GB1570475A - Creation of Inventions and Patents with Artificial Intelligence

GB1570475A – Bonded can and bonding composition
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GB1570475A – Bonded can and bonding composition
– Google Patents
Bonded can and bonding composition

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Publication number
GB1570475A

GB1570475A
GB474578A
GB474578A
GB1570475A
GB 1570475 A
GB1570475 A
GB 1570475A
GB 474578 A
GB474578 A
GB 474578A
GB 474578 A
GB474578 A
GB 474578A
GB 1570475 A
GB1570475 A
GB 1570475A
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GB
United Kingdom
Prior art keywords
phenol
bonded
resin
ditto
composition according
Prior art date
1978-02-06
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)

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Application number
GB474578A
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Toyo Seikan Kaisha Ltd

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Toyo Seikan Kaisha Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
1978-02-06
Filing date
1978-02-06
Publication date
1980-07-02

1978-02-06
Application filed by Toyo Seikan Kaisha Ltd
filed
Critical
Toyo Seikan Kaisha Ltd

1978-02-06
Priority to GB474578A
priority
Critical
patent/GB1570475A/en

1980-07-02
Publication of GB1570475A
publication
Critical
patent/GB1570475A/en

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legal-status
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Classifications

C—CHEMISTRY; METALLURGY

C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON

C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS

C08G8/00—Condensation polymers of aldehydes or ketones with phenols only

C08G8/04—Condensation polymers of aldehydes or ketones with phenols only of aldehydes

C08G8/08—Condensation polymers of aldehydes or ketones with phenols only of aldehydes of formaldehyde, e.g. of formaldehyde formed in situ

C08G8/24—Condensation polymers of aldehydes or ketones with phenols only of aldehydes of formaldehyde, e.g. of formaldehyde formed in situ with mixtures of two or more phenols which are not covered by only one of the groups C08G8/10 – C08G8/20

C—CHEMISTRY; METALLURGY

C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR

C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR

C09D163/00—Coating compositions based on epoxy resins; Coating compositions based on derivatives of epoxy resins

Description

(54) BONDED CAN AND BONDING COMPOSITION
(71) We, TOYO SEIKAN KAISHA LTD, a Japanese Body Corporate of 3-1, 1chome, Uchisaiwai-cho, Chiyoda-ku,
Tokyo, Japan do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a bonded can excellent in the resistance to hot water and also to an undercoating composition for use in manufacture of this bonded can.
More particularly, the invention relates to a bonded can in which a portion of a can body bonded by an adhesive is prevented from deterioration even under such conditions that the bonded portion falls in contact with hot water or steam, and also to an undercoating composition for use in manufacture of such bonded can.
In the field of manufacture of cans, socalled tin-free steels (TFS) such as chromium-plated steel plates and chromic acid-treated steel plates having a chromium oxide film on the surface have been widely used instead of tin-plated steel plates as metal materials in the production of cans.
Since soldering of these tin-free metal materials is very difficult, a side seam of a can body is mainly formed according to a method comprising bonding both the confronting side edges of a coated metal material for a can body to each other by an organic adhesive such as a polyamide.
According to this known bond-seaming method. as disclosed in the specification of
U.S. Patent No. 3,663,354 to Ueno et al., an undercoating composition such as an epoxyphenolic resin is first applied to both the surface of a metal material for a can body, melting a polyamide type adhesive located between both the confronting side edges of the metal material, and cooling and solidifying the melt under application of a pressure to bond both the side edges and form a can body. In short, according to this known method, in the bonded portion of the can body, the side edges are bonded to each other by the polyamide adhesive through the undercoat of an epoxy-phenolic resin coated on the surfaces of the metal material.
The bonded portion prepared according to such known method has a satisfactory bonding strength under normal conditions and can sufficiently resist the pressure of contents having a spontaneous pressure such as a carbonated drink. However, the bonded portion of this known can is still insufficient in its resistance to hot water. In the case of carbonated drinks, filling of contents into a can body is ordinarily carried out in a cold state, and since the contents per se are acidic, a severe sterilization treatment need not ordinarily be performed for preservation of the contents. Therefore, when a carbonated drink or the like is filled, the hot water resistance of the bonded portion is not particularly significant. However, in case of other various foods and drinks, for example, fruit juices and processed foods, from the viewpoint of preservation of the contents, it is necessary to perform a severe heat sterilization treatment or a retort sterilization treatment or to carry out filling of contents in a hot state, and therefore, the bonded portion is required to have a high resistance to hot water. More specifically, if the bonded portion has poor resistance to hot water, the can body is readily broken at the heat sterilization or air tightness is lost in the content during preservation. This tendency is conspicuous when the interior of the can is kept in vacuum or under a reduced pressure.
On the other hand, bonded cans are advantageous in various points. For example, the kind of the metal material to be used is not particularly critical, and the can manufacturing speed, namely the speed of formation of side seams, is very high and the productivity is therefore very high.
Accordingly, if a can body provided with a side seam formed by an adhesive and having a high hot water resistance be obtained, it is apparent that various advantages will be attained as regards the manufacturing cost
and the problem of the resource.
We found that the hot water resistance of
a bonded portion of a bonded can is most
influenced by an undercoating composition to be applied to a metal material and that in
each of bonded cans formed by using known
epoxy-phenolic undercoating compositions, the bonded portion cannot resist hot water
or steam heated at 1250C. at all but when a
novel undercoating composition comprising specific resol type phenolaldehyde resin and
epoxy resin described hereinafter at a
specific ratio is used for production of a bonded can, the bonded portion which can
resist hot water or steam heated above 125″C. can be obtained.
The higher is the temperature that can be resisted by the bonded portion of a bonded can, the more advantages can be attained with respect to maintenance of flavor and taste of the canned content. It is known that the time necessary for annihilating spores of bacteria may ordinarily be shortened as the sterilization temperature is increased and it also is known that the flavor and texture of the filled food and dyes and vitamins contained in the filled food are degraded and destroyed substantially in proportion to the sterilization time. Accordingly, as the temperature of hot water or steam that the bonded portion can resist is higher, the sterilization treatment can be performed at a higher temperature and completed in a shorter time, and it is possible to preserve the filled content for a longer period of time without degradation and destruction of the flavor and texture of the filled content and dyes and vitamins contained in the filled content.
The present invention provides a hot water-resistant undercoating composition which is applied to the surface of a metal
material prior to bonding of the metal
material for formation of a can body.
This undercoating composition can be used to provide a bonded can in which a portion bonded by an adhesive can resist
hot water or steam heated at high temperatures, especially high temperatures
exceeding 125″C. The reduction of the peel strength in the portion bonded by the
adhesive is extremely diminished at the heat
sterilization step or when the can is filled
with contents in a hot state and reduction of
the peel strength in the bonded portion with
the lapse of time is controlled to a very low
level while the content-filled can is
preserved for a long time.
In accordance with the present invention
there is thus provided an undercoating
composition for use in the production of a
bonded can having a high resistance to hot
water, which composition comprises (1) 50 to 959 by weight of an epoxy resin having a number average molecular weight of 800 to 5500, which is formed by condensation of an epihalohydrin with bisphenol A and (2) 50 to 5% by weight of a resol type phenolaldehyde resin having a number average molecular weight of 200 to 1000 and which has been obtained by reacting a mixed phenol comprising (a) 65 to 98% by weight of a dihydric phenol of the general formula:

