Manufacture of Draw-Redraw Cans Using Film Laminated or Extrusion Coated Steel Sheet Material

In one aspect, the present invention relates to the manufacture of metallic food and beverage cans and more particularly to the manufacture of 2-piece cans via a draw-redraw metal forming process. In another aspect, the present invention relates to the use of an extrusion coated or film laminated plastic/metal/plastic laminate as the can body feed stock in a draw-redraw steel can forming process.

Metallic cans of various types and sizes find widespread commercial use in the packaging of a wide variety of foods and beverages. In such food and beverage packaging usage, it is generally desired to avoid direct contact between the food or beverage to be packaged and the metallic substance from which the container is manufactured. To that end, metallic cans for food and beverage packaging are typically coated on at least their interior surfaces with a coating of a relatively inert organic substance.

Historically, such organic can coatings were typically deposited or applied from relatively low solids organic solvent-based solutions. However, in more recent times, environmental concerns and regulations requiring substantial reductions in airborne emissions from various industrial facilities have prompted a need for can coatings and can coating processes involving substantially less organic solvent usage (and thus substantially less potential for undesired airborne organic solvent emissions) than is normally encountered with the use of said low solids content organic solvent-based solutions.

In response to the aforementioned environmental concerns and the attendant need for reduced solvent emission, certain waterborne can coating compositions and high solids organic solvent-based can coating compositions have been developed. Unfortunately, while such waterborne and high solids organic systems appear to be potentially suitable for use in connection with certain types of can manufacturing operations (e.g., as coatings in the manufacture of 3-piece rolled and seamed cans and in coating the exterior and/or interior surface of pre-formed aluminum and/or steel cans made by the draw and iron can forming process), they are not generally satisfactory for use in connection with draw-redraw can forming processes in which the coating employed on the pre-coated steel can stock is required to withstand the rather severe mechanical stresses involved in the can body forming operation. Moreover, even the organic solvent or plasticizer-based polyvinyl chloride, epoxy and epoxy phenolic systems conventionally employed to prepare can sheet or can coil stock for use in draw-redraw can forming operations are somewhat limited in terms of the maximum depth to diameter draw ratio (or severity of draw) which can be tolerated without coating failure.

In view of the foregoing, it is an object of the present invention to provide an improvement in the manufacture of steel food or beverage cans by the draw-redraw process whereby the use of organic solvent-based coatings for the steel feed stock can be eliminated. In addition, it is also an object of this invention to provide a means by which retortable draw-redraw containers can be manufactured from film laminated or extrusion coated steel sheet material while still maintaining equal or greater depth to diameter of draw ratio capability relative to that currently possible using the conventional organic solvent or plasticizer based coating technology.

These and other objects are achieved in accordance with the present invention (A) by the use, as the coated steel feedstock in a draw-redraw process for the manufacture of steel food or beverage can bodies, of a laminated or extrusion coated steel sheet having adhered to both major planar surfaces thereof a multilayered synthetic thermoplastic resin coating comprising (1) an adhesive synthetic thermoplastic resin inner layer which comprises at least a major proportion of a normally solid random copolymer of ethylene with from about 1 to about 30 weight percent, based upon such copolymer, of an ethylenically unsaturated carboxylic acid comonomer and which is bonded directly to the surface of said steel sheet and (2) an outer layer of a synthetic thermoplastic polymer resin which has a crystalline melting point of about 250.degree. F. or more and which is adhered to said adhesive resin inner layer and (B) by post baking the coated steel can bodies formed in said draw-redraw process at a temperature above the crystalline melting point of the highest melting point resin employed in said thermoplastic resin coating for a period of time sufficient to relieve the stresses created in such coating during the formation of said can body.

Such invention is particularly beneficial in that it permits the preparation of retortable plastic coated draw-redraw beverage and food cans without the use of organic solvent or plasticizer based coatings systems. Moreover, such invention is especially beneficial and surprising in that it facilitates the preparation of retortable draw-redraw cans having equal or greater maximum draw ratio capabilities (i.e., capable of surviving equal or greater depth to diameter ratio draws) relative to that which is normally possible with coated steel can sheet stock prepared using conventional organic solvent or plasticizer based canstock coating systems. Further, the laminates employed can sometimes obviate the use of external lubricants in the can forming process employed.

FIG. 1 is a fragmentary sectional view of a coated steel sheet suitable for use in the practice of the present invention.

FIG. 2 is a sectional view of a schematic representation of a drawn-redrawn can body made in accordance with the present invention.

As has been indicated, and as is illustrated in FIGS. 1 and 2, the coated steel sheet employed in the practice of the present invention comprises a steel sheet (1) having adhered to both major planar surfaces thereof a multilayered synthetic thermoplastic resin coating (2) which in turn comprises an adhesive synthetic thermoplastic resin inner layer (3) bonded directly to the surface of said steel sheet and an outer layer of a synthetic thermoplastic polymer resin (4) which has a crystalline melting point of at least about 250.degree. F. and which is adhered to said adhesive resin inner layer.

