CA1054870A

CA1054870A - Method for coating metal container body interiors with aqueous dispersed resin coating materials

Info

Publication number: CA1054870A
Application number: CA210,493A
Authority: CA
Inventors: Ihab M. Hekal
Original assignee: Continental Can Co Inc
Current assignee: Continental Can Co Inc
Priority date: 1974-05-20
Filing date: 1974-10-01
Publication date: 1979-05-22
Also published as: IT1038242B; DK217275A; FR2278745A1; FR2278745B1; DE2514397A1; GB1445070A; BR7502829A; AU7680074A; ZA746386B; JPS534854B2; JPS50150750A

Abstract

ABSTRACT OF THE DISCLOSURE
A method for coating the interior of metal container bodies wherein at least the interior wall surfaces of an open ended container are contacted with an aqueous dispersion of a heat hardenable resin coating material. After application of the resin dispersion, the exterior walls of the container are contacted and rinsed with water to effect the removal of any of the resin coating material adhering to the exterior container walls as well as any excessive amounts of resin accumulated at the open end. Thereafter the container is heated to effect the removal of water and the hardening of the coating applied to the interior walls of the container body.

Description

~054870 This invention relates to the manufacture of metal con-tainers and more particularly to coating the interior of the container using an aqueous dispersion of resin.
A method of metal container manufacture which is being increasingly adopted by the metal container industry is the two piece can process. This process involves forming a drawn cup from a metal sheet and then deep drawing the cup into a can configuration. After the can body is completely configurated and decorated, but before the end is assembled onto the body, the interior surface of the can body is coated with a pro-tective coating of a synthetic resin material.
It has been the conventional practice in the metal con-tainer industry to apply, as the coating material, a heat hardenable resin mixed in a volatile organic solvent which is sprayed into the open end of the container body and onto the interior walls thereof. The wet coated container is then passed through an oven in which hot air is circulated to evaporate the solvent and harden the coating.
Although organic solvent based resin coatings are widely used in the manufacture of metal food and beverage containers, one drawback to the use of these coating systems is that in the step of heat hardening the resin coating material, the solvent with which it is mixed is driven off and exhausted into the atmosphere, thereby giving rise to an atmospheric pollution problem. Within the last few years there has been a great deal of attention directed to atmospheric pollution and the atmospheric contamination which results from industrial exhaust. One of the contaminants at which attention is now directed is the solvent exhausted from the curing ovens of metal container manufacturers, p ~054870 One method of avoiding the air pollution problem that has been considered by container manufacturers is to substitute an aqueous dispersion medium for the organic solvent presently used for container coatings. The substitution, however, of an aqueous dispersion medium for an organic solvent is not without its problems.
For example, in one method of coating two-piece cans with water dispersed heat hardenable coating materials, used by the present applicant, the open-ended can is caused to be passed in an inverted position over a reservoir of aqueous coating material which is flooded into the can interior.
Excess coating material is removed to a large extent from the interior of the can by draining. However, it has been found by the present applicant that when using this method, the viscosity and surface tension properties of the aqueous based coatings are such that after draining, there is often observed an excessive accumulation of coating material on the edge portions of the open end of the container which are to be flanged preparatory to the application of a closure for double seaming. This undesired excess accumulation of coating material in the flange area is generally large enough to interfere with the double seaming operations used to affix sealing ends and permits leakage of the closed container.
In accordance with the present invention, there is provided a method for coating the interior portions of metal containers with aqueous dispersions of heat hardenable resins which includes passing an open-ended container over a reservoir of an aqueous dispersion of a heat hardenable resin, contacting the container body with the dispersion to effect the application of the dispersion to at least the interior surfaces of the container, contacting the exterior walls of the container with an aqueous solvent such as water, allowing excess dis-~054870 persion material to drain from the interior of the containerand excess water to drain from the exterior walls of the container and then heating the container body to effect the evaporation of water remaining on the container walls and hardening of the coating material remaining on the interior wall surface of the container body.
More specifically, the invention relates to a method for coating the interior surfaces of metal containers comprising the steps of: providing a metal container having a body closed at one end and terminating in an open end integral wïth the body and adapted to receive an end closure, the container being in an upright, inverted position, spraying an aqueous dispersion comprised of a water dispersible, heat hardenable resin coating material into the open end of the container to coat the interior wall surfaces of the container with a wet layer of the aqueous dispersion, a portion of the wet layer flowing down the interior walls of the container and accumulating about the open end, applying water to the exterior walls of the container, the water being applied to the exterior walls of the container in such a manner as to substantially exclude the application of water to the inside surfaces of the container, allowing the water to drain down the exterior walls of the container and mix with and dilute any of the resin coating material which accumulates about the open end and then, heating the container to remove any water remaining on the container walls and effect hardening and permanent adherence of the resin coating material on the interior wall surfaces.
It has been unexpectedly found that by following the sequence of steps of the present invention and spraying or otherwise contacting the exterior walls of the container with water after the aqueous resin dispersion has been applied to the interior of the container, that substantial removal of the .

