JP6515522B2 - Polyester film excellent in folding retention, low shrinkage and transparency - Google Patents

Description

  The present invention relates to a polyester film having excellent folding retention that can be used as wrapping paper, handbags, origami, etc., and relates to a polyester film having excellent water resistance, low shrinkage under high temperature environment, and transparency.

  Paper is widely used in various wrapping papers, handbags, origami, etc. because of its excellent folding and holding properties. However, paper is inferior in water resistance, and when wet due to rain or the like, tears may occur or printing may be discolored. In addition, since the paper has no transparency, there is a problem that the contents can not be seen when used for packaging such as bags. Therefore, plastic films have been considered as alternatives to paper.

  Transparent cellophane has been used in the past as a film excellent in folding retention. However, since cellophane has hygroscopicity, its characteristics vary with seasons, it is difficult to supply it while maintaining the product quality constant, and its poor processability due to uneven thickness is considered as a drawback. It has

  On the other hand, polyethylene terephthalate film has excellent properties such as toughness, water resistance, transparency and the like, but has a disadvantage that the folding retention is inferior.

  As a method of eliminating such a defect, a polyethylene terephthalate film capable of maintaining good folding retention by lowering the density of the film is disclosed (see, for example, Patent Document 1).

  However, in the polyethylene terephthalate film of Patent Document 1, it is mentioned as a problem that the heat shrinkability is large. It is pointed out that when a bag or the like using such a film is left in a car in summer or in a warehouse without temperature control, the film shrinks and deforms and becomes unusable. Moreover, in the process process which requires high temperature, such as the printing process to a film, there also existed a problem which can not be processed by shrinkage of a film.

Patent No. 4308662 (Claim 1 etc.)

  The present invention aims to solve the problems of the prior art. That is, it is an object of the present invention to provide a polyester film having excellent folding and holding property, having extremely small heat shrinkability in a high temperature environment, and having excellent transparency.

The present invention is configured as follows.
1. A polyester film comprising an amorphous polyester containing an ethylene terephthalate unit and satisfying the following requirements (1) to (3).
(1) With respect to the stress at 40% elongation in each longitudinal direction and width direction of the film, the average value of the stress at 40% elongation in the longitudinal direction and the stress at 40% elongation in the width direction is 40 MPa to 110 MPa (2 The hot-shrinkage percentage in the longitudinal direction and the width direction in the case of treatment in warm water at 80 ° C. for 10 seconds is -10% or more and 10% or less (3) haze is 1% or more and 15% or less. The polyester film as described in the first above, characterized in that the folding and holding angle is 20 degrees or more and 50 degrees or less.
3. The polyester film as described in the first or second above, wherein the melting start temperature in the DSC temperature rising profile is 100 ° C. or more and 180 ° C. or less.
4. The polyester film according to any one of the above first to third, wherein the tensile fracture strength in the longitudinal direction and / or the width direction is 100 MPa or more and 300 MPa or less.
5. A method for continuously producing any of the first to fourth polyester films, wherein the melt-extruded, cooled and solidified unstretched sheet is stretched in the longitudinal direction and / or the width direction, and then the melting start of the polyester film is started. A method for producing a polyester film, comprising heat setting (hereinafter also referred to as heat treatment) at a temperature of not less than 240 ° C. but not less than the temperature.
(Here, the above third and fifth melting start temperatures are measured from the finished film, but neither the data at the unstretched stage nor the data differ significantly.)

  The polyester film of the present invention has excellent folding and holding properties, has small heat shrinkability in a high temperature environment, and is also excellent in water resistance, transparency, and printability, so it can be used in origami, handbags, It is possible to use suitably for paper substitute applications, such as a book cover, wrapping paper, etc.

It is a schematic diagram of the measuring method of folding holding angle. It is an example of the stress-strain curve in the tension test for evaluating stress at 40% elongation.

  The polyester film of the present invention is a film which can be used for paper substitutes, such as origami, paper bag, book cover, wrapping paper and the like, which are required to have foldability. It may or may not be printed. In addition, it may be used by laminating and laminating on a film excellent in folding retention. The polyester film is described below.

  The polyester composition which comprises the polyester film of this invention has an ethylene terephthalate unit as a main component. 40 mol% or more is preferable in 100 mol% of polyester structural units of a polyester composition, 50 mol% or more is more preferable, and 55 mol% or more of an ethylene terephthalate unit is more preferable. Other dicarboxylic acid components constituting the polyester composition include aromatic dicarboxylic acids such as isophthalic acid, naphthalenedicarboxylic acid and orthophthalic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid, and Alicyclic dicarboxylic acid etc. can be mentioned.

  Other diol components constituting the polyester include aliphatic diols such as 1,3-propanediol, 1,4-butanediol, neopentyl glycol and hexanediol, and alicyclic diols such as 1,4-cyclohexanedimethanol And aromatic diols such as bisphenol A.