Steel sheet materials suitably employed in the practice of the present invention include any steel sheet materials which exhibit good adhesion to the adhesive synthetic thermoplastic inner layer which is directly bonded thereto. Examples of such steel sheet materials include those types of chemically or electrochemically coated (e.g., electrolytically plated) steel sheetstocks already known in the art to be useful in the manufacture of food and beverage containers. Preferably, the steel sheet material employed herein is a non-ferrous metal coated steel sheet such as chromium/chromium oxide coated steel (also commonly referred to in the art as chrome/chrome oxide coated steel, tin-free steel and as electrolytically chrome coated steel or "ECCS") which bears a composite coating of chrome and chrome oxide on both major planar surfaces of said steel sheet and various species or versions of which are described in U.S. Pat. Nos. 3,475,295; 3,479,162; 3,526,486; 3,532,608; 3,567,599; 3,755,091; 3,816,082; 3,827,866; 3,828,628; 3,838,024 and Canadian Pat. No. 808,630, the pertinent teachings of which are hereby incorporated by reference.

The thickness of the steel sheet employed in the practice of the present invention corresponds to that employed in conventional draw-redraw can manufacturing operations and is typically in the range of from about 5 to about 15 mils. Preferably, such thickness is in the range of from about 7 to about 10 mils.

The adhesive synthetic thermoplastic resin employed to form the innermost layer of the above-noted multilayered synthetic thermoplastic resin coating (i.e., the layer directly bonded to the surface of the steel sheet material employed) comprises at least a major proportion of a normally solid random copolymer of ethylene with from about 1 to about 30 weight percent, based upon such copolymer, of an ethylenically unsaturated carboxylic acid monomer. Preferably, such adhesive synthetic thermoplastic resin exhibits an initial 90.degree. peel strength at ambient temperature (i.e., 25.degree. C.) of at least about 15 pounds per inch (preferably at least about 20 and most preferably at least about 25 pounds per inch) when measured by ASTM D 1876 in the form of a steel/adhesive layer/steel sandwich construction in which each steel layer has a thickness of 6 mils.

Examples of resins suitably employed as the major component of the aforementioned adhesive resin inner layers in the practice of the present invention include normally solid random copolymers of a major proportion of ethylene with from about 1 to about 30 (preferably from about 2 to about 20) weight percent, based upon such copolymer of an ethylenically unsaturated carboxylic acid monomer such as, for example, ethylenically unsaturated mono- or polybasic carboxylic acids per se (e.g., acrylic acid, methacrylic acid, crotonic acid, fumaric acid, maleic acid, itaconic acid, etc.); partial esters of polybasic ethylenically unsaturated acids (e.g., monomethyl maleate, monoethyl maleate, monomethyl fumarate, monoethyl fumarate, tripropylene glycol monomethyl ether acid maleate, ethylene glycol monophenol ether acid maleate, etc.); polybasic ethylenically unsaturated carboxylic acid anhydrides (e.g., maleic anhydride, etc.) and the like. In addition, partially neutralized versions of the aforementioned carboxylic acid containing polymers, generally referred to in the art as ionomers, can also be suitably employed. Further, the foregoing adhesive resins can consist essentially of ethylene and the aforementioned ethylenically unsaturated carboxylic acid monomers or can further contain a minor amount (e.g., up to about 20 weight percent) of other types of monomers copolymerizable therewith such as, for example, esters of acrylic acid and the like.

Preferably, the adhesive resin employed as at least the major component in the aforementioned adhesive resin inner layers is a normally solid random copolymer which comprises a major portion of ethylene copolymerized with from 1 to about 30 (especially from 2 to about 20) percent by weight of acrylic or methacrylic acid and which resin may optionally further comprise a minor proportion (e.g., up to about 20 percent) of other copolymerizable monomers such as, for example, lower alkanol esters of acrylic or methacrylic acid. The thickness of each of the individual adhesive resin inner layers on the plastic coated steel sheets used in the practice of the present invention is typically in the range of from about 0.1 to about 1.4 mils (preferably from about 0.2 to about 0.8 mils).

