WO2016002488A1

WO2016002488A1 - Polyester film with fold retention, low shrinkage and excellent concealment properties

Info

Publication number: WO2016002488A1
Application number: PCT/JP2015/067175
Authority: WO
Inventors: 慎太郎石丸; 雅幸春田
Original assignee: 東洋紡株式会社
Priority date: 2014-07-04
Filing date: 2015-06-15
Publication date: 2016-01-07
Also published as: JPWO2016002488A1; JP6641999B2

Abstract

The purpose of the present invention is to provide a polyester film, which has a superior fold retention angle, for which thermal shrinkage in high temperature environments is very low, and which has excellent content concealment properties. The present invention is a polyester film characterized in being obtained from non-crystalline polyester containing ethylene terephthalate units and in satisfying conditions (1) through (3) below. (1) With regard to stress at 40% elongation in respective tension tests in the longitudinal and width directions of the film, the mean values for the stress at 40% elongation for the longitudinal direction and the stress at 40% elongation for the width direction are 30 MPa - 90 MPa. (2) The hot water thermal shrinkage ratios in the longitudinal and width directions when treated for 10 seconds in hot water of 80°C were both -10% - 10%. (3) The total light transmittance is 20% - 50%.

Description

Polyester film with excellent folding retention, low shrinkage and hiding

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

Since paper has excellent folding retention properties, it is widely used for various types of wrapping paper, handbags, origami. However, paper is inferior in water resistance, and when it gets wet with rain or the like, tearing may occur or printing may be discolored. For this reason, plastic films have been studied as a substitute for paper.

In the past, transparent cellophane has been used as a film with excellent folding retention that can replace paper. However, since cellophane has hygroscopic properties, its characteristics vary depending on the season, it is difficult to supply while maintaining the product quality constant, and the poor workability due to uneven thickness is a disadvantage. I came.

On the other hand, the polyethylene terephthalate film has excellent properties such as toughness, water resistance, and transparency, but has a drawback of poor folding retention.

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

However, the polyethylene terephthalate film of Patent Document 1 has a problem of high heat shrinkability. It is pointed out that if a bag using such a film is left in a midsummer car or in a warehouse without temperature control, the film shrinks and deforms and cannot be used. Further, in a processing step requiring high temperature such as printing on a film or adhesion by hot melt, there is a problem that processing cannot be performed due to shrinkage of the film.

Furthermore, since the polyethylene terephthalate film of Patent Document 1 is transparent, there is a problem that it lacks the concealability of the contents. In order to solve such a problem, there has been proposed a polyester film having improved concealability by expressing fine cavities in the polyester film (Patent Document 2). Such a film has excellent film concealability and excellent low shrinkage, but has a drawback of lacking folding retention.

Japanese Patent No. 4308862 Japanese Patent No. 3080190

The present invention aims to solve the problems of the prior art. That is, an object of the present invention is to provide a polyester film having an excellent folding holding angle, extremely low heat shrinkability under a high temperature environment, and excellent contents concealment.

The present invention has the following configuration.
1. A polyester film comprising an amorphous polyester containing an ethylene terephthalate unit and satisfying the following requirements (1) to (3).
(1) About 40% elongation stress in the longitudinal and width directions 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 30 MPa or more and 90 MPa or less (2 ) When heated for 10 seconds in hot water at 80 ° C, the hot shrinkage in the longitudinal and width directions is -10% to 10%. (3) Total light transmittance is 20% to 50%. 2. 1. The folding holding angle is 15 degrees or more and 45 degrees or less. The polyester film as described in.
3. 1. The melting start temperature in the DSC temperature rise profile is 100 ° C. or higher and 220 ° C. or lower. Or 2. The polyester film as described in.
4). Above 1. To 3. A method for continuously producing any polyester film, wherein a melt-extruded, cooled and solidified unstretched sheet is stretched in a longitudinal direction and / or a width direction, and then a temperature of not less than the melting start temperature of the polyester and not more than 240 ° C. A method for producing a polyester film, characterized by heat-treating.

The polyester film of the present invention has excellent folding retention properties, low heat shrinkability under high temperature environment, excellent concealment of contents, and excellent water resistance, transparency and printability. Therefore, it can be suitably used for paper replacement applications such as origami, handbags, book covers, and wrapping paper.

It is an example of the stress-strain curve in the tension test for evaluating the stress at 40% elongation.

It is a schematic diagram of the measuring method of a folding holding angle.

It is an example of a DSC profile for evaluating the melting start temperature.

The polyester film of the present invention is a film that can be used for alternative uses of paper that requires folding retention, such as origami, paper handbags, book covers, and wrapping paper. Printing may or may not be performed. In addition, it may be used by laminating and laminating with a film having excellent folding retention. Hereinafter, the polyester film will be described.