wherein R represents a divalent bridging atom or group or is a direct bond, and (b) 35 to 20/, by weight of a monohydric phenol, with an aldehyde in the presence of a basic catalyst, wherein said components (1) and (2) may be present in the form of a precondensate according to need.
In accordance with another aspect of the present invention, there is provided a bonded can having a high resistance to hot water, which consists of a metal material provided with a layer of said undercoating composition, which is lap-seamed and bonded through a polyamide adhesive.
Referring to the accompanying drawing illustrating the structure of the bonded can of the present invention, the bonded can consists of a can body formed by bonding a can body blank 1 at a side seam 2. This side seam 2 includes side edges of the can body blank 1 lapped and bonded through a polyamide adhesive 3. This can body blank 1 is composed of a metal material 4 such as tin-free steel (TFS), on the surface of which a layer 5 of an epoxy-phenolic resin undercoating composition of the invention has been formed prior to bonding by the polyamide adhesive 3. Both the side edges of the can blank 1 are bonded together through this undercoating layer 5 by the polyamide adhesive.
The present invention will now be described in more detail.
Phenol-Aldehyde Resin Component
In the dihydric phenol (a) represented by the above general formula (I) that is used for the preparation of the resol type phenolaldehyde resin (2), R represents a divalent bridging atom or group or is a direct bond.
As the divalent bridging atom or group R, there can be mentioned, for example, an alkylidene group of the formula -CR1R2- in which R1 and R2 each represent a hydrogen or halogen atom or an alkyl or perhaloalkyl group having up to 4 carbon atoms, –00-, –SS-, a group -SO-, a group SO2 and a group -NR3- in which R3 represents a hydrogen atom or an alkyl group having up to 4 carbon atoms.
Among these atoms and groups, an alkylidene group or ether oxygen atom are preferred, As preferred examples of the dihydric phenol (a), there can be mentioned 2,2 – bis(4 – hydroxyphenyl)propane (bisphenol A), 2,2 – bis(4 hydroxyphenyl)butane (bisphenol B), 1,1 bis(4 – hydroxyphenyl)ethane, bis(4 hydroxyphenyl)methane (bisphenol F), 4 hydroxyphenylether and p – (4 hydroxyphenyl)phenol. Among them, bisphenol A and bisphenol B are especially preferred.
As the monohydric phenol (b) that is used for preparation of the resol type phenolaldehyde resin (2), any of monohydric phenols that have been used for production of resins of this kind can be used in the present invention. In general, however, it is preferred to use at least one member selected from bifunctional phenols of the general formula:

