IL25716A - Container closures - Google Patents

Description

COHAI ER CLOSURES The present invention relates to a process for applying sealing membranes to containers , an to a device for carrying out the process.

It is known that products such as dried milk powders and instant coffee powders, when placed in containers, require to be sealed by means of a membrane that prevents the ingress of both moisture and gases and that also has tamper-proof properties. The conventional method of effecting this seal is to fit the cl'osure caps with waxed pulpboard pads or wads to which are lightly heat-sealed double glassine membranes. In the course of application of the closure caps, the sealing surfaces or finishes (i,o» the mouths) of the containers are first made adhesive by passing them under glue-coated rollers* Gaps are then applied to the containers, and after the adhesive has had time to dry out, the caps are unscrewed and the membranes part from the waxed pads and remain adhered to the finishes of the containers* In those cases where metal foil membranes are provided on metal cans or tins, the membranes may be crimped in place.

It is, of course, essential that the membranes are intact when the closure caps are removed from the containers, thus providing evidence that the contents of the containers have not been tampered with. Membranes applied by means of adhesively coating the container finish possess the dis^ advantage that the adhesive takes some time to dry and the resultant water vapour is absorbed by the contents of the container. It is also difficult for the efficiency of the operation to be inspected at the end of the packing line.

This disadvantage has been overcome to a certain extent by a process which applies a flat disc of adhesive-backed metal foil cold to the finish of the container by the impact of a punch to attach it lightly, subjects it to the action of a heated pressure member to cause it to adhere to the glass, and then completes the capsuling by pressure alone applied by a resiliently mounted pressing member. This process also has its disadvantages, however, since it has to be carried out in three stages with three different pressing members, with the result that the sealing efficiency is impaired due to the heat not being applied at the same time as the resilient pressing member.

The crimping of metal foils to metallic tins or cans also has disadvantages, for instance filling disadvantages and the fact that the seal is not perfectly hermetic.

These disadvantages of sealing membranes removably to containers have been great enough for a suggestion to have been made of completely fusing a thermoplastic membrane to a container of thermoplastic material, this being effected by an inductive heating method. Such is far too drastic a sealing method for most purposes, since the seal cannot be broken without damaging or distorting the container neck, and this makes re-sealing or re-use difficult or impossible.

It is the object of the present invention to provide a method for applying membranes, both metallic and non-metallic, to containers, also metallic or non-metallic, which will give a hermetic gas-tight seal, which may be carried out in a single stage, which, in one embodiment, may be accompanied by the application of a closure cap, and which may lend itself to the highest closuring speeds attainable. Such membranes are to be readiljr removable without damaging or distorting the container.

According to the present invention a process for applying a sealing membrane to a container, wherein either the membrane or the container or both is or are of metal, comprises placing the membrane under sealing pressure onto the container finish (i.e. the mouth) and heating the metal mouth; of the membrane or container (SSii&k by means of an induced radio frequency current to a temperature sufficient to soften a thermoplastic material coated on or present in the material of the membrane, or coated on the material mouth I of the container (SSSSa., whereby the membrane is removably adhered to the container without distortion of the container.

By means of the process of the invention, metallic or non-metallic membranes may be applied to metallic or non-metallic containers, provided that either the membrane or the container in each instance is metallic. i'or instance, the process of the invention may be used to apply a metal membrane to a metal container, a glass container, a ceramic container, or a synthetic plastic container.

When the container is of metal and the membrane applied thereto is of metal, either the membrane or the finish of the container may have the coating of a thermoplastic material. When the membrane and the finish of the container are heated by the induced radio frequency current, the thermoplastic material softens sufficiently to adhere the membrane to the container.

When the metal membrane is being applied to a glass or synthetic plastic container, again either the membrane or the finish of the container may have the coating of a thermoplastic material. In the case of a synthetic plastic container, to avoid distortion of the container the thermoplastic coating must have a melting point lower than the material of the container.

By means of the invention also membranes of non-metallic material may be applied to metal containers. For instance, a paper membrane may be applied to a metal container by applying to the membrane or to the finish of the container the thermoplastic heat sealing coating. As before, this coating softens sufficiently to adhere the membrane to the container when the container finish is heated by the induced radio frequency current. Again, a thermoplastic membrane may be applied to a metal container, and here it is not absolutely necessary, although it may be sometimes desirable, to use a coating of thermoplastic adhesive.

