US3832962A

US3832962A - Precoating of aluminum can sheet

Info

Publication number: US3832962A
Application number: US00384039A
Authority: US
Inventors: R Rolles
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1971-08-23
Filing date: 1973-07-30
Publication date: 1974-09-03
Anticipated expiration: 1991-09-03

Abstract

The successive steps of coating aluminum can sheet with a resin capable of withstanding subsequent drawing and ironing, curing the resin coated on the sheet; drawing the sheet, for example, into the shape of a shallow can, and ironing the shallow can into a deep can of beverage size.

Description

D United States Patent 11 1 1111 3,832,962 Rolles Sept. 3, 1974 [54] PRECOATING 0F ALUMINUM CAN SHEET 3,206,848 9/1965 Rentmeester 1 13/120 A 3,268,620 8/1966 [75] Inventor: New Kensmgton, 3,360,157 12/1967 Bolt et a1. 113 120 A 73 Assigneez l i Company f America 3,497,466 2/1970 Markulin et a1. 220/64 Pittsburgh Pa. 3,539,480 11/1970 Groff et a1. 220/64 3,577,753 5/1971 Shah et a1. 113/120 [22] Filed: July 30, 1973 [21] Appl. No.: 384,039 Primary ExaminerRichard J. Herbst Related US Application Data Attorney, Agent, or FirmJohn P. Taylor [63] Continuation-impart of Ser. No. 174,136, Aug. 23, i

1971, abandoned, and a continuation of Ser. No. 805,829, March 10, 1969, abandoned. [57] ABSTRACT The successive steps of coating aluminum can sheet 2% 113/120 with a resin capable of withstanding subsequent draw- 72/46 ing and ironing, curing the resin coated on the sheet; 1 o earc drawing the sheet, for example, into the shape of a shallow can, and ironing the shallow can into a deep [56] References Cited can of bevera e Size UNITED STATES PATENTS g 2,856,094 10/1958 Gloyer et a1. 220/64 6 Claims, N0 Drawings PRECOATING OF ALUMINUM CAN SHEET CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 174,136 filed Aug. 23, 1971, now abandoned, as a streamlined continuation of application Ser. No. 805,829 filed Mar. 10, 1969, now abandoned.

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 US. 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 275380 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 2% 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 mar ring 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 theshallow 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 -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 elonga- I tion 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 crosslinking therewith. For example, urea-formaldehyde, triazine, melamine-formaldehyde, epoxy resins, phenolic resins or combinations thereof may be used for crosslinking and to obtain desired physical and chemical properties.

the cross-linking should not exceed about 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/ft 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:

EXAMPLE 1 To illustrate the prior art, chromium-phosphate conversion-coated 3004-1119 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/ft 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 21 1-12 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;

EXAMPLE 2 Aluminum Alloy 3004-1-119, 0.0145 was cleaned and pre-treatedwith 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/ft. The coated and waxed sheet was drawn, redrawn, sized and ironed without further heating or waxing to 211x408 beverage cans.

EXAMPLE 3 A sheet of Aluminum Alloy 3004-1-119, 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) 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 11121 1 Phosphoric acid catalyst 8 Paraffinic lubricant This coating composition was applied to one side of the sheet at 8 mg/in 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 reactiveportions of the resin solution. The coated sheet was then drawn into a cup, redrawn, sized and ironed into a 21 l 400 can without substantially disrupting the continuity of the coating on the inside side wall.

EXAMPLE 4 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/ft and subsequently coated with a high molecular weight noncross-linked epoxy resin. The epoxy resin (epichlorin-bisphcnol 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 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/in whereas film weight on the bottom surface was about 9 mg/in.

EXAMPLE 5 3004 conversion-coated aluminum sheet, 0.0145" was coated with a solution polymer consisting of a high molecular weight epoxy resin of the epichlorohydrinbisphenol type as in Example 4 except that the polymer was blended with 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.

EXAMPLE 6 3004 alloy conversion-coated aluminum sheet was coated with a solution polymer consisting of a medium molecular weight epoxy resin (epichlorohydrinbisphenol 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 ureaformaldehyde resin. The resin was dissolved in MEK/- toluol/isopropyl alcohol, applied to a dry film weight of 7 mg/in 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 mg/ft The sheet so prepared was drawn, redrawn, sized and ironed to obtain a 21 lX400 beverage can. The coating was somewhat milky after ironing but adhered well to the substrate.

EXAMPLE 7 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 21 1X408 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.

Having thus described my invention and certain preferred embodiments thereof, I claim:

1. In a process for forming shaped articles by drawing and ironing aluminum sheet or strip coated with an organic coating suitable for contact with comestibles, the successive steps comprising:

1. coating a conversion coated aluminum sheet or strip of a gauge adapted for drawing and ironing containers with a resin capable of withstanding substantial deformation in drawing and ironing operations while remaining integral without substantial fracture or exfoliation selected from the class consisting of a. vinyl resins having a vinyl chloride content of from 81-97% by weight and a vinyl acetate content of 3-14% by weight;

b. uncross-linked epoxy resins having a molecular weight of at least 36,000 and c. epoxy resins cross-linked with amino or vinyl resins, said epoxy resin having a minimum molecular weight of 5,000 and said cross-linked resin having a maximum cross-linkage of 2. curing the resin thus coated on said sheet or strip;

3. drawing the thus coated sheet with resin cured thereon into the shape of a shallow cup-shaped article and, thereafter, without first actively heating the drawn article;

4. ironing said shallow cup-shaped article into the form of an ironed article.

2. The process of claim 1 wherein said aluminum sheet is coated with a vinyl coating comprising vinyl chloride, vinyl acetate, and maleic acid.

3. The process of claim 1 wherein said aluminum sheet is coated with an uncross-linked epichlorohydrinbisphenol type epoxy resin.

4. The process of claim 1 wherein said aluminum sheet is coated with an epichlorohydrin-bisphenol type epoxy resin cross-linked with a urea-formaldehyde resin.

5. The process of claim 1 wherein said aluminum sheet is coated with an epichlorohydrin-bisphenol type epoxy resin cross-linked with a vinyl resin.

6. The process of claim 1 wherein said aluminum sheet is drawn and ironed from a gauge of at least about 0.0145 inch to a gauge of less than about 0.006 inch.

Claims

2. The process of claim 1 wherein said aluminum sheet is coated with a vinyl coating comprising vinyl chloride, vinyl acetate, and maleic acid.
2. curing the resin thus coated on said sheet or strip;
3. drawing the thus coated sheet with resin cured thereon into the shape of a shallow cup-shaped article and, thereafter, without first actively heating the drawn article;
3. The process of claim 1 wherein said aluminum sheet is coated with an uncross-linked epichlorohydrin-bisphenol type epoxy resin.
4. The process of claim 1 wherein said aluminum sheet is coated with an epichlorohydrin-bisphenol type epoxy resin cross-linked with a urea-formaldehyde resin.
4. ironing said shallow cup-shaped article into the form of an ironed article.
5. The process of claim 1 wherein said aluminum sheet is coated with an epichlorohydrin-bisphenol type epoxy resin cross-linked with a vinyl resin.
6. The process of claim 1 wherein said aluminum sheet is drawn and ironed from a gauge of at least about 0.0145 inch to a gauge of less than about 0.006 inch.