wherein R4 represents a hydrogen atom or an alkyl or alkoxy group having up to 4 carbon atoms with the proviso that two of the three R4’s represent a hydrogen atom and the other represents an alkyl or alkoxy group having up to 4 carbon atoms, and each R5 represents a hydrogen atom or an alkyl group having up to 4 carbon atoms.
As such bifunctional phenol, there can be mentioned, for example, o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3xylenol and 2,5-xylenol. In addition, trifunctional phenols such as phenol (carbolic acid), m-cresol, m-ethylphenol, 3.5-xylenol and m-methoxyphenol, monofunctional phenols such as 2,4-xylenol and 2,6-xylenol, and other bifunctional phenols such as p-tert-amylphenol, pnonylphenol, p-phenylphenol and p-cyclohexylphenol may be used singly or in combination with the above-mentioned bifunctional phenols represented by the general formula (II) for production of phenol-aldehyde resins.
In the undercoating composition that is used in the present invention, in order to improve the hot water resistance of the bonded portion, it is very important that a phenol-aldehyde resin obtained by using a combination (a) the dihydric phenol of the general formula (I) and (b) the abovementioned monohydric phenol at an (a)/(b) weight ratio of from 98/2 to 65/35, especially 95/5 to 75/25, should be used as one component of the undercoating composition. As will be apparent from the data shown in Table 2 given hereinafter, if the amount of the dihydric phenol (a) is outside the above range specified in the present invention, a bonded can formed by using the resulting undercoating composition tends to break down at a treatment with hot water above 125″C., and the peel strength of the bonded portion is drastically reduced by the hot water treatment and reduction of the peel strength of the bonded portion after the hot water treatment becomes conspicuous with the lapse of time. This tendency is similarly observed, as shown in Table I given hereinafter, when a dihydric phenol other than the dihydric phenol of the general formula (I), for example, resorcinol, is used as the dihydric phenol. In contrast, when a dicyclic dihydric phenol represented by the general formula (I) and a monohydric phenol are used at the above-mentioned specific ratio for production of the phenolaldehyde resin component of the undercoating composition according to the present invention, the bonded portion of the resulting bonded can has such a high hot water resistance that cannot be attained in any of conventional bonded cans at all.
Namely, the bonded portion can sufficiently resist a treatment with hot water above 125″C. and reduction of the peel strength by the hot water treatment or with the lapse of time can be controlled to a very low level.
It is known that an undercoating composition (primer) for a bonded can is formed by combining a resol-type phenolaldehyde resin containing a dicyclic dihydric phenol such as bisphenol A with an epoxy resin.
For example, the above-mentioned specification of U.S. Patent No. 3,663,354 discloses that a primer comprising an epoxy resin and a resol-type phenol-aldehyde resin formed by condensing a mixed phenol consisting of 50 to 10% by weight of bisphenol A with formaldehyde is very valuable as a primer for improving the adhesion between a metal and a polyamide adhesive, and it also is taught that when a resol-type phenol-aldehyde resin formed by by using bisphenol A alone instead of the above-mentioned mixed phenol is used and combined with an epoxy resin, the adhesion between a metal and a polyamide adhesive is not improved at all.
More specifically, an undercoating composition comprising an epoxy resin and a resol-type phenol-aldehyde resin formed by using a polycyclic dihydric phenol such as bisphenol A alone as the phenol component is defective in that peeling is readily caused between the undercoating and the polyamide adhesive. For this reason, in conventional undercoating compositions, the amount of a polycyclic phenol such as bisphenol A is reduced to a relatively low level such as up to 50% by weight based on the total phenol component.
We found that when a primer or undercoating composition comprising an epoxy resin and a resol-type phenolaldehyde resin obtained by using a larger amount of a monocyclic phenol and a smaller amount of a polycyclic phenol such as bisphenol A is used for manufacture of bonded cans, however, the bonding between the metal material and the undercoating is degraded at a retort sterilization conducted at high temperatures. On the other hand, when an undercoating composition is prepared by combining an epoxy resin with a resol-type phenol-aldehyde resin having a number average molecular weight of 200 to 1000, which is obtained by using a mixed phenol comprising (a) a dihydric dicyclic phenol and (b) a monohydric phenol at the abovementioned weight ratio, according to the present invention, and when it is used for manufacture of bonded cans, the undercoating composition has an excellent adhesion to both the polyamide adhesive and the metal material and this adhesion is hardly degraded by the retort sterilization or during storage after the retort sterilization. In short, we succeeded for the first time in providing an undercoating composition for bonded cans which has such excellent adhesion characteristics by using a mixed phenol comprising the abovementioned dihydric phenol (a) and monohydric phenol (b) at the abovementioned specific weight ratio for synthesis of a resol-type phenol-aldehyde resin to be combined with an epoxy resin.
Formaldehyde (or paraformaldehyde) is especially preferred as the aldehyde component of the phenol-aldehyde resin. Of course, other aldehydes such as acetaldehyde, butyraldehyde and benzaldehyde may be used singly or in combination with formaldehyde.
The resol-type phenol-aldehyde resin that is used in the present invention is prepared by reacting a mixed phenol having the above-mentioned specific composition with an aldehyde in the presence of a basic catalyst so that the number average molecular weight of the resulting resin is 200 to 1000, especially 250 to 800.
In the present invention, from the viewpoint of the use of the abovementioned dihydric dicyclic phenol of the general formula (I) in a specific amount, it is important that the number average molecular weight of the resulting phenolaldehyde resin should not exceed 1000. If the number average molecular weight of the resulting phenol-aldehyde resin exceeds 1000, peeling is readily caused between the polyamide adhesive and the undercoating layer and the bonding between the undercoating layer and the metal material is readily degraded by the retort sterilization or during storage.
In the instant specification and appended claims, the number average molecular weight is one determined by a vapor pressure osmometer.
The molecular weight of a phenolaldehyde resin is greatly changed by such factors as the kind of the phenol used, the amount used of the aldehyde, the kind of the catalyst, the reaction temperature and the reaction time, and it is very difficult to define these conditions collectively for production of a phenol-aldehyde resin having the above-mentioned number average molecular weight. Accordingly, practical reaction conditions are experimentally selected from the following general conditions so that the number average molecular weight of the resulting phenol-aldehyde resin is within the abovementioned range.
In general, the aldehyde is used in an amount of at least I mole, especially 1.5 to 3.0 moles, per mole of the mixed phenol.
Condensation is carried out ordinarily in an appropriate reaction medium and preferably in an aqueous medium. Any basic catalysts that have heretofore been used for production of resol-type resins can be used in the present invention. Among them, there are preferably employed ammonia and hydroxides, oxides and basic salts of alkaline earth metals such as magnesium hydroxide, calcium hydroxide, barium hydroxide, calcium oxide, basic magnesium carbonate, basic magnesium chloride and basic magnesium acetate. The basic catalyst is present in a reaction medium in a catalytic amount, especially 0.01 to 0.5 mole % based on the phenol component. Condensation conditions are appropriately selected from reaction temperatures of 60 to 1300C. and heating times of 10 minutes to 40 hours.
The resulting resin may be refined by known means. For example, the reaction product resin is extracted and separated from the reaction medium with a ketone, an alcohol or a hydrocarbon solvent or a mixture thereof, washing the recovered resion with water to remove unreacted substances according to need and removing water from the product by azeotropic distillation or precipitation to obtain a refined resol-type phenol-aldehyde resin that can be mixed with an epoxy resin.
It is ordinarily preferred that the so obtained resol-type phenol-aldehyde resin be combined with an epoxy resin as it is. If desired, however, it is possible to mix the phenol-aldehyde resin with an epoxy resin after it has been modified with at least one of known modifiers such as fatty acids, polymeric fatty acids, resin acids (rosins), drying oils and alkyd resins.
Epoxy Resin Component
In the present invention, an undercoating composition is prepared by combining the above-mentioned resol-type phenolaldehyde resin with an epoxy resin having a number average molecular weight of 800 to 5500, especially 1400 to 5500, which is synthesized by condensing an epihalohydrin with bisphenol A [2,2 – bis(4 hydroxyphenyl)propane]. This epoxy resin that is used in the present invention is represented by the following general formula:

