JP2003311827A - Biaxially stretched polyester film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially stretched polyester film keeping practical characteristics without losing heat resistance, aroma retention, water resistance or the like being excellent characteristics of a polyester stretched film, having good twist fixing properties and useful as a twist packaging film. <P>SOLUTION: The biaxially stretched polyester film is characterized in that residual strain is not less than 3.3% when stress in a tension relaxing process becomes zero in a 8% stretching hysteresis curve and a heat shrinkage factor in the longitudinal direction of the film at 150°C is not more than 5.0%. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a polyester film for twist packaging. More specifically, it is useful as a twist-wrapping film having good twist-fixing properties while maintaining the practical properties without losing the excellent properties of the stretched polyester film such as heat resistance, aroma retention, and water resistance. It relates to a stretched polyester film.

[0002]

2. Description of the Related Art Cellophane has been known as a film having excellent twistability. Cellophane is widely used for various packaging materials and adhesive tapes due to its excellent transparency, easy cutting property, and twisting property. But,
On the other hand, since cellophane has hygroscopicity, its characteristics fluctuate depending on the season, and it is difficult to always supply a certain quality. Further, a packaging bag using polyethylene terephthalate as a base film is used after being bought for its excellent properties such as toughness, heat resistance, water resistance and transparency of a stretched polyethylene terephthalate phthalate film. While having these excellent properties, it is difficult to cut, there are drawbacks that the mouth of the packaging bag is difficult to tear, the drawback that the adhesive tape is difficult to cut, and the twistability is inferior and it cannot be used for twist packaging. there were.

The solid packaging film found in twist packaging must be one that can be easily handled by the packaging machine, for example, 20 minutes per minute on one machine.
It must be able to pack 0 to 1000 solids. That is, generally, the area required to be wrapped must be cut out before the article is twist-wrapped, and the film must be perfectly flat before and after packaging. If the film after printing or vapor deposition has distortions due to wrinkles or dimensional changes, problems such as distortion during cutting or packaging and poor appearance of the packaged product occur.

Further, the film preferably has antistatic properties. A film having no antistatic property may cause troubles because it sticks to a machine or becomes slippery.

As a method for solving the above-mentioned drawbacks, a method in which a diethylene glycol component or the like is copolymerized (Japanese Patent Publication No. Sho 56)
-50692) or one using a low molecular weight polyester resin (Japanese Patent Publication No. 55-20514), or having a yield point in the stress-strain curve and having an average refractive index N _{0 of} the unstretched film of the copolymer, When the average refractive index of the biaxially stretched film is N ₁ , 0.003 ≦ N ₁ −N ₀ ≦ 0.0
An easily bendable polyester film (No. 2505474) characterized by satisfying No. 21 has been proposed.

[0006]

However, the method of copolymerizing a large amount of diethylene glycol component and the like in the above-mentioned prior art has a drawback that the original characteristics of polyethylene terephthalate are lost by the copolymerization. Furthermore, the method of using a low molecular weight polyester resin is not practical because it easily causes film breakage problems in the stretching step. When the stress-strain curve has a yield point and the average refractive index of the unstretched film of the copolymer is N ₀ and the average refractive index of the biaxially stretched film is N ₁ , 0.003 ≦ N _With the method of _1- N ₀ ≦ 0.021, the twisting property is insufficient, and wrinkles and dimensional changes of the film in the width direction occur due to shrinkage due to heat during processing such as printing and vapor deposition. , Could not withstand use.

[0007]

The present invention pays particular attention to the twistability among the characteristics of cellophane, and has these characteristics, and further, the excellent characteristics of the polyester film are heat resistance, moisture resistance, and transparency. It was achieved by conducting research for the purpose of obtaining a film having both aroma-retaining properties and the like.

That is, the biaxially stretched polyester film of the present invention has a residual strain of 3.3% or more when the stress becomes 0 in the tension relaxation process in the hysteresis curve of 8% elongation, and The thermal shrinkage in the longitudinal direction at 150 ° C. is 5.0% or less.

According to the present invention, in the hysteresis curve,
The residual strain when the stress becomes 0 is a certain amount or more, so that when the film is stretched, the film does not elastically recover, and the shape can be stably retained in the stretched state. It is due to the finding of.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The polyester film according to the present invention must have a residual strain of 3.3% or more when the stress becomes 0 in the hysteresis curve of 8% elongation.

