JP6693586B2 - Polyester film roll with excellent foldability, low shrinkage and transparency - Google Patents

Description

  The present invention relates to a polyester film excellent in foldability and holdability that can be used as wrapping paper, carrying bag, origami paper, etc., and is water-resistant, has low shrinkage under high temperature environment, and has excellent transparency. The present invention relates to a polyester film roll.

  Since paper has excellent foldability, it is widely used as various wrapping papers, carrying bags, origami papers, and the like. However, the paper has poor water resistance, and when it is wet with rain or the like, it may be torn or the print may be discolored. Further, since the paper is not transparent, there is a problem that the contents cannot be seen when used for packaging such as bags. Therefore, plastic films have been studied as an alternative to paper.

  Transparent cellophane has been used in the past as a film having excellent foldability. However, since cellophane has hygroscopicity, its characteristics fluctuate with the seasons, making it difficult to supply while maintaining the quality of the product constant, and poor workability due to uneven thickness is a drawback. Came.

  On the other hand, the polyethylene terephthalate film has excellent properties such as toughness, water resistance, and transparency, but has a drawback that it is inferior in foldability.

  As a method of solving such a defect, a polyethylene terephthalate film capable of maintaining good folding holding property by reducing the density of the film is disclosed (for example, refer to Patent Document 1).

  However, the polyethylene terephthalate film of Patent Document 1 has a problem that heat shrinkability is large. It is pointed out that if a bag or the like using such a film is left in a car in the midsummer or in a warehouse without temperature control, the film shrinks or deforms and becomes unusable. In addition, there is also a problem that processing cannot be performed due to the shrinkage of the film in a processing step requiring high temperature such as a printing step on the film.

Japanese Patent No. 4308662 (claim 1 etc.)

  The present invention aims to solve the above-mentioned problems of the prior art. That is, the present invention is intended to provide a polyester film roll obtained by winding a polyester film which has excellent foldability and has a very small heat shrinkability in a high temperature environment and is excellent in transparency.

The present invention has the following configurations.
1. It is made of amorphous polyester containing ethylene terephthalate unit, meets the following requirements (1) to (3), has a thickness of 3 μm or more and 80 μm or less, and has a folding holding angle of 20 ° or more and 50 ° or less. A polyester film roll, which is obtained by winding a polyester film used for a purpose into a roll shape.
(1) Regarding the stress at 40% elongation in each of the longitudinal and widthwise tensile tests 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 ) The heat shrinkage rate of hot water in the longitudinal direction and the 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 2. The melting point of the polyester film in the DSC temperature rising profile is 100 ° C. or higher and 180 ° C. or lower, and the polyester film roll according to the first aspect is characterized.
3. 3. The polyester film roll according to any one of the first and second aspects, wherein the polyester film has a tensile breaking strength in a longitudinal direction and / or a width direction of 100 MPa or more and 300 MPa or less.
(Here, the melting start temperature described in the above-mentioned third is measured from the completed film, but it is neither the data of the unstretched stage nor the difference in the data.)

  The polyester film constituting the polyester film roll of the present invention has excellent fold-holding property, has a small heat shrinkability in a high temperature environment, and is also excellent in water resistance, transparency and printability. Therefore, it can be suitably used for paper substitute applications such as origami paper, carrying bags, book covers, and wrapping paper.

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

  The polyester film constituting the polyester film roll of the present invention (hereinafter, may be referred to as the polyester film of the present invention) is required to have folding holdability for origami, a paper carrying bag, a book cover, wrapping paper, etc. It is a film that can be used as a substitute for paper. It may or may not be printed. Further, it may be used by laminating and laminating it with a film having excellent foldability. The polyester film will be described below.

  The polyester composition constituting the polyester film of the present invention has an ethylene terephthalate unit as a main constituent component. The ethylene terephthalate unit is preferably 40 mol% or more, more preferably 50 mol% or more, still more preferably 55 mol% or more in 100 mol% of the polyester constituent unit of the polyester composition. Other dicarboxylic acid components constituting the polyester composition, isophthalic acid, naphthalenedicarboxylic acid, aromatic dicarboxylic acid such as orthophthalic acid, adipic acid, azelaic acid, sebacic acid, aliphatic dicarboxylic acid such as decanedicarboxylic acid, and Examples thereof include alicyclic dicarboxylic acid.

