JP2021161151A - Biaxial oriented film - Google Patents

Abstract

To provide a biaxial oriented film excellent in folding resistance, thermal resistance, and workability.SOLUTION: A biaxial oriented film comprises, as a main component, a polybutylene naphthalate-based copolymer (A).SELECTED DRAWING: None

Description

The present invention relates to a biaxially stretched film having excellent heat resistance and folding resistance.

Polyester is highly versatile because it has excellent properties such as heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, and gas barrier properties, and it is also easily available at a reasonable price. It is a resin widely used for food containers, packaging materials, molded products, films, etc. The main polyester resin is polyethylene terephthalate (hereinafter sometimes referred to as "PET"), which has excellent mechanical properties, electrical properties, chemical resistance, etc. and has a wide range of uses, but has heat resistance and folding resistance. There are difficulties in sex.

Further, as an acid component using naphthalenedicarboxylic acid, polybutylene naphthalate obtained by polymerizing naphthalenedicarboxylic acid and 1,4-butanediol (hereinafter, may be referred to as "PBN") is known. Since the glass transition temperature is as low as about 75 ° C. and the melting point is as low as about 240 ° C., there is a problem in heat resistance. In addition, since the crystallization rate is too fast, there is a point that it is not suitable for film formation by extrusion molding.

On the other hand, in recent years, the need for a flexible display has been increasing, and there is a strong demand for a film having high heat resistance, excellent resilience, and excellent resistance to repeated bending.

For example, in Patent Document 1, a film resistant to repeated bending by a cyclic olefin resin film is studied.

Further, Patent Document 2 proposes a polyimide film as a film having excellent heat resistance and bending resistance.

Further, Patent Document 3 proposes a polyester copolymer having an improved crystallization rate and suitable for injection moldability.

Japanese Unexamined Patent Publication No. 2014-104687 International Publication No. 2016/060213 Japanese Unexamined Patent Publication No. 5-209044

However, the film disclosed in Patent Document 1 has a low level of repeated bending resistance and does not meet the demands of the market. Further, since the cyclic olefin resin has poor coatability and adhesiveness, it is considered that it is difficult to laminate it with other members as a member for a flexible display.

Further, although the polyimide film described in Patent Document 2 has bending resistance, it is poor in productivity and costly because it is a molding method by coating using a solvent in the manufacturing process thereof.

The polyester copolymer described in Patent Document 3 (diol component is 1,4-butanediol and 1,4-cyclohexanedimethanol, and acid component is 2,6-naphthalenedicarboxylic acid) improves the crystallization rate. Although the contents suitable for injection moldability are shown, if the crystallization rate is too fast, the film-forming property and stretchability of the film by extrusion molding deteriorate, so that the film is not suitable for stretched films.

An object to be solved by the present invention is to solve the above-mentioned problems and to provide a biaxially stretched film having excellent folding resistance, heat resistance and processability.

As a result of diligent studies to achieve the above problems, the present inventors have completed the present invention. The gist of the present invention is the following [1] to [10] in one aspect thereof.
The gist of the present invention is as follows.
[1] A biaxially stretched film containing a polybutylene naphthalate-based copolymer (A) as a main component.
[2] The biaxially stretched film according to the above [1], wherein the average value of the hysteresis loss rate when a tensile cycle test up to 5% tensile strain is performed in each of the MD and TD directions is 50.0% or less.
[3] The polybutylene naphthalate-based copolymer (A) contains a 2,6-naphthalenedicarboxylic acid unit as a dicarboxylic acid component (a-1) and 1,4-cyclohexane as a diol component (a-2). The biaxially stretched film according to the above [1] or [2], which contains a dimethanol unit and a 1,4-butanediol unit.
[4] The diol component (a-2) contains 10 mol% or more and 90 mol% or less of 1,4-cyclohexanedimethanol units, and 10 mol% or more and 90 mol% or less of 1,4-butanediol units. The biaxially stretched film according to any one of the above [1] to [3].
[5] Any one of the above [1] to [4], wherein the average value of the residual strain when the tensile cycle test up to 5% tensile strain is performed in each of the MD and TD directions is 1.00% or less. The biaxially stretched film according to.
[6] The biaxially stretched film according to any one of the above [1] to [5], wherein the glass transition temperature is 75 ° C. or higher and 150 ° C. or lower.
[7] The biaxially stretched film according to any one of the above [1] to [6], wherein the crystal melting temperature is 220 ° C. or higher and 300 ° C. or lower.
[8] The biaxially stretched film according to any one of the above [1] to [7], which has a thickness of 1 μm or more and 250 μm or less.
[9] The biaxially stretched film according to any one of the above [1] to [8] for display.
[10] A foldable display provided with the biaxially stretched film according to any one of the above [1] to [9].