wherein R stands for the condensation residue of 2,2 – bis(4 hydroxyphenyl)propane and n is a number selected so that the number average molecular weight of the resin is 800 to 5500.
When the number average molecular weight of the epoxy resin is lower than 800, as will be apparent from data shown in
Table 4 given hereinafter, the bonding strength between the undercoating layer and the metal material is low, and the bonded portion of the resulting bonded can is readily peeled if it falls in contact with hot water. When the number average molecular weight of the epoxy resin is higher than 5500, insufficient bonding strength can be obtained, and if the resulting bonded can falls in contact with hot water, the peel strength is drastically lowered and the can is readily broken. The molecular weight of the epoxy resin referred to is the average molecular weight. Accordingly, in the present invention, it is possible to combine an epoxy resin for undercoating compositions which has a relatively low degree of polymerization with a linear epoxy resin having a high molecular weight, namely a phenoxy resin (high-molecularweight thermoplastic epoxy resin), so that the average molecular weight is in the above-mentioned range, and to use the resulting mixture as the epoxy resin.
Undercoating Composition
In the present invention, in order to improve the adhesion of the undercoating layer to the metal material or polyamide adhesive and enhance the hot water resistance of the bonded can, it is very important that the above-mentioned bisphenol A type epoxy resin (1) should be combined with the above-mentioned resoltype phenol-aldehyde resin (2) at a (1)/(2) weight ratio of from 95/5 to 50/50, especially from 90/10 to 60/40, to obtain an undercoating composition. If the amount of the resol-type phenol-aldehyde resin is smaller than 5% by weight based on the sum of the resins (1) and (2), the curing speed of the undercoating composition is low and the degree of curing is insufficient, and as will be apparent from data shown in Table 3 given hereinafter, the peel strength of the resulting bonded can is low and it is often reduced to zero on contact with hot water.
When the amount of the phenol-aldehyde resin exceeds 50% by weight, the peel strength of the resulting bonded can is slightly higher than in the above-mentioned case, but it is often reduced to substantially zero just after contact with hot water or after passage of a certain time from contact with hot water.
In the present invention, the abovementioned epoxy resin and phenol-aldehyde resin may be used as the undercoating composition in the state mixed and dissolved in a ketone, an ester, an alcohol, a hydrocarbon solvent or a mixture thereof, but in general, it is preferred that the resins be precondensed in such dissolved state at 80 to 1300C. for about 1 to about 10 hours and the resulting precondensate solution be used as the undercoating composition.
Bonded Can
In the present invention, by selecting a resol-type phenol-aldehyde resin comprising the above-mentioned dicyclic dihydric phenol and monohydric phenol at a specific weight ratio and obtaining an undercoating composition by combining this resol-type phenol-aldehyde resin with an epoxy resin at a specific weight ratio, it is made possible to obtain a bonded can which can resist sufficiently hot water heated above 125″C., although such high hot water resistance cannot be attained at all by any of bonded cans prepared by using conventional undercoating compositions.
When a bonded can is prepared by using the undercoating composition of the present invention, the undercoating composition is first coated on the surface of a metal material for formation of a can body, and the resulting coating is then baked. As the metal material for formation of a can body, there can be used various metal materials for cans, such as steel plates (black plates), plated steel plates formed by plating the surface of a steel plate with zinc, tin, chromium or aluminum, and treated steel plates formed by subjecting the surface of a steel plate to a chemical treatment with chromic acid or phosphoric acid or to an anodic electrolysis treatment. Further, a composite metal material, formed by bonding and laminating a foil of a metal such as aluminum onto an organic substrate such as a film of a resin such as polyolefin or a paper board, can be used as the metal material. A specific kind of the metal material to be used is appropriately selected depending on the intended use of the resulting bonded can. In general, as the metal material excellent in the corrosion resistance and the adhesion to the coating, there is preferably employed a chromic acid-treated, anodically electrolyzed, steel plate comprising a steel plate substrate, a metallic chromium layer formed on the substrate and a hydrous chromium oxide layer formed on the metallic chromium layer, in which the metallic chromium layer is present in an amount of 0.1 to 2 mg/m2 and the hydrous chromium oxide layer is present in an amount of 0.05 to 5 mg/m2 as calculated as metallic chromium. The thickness of the metal material to be used for production of the bonded can is changed depending on the volume of the can body, the kind of the content and other factors, but it is generally preferred that the thickness be in the range of from 0.1 to 0.3 mm.
The above-mentioned metal material is degreased by trichloroethylene or other degreasing solvent according to need, and the undercoating composition of the present invention is applied in the form of a solution to the surface of the metal material according to known coating methods such as brush coating, spray coating, dip coating, roll coating, electrostatic coating and electrophoretic coating methods. The thickness of the undercoating layer is not particularly critical so far as the metal surface is uniformly coated with the undercoating composition, but in general, if the thickness of the undercoating layer is 1 to 15 microns, good results are obtained.
Then, the undercoating compositionapplied metal material is formed into a cylindrical shape by optional means, and both the confronting side edges are seambonded by using a known adhesive, for example, a polyamide adhesive to form a lap seam, lock seam or lap-and-lock seam. As the polyamide adhesive, there are employed linear homopolyamides, copolyamides and modified polyamides having a relative viscosity (ire) of at least 1.5, especially at least 1.8, as measured with respect a 1% solution in 98% concentrated sulfuric acid.
Polymer blends of two or more of these polyamides can also be used. As specific examples of such polyamide, there can be mentioned homopolyamides such as polyhexamethylene adipamide, polyhexamethylene sebacamide, polyhexamethylene dodecamide, polvdodecamethylene dodecamide, poly 6- aminocaproic acid, poly – 11 aminoundecanoic acid and poly – 12aminolauric acid, copolyamides consisting at least two members selected from constituent monomers of the foregoing homopolyamides, namely combinations of dicarboxylic acids and diamine salts or at least two els-aminocarboxylic acids, and the foregoing homopolyamides and copolyamides modified with polymeric fatty acids or the like From the viewpoint of the strength of the bonded portion, it is preferred that the polyamide adhesive to be used in the present invention be crystalline.
Various means may be adopted for interposing the polyamide adhesive between both the side edges of the coated metal material to be bonded. For example, a preformed tape of a polyamide adhesive is applied to both the side edges to be bonded or a molten polyamide in a tape-like form is melt-extruded and applied to the side edges to be bonded. Alternately, the polyamide adhesive can be applied in the form of a powder or solution to the side edges of the metal material to be bonded. The thickness of a layer of the polyamide adhesive applied to the side edges to be bonded is not particularly critical so far as the polyamide adhesive has uniform and tight contact with the undercoating layer formed on the metal material, but in general, it is preferred that the thickness of the polyamide adhesive layer be in the range of from 0.01 to 0.2 mm.
The polyamide adhesive may be applied to one or both of the side edges of the coated metal material prior to the bonding operation or it may be interposed between both the side edges at the bonding operation.
