Method of sealing container seams

[57] ABSTRACT

A method of forming a tight seal of a container end to a container body characterized by applying a heat foamable seam sealing compound, such as a heat foamable plastisol, to an end, seaming the end to the body so that the foamable seam sealing compound will be within the seam and heating the seam to cause foaming of the seam sealing compound within the seam to the constraints of the seam. The improvement in a container double seam of a mechanically undeformed foamed sealing compound, such as a foamed plastisol.

Claims, 6 Drawing Figures apply heat foamable PLASTISOL TO CONTAINER END

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PATENTED NOV? 7 1973

3^774,560

SHEET 1 CF 2

FIG. I

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INVENTOR.

THOMAS J. HARTZ

BY

PATENTED NOV 2 7 1973

3,774,560

SHEET 2 GF 2

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FIG, 6

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INVENTOR:

THOMAS J. HARTZ

BY

3,774,560

METHOD OF SEALING CONTAINER SEAMS

BACKGROUND OF THE INVENTION

This invention relates to the making of metal containers and more particularly to a method for forming an 5 excellent seal in locked or crimped seams such as those which join the various parts of a can, and to an improved seal in such seams.

One of the continuing problems in the container industry is the production of metal containers having 10 tight seam seals on a highly consistent basis. Even a small percentage of “leakers” is intolerable to canners for many obvious reasons, not the least of which is food , preservation. Leaks may occur in the seam formed between a can body and a can end. This seam is often 15 known as the “double seam.” Another seam area prone to leakage is what is commonly known as the side seam.

My invention, as will be described herein, improves the seal in any locked or crimped seam. Examples of such seams are the double seam and the locked side 20 seam. In sealing such seams, the primary need is not for structural strength as might be provided by an adhesive, but for sealing by filling voids within the seam.

In the past many new seaming methods have been developed to mechanically seal can components together. 25 However, the metal sealing itself, no matter how good, is grossly insufficient to effect a liquid-tight and airtight seal. Many sealants or seam sealing compounds of various types, such as solvent base rubber compounds, water base rubber compounds, and various plastisol ³⁰compositions have been used to supplement the mechanical seal formed by the seam. However, although seam sealing compounds have substantially improved the seal within a can seam, “leakers” still occur far too often and quality control is, therefore, a constant prob- ³³lem. Further, many of such seam sealing compounds have noxious solvents therein which can cause flavor problems to an enclosed food product. For example, hexane solvent systems may import a misflavor to an enclosed food product as the solvent evaporates from ⁴θ the seam sealing compound during storage. An important and preferred embodiment of my invention overcomes the sealing problem and avoids completely the use of compounds having noxious solvents.

A major problem in forming a proper seal is antici- ⁴^ pating the irregularities or voids within the mechanical seam and filling such irregularities or voids with a proper sealing compound. To accomplish this on a continuing basis has been extremely difficult or impossible because of numerous factors including varying metal conditions, the condition of the seaming machinery, the slight irregularities in positioning of the metal parts before seaming and even irregular placement of seaming compounds. The use of a seam sealing compound ₅₅does fill many such voids, but extreme voids in the metal seam will not be filled by a normal application of a seam sealing compound. Using a larger application of seam sealing compound will help in some instances, but can result in an unsightly excess of sealing compound ₆θ being extruded from the seam. This also aggravates the flavor problems caused by certain solvents in some sealing compounds.

Often, with certain seam irregularities, a seam sealing compound cannot properly perform its function because it is pre-set in position, for example, on a can end prior to seaming and has limited ability to move to fill voids. During the seaming, the seam sealing compound will make some movement to fill voids simply by virtue of mechanical squeezing. However, this movement is often grossly insufficient to effect a good seal. Further, flow of certain seam sealing compounds during a heating is minimal and in any case is insufficient to significantly improve the seam seal.

• The deformation of certain sealing compounds during the seaming operation may itself be harmful in the case of certain sealing compounds if it displaces compound from beneficial sealing position or breaks seal in any way.