The biaxially stretched film proposed by the present invention is excellent in folding resistance, heat resistance, and processability.

The profile of the stress-strain curve.

Hereinafter, the present invention will be described in detail. However, the content of the present invention is not limited to the embodiments described below.

<Biaxially stretched film>
The biaxially stretched film (hereinafter, may be referred to as “the present film”) according to an example of the embodiment of the present invention is a polybutylene naphthalate (hereinafter, may be referred to as “PBN”)-based copolymer (A). ) Is the main component.
In the present invention, a biaxially stretched film is obtained by using a PBN-based resin which has conventionally been unsuitable for a biaxially stretched film because the crystallization rate is too fast. Since this film is a biaxially stretched film, it is a thin film and has excellent folding resistance.

1. 1. Physical characteristics First, the physical characteristics of this film will be described.

(1) Hysteresis loss rate It is preferable that the average value of the hysteresis loss rate when the tensile cycle test up to 5% tensile strain is performed in each of the MD and TD directions of this film at 23 ° C. is 50.0% or less. , More preferably 47.0% or less, still more preferably 45.0% or less, even more preferably 40.0% or less, and particularly preferably 38.0% or less. The lower limit is not particularly limited, but is 0.100% or more.
When the hysteresis loss rate is 50.0% or less, the bending resistance of the film is kept within the practical range. Further, by setting the hysteresis loss rate as the average value in each of the MD and TD directions, it can be used as a characteristic index of the film as a whole. The hysteresis loss rate can be adjusted according to the stretching conditions and the like.
The hysteresis loss rate of this film can be measured by the method described in Examples according to JIS K 7312: 1996.
More specifically, when the stress-strain curve graph as shown in FIG. 1 is obtained by the tensile cycle test, the ratio of the area surrounded by abcef to the total area (abcda) is the hysteresis loss rate. Is defined as.
The MD means the flow direction of the film, and the TD means the direction perpendicular to the MD.

The hysteresis loss rate in the MD or TD direction of this film at 23 ° C. is preferably 50.0% or less, more preferably 47.0% or less, still more preferably 45.0% or less, and further. It is preferably 40.0% or less, and particularly preferably 38.0% or less. It is preferable that the hysteresis loss rate of one of MD and TD is within the above numerical range, and it is more preferable that the hysteresis loss rate of both MD and TD is within the above numerical range.

Further, the difference between the hysteresis loss rates in the MD and TD directions of this film at 23 ° C. is preferably 20.0% or less, more preferably 15.0% or less, still more preferably 10.0% or less. Yes, and even more preferably 5.0% or less.
When the difference between the hysteresis loss rates in the MD and TD directions is within the above numerical range, the bending resistance of the film and the anisotropy of various film characteristics are reduced, so that when the film is used as a member or secondary processing is performed. Sometimes it is not necessary to select a specific direction, and problems only in a specific direction due to anisotropy are unlikely to occur, so that the film is excellent in handleability.

(2) Residual Strain It is preferable that the average value of the residual strain of the film when a tensile cycle test up to 5% tensile strain is performed in each direction of MD and TD at 23 ° C. is 1.00% or less, which is more preferable. Is 0.900% or less, more preferably 0.850% or less, still more preferably 0.800% or less, and particularly preferably 0.750% or less. The lower limit is not particularly limited, but is 0.1% or more.
When the residual strain is 1.00% or less, the bending resistance of the film is kept within the practical range. Further, by setting the residual strain as the average value in each of the MD and TD directions, it can be used as a characteristic index of the film as a whole. The residual strain can be adjusted by stretching conditions and the like.
The residual strain of this film can be measured by the method described in Examples according to JIS K 7312: 1996.

