful utiiization of any composition for container liners, and which has not .

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1065~8~

been satisfactorily obtainable in water-based sanitary lining compositions known heretofore, includes the following:

(A) Properties oE the Cured Liner:

(1) Metal ~dhesion - Excellent adhesion to metals, including the aluminum, tin-free steel, blackplate and tinplate employed in beverage containers; this property permits the present CompOsitions to be utilized either as a one-coat liner or as a primary and/or a topcoat in a two-coat system.
(2) Taste Characteristics - Taste characteristics at least as good as the best "tasteless" vinyl polymers applied from solvent solu-tions and utilized extensively in the container industry at the present time,

(3) Turbidity Resistance - Beverages after packing, pasteuri-zation and storage must not develop undesirable turbidity and loss of appearance.

(4) Fabricating Properties - Fabricating properties represent a combination of flexibility, extensibility and adhesion so as to permit forming operations to be carried out on the coated mctal without crack-ing or otherwise lmpairing the coutinuity of the fil.m.

(5) Pasteurization Resistance - l~eer is generally pasteurized at a temperature oE 150F. for 15 to 40 minutes; occasionally during the pasteurization temperatures as high as 160F. to 180F. may be reacbed.

(6) Low rake Properties - The curing or baklng temperature in containers of the class described should not be excessively high because the exterior of some containers may be coated with lithograpllic coatings qnd inks which may discolor and lose their appearance at high temperatures.

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~06598B

In addition, some conLainers employ adhesive~s as bonding agents and such adhesives are adversely affected by higll baking temperatures.

(7) Extractability - No undesirable materials may be extracted from tlle liner during processing and storage.

(8) Intercoat Adllesion - In order to permit use of primer or base coat, if desired, or added coats to repair defects, tlle liner com-position should have good adllesion to itself and other conventionally utili~ed materials.

(B) ~roperties oL tlle Uncured Composition:

(1) Applicatioll Properties - ~pplication by equipment and methods conventionally employed in tlle coatings industry. Thus, tl-e composition sllould be capable of being applied by tnetllods such as dipping, roll coating, spraying and the like.
(2) Storage Stability - Tlle coating composition must be in a pl~ysical foml wllicll permits llandling and storage over varying conditions.
Water-based compositions in emulsion form, for example, usually are not storage-stable unless additives are employed whicll generally are undesirable in liners for contalners used for comestible produc~s.

.
Compo~itions containing amide lnterpolymers have been suggested for use on tlle exLerior and, to some extent, on tlle interior of containers, e.g., see U. S. Yatents Nos. 2,870,117 and 3,117,693 and Canadian Yatent No. 766,103. However, none of tl~ese patentsdescr1bes a water-based compo-sition for use as a sanitary liner.

`" ~0659~8 .
Summar of the Invention Y
The present invention relates to a metal container for containing a beverage and having its internal surface coated with an adherent sanitary liner, the coating which is intended to be in contact with said beverage being a cured layer of an amine-neutralized or partially-neutralized interpolyme~
formed in the presence of a vinyl polymerization catalyst and in the absence of mercaptan, said interpolymer being comprised of:
(1) from about 5 percent to about 25 percent of acryl-amide or methacrylamide in units of the structure:

Il R' - C - C - NH - CH2 - OR
C

where R' is methyl or hydrogen and R is hydrogen or lower alkyl with at least 50 percent of the groups represented by R being lower alkyl;
(2~ from about 3 percent to about 25 percent of units formed by vinyl polymerization of ethylenically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, crotonic acid and maleic acid and half esters of maleic and fumaric acids;
(3) from about 5 percent to about 75 percent of units derived by vinyl polymerization of hardening monomer selected from the group consisting of styrene, vinyl toluene and alkyl methacrylates having 1 to 4 carbon atoms; and (4) from about 5 percent to about 75 percent of units formed by vinyl polymerization of flexibilizing monomer selected from the group consisting of alkyl acrylates having up to 13 carbon atoms in each ~/
~5~

.

~06598~3 alkyl group and alkyl methacrylates having from 5 to 16 carbon atoms in each alkyl group;
wherein said interpolymer is solubilized by neutralizing or partially neutralizing the acid groups of said interpolymer with a sufficient amount of monomeric am me to produce at least 0.200 milliequivalents of salt per gram of resin solids.

-5a-1o65g88 In copending divisional application 318,452 filed December 21, 1978, there is provided a novel water-based coating composition for use as an internal sanitary liner for metal containers adapted for packing beverages, said composition especially adapted for spray application and consisting essentially of:
(A) from about 5 percent to about 60 percent by weight of an amine partially-neutralized interpolymer formed in the presence of a vinyl polymerization catalyst and in the absence of mercaptan, said interpolymer being comprised of:
(1) from about 5 percent to about 25 percent of acrylamide or methacrylamide in units of the structure l 11 R' - C - C - NH - CH2 - OR
C

wherein R' is methyl or hydrogen and R is hydrogen or butyl with at least 50 percent of the groups represented by R being butyl;
(2) from about 3 percent to about 25 percent of units formed from acrylic acid;
(3) from about 5 percent to about 75 percent of hardening monomer units derived from styrene; and (4)~ rom about 5 percent to about 75 percent of flexibilizing monomer units derived from ethyl acrylate;
wherein the acid groups of said interpolymer are partially neutralized with a sufficient amount of monomeric amine to solubilize the interpolymer but with an amount of amine which does not produce more than 0.764 milli-equivalents of salt per gram of ~esin solids; and ~ -6--- ~0655~88 (B) 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 in water wherein at least 60 percent by weight of the mixture is water.
The above compositions meet all the requirements for cured sanitary liners set forth above.

