JP3495466B2 - Polyester film for metal coating - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated metal sheet which is attracting attention as a substitute for coating a metal, and more particularly to a polyester for coating a raw metal sheet of a metal can in which a defect-free film is required.

[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. With a conventional metal plate, which is coated with a thermosetting resin paint and baked and cured,
Even if the coating amount is increased or the number of coatings is increased, it is difficult to completely eliminate metal exposure due to pinholes, etc. This is because it can be eliminated. Further, in the case of a coated plate, a large amount of solvent is used for coating and drying and baking, whereas thermal lamination of a heat-fusible thermoplastic resin enables coating without using a solvent. As the thermoplastic resin used for these coatings, polyester having excellent heat resistance, corrosion resistance, workability, 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, corrosion resistance, workability, gas barrier properties, etc., but has sufficient thermal laminating property to coat metal. Therefore, polyesters having improved thermal laminating properties by copolymerizing a copolymerization component such as isophthalic acid (for example, JP-A-4-105922) have been proposed so far.

Further, the die and punch are used for drawing,
When ironing with an ironing ring, if the coating film does not have sufficient slipperiness, the film may break due to stress concentration.By adding relatively large lubricant particles, surface protrusions may be formed. In order to impart slipperiness to a film, it has been performed. Since the film is oriented during drawing and ironing, the major axis of the anisotropic form of the lubricant is parallel to the film surface. It is desirable that spherical silica and the like have been used, as disclosed in, for example, JP-A-3-110124. However, since there is no material that has a high affinity with polyester that can be the core of protrusion formation, such lubricant particles serve as a core that causes voids in the can manufacturing process, and the thickness of the beverage can material is reduced, so the drawing ratio / ironing ratio is reduced. Nowadays, the problem is that voids grow to cracks and the defect-free property, which is the most important feature of film coating, is lost.

[0005]

As a result of various studies in view of the above-mentioned problems, the present inventor found a film having a high defect-free property with little void formation inside the film even when largely deformed. Has been completed.

That is, the gist of the present invention is to provide a surface layer made of polyester containing spherical particles having an average particle size of 1.0 to 4.0 μm and an intermediate layer made of polyester having an inorganic content of 0.10% by weight or less. A laminated polyester film having a maximum surface layer thickness among particles in the surface layer.
The particle size is 2/3 to 3 times the particle size, the melting point of the polyester forming the surface layer is 180 to 240 ° C., and the difference in glass transition temperature between the polyester forming the surface layer and the intermediate layer is 10 ° C. It exists in the polyester film for metal coating characterized by the following.

The present invention will be described in detail below. The polyester referred to in the present invention includes, for example, dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid and sebacic acid, ethylene glycol, 1,4-butanediol, diethylene glycol and triethylene glycol. A polymer obtained by polycondensing a diol such as ethylene glycol, 1,4-cyclohexanedimethanol and neopentyl glycol.

As the polyester of the present invention, ethylene terephthalate obtained by polycondensing terephthalic acid and ethylene glycol, which are the most widely used polyester raw materials, or ethylene obtained by polycondensing 2,6-naphthalenedicarboxylic acid and ethylene glycol. Those having naphthalate as a main repeating unit are preferable, and a copolyester obtained by copolymerizing a copolymer component in order to impart appropriate heat adhesiveness and moldability is preferable. Isophthalic acid, phthalic acid, sebacic acid, diethylene glycol, triethylene glycol, 1,4-butanediol and 1,4-cyclohexanedimethanol are recommended as the copolymerization component. Particularly, isophthalic acid, diethylene glycol, and triethylene glycol are inexpensive materials and are suitable for industrial production. In addition, the main repeating unit referred to herein is one that usually accounts for 70 mol% or more, preferably 80 mol% or more, of all ester units.