The residual strain of this film in the MD or TD direction at 23 ° C. is preferably 1.00% or less, more preferably 0.900% or less, still more preferably 0.850% or less, and even more preferably 0.850% or less. Is 0.800% or less, particularly preferably 0.750% or less, and more particularly preferably 0.700% or less. The residual strain of one of MD and TD is preferably within the above numerical range, and the residual strain of both MD and TD is more preferably within the above numerical range.

Further, the difference in residual strain in the MD and TD directions of this film at 23 ° C. is preferably 0.900% or less, more preferably 0.500% or less, still more preferably 0.200% or less. , More preferably 0.100% or less, and particularly preferably 0.050% or less.
When the difference between the residual strains in the MD and TD directions is within the above numerical range, the bending resistance of the film and the anisotropy of various film characteristics are reduced. Therefore, when the film is used as a member or during secondary processing. Since it is not necessary to select a specific direction and problems occur only in a specific direction due to anisotropy, the film has excellent handleability.

(3) Glass transition temperature The glass transition temperature (Tg) of this film is preferably 75 ° C. or higher and 150 ° C. or lower. It is more preferably 76 ° C. or higher and 140 ° C. or lower, and further preferably 77 ° C. or higher and 130 ° C. or lower. When the Tg is 75 ° C. or higher, the film is not deformed when used for a display, so that it can be said to have excellent heat resistance. On the other hand, when Tg is 150 ° C. or lower, the workability is also suitable.
The glass transition temperature (Tg) of this film is measured at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC) according to JIS K7121 (2012).
(4) Crystal melting temperature
The crystal melting temperature (Tm) of this film is preferably 220 ° C. or higher and 300 ° C. or lower.
In particular, it is more preferably 221 ° C. or higher and 295 ° C. or lower, further preferably 222 ° C. or higher and 290 ° C. or lower, and particularly preferably 223 ° C. or higher and 285 ° C. or lower. As long as the crystal melting temperature Tm of this film is within the range, this film has an excellent balance between heat resistance and melt extrusion moldability. Here, the crystal melting temperature Tm is measured for this film at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC) according to JIS K7121 (2012).

(5) Thickness The thickness of the present film is preferably 1 to 250 μm, more preferably 5 to 200 μm. By setting the film strength to 1 μm or more, the film strength is kept within the practical range. When it is 250 μm or less, folding resistance is likely to be exhibited. The thickness can be adjusted according to the film forming and stretching conditions.

2. Components Next, the components constituting this film will be described.

<Polybutylene naphthalate copolymer (A)>
This film contains a polybutylene naphthalate-based copolymer (A) as a main component.
Here, in the present invention, the "main component" means that the content of the PBN-based copolymer (A) in the present film is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, still more preferable. Is 80% by mass or more, more preferably 90% by mass or more (including 100% by mass).

It is generally known that glycol-modified polyethylene terephthalate obtained by introducing a copolymerization component into polyethylene terephthalate has improved impact resistance as compared with ordinary polyethylene terephthalate. On the other hand, there is a concern that the heat resistance may decrease due to the decrease in crystallinity. In addition, since it is necessary to raise the molding temperature by improving the glass transition temperature and the melting point, there may be a concern about thermal decomposition of the resin.
The present invention has made it possible to produce a biaxially stretched film by introducing a copolymerization component into polybutylene naphthalate, further improving impact resistance, and by extension, excellent bending resistance, heat resistance, and molding. We have found a film containing the PBN-based copolymer (A), which is also excellent in processability, as a main component.

