JP2908158B2 - Polyester film for metal plate lamination processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyester film for laminating and forming a metal plate, and more particularly, it exhibits excellent formability when laminating to a metal plate and performing can forming such as drawing. Polyester film for metal plate lamination that can be used to produce metal cans having good heat resistance, and excellent heat resistance, retort resistance, fragrance preservation and flavor, impact resistance, etc., such as beverage cans and food cans About.

[0002]

2. Description of the Related Art Metal cans are generally coated to prevent corrosion of the inner and outer surfaces. Recently, for the purpose of simplifying the process, improving hygiene, and preventing pollution, rust prevention is performed without using an organic solvent. Development of a method for obtaining the property has been advanced, and as one of the methods, coating with a thermoplastic resin film has been attempted. That is,
Studies have been made on a method of laminating a thermoplastic resin film on a metal plate such as tin, tin-free steel, or aluminum, and then forming the can by drawing or the like. Polyolefin films and polyamide films have been tried as this thermoplastic resin film, but they do not satisfy all of moldability, heat resistance, fragrance retention and impact resistance.

On the other hand, polyester films, particularly polyethylene terephthalate films, have been attracting attention as having balanced properties, and some proposals based on these have been made. That is, (A) a biaxially oriented polyethylene terephthalate film is laminated on a metal plate via a low-melting-point polyester adhesive layer and used as a can-making material (JP-A-56-1045).
No. 1, JP-A-1-192546). (B) Amorphous or extremely low-crystalline aromatic polyester film is laminated on a metal plate and used as a can-forming material (JP-A-1-192545, JP-A-2-57).
339). (C) A biaxially oriented polyethylene terephthalate film having a low orientation and thermally fixed is laminated on a metal plate and used as a can-making material (Japanese Patent Application Laid-Open No. 64-22530).

[0004] However, the present inventors' studies have revealed that no satisfactory characteristics can be obtained in any case, and that each has the following problems.

Regarding (A), a biaxially oriented polyethylene terephthalate film is excellent in heat resistance and fragrance retention,
In the process of making cans with insufficient molding workability and large deformation, whitening (generation of minute cracks) and breakage of the film occur.

As for (B), since it is an amorphous or extremely low-crystalline aromatic polyester film, the moldability is good, but the fragrance retention is inferior, and the printing and retort sterilization after can-making are performed. Such a film may be easily embrittled by post-treatments or storage for a long period of time, and may be changed to a film which is easily broken by an impact from the outside of the can.

As for (C), the effect is not exhibited in the intermediate region between the above (A) and (B), but it has not yet reached the low orientation applicable to can processing. Even if it can be processed in a region with a small degree of deformation, subsequent printing,
As in the case of (B), there is a possibility that the retort treatment for sterilizing the contents of the can makes the film brittle easily and the film is likely to be broken by an impact from the outside of the can.

In order to solve such a problem, the present inventors have considered various uses of a film made of a copolymerized polyester and have made various studies. As a result, the copolymerized polyester film is excellent in molding workability, heat resistance, retort resistance, and fragrance retention, but has an impact resistance, particularly
It has been found that the impact resistance at a low temperature of 5 ° C. or less is insufficient, and that when the metal can to which the film is bonded is dropped at a low temperature to give an impact, the film is likely to crack. Poor impact resistance at low temperatures poses a major problem when handled in a cooled state, such as metal cans for juices and soft drinks.

Further, it has been found that when a copolymerized polyester film is used, the fragrance retention is generally good, but when used in soft drinks and the like, there arises a problem that the taste of the contents deteriorates. .

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to improve the impact resistance of a copolymerized polyester film while maintaining the excellent moldability, heat resistance, retort resistance and fragrance retention. Another object of the present invention is to provide a polyester film for laminating a metal plate which is hardly cracked by an impact at a low temperature and does not deteriorate the taste of soft drink and the like.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above-mentioned object, and as a result, the film was formed into a laminated film in which both surface layers were each formed of a copolymerized polyester layer. By limiting the surface roughness of each layer to a specific range, it has been found that impact resistance and taste retention at low temperatures are improved, and that winding property can be prevented from deteriorating. Reached.

