JP3071573B2 - 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 processing a metal plate, and more particularly, to a metal can having excellent low-temperature impact resistance, for example, a metal can for producing beverage cans, food cans and the like. The present invention relates to a polyester film for lamination processing.

[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). (D) A copolymerized polyester film is laminated on a metal plate and used as a material for can production (Japanese Patent Laid-Open No. 3-86729).
issue).

[0004] These techniques have reached a practically practicable level, but there are problems in terms of molding workability, and in particular, the impact resistance is poor, and the film is liable to be broken by an impact from the outside of the can. There is.

In order to solve the problems of such a polyester film and obtain a film having good impact resistance, the inventors of the present invention use ethylene terephthalate having a melting point of 210 to 245 ° C. as a main repeating unit. A film comprising a polyester composition comprising 99 to 60% by weight of the copolymerized polyester (I) and 1 to 40% by weight of a polyester (II) having butylene terephthalate having a melting point of 180 to 223 ° C. as a main repeating unit is effective. I found that.

[0006]

However, even the film made of the above polyester composition has insufficient impact resistance at a low temperature of 15 ° C. or less, and particularly has an insufficient impact resistance at a low temperature of 15 ° C. or less. It has been found that when a metal can to which this film is attached is dropped at a low temperature to give an impact, the film tends 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.

[0007] An object of the present invention is to provide a polyester film for laminating and forming a metal sheet which has improved impact resistance while maintaining the excellent properties of the polyester film and is less likely to be cracked by impact at low temperatures. To provide.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, the surface of the polyester film to be bonded to the metal plate has been subjected to corona discharge treatment in advance. The present inventors have found that the impact resistance at low temperatures is remarkably improved, and completed the present invention.

That is, in the present invention, 99 to 60% by weight of a copolymer polyester (I) having ethylene terephthalate having a melting point of 210 to 245 ° C. as a main repeating unit, and a melting point of 180.
A polyester composition comprising 1 to 40% by weight of a polyester (II) containing butylene terephthalate as a main repeating unit at a temperature of 223 ° C. to 223 ° C., and a corona discharge treatment applied to one surface thereof; It is a polyester film for molding.

In the present invention, the copolymerized polyester (I)
Is a copolymer polyester containing ethylene terephthalate as a main repeating unit, and 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 2,6-naphthalenedicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decane dicarboxylic acid, and cyclohexanedicarboxylic acid. Examples of the alicyclic dicarboxylic acid and the like, and examples of the copolymerized alcohol component include an aliphatic diol such as butanediol and hexanediol, and an alicyclic diol 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, but as a result, the melting point of the polymer is 210 to 245 ° C., preferably 215 to 240 ° C., and more preferably 220 to 240 ° C.
It is a ratio that falls within the range of 35 ° C. Polymer melting point 210
If the temperature is lower than ℃, the heat resistance is inferior, so that it cannot withstand the heating by printing after can making. On the other hand, when the melting point of the polymer exceeds 245 ° C., the crystallinity of the polymer is too large, and the moldability is impaired.

In the present invention, the polyester (II)
Is a polyester having butylene terephthalate as a main repeating unit, and may be a homopolymer or a copolymer. The copolymer component in the copolymer may be an acid component or an alcohol component. Examples of the copolymerization component include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, and decane dicarboxylic acid; and cyclohexanedicarboxylic acid. Examples of the alicyclic dicarboxylic acid and the like, and examples of the copolymerized alcohol component include an aliphatic diol such as ethylene glycol and hexanediol and an alicyclic diol 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 180 to 223 ° C., preferably 200 to 223 ° C., more preferably 210 to 223 ° C.
It is a ratio falling within the range of 23 ° C. Polymer melting point 180
If the temperature is lower than ℃, the heat resistance is inferior, so that it cannot withstand the heating by printing after can making. The melting point of the polybutylene terephthalate homopolymer is 223 ° C.

Here, the melting point of polyester is measured by Du.
Pont Instruments 910 DSC
And determining a melting peak at a heating rate of 20 ° C./min. The sample amount is about 20 mg.

