CA1126670A - Bonded can and process for preparation thereof - Google Patents

Abstract

Abstract of the Disclosure A bonded can having a structure formed by lap-bonding a can blank composed of a tin-free steel by a polyamide type adhesive, wherein the can blank comprises in the lap-bonded portion an interposing adhesive layer composed of an epoxy resin and a polycyclic phenol-containing phenol-aldehyde resin, the can blank comprises an anti-corrosion protecting coating layer composed of a thermosetting resin at least on the inner face of a portion other than the lap bond portion, and the thickness Ta of the interposing adhesive layer and the thickness Tp of the protecting coating layer satisfy the following requirements:
Ta/Tp = 0.025 to 0.7, Ta = 0.5 to 9 microns, and Tp = 3 to 20 microns.

Description

'7~

Background of the Invention _ _ (1) ~ield of the Invention This invention relates to a bonded can having high heat resistance and a procc-~s_ for the preparation thereof, More particularly 9 the invention relates to a bonded can which has a high corrosion resistance and a high leak resistance 9 especially a high resistance to leak with lapse of time 9 in combination even if it is stored after -the inner face thereof has been caused to fall in contact with high-temperature hot water or water vapor as in case of a so-called hot-pack can or retort can 9 and also to a process for the preparation of such bonded can.

(2) Description of the Prior Art.
In -the can manufac-turing industry, so-called tin-free steels ( T~S ) such as chromium-plated steel plates and chromic acid-treated steel plates having a chromium oxide film on the surface have recently been vigorously used as metal blanks for cans inst~ad of tin-plated steel plates. Since soldering is very difficult in these metal blanks for cans 9 side seams of can bodies are exclusively formed according to the method in which both the confronting side edges of a can blank are bonded to each other by an organic ad-hesive such as a polyamide type adhesive.
According to the known process for productionof bonded cans, a primer acting as a bonding promotor and also as an undercoat enamel is first applied to ~ 7~

b~th the surfaces of a metal blank for a can9 the applied prime~is ~aked, a polyamide type adhesive located betweell bo-th the confronting side edges of the metal blank is then molten and the molten polyamide ~ype adhesive is cooled and solidified under compres-sion to bond both -the side edges of the metal blank to each other. The so formed can body is double-seamed with a can lid9 and a topcoat lacquer is applied -to thë inner face of the can be spraying or the like to obtain a pro~uct can, A can prepared according to the above-mentioned known process pr~vidies good results when a carbonated drink or the like is packed9 because such~ content is packed at low temperatures and no sterilization is necessary since the content is acidic. However9 when hot packing is performed for attaining a good preserving effect o~ the content or when retort sterilization is required after packing and sealing and the packed can is stored for a long time 9 no satisfactory results can be obtained in case of cans prepared according to the above-mentioned conventional process.
More specifically 9 since the topcoat lacquer for imparting a corrosion resistance is applied to a can body formed i'rom the rnetal blank, there is a require-ment that the baking -temperature of the topcoat lacquer should not exceed the melting point o~ the adhesive used. Accordingly 9 a lacquer of a thermoplastic resin such as a vinyl chloride resin has heretofore been J~7~

mainly us~d as the topcoa-t lacquer. This thermo-plastic resin lacquer9 however, is poor in the heat resistance arld the resis-ta1lce to hot water, and there-fore 9 degradation by hot wa-ter such as so-called blushing is causecl at reto~ s-terilization and other -troubles ;uoll as reduction of adheslon to -the under-coat are si;llilarly caused. .4ccordingly, a sufficient corrosion resis~tance canno-t be attained after the retort s'~erilization treatment.
Tin~free steel can blanks are quite different from -tin-plated steel plates customarily used for retort cans in the point that even if a defect is formed on one point of a protecting coating at the can manufacturing process 9 corrosion of the steel substrate advances from this defect to cause dissolution of the metal ( iron ) into the content and formation of pin-holes on the can body. In case of a can blank composed of a tin-plated steel plate 9 the tin-coating layer is sufficiently thick and SOIt9 and tin has such electro-~ 20 chemical characteristic that dissolved tin prevents - corrosion of the steel substrate and dissolution of iron Accordingly, in case of the tin-plated steel 9 even if defects are formed on the can blank at the can manufacturing process, these defects do not cause serious failure such ~s mentioned above. On the other hand, a chromium-containing coating layer in the TFS
- blank is much thinner than the tin-coa-ting layer and is hard and brittle Moreover, chromium does not i7~

possess sucl~ elec-',rochemical charactcristic as possessed by tin, Accordingly~ the chromium-containing coating layer is ;,usc~ptible -to scratch at the can manufacturing process a~d ,nce r`ol~med scra CC~2C. cause serious defects OWillg to lissollltiol of iron and corrosion of the ean body.
In th-? kno~l process for produetion of bonded cans 9 sinee -the -topcoat lacquer is applied in the form of a solution and is then baked 9 consequently9 an ~ 10 adhesive 9 sealing compound and undercoat enamel eonsti-- tuting the can body receive undesirable influences of heat and solven-t. More specifieally9 the undercoat enamel and adhesive are swollen under influenees of the solvent and heat and are thermally degraded. This therrnal degrada~tion resul-ts in weakening of bonding in the bonded area and eaus~s leakage of the eontent.
~his degradation of bonding is espeeially prominent in the fabrieated por-tions of the ean body9 for example9 a portion subjeet2d -to scvere fabrieated sueh as double seaming9 and wher, sueh c~n is subjected to retort sterilization 9 leakage of -the eontent is readily caused in such portion. ~urther 9 the degree of erystalliza-tion is inereased in thc adhesive by heating at the step OI baking the topcoat lacquer9 and the elongation at break of the adhesive eonstituting the bonded por--tion is redueed and also the elastieity is redueed. As a result 9 leakages are caused after the retort steri-lization treatment.

I.eal~s caused when bonàed cans are subjected to t.he re-tort steri,Lization treatment are classified in the followil~ three types~
(1) ~reakage o~'' Can Body.
'~ en de~radation OI borlding a-t the side seam is extreme brea~age of' the seam~ i.e,9 breakage of the can 'L)ody9 is caused at -the retort sterilization -treatmen-t.
(2) Microleakageo Even if rnicroscopic insufficient bonding is present at one point of the bonding interface of the side seam or double-seamed portion9 a very fine leak is caused jus-t af-ter -the retort steriliza-tion treat~
~.entO

(3) Leakage wi-th Lapse of Time ( Slow Leak ) o Even if tl1ere is no leak just after the retort sterilization trea~ment 9 since -the interior of the can maintained under such a low pressure as 20 to 60 cm Hg after the retort sterilization treatment9 if the adhesive or sealing compound is degraded by the retort sterilization -treatment or the elongation and elasticity are reduced by the retort sterilization treatment9 leakages are gradually caused wllile -~1e retor~ted ca med package is stored for a long time.
2~ According to -the conventional process for produc-tion of bonded c~ns9 a can body on which the topcoat lacquer has been coated should be subjected to the baking treatm nt at a relatively low temperature, ~ccordingl..y9 i-t so~.~etimes happens that a solvent is lef-t in tlle -topcoa-t layer in an amount very small but ul1rlegligible. In -this case, the flavor of the content is degIade~ by -thc resiclllal solven-t ~-t -the retort sterili7ation st~,r).

0.~
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I-t i.s therefore a prir,lary objec-t of this inven-tion to provide a bonde~i can that can be retorted and hot-packed9 which has high corrosion resistance and high leak resistanc~ 9 especially high resistance to leakage with lapse of -time, in combination even if it is stored after thc inncr face of the can has been caused to have contact with high-temperature hot water or stea.n, and a prccess for the preparation of such bonded can.
Another object of this invention is to provide a bonded can that can be retorted and hot-packed, in which an adhesive and soaling compound constituting tihe can body are protected from the actions of heat and solvent and dcgradation of properties of the adhesive and se21ing compound 9 especially the elongation 9 elasticity and softness 9 can be effectively preventcd 9 and a process ior tl1e preparation of such bonded can.
~ till another object of this invention is to provide a bonded can tha-t can be retorted and hot-packedj in which an optional food can be stored for along time withou-t deterioratioll of the flavor of the packed food, and a process for the preparation of such bonded can.

A further object of this invention is to provide a process for the preparation of bonded cans, in which bonded cans that can be retorted and hot-packed can be prepared at high efficiency by the reduced number of steps while eliminating a troublesome operation of coating a topcoat lacquer on inner faces of respective can bodies after the can body-manufacturing process.
Brief Summary of the Invention In accordance with one fundamental aspect of this invention, there is provided a bonded can having a structure formed by lap-bonding a can blank composed of a tin-free steel substrate by a polyamide adhesive, wherein the can blank comprises in the lap-bonded portion an interposed primer layer which is interposed between the tin-free steel substrate and the polyamide adhesive and is composed of a composition comprising (a) an epoxy resin and (b) a re-sol type phenol-aldehyde resin formed by a condensation reaction between formaldehyde and mixed phenol containing (1) a dihydric phenol represented by the following formula:

HO - ~ - R ~ - OH

wherein R stands for a direct bond or a divalent bridging group, and (ii) a monohydric phenol at a (i)/(ii) weight ratio of from 98/2 to 65/35, said epoxy resin (a) and said phenol-aldehyde resin (b) being present at an (a~/(b) weight ratio of from 95/5 to 50/50, and an anti-corrosion protective coating layer coated at least on the inner face of the whole portion of the can blank other than the lap-bonded portion prior to application of the polyamide ad-hesive, said protective coating layer being composed of a thermosetting resin having a gel proportion of at least 70% determined by extraction in 60C
chloroform for 60 minutes, and the thickness Ta of the interposed layer and the thickness Tp of the protective coating layer satisfy the following require-ments:
Ta/Tp = 0.025 to 0.7, Ta = 0.5 to 9 microns, and Tp = 3 to 20 microns.

L . ' ~31.,r~6~7~
According to anot]ler aspect of the present invention there is provided a process for the preparation of heat-resistant bonded cans, which comprises the steps of (I) coating an enamel comprising an epoxy resin and a phenol-aldehyde resin at least on a portion to be lap-bonded of a can blank composed of a tin-free steel to form a primer layer having a thickness Ta of 0.5 to 9 microns, said epoxy resin ~a) and said phenol-aldehyde resin (b) being present at a weight ratio of from 95/5 to 50/50, said phenol-aldehyde resin (b) being composed of a resol type phenol-aldehyde resin formed by condensation reaction between formaldehyde and a mixed phenol con*aining phenol containing (i) a dihydric phenol represented by the following formula:

HO _ ~ R ~ - OH

wherein R stands for a direct bond or a divalent group, and (ii) a monohydric phenol at (i)/(ii) weight ratio of from 98/2 to 65/35, (II) coating an anti-corrosion enamel comprising a thermosetting resin on the can blank except the portion to be lap-bonded to form an anti-corrosion protective layer having a whole thickness Tp of 3 to 20 microns, the ratio of Ta/Tp being in the range of 0.025 to 0.7, (III) applying a polyamide adhesive to the primer layer of the can blank, (IV) lapping the portion to be lap-bonded having thereon the polyamide type adhesive in the molten state, and (V) pressing the lap-bonded portion under cooling to effect lap bonding of the can blank.
Brief Description of the Drawing Figure 1 is a view illustrating the layout of the bonded can of this invention.

Figure 2 is an enlarged sec~ional view illustrating the bonded portion of the bonded can shown in Figure 1.
Figure 3 is a diagram illustrating one sequence of the steps of -8a-667~3 preparing the bonded can of this invention.
Figure 4 is a diagram illustrating another sequence of operations at the coating step.
Figure 5 is a diagram illustrating still another sequence of operations at the coating step.
Figure 6 is an enlarged sectional view showing an instance of the layout of the interposing adhesive layer, protecting coating layer and polyamide type _ g_ .,~

3L ~ f4 ~

adhesive il~ th~ bonded portion of the bonded can of this invention.
ll~ig-o 7 i s ar~ enlargrod s~ctional view illustrati-ng ailo-ther insi,ance o-; the layout of -the in-t~rposing adhesivc la~J r 9 protec-i,iYLg coatin" layer aIld polyamide c ~dllc~ -c, ~'`ig. r; i': ail enlarged sectional view illustrating s-t;ill an~-ther instence of the layout of t'rle interposing ~dh~siv_ ].ayer 9 protectin~ coating layer and polyamide t-ype adhesive.

