JPH0770341A - Polyester film for three-piece-can coating - Google Patents

Abstract

PURPOSE:To obtain a hygienically excellent three-piece can which is reduced in the amount of substances extractable with water or hot water. CONSTITUTION:The film, when used as a can coating formed by laminating to a metal with an adhesive, has a terminal carboxyl content lower than 50eq/t and a content of low-mol.wt. substances extracted by 1-hr retortion with 125 deg.C pure water of 2.0mug or lower per cm<2> of the coating.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to polyester films for coating three-piece cans.

[0002]

2. Description of the Related Art In recent years, the number of cases in which a thermoplastic resin film is coated on a metal plate and used as a material for a metal can is increasing. It is difficult to completely eliminate metal exposure due to pinholes, etc., even with a large amount of paint applied or double coating with a coated plate in which a thermosetting resin paint is applied to a conventional metal plate and baked and cured. On the other hand, in the case of a coated metal plate coated with a thermoplastic resin, metal exposure can be almost completely eliminated. Also, in the case of coated plates, a large amount of solvent is used for coating and dry baking,
This is because the printing layer can be provided by laminating a coating film that has been printed in advance. As the thermoplastic resin used for these coatings, polyester having excellent heat resistance, processability, gas barrier properties, etc. is generally used.

Polyethylene terephthalate, which is widely used as a raw material for plastic containers and stretched films, has excellent heat resistance, processability, gas barrier properties and the like as such a thermoplastic resin for metal coating, and is used like so-called PET bottles. It is also used as a structure for beverage containers. However, when it is used by coating it on a metal can, P
Unlike the case of an ET bottle or the like, it may be placed under high temperature under pressure for the purpose of sterilization or the like while being in contact with a liquid content that is substantially water. In this case, a so-called retort-treated form may be formed, and low molecular weight substances such as oligomers and aldehydes may be extracted into the liquid content and come out, and the flavor of the filled content may be impaired. Yes, it was not good for hygiene.

[0004]

The present invention has been achieved as a result of a study for solving such a problem. That is, the gist of the present invention is to laminate and coat metal with an adhesive.
The terminal carboxyl group concentration after can making is 50 equivalent / t.
Less and less than 2.0 μg per 1 cm ^{2 of} coated surface, which is extracted by a retort treatment with pure water at 125 ° C. for 1 hour after laminating and canning a metal with an adhesive. And a polyester film for covering a three-piece can.

The present invention will be described in detail below. The three-piece can referred to in the present invention is, for example, a can body in which a metal exposed portion welded and joined is fusion-coated with a coating film by a method described in JP-A-5-69485 without using an adhesive or the like. The can lid and the can bottom are configured such that the side of the film-coated steel sheet that comes into contact with the contents is the coating layer. Examples of the metal plate used for such an application include a surface-treated steel plate and a light metal plate such as aluminum. In the case of the surface-treated steel sheet, examples thereof include coating with tin that has been passivated by a general chrome treatment, or nickel- or zinc-plated steel. In particular, electrolytic chromium-coated steel having two layers of chromium metal and chromium oxide is preferable. In the case of aluminum, pure aluminum may be used,
It may be an alloy containing a small amount of manganese, magnesium, zinc, copper or the like. Further, both pure aluminum and aluminum alloy may be treated with chromic acid or chromic acid / phosphoric acid.

The polyester referred to in the present invention means an aromatic dicarboxylic acid or its ester-forming derivative as a difunctional acid component, specifically, terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, sebacic acid,
Adipic acid or the like, or one or more kinds of ester-forming derivatives thereof, and ethylene glycol, diethylene glycol, triethylene glycol, 1,3-as a glycol component.
Propanediol, 1,4-butanediol, 1,4-
Preferable are polyesters composed of one kind or two or more kinds such as cyclohexanedimethanol. Examples of typical polyesters include polyethylene terephthalate and its copolymers, polyethylene 2,6-naphthalene dicarboxylate and its copolymers, polycyclohexane dimethylene terephthalate and its copolymers, and the like.

