GB1565697A

GB1565697A - Propylene polymer adtered to enamel coated metal surface

Info

Publication number: GB1565697A
Application number: GB37898/76A
Authority: GB
Original assignee: Continental Group Inc
Current assignee: Continental Group Inc
Priority date: 1975-09-25
Filing date: 1976-09-13
Publication date: 1980-04-23
Also published as: DK432476A; AU510800B2; AU1743376A; GB1565698A; MX143441A; BR7606366A; SE7610532L; FI762739A; NO763253L; FR2333646A1; FR2333646B1

Description

(54) PROPYLENE POLYMER ADHERED TO ENAMEL COATED METAL SURFACE (71) We, THE CONTINENTAL GROUP, INC., a corporation organized and existing under the laws of the State of New York, United States of America, of 633 Third Avenue, New York, New York, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates generally to adhering a propylene polymer layer to an enamel coated metal surface and more particularly to effecting a heat seal bond between the surfaces.

Easy opening containers are known to the art. These containers are generally formed of metal and are provided with at least one 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 from the panel along the score line.

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

It has been proposed e.g., British Patent Specification No. 1,257,620, to repIace the metal tear strip with a plastics layered closure member to eliminate the cutting hazard as the removed portion will not have sharp edges. The closure member fabricated entirely from a thermoplastics resin, such as polypropylene, or a laminate of the resin and a metal foil such as aluminum or steel, is heat sealed to the surface of an enamel coated end panel having at least one preformed opening, the enamelled surface of the panel having been coated with a heat activatable, bond promoting layer containing a carboxyl modified polypropylene resin.

Although the plastics containing closures of British Patent Specification No.

1,257,620 could be effectively bonded to the metal can ends, problems arose in the application of the carboxyl modified polypropylene layer to the enamelled metal surface.

When the carboxyl modified polypropylene resin is utilized as an adhesion promoting layer for bonding the propylene polymer layer of a closure member to an enamel coated metal can end, the carboxyl modified polypropylene resin is usually applied as a dispersion in a volatile organic solvent such as kerosene. The carboxyl modified polypropylene resin used to prepare the dispersion generally has a particle size of 0.1 to 5 microns and is at the present time a relatively expensive resin material.

Although only small amounts of the modified resin are required in the adhesion promoting layer, the application of the modified resin in a dilute dispersion e.g., 10% solids in unsatisfactory as such dilute dispersions do not have the required physical properties such as viscosity and flow out, which are required for the application of the coating dispersion using conventional coating equipment, e.g. as by roll coating.

To obtain a coating dispersion having the properties necessary for commercial coating application, polypropylene resin powders of approximately the same particle size range, e.g. 0.1-5 microns are incorporated in the dispersion to raise the solids content to about 20% whereby the dispersion has the flow and viscosity characteristics required in commercial coating methods. One drawback to the use of polypropylene resins of such particle size is that the sub-micron size resin is difficult to manufacture and is presently in limited commercial supply.

According to the invention, a method of bonding a propylene polymer to a metal surface comprises: (a) applying to the metal surface an enamel coating composition E having incorporated therein 0.015% by weight, based on the solids content of the enamel, of a carboxyl modified polypropylene resin, (b) baking the enamel coated metal surface to cure and harden the coating composition, (c) heat sealing a propylene polymer layer to the hardened enamel coated metal surface and then (d) cooling the heat sealed assembly to ambient temperature.

Preferably the carboxyl modified polypropylene resin is added to an enamel coating formulation while said resin is dissolved in a solvent selected from an organic acid, alcohols and hydrocarbons having at least 10 carbon atoms, the solvent preferably being heated to a temperature above 1000C at the time of addition.

The amount of carboxyl modified polypropylene incorporated in the enamel coating composition is in the range of 0.01 to 5 percent by weight of the solids content of the enamel coating composition and preferably the carboxyl modified polypropylene resin is incorporated in the enamel coating composition at a concentration in the range of 0.1-3 percent by weight of the solids content of the enamel coating composition.

The method according to the invention eliminates the necessity for the application of special polypropylene/carboxyl modified polypropylene dispersions to effect the heat sealing of polypropylene polymer layers to enamelled metal surfaces.

Further, by the use of the method according to the invention, propylene polymer layers can be bonded directly to enamel coated metal surface without the need for a separate adhesion promoting layer between the propylene polymer layer and the enamel coated metal layer as has been the practice of the prior art.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a top plan view of a container having an easy opening structure provided with a laminated closure member containing a propylene polymer layer, the closure member being secured to the end panel surface in accordance with the present invention, Figure 2 is an enlarged cross-sectional view taken generally along the line 2-2 of Figure 1, Figure 3 is an elevational view of a container having a lapped side seam, Figure 4 is a cross-sectional view taken along the line 44 1 of Figure 3 showing the sideseam in detail.

The carboxyl modified polypropylene which is utilized in the practice of the present invention can be prepared by grafting an unsaturated dicarboxylic acid or anhydride onto a polypropylene backbone using high energy radiation or a peroxy catalyst as described in British Patent Specification No. 1,020,740. Unsaturated dicarboxylic acids or anhydrides which can be employed to prepare the carboxyl modified polypropylene resins include maleic, tetrahydrophthalic acid, fumaric acid, itaconic, nadic, methyl nadic and their anhydrides, maleic anhydride being preferred.