  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. , Aromatic diols such as bisphenol A, and the like.

  Further, the polyester composition constituting the film is an amorphous polyester, and one or more of which can be an amorphous component in 100 mol% of the polyhydric alcohol component or 100 mol% of the polycarboxylic acid component in the polyester. The total amount of the monomer components 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 amount of monomer components that can be amorphous components 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, 1, 3-propanediol, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2, Examples thereof 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 (for example, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, hexane). Since the melting start temperature can be lowered by incorporating a polyester elastomer containing diol, etc.), ε-caprolactone, tetramethylene glycol, etc., it is preferable to use at least one kind.

  For example, 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 polyester composition for the film is The content is preferably 1 mol% or more, more preferably 5 mol% or more, still more preferably 10 mol% or more, and most preferably 15 mol% or more. However, if too much of the above-mentioned component that lowers the melting start temperature is contained, the ethylene terephthalate unit responsible for the physical strength becomes relatively small, so that the film strength, heat resistance, etc. may be insufficient. Therefore, the amount is preferably 30 mol% or less, and 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 such as waxes, antioxidants, antistatic agents, crystal nucleating agents, viscosity reducing agents, heat stabilizers, and coloring agents may be added as necessary. Pigments, anti-coloring agents, ultraviolet absorbers and the like can be added.

  In the resin forming the polyester film of the present invention, it is preferable to add fine particles as a lubricant for improving the workability (sliding property) of the film. As the fine particles, any one can be selected, for example, as the inorganic fine particles, silica, alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate, etc., and as the organic fine particles, for example, acrylic resin. Examples thereof include particles, melamine resin particles, silicone resin particles and crosslinked polystyrene particles. The average particle size of the fine particles is in the range of 0.05 to 3.0 μm (when measured with a Coulter counter) and can be appropriately selected as necessary. 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, if the content of the lubricant is too large, the unevenness of the film surface may become large. Therefore, the content is preferably 3000 ppm or less, more preferably 1000 ppm or less.

  As a method of blending the above-mentioned particles in the resin forming the polyester film, for example, it can be added at any stage of producing the polyester resin, but it may be polycondensed after the esterification stage or after the transesterification reaction. It is preferable to add it as a slurry dispersed in ethylene glycol or the like before the start of the reaction to proceed the polycondensation reaction. Further, a method of blending a slurry of particles dispersed in ethylene glycol or water or the like with a polyester resin raw material using a kneading extruder with a vent, or using a kneading extruder, the dried particles and the polyester raw material It is also preferable to use a blending method or the like.

  The polyester film of the present invention may be subjected to corona treatment, coating treatment, flame treatment or the like in order to improve the adhesiveness of the film surface.

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

  First, the polyester film of the present invention, the 40% elongation stress in each tensile test in the longitudinal direction and the width direction of the film, the average value of the 40% elongation stress in the longitudinal direction and the 40% elongation stress in the width direction is It is preferably 40 MPa or more and 110 MPa or less. It is considered that when the film is folded, local compression occurs on the valley side of the fold and fracture occurs due to friction of the film itself, and at the same time, plastic deformation occurs on the mountain side of the fold due to local extension. The plastic deformation strain on the fold side varies depending on the film thickness and the degree of folding, but is considered to fall within the range of 20% to 60%. When 40%, which is the average value, is defined as plastic deformation strain, it is considered that the higher the stress at 40% elongation (deformation), the greater the restoring force when the film is folded, that is, the repulsion, and the foldability decreases. On the contrary, when the stress at 40% elongation is low, it is considered that the film easily yields, that is, folds are easily formed.

  In order to reduce the stress at 40% elongation in the tensile test in each of the longitudinal direction and the width direction of the film, it is preferable that the polyester composition constituting the film is an amorphous polyester as described above. A total of 13 mol% or more, preferably 14 mol% or more, of the one or more monomer components that can be amorphous components in 100 mol% of the polyhydric alcohol component or 100 mol% of the polycarboxylic acid component in the polyester, It is more preferably 15 mol% or more, and particularly preferably 16 mol% or more. The upper limit of the total amount of monomer components that can be amorphous components is not particularly limited, but the upper limit is preferably 30 mol%. Further, 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. Is more preferable. Further, in the final heat treatment step in the film forming step described later, it is necessary to perform heat treatment at a temperature of not lower than the melting start temperature of the polyester resin and not higher than 240 ° C. Through this heat treatment step, the molecular orientation of the film can be partially collapsed and the stress at 40% elongation can be reduced.