The hysteresis curve of 8% elongation in the present invention means that both ends of the material are gripped, a tensile strain of up to 8% is given at a constant speed, and then the strain is relaxed. , A curve drawn with the stress on the vertical axis.

In the present invention, the residual strain when the stress becomes 0 means that the stress becomes 0 when the tensile strain returns to its original value.
Is the residual strain when

In the present invention, the residual strain is 3.3% or more, more preferably 3.6% or more. If the residual strain is less than 3.3%, the twistability deteriorates.

According to the present invention, 30 at 30 ° C.
The heat shrinkage ratio when left to stand for 5.0 minutes or less is 3.0
% Or less is more preferable. If the heat shrinkage rate is more than 5.0% when left at 150 ° C for 30 minutes, wrinkles or irregularities in flatness may occur during post-processing such as printing or forming a vapor-deposition layer on the film. At the time of
This is not preferable because it causes mechanical troubles and the appearance of the packaged product deteriorates.

In the present invention, it is preferable that the yield point is between 3% and less than 5%. By stretching the film beyond the yield point, it is possible to obtain a film having a twist-fixing property which hinders elastic recovery of the film.

In the present invention, in order to prevent the contents from adhering to the film or clinging to the processing machine during processing, it is preferable to have an antistatic property. That is,
The surface resistivity of the polyester film of the present invention is 1 ×
10 ¹⁵ (Ω / □) or less, more preferably 1 × 1
It is 0 ¹² (Ω / □) or less. Surface resistivity is 1 × 10
^If it is higher than ¹⁵ (Ω / □), clinging to the processing machine becomes severe, which is not preferable.

As a method of imparting the antistatic property, a method of kneading an antistatic agent into a film, a method of coating the film with the antistatic agent, or the like is used.

The antistatic agent is not particularly limited as long as it reduces the surface specific resistance. For example, glycerin fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, alkyldiethanolamine, hydroxyalkylmonoethanolamine, poly Examples thereof include oxyethylene alkyl amine, alkyl diethanol amide, alkyl sulfonate, alkyl phosphate, tetraalkyl ammonium salt, alkyl betaine and alkyl imidazolium betaine.

The polyester film usable in the present invention can be formed by a known film forming method. As the film forming method, a method of performing a biaxial stretching method such as a simultaneous biaxial stretching method or a sequential biaxial stretching method, and then performing heat setting treatment can be used. For example, as a sequential biaxial stretching method, in addition to a method of sequentially performing longitudinal stretching and transverse stretching or lateral stretching and longitudinal stretching,
A stretching method such as a transverse-longitudinal-longitudinal stretching method, a longitudinal-lateral-longitudinal stretching method, or a longitudinal-longitudinal-lateral stretching method can be adopted. The simultaneous biaxial stretching method may be a conventional simultaneous biaxial stretching method.
In addition, you may carry out simultaneous biaxial stretching in multiple steps. Also,
In order to further reduce the heat shrinkage rate, a longitudinal relaxation treatment, a lateral relaxation treatment, etc. may be performed, if necessary.

Next, an example of a method for producing the film of the present invention will be described, but this is merely a specific example and does not restrict the content of the present invention.

First, a predetermined polyester composition is fed to an extruder, melt-extruded at a temperature equal to or higher than the melting point, extruded from a die and cooled and solidified to form an unstretched laminated film.

The unstretched film thus obtained is stretched at a temperature not lower than the glass transition temperature of the polyester resin or copolymer having the highest glass transition temperature among the constituents of the composition. The stretching ratio is 6 to 30 times, preferably 9 to 16 times in terms of stretching area.

The stretched film is heat treated. In this heat treatment, a relaxation treatment may be performed if necessary. Moreover, by performing these heat treatment and relaxation treatment,
A film having a heat shrinkage ratio in the longitudinal direction at 0 ° C. of 5.0% or less is obtained.

[0024]

In the hysteresis curve of 8% elongation, the one having rubber elasticity has a smaller residual strain when the stress becomes 0, and the one having twistability and bending fixability has a larger residual strain. At this time, by obtaining a biaxially stretched polyester film having a residual strain of 3.3% or more, when twisted, the twisted portion exists with residual elongation and maintains its shape stably. As a result, the desired twistability of the present invention can be obtained.

