JP3988228B2 - Biaxially oriented polyester film - Google Patents

Description

比較例１
実施例１で用いたリニアー駆動方式による同時二軸延伸装置の代わりに通常のパンタグラフ式同時二軸延伸装置を用いて、巻き取り速度１２０ｍ／分の低速延伸する以外は実施例１と全く同じにして、厚さ１２μｍの二軸延伸フィルムを得た。その結果とフィルム評価結果を表２に示した。
【００４５】
表２に示したように、延伸速度を下げると厚み方向の圧縮弾性率が大きくなり、硬いフィルムになるばかりか、厚みむらの悪いフィルムしか得られないことがわかる。
【００４６】
比較例２
実施例１で用いたキャストフィルムをそのまま逐次二軸延伸装置にて二軸延伸した。すなわち、一段目の長手方向延伸は、従来のロール式延伸機を用いて、９６℃で４倍延伸後、いったんガラス転移温度以下に冷却した後、テンターにて幅方向に１００℃で４．２倍延伸し、これを再度、ロール式再延伸装置にて長手方向に１３５℃で１．５倍で延伸し、テンターで２３０℃の熱処理を行い、その後均一に両軸方向に５％のリラックスをさせながら徐冷して室温まで冷却して、約５７０ｍ／分という超高速で巻き取り、厚さ１２μｍの二軸配向フイルムを得た。かくして製膜した状況、および得られたフィルム特性を、実施例１の結果とともに表２に示した。
【００４７】
このように延伸方式を逐次二軸延伸という汎用延伸方式では、たとえ延伸速度を速めても圧縮弾性率の低いサンプルは得られないばかりか、厚みむらの悪い、熱寸法安定性の悪いフィルムしか得られないことがわかる。このことは、感熱転写受容紙などの用途に使うと印字性の悪い画像しか得られないことになり、好ましくないものである。
【００４８】
比較例３
比較例２で延伸した長手方向延伸倍率、幅方向延伸倍率、および再度の長手方向延伸倍率を、それぞれ４倍から２．５倍に、４．２倍から３倍に、さらに１．５倍から１．０倍に変更して、厚さ１２μｍの二軸延伸フィルムを製膜速度として約４００ｍ／分で巻き取った。
【００４９】
この様に延伸倍率を下げたフィルムの圧縮弾性率はある程度低いものの、それよりも厚みむらが極端に悪く、受容紙等の用途に全く利用できない。
【００５０】
【表２】

【００５１】
【発明の効果】
以上、説明したように、本発明に係る同時二軸延伸ポリエステルフィルムは、従来の二軸延伸ポリエステルフィルムが有していない、厚み方向に柔らかく、かつ良好なクッション性を兼備するとともに、厚み均質性にも優れたフィルムである。このフィルムは、良好なクッション性が必要とされる分野用フィルムあるいはベースフィルムとして好適なものであり、例えば、受容紙用のベースフィルム等として好適に使用でき、かつ、生産性にも優れた製造法によって製造することができるものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a biaxially oriented polyester film having a small compressive elastic modulus in the thickness direction. For example, the biaxially oriented polyester film preferably used particularly for a base film for receiving paper, etc. The present invention relates to a simultaneous biaxially oriented polyester film.
[0002]
[Prior art]
Biaxial stretching of the polyester film is usually performed by a so-called sequential biaxial stretching method in which the polyester film is stretched in the longitudinal direction and then stretched in the width direction. Of course, when the stretching order is reversed, or after biaxial stretching, further stretching in the longitudinal direction and / or the width direction has been performed.
[0003]
[Problems to be solved by the invention]
However, such a normal sequential biaxial stretching method has a drawback that the compression elastic modulus in the thickness direction is inevitably increased, resulting in a so-called hard film. On the other hand, if the biaxial stretching ratio is successively lowered and the film is stretched, a film having a small compression elastic modulus can be obtained. However, the resulting film has a large thickness unevenness and poor flatness. It cannot be used as a material.
[0004]
  Therefore, the present invention has excellent characteristics equivalent to those obtained by normal biaxial stretching in the mechanical characteristics and thickness accuracy in the in-plane direction, but the compressive modulus in the thickness direction is small, and soft touch properties are achieved. Biaxial withOriented polyesterIt is intended to provide a film.