  Regarding the stress at 40% elongation by tensile tests 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 crease will open when you fold it, and you will not be able to obtain a beautiful appearance. It is preferably 105 MPa or less, more preferably 100 MPa or less. The lower the stress at 40% elongation is, the more easily the film is liable to be mechanically broken or deformed and the better the foldability, which is preferable, but at the current technical level, 40 MPa is the lower limit.

  In order to adjust the melting start temperature of the film from 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 that 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- Examples thereof include n-butyl-1,3-propanediol and hexanediol. Among these, neopentyl glycol, 1,4-cyclohexanedimethanol, and isophthalic acid are preferable. The total of one or more types of monomer components that can be amorphous components is 13 mol% or more, preferably 14 mol% or more, more preferably 15 mol% or more, and particularly preferably 16 mol% or more. The total upper limit of the monomer components that can be amorphous components is not particularly limited, but the upper limit is preferably 30 mol%. Further, 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.), By including a polyester elastomer containing ε-caprolactone, tetramethylene glycol, or the like, the melting start temperature can be lowered, and therefore it is preferable to use at least one kind. For example, 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 polyester composition for the film is The content is preferably 1 mol% or more, more preferably 5 mol% or more, still more preferably 10 mol% or more, and most preferably 15 mol% or more. However, when too much of the above-mentioned component that lowers the melting start temperature is contained, the ethylene terephthalate unit responsible for the physical strength is relatively reduced, and the film strength and heat resistance may be insufficient. Therefore, the amount is preferably 30 mol% or less, and more preferably 25 mol% or less, based on the total polyester units of the polyester composition constituting the film.

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

  In order to make the hot water heat shrinkage rate within the range of -10% to 10%, heat treatment should be performed at a temperature of not less than the melting start temperature of the polyester resin and not more than 240 ° C in the final heat treatment step in the film forming step described later. Is preferred. If the heat treatment temperature is lower than the melting start temperature, the shrinkage ratio exceeds 10%, which is not preferable. Further, the content of the above-mentioned monomer that can be an amorphous component is preferably 30 mol% or less, and 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 in the heat treatment at the melting start temperature or higher, which is not preferable.

  The polyester film of the present invention preferably has a haze of 1% or more and 15% or less. If it exceeds 15%, the transparency of the film is deteriorated, so that the visibility of the content is deteriorated when used as a packaging material such as a bag. The upper limit of 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 state of the art, the lower limit is 1%, and even if it is 2% or more, it can be said to be 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 irregularities may become large. It is preferably 3000 ppm or less, more preferably 1000 ppm or less. Further, if the heat treatment temperature in the final heat treatment step exceeds 240 ° C., the film surface becomes rough due to heat and the haze increases, 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 as measured by the method described below. When the angle is 50 degrees or less, the folds are opened when folded with origami or packaging, and a beautiful appearance can be obtained, which is preferable. The folding holding angle is more preferably 45 degrees or less, and further preferably 40 degrees or less. Also, the smaller the folding holding angle is, the more preferable, but the lower limit of the range of the present invention is 20 degrees, and it can be said that it is practically preferable even if it is 25 degrees or more.

