JPH04105922A - Polyester film for use in metal bonding process - Google Patents

Abstract

PURPOSE:To improve the deep drawing ability of polyester film attached to a can at the time of its manufacture and the impact resistance and heat resistance thereof after the can manufacture by a method wherein the polyester film contains a lubricant having a specific average particle size, consists of a copolymer polyester having a melting point within a specific range and has a refraction factor within a specific range in the film thickness direction and in all the film surface directions. CONSTITUTION:As copolymer polyester, a copolymer polyethylene terephthalate is used and its copolymer components are mixed in such a proportion that the polymer melting point is brought within the range of 210-245 deg.C. If the polymer melting point is less than 210 deg.C, since the heat resisting performance is low, the polymer cannot withstand the heating in printing after manufacture of a can. If it exceeds 245 deg.C, the crystallinity of the polymer is so large that its formability is impaired. The copolymer polyester contains a lubricant having an average particle size of at most 2.5mum in diameter and has a refraction factor of 1.505 to 1.545 in the film thickness direction and 1.61 to 1.66 in all the film surface directions.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a polyester film for metal lamination molding, and more specifically to a polyester film that is excellent in forming when laminated with a metal plate and used in can manufacturing processes such as drawing. The present invention relates to a polyester film for metal lamination molding processing, which can be used to produce metal cans, such as beverage cans and food cans, which exhibits excellent heat resistance and aroma retention.

<Prior art and its problems> Metal cans are generally coated with paint to prevent corrosion on the inside and outside surfaces, but recently, for the purpose of simplifying the process, improving hygiene, and preventing pollution, it has become possible to paint cans without using organic solvents. The development of methods to obtain rust prevention is progressing, and one of the methods is to try coating with a thermoplastic resin film. That is, studies are underway on a method of laminating a thermoplastic resin film onto a metal plate such as tinplate, tin-free still, aluminum, etc., and then producing cans by drawing or the like. Polyolefin films and polyamide films have been tried as thermoplastic resin films, but they have poor moldability.

It does not satisfy both heat resistance and fragrance retention.

On the other hand, polyester films, particularly polyethylene derephthalate films, have attracted attention as having well-balanced properties, and several proposals have been made based on them. That is, (A) a biaxially oriented polyethylene terephthalate film is laminated to a metal plate via an adhesive layer of low melting point polyester and used as a V-can material (JP-A-56-10451, JP-A-1-1). No. 192546) 1° (B) A non-quality or extremely low crystallinity aromatic polyester film is laminated onto a metal plate and used as a can-making material (Japanese Patent Application Laid-open No. 1-192545).

JP-A No. 2-57339).

(C) A heat-set biaxially oriented polyethylene terephthalate film with low orientation is laminated onto a metal plate and used as a can-making material (Japanese Patent Laid-Open No. 6422530).

However, inspection by the present inventors revealed that neither of them had sufficient characteristics, and each had the following problems.

Regarding (A), the film made of biaxially oriented polyethylene terephthalate 1 has excellent heat resistance and aroma retention, but its moldability is insufficient, and when the can manufacturing process involves large deformation, the film becomes white (microcracks occur). ), rupture occurs.

Regarding (B), since it is an aromatic polyester film with poor quality or extremely low crystallinity, it has good molding processability, but it has poor fragrance retention, and it is also subjected to printing after can manufacturing, and sterilization treatment. There is a risk that the film will become brittle due to post-processing, and may deteriorate into a film that is easily broken by impact from the outside of the can.

(C) is intended to be effective in the intermediate region between (A> and (B)), but since the isotropy of the film surface is not guaranteed, deformation in all directions is required as in can manufacturing. If this is done, the film may have insufficient moldability in certain directions.

<Means for Solving the Problems> The present inventors further conducted intensive research to develop a polyester film for can manufacturing processing that does not have these problems, and as a result, they arrived at the present invention.

That is, the present invention comprises a copolyester containing a lubricant with an average particle size of 2.5 μm or less, a melting point of 210 to 245°C, and a refractive index in the film thickness direction of 1.505 to 1.
．． 545 and a refractive index of 1.61 to 1.66 in all directions.