[0005]
[Means for Solving the Problems]
  The biaxially stretched polyester film of the present invention that achieves the above-mentioned object has a compression modulus of 95 mg / μm² 300 mg / μm or more²And thickness unevenness in the film longitudinal direction5% or moreIt is characterized by being 6% or less.
[0006]
  The film is preferably used as a stretching speed, in particular, after being closely cooled and solidified in a cooling medium while applying an electrostatic charge to the molten polyester film.39000A biaxially oriented polyester film characterized by being obtained by a method such as simultaneous biaxial stretching in the longitudinal direction and the width direction at a high speed of at least% / min.
[0007]
Further, in the present invention, when it is obtained by simultaneous biaxial stretching, the simultaneous biaxial stretching is performed by a tenter provided with a clip driven by a linear motor system. It is an axially oriented polyester film.
[0008]
In the present invention, the biaxially oriented polyester film is preferably used as a base film for receiving paper.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
  The biaxially stretched polyester film of the present invention has a compression modulus of 95 mg / μm.²300 mg / μm or more²And thickness unevenness in the film longitudinal direction5% or moreIt is characterized by being 6% or less, and the compression elastic modulus is 95 mg / μm.² 300 mg / μm or more²The following biaxially oriented polyester film is a supple film having cushioning properties in the thickness direction as compared with a normal biaxially stretched film. However, the so-called cellular film, which has a reduced density by providing voids inside the film, has weak mechanical strength such as tensile strength and bending strength, is opaque, and has limited use, thermal transfer receiving paper In reality, it cannot be used as most industrial materials.
[0010]
  The film of the present invention is not inferior to the conventional polyester film in properties such as transparency and in-plane mechanical strength, but only the compressive modulus in the thickness direction is smaller than that of the conventional film and provides a cushioning film. To do. As compression modulus,95 (mg / μm ² ) more than300 (mg / μm²), Preferably95 (mg / μm ² ) more than200 (mg / μm²), More preferably95 (mg / μm ² ) more than100 (mg / μm²) The following biaxially oriented polyester film. The polyester in the present invention is a compound having an ester bond in the polymer main chain, specifically, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene-2,6-naphthalate (PEN), poly Methylene terephthalate (PPT), polyethylene-p-oxybenzoate (POB), poly-1,4-cyclohexylene dimethylene terephthalate (PCT), and copolymerization components such as diethylene glycol, neopentyl glycol, polyalkylene glycol, etc. Polyester resins obtained by copolymerization of diol components, dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid. Among them, polyethylene-2,6-naphtha Over preparative (PEN) or polyethylene terephthalate (PET) it is particularly preferred. In addition, various additives such as lubricants, antiblocking agents, antioxidants, antistatic agents, crystal nucleating agents, thinning agents, heat stabilizers and the like are added to the polymer compound. A compound or the like may be added. Typical additives include silicon dioxide, alumina, calcium carbonate, zirconium oxide, talc, kaolin, clip, barium sulfate, titanium oxide, crosslinked polystyrene resin, crosslinked polyester resin, and mixtures thereof. is there.
[0011]
  After applying the electrostatic charge to the above-mentioned polyester resin melt film and allowing it to cool and solidify in a cooling medium, and subjecting the resulting film to normal sequential biaxial stretching and heat treatment steps, the film with a small compression modulus is I can't get it. However, the film is simultaneously biaxially stretched and the simultaneous stretching speed is increased.39000% / min or more,Preferably, it can be obtained only by simultaneous biaxial stretching at a high speed of 100,000% / min or more. The reason for this is not clear, but it is thought that the molecular orientation is firmly fixed by cooling below the glass transition temperature of the film for each stretching, as in sequential biaxial stretching. In biaxial stretching, the molecules may be stretched and oriented while being completely relaxed.