Detailed Descri tiOn of the Invention ~ -p In formulating a coating composition for use as an internal sani-tary liner for metal containers in which beverages are to be stored, it is extremely important that cured films-produced from such coating compositions do not contain certain materials, even in residual amounts, which can be extracted by the beverage from the cured film. Thus, it has been found that certain additives commonly employed in the preparation of prior art coating compositions may remain in residual amounts in cured films produced from such compositions and that even residual amounts of such additives can adversely affect the characteristics of beverages in contact with such films.
For example, residual amounts of such materials as mercaptan chain transfer agents commonly employed in polymerizing interpolymers such as those used and described herein; external surfactants or disperslon st~bili~ers; and external crosslinking agents sucll as amine-aldehyde resins in cured films - 6a-~0659~

employed as sanitary liners for beverages such as beer have been found to exert adverse effects on the turbidity and/or taste characteristics of the beer. Accordingly, in formulatlng the compositions of tllis invention, such materials are avoided.
As indicated above, the term "beverage" as used throughout the specification and claims refers to beer, carbonated and non-carbonated soft drinks, fruit Juices, and ~he like.
Among tlle units in the interpolymers herein are units derived from acrylamide or methacrylamide. These units may be formed Erom substituted amides, suc~l as N-(alkoxyalkyl)acrylamide or meth-acrylamide. N-(alkoxyalkyl)acrylamides or methacrylamides in wllicll the alkoxy group is butoxy (i.e., where R in the formula above is butyl) are especially preferred since interpolymers prepared from such substituted amides exhibit the best balance of stability and efEicient curing. In addition, mixtures of N-(alkoxyalkyl)acrylamlde and N-methylolamide can also be used. 'rhese substituted amides can be prepared by several methods. One preferred metllod is to react an unsaturated amide (e.g., acrylamide) with formaldellydc anù an all~anol (e.~,., butanol) un(ler acidic conditions and in the presence of a polymerization inhibitor. For a detailed description oE this method, see Roger M. Christenson et al's U.S.Patent No. 3,079,434, issued Feb. 26, 1963. The resultsnt N-(alkoxyalkyl)acrylamide [e.g., N-(butoxymethyl)acrylamide] is tllell interpolylllerized with the other monomers (ùescribed below) to form the interpolymer containing the substi-tuted amide units.
As indicated, in the above metllod tlle substituted amide Utlits of the interpolymer herein are formed by first separately preparing Llle substituted amide and then interpolymerizing it with tlle other monomers.
Alternativcly, these substituted amide unlts can be formed in situ, that '~J ' 106591~3~

is, the substituted amide units can be formed subsequent to the formation of the interpolymer. Thus, the unsaturated amide (e.g., acrylamide or metllacrylamide) can first be interpolymerized witl~ the other monomers and tlle resultant product then reacted with the aldellyde (e.g., formal-dehyde) and alkanol (e.g., butanol). For a more detailed discussion of tllls reaction, see Roger M. Christenson's U.S. Patent No. 3,037,963 issued June 5, 1962. Th1s method has the disadvantage of requiring removal of any excess formaldel~yde wl~icll has a troublesome odor, and in tllis method the degree of etherification of tlle methylol groups is more difficult to control; thus, in the above structure, the proportion oE R groups which are alkyl ratller tl-an hydrogen may not be as desired. For good stability at least 50 percent of these groups sllould be alkyl, and usually it is preferred that all or nearly all be al~yl.
As described above, the acrylamide or methacrylamide is in units of polymerized ami~e substituted wlth alkoxymethyl groups, with tlle alkoxymethyl substituent introduced either as part of the amide reactant or by reacting the polymeri~ed amide with formAl~el-yde and alkanol. It has been the practice in the art to describe tlle proportlon of amicle in these interpolymers by referellce to the unsubstituted ami~!e, e.g., acrylamlde or methacrylamide. l`his is because the alkoxymethyl groups may or may not be present in tl;e monomer mixture used to form the interpolymer, and if present may be oE difEerellt m(>lecular weigllt depcnding on the particular group, and also because these groups split off and are substantially lost during the baking of the interpolymer coating. That practice is followed throughout tl-e sl~ecification and claims herein in referring to tbe propor-tion of amide. Thus, for example, an interpolymer described as containing lO percent acrylamide may be formed from a monomer mixture containing lO
yercent acrylamide itself (and the interpolymer later reactcd witll 'Z`
0,~,~ 5, .