  Further, the polyester composition constituting the film is an amorphous polyester, and it can be an amorphous component in 100 mol% of the polyhydric alcohol component or 100 mol% of the polyvalent carboxylic acid component in the polyester. The total amount of the monomer components is at least 13 mol%, preferably at least 14 mol%, more preferably at least 15 mol%, particularly preferably at least 16 mol%. The upper limit of the total of the monomer components that can be an amorphous component is not particularly limited, but the upper limit is preferably 30 mol%.

  In the present invention, specific examples of the monomer that can be an amorphous component include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 3-propanediol, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2, Examples include 2-di-n-butyl-1,3-propanediol and hexanediol. Among these, neopentyl glycol, 1,4-cyclohexanedimethanol and isophthalic acid are preferable.

  In the present invention, cyclic diols such as 1,4-cyclohexanedimethanol and diols having 3 to 6 carbon atoms (eg, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, hexane) It is preferable to use at least one or more of them, since the melting start temperature can be lowered by containing a polyester elastomer containing a diol, etc.), ε-caprolactone, tetramethylene glycol, etc.

  For example, when a polyester elastomer containing 1,3-propanediol, 1,4-butanediol, ε-caprolactone, tetramethylene glycol or the like is included to lower the melting start temperature, the polyester composition constituting the film may be used. The content is preferably 1% by mol or more, more preferably 5% by mol or more, still more preferably 10% by mol or more, and most preferably 15% by mol or more. However, if too many components to lower the melting start temperature are contained, the number of ethylene terephthalate units responsible for the physical strength decreases relatively, which may result in insufficient film strength, heat resistance, etc. The amount is preferably 30 mol% or less, more preferably 25 mol% or less, based on the total polyester units of the polyester composition constituting the film.

  Among the resins forming the polyester film of the present invention, various additives, for example, waxes, antioxidants, antistatic agents, crystal nucleating agents, viscosity reducing agents, heat stabilizers, for coloring, as needed A pigment, a coloring inhibitor, an ultraviolet absorber, etc. can be added.

  In the resin forming the polyester film of the present invention, it is preferable to add fine particles as a lubricant to improve the workability (slidability) of the film. The fine particles may be selected arbitrarily, for example, as the inorganic fine particles, for example, silica, alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate and the like, and as the organic fine particles, for example, acrylic resin Particles, melamine resin particles, silicone resin particles, crosslinked polystyrene particles and the like can be mentioned. The average particle diameter of the fine particles can be appropriately selected as needed within the range of 0.05 to 3.0 μm (as measured by a Coulter counter). The fine particles as a lubricant are preferably contained in an amount of 50 ppm or more, more preferably 100 ppm or more, based on the total weight of the film. However, when the content of the lubricant is too large, the surface roughness of the film may be large, so the content is preferably 3000 ppm or less, more preferably 1000 ppm or less.

  As a method of compounding the above-mentioned particles in the resin which forms the polyester film, for example, although it can be added at any stage of producing the polyester resin, it is polycondensed at the stage of esterification or after the end It is preferable to add as a slurry dispersed in ethylene glycol or the like at the stage before the start of the reaction to advance the polycondensation reaction. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a polyester-based resin raw material using a vented kneading extruder, or dried particles and a polyester raw material using a kneading extruder It is also preferable to carry out by a blending method or the like.

  The polyester film of the present invention can also be subjected to corona treatment, coating treatment, flame treatment, etc. in order to improve the adhesion of the film surface.

  Next, various properties of the polyester film of the present invention will be described.

  First, for the polyester film of the present invention, the average value of the stress at 40% elongation in the longitudinal direction and the stress at 40% elongation in the width direction is about the stress at 40% elongation in each of the film longitudinal and width tensile tests. The pressure is preferably 40 MPa or more and 110 MPa or less. When the film is folded, it is believed that the valley side of the fold is fractured by the local compression and the friction of the film itself, and the crest side of the fold is plastically deformed by the local elongation. The plastic deformation strain on the crease peak side is considered to be within 20% to 60%, though it changes depending on the thickness of the film and the degree of folding. Assuming that the plastic deformation strain is 40%, which is the average value, the higher the stress at 40% elongation (deformation), the greater the resilience when the film is folded, ie, the greater the rebound, and the lower the foldability. On the contrary, it is considered that if the stress at 40% elongation is low, the film tends to easily yield, that is, it tends to crease.