  The folding holding angle decreases as the stress at 40% elongation decreases, so that the total of one or more monomer components that can be an amorphous component is 13 mol% or more, preferably 14 mol% or more, as described above. The polyester resin is preferably used in an amount of 15 mol% or more, particularly preferably 16 mol% or more, and is heat-treated at a temperature of not less than the melting start temperature of the polyester resin and not more than 240 ° C. in the final heat treatment step in the film forming step. Preferably. Under 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 of 100 ° C. or higher and 180 ° C. or lower obtained from the DSC heating profile. The melting start temperature is more preferably 175 ° C or lower. In the heat treatment step of the film described later, it is preferable that the film can be partially melted, and in this case, the melting start temperature has an influence. When the melting start temperature is 180 ° C. or lower, the crystallinity of the polyester resin is not too high, and crystallization is less likely to be promoted during heat treatment, and partial melting is likely to occur, which is preferable. A film made of a polyester composition in which crystallization is not promoted and which has a large amount of an amorphous portion is preferable because the repulsion at the time of folding is small and the fold-holding property is maintained well. When the melting start temperature is 100 ° C. or higher, heat resistance is sufficient even when the film is left in a high temperature environment, which is preferable. The melting start temperature is more preferably 120 ° C. or higher.
The polyester film of the present invention preferably has a tensile fracture strength in the longitudinal direction and / or the width direction of 100 MPa or more and 300 MPa or less. When the pressure is 100 MPa or more, there is little risk of tearing during processing such as printing or 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, further preferably 140 MPa or more. The higher the tensile fracture strength, the more preferable, but in the present invention, since it does not exceed 300 MPa, the upper limit is 300 MPa. If it is 100 MPa or more, it can be said that it is practically preferable even if it is 200 MPa or less.

  Since the tensile breaking strength is reduced by using the amorphous polyester resin, the content of the monomer that can be an amorphous component should be 30 mol% or less based on all the polyester units of the polyester composition constituting the film. Is preferable, and more preferably 25 mol% or less. In addition, the tensile breaking strength tends to increase as the draw ratio increases during the film forming process. In order to obtain a tensile fracture strength of 100 MPa or more in either the machine 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, which is preferable, but in reality, there is a concern that the film may break, so it is preferably set at 5.5 times or less. Further, the tensile strength at break seems to be related to disturbance of the molecular orientation by heat treatment at the melting start temperature or higher in the final heat treatment step, but as a result, it decreases. Considering the above-mentioned stress and shrinkage at 40% elongation, it is preferable to perform heat treatment at a melting start temperature or higher, and excessive heat treatment extremely reduces tensile fracture strength, so the upper limit is set to 240 ° C. preferable.

  The thickness of the polyester film of the present invention is not particularly limited, but is preferably 3 μm or more and 100 μm or less. If the thickness of the film is less than 3 μm, processing such as printing may be difficult. Further, if the film thickness is thicker than 100 μm, the foldability is deteriorated, the weight of the film used is increased, and the cost is increased, which is not preferable. The thickness of the film is more preferably 5 μm or more and 90 μm or less, and further preferably 7 μm or more and 80 μm or less.

  The above-mentioned polyester film of the present invention is an unstretched film by melt-extruding a single or a plurality of polyester raw materials with an extruder to obtain the above-mentioned polyester composition constituting the film. It can be obtained by uniaxially stretching or biaxially stretching by a predetermined method shown in 1) and then further heat treatment. The polyester can be obtained by polycondensing the above-mentioned suitable dicarboxylic acid component and diol component by a known method. Further, it is usually preferable to mix two or more kinds of chip-shaped polyesters and use them as a raw material for a film.

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

  Then, an unstretched film can be obtained by rapidly cooling the sheet-shaped molten resin after extrusion. As a method of rapidly cooling the molten resin, 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 can be suitably adopted.

  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 lateral (width) direction. In the following, a longitudinal stretching-horizontal stretching method in which first longitudinal stretching and then lateral stretching are performed will be described. However, even in transverse stretching-longitudinal stretching in which the order is reversed, only the main orientation direction changes. I do not care. Further, the stretching direction may be only the longitudinal direction or the lateral direction.

  First, stretching in the longitudinal direction is performed. It is preferable that the substantially unoriented film is longitudinally stretched at a temperature of Tg or more and Tg + 30 ° C. or less so as to have a draw ratio of 3.0 times or more and 4.5 times or less. When the stretching ratio in the machine direction is increased, the orientation in the machine direction is excessively strengthened, and breakage occurs in the machine direction or in the transverse process in the next step. The preferable upper limit of the longitudinal stretching ratio is 4.2 times or less, and more preferably 3.9 times or less. After the longitudinal stretching, the final stretching ratio can be controlled by relaxing the film in the longitudinal direction. Relaxation is performed by heating the film after longitudinal stretching at a temperature of Tg or higher and Tg + 90 ° C. or lower, and relaxing the film at an arbitrary ratio in the longitudinal direction by using a means such as using a speed difference of rolls. be able to. As the heating means, any of rolls, near infrared rays, far infrared rays, hot air heaters and the like can be used.

