Bonding Propylene Polymers To Metal Surfaces

Abstract

This disclosure relates to bonding a propylene polymer article, such as a removable closure member for a container, to a metal surface, such as the metal end portion of the container, which contains an opening for removal of the contents of the container. The metal surface is first coated with a thermosetting enamel coating composition. Next there is applied to the enamel coated surface a heat activatable adhesion promoting layer containing a carboxyl modified polypropylene resin having a particle size of less than 5 microns. The propylene polymer article is heat sealed to the carboxyl modified polypropylene containing coated surface and the resultant assembly is then immediately cooled to ambient temperature.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to bonding propylene polymer articles to metal surfaces, and more particularly to bonding closure members fabricated from propylene polymers to the surfaces of metal containers.

2. The Prior Art

Easy opening containers are known to the art. These containers are generally formed of metal and are provided with a pour opening. The pour opening generally occupies only a portion of the end panel of the container. Heretofore, the pour opening has generally been formed by scoring to define a tear strip. A pull tab is attached to the tear strip, and upon the application of a force, the pull tab is operative to separate the tear strip along the weakening line from the panel.

Although easy opening containers have been readily accepted by the public, deficiencies still remain in this type of container. One of these deficiencies is that the removable tear strip which is torn from the can end in the opening of the can has sharp edges, and when thrown on the ground or otherwise improperly disposed of, remains as a nuisance on which an individual may be readily cut.

It has been proposed to replace the metal tear strip with a plastic closure member which will eliminate the cutting hazard in that the removed portion does not have sharp edges on which a person may be cut. The plastic closure member fabricated from a thermoplastic resin, such as polypropylene, is detachably and sealably secured to the outer periphery of the pour opening in the end panel by bonding with a suitable adhesive. Container end panels used for beverages and the like have been coated with a heat activatable adhesive layer to provide a surface for bonding a plastic closure member to the container end panel to seal the pour opening. In this manner, the plastic closure member is heat sealed to and fixed about the periphery of the opening until a pulling force is applied thereto.

Although sealing the pour opening of a container with a plastic closure of this type has been proven feasible, the rigid standards of the food industry require that the sealing bond between the plastic closure and the end panel be greater than has heretofore been achieved.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method of improving the bond strength of propylene polymers heat sealed to metal surfaces which comprises first coating the metal surface with a thermosetting enamel coating, followed by coating the enameled surface with heat activatable, adhesion promoting layer containing a carboxyl modified polypropylene resin having a particle size of less than 5 microns, to which is heat sealed the propylene polymer. The heat sealed surface is then immediately cooled to ambient temperature.

As will hereinafter be illustrated, the combination of the use of a carboxyl modified polypropylene resin having a particle size of less than 5 microns and the immediate cooling of the metal surface after heat sealing the propylene polymer layer thereto causes an unexpected and substantial increase in the bond strength of the resultant assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plane view of a container having an easy opening structure provided with a propylene polymer closure member.

FIG. 2 is an enlarged cross-sectional view taken generally along the lines 2-2 of FIG. 1 showing the propylene polymer closure member secured to the end panel surface in accordance with the present invention.

PREFERRED EMBODIMENTS

In the process of the present invention, any of the thermosetting resins or mixtures of thermosetting resins conventionally used in the coating art may be employed for enamel coating the metal substrate.

Typical thermosetting resins which may be employed as enamel coatings include epoxy resins of the type which are polymeric reaction products of polyfunctional halohydrins with polyhydric phenols having the structural formula:

wherein x represents the number of molecules condensed. Typical polyfunctional halohydrins are epichlorohydrin, glycerol dichlorohydrin and the like. Typical polyhydric phenols are resorcinol and a 2,2-bis(4-hydroxyphenyl)alkane, the latter resulting from the condensation of phenols with aldehydes and ketones, including formaldehyde, acetaldehyde, propionaldehyde, acetone, methyl ethyl ketone and the like, which result in such compounds as 2,2-bis(4-hydroxyphenyl)propane and the like compounds. These epoxy resins normally contain terminal epoxy groups but may contain terminal epoxy groups and terminal hydroxyl groups.

In place of or in admixture with the epoxy resins, any of the well-known class of heat-hardenable phenolic resins produced by condensing a phenolic compound with an aldehydic compound may be employed as thermosetting enamel coating materials.

Exemplary phenols suitable for the preparation of phenol/formaldehyde resins include phenol itself, the ortho, para and metacresols, the xylenols, the dihydroxy benzenes, such as resorcinol, and polynuclear phenols, such as the naphthols, and the various alkylated, aralkylated, carboxylated, alkylolated, etc., derivatives of these types, such as o-ethyl phenol, carvacrol, salicylic acid and the like.

