US2215144A - Coating metal containers - Google Patents

Description

106. comPosiiiciis,

8 Patented Sept.

comma 0R PLASTlC.

- UNITED STATES Cross Reference PATENT OFFICE COATmG METAL CONTAINERS a British company No Drawing. Application February 9, 1938, Se-

rial No. 189,588. In Great Britain February 18,

Claims.

This invention relates to the coating of metal, especially metal containers more particularly intended for beverages such as beer and wine, which may be preserved by heat treatment at a relatively low temperature, and to an improved method of coating such containers.

It is well known that in order to prevent the development of turbidity and other defects, it is necessary to protect the beverage from direct contact with the metal surface of the container. The ordinary lacquer coatings used for food cans are unsuitable for this purpose, since an undesired flavour is imparted to the beverage. Certain waxes or mixtures of waxes with one another or with other constituents may be used, but when it is attempted to coat the metal by direct application of molten wax the results are unsatisfactory. This is principally due to the tendency of the wax to drain away from the convex portions of the metal surface, such as seams or corrugations. In order to overcome this difficulty a method is in use whereby the metal surface is first coated with a lacquer and afterwards molten wax is applied so as to cover the lacquer surface and any exposed small areas of metal.

According to the present invention the metal surface to be coated is brought into contact with an aqueous dispersion of the wax or like coating material and forms the anode in an electric circuit, the cathode also being in contact with the aqueous dispersion and in such a position relative to the surface to be coated that the desired coating of wax is obtained by electrode position directly upon the metal surface. The coated surface is freed from excess of the aqueous dispersion by drainage, followed if necessary by rinsing, and the moisture is removed from the deposited film. Removal of moisture may be effected by warm air, preferably at a temperature below the melting point of the wax. At this stage the deposit appears opaque and granular, and is porous. The temperature is then raised sufficiently to melt the wax, causing coalescence of the deposited particles and-allowing any entrained gas to escape, whereby on cooling a smooth, impervious coating is obtained, .which has the normal appearance of a wax film.

No intermediate coat of lacquer is necessary and stoving at high temperature is eliminated. 1

Satisfactory coatings may be obtained when the dispersion is at room temperature, but if desired, electrodeposition may be carried out at an elevated temperature, for; example, above the melting point of the-wax.

In certain cases the step of rinsing or washing QOy f/ the deposit may be omitted, a dispersion being used which is of sufiiciently low viscosity to leave only a small amount of dispersion clinging to the surface after draining. For example a wax containing acidic constituents, such as beeswax, or a mixture of such a wax with a neutral wax, may be dispersed in dilute aqueous ammonia. After electro-deposition, the excess of ammonia may be removed by application of heat alone. The omission of rinsing obviates risk of disintegrating the deposit. In other cases the dispersion medium may be such that washing 1s necessaryin order to remove the excess, and in such cases it is especially desirable that the deposited wax particles should adhere well to one another and to the metal surface, in order to withstand the impact of the wash liquor. Complete coalescence of the deposited particles does not occur until the coating is melted, preferably after drying ofi' residual moisture.

A further desirable feature of the dispersion is that it should possess a property known in the terminology commonly used in connection with electro-deposition processes as throwing power. By this is meant the property whereby different areas of the electrode to be coated, in the present case the anode, receive substantially the same density of deposit even through they lie at appreciably different distances from the other electrode, in this case the cathode. A major reason for the importance of this property lies in the fact that in a fabricated container, the seams present crevices which may be relatively deep and narrow, and the electrolytic resistance which has to be overcome in treating areas of metal within the crevices may be considerably greater than that involved in treating freely exposed areas nearer to the cathode. Unless the dispersion has adequate throwing power, metal in the crevices may be imperfectly covered.

Where the container is of uniform cross-section, or is not narrower at the orifice than below, it is possible to employ a cathode of such contour that the distance between the cathode and the container surface is everywhere practically the,

same, and provided that the throwing power of the dispersion is suflicient to give satisfactory coating within crevices, a substantially uniform coating over the whole surface may be readily obtained. Certain types of containers used for beverages, however, have conical tops with narrow orifices, andii'i such cases it is not possible by simple means to obtain a uniform separation of the electrodes, when the container is coated after fabrication. .If the throwing power of the disper- Example 1 White beeswax. 220 grms. 1% sodium silicate 2 litres Initial current density---" 30-40 amps./sq. ft. 5 Final current density about 4 amps/sq. it. Time of treatment 6 secs.

