JP2024525946A - Manufacturing method of biaxially oriented polyester film - Google Patents

Abstract

The present application relates to a biaxially oriented polyester film and a manufacturing method thereof. According to the present application, there is provided a polyester film in which the bowing phenomenon is suppressed, the deviation of retardation is reduced, and at the same time, the film uniformity and mechanical properties are improved.

Description

This application relates to a method for producing a biaxially oriented polyester film.

Optical films are films used as optical materials for displays, and are used for surface protection of various optical displays such as LCD BLUs (Back Light Units) or touch panels, and as optical materials (components) for process carriers. Polyester films can be used as such optical films. For example, when polyester films are used in the manufacturing process of polarizing plates, the polarizing plate is inspected for defects by arranging the orientation axis of the polarizing plate to which the release film is attached perpendicular to the crystal axis of the inspection machine. Therefore, only when the orientation angle of the polyester film, which is the base material of the release film, is low can color distortion be prevented during inspection and inspection sensitivity be improved.

Meanwhile, simultaneous biaxial stretching and sequential biaxial stretching are used when manufacturing polyester films such as PET. For example, when a polyester film is manufactured by the sequential biaxial stretching method, the unstretched film is stretched in the machine direction (MD) and then stretched in the transverse direction (TD), and heat treatment can be performed in a facility called a tenter. However, such heat treatment causes a so-called bowing phenomenon. The bowing phenomenon refers to a phenomenon in which a straight line drawn in the width direction of the film before stretching is deformed into a bow shape (bent) after stretching and heat treatment. This phenomenon is related to the deviation of the phase difference in the film. The deviation of the phase difference causes non-uniformity, spots, and interference colors in the film, so a film with a large deviation of the phase difference is not suitable as an optical film.

In the prior art, in order to suppress the bowing phenomenon, a method was considered in which the degree of orientation in the machine direction (MD) was reduced and the shrinkage deviation in the transverse direction (TD) was minimized during heat treatment in a tenter facility. For example, a film stretched in the MD direction is heat treated in a tenter facility with both sides fixed by clips. In this process, if the degree of MD orientation is reduced (i.e., the MD stretch ratio is reduced), the polymer chains oriented in the longitudinal direction (MD direction) can reduce the difference in shrinkage behavior due to heat in the longitudinal direction between both sides of the film fixed by clips in the tenter and the center of the film not fixed by clips, and as a result, the bowing phenomenon was suppressed.

However, when the degree of orientation in the machine direction (MD) is reduced, that is, when the stretch ratio in the machine direction is reduced, the orientation in the machine direction is reduced, and there is a problem that it is difficult to ensure uniformity of stretching. Considering the characteristics of polyester film, in which the greater the stretching, the more uniform the thickness and the higher the mechanical rigidity, improvements are needed over the conventional technology of reducing the degree of orientation in the machine direction. In addition, when the degree of orientation in the machine direction is reduced, polarization spots occur and poor heat wrinkle quality occurs due to insufficient in-plane orientation, so improvements are also needed to address these issues.

One objective of this application is to solve the above-mentioned problems of the prior art that arise in connection with the stretching and heat treatment processes carried out during the production of biaxially oriented polyester film.

Another object of the present application is to provide a polyester film in which the bowing phenomenon is suppressed and a method for producing the same.

Yet another object of the present application is to provide a polyester film with reduced deviation in retardation and a method for producing the same.

Yet another object of the present application is to provide a polyester film suitable for use as an optical or display component, and a method for producing the same.

The above and other objects of this application are all achieved by the application as described in detail below.

Specific examples of the present application provide a method for producing a biaxially oriented polyester film and a film produced therefrom.

Specifically, the inventors of the present application experimentally confirmed the relationship between the stretching speed and stretching temperature that does not sacrifice the uniformity of the film due to low-magnification stretching when low-magnification stretching (e.g., MD stretching) is performed so as to suppress the bowing phenomenon, and thus completed the present invention. According to the present invention described below, not only is the bowing phenomenon suppressed, but the uniformity of the film can be ensured even in low-magnification stretching, and accordingly, a film with reduced deviation in the width-direction retardation can be provided.

The present invention is explained in more detail below.

In one embodiment, the present application relates to a method for producing a polyester film.