As has been noted, each of the outer layers in the aforementioned multilayered thermoplastic resin coatings is composed of a synthetic thermoplastic resin which has a crystalline melting point of about 250.degree. F. or more. Preferably, the crystalline melting point of such synthetic thermoplastic resin is greater than 250.degree. F. This feature is important in order that the layer composed of such resin does not become a flowable melt under the conditions employed in the food processing operation but instead maintains its integrity as a protective coating layer during such operation. In addition, it is also important that the synthetic thermoplastic resin employed in such layer be relatively gouge resistant (at least at ambient temperatures and both before and after exposure to steam and/or grease at 250.degree. F.) as well as being relatively inert or impervious to water, grease and the various foodstuffs and/or beverages to be ultimately packaged in the subject draw-redraw cans both at the elevated temperatures typically employed during the post-canning processing operations (e.g., in retorting operations) and at the ambient temperatures encountered during storage pending final consumption of such food or beverage contents. Further, it is also important that the thermoplastic resin employed in such outer resin layer possess (a) sufficient strength and toughness to withstand the rather severe stresses imparted thereto during formation of the draw-redraw can body from the instant plastic/metal/plastic laminates as well as (b) sufficient adhesiveness to the above-described adhesive resin inner layer to prevent delamination therefrom during such can forming operation and during the subsequent food or beverage canning and/or processing operations.

Examples of synthetic thermoplastic resins suitable for use in the aforementioned .gtoreq.250.degree. F. melting point outer layers include high density polyethylene (e.g., having a density in the range of from 0.950) to about 0.965 and a melt index in the range of from 0.05 to about 5) polypropylene, polyamides, polyesters, ethylene/propylene copolymers and the like. Further, blends of these <250.degree. F. melting point resins with resins of lower melting points (e.g., low density polyethylene, ethylene/acrylate copolymers, ethylene/vinyl acetate copolymers, random ethylene/acrylic or methacrylic acid copolymers, ionomers, chlorinated polyethylene, etc.) can also be employed so long as the effective melting point of the resulting blend remains at above about 250.degree. F. Indeed, in those instances where high density polyethylene resin is employed as the predominant component of the outer resin layer, it has been found to be especially advantageous to employ a blend of such high density polyethylene with up to about 30 weight percent, based upon the blend weight, of a low density polyethylene therein. Resins which are particularly preferred for use as the outermost coatings layers include high density polyethylene having densities of from 0.960 to about 0.965 g/cc and melt indexes of from about 0.05 to about 3, blends of such high density polyethylenes with up to about 30 weight percent of low density polyethylene, and polyamide resins such as nylon 6, nylon 6/9 and nylon 6/12.

The thickness of each of the indicated .gtoreq.250.degree. F. softening point outer resin layers is typically from about 0.1 to about 1.4 mils (preferably from about 0.2 to about 0.8 mils).

As has been noted, one of the primary objects of the present invention is to provide a means of avoiding the use of organic solvent-based coatings in the manufacture of draw-redraw food and beverage cans. Accordingly, the hereinbefore described multilayered plastic coated steel container stock employed in the practice of the present invention is prepared using conventional extrusion coating or film laminating techniques. In such coating operations, the individual layers of the multilayered plastic coatings (i.e., the adhesive inner layers and the .gtoreq.250.degree. F. melting point outer layers) can be applied separately or they can be applied simultaneously either via coextrusion technology or by lamination of a previously prepared multilayered film of the type desired for the multilayered plastic coating.

Preferably, the individual layers of the multilayered plastic coatings are applied simultaneously by either coextrusion or multilayer film lamination techniques. However, in an alternative embodiment, it is also possible to arrive at the desired type of multilayer plastic coated can stock without the use of organic solvent-based coating systems by first applying the adhesive synthetic thermoplastic resin inner layers to the steel substrate from an aqueous dispersion of such resin (e.g., such as the ammoniated aqueous dispersions described in U.S. Pat. No. 3,799,901) and thereto after applying the .gtoreq.250.degree. F. melting point resin outer layers to the exposed surfaces of the adhesive resin inner layers via either extrusion coating or film lamination techniques.

Regardless of how the above-noted multilayered coatings are applied in the present invention, it is generally preferable to post-heat the resulting laminate prior to the can forming step at a temperature above the melting point of the highest melting resin employed in said multilayered coating for a short period of time such as, for example, for a period of 5 minutes or less.

The use of the hereinbefore described multilayer plastic coated can stock in the actual can forming operations is conducted generally pursuant to conventional draw-redraw can forming techniques and, naturally, such operation can consist of either a single draw or multiple drawing steps depending upon the ultimate depth of draw (or draw ratio) required for the particular type of can to be formed in such operation.

The formation of can bodies (especially deeply drawn can bodies) by the draw-redraw forming process imparts a high degree of stress to the container stock employed and a significant amount of unrelieved residual stress can remain in the plastic coating employed on such container stock following can body formation therefrom. It is therefore important in the practice of the present invention that the multilayer plastic coating employed has sufficient strength and adhesion at ambient temperatures to withstand such residual stresses without coating failure during ambient temperature storage of foods and beverages therein. In addition, since food and/or beverage canning operations often involve processing at elevated temperatures (e.g., such as steam processing at about 250.degree. F.) for prolonged periods of time (e.g., as much as an hour or an hour and a half at such elevated temperatures), it is similarly important that the multilayered plastic coating employed have sufficient strength and adhesion to avoid coating failure under such elevated temperatures.

