JP3022683B2 - Polyester film used for coating the outer surface of metal plates for cans - 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 used for coating a metal plate for a can on the outer surface of a can, and more particularly, to a polyester film for coating the outer surface of a can and a polyester film for coating the inner surface of a can. The present invention relates to a polyester film used for coating the outer surface of a metal plate for cans, which is useful for forming a metal can after laminating on both surfaces of the metal can.

[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, coating with a thermoplastic resin film has been attempted as a method of imparting rust prevention without using an organic solvent.

That is, studies are being made on a method of laminating a thermoplastic resin film on a metal plate such as tinplate, tin-free steel, aluminum or the like, and then forming the can by drawing or the like.

[0004] As the thermoplastic resin film, it is gradually becoming clear that a copolyester film is suitable in terms of can processability, heat resistance, impact resistance, fragrance retention and the like.

[0005]

On the other hand, on both sides of a metal plate, a polyester film having the same properties (however, the polyester film for coating the outer surface of the can is provided with a white light-shielding property for printing, After laminating, the polyester film of the outer surface of the can is peeled off during molding, retorting, or when an impact is applied.
Alternatively, it has been found that a problem such as cracking of the film occurs. This phenomenon appears remarkably in the upper part of the can body that has undergone large deformation during can manufacturing.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art, and to peel off or crack a metal can even when a polyester film is bonded to both sides of a metal plate and can-forming is performed. An object of the present invention is to provide a polyester film used for coating the outer surface of a metal plate for cans without a can.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, have determined the melting point of a polyester film used for coating the outer surface of a metal plate for can making with the inner surface of the can. It was found that the melting point should be lower than the melting point of the polyester film used in the present invention, and the present invention was reached.

That is, the present invention relates to a polyester film used for laminating and coating the outer surface of a metal plate for a can,
When the film has a white pigment having an average particle size of 2.5 μm or less,
A copolyester containing 50% by weight and having a melting point in the range of 210 to 245 ° C., which is lower by 0.5 to 15 ° C. than the polymer melting point of the polyester film covering the inner surface of the can; Wherein the degree of crystal orientation of the polyester film is in the range of 0.2 to 0.6.

The copolymerized polyester constituting the polyester film (hereinafter sometimes referred to as the polyester film for coating the outer surface of the can) used for coating the outer surface of the metal plate for the can in the present invention is copolymerized polyethylene terephthalate. As an example. This 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, 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 for coating the inner surface of the metal can is formed. The ratio is 0.5 to 15 ° C, preferably 0.5 to 5 ° C, more preferably 0.5 to 2 ° C lower than the polymer melting point. When the melting point of the polymer is less than 210 ° C., the heat resistance is inferior, so that it cannot withstand the heating in printing after can-making. On the other hand, when the polymer melting point is 2
If the temperature exceeds 45 ° C., the crystal size of the polymer is too large, and the workability of the can is impaired.

In the present invention, it is particularly important that the copolymerized polyester has a melting point lower than the polymer melting point of the polyester film covering the inner surface of the metal can. When the melting point difference is less than 0.5 ° C., the workability of the can is insufficient, and the film is easily cracked and peeled from the metal can. On the other hand, when the melting point difference exceeds 15 ° C., the heat resistance decreases, embrittlement occurs, and further, the color tone when printed changes.

Here, as the polyester film for coating the inner surface of the metal can, a film made of the same copolymer polyester as the above-mentioned polyester film for forming the polyester film for coating the outer surface of the can is used.
Preferably, the polymer melting point is in the range of 210.5-245.5 ° C. In particular, in order to obtain sufficient fragrance retention, a copolymerized polyethylene terephthalate having a secondary transition point (Tg) of 70 ° C. or more 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 Du Pont Instruments 910 DSC to determine a melting peak at a temperature rising rate of 20 ° C./min. The sample amount is about 20 mg.

The copolymer polyester constituting the polyester film for coating the outer surface of a can of the present invention has an average particle size of 2.5.
3 to 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 increase the white light shielding property of the film, a pigment having a particle refractive index of 1.5 or more is preferable. When the average particle size of the white pigment exceeds 2.5 μm,
Coarse particles (for example, particles having a size of 10 μm or more) serve as a starting point at a portion deformed by can-making such as deep drawing, which is not preferable because pinholes or breaks sometimes occur.