Specifically, the method includes a first stretching step in which an unstretched polyester sheet is stretched in a first direction, and the stretching in the first direction is performed under conditions that satisfy the following relationship:

[Relational expression]
0.095<y<0.110

In the above formula, y = (a/b) x 100, a is the stretching temperature in the first direction (°C), b is the stretching speed in the first direction (%/min), and y is treated as a dimensionless constant.

Here, the stretching speed in the first direction is calculated by the following formula 1:

[Formula 1]
Stretching speed in first direction (%/min)=ΣS _n /n

In the above formula 1, n is an integer of 1 or more, and _Sn means a stretching speed (%/min) for each section in an n-stage stretching process having n+1 stretching rolls, and the stretching speed for each section is calculated by the following formula 2.

[Formula 2]
S _n = E _n / [L _n / {(R _n+1 - _{R n} ) / 2} ]

In the above formula 2, E _n is the amount of stretching (%) in each section in the process in which n-stage stretching from the first section to the nth section is performed, R _n and R _n+1 are the rotation speeds (m/min) of the n+1th stretching roll and the nth stretching roll forming the nth section, and L _n is the distance (m) between the n+1th stretching roll and the nth stretching roll forming the nth section.

Here, the stretch amount is used in the same sense as the stretch ratio used in the technical field of polyester film stretching. For example, a stretch amount (%) of 300% in the first direction means that the stretch ratio in the first direction is 3.00 times. Usually, the stretch ratio means the ratio of the length after stretching to the length before stretching (length after stretching/length before stretching), but in roll-to-roll stretching, the ratio of the roll speed after stretching to the roll speed before stretching is sometimes used as the stretch ratio.

In a specific example of the present application, when the stretching in the first direction is performed in a two-stage stretching process, the above formulas 1 and 2 are expressed as follows:

[Formula 1-1]
Stretching speed in the first direction=(S _1st +S _2nd )/2

In the above formula 1-1, S _1st and S _2nd are the stretching speeds (%/min) of the first and second sections represented by formulas 2-1 and 2-2 in a two-stage stretching process equipped with first to third stretching rolls,

[Formula 2-1]
S _1st = E ₁ / [L ₁ / {(R ₂ - R ₁ )/2}]

[Formula 2-2]
S _2nd = E ₂ / [L ₂ / {(R ₃ - _{R 2} )/2} }

In formulas 2-1 and 2-2, _E1 and _E2 are the draw ratios (%) in each section of the two-stage drawing step, _R1 , _R2 , and _R3 are each independently the rotation speeds (m/min) of individual drawing rolls (Roll, R/L), _L1 is the distance (m) between the first drawing roll and the second drawing roll, and _L2 is the distance (m) between the second drawing roll and the third drawing roll.

In one example, the lower limit of the numerical value of the above relational expression may be 0.096 or more, 0.097 or more, 0.098 or more, 0.099 or more, 0.100 or more, 0.101 or more, 0.102 or more, 0.103 or more, 0.104 or more, or 0.105 or more. And the upper limit may be, for example, 0.109 or less, 0.108 or less, 0.107 or less, 0.106 or less, 0.105 or less, 0.104 or less, 0.103 or less, 0.102 or less, 0.101 or less, or 0.100 or less.

According to a specific example of the present application, as described below, sequential biaxial stretching is performed in which the first stretching in the first direction is performed first. As seen through the following experimental example, it has been confirmed that when the process conditions for the first direction stretching satisfy the range of the above-mentioned relational expression, it is effective in solving the problems of the prior art described above. This is thought to be because the main cause of problems such as bowing in sequential biaxial stretching is the degree of orientation in the first direction (e.g., MD direction). As confirmed through experiments by the inventors of the present application, control of the stretching conditions (e.g., stretch ratio) in the second direction (e.g., TD direction) cannot provide significant results in improving the problems of the prior art for the stretching conditions in the first direction that satisfy the above-mentioned relational expression.

As long as the above-mentioned relationship is satisfied, there are no particular limitations on the stretching temperature (°C) in the first direction.

For example, the first stretching may be performed at a temperature equal to or higher than the glass transition temperature (Tg) of the polyester to be stretched. According to a specific example of the present application, the stretching in the first direction may be performed at, for example, 80°C or higher, 85°C or higher, 90°C or higher, 95°C or higher, or 100°C or higher. The upper limit is not particularly limited, but may be, for example, 120°C or lower, 115°C or lower, 110°C or lower, 105°C or lower, 100°C or lower, or 95°C or lower.

As long as the above-mentioned relationship is satisfied, there are no particular limitations on the stretching speed (%/min) in the first direction.