The PBN-based copolymer (A) in the present invention contains a dicarboxylic acid component (a-1) and a diol component (a-2).
As the dicarboxylic acid component (a-1), 2,6-naphthalenedicarboxylic acid is an essential component, and other copolymerization components are added as necessary. Other copolymerization components include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-frangicarboxylic acid, 3 , 4-Frangicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid and other aromatic dicarboxylic acids; cyclohexanedicarboxylic acid, oxalic acid , Malonic acid, succinic acid, glutaric acid, adipic acid, pimelli acid, suberic acid, azelaic acid, sebacic acid, dimeric acid and other aliphatic dicarboxylic acids; oxycarboxylic acids such as p-oxybenzoic acid and the like. Of these, isophthalic acid, 2,5-frangicarboxylic acid, 2,4-frangicarboxylic acid, and 3,4-frangicarboxylic acid are preferable from the viewpoint of moldability.
These copolymerization components may be used alone or in combination of two or more.

On the other hand, as the diol component (a-2), 1,4-butanediol is an essential component, and if necessary, as other copolymerization components, 1,4-cyclohexanedimethanol, ethylene glycol, diethylene glycol, etc. Examples thereof include propylene glycol, neopentyl glycol, polytetramethylene ether glycol, dimerdiol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F or bisphenol S or derivatives thereof, or ethylene oxide adducts thereof). One type or two or more types can be used. Of these, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimerdiol, and bisphenols are preferable. In particular, from the viewpoint of maintaining the film strength, it is preferable to use 1,4-cyclohexanedimethanol and bisphenols. In particular, it is preferable to use 1,4-cyclohexanedimethanol.

As described above, the PBN-based copolymer of the present invention has a dicarboxylic acid component (a-1) and a diol component (a) with respect to PBN obtained from 2,6-naphthalenedicarboxylic acid and 1,4-butanediol. -2) At least one of them contains a copolymerization component. By including such a copolymerization component, biaxial stretching, which has been difficult until now, can be made possible, and bending resistance, heat resistance, and molding processability can be further improved with respect to PBN.

The PBN-based copolymer (A) used in the present invention contains 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1) and 1,4-cyclohexanedimethanol units as the diol component (a-2). It is preferably a polybutylene naphthalate-based copolymer containing 1,4-butanediol unit.

The PBN-based copolymer (A) contains 2,6-naphthalenedicarboxylic acid units in the dicarboxylic acid component (a-1) in an amount of preferably 90 mol% or more, more preferably 92 mol% or more, still more preferably 94 mol. % Or more, more preferably 96 mol% or more, particularly preferably 98 mol% or more, and all (100 mol%) of the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid. Especially preferable.
By setting the content of the 2,6-naphthalenedicarboxylic acid unit in the dicarboxylic acid component (a-1) within the above numerical range, the glass transition temperature and melting point of the polybutylene naphthalate-based polymer are improved, and thus the book. The heat resistance of the film is improved.

The PBN-based copolymer (A) contains 1,4-cyclohexanedimethanol in the diol component (a-2), preferably 10 mol% or more and 90 mol% or less, more preferably 20 mol% or more and 85 mol% or less. , More preferably 30 mol% or more and 80 mol% or less, still more preferably 40 mol% or more and 75 mol% or less, and particularly preferably 50 mol% or more and 70 mol% or less.
By keeping the content of 1,4-cyclohexanedimethanol in the diol component (a-2) within the above numerical range, the glass transition temperature and melting point of the PBN-based copolymer (A) are improved, and thus the present film. Heat resistance is improved. Further, since the crystallinity can be controlled, the crystallization rate can be slowed down, and the extrusion moldability and stretchability of the film can be improved. Further, when the content is 90 mol% or less, the melting point does not become too high. Therefore, it is not necessary to set the molding temperature high, and there is no concern about thermal decomposition.

The PBN-based copolymer (A) contains 1,4-butanediol in the diol component (a-2), preferably 10 mol% or more and 90 mol% or less, more preferably 15 mol% or more and 80 mol% or less. It is more preferably 20 mol% or more and 70 mol% or less, still more preferably 25 mol% or more and 60 mol% or less, and particularly preferably 30 mol% or more and 50 mol% or less.
By keeping the content of 1,4-butanediol in the diol component (a-2) within the above numerical range, the crystallinity of the polyester resin (A) is maintained, and the heat resistance of this film is improved. .. On the other hand, if the content exceeds 90 mol%, the crystallization rate may be too fast and the extrusion film-forming property and the film stretchability may be deteriorated.