The copolymerized polyester (I) in the present invention
And polyester (II) are not limited by their production methods. 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 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 the polyester, if necessary, other additives such as a lubricant, an antioxidant, a heat stabilizer, an ultraviolet absorber, and an antistatic agent can be added.

The polyester composition of the present invention is a composition comprising 99 to 60% by weight of the copolyester (I) and 1 to 40% by weight of the polyester (II). When the copolymerized polyester (I) is more than 99% by weight and the polyester (II) is less than 1% by weight, when the film is used for a can to which an internal pressure is applied, the film is easily broken by an impact from the outside of the can, and a can of excellent quality is obtained. Can not be obtained. When the copolymerized polyester (I) is less than 60% by weight and the polyester (II) is more than 40% by weight, the heat resistance of the film decreases and the impact resistance becomes insufficient.

In order to obtain good deep drawability, the film comprising the polyester composition has a plane orientation coefficient of 0.08 to 0.16, preferably 0.09 to 0.15,
More preferably, it is desirable to be 0.10 to 0.14.

Here, the plane orientation coefficient is defined by the following equation.

[0019]

F = [(nx × ny) / 2] −nz In the above equation, f: plane orientation coefficient, nx, ny, nz:
These are the refractive index in the horizontal, vertical and thickness directions of the film, respectively. The refractive index is measured as follows.

A polarizing plate analyzer is attached to the eyepiece side of the Abbe refractometer, and each refractive index is measured with a monochromatic NaD line. The mounting solution uses methylene iodide, and the measurement temperature is 25 ° C.

The polyester film of the present invention has a heat shrinkage at 150 ° C. of 10% or less in order to prevent defects such as wrinkling when the film is bonded to a metal plate.
It is preferably at most 7%, particularly preferably at most 6%.

Here, the heat shrinkage rate is determined by setting two points (at intervals of about 10 cm) on the sample film at room temperature,
Hold in a hot air circulating oven at 0 ° C. for 30 minutes, then return to room temperature, measure the distance between the mark points, find the difference before and after holding the temperature at 150 ° C., and compare this difference with the temperature before holding at 150 ° C. Calculated from the reference point interval. And it is represented by the heat shrinkage in the longitudinal direction of the film.

The plane orientation coefficient and the heat shrinkage ratio (150
In order to obtain a polyester film satisfying (C), for example, in sequential biaxial stretching, the longitudinal stretching ratio is from 2.5 to 3.6.
From the range of 2.7 to 3.6 times, the heat setting temperature is 150 to 220 ° C, preferably 160.
It is good to select from the range of ~ 200 ° C and combine them.

Further, in the polyester film of the present invention, a white pigment may be added for the purpose of omitting the undercoating of a white paint before printing.

In this case, the average particle size is 0.1 to 2.5 μm
Of the white pigment in the range of 3 to 50% by weight.

The white pigment may be inorganic or organic, but inorganic is preferred. 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 refractive index of 1.5 or more is preferable, and the average particle diameter of the pigment is preferably 0.1 μm or more and 2.5 μm or less. When the average particle size of the white pigment exceeds 2.5 μm, coarse particles (for example, particles of 10 μm or more) serve as a starting point in a portion deformed by processing such as a deep drawing can, and pinholes are generated, and in some cases, It is not preferable because it causes breakage.

Before incorporating such a white pigment into the (copolymerized) polyester, a particle size adjustment,
It is preferable to remove coarse particles. Industrial means of the refining process include jet mills as grinding means,
A ball mill and the like are mentioned, and as a classification means, for example, a dry or wet centrifugal separation method and the like are mentioned. Of course, these means may be used in combination of two or more, and may be purified stepwise.

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

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

In the case of a polyester film containing a white pigment, the degree of crystal orientation in the thickness direction of the film is from 0.2 to 0.2.
0.6, particularly preferably 0.25 to 0.55. When the degree of crystal orientation exceeds 0.6, the formability becomes insufficient, and the film tends to break 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. Further, for the same reason as described above, the heat shrinkage at 150 ° C. is preferably 10% or less.

The degree of crystal orientation is measured as follows.

Using a X-ray diffractometer, crystal orientation index <cos ² Φ _j , 100 in three directions (longitudinal direction MD, width direction TD, thickness direction ND) of the crystal plane (100) of the film.
>, And the degree of crystal orientation ^{fi, k} is determined from the following equation.

