CA1086148A - Method of manufacturing coated metal containers - Google Patents

Abstract

Containers and Method of Manufacture Abstract of the Disclosure Coated metal containers and method for their manufacture are provided. The containers are manufactured from metal pre-coated with an organic resin coating in which the resin is partially cured to have sufficient viscoelastic properties to remain substantially intact during multiple forming steps without the need to actively heat the coating between the forming steps. After forming the containers are subsequently subjected to an elevated temperature for a period sufficient to maximize final properties of the coated container.

Description

~3a ckE~n :1 o ~ t h_I n ~?n L I o~l ~wo-piece metal containers of tlle mulciple clrawn and d~awn and ironed type are increaslngly seen in the ~arketplace for such items as beer and beverages, foods, etc. Such con~ainers are formed by forcing a metal blank into a die or series of the same on a punch or mandrel while the blank is preven~ed from wrinkling by pressure exerted orl a clamp plate. The ~orming steps may vary in number and may include multiple dra.1ing steps or a series of drawing and ironing steps depending on the type of container being formed. In recent years, various organic coatings have been used in drawing, multipLe drawLor deep draw procedures wherein metal is precoated with various substances and then formed. In many of said methods, the coating is used primarily as a lubricant during the forming steps with little or no coating remaining on the formed con-tainer. In my U.S. Patent 3,206,848, dated September 21, 1~65 and commonly assigned herewith, deep drawn containers are produced having an organic coating thereon from metal stock precoated with an organic coating by a method of essentially baking the coating, drawing the metal, rebaking ~`?~

= , , ,, ~ ~. -; ~j' :, the coating at a temperature 10-30F below the initial hardening temperature, redrawing the metal, etc. In this method, it was necessary to relieve the stresses in the coating between each forming step and thus reduce the susceptibility of the coating to fracture during the forminy steps. For example, to produce a container having a diameter of about 2 1/2 inches and a depth of about 2 1/4 inches, a precoated blank was forced through a die to form a shallow cup having a diameter of approximately three inches and a depth of less than two inches; the cup was removed from the press~ heated to stress relieve the coating, the cup was cooled and forced through a second die to the final depth and d.iameter. Additional drawing operations are repeated as desired with baking steps between each of the draws resulting in coated containers in which the coating undergoes substantial deformation without substantial fracture or exfoliation.
In Canadian patent 1,058,454 issued July 17, 1979 in the names of Kenneth R. Rentmeester and Richard R. Bolt, there is described a method for forming drawn and ironed containers from metal sheet precoated with an organic coating that can withstand the drawing and ironing steps without substantial exfoliation or fracture and without actively heating between the drawing and ironing steps. A similar procedure for aluminum is also disclosed in UOS. patent 3,832,962 issued September 3, 1974 to Rolles.
Such methods as represented by the above referred to disclosures produce containers in which the coating on the finished container as formed is without substantial ex-foliation. The coating in these methods are applied tothe metal and partially cured to build in sufficient

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l flexibility (or other viscoelastic properties) to enable it to withs tand the deformation imposed by the various forming steps. While the containers resulting from such methods are coated as formed and permit a substantial reduction'in the number of subsequent operations necessary to produce a commercial can, the performance of such containers is usually directly proportional to the damage which the coating has undergone during forming. Where highly acidic and/o~ corrosive products are to be contained in steel cpntainers or where the containers are filled and subjected to high humidity conditions as in sterilizatlon or pasteurization processes, such factors as corrosion, adhesion, metal ion dissolution, sulfide staining or buildup, etc. are directly proportional to the number of fractures in the coating. Because the coating has been undercured to withstand the forming steps and its maximum properties of adhesion, gloss, etc. have not been realized, it is customary to view the "as drawn" coating as a base coat and to apply one or several top coats to such containers as formed to minimize exposure of the metal to the contents and to improve the performance and appearance of the container, A method for providing containers from precoated metal sheet wherein sufficient viscoelastic properties are built ' ' -into the coating initially so that baking between forming steps is not necessary and wherein the 1exible as formed coating has improved adhesion, resistance to corrosion and staining without the need for a top coat or other repair of the container i5 a primary object of this invention.
- It is another object of this invention to provide a 30 method of forming coated cup-shaped metal containers from precoated metal in which it is not necessary to bake the coating between successive forming steps.