In the instant specification and appended claims, by the term " can ;' or ;~ can body " is meant a packaging metal vessel 9 whether or not a content is packed9 ancl by the term i' calmed package i' is meant a package in which a content is packed and sealed.
The bonded can of this invention and a process for the prepar~tion thereoi ~Jill now be described in detail, [ Structure of Bonclecl Can ~
The bonded can OI this invention consists of a can body formed by bonding a can bl~lk 1 at a side seam 2 as shown in Fig~. 1. This side seam 2 is constructed by lap boncling using a polyamide type adhesiv~ 3.
This can blan~ 1 is composed of a tin~free steel ( TFS ) 4, and -the portion corresponding to the side seam 2 has an in-terposing adhesive layer 5 composed of an epoxy r~sin and a polycyclic phenol-containing phenol-a]dehyde resin 9 -~hich has been applied prior to lap ~ 10 --6&i7a~

~ondiY~; by the polyami~1e type adhesive 3, ~ 'u~-tl^-r 9 -thi.s ca~ blank 1 has at least on the inn?r fa~e of a portio.l othe.~ than the lap-bond portion ;~. an anti-corrosio.n protecti,ng coa-ting layer 5 composed ol ~ therriose-t-(.io,g resill., wl-lic`l-l has been appiied prior to lap kond:irl~g by the polyar.,de -type adhesive 3.
One o:î -the irnoortan-t featllres of -the bonded can of this invention is that in order to form bondingrr having a sufficient resis-tance to retort sterilization and not causing leakage with lapse of tirne 9 a certain specific material should be chose-n f'or the interposing adh.esive layer 5 and a specific t,hickness should be given to this layer 5 9 and that in order to preven-t corrosion of the can blank at the retort sterilization process and during the storage 9 a certain specific ma-terial shoulcl bi chosen for the anti-corrosion protecting layer 6 and a specific -thickness should be given to this layer G, More specif'ically9 in this invention 9 ~n enamel comprisin~ an epo~,~y resin and a polycycli.c phenol-containiilg phenol-aldehyde resin is used for -forrrlation of the interposing adhesive layer 5 and an enamel cornprising an anti-corrosiol thermosetting resin is used for ~ormation of the anti-corrosiorl protec-ting layer lnstead of a thermoplastic resin lac~,uer lleretofore usedO Further, as seèn from the enlarged view of ~ig. 2 9 the thickness Ta of the in-terposing adh~sive layer 5 and -the thickness Tp of -th~ anti-corrosion protecting layer 6 are arranged so ~ '7~' -tha~ t1l :L o~Llowiil,r, reqtlirem-n-ts are sa-tisfied, Ta/Tp - 0,025 to 0.7? especially 0.05 to 0,69 Ta = 0.5 to ~ microns 9 c~specially 1 to 6 microns 9 and Tp a 3 to 20 microns 9 especially 5 to 12 r;!icrons, This invc-n-tion is based on -the novel findillg that ~hen thc interposing adhesive layer 5 and ant,i-corrosion protect ng coating layer 6 composed of -the above-meiltioned specific materials are formed so as to satisfy t~ie above thickness requirements 9 though thesc layers 5 and 6 formed on a TFS ma-terial prior to application o~ the polyamide type adhesive 3 and bonding by the po~Lyamide -type adhesive 3 9 it is possible to impart to the f`inal bonded can excellent corrosion resistance and excellent leak resistance after retort steriliza-tion 9 and the troublesome operation of applying a topcoat lacquer to respective cans separately need not be conduc-ted and various -troubles mentioned above can be eliminat~dO
Incidentallyy the thickness of the interposing adhesive layer 5 may be equal or different be-tween the inside and outside of the bonded portion 2, In bonded cans 9 in order to form a strong, hot water resistant bondin.r; sufficient to resist retort sterilization and prevent leakages under such a reduced pressure as 15 to 50 cm H~^9 it is necessary that the thickness of -the interposing adhesive layer 5 should ke relatively small 9 and in order to impart a sufficient resistance to various proce,s~ings at t,he can manufact-uring process and preve~nt corrosion of the TFS material 7~3 n~t oilly a~t -the retort sterilization proces~ but also durill, th. suhsequellt storage9 it is necessary ~ha-t thc thic~s]-loss of the anti-corrosion pro-tecting làyer 6 should be r~latively large. In this inventiorl, -these two conditions are effectively satisfied by for]-ning the interposlrlg adhesive layer ~ and protecting coa-t ing layer 6 in sllch thic~nesses as sa-tisIying the above~mentioned three thickness requirements, l~ccording to the conventional tecl~iqueg a pro tecting lacquer is applied to a can body after formation thereof 9 and i-t has been believed that high-temperature baking of the protecting lacquer will result in degra~
dation of the adhesive or sealing compound. Therefore9 it has not been considered to use industrially a thermosetting resin enamel as the protecting coating enamel. According to this invention9 on the other h&nd9 it has been found -that when a thermosetting resin enamel is applied as the protecting coating layer 6 prior to application of a polyar.lide type adhesive or bonding by this adhesive9 if the thickness of -this layer 6 satisfies the above-mentioned -thickness require-rnents9 the layer can be a protecting layer having suffi-cient resistance -to various processings and re-tort sterilization. Based on this finding~ we have succeeded for -the firs-t time in USillg a thermose-t-ting resin as enarrl-l for forming a protecting coating layer ( namely, an inner face-coating enamel ) in the manufacture of bonded cans.

7~3 ~ loreov~?r9 .in -this invention9 tne thermosetting resin ellamel can be sufficiell-tly baked so as to form a protectin~ coa-til:lg :layer ~.uch more excellent in the resis-tance to plocessings and retort sterilization and the corrosion r sistance without degradation of the polyamid.e type adhesive or sealing compound by heat and/or solvell-tO Namely 9 such troubles as degra-da-tion of the adhesive or sealing compound can be eliminated and the troublesome operation of applying a protecting enamel to formed can bodies can be omitted. Therefore 9 according to this invention 9 there can be attained various great advantages with respec-t to not only the properties of bonded caas but also the step number and manufacturing costO
[ Respective Materials ]
(1) TFS MaterialO
Any of known tin-free steel ( TFS ) materials can be used in this inventionO As the TFS material 9 there is kno~ a structure comprising a substra-te o F a steel plate such as a rolled steel plate and a chromium~
containing coa~ting layer composed of metallic chromium9 non-metallic chromium or a mixture thereof9 which is formed on -the surface of the substrate9 and this coated s-teel pla-te is conveniently used in this inventionO
A coated plate having a chromium-con-taining layer in a thickness of 0O06 to 3O6 mg/dm29 especially 0.1 to 2.5 mg/lm2 3 as chromium is readily available and is suitably used in this inventionO Of course9 the material that -- lLL --R~f'2~7 ~

can be used in this invention is not limited to such chromium coa-ted steel plate~ Namely9 aluminurn-plated steel plates9 electrolytically ~inc-plated steel plates9 cold-rolled steel plates and the like can be used appropriately depending on the intended uses.
As a coated steel plate especially excellent in the corrosion resistance9 there is known a coated steel plate comprising a substrate of a steel plate 9 a metallic chromiun layer formed on the substrate and a non-metallic chromium layer ( composed of chromium oxide and/or hydra-ted chromium oxide ) formed on the metallic chrornium layer9 in which the-thickness of the metallic chromium layer is 0.05 to 3.0 mg/dm29 especially 0.1 -to 2.0 mg/dm2 and the non-metallic chromium layer has a thickness of 0.01 to 0.6 mg/dm29 especially 0.05 to 004 mg/dm29 as chromium. Also this TFS
material can be suitably used in this invention, In this invention9 it is preferred that the entire thickness of the TFS ma-terial to be used be in the range of 0.12 to 0.40 mm9 especially 0.14 to 0.36 mm9 though the preferred thickness differs to some ex-tent according to the intended use of the bonded can. When the thickness of the TFS material is smaller than 0.12 mm9 deformation is readily caused during -the can manufacturing process or storage. When the thickness exceeds the above range, processing such as double deaming becomes difficult.
(2) Interposing Adhesive Layero The kind of the material constituting the interposing '7~

ad'losi~T~ layer is not particularly critical in this invention, ,o far as it cor.~prises (a) an epoxy resin and (b) a polycyclic phenol~containing phenol-aldehyde resin.
All oi` epoxy resins that are customarily used as epoxy resin components in so-called phenol-epoxy enamels can be used in tln:is invention as the epoxy resin compo~-nent (a). For exar.ple9 there can be mentioned an epoxy resin havin~ an average molecular weight of 800 to 5500, preferably 1400 to 55009 which is prepared by condensation of bisphenol A [ 292 bis(4-hydroxyphenyl)-propane 1 with an epihalohydrin. This epoxy resin is used especially suitably for attaining the objects of this invention. This epoxy resin is represented by the following general formula.

\2/ 2 O ~R-O-CIH-CH2-O ~nR-O-CH2-CH-CH (I) O OH O

w~erein R stands for a condensation residue of 292-(4-hydrox~JTphenyl)propane and n is a number selected so as -to provide a resin having an average molecular weight of ~00 to 5500.
The molecular weight of the above-mentioned epoxy resin is an average molecular weight. Accordin~ly, in this invention9 it is possible to use an epoxy resin for an enamel 9 which has a relatively low degree of polymerization9 and a high-molecular~weight linear epoxy resin9 namely a so-called phenoxy resin9 in combination ~ ~?~j~ 7~

in such amounts tha-t -the ave-L-a e molecular ~Jeight is within the above-mentiorled range~
Any of phenol~aldehyde resins can be used in this invention as tlle resiYl component (b) 9 SO Iar as a poly-cyclic phenol is contained in the resin s-eleton.
3y the term i polycyclic phenol ;' used in the instant specifica-tion and claims is ~!leant a phenol containing a plurality of rings to whicll a phenolic hydroxyl group is bo-nded. As a typical instance of such polycylcic phenol~ there is knol~m a dihydric phenol represented by the following formulaO
HO ~ R ~ OH (II) wherein R stands for a direct bond or a divalent bridging group~
This phenol can be suitably used for attaining the objects of this invention. As the divalent bridging group of the dihydric phenol of the formula (II) 9 there can be mentioned 9 for example 9 an alkylidene group -CRlR2- in which Rl and R2 s-tand for a hydrog~en or halogen atom or an alkyl or perhaloalkyl group having up to 4 carbon atoms 9 -O- 9 -S- 9 -SO- and a group -NR3 in which R3 stands for a hydrogen a-tom or an alkyl group having up to 4 carbon atoms. In general 9 the alkylidene group and ether group are preferred as the divalent bridging group. Suitable examples of the dihydric phenol (a) include 2 9 2-bis(4-hydroxyphenyl)propane ( bisphenol A ) 9 2 9 2-bis(4-hydroxyphenyl)butane ( bisphenol B ) 9 1 91-bis(4-~ 7 ~

hydroxyphenyl)ethane 9 bis(L~ ydroxyphenyl)rnethane ( bisphenol F ) 9 4-hydroxyphenyl ether and p-(4-P~' 1 hydr ~ ne~1ol. .-'~mong thel1l0 l~isphenol .. and bisphenol B
are most preferred.
Such polycyclic phenol is reacted singly or together Wit}1 other phenol wi-th formaldehyde to form a resol type phenol-aldehyde resin. AS -the other phenol to be com-bined wi-th the polycyclic phenol, all of monohydric phenols customarily used for production of resins of this type can be used. In general 9 however, it is preferred to use a-t least one mernber selected from bifunctional phenols represented by the following general formulaO