It is necessary that the amount of the white pigment contained in the copolymerized polyester is 3 to 50% by weight. If the amount is less than this, the white light-shielding property becomes insufficient, while if it is too large, the processability in can making deteriorates. . The white pigments may be used alone or in combination of two or more. In the present invention, rutile-type titanium dioxide is preferably used because of its high concealing property and shielding effect and low cost.

The white pigment in the present invention is preferably subjected to particle size adjustment, removal of coarse particles, etc. by using a purification process before being contained in the copolymerized polyester. Industrial means of the refining process include, for example, a jet mill and a ball mill as a pulverizing means, and a dry or wet centrifuge as a classifying means. Of course, these means may be used in combination of two or more, and may be purified stepwise.

Various methods can be used for incorporating a white pigment into the copolymerized polyester. A typical method is as follows. (A) A method of adding before the end of transesterification or esterification reaction at the time of synthesizing the copolyester, or adding before the start of the polycondensation reaction. (A) A method of adding to the copolymerized polyester and melt-kneading. (C) A method in which a master pellet containing a large amount of a white pigment is produced in the above methods (a) and (b), kneaded with a copolymer polyester containing no white pigment, and a predetermined amount of the white pigment is contained.

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

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 obtain a copolymerized polyester, or a transesterification reaction of dimethyl terephthalate, ethylene glycol and a copolymer component. And then subjecting the resulting reaction product to a polycondensation reaction to form a copolymerized polyester. In the production of copolymerized polyester,
If necessary, other additives such as a fluorescent whitening agent, an antioxidant, a heat stabilizer, an ultraviolet absorber, an antistatic agent and the like can be added. In particular, when the whiteness is to be improved, the addition of a fluorescent whitening agent is effective.

The polyester film for coating the outer surface of a can of the present invention is obtained by melting the above-mentioned white pigment-containing copolymerized polyester, discharging it from a die, forming it into a film, and heat-setting it by, for example, biaxial stretching. The degree of crystal orientation in the thickness direction of the film is in the range of 0.2 to 0.6, preferably in the range of 0.25 to 0.55. When the degree of crystal orientation exceeds 0.6, the workability in can making becomes insufficient, and the film is easily broken during deep drawing. On the other hand, when the degree of crystal orientation 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 a X-ray diffractometer, crystal orientation index <cos ² Φ _j , 10 in three directions of crystal plane (100) (three directions of longitudinal direction MD, width direction TD, thickness direction ND)
0> is determined, and the degree of crystal orientation ^{fi, k} is determined 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 degree of crystal orientation in three directions is The measurement is performed using a pole sample table.

However, when the white pigment is titanium dioxide,
Since the reflection peak due to the titanium dioxide particles is close to the copolyester (100) for anatase (101) and rutile (110), the reflection peak of (100) for the copolyester with α = 0 in the pole figure. As a result of the reflection intensity (I _Tio2, α _{= 0} ) of titanium dioxide,
The intensities of all positions α and β up to α = 90 ° are _represented by I _Tio2,
The degree of crystal orientation is calculated by subtracting α _{= 0} .

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

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

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

As described above, the copolyester is melted, discharged from a die, formed into a film, and immediately cooled to obtain a substantially amorphous copolyester sheet. Next, the sheet is heated to a predetermined temperature by roll heating, infrared heating, or the like, and is stretched in the longitudinal direction. At this time, the stretching temperature is set to 20 to 4 from the secondary transition point (Tg) of the copolymerized polyester.
It is preferable that the temperature is 0 ° C. higher and the stretching ratio is 2.7 to 3.6 times. The transverse stretching is started at a temperature 20 ° C. or more higher than Tg, and the melting point (Tm) of the copolymerized polyester is obtained.
It is preferable to perform the heating while raising the temperature to a temperature lower by 100 to 130 ° C. It is preferable that the magnification of the transverse stretching is 2.8 to 3.7 times. The heat setting temperature is 150-205 ° C.
In order to adjust the film quality in accordance with the melting point (Tm) of the copolymerized polyester.

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

As a metal plate for a can to which the polyester film for coating the outer surface of a can of the present invention is bonded, a plate of tin, tin-free steel, aluminum or the like is suitable.
Lamination of the polyester film on the metal plate, for example,
The method can be carried out by stacking and heating a metal plate and a film, cooling the film after laminating the film, and amorphizing and fusing the surface layer (thin layer) of the film in contact with the metal plate. In this case, the polyester film for coating the inner surface of the can,
It can be bonded to the other surface of the metal plate and fused at the same time.

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 respectively bonded to both surfaces of the metal plate, and then formed by drawing or the like to form a metal can.