Bonding of both the side edges of the metal material is accomplished by melting the polyamide adhesive located between both the confronting side edges of the metal material formed in a cylindrical shape, pressing both the side edges of the coated metal material under cooling and thereby solidifying the polyamide adhesive. A can body formed by such side-seam bonding is then subjected to known can-manufacturing processings such as notching processing, flanging processing and lid seaming processing, and a final can body is thus obtained.
When the undercoating composition of the present invention is used for manufacture of bonded cans, as will be apparent from the results of Examples given hereinafter, the side edges of the metal material are tightly bonded to each other by the adhesive through this undercoating layer and a bonded portion having a very high peel strength is formed, and this bonded portion can resist sufficiently such a severe hot water or steam sterilization treatment as carried out at 1250C. for 60 minutes. Further, even if the bonded can subjected to such severe sterilization treatment is then preserved for a long time, reduction of the peel strength with the lapse of time can be controlled to an extremely low level.
In retort-sterilized canned foods or drinks formed by using a bonded can, the bonding state is greatly influenced by the preservation temperature. For example, when such retorted canned foods or drinks are preserved at relatively low temperatures, e.g., 200 C. or lower, degradation of the bonding with lapse of time is not so conspicuous, but when the preservation temperature is relatively high, e.g., 35″C. or higher, degradation of the bonding with the lapse of time is conspicuous. According to the present invention, however, by forming an undercoating layer of a specific composition on the metal material, even if the preservation temperature is relatively high, degradation of the bonding with the lapse of time can be remarkably controlled.
The bonded can prepared by using the undercoating composition of the present invention is therefore very valuable as a can to be subjected to a retort sterilization treatment conducted at high temperatures, for example, a sterilization treatment cbnducted at a temperature of 122 to 1350C. for 10 to 150 minutes under a pressure of 1.1 to 2.3 Kg/cm2 (pressure gauge). Namely, the bonded can according to the present invention is very valuable as a can for preservation of various drinks such as fruit juices, fruit juice-mixed drinks and coffee, fruits, vegetables, marine products, meat products and processed foods thereof. Of course, the foregoing advantages can be similarly attained when the bonded can according to the present invention is used as a can subjected to a heat sterilization treatment conducted at a temperature lower than the temperature adopted in the abovementioned high-temperature sterilization, for example, a sterilization treatment conducted at 110 to 1210C., or a can in which the content is filled in a hot state (for example, at the boiling temperature).
The present invention will now be described in detail by reference to the following Examples that by no means limit the scope of the invention.
Example I
This Example illustrates influences of the kinds and amounts of the dihydric phenol and monohydric phenol in a mixed phenol that is used for obtaining a resol-type phenol-aldehyde resin, on properties of the resulting bonded cans.
The metal plate used as the metal material was a chromic acid treated steel plate having a thickness of 0.24 mm (“Hi
Top” manufactured and sold by Toyo
Kohan).
The adhesive used was a linear polyamide derived from 12-aminolauric acid and having a relative viscosity of 2.7 as measured with respect to a 1% solution in 98% concentrated sulfuric acid.
The undercoating composition was prepared in the following manner.
A 37% aqueous solution of formaldehyde was added to a mixed phenol indicated in
Table 1. The amount of formaldehyde was 2.0 moles per mole of the mixed phenol.
Then, the temperature was elevated to 65 CC. to dissolve the mixed phenol in the aqueous solution. Then, a basic catalyst (magnesium hydroxide) was added to the solution, and reaction was carried out at 95″C. for several hours.
The reaction product was extracted with a mixed solvent comprising a ketone, an alcohol and a hydrocarbon and washed with water and water was removed by azeotropic distillation or precipitation. The so obtained phenol-aldehyde resin was found to have a number average molecular weight of 250 to 500.
The so obtained solution of the resol-type phenol-aldehyde resin was mixed with a solution separately prepared by dissolving an epoxy resin having a number average molecular weight of 3750 (“Epikote 1009” manufactured and sold by Shell Chemicals, “Epikote” is a Registered Trade Mark) in a mixed solvent comprising a ketone, an ester, an alcohol and a hydrocarbon. The epoxy resin/phenol-aldehyde resin mixing weight ratio was 75/25. The resulting mixture was heated under reflux (1 l00C.) to effect precondensation. Thus, the undercoating composition was prepared.
The so prepared undercoating composition was coated on both the surfaces of the above-mentioned steel plate (Hi-Top) and cured at 150 to 2300C. for 1 to 20 minutes to obtain a cured coating layer having a thickness of 1 to 15y.
The so coated metal plate was cut in a can blank for a can body having a diameter of 3 1/12 in. and an inner volume of 454.4 ml, and the can blank was rounded by three rolls, Then, a tape of the above-mentioned polyamide adhesive having a thickness of about 100N and a width of about 8 mm was inserted between lapped side edges to be bonded. The side edges of the can blank to be bonded were heated at 230″C., compressed for about 50 milliseconds and then cooled to obtain a metal can body. The lap width of the bonded portion was 5 mm.
A lid was double-seamed to one opening of the flanged can body, and warm water maintained at 950C. was filled and another lid was double-seamed to the other opening.
Then, the filled can was heat-treated at 125″C. for 60 minutes. This heat treatment test was conducted with 20 can bodies for each undercoating composition. The presence or absence of broken can bodies was examined, and the peel strength of the bonded portion was measured at 25″C. before and after the heat treatment.
Further, the peel strength of the bonded portion after storage of 6 months, I year or 2 years at 370C. was measured at 250C.
Obtained results are shown in Table 1.
TABLE 1
Mixed Phenol
Dihydric phenol Monohydric phenol
Run Amount Amount
No. Kind (wit.%) Kind (wit.%) 1 2,2-bis(4-hydroxyphenyl)propane 90 p-cresol 10
2 ditto 80 o-cresol 20
3 ditto 70 p-cresol 15
o-cresol 15
4 ditto 75 p-tert-butylphenol 25
5 ditto 80 p-ethylphenol 20
6 ditto 80 phenol 20
7 ditto 80 m-cresol 20
8 2,2-bis(4-hydroxyphenyl)butane 85 p-cresol 15
9 ditto 80 2,3-xylenol 20
10 1.1-bis(4-hydroxyphenyl)ethane 95 o-cresol 5
11 ditto 70 p-phenylphenol 30
12 bis(4-hydroxyphenyl)methane 80 p-tert-amylphenol 20
13 4-hydroxyphenylether 75 2,5-xylenol 25
14 ditto 70 p-cresol 30
15 p-(4-hydroxyphenyl)phenol 80 p-cresol 20
16 resorcinol 70 p-cresol 30
17 2,2-bis(4-hydroxyphenyl)propane 40 o-cresol 60
18 ditto 30 o-cresol 35
ditto phenol 35
19 ditto 20 phenol 80
TABLE I (Continued) Peel Strength (Kg/cm) of bonded portion Can Number of
Can Bodies Aerheat Broken at
Before treatment After 6 After 1 After 2 heat treat
Run heat at 1250C. months’ year’s years’ ment at 1250C.
No. treatment for 60 min. storage storage storage for 60 minutes Remarks
1 23 22 20 16 10 0/20 present
invention
2 24 22 20 17 11 0/20 ditto
3 25 24 21 17 11 0/20 ditto
4 25 23 20 17 10 0/20 ditto
5 23 22 20 16 11 0/20 ditto
6 25 24 22 18 13 0/20 ditto
7 25 23 21 17 10 0/20 ditto
8 24 22 20 16 11 0/20 ditto
9 24 22 20 16 11 0/20 ditto
10 24 22 21 17 12 0/20 ditto
11 25 22 19 16 10 0/20 ditto
12 24 22 20 17 12 0/20 ditto
13 23 21 18 15 10 0/20 ditto
14 23 21 19 16 11 0/20 ditto
15 24 20 18 15 10 0/20 ditto
16 20 10 6 3 0 13/20 comparison
17 24 0 0 0 0 20/20 ditto
18 23 0 0 0 0 20/20 ditto
19 23 13 8 4 2 10/20 ditto
Example 2