OH
R4 (III) wherein R4 stands for a hydrogen atom or an alkyl or alkoxy group having up to 4 carbon atoms 9 wi-th the pro~iso that of three R4 groups9 two are hydrogen atoms and -the remaining one is an alkyl or alkoxy group 9 and R5 stands for a hydrogen atom or an alkyl group.
As typical instances of the bifunctional phenol represented by -the above general formula (III) 9 there can be mentioned o-cresol 9 p-cresol 9 p-tert-butylphenol 9 p ethylphenol 9 2 9 3-xylenol and 2 9 5-xylenol. In addition 9 trifunctional phenols such as phenol ( carbolic acid ) 9 ~ 18 -6S17~3 ;i- cr esol 9 m-etllylp}l.nol9 395-xylenol and m-methoxyph~nol 9 mOnOfUnCtiOna1 Pl1enO1S SUCh as 2,4-xylenol and 295-XY1t`~'01 9 a11d Other bifunctiolal phenols such as p-t;ert~amylphello1 9 p-nonylphenol9 p-phenylphenol and p-cycloh;exylphellol may be used singly or in combination With the bifunctional phenol of the formula (III) for production of phenol-aldehyde resins.
~ s poin-ted out h~reinbefore 9 in order to impar-t -to a bonded can a sui`ficicnt resistance -to retor-t steriliza~
tion and prevent leakages with lapse of -time during storage after the retort sterilization treatment 9 in -this invention it is very important that an enamel comprising an epoxy resin and a PO1YGYC1iC phenol-con-~ taining phenol~aldehyde resin should be for the inter posing adhesivc layer. ~en an enamel comprising anepoxy resin and a polycyclic phenol-free phenol-aldehyde resin is used for the int~rposing- adhesive layer 9 it is very difficult to form a bonded portion having a suffi-cient resistance to retort sterilization and breakage of the can body or microleakage is readily caus~d.
The amount of the polycyclic phenol in the phenol-aldehyde resin i, at le~st 13 ~09 especially at least 30 ~'0, FrOM the viewpoint of -the resis-tance to the retort sterilization treatinent9 it is preferred that the poly-hydric phenol (x) be used in combination with -the above-men-tioned monohydric phenol (y) at the (x)/(y) weight ratio of from 98/2 to 65/35 9 especially from 95/5 to 75/25.

~ 19 -~ '7~

Formaldehyde ( or paraIormaldehyde ) is especially suita~le as the aldehyde component of the phenol-aldehyde resin. Of course 9 other aldehydes such as acetaldehyde, butylaldehyde and benzaldehyde can be used sin~ly or in combination with fOrlllaldehy(ie, The rt~-.ol type phenol-aldehyde resin that is used in this in~entio~ can 'oe obtained by reacting the above mentloned phellol -~ith the aldehyde in the presence of a basic catalystO The amount of the aldehyde used for the phenol is not particularly critical, and amounts customarily used in production of resol type resins can be adopted. ~or example 9 -the aldehyde is used in an amount of at leas-t l mole per mole of the phenol 9 preferably 1.5 to 3.0 moles per mole of the phenol, However9 any particular disadvantage is not brought about if the aldehyde is used in an amount smaller than l mole per mole of -the phenol.
It is ordinarily preferred that the condensation be carried ou~t in an appropriate reaction medium9 especially an aqueous medium. Any of basic catalysts customarily used for production resol type resins, l~mong them9 there are especially preferably employed ammonia9 and hydroxides, oxides and basic salts of all~aline earth metals, such as magnesium hydroxideg calcium hydroxide9 barium hydroxide9 calcium oxide9 magnesium carbonate9 magnesium chloride and magnesium ace-tate. ~uch basic catalyst is made present in the reaction medium in a catalytic amount9 especially 0.01 to 0.5 mole /0. The condensation reaction eollditiolls are llot; particularly critic~l. Ordinarily7 -the colldL~lsatlon is carricd ou-t at 80 to 130C, for 1 to 10 hollr,.
The resultillg resi}l can be purifiod by known rneans, l;`OI' exa!lple 9 th-~ resin cOmpoacQt as the reaction product can be ~ tracted al(l,scl)arated ~'rorn the reaction rmedium by a ketolle 9 an alcohol 9 a hydrocarbon solve~nt or a mix-ture ther~ofg and lf nc-ces,,ary9 the unreacted reactants are removed by washing with water. Then9 wa-ter is removed by azeotropic distillation or decanta-tion. Thus 9 there can be obtained a resol type phenol aldehyde resin in the forrn that can bc rrlixed with an epoxy resin.
The epoxy resin component (a) and phenol-aldehyde resin component (b) can be used at any of mixing ratios cus-tomarily adopted for enamels of this type, and the mixing ratio :is no-t particularly critical in this inven~
tion. From the vie~oillt of the resistance of the bonded portion -to -the retort steriliza-tion treat~ent9 it i.s preîerred that both the components (a) ~nd (b) be r~ixed at a mixing (a)/(b) weigh-t ratio of from 35/5 to 50/50 9 especi~lly from 90/10 to 60/40 9 for production of an enarnel for formin~ the in-terposing adhesive l~yer.
In -this invention, the epoxy resin and phenol-aldehyde resins are rnixed in the state dissolved in a Xetone9 an ester 9 ~1 alcohol9 a hydrocarbon solvent or a mixture thcreof and the resul-ting solution may be directly used as ai~ enamel for formation of -the interposin~ adhesive layer. 0rdinarily, however9 it is preferred that the - 21 ~

~,4~i~ 7~

ix~d resin solu-tion be preliminarily condensed a-t ~0 to 130C. for abou-t l -to abou-t lO hours and then used as ail e~la:.~el i~or forration of t.he :interposing adhesive layer.
Ins-tea~ of -the above I ea-ture where the epoxy resin and phenol-lldeh-yde resin are used in the form of a two-componen-t -t~pe enamel9 there can be adopted a method in which -the phenol.-aldehyde resin is moclilied ~ri-th at least one known modifier 9 for example, a fatty acid9 a polymerized fatty acid9 a resin acid ( or rosin ) 7 a drying oil or an alkyd resin to such an extent that inherent properties of the resol are not lost 9 and the modified phenol-aldehyde resin is then mixed with the epoxy resinO Of course 9 both the resins may be modified with a polyvinyl acetate or butyral resin 9 an arnino resin 9 a xylene resin 9 an acrylic resin9 a silicone resin9 a wax or phosphoric acid if desired.
(3) Protecting Coating Layer In this invention9 for formation of the pro-tecting coating layer 9 any of thermosetting resin enamels can be used so far as ~i anti-corrosion coating excellent in the heat resistance9 hot water resistance and adhe-sion is formed.
~s the thermoset-ting resin enamel that can be used in this invention 9 there can be mentioned 9 for example, phenol-aldehyde resins 9 furan resins 9 xylene-formaldehyde resins 9 ketone-formaldehyde resins 9 urea-aldehyde resins 9 melamine-aldehyde resins 9 aniline-aldehyde resins 9 alkyd 22 ~

i'7~

r esins ~ g~uana~ e-~aldehyde resi]ls 9 unsaturated polyester resins 9 epo~y reslns 9 therrnos2-tting acrylic resins, triallyl cyamlra-te resinsp his!naleilnide resins ~nd oleoresinous varnishes. These enar;lels may be used singly or in the forn of a mix-ture of -two or more of them.
0f course, ther.Qosettin~, resins that can be used in this invention are no-t limit~d to those exemplified above.
In -this inverltion9 i-t is preferred -that -the thermo-setting resin for formation of the anti-corrosion protec-ting coatin~ layer be able -to be highly crosslinked such that the gel proportion de-termined according to the~
method described in examples given hereinafter is at least 70 S;~ 9 especially at least 75 C/o 9 though the degree of crosslinking differs to some extent according to baking conditions described hereinafter. This gel proportion has close relations to the scratch resistance of -the can blank composed o~ a coated TFS material at the can manufacturi~g process and the adhesion and hot water swelling resis-tance of the coating at the retort sterili-zation treatrnent or during storage after the retorttreatmen~t. When the gel proportion is main-tained at the above-mentioned lcvel, scratching oî the coating layer at the can manufac-turing process and degrada-tion of adhesion 9 swelling and other degradation of the coa-ting layer at the retort treatment or during -the subsequent stora~e can be effectively prevented.
Color pigmelts, fillers, anti-corrosion agents and other additives may be incorporated in the protecting 67~

coating layer if desired. L`or example 9 a-t least one rnembcr selected f om aluminurll powder 9 zinc oxide 9 zinc powder9 tin pow(1~r 9 nickel po~der 9 titanium dioxide 9 talc9 clay9 collo:idal silica9 calcium carbonate9 zinc phosphate ~ d carbon black ma-y be incorporated in an ar,loul-lt of 0.1 to 100 //9 especially 1 to 50 ~09 based oll the resinO
It is l~nown that amonV the foregoing resin enarnels 9 epoxy-modified enamels such as epoxy resin/phenol-aldehyd resin enamels 9 epoxy resin/urea resin enamels 9 epoxy resin/melamine resin enamels and vinyl resin modified products thereof 9 alkyd resin enamels 9 thermosetting acrylic resin enamels 9 oleoresinous varnishes and acrylic resin/epoxy resin enamels are excellent in both the heat resistance and the corrosion resistance. These enamels are advantageously used for attainin~ the objects o~ -this inven-tionO
The protecting coating layer forrned on the TFS
material rnay have a sin~le layer structure or a multi-layer structure includi-ng at least two layers. For example 9 -the protecting coating layer on the TFS material rnay be -the same or similar epoxy resin/phenol-aldehyde resin enarrlel as that of the interposing adhesive layer described in detail hereinbefore. Further9 this pro tectin" coatil?g layer may have a multi layer structure including an undcrcoat layer and a topcoat layer9 in which the undercoat layer is composed of the sarne epoxy resin/phenol-aldehyde resin as -that of the interposing adhesive layer and -the topcoat layer is corrposed of -the same or similar epoxy resin/phenol-aldehyde resin ena~!el or a differen-t therr.losetting resin9 for example7 a furan resin, a xylene-formaldehyde resin 7 a ketone-formaldehyde resin 9 a urea~aldehyde resin, a melamine-alde'lyde resin~ a guanam~irle-aldehyde resin~ an aniline-aldehyde resin 9 an alkyd resi,n 9 an unsaturated polyester resin9 an o:leoresil.ous varnish or an unsaturated hydro-carbon resin.
In this invention 9 it is preferred that among the foregoing -therlnosetting resin enamels, an epoxy resin/
phenol-aldehj~de resin enamel satisfying at least one of the following two requirements be chosen and used for formation of the protecting coating layer:
(l) The phenol~-aldehyde resin content is higher than the enamel constituting the interposing adhesive layer.
- (2) The f'unctional index ( IoF. ) represent~d by the following formulao l.F, = ,~I2 ~ 3M3 ~- ~14 wherein M2 stands for the mole number of the bifunctional monohydric phenol in lO0 g of the total phenols contained in the phenol-aldehyde resil19 M3 stands for th~ mole number of the tri-functional monohydric ph@nol in lO0 g of the total phenols contained in the phenol-aldehyde resin and M4 s-tands for the mole number of the poly-cycllc dihydric phenol in lO0 g of the total phenols contained in the phenol-aldehyde resin, ~L~,f,~jt~

is higher -lhan -the functional index ( I.Fo ) of the enamel constituting the interposing adhesive layer.
In order to Lorm a stro;.g bonding having a hot ~Jater rosistance sufficiellt 'co re.;ist retor-t s-teriliza-tion and prevent leal~age with lapse ol time under such a reduced pressure as 15 to 50 cm Hg 9 as pointed out hereinbefore 9 it is required to use an enamel for the interposing adhesivt~ layer in ~hich the content of the epoxy resin content is rela-tively high9 namely -the content of the phenol aldellyde resin is low. In other words 9 it is required to use an enamel -the crosslinking density is relatively low in -the resulting coa-ting. On the other hand9 from the viewpoint OI the corrosion resistance of the TFS material or the extraction resistance of the coating at the retort trea-tment or during storage9 it is preferred to use as the erlam~l for the anti-corrosion protecting coating layer an enamel that can be highly crosslinked to providc- a coating either mechanically or chemically compact and dense. In other words 9 the enamel for formation of the protecting coating layer is required -to have a higher phenol-aldehyde resin content.
Also the functlonal characteristic of the phenols to formaldehyde in the phenol-formaldehyde resin is impor-tant for imparting the above-mentioned preferred properties to the coa-tingsO The abov~-mentioned.dicyclic dihydric phenol is tetra-functional 9 but o-cresol 9 p~cresol or the like is bifunctional ~nd carbolic acid 9 fil-CreSO
or the like is trifunctional. In combination of a - 2~