Further, the heat shrinkage ratio at 150 ° C. for 30 minutes is 5%.
By the following, the heat resistance which is the object of the present invention can be obtained without the occurrence of wrinkles and the deterioration of flatness during printing or vapor deposition.

[0026]

The present invention will be described with reference to the following examples. The evaluation methods in Examples and Comparative Examples are (a) to
It carried out by the method of (d).

(A) Residual strain In accordance with JIS-C2318, a hysteresis curve in the longitudinal direction of the film was prepared, and the residual strain when the stress became 0 was determined. At this time, the tensile speed of the test piece is 50 mm / min
And

(B) Twistability A sensory test called "twist return angle" was conducted. First, a 100 mm square sample is cut out and wound in a longitudinal direction so as to protrude by 50 mm on a round bar having a diameter of 20 mm. Then, the protruding portion was twisted by 360 °, and the angle returned from 360 ° was measured. These measurements were performed 100 times, the average value was obtained, and the following three-stage evaluation was performed. ◯: Twist return angle is 75 ° or less Δ: Twist return angle is 75 to 85 ° ×: Twist return angle is 85 ° or more By the way, the twist return angle measured by the same method with cellophane currently used for twist packaging is 70. It was °.

(C) Heat shrinkage ratio A film having a width of 10 mm and a length of 250 mm was cut out in the longitudinal direction of the film and marked at intervals of 200 mm to give 5 g.
Measure the interval A with a constant tension of. Then, it was left in an oven in an atmosphere of 150 ° C. for 30 minutes without load. Remove from the oven and cool to room temperature with a constant tension of 5g.
Was calculated, and the heat shrinkage ratio was calculated by the following formula. Thermal contraction rate = (A−B) / A × 100 (%)

(D) Heat resistance The flatness after vapor deposition processing was evaluated. The film is sent to a vacuum deposition device and the inside of the chamber is 1 × 10 ⁻⁵ Torr.
Maintaining the pressure of 70% by weight of SiO ₂ and 30% of Al ₂ O ₃
The mixed oxide (wt%) was evaporated by heating with an electron beam of 15 kw to form a colorless transparent inorganic vapor-deposited layer having a thickness of 200 Å on the adhesion-modified layer. The film after vapor deposition was unwound from a roll, the flatness of the film was visually observed, and the following three-stage evaluation was performed. ◯: The flatness is good even when no tension is applied. Δ: Flatness is good when a little tension is applied to the film,
There is no problem in practical use. X: There is a problem in practical use because the plane is disturbed even when tension is applied.

Example 1 42% by weight of polyethylene terephthalate (A), 100 m of terephthalic acid as an acid component
ol%, ethylene glycol 70 as the glycol component
A polyester composition composed of 38% by weight of a copolyester (B) composed of 30% by mol of neopentyl glycol and 20% by weight of a polybutylene terephthalate (C) was melt extruded at 285 ° C. and rapidly cooled by a cooling drum at 30 ° C. To obtain an unstretched sheet.

The unstretched film is first stretched in the machine direction at 90 ° C.
3.7 times and then 110 ° C in the transverse direction in the tenter
And draw it to 4.0 times, and then relax it by 3% and 210
A heat treatment was performed at a temperature of ° C to obtain a film of 20 µm.

The film thus obtained had a residual strain of 4.4% and a heat shrinkage of 3.0%.

The twist return angle of the film was 65 °, and good twistability could be obtained. Further, the flatness after the vapor deposition process was also good.

(Example 2) Polyester (A) / (B)
A film having a thickness of 20 μm was obtained by the same raw material and method as in Comparative Example 1 except that the compounding ratio of / (C) was 65/25/10 wt% and the heat setting temperature was 220 ° C. The residual strain of this film was 3.4%, and the heat shrinkage rate was 3.8%, which was in agreement.
The twist return angle of the film is as good as 71 °,
The flatness after vapor deposition was also good.

Comparative Example 1 Polyester (A) / (B)
A film having a thickness of 20 μm was obtained by the same raw material and method as in Example 1 except that the mixing ratio of / (C) was 67/23/10 wt% and the heat setting temperature was 200 ° C. The residual strain of this film was 3.1, and the thermal shrinkage was 4.5%. The twist return angle of the film was 92 °, and the twistability was inferior.