[0012]
The biaxially oriented film in the present invention refers to a film that is stretched in the longitudinal direction and the width direction and has at least molecular orientation in the biaxial direction. Although it does not specifically limit as a biaxial stretching system, The simultaneous biaxial stretching system which extends | stretches a longitudinal direction and the width direction simultaneously is especially preferable. Note that simultaneous biaxial stretching refers to a stretching method for simultaneously orienting the film in the longitudinal direction and the biaxial direction of the width direction. For example, while holding both ends of the film with clips using a simultaneous biaxial tenter. It is the method of conveying and extending | stretching to a longitudinal direction and the width direction. Of course, it is sufficient if there is a portion in which the stretching in the longitudinal direction and the width direction are simultaneously stretched in time, and after the stretching in the width direction or the longitudinal direction alone, the longitudinal direction and the width direction are simultaneously performed. A stretching method and a method of further stretching independently in the width direction or the longitudinal direction after simultaneous biaxial stretching are also included. Incidentally, in the case of the present invention, a method of re-stretching after simultaneous biaxial stretching is particularly preferable. The film temperature during each of these stretching steps is preferably maintained at or above the glass transition temperature of the polyester film.
[0013]
In addition, since the stretching speed in each direction can be as fast as 10,000% / min or more, and the stretching direction and stretching ratio can be freely changed, the simultaneous biaxial tenter clips are individually separated by the linear motor system. It is better to use a system driven by Conventional simultaneous biaxial stretching methods have used a screw method in which a clip is placed in a screw groove to increase the clip interval, and a pantograph method in which the clip interval is increased using a pantograph. However, with this method, there were problems such as slow film formation and difficulty in changing conditions such as stretch ratio, but each clip was controlled independently using the principle of linear motors. If a linear drive tenter that adjusts the clip interval is used, the film forming speed can be increased more than the conventional sequential biaxial stretching method, and an arbitrary stretching method / form can be incorporated. For example, in order to form a highly oriented film in the longitudinal direction, it can be stretched in multiple stages in each stretching direction, or can be further stretched in the longitudinal direction or the width direction after simultaneous biaxial stretching, and further biaxial stretching Later, a relaxing process of returning the dimensions in the longitudinal direction and / or the width direction is possible in the heat treatment step, so that a film having excellent dimensional stability and a film having no physical property unevenness (no bowing) in the width direction can be obtained. it can. That is, it is possible to form various excellent films with high productivity by using a linear drive tenter using clips that can be individually controlled. In order to be able to take such a free stretch format, the film end thickness is about 2 to 3 times thicker than the center, or the molecular weight of the end is made higher than the center. It is effective to use a film having an end portion.
[0014]
The magnification of simultaneous biaxial stretching varies depending on the type of resin, but is usually about 2 to 12 times in each direction.
[0015]
In addition, in order to remove the distortion after stretching, a relaxation heat treatment (heat setting) is often performed. At this time, the glass of the stretched film is not temporarily heat treated immediately after stretching, but once stretched. Heating after cooling to the transition temperature Tg or lower is preferable because a film having uniform physical properties in the width direction can be obtained. In addition, as temperature of heat processing, various temperature from extending | stretching temperature to the melting | fusing point vicinity of resin can be employ | adopted suitably according to a use.
[0016]
It is relatively easy to cause the orientation main axis of the simultaneous biaxially stretched film thus obtained to exist in the longitudinal direction and the width direction, and to produce a film in which the orientation is balanced on both axes.
[0017]
Next, although the manufacturing method of the film of this invention is demonstrated, this invention is not necessarily limited to this.