The amount of unsaturated dicarboxylic acid or anhydride which can be grafted onto the polypropylene backbone preferably ranges from 0.05 to 10 percent by weight based on the total weight of the grafted polymer and more preferably the amount of grafted dicarboxylic acid or anhydride ranges from 0.1 to 5.0 percent, most preferably from 0.5 to 5.0 percent.

When the carboxyl modified polypropylene resin is utilized as an adhesion promoting adjuvant for enamel coatings, the resin can be of any particle size; the particle size is generally from 0.05 to 50 microns, more preferably from 35 to 40 microns.

The enamel coating in which the carboxyl modified polypropylene resin is incorporated is preferably an epoxy resin coating formulation containing a heat activatable aminoplast cross-linking resin. Such enamel coating compositions are the subject of our co-pending application No. 23121/78 (Serial No. 1,565,698).

The preferred aminoplast condensates are urea-aldehyde and triazine aldehyde resins and alcohol modified derivatives thereof, that is, alkylated amino resins wherein the alkyl radical contains from 2 to 8 carbon atoms. Such aminoplast resins are the reaction products of aldehydes, for instance formaldehyde, or acetaldehyde with urea, substituted ureas, thioureas, ethylene urea, melamine, benzoguanamine, or acetoguanamine. The resulting methylol substituted products are etherified with alcohols, for example isopropanol, butanol and 2-ethyl hexanol, in order to obtain stability and organic solubility. Such organic soluble aminoplast resins are contemplated for use in this invention, butylated urea formaldehyde resins being preferred.

The epoxy resins used for preparing the preferred enamel coating compositions are the polymeric reaction products of polyfunctional halohydrins with polyhydric phenols. These preferred epoxy resins have the structural formula:

wherein X represents the number of molecules condensed. Typical polyfunctional halohydrins are epichlorohydrin and glycerol dichlorohydrin. 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 and methyl ethyl ketone, which result in such compounds as 2,2-bis(4-hydroxyphenyl)propane. These epoxy resins normally contain terminal epoxy groups but in some instances they may contain terminal epoxy groups and terminal hydroxyl groups.

The molecular weight of the epoxy resins may be controlled by the relative proportions of the reactants as well as by the extent to which the polymerisation reaction proceeds.

It is preferred that epoxy resins which are of relatively high molecular weight are utilized in preparing the enamel coatings. Generally, epoxy resins having an average molecular weight in the range of 1400 to 5000 may be used.

Epoxy resins are available commercially. Preferred examples are Epon 1004 and Epon 1007, products of Shell Chemical Company, which are the condensation products of epichlorohydrin and Bisphenol A (dihydroxy-diphenyl-dimethyl methane) and have respective epoxy assays of 875 to 1025 and 2500 to 4000 grams of sample per gram mole of epoxy group (gram/gram mole). In general, the average molecular weight of an epoxy resin is approximately twice the epoxy assay.

The solids content of the enamel coating compositions used in the method and laminate of the present invention preferably comprises 60 to 95, preferably 70 to 90, percent by weight of the epoxy resin; 5 to 40, preferably 10 to 30, percent by weight of the aminoplast resin; and 0.01 to 5, preferably 1.0 to 3, percent by weight of the carboxyl modified polypropylene resin.

Generally, to prepare the enamel compositions, the epoxy resin and aminoplast resin are dissolved in a mixture of solvents such as a mixture of a ketone and an aromatic hydrocarbon until the resins are completely dissolved.

Examples of ketones which can be employed as solvents for the epoxy resin aminoplast resin-based enamel coating formulations include methyl ethyl ketone, methyl isobutyl ketone, isophorone, cyclohexanone, do acetone alcohol and diisobutylketone. Examples of aromatic hydrocarbon useful as solvents for epoxy resin aminoplast resin based-enamel coating formulations include benzene, toluene, xylene, and commercially available aromatic naphtha mixtures, such as "Solvesso" 100 or 150.

"Solvesso" is a registered Trade Mark. Other useful solvents are ether alcohols such as butyl cellosolve and ether alcohol esters such as "Cellosolve" acetate.

Antioxidants and thermal stabilizers may also be incorporated in the enamel coating composition to inhibit oxidation of the carboxyl modified polypropylene resin during the baking and curing of the enamel coating after its application to a metal surface. Antioxidant compounds which have been found useful in the practice of the present invention include hindered phenolic compounds such as Irganox 1010, tetrakis [methylene 3-(3',5'4i-tert-butyl-4-hydroxyphenyl)propionate]; these antioxidants are usually incorporated in the enamel coating compositions at concentrations in the range of 0.1 to 1.00/ by weight based on the solids content of the enamel.

Lubricants, such as polyethylene dispersions which are required during forming of the enamel coated metal sheet in container end manufacture may also be incorporated in the enamel.

To improve the heat sealability of the baked enamel composition, the carboxyl modified polypropylene resin is preferably first dissolved in a hot, e.g. greater than 100 C, organic solvent which is an aliphatic compound containing at least 10 carbon atoms such as an alcohol, an acid or a hydrocarbon.

As will hereinafter be illustrated if a solvent other than an aliphatic compound containing at least 10 carbon atoms is used as a solvent for the carboxyl modified polypropylene resin in this preferred procedure, the incorporation of the modified polypropylene resin in an enamel formulation dissolved in such solvents will not materially improve the heat seal bond between a propylene polymer and the baked enamel coating.