The polyester resin (A) may be copolymerized with less than 10 mol% of a diol component other than 1,4-cyclohexanedimethanol and 1,4-butanediol for the purpose of improving moldability and heat resistance.
Specifically, 1,2-propanediol, 1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, ethylene glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, Examples thereof include 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, hydroquinone, bisphenol, spiroglycol, 2,2,4,4-tetramethylcyclobutane-1,3-diol, isosorbide, and the like. Of these, ethylene glycol, diethylene glycol, 1,3-propanediol, and 1,3-cyclohexanedimethanol are preferable from the viewpoint of moldability.

The amount of heat of crystal fusion ΔHm of the PBN-based copolymer (A) is preferably 15 J / g or more and 60 J / g or less, and more preferably 20 J / g or more or 50 J / g or less. As long as ΔHm (A) is in such a range, the PBN-based copolymer (A) has appropriate crystallinity excellent in heat resistance, moisture heat resistance, melt moldability, and stretchability.
The amount of heat of crystal melting ΔHm (A) of the PBN-based copolymer (A) can be measured at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC) according to JIS K7121 (2012). ..

The crystal melting temperature Tm (A) of the PBN-based copolymer (A) is preferably 220 ° C. or higher and 300 ° C. or lower, more preferably 225 ° C. or higher or 290 ° C. or lower, and 230 ° C. or higher or 280 ° C. or lower. It is more preferable that the temperature is 235 ° C. or higher and 270 ° C. or lower. As long as the crystal melting temperature Tm (A) of the PBN-based copolymer (A) is within the range, the PBN-based copolymer (A) has an excellent balance between heat resistance and melt moldability.
The crystal melting temperature Tm (A) of the PBN-based copolymer (A) can be measured at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC) according to JIS K7121 (2012). ..

The glass transition temperature Tg (A) of the PBN-based copolymer (A) is preferably 75 ° C. or higher and 150 ° C. or lower, more preferably 77 ° C. or higher and 145 ° C. or lower, and 80 ° C. or higher and 140 ° C. or lower. It is more preferable that there is. When the glass transition temperature Tg (A) of the PBN-based copolymer (A) is within such a range, the balance between heat resistance and melt moldability is excellent.

In the present invention, it is permissible for the present film to contain a resin other than the PBN-based copolymer (A) as long as the effects of the present invention are not impaired.
Other resins include polystyrene-based resins, polyvinyl chloride-based resins, polyvinylidene chloride-based resins, chlorinated polyethylene-based resins, polyester-based resins, polycarbonate-based resins, polyamide-based resins, polyacetal-based resins, acrylic resins, and ethylene acetic acid. Vinyl copolymer, polymethylpentene resin, polyvinyl alcohol resin, cyclic olefin resin, polylactic acid resin, polybutylene succinate resin, polyacrylonitrile resin, polyethylene oxide resin, cellulose resin, polyimide resin , Polyurethane resin, polyphenylene sulfide resin, polyphenylene ether resin, polyvinyl acetal resin, polybutadiene resin, polybutene resin, polyamideimide resin, polyamide bismaleimide resin, polyetherimide resin, polyether ether ketone Examples thereof include resins, polyether ketone resins, polyether sulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine resins.

Further, in the present invention, in addition to the above-mentioned components, the present film may appropriately contain additives generally blended as long as the effects of the present invention are not significantly impaired. Examples of the additive include recycled resins generated from trimming loss of ears and the like, which are added for the purpose of improving and adjusting molding processability, productivity and various physical properties of porous films, silica, talc, kaolin and calcium carbonate. Inorganic particles such as titanium oxide, pigments such as carbon black, flame retardants, weather resistance stabilizers, heat stabilizers, antistatic agents, melt viscosity improvers, cross-linking agents, lubricants, nucleating agents, plasticizers, anti-aging agents, etc. Additives such as antioxidants, light stabilizers, UV absorbers, neutralizers, antifogging agents, antiblocking agents, lubricants, and colorants can be mentioned.