Example 2 White beeswax 47 grms. Parafiin wax 453 grms. 0.375% sodium aluminate 2 litres Initial current density"--- 25-30 amps/sq. ft. Final current density about 3 amps./sq. it. Time of treatment 6 secs.

Example 3 White beeswax. 47 grms. Ceres 453 grms. Sodium aluminate 5.65 grms. 20 Sodium silicate 9.35 grms. (calculated as solid) ater 2 litres Initial current density 30-50 amps/sq. ft. Final current density about 8 amps/sq. ft. Time of treatment 6 sees.

25 The coating material may contain substances such as resins, bitumens, pitches, plasticizers or solvents to improve the adhesion between individual particles in the deposited film and be- 30 tween the film and the can. Similarly the dispersion medium may contain additional materials intended particularly for controlling the stability of the dispersion, for example, emulsifying agents. 35 he inven ion may be applied to completed containers or to portions thereof as a step in manufacture. For example, the bodies and ends may be coated separately before being seamed, or when partially seamed together.

We declare that what we claim is:

1. A process of coating the interior of metal containers, which comprises making the container the anode in an alkaline electrolytic bath containing a foodstuff-permissible salt of alkaline reaction having an anion such that addition of acid would produce an insoluble oxide or hydroxide said salt being present in an amount to give the dispersion suflicient throwing power to deposit the wax substantially uniformly on the container wall, and waxy material in dispersion and passing an electric current therethrough to deposit waxy material directly on the metal.

2. A process of coating the interior of metal containers, which comprises making the container the anode in an alkaline electrolytic bath containing a mixture of foodstuff-permissible salts of alkaline reaction having anions such that addition of acid would produce insoluble oxides or hydroxides said salts being present in an amount to give the dispersion sufiicient throwing power to deposit the wax substantially uniformly on the container wall, and waxy material in dispersion and passing an electric current therethrough to deposit waxy material directly on the metal.

3. A process of coating the interior of metal containers, which comprises making the container the anode in an alkaline electrolytic bath containing sodium aluminate and waxy material in dispersion the aluminate being present in an amount to give the dispersionsufficient throwing power to deposit the wax substantially uniformly on the container wall and passing an electric current therethrough to deposit waxy material directly on the metal.

4. A process of coating the interior of metal containers, which comprises making the container the anode in an alkaline electrolytic bath containing sodium silicate and waxy material in dispersion the silicate being present in an amount to give the dispersion sufficient throwing power to deposit the wax substantially uniformly on the container wall and passing an electric current therethrough to deposit waxy material directly on the metal.

5. A process of coating the interior of metal containers, which comprises making the container the anode in an alkaline electrolytic bath containing a mixture of sodium aluminate and sodium silicate; and waxy material in dispersion the aluminate and silicate being present in an amount to give the dispersion sufiicient throwing power to deposit the wax substantially uniformly on the container wall, and passing an electric current therethrough to deposit waxy material directly on the metal.

6. A process as in claim 1 in which the waxy material is deposited from a dispersion at a temperature above the melting point of the wax.

7. A process of coating a metal object which comprises making the said object the anode in an alkaline electrolytic bath containing waxy material in dispersion, said waxy material having acidic constituents said dispersion containing an alkaline salt selected from the group consisting of sodium aluminate and sodium silicate in an amount to give the dispersion sufficient throwing power to deposit the wax substantially uniformly on the container wall, and passing an electric current therethrough to deposit waxy material directly on the metal.

8. A procas of coating a metal object which comprises making the said object the anode in an alkaline electrolytic bath containing a dispersion of a. waxy material and a foodstufi-permissible salt of alkaline reaction having an anion such that addition of acid will produce an insoluble oxide or hydroxide the salt being present in an amount to give the dispersion sufficient throwing power to deposit the wax substantially uniformly on the container wall, the waxy material containing an amount of acidic wax below that which gives an unstable dispersion in the presence of said salt of alkaline reaction, and an amount of neutral wax to increase the wax phase of the dispersison, and passing an electric current therethrough to deposit waxy material and insoluble oxide or hydroxide directly on the anode.

9. A process as set forth in claim 1 in which the deposited coating is heated to cause coalescence of said coating to a smooth impervious wax deposit.

10. A process as set forth in claim 1 in which the initial anodic current density is of the order of 25 to 50 amperes per square foot, followed by a subsequent anodic current density of the order of 3 to 8 amperes per square foot.

WILLIAM CLAYTON. ROBERT IAN JOHNSON. CYRIL GORDON SUMNER.