  In order to reduce the stress at 40% elongation by the tensile test in the longitudinal direction and the width direction of the film, as described above, it is preferable that the polyester composition constituting the film is an amorphous polyester. And 13 mol% or more, preferably 14 mol% or more, of the total of at least one monomer component that can be an amorphous component in 100 mol% of the polyhydric alcohol component or 100 mol% of the polyvalent carboxylic acid component in the polyester. More preferably, it is at least 15 mol%, particularly preferably at least 16 mol%. The upper limit of the total of the monomer components that can be an amorphous component is not particularly limited, but the upper limit is preferably 30 mol%. In addition, when the melting start temperature is lowered by including a polyester elastomer containing 1,3-propanediol, 1,4-butanediol, ε-caprolactone, tetramethylene glycol, etc., the stress at 40% elongation is further lowered. More preferred. In addition, in the final heat treatment step in the film forming step of the film described later, it is necessary to perform the heat treatment at a temperature higher than the melting start temperature of the polyester resin and 240 ° C. or less. Through this heat treatment step, the molecular orientation of the film can be partially collapsed and the stress at 40% elongation can be reduced.

  For the stress at 40% elongation by the tensile test in the longitudinal direction and width direction of the film, if the average value of the stress at 40% elongation in the longitudinal direction and the stress at 40% elongation in the width direction exceeds 110 MPa, origami, packaging, etc. It is not preferable because the folds open when folded at the end, and a beautiful aesthetic appearance can not be obtained. 105 MPa or less is preferable, and 100 MPa or less is more preferable. The lower the stress at 40% elongation, the more likely mechanical fracture and deformation of the film will occur and the foldability will be good, but at the present state of the art the lower limit is 40 MPa.

  In order to adjust the melting start temperature of the film to 100 ° C. to 180 ° C., it is preferable to use an amorphous polyester resin as a raw material as described above. Specific examples of the monomer which can be an amorphous component include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,3-propanediol 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di- n-Butyl-1,3-propanediol, hexanediol can be mentioned. Among these, neopentyl glycol, 1,4-cyclohexanedimethanol and isophthalic acid are preferable. The total amount of one or more monomer components that can be an amorphous component is 13 mol% or more, preferably 14 mol% or more, more preferably 15 mol% or more, and particularly preferably 16 mol% or more. The upper limit of the total of monomer components that can be an amorphous component is not particularly limited, but the upper limit is preferably 30 mol%. In addition, cyclic diols such as 1,4-cyclohexanedimethanol, and diols having 3 to 6 carbon atoms (for example, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, hexanediol, etc.), The melting start temperature can be lowered by containing a polyester elastomer containing ε-caprolactone, tetramethylene glycol or the like, and therefore, it is preferable to use at least one or more kinds. For example, when a polyester elastomer containing 1,3-propanediol, 1,4-butanediol, ε-caprolactone, tetramethylene glycol or the like is included to lower the melting start temperature, the polyester composition constituting the film may be used. The content is preferably 1% by mol or more, more preferably 5% by mol or more, still more preferably 10% by mol or more, and most preferably 15% by mol or more. However, if too many components to lower the melting start temperature are contained, the ethylene terephthalate unit responsible for the physical strength decreases relatively, and there is a fear that the film strength, heat resistance and the like may become insufficient. The amount is preferably 30 mol% or less, more preferably 25 mol% or less, based on the total polyester units of the polyester composition constituting the film.

  The polyester film of the present invention preferably has a heat shrinkage ratio of -10% to 10% both in the width direction and in the longitudinal direction when treated in warm water at 80 ° C for 10 seconds. If it exceeds 10%, the film shrinks and deforms unfavorably in a high temperature environment such as in a car in summer or in a warehouse without temperature control management. The upper limit of the hot water heat shrinkage ratio is more preferably 8% or less, and still more preferably 6.5% or less. On the other hand, when the hot water heat shrinkage rate is less than -10%, the film can not easily maintain its original shape as in the case where the shrinkage rate is high, which is not preferable. The lower limit value of the hot water heat shrinkage ratio is more preferably -5% or more, and further preferably -2% or more.

  In order to make the hot water heat shrinkage rate be in the range of -10% or more and 10% or less, in the final heat treatment step in the film forming step of the film described later, heat treatment at a temperature above the melting start temperature of polyester resin Is preferred. When the heat treatment temperature is equal to or lower than the melting start temperature, the shrinkage ratio is more than 10%, which is not preferable. Furthermore, the content of the above-mentioned monomer that can be an amorphous component is preferably 30 mol% or less, more preferably 25 mol% or less, based on the total polyester units of the polyester composition constituting the film. If it exceeds 30 mol%, the shrinkage rate becomes too high even with heat treatment above the melting start temperature, which is not preferable.

  The polyester film of the present invention preferably has a haze of 1% to 15%. If it exceeds 15%, the transparency of the film will be poor, and the visibility of the contents will be poor when used as a packaging material such as a bag. The upper limit of the haze is more preferably 13% or less, further preferably 10% or less, particularly preferably 7% or less, and most preferably 3% or less. The lower the haze, the higher the transparency, which is preferable, but at the current technical level, the lower limit is 1%, and it can be said that 2% or more is sufficient for practical use.