Further, in the present invention, in addition to the above-mentioned additives, a coating layer can be provided on the present film within a range that does not significantly impair the effects of the present invention. Functions of the coating layer include hard coat property, antistatic property, peeling property, easy adhesive property, printability, UV cut property, infrared ray blocking property, gas barrier property and the like. The coating layer may be formed by an in-line coating that treats the film surface during the stretching process, or an offline coating that is applied outside the system on the once manufactured film may be adopted, or both may be used in combination. May be good.

<Manufacturing method of this film>
The method for producing a biaxially stretched film of the present invention will be described, but the following description is an example of a method for producing the present film, and the present film is not limited to the film produced by such a production method.

The method for producing the present film according to an example of the embodiment of the present invention is a method for producing a resin composition containing the PBN-based copolymer (A) as a main component, which is formed into a film and biaxially stretched.

The method for obtaining the resin composition by kneading the PBN-based copolymer (A), other resins, and additives is not particularly limited, but in order to obtain the resin composition as easily as possible, an extruder is used. It is preferably produced by melt-kneading. In order to uniformly mix the raw materials constituting the resin composition, it is preferable to melt-knead using a twin-screw extruder in the same direction.
The kneading temperature must be equal to or higher than the glass transition temperature of all the polymers used, and for the crystalline resin, to be equal to or higher than the crystal melting temperature of the polymer. When the kneading temperature is as high as possible with respect to the glass transition temperature and crystal melting temperature of the polymer to be used, a transesterification reaction of a part of the polymer is likely to occur and the compatibility is likely to be improved, but the kneading temperature is higher than necessary. If the temperature is high, the resin is decomposed, which is not preferable. From this, the kneading temperature is preferably 255 ° C. or higher and 340 ° C. or lower, more preferably 260 ° C. or higher and 330 ° C. or lower, further preferably 270 ° C. or higher and 320 ° C. or lower, and particularly preferably 280 ° C. or higher and 310 ° C. or lower. As long as the kneading temperature is high, compatibility and melt moldability can be improved without causing decomposition of the polymer.

The obtained resin composition can be molded by a general molding method, for example, extrusion molding, injection molding, blow molding, vacuum molding, pressure molding, press molding, or the like to produce a biaxially stretched film. In each molding method, the apparatus and processing conditions are not particularly limited.
This film is preferably produced, for example, by the following method.

From the resin composition obtained by mixing, a film having a substantially amorphous shape and not oriented (hereinafter, may be referred to as "unstretched film") is produced by an extrusion method. The production of this unstretched film employs, for example, an extrusion method in which the raw material is melted by an extruder, extruded from a flat die or an annular die, and then rapidly cooled to form a flat or annular unstretched film. Can do things. At this time, depending on the case, a laminated structure using a plurality of extruders may be used.

Next, the above unstretched film is usually placed in at least one direction in the film flow direction (longitudinal direction, MD) and in the direction perpendicular to the film flow direction (horizontal direction, TD) in terms of stretching effect, film strength, and the like. Stretch in the range of 1.1 to 5.0 times, preferably 1.1 to 5.0 times in each of the vertical and horizontal directions.

As the biaxial stretching method, conventionally known stretching methods such as tenter-type sequential biaxial stretching, tenter-type simultaneous biaxial stretching, and tubular simultaneous biaxial stretching can be adopted. For example, in the case of the tenter-type sequential biaxial stretching method, the unstretched film is heated to a temperature range of Tg to Tg + 50 ° C. with the glass transition temperature of the resin composition as Tg, and the unstretched film is heated in the longitudinal direction by a roll-type longitudinal stretching machine. It can be manufactured by stretching 1.1 to 5.0 times in the transverse direction and then stretching 1.1 to 5.0 times in the lateral direction within the temperature range of Tg to Tg + 50 ° C. by a tenter type transverse stretching machine. can. Further, in the case of the tenter type simultaneous biaxial stretching method or the tubular type simultaneous biaxial stretching method, for example, in the temperature range of Tg to Tg + 50 ° C., the film is stretched 1.1 to 5.0 times in each axial direction at the same time in the vertical and horizontal directions. Can be manufactured by.