- i3 -~(~6S988 formaldehyde and alkanol), or the interpolymer may be formed from a monomer mixture containing N-(alkoxymethyl)acrylamide in an amount which would provide an interpolymer containing 10 percent of the acrylamide moiety if the aikoxymetllyl groups were removed. Where the latter is the case in the examples, it is so indicated by stating the component as "Acrylamide as NBMA".
The interpolymer of the compositions of this invention conta`ins from about 5 percent to about 25 percent of acrylamide or methacrylamide, with the preferred range being from about L0 percent to about 15 percent.
For similar reasons, the levels of salt groups in the partially-neutralized interpolymers, as described in detail below, are calculated herein based on interpolymers containing unsubstituted amide~ i.e., not having alkoxymethyl substituents. The calculated values correspond closely to experimental values based on the non-volatile solids content of the interpolymer as obtained by lleating at 150C. for 2 hours, thus simulating the losses incurred in the baking operation during which alkoxymethyl groups are evolved.
The units of the interpolymer derived from ethylenically unsaturated carboxylic acid are best formed from acrylic acid or metll-acrylic acid, but itaconic acid, crotonic acLd, and maleic acid, and half esters of maleic and fumaric acids may also be used. In the half esters, one of the carboxyl groups is esterified with an alcohol, the identity of which is not significant so long as it does not prevent polymerization or preclude the desired utilization of the product. ~utyl hydrogen maleate and ethyl hydrogen fumarate are examples.
The interpolymer of the compositions of this invention may contain from about 3 to about 25 percent of such acid units. llowever, for the best balance of curing and ease of solubilization, it is preferred that tlle interpolymer contain from 5 to ]5 percent of the acid units. It shall be _ g _ ~0655~8~3 noted that interpolymers of the compOsitiOns in some instances con-tain more acid than is normally required for solubilization purposes.
This is often desirable in order to obtain satisfactory curing of the interpolymer.
The other units in the interpolymer are derived from a combina-tion of hardening and flexibilizing monomers, to provide the desired combination of properties. The hardening monomer is generally styr~ne"
but otl-ers such as vinyl toluene or alkyl methacrylates having from 1 to 4 carbon atoms can also be used.
The interpolymer of the compositions of this invention may contain from about S to about 75 percent of units derived from hardening monomers with a preferred range being from about 40 to about 60 percent of such units.
The flexibilizing monomer component is one or more alkyl or substituted alkyl esters of acrylic acid or methacrylic acid, the alkyl groups having l to 13 carbon atoms in the case of acrylic esters and 5 to 16 carbon atoms in the case of methacrylic esters. Ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate, 2-ethyl.tlexyl methacrylate, decyl methacrylate and lauryl metllacrylate arc examllc~. Uthyl acrylate is especially preferred. The interl)olymer of the compositions of this invention may contain from about 5 to about 75 percent of units derived from flexibilizing monolners with a preferred range being from about 20 to about 50 percent of such units.
The com~?ositions of this invention may contain from about 5 percent to about 60 percent by weight, preferably 15 to 40 percent by weigllt, of tbe interpolymer, the balance being the liquid medium.
The interpolymer is formed by polymerization in the presence of a vinyl polymerization catalyst. Thc preferred catalysts are azo compounds, 1~)6S98~

such as, for example, alpha, alpha'-azobis(isobucyronitrile). Otl-er useful catalysts are tertiary-butyl perben~oate, tertiary-butyl pivalate, isopropyl percarbonate and similar compounds. In some instances, Gther free radical catalysts such as benzoyl peroxide and cumene hydroperoxide may also be useful.
As indicated above, the polymerization is carried out in the absence of a mercaptan. This is an important consideration because it has been tlle practice, as illustrated by the above patents, to employ small amounts (e.g., one percent to 3 percent) of a mercaptan as a chain transfer agent in the preparation of amide interpolymers of this desired type. It ilas been found that the presence of even these small amounts of mercaptan in the interpolymer composition results in compositions which are unsuitable for use in sanitary liners for beer and similar beverages.
When a mercaptan is present, the lining imparts an undesirable taste characteristic to the beverage, especially on storage for a period of 6everal weeks. Since packed beverages are normally packed for some period of time before they are used, this precludes the utilization of such interpolymer compositions ss liners for the~e containers.
The compositions of thls lnvention are composed of dispersions of the above-described interpolymers in a liquid medium. The liquid medlum may consist entirely of water in some cases but, more commonlyj will conslst of a mixture containing a major proportion of water and a minor proportion of water-soluble or water-miscible organic solvents.
Sultable organic solvents are the etl~er type alcohols, such as ethylelle glycol monobutyl ether (butyl Cellosolve~), ethylene glycol mono-ethyl ether ~ethyl Cellosolve~) and the like, and lower alkanol~ having ~ to 4 carbon atoms ~uch as ethanol, lsopropanol, butanol, and the like. ~lnor proportlons o~ hydro-carbon solvent~ such as xylene, toluene, and the like may also De pre~ ln tlle liq~id medium. Mixtures * Trade Mark~