This Example illustrates influences of the mixing ratio of the dihydric phenol and monohydric phenol in a mixed phenol that is used for the preparation of a resol-type phenolaldehyde resin, on properties of the resulting bonded can.
As the dihydric phenol, 2,2-bis(4hydroxyphenyl)-propane (bisphenol A) was used and p-cresol was used as the monohydric phenol, and they were mixed at a weight ratio shown in Table 2.
A resol-type phenol-aldehyde resin having a number average molecular weight of 200 to 450 was prepared from the above mixed phenol in the same manner as described in Example 1. An epoxy resin having a number average molecular weight of 2650 (“Epikote 1007” manufactured and sold by Shell Chemicals) was mixed with the so prepared phenol-aldehyde resin. The epoxy resin/phenol-aldehyde resin mixing weight ratio was 75/25.
By using the so prepared undercoating composition, a bonded can was prepared in the same manner as described in Example 1.
The metal plate and polyamide adhesive used were the same as those used in
Example 1. The resulting bonded can was tested in the same manner as described in
Example 1 to obtain results shown in Table 2.
TABLE 2
Mixed phenol
Dihydric phenol Monohydric Phenol
Run Amount Amount
No. Kind (wt.%) Kind (wt.%)
1 2,2-bis(4-hydroxy- 100 p-cresol 0
phenyl)propane
2 ditto 98 ditto 2
3 ditto 95 ditto 5
4 ditto 90 ditto 10
5 ditto 80 ditto 20
6 ditto 75 ditto 35
7 ditto 60 ditto 40
8 ditto 50 ditto 50
9 ditto 30 ditto 70
10 ditto 10 ditto 90
TABLE 2 (Continued)
Peel Strength (Kg/cm) of Bonded Portion
After heat Number of can
treatment bodies broken
Before at 1250C. After 6 After 1 After 2 by heat treat
Run heat for 60 months’ year’s years’ ment at 1250C.
No. treatment minutes storage storage storage for 60 minutes Remarks
1 18 13 10 7 3 5/20 comparison
2 20 17 15 12 9 1/20 present
invention
3 22 19 17 15 11 0/20 ditto
4 23 22 20 16 12 0/20 ditto
5 24 22 19 15 10 0/20 ditto
6 25 21 17 12 8 0/20 ditto
7 26 17 12 8 3 5/20 comparison
8 26 15 10 5 2 15/20 ditto
9 25 0 0 0 0 20/20 ditto
10 25 0 0 0 0 20/20 ditto
Example 3
This Example illustrates influences on the mixing ratio of the resol-type phenolaldehyde resin and epoxy resin in an undercoating composition on properties of the resulting bonded can.
The resol-type phenol aldehyde resin was prepared by reacting a mixed phenol comprising 80% by weight of 2,2-bis(4hydroxyphenyl)butane (bisphenol B) and 20% by weight of o-cresol with a 37% aqueous solution of formaldehyde in the presence of a basic catalyst. The amount of formaldehyde was 2.5 moles per mole of the mixed phenol. The number average molecular weight of the resulting resol-type phenol-aldehyde resin was 350.
The so prepared phenol-aldehyde resin was mixed with the same epoxy resin as used in Example 1 at a mixing ratio shown in
Table 3. By using the so obtained undercoating composition, a bonded can was prepared in the same manner as described in Example 1. The metal plate and polyamide adhesive used were the same as those used in Example I. The resulting bonded can was tested in the same manner as described in Example 1 to obtain results shown in Table 3.
TABLE 3
Peel strength (Kg/cm) of bonded portion
Number of can bodies
Expoxy resin/ broken at
Phenol-aldehyde After heat treatresin mixing Before treatment After 6 After 1 After 2 ment at
Run ratio heat at 125 C. months year’s years’ 125 C. for
No. (weight) treatment for 60 min. storage storage storage 60 minutes Remarks 1 98/2 7 0 0 0 0 20/20 comparison 2 95/5 10 8 7 6 6 3/20 present invention 3 90/10 17 14 13 11 10 0/20 ditto 4 80/20 25 23 21 17 13 0/20 ditto 5 70/30 26 24 22 18 14 0/20 ditto 6 50/50 23 18 15 11 8 1/20 ditto 7 30/70 15 0 0 0 0 20/20 comparision 8 10/90 10 0 0 0 0 20/20 ditto Example 4
This Example illustrates influences of the number average molecular weight of the epoxy resin in an undercoating composition on properties of the bonded can.
The same phenol-aldehyde resin as used in Example 3 was mixed with an epoxy resin having a number average molecular weight indicated in Table 4 to obtain an undercoating composition. By using the so prepared undercoating composition, a bonded can was prepared in the same manner as described in Example 1. The metal plate and polyamide adhesive used were the same as those used in Example 1.
The bonded can was tested in the same manner as described in Example 1 to obtain results shown in Table 4.
TABLE 4
Peel strength (Kg/cm) of bonded portion
Number of can
Number average After heat bodies broken at molecular Before treatment After 6 After 1 After 2 heat treatment
Run weight of heat at 125 C. months’ year’s years’ at 125 C. for
No. epoxy resin treatment for 60 min. storage storage storage 60 min. Remarks 1 500 5 0 0 0 0 20/20 comparison 2 800 15 10 8 7 7 1/20 present invention 3 1,400 20 15 12 10 8 0/20 ditto 4 2,650 24 23 21 18 13 0/20 ditto 5 3,750 25 23 20 17 12 0/20 ditto 6 4,500 23 21 18 15 12 0/20 ditto 7 5,500 20 17 15 13 10 0/20 ditto 8 10,000 12 8 3 1 0 7/20 comparison 9 30,000 10 3 0 0 0 20/20 ditto Example 5
This Example illustrates influences of the number average molecular weight of the resol-type phenol-aldehyde resin on an undercoating composition in properties of the bonded cans.
A 37% aqueous solution of formaldehyde was added to a mixed phenol comprising 75% by weight of 2,2-bis(4hydroxyphenyl)propane (bisphenol A) and 25% by weight of p-cresol. The amount of formaldehyde was 2.0 moles per mole of the mixed phenol. The temperature of the mixture was elevated to 650C. to dissolve the mixed phenol in the aqueous solution.
Then, 25% aqueous ammonia was added to the solution. The amount of ammonia was 0.1 mole per mole of the mixed phenol.
Reaction was carried out at 950 C. for 10 minutes to 8 hours to obtain a resol-type phenol-aldehyde resin having a number average molecular weight indicated in Table 5. The so obtained phenol-aldehyde resin was mixed with an epoxy resin having a number average molecular weight of 3750 (“Epikote 1009″ manufactured and sold by
Shell Chemicals) to form an undercoating composition. The epoxy resin/phenolaldehyde resin mixing weight ratio was 80/20. By using the so prepared undercoating composition, a bonded can was prepared in the same manner as described in Example 1. The metal plate and polyamide adhesive used were the same as those as used in Example 1. The bonded can was tested in the same manner as described in Example 1 to obtain results shown in
Table 5.
TABLE 5
Peel strength (Kg/cm) of bonded portion
Number average
Number of can molecular weight of After heat bodies broken at resol-type Before treatment After 6 After 1 After 2 heat treatment
Run phenol-aldehyde heat at 125 C. months’ year’s years’ at 125 C. for
No. resin treatment for 60 min. storage storage storage 60 minutes Remarks 1 190 8 0 0 0 0 20/20 comparison 2 200 23 22 20 18 13 0/20 present invention 3 255 26 24 22 20 15 0/20 ditto 4 380 26 24 22 20 14 0/20 ditto 5 540 23 21 19 17 12 0/20 ditto 6 660 20 17 15 13 10 0/20 ditto 7 850 18 15 13 11 8 0/20 ditto 8 1,040 15 10 8 6 3 2/20 comparison 9 1,400 10 0 0 0 0 20/20 ditto Example 6
A 37 n aqueous solution of formaldehyde was added to a mixed phenol comprising 85% by weight of 2,2-bis(4hydroxyphenyl)propane (bisphenol A) and 150-n, by weight of o-cresol. The amount of formaldehyde was 2.0 moles per mole of the mixed phenol. The temperature of the mixture was elevated to 650C. to dissolve the mixed phenol in the aqueous solution.
Then, 25 ‘n aqueous ammonia was added to the solution and reaction was carried out at 95 C. for 3 hours. The amount of ammonia was 0.1 mole per mole of the mixed phenol.
The resulting condensation product was extracted with a mixed solvent comprising 30 parts of methylisobutyl ketone, 20 parts -of cyclohexanone and 50 parts of xylene and washed with water. Water was removed by precipitation.
In the so prepared resol-type phenolaldehyde resin solution, the solid content was 30 2” by weight, and the resin was found to have a number average molecular weight of 375.
Separately, an epoxy resin having a number average molecular weight of 3750 (“Epikote 1009” manufactured and sold by
Shell Chemicals) was dissolved in the same mixed solvent as described above.
Both the solutions were mixed to form a solution having a solid content of 33%, in which the epoxy resin/phenol-aldehyde resin weight ratio was 75/25 The solution was heated at 1100 C. for 3 hours to effect precondensation and obtain an undercoating composition in the form of a solution.