. ~
A ~ . 3 -excess coating material accumulated in the open end area of the container is effected. In attempting to explain this unexpected phenomenon, it is the applicant's belief that due to the differ-ence in surface tension between the aqueous dispersion accumulat-ed on the interior of the open end area and the water draining past the exterior of this area, a portion of the water draining past the exterior portion of the open end area is pulled by capillary action towards the interior of the container whereby it intermixes with and dilutes the resin dispersion accumulated in the interior open end area. The so diluted resin coating dispersion is of such lowered concentration that upon drying the hardened coating is of an acceptable thickness.
Figure 1 is a side elevation of an apparatus which may be utilized for coating containers in accordance with the method of the present invention.
Figure 2 is a fragmentary longitudinal cross-section of a container after application of an aqueous resin dispersion to the interior walls of the container.
Figure 3 is a similar view of the container after application of water to the exterior walls of the container.

- 3a -~054870 Figure 4, is a further similar view of the container after drainage of water down the exterior walls of the container is completed.
Referring more particularly to Figure 1, there is shown an apparatus generally indicated at 10 which may be used to carry out the method of the present invention. Metal con-tainers 11 which have been previously cleaned and washed (in an apparatus not shown) to remove any traces of extraneous metal, lubricating oil and other contaminants derived from the container fabricating process,~ are delivered while still wet, in an upright, inverted position to coating tunnel 12 by conveyor or endless belt 13 and are there transferred to an endless reticulated belt 14. The containers 11 are comprised of side walls 15 closed at one end with end member 16 (which may be integral with the container sidewalls) and having a remaining open portion 17 having an end 18. If desired, prior to entering the coating tunnel 12, the containers 11, especially if they are fabricated from aluminum, have a chromate-phosphate conversion layer applied to the interior surface 19 of the container.
Upon entering the coating tunnel 12, the closed ends 16 of the containers 11 are contacted with a second reticulated belt 21. The belt 21 serves to support the containers 11 during their travel through the coating tunnel 12 and prevents the con-; tainers 11 from falling as they are subjected to coating resinsprays. As the containers 11 travel through the coating tunnel 12, the interior surface walls 19 of the containers are contacted with an aqueous dispersion of a heat hardenable resin by means of a ~~` spraying member 22 provided with a plurality of nozzles 23 which are disposed under the belt 14 and which are effective todirect an atomized spray of wet resin coating material onto the interior container walls 19. The spraying member 22 communicates 10548~0 with any suitable means, such as pump 24, which supplies aqueous resin dispersion material 25 under pressure to the spray member 22 from storage tank 26 through pipe 27. Excess coating material is allowed to drain from the container 11 and to flow by ~ravity back into tank 26. Coating material 25 depleted from the tank 26 is replaced with water and resin material supplied to the tank 26 through pipes 26a and 26b from respective water and resin storage means (not shown).
After the aqueous resin dispersion 25 is applied to the interior surfaces of the container 11, the containers ll are passed under a water spray member 28 connected by pipe 29 to a source of water ~not shown). The spray member 28 contains nozzles 30 by means of which the exterior walls 20 of the containers 11 are sprayed with water. This water may contain small amounts, e.g. about 1% by weight of resin coating material.
As the water applied to the container drains down the exterior walls of the container, the water is caused to intermix and dilute (as shown in Figures 2-4) any coating material accumulated on or in the interior area of the open end 18. The dilution of the accumulated coating material causes the excess coating material to be released from the interior surface of the container and drain away from the open end area 18.
Associated with the water spray member 28 is collection tank 31 in which the water effluent which drains off the con-tainer walls is collected. The drained water effluent contains a small but economically valuable amount of resin material. To recover this resin material, the effluent collected in tank 31 is pumped (by means not shown) through pipe 32 to ultrafilter 33.
The ultrafilter 33 is equipped with a semi-permeable membrane 34 adapted to separate high molecular weight resins from aqueous media. The effluent from tank 31 is pumped to the "upstream" or feed zone of the membrane 34. The water portion of the effluent passes through the membrane 34 and the resin is retained whereby a concentrated dispersion of resin solids is formed in the feed zone of the membrane. This concentrated resin dispersion is pumped from the ultrafilter 33 by means of pump 35 through pipe 36 to storage tank 26. The water separated from the effluent referred to as "permeate" is pumped by pump 37 through pipe 38 to the spray member 28 to be reemployed in the washing of the exterior walls of the container bodies.
If desired or necessary, to insure that all superfluous coating material has been removed from the exterior body walls and end area of the containers 11, the containers may be passed under a second or post water spray member 40 and washed again. Effluent drained from the container bodies in this second optional washing step may be colleced in tank 41. When sufficient effluent has been collected in tank 41, the effluent is pumped to the feed zone of ultrafilter 33 through pipe 42 by pump 43 in order that any residual resin material present in the effluent of the second washing step can be recovered in the ultrafiltration unit 33 and returned to coating material tank 26 and the permeate to spray member 28.
After the containers 11 have been rinsed with water, they are conveyed out from the coating tunnel 12 and transferred ~ to suitable conveyor means 48 whereby the containers 11 are con-¦ ducted to a drying oven (not shown) to effect hardening of the coating material applied to the interior walls of the container body as well as to evaporate any water remaining on the container bodies.
In an attempt at a fuller explanation of the phenomenon by which excess coating material is removed from the container open end area in accordance with the method of the present invention, there is shown in Figure 2 the open end 18 of container 11 as it appears when inverted on the belt 14 immediately after the resin dispersion has been applied to the container interior as shown in Figure 1. The view of Figure 2 shows the increased thickness 50 of the coating material which has drained down the interior walls and accumulated about the interior portion of open end area 18 as well as any tramp coating material 51 adhering to the exterior portion of the open end area.
In Figure 3, there is further shown the open end area 18 of container 11, the view being taken after the exterior walls 20 of the container 11 have been rinsed with water and shows the removal of the tramp coating material from the exterior portion of the open end area and the initiation of the intermixing of the water 52 draining down the walls 20 with the excess coating material 50 to form diluted coating material 53 in the area immediate to the interior portion of the open end 18.
In Figure 4, there is still further shown the open end 18 of the container 11, the view being taken after the container 11 has undergone the water rinse step of the present invention and showing the coating material remaining on the open end after the water has been allowed to drain from the exterior walls 20 of the container 11. The view graphically illustrates the extent to which the undesired accumulation of resin material has been caused to be removed from the interior of the container and to restore a uniform thickness over the entire interior container surface.
Although the process of the present invention generally provides the normal coating thickness in the open end area, it is not deleterious to the usefulness of the container if the coating applied in the open end area is attenuated by the water rinse step to less than the normal thickness of the interior coating as this open end area during the closing operation is double seamed with the end closure and is thereby removed from contact with the packaged products.