A representative example of the copolymerized polyester in the present invention is copolymerized polyethylene terephthalate. This copolymerization component may be an acid component or an alcohol component. The acid component includes aromatic dibasic acids such as isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, and cyclohexanedicarboxylic acid. Examples include alicyclic dicarboxylic acids, and examples of the alcohol component include aliphatic diols such as butanediol and hexanediol, and pre-alicyclic diols such as cyclohegysanedimethanol.

These can be used alone or in combination of two or more.

The proportion of copolymerized components depends on the type, but as a result, the polymer melting point is 210-245°C, preferably 215-245°C.
The temperature is 40°C, more preferably 220-235°C. If the polymer melting point is less than 210'C, the heat resistance will be poor and it will not be able to withstand the heating that occurs during printing after can manufacturing.

On the other hand, if the polymer melting point exceeds 245°C, the crystallinity of the polymer will be too large and moldability will be impaired.

Here, the melting point of the polyester is measured using a DSC-3SC1580 manufactured by Seiko Electronics Inc. and a method of determining the melting peak at a heating rate of 20° C./min.

The copolymerized polyester I'J in the present invention has an average particle size of 2
, 511m or less of lubricant. This lubricant may be inorganic or organic, but inorganic lubricants are preferred. Inorganic lubricants include silica and alumina.

Examples include titanium oxide, calcium carbonate, barium sulfate, etc., and examples of organic lubricants include silicone particles. Both require an average particle size of 2.5 μm or less. If the average particle size of the lubricant exceeds 2.5 μm, coarse lubricant particles (
For example, particles of 10 μm or more) act as starting points, causing pinholes or even breakage, which is not preferable.

In particular, preferred lubricants from the point of view of pinhole resistance are those with an average particle size of 2
．． 5 μm or less, and the particle size ratio (major axis/minor axis) is 1.
．． It is a monodisperse lubricant having a molecular weight of 0 to 1.2.

Examples of such lubricants include spherical silica, spherical titanium oxide, spherical zirconium, and spherical silicone particles.

Here, the average particle size and particle size ratio of the spherical monodisperse lubricant can be determined by first depositing metal on the particle surface and then using an electron microscope to obtain an image magnified from 10,000 to 30,000 x 8. It is calculated by finding the area circle equivalent diameter and then applying these to the following formula.

Average particle diameter - Sum of area circle equivalent diameters of the measured particles / Number of particle diameter ratio of the measured particles - Average major axis of the particles / Average minor axis of the particles Further, it is preferable that the spherical lubricant particles have a sharp particle size distribution,
The relative standard deviation representing the steepness of the distribution is preferably 0.5 or less, more preferably 0.3 or less.

This relative standard deviation is expressed by the following formula.

Here, D: Area circle equivalent diameter of each particle (μ7y1): Average value of the area circle equivalent diameter Σ D (= i =1) (l1m)n: Represents the number of particles.

The strength of the lubricant in the copolyester may be determined by the winding properties in the film manufacturing process.

Generally, it is preferable to add a small amount of large particles and a large amount of small particles. For example, in the case of silica having an average particle size of 2.0 μm, it is preferable to add about 0.05% by weight, and for titanium dioxide having an average particle size of 0.3 μm, it is preferable to add about 0.3% by weight. It is also possible to make the film opaque by intentionally adjusting the lubricant content. For example, 1 titanium dioxide
By adding 0 to 15% by weight, a white film can be obtained.

The copolymerized polyester in the present invention is not limited by its manufacturing method. For example, by esterifying terephthalic acid, ethylene glycol, and a copolymer component,
A method in which the resulting reaction product is then subjected to a polycondensation reaction to obtain a copolymerized polyester, or dimethyl terephthalate,
Preferably used is a method in which ethylene glycol and a copolymer component are subjected to a transesterification reaction, and then the resulting reaction product is subjected to a polycondensation reaction to obtain a copolymerized polyester.

In the production of the copolyester, other additives such as antioxidants, heat stabilizers, external radiation absorbers, antistatic agents, etc. may be added as necessary.

The polyester film of the present invention is made by melting the lubricant-containing copolyester described above, extruding it from a die, forming it into a film, biaxially stretching it, and heat-setting it.

