JPH0649234A - Polyester film to be used for covering outside of can of metal plate for manufacturing can - Google Patents

Abstract

PURPOSE:To obtain a polyester film to be used for covering the outside of a can of a metal plate for manufacturing cans, causing neither peel from metal can nor cracks even in the case of laminating polyester films to the sides of the can manufacturing metal plate and producing cans. CONSTITUTION:A polyester film for laminating and covering the surface of the outside of can of a metal plate for manufacturing cans contains 3-50wt.% white pigment having <=2.5mum average particle diameter, has 210-245 deg.C melting point, comprises a copolyester having a melting point 0.5-15 deg.C lower than the melting point of the polymer of a polyester film covering the inside of the can and has 0.2-0.6 degree of orientation of crystal in the thickness direction of the film to give a polyester film for covering the outside of a can of a metal plate for manufacturing cans.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyester film used for coating the outer surface of a can of a metal plate for can making, and more specifically, a polyester film for coating the outer surface of the can and a polyester film for coating the inner surface of the can, respectively. The present invention relates to a polyester film used for coating the outer surface of a metal plate for a can, which is useful for forming a metal can after being laminated on both sides of the can.

[0002]

2. Description of the Related Art Metal cans are generally coated to prevent corrosion on the inside and outside. Recently, for the purpose of simplifying the process, improving hygiene, preventing pollution, etc., coating with a thermoplastic resin film has been attempted as a method of imparting rust preventive properties without using an organic solvent.

That is, a method of making a can by laminating a thermoplastic resin film on a metal plate such as tin plate, tin-free steel, aluminum or the like and then making a can is under study.

[0004] As this thermoplastic resin film, it is gradually becoming clear that a copolyester film is suitable from the viewpoints of can-making processability, heat resistance, impact resistance, aroma retention and the like.

[0005]

On the other hand, on both sides of the metal plate, a polyester film having the same characteristics (however, the polyester film for coating the outer surface of the can is used for the purpose of imparting a white light-shielding property for printing, After bonding (containing a white pigment), if molded into a can, during molding, during retort treatment, or when shock is applied, the polyester film on the outer surface of the can peels off,
Alternatively, it has been found that there is a problem that the film is cracked. This phenomenon remarkably appears in the upper part of the can body that has undergone great deformation during can manufacturing.

An object of the present invention is to solve the above-mentioned problems of the prior art, and even when a polyester film is stuck on both sides of a metal plate to form a can, peeling or cracking occurs from the metal can. It is an object of the present invention to provide a polyester film used for coating an outer surface of a can-free metal plate for a can.

[0007]

Means for Solving the Problems As a result of intensive studies to achieve the above-mentioned object, the present inventors have found that the melting point of a polyester film used for coating a can outer surface of a metal plate for a can is coated with the inner surface of the can. The inventors have found that it is sufficient to lower the melting point of the polyester film used for the above, and have reached the present invention.

That is, the present invention relates to a polyester film used for laminating and coating a can outer surface of a metal plate for can making,
The film contains 3 to 3 white pigments having an average particle size of 2.5 μm or less
A copolymerized polyester containing 50% by weight and having a melting point within the range of 210 to 245 ° C. and 0.5 to 15 ° C. lower than the polymer melting point of the polyester film coating the inner surface of the can, and the thickness direction of the film. Is a polyester film used for coating the outer surface of a can with a metal plate for a can, which has a crystal orientation degree of 0.2 to 0.6.

Copolymerized polyethylene terephthalate is a typical example of the copolyester constituting the polyester film used for coating the can outer surface of the metal plate for can making in the present invention (hereinafter, also referred to as a polyester film for can outer coating). Take as an example. The copolymerization 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 decanedicarboxylic acid, and alicyclic compounds such as cyclohexanedicarboxylic acid. Examples thereof include dicarboxylic acid, and examples of the alcohol component include aliphatic diols such as butanediol and hexanediol, and alicyclic diols such as cyclohexanedimethanol. These may be used alone or in combination of two or more.

The proportion of the copolymerization component depends on the kind thereof, but as a result, the melting point of the polymer is in the range of 210 to 245 ° C., preferably 215 to 235 ° C., and the polyester film coating the inner surface of the metal can. The ratio is 0.5 to 15 ° C, preferably 0.5 to 5 ° C, more preferably 0.5 to 2 ° C lower than the melting point of the polymer. When the melting point of the polymer is less than 210 ° C., the heat resistance is inferior, so that it cannot withstand the heating during printing after can making. On the other hand, the melting point of the polymer is 2
If it exceeds 45 ° C., the crystal of the polymer is too large, and the workability in can making is impaired.