In preparing solutions of the carboxyl modified polypropylene resin, for incorportion in the enamel coating formulation by the preferred procedure, the resin is added to the organic alcohol, acid or hydrocarbon solvent at a concentration of 1 to 30% by weight, preferably 2 to 10% by weight. After the resin is added to the solvent, the resultant mixture is heated to a temperature above 1000C until the resin completely dissolves in the solvent. The resin solution is then added to the enamel coating formulation, preferably one which is an organic solvent solution of the said epoxy resin and a cross-linking material such as the said heat activatable aminoplast resin. It is preferred that the resin solution be at a temperature above 100 C when added to the enamel coating formulation.As will hereinafter be illustrated, if the temperature of the resin solution is 100 C or less, gel formation is observed and the strength of the heat seal bond which is subsequently obtained between the polypropylene polymer and enamel coated metal surface is materially diminished.

Examples of suitable organic alcohols used to prepare solutions of the carboxyl modified polypropylene resin for incorporation in the enamel coating formulations in accordance with the preferred practice of present invention are long chain, saturated or unsaturated, aliphatic monohydroxy alcohols having a general formula R-OH wherein R is a straight or branched-chain saturated or olefinically unsaturated hydrocarbon group having from 10 to 30, preferably from 12 to 22, carbon atoms. Illustrative of such alcohols are decyl alcohol, tridecyl alcohol, lauryl alcohol, tetradecyl alcohol, cetyl alcohol, oleyl alcohol, linoleyl alcohol, palmitoleyl alcohol, arichidyl alcohol, stearyl alcohol, benhenyl alcohol, arachidonyl alcohol, myristoleyl alcohol and mixtures of these alcohols.

Examples of suitable organic acids which may be used as solvents for the carboxyl modified polypropylene resin are saturated and olefinically unsaturated aliphatic acids having 10 or more carbon atoms, preferably 12 to 22 carbon atoms. Examples of such acids are the fatty acids: capric acid, lauric acid, myristic acid palmitic acid, isostearic acid, stearic acid and arachidic acid, undecylemic acid, myristoleic acid, palmitoleic acid, oleic acid, cetoleic acid and erucic acid and mixtures of these acids.

Examples of suitable aliphatic hydrocarbons having 10 or more carbon atoms which may be used as solvents in the practice of the present invention include saturated hydrocarbons such as decane, iodecane, pentadecane, nepthadecane, nonadecane and mixtures of such hydrocarbons such as kerosene and mineral oil, as well as unsaturated hydrocarbons and particularly unsaturated hydrocarbons having olefinic unsaturation such as undecene, tridecene and pentadecene, and mixtures thereof such as a mixture of C11 to C11-olefinically unsaturated hydrocarbons.

The enamel compositions can be satisfactorily applied at a solids content ranging from 20% to 70 Ó by weight, based on the total weight of the liquid coating composition. Generally, a solids content of 30 to 50 fO by weight is preferred.

The enamel coating containing the adhesion promoting carboxyl modified polypropylene resin can be satisfactorily applied by any of the conventional methods employed in the coating industry. However, for coating of sheet metal used in container manufacture, 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 enamel coating, it is cured and hardened preferably by heating the coated substrate at a temperature of 3500F to 6000F for a period of 20 minutes to 1 minute, the preferred conditions being 8-10 minutes at 4000F.

The preferred coating weight for coating metal ends to which a propylene polymer closure may be heat sealed is in the range of 2.5 to 10.0 milligrams of dry coating per square inch of substrate surface to provide an enamelled surface to which a propylene polymer layer can be heat sealed.

Propylene polymer layers which may be bonded to the enamel coated surface in accordance with the practice of the present invention include polypropylene, and propylene/ethylene copolymers containing from 1 ,b to 10% ethylene.

The propylene polymer layer is bonded to the enamel coated metal surface by heat sealing preferably at a temperature range of 3500 to 4500F, more preferably at a temperature of 3750 to 4000F. Heat sealing may be accomplished by any means known to the art, such as a hot platen press or metal jaws heated by resistance wire or by induction heating, using dwell times varying from 0.1 seconds to 5 seconds.

After the propylene polymer layer is heat sealed and bonded to the enamel coated metal surface, the assembly is allowed to cool to ambient temperature.

Referring to the drawing, and in particular to Figure 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 11 having a bead formed adjacent the outer periphery from which there depends a peripheral flange 12. The peripheral flange 12 is arranged to be curled and double seamed with an outwardly extending flange at the upper end of a container body in the usual manner.

The panel 11 in the embodiment illustrated in the drawing is shown with a plurality of pour openings 13 through which the contents of the container are poured.

It is to be understood that the openings 13 may assume any configuration, and is not limited to the pour opening arrangement shown in the drawings.

The top surface of the end assembly 10 is coated with a layer of a thermosetting enamel coating, such as an epoxyurea/formaldehyde resin 14 having incorporated therein an adhesion promoting carboxyl modified polypropylene resin, such as a polypropylene/maleic anhydride graft copolymer. Peelably heat sealed to the enamel layer 14 is a laminated closure member 15 constructed of an aluminum foil outer layer 16 and a polypropylene inner layer 17. The closure member 15 has a sealing flap 18 for closing the openings 13. Integrally extending from the sealing flap 18 is a pull ring portion 19.