A can blank for a can body having a diameter of 2 11/12 in. and an inner volume of 318.2 ml was formed by cutting a chromic acid treated steel plate having a thickness of 0.23 mm (“Hi-Top” manufactured and sold by Toyo Kohan), and the above undercoating composition was coated on both the surfaces of the can blank in a thickness of about 5, and was cured at 205 C. for 10 minutes.
A film of poly- 1 2-aminolauric acid having a relative viscosity of 2.3, which had a thickness of 50,u and a width of 8 mm, was located on both the side edges of the coated can blank to be bonded, and the film was compressed at 2300 C. for 2 seconds and then cooled.
The resulting can blank having a polyamide coating on both the side edges to be bonded was rounded by three rolls so that the polyamide-coated end faces confronted each other. Then, the polyamide-coated faces were lapped,
heated for 50 milliseconds by high frequency heating, compressed and cooled
to obtain a can body.
T peel pcsl ;tren.th of the lap sear; of the bonded can was 24 Kg/cm. A lid was double-seamed to one opening of the can body, and warm water maintained at 95 C. was filled in the can body and a lid was double-seamed to the other opening.
When the filled can was subjected to a heat treatment at 1300C. for 60 minutes, the number of broken can bodies was zero (1000 can bodies were tested). The peel strength of the bonded portion after the heat treatment was 22 Kg/cm as measured at 20 C. The peel strengths of the bonded portion after 6 months’ storage, 1 year’s storage and 2 years’ storage at 370 C. were 20, 18 and 13 Kg/cm, respectively, as measured at 200 C. In each case, breakage of the bonded portion was not observed, and a good vacuum state was maintained in the can and the inner pressure was lower by at least 40 cmHg than the atmospheric pressure.
Example 7
An undercoating composition was prepared in the same manner as described in Example 6 except the following changes.
A mixture comprising 90% by weight of bis(4-hydroxyphenyl)methane (bisphenol F) and 10% by weight of p-tert-butylphenol was used as the mixed phenol composition and sodium carbonate was used in an amount of 0.01 mole per mole of the mixed phenol as the catalyst. The number average molecular weight of the resulting phenolaldehyde resin was 320. An epoxy resin having a number average molecular weight of 2650 (“Epikote 1007” manufactured and sold by Shell Chemicals) was used as the epoxy resin. The epoxy resin/phenolaldehyde resin mixing weight ratio was 70/30. Precondensation was carried out at 110 C, for 5 hours.
The undercoating composition was coated on both the degreased surfaces of a cold-rolled steel plate and cured at 210 C. for 10 minutes. By using a film of poly-6aminocaproic acid having a relative viscosity of 2.25 as a linear polyamide adhesive, a can body was prepared in the same manner as described in Example 6.
The peel strength of the bonded portion was 26 Kg/cm as measured at 370C. A lid was double-seamed on one opening of the can body, and warm water maintained at 95″C. was filled and a lid was double-seamed to the other opening. The filled can was subjected to a heat treatment at 1250C. for 90 minutes. Breakage of the can body was not caused at all by this heat treatment. The peel strength of the bonded portion after the heat treatment was 22 Kg/cm as measured at 20 C. The peel strengths of the bonded portion after 6 months’ storage, 1 year’s storage and 2 years’ storage at 370C were 19 Ki:/cm 16 KgJcm and 1.3 Kg/cr.,. respectively, as measured at 20″C. In each case breakage of the bonded portion was not observed, and a good vacuum state was maintained in the can and the inner pressure was lower by at least 40 cmHg than the atmospheric pressure.
Example 8
Lids were double-seamed to one openings of bonded cans prepared by using the undercoating compositions obtained in
Example 1, respectively, and warm water maintained at 950C. was filled in the cans and lids were double-seamed to the other openings. The resulting filled can bodies were then subjected to a heat treatment at 115″C. for 60 minutes. The number of can bodies broken by the heat treatment was checked. After the heat treatment, the filled can bodies were allowed to stand still at room temperature for 1 week or stored for I year and the inner vacuum [atmospheric pressure (76 cm Hg)-inner pressure (cm
Hg absolute)] of cans was determined to examine the leakage shortly after the heat treatment and the delay leakage by longtime storage. Obtained results are shown in Table 8.
In Table 8, the run numbers correspond to those in Table 1.
TABLE 8
Number of can
bodies broken Inner vacuum Inner vacuum
at heat of cans after of cans after
treatment at 1 week’s 1 year’s 1150C. for 60 storage storage
Run minutes (cmHg) (cmHg)
No. (n=100) (n=20) (n=20) Remarks
1 0 45 42 Present invention
2 0 44 41. ditto
4 0 46 42 ditto
6 0 47 43 ditto
7 0 48 44 ditto
8 0 45 42 ditto
10 0 46 43 ditto
12 0 45 42 ditto
13 0 44 42 ditto
16 0 40 0 Comparison
17 0 40 5 ditto
18 0 38 0 ditto
19 0 35 0 ditto
Example 9
Lids were double-seamed to one opening of the bonded cans prepared by using the undercoating compositions obtained in
Example 2, and warm water maintained at 95″C. was filled in the cans. The, lids were double-seamed to the other opening, and the filled can bodies were heat-treated at 115″C. for 60 minutes. The number of the can bodies broken by the heat treatment was checked. After the heat treatment, the filled can bodies were stored for 1 week or 1 year, and the inner vacuum (cm Hg) was determined to examine the leakage shortly after the heat treatment and the delay leakage by longtime storage. Obtained results are shown in Table 9.
Run numbers in Table 9 correspond to those in Table 2.
TABLE 9
Number of can Inner vacuum Inner vacuum
bodies broken of cans after of cans after
at heat treat- 1 week’s 1 year’s ment at 1 l50C. storage storage
Run for 60 minutes (cmHg) (cmHg)
No. (n=100) (n=20) (n=20) Remarks
1 0 40- 0 Comparison
2 0 45 40 present invention
3 0 47 43 ditto
4 0 46 44 ditto
5 0 48 44 ditto
6 0 45 41 ditto
7 0 42 20 Comparison
8 0 40 10 ditto
9 0 38 5 ditto
10 0 38 3 ditto
Leakages caused when bonded can bodies are subjected to retort sterilization are classified into the following three kinds.
(1) Can Body Breakage:
When the bonding strength is drastically reduced in the side seam portion by retort sterilization, the can body is broken from the side seam during retort sterilization and leakage of the content is caused.
(2) Micro-Leakage:
When the bonding strength is reduced at some points on the bonding interface of the side seam portion, though the abovementioned can body breakage is not caused, micro-leakage is caused from such points after retort sterilization.
(3) Delay Leakage:
Even if leakage is not caused shortly after retort sterilization, in the area close to the double-seamed portion of the side seam, the bonding between the metal material and the undercoating composition becomes insufficient with the lapse of time owing to processing and retort sterilization. In this case, leakage gradually is caused during long-time storage.
In case of bonded cans prepared by using an undercoating composition comprising a phenol-formaldehyde resin formed by using a mixed phenol in which the amount of the dicyclic phenol is outside the range specified in the present invention and an epoxy resin, heat sterilization is possible (breakage of can bodies is not caused) when the temperature is relatively low, e.g., 11 50 C., but as is seen from the results shown in Tables 8 and 9, micro-leakage and delay leakage are considerable in case of such can bodies. Further, in case of bonded can bodies prepared by the above-mentioned undercoating composition, breakage of can bodies become conspicuous when the sterilization temperature is high, e.g., 125″C.
In contrast, in case of bonded cans prepared by using an undercoating
composition comprising phenol
formaldehyde resin formed by using a mixed
phenol in which the amount of the dicyclic
phenol is in the range specified in the
present invention and an epoxy resin, even
such micro-leakage or delay leakage is not
caused at all, and they can sufficiently resist the heat sterilization conducted at such a
high temperature as 1250C.