In the present invention, a particularly important point is to set the melting point of the copolyester to be lower than the polymer melting point of the polyester film coating the inner surface of the metal can. If the difference in melting point is less than 0.5 ° C, the can manufacturing processability is not sufficient, and the film tends to crack and peel from the metal can. On the other hand, if the difference in melting point exceeds 15 ° C., the heat resistance is lowered, embrittlement occurs, and further, the hue when printed is changed.

As the polyester film for coating the inner surface of the metal can, a film made of a copolyester similar to the copolyester used for forming the polyester film for coating the outer surface of the can is used.
The polymer melting point is preferably within the range of 210.5 to 245.5 ° C. Particularly, in order to obtain sufficient aroma retention, copolymerized polyethylene terephthalate having a secondary transition point (Tg) of 70 ° C. or higher, which is obtained by copolymerizing 10 to 14 mol% of isophthalic acid, is preferably used.

Here, the melting point of the copolymerized polyester is measured by a method using a Du Pont Instruments 910 DSC to obtain a melting peak at a temperature rising rate of 20 ° C./min. The sample amount is about 20 mg.

The copolymerized polyester constituting the polyester film for coating the outer surface of the can of the present invention has an average particle size of 2.5.
3-50% by weight, preferably 10
˜30% by weight.

The white pigment may be inorganic or organic, but is preferably inorganic. Preferred examples of the inorganic pigment include alumina, titanium dioxide, calcium carbonate, barium sulfate and the like. In order to improve the white light shielding property of the film, a pigment having a particle refractive index of 1.5 or more is preferable. If the average particle size of the white pigment exceeds 2.5 μm,
Coarse particles (for example, particles having a particle size of 10 μm or more) are used as a starting point in a portion deformed by a can forming process such as deep drawing, which causes pinholes and, in some cases, breakage, which is not preferable.

The amount of the white pigment contained in the copolyester is required to be 3 to 50% by weight. If it is less than this, the white light-shielding property becomes insufficient, while if it is too much, the can-making processability deteriorates. . The white pigment may be used alone or in a combination of two or more kinds. In the present invention, rutile type titanium dioxide is preferably used because it has a large concealing property and shielding effect and is inexpensive.

The white pigment in the present invention is preferably subjected to a particle size adjustment, removal of coarse particles and the like by using a refining process before being incorporated into the copolyester. As an industrial means of the purification process, for example, a jet mill, a ball mill or the like can be used as a pulverizing means, and a dry or wet centrifuge or the like can be used as a classification means. It is needless to say that these means may be used in combination of two or more and purified in stages.

Various methods can be used for incorporating the white pigment into the copolyester. The following method can be given as a typical method. (A) A method of adding before the completion of the transesterification or esterification reaction during the synthesis of the copolyester, or before the start of the polycondensation reaction. (A) A method of adding to the copolyester and melt-kneading. (C) A method of producing a master pellet containing a large amount of a white pigment, kneading with a copolyester containing no white pigment, and incorporating a predetermined amount of the white pigment in the methods (a) and (b).

When the method of adding a white pigment during the polyester synthesis of (a) is used, it is preferable to disperse the pigment in glycol and add it to the reaction system as a slurry.

The copolyester used in the present invention is not limited by its production method. For example, a method of subjecting terephthalic acid, ethylene glycol and a copolymerization component to an esterification reaction, and then subjecting the resulting reaction product to a polycondensation reaction to form a copolymerized polyester, or a transesterification reaction of dimethyl terephthalate, ethylene glycol and a copolymerization component. Then, the resulting reaction product is subjected to a polycondensation reaction to obtain a copolyester, which is preferably used. In the production of copolyester,
If necessary, other additives such as a fluorescent whitening agent, an antioxidant, a heat stabilizer, an ultraviolet absorber and an antistatic agent can be added. The addition of a fluorescent whitening agent is particularly effective in order to improve whiteness.

The polyester film for coating the outer surface of the can of the present invention is obtained by melting the above-mentioned white pigment-containing copolyester, discharging it from a die and molding it into a film, for example, biaxially stretched and heat-set. The degree of crystal orientation in the thickness direction of this film is in the range of 0.2 to 0.6, preferably 0.25 to 0.55. If this crystal orientation degree exceeds 0.6, the can-making processability becomes insufficient, and the film tends to break during deep drawing. On the other hand, when the crystal orientation degree is less than 0.2, that is, when the orientation is excessively low, the heat resistance becomes insufficient.

The degree of crystal orientation is measured as follows.

Using an X-ray diffractometer, crystal orientation indices <cos ² Φ _j , 10 in three directions (longitudinal direction MD, width direction TD, thickness direction ND) of the crystal plane (100) of the film.
0> is obtained, and the crystal orientation degree f ^{i, k} is obtained from the following equation.