The polyester composition used for 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, and even more preferably 55 mol% or more in 100 mol% of the polyester constituting unit of the polyester composition. When the ethylene terephthalate unit is less than 40 mol% with respect to all the polyester units of the polyester composition constituting the film, the film strength and heat resistance are insufficient, which is not 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 An alicyclic dicarboxylic acid etc. can be mentioned.

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

Further, the polyester composition constituting the film is an amorphous polyester, and one or more kinds that can be an amorphous component in 100 mol% of a polyhydric alcohol component or 100 mol% of a polyvalent carboxylic acid component in the polyester. The total 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. Further, 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, 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 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, a cyclic diol such as 1,4-cyclohexanedimethanol, or a diol having 3 to 6 carbon atoms (for example, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, hexane) Diol, etc.) and a polyester elastomer containing ε-caprolactone, tetramethylene glycol, etc. can be used to lower the melting start temperature (details will be described later). Therefore, it is preferable to use at least one kind.

For example, in the case where 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 is reduced. The amount of the polyester elastomer 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 elastomer component that lowers the melting start temperature is contained, the ethylene terephthalate unit responsible for physical strength is relatively reduced, so that the film strength, heat resistance, etc. are insufficient. Because of fear, the amount of the polyester elastomer is preferably 30 mol% or less, more preferably 25 mol% or less with respect to all the polyester units of the polyester composition constituting the film.

In the present invention, in order to improve the concealability of the film, it is preferable to contain fine cavities inside. For example, a foam material may be mixed and extruded, but a preferable method is to mix a polyester in a thermoplastic resin that is incompatible with the polyester, and at least uniaxially stretch to obtain a cavity. . The thermoplastic resin incompatible with the polyester used in the present invention is arbitrary, and is not particularly limited as long as it is incompatible with the polyester. Specific examples include polystyrene resins, polyolefin resins, polyacrylic resins, polycarbonate resins, polysulfone resins, and cellulose resins. In particular, a polystyrene resin or a polyolefin resin such as polymethylpentene or polypropylene is preferable because of the formation of cavities.

Polystyrene resin refers to a thermoplastic resin containing a polystyrene structure as a basic component, and grafted or block copolymerized with other components in addition to homopolymers such as atactic polystyrene, syndiotactic polystyrene, and isotactic polystyrene. Modified resins, such as impact-resistant polystyrene resins and modified polyphenylene ether resins, and mixtures of thermoplastic resins having compatibility with these polystyrene resins, such as polyphenylene ether, are included.

The polymethylpentene resin is a polymer having units derived from 4-methylpentene-1 at 80 mol% or more, preferably 90 mol% or more, and other components include ethylene units, propylene units, butene- Examples are units derived from 1 unit, 3-methylbutene-1, and the like.

The polypropylene resin in the present invention includes modified resins obtained by grafting or block copolymerizing other components in addition to homopolymers such as isotactic polypropylene and syndiotactic polypropylene.

The thermoplastic resin that is incompatible with the polyester of the present invention exists in various forms such as a spherical shape, an elliptic spherical shape, or a thread shape in the polyester.

For the mixture of the polyester of the present invention and the thermoplastic resin incompatible with the polyester of the present invention, various additives such as waxes, antioxidants, antistatic agents, crystal nucleating agents, viscosity reducing agents are added as necessary. An agent, a heat stabilizer, a coloring pigment, a coloring inhibitor, an ultraviolet absorber, and the like can be added. Further, it is preferable to add fine particles as a lubricant for improving the workability (slidability) of the film and fine particles as a concealing aid for reducing the total light transmittance. As the fine particles, any one can be selected. For example, as inorganic fine particles, silica, alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate, etc. As organic fine particles, for example, acrylic resin Examples thereof include particles, melamine resin particles, silicone resin particles, and crosslinked polystyrene particles. The average particle diameter 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 film surface unevenness may become large. Therefore, it is preferably set to 3000 ppm or less, more preferably 1000 ppm or less.

As a method of blending the above particles into the resin forming the polyester film, for example, it can be added at any stage of producing the polyester resin. 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 using a vented kneading extruder and a polyester resin raw material, or a dried particle and a polyester resin raw material using a kneading extruder It is also preferable to carry out by blending method.