This film must satisfy the following requirements (1) and (2).

(1) The refractive index in the thickness direction of the film is 1,505
More than 1.545, preferably more than 1.510 and less than 1
,540 or less, more preferably over 1.510
1.530 or less. If this refractive index is less than 1.505, moldability will be insufficient, while if it exceeds 1.545 (i.e., a platform with excessively low orientation), the structure will be close to amorphous, resulting in poor heat resistance. becomes insufficient.

(2) The refractive index in the film plane direction is 1 in all directions.
．． 61 or more and 1.66 or less, preferably 1,615 or more and 1
．． It is within the range of 655 or less. In deep drawing and drawing-sealing processes that are frequently used in can manufacturing, deformation must be uniform in all directions, and it must be possible to follow this deformation no matter what part of the film is used. Formability is good in directions where the film plane refractive index is less than 1.61, but heat resistance is poor, and formability is poor in directions where the -octatal refractive index exceeds 1.66, which makes it difficult to perform deep drawing. Whitening and breakage of film occur 4'? lru.

Note that the refractive index in the film thickness direction and in the plane direction is measured as follows.

Attach a polarized root analyzer to the eyepiece side of the refractive index and measure each refractive index using a monochromatic NaD line.

Maun] - The solution used was medilene iodide, and the measurement temperature was 25
It is ℃.

In order to obtain such a refractive index in the thickness direction and a refractive index in the film plane direction, in biaxial stretching, particularly in sequential biaxial stretching, the longitudinal stretching ratio is 2.5 to 366 times, and the transverse stretching ratio is 2.7 to 3. ．．
Stretching heat treatment is preferably carried out at a heat setting temperature of 150 to 230°C. More preferably, among these conditions,
The refractive index in the thickness direction is 1,505 or more and 1.545 or less, the refractive index distribution on the film surface is 1.61 or more and 1.66 or less, and the film density is less than 1.385!7/cJ, furthermore, 1, It is preferable to find a condition of 380 to 1,384 g/crd and perform the biaxial stretching heat setting treatment.

The polyester film of the present invention preferably has a thickness of 6
~75 μm. Furthermore, 10 to 75 μm1 especially 15 to
Preferably, it is 50 μm. If the thickness is less than 6 .mu.m, breakage is likely to occur during processing, while if it exceeds 75 .mu.m, it is of excessive quality and is uneconomical.

As the metal plate for can manufacturing to which the polyester film of the present invention is laminated, plates of tinplate, tin-free steel, aluminum, etc. are suitable. The polyester film can be laminated to the metal plate by, for example, the following methods (1) and (2).

■ The metal plate is heated above the melting point of the film, the film is laminated, and then rapidly cooled to amorphize the surface layer (thin layer) of the film that comes into contact with the metal plate and make them adhere tightly.

■ Primer-coat the film with an adhesive layer in advance, and laminate this surface to the metal plate. As the adhesive layer, known resin adhesives such as poxy adhesive, epoxy ester adhesive, algide adhesive, etc. can be used.

<Example> The present invention will be further explained below with reference to Examples.

Examples 1 to 4 and Comparative Examples 1 to 3 Spherical monodisperse silica with an average particle size of 1.5 μm or 2.0 μm (particle size ratio 1.07, relative standard deviation 0.1) was added and contained, and Copolymerized polyethylene terephthalate (intrinsic viscosity: 0.60) obtained by copolymerizing the components shown in Table 1 was melt-extruded at the temperature shown in the same table, and solidified by rapid cooling to obtain an unstretched film.

Next, this unstretched film was longitudinally stretched and transversely stretched under the conditions shown in the same table, followed by heat setting treatment to obtain an axially oriented film.

The properties of this film are shown in Table 4.

Example 5 and Comparative Example 4 Polyethylene terephthalate copolymerized with 12 mol % of isophthalic acid (intrinsic viscosity: 0.5%) containing the lubricant shown in Table 2.
60) was melt-extruded at 280°C, rapidly cooled and solidified to obtain an unstretched film, and then the unstretched film was longitudinally stretched at a temperature of 100°C and a longitudinal stretching ratio of 3.0 times.