Another factor, the minor swelling of some seam sealing compounds upon contacting the container contents, also gives only minor improvement since it does not occur fast enough or to a sufficient degree. More importantly, the compound must be matched to specific products to cause any swelling whatsoever.

To summarize, the existence of leakers in seams is still a problem within the container industry. Improved seaming methods and seam sealing compounds have failed to satisfactorily alleviate the problem because of continued seam irregularities and severe limitations on what seam voids seam sealing compounds can fill.

DESCRIPTION OF THE INVENTION

The aforementioned problems have been alleviated by providing a heat foamable seam sealing compound within the locked or crimped seams joining the various parts of the metal containers. By applying heat after the heat formable seam sealing compound has been located within the confines of such a seam, the seam sealing compound will expand within the constraints of the seam and completely fill all voids within the seam. Thus, such a seam sealing compound will not be deficient because of failure to fill seam voids. Further, the aforementioned problems may be alleviated without any flavor problem caused by solvents. The highly preferred type of heat foamable seam sealing compounds of this invention are heat foamable plastisols which may be as high as 100 percent solids with no noxious solvents necessary. I have conducted specific tests with such heat foamable plastisols and have obtained both extremely tight and consistent seals and no flavor problems. Another important feature of my invention is that the method of my invention may be practiced with few, if any, changes, and no radical changes, in the typical can making process and apparatus.

A further important advantage resulting from my invention is the increases acceptability of seamed containers having slightly imperfect seams. Heretofore, only very minor imperfections could result in “leakers” which would be rejected upon testing. As a result of my invention, many containers which would have been rejected are now entirely satisfactory. Structural strength is not in question for most containers with imperfect seams. Most seam imperfections result only in voids which cause leakage. Such voids will be filled by the expansion of sealing compound within the seam forming an excellent seal and thus my invention makes acceptable heretofore unacceptable containers. The container will pass inspection and testing and be entirely satisfactory from both a structural and functional standpoint.

The foaming of compound within the seam provides an undeformed foamed sealing compound which avoids the problems which may result from deformation.

3,774.

Accordingly, the primary object of my invention is to provide a method for making metal containers which are devoid of the aforementioned problems.

Another object of my invention is to provide a method for making metal containers having seams with seals of very consistent high reliability.

It is yet another object of my invention to provide a method for sealing the locked or crimped seams of metal containers which provides sealant conforming to the constraints of the seam.

It is still another object of my invention to provide a method for making a sealed seam of high reliability without adding significantly to the apparatus and operations in the typical can making process.

It is yet a further object of my invention to provide a method which solves the aforementioned problems without using a sealing compound which has a noxious solvent which may be harmful to an enclosed food product from a flavor and purity standpoint.

It is yet another object of my invention to provide a method for sealing container seams which results in a drastic reduction of rejects because of seam imperfections.

It is still a further object of my invention to provide an improvement in a double seam of undeformed foamed sealing compound.

These and other important objects of my invention will become apparent from the following description and from the drawings showing preferred embodiments wherein:

FIG. 1 is a partial cross-sectional view of an edge of a container end prior to double seaming thereof to a container body, but after application of a heat foamable seam sealing compound.

FIG. 2 is a cross-sectional view of a double seam showing a heat foamable seam sealing compound therein prior to the curing thereof.

FIG. 3 is a cross-sectional view of the double seam of FIG. 2, but after the curing thereof, showing the filling of seam voids accomplished by foaming of the seam sealing compound.

FIG. 4 is a cross-sectional view of a slightly imperfect double seam showing a heat foamable seam sealing compound therein prior to the curing thereof.

FIG. 5 is a cross-sectional view of the double seam of FIG. 4, but after the curing thereof, showing the filling of seam voids accomplished by the foaming of seam sealing compounds.

FIG. 6 schematically illustrates a preferred embodiment of the method of my invention.