Claims (12)

WHAT WE CLAIM IS:

1. An undercoating composition for use in the production of bonded cans having a high hot water resistance, which composition comprises 50 to 95% by weight of an epoxy resin having a number average molecular weight of 800 to 5500, which is obtained by condensation of an epihalohydrin with bisphenol A, and 50 to 5% by weight of a resol-type phenolaldehyde resin having a number average molecular weight of 200 to 1000 and which has been obtained by reacting a mixed phenol comprising 65 to 98% by weight of a dihydric phenol of the general formula:

wherein R represents a divalent bridging atom or group or is a direct bond, and 35 to 2% by weight of a monohydric phenol with an aldehyde in the presence of a basic catalyst.

2. A composition according to claim 1 wherein said dihydric phenol is a dihydric phenol represented by the general formula:

wherein R is a direct bond or represents –OO- or CR1R2- in which R1 and R2 represent a hydrogen or halogen atom or an alkyl or perhaloalkyl group having up to 4 carbon atoms.

3. A composition according to claim 1 wherein said dihydric phenol is 2,2 – bis(4 hydroxyphenyl)propane or 2,2 – bis(4 – hydroxyphenyl)butane.

4. A composition according to claim 1,2 or 3 wherein said monohydric phenol is a bifunctional or trifunctional monohydric phenol.

5. A composition according to claim 4 wherein said monohydric phenol is a bifunctional phenol represented by the general formula:

6. A composition according to any one of the preceding claims wherein in the resoltype phenolaldehyde, said dihydric phenol (a) and said monohydric phenol (b) are present in an (a)/(b) weight ratio of from 95/5 to 75/25.

7. A composition according to any one of the preceding claims wherein the resol-type phenolaldehyde resin is a resin having a number average molecular weight of 250 to 800 and has been obtained by condensing 1 mole of said mixed phenol with 1.5 to 3 moles of formaldehyde.

8. A composition according to any one of the preceding claims wherein said epoxy resin has a number average molecular weight of 1400 to 5500.

9. A composition according to any one of the preceding claims wherein in said epoxyphenolic undercoating composition, the epoxy resin/resol-type phenol-aldehyde resin weight ratio is from 90/10 to 60/40.

10. A composition according to claim 1 substantially as described in any one of the examples.

11. A bonded can having a high hot water resistance, which consists of a metal material having confronting side edges bonded together by a linear polyamide adhesive through an epoxy-phenolic resin undercoating composition, wherein said epoxy-phenolic resin undercoating composition is as claimed in any one of the preceding claims.

12. A bonded can according to claim 11 substantially as described herein with reference to, and as illustrated by, the accompanying drawings.

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