The peelable heat sealed bond that is formed due to the presence of the carboxyl modified polypropylene resin in the enamel layer 14 permits the fiap 18 to be heat sealed to the metal end panel and thereafter separated cleanly from the metal end by the application of a pulling force at the ring portion 19. Preferably, the pull ring portion 19 is formed with an opening size to receive the finger of a user. The heat to achieve bonding is preferably applied by induction heating of the metal surface. In this manner, the sealing flap 19 is firmly heat sealed but peelably fixed about the openings 13 and remains adhered thereto until pulled and separated from the container end surface.

In addition to promoting the adhesion of propylene polymer coated closures to enamel coated container ends, the method of the present invention has other applications in the container fabrication art and the metal adhesion art generally.

Another example of the use of the method of the present invention in the container fabrication art is in the fabrication of container bodies. In one method of metal container body manufacture, a sheet of enamel coated metal is formed into a tubular shape and the edges of the blank are brought together in lapped relation. A sealed joint is made by interposing a bonding agent between the laps and then heating the joint and pressing the laps together to obtain the desired bond of the metal sections. Propylene polymers have not been utilized as adhesive materials due to their poor bonding adhesion to enamelled metal surfaces.By modifying the enamel coating applied to the blang with a carboxyl modified polypropylene resin in accordance with the present invention, the adhesion of propylene polymers to the enamelled metal surface is improved to a degree that the propylene polymer can be used as a bonding agent for the lapped portions of the container body.

Thus, in Figure 3, there is shown a container 20 having body 21, and end 22 and a longitudinal side seam 23. Figure 4 shows the side seam 23 in detail. The side seam is composed of metal layers 25, 26, each having applied thereto an enamel coating 27, the enamel coating having incorporated therein an adhesion promoting amount of a carboxyl modified polypropylene resin. The seam 23 has a laminate structure consisting of the overlapped edges of enamel coated metal surfaces 25, 26 having a propylene polymer layer 28 interposed therebetween bonding the overlapped edges together.

To illustrate the manner in which the present invention may be carried out, the following Examples are given.

In the Examples, parts and percentages are by weight, unless specified otherwise.

EXAMPLE I An epoxy resin based enamel coating formulation was prepared composed of a 40% by weight solids consisting of 80 parts of the diglycidyl ether of Bisphenol A and 20 parts of a butylated urea formaldehyde suspended in an organic solvent mixture of approximately equal parts of xylene, methyl isobutyl ketone, diacetone alcohol and butyl alcohol.

To the enamel coating formulation was added Hercoprime A-35 (trademark) in the form of a 10% dispersion in kerosene. Hercoprime A-35 is a maleic anhydride modified polypropylene resin having an inherent viscosity of about 1.7, a carboxyl content of 0.5% to 1.0%, a particle size range of 35 to 40 microns and a specific gravity of 0.9. The final solids content of the modified enamel formulation consisted of 98.5% by weight of the epoxy resin/urea formaldehyde enamel coating solids and 1.5% by weight of the Hercoprime A-35. In adding the Hercoprime dispersion to the enamel coating formulation, the Hercoprime dispersion was slowly added to the epoxy enamel formulation at ambient room temperature (250C) and with vigorous stirring. Stirring of the modified enamel suspension was continued for another five minutes or until a homogeneous mixture has been achieved.

The Hercoprime modified enamel formulation was applied by means of a laboratory coating rod to the surface of a sheet of tin-free steel at a coating weight of 2.5 mgs/in2. After application of the enamel coating formulation, the wet sheet was baked at 2050C for 8 minutes in a hot-air oven to volatize the solvent and cure the enamel to a hard film. Strips 4" x 1" were cut from the coated sheet to test the bonding characteristics of the enamel. A similar sized strip of a polypropylene/ aluminum foil laminate constructed of 4 mil aluminum foil having adhered thereto a 2 mil polypropylene resin having a melt index 0.5 and a density of 0.905 was heat sealed to the enamel coated steel strip with the polypropylene layer in direct contact with the enamel surface using a laboratory Sentinal heat sealer.Bonding was achieved with the sealer set at 2050C, 40 pounds per square inch jaw pressure and a dwell time of 4 seconds. The heat sealed structure was then allowed to cool to ambient room temperature.

The heat sealed structure was then tested to determine the peel force required to separate polypropylene coated aluminum foil from the enamel coated strip. The test was performed using an Amthor peel tester which applied a constant strain rate of 12 lineal inches per minute on the structure. The peel forces required to effect separation of the heat sealed layers was determined to be 3 A pounds/inch (Ibs/in).

For purposes of comparison, the procedure of Example 1 was repeated with the exception that the carboxyl modified polypropylene resin was not incorporated in the enamel coating-no measurable bond was obtained in the heat sealed structure.

EXAMPLE II A sheet of tin-free steel was coated with an epoxy resin based enamel coating modified with Hercoprime in accordance with the procedure of Example I. Strips of the enamel coated steel were bonded together by interposing a film of polypropylene between the enamel coated surfaces and heat sealing under the conditions of Example I. The peel force required to effect separation of the heat sealed strips was determined to be 66 lbs/in.