~)6598~

of the ether type alcohols and lower alkanols can also be used. The preferred water-soluble, or water-miscible organic solvents are mixtures of butyl Cellosolve and isopropyl alcohol. The liquid medium portion of the CompositiOnS of this invention may contain from about 60 percent to about 100 percent, preferably 80 percent, by weight of water and from about 0 percent to about 40 percent, preferably 20 percent, by weight of water-soluble or water-miscible organic solvent. In any event, thç
liquid medium will contain at least 60 percent by weight of water. It should be noted that while ordinary tap water can be used in the composition of this invention, deionized or distilled water is preferred. Based on total weight of tl-e composition, the composition may contain from ahout 40 percent to a~out ~5 percent, preferably 60 to 85 percent, by weight of the liquid medium.
The interpolymer herein is rendered water-reducible or disper-sible by the addition of a monomeric amine solubilizing agent. As is known in the art, acid group-containing interpolymers such as those employed in tlle present compositions can be rendered water-soluble or water-reducible by neutralizing or partially neutralizing the acid groups therein with amines. In general, the monomerLc amines employed herein for tllat purpose may be any of the allllnes used for solubilizing resin systems known heretofore, including an~nonia, ethyl amine, butylamine, dimethylamine, cycloliexylamine, morpholine, monocthanolallline, diethanol-amine, diethylethanolamine, and the like. The preferred amines herein are monoethanolamine and dimethyletllanolamine with dimethylethanolamine being particularly preferred since it has been found to provide the best results. While, as indicated above, amines may in general be used, there are certain considerations which should be taken into account in choosing the amine used to neutralize any particular system; for example, some amines are morc effective than others in providing stable compositions, depending upon the degree of neutralization of the interpolymer.

, ~ - 12 -~0655~8~

The term "degree of neutralization" refers to the amount of acid in the interpolymer (as determined theoretically) which has been neutralize~ with amine. Thus, for example, if 20 percent of the theoretical amount of acid in the interpolymer has been neutralized with amine, the degree of neutralization is 20 percent theoretical neutrali-- -zation; if 30 percent of the theoretical amount of acid has been neutralized, the degree of neutralization is 30 percent theoretical neutralization, and so forth. In all cases, the degree of neutralization and the amount of amine is sufficient to provide a stable composition, which is defined as one which does not separate into distinct phases, and retains its application properties on storage. When it is desired that this be accomplished at a relatively low degree of neutralization, i.e., below about 30 percent, tlle preferred amines, dimethylethanolamine and monoethanolamine, should be used. At higher degrees of neutralization (i.e., 30 percent or above), other amines are effective. Thus, for example, at 30 percent theoretical neutralization, diethylethanolamine produces good results and at say 50 percent theoretical neutralization, essentially all of the above-listed amines are suitable.
The amount of amine employed in producing the water-based coating compositions of tllis invelltioll is an impot~allt asl)ect of the invention.
This amount is partlcularly sigllificant at lower degrees of neutralization of the interpolymer if a stable composition is to be obtained.
The amount of amine employed in producing the water-based coating composition of tllis invention may in some cases be as low as an amount sufficient to produce 0.180 milliequivalent of salt per gram of resins solids, but ordinarily will be an amount sufficient to produce at least 0.200 milliequivalent of salt per gram of resin solids. The term "milli equivalents of salt Rer gram of resin solids" refers to the number of acid milliequivalents per gram of resin solids of the interpol~ner which ' . , . . , .. . _ . _ _ _ ~O~;S~8 have been neutralized with a base (i.e., an amine). The number of milliequivalents of salt per gram of resin solids can be determined by calculation using the following formula:

WeiglIt fraction of acid x 10 N ~lilliequivalents of salt per Equivalent weight of acid gram of resin solids wherein N is the degree of neutralization. A degree of neutrali~ation greater than 100 percent indicates that excess base has been used. In this latter situation, tlle value of 100 percent is used in the above calculation. As indicated above, the compositions of this invention will ordinarily contain at least 0.200 milliequivalents of salt per gram of resin solids.
The upper limit in the amount of amine employed is somewhat less important and is governed to a large degree by the method of application.
Thus, for example, if the composition is to be applied by brushing, roll coating or dipping, the amine may be used in an amount which will produce as much as 3.47 milliequivalents of salt per gram of resin solids. IIowever, preferably, even in the above metllods the amount of amine employed does not exceed an amount which will produce 1.910 milliequivalents of salt per gram of resin solids. Moreover, when the composition is to be applied by spraying, the amount of amine employed ordinarily should not produce more than 0.764 milliequivalents of salt per gram of resln solids although in some instances an amount of amine whicll will produce ~Ip to about 1.40 milliequiva-lents of salt per gram of resin solids may be used.
The preferred method for producing water-based coating composi-tlons of this invention comprises the steps of (a) polymeri~ing the substituted amide, ethylenically-unsaturated acid, hardening and flexibil-i~inb monomers in an escess of a mixture of water-soluble or water-miscible organic solvents, the solvent mixture consisting essentially of an ether-type alcohol selected from the group consisting of ethyl Cellosolve and butyI Cellosolve and a lower alkanol selected from the group consisting of . : .