Referring now in detail to the figures and specifically to FIG. 1, a partial cross-sectional view of a container end, identified by reference numeral 11, is shown prior to seaming to a container body. The container is of the type having a body, two ends joined to the top and bottom edges of the body by what is known as double seams of the body edges to the peripheries of the ends. Reference numeral 13 identifies what will become the outside surface of the container end, that is, the topsurface or bottom-surface of a can. Reference numeral 15 identifies the inside surface, that is, the surface which will be in contact with a contained product. That portion of container end 11 to the left of numeral 17, as shown in FIG. 1, will become part of the seam with a container body. Flange portion 19 which will become the cover hook which rests upon body flange 20 of container body 25 and will become the body hook of the

560 container body. A generally normal double seam between a container end and container body is illustrated by the partial cross-sectional view shown in FIG. 2.

In the method of my invention, heat foamable seam 5 sealing compound 21, illustrated in FIG. 1 by the dotted cross-hatched area, is deposited within groove 23 in end 11 prior to the seaming operation. Groove 23 is on the outer circumferential portion of one side of container end 1,1, namely on inside surface 15. Equipment 10 such as that normally used for depositing a seam sealing compound on a container end may be used in the method of my invention.

The selection of a heat foamable compound for use as a sealing compound depends on the container and 15 the purposes for which it will be used. Many common sealing compounds in use today may be made foamable and have application in the method of my invention. In any case, a sealing compound with a blowing agent which will decompose in a medium of fairly high viscos20 ity in order to form small closed cell structures is required. Various sealing compounds may be made heat foamable by the addition of certain blowing agents. Certain of these heat foamable sealing compounds will require a pre-cure operation after application thereof 25 to the container end. The pre-curing operation partially cures the sealing compound and is done at a temperature and for a time insufficient to cause activation of the blowing agent. This step is necessary to avoid incorrect distribution of “wet” sealing compounds because ³θ of excessive flow prior to or during the seaming operation, and, in the instant invention, to provide some finning of the polymeric materials to constrain the gases liberated by the blowing agent. With some heat foamable sealing compounds, such as some solvent 35 base compounds, the pre-curing operation is not necessary as the compound will air dry without the necessity of adding heat. Various heat foamable compounds satisfactory for use as sealing compounds in the method of my invention will be discussed in detail later. These ⁴θ include the highly preferred heat foamable plastisols with which I have worked and which I have found totally satisfactory and very useful in the method of my invention.

If a pre-curing operation is necessary, container end ⁴5 11 with the applied heat foamable compound 21 will be heated for a short period of time and at a temperature sufficient for this purpose but insufficient to activate the blowing agent to cause foaming of compound 21. A specific example will be given hereinafter. Equipment capable of performing the pre-curing operation is well-kriown in the art and may readily be used for this purpose with heat foamable compounds on container ends.

Referring to FIG. 2, a cross-sectional view of the seam 24 formed by container end 11 and a container body 25 is shown. More specifically, container end 11 is fitted onto the end of container body 25 so that flange portion 19 of container end 11 rests outside container body flange 20 (as best shown in FIG. 1). The edge of container end 11 is crimped thus forming a double seam metal configuration widely used in the container industry.

The heat foamable sealing compound 21 which had been applied to container end 11 is now sandwiched within the seam forming members 11 and 25 and is illustrated by the dotted and cross-hatched area. Because of the mechanical manipulation during the seam3,774,560 ing operation sealing compound 21 has been squeezed and rearranged somewhat. Note, however, that sealing compound 21 has only partially filled voids within the seam. Substantial voids, such as those identified by reference numeral 27, remain within the seam.

FIG. 4 is a cross-sectional view of an improperly formed double seam. Such a seam, although quite imperfect, may be satisfactory from a structural standpoint. That is, body hook 29 may have engaged cover hook 31 sufficiently to hold the container end to the container body. However, the extreme voids 27 would very likely produce a “leaker.” The voids within a well formed seam, such as is shown in FIG. 2, and the imperfect seam, as shown in FIG. 4, and the consequent leaks, are the specific problems that the method of my invention overcomes.

In the method of my invention, heat foamable seam sealing compound 21 will expand upon heating to form a solid gel-like structure which will completely fill the voids within seam 24. The heating of the heat foamable compound activates the blowing agent, causing the release of gases such as oxygen, carbon dioxide or nitrogen, depending on the blowing agent, to form closed cells within the compound. The forming of cells increases the volume of the compound and this expansion results in the filling of voids within the seam as such voids are the only available area of expansion. The sealing compound expands to the constraints of the pre-formed seam and thereby eliminates voids and forms an excellent seal of said seam. FIG. 3 illustrates the complete filling of voids within the normal double seam shown in FIG. 2. FIG. 5 illustrates the complete filling of the large voids within the imperfect double seam shown in FIG. 4. Numeral 33 identifies the foamed sealing compound. In FIG. 5, the likelihood of leakage due to imperfect seam configuration is overcome by the sealing effect provided by the foamed sealing compound which has foamed to the constraints of the seam, as illustrated by the dotted and cross-hatched area. Further, it should be noted that the foamed sealing compound in the final product is undeformed. Thus the foam cell structure is completely intact and provides the best possible sealing effect.

My invention may be practiced with any heat foamable compound which will become pliably firm such that it may serve as a sealing compound. Many elastomeric compounds which are widely available are suitable when combined with a blowing agent to cause foaming during a heating process. Polymers, such as plasticized vinyls, with a blowing agent, are also adaptable to my invention.

The highly preferred heat foamable sealing compound of my invention, and the type of compound with which I have performed experiments which indicate successful use in my invention, are heat foamable vinyl plastisols. I have found excellent sealing properties with these heat foamable plastisols. I have also found that heat foamable plastisols may be chosen such that foaming will occur at temperatures already being used for other purposes in the can making process. This will be referred to in more detail hereinafter.

Plastisol compositions consist generally of a dispersion of solid resin particles suspended in a plasticizer. The resin particles may have a protective coating such as soap film to protect the resin from premature combining with the plasticizer. Upon heating the plasticizer and resin combine to form a solid substance. This may be accomplished in a pre-curing step. When combined with a blowing agent, such as sodium bicarbonate, upon further heating the blowing agent will decompose and form closed cells in the plastisol. Provided the viscosity of the plastisol is proper when matched with a blowing agent and a temperature and time for heating, a sponge-like substance having a significantly higher volume than the unfoamed plastisol will be formed. The closed cells of the foamed plastisol maintain the beneficial sealing properties of the substance while the increased volume enables better coverage.

Resinous materials which may be used in this invention include the polyvinyl chlorides such as the commerically available Opalon 410, VR 50 and Geon 121. Many of such commerically available polyvinyl chlorides are mixtures of resins having different characteristics such as varying solvation rates. Such blends can provide favorable foaming characteristics and may be adjusted according to the specific application. Other well known vinyl resins are suitable for use in preparing heat foamable plastisols for use in this invention. These include copolymers of vinyl chloride and vinyl acetate and copolymere of vinyl chloride and ethyl maleate.

Many well known plasticizers may be used in constituting a heat foamable plastisol for use in my invention. Monomeric plasticizers such as dioctyl phthalate, glycollic acid esters, acetyl-tributylcitrate and Harflex 500 are acceptable for making heat foamable plastisols. Polymeric plasticizers such as the “Plastolein” plasticizers, which are primarily composed of esters and polyesters of azelaic and pelargonic acids, and polymeric plasticizers such as Harflex 300 and Harflex 330 may be used. When a food product is to be enclosed within a container sealed by the method of my invention is used, it is highly desirable to use plasticizers or mixtures of plasticizers that are not readily subject to extraction by the food or beverage and which are nontoxic.