  In the resin forming the polyester film of the present invention, it is preferable to add fine particles as a lubricant as described above, but if the content of the lubricant is too large, the film surface unevenness may become large. And preferably 3000 ppm or less, more preferably 1000 ppm or less. Furthermore, when the heat treatment temperature in the final heat treatment step exceeds 240 ° C., the film surface becomes rough due to heat and the haze becomes high, which is not preferable.

  The polyester film of the present invention preferably has a folding holding angle of 20 degrees or more and 50 degrees or less, which is measured by a method described later. When it is 50 degrees or less, a fold is opened when folded with an origami, a package, etc., and a beautiful beauty is obtained and it is preferable. The folding and holding angle is more preferably 45 degrees or less, further preferably 40 degrees or less. The smaller the folding holding angle, the better. However, the lower limit of the range of the present invention is 20 degrees, and it can be said that 25 degrees or more is practically preferable.

  The lower the stress at 40% elongation, the lower the folding retention angle, and the total of one or more monomer components that can be an amorphous component as described above is at least 13 mol%, preferably at least 14 mol%, The polyester resin is preferably 15 mol% or more, particularly preferably 16 mol% or more, and heat treatment is performed at a temperature higher than the melting start temperature of the polyester resin and 240 ° C. or less in the final heat treatment step in the film forming step. Is preferred. By these conditions, the stress at 40% elongation can be adjusted to 40 MPa or more and 110 MPa or less, and the folding angle can be adjusted to 20 degrees or more and 50 degrees or less.

The polyester film of the present invention preferably has a melting start temperature obtained from a temperature rising profile of DSC of 100 ° C. or more and 180 ° C. or less. The melting start temperature is more preferably 175 ° C. or less. In the heat treatment process of the film described later, it is preferable that the film can be partially melted, and in this case, the melting start temperature influences. It is preferable that the melting start temperature is 180 ° C. or less, because the crystallinity of the polyester resin is not too high, so that crystallization is hardly promoted at the time of heat treatment and partial melting is likely to occur. A film composed of a polyester composition having a large amount of amorphous parts without promoting crystallization is preferable because the repulsion at the time of folding is small and the folding retention is well maintained. It is preferable that the melting start temperature is 100 ° C. or more, because the heat resistance is sufficient even when the film is left in a high temperature environment. The melting start temperature is more preferably 120 ° C. or more.
The polyester film of the present invention preferably has a tensile breaking strength in the longitudinal direction and / or the width direction of 100 MPa or more and 300 MPa or less. When it is 100 MPa or more, the risk of tearing is small at the time of processing such as printing or when used as a packaging material such as a bag, which is preferable. The tensile fracture strength in the longitudinal direction and / or the width direction is more preferably 120 MPa or more, and further preferably 140 MPa or more. The higher the tensile breaking strength, the better, but in the present invention the upper limit is 300 MPa, because the tensile strength does not exceed 300 MPa. If it is 100 MPa or more, even if it is 200 MPa or less, it can be said that it is practically preferable.

  The tensile strength at break is lowered by using the amorphous polyester resin, so that the content of the monomer which can be an amorphous component should be 30 mol% or less with respect to the total polyester units of the polyester composition constituting the film. Is more preferable, and more preferably 25 mol% or less. In addition, the tensile breaking strength tends to increase as the draw ratio in the film forming process increases. In order to obtain a tensile breaking strength of 100 MPa or more in either the longitudinal direction or the transverse direction, the draw ratio in the corresponding direction is preferably 2.0 times or more, more preferably 2.5 times or more. The larger the draw ratio, the higher the tensile strength at break and the more preferable. In addition, tensile fracture strength seems to be related to the fact that molecular orientation is disturbed by heat treatment above the melting start temperature in the final heat treatment step, but it decreases as a result. In consideration of the stress and shrinkage at the time of 40% elongation described above, heat treatment is preferably performed at or above the melting start temperature, and excessive heat treatment is not preferable because it extremely reduces the tensile fracture strength, and the upper limit is 240 ° C. preferable.

  Although the thickness of the film in the polyester film of the present invention is not particularly limited, it is preferably 3 μm or more and 100 μm or less. If the thickness of the film is smaller than 3 μm, processing such as printing may become difficult. If the thickness of the film is more than 100 μm, the folding property is lowered, and the weight of the film is increased to increase the cost, which is not preferable. The thickness of the film is more preferably 5 μm or more and 90 m or less, and still more preferably 7 μm or more and 80 μm or less.