The film in the present invention is preferably provided with a layer B that does not contain cavities on at least one side of the layer A that contains many cavities inside. By doing so, the roughness of the surface is reduced, and the film does not impair the aesthetic appearance when printed. In addition, since there are portions where there are not many cavities in the film, the strength of the film is not significantly impaired. In order to achieve this configuration, different raw materials are put into different extruders A and B, melted, bonded in a molten state before or in the T-die, and solidified in close contact with the cooling roll, and thereafter It is preferable to stretch by the method described. A particularly preferable laminated structure is a B / A / B type two-type three-layer structure. By adopting the above-mentioned two-type three-layer structure, it is possible to prevent the production equipment from being soiled by fuming generated when a thermoplastic resin incompatible with polyester is heated, and both surfaces of the product film can be smoothed.

The ratio of the incompatible resin in the layer A (void-containing layer) is 0% or more and 20% or less by weight. 0% indicates that no incompatible resin is contained. In this case, the concealability is exhibited by adding fine particles as a concealing aid that reduces the total light transmittance described above. Further, if the incompatible resin is contained in an amount of 20% or more, the void content in the A layer is increased, and the physical strength of the film is lowered, which is not preferable. More preferably, it is 5% or more and 15% or less.

In addition, when the hiding property by the cavity developing agent such as polyolefin resin is insufficient, it is common to use inorganic particles such as titanium dioxide, calcium carbonate and barium sulfate together. The amount of these inorganic particles in the A layer is 2 to 25% by weight, preferably 5 to 22% by weight from the viewpoint of concealability. When the added amount of the inorganic particles is less than 2% by weight, the concealability is insufficient, which is not preferable. On the other hand, when the added amount of inorganic particles is more than 22% by weight, film formation becomes unstable and breakage frequently occurs, which is not preferable.

In the B layer, it is preferable to add fine particles as fine particles as a lubricant for improving the slipperiness of the film. As the fine particles as the lubricant, inorganic particles such as silica, alumina, titanium dioxide, calcium carbonate, kaolin, and barium sulfate can be used. The amount of the inorganic particles is preferably 50 ppm or more, more preferably 100 ppm or more based on the weight of the B layer film. However, if the content of the lubricant is too large, the film surface unevenness may become large. Therefore, it is preferably set to 3000 ppm or less, more preferably 1000 ppm or less.

Furthermore, the polyester film of the present invention can be subjected to corona treatment, coating treatment, flame treatment or the like in order to improve the adhesion of the film surface.

Next, the various characteristics of the polyester film of the present invention will be described. First, the polyester film of the present invention has a 40% elongation stress in the longitudinal direction and a 40% elongation stress in the longitudinal direction. The average value of stress at 40% elongation in the width direction is preferably 30 MPa or more and 90 MPa or less.

When the film is folded, it is considered that the valley side of the crease is broken due to local compression or the friction of the film itself, and at the same time, plastic deformation due to local stretching occurs on the crest side of the fold. The plastic deformation strain on the crease peak side varies depending on the thickness of the film and the degree of folding, but is considered to be within 20% to 60%. When this average value of 40% 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 lower the folding property. On the contrary, if the stress at 40% elongation is low, it is considered that the film easily yields, that is, is easy to be creased. The stress at 40% elongation is a value read from the point that the horizontal axis strain is 40% in the stress-strain curve obtained from the tensile test shown in FIG.

In order to reduce the stress at 40% elongation, as described above, the polyester composition constituting the film is preferably an amorphous polyester, and in 100 mol% of the polyhydric alcohol component in the polyester or The total of at least one monomer component that can be an amorphous component in 100 mol% of the polyvalent carboxylic acid component is 13 mol% or more, preferably 14 mol% or more, more preferably 15 mol% or more, particularly preferably 16 mol. % Or more. Further, 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, if 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 stress at 40% elongation is further reduced. More preferable. Further, in the final heat treatment step in the film forming step described later, it is necessary to heat at a temperature 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 is partially collapsed, and the stress at 40% elongation can be reduced.

If the average value of the stress at 40% elongation in the longitudinal direction of the film and the stress at 40% elongation in the width direction exceeds 90 MPa, it is not preferable because a crease opens when folded with origami or packaging, and a beautiful aesthetic cannot be obtained. The average value of the 40% stretching stress in the film longitudinal direction and the 40% stretching stress in the width direction is preferably 85 MPa or less, more preferably 80 MPa or less. The lower the stress at 40% elongation, the easier it is to cause mechanical fracture / deformation of the film and the better the folding property. However, the lower limit is 30 MPa in the current technical level.