[0033]

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

[0034]

REFERENCE EXAMPLE Polyethylene terephthalate copolymerized with 12 mol% of isophthalic acid (intrinsic viscosity 0.64, containing 0.3% by weight of titanium dioxide having an average particle diameter of 0.3 μm, melting point 2)
28 ° C.) at 280 ° C., quenched and solidified to give an unstretched film, and then stretches the unstretched film in the machine direction by 10 mm.
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 20 μm-thick polyester film for coating the inner surface of a can.

[0035]

Examples 1-4 and Comparative Examples 1-2 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, quenched and solidified to form an unstretched film, and then the unstretched film was stretched longitudinally and transversely stretched under the conditions shown in the table,
Subsequently, it was heat-set to obtain a 20 μm-thick polyester film for coating the outer surface of a can.

The polymer melting points and crystal orientations of these films are 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 and adding 10% by weight of rutile type titanium dioxide having an average particle size of 0.27 μm is shown in the same table. It was melt-extruded under film-forming conditions, quenched and solidified to obtain an unstretched film, and then the unstretched film was stretched longitudinally and transversely, and then heat-set to obtain a 20 μm-thick polyester film for coating the outer surface of a can.

[0039]

[Table 2]

A total of nine kinds of polyester films for coating the outer surface of the can obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were 0.2%.
Laminated on one side of 5mm thick tin-free steel,
On the other surface, the polyester for coating the inner surface of the can obtained in Reference Example was bonded, heated and fused at 230 ° C., cooled with water, cut into a 150 mm-diameter disc, and drawn with a drawing die and punch. Deep drawing was performed in four stages to create a 55 mm diameter side seamless seam can.

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

(1) Deep drawing workability -1 ○: Both inner and outer surfaces are processed into a film without any abnormality, and no whitening, peeling or breakage is observed on the film on the inner and outer surfaces of the can. Whitening is observed ×: Film peeling and breakage are observed in a part of the film on the inner and outer surfaces of the can

(2) Deep drawing processability-2 ○: Both inner and outer surfaces were processed without any abnormality, rust prevention test on film surface in can (1% NaCl water was put in the can, electrodes were inserted, and the can was The current value when a voltage of 6 V is applied to the anode is measured.
X: The film has no abnormality on both the inner and outer surfaces, but the current value is 0.1 mA or more in the ERV test, and pinhole-shaped cracks starting from the coarse lubricant are observed in the film when the energized portion is observed under magnification.

(3) Impact resistance For cans with good deep drawability, water was fully poured, and 10 pieces of each test were dropped from a height of 10 cm on a PVC tile floor for each test, and then an ERV test in the can was performed. 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 pieces or more, or Cracks were found

(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). Δ: 0.1 mA or more for 1 to 5 pieces ×: 0.1 mA or more for 6 pieces or more, or cracking of the film already observed after heating at 200 ° C. × 5 minutes Was

Table 3 shows the results of the above four evaluations.

[0047]

[Table 3]

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

[0049]

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

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Susumu Onizuka 3-37-19 Koyama, Sagamihara-shi, Kanagawa Teijin Limited Sagamihara Research Center (56) References JP-A-4-117427 (JP, A) JP-A-5-339391 (JP, A) JP-A-5-331301 (JP, A) JP-A-5-170942 (JP, A) JP-A-52-9084 (JP, A) JP-A-5-287090 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C08J 5/18 B32B 15/08 B65D 1/12 C08K 3/22 C08L 67/02

Claims (3)

(57) [Claims] 1. A polyester film used for laminating and coating the outer surface of a metal plate for a can on the outer surface of a can, wherein the film contains 3 to 50% by weight of a white pigment having an average particle size of 2.5 μm or less, and has a melting point of Consisting of a copolyester which is in the range of 210 to 245 ° C and is lower than the polymer melting point of the polyester film covering the inner surface of the can by 0.5 to 15 ° C,
And the degree of crystal orientation in the thickness direction of the film is 0.2 to 0.2
A polyester film used for coating a metal plate for can making on the outer surface of the can, which is in the range of 0.6. 2. A polymer having a melting point of 210.5 to 245.5.
The polyester film used for coating a metal plate for can making on the outer surface of a can according to claim 1, which is used in combination with a polyester film for coating the inner surface of a can comprising a copolyester having a temperature within the range of ° C. 3. The polyester film used for coating the outer surface of a metal plate for a can according to claim 2, wherein the copolymerized polyester is polyethylene terephthalate copolymerized with isophthalic acid.