  The polyester film of the present invention described above is formed by melt-extruding a single or a plurality of polyester raw materials by an extruder to form the above-described polyester composition constituting the film to form an unstretched film. It can obtain by uniaxially or biaxially stretching by the predetermined method shown to, and heat-processing after that. In addition, polyester can be obtained by polycondensing said suitable dicarboxylic acid component and diol component by a well-known method. In addition, it is usually preferable to use two or more types of chip-like polyester as a raw material of the film.

  When melt-extruding the raw material resin, it is preferable to dry the polyester raw material using a hopper dryer, a dryer such as a paddle dryer, or a vacuum dryer. After drying the polyester material as such, it is melted at a temperature of 200 to 300 ° C. and extruded as a film by using an extruder. For extrusion, any existing method such as T-die method or tubular method can be adopted.

  And an unstretched film can be obtained by quenching the sheet-like molten resin after extrusion. In addition, as a method of quenching the molten resin, it is possible to suitably adopt a method of obtaining a substantially unoriented resin sheet by casting the molten resin from a die onto a rotating drum and rapidly solidifying it.

  In order to achieve the object of the present invention, the stretching direction of the film may be either the film longitudinal (longitudinal) direction or the transverse (width) direction. In the following, the longitudinal stretching-lateral stretching method in which longitudinal stretching is performed first and then transverse stretching will be described. However, even in the case of horizontal stretching-longitudinal stretching in which the order is reversed, only the main orientation direction changes. I do not care. Also, the stretching direction may be only the longitudinal direction or only the lateral direction.

  First, longitudinal stretching is performed. The substantially unoriented film is preferably longitudinally stretched so as to have a magnification of 3.0 times or more and 4.5 times or less at a temperature of Tg or more and Tg + 30 ° C. or less. When the draw ratio in the longitudinal direction is high, the orientation in the longitudinal direction is excessively strengthened, and breakage occurs in the draw in the longitudinal direction or in the transverse drawing in the next step. The upper limit of the longitudinal stretching ratio is preferably 4.2 times or less, more preferably 3.9 times or less. After longitudinal stretching, the film can be relaxed in the longitudinal direction to control the final draw ratio. Relaxing is performed by heating the film after longitudinal stretching at a temperature of Tg or more and Tg + 90 ° C. or less and using a roll speed difference or the like to relax the film at an arbitrary magnification in the longitudinal direction. be able to. As a heating means, any of a roll, near infrared rays, far infrared rays, a hot air heater and the like can be used.

  In addition, if the longitudinal stretching ratio is lower than 3.0 times, thickness unevenness in the film longitudinal direction may be deteriorated, and problems such as misalignment may occur when the film is wound as a roll, which is not preferable. When the final stretching ratio in the longitudinal direction is to be 3.0 times or less, it can be controlled by performing the longitudinal stretching at 3.0 times or more and then performing relaxation. The lower limit of the longitudinal stretching ratio is preferably 3.2 times or more, more preferably 3.4 times or more. As described above, longitudinal stretching may not be performed if transverse stretching is performed.

  Next, lateral stretching is performed. Stretching in the transverse direction is preferably performed at a temperature of Tg or more and Tg + 30 ° C. or less and about 3.5 to 5.0 times in a state in which both ends of the film width direction in the tenter are gripped by clips. It is preferable to perform preheating at a temperature of Tg-10 ° C. or more and Tg + 50 ° C. or less before stretching in the transverse direction.

  After the transverse stretching, the film is heat-treated. The heat treatment temperature is preferably equal to or higher than the melting start temperature obtained from the DSC temperature rise profile of the polyester resin used as the raw material. During the heat treatment, a relaxation process (different from the longitudinal relaxation process described above) for reducing the distance between the clips in the lateral direction may be performed at an arbitrary magnification. By heat treatment at a temperature higher than the melting start temperature, the crystal orientation of the film produced by stretching can be partially collapsed to adjust the stress at 40% elongation, which has the effect of reducing the folding angle. At the same time, the heat treatment at or above the melting start temperature can also collapse the amorphous orientation of the film produced by stretching and reduce the shrinkability. When the heat treatment temperature is lower than the melting start temperature, heat treatment is performed in the region where the film is crystallized, and although the shrinkability of the film is reduced, the folding retention angle is undesirably increased. On the other hand, when the heat treatment temperature exceeds 240 ° C., the shrinkage of the film becomes large because the amorphous polyester resin is used in the present invention. Since the thickness spot of the film in film forming worsens by this and further overheating advances, since a film melts completely and it breaks, it is unpreferable.

  After that, a polyester film roll can be obtained by winding the film while cutting and removing both ends of the film.

  EXAMPLES Next, the present invention will be specifically described using Examples and Comparative Examples, but the present invention is not limited to the embodiments of the Examples in any way, and within the scope of the gist of the present invention, as appropriate. It is possible to change.