When the film is left in a high temperature environment, the film has sufficient heat resistance, and the folding start temperature of the film is preferably 100 ° C. to 220 ° C. in order to maintain good folding retention. As described above, in order to adjust the melting start temperature of the film from 100 ° C. to 220 ° 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 at least one monomer component 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 amorphous components is not particularly limited, but the upper limit is preferably 30 mol%. In addition, cyclic diols such as 1,4-cyclohexanedimethanol, 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. Therefore, it is preferable to use at least one kind. For example, in the case where 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 is reduced. 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 component that lowers the melting start temperature is contained, the ethylene terephthalate unit responsible for physical strength is relatively reduced, and the film strength, heat resistance, etc. may be insufficient. Since it arises, it is preferable to set it as 30 mol% or less with respect to all the polyester units of the polyester composition which comprises a film, More preferably, it is 25 mol% or less.

Further, the polyester film of the present invention preferably has a hot water heat shrinkage in the width direction and the longitudinal direction of not less than -10% and not more than 10% when treated in hot water at 80 ° C. for 10 seconds. If it exceeds 10%, the film shrinks and deforms in a high temperature environment such as in a midsummer car or in a warehouse without temperature control, which is not preferable. The upper limit of hot water heat shrinkage is more preferably 9% or less, and even more preferably 8% or less. On the other hand, if the hot water heat shrinkage rate is less than -10%, it is difficult to maintain the original shape of the film as in the case where the shrinkage rate is high. The lower limit value of the hot water heat shrinkage is more preferably −7% or more, and further preferably −4% or more.

In order to keep the hot water heat shrinkage in the range of -10% to 10%, heat treatment should be performed at a temperature not lower than the melting start temperature of the polyester resin and not higher than 240 ° C in the final heat treatment step in the film forming step described later. Is preferred. When the heat treatment temperature is not higher than the melting start temperature, the shrinkage rate exceeds 10%, which is not preferable. Further, the content of the monomer that can be an amorphous component is preferably 30 mol% or less, more preferably 25 mol% or less, based on all the polyester units of the polyester composition constituting the film. If it exceeds 30 mol%, the shrinkage rate becomes too high even in a heat treatment at a temperature higher than the melting start temperature.

The total light transmittance of the polyester film of the present invention must be 20% or more and 50% or less. If the total light transmittance is 50% or more, the concealability of the film is inferior, and an object to be packaged can be seen when used as a packaging material. The total light transmittance of the film is preferably less than 20%, but in the present invention, 20% was the limit, so 20% was made the lower limit.

The polyester film of the present invention preferably has a folding holding angle of 15 degrees or more and 45 degrees or less measured by a method described later (FIG. 2). When it is 50 degrees or less, the crease opens when folded with origami or packaging, and a beautiful aesthetic appearance can be obtained. The folding holding angle is more preferably 40 degrees or less, and further preferably 35 degrees or less. Further, the smaller the folding holding angle, the better. However, the range of the present invention is 15 degrees as a lower limit, and even if it is 20 degrees or more, it can be said that it is practically preferable.

Since the folding holding angle decreases as the stress at 40% elongation decreases, the total of at least one monomer component that can be an amorphous component as described above is 13 mol% or more, preferably 14 mol% or more. The polyester resin is preferably used in an amount of 15 mol% or more, particularly preferably 16 mol% or more, and in the final heat treatment step during the film forming step, heat treatment is performed at a temperature not lower than the melting start temperature of the polyester resin and not higher than 240 ° C. It is preferable. Under these conditions, the stress at 40% elongation can be adjusted from 30 MPa to 90 MPa, and the folding angle can be adjusted from 15 degrees to 45 degrees.

In the polyester film of the present invention, the melting start temperature obtained from the DSC temperature rise profile is preferably 100 ° C. or higher and 220 ° C. or lower. The melting start temperature is more preferably 210 ° C. or lower. In the film heat treatment step described later, it is preferable that the film can be partially melted, and in this case, the melting start temperature affects. It is preferable that the melting start temperature is 220 ° C. or lower because the crystallinity of the polyester resin is not too high, and crystallization is not easily promoted during heat treatment, so that it partially melts. A film made of a polyester composition that is not accelerated to crystallize and has a large number of amorphous parts is preferable because the rebound upon folding is small and the folding retention is maintained well. It is preferable that the melting start temperature is 100 ° C. or higher 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 higher. The melting start temperature can be determined from the profile obtained from the DSC measurement shown in FIG. The melting start temperature is the inflection point of the endothermic curve shown on the high temperature side after 100 ° C.

In the polyester film of the present invention, the thickness of the film is preferably 3 μm or more and 200 μm or less. If the thickness of the film is less than 3 μm, processing such as printing may become difficult. On the other hand, when the film thickness is greater than 200 μm, the film is not foldable, and the weight of the film used increases and the cost increases, which is not preferable. The thickness of the film is more preferably 5 μm or more and 190 m or less, and further preferably 7 μm or more and 180 μm or less.