Normally containers provided with sealing membranes are also supplied with closure caps, although this is not always the case. The closure caps may also be provided with resilient wads such as pulp board or composition cork wads. Basically there are two types of closure cap for use with such containers as the present invention involves, and these may be classified broadly as the "snap-on" type and the "screw-on" type. The screw-on type may have a continuous thread for engagement with a corresponding thread on the container neck, or it may be provided with a series of lugs which engage a rim on the container neck or co-operate with a multi-start thread formed on the container neck. The screw-on type caps may be formed from metal, synthetic thermosetting materials such as urea-formaldehyde resins, and synthetic thermoplastic materials such as polystyrene, high density polythene and polypropylene.

The snap-on type caps are normally formed from metal or flexible plastic material, but can be formed of paper.

When a screw-on plastic closure cap is used, the cap_ may be fitted with a resilient wad and the metal foil membrane. The cap may then be screwed on to the container to develop sealing pressure on the membrane, after which the radio frequency heating current will be induced into the foil membrane. It is possible for the resilient wad and the metal foil membrane to be formed as a unitary article. For instance a sheet of wad material, e.g. composition cork, may be stuck to a sheet of metal foil, and then discs may be cut from the laminate so formed. It is important, however, that the bond between the wad and the foil membrane should be less than that between the membrane and the container, so that on removing the closure cap, the wad parts from the membrane and may be used to reseal the container when the membran*e has been discarded. It is to be noted that if a resilient wad is not used the foil membrane should be provided with a backing of paper or the like so that the membrane does not adhere to the inside of the closure cap.

When a snap-on plastic or paper closure cap is used, a particular advantage of the invention is that the cap, a wad, for example of pulpboard, and the membrane may all be applied to the container at the same time.

In the case of metal caps, both of the screw-on type and the snap-on type, neither of the above procedures may be adopted, since a metal cap would immediately absorb all the radio frequency energy, become heated in the process, and act as an effective shield to the membrane or the rim of the container, one of which must be metallic. Thus, if it is required to use a metallic cap, this must be applied after application of the sealing membrane.

As mentioned, the sealing membrane may be metallic or non-metallic. It may be of any metallic foil, but we have found that aluminium foil is the most economical, and tin foil also makes a very good membrane. Another example is a tin-antimony foil. Before selecting the metal for the membrane consideration should, of course, be had to the contents of the container to be sea-led. The metal of the membrane must be inert with respect to the contents of the container. The thickness of the foil membrane should be as thin as possible consistent with it being free from pin holes and readily handleable. Normally this will be between 0.025 and 0.05 millimetres. Non-metallic membranes include, inter alia, those made from paper, glassine, polythene, polypropylene, polystyrene, polyvinylchloride, styrene copolymers, cellulose acetate, polycarbonates, and phen-oxy resins.

When it is required not to use a separate closure cap, it being sufficient for the container to be closed by the membrane only, the membrane may have the form of a disc, to seat on the container finish, with a continuous flange on the periphery of the disc for engagement with the container below its finish.

The thermoplastic coating may be, for example, of a vinylite resin, a natural or synthetic wax, or a micro-crystalline wax mixture. It may also be of polythene or polypropylene, or one of the many heat sealing coatings which are readily available, and which consist substantially of mixtures of plastic materials and resins. Examples include those which are sold under the trade names "Telstic',' "Delseal", "Imseal", "Thermostik" and "Elvax". The thermoplastic material may be coated on the whole of the underside of the membrane, or it may be coated in the form of an annular ring on the periphery thereof. The former is preferred when the membranes are stamped in large numbers from sheet material, since the sheet material may suitably' be coated beforehand.

Plastic containers onto which membranes may be sealed by means of the present invention include those made from polythene, polyvinylchloride , polystyrene, styrene copolymers for example acrylo-nitrile-butadiene-styrene copolymers, polypropylene, cellulose acetate, polycarbonates and phenoxy resins such as high molecular weight polyhydroethers.

Since the sealing surfaces of containers are rarely uniform enough to ensure even adhesion over the whole of the rim of the container, a suitable tool for use in applying sealing pressure to the membrane, and to the closure cap and pulpboard wad when such are usedF includes a pad of resilient insulating material. This may conveniently be in the form of an annulus corresponding to the sealing surface of the container. The tool is made of electrically insulating material, and suitably it is this tool which is provided with the source of radio frequency current „ This may, for instance, be a coil or series of coils surrounding the sealing head of the tool, the coils being water cooled so that they do not heat up to any appreciable extent. No separate pressing tool is required when sealing pressure is provided by a plastic screw-on cap. The resilient sealing pressure, which is most important if the membrane is to bed down properly on the container finish, is there provide by the resilient wad in combination with the screw cap The insulating material of which the sealing head is made may be that sold under the trade mark "Tufnol" . The resilient pad may be of any insulating material which has resilient properties, for instance natural and synthetic rubbers, foamed and expanded rubbers, polyurethanes and other plastic materials, natural cork and composition cork.