The polyester film of the present invention is usually made of the above polyester and is biaxially stretched or heat-set, and has a degree of plane orientation (ΔP) of at least 0.10.
0 to 0.200, more preferably 0.110 to 0.190, and particularly preferably 0.130 to 0.180, the crystallinity χc
Is preferably at least 30 to 70%, more preferably 35 to 60%, and particularly preferably 40 to 60%. When ΔP is less than 0.100, sufficient gas barrier properties tend not to be obtained. Similarly, when χc is less than 30%, sufficient gas barrier properties tend not to be obtained. ΔP is 0.200
When the value exceeds 50% or when χc exceeds 70%, the film becomes too rigid and is laminated on a metal plate, then the film peels from the metal plate or is cracked when forming and forming a can. However, the dent resistance of the metal can may decrease after it is formed.

The polyester film of the present invention is usually formed with a large number of surface protrusions whose core is a filler contained in the film and has a dynamic friction coefficient μd of 0.02.
~ 0.50, more preferably 0.05-0.45, especially 0.07
~ 0.40 is preferred. When μd is less than 0.02, surface irregularities tend to increase, and the appearance may be impaired. When μd exceeds 0.50, blocking tends to occur when the film-covered portions come into contact with each other in the can-making process.

The greatest feature of the polyester film of the present invention is that the concentration of terminal carboxyl groups is low when a laminated coated metal can is used, and the amount of low molecular weight substances extracted by immersion in water or an aqueous ethanol solution and heating is extremely small. Is. That is, the terminal carboxyl group concentration after the metal is laminated and coated with an adhesive to form a can is less than 50 equivalents / t, preferably less than 40 equivalents / t, further preferably 3
It is less than 0 equivalent / t. Terminal carboxyl group concentration is 50
If it exceeds the equivalent weight / t, the number of terminal carboxyl groups, which are the starting points of thermal degradation decomposition, becomes too large, so that a new low molecular weight substance is newly formed when heated after the coating can is not preferable. In addition, a low molecular weight substance extracted by applying a retort treatment with pure water at 125 ° C for 1 hour after being laminated and metal-coated on a metal with an adhesive is a polyester-coated metal 1 cm ²
The amount is 2.0 μg or less, preferably 1.5 μg or less, and more preferably 1.0 μg or less. The low molecular weight substance extracted by such retort treatment is polyester-coated metal 1
When it exceeds 2.0 μg per cm ² , the flavor of the filled contents is impaired when heated for the purpose of sterilization and the like, which is not preferable in terms of hygiene.

As a method for reducing the amount of low molecular weight substances extracted by the retort treatment, 1) a polyester film is formed from polyester in which the internal amount of low molecular weight substances is reduced by solid-state polymerization during polycondensation, 2) polycondensation The polyester resin obtained in this way is preliminarily soaked in warm water to form a polyester film from a polyester having a reduced amount of low molecular weight substances. 3) The polyester film obtained by a conventional method is soaked in warm water to reduce Examples thereof include a method of reducing the intrinsic amount of the molecular weight substance, and these methods may be used alone or in combination to obtain the polyester film of the present invention. Further, each method will be described by showing an actual example. A method for reducing low molecular weight substances by solid phase polymerization is
In the usual polyester production, where polymerization is carried out until the intrinsic viscosity [η] becomes 0.60 or more,
The polymerization is stopped at 0.40 to 0.50, and the obtained resin is heated at 240 ° C. for 10 hours under a high vacuum of 0.3 mmHg.

The method of preliminarily dipping the polyester in hot water to reduce the amount of the low molecular weight substances present is carried out by bringing the polyester into contact with water, steam, a steam-containing inert gas, steam-containing air or the like. The polyester is immersed in water at room temperature to 150 ° C. for 1 minute to 100 hours or more, preferably 1 minute to 20 hours, more preferably 5 minutes.
It is performed by immersing for 10 minutes to 10 minutes. Desirably, the polyester is heated in hot water at 50 to 150 ° C. for 1 minute to 1
It is performed by soaking for 0 hour. More desirably, the polyester is immersed in hot water at 70 to 110 ° C. for 3 minutes to 5 hours. Particularly preferably, it is carried out by immersing the polyester in hot water at 80 to 100 ° C. for 5 minutes to 3 hours.

The contact between the polyester and the steam or the steam-containing gas or the steam-containing air is usually room temperature to 150 ° C., preferably 50 to 150 ° C., more preferably 70 to 110 ° C. Alternatively, the steam-containing air is preferably supplied in an amount of 0.5 g or more per 1 kg of polyester, or is allowed to exist and the polyester and steam are brought into contact with each other. The contact between the polyester and steam is usually performed for 1 minute to 50 hours, preferably 5 minutes to 10 hours. The method of subjecting the polyester film to the hot water immersion treatment to reduce the internal content of the low molecular weight substance is similar to the warm water immersion treatment applied to the polyester resin. The polyester film of the present invention is produced according to a conventional method,
An example is shown below.