~ 8 1 I~ is a fur~her object of this invention to provide a n!ethod of maximizing the properties of precoa~ed me~al con-tainers as formed.
Another object of this invention is to provide coated metal containers from precoatecl metal that have improved resistance to corrosion, sulfide staining and improved adhesion.
It is another objec~ of this invention to provide a method of producing formed metal containers from metal precoated with an organic resin capable of withstanding the forming steps without substantial exfoliation and wherein ~the ormed container is post-treated to improve the performance thereof thereby eliminating the need for a top-coat and other conventional post-repair steps.
These and other objects and advantages of the invent.ion will be apparent as they are better understood from the description which follows.

Detailed_Description of the Invention In accordance with this invention the method of forming containers broadly comprises the steps of:
a) applying an organic resin to the surface of flat metal strip or sheet;
b) subjecting said sheet carrying said resin to an ele~ated temperature for a period of time sufficient to effect adhesion to the metal and a partial curing of the resin to the extent that it is capable of withstanding the subsequent forming steps without exfoliation;
c) forming a workpiece from said organic-resin carrying metal sheet;

, 1 d) forcing said workpiece through sui~able dies to form a coated article without actively heating during or between the forming steps and e) subjecting said coated article to an elevated temperature for a period of time sufficie~t to further cure the coating and improve resistance properties thereof, As employed herein, curing is meant to indicate hardening an~ is applicable to either thermosetting resins which are cured usually through reaction with a suitable material with crosslinking and/or further polymerization or to thermoplastic resins which are cured or hardened usually by evaporation of solvents.
More specifically, an organic resin is applied to metal sheet or strip which may be steel including blackplate tinplate, chemically treated steel such as TFS-CT, aluminum including alodine or other conversion coated aluminum, etc., after which the resin-carrying metal is subjected to an elevated temperature, for example as by baking in an oven, for a time sufficient to cure the resin to the extent that it is capable of withstanding the forming steps, which may be drawing, multiple drawing including reverse drawing - and/or drawing and ironing, without exfoliation during and after the forming steps. A workpiece is then formed from the treated metal sheet or strip and may be a circular blank which is forced through one or more dies or a circular blank which is first drawn to a shallow cup which is then forced through one or more dies. The entire ~orming operation is performed without actively heating the workpiece and/or coating, i.e. no additional baking steps are performed during or between the forming steps. The formed coated article is ..

