BACKGROUND OF THE INVENTION
This invention relates to the manufacture of drawn and ironed containers from precoated aluminum sheet. More particularly, it relates to a process in which a particular coating is applied to and cured on aluminum sheet prior to drawing and ironing. The term "aluminum" as used herein includes aluminum and its alloys containing at least 50 percent by weight aluminum
U.S. Pat. No. 3,206,848 discloses precoating of aluminum sheet with vinyl organosols, solution vinyls, vinyl phenolics and epoxy phenolics prior to drawing the coated sheet into a shallow cup, heating the shallow cup to a temperature of around 275°-380° F which is below the initial hardening temperature of the coating for 1-20 minutes to relieve the stresses in the coating and the aluminum alloy and thereby providing greater coating elongation and minimizing the possibility of subsequent fracture, cooling and then drawing again to a depth of about 21/4 inches. According to the patentee, additional drawing operations may be performed provided there are baking steps between each of the draws. Such a baking operation tends to result in possible marring or decomposition of the coating as well as reducing the tensile properties of the side wall of the shallow cup formed. Therefore, development of a process which would minimize possible coating and metal partial anneal and also permit formation of a deep cup from precoated sheet represents a highly desirable result.
SUMMARY OF THE INVENTION
It is accordingly one object of this invention to provide a method of forming a deep article such as a beverage container from precoated aluminum sheet. A further object is to provide a method of minimizing the formation of cracks and fractures in the coating of precoated aluminum articles such as cans of beverage depth during or subsequent to the drawing and ironing steps used in forming such articles. Further objects will be apparent from the description and claims which follow.
DESCRIPTION OF THE INVENTION
In its broader aspects, my invention involves precoating aluminum sheet with a preselected composition capable of withstanding substantial deformation in subsequent drawing and ironing steps without substantial fracture or exfoliation, drawing the cured coated sheet into shallow cup and then ironing the shallow cup to a can of beverage can depth. I have found surprisingly that by ironing the precoated shallow cup to a deeper can size, I produce a can that retains a smooth, substantially unfractured coating without any heating above room temperature of about 20°-30° C except when the coating is thermosetting and requires heating for curing before the drawing step as applied to the sheet. This is particularly suprising in view of the fact that it has heretofore been generally believed that heating prior to or during forming is necessary to permit sufficient elongation of the metal and flexibility for the coating to assume the shape of the can into which the blank sheet is formed without a substantial amount of cracking or removal of coating, especially when ironing of the side wall is involved rather than metal drawing.
The composition which I use for precoating according to my invention must be able to accept the strain of deformation resulting from drawing followed by ironing, yet remain integral without undergoing substantial fracture or exfoliation. Resins which I have found to have such a property and be especially useful according to my invention are certain specific vinyl or epoxy resin types which apparently allow the required degree of coating elongation or coating flow (without heating) during the drawing and ironing steps which follow the curing of the resin on the aluminum sheet.
Representative vinyl resins useful according to my invention include those having a vinyl chloride content of 81 to 97% and a vinyl acetate content of 3 to 14%. Such vinyl polymers may be modified with appropriate dibasic acids such as maleic acid or anhydride or vinyl alcohol. The foregoing resins may also be modified with plasticizers, stabililizers, solvating agents or pigments as desired. When cross-linking is desired prior to coating, it can be obtained by modifying the vinyl polymers, particularly those containing free hydroxyl or carboxyl groups, with a resin which is capable of cross-linking therewith. For example, urea-formaldehyde, triazine, melamine-formaldehyde, epoxy resins, phenolic resins or combinations thereof may be used for cross-linking and to obtain desired physical and chemical properties.
Uncross-linked epoxy resins having a molecular weight of at least 50,000 or more are such as epichlorohydrin-bisphenol polymers also useful in practicing my invention.
Lower molecular wieght epoxy resins of about at least 5,000 molecular weight can also be used in accordance with the invention when cross-linked with amino resins such as urea-formaldehyde or melamine-formaldehyde or with vinyl resins such as described above. However when such cross-linked resins are used the cross-linking should not exceed about 80% of the theoretical equivalency. This helps maintain elongation of film under stress and compression and prevents excessive brittleness.