First, a raw material polyester is melt-extruded to obtain a substantially amorphous sheet. At this time, in order to suppress the increase in the amount of the low molecular weight substance contained due to heat deterioration, at least 3
Melt extrusion is performed at a melting temperature of 00 ° C. or lower, preferably 290 ° C. or lower, more preferably 280 ° C. or lower, for a residence time of at most 15 minutes, preferably 10 minutes, more preferably 5 minutes. The obtained amorphous sheet was stretched by a roll stretching machine at a glass transition point of polyester up to 160.
Stretching in the machine direction at a temperature of ° C. The obtained uniaxially stretched film is coated with a coating liquid for improving various printability. Further, using a tenter stretching machine, the coating liquid is dried at a temperature of from the longitudinal stretching temperature to 170 ° C. and stretched in the transverse direction while being crosslinked and polymerized,
Subsequently, heat treatment is performed in a tenter at a temperature of 100 to 240 ° C. to obtain a biaxially stretched film. If necessary, a roll stretching machine and a tenter stretching machine are used to respectively re-longitudinal stretching,
Re-transverse stretching is performed.

An example of a method for producing a three-piece can coated with the polyester film of the present invention will be shown. Three-piece cans are roughly classified into solder cans, adhesive cans, and welded cans depending on the method of joining the can bodies, and the polyester film of the present invention is particularly suitable for adhesive cans and welded cans. Adhesive cans have been used for a long time in general cans, and are a method of injecting a molten adhesive into a folded portion. As a manufacturing method of a welding can, a cono weld method and a sudronic weld method can be mentioned. The former is TFS (Tin Free St).
eel) A method of peeling off the chromium film on the surface and welding the steel base material with a roll electrode. The latter is intended for tinplate, and is a method of welding by using a copper wire as an intermediate electrode between the roll electrode and the tinplate without polishing the weld end surface of the blank.

First, a polyester-coated steel sheet forming a three-piece can is manufactured. The polyester film of the present invention is pressure-bonded to both surfaces of a chrome surface-treated steel plate, a tin-plated steel plate or an aluminum plate by using various adhesives such as epoxy resin adhesives. At this time, the steel plate for forming the can body may be coated with the film of the present invention which has been subjected to various printings in advance. Chromium surface-treated steel sheets and tin-coated steel sheets coated with polyester are used as raw materials for the can body and the bottom of the can, and aluminum sheets coated with polyester are used as raw materials for the can lid. In any case, cut out to a suitable shape 3
Used as a component of a piece can. First, the coated steel sheet for a can body is rolled into a tubular shape, and the can body is made by the above-mentioned joining method.
A coated steel plate for a can bottom is attached to this can body. A rubber-based sealing compound is applied as a sealing material to the winding edge of the can body and dried to double-tighten the can bottom. Usually, the molded body thus obtained is filled with contents such as juice and then a polyester-coated aluminum plate for a can lid is wound in the same manner as the can bottom to form a three-piece can.

[0016]

EXAMPLES The present invention will be described in more detail with reference to examples, but the invention is not limited to the following examples without departing from the gist of the invention. The evaluation methods in the examples are as follows. In Examples and Comparative Examples, "part" means "part by weight". (1) Intrinsic viscosity [η] 1 g of a polymer was added to phenol / tetrachloroethane = 50
Dissolve in 100 ml of a mixed solvent of 50/50 (weight ratio) and
It was measured at 0 ° C.

(2) Low molecular weight extract amount (μg / cm 2) A sample film was pressure-bonded to a 0.30 mm-thick steel plate plated with chromium using an epoxy resin adhesive to obtain a polyester-coated steel plate. However, the resin was coated only on one side of the steel sheet. The surface coated with polyester is set to be the inner side, and the method is disclosed in JP-A-5-69.
According to the method disclosed in Japanese Patent Publication No. 485, a 350 ml juice can body in which a metal exposed portion welded and joined is fusion-coated with a coating film without using an adhesive or the like is manufactured. A can lid and a can bottom were manufactured so that the side in contact with the contents was the coating layer. A three-piece can consisting of the can body, the can lid, and the can bottom was manufactured, and 300 ml of pure water was sealed inside. This polyester-coated 3-piece can is heated to 125 ° C. and subjected to retort treatment for 1 hour. The water after the retort treatment is recovered, the water is distilled off, and the mass M (μg) of the obtained non-volatile matter is measured.
The value M / S (μg / cm ² ) divided by the internal surface area S (cm ² ) of the sample three-piece can used is taken as the amount of low molecular weight extract.