  The evaluation method of the film is as follows. In the case where the longitudinal direction and the width direction can not be identified immediately because the area of the film is small, etc., the longitudinal direction and the width direction may be determined and measured temporarily, and the provisionally determined longitudinal direction and width direction are true. There is no problem in particular because it is wrong 90 degrees in the longitudinal direction and the width direction.

[Stress at 40% elongation]
Assuming that the measurement direction is the film width direction, a strip-shaped test piece of 140 mm in the width direction and 20 mm in the direction (film longitudinal direction) orthogonal to the measurement direction was produced. Using a universal tensile tester “DSS-100” (manufactured by Shimadzu Corporation), the chuck holds 20 mm each from the both ends of the test piece by 20 mm (chuck distance 100 mm) and has an ambient temperature of 23 ° C. and a tensile speed of 200 mm / min. A tensile test was performed under the conditions. From the obtained stress-strain curve, the stress at 40% strain was taken as the stress at 40% elongation. The measurement in the longitudinal direction was performed by changing the measurement in the width direction and the direction of preparation of the sample piece by 90 degrees. When the film was broken before the strain reached 40%, the stress was set to 0 MPa. Finally, the average value of the stress obtained from the width direction and the longitudinal direction was used as the stress at 40% strain.

[Folding holding angle]
The film is left for 24 hours in a temperature-controlled room at 28 ° C. and 50% RH. Immediately thereafter, each film was cut into 10 cm × 10 cm squares in a 20 ° C., 65% RH environment and folded in four (5 cm × 5 cm squares). When the film was folded, the short side of the first two-folded rectangle was oriented vertically. After that, a 5 kg weight of 3 cm × 3 cm in size at the bottom was placed on the film for 4 seconds for 20 seconds. After removing the weight, the four-folded film was left for 30 minutes. After that, it was determined by measuring the angle at which the folded film was opened (the fully folded state was taken as 0 degree). Further, the folding holding angles in the longitudinal direction and the transverse direction when the film was folded were measured, and the value with the larger angle was taken as the folding holding angle.

[Hot water heat shrinkage rate]
The film is cut into a square of 10 cm × 10 cm, immersed in warm water at 80 ± 0.5 ° C. for 10 seconds under no load and heat shrunk, then immersed in water at 25 ° C. ± 0.5 ° C. for 10 seconds, and pulled out from the water The dimensions of the film in the longitudinal and transverse directions were measured, and the shrinkage was determined in accordance with the following equation 1:

  Shrinkage ratio = {(length before contraction−length after contraction) / length before contraction} × 100 (%) Formula 1

[Haze]
According to JIS-K-7136, it measured using a haze meter (made by Nippon Denshoku Industries Co., Ltd., 300A). In addition, the measurement was performed twice and the average value was calculated | required.

[Tensile breaking strength]
Assuming that the measurement direction is the film width direction, a strip-shaped test piece of 140 mm in the width direction and 20 mm in the direction (film longitudinal direction) orthogonal to the measurement direction was produced. Using a universal tensile tester “DSS-100” (manufactured by Shimadzu Corporation), the chuck holds 20 mm each from the both ends of the test piece by 20 mm (chuck distance 100 mm) and has an ambient temperature of 23 ° C. and a tensile speed of 200 mm / min. A tensile test was conducted under the conditions, and the stress at the time of tensile failure was taken as the tensile failure strength. The measurement in the longitudinal direction was performed by changing the measurement in the width direction and the direction of preparation of the sample piece by 90 degrees.

[Glass transition point (Tg)]
It calculated | required according to JISK7121 using the Seiko Instruments Inc. differential scanning calorimeter (model | form: DSC220). 10 mg of the unstretched film was heated from -20 ° C to 120 ° C at a temperature rising rate of 10 ° C / min to obtain a temperature rising profile. A tangent was drawn before and after the inflection point of the obtained profile, and the intersection point was read as a glass transition point (Tg; ° C.).

[Melting start temperature]
The extrapolation melting start temperature on the low temperature side was determined according to JIS K 7121 using a differential scanning calorimeter (model: DSC 220) manufactured by Seiko Instruments Inc. First, 10 mg of film was heated from 20 ° C. to 300 ° C. at a heating rate of 10 ° C./min to obtain a temperature rising profile. In the temperature rising profile, the temperature at the intersection point of a straight line extending the high temperature side of the low temperature side and a tangent drawn at a point where the slope is maximum on the low temperature curve of the melting peak was read as the melting start temperature.