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

  Next, stretching in the lateral direction is performed. Stretching in the transverse direction is preferably carried out at a temperature of Tg or more and Tg + 30 ° C. or less about 3.5 to 5.0 times in a state where clips are held at both ends in the film width direction in a tenter. Before stretching in the transverse direction, it is preferable to perform preheating at a temperature of Tg-10 ° C. or higher and Tg + 50 ° C. or lower.

  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 rising profile of the polyester resin used as the raw material. During the heat treatment, a relaxing process for reducing the distance between the clips in the lateral direction (different from the relaxing process in the longitudinal direction) may be performed at an arbitrary ratio. By heat-treating at a temperature above the melting start temperature, the crystal orientation of the film produced by stretching can be partially collapsed, the stress at 40% elongation can be adjusted, and the folding angle can be reduced. At the same time, the amorphous orientation of the film produced by the stretching can be destroyed by the heat treatment at the melting start temperature or higher, and the shrinkability can be reduced. When the heat treatment temperature is lower than the melting start temperature, the film is heat treated in the region where the film is crystallized, and the shrinkability of the film is lowered, but the folding holding angle is increased, which is not preferable. On the other hand, when the heat treatment temperature exceeds 240 ° C., the amorphous polyester resin is used in the present invention, so that the film shrinks greatly. As a result, unevenness in the thickness of the film during film formation is aggravated, and further progress of overheating causes the film to completely melt and break, which is not preferable.

  After that, a polyester film roll is obtained by winding the film while cutting and removing both end portions of the film.

Next, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the modes of the Examples, and may be appropriately performed without departing from the spirit of the present invention. It is possible to change.

  The evaluation method of the film is as follows. If the longitudinal direction and the width direction cannot be immediately specified because the area of the film is small, the longitudinal direction and the width direction may be temporarily determined and the measurement may be performed. There is no particular problem even if it is wrong by 90 degrees in the longitudinal direction and the width direction.

[Stress at 40% elongation]
A strip-shaped test piece of 140 mm in the width direction and 20 mm in the direction orthogonal to the measurement direction (longitudinal direction of the film) was prepared, where the measurement direction was the film width direction. Using a universal tensile tester “DSS-100” (manufactured by Shimadzu Corporation), grip each 20 mm from both ends of the test piece (distance between chucks: 100 mm) at an ambient temperature of 23 ° C. and a pulling speed of 200 mm / min. A tensile test was conducted 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 preparation direction of the sample piece by 90 degrees. When the film broke before the strain reached 40%, the stress was set to 0 MPa. Finally, the average value of the stress obtained from each of the width direction and the longitudinal direction was used as the stress at 40% strain.

[Folding 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 a 10 cm × 10 cm square in an environment of 20 ° C. and 65% RH, and folded in four (5 cm × 5 cm square). When folding the film, the short side of the first two-fold rectangle was oriented vertically. Then, a 5 kg weight having a bottom size of 3 cm × 3 cm was placed on the four-fold film for 20 seconds. After removing the weight, the four-fold film was left for 30 minutes. After that, the angle at which the folded film was opened (the completely folded state was defined as 0 degree) was measured and determined. Further, the folding holding angle in both the longitudinal direction and the lateral direction when the film was folded was measured, and the larger value was used as the folding holding angle.

[Hot water heat shrinkage rate]
Cut the film into squares of 10 cm x 10 cm, immerse it in hot water of 80 ± 0.5 ° C for 10 seconds with no load for heat shrinkage, then immerse it in water of 25 ° C ± 0.5 ° C for 10 seconds, and pull it out from the water. The dimensions of the film in the longitudinal and transverse directions were measured, and the shrinkage rate was calculated according to the following formula 1.

  Shrinkage rate = {(length before shrinkage-length after shrinkage) / length before shrinkage} × 100 (%) Equation 1

[Haze]
According to JIS-K-7136, it measured using the haze meter (Nippon Denshoku Industries Co., Ltd. make, 300A). The measurement was performed twice and the average value was calculated.