For example, according to a specific example of the present application, the stretching speed in the first direction may be 80,000%/min or more, 81,000%/min or more, 82,000%/min or more, 83,000%/min or more, 84,000%/min or more, 85,000%/min or more, 86,000%/min or more, 87,000%/min or more, 88,000%/min or more, 89,000%/min or more, 90,000%/min or more, 91,000%/min or more or 92,000%/min or more. The upper limit can be, for example, 95,000%/min or less, 94,500%/min or less, 94,000%/min or less, 93,500%/min or less, 93,000%/min or less, 92,500%/min or less, 92,000%/min or less, 91,500%/min or less, 91,000%/min or less, 90,500%/min or less, or 90,000%/min or less.

Experimental confirmation has shown that if the first stretching is performed within this temperature and stretching speed range, assuming the above-mentioned relationship is satisfied, the uniformity of the film does not decrease.

According to a specific example of the present application, the stretch amount (%) in the first direction may be lower than the amount of stretching generally performed to ensure uniformity of the film. For example, in recent technology related to sequential biaxial stretching, the first direction (e.g., MD) stretching is performed at a level of about 350% or more, such as about 330 to 400%, but the manufacturing method of the present application can obtain a lower level of stretching amount in the first direction.

For example, the stretching amount in the first direction may be 320% or less, 310% or less, 300% or less, 290% or less, 280% or less, or 270% or less. As described above, in the prior art, lowering the stretching amount has been used as one means of suppressing the bowing phenomenon, but there is a problem that a low stretching amount (stretching ratio) deteriorates the uniformity of the polyester film. However, in the present application, the relationship between the stretching speed and the stretching temperature is controlled as in the above relational expression, so that the uniformity of the film due to low-ratio stretching is not sacrificed. The lower limit of the stretching amount in the first direction is not particularly limited, but if the stretching is too low, it is difficult to increase the strength of the film, so taking this into consideration, the lower limit of the stretching amount in the first direction may be 250% or more, 260% or more, 270% or more, 280% or more, 290% or more, 300% or more, or 310% or more.

In one example, the first direction in which the stretching occurs may be the machine direction (MD) (or longitudinal direction). However, without being limited thereto, the first direction may be the transverse direction (TD) (or transverse direction) perpendicular to the machine direction.

In one example, the method may be a method of sequentially performing biaxial stretching. Specifically, the method may further include, after the first stretching step, a second stretching step of stretching the film stretched in the first direction in a second direction intersecting the first direction. The angle at which the first and second directions intersect is not particularly limited, but may be a right angle.

According to a specific example of the method of the present application for sequential biaxial stretching, the first direction can be the machine direction (MD) and the second direction can be the transverse direction (TD).

In one example, the method may further include a step of preheating the film stretched in the first direction after the first stretching step and before the second stretching step. The temperature at which the preheating is performed is not particularly limited, but for example, the preheating may be performed at a temperature equal to or higher than the glass transition temperature (Tg) of the polyester film. The glass transition temperature of the polyester film varies depending on the intrinsic viscosity (I.V.) depending on the degree of polymerization, but may generally be in the range of 65 to 85°C. In consideration of this, the preheating may be performed at, for example, 65 to 95°C, but the preheating temperature is not limited to this temperature range.

The temperature at which stretching in the second direction is performed is not particularly limited. In a specific example of the present application, the temperature at which stretching in the second direction is performed may be 80°C or more, 85°C or more, 90°C or more, 95°C or more, 100°C or more, 105°C or more, or 110°C or more, and the upper limit may be, for example, 140°C or less, 135°C or less, 130°C or less, 125°C or less, 120°C or less, 115°C or less, 110°C or less, or 105°C or less.

In one example, stretching in the second direction can be performed at a higher temperature than stretching in the first direction.

When second direction stretching is performed under these temperature conditions, the process can be carried out stably without breaking the film.

In a specific example of the present application, the stretching in the second direction is performed in a tenter, and in this case, the stretching speed (%/min) in the second direction may be in the range of 4,500%/min to 5,500%/min. Here, the stretching speed in the second direction may be calculated according to the following Equation 3.

[Formula 3]
Stretching speed in the second direction (%/min)=E/(L/LSP)

In the above formula 3, E is the amount of stretching (%) in the multiple stretching zones in the tenter-type stretching process, L is the total length (m) of the multiple stretching zones, and LSP is the line speed (m/min) in the tenter-type stretching process.