The thickness ratio of the A layer in the entire film is determined by the amount of thermoplastic resin and inorganic particles that are incompatible with polyester, but from the viewpoint of maintaining the strength of the film and concealing properties, it is 20% to 80% of the total film thickness. Is preferably 25% or more and 75% or less, and more preferably 30% or more and 70% or less. When the thickness of the A layer is less than 20% of the total thickness of the film, the amount of thermoplastic resin and inorganic particles that are incompatible with the polyester that needs to be added to the A layer becomes large, and film formation becomes difficult. On the other hand, when the thickness of the A layer is larger than 80%, the thickness of the B layer is relatively decreased and the film strength is decreased, which is not preferable.

The polyester-based film of the present invention described above is obtained by melting and extruding the above-described polyester raw material with an extruder to form an unstretched film, and then stretching the unstretched film uniaxially or biaxially by a predetermined method shown below. Further, it can be obtained by heat treatment. The polyester can be obtained by polycondensing the above-described preferred dicarboxylic acid component and diol component by a known method. In general, it is preferable to use two or more kinds of chip-like polyester as a raw material for the film.

When the raw material resin is melt-extruded, the polyester raw material is preferably dried using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer. After the polyester raw material is dried in such a manner, it is melted at a temperature of 200 to 300 ° C. and extruded into a film using an extruder. In extruding, any existing method such as a T-die method or a tubular method can be employed.

Then, an unstretched film can be obtained by quenching the extruded sheet-like molten resin. In addition, 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 employed.

In order to achieve the object of the present invention, the film stretching direction may be either the film longitudinal (longitudinal) direction or the lateral (width) direction. In the following, the longitudinal stretching-lateral stretching method in which longitudinal stretching is performed first and then lateral stretching will be described. However, even in the case of transverse stretching-longitudinal stretching in which the order is reversed, only the main orientation direction is changed. I do not care. Moreover, even if the extending | stretching direction is only a vertical direction or only a horizontal direction, as long as it does not deviate from the structural requirements of this invention, it does not matter.

First, longitudinal stretching is performed. The substantially unoriented film is preferably stretched longitudinally at a temperature of Tg or more and Tg + 30 ° C. or less so as to have a magnification of 3.0 to 4.5 times. When the stretching ratio in the longitudinal direction is increased, the orientation in the longitudinal direction is excessively strengthened, and breakage occurs in stretching in the longitudinal direction or in the lateral stretching in the next step. A preferable upper limit of the longitudinal draw ratio is 4.5 times or less, and more preferably 4.4 times or less. It is also possible to control the specific gravity by relaxing the film in the longitudinal direction after the longitudinal stretching. Relaxing is performed by relaxing the film at an arbitrary magnification in the longitudinal direction by heating the film after longitudinal stretching at a temperature of Tg or more and Tg + 90 ° C or less, and using means such as using a speed difference between rolls. be able to. As the heating means, any of a roll, a near infrared ray, a far infrared ray, a hot air heater and the like can be used.

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

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

After the transverse stretching, heat treatment of the film is performed. 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 a raw material. During the heat treatment, a relaxation process for reducing the distance between the clips in the lateral direction (different from the relaxation process in the longitudinal direction) may be performed at an arbitrary magnification. By heat-treating at or above the melting start temperature, the crystal orientation of the film produced by stretching can be partially collapsed to adjust the stress at 40% elongation, and has the effect of reducing the folding angle. At the same time, the heat treatment at the melting start temperature or higher can also collapse the amorphous orientation of the film produced by stretching, thereby reducing shrinkage. When the heat treatment temperature is lower than the melting start temperature, heat treatment is performed in the region where the film crystallizes, and the shrinkage of the film decreases, but the folding holding angle increases, 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 shrinkage of the film is increased during the heat treatment. As a result, the thickness unevenness of the film during film formation deteriorates, and further overheating is not preferable because the film completely melts and breaks.

Then, if it winds up, removing both ends of a film, a polyester-type film roll will be obtained. *

Next, the present invention will be specifically described by way of examples and comparative examples. However, the present invention is not limited to the modes of the examples, and may be appropriately selected without departing from the spirit of the present invention. It is possible to change.

The evaluation method of the film is as follows. In addition, when the longitudinal direction and the width direction cannot be specified immediately because the film area is small, the longitudinal direction and the width direction may be determined. Even if the longitudinal direction and the width direction are wrong by 90 degrees, there is no particular problem.