[Longitudinal thickness spots]
The film is sampled in a long roll having a length of 12 m and a width of 40 mm, and continuously measured along the longitudinal direction of the film at a measurement speed of 5 m / min using a continuous contact thickness meter manufactured by Micron Measuring Instruments Co., Ltd. The thickness was measured (measurement length is 10 m). The maximum thickness at the time of measurement was Tmax., The minimum thickness was Tmin., And the average thickness was Tave. The thickness unevenness of the film in the longitudinal direction was calculated from the following Formula 2.

Thickness unevenness = {(Tmax.-Tmin.) / Tave.} × 100 (%) · · · Formula 2

[Width direction thickness spots]
The film is sampled in a wide band of 40 mm long × 1.2 m wide, and continuously measured along the width direction of the film sample at a measurement speed of 5 m / min using a continuous contact thickness meter manufactured by Micron Measuring Instruments Co., Ltd. The thickness was measured (measurement length is 500 mm). The maximum thickness at the time of measurement was Tmax., The minimum thickness was Tmin., And the average thickness was Tave. The thickness unevenness of the film in the width direction was calculated from the above equation 2.

  Moreover, the polyester used for the Example and the comparative example is as follows.

Polyester 1: Copolymerized polyester consisting of 70 mol% ethylene glycol, 30 mol% neopentyl glycol and terephthalic acid Polyester 2: Polyethylene terephthalate (PET)
-Polyester 3: polyester masterbatch in which 7000 ppm of SiO ₂ is added to polyester 2-Polyester 4: polybutylene terephthalate (PBT)
· Polyester 5: 54 mol% of polybutylene terephthalate, 46 mol% of poly ε-caprolactone · Copolymer · Polyester: Copolymer polyester consisting of 70 mol% of ethylene glycol, 30 mol% of 1.4-cyclohexanedimethanol and terephthalic acid

Example 1
The above-mentioned polyester 1, polyester 2, polyester 3 and polyester 4 were mixed at a weight ratio of 60: 15: 5: 20, mixed and charged into the extruder. The mixed resin is melted at 270 ° C., extruded from a T-die, wound around a rotating metal roll cooled to a surface temperature of 25 ° C., and quenched to obtain an unstretched film with a thickness of 250 μm. The glass transition temperature of the unstretched film was 65 ° C., and the melting start temperature was 130 ° C. The obtained unstretched film is guided to a longitudinal stretching machine in which a plurality of roll groups are continuously disposed, and is preheated by a preheating roll until the film temperature reaches 70 ° C. Thereafter, the low-speed rotation set to the surface temperature 80 ° C. Between the roll and a high-speed rotating roll set to a surface temperature of 80 ° C., stretching was made 3.5 times in the longitudinal direction by using the difference in rotational speed. After that, with the film after longitudinal stretching held in the tenter with clips at both ends in the width direction, first, preheating is performed until the surface temperature of the film reaches 90 ° C., and then in the lateral direction at 85 ° C. It stretched to 4.0 times. The film after transverse stretching is led to the heat treatment zone in the tenter in a state of being gripped by the clips at both ends in the width direction, and heat treated for 10 seconds at a temperature of 140 ° C. in the heat treatment zone and then cooled. Thereafter, by cutting off both edges and winding in a roll of 400 mm in width, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length.

The properties of the obtained film were evaluated by the method described above. The obtained stress-strain curve is shown as an example in FIG. Also, the evaluation results are shown in Table 1. The obtained biaxially stretched film became a film with a preferable folding angle, a low heat shrinkage, and a preferable haze, and was totally highly preferable.

(Example 2)
The same polyester raw material as in Example 1 is melt extruded in the same manner as in Example 1, and film formation is carried out in the same manner as in Example 1 except that the heat treatment temperature is 170 ° C. Were produced continuously over a predetermined length. And the characteristic of the obtained film was evaluated by the above-mentioned method. The evaluation results are shown in Table 1. As in Example 1, a good film was obtained.

(Example 3)
After mixing the above-described polyester 1, polyester 2, polyester 3 and polyester 4 in a weight ratio of 50: 35: 5: 10, melt extrusion was performed in the same manner as in Example 1. The glass transition temperature of the unstretched film was 70 ° C., and the melting start temperature was 170 ° C. Thereafter, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length by forming a film in the same manner as in Example 1 except that the heat treatment temperature was 170 ° C. The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 1. As in Example 1, a good film was obtained.

(Example 4)
After mixing the above-described polyester 1, polyester 2, polyester 3 and polyester 5 at a weight ratio of 50: 35: 5: 10, melt extrusion was performed in the same manner as in Example 1. The glass transition temperature of the unstretched film was 65 ° C., and the melting start temperature was 150 ° C. Thereafter, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length by forming a film in the same manner as in Example 1 except that the heat treatment temperature was 150 ° C. The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 1. As in Example 1, a good film was obtained.