Although not particularly limited, in connection with such stretching in the second direction, the multiple stretching zones may be two or more, such as, for example, two-stage stretching with first to third stretching rolls.

In one example, the stretching in the second direction may be performed at a stretch amount (%) of 500% or less. For example, the stretch amount of the stretching in the second direction may be 490% or less, 480% or less, 470% or less, 460% or less, 450% or less, 440% or less, 430% or less, 420% or less, 410% or less, or 400% or less. And the lower limit may be, for example, 200% or more, 250% or more, 300% or more, 350% or more, or 400% or more.

The amount of stretching in the second direction can also be used in the same sense as the stretch ratio. As mentioned above, the stretch ratio means the ratio of the length after stretching to the length before stretching (length after stretching/length before stretching), but in the case of tenter-type stretching, the stretch ratio is sometimes defined by the ratio of the width W1 (m) at the entrance side of the tenter to the width W2 (m) at the exit side.

When the stretching speed and stretching amount in the second direction are satisfied, it is advantageous to produce a film without deterioration of physical properties or breakage.

In one example, the method can further include a heat treatment (or heat setting) step performed after the second stretching step. The heat treatment involves exposing the film to a warm or heated atmosphere, and can increase the mechanical properties (e.g., stiffness, etc.) of the film.

Although not particularly limited, the heat treatment may be performed at a temperature higher than the temperature at which the first stretching and the second stretching are performed.

In one example, the heat treatment may be performed at a temperature in the range of 120 to 250°C. Specifically, the lower limit of the heat treatment temperature may be, for example, 130°C or more, 140°C or more, 150°C or more, 160°C or more, 170°C or more, 180°C or more, 190°C or more, or 200°C or more, and the upper limit may be, for example, 240°C or less, 230°C or less, 220°C or less, 210°C or less, or 200°C or less.

According to a specific example of the present application, the heat treatment may be performed in stages at different temperatures. Specifically, the heat treatment stages may include a first heat treatment stage performed at a relatively low temperature and a second heat treatment stage performed at a relatively high temperature.

The temperatures of the first and second heat treatments can be appropriately adjusted taking into consideration the mechanical properties (e.g., rigidity) of the film, and for example, the first heat treatment can be performed at 120 to 180°C. The second heat treatment can be performed at a temperature of, for example, 160°C or higher, specifically, 180 to 250°C. Experimental results show that controlling the temperature of the second heat treatment within the above range is more advantageous in terms of ensuring the rigidity of the polyester film.

Although not particularly limited, the first and second heat treatments performed sequentially can be performed, for example, by passing the biaxially stretched film sequentially through areas in a tenter device in which the temperatures are set to be different from each other.

In one example, the method may further include a step of relaxing the heat-treated film after the heat treatment step. If the film that has been stretched and heat-treated in this way is directly processed, there is a problem that dimensional changes occur due to heat. The relaxation treatment can prevent such dimensional changes.

Although not particularly limited, such a relaxation treatment can be performed in a known manner, for example, by shortening the rails of the clips holding the film in a tenter in the width direction or shortening the distance between the clips in the machine direction (MD).

The relaxation process also requires a certain amount of heat, so the relaxation process may be performed together with the heat treatment described above. However, the method of performing the relaxation process is not limited to this method, and the relaxation process may also be performed through a cooling section (cooling zone) that is distinct from the heat treatment section, if necessary.

In one example, the relaxation can occur in a first direction and/or a second direction.

In one example, the relaxation treatment is carried out to achieve a relaxation rate in the range of 2.0 to 5.0%. Here, the relaxation rate can be used in the same sense as the total relaxation rate used in the technical field of polyester film stretching.

Although not particularly limited, processes after the first stretching step, such as preheating the film stretched in the first direction, stretching in the second direction, heat treatment, and relaxation, can be carried out through equipment known as a tenter. In this case, the space of the tenter can include or be partitioned into separate areas in which each process or treatment can be carried out, such as a preheating zone, a second direction stretching zone, a heat treatment zone, and other cooling zones. The film that has been subjected to the relaxation treatment is discharged from the tenter equipment.

In one example, the method may further include a step of winding the relaxed film after the relaxation step. The method and device used for winding are not particularly limited, and may be appropriately performed using a known method.

There are no particular limitations on the method for producing the unstretched sheet that undergoes the process of stretching in the first direction. For example, the unstretched sheet may be produced by (melt) extruding a polyester resin or a composition or chips containing the polyester resin. The extrusion is performed by melt extrusion using a T-die, which is widely used in conventional technology, and the extruded unstretched sheet may be cooled.