The biaxially stretched film stretched by the above method is subsequently heat-fixed. Dimensional stability at room temperature can be imparted by heat fixing. The treatment temperature in this case is preferably selected in the range of the crystal melting temperature Tm-1 to 50 ° C. of the resin composition. If the heat fixing temperature is within the above range, heat fixing is sufficiently performed, stress during stretching is relaxed, sufficient heat resistance and mechanical properties are obtained, and there are no problems such as breakage or whitening of the film surface. The film is obtained.

In the present invention, in order to relax the stress of crystallization shrinkage due to heat fixing, it is preferable to perform relaxation in the range of 0 to 15%, preferably 3 to 10% in the width direction during heat fixing. Since the film is sufficiently relaxed and relaxes uniformly in the width direction of the film, the shrinkage rate in the width direction becomes uniform, and a film having excellent room temperature dimensional stability can be obtained. In addition, since the film is relaxed following the shrinkage of the film, there is no film tarmi or fluttering in the tenter, and there is no breakage of the film.

<Use of this film>
Since the biaxially stretched film of the present invention is excellent in folding resistance and heat resistance and also excellent in transparency, it can be used as a display film, a packaging film and various protective films. Among them, from the viewpoint of folding resistance, it can be suitably used for a foldable display.

Examples are shown below, but these do not limit the present invention in any way.

(1) Hysteresis loss rate
According to JIS K 7312: 1996, the average value of the hysteresis loss rate at 23 ° C. was determined by the following method.
A tensile tester (Tensile tester AG-1kNXplus manufactured by Shimadzu Corporation) was used as the measuring device. The test piece used was cut out from this film into a rectangle having a length of 100 mm and a width of 10 mm in the measurement direction. One cycle of tension in which both ends of the test piece in the length direction are chucked at a distance between chucks of 50 mm, the strain is increased to 5% at a crosshead speed of 0.5 mm / min, and then lowered to the initial position at the same speed. The stress-strain curve obtained from the cycle test was obtained. The stress-strain curve has a profile as shown in FIG. 1, and the hysteresis loss rate is obtained from the obtained stress-strain curve by the area A1 (abcda) of the curve obtained by the ascending operation and the area A1 and the descending operation. It was calculated by the following formula 1 using the area A2 (abcef) which is the difference between the areas of the obtained curves. The test was measured three times and the average value was calculated. The above tension cycle test was carried out on MD and TD of the film, respectively, and the average value was determined.
Hysteresis loss rate = A2 / A1 × 100 (Equation 1)

(2) Residual strain According to JIS K 7312: 1996, the average value of the residual strain at 23 ° C. was determined by the following method.
A tensile tester (Tensile tester AG-1kNXplus manufactured by Shimadzu Corporation) was used as the measuring device. The test piece used was cut out from this film into a rectangle having a length of 100 mm and a width of 10 mm in the measurement direction. One cycle of tension in which both ends of the test piece in the length direction are chucked at a distance between chucks of 50 mm, the strain is increased to 5% at a crosshead speed of 0.5 mm / min, and then lowered to the initial position at the same speed. From the stress-strain curve obtained from the cycle test, the strain at the point where the stress disappeared was defined as the residual strain. The test was measured three times and the average value was calculated. The above tension cycle test was carried out on MD and TD of the film, respectively, and the average value was determined.

(3) Glass transition temperature The obtained film is once heated to the melting temperature at a heating rate of 10 ° C./min according to JIS K7121 (2012) using a Diamond DSC (manufactured by Perkin Elmer Japan). After that, the temperature was lowered at a heating rate of 10 ° C./min, and the glass transition temperature in the heating process of a heating rate of 10 ° C./min was measured.

(4) Crystal melting temperature For the obtained film, use Diamond DSC (manufactured by PerkinElmer Japan Co., Ltd.) to determine the crystal melting temperature in the heating process at a heating rate of 10 ° C./min according to JIS K7121 (2012). It was measured.

(5) Formability When a transparent film is obtained without crystallizing and whitening when cooling and solidifying with a cast roll in extrusion molding to obtain a pre-stretched sheet, ○, a crystallized and whitened film is obtained. When it was, it was evaluated as x.