[0018]
First, raw material pellets containing additives necessary for the polymerization step of the polyester resin or the extruder process are prepared, and the raw material is dried in hot air or under vacuum and supplied to the extruder. In the extruder, in order to minimize the melting point or the thermal decomposition product or gelled product of the resin as much as possible, remove the foreign matter through the filter while the resin is flowing, and connect a gear pump etc. The uniformity of the extrusion amount is improved and the thickness unevenness is reduced. The resin sent from the extruder to the die is discharged after being formed into a target shape by the die. The resin temperature at the time of discharge is usually equal to or higher than the melting end temperature (Tme). However, the molten resin is cooled in, for example, the second stage of the tandem extruder to be in a supercooled state, or is cooled in the land portion immediately before the die exit, which is a portion where the gap is narrow and the flow velocity is high, to be in the supercooled state. By extruding, it is possible to obtain a film with good thickness uniformity with less generation of oligomers with less thermal decomposition and less melt vibration. That is, the temperature of the resin discharged from the die is usually equal to or higher than the melting end temperature (Tme), but a temperature range below the melting end temperature (Tme) and above the temperature-falling crystallization start temperature (Tcb). In this state, since the resin has a high viscosity and is stable against membrane vibration and disturbance between the die and the cooling drum, a film with small thickness unevenness can be obtained, and a film with few thermal decomposition products can be obtained. As a cooling means for the land portion of the die, for example, there is a method in which a hole for cooling is provided in the land portion and a coolant is passed through the hole. As the refrigerant, various liquid refrigerants such as air or water can be used, and the desired temperature can be set by controlling the temperature and flow rate of the refrigerant. Furthermore, it is preferable to use a heat pipe such as water together for temperature equalization. If such an extruder or die is used, the resin and apparatus used for the production of conventional films can be used as they are. When the temperature is lower than Tcb, the resin starts to crystallize, and the extruded film becomes rough, causes abnormal extrusion, uneven flow, or solidifies over time, which is not preferable because it becomes difficult to extrude with a normal extruder. . Utilizing the supercooled state of the polymer, it is important to extrude in the liquid phase state although it is below the melting point. The melting end temperature (Tme) and the temperature drop crystallization start temperature (Tcb) can be determined by a differential scanning calorimeter (DSC). DSC is usually used in thermal analysis, and is a method of measuring endotherm and exotherm accompanying state changes such as melting, crystallization, phase transition, and thermal decomposition of substances. Various methods can be used when measuring the melting temperature at the time of temperature rising of the thermoplastic resin and the crystallization temperature at the time of temperature falling by DSC. By adopting the supercooling extrusion method in this way, it is possible to easily stretch at a high simultaneous biaxial stretching ratio by forming a cast film with less gelation and small thickness unevenness, as well as uniform quality. It becomes a good film.
[0019]
  Static electricity is applied to the sheet-like film discharged from the die, and the sheet-like film is closely cooled and solidified on the cast drum. At this time, if a very thin water film or water droplet of about 0.1 μm is placed on the drum, the adhesion to the drum is further improved, and a completely amorphous and uniform film can be obtained. It is preferable because it is possible. Note that the shape of the thermoplastic resin film is to adjust the gap between the caps and reduce the end thickness to about three times the thickness of the central portion in order to improve stretchability and to improve the film formation yield. good. In addition, the stretchability is further stabilized by increasing the molecular weight only at the end of the film, but for this purpose, a raw material having a molecular weight higher than the molecular weight at the center is supplied by using another extruder, and before or inside the die. And coextrusion so as to be laminated only on the film edge. Stretching from an amorphous state is preferable, but thermal dimensional stability can be achieved by forming a film having a slight crystallinity, that is, a crystallinity of about 0.5 to 15 (%) as required. It is preferable because a film having good properties and slipperiness may be obtained. The thickness unevenness in the longitudinal direction of the cast film before stretching thus obtained is5% or more and 6% or less.
[0020]
The unstretched film is subjected to a simultaneous biaxial tenter and simultaneously biaxially stretched, but may be simply started and terminated simultaneously in the longitudinal direction and the width direction, but before and after the simultaneous biaxial stretching, the longitudinal direction and Alternatively, prior stretching in the width direction or re-stretching after simultaneous biaxial stretching may be performed. In particular, by stretching again in the longitudinal direction after simultaneous biaxial stretching, the film shape in the vicinity of the edge becomes linear, and not only the cutting rate decreases, but also the strength in the longitudinal direction is strong (F5 value is 14 kg / mm).²As described above, a film having good flatness and small thickness unevenness is preferable. The simultaneous biaxial stretching temperature is preferably not less than the glass transition temperature of the thermoplastic resin and less than (glass transition temperature Tg + 100 ° C.). Here, when the stretching is less than the glass transition temperature Tg, the film is broken and the stretching becomes impossible. On the other hand, if it exceeds (glass transition temperature Tg + 100 ° C.), the molecules cannot be effectively oriented at the time of stretching, the film is torn or the thickness unevenness becomes large, and uniform stretching becomes impossible.
