US2175972A - Container and method of making same - Google Patents

Description

Patented Oct. 10, 1939 UNITED STATES PATENT OFFICE William Charles Rainer, Baltimore, Md., assignor to Crown Cork & Seal Company, Inc., Baltimore, Md., a corporation of New York No Drawing. Application May 7, 1937,

- Serial No. 141,376

2 Claims.

This invention relates to containers and particularly to a lining for metal containers or cans. While particularly useful as a container for beer, the can. of the present invention may be used t with ginger ale, carbonated water and carbonated beverages as well as for a wide variety of foods,

drugs and chemicals.

The lining of the present invention is useful with flat top cans as well as cans having a suitable 1c cap receiving finish, e. g., a can having a cone top with a lip or head for receiving the usual crown or crimped type of cap.

It has been consistently a problem in the packing of beer in metal cans as well as the other 15 products mentioned to some extent, to provide a lining thoroughly protective of the contents from chemical reaction with the metal wall of the can. Such lining, in the case of beer, must be insoluble, odorless, resistant to pasteurizing tem- 20 peratures as well as refrigeration temperatures, and particularly, must afford a continuous phase coating over the entire interior of the can. For

' the various other products, the lining must possess physical and chemical characteristics which will make it suitable for the particular purpose.

It should be understood, that in the case of beer as well as other of the products mentioned, chemical reaction between the contents and the metal wall produces such off-flavors, discolorations and contaminations, as render the canned product frequently unsaleable.

It is the aim of the present invention, to provide a continuous protective lining which will eliminate such chemical reaction and, at the same A time, possess the required physical characteristics.

It has been proposed to apply a lacquer or enamel lining, to the interiorv of a can, but such procedure is expensive because of the cost of the 7 material, the equipment and space required, and the heating and time necessary for a successful result, since it is essential to thoroughly bake such a coating.

Also, it has been proposed to use a suitable wax for the purpose, but the presence of the seams or the sealing bead has made it difllcult to form a continuous coating and one free from fracture.

It will be appreciated that not only is there the problem of providing a satisfactory protective coating for the interior of the can, but likewise for commercial production, it is equally important that such coating be applied in an economical manner.

To this end, I have investigated the problem and believe that the tendency of wax to move or draw away from the sharp edge portions of the container,e.g.,the seams and bead and thus cause fractures is due primarily to the conditions of surface tension which exist and to the presence of the sharp edges. That is to say, in the usual waxing operation, after the wax has been applied, there is a difference in surface tension between the wax about the seam for example and the wax on the adjacent body portion of the heated can because of the laminated character of the seam 10 causing it to hold heat for a longer period, and hence, maintain the wax coating at the seam portion relatively fluid for a longer time. Under this condition, the wax at the seam has a lower surface tension and tends to thin out, and by 3 reason of the difference in surface tension between the adjacent wax coated area of the can and the seam and the presence of sharp edge portions, there results a fracture of the wax coating on cooling. This condition takes place also at the sealing head or lip where it appears that the bead portion cools quicker than the adjacent wall of the container by reason of its proximity to the atmosphere in which the can is cooled, with the result that the surface tension of the wax is higher on the bead than on the adjacent area of the container.

I have found a wax coating to be particularly useful for beer as well as other products, and in order to remedy the condition just explained in aneconomical manner which will be suitable for large scale production, have discovered that if a coating of shellac be first applied, a continuous subsequent wax coating may be satisfactorily formed. That is to say, it appears-that the wax has its elastic characteristic so considerably .enhanced by the shellac notwithstanding the tendency of the wax to draw away from the seam or I head, that there does not result any such fracturing of the wax coating heretofore experienced 0 as would enable the contents to contact with the metal wall of the can. The shellac apparently serves to increase the stretchability of the wax so that notwithstanding differences in surface tension, a continuous phase wax coating is provided on the entire interior of the container andforms a substantial protective coating for the seam areas and bead as well as the uninterrupted body area of the container.