.

_ _ .. ., . .. .. .... . . _ _ ~ . .. .. . _ -~O~S5~8~

ethanol, propanol, isopropanol and butanol, wherein the excess solvent in the mixture is essentially the lower alkanol, in the presence of a polymerization catalyst to form the interpolymer in solution; (b) strip-! ping off the e~cess solvent consisting essentially of lower alkanol by distillation at atmospherlc pressure, under vacuum, or both; (d) adding monomeric amine to tlre interpolv.ner solution; (d) adding water to the .
interpol~mer solution under conditions of agitation to form a dispersion of the interpolvmer; and (e) heat aging the dispersion at temperatures of from about 70C. to about 90C., while maintaining conditions of agitation until the viscosity of the dispersion is substantially constant.
Tlle term "excess" as employed with reference to the organicsolvent mixture in step (a) above means that the total amount of organic solvent mixture employed in the polvmerization process exceeds the total amount of organic solvent desired in the final composition. The period of heat aging, i.e step (e), depends upon several factors, such as the composition of the interpolymer, the solids content of the composition, the initial viscosity of the interpolymer formed, the degree of agitation and the like. In general, the heat aging period can range from one hour or less up to about 11 hours.
In a partlculaLLy preferred emL~odL~nent oE the above method, the above-described monomers are polymeri%ed in an excess of a solvent mixture consisting essentially of butyl Cellosolve and isopropanol, wllerein the excess solvent of the mixture is essentially isopropanol, in tlle presence of an azo polymerization catalyst to form the interpolymer in solution.
Then, the excess solvent consisting essentially of isopropanol is stripped off by distillation at atmospheric pressure or under vacuum, or both, following which the monomeric amine is added to the interpolymer solution.
Then deionized vater at a temperature ranging from 50C. to 99C. (i.e., , , 1()65~88 hot water) is added to the solution to form a dispersion of the inter-polvmer and finally the interpolymer dispersion is heat aged as in step (e) above.
The preferred embodiment of preparing the water-based coating compositions of this invention provides a number of significant and important advantages. First, the use of butyl Cellosolve and isopropanol as tlle solvent mixture provides a significant advantage because of tile wide difference in the boiling points of these materials (i.e., butyl Cellosolve = 171.2C., isopropanol ~ 8Z.4C.). This difference is boiling points permits the ready removal (e.g., by distillation) of isopropanol from the mixture, Accordingly, where as here, the excess solvent (as defined above) employed in tlle polymerization procedure is essentially isopropanol, such excess is readily removable and the total solvent content of tlle final composition is more easily controlled. Secondly, the iso-propanol in the solvent mixture functions as a chain-transfer and molecular weight control agent in this system. I~ence, in the preferred embodiment, isopropanol effectivel~ controls the molecular weight of the interpolymer without the necessity of using an external or adcled cllain transfer agent, such as the mercaptall chain transfer agellt~s normal?y cmpLoyed in polymer-izing lntcrpolylllers oL thls type. l~lnlllly, the adtllt:loll of hot water to the lnterpolylller solutioll is advantageous in that lt aids in the formation of the interpolvmt!r disperslon and permits thc morc rapid attaimnent of coating composition viscosity.
As indicated, the above-described methods are dirccted to the preparation of the preferred water-based coating compositions in which a portion of the or~anic solvent originally employed in the polymerization of the interpolymer remains in the final resin composition. Ilowever, as indi-cated previouslv in the specification, water-based coating compositions in whicll tl-e liquid medium is entirely water are also contem~)lated as being , witllin the scope of this invention. Such compositions may be obtained by following the above-described metllods and then employing an additional separation procedure following step (e). Thus, for example, the organic solvent remaining in the composition after step (e) can be removed bv various separation procedures such as distillation, liquid cllromatography and the like.
The water-based coating compositions of this invention can b'e applied by methods conventionally employed in the coatings industry, such as brushing, dipping, roll coating, spraying, and the Like and they are particularly adapted to be applied by the metllods used to coat containers. -In this regard, it should be observed that the exact formulation of the coating composition employed herein (e.g., total solids, viscosity, etc.) will depend upon the desired manner in which the composition is to be applied. Thus, within the compositional limitations set forth in the specification above, compositions having suitable application properties (e.g., solids contents, viscosity, etc.~ can be readily selected for the the method of applicatlon desired. Such a determLnation is well within the skill of those in the coating art.
After applicatlon, the compositioll~ are ordLn~rily dried and cured by baklng at elevated temperatures to yroduce a hard, thermoset film. 'l'he baking schedules depend upon the nature of the particular composition, the nature of the substrate, and the manner in which it is to be used. In general, the water-based coating compositions of this invention can be cured by using typical baking schedules employed in the container industry. Typical baking schedules for such container coatings employ temperatures ranging from about 300~F. to about 420F. and times ranging from about 2 minutes to about 15 minutes.
The following examples are submitted to further illustrate the nature oE the present invention and are not intended as a limitation on ... . , . ~

the scope thereof. All parts and percentages used in the examples and throughout the specification, are by weight unless otherwise indicated.
The solids contents reported in the examples were run at 105C. as a quality control test.