Formaldehyde is the aldehydic compound preferred for condensation with the phenolic compound, but in general, any methylene-containing agent, such as formaldehyde, paraformaldehyde, hexamethylene tetramine, acetaldehyde, and the like may be used.

Amine-aldehyde resins may also be employed in preparing thermosetting enamel coatings, either separately or in combination with epoxy and/or phenolic resins.

The term "amine-aldehyde" resin includes aldehyde condensation products of melamine, urea, aceto-guanamine, or a similar compound. Generally, the aldehyde employed is formaldehyde, although useful products can be made from other aldehydes, such as acetaldehyde, crotonaldehyde, acrolein, benzaldehyde, furfural, and others. Condensation products of melamine or urea are the commonly employed amine-aldehyde resins.

For application to metal surfaces, the above-described resins or mixtures of these resins are dissolved in suitable solvent systems, such as organic ketones, such as methyl ethyl ketone, methyl isobutyl ketone, isophorone, cyclohexanone, and aromatic hydrocarbons, such as xylene and toluene, and mixtures thereof, to provide a coating solution of the necessary viscosity for application to the metal surfaces.

The thermosetting enamel coating compositions are applied as a liquid solution to the metal surface by any of the conventional methods employed by the coating industry. In the coating of metal sheet used in container fabrication, roller coating is a preferred method as the deposited coating weight is easily and conveniently applied in a single coat. For general coating purposes, spraying, dipping, and flow coating are useful methods of application.

The applied coating after substantial volatile loss of solvent is cured to a hard film by heating the coated substrate at a temperature between about 150.degree. and about 250.degree.C for about 1 to 10 minutes. The preferred coating weight for use as an enamel for containers is in the range of about 1 to 5 milligrams of dry coating per square inch of substrate surface.

The metal sheet stock, coated with the hardened enamel coating, in accordance with the present invention, is coated with an adhesion promoting layer containing a carboxyl modified polypropylene resin.

The carboxyl modified polypropylene which is utilized in the practice of the present invention is prepared by grafting an unsaturated dicarboxylic acid or anhydride onto a polypropylene backbone. Unsaturated dicarboxylic acids or anhydrides which can be employed to prepare the carboxyl modified polypropylene resins of the present invention include maleic, tetrahydrophthalic acid, fumaric acid, itaconic, nadic, methyl nadic and their anhydrides.

The amount of unsaturated dicarboxylic acid or anhydride which can be grafted onto the polypropylene ranges from about 0.05 to about 10 percent by weight based on the total weight of the grafted polymer. It has been found, however, that only small amounts of the acid or anhydride are required to impart adhesion promoting properties to the polymer and generally, the amount of grafted dicarboxylic acid or anhydride ranges from about 0.5 to about 5.0 percent.

The modified polymers can be prepared by reacting the unsaturated dicarboxylic acid, or anhydride such as maleic anhydride, with a polypropylene containing active sites which are capable of anchoring the dicarboxylic acid or anhydride thereon. Active centers at which anchoring will occur can readily be induced on the polypropylene in known ways, as for example, by subjecting the polypropylene to the action of high energy ionizing radiations or by contacting the polymer, either as a solid or in solution in a solvent, with a free radical producing material, such as dibenzoyl peroxide, dilauroyl peroxide, dicumyl peroxide, t-butyl perbenzoate and the like. Preferably, the carboxyl modified polypropylene resin is prepared by reacting maleic anhydride with a solution of the polypropylene in an organic solvent containing a free radical producing material, such method being described in Belgian Pat. No. 607,269. The polypropylene resin used as a starting material for the preparation of the carboxyl modified polymers can either be the amorphous polymer, otherwise known as atactic polypropylene, or a crystalline polymer, otherwise known as a syndiotactic or isotactic polypropylene. In the case of maleic anhydride modified polypropylene, amorphous polypropylene is generally preferred as the starting material as the carboxyl modified product is soluble at ordinary temperatures in a variety of common organic solvents and thus, the application of the modified polymer by ordinary solution techniques is possible. Maleic anhydride modified crystalline polypropylene is not soluble in common solvents, except at relatively high temperatures, and therefore is generally applied as a dispersion following which the coating is baked to effect fusion.

It is a critical and essential feature of the present invention that the particle size of the adhesion promoting carboxyl modified polypropylene layer applied to the enamel metal surface be less than 5 microns and preferably be in a particle size range of 0.1 to 4 microns. As will hereinafter be illustrated, when the particle size of carboxyl modified polypropylene is substantially above 5 microns, the bond strength between a propylene polymer bonded to a carboxyl modified polypropylene coated surface is substantially reduced.