The coating resins useful according to my invention may be applied by direct or reverse roller coat application, by spraying or by curtain coating. Conventional catalysts may also be used to facilitate curing. When solvents are used, heating is desirable to assist solvent evaporation and cross-linking. A lubricant such as paraffin wax, low-molecular weight polyethylene, or the like, may be incorporated in the coating resin. Preferred range of lubricant concentration is from 1% to 5% based on resin solids composition. The lubricant may be applied as a separate step by dispersion in a suitable liquid. It may be applied by roller coating, spraying or cascading. The preferred range of lubricant on the coating surface is from 5 to 40 mg/ft2 of coated surface.
The ironing step according to my invention is capable of reducing the metal thickness from as great as 17 mils to as little as 4 mils. The aluminum which I use may be either of the hard (H) or annealed (O) temper, and the resin coating may be applied to one or both sides of the sheet.
If desired, before the resin is applied, the aluminum sheet may be precleaned and be subjected to a conversion coating treatment. An alkaline or acid-base cleaner may be used. It may contain sodium or potassium hydroxide, tetrasodium pyrophosphate or other effective alkaline salts combined with conventional wetting agents, buffers and chelating chemicals. If of acid base, the cleaner may contain nitric or sulfuric acid, for example. The conversion coating may be an electrochemically produced aluminum oxide film such as produced by sulfuric acid anodizing or a complex metal oxide film such as produced by chemical conversion coating of the chromium-phosphate type, for example, as described in U.S. Pat. No. 2,438,877. The anodically produced coating may be conventionally sealed in boiling, de-ionized water but is preferably left unsealed to obtain maximum adhesion of the subsequently applied resin coating.
The drawing prior to ironing may be in more than one step. For example, a reverse drawing step may be used.
The following examples are illustrative of my invention:
To illustrate the prior art, chromium-phosphate conversion-coated 3004-H19 0.0145" aluminum was used for this example. Six-inch strip was cleaned with an alkaline soidum pyrophosphate cleaner and coated with a chromium-phosphate conversion coating solution to obtain a film weight of 14 mg/ft2. The strip was blanked, drawn, redrawn and sized, and then ironed into beverage cans. The finish was bright on the outer wall and wall thickness was equivalent to uncoated cans (about 0.0055"). The cans were solvent cleaned to remove excess lubricant.
To study metal flow and conversion coating uniformity, X-ray fluorescence and microprobe surface studies were made on the bottom and side wall exterior and interior of the 211-12 oz. container. These tests showed very uniform metal and surface flow. Distribution of conversion coating was substantially uniform when comparing variations of chromium between the unworked bottom and the worked side wall. There was a 3:1 surface conversion coating reduction. There were little differences between metal flow on the mandrel side (interior) and the ring side (exterior) of the can. To assess the surface optically for conversion coating uniformity, cans were heated to 800° F for 15 minutes to develop color in the conversion coating. This converted the transparent chromium-phosphate film to a strongly colored layer. Color uniformity on the side wall was excellent and in agreement with the shade attained on surfaces with similar conversion coating thickness not mechanically worked. Microscopic studies showed that the surface of the pretreated drawn and ironed can was somewhat more broken up than the surface of the drawn and ironed can post-treated with chromium-phosphate.
Aluminum Alloy 3004-H19, 0.0145" was cleaned and pre-treated with a chromium-phosphate conversion coating and one side of this sheet was subsequently coated with a solution vinyl coating. The vinyl coating consisted of a mixture of a vinyl chloride resin having about 87% vinyl chloride content and 13% vinyl acetate content and a second resin having 86% vinyl chloride content, 13% vinyl acetate content and 1% maleic acid content. Equal portions of each resin were blended with about 20% dioctyl phthalate to plasticize the coating, and this mixture was dissolved in methyl ethyl ketone/toluol to obtain a suitable solution coating. This solution coating was applied at 7 mg/in and the solvent portion of this coating was evaporated by force drying the coating at 300° F for 5 minutes. The resulting sheet was coated with a paraffin wax dissolved in hexane to obtain a wax weight of 20 mg/ft2. The coated and waxed sheet was drawn, redrawn, sized and ironed without further heating or waxing to 211×408 beverage cans.