[0021]
After simultaneous biaxial stretching, it is preferable that the resin constituting the film is cooled to a temperature below the glass transition point, preferably to a temperature below the glass transition point of −10 ° C., and then heat-treated for imparting thermal dimensional stability. . When heat treatment is performed at the same temperature immediately after stretching, the softened film in the heat treatment process is dragged into the stretching process due to stress and heat shrinkage stress during stretching, causing a bowing phenomenon, and the refractive index ellipsoid of the film is distorted. As a result, a physical property distribution in the width direction is generated. Therefore, in order to avoid these bowing phenomena, after biaxial stretching, the resin is once cooled to a temperature below the glass transition point of the resin, a hard part is provided between the stretching process and the heat treatment process, and the heat treatment is performed after separating each process. By doing so, it becomes possible to suppress the Boeing phenomenon.
[0022]
Furthermore, it is preferable to perform a relaxation treatment that reduces the dimensions during and after the heat treatment because higher thermal dimensional stability can be obtained. However, if the heat treatment temperature is too high in pursuit of thermal dimensional stability, or if the heat treatment is performed at a relaxation rate that is too high, properties such as strength and flatness are deteriorated.
[0023]
The film thus obtained is gradually cooled to room temperature and then taken up by a winder. The film thus obtained is effective not only for thermal transfer receiving paper, but also for thermal stencil sheet, condenser sheet, thermal transfer ribbon sheet, magnetic tape sheet, packaging sheet, etc. It can be used.
[0024]
[Method for evaluating physical properties]
1. Thermal properties
As a differential scanning calorimeter, a robot DSC “RDC220” manufactured by Seiko Denshi Kogyo Co., Ltd. is used. As a data analysis device, a disk station “SSC / 5200” manufactured by the same company is used. About 10 mg of a sample is placed on an aluminum tray at 300 ° C. After melting and holding for 5 minutes and rapidly solidifying with liquid nitrogen, the temperature was raised from room temperature at a rate of temperature rise of 20 ° C./min. At this time, the observed glass transition temperature peak temperature was Tg, the melting endothermic peak start temperature was Tmb, the peak temperature was Tm, and the peak end temperature was Tme. Further, after 5 mg of the sample was melted and held at 300 ° C. for 5 minutes, the temperature was lowered at a temperature lowering rate of 20 ° C./min. The starting temperature of the cooling crystallization exothermic peak observed at this time was Tcb, the peak temperature was Tc, and the peak end temperature was Tce.
[0025]
2. Uneven thickness in the longitudinal direction of the film
Using an Anritsu Co., Ltd. film thick tester “KG601A” and an electronic micrometer “K306C”, the thickness of a film sampled 30 mm wide and 10 m long in the longitudinal direction of the film is continuously measured. The conveyance speed of the film was 3 m / min. From the maximum thickness Tmax (μm) and the minimum value Tmin (μm) at a length of 10 m,
R = Tmax-Tmin
From R and average thickness Tave (μm) of 10m length
Unevenness of thickness (%) = R / Tave × 100
As sought.
[0026]
3. Birefringence
The retardation Rd of the film was determined by a polarizing microscope equipped with a Berek compensator. Rd was divided by the thickness of the film to obtain birefringence.
[0027]
4). Crystallinity
A density gradient tube with an aqueous sodium bromide solution is made and the density of the film at 25 ° C. is measured. From this density d,
Crystallinity (%) = (d−da) / (dc−da) × 100
It was.
[0028]
Here, da is the amorphous density, dc is the complete crystal density, and in the case of polyethylene terephthalate, from the literature values, da = 1.335, dc = 1.455 g / cm.^ThreeIt was.
[0029]
5. Young's modulus, F5 value
Measurement was performed at a sample width of 10 mm, a sample length of 100 mm, and a tensile speed of 300 mm / min using an orientation-tech automatic film strength measuring device MODEL AMF / RTA-100.
[0030]
6). Heat shrinkage
The film is sampled to a width of 10 mm and a length of 220 mm, and two points with an interval of about 200 mm are marked on the film. The distance between these two points is accurately measured and is defined as L0 (mm). Next, this sample is allowed to stand for 30 minutes in a hot air oven heated to 150 ° C., then taken out and left to reach room temperature. When the sample is completely cooled, the interval between the two points is measured again and is defined as L (mm).