[0033]

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 an electric pole sample stand.

However, when the white pigment is titanium dioxide,
Since the reflection peak due to the titanium dioxide particles is close to the (copolymerized) polyester (100) in anatase (101) and rutile (110), α = 0 in the pole figure.
_Assuming that the (100) reflection peak of the (copolymerized) polyester is due to the reflection intensity of titanium oxide ( _ITio2 , α _{= 0} ), the intensity at all positions α and β up to α = 90 ° is I
_The degree of crystal orientation is calculated by subtracting _Tio2 , α _{= 0} .

Here, I _Tio2 , α _{= 0} = 1/2 (I _Tio2 ,
α _{= 0} , _MD + _ITio2 , 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 the thickness direction N
Expressed by the value of D.

In order to obtain a polyester film satisfying the above-mentioned degree of crystal orientation and heat shrinkage (150 ° C.), for example, in sequential biaxial stretching, a temperature of 20 to 40 ° C. higher than the glass transition temperature of the polyester is required to be 2.5 to 2.5 ° C. Stretched 3.6 times in the machine direction, then 2.7 to 3.6 times in the transverse direction,
What is necessary is just to heat-set at 150-220 degreeC.

In the polyester film of the present invention, a corona discharge treatment is applied to the surface of the above-mentioned various films to which the metal plate is attached.

For the corona discharge treatment, a conventionally known method such as a self-discharge method, a DC discharge method, and an AC discharge method may be employed. The degree of the corona discharge treatment is such that the contact angle of the treated surface of the film with distilled water is from θ-5 ° to θ-45 ° (where θ is the contact angle of the untreated film surface with distilled water), and particularly preferably θ-10 °. It is preferable that the treatment is performed so as to be? -45 degrees. The voltage for the discharge treatment is usually 1 to 30 KV, and it is not always necessary to carry out the treatment in a special gas atmosphere, but the treatment can be carried out in air. Further, the processing strength is suitably from 10 to 50 W · min / m ² . In addition,
The treatment intensity F (W · min / m ² ) is equal to the discharge electrode voltage V (V
lt), current A (Amp), electrode width W (m), and film running speed S (m / min), and have the following relationship.

F = V. A. W / S The contact angle was measured at 23 ° C. and 60% R using distilled water.
H is performed in the environment. After the corona discharge treatment, the contact angle changes with time, and the contact angle is a value measured one hour after the discharge treatment.

The attachment of the film to the metal plate does not necessarily have to be performed immediately after the corona discharge treatment, but may be performed after a long time (for example, several months) after the treatment. The contact angle of the film treated surface with distilled water changes over time,
No change with time is observed in the low-temperature impact resistance after being attached to the metal plate.

The polyester film of the present invention can be used as a multilayer film by laminating one or more other films on the side opposite to the surface subjected to the corona discharge treatment, if necessary.

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.

As a metal plate for cans to which the polyester film of the present invention is bonded, a plate of tin, tin-free steel, aluminum or the like is suitable. For bonding a polyester film to a metal plate, for example, heating the metal plate to a temperature equal to or higher than the melting point of the film, bonding the film, rapidly cooling the film, and contacting the metal plate with the surface layer (thin layer)
Can be performed by amorphizing and closely adhering.

[0045]

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

[0046]

EXAMPLES 1-5 AND COMPARATIVE EXAMPLES 1-5 Average particle size 0.2 μm
m copolymerized with the components shown in Table 1 and containing 0.4% by weight of titanium dioxide (copolyester (I), intrinsic viscosity 0.60), and polybutylene terephthalate (polyester (II)). , An intrinsic viscosity of 0.80) at a ratio shown in Table 1 to prepare a polyester composition.
And quenched and solidified to obtain an unstretched film.

Next, the unstretched film was subjected to longitudinal stretching, transverse stretching and heat setting under the conditions shown in the same table to obtain a film having a thickness of 25 μm.
m was obtained.