That is, the present invention has a melting point of 210-245 ° C.
And a copolymer polyester layer (B) having a melting point of 210 to 245 ° C., and the surface roughness of the isophthalic acid copolymer polyester layer (A) Ra) is not more than 15 nm, and the polyester layer (B) has a surface roughness (Ra) of not less than 15 nm.

In the present invention, copolymerized polyethylene terephthalate is a representative example of the copolymerized polyester used in the copolymerized polyester layer (B). The copolymer component may be an acid component or an alcohol component. Examples of the acid component include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid and naphthalenedicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decane dicarboxylic acid, and alicyclic groups such as cyclohexanedicarboxylic acid. Examples of the dicarboxylic acid include aliphatic alcohols such as butanediol and hexanediol, and alicyclic diols such as cyclohexanedimethanol. These can be used alone or in combination of two or more.

The proportion of the copolymer component depends on the type thereof, and as a result, the melting point of the polymer is in the range of 210 to 245 ° C., preferably 215 to 235 ° C. Melting point 2
If it is lower than 10 ° C., the heat resistance will be poor. On the other hand, when the melting point exceeds 245 ° C., the crystallinity of the polymer is too large, and the moldability is impaired.

Here, the melting point of the copolyester was measured by Du Pont Instruments 910.
A method in which a melting peak is determined at a heating rate of 20 ° C./min using DSC. The sample amount is about 20 mg.

The intrinsic viscosity of the copolymerized polyester is preferably 0.52 to 0.80, more preferably 0.54 to 0.70, and particularly preferably 0.57 to 0.70.
0.65.

In the present invention, the copolymer polyester used for the isophthalic acid copolymer polyester layer (A) is
Among the above-mentioned copolyesters used for the copolyester layer (B), a copolyester containing an isophthalic acid component as an acid component, and a copolymerized polyethylene terephthalate obtained by copolymerizing an isophthalic acid component (isophthalic acid copolymerized polyethylene terephthalate) ) Is a representative example. This isophthalic acid copolymerized polyethylene terephthalate has a copolymerization component other than isophthalic acid, such as the copolymerization component exemplified as the copolymerization component of the copolymerized polyester layer (B). It may be copolymerized at a ratio of 3 mol% or less based on all alcohol components.

The proportion of the copolymer component containing isophthalic acid is as follows:
Polymer melting point 210-245 ° C, preferably 215-245 ° C
This is a ratio falling within the range of 235 ° C. If the melting point is lower than 210 ° C., the heat resistance will be poor. On the other hand, when the melting point exceeds 245 ° C., the crystallinity of the polymer is too large, and the moldability is impaired.

In the present invention, the isophthalic acid copolymerized polyester layer (A) must have a surface roughness (Ra) of 15 nm or less, preferably 12 nm or less, more preferably 2 nm or more and 10 nm or less. This copolyester layer (A) is a layer located on the side in contact with the contents of the can when the laminated film is bonded to a metal plate and formed into a can, and has a surface roughness (Ra) of 15 nm or less. By doing so, it is possible to prevent the taste of the contents, especially soft drinks such as carbonated drinks, from being deteriorated, and to improve the impact resistance at low temperatures. However, this surface roughness (R
When a) is 2 nm or less, the surface of the film is easily damaged and requires careful handling.

In order to reduce the surface roughness (Ra) to 15 nm or less, the average particle size and the amount of the lubricant to be added to the isophthalic acid copolymerized polyester may be appropriately selected. For example,
In the case of using monodispersed spherical silica as a lubricant, 0.04% by weight or less when the average particle diameter is 1.5 μm and 0.36% by weight or less when the average particle diameter is 0.8 μm. Is preferred. When the addition amount is 0.2% by weight, the average particle size of the monodispersed spherical silica is 0.95 μm or less, and when the addition amount is 0.05% by weight, the average particle size of the monodispersed spherical silica is Preferably, the diameter is 1.33 μm or less. In these cases, lubricants having different types and different average particle sizes may be mixed and used.

