JPH02182432A - Biaxially stretched polyester film for seal bag - Google Patents

Abstract

PURPOSE:To obtain a seal bag easy to tear by using a biaxially stretched polyester film having specific physical properties. CONSTITUTION:A polyester film to be used is a biaxially stretched film wherein polyethylene terephthalate occupies 80mol% or more and simultaneously satisfies formulae I-IV when theta is set to the angle formed by the main orientation direction within a film plane and the longitudinal direction of the film, DELTAn is set to the double refractive index within the film plane, nalpha is set to the refractive index of the film in the thickness direction, [eta] is set to the extreme viscosity of the film and n is set to the mean refractive index of the film. This film has strength sufficient from a practical aspect as a seal bag and excellent preservability and is broken at the time of opening but breakage is not propagated in an unexpected direction.

Description

[Detailed description of the invention] [Industrial application field] The present invention provides a biaxially stretched polyester film for seal bags,
More specifically, the present invention relates to a biaxially stretched polyester film for sealable bags that is easy to tear.

[Prior art and problems to be solved by the invention] Conventionally, two-wheel stretched polyester films have been used for various purposes due to their excellent mechanical properties, water resistance, chemical resistance, dimensional stability, transparency, etc. It is also widely used as a packaging material due to its low permeability to oxygen, fragrance, and oil, and its lack of additives harmful to food hygiene.

This biaxially stretched polyester film is often used as a seal bag (for example, an imine-based or urethane-based adhesive is applied to a two-wheel stretched film about 10-odd μm thick, and then polyethylene of about 30 μm thick is laminated. After that, the bag is heat-sealed using the polyethylene layer as a sealing surface.

When opening this sealed bag, a large number of scars are provided on the fused portion of the bag so that the bag can be opened from anywhere.

However, many of these types of sealed bags are difficult to tear, or even if they are torn, they often tear diagonally, causing problems such as the contents being scattered around.

[Means to solve the problem]

As a result of intensive studies in view of the above problems, the present inventors discovered that a polyester film having certain specific physical properties is useful for sealable bags, and completed the present invention.

That is, the gist of the present invention resides in a biaxially stretched polyester film for sealable bags, which is characterized in that it satisfies the following formulas [1] to [4] at the same time.

(90-θ)(Δnxlo'-30)≦200-...[
1] 1.483≦nα≦1.501 −・
・■0.45≦[η]≦0.64 ・・
・■1.602≦n≦1.608 ・
,, ■ (However, in the above formula, θ is the angle (0 to 90°) between the main orientation direction in the film plane and the longitudinal direction of the film, Δ
(n is the in-plane birefringence of the film, nα is the refractive index in the film thickness direction, [η] is the intrinsic viscosity of the film, and n is the average refractive index of the film).The present invention will be described in detail below.

The polyester film used in the present invention is 8
A biaxially stretched film containing 0 mol% or more of polyethylene terephthalate, the film composition including diethylene glycol, ethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol,
Diol components such as polyoxyethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, and pentaerythritol, dicarboxylic acids such as isophthalic acid, phthalic acid, terephthalic acid, adipic acid, and sebacic acid, and hydroxyl such as P-hydroxybenzoic acid. Carboxylic acids and the like may be copolymerized, or polyesters made of these diols, dicarboxylic acids, and oxycarboxylic acids may be added or compounded. Further, polymers such as nylon, polypropylene, polyethylene, polyethylene oxide, and propylene oxide copolymers, and modifiers such as non-aluminum fine particles may be added to the film composition.

Note that the surface of the film may be subjected to corona treatment, coating, printing, etc., if necessary.

The polyester film used in the present invention has the following formula: 0 Formula % Here, θ represents the angle (90° or less) between the main in-plane orientation direction (γ direction) of the film and the longitudinal direction of the film. Δn is the in-plane birefringence of the film and the main orientation direction (γ
It is expressed as the difference between the refractive index in a direction) and a direction perpendicular to the direction (β direction). A film with higher strength in the horizontal direction than the vertical direction generally has θ of about 90°, and a film with higher strength in the longitudinal direction than in the horizontal direction generally has θ of about 06°.

In a region where the formula 0 is not satisfied, it is not preferable because it is extremely difficult to tear, or it is easy to tear but it tears diagonally.

In addition, in the present invention, the refractive index nα in the thickness direction of the film is
is in the range of 1.483 to 1.501.

When nα months are less than 483, the film becomes extremely easy to tear and is unsuitable for packaging. On the other hand, nα is 1.501
A film exceeding 0 is not preferable because it becomes difficult to tear even if the film satisfies the formula 0.

Furthermore, in the present invention, the intrinsic viscosity [η] of the film is 0.
It is in the range of 45 to 0.64. When [η] is less than 0.45, it is easy to tear and is unsuitable for packaging; on the other hand, [η]
A film with a value exceeding 0.64 is unsuitable because it is difficult to tear even if it satisfies the 0 formula and nα.

Preferably 0.50 to 0.60, more preferably 0.5
It is in the range of 6 to 0.59.