[0048]

[Table 1]

The obtained biaxially oriented film was subjected to a corona discharge treatment so as to have a treatment strength of 30 W · min / m ² , and was compared with an untreated film. Table 3 shows the surface orientation coefficient of the film, the dry heat shrinkage at 150 ° C., and the contact angle of the film surface with water.

[0050]

Examples 6 to 10 and Comparative Examples 6 to 10 Copolymerized polyethylene terephthalate (copolyester (I), intrinsic viscosity 0.62) obtained by copolymerizing the components shown in Table 2, and polybutylene terephthalate (polyester (II)) And an intrinsic viscosity of 1.10) in a proportion shown in Table 2 to prepare a polyester composition, to which 10% by weight of titanium oxide having an average particle size of 0.27 μm and a density of 39.8 g / cm ³ was added. Then, it was melt-extruded at 280 ° C. and solidified rapidly to obtain an unstretched film.

Next, the unstretched film was subjected to longitudinal stretching, transverse stretching and heat setting under the conditions shown in the same table to obtain a film having a thickness of 25 μm.
m was obtained.

[0052]

[Table 2]

The obtained biaxially oriented film was subjected to a corona discharge treatment so as to have a treatment intensity of 30 W · min / m ² , and was compared with an untreated film. The degree of crystal orientation in the thickness direction of the film, the dry heat shrinkage at 150 ° C., and the contact angle of the film surface with water were as shown in Table 3.

A total of 20 kinds of films obtained in Examples 1 to 10 and Comparative Examples 1 to 10 were applied to both sides of a 0.25 mm-thick tin-free steel heated to 230 ° C. on a corona discharge treated film. After laminating so as to be in contact with the steel surface of the treated surface, cooling with water, cutting into a disk shape of 150 mm diameter, deep drawing in four stages using a drawing die and a punch, and a sideless seamless container of 55 mm diameter ( Hereinafter, it is abbreviated as can).

The impact resistance of this can was evaluated according to the following criteria.

Impact resistance After cans were filled with water and cooled to 5 ° C., 1
After dropping 0 pieces from the height of 30 cm on the PVC tile floor, put 1% NaCl water in the can, insert the electrode, measure the current value when applying 6V voltage with the can body as the anode. I do.

：: 0.1 mA or less for all 10 pieces.

Δ: 0.1 mA or more for 1 to 5 pieces.

X: The value is 0.1 mA or more for 6 or more pieces, or cracks of the film have already been observed after dropping. The evaluation results were as shown in Table 3.

[0060]

[Table 3]

As is clear from the results shown in Table 3,
In the can using the polyester film of the present invention, the impact resistance at low temperatures was improved.

[0062]

The polyester film for laminating and processing metal sheets of the present invention has remarkably improved impact resistance at low temperatures, and can be used at a low temperature after cooling such as for juices and soft drinks. It is particularly suitable for use by bonding to many metal cans.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI // C08L 67/02 C08L 67/02 B29K 67:00 B29L 7:00 C08L 67:02 (56) References JP-A-3-3 73337 (JP, A) JP-A-2-305827 (JP, A) JP-A-3-86729 (JP, A) JP-A-64-9244 (JP, A) JP-A-59-191741 (JP, A) JP-A-63-57227 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08J 7/00 303 C08J 5/18 B32B 15/08 B29C 55/12

Claims (3)

(57) [Claims] 1. A copolymer (II) containing ethylene terephthalate having a melting point of 210 to 245 ° C. as a main repeating unit and 99 to 60% by weight of a copolymer polyester (I) having a melting point of 180 to 223 ° C. as a main repeating unit. 1) A polyester film for laminating and forming a metal plate, comprising a polyester composition containing 1 to 40% by weight, and having one surface subjected to a corona discharge treatment. 2. A plane orientation coefficient of 0.08 to 0.16, 15
The polyester film for metal plate lamination processing according to claim 1, wherein the heat shrinkage at 0 ° C is 10% or less. 3. A heat-shrinkable material containing 3 to 50% by weight of a white pigment having an average particle size of 0.1 to 2.5 μm, a crystal orientation degree in the thickness direction of the film of 0.2 to 0.6, and 150 ° C. Rate is 10%
The polyester film for laminating and processing a metal plate according to claim 1, which is: