GB2113118A - A coated metal container and a method of making the container - Google Patents

Abstract

A metal container has a seamless body with an integral bottom, formed by drawing and redrawing. At least the inside surface of the body has a resin undercoat and a top coat thereon, the latter being a reaction product of a citric acid ester and a heat-curable resin selected from epoxy, acrylic, polyester and vinyl resins, save that vinyl resins are not used if the undercoat is a thermoset epoxy resin. In making the container, blanks are cut from tinplate or tin-free steel, the undercoat is applied and cured and then the citric acid ester is applied as a lubricant for the drawing process. The top coat is applied without first removing the lubricant.

Description

SPECIFICATION A coated metal container and a method of making the container The present invention relates to a coated metal container and a method of making the container.

The well known, three-piece container for commestibles is made from sheet metal and is fast being supplanted by a two-piece container. This container has a seamless body with one integral end wall; another end wall is secured to the body by means of a double seam after filling the container.

Such containers for beer and soft drinks are made by the drawing and ironing process. Containers for fruits and vegetables which require heavier and sturdier side walls than drinks containers and are beginning to be made by a newer process known as the draw/redraw process.

Although the production of sheet metal containers by means of multiple draws is not new, the draw/redraw process is an improvement over the older drawing process in that it is done at higher speed with greater draw ratios in each drawing step. Achieving the higher productivity-rate requires special machines and, inter alia, special lubricants for the metal-working operation; and to maintain this higher productivity-rate, subsequent processing steps performed on the formed container must also be accomplished at commensurate speeds or eliminated where possible.

U.S. Patent No. 4,287,741 describes a class of citric acid esters which are effective as lubricants in the drawing and ironing process. We have found that the same citric acid esters are effective lubricants in the draw/redraw process described in relation to the present invention.

The present invention involves the discovery that the citric acid esters disciosed in U.S. Patent No. 4,287,741 not only act as effective lubricants in the drawing and redrawing of ferrous metal stock to form a container, but that these lubricants, unlike metal-working lubricants used heretofore, need not be removed from the surface of the formed container, but may be made to react with a subsequently applied synthetic resin top coat by heating to form a hardened barrier layer over the interior surface of the container.

The citric acid esters useful in the present invention are disclosed in U.S. Patent No. 4,287,741 to which reference should be had for further details. This patent gives the following structural formula for these esters:

wherein R1, R2 and R3 are selected from the group consisting of hydrogen and alcoholic residues containing 1-10 carbon atoms, R4 is selected from the group consisting of hydrogen and carboxylic acid radicals containing 1-10 carbon atoms, and at least one of R1, R2 and R3 is an alcoholic residue.

We have found that the present invention is equally applicable to tinplated ferrous metal, e.g.

electrolytic tinplate, and to tin-free, low carbon sheet steel. Although the common prior art tinplate referred to in column 2, lines 50-56, of U.S. Patent No. 4,287,741 is usable in the present invention, non-reflowed, matte finish tinplate described in U.S. Patent No. 3,360,1 57 is preferred for the reasons set forth therein in respect of the drawing and ironing process for making containers. The tin weight on the steel basis metal may vary from 0,10 Ibs to 1.0 Ibs per base box (2.24x 10-3 to 2.-24x 10-2 kg/m2).

According to the present invention, there is provided a metal container including a seamless side wall with an integral bottom wall, the side wall being substantially the same thickness as or thinner than the bottom wall, and the metal from which the container is made being selected from electrolytic tinplate and tinless, low-carbon steel, the container having a coating on at least the interior of said container and including a hardened top coat covering the said walls, the top coat consisting essentially of a reaction product of a citric acid ester and a synthetic resin selected from an epoxy resin, an acrylic resin, a polyester resin and a vinyl resin, provided however that where a vinyl resin is selected, it is not applied over a thermoset epoxide resin base coat.

The invention further provides the method of forming a coated metal container, comprising the steps of applying a citric acid ester lubricant to a metal selected from electrolytic tin plate and tinless, low carbon steel, forming a circular flat blank from the metal, drawing the blank into a shallow cup, subjecting the tup to two additional draws in rapid sequence to form a seamless container body, applying a synthetic resin coating over the interior of said container on top of the lubricant on said container inside surface and heating the so-coated container body to harden the coating and adhere it to its substrate.

The preferred tin-free steel has a chromium surface treatment. While it is still in flat sheet form prior to its formation into a container, a synthetic resin base coat is applied and adhered to this treated surface. The most usual compositions for application as a base coat are those containing an epoxy resin or vinyl resin or polyester resin.

The present invention will now be described in more detail by way of example.

A tin-free steel preferred for use in the present invention is aluminium-killed, continuous cast steel with a chromium/chromium oxide surface treatment. The chromium in the oxide is present at about 0.5 to 2.0 mg per square foot (5.38 to 21.5 mg per square metre), and the chromium metal at about 3 to 13 mg per square foot (32.3 to 140 mg per square metre). The material described is known in the art as TFS-CT for tin-free steel, chromium type. The treatment is described in a paper published in the Journal of the Electrochemical Society, Vol. 116, No. 9, pp. 1299-1305.

The preferred tinplate is steel plate of the same composition as the steel set forth above. At the steel mill, the steel - in a well known manner - has tin applied to its surface electrolytically in various amounts, for example, 0.25 Ib per base box (5.6x 10-3 kg/m2). As mentioned previously, this tinplate is left in a matte condition, i.e., is not flow brightened, is oiled for rust inhibition and coiled for shipment to a container- making installation.

When received in the can-making plant, the tin-free steel has applied to its surfaces a base coating. The preferred coating contains an epoxy resin based on bisphenol A with an epoxide equivalent weight of 2300 to 4000, a urea-formaldehyde cross-linker, a sulfonic acid catalyst, and a high-melting polyethylene internal lubricant. The coating has a solids content of 28 percent. Both vinyl and polyester resin compositions have also been used as base coats.

This base coating applied to both sides of the steel may be applied while the steel is still in coil form. Alternatively, the steel may be cut into sheets and the coating applied individually. After application the coating is baked to form a tough, adherent base coat on the tin-free steel.

The citric acid ester used in the present invention is dip coated onto both surfaces of individual sheets of the base coated tin-free steel, the excess ester is removed and the sheets are then fed into a blanking and cupping press which cuts from the sheet one or more circular discs of 7.947 inches (20.18 cm) in diameter, and draws the disc into a cup of 5.007 inches (12.72 cm) in diameter and 1.850 inches (4.70 cm) in side wall height. In two subsequent operations, the cup is successively reduced in diameter with concurrent lengthening of its side wall, i.e. drawn; and simultaneously this side wall is slightly thinned, i.e., to about 10% less than the starting gauge, and further elongated, i.e., ironed, in the manner similar to that described in U.S.Patent 3,360,1 57. The final diameter and side wall height accomplished in the drawing plus ironing are 3.060 in. and 4.450 in. (7.77 and 11.30 cm) respectively and are accomplished in a few seconds. The diameter of the starting blank and the height to diameter ratios, draw ratios, in the ensuing metal working process may be varied depending upon the desired size of the finished can. Also, as between different draw/redraw systems, the amount of draw in each step may be varied provided the cumulative effect of the plural draws produces the can of desired height and diameter.

It is readily apparent that a draw/redraw system with ironing is a more severe metal working process than a draw/redraw system without ironing. The citrate ester lubricant of the present invention performs equally well in both systems.

The particular citric acid compound used in the above example is acetyl tributyl citrate. It may be applied in any suitable way such as by roller coating, immersion coating with excess suitably removed, electrostatic deposition which is accurate in application both as to weight and placement, or by hot spraying from an oxy-dry unit or cold spraying if the ester is reduced with a suitable solvent.

The amount of lubricant applied over the base coat can vary from 10 to 40 (108 to 430 and preferably 10 to 20 milligrams per square foot (108 to 21 5 mg/m2) of total surface, i.e., both sides, of the sheet being fed into the draw/redraw apparatus. It has been found that the lubricating effect falls off appreciably below 10 mgs/ft2 (108 mg/m2) and for most operations 20 mgs/ft2 (21 5 mg/m2) is sufficient to achieve high speed, trouble free, multiple draws from flat blank to formed container. Due to the severity of the metal-working operation, i.e., the appreciable draw ratios and draw speed plus ironing, substantial heat is generated on the surfaces being worked.While not wishing to be bound by any particular theory, it is possible that this generated heat causes at least a partial decomposition of the lubricant on the worked surfaces, thereby liberating reactive substances such as those having carboxyl or acetyl functional groups. For example, in the case of the preferred acetyl tributyl citrate, acetic acid would be liberated. These functional moieties are believed to attach themselves to the base coat and/or be available for reaction with a postsprayed top coating.

At the completion of the draw/redraw operation, the container is beaded to impart strength to the side and bottom walls before being fed into a device for applying a top coat to the container's inside surface. Most usually this device involves a turret which revolves the container past a reciprocating spray gun which enters the interior of the container which is spun on its longitudinal axis while the spray gun, as it is retracted from the container body, emits a 360 degree spray of a synthetic resin solution to coat the entire interior surface of the container.