EXAMPLE Ill Hot solvent solutions of a carboxyl modified polypropylene resin were prepared by adding 0.4 parts Hercoprime A-35 (trademark) to varying amounts of oleyl alcohol to prepare resin solutions ranging in concentration from 4 to 20 percent by weight. Hercoprime A-35 is a maleic anhydride modified polypropylene resin having an inherent viscosity of about 1.7, a carboxyl content of 0.5% to 1.0%, a particle size range of 35 to 40 microns and a specific gravity of 0.9.

The oleyl alcohol-Hercoprime mixture was heated to 155oC for 15 minutes during which time the Hercoprime dissolved in the oleyl alcohol. The hot solution at 1550C was then added slowly to a rapidly stirred solution of 100 parts of an epoxy resin based enamel coating formulation composed of a 40% by weight solids con sisting of 85 parts of a diglycidyl ether of Bisphenol A and 15 parts of a butylated urea formaldehyde suspended in an organic solvent mixture of approximately 25 parts xylene, 25 parts methyl isobutyl ketone, 30 parts diacetone alcohol and 20 parts butyl alcohol.To the enamel coating formulation was also added 30 parts of "Cellosolve" acetate as a diluent to adjust the flow and viscosity properties of the modified enamel formulation to that required for commercial coating specifications and 1.8 parts of a polyethylene dispersion for lubricant properties. The resultant enamel formulation had a Hercoprime concentration of 1 % by weight (based on enamel solids).

The modified enamel coating formulation was applied, by means of a draw bar, to the surface of a sheet of 95 lb. tin-free steel at a dried film weight of 3.5-4.5 mg/sq.

in. of steel surface.

After application of the modified enamel coating formulation, the coated sheet was baked at 3700F for 8 minutes to volatilize the solvent mixture and to cure the epoxy-urea formaldehyde-carboxyl modified polypropylene resin solids mixture to a hard enamel film.

After cooling, the tin-free steel sheet was cut into 1x4 inch strips. A closure member having a laminate structure of 3.0 to 4.0 mil aluminum foil coated with a 1.5 to 2.0 mil layer of a polypropylene resin having a melt index of 0.55 and a density of 0.90 was heat sealed to the enamel coated strips at 4000F with a poly- propylene layer in contact with the enamel coated surface at a 4 second dwell time and 40 Ibs/sq. in. pressure. The closure member heat sealed to the strip was allowed to cool to room temperature.

The heat sealed portion of the closure member was then tested to determine the peel force required to separate the closure member from the enamel coated strip.

The peel strengths necessary for commercial acceptance generally should be in excess of 1.5 Ibs/lineal inch (PLI). The test was performed using an Amthor peel tester which applied a constant strain rate of 12 lineal inches per minute on the closure member. The peel forces required to separate the closure member are summarized in Table 1 below.

For purposes of comparison, the procedure of Example III was repeated with the exception that the carboxyl modified polypropylene resin was incorporated in the enamel coating directly without first dissolving the resin in oleyl alcohol. The peel force required to separate the closure member in this comparative run is also listed in Table 1 designated by the symbol "C".

TABLE I Parts Hercoprime Parts Oleyl Peel per 100 parts Alc. per 100 Strength Run No. Oleyl Alcohol Parts of Enamel (PLI) 1 20.0 2 1.9 2 10.0 4 3.3 3 6.7 6 5.5 4 5.0 8 7.5 5 4.0 10 9.2 C 0 0 No Bond The data in Table 1 indicates that dissolving the carboxyl modified polypropylene resin in oleyl alcohol materially improves the bond strength of the heat sealed polypropylene layer of the closure member to the enamel coated steel surface and that the more dilute the solution of the resin or the greater the amount of alcohol used to dissolve the resin, the greater is the improvement in bond strength.

EXAMPLE IV The procedure of Example III was repeated with the exception that Alfol 1218 (trademark), a mixture of C12-C,8 aliphatic alcohols was used to dissolve the Hercoprime. Alfol 1218 has the following composition: Alcohol Wt. % Dodecyl (C1,) 40 Myristyl (Cl4) 30 Cetyl (C1 ,) 20 Stearyl (C1,) 10 Alfol 1218 solvent solutions containing Hercoprime A-35 were prepared by adding 0.34 parts Hercoprime to varying amounts of Alfol 1218 to prepare resin solutions containing varying ratios of Hercoprime and Alfol 1218. In dissolving the Hercoprime, the Alfol 1218 was heated to 1000--1200C and the Hercoprime resin powder was added.Heating was continued to 150cl 600C and the hot solution was added to 100 parts of a rapidly stirred epoxy-urea formaldehyde resin mixture (40 Ó solids in mixed solvent solution) at room temperature. To the enamel formulation was then added 0.1 part Irganox 1010 as a 10 ,ó solution in "Cellosolve" acetate as well as additional (20 parts) "Cellosolve" acetate diluent. The resultant enamel coating formulation contained 0.9 % by weight (based on enamel coating solids) Hercoprime.

The peel strengths of polypropylene coated aluminium closure members heat sealed to 1 x 4 inch enamel coated tin-free steel strips are recorded in Table II below.

For purposes of comparison, the procedure of Example IV was repeated with the exception that the carboxyl modified polypropylene resin was incorporated in the enamel coating directly without first dissolving the resin in Alfol 1218. The peel force required to separate the closure member in this comparative run is also listed in Table II designated by the symbol "C".