The heat hardenable resin of the aqueous dispersion used in the practice of the process of the present invention is advantageously comprised of a carboxyl containing polymer in admixture with a water-soluble cross-linking agent or resin.
Illustrative examples of carboxyl containing polymers include copolymers of acrylic or methacrylic acid with lower (1 to 5 carbon atom) alkyl esters of either or both of these acids or another alkyl ester. Other useful carboxyl containing polymers include copolymers of alpha, beta-ethylenically unsaturated acids such as acrylic, methacrylic or maleic acid with alp~a-olefins such as ethylene and propylene. Still other useful carboxyl containing polymers are polyester resins having-excess carbo~yl groups, e.g. a polyester prepared from phthalic an-hydride and neopentyl glycol. Carboxyl containing polymers generally contain about 5-40% by weight of the acid component and are converted to water dispersible materials by neutraliza-tion with ammonia, amine compounds or an alkali metal hydroxide such as NaOH.
Preferred carboxyl containing polymers include co-polymers of acrylic or methacrylic acid with one or more alkyl acrylates such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate as well as ethylene/
acrylic acid copolymers, ethylene/methacrylic acid copolymers, ethylene/propylene/acrylic acid copolymers and ionomer resins, i.e. inorganic salts (Na, K,) of ethylene/acrylic acid or ethylene/methacrylic acid. A representative and preferred carboxyl containing polymer is Carboset 514* which is an ethyl acrylate/methyl methacrylate/methacrylic acid terpolymer having 10% reactive groups manufactured by B.F. Goodrich.
Thermosetting resins are suitable cross-linking agents for use in combination with the carboxyl containing polymers.
Thermosetting resins which are used in admixture with the * trademark A