The film was successively biaxially stretched at a transverse stretching temperature of 110° C. and a transverse stretching ratio of 3.1 times, and then heat-set at the temperature shown in the same table.

Table 4 shows the properties of the obtained biaxially oriented film.

Comparative Examples 5 to 8 Spherical monodispersed silica with an average particle size of 2.0μ (particle size ratio 1.0
7, relative standard deviation 0.1) 12 mol% isophthalic acid copolymerized polyethylene prephthalate (melting point 229°C, intrinsic viscosity 0.60) containing 0.05% by weight (relative standard deviation 0.1)
was melt-extruded at 280°C and rapidly solidified to obtain an unstretched film. Next, this unstretched film was longitudinally stretched and transversely stretched under the conditions shown in Table 3, and then heat-set to obtain a biaxially oriented film.

The properties of this film are shown in Table 4.

Table 2 Table 3 Above Examples 1-5. A total of 13 types of films obtained in Comparative Examples 1 to 8 were laminated on both sides of a 0.25 mm thick tin-free steel plate heated to 260°G, and after cooling with water,
It was cut into a disk shape with a diameter of 0 mm and deep drawn in two stages using a drawing die and a punch to create a container with seamless sides (hereinafter referred to as a can) with a diameter of 55 mm.

The following observations and tests were performed on this container, and each was evaluated based on the following criteria.

(1) Deep drawing workability-1 0: The film is processed without any abnormality on both the inner and outer surfaces of the can, and no whitening or breakage is observed in the film on the inner and outer surfaces of the can.

Δ: Whitening is observed on the top of the film on the inside and outside of the can.

×: Film breakage is observed in part of the film on the inside and outside of the can.

(2) Deep drawing workability-2 0: Both the inner and outer surfaces were processed without any abnormalities, and the rust prevention test on the film surface inside the can (1% NaCR water was put into the can, an electrode was inserted, and the can body was used as an anode. Measure the current value when the voltage of 6■ is applied.

It shows 0.2IIIA or less in the ERV test (hereinafter abbreviated as ERV test).

×: There is no abnormality in the film on both the inner and outer surfaces, but the current value was 0.2 mA or more in the ERV test, and when the energized area was observed under magnification, pinhole-shaped cracks originating from the coarse lubricant were observed in the film.

(3) For cans with good impact cracking resistance and deep drawing, fill them with water and
After dropping 10 pieces at a time from a height of 1 TrL onto the PVC tile floor surface (1=), an ERV test was conducted inside the can, and the results were as follows: ○: 0.2 mA or more for all 10 pieces.

Δ: 0.2 mA or more for 1 to 5 pieces.

×: 0.2 mA or more for 6 or more pieces, or cracks in the film were observed in some pieces after being dropped.

(4) Heat embrittlement resistance Cans with good deep drawing were heated and held at 210°C for 5 minutes, and then the impact cracking resistance was evaluated as described in (3). ○: For all 10 cans. It was 0.2111A or less.

Δ: 0.2 mA or more for 1 to 5 pieces.

×: 0.2IIIA or more for 6 or more pieces, and cracks in the film were already observed after heating at fc or 210° C. for 5 minutes.

Table 4 shows the evaluation results for the above four types.

=19 From the results in Table 4, it can be seen that the films of Examples are excellent in all of the deep drawability, impact cracking resistance, and heat resistance.

<Effects of the Invention> The polyester film for metal lamination molding processing of the present invention has good deep drawability and durability after lamination with a metal plate when forming metal cans by performing can making processing, for example, deep drawing processing. It has excellent impact resistance and heat resistance, and is extremely useful for metal containers.

Patent applicant Teijin Kaisha Ltd.

Claims (1)

[Claims] Contains a lubricant with an average particle size of 2.5 μm or less and has a melting point of 210
It is made of copolymerized polyester at ~245°C, and has a refractive index in the film thickness direction of 1.505 to 1.545, and a refractive index in the film surface direction of 1.61 to 1.5 in all directions.
66. A polyester film for metal bonding and molding processing, characterized by having a molecular weight of 66.