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l then stripped from the dies and p~mch an(l subjected to an elevated temperature for a period of time sufficient ~o further cure the coa~ing, to repair any frac~ures therein and to improve the adhesion o the coating to the ~etal.
There are thus obtained ar~icles exhibiting improved performance with highly corrosive materials and other improved properties.
The resins utilized herein may be any thermoplastic or thermosetting resin or combinations thereo that are capable of withstanding the forming steps without exfoliation, that exhibit adequate elongation, compression and plastic flow under the orming condi~ions, exhibit malleability with good adhesion to metal without adhesion to tooling; that are abrasion resistant; that will not impart off-flavor or odor or other detrimental affects to comestibles contacted therewith and that are capable of conversion to a corrosion and abuse-resistant coating upon being subjected to an elevated temperature af~er forming. It has been found tha~ vinyl resins such as polyvinyl chloride organosols, solution vinyls and combinations thereof are particularly effective as inside coatings in the process and exhibit the optimum requisite properties in accordance with the method of the invention. Suitable vinyl organosols include known compositions of polyvinyl chloride resins of relatively high molecular weight, usually at least about 15,000, which resins are substantially insoluble in the usual solvents and are designed to be dispersed in the liquid ingredients of the organosol. The high molecular weight resins are in a finely divided state, generally of a particle size of less than 5 microns. The term "vinyl organosol" as employed herein indicates dispersions of vinyl chloride resins including not 1 only the homopolymer bu~ also copolymers o~ vinyl chloride ~ith a vinyl carboxylate including vinyl acetate, vinyl butyrate, etc. usually containing at least.50% vinyl chloride in the vinyl copolymer structure. Dispersants for such resins are well known and include any substance in~which dispersions of the resins can be ~ormed and maintained with-out solution and/or gelation. Suitable dispersants include oxygen-containing polar solvents including water, ketones, ether alcohols, glycol ethers, esters and hydrocarbons, examples of which include dlisobutyl ketone, isophorone, 2-butoxy ethanol, ethylene glycol monobutyl ether, benzene, toluene ar.d mixtures of such solvents.
Solution vinyls are also a well known class of resin compositions and include vinyl chloride homopolymers as well as copolymers thereof with vinyl carboxylates such as vinyl acetate and terpolymers thereof with maleic acid or other dicarboxylic acids. These resins may also be dissolved in suitable solvents including those lis~ed above as dispersants for the organosols and particularly ketones such as methyl ethyl ketone, methyl isobutyl ketone and mixtures thereof with suitable hydrocarbons such as toluene, xylene, etc.
The vinyl resins may advantageously be admixed with ` suitable adhesion promoting solution resins, if desired, such as epoxy resins, melamines, acrylic acid resins, phenol formaldehydes, urea formaldehydes, etc. A particularly preferred composition may be illustrated by a composition comprising about 80% polyvinyl chloride admixed with a 20% solution resin mixture comprising epoxy, polyester and melamine resins. Other suitabLe resins may be illustrated by vinyl chloride-vinyl acetate copolymers and mixtures thereof, for example, copolymers comprising approximately .. . .

~W ' l 85 to 90% vinyl chloride and about S ~o 15% vinyl acetate, ~lso including such resins containing interpolymerized maleic acid. Such resins may also be admixed with other resins including epoxy, acrylic, phenol ~ormaldehyde, etc.
The vinyl resins may be applied to one or both~ sides of the metal sheet. In the instance where such resins are applied to only one side, it is preerred that they be applied to that surface of the metal that is to form the inside of the container to convey the highly beneficial corrosion-resistance and o~her properties when in contac~ wi~h thecontents of the container.
Other resins such as epoxy-urea formaldehydes and epoxy-phenolics are also suitable for use as outside and inside coatings. Such resins are well known in the art and include reaction products of the classic epoxy resin obtained by reaction of bisphenol A and epichlorohydrin, known in the art as diglycidyl ethers of bisphenol A, also referred to as DGEBA resins and other resins of this type derived from reaction of polyhydric phenols and epichloro-hydrins with phenol-formaldehyde resins. Preferred DGEBA
reactants are diglycidyl ethers of bisphenol A having average molecular weights of from about 900 to 12,000 and epoxide equivalents of from about 425 to about 6,000. Such resins may be reacted with phenolic components such as methylol ethers in which the hydrogen of the hydroxyl group attached to the phenyl group is substituted by an alkyl, alkenyl or cycloalkyl group or by an aralkyl or aralkenyl group as well as the halogenated derivatives thereof. These resins are A stage methylol phenol resins, i.e. soluble and fusible, and are disclosed and described in U.S. Patent 2,579,330.
The preferred resin is l-allyloxy-2,4-trimethylol benzene.

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Epoxy-urea formaldehycles are epoxy-amino resins derived by reaction of DGEBA resins having epoxide equivalents of 425 to 6,000. An example of a suitable epoxy-phenolic resin to be ùtilized herein may be illustrated by a ~ormulation comprising about 50 to ~0%, preferably 70% Epon 1007 [trademark], a DGEBA resin having an epoxy equivalent weight of about 2,000-2,500, about 5-50~ 1-allyloxy-2,4,6-trimethyl- , olbenzene and about l to 8% polyvinyl butyral. A preferred epoxy-urea formaldehyde may be illustrated by a mixture o~
DER667, a DGEBA resin having an epoxy equivalent weight of 1,600-2,000 and Plaskon 3300 [trademark]~ urea-formaldehyde resin.
Combinations of such resins may be employed as desired.
In a preferred embodiment, the inside coating is a vinyl resin and the outside coating is an epoxy-phenolic. It is also possible to apply a double coat system to either or hoth sides of the metal sheet prior to forming including a base coat and an overcoat of any of the above type resins and combinations of the same.
The organic resins identified herein above may be formu-lated in suitable solvents or dispersants with pigments and/or fillers and/or internal lubricants and/or plasticizers as desired, by means well known in the art. The particular additives, whether solvents or dispersants, etc. are not especially critical. It is necessary, however, that the solvents or dispersants be volatile at the elevated tempera-ture indicated and that they be compatible with all ingredients of the composition in their useful concentration.
The coating may be applied by spraying, dipping, coil ~;
coating, electrocoating, roller coating or by other means well known in the art.
Any suitable method may be used to heat the coated _ g _ ~ . ~
.

1 metal to the clesired telllperature both prior to forming and subsequently. Hot air ovens as well as HTHV (high temperature-high velocity) ovens have been used with success. Additionally, the coated sheet may be heated by induction, flamè, or infra-red means, etc.
The temperature and time to whicll the resin-carrying sheet is subjected prior to forming will vary depending on the particular resin and coating weight involved. Similarly, the time-temperature relationship to which the coated formed container is subjected after forming will likewise vary depending on the particular resin and coating weight. Short-high bakes in a high temperature-high velocity oven, for example or lower bakes for longer periods may be used with equivalent results. For the vinyl resins, conditions or the initial bake of the coated metal may vary rom subiecting the coated metal to a temperature of about 300F for about 20 minutes to subjecting the same in a high temperature-high velocity oven at 600F for about 5 seconds. Optimum results have been obtained with a sheet bake of about 390F for 6 to 8 minutes as well as with a coil bake in the HTHV oven at 575F for about lS seconds. With the epoxy-phenolics and epoxy-urea formaldehydes, the time-temperature relationship may vary from a sheet bake of 270F for about 20 minutes to a coil HTHY bake up to about 700F for about 5 seconds, optimum results being obtained at 400F for 6 minutes as well as 575F for 15 seconds. Post bakes varying from 300F
at 10 minutes to 675F for 2 seconds have been found to be effective with all of the resins, with optimum results being obtained with 400F for 5 minutes or at 500F for 6 seconds in the HTHV oven. It is to be understood, however, that other time and temperature relationships may be used to give .

1 equivalent resul~s. It is essential that th~ po5t bake not ~e so high as to degrade the resin. Similarly, post baking at too low a temperature leads to adhesi.on.loss and blistering and will not improve the performance of the container. At the same time, the initial bake temperature must be, suffi-ciently high to partially cure the resin and effect adhesion thereof to the metal but insuEicient to embrittle ~he r~sin to the extent that it is deficient in flexibility and cannot withstand the forces of deformation during formin~.
It is a particular feature of this invention that the post-bake step given to the formed coated container results in improved coating performance that is not seen when this step is omitted and the improved performance is observed regardless of the specific metal involved and regardless of the number of forming steps that have been performed on the coating as will be illustrated further hereinbelow.
While the exact reason for the improved performance is not known, it has been found, for example, with double drawn 208 x 207 alum.inum cans by examination of samples of the "as formed" and "post baked" coatings and metal by electron microscopy that the top surface o the i'as formed" sample has basically a very fine grained surface with portions of what appear to be resin particles scattered randomly on the surface while the post-baked sample differs in that sot ridges are observed, the resin particles are reduced in size and have a tendency to line up on the crests of the ridges.
The under surface of the "as formed" coating showed closely spaced ridges and furrows while the post-baked sample revealed more separation between ridges. The metal surface as post-baked had ridging and in general resembled the underside of the post-baked coating surface. These results .

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l are believed ~o inclica~e ~ha~ the post bake serves ~o relieve the stresses built into the coating during ~orming allowing the coating to relax and settle closer to the surface of the metal resulting in increased adhesion of coating and metal and improved resistance characteristics.
The following examples will serve to ~urther illustrate the invention;
Example 1 r An epoxy-phenolic resin coating was applied as an outside coating to TFS-CT ~tin free steel having a chrome-chrome oxide sur~ace treatment) sheet at a coating weight of 12 mg/4 in.2, baked for 6 minutes at a metal temperature of 400 after which an aluminum pigmented polyvinyl chloride organosol was applied as an inside coating at 30 mg./4 in.2 and baked for 6 minutes at a temperature of 390F. Both sides of the coated sheet were lubricated with pe~rolatum.
The treated sheet was placed over an annular die, within a hydraulic press and forced through a drawing die thereby forming a cup having a diameter of approximately 5.312 inches and a depth of 2.425 inches. Af ter the cup was formed by the initial drawing operation, it was placed over a second annular drawing die having a diameter less than the first.
A punch forced the coated metal cup through the die to form a 404 x 307 inch elongated, cylindrical cup, ater trimming. The cup is then subjected to a final bake step in a hot air oven for approximately 5 minutes at 400F.
The final cup is a glossy, smooth, coated article having improved adhesion and corrosion resistance.
Additional drawing operations may be performed after the second drawing operation and before the post bake ~0 resulting in a deeper ar~icle of reduced diameter. Should ~ ~ ~ 6'~ ~

1 such additional draws be desired, ~he Eollowing average reductions in diame~er for each draw is suggested where D
is the diameter: .
Circular blank D (diameter) Reduction in Diameter First Operation cup-Dl 42V/o~D
Second Opera~ion cup-D2 25% Dl Third Operation cup-D3 18% D2 To illustrate the effect of the post-bake on the performance of the contai.ner, a number o~ experiments and ~ tests were performed.

Example 2 A. 404 x 309 double drawn'TFS cans were produced employing the method and coatings of Example 1 but varying the extent of the post-bake as indicated hereinbelow. The - containers were filled with water and the filled containers were contacted with steam at 250F for 130 minutes to simula~e the conditions present in a sterilization procedure. The processed containers were examined for condition of the,inside coating with the results as listed in Table I which follows:

- 13 - ' l Table I
Con~ainer Pos~-Bake Observa~ion . _ _ 1. None Severe sca~tered blister-i~g at sidewalls and flange area.
2. 1 min. at 400F Moderate to severe blistering`at side-walls and flang~ area.

3. 2 min. at 400F Moderate scattered blister-ing at side~alls and flange area.

4. 3 min. at 400F Slight to moderate scat-tered blistering at side-walls and flange area.

5. 4 min. at 400F A trace of scattered blistering at sidewalls and flange area - Acceptable.

6. 5 min. a~ 400F No trace of blistering at sidewalls or flange area - coating in good condition - acceptable.

B. 300 x 402 containers were produced by the procedure of example 1 employing an aluminum pigm~nted vinyl organosol comprising about 60% polyvinyl chloride dispersion admixed with about 40% solution resins (Tests 1 and 2) and a vinyl ~ organosol containing about 60% of a polyvinyl chloride dispersion admixed with about 40% vinyl chloride-vinyl acetate copolymers containing titanium dioxide and lanolin (Tests 2 and.3) as the respective inside coatings.
Metal was coated and treate~ as indicated below, where the bake time (6' ~ 3'~ employed indicates that the coating was baked for 6 minutes after allowing 3 minutes for the oven to reach the indicated temperature.
Finished containers were water-packed, double seamed and processed in steam at 250F for 60 minutes after which the coating condition was evaluated. The results were as follows:

l Test l ~ 300 x 402 c~ns from coa~ed TlS at 30 mg/4 in. , initially baked 6' ~ 3' a~ 400F.

After processing, poor coating adhesion and - low gloss.

Test 2 - Same a,s Test 1 but given an 8' bake at 400 after can fabrication.
Excellen~ process resistance with improved gloss and coating continuity.
Test 3 - 211 x 300 cans from coated TFS at 45 m~,/4 in.2, 575O coil bake for 15 seconds.
, Poor coating adhesion after process.

Test 4 - Same as Tes~ 3 but given a 6' bake at 400F after can fabrication.
After processing, improved adhesion of coating.

The above findings clearly illustrate a general correlation between post-bake and coating performance on the finished containers illustrating that for the coatings util'ized a 5-minute bake at 400F'or equivalent time-temperature re--lationship gives a markedly improved container.

, ' Example 3 i~ To illustrate the ef~ect of the initial bake and post-bake temperatures on the finished container, ~75 lb. TFS-CT
was coated with the polyvinyl chloride organosol resin of Example 1 as the inside coating and an 'epo~y-phenolic coating as the outside coating. The vinyl resin applied to the inside surface included a variety of weighti~ and time-temperature relationships as indicated in Table II which ollows. Double drawn 404 x 307 cans were fabricated by the procedure of Example 1. Samples of cans from each coating variable were 1) given no post-bake treatment or 2) treated for 3 5 minutes at ~00F or 3) treated in the HTHV oven at 625F

1 for 8 seconds. The cans thus treated werc filled wi~h water, ~ouble seamed, subjected to a hot wa~er process ~or 60 mi.nutes at 240F, cut open,scotch tapcd to evaluate adhesiorl of the coating and painted with an indicator for iron which makes visible fractures or imperfections in the coating. ~The results were as indicated in Table II. In the Table, the coa~ing weights are ~he semi-wet weights excep~ ~or con~ainers 9 and 10 where the dry coating weight is used. Redraw refers to the portions of the container subjected to the mos~ defor-mation while bottom refers to the bottom end.

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_ _ _ _ _ 1 The effect o.E ~he time-~emperature relationship of the initial bake as it affects the adhesion properties and con-tinuity of the "as formed" coating as well as the ef~ect of the post-treatment on adhesion and continuity may be readily . seen from the results illustrated ln Table II. Thus, container numbers 1,2,3,7 and 8 were initially baked at 400F for 9 to - 13 minutes and exhibited the best properties on ~he "as ormed"
coating under severe condi.tions of heat and humidity while . containers 5,6,9 and 10, ini.tially baked at 3~0-390 for 9 to 11 minutes or in the HTHV oven for approximately 15 seconds exhibited the worse properties initially under these conditions.
All of such containers however, were improved in both adhesion and coating continuity after the 400F post-bake treatment while the HTHV treatment resulted in improved adhesion in all cases except the 400F - 10'+3' relationship. That overbaking initially is to be avoided is evidenced by container 4 where baking at 415F for 6'~3' showed adhesion loss and discontinuity which could not be improved by ei.ther of the treatments.

Example 4 208 x 207 double drawn aluminum cans were produced using the procedure of Example 1. Both plain and alodine treated aluminum sheet and strip were coated with an epoxy-urea formaldehyde resin coating as the outside coating, with the sheet being baked at 300F for 8 minutes and the strip being baked in an oven for 60 seconds at 650F. To each of the sheets and strip thus coated, an epoxy-phenolic coating containing about 1% lanolin was applied as the inside coating, the sheet being baked for 8 minutes at 370 and the strip for 60 seconds at about 600 to 650F. Containers were fabricated from each of the variables and were beaded and post-baked at l 380F for S minutes or beaded with no post-bake. 'IAs formed" containers were also evaluated as controls. Adhesion of the inside coating on each of the variables was evaluated before and after processing ~or 30 minu~es with 250.F steam.
The results were as given in Table III.

Table III
Metal Coatin~ Before Process Coating A.f~er Process As Beaded As Beaded Drawn Beaded Post-Baked Drawn Beaded Post-Baked Al Sheet S S S , B U S
Al + Alodine Sheet S S S . B- S S
Al Strip (650) S S S B U S
Al + Alodine Strip (650~ S S S B- B- S
Al Strip (600) S S S S U S
Al + Alodine Strip (600) S S S S B- S

In the table, S indicates sat.isfactory coating conditions;
B indicates a borderline,condition with traces of blush and/or loss of adhesion and U indicates unsatisfactory condition with blisters and/or loss of adhesion making the container unacceptable~ It will be seen from the above that while all containers were satisfactory before processing, only the post-baked containers were consistently satisfactory even after beading which further deforms the coating.

Example 5 3o 307 x 113 single draw containers were produced by the procedure of Example,l from tinplate coated with epoxy-phenolic - 19 - .
' 1 coatings. Bo~h coatings were applied as inside coatings at 16~ mgs. and baked Eor 8 minutcs at 400F. Containers thus produced were post-baked for 8 minutes at 380~ retaining some "as formed" containers without a post-bake treatment as controls, The post-baked and "as formed" containers were pack~ed with tuna, stored or several days after which they were opened, evaluated for sulfide staining and rated on a scale of 10 to 0 with 10 indicating severest staining. The results were as ~ollows:
10 Test 1 Ratin~
Container As Drawn: Severe staining 9 Container (1) Post-baked: Slight staining 3 Container (2) Post-baked: Slight staining 4 Test 2 Container As Drawn: Severe s~aining 7 Container ~1) Post-baked: Trace of staining 2 Container (1) Post-baked: Trace of staining Example 6 211 x 400 triple drawn containers were produced from 75 lb. TFS-CT with each of various coatings by the procedure of Example 1 after which containers from each variable were post-baked at 400F for 5 minutes. The containers were hot filled with 180F water, double seamed and processed for 50 minutes at 250F af~er which adhesion was evaluated with Scotch Tape. The coatings and results were as follows in Table IV.
In the table, the outside coating on all variables was an epoxy-phenolic resin. For purposes of comparison, an equal 3 number of containers not post-treated were filled, processed and evaluated under the same conditions.

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1 It will be evident ~rom the above that containers produced by the instant method and subjected ~o the post bake as described herein have improved resistance to corrosion under severe conditions of heat and humidity, are of improved gloss, coating continuity and adhesion.

It will be evident from the aforegoing that although the method has primarily been described in terms of drawing and multiple drawing steps, other steps may be used including drawing and ironing, flanging, beading~ curling, erimping, etc.
It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the steps of the method described and their order of accom-plishment without departing from the spirit and scope of the invention, the form hereinbefore described being merely a preferred embodiment thereof.

,

Claims (21)

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

1. A method of manufacturing a coated metal container having improved properties comprising the steps of:
(a) applying an organic resin coating to at least the inside surface of metal sheet or strip;
(b) subjecting said metal carrying said coating to an elevated temperature for a period of time sufficient to effect adhesion to the metal and a partial curing of the resin to the extent that it is capable of withstanding subsequent forming steps without exfoliation;
(c) forming a coated multiple drawn or drawn and ironed container from the thus treated metal without actively applying heat and (d) subjecting the container to an elevated temperature for a period of time sufficient to further cure the coating and to convey at least improved coating adhesion and resistance to corrosion to the coated formed container.

2. The method of claim 1 wherein said forming step includes multiple drawing of the metal to an elongated cylin-drical container.

3. The method of claim 2 wherein organic resin coatings are applied to both surfaces of the metal.

4. The method of claim 1 wherein said organic resin coating comprises a vinyl organosol.

5. The method of claim 3 wherein said organic resin coating applied to the inside surface comprises a vinyl organosol and said organic resin coating applied to the out-side surface comprises an epoxy-phenolic resin.

6. The method of claim 3 wherein said organic resin coating applied to the inside surface comprises an epoxy-urea formaldehyde resin and said organic resin coating applied to the outside surface comprises an epoxy-phenolic resin.