In the case of the hereinbefore described random ethylene/carboxylic acid copolymer adhesive resins, it has been found that, while such resins exhibit adequate ambient temperature (e.g., normal room temperature such as 20.degree. to 25.degree. C.) strength properties to generally withstand the aforementioned residual stresses at normal room temperatures, the elevated temperature strength properties of such resins are not normally sufficient to withstand such residual stresses at the elevated temperatures (e.g., usually about 250.degree. F.) normally employed in food or beverage canning or processing operations. Fortunately, however, it has also been found that the can bodies formed by the draw-redraw process from the above-described multilayered plastic coated steel sheet material (i.e., using the indicated adhesive resin inner layers therein) can be caused to successfully withstand the aforementioned type of elevated temperature canning and/or processing operations by relieving the aforementioned residual stresses (i.e., those residing in the multilayered coating after draw-redraw can body formation) by subjecting the multilayered plastic coated draw-redraw can body, after its formation but prior to any food or beverage filling and processing operations, to a temperature above the crystalline melting point of the highest melting point resin employed in said multilayered coating for a time sufficient to relieve the stresses created in such coating during the can body formation. Typically, when such stress-relieving step is conducted in a hot air oven, the oven temperature will be in the range of from about 300.degree. to about 500.degree. F. (especially from about 375.degree. to about 450.degree. F.) and the can bodies to be treated will have a residence time in said oven of from about 0.2 to about 5 minutes (especially about 1 minute or so). On the other hand, when more rapid heating means are employed, such as, for example, induction heating techniques, much shorter heating cycles such as, for example, on the order of only a few seconds or so can be sufficient to raise the temperature of the multilayered coating to a level above the crystalline melting point of the highest melting point component thereof.

Another feature to note in the practice of the present invention is that the interlayer adhesion between each of the above-described outer .gtoreq.250.degree. F. melting point resin outer layers and its corresponding adhesive resin inner (metal contacting) layer can be a determining factor in the ability of the overall multilayer structure to withstand the rather severe stresses imparted during draw-redraw can body formation and/or any residual stress remaining during the subsequent canning/processing operations. In some cases (i.e., in the case of a high density polyethylene or nylon outer layer and the aforementioned random ethylene/ethylenically unsaturated carboxylic acid copolymer inner layers) the direct adhesion between such layers has been found to generally be sufficient to withstand such stresses. However, in any cases where the direct adhesion between such layers is not alone sufficient to withstand the indicated stresses, it may be necessary or desirable to employ an intermediate adhesive or compatibilizing layer of a third thermoplastic resin (or of a resin blend) which exhibits good adhesion to both of the resins employed, respectively, in the outer and inner layers of the subject multilayer thermoplastic resin coating.

Naturally, after the draw-redraw cans have been prepared in accordance with the present invention, they are employed in the conventional fashion in their intended ultimate end-use for the canning, processing and storage of various food and/or beverage products.

The practice of the present invention is further illustrated by reference to the following non-limiting working examples.

In these examples, 2 different multilayered plastic/chrome-chrome oxide coated steel/multilayered plastic laminates are prepared by laminating a previously prepared multilayered plastic film to both sides of a 7.5 mil thick chrome-chrome oxide coated steel sheet. The lamination is conducted by preheating the steel sheet to about 400.degree. F. and passing said preheated steel sheet through closed nips already threaded with the two multilayered films and by then post-heating the so-formed laminate in a hot air oven at an elevated temperature (i.e., 400.degree. F. for Example 1 and about 430.degree. F. for Example 2) for 5 minutes. The resulting laminate is then formed into a 307.times.303 size can body via a draw-redraw operation and the resulting can body is thereafter post-baked for about 1 minute in a hot air oven maintained at a temperature of about 400.degree. F. in the case of Example 1 and at about 430.degree. F. in the case of Example 2, and is subsequently (i.e., after cooling to room temperature) subjected to a grease and steam resistance testing by filling the can body with grease (i.e., lard) and exposing the filled can to a temperature of 250.degree. F. for 1 hour and by exposure to steam in an autoclave at 250.degree. F. for 1 hour. The multilayered film employed in the individual examples, and the steam and grease resistance results for the resulting cans formed from laminates made therewith, are summarized in Table 1, below.

For comparative purposes, two additional runs (i.e., Controls 1 and 2) are made in the same fashion (i.e., using the same plastic/steel/plastic laminates as in Examples 1 and 2, respectively) except that the indicated post-baking of the formed can bodies is eliminated.

While the present invention has been herein described with reference to various specific embodiments and examples thereof, such examples and embodiments are not to be understood as limiting the scope of the instantly claimed invention.