TABLE II Parts Hercoprime Parts Alfol Peel per 10G parts pert 100 Strength Run No. Alfol Parts Enamel (PLI) 1 5.3 6 7.7 2 2.7 12 21.9 3 1.8 18 16.9 4 1.3 24 4.9 C 0 0 No Bond The data in Table II indicates that dissolving the carboxyl modified polypropylene resin in a mixture of C,2-C,8 alcohols materially improves the bond strength of the heat sealed polypropylene layer of the closure member to the enamel coated steel surface and that the strongest peel strengths are achieved when a solution containing about 2.7% by weight of the resin is used, the bond strengths diminishing as more dilute solutions are employed.

EXAMPLE V The procedure of Example III was repeated using a variety of C12-C2 organic alcohols and acids and C10 and greater hydrocarbons having boiling points above 1500C as solvents for the Hercoprime. The solvent/Hercoprime weight ratio was maintained constant at 15:1. In dissolving the Hercoprime, the solvent material and the Hercoprime powder mixture was heated to 150-1600C. The hot solution was added to the rapidly stirred epoxy-urea formaldehyde enamel at room temperature. To the enamel formulation was then added an Irganox 1010 as a 10% solution in "Cellosolve" acetate as well as additional "Cellosolve" acetate diluent.The resultant enamel coating formulation contained 1% by weight (based on enamel coating solids) Hercoprime and had the following composition: Solids Content Grams (grams) Epoxy-urea/formaldehyde suspension 100.0 40 Hercoprime 0.4 0.4 Solvent for Hercoprime 6.0 0 Irganox 1010 1.0 0.1 Cellosolve acetate diluent 25.0 0 The peel strengths of polypropylene coated aluminum closure members heat sealed to 1X4 inch enamel coated tin-free steel strips are recorded in Table III below, the enamel coatings having Hercoprime incorporated therein using different solvents within the scope of the appended claims.

For purposes of comparison, the procedure of Example V was repeated with the exception that the Hercoprime resin was incorporated in the enamel coating formulation dissolved in a solvent outside the scope of the appended claims. The peel forces required to separate the closure member heat sealed to these comparative enamel coaungs also listed in Table III. These comparative tests are designated by the symbol "C".

TABLE III Temperature at which Hercoprime Peel Resin dissolves Strength Run No. Solvent Chemical Type ( C) (PLI) 1 Oleyl alcohol C18 Olefinic alcohol 120 48.0 2 Alfol 1218 C,OCl9 Aliphatic alcohol mixture 120 10.5 3 Cetyl alcohol C,, Aliphatic alcohol 120 8.2 4 Stearyl alcohol C18 Aliphatic alcohol 120 5.1 5 Tridecyl alcohol Cl3 Aliphatic alcohol 120 3.5 6 Arachidyl alcohol C Aliphatic alcohol 120 2.2 7 Dedecyl alcohol C12 Aliphatic alcohol 120 1.9 8 Isostearic Acid C18 Aliphatic Acid 130 2.9 9 Oleic acid C18 Unsaturated 125 2.4 carboxylic acid 10 Marine fatty acid Cl4C22 Fatty acid 120 2.2 mixture 11 Mineral oil C20-C26 Aliphatic 120 10.0 hydrocarbons 12 Kerosene CloCl3 Aliphatic 135 5.1 hydrocarbons 13 Chevron Alpha C11-C13 Olefin hydro- 120 4.1 Olefins carbon mixture 14 l-Dodecene C12 Unsaturated 115 1.1 hydrocarbon C1 Ethyl Hexyl Cs Aliphatic alcohol 125 0 alcohol Butyl alcohol C4 Aliphatic alcohol Resin insoluble at boiling point C3 Isopropanol Ca Aliphatic alcohol Resin insoluble at boiling point C8 Diacetone alcohol Aliphatic ketoalcohol Resin insoluble at boiling point C, Butyl cellosolve Aliphatic glycolether Resin partly soluble at 160 C C8 Xylene Aromatic hydrocarbon 1 100C 0 C, 1,2,3,4-Tetrahydro- Aromatic hydrocarbon 900C 0 napthalene (tetralin) Cs Diisobutyl ketone Aliphatic ketone Resin insoluble at boiling point Ce Hydroxy ethyl ,aB-Hydroxy ethyl fatty Resin insoluble azeliate acid ester at 150-1600C C10 Glyceryl monooleate C18 Olefinic ester Resin insoluble The data in Table III shows Hercoprime when added to an enamel coating formulation dissolved in C12C20 aliphatic alcohols and acids and C10 or more aliphatic hydrocarbons results in enamel surfaces to which polypropylene may be heat sealed.Although Hercoprime is soluble in hot xylene, tetralin and ethyl hexyl alcohol3 the enamel coatings, which were prepared using these hot solutions, did not form any heat seal bonds with polypropylene surfaces. Lower aliphatic alcohols such as butanol and isopropanol and other solvent materials such as butyl cellosolve, diacetone alcohol and methyl isobutyl ketone could not be used to incorporate Hercoprime in enamel coating formulations as Hercoprime is not soluble in these common organic solvents at their boiling points.

EXAMPLE VI The procedure of Example V was repeated with the exception that the heat sealed structure was baked at 4000F for 15 seconds to stimulate conditions encountered during manufacture of metal container ends. The results of these tests are recorded in Table IV below.