(Example 5)
After mixing the above-described polyester 1, polyester 2, polyester 3 and polyester 4 in a weight ratio of 75: 10: 5: 10, melt extrusion was performed in the same manner as in Example 1. The glass transition temperature of the unstretched film was 70 ° C., and the melting start temperature was 140 ° C. Thereafter, a film was formed in the same manner as in Example 1 except that the heat treatment temperature was changed to 160 ° C., whereby a biaxially stretched film of about 18 μm was continuously produced over a predetermined length. The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 1. As in Example 1, a good film was obtained.

(Example 6)
The same polyester raw material as in Example 1 was melt extruded in the same manner as in Example 1 to obtain an unstretched film with a thickness of 70 μm. The unstretched film was first preheated until the surface temperature of the film reached 85 ° C. in a state in which both edges in the width direction were gripped by a clip in the tenter, and the film was stretched 4.0 times in the transverse direction at 80 ° C. The film after transverse stretching is led to the heat treatment zone in the tenter in a state of being gripped by the clips at both ends in the width direction, and heat treated for 10 seconds at a temperature of 140 ° C. in the heat treatment zone and then cooled. Thereafter, in the same manner as in Example 1, a transverse uniaxially stretched film of about 18 μm was continuously produced over a predetermined length. And the characteristic of the obtained film was evaluated by the above-mentioned method. The evaluation results are shown in Table 1. As in Example 1, a good film was obtained.

(Example 7)
After mixing the polyester 3, the polyester 4 and the polyester 6 described above at a weight ratio of 5:20:75, melt extrusion was performed in the same manner as in Example 1. The glass transition temperature of the unstretched film was 65 ° C., and the melting start temperature was 140 ° C. Thereafter, by forming a film in the same manner as in Example 1, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length. The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 1. As in Example 1, a good film was obtained.

(Comparative example 1)
After mixing the polyester 2 and the polyester 3 described above in a weight ratio of 95: 5, melt extrusion was performed in the same manner as in Example 1. The glass transition temperature of the unstretched film was 75 ° C., and the melting start temperature was 200 ° C. Thereafter, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length by forming a film in the same manner as in Example 1 except that the heat treatment temperature was set to 200 ° C. The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 1. The stress at 40% elongation and the folding and holding angle are out of the specified range of the present invention, resulting in an undesirable film.

(Comparative example 2)
The same polyester raw material as in Example 1 was melt extruded in the same manner as in Example 1 to obtain an unstretched film. Thereafter, a film was formed under the same conditions as Comparative Example 1 except that the heat treatment temperature was set to 90 ° C., whereby a biaxially stretched film of about 18 μm was continuously manufactured over a predetermined length. And the characteristic of the obtained film was evaluated by the above-mentioned method. The evaluation results are shown in Table 1. The hot water heat shrinkage rate at 70 ° C. was large, resulting in an undesirable film.

(Comparative example 3)
After mixing the above-described polyester 1, polyester 2 and polyester 3 in a weight ratio of 25: 70: 5, melt extrusion was performed in the same manner as in Example 1. The glass transition temperature of the unstretched film was 75 ° C., and the melting start temperature was 190 ° C. Thereafter, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length by forming a film in the same manner as in Example 1 except that the heat treatment temperature was 190 ° C. The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 1. The folding angle was out of the specified range of the present invention, and a non-qualified film was obtained.

  The polyester film of the present invention has excellent folding retention, low shrinkage, and excellent transparency. Therefore, it can be preferably used for paper substitutes such as wrapping paper, paper bag, book cover, origami and the like.

Claims (4)

Consists amorphous polyester containing ethylene terephthalate units, with satisfying from the following requirements (1) (3), a thickness of 3μm or more 80μm or less, and folding the holding angle is 50 ° or less than 20 degrees, a bag applications or A polyester film characterized by being used for origami applications .
(1) With respect to the stress at 40% elongation in each longitudinal direction and width direction of the film, the average value of the stress at 40% elongation in the longitudinal direction and the stress at 40% elongation in the width direction is 40 MPa or more and 85 MPa or less 2) Hot water heat shrinkage ratio in the longitudinal direction and width direction when treated in hot water at 80 ° C. for 10 seconds is -10% or more and 10% or less (3) haze is 1% or more and 15% or less The polyester film according to claim 1 , wherein a melting start temperature in a DSC temperature rising profile is 100 ° C or more and 180 ° C or less. The polyester film according to any one of claims 1 and 2 , wherein the tensile breaking strength in the longitudinal direction and / or the width direction is 100 MPa or more and 300 MPa or less. A method for continuously producing a polyester film according to any one of claims 1 to 3 , which is a melt start temperature of a polyester film after drawing a melt-extruded, cooled and solidified unstretched sheet in a longitudinal direction and / or a width direction A method for producing a polyester film, comprising heat setting at a temperature of not less than 240 ° C.