In addition, in the present invention, the average refractive index n of the film is 1.6
It is in the range of 02 to 1.608. Films with n less than 1.602 are difficult to tear even if other conditions are satisfied, and films with n greater than 1.608 are unsuitable for packaging because of their low strength.

n is preferably 1.604 to 1.60? , more preferably in the range of 1.605 to 1.607.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof.

The method for evaluating the characteristics of the film of the present invention is as follows.

■ Angle θ between the main orientation direction in the film plane and the longitudinal direction of the film. Using a polarizing microscope, find the direction showing the maximum refractive index in the film plane, that is, the main orientation direction (T direction), and calculate the angle θ between the γ direction and the longitudinal direction of the film. The angle (less than 90°) made with θ is defined as θ.

■ Using a Na-D line with a refractometer with refractive index, birefringence Δn, and average refractive index nAbbe, measure the refractive index nT in the γ direction, the refractive index nβ in the direction orthogonal to the γ direction within the film plane, and the thickness direction. The refractive index nα was measured and determined according to the following formula.

Δn=n7−nβ n=1/3 (nα+Ωβ+nγ) ■ Intrinsic viscosity [η] Film Ig is phenol/tetrachloroethane-50
150 (weight ratio) in a mixed solvent of 100, 3
Measured at 0°C.

■ Tearability of the seal part After sealing into a bag shape, tear it by hand. If the tearability is good and the direction of tearing is parallel to the width direction, it is ○, if it is difficult to tear, it is △, if it tears but the direction of tearing is diagonal. Those that result in the following are marked as ×.

Examples 1 to 3 Silica 0.0 with an intrinsic viscosity of 0.60 and an average particle size of 1.5 μm
Polyethylene terephthalate containing 8 wt% was melt-extruded into a sheet through a die slit using an extruder and a T-die, and cooled with a casting drum to obtain an unstretched film.

The obtained unstretched film was first stretched 3.0 times in the machine direction at 107°C, further stretched 1.3 times in the machine direction at 98°C, and then stretched 3.9 times in the transverse direction at 110°C. Heat fixation was performed at 230°C. The films thus wound were sampled and evaluated for the central product (Example 1), the 5:h product (Example 3), and the intermediate portion (Example 2).

Example 4. Comparative Example 1, Comparative Example 2 Using the unstretched film obtained in Example 1, 8
A film was formed in the same manner as in Example 1, except that it was stretched 2.9 times at 5°C and further 1.35 times in the longitudinal direction at 78°C. The center product (Example 4), edge product (Comparative Example 2), and intermediate product (Comparative Example 1) of the obtained film were evaluated.

Comparative Example 3 A film was formed in the same manner as in Example 4, except that the longitudinal stretching ratio in the second stage was 1.4 times, and the central product was evaluated.

Examples 5 and 6.7 A film was formed in the same manner as in Example 4 except that the stretching ratio of the second stage was 1.28 times, and the central product (Example 5
), a scrap product (Example 7), and an intermediate product (Example 6) were evaluated.

Example 8. Comparative Examples 4 and 5 Using the unstretched film obtained in Example 1, 8
After stretching 3.0 times at 5°C and then 4.3 times in the transverse direction at 95°C, the center product (Example 8), the edge product (Comparative Example 5), and the intermediate product (Comparative Example 4) were evaluated. .

Comparative Example 6.7 Film viscosity of 0.68 (Comparative Example 6) and 0.40
A film was formed in the same manner as in Example 1 except that (Comparative Example 7) was used, and the central product was evaluated.

Comparative Examples 8 and 9 Films were formed and evaluated in the same manner as in Example 1, except that heat treatment was performed at 250°C (Comparative Example 8) and 200°C (Comparative Example 9).

Comparative Example 10 A film was formed in the same manner as in Example 1 except that the lateral stretching ratio was 3.2 times, and the central product was evaluated.

Comparative Example 11 A film was formed in the same manner as in Example 4 except that the lateral stretching ratio was 4.5 times, and the central product was evaluated.

The results obtained above are summarized in Table 1.

〔Effect of the invention〕

The film of the present invention is an excellent material that has sufficient strength for practical use for seal bags, has excellent preservability, and does not spread in unwanted directions when opened. The value is high.

Claims (1)

[Claims] (1) A biaxially stretched polyester film for seal bags, which satisfies the following formulas [1] to [4] at the same time. (90-θ) (Δn×10^3-30)≦200...
[1] 1.483≦nα≦1.501・・・・・・・・・・・・
[2] 0.45≦[η]≦0.64・・・・・・・・・・・・
[3] 1.602≦@n@≦1.608・・・・・・・・・・・・
[4] (However, in the above formula, θ is the angle (0 to 90°) between the main orientation direction in the film plane and the longitudinal direction of the film, Δn is the birefringence index in the film plane, and nα is the angle in the film thickness direction. Refractive index, [η] is the intrinsic viscosity of the film, @n@ is the average refractive index of the film)