~ 7~

phellOliC re,-~ aVi l/~- ~'7 relatively high functional index ( I, F . ) with an epoxy resill9 as se~-~n frorn Examples given herei3laI`ter9 the enai,lel is highly crosslink~-;d arîd all anti-corrosioll protecti3lg coatin~ layer can be formed.
On th~ l~ther halld 9 frjm combination of a phenolic resin having 2 lo~ f~mc-tional inde~ ( I.F. ) with an epoxy resin 9 -there can be for~lod a-l interposing adhesive layer suitable f)~llea-t bonding by a polyamide -type ~dhesive as seen frorl Examples given hereinafter, Ordinarily9 it is preferred that the functional index ( I.F. ) of the phenolic resin for the in-terposing adhesive layer be 1060 to 1.95 9 especially 1.70 to 1.909 and that the functional index ( I.F. ) of the phenolic resin for the pro-cectil3g coating layer be 2.00 to 3.009 especially 2.05 to 2.50. Further9 it is preferred that -the functional index ( I.F. ) of -the phenolic resin for the protec-ting coating layer be higher by at least 0,2 than the func-tional index ( I.F. ) o:~ the phei1olic resin f'or the in-terposing adhesive layer.

(4) Polyamide Type ~dhesiveo Any of polyamide type adhesives customarily used for production of bonded cans can be used in this inven-tion. Linear homopolyamides, copolyamides 9 modified polyamides and mixtures thereof ? having a relative vis-cosity ( ~rel ) of at least 1.59 especially at least 1.89 as measured ~.~i-th respect -to a 1 ,jb solution in 98 ~' concentrated sulfuric acid9 are preferably emplo~yed.
Suitable exanples of tne polyamide tha-t can be used in -tl~is i~ o~rtiol incllldcl~lomo}~olyal~i(les such as poly( h~ am~thy~ e adipall~ide) 9 pol~(hexametllylene sebacami~e) 9 poLy(he~ale~t~lylene dodecarnid ) 9 poly(clodecamethylene dodecal,~ide)9 5~amino-caproic acill pol-~ner9 11 ar,~ino~
undecarloic acid polymer a(ld 12-amino-lauric acid polymer, copolyamides corlpos~d of a-t least two m~mbers selected from constituent Illollomers of the foregoing homopolyamides 9 namely dicarboxylic acicl diamine sal-ts and w-aminocarbo-xylic acids 9 and products formed by modifying these homopolyamides and copolyamides with polymerized fatty acids and the like. From the viewpoint of the s~trength of the bonded portion9 it is preferred that crystalline polyamide type adhesive be used.
[ Production of Bonded Cans ]
Referring to Fig 3 illustrating the sequence of steps for p-roduction of the bonded can o~ this inven~-tion7 an enamel corllprising an epoxy resin and a poly-cyclic phenol-containing phenol-aldehyde resin is coated on the entire surface of a TFS blank Lr9 which may be cleaned by a solven-t sucn a.s trichlene if necessary, according to known r~eans such as brush coating9 spray coating9 dip coating 9 roll coa-ting 9 doctor coating 9 electi~ostatic coating or electrophoretic coating 9 and the applied enar,~el is baked to form an interposing adhesive lay~r in a portion to be lap~bonded ( ordinarily, a side edge portion ) and a protecting undercoat layer ~' in other portLon. Of course 9 the interposing adhesive layer 5 is continuous to the undercoa-t layer 6' and has ~,'2~ci7~

the samo thicknoss as -that of -the undercoat layer 6'.
The baking conditions dif~er depending on the com-position of the enaMel, However 9 in general 9 conditions for causing appropriate curil-lg in the coating are- experi-mentally chosen frorll baking temperatures of 150 to 400C, and baking time of 1 s~cond to 20 r!linutes.
Then~ an anti-corrosion thermosetting resin enamel is applied to the portion other than the portion of the interposing adhesive layer 5 to be lap-bonded, namely the portion of the protecting undercoat layer 6 19 and the applied enamel is baked to for~. a protecting topcoat layer 6'1. Coating and baking of the protecting topcoat layer 6" are performed according to procedures described above with respect to t,he undercoat layer 6'.
The amounts coated of the foregoing enamel are determined so that the above-mentioned thickness requirements are satisfied by the interposing adhesive layer 5 and the protecting coating layer 6 ~ including the undercoat and topcoat layers 6' and 6i7 ).
Instead of the above~mentioned two-staged baking methcd in which the enamel of the protecting topcoat layer 6'i is applied to the baked protecting undercoat layer 6' and the applied enamel is then baked7 there can be adopted a one-staged baking method in which the protecting topcoat layer 6'i is ~ormed on the unbaked protecting undercoat layer 5' in the so-called wet-on-wet state and both the undercoat layer 6' and topcoat layer 611 are simultaneously baked. According to this 7~

~,nethod 9 one cycle of passing -the coa-ted can blanlc througll a bakinb furnace can be omitted and there can be attained various industrial advan-tages such as saving of heat energy, rcduction of th~ equipmen-t cost or space of th~ bal~ing furnace and r~duction of th~ st2p number, Then9 a polya~ide type adhesive 3 is applied to the portion of the can blallk to be bonded9 namely the portion of the interposing adhesive layer 5. Various means can be adopted for introducing the polyamide type adhesive 3 between the side edges of the can blank 1 tc be bonded, namely between the interposing adhesive layers 5 to be faced each otherO For exampley a preformed tape of the polyamide type adhesive 3 is applied to each of both the side edges of the c&n blank, or a molten tape of the polyamide type adhesive is extruded between both the side edges of the can blank. Further, the polyamide type adhesive 3 can be applied to the portion of the can blank to be bonded in the powdery form or in the form of a solution. The thickness of the layer of the so applied polyamide type adhesive 3 is not particularly critical, so far as the polyamide type adhesive 3 _s intlmately contacted with the interposing adhesive layer on the can blank, In general 9 however 9 it is preferred that the thickness of the layer of the adhesive 3 be 0.01 -to 0,2 mmO The polyamide type adhesive may be applied to one or both of the side edges of the can blank to be bonded prior to the bonding operation or it may be located between both the side edges at the bonding step.

t'~

Bonding of both the side edges of the can blank 1 is accomplished by melting the polyamide type adhesive located between both the confronting side edges of the can blank 1 formed iIl a cylinderical shape 9 pressing both the side edges of the can blank 1 under cooling and thereby solidifying the polyamide t~pe adhesive, can body formed by such side seam bonding 2 is then subjected to known can manufacturing process such as notching process 9 flange process and double seaming with a can lid provided with a known sealing compound.
Referring to Fig. 4 illustrating the step sequence in another embodiment of the bonded can manufacturing process, the thermosetting resin enamel is applied -to the surface of the TFS blank 4 9 which is to constitute the inner face of the resulting can 9 except the portion to be lap-bonded 9 and the interposing adhesive layer-forming enamel ( epoxy resin/phenol-aldehyde resin enamel) is applied to said portion to be lap-bonded in the same thickness as that of the coating layer of the thermoset-ting resin enamel, Then9 both the applied enamels arebaked and cured to form an interposing adhesive layer 5 and a protecting coating layer 6.
In -the foregoing embodiment 9 instead of the above-mentioned two-staged coating method 9 there can be adopted a method in which both the interposing adhesive coating layer 5 and the anti-corrosion protecting layer 6 can be applied by one coating operation, For exarnple 9 as shown in Fig. 5 9 a relatively thin interposing adhesive ;ti7~

layer 5 and a relatively thick anti-corrosion protecting coating layer 6 are formed from one epoxy resin/phenol-aldehyde resin enamel such a, mentioned above by using a stepped coating roller 8 or doctor.
According to the bonded can-preparing process of this invention 9 coating and baking of all the enamels are accomplisl~ed on a metal blank prior to coating of an adhesive or bonding with the adhesive. Therefore 9 the step number can be reduced, and treatment operations can be remarkably facilitated and equipment costs can be reduced. Moreover 9 degradation of the adhesive or sealing compound can be effectively prevented~ There-fore 9 the process of this invention is very advantageous from the industrial viewpoint, Various arrangements of the interposing adhesive layer 5 9 protecting coating layer 6 and polyamide type adhesive 3 can be adopted for the portion to be lap-bonded in the bonded can of this invention.
For example 9 as shown in Fig. 6 9 on the side edge of the can blank 1 on the outside of the portion to be lap-bonded 9 a shoulder 9 between the interposing adhesive layer 5 and the protecting coating layer 6 may be precisely fitted for the edge of the adhesive 3 9 or as shown in Fig. 7 9 it may be located on a slightly inner side than the edge of the adhesive 3. When these two arrangements are adopted 9 the~re can be provided retort bonded cans especially excellent in the corrosion resistance during a long-time storage.

i7~

~ urtliler9 as shown in ~ig. ~ 9 the shoulder 9 may be locatæd on a sligh~ly outer sicle than the edge of the adhesive ,. If -this layout is adopted9 the adhesive layer 3 and pro-tecting coating layer 6 applied to the side edge of the can blank ac-t as a r~echanical barrier for the exposed interposing adhesive layer 5 9 and scratching of the exposed layer 5 can be effectively prevented at the can manufacturing processO
The exposed metal on the cut end face of the lap-bonded portion of the can body can be protected by knownmeans. For example 9 there can be adopted a method in which a filmy adhesive is applied prior to bonding so that the filmy adhesive is folded back on such end face ( see Example 1 )9 a method in which a known polymer is applied in the molten state or in the form of a powder to the end face of the lap-bonded portion after forma-tion of the can body 9 or a method other thermoplastic or thermosetting enamel is applied to the end face of the lap bonded portion according to brush coating 9 spray coating9 dip coating9 roll coating or electrostatic coating.
[ Uses ]
By virtue of the abo-ve-mentioned characteristic properties 9 breakage of a can body or micro-leakage is not caused in the bonded can of this invention even if it is subjected to such a severe hot water or hot steam sterilization treatment as conducted at 125C. for 60 minutes 9 and leakage with lapse of time ( slow leak ) ~26~7~3 is not caused even if it is stored for a long time after such severe sterilization treatment. Further9 the corro-sion resistance of the can ~ody is very higho Thus, i-t will readily be mderstood -that the bonded can of this invention has novel excellent proper-ties that are not possessed by conventional bonded cans a-t all.
The bonded can of this invention is especially suitable as a can to be subjected to a high temperature retort sterilization treatment9 for example9 a steriliza-tion treatment conducted at a temperature of 122 to 130C.
under a pressure of 1.1 to 1.8 Kg/cm2 ( gauge ) for 10 to 150 minu-tesO ~ccordingly9 the bonded can of this invention can be effectively used for storing for a long time a drink such as a fruit juice9 a fruit juice-incorporated drink or coffee, a fruit, a vegetable, a marine product, a meat product or a processed food thereof. Of course 9 it must be noted that the fore-going advantages can be similarly attained when the bonded can is subjected to a sterilization treatment conducted at a temperature much higher than ~the tempe-rature adopted for the above-mentioned high temperature sterilization, for example9 a steriliza-tion treatment conducted at 110 to 121C., or when the bonded can is used for packing at a high temperature ( for example9 the boiling temperature of the content ).
This invention will now be described in detail by reference to the following Examples that by no means limit the scope of the invention.