, This example illustrates the preparation of a water-based coating composition of this invention.
Into a reactor equipped with reElux condenser, heating means, stirrer, thermometer and nitrogen feed liner were charged 159.0 grams of ethyl Cellosolve, 325.0 grams of isopropanol, and 290 grams of a monomer mixture consisting of 5.9 percent glacial acrylic acidj 28.2 percent of a 61.5 percent solids solution of N-(butox~methyl)acrylamide (NBMA) in a 1:3 solvent mixture of toluene and butanol, 32.I percent of styrene, 32.5 percent of ethyl acrylate and 1.3 percent alpha, alpha'-azobis(isobutyro-nitrile) catalyst (based on monomer solids, the monomer charge contains 10 percent acrylamide as N~MA, 41 percent styrene, 41.5 percent ethyl acrylate and 7.5 percent acrylic acid). The charged mixture was then heated under nitrogen to reflux temperature (about 90C-93C.) in a period of about 45 minutes. Afte.r reflux llad hesill, nn ad~itional ln81 grams of the above mOIlomer m:Lxture were ad~e(l to the reactor over a perlod of about 3 hours. Thell 12.0 grams of t-butyl perbenzoate were added in 3 equal increments (i.e., 4 grams eacll) over a period of about 6 hours with each increment being added at two-hour intervals. At the end of the six-llour period, 337 grams of solvent (slightly more than the ori~inal amount of isopropanol used) were stripped from the resultant interpolymer solu-tion by distillation at atmospheric pressure at a temperature of about 94C. over a period of about 75 minutes. At this point, a sample of the reaction mixture was analyzed for total solids content and found to have a solids content measured at 150C. of 73 percent by welght. Then 49.8 grams (50 percent theoretical neutralization) of dimethyl ethanolamine , ~06S~88 were added to the reaction mixture (temperature of the mixture about 97C.). After addition of the dimethyl ethanolamine was completed, 1939 grams of deionized water preheated to 75C. was added to the reaction mixture with stirring and the stirring was continued for about one hour after the addition of deionlzed water was completed.
The water-based coating composition resultlng from the above procedure is a dispersion of interpolymer in a liquid medium, the compo-sition has the following formulation:

Interpolymer Units Interpolvmer (Percent) Acrylamide as N~ 10.0 Styrene 41.0 ¢thyl acrylate 41.5 Glacial acrylic acid 7.5 Liquid Medium Percent by Weight Deionized water 83.8 Organic solvents** 16.2 * - N-(butoxymethyl)acrylamide ** - A mixture consisting of ethyl Cellosolve, isopropanol, toluene and butanol.

The composition ha8 thc foLlowing properties:

Polymer solids: 33.4 percent by weight of total composition Liquid Medium: 65.6 percent by weigllt of total composition Viscosity: 140 centipoises, Brookfield viscosity at 77F., Spindle #4 at 20 r.p.m.
Acid No. 16.1 Milliequivalents of salt per gram of resin solids: 0.521 Stability: Good 106S9~

The composition was drawn down on a metal substrate and baked for two minutes at 375F., producing a smooth, hard and durable film.

EXAMPI,E 2 In this example, Example 1 was repeated except that the amount of dimethyl ethanolamine used to neutralize the interpolymer was reduced by 40 percent (to 30 percent theoretical neutralization) and the amount of isopropanol increased by 10 percent to adjust the viscosity. The finis~led composition had the following properties:
Polymer solids: 35 percent by weight of total compositlon Liquid medium*: 65 percent by weight of total composition Viscosity 100 centipoises, Brookfield viscosity at 77F., #4 spindle at 20 r.p.m.
Acid No.: 17.43 Milliequivalents of salt per gram of resin solids: 0.314 Stability: Good * - 85 percellt bv welgllt of delonized water and 15 percent by weight of organlc solvents.

The composition when drawn down on a metal substrate and baked for two minutes at 375F. produced a smooth, hard and durable film.

~0~:;59~

~ ~IPLE 3 This example illustrates tlle preparation oE a preferred water-based coating ccmposition of the invention in whicll Lhe ether-type alcohol employed was butyl Cellosolve.
Into a reactor having the same equipment as in Example 1 were cllarged 477 grams butyl Cellosolve, 975 ~rams of isopropallol and 870 grams of the monomer mixture of ~xample 1 with ~he alpha, alpha'-azobis(isobutyro-nitrile) catalyst removed. The chargcd Tnix~ure was tllen heated under nitrogen to reflux employing the same conditions as in Example 1. After the onset of reflux, an additional 3189 grams of the monomer mixture of Example 1 and -54 grams of alpha, alplla'-azobis(isobutyronitril.e) catalyst were added to the.
reactor over a three-llour period as in Example 1. Following this addition, 12.0 grams of.t-butyl perbenzoate were added to the reactor using tl~e addition procedure of Example 1. Then 981 grams of organic solvent were removed by distillation at atmospherlc yressure at temperattlres ranging from about 95C.
to about 120C. Subsequently, 89.4 grams (30 percent theoretical neutraliza-tion) of dimethyL ethanolamlne were added to tl~e reaction mixture. After this addiLioll~ 5085 grams of deionized water preheated to 75C. were added to the reaction mi.xture with stirring as in Example 1.
The resultant water-based coating composition has tl~e following formulation:

nterpolymerYercent by welgl~t to~al solids ~ 38.5 Interpolymer Units (E~ercent) Acrylalllide as NB~I~ 10.0 Styrene 41.0 Ethyl acry.l.ate 4L.5 Glacial acrylic acid7.5 . .