On the other hand, the copolymer polyester layer (B) needs to have a surface roughness (Ra) of 15 nm or more. When the surface roughness (Ra) is less than 15 nm,
The film is unsuitable because the handleability (winding property) of the film deteriorates. When the film is bonded to a metal plate and formed into a can, the copolyester layer (B) is a layer adhered to the metal can and does not directly contact the contents of the can. Even if the surface roughness (Ra) is 15 nm or more, the taste of the contents does not deteriorate.

In the case of the copolymerized polyester layer (B), a desired surface roughness (Ra) can be obtained by appropriately controlling the average particle size and the amount of the lubricant added to the copolymerized polyester.

Isophthalic acid copolymerized polyester layer (A)
The lubricant to be added to the copolymer or the polyester layer (B) may be inorganic or organic, but is preferably inorganic. Examples of the inorganic lubricant include silica, alumina, kaolin, titanium dioxide, calcium carbonate, and barium sulfate. Examples of the organic lubricant include silicone particles, crosslinked polystyrene particles, and crosslinked acrylic resin particles. In particular, a lubricant which is preferable in terms of pinhole resistance is a monodispersed lubricant having a particle size ratio (major axis / minor axis) of 1.0 to 1.2. Examples of such a lubricant include spherical silica and spherical silicone particles.

The surface roughness (Ra) of the film is J
This is the center line average roughness determined according to IS-B0601, and a portion of the measured length L is extracted from the film surface roughness curve in the direction of the center line, the center line of the extracted portion is defined as the X axis, and the longitudinal magnification Is the Y axis, and the roughness curve is Y =
When represented by f (x), the value (R
a: nm) is defined as the film surface roughness.

[0025]

(Equation 1)

In the present invention, when the reference length is 2.5 mm, 5
The number is measured, and Ra is represented as an average of four values excluding one from the larger value.

The copolymerized polyester in the present invention is not limited by its production method. For example, a method of subjecting terephthalic acid, ethylene glycol and a copolymer component to an esterification reaction and then subjecting the resulting reaction product to a polycondensation reaction to form a copolymerized polyester, or a method of transesterifying dimethyl terephthalate, ethylene glycol and a copolymer component. A method of reacting and then subjecting the resulting reaction product to a polycondensation reaction to form a copolymerized polyester is preferably used. In the production of the copolymerized polyester, if necessary, other additives such as an optical brightener, an antioxidant,
Heat stabilizers, ultraviolet absorbers, antistatic agents and the like can also be added.

The polyester film of the present invention has a structure in which an isophthalic acid copolymerized polyester layer (A) and a copolymerized polyester (B) are laminated. The copolyesters constituting are separately melted and co-extruded from a die, and after lamination and fusion before solidification, biaxial stretching, a method of heat setting, each copolyester is melted separately, extruded to form a film, The unstretched state or after the stretching can be produced by a method of laminating and fusing the two. At that time, in order to increase the laminating adhesive force of the isophthalic acid copolymerized polyester layer (A) and the copolymerized polyester layer (B) and improve the impact resistance, the melting point difference between the polymers constituting both layers is 10 ° C.
Hereinafter, it is more preferably 7 ° C. or lower, particularly preferably 5 ° C. or lower.

The polyester film of the present invention may be an unstretched film, but is usually used in a state of being biaxially stretched and heat set. In this case, the refractive index in the thickness direction of the isophthalic acid copolymerized polyester layer (A) is 1.49.
It is preferably 0 to 1.550, and more preferably more than 1.505 and not more than 1.540. If the refractive index is too low, the moldability will be insufficient, while if it is too high, the structure will be close to amorphous and the heat resistance may decrease.

The polyester film of the present invention preferably has a thickness of 6 to 75 μm. 10-75μ
m, particularly preferably 15 to 50 μm. If the thickness is less than 6 μm, breakage or the like is likely to occur during processing, while if it exceeds 75 μm, the quality is excessive and uneconomical.