The laminated polyester film of the present invention comprises a surface layer containing lubricant particles to impart slipperiness to the film, and an intermediate layer containing a small amount of various inorganic additives. The particle size of the particles contained in the surface layer is 1.0 to 4.0 μm,
It is preferably 2.0 to 3.5 μm, more preferably 2.
It is 6 to 3.0 μm. This has a larger particle size than the lubricant particles contained in the polyester film used for general purposes, and forms a sharp projection on the film surface.
If the particle size does not reach 1.0 μm, the height of protrusions formed by drawing and ironing is too low to provide sufficient slipperiness to the film, which is not preferable. If the particle size of the particles contained in the surface layer exceeds 4.0 μm, the formation of voids / cracks cannot be suppressed even if the processing conditions are optimized, and defects eventually result, which is not preferable.

[0010] particles the surface layer contains the Ru Oh spherical particles having a suitable slip properties. The spherical particles referred to here are, for example, when the particles are cut along a plane such that the cross-sectional area of the particles is maximized, a true circle having the same area as the cross-sectional area, and a region where the cross-sections overlap each other. The area is 63% or more of the cross-sectional area. Further, when the particles functioning as a lubricant are particles in an agglomerated state, the shape of the agglomerate is preferably spherical. As materials for the spherical particles, silicon dioxide, titanium dioxide, calcium carbonate, styrene-based cross-linked polymer, etc. are recommended.

[0011] surface layer thickness is 2/3 to 3 times the grain size of the largest particle size of the particles containing organic. If the surface layer thickness is less than ⅔ of the maximum lubricant particle size, particles may fall off during drawing or ironing, or voids generated around the particles may cause interfacial delamination between the surface layer and the intermediate layer. is there. When the surface layer thickness exceeds 3 times the maximum particle size, a region containing particles that can hardly contribute to the formation of surface protrusions is generated, and the possibility of occurrence of defects due to voids increases.

The thicknesses of the two surface layers are not necessarily the same, but when the polyester constituting the surface layer and the polyester constituting the intermediate layer have remarkably different shrinkage relaxation characteristics, curling may occur depending on the thermal history if the thicknesses of both surface layers are different. Therefore, it is desirable that both surface layers have substantially the same thickness. The term “substantially the same thickness” as used herein means that the thickness ratio is 1.0 to 1.4. The intermediate layer is a main constituent layer of the film thickness, and is usually 65% or more, preferably 75% or more of the total thickness, and the frequency of occurrence of film defects unless the ratio of the intermediate layer to the total thickness reaches 65%. Tends to be high.

The amount of inorganic substances contained in the intermediate layer is 0.10% by weight or less, preferably 0.05% by weight or less. The most characteristic feature of the present invention, the defect-free property, is that the content of the inorganic material having high rigidity, which can cause voids in the intermediate layer, which is the main constituent layer, in the drawing and ironing processes is small. In industrial production, since various recycled polyesters must be mixed for the purpose of environmental protection or cost reduction, it is difficult to make the inorganic content substantially 0 because inorganic substances derived from these recycled polyesters are mixed. However, if the inorganic content exceeds 0.10% by weight, defects such as cracks may be induced in the can manufacturing process, which is not preferable. Further, when the amount of the inorganic substance contained in the intermediate layer is large, even if the amount is small, it may be a small number of nuclei for defects, which is not preferable, and the average particle size of the inorganic substance is preferably not more than 4.0 μm as in the surface layer. The intermediate layer does not necessarily have to be a single layer, and may be a plurality of layers as long as they are within the range specified for the intermediate layer of the present invention.