The adhesion promoting carboxyl modified polypropylene resin is generally applied to the enamel coated metal article surface as a dispersion in a high-boiling volatile organic solvent, such as kerosene, in order to achieve satisfactory coalescence of the resin particles. Preferably, the carboxylated polypropylene is applied to the enamel coated substrate in admixture with a polypropylene resin having approximately the same particle size, i.e., 0.1 to 5 microns. The incorporation of the polypropylene resin in the dispersion improves the flow and viscosity characteristics of the dispersion required in commercial coating methods. Generally, the polypropylene resin is incorporated in the dispersion at a weight ratio to the carboxyl modified polypropylene resin in a range from about 85:15 to 99:1.

The dispersion containing the carboxyl modified polypropylene resin can be satisfactorily applied at a solids content ranging from about 15 percent to about 25 percent by weight, based on the total weight of the dispersion composition. Generally, a solids content of 18 percent to about 23 percent by weight is preferred.

The dispersion containing the adhesion promoting carboxyl modified polypropylene composition can be satisfactorily applied by any of the conventional methods employed by the coating industry. However, for coating of enameled sheet metal used in container fabrication, gravure or direct roller coating are preferred methods, as the desired coating weight is easily and conveniently applied in a single coat. Spraying, dipping and flow coating are also useful methods of applying the coating dispersion.

After applying the dispersion, the solvent is volatilized by heating the coated substrate. Generally, the substrate is heated at a temperature of about 175.degree. to about 200.degree. C. for about 2 to about 6 minutes to volatilize the solvent.

In the case of propylene polymer closure members, to provide enamel coated metal sheet substrates with an adequately adhesion promoting coating containing the carboxyl modified polypropylene resin, the dry film weight of the bond promoting coating should be in the range of 1.5 to 3.5 milligrams of dry coating per square inch of enamel coated surface.

Propylene polymers which are bonded to the carboxyl modified polypropylene coated metal surface in accordance with the practice of the present invention include polypropylene, and propylene/ethylene copolymers containing about 1 percent to 10 percent ethylene.

The propylene polymer is bonded to the carboxyl modified polypropylene coated metal surface by heat sealing at a temperature of about 175.degree. to about 225.degree. C., and preferably at a temperature of 180.degree. to 200.degree. C. Heat sealing may be accomplished by any means known to the art, such as heated bars or wires, induction and RF heating.

After the propylene polymer is heat sealed and bonded to the carboxyl modified polypropylene coated metal surface, the assembly is immediately and rapidly cooled or chilled to ambient temperature, as, for example, room temperature, in any suitable manner, as by air cooling, flooding with water, plunging into a water bath, or by any other appropriate procedure that does not disturb the applied propylene polymer.

Referring now to the drawings, and in particular to FIG. 1, there is shown a container top end assembly 10 of a container. The end assembly 10 is made of metal such as tin plate, tin-free steel or aluminum. The end assembly 10 comprises a central panel 12 having a bead formed adjacent the outer periphery from which there depends a peripheral flange 13. The peripheral flange 13 is curled and double seamed with an outwardly flap flange at the upper end of a container body in the usual manner.

The panel 12 in the embodiment is shown with a pour opening 14 through which the contents of the container are poured.

The opening 14 may be formed by blanking or the like. It is to be understood that the opening 14 may assume any configuration, and is not limited to the conventional tear drop design shown.

The top surface of the end assembly 10 is coated with a first layer of a thermosetting enamel coating, such as an epoxy-urea/formaldehyde resin 15. To the thermosetting enamel coating is applied a second layer of an adhesion promoting layer 16 containing a carboxyl modified polypropylene resin, such as a polypropylene/maleic anhydride graft copolymer.

Detachably and sealably secured to the periphery of the opening 14 is propylene polymer closure member 17. The propylene polymer closure member 17 has a sealing flap 18 for closing the opening 14. Integrally extending from the sealing flap 18 is a pull ring portion 19. The sealing flap 18 of closure member 17 is heat sealed to the coated periphery of the opening 14 at a temperature of 180.degree. to 200.degree. C.

The bond that is formed permits the closure member or flap to be separated by the application of a pulling force at the ring portion 19. Preferably, the pull ring portion 19 is formed with an opening sized to receive the finger of a user. The heat to achieve bonding is preferably applied by RF heating of the metal surface. RF heating is used so as to localize the heat in the peripheral edge portion of the opening 14, thereby to minimize the buckling of the relatively thin metal from which the end 10 is made. In this manner, the sealing closure member 17 is firmly fixed about the peripheral portion of the opening 14 and remains adhered thereto.

After the closure member 17 is heat sealed to the peripheral portion of the opening, the end assembly 10 is rapidly cooled to room temperature.

To illustrate the manner in which the present invention may be carried out, the following example is given. It is to be understood, however, that the example is for the purpose of illustration, and the invention is not to be regarded as limited to any of the specific materials or conditions recited therein.