The polyester resin contained in the composition or chips may be obtained by a reaction between known components, such as an acid component (e.g., a dicarboxylic acid component) and a glycol component.

The specific type of dicarboxylic acid that can be used is not particularly limited, but for example, terephthalic acid or its alkyl esters or phenyl esters can be used, and in some cases, difunctional carboxylic acids such as isophthalic acid, oxyethoxybenzoic acid, adipic acid, sebacic acid, and 5-sodium sulfoisophthalic acid, or those substituted with their ester-forming derivatives, can be used.

The type of glycol component that can be used is not particularly limited, but for example, ethylene glycol and one or more components selected from propylene glycol, neopentyl glycol, trimethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 1,4-bisoxyethoxybenzene, bisphenol, and polyoxyethylene glycol as other mixed components can be used.

In one example, the polyester resin may be polyethylene terephthalate (PET). Specifically, the polyester resin may be polyethylene terephthalate produced using terephthalic acid as the dicarboxylic acid component and ethylene glycol as the glycol component.

In one example, the polyester resin may have an intrinsic viscosity of 0.6 to 0.7 dl/g. A polyester resin that satisfies the intrinsic viscosity range is advantageous in providing heat resistance to the film and in ensuring the stability of the interlayer interface that forms the film, stable lamination, and running stability.

In one example, the unstretched polyester sheet may further include other components in addition to the polyester resin. That is, the composition or chips used to manufacture the unstretched polyester sheet may include other components in addition to the polyester resin. The other components may be, for example, various known additives or particles.

The type of additive contained in the film is not particularly limited, but may be, for example, one or more selected from a pinning agent, an antistatic agent, an ultraviolet stabilizer, a waterproofing agent, a slip agent, and a heat stabilizer. In the case of a light stabilizer, for example, a benzophenone-based compound, a benzotriazole-based compound, a benzoxazinone-based compound, a benzoate-based compound, a phenyl salicylate-based compound, or a hindered amine-based compound may be used.

The particles contained in the film may be organic or inorganic particles. Such particles may be, for example, organic particles, inorganic particles, or hybrid particles that function as anti-blocking agents for the film manufacturing process. The inorganic particles may be, for example, one or more selected from calcium carbonate, silica, titanium dioxide, kaolin, barium sulfate, alumina silicate, and calcium carbonate, but are not limited thereto. The organic particles may be, for example, one or more selected from silicone resin, cross-linked divinylbenzene polymethacrylate, cross-linked polymethacrylate, cross-linked polystyrene resin, benzoguanamine-formaldehyde resin, benzoguanamine-melamine-formaldehyde resin, and melamine-formaldehyde resin, but are not limited thereto. The hybrid particles may refer to, for example, particles in which two different components form a core and a shell (coating component), respectively. When particles are used, the content (by weight) may be, for example, in the range of 200 to 2,000 ppm or 400 to 1,000 ppm based on the total film. In addition, the particles used may have a length in the longest dimension within the range of, for example, 0.01 to 5 μm or 0.1 to 3 μm, or an average particle size within the range.

In one example, the unstretched sheet can be a monolayer or multilayer film. A monolayer film can contain the polyester resin component described above, and a multilayer film can have at least one or more of the two layers forming the multilayer film contain the polyester resin component described above.

In a specific example of the present application, when the unstretched sheet is a multilayer film, the multilayer film may be produced by co-extruding two or more layers formed from two or more chips each having a different composition.

When the unstretched sheet is a multilayer film, the film may include a core layer and at least one skin layer laminated on each side of the core layer. These may be co-extruded to form the unstretched sheet.

Although not particularly limited, the skin layer may contain particles with anti-blocking function, and the core layer may not contain particles with the same function. Here, the multilayer film may include a core layer that accounts for 70 to 90% by weight of the entire film, and a skin layer that accounts for 10 to 30% by weight.

In one example, the method may further include a step of melting the core layer-forming chips and the skin layer-forming chips and then co-extruding them to produce a multi-layer unstretched sheet. At this time, the temperature of the co-extrusion is not particularly limited, but may be 260 to 300°C, taking into consideration processability, etc. In some cases, a cooling process may be additionally performed on the co-extruded film. The temperature at which cooling is performed may be, for example, 40°C or less or 30°C or less.