The film for the above-mentioned use does not show superiority or inferiority in characteristics as long as it is thick enough to cover the metal surface without cracks or pinholes, but in industrial production, it is necessary to coat the film with a thickness larger than the necessary thickness. Is not preferable in terms of cost, and a thickness of 5 to 30 μm is desirable. The polyester constituting the surface layer and the intermediate layer need not be the same, but the melting point of the polyester constituting the surface layer is 180 to 240 ° C, preferably 195 to 230 ° C. The melting point of the polyester constituting the surface layer is 180
If the temperature does not reach ℃, the film is softened during printing of the ink in the printing process, and if it is severe, deterioration is caused, which is not preferable. The melting point of the polyester constituting the surface layer is 24
When the temperature exceeds 0 ° C, sufficient thermal adhesiveness cannot be obtained, which is not preferable. It should be noted that both the surface layer and the intermediate layer may not only consist of polyester of a single component, but may consist of a mixture of a plurality of polyesters, but in this case, if a plurality of crystal melting temperatures occur, both It must be within the scope of the invention.

Further, in the present invention, the difference in glass transition temperature between the polyester constituting the surface layer and the intermediate layer is 10 ° C. or less. The difference in glass transition temperature between the surface layer and the intermediate layer is 10 ° C.
If it exceeds, the difference in deformation efficiency at the same temperature becomes large, so the orientation of the surface layer and the intermediate layer will differ during drawing and ironing, which may cause stress concentration at the interface and interface delamination, which is not preferable. . When there are a plurality of intermediate layers, the difference in glass transition temperature between the surface layer and all the intermediate layers must be within 10 ° C.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The laminated polyester film of the present invention is not limited in its production method as long as it does not exceed the gist of the present invention, but one example of the recommended production method is shown below for reference. First, the raw material polyester is melted by an extruder and extruded through a T die to obtain a film.
You may obtain the laminated polyester film of the present invention by heat-laminating the surface layer and the intermediate layer each separately as a film, but in order to reduce the risk of interfacial peeling with sufficient interfacial compatibility, It is recommended to obtain a film in which the surface layer and the intermediate layer are laminated by a so-called coextrusion method. In addition, the film thickness obtained by extrusion molding is usually several hundreds μm, but a film thickness of several μm to several tens of μm is sufficient as a substitute for coating which is the purpose of the film of the present invention. Fixing is also preferable. Stretching has the effect of reducing unevenness in film thickness and suppressing stress concentration during drawing and ironing. In order to perform stretching, the molten extrudate is rapidly cooled using a cooling drum or the like during extrusion molding to be in a substantially amorphous state.

The amorphous film thus obtained is stretched in the machine direction and / or the transverse direction at a temperature not lower than the glass transition temperature. When stretching is performed in both directions, the order does not matter. Further, if necessary, re-stretching may be performed in each direction more than once. After applying the necessary stretching,
It is heat-fixed to improve dimensional stability. Since only a few crystals are formed only by stretching, the thermal adhesiveness is high, but when the strain imparted by stretching is not fixed and the temperature reaches the glass transition temperature or higher, heat shrinkage occurs and the thickness increases. It is preferable to heat-fix because the spots are also aggravated. In the case of the copolymerized polyethylene terephthalate of the present invention, the crystals that are mainly formed are composed of unit cells composed of ethylene terephthalate, so it may be heat-set at a temperature of 180 to 230 ° C.

[0018]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. The measuring methods and definitions of various physical properties and characteristics in the present invention are as follows. (1) Layer thickness [μm] A sample film is embedded with an epoxy resin, a 150 nm-thick section is prepared with an ultramicrotome, and this is observed with a transmission electron microscope. If necessary, a dyeing process is performed and the thickness of each laminated layer is measured.

(2) Ester unit [mol%] The sample film is subjected to plasma etching treatment to be ashed to obtain only the target layer. The obtained polyester film is dissolved in a deuterium solvent capable of dissolving it, for example, a deuterium solvent of trifluoroacetic acid, and ¹ H-NMR and ¹³ C-NMR spectra are obtained. Calculate the molar ratio of the corresponding dicarboxylic acid and diol from the integral curve. (3) Melting point [° C] The sample film is subjected to plasma etching treatment to be ashed to obtain only the target layer. The crystal melting peak of the obtained film is determined by a differential scanning calorimetry method, and the temperature at the peak top is taken as the melting point. In this example, the measurement was performed by heating from room temperature at a temperature rising rate of 4 ° C / min.