After completion of the top coating operation, the container is then subjected to a temperature of 4000F (2040C) for 4 minutes to harden and cure the top coat. Unlike prior art procedures which require the lubricant to be removed before application of the top coat, to avoid contamination and improper curing of the top coat, the procedure of the present invention not only eliminates the time-consuming step of removing the lubricant and permits the application of the top coat directly to a still-lubricated internal surface of the container but actually assists in firmly adhering the top coat to the base coat.

Again, not wishing to be bound by any particular theory, it is believed that the reactive moieties liberated from the applied lubricant previously and/or during the above mentioned baking operation react with or at least anchor themselves into the top coat as well as the base coat thereby forming a strong adhering medium between the base coat and top coat. The cured top coat is a reaction product of the lubricant and the top coating composition.

The citric acid ester lubricant and what remains thereof after the draw/redraw operations are soluble in organic solvents such as butanol, butyl Cellosolve (RTM), di-isobutyl ketone, Cellosolve acetate and Solvesso 1 50. Therefore, resins for top coats which are also soluble in these same solvents and provide inert, continuous, resin films upon thermal curing are preferred. Examples are epoxy resins and acrylic resins. Vinyl resins are usable if they are applied over a base coat which is other than a thermoset epoxy resin. The citric acid ester may also be used in water-base coating compositions provided the liquid system contains a solvent for the citrate ester which is miscible with the water of the system.

Evaluation of top coats applied over an epoxy-UF base coat is done by testing process resistance as well as intercoat adhesion between the base coat and the top coat. The foliowing table summarizes the results of these tests: Evaluation of postsprayed top coats over an epoxy type base coat Process Intercoat Top coat resistance adhesion acrylic passed passed vinyl passed failed epoxy-phenolic passed passed epoxy-acrylic passed passed epoxy-phenolic passed passed epoxy-phenolic passed passed epoxy-phenolic passed passed epoxy passed passed The epoxy-phenolic resin systems in the above table differ from each other in the proportion of epoxy to phenolic.

Process resistance is checked by filling the container with deionized water followed by steam processing at 2650F (1290 C) for 90 minutes. After water cooling and standing overnight, the cans are emptied, cut open, cross hatched, and intercoat adhesion tested (i.e. peel tested using pressure sensitive tape).

Other suitable base coat/top coat combinations include vinyl and polyester resins as base coats, examples of which are in the table below: Intercoat adhesion after process Top coat Base coat Acrylic Vinyl Epoxy polyester passed passed passed vinyl passed passed passed epoxy passed failed passed epoxy passed failed passed vinyl passed passed passed epoxy passed failed passed The failures noted in the above table result from an attempt to adhere a thermoplastic, vinyl resin top coat to a thermoset, epoxy resin base coat. These failures are independent of the citrate ester lubricant used in practising the present invention and would have occurred were the citrate ester not present.

Although the invention has been described above with respect to a base coated tin-free steel, it is also applicable with matte-finish, electrolytic tinplate used as the metal starting material. As set forth in U.S. Patent 3,360,1 57, the matte finished tinplate and a liquid lubricant act synergistically with each other and an analogous situation obtains in the present invention between the matte tin and citrate ester to augment the lubricant system during the draw/redraw process for forming the container.

Further, as theorized above, it is believed that functional groups, e.g., carboxyl and/or acetyl, formed either during the metal-working process or during the thermal hardening of the top coat, or in both instances, form in effect anchoring chains to hold the top coat firmly adhered to the tinplate surface.

Consistent with this theory are the results of gas chromatographic and mass spectrometric anaiyses which indicate that acetic acid and butanol are formed and liberated from thermal degradation of acetyl tributyl citrate when it is heated to 19500 (900 C) the approximate temperature attained in the thermal cure of the top coat.

Furthermore, acetic and citric acids and butanol are produced by hydrolysis of the acetyl tributyl citrate with an amine, such as 2-amino-2-methyl propanol, a solvent generally used as a component of water-base coatings.

Comparative tests were run forming a base coated tin-free steel into containers by means of the draw/redraw with ironing process using various metal working lubricants. In each instance except for the acetyl tributyl citrate (ATBC) of the instant invention, the lubricant either was inferior to the acetyl tributyl citrate as a metal-working lubricant as exemplified by the required longer time to produce the container or it was incompatible with the top coat preventing the top coat from forming a continuous protective film.The table set forth immediately below shows the results of these comparative tests: Effect of lubricant type on draw/redraw and ironing tin-free steel and electrolytic tinplate Adhesion Lubricant Fabrication of top coat Petrolatum (ETP) Good Very poorl Butyl Stearate2 (ETP) Good Good Dioctyl Stearate3 (ETP) Good Good Neodene (a-olefin) (TFS) Cracked domes; Good Broken radii Phosphate Ester (TFS) Failure on Second Operation Acetyl Tributyl Citrate (ETP) Good Good ATBC with Carnauba wax (TFS) Good Good Acetyl trihexyl citrate (TFS) Good Good Notes 1No adhesion before or after process.