Isophthalic acid copolymerized polyester layer (A)
The thickness T _B of the thickness T _A, copolyester layer (B)
The ratio of the (T _A / T _B) is preferably about 0.02 to 1.5, more preferably 0.04 to 0.67, particularly preferably 0.04 to 0.25. Specifically, for example, in the case of a polyester film having a thickness of 25 μm, the thickness of the isophthalic acid copolymerized polyester layer (A) is set to 0.5 to 15 μm.
μm, preferably 1 to 10 μm, more preferably 1 to 5
μm.

As a metal plate to which the polyester film of the present invention is bonded, in particular, a metal plate for can making, a plate of tin, tin-free steel, aluminum or the like is suitable.
The bonding of the polyester film to the metal plate can be performed, for example, by the following method.

The metal plate is heated to a temperature equal to or higher than the melting point of the film, the film is bonded, and then cooled, and the surface layer (thin layer) of the film in contact with the metal plate is made amorphous and adhered.

An adhesive layer is coated on the film with a primer in advance, and this surface is bonded to a metal plate. As the adhesive layer, a known resin adhesive, for example, an epoxy-based adhesive,
Epoxy-ester adhesives, alkyd adhesives and the like can be used.

When the polyester film of the present invention is bonded to a metal plate, the side of the copolymerized polyester layer (B) is bonded to the metal plate.

Further, in the polyester film of the present invention, if necessary, another additional layer may be laminated between the copolymerized polyester layers (A) and (B).

[0037]

The present invention will be further described with reference to the following examples.

[0038]

Examples 1 to 6 and Comparative Examples 1 to 5 Polyethylene terephthalate copolymerized with the components shown in Table 1 (intrinsic viscosity 0.6
4, containing 0.1% by weight of spherical silica having a particle size ratio of 1.1 and an average particle size of 0.3 μm) is a copolymerized polyester layer (A),
Polyethylene terephthalate (containing 0.1% by weight of spherical silica having an intrinsic viscosity of 0.64, a particle size ratio of 1.1, and an average particle size of 1.5 μm) copolymerized with the components shown in Table 1 To obtain (B), each was dried and melted separately by a common method, then co-extruded from dies adjacent to each other, laminated, fused and quenched and solidified to prepare an unstretched laminated film.

Next, the unstretched film was longitudinally stretched 3.0 times at 110 ° C., and then transversely stretched 3 times at 120 ° C.
It was heat-set at 190 ° C. to obtain a biaxially oriented laminated film.

The thickness of the obtained film was 25 μm, the thicknesses of the copolymerized polyester layer (A) and the copolymerized polyester layer (B) were 5 μm and 20 μm, respectively, and the surface roughness (Ra) was 5 nm and 23 nm, respectively. Met.

The surface roughness (Ra) was measured using a stylus type surface roughness meter (SURFCORDER S, manufactured by Kosaka Laboratory Co., Ltd.).
E-30C) using a stylus radius of 2 μm and a measurement pressure of 0.0
The measurement was performed under the conditions of 3 g and a cutoff value of 0.25 mm.

[0042]

[Table 1]

[0043]

Comparative Examples 6 and 7 In Example 2, only the copolymerized polyester layer (A) was used (Comparative Example 6) without using a two-layer laminated structure.
And the thickness 25 of only the polyester layer (B) (Comparative Example 7)
A μm monolayer film was prepared.

[0044]

Examples 7 to 8 and Comparative Examples 8 to 9
Table 2 shows the average particle size and amount of spherical silica contained in the copolymerized polyester layer (A) and the copolymerized polyester layer (B).
And the other conditions were the same as in Example 2 to obtain a biaxially oriented laminated film.

[0045]

[Table 2]

A total of 17 kinds of films obtained in the above Examples 1 to 8 and Comparative Examples 1 to 9 were heated at 230 ° C. to a thickness of 0.1 mm.
Affixed to both sides of a 25 mm tin-free steel so that the surface of the polyester layer (B) is in contact with the tin-free steel, and after cooling with water, cut into a 150 mm diameter disk.
Deep drawing in 4 stages using a drawing die and punch
A 5 mm-diameter side-seamless container (hereinafter abbreviated as can) was prepared.