'7~

l~n enamel for f`ormation of an interposing adhesive layer was prepared in the following manner.
1~ 37 /D aqueous solution of formaldehyde was added to a mixed phenol comprising 70 par-ts of bisphenol A
~ 292-bis(i~-hydrox~phenyl)propane ] 9 20 parts of p-cresol and 10 parts of o-cresol so that the amount of form~
aldehyde was 1.5 mole per mole of the mixed phenol.
Tne mixture was heated at 65C. to dissolve the mixed phenol in the aqueous solution. Then 9 a basic catalyst was added to the solution and reaction was carried out at 95~C. The reaction mixture was extracted with a mixed solvent of a ketone 9 an alcohol and a hydrocarbon and washed with water. Then9 water was removed from the reaction product by azeotropic distillation or decanta-tion to obtain a resol type phenolic resin having a functional index of 1078O
A solution of -the so formed resol type phenolic resin was mixed with a solution of an epoxy resin ( Epikote 1009 manufactured by Shell Chemical Co. and having a number average molecular weight of 3750 and an epoxy equivalen-t of 2700 formed by condensation of bis-phenol A with epichlorohydrin ) in a mixed solvent of a ketone, an ester, an alcohol and a hydrocarbon so that the weight ratio of the phenolic resin to the epoxy resin was 25/75 9 and the -temperature was elevated to 110C.
and precondensation was carried out for 3 hours to obtain an epoxy resin/phenolic resin mixed enamel A

~2~i~7~

( epoxy~phenolic enamel h ) for formation of an inter-posing adhesive layer.
'~n enamel for formation of a protecting topcoat enamel was prepared in the same manner as above except that a mixed phenolic resin ( having a functional index of 2.07 ) derived from 35 parts of p cresol 9 35 par-ts of p-tert~butyl phenol and 30 parts of phenol was used as the mixed phenolic resin and Epikote 1007 ( manufac-tured by Shell Chemical Co. and having a number average molecular weight of 2640 and an epoxy equivalent of 1900 ) was used as the epoxy resin. Incidentally 9 the mixing weight ratio of the epoxy resin to the mixed phenolic resin was 60/40. Thus 9 there was obtained a thermosetting protecting topcoat enamel B ( epoxy-phenolic enamel B ).
Can bodies used at the tests described below were prepared in the following manner.
The interposing adhesive layer-forming enamel ( epoxy-phenolic enamel A ) described above was coated on one surface of an electrolytically chromic acid-treated steel plate ( TFS ~ having a large size of 827mm ( length ) x 1020 mm ( width ) and a thickness of 0.23 mm by using a roll coater so that -the thickness of the coating after baking was 2 ~ 9 and baking was carried out at 190C. for 10 minutes~ The interposing adhesiv@
z5 layer-forrning enarnel was similarly coated on -the othe surface of--:the steel plate 9 which was to constitute the inner face of the resulting bonded can 9 by means of a roll coater so that the thickness of the coating after ~ '7~

ba~ing was l ~ aking was carried out at 210C. for 10 minu-tes. Thus 9 there was formed an interposing adhesive layer on each of the sur~`aces of the steel plate.
In case of samples 1-1 -to 1-3 shown in Table 19 the above-mentioned topcoat~forming enamel ( epoxy-phenolic enamel B ) was margin-coated ( roll-coated ) on the surface of -the above coated steel 9 which was to consti-tute the inner face of the resulting bonded can 9 e~cept a portion of a width of 2,0 + 0.5 mm from one side edge in the longitudinal direction of a strip which was to be formed by cutting the coated steel plate into strips having a width corresponding to the height of the resulting bonded can and a portion of a width of 5.5 +
0.5 mm from the other side edge of said strip 9 SO that the thickness of the coating after baking was 5 ~.
Baking was carried out at 205C. for 10 minutes to cure the applied enamel. The other surface of the coated steel, which was to constitute the outer face of the bonded can 9 was printed and coated with a finishing varnish according to customary procedures. Then, the coated steel plate was cut along the coating direction into strips having a width of 170.40 mm and a length of 827 mmO
Each of both the side edges in the longitudinal direction of the strip was pre-heated along a width of about 7 to about 8 mm at about 270C. according to the high frequency heating method. Then 9 a tape of the nylon 12-type adhesive having a thickness of 50 ~ and a width of G m!n was roll-pressed for 35 milliseconds on one side edge of -the strip 9 that was to constitute -the bonded portion on the inner face side ot the bonded can 9 and the applied -tape was then cooled. Simultaneously 9 a tape of the same adhesive having a width of 8 mm was applied along 2.5 r.~m to the faGe to constitute -the bonded portion of the inner face side of the bonded can 9 along

5 mm to -the face to consti-tute the bonded portion on the outer facel side oi: the bonded can and to the cut end face of the strip for protection thereofg and the tape was roll~pressed and cooled under the same conditions as described above~ In -this case 9 the adhesive tape was arranged so that the 2,5 mm width portion of the tape was located on the side where the width of the portion not coated with the topcoat layer-forming enamel was 2.0 + 0.5 mm ( the innermost face of the bonded portion ).
The ratio of the thickness Ta of the interposing adhesive layer to the thickness Tp of the protecting coating layer 9 namely the Ta/Tp ratio 9 was 0.44 9 and the thick-ness Ta was 4 ~ and the thickness Tp was 9 ~. Then9 the adhesive applied strip was cut in the rectangular direc-tion to form a can blank having the adhesive tape applied to both the side edges and a size of 136.53 mm x 170/40 mm.
The can blank was formed into a cylindrical shape having a can helght of 136.53 mm by using a customary can making machine 9 and both the adhesive-applied side edges were heated at 250C. according to the high fre~uency - 3~ -

6~

heatin~ metllod. The C~.lll blanl~ was pressed for 30 milli-secon.cis so that -the adhesive-applied edges were faced each other, and -then cooled to obtain a can body. The lap width o' tne Searn O:L the can body was 5 mm.
The so prepared can body was subjected to a customary flange proc,ss 9 arld th_ can body was double seamed at a rate of 550 cans per minute according to a customary method with a 202-diameter aluminum lid obtained by blanking a coated aluminum plate having an organosol type coating of a thickness of 13 ~1 on the inner face side according to a customary method 7 coating an SBR type water-based compound on the cut-out disc of the aluminum plate and drying the applied compound. Thus9 there was obtained a can product ( empty can ) of this invention ( sample 1-1 ) having an inner capacity of 250 m~.
Samples 1-2 and 1-3 of this invention were prepared in the same manner as described above by using different nylon 12--type adhesivesO
Comparati,ve sample 1-1 was prepared in the same manner as described above excep-5 -that the undercoat layer-forming enamel was roll-coated on the entire surface. Accordingly9 in this comparative sample9 the topcoat layer was present also on the bonded portion.
Comparative sample 1-2 was prepared in the same manner as described above except that a can body was formed from the steel plate having both the surfaces entirely roll-coated only with the interposing adhesive layer-formirlg plate 9 a vinyl chloride-vinyl acetate

7~

copolyl;lell solu-ti(lL in a mixed solvent of a ketone and a hydr OcarbOz~ ~ wlîich had a solid content of 17,5 ,~ 9 was spray-coa'L,~d on th- can bod~ so that ~the thickness of the dry coating was 5 ~l and the coated can body was theIl dried a-t 165C. Ior 4 minutes. The same polyamide type adhesive as described above -was used for formation of these comparative samples.
Each of -the so obtained empty cans was filled with 250 m~ of r.lilk coffee~ ~ he can was double seamed by means of a custor.lary seamer with a 202-diameter top lid formed by blanking a TFS plate having both the surfaces coated with a thermosetting epoxy-phenolic enamel by a customary lid forming press, coating the above-mentioned water-based compound on the lid and drying in the same manner according to customary procedures.
With respect to each sample 9 100 of the so packed cans were subjected to a retort sterilization treatment in a cus-tomary retort at 1~0C, for 90 minutes, and leakage of the con-tent and occurrence of breakage of can bodies were examinedO
In connection with physical properties of the adhe-sive 9 the adhesive was peeled from each bonded can prepared above 9 and the degree of crystallization 9 strength at break and elongation were determined at 25Co A specimen was cut out from the body portion of the empty can product and was extracted with chloroform at 60C. for 60 minutes. The weight W0 of the protecting coating layer before the extraction and the weight W

l~.f '~ 7i~i of tlle pl~otectiYIg coatin layer after -the extraction were measured9 and tlle ~el proportion ( GP ) of the protecting coatin~ la~yer was calculate(~ according -to the following formula o l/WO ) X 100 Obtained results are shown in Table 1.

o N gO~O g OO O

O O O Cl) O N
~ ~U
~ +) _~

;~ ~0 ~ O Ll~ Lr~ O
~ ~ ~D L~ O
C~
~ U~ ~

I
~1 ~) rl _~
o ~1 r~ ~ ~ (U t-- Lr`~

~U

~ I 1 ~0 (U

~ o ~ $ +' o oLr~ LO U~Lr co co a~c~
o a) ,, o ~0 ~0 ~0 D~O I 5 C C C o oCU C ~ ~U ~U ~
) hu ~+~$ $ ~ 1 $ ~ ~- C
~ ~ h ~d O ~ d O td ~I h ~1 ~ ~! t~
O t~ 1~ 0 h O ~ ~ 5:1 0 h O ~ O ~ O ~ +~
E~ ~1 ~ cu c~ r~ " ~u p~ ~, ~
1 C~ O U~ o ~0 I ~ 0 ,~
O ~ C~ ¢ ~O)$ $ $ +~
P~ -l i ~1 h u~ h ~ O ~ ~1 X ,C
J .'~ 51 C h H ¢ ~ ~U Q, aJ
~u (u al ~u l l h a) h ~u ~L ~ ~ ~ C~l ~ ~ ~ ~ ~I ~1] ~1 OJ
~. ~ u Q, U~ r~ V~ U~ 0 ~ ~ 0 l ~I N 1~ ~ Ll~

- 42 ~

r26c~

o ~1 1 O
,Q
~

.~ ~o p h XE~
~) ~ '~
O O o O ~ $~ S: O O O O ~
~1 ~ ~

~: Z I

7~3 In this Example 9 roll coating and spray coating were compared with each other by using various ~inds of sealing compounds .
~ reso~L -ty~e phenolic resin ( having a fullctional index of 1.~ ) prepared i`rom a r;lixed pheaol comprising 85 parts of bisphernol B [ 292-bis(4-hydroxyphen~l)butane ]
and 15 parts of phei~ol in -the same rnanner as described in Example 1 was used for forrllation of the interposing adhesive layer-forming ename~ Epikote 1007 ( manufactured by Shell Chernical Co. and having a number average mole-cular weight of 2700 and an epoxy equivalen-t of 2000 ) was used as the epoxy resin, The phenolic resin was mixed with the epoxy resin at a weight ratio of 20~80 to form an epoxy-phenolic resin enamel for formation of the inter-posing adhesive layer.
The same epoxy-phenolic enamel B as used in Example 1 was used as the topcoat layer-forming enamel.
Bonded cans tested were prepared in the following manner.
The in-terposin~ adhesive layer-forming enamel was coated and cured on one surface of -the same TFS plate as used in Example 1~ and -~ie same in-terposing adhesive layer-forming enamel was roll-coated entirely on the other surface so that the thickness of the coating after baking was 4 ~ The above-mentioned topcoat layer-forming enam_l was margin-coated ( roll-coated ) on said coated other surface in the wet-on-wet state except a ~ ~f~ 7~

portion of a ~l~dth of abou-t 6 m~9 ~jhich was to consti-tu-te tne bonded por-tion o-f` the bonded can 9 SO that the thickness of the coating al`t l bakin,~ ~as 5 Il. Then 9 bakin~ was carried ou' at 205C. for 10 mlnutes. The Ta/~p ratio was 0044 and -the ~el proportion of the pro-tecting coating layer was 80 ~0.
The other surface of the plate 9 which was to constitute the outer face of the bonded can 9 was prin-ted and coated with a finishing varnish according to customary procedures.
The plate was cut along the coating direction into s-trips having a length of 827 mm and a width of 170.40 mm. One side edge of each strip was heated at about 270C. by high frequency heating 9 and a nylon ll-type adhesive was coated on the heated side edge along a wid-th of 5 mm in a thickness of 80 ~ at an extrusion tempera-ture of 250C. by using a customary extruder, and the applied adhesive was pressed and cooled by a cold roll main-tained a-t ~ 20C.
The so adhesive-applied strip was cut into a can blank in -the same malmer as in Example 1 and formed into a cylindrical shape by a cus-tomary carlinc-lking machine. Both the side edges -to be bonded were heated at 270C. and they were lapped so tha-t the seam had a width of 5 mm 9 a~nd then, they were pressed for 35 milliseconds and cooled -to form a can bod-y. Immediately, an epoxy-polyamide type enamel ( an enamel composed of a mixture of an epoxy resin and a polyamide resin ) was spray-coated on -the cut end face exposed to the inner tj~l7(J

face oi` the call9 and the appLi~d enamel was dried and c~lred by -~he l~eat left 011 the bonded por-tion -to protect the cut ~nd face. Thus~ a can body for the bonded c~
was obtainec'.
The can body was subjected to a flange process according to a custormary me-thod 9 and was -then double-seamed wi-th the same 202-diam~ter aluminum lid as used in Example 1 to obtain an ~mpty bonded can having an inner capacity of 250 m~.
In case of cornparative samples 9 a can body was prepared from the TFS plate having both the surfaces coated with the interposing adhesive layer-forming enamel 9 in the same manner as described above. The can body was sub.jected to a flange process and double-seamed with the above-mentioned aluminum lid. By using a spray coating device 9 a vinyl chloride-vinyl acetate copol~mer lacquer was coated on the can body and a lacquer ol a vinyl chloride-vinyl aceta-te copolymer modified witll an epoxy resin and a benzoguanamine~ -aldehyde resin was coated on the aluminurn lid, Baking was carried out at 150C. for about 10 minutes to obtain an empty bonded can, Each of the empty cans was filled wi-th 250 mB of black coffec heated at 93C. and the packed can body was double seamed with the same 202-diameter TFS lid as used in Example 1.
With respect -to each sample, 100 cans were subjected to a retort treatment at 130C. for 90 minutes and stored ~ 7~

at 50C -f`or ~ rl~onths. Then 7 t.he cans were allowed to s-tand at room -temperature for 1 week, and the degree of vacuuil ( cin ~g ) in the c~n was measured ancl leakage with l~ps~ of ti!l~ was ~?xaMill~d.
The sealing compo~md used for the alurrlinum and TFS
lids of sam~le 2 1 of this invention and comparative sample 2-1 was an SBR-~type wa-ter~based cornpound, and the sealing compound used for the aluminum and TFS lids of sample 2-2 of this invention and cornparative sample 2-2 was an SBR~-type solvent-based compound.
The aluminum lid was rernoved from each can, and the adhesion between the compound and can body was exarnined ( evaluated according to the 5-ranked method, 50 ~ood 9 1 r bad ). The compound was peeled off and the elongation (%) at break, 10 % strain modulus ( elasticity ) and Mooney viscosity of the peeled compound were mea-sured at 25C.
Obtained results are shown in Table 2.

~ 1A~6~7~

(1) ~ rC ~12 ~) ~ 1.~ 0 ~
l ~ Ir~

rc~

C r~~1 O C ~
r~ a) ~; O
O !~ ~ ~1 ~1 ~O 3 Ql C~ ~ C
o ~5 r~ ~ C
~ O Lr~ O ~ ~
~t N L'-\ N
C O r~
~ ~ Ir~
N Q O
~O ~ ~
F5 O ~ ~ L'`\ ~ C~) ~
o u~ ~ o ~ o r, o r~ a) a N

a) 5 r~ r5 1 ~ ~
t~5 O O L~ O ~ O
~ ~ ) C~
O ~ ~ ~ X
c) a) r~ O
~I N '~ ~1 '' N $ ~
r~ I r1 1 ~ 5 N N _L) N +~ ~`J ~ ~ o r~
h5 ~ h5 r-l m m ~ ~o ~
rl a~ F~ ~ ~ oo D~ 0~ O> 00 u~ ~5 ~r5 ~r5 o ~T5 r~~ ~\1 l~ ~ If' U~ ~ V ~t~ V U ~ * ~r * * I-c ~
:~ l ~I N 1~ ii -- 4(3 --.~i~ 7~

Exampl e Bonded CallS were prepared by usin,.o vario~s -tGpcoat layer-formin~ enalllels and adoptin~ the roll. coating method. an~ pray COa-tirlg methods in combillation. Boiled sardine ~.ras packeà in -these cans and the packed C~lS were subJected to the re-tort sterilizatiol treatr,ien~t 9 and proper-ties of tht oonded cans were cvaluated.
I~n interposin,^; adllcsi-v~ layer~forming enamel was prepared in the sai,le rnanner as described in Example 1 except that a resol type phenolic resin ( having a functional index OI 1, 72 ) prepared from a mixed phenol comprisin~ 65 parts of bisphenol B [ 292-bis(4-hydroxy-phenyl~butarle ] and 35 parts of o-cresol was used as the phenolic resin. The epoxy r~sin used was the same as that used in Example 1 and the rnixing weight ratio of the phenolic resin to the epoxy resin wa~s the same as in Example 1.
Various topcoat layer-formin~ enamels were pre-pared according to the follo~r~ring procedures.
~ thermosetting epoxy-phenolic ena!nel as one topcoat layer-forrning ename~l was prepared in the sarne manner as described in Exar;~ple 1 --xcept that '7 resol type phenolic resin ( havin~ a functional index of 2.23 ) prepared from a mixed phenol comprising 70 parts of m-cresol and 30 parts of p-octylphenol was v.sed as the phenolic resin, Epikote 1009 ( rr,lanufactured by Shell Chemical Co. and having a rlumber average rnolecular wei~ht of 3700 an1 an epoxy equivalent of 2650 ) ~.vas used -- ~r9 ~

7~

as the el-,oxy resin ~s -the collclensation rcaction product oi bis~henol ~ wil.h epichlorohydrin and. the mixin~
weig~lt ratio of the phenolic .resin to the epoxy resin w~s 30/70.
An epoxy-urea enamel ( epoxy resin-urea aldelîyde resin mixed enam~.-l ) a anothe:r -topcozt layer-formin~
enamcl w s prepare-L in the following manner ,~ mix-ture of 2~o3 parts cf urea 9 100 parts of a 37 % aqueous solution of formaldehyde and 60 parts of n-butanol was maintained. at 95C. for about 20 minutes and reacted under reflux in the presence of formio acid as a catalyst Water was removed by a7eotropic distil-lation to obtain a butanol solution of a butyl-etherifled urea-formaldehyde resin ( havin~ a resin content of 60 % ) This urea-aldehyde resin solution was mixed with a solution of n epoxy resin ( Epikot~ 1007 manufacturecl by Shell Chemical Co. and havin~ a number average mole-cular weight of 2600 and an epoxy equiv~lent of 1860 ) in a mixed solvent of a ke-tone, an ester 9 an alcohol and a hydrocarbon so that thc mixin~ wei~ht ratio of the urea-ald~hyde resin to the epoxy resin was 20/~0 9 whereby an epoxy resin-urea-alclehydc resin nixed enamel was obtained.
An epoxy resin-melamine-aldehyde resin mixed enamel ( epoxy-melamine enamel ) as still another topcoat layer-formin~ enamel was preparecl in -tle following manner.
mixture of 126 par-ts of melamine and 4~0 parts of ~ 2~

a 37 c,~ aql~eous solu-tion of ~ormaldeh~de was reacted at a p~ of 8.0 in the presence of an alkali ca-talyst at 70C. llhen dissolution of melamine was completed, 420 parts of butc~nol was added and reac-tion was con-ducted at 80 to 95C. The reaction product was washedwith water and water was removed by azeotropic distil-lation. The residue was concentrated under reduced pressure -to obtain a butanol solution of a bu-tyl-etherified melamine~formaldehyde resin ( having a resin content of 60 ~ ). The melamine resin solution was mixed with a solution of an epoxy resin ( Epikote 1009 manufactured by ,~hell Chemical Co. and having a number average molecular weight of 3800 and an epoxy equivalent of 2590 ) in a mixed solvent of a ketone, an ester, an alcohol and a hydrocarbon so that the mixing weight ratio of the melamine-aldehyde resin to the epoxy resin was 15/85, whereby an epoxy resin-melamine-aldehyde resin mixed enamel was ob-tained.
An acrylic resin-epoxy resin mixed enamel ( acrylic-epoxy enamel ) as still another topcoat layer-~orming enamel was prepared in -the following manner.
A solution of 50 parts of methyl methacrylate, 35 parts of butyl acrylate and 15 parts of methacrylic acid in 100 parts of an ester type solvent was heated at 90C.
and a peroxide catalyst was added little by little to the solution to advence -the reaction. The so formed solution of the acrylic copolymer resin ( having an average degree of polymerization of 200 ) was mixed in ~ '7~

A solution of an epoxy resin as -the condensatlon reac-tion product between bisphenol .~ and epichloro-hydrin ( Epii~ote 1001 manufacture~ by ~nell Chemical Co.
and having a number average i-s]ecular weiglt o~ 1000 and an epoxy equivalent of 4~30 ) in a mixed solvent OI
a ketone 9 an ester 9 an alcGt^~ol and a [lydrocarbon so that the mixing weight ratio of the acrylic copolyrner to the epoxy resin was 50/50 9 whereby an acryIic~epoxy enamel was obtained.
An oleoresinous enamel as still ano-ther topcoat layer-forming enamel was prepared by reaction 250 parts of tung oil 9 100 parts of a rosin ester of maleic acid and 120 parts of a resol -type phenolic resin derived from p-phenylphenol and formaldehyde at a temperature higher than 200C., dissolving the reaction product in a mixed solvent of a ketone 9 an ester 9 an alcohol and a hydrocarbon and adding a small amount of a metal salt of naphthenic acid.
A vinyl chloride resin type topcoat layer-forming enamel used as a comparative enarnel was prepared by dissolving a vinyl chloride-vinyl acetate copolymer ( having a vinyl chloride content of 87 ~0 9 a vinyl acetate content of 13 ~' and an average degree OI poly-merization of 400 ) in a mixed solvent of a ketone and a hydrocarbon.
Bonded cans tested were prepared according to the following procedures~
The interposing adhesive layer-forming enamel was roll-coated en-t1rely on one ,urface of a TF~ -plate having a thicknes~ of 0~21 mm9 a length of 836 mm ~d a width of 1026 mm 9 SO -that '-he thickness of -the coating after baking was 2 ~! . Baking was carried out at 210C.
îor 10 mimltes to form an i;rterposing adhesive layer.
In case of the roll coatirlg of the topcoa-t layer-forming enarllel 9 ill the same manner as described in Example 19 -the topcoat layer-forming enamel mentioned above was margin-coated on the surface of the TFS plate which was to cons-titute the inner face of the bonded can and on wh ch the interposing adhesive layer had been formed in -the same manner as described above.
This coating was carried out so that the thickness of the coating after baking was 4 ~. Baking was carried out at 200C. for 10 minutes to form a topcoat layer.
The thickness Ta of the interposing adhesive layer was 4 ~ and the thickness Tp of -the pro-tecting coating layer was 8 ~, and the thickness ratio Ta/Tp was 0.5. The surface of the plate 9 which was to constitute the outer face of the bonded can 9 was printed ancl coatecd wi-th a finished varnish.
The so prepared coated plate was cut in-to strips having a length of ~36 mm and a width of 170.4 mm in the same mannér as in Example 1. Then 9 in -the same manner as described in ~xample 1, an adhesive tape was applied to both the side edges of the strip and tne strip was cut to form can blanks having a size of 92.08 mm x 170,40 mm~ The can blank was formed into a can body in 7~

the same manner as described in Example 1 except that the side ed~es were heated at ~40C, by high frequency heatin~, The c~l body was then subjectcd.-to a flange process and double seaming by a customary double seamer with a 202-diameter lid ob-tained by blankin~ a coatc-d TFS nlate havin~ both the surfaces coated with a cured thermosettin~ epo~y-urea-aldehyde el1am21 by a customary lid forming machine and coating on -the cut-out lid an SBR type seali.ng compound~ whereby an empty bonded can 0 OI a Small Size No. 1.
In case Oî the spray coating of -the topcoa-t layer-forming lacquer 9 the above-mentioned interposing adhesive layer-forming enamel was coated, dried and cured on both the surface of the TFS plate in the same manner as described above, and the surface of the TFS plate, which was to constitu-te -the outer face of the bonded can, was printed and coa-5ed with a finishing varnish in the same mam1er as described above, /~ can body was prepared from this coated TFS pla-te according to the same me-thod as described above. Then 9 the above~mentioned topcoat layer-formin~S lacquer was spray-coated on the can body accordin~ to a customary method and the applied coatin~S
was driecl and baked at 165C. for 4 minutes. Then9 the can body was double-seamed with the above~mentioned lid to ob-tain an empty bonded can.
Boiled sardine was packed in the so obtained empty bonded cans according to customary procedures, and the cans were double-seamed with the above-mentioned 202 5~ -7lv~

diar.leter li.ds by a vacuum seamer to obtain canned packages, l~ th respect to each sample9 100 cans were subjected to a retort treatment at 125C, for 90 minutes in an ordinary retort9 and occurrence of breaka~e of cans was examined and in the same m~ler as described in Example 29 leakage with lap.se of time was examined~
fter the retor-t treatment 7 the lid was removed and the condition of -the coating in the lap-bonded portion was examined to evalua-te the adhesion between the interposing adhesive layer and topcoat layer and the adhesion between the interposing adhesive layer and metal sub-strate according to a 5-ranked evaluation method ( 50 ~ood9 1~ bad ). Further, the degree of blushing in the topcoat layer (~ good9 X o bad )9 the degree of sulfide stain (~ : good, X o bad ) and the s-tate of generation o: rust in the double-seamed portion (~ o good~ X o bad ) wcre examined. The gel proportion of the protecting coating layer was determined in the same mam~er as in Exam~le 1.
Obtained results are shown in Table 3.

G) r~ ~
+~ ri '~ ~
~ o ~3 .,~ o ~ ~
al O ~ ~ ~1 ~ ~ ~ ~
o ~ Ql o ,n,~ ut o O o ~D o c-- O ~ O C~ O O
LS~
~; m v O ~0 ~0 L~
,~ I D S~ Ll) S~ L`~
a) ~1 ~) c~ ~r I ~ r1 ~ r I ~ r~ S~ r~
~1 a) ,A) r~ r1 ~> r~ ~ r~ ~) r~ +) r~ +~
,n ~ ~ ~
O ~ O ~ O t~l o ~ o ~ o 5~ L~D C) ~ t ) ~C r~ r-~
.~ O O ~ O ~ O ~ O ~ O ~ O
h V h u~ h u~ h U2 h u~
o .
I t~ ~1J
h ~r~ r~
~ ~ "~ OO
I--l S ~ ,~$ h $ (~ $ ~ r~ $ S~ $
~ Ql ¦ r~ ¦ r~ ¦r~ ~r~ r~ a) r~ ~r~
o ~ ~ ~ ~ ~ ~ ~ ~ h rc~
O ~ ~ri ~ o o o ~ a) ~ ~q O Q~ I ri a) Q) a) a) ~ o ~ h cJ a~
a) ¦ ~r~ r~ r ¦¦ r~ r~
~1 ~ J
S~ a~ aJ h c) a) h ~ a~ h a~ ~ h a) S~ ~ h a) k r~ ~I tll r-l r-l ~ r I r I ~ r~ ) r~ (~; r I (d r~J
~Q ~ . k ~ 3 k O ~ ~ O ~ ~ O ~ O ~ ~O ~ O ~ O ~
~Q V lJ~ ~Q V U~ U~ V V2 U~ V CQ U~ V IQ V U~ V ~Q

~;1 ~ ~ ~ Lr~ ~O ~ ~ r~ r~ r~

~ 56 -~ ~&i67~

r~
r~ r O ~ O
ri~ ~ C (~ O ~1 O ~1 O x ~ t O ~0 ~ ~a ~X ~a~x~
cq ~-J ,-1 ~
rn U'~ ~
C) ~ ~i ~_ O

h O ¦ ~
4 ~1^
J~c;o ~ O CO O O Lr~ ~ O ~OO Lr O
r~ I~ o ~ ~ C~) ~ ~\ ~ C)~ u~ a) ~ J ~ ~>

~1 +' L~

Ll~ ~ L~ ~ L~ ~ Lr~ Lr~ ~J L~\ (\I
rr~ ,.
o ~
o ~ o ~n ~, rn a) o a) rn o ~ ,~o c L~ r~l L~ ~ Lr~ ~ ~ r~ ~ ~ ~ ~ +~
o Q~ O O

.
0~ \1 ~ ~ Lr~ O r-l r~j ol ~ 57 ~

l e J+
Influellces of -~he thicl~nesses of the interposinp;
adhesive layer and pro-tectinJ coating layer on the properties oI the coatirig and the lea~age wi-ch lapse of time ancl corrosiorl resistance in bonded cans were examined, The same epoxy-phel~olic enamel as used in Example 3 was used for Iormation of` the interposing adhesive layer 9 and the same topcoa-t layer-forming epoxy-phenolic enamel as used in Example 2 was used for formation of the topcoat layer. For lormation of can bodies 9 the same TFS plate as used i-'l Example l. Coating9 baking and curing were carried out in the same malmer as described in Example l whilc- changing the -thicknesses of the interposing adhesive layer and protectin~ coating layer as indicated in Table 4, Bonded cans having an inner capacity of 250 m~ were prepared from the so coated TFS
plate in the same manner as in ~xample lo The same TFS
bottom lid as used in Example 3 was employed.
Each of the so obtained cans was filled with 250 m~
of beef consomme soup 9 and the packed can was double-seamed wi-th a -top l~d prepared in the same mal~1er as -the bot-tom lid. ~ith respect to each sample, lO0 cans were subjected to a retort treatment at 130C. for 60 minutesO
Properties of the coating and leakage with lapse of time were examined. Further, a~ter l year's storage at 37C. 9 the corrosion of the inner face of the can was - 5~ -e~amini~d, The condition O-r the coating ( cracked or not crackcd ) after the bending processing was exami-ned in th_ fOllOWi!lg manner.
~fi~ specimen naving; a Si~-~ of 30 rQm x 30 mm was cut out from -ti~e body portion of an empty bond~d can and was bent at an inner diameter of 1.5 rmm ~. Onc(lT) or three(~T) of plates having the same thickIle,s were gripped between the ben. portions 9 and a weight of 1 Kg was let -to fall do~n -thereon from a height of 50 cm to make an impact deformation. Then, the specimen was dipped in a 1 %
aqueous solu-tion of sodium chloride and a voltage of 7 V ~as applied between the specimen and a counter electrode. l~fter passage of 1 minute 9 the current ( mA ) was measured to evaluate -the crack resistance at the bending processing, Obtained results are shown in Table 4, ~ 59 -'7~

~ô
C~ I
o b~h .,1 r~, ~J
+) rl ~
~ Lr`\ C-- ~J~ O ~'~I Ll~ O O C' ~ t\l ~D O
O ~ ill~ C-- C-- C-- O.~ r~l 0~) L-3 ~ C-- C-- C~ C-- ) ~4 h o ~5 o ~

r~ ~ ~ O ~ L-~ ~ C-- C:) ~ L~ O
O LXi ~D L~ ~ ~1 0 L~ ~D t-- O O L~
~1 ~ O OOOOO O OOOO O ~i0 ~ I
_~ ~ h~

L~ ~D CO O Lr\ O ~r~ i~ C~ ~ ~ O Lr~ O
o ~ ~
a) S ~ o ~1 ~ O ~) E~
`_~
U~ L~
h ~ L~ Lr~ L~ Lr~ L~ Lr~ L~ O O C-- C~ L o o H
E~ o~

Q) ~
'? r-J ~ f`~l i~ f~ ~ Lf~ > LJ~ 0 r-l +~ J ~:) t ~ ~ ~ t +) t +~ ~ t ~ t ~ t +~ ~ t f~ Q~ ~ fl) h fi) f~U f~ f~l fU f;i ~Q f~ r fd r r r r r f~ f f;S I--I r r r fd I--I f~ r f~ fJ I--I
O f.~ O (~ ~J r~ fl~ fu O t~ O fd n;l fi I ~f~ O f~ O O fi:5 0 t~
V u~ V r,? U~ u~ r,? r,? V u~ V u~ V Ul V u~ V rJ~ V rJ~

01 r~ N ~ ~ Lf~ ~0 t-- 0 i~ O r~ f~ t Lf~ :
_. ~I r-l ~I r-l r~ ri r-l ~ 60 ~

. 667~

~ ;
oi ,~
,`-, ~
.
?~ ~ J ~ r 1 2 L~
O CH ~ ~ i~ L~ ~I Lr~ L'~ J
+> ~ $ ~ O Oi Oi L~ ~ O ~i (i Oi ~ ~ ~O
r-l ~I r-l O ~I q ~q H ,~
_~ ~q a o~;

+~
o a) I
,~
~_ ~ c~
,~
~ O L~) L~ ~D ~ Lr~ ; r~l ~ ~D L'~ ~1 ~D vl r~l ~C) ~ ~ ~ ~ ~ ~ ~ C~l ~ ~ ~ ~
r!) ~ r ~) O O O O
Q~~~ ~q ~q ~q ~q E~ !~ ~

~0 c ~) O i~ ~1 1~\ L~ OL'~ \1 0 Lrl O O
O C 1 i~ (a i~ ~ Oi O ~ I O O O
~ i~l o a)-, m r,q r,q r,q s~ a) a) o rl ~ _~ L~ O L~ i~ 20 i (:~ il i~ O~ O Lr~ ~

~I N i~~~ ~ L~ ~ 1~ C:) ~ 2 r I N i` \ ~ L

~ 61 --'7 L xample 5 The same in-terposing adhesive layer- orming enarQel and topcoat layer-for3lirlg erl-~mel as u,.ed in ~xamplc 3 were employed. l~X tlle T~ plate 9 ~there Wcl S -'mplo-~red a TFS pla-te h~ving 2 thiCkIleSS OI 0.17 rml7 a leng-th of ~27 mm and a width of 102G m,. The can body was made in -the same manner in ~xample l, The obtained can body was subjected to beadin&r process 9 and i-t was -then sub-jec-ted -to iîneck in~i and flange process according to customary procedures. The can body was double-seamed with a 202-diameter lid composed of a coated TFS plate as a bottor,l:Lid by an ordinary double seamer, Then 9 the can was packed with tuna dressing and was then double-seamed with the same lip as that used as the bottom lid by a vacuum seamer, Wi-th respec-t to each sample 9 100 packed cans were subjected to a retort -treatment at 125C.
for 90 minutesO BreakaOre o~ can bodies and occurrence of micro-leakages by the retort treatment were examined, and af-ter 6 mon-ths' storage at 50C, 9 leakarr~e with lapse of time and degradation of the coa-ting on -the inner face of the can were examined, Fur-tller, the gel proportion of the protecting coating layer was measured, Obtained results are shown in Table 5.