(4) Glass transition temperature [° C.] A sample film is subjected to plasma etching treatment to be ashed to obtain only a target layer. The glass transition temperature of the obtained film was determined by the differential scanning calorimetry method. In this example, the measurement was performed by heating from room temperature at a temperature rising rate of 4 ° C / min. (5) Particle size [μm] The sample film is sectioned and the cross section is observed with a scanning electron microscope to measure the particle size. The particle diameter of the non-spherical particles contained in the intermediate layer is the diameter of a circle having the same area as the observed particle cross-sectional area.

(6) Spherical index [%] The sample film is sectioned and the cross-section is photographed with the scanning electron microscope. A spherical index is defined as a percentage obtained by dividing the overlapping area by overlapping a true circle having the same area as the particle cross-sectional area on the particle image so that they overlap each other most. (7) Content of inorganic material in intermediate layer [wt%] The surface layer is ashed and removed by plasma etching, and the mass W1 of the intermediate layer is measured. After that, the intermediate layer is burnt and ashed, and the mass W2 of the remaining inorganic matter is measured. Let W2 / W1 be the inorganic content of the intermediate layer.

(8) A non-defect sample film heat-laminated on an aluminum plate is squeezed and ironed, and the thickness of the can body is 3 times the original plate thickness.
Obtain a squeezed ironed can that is 7%. However, the film coated surface should be the inner surface. Pour 2N hydrochloric acid into the can and let it stand for 3 hours at room temperature and pressure. The amount of liquid is adjusted so that the inner area of the can which is in contact with hydrochloric acid is 300 cm ² . Rank defect-free according to the following criteria: ◯: Aluminum has not been corroded at all Δ: Corrosion points where the area per piece is 5 mm ² or less are seen ×: Corrosion points where the area per piece exceeds 5 mm ²

Example 1 0.10 weight of spherical silica having a particle size of 2.6 μm was added to a polyester (hereinafter polyester A) obtained by polycondensing 87 mol% of terephthalic acid residues and 13 mol% of isophthalic acid residues with ethylene glycol. % Of the terephthalic acid residue, 12 mol% of isophthalic acid residue and ethylene glycol polycondensed with 0.01% by weight of spherical silica having a particle diameter of 2.6 μm. % (= 150 ppm) is laminated by a co-extrusion method, and a layer made of polyester A in both surface layers and a layer made of polyester B in the intermediate layer have a thickness ratio of 3: 1 respectively.
A laminated amorphous film of 9 was obtained. Subsequently, the film was stretched 3 times in the machine direction at 80 ° C. and 4 times in the transverse direction at 95 ° C., and then heat set at 210 ° C. to obtain a laminated film having a thickness of 25 μm.

Example 2 Polyester obtained by polycondensing 87 mol% of 2,6-naphthalenedicarboxylic acid residues and 13 mol% of isophthalic acid residues with ethylene glycol in place of polyester A constituting the surface layer (polyester C ), The polyester constituting the intermediate layer is replaced with polyester B, and 2,6
Example 1 except that a polyester (polyester D) obtained by polycondensing 88 mol% of naphthalene dicarboxylic acid residues and 12 mol% of isophthalic acid residues with ethylene glycol was used.
A 25 μm-thick laminated film was obtained in the same manner as in.

Example 3 The surface layer was composed of polyester B containing 0.06% by weight of spherical particles of styrene-based cross-linked polymer having a particle size of 2.7 μ, and the intermediate layer was 79 mol% of terephthalic acid residues and isophthalic acid. Polyester obtained by polycondensing 21 mol% of acid residue and ethylene glycol (polyester E) has an average particle size of 1.3.
A 25 μm-thick laminated film was obtained in the same manner as in Example 1, except that 160 μm of amorphous silica of 160 μm was contained.