[Tensile fracture strength]
A strip-shaped test piece of 140 mm in the width direction and 20 mm in the direction orthogonal to the measurement direction (longitudinal direction of the film) was prepared, where the measurement direction was the film width direction. Using a universal tensile tester "DSS-100" (manufactured by Shimadzu Corporation), grip each 20 mm from each end of the test piece with a chuck (distance between chucks: 100 mm) at an ambient temperature of 23 ° C and a pulling speed of 200 mm / min. A tensile test was performed under the conditions, and the stress at the time of tensile fracture was defined as the tensile fracture strength. The measurement in the longitudinal direction was performed by changing the measurement in the width direction and the preparation direction of the sample piece by 90 degrees.

[Glass transition point (Tg)]
It was measured according to JIS K7121 using a differential scanning calorimeter (model: DSC220) manufactured by Seiko Instruments Inc. An unstretched film (10 mg) was heated from −20 ° C. to 120 ° C. at a heating rate of 10 ° C./min to obtain a heating profile. A tangent line was drawn before and after the inflection point of the obtained profile, and the intersection was read as the glass transition point (Tg; ° C).

[Melting start temperature]
Using a differential scanning calorimeter (model: DSC220) manufactured by Seiko Denshi Kogyo Co., Ltd., the extrapolation melting start temperature on the low temperature side was determined according to JIS K7121. First, 10 mg of the film was heated from 20 ° C. to 300 ° C. at a heating rate of 10 ° C./min to obtain a heating profile. In the temperature rising profile, the temperature at the intersection of the straight line extending the low temperature side to the high temperature side and the tangent line drawn at the point where the gradient of the low temperature side of the melting peak has the maximum was read as the melting onset temperature.

[Thickness variation in the longitudinal direction]
The film is sampled into a long roll with a length of 12 m and a width of 40 mm, and it is continuously measured along the longitudinal direction of the film at a measuring speed of 5 m / min using a continuous contact type thickness gauge manufactured by Micron Measuring Instruments Co., Ltd. The thickness was measured (measurement length was 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 in the longitudinal direction of the film was calculated from the following formula 2.

Thickness variation = {(Tmax.-Tmin.) / Tave.} X 100 (%) ・ ・ Equation 2

[Thickness variation in width direction]
The film is sampled into a wide strip with a length of 40 mm and a width of 1.2 m, and is continuously measured along the width direction of the film sample at a measurement speed of 5 m / min using a continuous contact thickness gauge manufactured by Micron Measuring Instruments Co., Ltd. The thickness was measured (measurement length was 500 mm). At the time of measurement, the maximum thickness was Tmax., The minimum thickness was Tmin., And the average thickness was Tave.

  The polyesters used in Examples and Comparative Examples are as follows.

・ Polyester 1: Copolyester 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: Polybutylene terephthalate 54 mol%, poly ε-caprolactone 46 mol% copolymer Polyester 6: Ethylene glycol 70 mol%, 1.4-Cyclohexanedimethanol 30 mol% and terephthalic acid copolymerized polyester

(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 put into an extruder. This mixed resin was melted at 270 ° C., extruded from a T-die, wound on a rotating metal roll cooled to a surface temperature of 25 ° C., and rapidly cooled to obtain an unstretched film having a thickness of 250 μm. The glass transition temperature of the unstretched film was 65 ° C, and the melting onset temperature was 130 ° C. The resulting unstretched film is guided to a longitudinal stretching machine in which multiple roll groups are continuously arranged, preheated by a preheating roll until the film temperature reaches 70 ° C, and then rotated at a low speed set to a surface temperature of 80 ° C. Using the difference in rotation speed between the roll and the high-speed rotation roll whose surface temperature was set to 80 ° C., it was stretched 3.5 times in the machine direction. Then, the longitudinally stretched film is preheated until the surface temperature of the film reaches 90 ° C in the state where the widthwise ends of the film are held by clips in the tenter, and then in the transverse direction at 85 ° C. It was stretched 4.0 times. The laterally stretched film was guided to a heat treatment zone in a tenter while holding both edges in the width direction with clips, and in the heat treatment zone, heat treatment was performed at a temperature of 140 ° C. for 10 seconds, and then the film was cooled. After that, both edges were cut off and wound into a roll with a width of 400 mm to continuously produce a biaxially stretched film of about 18 μm over a predetermined length.