Comparative Example 2 Polyester (A) / (B)
A film having a thickness of 20 μm was obtained by the same raw material and method as in Example 1 except that the compounding ratio of / (C) was changed to 82/8/10% by weight. The residual strain of this film is 3.2%,
The heat shrinkage ratio was 3.2%. The twist return angle of the film was 85 °, and the twistability was inferior.

(Comparative Examples 3 and 4) Polyester (A) /
A film having a thickness of 20 μm was obtained by the same raw material and method as in Example 1 except that the mixing ratio of (B) / (C) was 37/53/10 wt% and the heat setting temperature was 200 ° C. and 180 ° C., respectively. It was The residual strain of this film is 4.2, respectively.
% And 4.4%. These films had good twistability, but had poor heat shrinkage and poor flatness after vapor deposition.

(Example 3) Polyester (A) / (B)
A film having a thickness of 20 μm was obtained by the same raw material and method as in Example 1 except that the compounding ratio of / (C) was 67/23/10% by weight and the heat setting temperature was 220 ° C. The residual strain of this film is 3.6%, and the thermal shrinkage is 2.0.
%Met. The twist return angle of the film was 70 °, and the flatness after vapor deposition was also good.

Example 4 A block comprising polyethylene isophthalate (C) having a melting point of 210 ° C., 100 mol% of terephthalic acid as a dicarboxylic acid component, 85 mol% of butanediol as a diol component, and 15 mol% of polytetramethylene glycol having a molecular weight of 1000. A polyester resin layer (A) made of a mixture of the copolyester resin (D) at a ratio of 95/5 wt% and a melting point of 260.
The polyester resin layer (B) composed of polyethylene terephthalate at ℃ was melted by a separate extruder at a temperature of 285 ° C., and the melts were joined by a composite adapter and then extruded from a T die (B / A / B). = 15% /
70% / 15%) of the three-layer unstretched laminated film,
Stretching and heat setting were carried out in the same manner as in Example 1 to obtain a 20 μm film. The residual strain of this film was 3.4%, and the thermal shrinkage was 3.2%. The twist return angle of the film was 75 °, and the flatness after vapor deposition was also good.

Comparative Example 5 A 20 μm film was obtained in the same manner as in Example 1 except that polyethylene terephthalate was used as the polyester composition. The residual strain of the film is 2.7%, and the twist return angle is 200 °.
That's it, and the twist came off immediately.

(Comparative Example 6) As a polyester composition,
Example 1 except that a polyester copolymer using 87.5 mol% of terephthalic acid, 12.5 mol% of isophthalic acid as an acid component, and 100 mol% of ethylene glycol as a glycol component was used, and the heat setting temperature was 180 ° C. A 20 μm film was obtained in the same manner as in (1). The residual strain of the film is 2.9%, and the twist return angle is 120.
It was inferior to °. Further, the heat shrinkage was 9.0%, and the film after the vapor deposition process had irregularities, which made it difficult to use.

The evaluation results of the films obtained in Examples and Comparative Examples are shown in Tables 1 and 2.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

As described above, a polyester film having good twisting property and heat resistance can be obtained, and it is understood that it is effective for twisting packaging.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C08L 67:00 C08L 67:00 F term (reference) 3E086 AC07 AC22 AC34 AD15 BA02 BA15 BB02 BB21 BB41 BB51 BB90 DA08 4F071 AA44 AH04 BA01 BB06 BB08 BC01 4F210 AA24 AE03 AG01 QC05 QC06 QG01 QG18

Claims (4)

[Claims] 1. A hysteresis curve of 8% extension,
The residual strain when the stress becomes 0 in the tension relaxation process is 3.3% or more, and the thermal shrinkage in the longitudinal direction at 150 ° C. is 5.0% or less. Biaxially stretched polyester film. 2. The biaxially stretched polyester film according to claim 1, wherein the biaxially stretched polyester film has a yield point between an elongation of 3% and less than 5%. 3. The biaxially stretched polyester film according to claim 1 or 2, wherein the surface resistivity of at least one surface is 1 × 10 ¹⁵ (Ω / □) or less. the film. 4. The biaxially stretched polyester film according to claim 1, wherein the biaxially stretched polyester film is used for twist packaging.