EXAMPLE

A solution of a mixture consisting of 85 percent by weight of an epoxy resin EPON 1009, available from the Shell Chemical Company, and 15 percent of a urea/formaldehyde resin BEETLE 216-18, available from the American Cyanamid Company, in a volatile solvent was applied to the surface of a chrome plated steel sheet by means of a roller to deposit an enamel coating at a coating weight of 3.5 mg/in.sup.2 of steel surface. The coating was heated for 8 minutes at 213.degree. C. to remove the solvent and to cure the epoxy resin-urea/formaldehyde resin mixture to a hard enamel film.

A bond-promoting coating formulation was prepared using a carboxyl modified polypropylene resin admixed with a finely divided unmodified polypropylene resin at a weight ratio of 5:95. The carboxyl modified polypropylene resin was HERCOPRIME PA-672 available from Hercules, Inc., which is a maleic anhydride modified polypropylene having an inherent viscosity of about 1.7, a carboxyl content of 0.6 percent to 1 percent, and a particle size range of 0.1 to 3 microns. The polypropylene resin was HERCOTUF PB-681, available from Hercules, Inc., having a melt index of 6 and a particle size range of from 0.1 to 2 microns. A 20 percent solids dispersion of the carboxyl modified polypropylene-unmodified polypropylene resin mixture in kerosene was applied to the enamel coated surface of the steel.

After application of the carboxyl modified polypropylene-unmodified polypropylene dispersion, the coated sheet was baked at 180.degree. C. to volatilize the solvent, fuse and coalesce the dispersed particles into a continuous film. A 1inch .times.1 inch .times.0.050 inch strip molded from SHELL V521, a propylene/ethylene copolymer having an ethylene content of 4.0 percent, a melt index (D1238) of 0.55, and a density of 0.895 to 0.905 (D1505) was inserted between the carboxyl modified polypropylene coated sides of two 1 inch .times.1inch .times.0.006 inch strips cut from the coated metal sheet. The assembly was then laminated by heat sealing at a temperature of 240.degree. C. for a 3.5 second dwell time at 20 p.s.i.

Immediately after heat sealing, the laminated assembly was chill quenched with water to cool the assembly to room temperature.

The above procedure was repeated using a laminated assembly wherein the intermediate layer was fabricated from DIAMOND 8620, a polypropylene resin having a melt index of 4.0 and a density of 0.905, available from the Diamond Shamrock Chemical Company.

The peel strength of the cooled assembly was then determined. The peel strength of the assembly, i.e., a measure of the load required to peel apart the laminate assembly was determined by pulling the outer steel layers of the assembly apart at an angle of 180.degree. to the bond in an Instron machine at a rate of 0.5 inches per minute.

For purposes of contrast, the above procedure was repeated with the exception that in separate runs, the heat sealed assembly was not chilled, or a carboxyl modified polypropylene having a particle size greater than 5 microns was used.

The peel strengths of the assemblies are summarized in the table below. ##SPC1##

By referring to the table, it is at once apparent that the peel strengths of propylene polymers bonded to metal surfaces coated with an adhesion promoting layer containing a carboxyl modified polypropylene resin in accordance with the present invention (Run Nos. 1 and 4) are substantially greater than the peel strengths over the contrasting or control runs (Run Nos. 2, 3, 5 and 6) in which the heat sealed assembly was not cooled immediately after heat sealing or a carboxyl modified polypropylene having a particle size greater than 5 microns was used.

Claims

What is claimed is:

1. A method of preparing a metal article having a propylene polymer article adhered to the surface thereof which comprises the sequential steps of:

a. applying an enamel coating composition to the surface of the metal;

b. applying to the enamel coated metal surface a heat activatable thermoplastic bond promoting layer containing a carboxyl modified polypropylene resin, said resin being the graft copolymer of polypropylene and an unsaturated dicarboxylic acid or anhydride having a particle size less than 5 microns;

c. heat sealing to the bond promoting layer, a layer of a propylene polymer; and then

d. rapidly cooling the heat sealed assembly to ambient temperature immediately after heat sealing.

2. The process of claim 1 wherein the carboxyl modified polypropylene resin contains about 0.05 percent to about 5 percent carboxyl groups.

3. The process of claim 1 wherein the unsaturated anhydride is maleic anhydride.

4. The process of claim 1 wherein the carboxyl modified polypropylene resin has a particle size ranging from 0.1 to 3 microns.

5. The process of claim 1 wherein the bond promoting layer is an admixture of a carboxyl modified polypropylene resin and a polypropylene resin, both having a particle size less than 5 microns.

6. The process of claim 1 wherein the propylene polymer is polypropylene.

7. The process of claim 1 wherein the propylene polymer is a propylene/ethylene copolymer.