TABLE IV Percent Hercoprime Based Hercoprime Peel Strengths (PLI) Run No. on Enamel Solids Solvent Before Bake After Bake 1 0.6 Oleyl alcohol 9.5 11.5 2 0.9 Oleyl alcohol 26.1 39.0 3 0.8 C1Cl8 Aliphatic* alcohol blend 1.4 3.4 4 1.0 Stearyl alcohol 5.1 7.3 * Alfol 1218 The results in Table IV show that the bond strength of a polypropylene layer heat sealed to an enamel coating modified in accordance with the method of the present invention increases if the structure receives additional heating.

EXAMPLE VII The procedure of Example V was repeated to modify enamel coating formulations with oleyl alcohol solutions of Hercoprime A-35 with the exception that the tem- perature of the Hercoprime solution at the time of its addition to the enamel coat ing formulation was varied from 230C to 155 C. The peel strengths of polypropylene coated aluminum closure members heat sealed to 1 X 4 tin-free steel strips coated with the enamel coating formulations are recorded in Table V.

TABLE V Temperatures at which Hercoprime solution Peel Strengths added to enamel (PLI) Comments 1550C 48 Solution 1000C 6.1 Gel 230C 6.4 Gel The results recorded in Table V indicate that superior bonding results are obtained with enamels modified with Hercoprime solutions heated to temperatures above 1000C at their time of addition to the enamel formulation. If the Hercoprime solution is cooled to temperatures of 100CC or less at time of addition to the enamel formulation, the Hercoprime crystallizes out as gel which materially affects the bonding results with the enamel.

EXAMPLE VIII The procedure of Example V was repeated with the exception that Hercoprime G-35 was substituted for Hercoprime A-35. Hercoprime G-35 differs from Herco prime A-35 in that Hercoprime G-35 has a carboxyl content of 3.0 to 4.0% and has a lower inherent viscosity. In dissolving Hercoprime G-35 in oleyl alcohol, the alcohol was heated to 1200--1300C and the Hercoprime powder added. Heating was con tinued to 1500--1600C and the hot solution added to the rapidly stirred epoxy-urea formaldehyde enamel at room temperature. To the enamel formulation was then added Irganox 1010 as well as "Cellosolve" acetate diluent.The resultant enamel coating formulation contained 0.5-1 % by weight (based on enamel coating solids) Hercoprime G-35 and had the following composition: Solids Content Grams (Grams) Epoxy-urea/formaldehyde suspension 100.0 40 Hercoprime G-35 0.2)A 0.2OA Oleyl alcohol 6.0 0 Irganox 1010 1.0 0.1 Cellosolve acetate diluent 25.0 0 The peel strengths of polypropylene coated aluminum closure members heat sealed to 1X4 inch enamel coated tin-free steel strips are recorded in Table VI.

TABLE VI OÓ Hercoprime G-35 Peel Strength Added to Enamel (PLI) 1.0 42 0.5 45 The words "Irganoxí' and "Cellosolve" used in this specification are registered Trade Marks.

WHAT WE CLAIM IS:- 1. A method of bonding a propylene polymer to a metal surface which comprises: (a) applying to the metal surface an enamel coating composition E having in corporated therein 0.015% by weight, based on the solids content of the enamel, of a carboxyl modified polypropylene resin, (b) baking the enamel coated metal surface to cure and harden the coating composition, (c) heat sealing a propylene polymer layer to the hardened enamel coated metal surface and then (d) cooling the heat sealed assembly to ambient temperature.

2. A method according to Claim 1, wherein the enamel coated metal surface having the propylene polymer sealed thereto is heat sealed to a second metal surface which has also had applied thereto an enamel coating composition E as defined in

Claims

claim 1.

3. A method according to Claim 1 or Claim 2, wherein the metal surface and/or the second metal surface is of steel.

4. A method according to Claim 1, 2 or 3, wherein the carboxyl modified polypropylene resin is the reaction product of polypropylene and an unsaturated dicarboxylic acid or anhydride and contains 0.1% to 5.0% by weight of carboxyl groups.

5. A method according to Claim 4, wherein the unsaturated anhydride is maleic anhydride.

6. A method according to any one of the preceding claims, wherein the enamel coating composition E comprises an epoxy resin and a urea formaldehyde resign.

7. A method according to Claim 6, wherein the solids content of the enamel coating composition E comprises 60 to 95% by weight of the epoxy resin, 5 to 40% by weight of the urea formaldehyde resin and 0.01 to 5% by weight of the carboxyl modified polypropylene resin.

8. A method according to Claim 7, wherein the said solids content comprises from 70 to 90 percent by weight of the epoxy resin, from 5 to 25 percent by weight of the urea formaldehyde resin and from 0.1 to 5% by weight of the carboxyl modified polypropylene resin.

9. A method according to any one of the preceding claims, wherein the propylene polymer is polypropylene.

10. A method according to any one of the preceding claims, wherein the enamel coating composition E is prepared by dissolving the carboxyl modified polypropylene resin in a solvent heated to a temperature greater than 1000C to form a solution thereof, the solvent being selected from saturated or olefinically unsaturated aliphatic acids, alcohols having 10 to 22 carbon atoms and saturated or unsaturated hydrocarbons having 10 to 30 carbon atoms, and the resultant resin solution is added to an enamel coating formulation.