Example 4 Particles contained in the surface layer / intermediate layer have an average particle size of 2.6 μm.
Thickness 2 in the same manner as in Example 1 except that amorphous silica is used.
A laminated film of 5 μm was obtained. Example 5 Example 1 except that the thickness ratio of the surface layer to the intermediate layer was 4:17.
A 25 μm-thick laminated film was obtained in the same manner as in.

Comparative Example 1 A laminated film having a thickness of 25 μm was obtained in the same manner as in Example 1 except that the particles contained in the surface layer were spherical silica having a particle size of 0.8 μm. Since the slipperiness was insufficient, it was observed that the film surface was scraped by the ironing ring in the process of making a drawn and ironed can for the purpose of evaluating defect-free property. Comparative Example 2 A laminated film having a thickness of 25 μm was obtained in the same manner as in Example 1 except that the particles contained in the surface layer were spherical silica having a particle size of 4.2 μm. Many voids and cracks caused by the voids were generated in the process of drawing and ironing a can for the purpose of evaluating the defect-free property, and the defect-free property was remarkably inferior.

Comparative Example 3 Polyester (polyester F) obtained by polycondensing 67 mol% of terephthalic acid residue and 33 mol% of isophthalic acid residue with ethylene glycol, which constitutes the surface layer,
The polyester constituting the intermediate layer is made of terephthalic acid residue 70
Polyester (polyester G) obtained by polycondensation of ethylene glycol with mol% and 30 mol% of isophthalic acid residue
Was biaxially stretched in the same manner as in Example 1, except that the heat setting temperature was 150 ° C. to obtain a laminated polyester film having a thickness of 25 μm. Such a film caused thermal shrinkage at the free end in an atmosphere of 200 ° C. for 30 seconds, which is equivalent to the drying temperature after cleaning the can-making lubricating oil, and the thickness unevenness was extremely deteriorated.

Comparative Example 4 70 mol% of ethylene glycol residues and 30 mol% of triethylene glycol residues were used for the polyester constituting the intermediate layer.
And a terephthalic acid polycondensed polyester (polyester H) except that the thickness is 25 μm in the same manner as in Example 1.
A laminated film of m was obtained. The interface between the surface layer and the intermediate layer was peeled off when a squeezed ironing can was manufactured for defect-free evaluation. Comparative Example 5 The same as Example 1 except that the content of the spherical silica contained in the intermediate layer was 0.2% by weight. A large number of void cracks were formed in the intermediate layer, and a remarkable reduction in defect-free property was observed. The results obtained above are summarized in Table 1 below.

[0030]

[Table 1]

[0031]

[Table 2] In Table 1, IPA represents isophthalic acid and TEG represents triethylene glycol.

[0032]

INDUSTRIAL APPLICABILITY The laminated polyester of the present invention is suitable for use in coating a metal plate as a paint substitute, and such a coated metal plate is widely used as a beverage can, a building material, a home electric appliance part, etc., and has an industrial value. Very expensive.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-51-8387 (JP, A) JP-A-62-225345 (JP, A) JP-A-2-77431 (JP, A) JP-A-7- 101015 (JP, A) JP-A-6-218894 (JP, A) JP-A-6-218893 (JP, A) JP-A-3-110124 (JP, A) JP-A-4-105922 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B32B 1/00-35/00

Claims (2)

(57) [Claims] 1. A laminated polyester film having a surface layer made of polyester containing spherical particles having an average particle diameter of 1.0 to 4.0 μm and an intermediate layer made of polyester having an inorganic content of 0.10% by weight or less. The surface thickness is
2/3 to 3 of the maximum particle size among the particles in the surface layer
And the melting point of the polyester constituting the surface layer is 180
A polyester film for metal coating, wherein the polyester film constituting the surface layer and the intermediate layer has a glass transition temperature difference of 10 ° C or less. 2. The polyester film for metal coating according to claim 1 , wherein the polyester constituting the surface layer and the intermediate layer mainly comprises ethylene terephthalate and / or ethylene naphthalate as a constituent unit.