(3) Appearance of extraction The water after the retort treatment was recovered, and the Tyndall phenomenon was observed using a xenon lamp as a light source, and ranked according to the following criteria. ◎: No turbidity of the optical path was observed immediately after the retort or after standing for 1 week. ○: No turbidity of the optical path was observed immediately after the retort. Indicates a Tyndall phenomenon. △: Immediately after the retort, it is transparent under the amount of room light, but
Tyndall phenomenon is observed in the xenon optical path, and underwater light is visually observed even under room light after standing for 1 week. ×: Underwater under room light under naked light, both immediately after retort and after standing for 1 week. Floating matter is observed

(4) Terminal carboxyl group concentration (equivalent /
t) Makromol. chem. , 26, 226 (195
A. described in 8). It was measured according to the method of Conix. (5) Degree of plane orientation (ΔP) The maximum value a and the minimum value b of the refractive index in the film plane and the refractive index c in the film thickness direction are measured using an Abbe refractometer manufactured by Atago Optical Co., Ltd. The plane orientation was calculated by. In addition, the measurement of the refractive index uses a sodium D line at 23 ° C. 5
It was performed under 0RH% atmosphere. ΔP = (a + b) / 2−c

(6) Crystallinity χ c (%) A mixed solution of n-heptane and carbon tetrachloride that has density gradients. A standard density float is placed between the density gradients, and the sample is Density ρ of sample depending on position
And Substituting the obtained density ρ into the following equation, the crystallinity χc
Got

## EQU1 ## χ c = (ρ-1.335) × 100 / (1.4
55-1.335) (7) Dynamic friction coefficient μd Two pieces of film cut into a width of 15 mm and a length of 150 mm are stacked on a smooth glass plate, a rubber plate is placed on it, and a load is further placed on it. With the contact pressure between the two films set to 2 g / cm ² , the friction force was measured by sliding the films at 20 mm / min, the friction force was divided by the contact pressure, and the result was divided by the contact area. And The coefficient of friction at the point of sliding by 5 mm was defined as the dynamic coefficient of friction μd.

Example 1 [Production of polyester] (Polyester a) 100 parts of dimethyl terephthalate, 60 parts of ethylene glycol and 0.1 part of calcium acetate monohydrate were placed in a reactor to carry out a transesterification reaction.
That is, the reaction start temperature was set to 170 ° C., the reaction temperature was gradually raised with the distillation of methanol, and after 23 hours, 23
The temperature was raised to 0 ° C. to substantially end the transesterification reaction. Then, after adding ethylene glycol slurry containing amorphous silicon oxide having an average particle diameter of 3 μm and 0.04 part of phosphoric acid, 0.04 part of antimony trioxide was added, and polycondensation reaction was carried out by a conventional method. That is, the temperature is gradually increased and the pressure is gradually reduced, and after 2 hours, the temperature is 280
The temperature was set to 0 ° C. and the pressure to 0.3 mmHg, and the reaction was stopped after a lapse of 2 hours to obtain a polyester containing 0.05 part of silicon oxide and having an intrinsic viscosity of 0.62.

(Polyester b) 100 parts of dimethyl terephthalate, 60 parts of ethylene glycol and 0.1 part of calcium acetate monohydrate were placed in a reactor and transesterified. That is, the reaction start temperature was set to 170 ° C., the reaction temperature was gradually raised along with the distillation of methanol, and the temperature was raised to 230 ° C. after 4 hours to substantially end the transesterification reaction. Then, an ethylene glycol slurry containing amorphous silicon oxide having an average particle diameter of 3 μm and phosphoric acid of 0.1.
After adding 07 parts, 0.04 part of antimony trioxide was added, and polycondensation reaction was carried out by a conventional method. That is, the temperature was gradually increased and the pressure was gradually decreased, and after 2 hours, the temperature was 280 ° C., the pressure was 0.3 mmHg, the reaction was stopped after 1 hour, and 0.05 part of silicon oxide was contained. A polyester having an intrinsic viscosity of 0.45 was obtained.
This polyester was further subjected to solid phase polymerization at 225 ° C. and 0.3 mmHg for 10 hours to obtain a polyester having an intrinsic viscosity [η] of 0.75.