~054870 carboxyl containing polymer are generally water-soluble con densation products of resin forming character including amino-plasts such as urea-formaldehyde resins, melamine-formaldehyde resins, ethylene-urea-formaldehyde resins and aminoplasts derived from the formaldehyde condensates just named and modified with methanol or ethanol, uron resins such as N, Nl-bis(methoxymethyl) uron and polyepoxides of water-soluble or water dispersible character, especially the polyether condensa-tion products of epichlorohydrin and a polyhydric phenol such as Bisphenol A (p,p'-isopropylidinediphenol). A representative and preferred epoxy resin is a commercial resin identified as EPON 828* sold by the Shell Chemical Co. EPON 828* is the re-action product of epichlorohydrin and Bisphenol A and is a viscous liquid at room temperature and has an average epoxide equivalent of one gram equivalent of epoxide per 200 grams of resin.
An especially useful class of cross-linking agents which may be used in combination with the carboxyl containing polymers axe aliphatic polyamines of the formula NH2 (C2H4~H)X
C2H4NH2 where x is an integer of 0 to 20. Included in this class of cross-linking agents are diethylene triamine, triethylene tetramine and tetraethylene pentamine. These coating compositions are more fully described in U.S. patent ~o. 3,904,569, Ihab M.
Hekal et al, September 9, 1975.
In preparing the aqueous resin dispersions useful in the practice of the present invention, the dispersions generally contain about 5 to 15% solids comprised of about 75 to 95% by weight of the carboxyl containing polymer and about 5 to 25% by weight of the cross-linking agent. These dispersions when appli-ed to the interior surface of a container may be dried by heating in an air oven for 0.5 to 5 minutes at 300 to 400F.
An aqueous dispersion containing 10% by weight solids comprised of 84% by weight Carboset 514 and 16% by weight EPON
* trademark A g 105487~

828 neutralized to clarity with 28% ammonia has been found to be especially useful in coating the interiors of aluminum containers by the process of the present invention.
The water rinse step whereby the exterior walls of the container bodies are washed is performed at an ambient temperature of about 50 to 150F.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method for coating the interior surfaces of metal containers comprising the steps of:
providing a metal container having a body closed at one end and terminating in an open end integral with the body and adapted to receive an end closure, the container being in an upright, inverted position, spraying an aqueous dispersion comprised of a water dispersible, heat hardenable resin coating material into the open end of the container to coat the interior wall surfaces of the container with a wet layer of the aqueous dispersion, a portion of the wet layer flowing down the interior walls of the container and accumulating about the open end, applying water to the exterior walls of the container, the water being applied to the exterior walls of the container in such a manner as to substantially exclude the application of water to the inside surfaces of the container, allowing the water to drain down the exterior walls of the container and mix with and dilute any of the resin coating material which accumulates about the open end and then, heating the container to remove any water remaining on the container walls and effect hardening and permanent adherence of the resin coating material on the interior wall surfaces.

2. The process of claim 1 wherein the interior walls of the container are wet with wash water prior to the application thereto of the coating dispersion.

3. The method of claim 1 where the heat hardenable resin is a mixture of 75 to 95% by weight of a carboxyl containing polymer and S to 25% by weight of a cross-linking agent.

4. The method of claim 3 wherein the carboxyl containing polymer is a copolymer of an acid selected from the group consist-ing of acrylic and methacrylic acid and at least one lower alkyl ester, the alkyl group containing 1 to 5 carbon atoms.

5. The method of claim 3 wherein the carboxyl containing copolymer is an ethyl acrylate/methyl methacrylate/methacrylic acid terpolymer containing 10% reactive groups.

6. The method of claim 3 wherein the carboxyl containing polymer is a copolymer of an alpha-olefin and an acid selected from the group consisting of acrylic and methacrylic acid.

7. The method of claim 6 wherein the carboxyl containing copolymer is an ionomer resin.

8. The method of claim 3 wherein the cross-linking agent is a thermosetting resin.

9. The method of claim 8 wherein the thermosetting resin is selected from the group consisting of polyepoxides, urea/
formaldehyde, uron and melamine/formaldehyde resins.

10. The method of claim 3 wherein the cross-linking agent is a polyamine having the formula NH2(C4NH)xC2H4NH2 where x is an integer from 0 to 20.

11. The method of claim 8 wherein the thermosetting resin is a reaction product of epichlorohydrin and p,p'-isopropylidine-diphenol.

12. The method of claim 3 wherein the carboxyl containing polymer is a polyester resin having an excess of carboxyl groups.

13. The method of claim 12 wherein the polyester resin is prepared from phthalic anhydride and neopentyl glycol.