2Application problems due to rapid viscosity changes as solidification point (650F or 1 80C) is approached.

3Lower stability due to voiatilization and oxidation (Ref: U.S. Patent 3,923,471).

We believe another factor contributing to the effectiveness of acetyl tributyl citrate is its ability to increase the wettability of the TFS or ETP surface by the top coating as shown by the contact angle measurements in the table below. The measurements were taken by applying a coating of the listed lubricant to the metal surface, placing a drop of water on the lubricated surface and then measuring the angle between the lubricated surface and an intersecting line tangential to the curved surface of the water droplet. The greater the wettability of the lubricant, the flatter the water droplet and the smaller the angle of contact.

Contact angles Lubricant Plate Contact angle None TFS 83.5 Petrolatum TFS 96.7 ATBC TFS 74.5 None ETP 72.4 Petrolatum ETP 84.5 ATBC ETP 70.5 ATBC with Carnauba Wax ETP 70.6 Butyl Stearate ETP 77.0 Dioctyl Stearate ETP 73.5 Neodene ETP 79.5 High melting waxes can be added to the citrate ester lubricant in concentrations of up to about 2% by weight of the ester solids to improve lubricity without adverse effects on intercoat adhesion.

It is understood that the invention is not confined to any particular embodiment described herein as illustrative of the invention, but embraces all modifications as may come within the scope of the following claims.

Claims (13)

Claims

1. A metal container including a seamless side wall with an integral bottom wall, the side wall being substantially the same thickness as or thinner than the bottom wall, and the metal from which the container is made being selected from electrolytic tinplate and tinless, low-carbon steel, the container having a coating on at least the interior of said container and including a hardened top coat covering the said walls, the top coat consisting essentially of a reaction product of a citric acid ester and a synthetic resin selected from an epoxy resin, an acrylic resin, a polyester resin and a vinyl resin, provided however that where a vinyl resin is selected, it is not applied over a thermoset epoxide resin base coat.

2. The metal container according to claim 1 , wherein the citric acid ester is acetyl tributyl citrate.

3. The metal container according to claim 1 or claim 2, wherein the top coat contains up to about 2% of high melting point wax based on the total weight of the said ester and wax solids.

4. The metal container of claim 1, 2 or 3, wherein the metal of the container is non-reflowed, matte finish tinplate.

5. The metal container of claim 1,2 or 3, wherein the metal of the container is tin-free steel.

6. The metal container according to claim 5, wherein a hardened synthetic resin base coat is interposed between the tin-free steel surface and the hardened top coat, which base coat covers and adheres to the steel surface, and the base coat is selected from an epoxide resin, a polyester resin and a vinyl resin.

7. The metal container according to claim 6, wherein the base coat is an epoxide resin and the top coat is selected from an epoxide resin, an acrylic resin and a polyester resin.

8. A coated metal container according to claim 1 and substantially as herein described by way of example.

9. The method of forming a coated metal container, comprising the steps of applying a citric acid ester lubricant to a metal selected from electrolytic tin plate and tinless, low carbon steel, forming a circular flat blank from the metal, drawing the blank into a shallow cup, subjecting the cup to two additional draws in rapid sequence to form a seamless container body, applying a synthetic resin coating over the interior of said container on top of the lubricant on said container inside surface and heating the so-coated container body to harden the coating and adhere it to its substrate.

10. The method according to claim 9, wherein the side wall of the cup is subjected to ironing during the additional draws, whereby the side wall of said container is thinner than the bottom wail of said container.

11. The method according to claim 8 or claim 9, wherein a synthetic resin base coating is applied to and adhered to the metal prior to the application of the citric acid ester lubricant.

12. A method according to any of claims 9, 10 and 11, wherein the lubricant employed is acetyl tributyl citrate.

13. A method according to any of claims 9 to 12, wherein the lubricant contains up to 2% by weight of a high melting point wax.

1 4. A method according to any of claims 9 to 13, wherein the metal is provided with a cured resin base coat before the lubricant-application step, and the coating applied after draw-forming is selected from epoxy resins, acrylic resins, polyester resins and vinyl resins, save that vinyl resins are not selected where the base coat is a thermoset epoxide resin.

1 5. A method of making a container substantially as herein described by way of example.