7~

, G
O ~0 h ~1 ,_, + ~1 ~
O ~ ~ ~ Lr~ ~ O Ll~. O
0 ~ C(`O 1~

~ h-,-r~
c) ;~ O
C~ O ~) ~ 1.. 0 ~0 ~0 --I +~r~ +,~ ~0 r-c) Q~-1~ ,L~ ~
E~ ~ 0 o t~:l O t~ o ~ O O O ~> O
5~' ~ c) o c~
~:1 r~
b~ +~r~ h r h r-l O Q~ O Ql O Q
h O h u~ h u2 h u~
Q~) O
r ~
~J h ~1 Q~
.,~ r~
~1 r~ r I ., I ~
r-' ~ ho $
0o +~ ! QJ r~ $ r~ Q~ h $
t~ I J ~ ~ r~
O p~ ~ r~ r~ r~
O O r~ ~

Q.) a) L'l) r~l ~ r ~ ~I r-~ ~ r~
r~ Ll +)LI Ll+ 1I Ll +~LI\
~ `~J h G~ a)h ~ a~ h a~
E r-l ~3 r~ r~ ~I r~ r~ 0 r~
0 ~ ~ P~{ i P1 ~QI ~ ~ ~

~3 0 ~'J ~'5 0 ~3 ~ O `3 V~ V U~ U~V V~ U~ V U~
.

o ~ r~ C~l r~
a~

.2~6~g3 ~o s~ ~o ~?
c :~, ~ ~, ~a s ,? .~ ~ ~ ~ " h ~ O o ,~ C
i, ~> ~o ~ ~n ~n 4~ ~ ~ u7 hO o ~ ~ ~ ~ ~d ~d -~n ~ ~4 Q .~ ~ ~
[n u~ E3 $ ~o c C C ~

C ~~ ~ N ~ o D ~o C

~_ ~ Q o a) ~

D~ D ~ C
,~ O Lr~ O ~O o ~ ~n 1l ~ ~ ~ C) V o r~ d 5 ~ ~+~~
,~ . ~ r~ ~r~ hO ~ hO
-,~
r~
(1) +) ~ ~ ~) C) ~ C) h ~ h p, a) o -,1 o r~ ,C ~ ^ ~ a h Q ~ O~ hO i) hO
tn a~ ~n-,l ~ o o ~ o ~r o o t~ d (~ ~d O r~
h ~I ~d O o ~0 ~10 ~ Ch ~ h r, u~
O r~ *
~l ri (\~ ~ ~ L'~ ~O

i7~

The ~ame epoxy~-phenolic resin enarlel i. as used in Exal~ple 1 was ,entirely coated on one sur,face ol the sars2 TFS pla-te 'lS used in Exampl~ 1 so that the -thicknec,s of the coa-ting aIter baking w~s 2 ,I. I)rying a.nd baki~g were car~ried ou-t under -thc sa;:le conditions as in 2x.~mple 1.
The other surfac2 of -the T~ plate 9 that ~;~as to constitu-te the inner face oI` tlle bor~ded can 9 was coa-ted in a manner as described belo~.
(1) The epoxy-phenolic resin enamel ~-\ was roll~coated for forma-tioll o-f the in-te~rpos~ng adhesive layer and protecting coa-ting la~yer on the surface tha-t was to constitute the inner face of the bonded can9 by using a stepped roll so that the thickness of the coating was 3 l~ in the portion to be lap-bonded and 8 ~ in other portion. Baking was carried out at 210C. for 10 minutes to form a coated plate for production of a can body.
F'rom this coated pla-te 7 an empty bonded can was pre-pared in th~ same i~anner as in Example 1-1, The Ta/Tp thickness ratio was 0.339 and t'ne gel proportion in the protecting coating la~er was 80 5S.
(2) The epoxy-phenolic resin enar.lel B for fori~ation of the pro-tecting coa~ting layer was margin-coated on the surface to constitute the imler f~ce o-F the bonded c~n except a portion to be lap~bonded 9 SO that the thickness of the coating after baking was 5 ~,. Then, the epoxy-phenolic resin enamel ~-~ for formation of the interposing adhesive l~yer and protecting coating layer ~ G5 -'7(~

was roll coa-ted erltirely on -the same surface iiî the wet-Oll-WC-t s-tat~ so tha-t the thickness of the coating after baking w as L, !~. Bakirlg and curing were carried out a-t 210C. for 1.0 ninutes to a coated pla-te for produc-tion of a can body.
IrL the .same manner as (,l(escribed in r.xa~ple 1-19 an empty bonded can was pr~7j~ared fronm th~ so obtained coated plate. The Ta~Tp thickness ratio was 0.44 and the gel proportion of the pro-tecting coatin,~ layer was 82 %.
(3) The epoxy-phenolic enam~l 3 was margin coated on -the surface to constitute th~ inner face of -the bonded can except a portion to be lap~bonded 9 SO that the th.ickness of the coating after baking was 4 ll. Baking was carried out at 200C, for 10 minutesO
The epoxy-phenolic resin h for formation of the interposing adhesive layer and the protecting coating layer was entirely coated on the same surf'ace so that the thickness after baking was 4 ~.. Then9 bakin~ and curing were carried out in the same manner in Example 1-1 to obtain a coa-ted plate for production of a can body. From -this coated plate, an empty bonded can was prepared in the same malmer as in Example 1~1. The Ta/Tp thickness ratio WLS 0.50 and the gel proportion of the protec-ting coating layer was ~4 %.
The so obtained three kind of empty bonded cans were filled ~rith milk coffee 9 and each of them was double-seamed with the same 202-diameter TFS lid as used in Exal3lpl~], ~lith respec-t -to each sal~ple 9 100 cans werc slllJ~ cted -to a rc.ort tr~tment a-t 130C, for 90 minutes.
In each sample~ Lrea'ra~re o:~ can bodies w~s not caused l)y -the r^tort trea+,r;~ tO Wher ~these sam~les were s-tored a+ ,7C1. for 1 year .md a] 1O~JC?d to keep at room teinperature for ] wee~ 9 i3~ ~'`?aCh sample the cle~ree of vacuu3n in the can was, 30 cm H~ or lower.
No leakag~-? with lapse of time was observed. Further, 10 rus-t on the im~ er face o c the can was not observed in each sample,

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A bonded can having a structure formed by lap-bonding a can blank composed of a tin-free steel substrate by a polyamide adhesive, wherein the can blank comprises in the lap-bonded portion an interposed primer layer which is interposed between the tin-free steel substrate and the polyamide adhesive and is composed of a composition comprising (a) an epoxy resin and (b) a resol type phenol-aldehyde resin formed by a condensation reaction be-tween formaldehyde and a mixed phenol containing (1) a dihydric phenol rep-resented by the following formula:
wherein R stands for a direet bond or a divalent bridging group, and (ii) a monohydric phenol at a (i)/(ii) weight ratio of from 98/2 to 65/35, said epoxy resin (a) and said phenol-aldehyde resin (b) being present at an (a)/(b) weight ratio of from 95/5 to 50/50, and an anti-corrosion protective coating layer coated at least on the inner face of the whole portion of the can blank other than the lap-bonded portion prior to application of the polyamide ad-hesive, said protective coating layer being composed of a thermosetting resin having a gel proportion of at least 70% determined by extraction in 60°C
chloroform for 60 minutes, and the thickness Ta of the interposed layer and the thickness Tp of the protective coati.ng layer satisfy the following re-quirements:
Ta/Tp = 0.025 to 0.7, Ta = 0.5 to 9 microns, and Tp = 3 to 20 microns.

2. A bonded can as set forth in claim 1 wherein the thicknesses Ta and Tp satisfy the following requirements:
Ta/Tp = 0.05 to 0.6, Ta = 1 to 6 microns, and Tp = 5 to 12 microns.

3. A bonded can as set forth in claim 1 wherein the tin-free steel comprises a steel plate substrate and a chromium-containing coating layer formed on the substrate, said coating layer being composed of metallic chrom-ium, non-metallic chromium compound or a mixture thereof and being present in an amount of 0.06 to 3.6 mg/dm2 as chromium.

4. A bonded can as set forth in claim 3 wherein said coating layer includes a metallic chromium layer formed on the steel substrate and a non-metallic chromium compound layer formed on the metallic chromium layer, said metallic chromium layer being present in an amount of 0.05 to 3.0 mg/dm2 and said non-metallic chromium compound layer being present in an amount of 0.05 to 0.4 mg/dm as chromium.

5. A bonded can as set forth in claim 1 wherein the epoxy resin in the interposed layer is an epoxy resin having a number average molecular weight of 800 to 5500, which is formed by condensation of bisphenol A with an epihalohydrin.

6. A bonded can as set forth in claim 1 wherein the anti-corrosion protective coating layer has a single-layer structure.

7. A bonded can as set forth in claim 1 wherein the anti-corrosion protective coating layer has a two-layer structure including an undercoat layer and a topcoat layer, the undercoat layer is composed of an epoxy resin/
phenol-aldehyde resin mixture having the same composition as that of the in-terposing adhesive layer and the topcoat layer is composed of a thermosetting resin having a composition different from that of the undercoat layer.

8. A bonded can as set forth in claim 7 wherein the undercoat layer of the anti-corrosion protective coating layer and the interposed layer are composed of an epoxy resin/phenol-aldehyde resin mixture applied in the form of a single-layer continuous film.

9. A bonded can as set forth in claim 7 wherein the topcoat layer of the anti-corrosion protective coating layer is composed of an epoxy resin/
phenol-aldehyde resin mixture satisfying at least one of the following two requirements.
(1) the phenol-aldehyde resin content is higher than the resin composition constituting the interposing adhesive layer, and (2) the functional index (I.F.) represented by the following formula:
I.F = 2M2 + 3M3 + 4M4 wherein M2 stands for the mole number of the bifunctional monohydric phenol in 100 g of the total phenols contained in the phenol-aldehyde resin, M3 stands for the mole number of the trifunctional monohydric phenol in 100 g of the total phenols contained in the phenol-aldehyde resin, and M4 stands for the mole number of the polycyclic dihydric phenol in 100 g of the total phenols contained in the phenol-aldehyde resin, is higher than the functional index (I.F.) of the resin composition constituting the interposed layer.

10. A bonded can as set forth in claim 9 wherein the phenol-aldehyde resin constituting the interposed layer has a functional index (I.F.) of 1.60 to 1.95 and the phenol-aldehyde resin constituting the anti-corrosion protecting coating layer has a functional index (I.F.) of 2.00 to 3.00.

11. A bonded can as set forth in claim 1 wherein the polyamide type adhesive has a relative viscosity (? rel) of at least 1.5 as measured with respect to a 1% solution in 98% sulfuric acid.

12. A process for the preparation of heat-resistant bonded cans, which comprises the steps of (I) coating an enamel comprising an epoxy resin and a phenol-aldehyde resin at least on a portion to be lap-bonded of a can blank composed of a tin-free steel to form a primer layer having a thickness Ta of 0.5 to 9 microns, said epoxy resin (a) and said phenol-aldehyde resin (b) being pres-ent at a weight ratio of from 95/5 to 50/50, said phenol-aldehyde resin (b) being composed of a resol type phenol-aldehyde resin formed by condensation reaction between formaldehyde and a mixed phenol containing phenol containing (i) a dihydric phenol represented by the following formula:

wherein R stands for a direct bond or a divalent bridging group, and (ii) a monohydric phenol at (i)/(ii) weight ratio of from 98/2 to 65/35, (II) coating an anti-corrosion enamel comprising a thermosetting resin on the can blank except the portion to be lap-bonded to form an anti-corrosion protective layer having a whole thickness Tp of 3 to 20 microns, the ratio of Ta/Tp being in the range of 0.025 to 0.7, (III) applying a polyamide adhesive to the primer layer of the can blank, (lV) lapping the portion to be lap-bonded having thereon the polyamide type adhesive in the molten state, and (V) pressing the lap-bonded portion under cooling to effect lap bonding of the can blank.

13. A process according to claim 12 wherein an enamel comprising an epoxy resin and a polycyclic phenol containing phenol-aldehyde resin is coat-ed on the entire surface, inclusive of the lap-bonded portion of the can blank so that the thickness Ta after drying is 0.5 to 9 microns, and an anti-corrosion thermosetting resin enamel was coated on the so formed first coat-ing layer except the lap-bonded portion so that the thickness Tp after dry-ing is 3 to 20 microns, the ratio of Ta/Tp being in the range of from 0.025 to 0.7.