The following observations and tests were performed on the cans, and the cans were evaluated according to the following standards.

(1) Deep drawing processability -1 ：: The film was processed without any abnormality, and no whitening or breakage was observed in the film. Δ: Whitening was observed in the upper part of the film can. ×: The film was partially broken in the film. Is recognized

(2) Deep drawing workability-2 ○: Processed without any abnormality, rust prevention test on film surface in can (1% NaCl water is put in the can, electrodes are inserted, and the can body is used as an anode. The current value when a voltage of 6 V is applied is measured.
X: No abnormality in the film, but the current value was 0.1 mA or more in the ERV test, and when the energized portion was enlarged and observed, a pin starting from a coarse lubricant was found on the film. Hole-shaped cracks are observed

(3) Impact resistance For cans having good deep drawing, pour water thoroughly and cool to 10 ° C.
ERV test was performed in the can after dropping on a PVC tile floor surface from the test results. A: 0.3 mA or less for all 10 pieces. Δ: 1 mA or more was 0.3 mA or more. X: 0.3 mA or more for 6 or more pieces, or cracks of the film were already observed after dropping.

(4) Heat Embrittlement Resistance The can which had been subjected to good deep drawing was heated and maintained at 200 ° C. for 5 minutes, and then subjected to the impact resistance evaluation described in (3). Was 0.3 mA or less. Δ: 1 mA or more was 0.3 mA or more. C: Cracks were already observed in the film after heating for 6 or more at 0.3 mA or more or at 200 ° C. for 5 minutes.

(5) Retort resistance The cans having good deep drawability were filled with water, retorted in a steam sterilizer at 120 ° C. for 1 hour, and then stored at 50 ° C. for 30 days. After dropping 10 cans obtained for each test from a height of 1 m onto a PVC tile floor, an ERV test in the cans was performed. ：: 0.3 mA or less for all 10 pieces Δ: 0.3 mA or more for 1 to 5 pieces ×: 0.3 mA or more for 6 pieces or more, or cracks of the film were already observed after dropping Was

(6) Scent Retention The cans having good deep drawing were filled with cider and sealed. After being kept at 37 ° C. for 30 days, the can was opened and the change in fragrance was examined by sensory test. ：: no change in scent Δ: slight change in scent was observed ×: change in scent was observed

(7) Taste preservation In the same manner as in (6), the change in taste was examined by a sensory test. ：: No change in taste. Δ: Slight change in taste was observed. ×: Change in taste was observed.

The evaluation results and winding properties of the above seven types are as shown in Table 3.

[0056]

[Table 3]

As is clear from the results shown in Table 3, cans using the polyester film of the present invention are excellent in deep drawing workability, heat embrittlement resistance, retort resistance, fragrance retention and impact resistance. In particular, it has excellent impact resistance at low temperatures, does not deteriorate the taste of soft drinks and the like, and has good winding properties.

[0058]

The polyester film for laminating and processing metal sheets of the present invention has excellent moldability, heat resistance, retort resistance and fragrance retention, and also has impact resistance, especially impact resistance at low temperatures. The properties are improved, the taste of soft drinks and the like are not deteriorated, and the winding property is also good. Therefore, it is particularly suitable for being used after being cooled and stuck to a metal can for soft drinks and the like which is often handled at a low temperature.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-338103 (JP, A) JP-A-6-172556 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B32B 1/00-35/00

Claims (1)

(57) [Claims] 1. An isophthalic acid copolymerized polyester layer (A) having a melting point of 210 to 245 ° C., and a melting point of 210 to 245 ° C.
The copolymerized polyester layer (B) at 45 ° C. is laminated, and the surface roughness (R) of the copolymerized polyester layer (A) is
(a) is 15 nm or less, and the surface roughness (Ra) of the copolymerized polyester layer (B) is 15 nm or more.