11. A method according to Claim 10, wherein the resin solution is at a temperature greater than 1000C when added to the enamel coating formulation.

12. A method according to Claim 10, wherein the enamel coating composition E comprises an epoxy resin and a butylated urea formaldehyde resin.

13. A method according to Claim 10, 11 or 12, wherein the solvent is oleyl alcohol.

14. A method according to Claim 10, 11 or 12, wherein the solvent is dodecyl alcohol.

15. A method according to Claim 10, 11 or 12, wherein the solvent is myristyl alcohol.

16. A method according to Claim 10, 11 or 12, wherein the solvent is cetyl alcohol.

17. A method according to Claim 10, 11 or 12, wherein the solvent is stearyl alcohol.

18. A method according to Claim 10, 11 or 12, wherein the solvent is tridecyl alcohol.

19. A method according to Claim 10, 11 or 12. wherein the solvent is arachidyl alcohol.

20. A method according to Claim 10, 11 or 12, wherein the solvent is oleic acid.

21. A method according to Claim 10, 11 or 112, wherein the solvent is isostearic acid.

22. A method according to Claim 10, 11 or 12, wherein the solvent is mineral oil.

23. A method according to Claim 10, 11 or 12, wherein the solvent is kerosene.

24. A method according to Claim 10, 11 or 12, wherein the solvent is l-dodecane.

25. A method according to Claim 10, 11 or 12, wherein the solvent is a mixture of C1,-C1, olefinically unsaturated hydrocarbons.

26. A method according to Claim 1 substantially as described herein with reference, and as illustrated in, the accompanying drawings.

27. A method according to Claim 1 substantially as described in any one of the foregoing Examples I to VIII.

28. A propylene polymer/metal substrate composite whenever prepared by a method as claimed in any one of the preceding claims.

29. A method for securing metal substrates together comprising: (a) applying to each of the metal substrates an enamel coating composition E having incorporated therein 0.01 to 5% by weight, based on the solids con tent of the enamel, of a carboxyl modified polypropylene resin, (b) baking the enamel coating to cure and harden the coating, (c) positioning a propylene polymer layer between portions of the enamel coated metal substrates to be secured, (d) heat sealing the propylene polymer layer to each hardened enamel coated metal surface by heating the propylene polymer layer and then applying to the metal substrate portions sufficient pressure to bond the metal sub strates to the propylene polymer layer.

30. A method according to Claim 29, wherein the carboxyl modified polypropylene resin is the reaction product of polypropylene and an unsaturated dicarboxylic add or anhydride and contains 0.1% to 5.o Ó by weight of carboxyl groups.

31. A method according to Claim 30, wherein the unsaturated anhydride is maleic anhydride.

32. A method according to any one of Claims 29 to 31, wherein the enamel coating composition E comprises an epoxy resin and a urea formaldehyde resin.

33. A method according to Claim 32, wherein the solids content of the enamel coating composition E comprises 60 to 95% by weight of the epoxy resin, 5 to 40 by weight of the urea formaldehyde resin and 0.01 to 5% by weight of the carboxyl modified polypropylene resign.

34. A method according to Claim 33, wherein the said solids content comprises from 70 to 90% by weight of the epoxy resin, from 5 to 20% by weight of the urea formaldehyde resin, and from 0.1 to 5% by weight of the carboxyl modified polypropylene resin.

35. A method according to any one of Claims 29 to 34, wherein the propylene polymer is polypropylene.

36. A method according to any one of Claims 29 to 35, wherein the metal substrate is steel.

37. A method according to any one of Claims 29 to 36, wherein the enamel coating composition E is prepared by a process as defined in any one of Claims 10 to 25.

38. A method according to Claim 29 substantially as described herein with reference to, and as illustrated in, Figures 3 and 4 of the accompanying drawings.

39. A composite of two or more metal substrates secured together by a method as claimed in any one of Claims 29 to 38.

40. An assembly of overlapped metal substrates, each coated with an enamel coating having a carboxyl modified polypropylene resin incorporated therein, the substrates being bonded with a propylene polymer interposed between the opposed enamel coated substrates, the said coating and bonding being performed in accordance with a method as claimed in any one of Claims 1 to 27 and 29 to 38.

41. An assembly of metal substrates according to Claim 40, substantially as hereinbefore described with reference to, and as illustrated in, Figures 3 and 4, of the accompanying drawings.

42. An easy opening container member comprising a container end panel having a preformed opening, the end panel having coated therein an enamel coating having a carboxyl modified polypropylene resin incorporated therein, a removable closure member closing the opening, the closure member including a sealing surface of a propylene polymer heat sealed to the enamel coated surface, the said coating and heat sealing being performed in accordance with a method as claimed in any one of Claims 1 to 27.

43. An easy opening container member according to Claim 42, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

44. A tubular sheet metal can body having a longitudinally extending lap seam, the opposed surface portions of the sheet metal included within said side seam having coated thereon an enamel coating having incorporated therein a carboxyl modified polypropylene resin and a propylene polymer layer heat sealed and securing together the opposed surface portions of the enamel coated sheet, the said coating and heat sealing being performed in accordance with a method as claimed in any one of claims 1to27and29to38.

45. A tubular sheet metal can body according to Claim 44, substantially as hereinbefore described with reference to, and as illustrated in Figures 3 and 4 of the accompanying drawings.