A sheet of Aluminum Alloy 3004-H19, 0.0145" was cleaned and pretreated as in Examples 1 and 2 and subsequently coated with a vinyl resin dispersion-type coating of the following composition.
______________________________________ Parts by Weight Components ______________________________________ 100 Vinyl chloride dispersion resin (fine particle size) 60 Vinyl solution resin (86% vinyl chloride, 13% vinyl acetate, 1% maleic acid) 45 Med MW epichlorohydrin-bisphenol resin 35 Triazine resin 25 Non-reactive polymeric plasticizer 200 Titanium dioxide pigment 600 Methylethyl ketone (MEK):toluol:xylol 1:1:1 1 Phosphoric acid catalyst 8 Paraffinic lubricant ______________________________________
This coating composition was applied to one side of the sheet at 8 mg/in2 dry film weight, and the coating was heated for 1 minute at 500° F oven temperature to facilitate solvent evaporation and cross-linking of the reactive portions of the resin solution. The coated sheet was then drawn into a cup, redrawn, sized and ironed into a 211×400 can without substantially disrupting the continuity of the coating on the inside side wall.
A sheet of Aluminum Alloy 3004-H19, 0.0145" was cleaned and pretreated with a chromium-phosphate conversion coating having a film weight of 25 mg/ft2 and subsequently coated with a high molecular weight noncross-linked epoxy resin. The epoxy resin (epichlorin-bisphenol type) had an average molecular weight of 36,000 and was dissolved in a mixture of methyl ethyl ketone (MEK) and cellusolve acetate, the final coating having a resin content of 18 per cent by weight and a solvent content of 82 per cent by weight. The resin was applied to obtain a dry coating film weight of 8 mg/in2. The resulting coating was heated for 5 minutes at 400° F to facilitate solvent evaporation and curing on the aluminum sheet.
A solution lubricant consisting of paraffin dissolved in hexane was subsequently applied by roller coat application. The sheet containing the lubricated cured resin was drawn into a cup, redrawn, sized and ironed without reheating. The resulting surface inside the can was substantially free of pinholes. Adhesion was judged excellent as measured by scotch tape adhesion tests, and the coated metal surface withstood beer pasteurization of 20 minutes at 180° F in beer without loss of coating adhesion. Film weight measurements on the side wall were approximately 3 mg/in2, whereas film weight on the bottom surface was about 9 mg/in.
3004 conversion-coated aluminum sheet, 0.0145" was coated with a solution polymer consisting of a high molecular weight epoxy resin of the epichlorohydrin-bisphenol type as in Example 4 except that the polymer was blended with 10% by weight urea-formaldehyde resin (Uformite F240) to facilitate partial cross-linking. The resin was dissolved in MEK/butyl cellusolve to facilitate application. The coating was applied at 8 mg/in and heated for 5 minutes at 425° F to facilitate solvent evaporation and resin cross-linking. The resulting precoated sheet was then handled substantially as described in Example 3 with substantially the same results.
3004 alloy conversion-coated aluminum sheet was coated with a solution polymer consisting of a medium molecular weight epoxy resin (epichlorohydrin-bisphenol type) modified with a urea-formaldehyde resin as in Example 3 except for modification to obtain a cross-linking of 70% of that theoretically available based on the equivalent weight of the epoxy and urea-formaldehyde resin. The resin was dissolved in MEK/toluol/isopropyl alcohol, applied to a dry film weight of 7 mg/in2 and heated for 5 minutes at 400° F to facilitate solvent evaporation and cross-linking. A lubricant consisting of paraffin wax dissolved in hexane was applied to obtain a dry lubricant weight of 15 mg/ft2. The sheet so prepared was drawn, redrawn, sized and ironed to obtain a 211×400 beverage can. The coating was somewhat milky after ironing but adhered well to the substrate.
To both sides of 3004-H19, 0.0145" pretreated aluminum sheet was applied a solution vinyl coating as in Example 2. The sheet was lubricated and subsequently drawn and ironed to a 211×408 beverage can. Both inside and outside coating adhered well to the pretreated sheet. Film continuity and dry adhesion were acceptable for commercial use.
It is believed apparent from the foregoing description and examples that by my invention I have provided a method which permits economical use of precoated aluminum sheet in the manufacture of drawn and ironed beverage cans.
While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.