[Stress at 40% elongation]
When 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 orthogonal to the measurement direction (film longitudinal direction) was produced. Using a universal tensile tester “DSS-100” (manufactured by Shimadzu Corporation), grip 20mm each from both ends of the test piece (distance between chucks: 100mm), with an ambient temperature of 23 ° C and a tensile speed of 200mm / min. A tensile test was performed under the conditions. From the obtained stress-strain curve, the stress at 40% strain was defined as the stress at 40% elongation. The measurement in the longitudinal direction was carried out by changing the measurement in the width direction and the preparation direction of the sample piece by 90 degrees. In addition, when the film broke before the strain reached 40%, the stress was set to 0 MPa. Finally, the average value of stress obtained from each of 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 constant temperature 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 into four (2.5 cm × 2.5 cm square). When folding the film, the short side of the rectangle made by the first two folds was vertical. After that, a 5 kg weight with a bottom size of 3 cm x 3 cm was placed on a four-fold film for 20 seconds. After removing the weight, the four-fold film was left for 30 minutes. Thereafter, the angle at which the folded film was opened (the completely folded state was defined as 0 degree) was obtained by measurement. Further, the folding holding angle in both the vertical direction and the horizontal direction when the film was folded was measured, and the value with the larger angle was defined as the folding holding angle.

[Hot water heat shrinkage]
The film is cut into a 10cm x 10cm square, immersed in 80 ± 0.5 ° C warm water for 10 seconds under no load and heat-shrinked, then immersed in 25 ° C ± 0.5 ° C water for 10 seconds and pulled out of the water. Then, the vertical and horizontal dimensions of the film were measured, and the shrinkage rate was determined according to the following formula 1.
Shrinkage rate = {(length before shrinkage−length after shrinkage) / length before shrinkage} × 100 (%) Formula 1

[Total light transmittance]
Based on JIS-K-7136, it measured using a haze meter (Nippon Denshoku Industries Co., Ltd., 300A). The measurement was performed twice and the average value was obtained.

[Glass transition point (Tg)]
It calculated | required according to JISK7121 using the differential scanning calorimeter (model | form: DSC220) by Seiko Denshi Kogyo Co., Ltd. 10 mg of unstretched film was heated from −20 ° C. to 120 ° C. at a heating rate of 10 ° C./min to obtain a temperature rising profile. The temperature at the intersection of the base line extension below the glass transition temperature and the tangent indicating the maximum slope at the transition was taken as the glass transition temperature.

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

[Longitudinal thickness unevenness]
The film is sampled into a long roll with a length of 12m and a width of 40mm, and continuously along the longitudinal direction of the film at a measurement speed of 5m / min using a continuous contact 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., The average thickness was Tave.
Thickness unevenness = {(Tmax.−Tmin.) / Tave.} × 100 (%) ・・ Formula 2

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

<Preparation of polyester raw material>
Synthesis example 1
In a stainless steel autoclave equipped with a stirrer, a thermometer and a partial reflux condenser, 100 mol% of dimethyl terephthalate (DMT) as a dicarboxylic acid component and 100 mol% of ethylene glycol (EG) as a polyhydric alcohol component, Charge ethylene glycol to a molar ratio 2.2 times that of dimethyl terephthalate, 0.05 mol% of zinc acetate (based on the acid component) as a transesterification catalyst, 0.225 mol of antimony trioxide as a polycondensation catalyst % (Based on the acid component) was added, and the ester exchange reaction was carried out while distilling off the produced methanol out of the system. Thereafter, a polycondensation reaction was performed at 280 ° C. under a reduced pressure of 26.7 Pa to obtain polyester 1 having an intrinsic viscosity of 0.75 dL / g. Polyester 1 contains 1% by-product diethylene glycol (DEG). The composition is shown in Table 1.

Synthesis examples 2-5
In the same manner as in Synthesis Example 1, polyesters 2 to 5 shown in Table 1 were obtained. In the table, TPA is terephthalic acid, IPA is isophthalic acid, EG is ethylene glycol, BD is 1,4-butanediol, NPG is neopentyl glycol, and ε-CL is ε-caprolactone. The intrinsic viscosities of the respective polyesters were as follows: Polyester 2: 0.73 dL / g, Polyester 3: 0.80 dL / g, Polyester 4: 1.20 dL / g, Polyester 5: 0.78 dL / g. Each polyester was appropriately formed into a chip shape.