Tests conducted with the product of this invention using copper sulphate solution disclosed no indication of exposed metal areas such as would produce the disclorations, contaminations and off-flavors heretofore encountered.

In carrying out the invention, I first prepare a relatively inexpensive solution of shellac in a suitable solvent, preferably alcohol. The present invention is particularly economical in that a solution of as low as about 10% shellac may be satisfactorily used, although higher concentrations may be employed as desired, e. g., up to or These or higher concentrations are not preferred, but rather a concentration below 20% is usually suitable. This solution is sprayed into the finished can and the coated can is then air dried which usually requires about a minute and the product is then ready to be wax coated.

In some cases, dependent upon the concentration of the shellac after the air drying operation, the can is heated for a brief interval, namely, 1 to 3 minutes which has the function of setting the shellac if necessary. The can, while still warm from this heating operation or in a cooled state, may then be given the wax coating which is sprayed into the interior of the can in the same manner as the shellac.

Relative to the wax employed, I use any of those recommended for the purpose and which are commercially available in the market, e. g., Petrowax and Parafiin wax. It is, of course, understood that the composition of these waxes will vary with the particular contents. For example, with beer, a wax is used which will not flow under pasteurizing temperatures or flake off at normal refrigeration temperatures.

In the case of both the shellac and the wax, I preferably use white colored products, and also ones which are free from odor.

After the wax had been sprayed into the can, the can is heated so that excess wax may drain off, following which the heated can is rotated under cooling conditions, e. g., at room temperature, to assure thorough distribution of the wax while cooling the can.

The heating of the can, following by this subsequent cooling, produces a situation where the laminated metal areas constituting the seams retain the heat for a longer period than the single ply can body. As a result, the wax coating on the seam areas is more fiuid and has a-lower surface tension than the coating upon the adjacent cooler can body surface. The presence of the shellac under these conditions serves to prevent the movement of the wax away from the seams in a manner which would cause fracture of the coating. In fact, it appears that the shellac enhances the elasticity of the coating during its change of state from the liquid phase through the more plastic stage to a final substantially solid stage so that the continuity of the coating is preserved.

This same result takes place at the bead where, as explained, it appears that the wax coating on the bead by reason of its proximity to the atmosphere cools quicker and has a higher surface tension than the Wax coating on the adjacent area of the container.'

As will be understood, the presence of a shellac coating formed from a dilute solution assures the production of a continuous phase coating of wax on the interior of the container and notwithstanding the presence of sharp edge portions at the head or the seams, there is no fracturing of the wax coating such as might give rise to objectionable contact of the contents with the metal of the can.

Further, it will be appreciated that the shellac coating is considerably thinner than the wax coating, the shellac being present in amount, however, to overcome the surface tension differential and congealing force of the wax whereby the continuous phase coating is produced.

I have referred herein to the bead and seam or seams as constituting sharp edge portions. By that I mean that the interior wall of the container has surface irregularities constituted by the seam portions or bead which have heretofore caused fractures of the wax coating. The irregularities need not necessarily be of a sharp character, but are of such angularity as to normally produce the fractures which the present invention successfully overcomes.

I claim:

1. A metal container having its interior surface coated with a thin layer of shellac, and a relatively thicker continuous layer of wax on said shellac coating, said shellac layer being present in amount to overcome the surface tension differential and congealing force of the wax and to enhance the elasticity thereof during the change of state of the wax from the liquid phase through the more plastic stage to a final substantially solid stage, whereby the continuity of the wax coating is preserved and the same is free of fracture at the seam portion of the container as well as at the mouth thereof.

2. The method of coating metal containers comprising forming on the interior of the container a relatively thin coating of shellac, drying the shellac and forming over said coating a relatively thick layer of wax, the shellac being present in amount to overcome the surface tension differential and congealing force of the wax and to enhance the elasticity thereof during the change of state of the wax from the liquid phase through the more plastic stage to a final substantially solid stage, whereby the continuity of the wax coating is preserved and the same is free of fracture at the seam portion of the container as well as at the mouth thereof.

WILLIAM C. RAINER.