  The characteristics of the obtained film were evaluated by the methods described above. The obtained stress-strain curve is shown in FIG. 2 as an example. The evaluation results are shown in Table 1. The obtained biaxially stretched film was a film having a preferable folding angle, a low heat shrinkage ratio, and a preferable haze, and it was very preferable overall.

(Example 2)
The same polyester raw material as in Example 1 was melt extruded in the same manner as in Example 1, and a film was formed by the same method and conditions as in Example 1 except that the heat treatment temperature was 170 ° C., thereby producing a biaxially stretched film of about 18 μm. Was continuously manufactured over a predetermined length. Then, the characteristics of the obtained film were evaluated by the methods described above. The evaluation results are shown in Table 1. A good film was obtained in the same manner as in Example 1.

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

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

(Example 5)
The above polyester 1, polyester 2, polyester 3 and polyester 4 were mixed at a weight ratio of 75: 10: 5: 10, and then melt-extruded in the same manner as in Example 1. The glass transition temperature of the unstretched film was 70 ° C, and the melting onset temperature was 140 ° C. After that, a biaxially stretched film of about 18 μm was continuously produced over a predetermined length by forming a film by the same method and conditions as in Example 1 except that the heat treatment temperature was 160 ° C. The properties of the obtained film were evaluated by the methods described above. The evaluation results are shown in Table 1. A good film was obtained in the same manner as in Example 1.

(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 having a thickness of 70 μm. The unstretched film was first preheated until the surface temperature of the film reached 85 ° C. in a state where the widthwise ends of the unstretched film were held by clips in a tenter, and stretched 4.0 times in the transverse direction at 80 ° C. The laterally stretched film was guided to a heat treatment zone in a tenter while holding both edges in the width direction with clips, and in the heat treatment zone, heat treatment was performed at a temperature of 140 ° C. for 10 seconds, and then the film was cooled. Then, in the same manner as in Example 1, a laterally uniaxially stretched film of about 18 μm was continuously manufactured over a predetermined length. Then, the characteristics of the obtained film were evaluated by the methods described above. The evaluation results are shown in Table 1. A good film was obtained in the same manner as in Example 1.

(Example 7)
After the above-mentioned polyester 3, polyester 4 and polyester 6 were mixed 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 onset temperature was 140 ° C. Then, a biaxially stretched film of about 18 μm was continuously manufactured over a predetermined length by forming a film under the same method and conditions as in Example 1. The properties of the obtained film were evaluated by the methods described above. The evaluation results are shown in Table 1. A good film was obtained in the same manner as in Example 1.

(Comparative Example 1)
After the above polyester 2 and polyester 3 were mixed at 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 the film by the same method and conditions as in Example 1 except that the heat treatment temperature was 200 ° C. The properties of the obtained film were evaluated by the methods described above. The evaluation results are shown in Table 1. The stress at 40% elongation and the folding holding angle were out of the specified range of the present invention, and the film was not preferable.

(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. Then, a film was formed under the same conditions as in Comparative Example 1 except that the heat treatment temperature was 90 ° C., whereby a biaxially stretched film of about 18 μm was continuously manufactured over a predetermined length. Then, the characteristics of the obtained film were evaluated by the methods described above. The evaluation results are shown in Table 1. The heat shrinkage rate of hot water at 70 ° C was large and the film became unfavorable.

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

  The polyester film of the present invention has excellent folding-holding property, low shrinkage property, and excellent transparency. Therefore, it can be preferably used as a substitute for paper such as wrapping paper, paper carrying bag, book cover, origami.

Claims (3)

It is made of amorphous polyester containing ethylene terephthalate unit, meets the following requirements (1) to (3), has a thickness of 3 μm or more and 80 μm or less, and has a folding holding angle of 20 ° or more and 50 ° or less. A polyester film roll, which is obtained by winding a polyester film used for a purpose into a roll shape.
(1) Regarding the stress at 40% elongation in each of the longitudinal and widthwise tensile tests 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 ) The heat shrinkage rate of hot water in the longitudinal and width directions 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 roll according to claim 1, wherein a melting start temperature of the polyester film in a DSC heating profile is 100 ° C or higher and 180 ° C or lower.   The polyester film roll according to claim 1 or 2, wherein the polyester film has a tensile breaking strength in a longitudinal direction and / or a width direction of 100 MPa or more and 300 MPa or less.