[Table 1]

[Example 1]
Polyester 1, Polyester 2, and Polyester 3 are mixed at a weight ratio of 10:80:10, and SiO ₂ (Silicia 266 manufactured by Fuji Silysia) is added as a lubricant to a concentration of 50 ppm with respect to the polyester mixture. Layer raw material. The raw material for layer A is polyester 1, polyester 2 and polyester 3 mixed at a weight ratio of 10:80:10, and polystyrene resin (G797N manufactured by Nippon Polystyrene) 10% by weight and titanium dioxide (TA-300 manufactured by Fuji Titanium) 10 wt% was added and mixed. The raw materials of layer A and layer B are put into separate twin screw extruders, mixed, melted, and joined by a feed block, melt extruded at 280 ° C from a T-die, and cooled to a surface temperature of 30 ° C. The film was wound around a rotating metal roll and quenched to obtain an unstretched film having a thickness of 240 μm and a B / A / B laminated structure (layer ratio B / A / B = 1/2/1). Then, the 240 μm-thick unstretched film obtained as described above was guided to a longitudinal stretching machine in which a plurality of roll groups were continuously arranged, and stretched in the longitudinal direction using the difference in the rotational speed of the rolls. That is, after pre-heating an unstretched film on a preheating roll until the film temperature reaches 85 ° C., between a low-speed rotating roll set to a surface temperature of 85 ° C. and a high-speed rotating roll set to a surface temperature of 30 ° C. The machine was stretched longitudinally 3.5 times using the difference in rotational speed.

Thereafter, the film after longitudinal stretching is preheated until the surface temperature of the film reaches 100 ° C. with the clips held at both ends in the width direction in the tenter, and then in the transverse direction at 90 ° C. Stretched 4.0 times. The laterally stretched film was guided to a heat treatment zone in the tenter with both ends in the width direction held by clips, and cooled in the heat treatment zone at a temperature of 200 ° C. for 10 seconds. After that, by cutting and removing both edges and winding up into a roll with a width of 400 mm, a biaxially stretched film of about 20 μm was continuously produced over a predetermined length (the film finally obtained was Since it contains voids inside, it does not equal the value obtained by dividing the thickness of the unstretched film by the stretch ratio).
The properties of the obtained film were evaluated by the method described above. The evaluation results are shown in Table 2.
The obtained biaxially stretched film was a film having a low folding angle, hot water shrinkage, and total light transmittance, and 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 so that the thickness of the unstretched sheet was 280 μm (layer ratio B / A / B = 1/2/1), and the draw ratio was 4.0. Except for the above, the film was stretched in the machine direction in the same manner as in Example 1, and then a biaxially stretched film of about 20 μm was continuously formed over a predetermined length by forming a film under the same conditions as in Example 1. Manufactured. And the characteristic of the obtained film was evaluated by the above-mentioned method. The evaluation results are shown in Table 2. A good film was obtained as in Example 1.

Example 3
The same polyester raw material as in Example 1 was melt extruded in the same manner as in Example 1, and longitudinal stretching and lateral stretching were performed in the same manner as in Example 1. A biaxially stretched film having a thickness of about 20 μm was continuously produced over a predetermined length by forming a film under the same conditions as in Example 1 except that the film after transverse stretching was heat-treated at 180 ° C. And the characteristic of the obtained film was evaluated by the above-mentioned method. The evaluation results are shown in Table 2. A good film was obtained as in Example 1.

Example 4
In Example 1, heat shrinkage was carried out in the same manner as in Example 1 except that 10% by weight of the polystyrene resin added to the raw material of the A layer was changed to 10% by weight of crystalline polypropylene resin (FO-50F ground polymer). Film was continuously produced. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 2.
A good film was obtained as in Example 1.

Example 5
In Example 1, the heat-shrinkable film was continuously formed in the same manner as in Example 1 except that the polyester 3 was used as the raw material polyester 3 for the A layer and B layer to be fed into the extruder and the weight ratio was the same as in Example 1. Manufactured. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 2.
A good film was obtained as in Example 1.

Example 6
Polyester 1, Polyester 2, Polyester 3, and Polyester 5 are mixed at a weight ratio of 20: 50: 10: 20, and SiO ₂ (Silicia 266 manufactured by Fuji Silysia) as a lubricant is 50 ppm with respect to the polyester mixture. To make a raw material for the B layer. The raw material resin for layer A is polyester 1, polyester 2, polyester 3, and polyester 5 mixed at a weight ratio of 20: 50: 10: 20, 10% by weight of polystyrene resin (G797N made by Nippon Polystyrene) and titanium dioxide (TA- (300 Fuji Titanium) 10% by weight was added and mixed. Thereafter, a film was continuously produced by the same method as in Example 1. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 2. A good film was obtained as in Example 1.

Example 7
The above polyester 1, polyester 2, and polyester 3 are mixed at a weight ratio of 30:60:10, and SiO2 (Silicia 266 manufactured by Fuji Silysia) as a lubricant is added so as to be 50 ppm with respect to the polyester mixture. As a raw material. The raw material resin for layer A is polyester 1, polyester 2 and polyester 3 mixed at a weight ratio of 30:60:10. Polystyrene resin (G797N made by Nippon Polystyrene) 10% by weight and titanium dioxide (TA-300 made by Fuji Titanium) 10 wt% was added and mixed. Thereafter, a film was continuously produced by the same method as in Example 1. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 2. A good film was obtained as in Example 1.