F ^{i, k} = 2/3 <cos ² Φ _{j, k} > -1/2 (where i = MD, TD or ND, k = 100) where the crystal orientation in three directions is Rigaku Denki Measure using a pole-making sample stand.

However, when the white pigment is titanium dioxide,
Since the reflection peaks due to the titanium dioxide particles are near an anatase (101) and a rutile (110) of the copolyester (100), the (100) reflection peak of the copolyester of α = 0 in the pole figure is shown. Based on the reflection intensity (I _Tio2, α _{= 0} ) of titanium dioxide,
The intensity at all α and β positions up to α = 90 ° is I _Tio2,
The crystal orientation degree is calculated by subtracting α _{= 0} .

Where I _Tio2, α _{= 0} = 1/2 (I _Tio2,
α _{= 0, MD} + I _Tio2, α _{= 0, TD} ).

In the above, α is the pole sample stand, and α = 9
0 ° shows the case where (100) is arranged parallel to the film surface, and α = 0 ° shows the case where it is arranged perpendicularly to the film surface. Further, β represents the direction in the MD and TD planes of the film, β = 0 was the MD, and β = 90 ° was the TD direction. The degree of crystal orientation referred to in the present invention is represented by the value in the thickness direction ND.

As an example of a method for producing a film satisfying such requirements, a method of biaxial stretching, particularly sequential biaxial stretching will be described below, but the present invention is not limited to this method.

As described above, the copolyester is melted, discharged from a die to be formed into a film, and immediately cooled to obtain a substantially amorphous copolyester sheet. Next, this sheet is heated to a predetermined temperature by roll heating, infrared heating or the like and stretched in the longitudinal direction. At this time, the stretching temperature is 20 to 4 from the second-order transition point (Tg) of the copolyester.
It is preferable to set the temperature at 0 ° C. higher and the draw ratio to be 2.7 to 3.6 times. Stretching in the transverse direction starts at a temperature higher than Tg by 20 ° C. or more, and the melting point (Tm) of the copolyester
It is preferable to perform it while raising the temperature to 100 to 130 ° C. lower. The transverse stretching ratio is preferably 2.8 to 3.7 times. Also, the temperature for heat setting is 150 to 205 ° C.
The range is selected to adjust the film quality according to the melting point (Tm) of the copolyester.

The polyester film for coating the outer surface of the can of the present invention preferably has a thickness of 6 to 75 μm. 1 more
It is preferably 0 to 75 μm, and particularly preferably 15 to 50 μm. If the thickness is less than 6 μm, tearing or the like is likely to occur during can manufacturing, while if it exceeds 75 μm, it is uneconomical because of excessive quality.

A tin plate, tin-free steel, aluminum plate or the like is suitable as a metal plate for a can, to which the polyester film for coating an outer surface of the can of the present invention is attached.
Lamination of the polyester film to the metal plate, for example,
The method can be carried out by a method in which a metal plate and a film are superposed and heated, the films are laminated and then cooled, and the surface layer part (thin layer part) of the film in contact with the metal plate is made amorphous and fused. In this case, the polyester film for coating the inner surface of the can also
It can be attached to the other surface of the metal plate and simultaneously fused.

In this way, the polyester film for coating the outer surface of the can and the polyester film for coating the inner surface of the can are laminated on both surfaces of the metal plate, and then formed by drawing or the like to form a metal can.

[0033]

EXAMPLES The present invention will be described in more detail with reference to examples.

[0034]

[Reference Example] Polyethylene terephthalate copolymerized with 12 mol% of isophthalic acid (contains 0.3% by weight of titanium dioxide having an intrinsic viscosity of 0.64 and an average particle diameter of 0.3 μm and has a melting point of 2).
28 ° C.) at 280 ° C. and rapidly solidified to form an unstretched film.
The film was stretched 3.0 times at 0 ° C. and 3.0 times at 120 ° C. in the transverse direction, and then heat set at 190 ° C. to obtain a polyester film for coating a can inner surface having a thickness of 20 μm.

[0035]

Examples 1 to 4 and Comparative Examples 1 and 2 10% by weight of rutile type titanium dioxide having an average particle size of 0.27 μm was added to copolymerized polyethylene terephthalate (intrinsic viscosity 0.64) obtained by copolymerizing the components shown in Table 1. Then, melt-extruded under the film forming conditions shown in the same table, rapidly solidified into an unstretched film, and then the unstretched film was longitudinally stretched under the conditions shown in the same table, and laterally stretched,
Then, it was heat-set to obtain a polyester film for coating the outer surface of a can having a thickness of 20 μm.

Polymer melting points and crystal orientations of these films were as shown in Table 3.