7. The method of claim 1 wherein said metal is aluminum.

8. The method of claim 1 wherein said metal is steel.

9. The method of claim 1 wherein said metal is tinplate.

10. A method of manufacturing a coated, elongated cylindrical container having improved coating continuity, adhesion, and gloss and resistance to corrosion and staining comprising the steps of:
(a) applying an organic resin coating to at least the inside surface of metal sheet or strip;
(b) subjecting said metal carrying said coating to an elevated temperature for a period of time sufficient to effect adhesion to the metal and a partial curing of the resin to the extent that it is capable of withstanding subsequent forming steps without exfoliation;
(c) forming a coated shallow seamless cup from said coated sheet;
(d) forming said shallow cup into a coated triple drawn seamless container without actively heating between the forming steps;
(e) subjecting said coated seamless triple drawn container to an elevated temperature for a period of time sufficient to further cure the coating and to convey at least improved coating adhesion and resistance to corrosion to the coated formed container.

11. The method of claim 10 wherein organic resin coatings are applied to both surfaces of the metal.

12. The method of claim 10 wherein said organic resin comprises a vinyl organosol.

13. The method of claim 11 wherein the organic resin coating applied to the inside surface comprises a vinyl organosol and the coating applied to the outside surface comrpises an epoxy-phenolic resin.

14. The method of claim 13 wherein said formed coated container is heated to a temperature of about 300°F. to 415°F. for about 5 to about 15 minutes.

15. The method of claim 13 wherein said formed coated container is heated in a high temperature-high velocity oven at a temperature of about 500 to 575°F. for about 2 to about 15 seconds.

16. In a method of manufacturing a coated metal cylindrical container wherein an organic coating is applied to the surface of metal sheet or strip, the organic coating is hardened and said container is shaped from said sheet, the improvement comprising the steps of partially curing the coating on the sheet to the extent that it is capable of withstanding the shaping without exfoliation and after shaping, heating the shaped coated container at a temperature whereby the coating is further cured and the adhesion, gloss and resistance properties of the organic coating are improved relative to the coating after shaping and before said heating step.

17. The method as described in claim 15 wherein the shaping includes multiple drawing steps.

18. A metal container having an organic resin film on the bottom end and sidewalls thereof, said organic resin film having been subjected to the forming and post-forming steps of claim 1.

19. A coated metal container of improved adhesion, coating continuity, gloss and resistance to corrosion, said container having an organic resin film on the bottom end and sidewalls thereof, said organic resin having been subjected to the forming and post-forming steps of claim 10.

20. A method of manufacturing a coated metal container having improved properties comprising the steps of:
(a) applying an organic resin coating to at least the inside surface of metal sheet or strip, said organic resin being selected from the group consisting of:
(1.) an epoxy-phenolic comprising from about 50% to about 90% of diglycidyl ether of bisphenol A having an epoxy equivalent weight of about 2,000-2,500;
about 5 to 50% of 1-allyloxy-2,4,6-trimethylol benzene and from about 1 to about 8% polyvinyl buytryal;
(2.) an epoxy-urea formaldehyde comprising a diglycidyl ether of bisphenol A having an epoxy equivalent weight of about 1,600 to 2,000 and urea-formaldehyde resin;
(3.) a polyvinyl chloride organosol comprising at least about 60% polyvinyl chloride.
(4.) vinyl solution resins comprising about 85 to 90%
vinyl chloride and about 5 to 15% vinyl acetate; and mixtures of said polyvinyl chloride and vinyl solution resins;
(b) subjecting said metal carrying said coating to an elevated temperature for a period of time sufficient to effect adhesion to the metal and a partial curing of the resin to the extent that it is capable of withstanding subsequent forming steps without exfoliation;
(c) forming a coated multiple drawn or drawn and ironed container from the thus-treated metal without actively applying heat and (d) subjecting the container to an elevated temperature for a period of time sufficient to further cure the coating and to convey at least improved coating adhesion to the coated formed container.

21. The method of claim 20 wherein the metal is steel.