1065S~88 Liquid ~ledium Percent by weight total liquid = 61.5 Percent by wei~
Deionizecl water 83.5 Organic solvents* 16.5 *Consisting of butyl Cellosolve, isopropanol, toluene and - butanol.

' The composition had the following properti,es:
Total solicls: ' 38.5 percent Acid number: 17.7 Visc06ity: 300 centipoises, l3rookEielcl viscosity at 77F., No. 4 Spindle at 20 r.p.m.
~lilliequivalents of salt per gram of resin solicls: 0.313 Stability: Good The composition when drawn down on a metal substrate and cured as in the preceec!ing examples produced a smooth, har(l and durable film.

- EX~'LE 4 In this example a water-~ased eoating omposltlatl was prepared ln accordanee ~ h the manner used in Example 3, except tllat additional butyl Cellosolve wns empLoyecl to increase the orgnnic solvent content of tlle composition. The COIllpOsitioll had the following Eormulation:
Total Weigllt Interpolymer as in Example 3 37.$X
Liquld medium 62.5X
Percent by Weigllt Deionized water 79.5 Organic Solvents 20.5 .. . .

1(~6S~88 The composition had the following ?roperties:

Total solids: 37.5 percent Viscosity: 16.5 seconds, #4 Ford cup Acid No. 18.6 Milliequivalents of salt per gram of resin solids: 0.313 Stability Good The CompositiOn when applied to a metal substrate and cured as in Examples 1-3 produced a smooth, hard and durable rilm.

EX~MPLES 5-8 These examp]es illustrate tile crltical efrcct of the degree of neutralization and thus the amount of salt procluced per gram of resin solids on the stability of the water-based coating compositions of this invention.
In this evaluation, compositions were prepared in substantially the same manner and using the same interpolymer as in Examples 3-4 except that the amount of dimethyl ethanolamine employed was such that the degree of neutralizatioll of the interpolymer was 30 percent of theoretical neutralization (hereafter TN) in Example 5, 25 pereent TN in Ex;llllplc 6, 20 percent TN in ~xample 7 an~ 15 percellt 'I'N in i~xample 8. ~8lllce tha lower dagrees of neutrali-zation resulted in higll composition viscosity (i.e., Examples 6-8), tlle solids content o~ these compositions were recluced by aclding adclit:ional solvent in the form of an 80/20 mixture of deionized water anci butyl Cellosolve. The resul-LanL co~pDtitions ha~l the follouing Lormal~Lions and properties:

:' .' ' - , .

~0659-88 E X A M P L E
5 _ 6 7 8 Interpolymer ~ (As in ~xample 4, above)~
Percent Lheoretical neutralization 30 25 20 15 Total s()lids (weight percent) 36.4 33.1 18.0 25 Total liquid medium (weight percent) 63.6 66.9 82.0 75 Viscosi~y, #4 Ford cup, seconds at 77~ 26 31 25 __ Acid number 18.G 19.4 12.9 12.1 Millie(luivalents of salt per gram resin solids 0.312 0.261 0.20~ 0.156 Stability Good Cood ~air-Slight Bad - two settling phase separation As can be seen from these examples, the stability of the composition depends upon the degree of neutralization and thus the amount of salt per gram oE resin solids produced.
lhe above examples were repeated Usillg monoethanolamine in place of dimethyl ethanolamille and showed similar stabi]ity results.

` ~XA~II1.1~ 9 This example illustrates a water-based coating composition of this invelltion having Low temperature bal~ing propertieY.
ln thiY example, the composition was prepared in substantialLy the same manner as in ~xamples 1-8 except that tl~e monomer charge consisted of 10 percent acrylamide as N~IA, 10 percent acrylic acid, 30 percent etllyl acrylate alld 50 percellt styrene; the stripping procedure was carried out under vacuum, and the degree of neutralization of the acid groups of tl-e interpolymer was 27 percent TN. Tl~e resultant composition had tlle following formulation and proPerties:

~065~38 Interpolymer (composition as above): 35 percent by weight Liquid medium*: 65 percent by weight Viscosity: 340 centipoises, Brookfield viscosity at 77F., #4 spindle at 20 r.p.m.
Acid number: 25.3 Milliequivalents of salt per gram of resin solids: 0.375 Stability: Good * - 82.3 percent by weight of deionized water and 17.7 percent by weight of organic solvents consistlng of butyl Cellosolve, isopropanol, butanol and xylene.