[0031]
Here, the thermal contraction rate was set to thermal contraction rate (%) = (L0−L) / L0 × 100.
[0032]
7. Intrinsic viscosity of polyester [η]
It calculated | required from following Formula at 25 degreeC by using o-chlorophenol as a solvent.
[0033]
[Η] = lm [ηsp / c]
The specific viscosity ηsp is obtained by subtracting 1 from the relative viscosity ηr. c is the concentration. The unit is represented by dl / g.
[0034]
8). Compression modulus (mg / μm²)
Using a thin film hardness meter MHA-400 (manufactured by NEC), the load is changed to 0 to 0.3 g, and the measurement depth is changed to 0 to 0.3 μm for measurement. As the indenter, a triangular indenter having an opposite ridge angle of 80 ° is used, and the pushing speed is 10.5 nm / sec.
[0035]
Three or more measurement points are measured within a sample length of 100 μm. The square of the indentation depth at this time (μm²) And the compression elastic modulus is obtained from the initial inclination.
[0036]
【Example】
In order to describe the present invention more specifically, examples and comparative examples are shown below.
[0037]
Examples 1 and 2 and Comparative Examples 1 to 3
Polyethylene terephthalate (PET) having intrinsic viscosities of 0.65 and 0.72 was used as the polyester resin. DSC was used to measure the thermal properties of polyethylene terephthalate having an intrinsic viscosity of 0.65. Tg: 69 ° C., Tmb: 240 ° C., Tm: 255 ° C., Tme: 268 ° C., Tcb: 203 ° C., Tc: 188 ° C. Tce: 174 ° C., polyethylene terephthalate having an intrinsic viscosity of 0.72 has the following thermal properties: Tg: 69 ° C., Tmb: 240 ° C., Tm: 256 ° C., Tme: 270 ° C., Tcb: 210 ° C., Tc: 190 ° C. , Tce: 178 ° C. Each polyethylene terephthalate pellet was vacuum dried at 180 ° C. for 3 hours. First, a raw material having an intrinsic viscosity of 0.65 is supplied to a 90 mm tandem extruder, and is uniformly melted at 285 ° C. in a first stage extruder, and then polymer temperature is increased by a heating medium in a second stage extruder. Was uniformly cooled to 245 ° C. After a fixed amount of this supercooled melt was supplied with a gear pump, the melt was passed through a sintered filter for removing foreign matters of 10 μm or more and introduced into a T die die.
[0038]
On the other hand, a polyethylene terephthalate (PET) raw material having an intrinsic viscosity of 0.72 was also dried like the PET raw material having an intrinsic viscosity of 0.65, supplied to a 40 mm tandem extruder, melted at 285 ° C., and melted at 235 ° C. After the supercooled state, a fixed amount was discharged, and this was laminated in three layers in the width direction within the die at a width of 35 mm only at both ends of a PET supercooled melt sheet having a limiting viscosity of 0.65.
[0039]
A supercooled film extruded from a die is maintained at a surface temperature of 55 ° C. while applying static electricity of 12,000 V, and a 0.1 μm diameter fine water droplet formed on the surface layer by a condensation method. Was taken up at a high speed of 100 m / min on a mirror-casting drum whose entire surface was covered with, and rapidly cooled and solidified.
[0040]
The die lip gap was adjusted so that the average thickness at the film edge of the cast film thus obtained was 2.5 times the thickness at the center. In addition, the intrinsic viscosity of the end [η_e] Is 0.68, the intrinsic viscosity of the film center [η_c] Was 0.62. The thickness unevenness in the longitudinal direction / width direction was 3% / 4%, the birefringence was 0, and the crystallinity was 0%.
[0041]
The film is supplied to a simultaneous biaxial stretching machine in which the clip moves linearly and is stretched at a high speed of 65000% / min in the longitudinal direction and the width direction in an atmosphere heated to 95 ° C., and 4 in each direction. The film was biaxially stretched 5 times at a time, and further stretched 1.5 times at 30000% / min at 105 ° C. in the longitudinal direction and the width direction. Then, it is once cooled to 65 ° C., heat-treated at 230 ° C. in the same tenter, and then gradually cooled to room temperature while relaxing 5% uniformly on both axes, and is about 640 m / min. The film was wound at an ultra high speed to obtain a biaxially oriented film having a thickness of 12 μm. The state of film formation in this way and the obtained film characteristics are shown in Table 1.