[Comparative Example 1]
In both layers A and B, only the above-mentioned polyester 1 was used as a raw material, and SiO2 (Silicia 266 manufactured by Fuji Silysia) was added as a lubricant to layer B so as to be 50 ppm with respect to the polyester. The raw materials of the A layer and the B layer were extruded in the same manner as in Example 1 and then rapidly cooled to obtain an unstretched film having a thickness of 180 μm and a B / A / B laminated structure (layer ratio B / A / B = 1/2/1). The unstretched film having a thickness of 180 μm obtained as described above was longitudinally stretched in the same manner as in Example 1 except that the stretching temperature was 94 ° C. and the stretching ratio was 3.0 times.
Thereafter, transverse stretching was performed in the same manner as in Example 1 except that the preheating temperature and the stretching temperature were 100 ° C. and the stretching ratio was 3.0 times. The temperature of the subsequent heat treatment zone was set to room temperature, and the zone was passed through without being subjected to the heat treatment. Thereafter, both edges were cut and removed and wound into a roll having a width of 400 mm, whereby a biaxially stretched film of about 20 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 2.
The obtained biaxially stretched film had high hot water shrinkage and total light transmittance, and an unfavorable film for the present invention was obtained.

[Comparative Example 2]
SiO ₂ (Silicia 266 manufactured by Fuji Silysia) as a lubricant was added to the above polyester 1 so as to be 50 ppm with respect to the polyester mixture, and used as a raw material for the B layer. The raw material for layer A was

polyester

1 and 20% by weight of polystyrene resin (G797N manufactured by Nippon Polystyrene) and 20% by weight of titanium dioxide (TA-300 manufactured by Fuji Titanium). The raw materials of the A layer and the B layer were extruded in the same manner as in Example 1 and then rapidly cooled to obtain an unstretched film having a thickness of 180 μm and a B / A / B laminated structure (layer ratio B / A / B = 1/2/1). The unstretched film having a thickness of 190 μm obtained as described above was stretched in the same manner as in Example 1 except that the stretching temperature was 90 ° C.

Thereafter, the film after longitudinal stretching was stretched in the same manner as in Example 1 except that the preheating temperature and stretching temperature were 135 ° C. and the stretching ratio was 3.5 times. The film after transverse stretching was subjected to heat treatment and cooling in the same manner as in Example 1 except that the heat treatment temperature was 220 ° C. Thereafter, both edges were cut and removed and wound into a roll having a width of 400 mm, whereby a biaxially stretched film of about 20 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 2.
Since the obtained biaxially stretched film could not be measured because the film was cracked when evaluating the folding angle, an unfavorable film was obtained as the present invention.

[Comparative Example 3]
The same polyester raw material as in Example 1 was melt extruded in the same manner as in Example 1, and longitudinal stretching and lateral stretching were performed in the same manner as in Example 1. A biaxially stretched film of about 20 μm was continuously produced over a predetermined length by forming a film under the same conditions as in Example 1 except that the film after transverse stretching was heat-treated at 120 ° C. And the characteristic of the obtained film was evaluated by the above-mentioned method. The evaluation results are shown in Table 2. The obtained film had a high hot water shrinkage rate, and an unfavorable film was obtained in the present invention.

[Table 2]

1 ... folding holding angle 2 ... melting point start temperature

Claims

A polyester film comprising an amorphous polyester containing an ethylene terephthalate unit and satisfying the following requirements (1) to (3).
(1) About 40% elongation stress in the longitudinal and width directions 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 30 MPa or more and 90 MPa or less (2 ) When heated for 10 seconds in hot water at 80 ° C, the hot shrinkage in the longitudinal and width directions is -10% to 10%. (3) Total light transmittance is 20% to 50%.
The polyester film according to claim 1, wherein the folding holding angle is 15 degrees or more and 45 degrees or less.
The polyester film according to claim 1 or 2, wherein the melting start temperature in the DSC temperature rising profile is 100 ° C or higher and 220 ° C or lower.
A method for continuously producing a polyester film according to any one of claims 1 to 3, wherein an unstretched sheet that has been melt-extruded and cooled and solidified is stretched in the longitudinal direction and / or the width direction, and then the melting start temperature of the polyester or higher. A method for producing a polyester film, wherein the heat treatment is performed at a temperature of 240 ° C. or lower.