The above composition when applied to a metal substrate can be cured in 4 minutes at 320F. to produce a smooth, hard and durable film.
As indicated above, the compositions of this invention are especi-al;y adapted for use as internal sanitary liners for beverage containers.
- As indicated at the beginning of the specification, the sanitary liner of such beverage containers are subjected to a number of tests.
These tests are performed on the liner both prior to and after the container is backed with beverages te.g., beer). When such a container is packaged with beer and sealed, the beer ls sub~ected to pasteurization and other normal processing operations. After the containers have been subjected to storage, they are opened and both the beer and the liner are evaluated. The following are some of the more important tests which have been developed to measure the sanitary liner compositions for beverage containers:
(a) metal adhesion;
(b) turbidity;
(c) extractability;
(d) taste; and (e) pasteurization resistance.

.
.. . . .

~06~ 8~

In the following examples, designated 10-13, samples of the water-based coating compositions of Examples 1, 2 ? 4 and 9 (Examples 10-13 below) were applied to beverage container stock used to package beer and cured as indicated above. The cured sanitary liner compositions of the invention were then evaluated to determine their effectiveness in the above tests.
Results are shown below.

E X A M P L E

Formulation Ex. 1 Ex. 2 Ex. 4 Ex. 9 Type Test and Result:
Metal adhesion P P P P
Turbidity P P P P
Extractability P P P P
Taste P P P P
Pasteurization resistance P P P P

P = Passed.

As can be seen from the above, the compositions of this invention passed all of the important testæ for sanitary liners.
It should be observed that a composition similar in formulation to Example 1, except that it contained 2.5 percent mercaptoethanol chain transfer agent, falled to pass the taste test.
According to the provisions of the Patent Statutes, there are des-cribed above the invention and what are now considered to be its best embodiments. However, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically descrlbed .
,

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A metal container for containing a beverage and having its internal surface coated with an adherent sanitary liner, the coating which is intended to be in contact with said beverage being a cured layer of an amine-neutralized or partially-neutralized interpolymer formed in the presence of a vinyl polymerization catalyst and in the absence of mercaptan, said inter-polymer being comprised of:
(1) from about 5 percent to about 25 percent of acryl-amide or methacrylamide in units of the structure:

where R' is methyl or hydrogen and R is hydrogen or lower alkyl with at least 50 percent of the groups represented by R being lower alkyl;
(2) from about 3 percent to about 25 percent of units formed by vinyl polymerization of ethylenically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, crotonic acid and maleic acid and half esters of maleic and fumaric acids;
(3) from about 5 percent to about 75 percent of units derived by vinyl polymerization of hardening monomer selected from the group consisting of styrene, vinyl toluene and alkyl methacrylates having 1 to 4 carbon atoms; and (4) from about 5 percent to about 75 percent of units formed by vinyl polymerization of flexibilizing monomer selected from the group consisting of alkyl acrylates having up to 13 carbon atoms in each alkyl group and alkyl methacrylates having from 5 to 16 carbon atoms in each alkyl group;
wherein said interpolymer is solubilized by neutralizing or partially neutralizing the acid groups of said interpolymer with a sufficient amount of monomeric amine to produce at least 0.200 milliequivalents of salt per gram of resin solids.

2. A metal container as claimed in Claim 1 in which the lower alkyl groups represented by R are butyl.

3. A metal container as claimed in Claim 1 in which said ethylenically-unsaturated acid is acrylic acid.

4. A metal container as claimed in Claim 1 in which said hardening monomer is styrene.

5. A metal container as claimed in Claim 1 in which said flexibilizing monomer is ethyl acrylate.

6. A metal container as claimed in Claim 1 wherein the monomeric amine employed in neutralizing the acid units of the interpolymer is dimethyl-ethanolamine or monoethanolamine.

7. A metal container as claimed in Claim 1 wherein the interpolymer forming the coating is comprised of:
(1) from about 5 percent to about 25 percent of acrylamide or methacrylamide in units of the structure wherein R' is methyl or hydrogen and R is hydrogen or butyl with at least 50 percent of the groups represented by R being butyl;

(2) from about 3 percent to about 25 percent of units formed from acrylic acid;
(3) from about 5 percent to about 75 percent of hardening monomer units derived from styrene; and (4) from about 5 percent to about 75 percent of flexibilizing monomer units derived from ethyl acrylate;
wherein the acid groups of said interpolymer are partially neutralized with a sufficient amount of monomeric amine to solubilize the interpolymer but with an amount of amine which does not produce more than 0.764 milliequivalents of salt per gram of resin solids.

8. A metal container as claimed in Claim 7 wherein the monomeric amine employed in neutralizing the acid units of the interpolymer is dimethyl-ethanolamine or monoethanolamine.

9. A metal container as claimed in Claim 7 wherein the interpolymer comprises from about 10 percent to about 15 percent of acrylamide or methacrylamide in the form of units of said structure, from about 5 percent to about 15 percent of said acid units, from about 40 percent to about 60 percent of said hardening monomer units and from about 20 percent to about 50 percent of said flexibilizing monomer units.