The invention is illustrated by means of the accompanying drawing, which is an exploded, part sectional elevation of an apparatus for applying, simultaneously, a membrane, a pulp-board wad, and a closure cap to a container.

Referring to the drawing, there -is shown a glass jar 1 to which is to be applied a sealing membrane of aluminium foil 2 having a coating of thermoplastic adhesive on its underside J, a pulpboard wad 4, and a snap-on closure cap 5? The vertically reciprocable pressing tool, designated generally 6, comprises a chuck of insulating material 7? a resilient rubber ring or annulus 8, and water-cooled radio frequency heating coils 9 surrounding the chuck 7» The apparatus also comprises an air press 10 and a pressure gauge 11, In practice, the membrane 2 , the pulpboard wad 4 and the closure cap 5 are pressed against the container finish by means of the tool 6, whereafter the radio frequency current is switched on immediately the tool has attained its prescribed capping pressure, The aluminium foil is heated fiwFmeans of the radio frequency eddy currents induced in it, and this causes the thermoplsstic coating to become soft and thus to cause adhesion. The heating time is normally of very short duration, generally considerably less than 1 second. We have found a frequency of 20 megacycles/sec and a power source of about l-g- Kw to be very satisfactory.

It should be noted that the heat sealing operation described herein may be carried out by a simple manually-controlled machine, or it can be effected by incorporating extra machinery into a single-or multi-head semi- or fully' automatic capping machine. For instance, a plurality of capping heads such as that illustrated may be used, or a single capping head mounted on an intermittently rotating table. Again, especially when plastic screw-on closure caps are applied to the containers, the containers may be fitted with their membranes and screw-on caps and passed, a conveyor belt, through a continuous induced radio frequency field. This last method permits very high speeds to be attained.

Claims (1)

1. 25716/2/3 Claims 1 · A process for applying a sealing membrane to a container* wherein either the membrane or the container or both is or are of metal, which comprises plaoing the membrane under sealing pressure onto the container mouth and heating the metal of the membrane or container mouth by means of an induced radio frequency current to a temperature sufficient to soften a thermoplastic material coated on or present in the material of the membrane* o coated on the material of the container mouth* whereby the membrane is removably adhered to the container without distortion of the container* 2. A process as claimed in claim 1 wherein a metal membrane is applied to a non-metallic container, the membrane having a coating of thermoplastic material. 3» A process as claimed in claim 1 wherein a metal membrane is applied to a glass container, the membrane having a coating of thermoplastic material* It* A process as claimed in claim 1 wherein a metal membrane is applied to a container made from thermoplastic synthetic material, the membrane having a coating of thermoplastic material. 5. A process as claimed in claim wherein a paper membrane is applied to a metal container, mouthy the membrane or the/3¾5ek of the container being supplied with a coating of thermoplastic material. 25716/2 6· A process as claimed in claim 1 wherein a membrane of synthetic thermoplastic material is applied to a metal container. 7· A process as claimed In claim 6 wherein either the membrane or the container Is supplied with a coating of thermoplastic material. 8· A process as claimed in any of claims 1 to 7 wherein the sealing membrane is in the shape of a closure cap, 9· A process as claimed in any of claims 1 to 7 wherein a non-metallic snap-on closure cap is applied to the container simultaneously with the membrane· 10· A process as claimed In claim 9 wherei a wad of resilient material Is also applied to the container Intermediate the membrane and the closure cap* 11· A process as claimed in any of claims 1 to 7 wherein a screw-on cap is fitted with a wad of resilient material to which is lightly attached the membrane* the eap is screwed onto the container, and thereafter the radio frequency current is switched οη· 12. An apparatus for applying a sealing membrane to a containe In accordance with any of the previous claims comprising a pressing head of insulating material surrounded by at least one coil for the supply of radio frequency current· 25716/2 13· An apparatus as claimed in claim 12 wherein the head is vertically reeiprocable. 1 An apparatus for applying a sealing membrane to a container substantially as hereinbefore described with reference to the accompanying drawing* 15· A container, whenever sealed by a membrane in accordance with the process claimed in any of elaims 1 to 11 and/or using an apparatus as claimed in any of claims 1 to tlw