(Polyester c) Polyester a was placed in a pressure vessel containing pure water, the internal temperature was controlled at 90 ° C., the dipping treatment was carried out by holding for 4 hours, and then water was distilled off to give polyester c. Got

(Polyester d) Polyester b is placed in a pressure vessel containing pure water, the internal temperature is controlled at 90 ° C., the temperature is maintained for 4 hours for immersion treatment, and then water is distilled off to produce polyester d. Got

(Polyester e) 78 parts of dimethyl terephthalate, 12 parts of dimethyl isophthalate, 60 parts of ethylene glycol and 0.1 part of calcium acetate monohydrate were placed in a reactor for transesterification. That is, the reaction start temperature was set to 170 ° C., the reaction temperature was gradually raised along with the distillation of methanol, and the temperature was raised to 230 ° C. after 4 hours to substantially end the transesterification reaction. Then
After adding ethylene glycol slurry containing amorphous silicon oxide having an average particle diameter of 3 μm and 0.04 part of phosphoric acid, 0.04 part of antimony trioxide was added and polycondensation reaction was carried out by a conventional method. That is, the temperature is gradually increased and the pressure is gradually reduced, the temperature is 280 ° C. and the pressure is 0.3 mmHg after 2 hours, the reaction is stopped after 2 hours, and 0.05 part of silicon oxide is contained. Intrinsic viscosity is 0.7
A polyester of 0 was obtained.

[Production of Polyester Film] Polyester b was melt-extruded from a T-die at 280 ° C. for a residence time of 5
Melt extruded in minutes. After being extruded from the T-die, it was rapidly cooled to a temperature below the glass transition point on a cast drum to obtain a substantially amorphous sheet. This sheet was stretched 3.6 times in the machine direction at 80 ° C. using a roll stretching machine. Next, it was stretched 4.2 times in the transverse direction at 100 ° C. using a tenter stretching machine, and subsequently heat-set at 230 ° C. in a tenter,
A film having a thickness of 25 μm was obtained. [Production of 3-piece can] Chromium plating of the obtained film using an epoxy resin adhesive, thickness 0.30
A polyester-coated steel plate was obtained by pressure-bonding to a mm steel plate. However, the resin was coated only on one side of the steel sheet. The exposed surface of the welded metal is fusion-coated with a coating film by the method described in JP-A-5-69485 so that the surface coated with polyester is the inside. 35
A 0 ml juice can barrel was manufactured, and a can lid and a can bottom were separately manufactured from the above polyester-coated steel sheet so that the side in contact with the contents became the coating layer. A three-piece can comprising the can body, the can lid, and the can bottom was manufactured.

Example 2 Example 1 except that polyester b was changed to polyester c
A three-piece can was manufactured in the same manner as in. Example 3 Example 1 except that polyester b was changed to polyester d
A three-piece can was manufactured in the same manner as in. Example 4 The same as Example 3 except that the film after biaxial stretching and heat setting was obtained in Example 3 and then the film was immersed in pure water at 125 ° C. for 1 hour and then dried and used. And then
Three-piece cans were manufactured.

Comparative Example 1 Example 1 except that polyester b was changed to polyester a
A three-piece can was manufactured in the same manner as in. Comparative Example 2 Example 1 except that polyester b was changed to polyester e
A three-piece can was manufactured in the same manner as in. The results obtained above are summarized in Table 1 below.

[0029]

[Table 1]

[0030]

The polyester film for coating a three-piece can of the present invention has a small amount of water or hot water extractable components,
It is excellent in hygiene and its industrial value is high.

Claims (1)

[Claims] 1. The concentration of a terminal carboxyl group after being laminated and coated on a metal with an adhesive is less than 50 equivalents / t, and at 125 ° C. after being laminated and coated on a metal with an adhesive and produced. Low molecular weight substances extracted by pure water retort treatment for 1 hour are 2.0 per cm ^{2 of} coated surface.
A polyester film for covering a three-piece can, which is characterized in that it is not more than μg.