[0037]

[Table 1]

[0038]

Comparative Examples 3 to 5 Copolymerized polyethylene terephthalate (intrinsic viscosity 0.64) obtained by copolymerizing the components shown in Table 2 with 10% by weight of rutile type titanium dioxide having an average particle size of 0.27 μm is shown in the same table. Melt extrusion under film forming conditions, rapid cooling and solidification to obtain an unstretched film, then the unstretched film was longitudinally stretched, laterally stretched, and then heat set to obtain a polyester film for coating the outer surface of a can having a thickness of 20 μm.

[0039]

[Table 2]

Nine kinds of polyester films for coating the outer surface of a can obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were 0.2
Laminated on one side of 5 mm thick tin-free steel,
On the other surface, the polyester for can inner surface coating obtained in Reference Example was laminated, heat-fused at 230 ° C., water-cooled, and then cut into a disc shape with a diameter of 150 mm, using a drawing die and a punch. Deep drawing was carried out in four steps to form a side surface seamless can having a diameter of 55 mm.

The following observations and tests were carried out on the cans, and each was evaluated according to the following criteria.

(1) Deep drawing workability-1 ○: The film was processed without any abnormality on both the inner and outer surfaces, and no whitening, peeling, or breakage was observed on the film on the inner and outer surfaces of the can. Δ: On the upper part of the film on the inner and outer surfaces of the can. Whitening is observed. X: Film peeling and breakage are observed on a part of the film on the inside and outside of the can.

(2) Deep drawing workability-2 ○: Both the inner and outer surfaces were processed without any abnormality, and the rust prevention test of the film surface inside the can (1% NaCl water was put in the can, the electrode was inserted, and the can body was opened. The current value is measured when a voltage of 6 V is applied as an anode, which is 0.1 mA in the ERV test.
Shown below: There is no abnormality in the film on both the inner and outer surfaces, but the current value is 0.1 mA or more in the ERV test, and pinhole-like cracks originating from the coarse lubricant are observed in the film when the energized portion is enlarged and observed.

(3) Impact resistance For cans with good deep drawing, water was fully poured, and 10 bottles for each test were dropped from a height of 10 cm onto the PVC tile floor surface, and then an ERV test in the can was conducted. As a result, ◯: 0.1 mA or less for all 10 pieces: Δ: 0.1 mA or more for 1 to 5 pieces: 0.1 mA or more for 6 or more pieces, or after dropping Cracks were observed

(4) Heat embrittlement resistance After the cans which had been well-formed by deep drawing were heated and held at 200 ° C. for 5 minutes, the impact resistance evaluation as described in (3) was performed. Was 0.1 mA or less Δ: 0.1 mA or more for 1 to 5 pieces: 0.1 mA or more for 6 or more pieces, or cracks of the film were already observed after heating at 200 ° C. for 5 minutes Was

Table 3 shows the evaluation results of the above four types.

[0047]

[Table 3]

As is clear from the results shown in Table 3, the metal can using the polyester film of the present invention is excellent in deep drawability, impact resistance and heat embrittlement resistance.

[0049]

According to the present invention, the polymer melting point of the polyester film for coating the can outer surface is set to be 0.5 to 15 ° C. lower than the polymer melting point of the polyester film for coating the can inner surface. When the films are laminated and heat-sealed, the polyester film for covering the outer surface of the can has a low melting point, so that the film is sufficiently fused to the metal plate without peeling or cracking, and the film for covering the inner surface of the can. Since has a high melting point, a certain degree of crystal orientation remains and sufficient aroma retention can be obtained. Further, it is also excellent in deep drawability, impact resistance after can making and heat embrittlement, and is extremely useful as a film for metal cans.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Susumu Onizuka 3-37-19 Oyama, Sagamihara City, Kanagawa Teijin Limited Sagamihara Research Center

Claims (3)

[Claims] 1. A polyester film used for laminating and coating an outer surface of a metal plate for a can, the film containing 3 to 50% by weight of a white pigment having an average particle diameter of 2.5 μm or less, and a melting point. Is in the range of 210 to 245 ° C. and is 0.5 to 15 ° C. lower than the polymer melting point of the polyester film that coats the inner surface of the can.
The crystal orientation in the thickness direction of the film is 0.2 to
A polyester film used for coating the outer surface of a can of a metal plate for can manufacturing, which is in the range of 0.6. 2. A polymer melting point of 210.5 to 245.5.
The polyester film used for coating the can outer surface of a metal plate for can manufacturing according to claim 1, which is used in combination with a polyester film for can inner surface coating made of a copolyester in the range of ° C. 3. A polyester film used for coating an outer surface of a can with a metal plate for can manufacturing according to claim 2, wherein the copolyester is isophthalic acid copolyethylene terephthalate.