[0042]
Example 2
A biaxially stretched film was formed in exactly the same manner as in Example 1 except that the casting speed used in Example 1 was changed from 100 m / min to 60 m / min to reduce only the stretching speed, and the results and film The evaluation results are shown in Table 1.
[0043]
As can be seen from Table 1, according to the present invention, a soft, soft biaxially stretched film can be obtained, which is optimal for uses such as a receiving paper for thermal transfer.
[0044]
[Table 1]

Comparative Example 1
Except for the simultaneous biaxial stretching apparatus using the linear drive system used in Example 1, a normal pantograph type simultaneous biaxial stretching apparatus was used, and the process was exactly the same as in Example 1 except that the winding speed was 120 m / min. Thus, a biaxially stretched film having a thickness of 12 μm was obtained. The results and film evaluation results are shown in Table 2.
[0045]
As shown in Table 2, it can be seen that when the stretching speed is lowered, the compressive modulus in the thickness direction increases, and not only a hard film but also a film with poor thickness unevenness can be obtained.
[0046]
Comparative Example 2
The cast film used in Example 1 was biaxially stretched with a sequential biaxial stretching apparatus as it was. That is, in the first-stage longitudinal stretching, a conventional roll-type stretching machine is used to stretch 4 times at 96 ° C., once cooled to the glass transition temperature or lower, and then 4.2 in the width direction at 100 ° C. with a tenter. The film is stretched twice and stretched again in the longitudinal direction at 135 ° C by 1.5 times with a roll-type re-stretching device, heat treated at 230 ° C with a tenter, and then uniformly relaxed by 5% in both axial directions. The film was gradually cooled to room temperature, and wound at an ultra high speed of about 570 m / min to obtain a biaxially oriented film having a thickness of 12 μm. The state of film formation in this way and the obtained film characteristics are shown in Table 2 together with the results of Example 1.
[0047]
In this way, in the general-purpose stretching method called sequential biaxial stretching, not only low compression modulus samples can be obtained even if the stretching speed is increased, but only films with poor thickness variation and poor thermal dimensional stability are obtained. I can't understand. This is undesirable when used in applications such as thermal transfer receiving paper, because only images with poor printability can be obtained.
[0048]
Comparative Example 3
The longitudinal stretch ratio, the width direction stretch ratio, and the longitudinal stretch ratio again stretched in Comparative Example 2 were each increased from 4 times to 2.5 times, from 4.2 times to 3 times, and further from 1.5 times. The thickness was changed to 1.0, and a biaxially stretched film having a thickness of 12 μm was wound up at a film forming speed of about 400 m / min.
[0049]
Although the compression elastic modulus of the film with the draw ratio thus lowered is somewhat low, the thickness unevenness is extremely worse than that, and it cannot be used at all for applications such as receiving paper.
[0050]
[Table 2]

[0051]
【The invention's effect】
As described above, the simultaneous biaxially stretched polyester film according to the present invention is soft and has a good cushioning property in the thickness direction, which the conventional biaxially stretched polyester film does not have, and thickness uniformity. It is also an excellent film. This film is suitable as a field film or base film that requires good cushioning properties. For example, it can be suitably used as a base film for receiving paper, and has excellent productivity. Can be manufactured by the law.

Claims (4)

A biaxially oriented polyester film having a compressive modulus of 95 mg / μm ² or more and 300 mg / μm ² or less, and thickness unevenness in the film longitudinal direction of 5% or more and 6% or less. Obtained by simultaneously biaxially stretching the film in the longitudinal direction and the width direction at a high speed of 39000% / min or higher as the stretching speed after applying the electrostatic charge to the molten polyester film and allowing it to cool and solidify in the cooling medium. The biaxially oriented polyester film according to claim 1. 3. The biaxially oriented polyester film according to claim 2, wherein the simultaneous biaxial stretching is performed by a tenter having a clip driven by a linear motor system. The biaxially oriented polyester film according to any one of claims 1 to 3, which is used as a base film for receiving paper.