Suitable blowing agents for use in my invention to cause the foaming of seam sealing compounds, including the preferred plastisols, may be any gas releasing compound which will readily be combined with the sealing compound and which will release a gas during heating to cause foaming of the sealing compound. Oxygen releasing blowing agents, carbon dioxide releasing blowing agents and nitrogen releasing blowing agents are well known for this purpose and may be used in my invention. Suitable oxygen releasing blowing agents include the alkali metal perborates, such as sodium perborate, with an acidic component. The acidic component may be, for example, the acid salts of polybasic inorganic acids such as alkali metal hydrogen phosphates, alkali earth metal hydrogen phosphates and metal salts of higher molecular weight organic carboxylic acids and weaker bases such as calcium stearate, aluminum stearate, zinc stearate, magnesium stearate and the like. Carbon dioxide releasing blowing agents include sodium bicarbonate, sodium bicarbonate with calcium oxide and azodicarbonamide. Nitrogen releasing blowing agents which may be used in my invention include various types of azobisformamide, such as those known as Kempore 60, Kempore 125 and Kempore 200.

In addition to the main ingredients in the preferred heat foamable plastisols for use with my invention, there may be added absorbing agents such as the oxygen absorbing agents anhydrous aluminum oxide, silica

3,774,560 gel, and activated carbon. Such absorbing agents may be used to control the rate of foaming. Other ingredients may be used to impart various characteristics to the sealing compounds, Fillers may be used to help maintain the foamed structure after the foaming step. A coloring agent might be useful in some cases, at least insofar as the seam sealing plastisol of this invention were to expand to escape the confines of the seam such that they would be visible.

An example of a heat foamable plastisol which may be used, and which I have used in my invention, has the following composition: 25-35% dioctyl phthalate for a plasticizer, 65-75% vinyl resin and 1% Kempore 200.

Such a foamable plastisol composition is applied to a container and within the groove as shown in FIG. 1. The end is then heated to a temperature of approximately 125° to 200°F. for a short period of time, for example, approximately two minutes to pre-cure the heat foamable plastisol. This process is at a temperature insufficient to cause the foaming of the plastisol but is a highly desirable step since it reduces or eliminates the problems of flow of the sealing compound prior to and during seaming to a container body and provides crosslinking of the polymeric materials to constrain the gases liberated by the blowing agent during the further heating.

The container end with the applied plastisol composition referred to above is then seamed to the container body. The plastisol within the seam then cured by baking the container at a temperature of approximately 300°-450°F. for one to ten minutes, which causes the release of gas within the plastisol and the foaming thereof. The foaming causes the filling of voids within the seam to form an excellent seal within the seam.

The baking, or curing, temperature depends upon the specific composition used. However, there may be limitations due to the metal used in the container. For example, aluminum cannot withstand the higher temperatures while steel can.

In the making of cans, a coating composition is often applied to the inside surface of the container after the double seaming of one end to the body. This composition, which may be a vinyl composition, is often sprayed on the inside walls. To cure, or dry, the coating composition, it is necessary to heat the container to a fairly hot temperature. When such an inside coating composition is used and a heating step is required, my invention may be practiced without adding additional steps to the can making process. This is done by simultaneously causing the foaming of the plastisol within the seam and the curing of the coating composition. The heat foamable sealing compound, such as the preferred heat foamable plastisols, may be designed to provide proper foaming at the temperatures required for the curing or drying of the inside coating composition. For example, in the case of the preferred plastisols, a particular blowing agent may be matched with a resin and plasticizer having the proper viscosity and other characteristics such that the two operations occur simultaneously in a satisfactory manner. One specific example of this process which was performed used the foamable plastisol composition described above. After application to a container end, the plastisol was pre-cured at a temperature of 200°F. for two minutes. The end was double seamed to a container body and the inside was sprayed with a solution vinyl coating. The can was then baked at 375°F. for 5 min8 utes. The plastisol composition foamed within the seam filling the voids therein while the vinyl coating was cured.

Although it is highly preferred to cause the foaming of the sealing compound by baking of the whole container, it is not necessary to heat the whole container to cause proper foaming within the seam. Thus, an alternative method would be by applying heat directly to the seam. This could be done by a flame of intense heat directed at the seam for a short period of time or could be done by electrical means. Anyway in which the heat foamable sealing compound is heated within the seam would provide satisfactory results.

The method of my invention may be used in the manufacture of a wide variety of metal containers. The invention is applicable to any locked or crimped seam as indicated above. The experiments which I have performed indicate that seams with the improvement of my invention are consistently well sealed.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.