[PBN-based copolymer (A)]
As the PBN-based copolymer (A) -1, the dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, the diol component (a-2): 1,4-cyclohexanedimethanol = 60. Mol%, 1,4-butanediol = 40 mol% was used. The Tm of the PBN-based copolymer (A) -1 was 251 ° C, and the Tg was 101 ° C.
As the PBN-based copolymer (A) -2, the dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, the diol component (a-2): 1,4-cyclohexanedimethanol = 60. Mol%, 1,4-butanediol = 40 mol% was used. The Tm of the PBN-based copolymer (A) -2 was 254 ° C, and the Tg was 96 ° C.
As the PBN-based copolymer (A) -3, the dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, the diol component (a-2): 1,4-butanediol = 100 mol. % Was used. The Tm of the PBN-based copolymer (A) -3 was 243 ° C, and the Tg was 78 ° C.

(Example 1)
Pellet-shaped (A) -1 simple substance is melt-kneaded with a Φ25 mm twin-screw extruder set at 285 ° C, extruded as a film from inside a T-die with a gap of 1.0 mm, taken up by a cast roll at 100 ° C, and cooled and solidified. , A film-like material having a thickness of about 450 μm was obtained.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 120 ° C. Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 3.0 times in the transverse direction (TD) at a preheating temperature of 110 ° C., a stretching temperature of 115 ° C., and a heat fixing temperature of 165 ° C., and then. The film was relaxed by 5% in the tenter. Table 1 shows the results of measurement on the obtained film.

(Example 2)
(A) -2 was used instead of the PBN-based copolymer (A) -1, and a cast film was obtained in the same manner as in Example 1.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 2.9 times in the longitudinal direction (MD) at 110 ° C. Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) to perform 3.1-fold stretching in the transverse direction (TD) at a preheating temperature of 107 ° C., a stretching temperature of 110 ° C., and a heat fixing temperature of 150 ° C., and then. The film was relaxed by 5% in the tenter. Table 1 shows the results of measurement on the obtained film.

(Comparative Example 1)
An attempt was made to form a cast film by the same method as in Example 1 except that (A) -3 was used instead of the PBN-based copolymer (A) -1 and the temperature of the cast roll was set to 75 ° C. However, whitening occurred due to crystallization, and an amorphous film could not be collected.

The films of Examples 1 and 2 have high crystal melting temperature and glass transition temperature, and are excellent in heat resistance.
On the other hand, in Comparative Example 1, since the crystallization rate was too fast, the amorphous pre-stretched sheet could not be collected, and there was a problem in moldability.

Claims (10)

A biaxially stretched film containing a polybutylene naphthalate copolymer (A) as a main component. The biaxially stretched film according to claim 1, wherein the average value of the hysteresis loss rate when a tensile cycle test up to 5% tensile strain is performed in each of the MD and TD directions is 50.0% or less. The polybutylene naphthalate-based copolymer (A) contains 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1) and 1,4-cyclohexanedimethanol units as the diol component (a-2). The biaxially stretched film according to claim 1 or 2, which comprises 1,4-butanediol unit. A claim that the diol component (a-2) contains 10 mol% or more and 90 mol% or less of 1,4-cyclohexanedimethanol units and 10 mol% or more and 90 mol% or less of 1,4-butanediol units. Item 2. The biaxially stretched film according to any one of Items 1 to 3. The biaxial stretching according to any one of claims 1 to 4, wherein the average value of the residual strain when the tensile cycle test up to 5% tensile strain is performed in each of the MD and TD directions is 1.00% or less. the film. The biaxially stretched film according to any one of claims 1 to 5, wherein the glass transition temperature is 75 ° C. or higher and 150 ° C. or lower. The biaxially stretched film according to any one of claims 1 to 6, wherein the crystal melting temperature is 220 ° C. or higher and 300 ° C. or lower. The biaxially stretched film according to any one of claims 1 to 7, wherein the thickness is 1 μm or more and 250 μm or less. The biaxially stretched film according to any one of claims 1 to 8, which is for a display. A foldable display comprising the biaxially stretched film according to any one of claims 1 to 9.

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