In a specific example of the present application, the biaxially stretched film may have a thickness in the range of 20 to 60 μm. Specifically, the lower limit of the thickness may be, for example, 25 μm or more, 30 μm or more, 35 μm or more, or 40 μm or more, and the upper limit may be, for example, 55 μm or less, 50 μm or less, 45 μm or less, 40 μm or less, or 35 μm or less.

The film produced by the above method can have a low orientation angle, small in-plane retardation, small deviation in in-plane retardation and thickness deviation, etc.

In a specific example of the present application, the biaxially stretched film may have an orientation angle of 12° or less, 11° or less, or 10° or less. Here, the orientation angle is the main orientation angle of the molecules in the transverse direction (TD), which means the direction in which the refractive index of the film is the largest, and may be measured or calculated by the method described in the following experiment. If a polyester film exceeding the above range is used as an optical component, for example, as a base film for a release film of a polarizing plate, the inspection sensitivity may be low in the inspection of foreign matter and defects in crossed Nicols.

In a specific example of the present application, the biaxially stretched film may have a standard deviation of in-plane retardation of 100 nm or less. The standard deviation of the in-plane retardation may be measured or calculated by the method described in the following experiment. Specifically, the standard deviation of the in-plane retardation may be, for example, 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, or 50 nm or less. If it exceeds the above range, the uniformity of light transmission is insufficient, and therefore it is not suitable for use as a high-end optical component.

In a specific example of the present application, the biaxially stretched film may have an in-plane retardation deviation of 5% or less. The in-plane retardation deviation may be measured or calculated by the method described in the following experiment. Specifically, the in-plane retardation deviation may be, for example, 4.9% or less, 4.8% or less, 4.7% or less, 4.6% or less, 4.5% or less, 4.4% or less, 4.3% or less, 4.2% or less, 4.1% or less, 4.0% or less, 3.9% or less, 3.8% or less, 3.7% or less, 3.6% or less, or 3.5% or less. If the above range is satisfied, it is advantageous for suppressing polarization spots.

Although not particularly limited, the average in-plane retardation of the biaxially stretched film of the present application may be in the range of 0 to 1700 nm, provided that the standard deviation (nm) of the in-plane retardation and the deviation (%) of the retardation are satisfied. The average in-plane retardation may be measured or calculated by the method described in the following experiment. Specifically, the average in-plane retardation may be, for example, 1650 nm or less, 1500 nm or less, 1450 nm or less, 1400 nm or less, 1350 nm or less, 1300 nm or less, 1250 nm or less, or 1200 nm or less.

In a specific example of the present application, the biaxially stretched film may have a thickness deviation of 1.0 μm or less. The thickness deviation is measured or calculated by the method described in the following experiment. Specifically, the thickness deviation of the film may be, for example, 0.9 μm or less, 0.8 μm or less, 0.7 μm or less, 0.6 μm or less, or 0.5 μm or less. If the thickness deviation is small, it can contribute to improving the performance of the film when used as the above-mentioned polarizing plate member.

The applications of the biaxially stretched film are not particularly limited. For example, it can be used for surface protection of optical displays, as a carrier during processing, or as a release film for optical films such as polarizing plates.

The present application can solve the above-mentioned problems of the prior art that occur in relation to the stretching and heat treatment processes carried out during the manufacture of biaxially stretched polyester films. Specific examples of the present application provide polyester films that suppress the bowing phenomenon, reduce the deviation in retardation, and have excellent film uniformity and mechanical properties (e.g., rigidity). The present application also has the effect of providing a polyester film that is suitable as a component for optical or display applications.

1 is a schematic diagram for explaining the stretching speed in the first direction, showing the lengths L1 and L2 of two sections in which three rollers R1, R2, and R3 are used, where the three rollers may have rotational speeds of V _R1 , V _R2 , and V _R3 , respectively. 1 is a schematic diagram for explaining the stretching speed in the second direction, showing the stretching zone in a tenter-type stretching equipment, where the widths of the inlet and outlet sides are W1 and W2, respectively, in the tenter-type TD stretching, and the total length of the stretching zone is L. 1 is an image to illustrate a methodology involved in assessing the presence or absence of iridescence in a film. 1 is an image for explaining a method related to evaluating the presence or absence of heat wrinkles during release coating.

The following provides a more detailed explanation of the function and effect of the invention through specific examples of the invention. However, these are presented as examples of the invention and do not limit the scope of the invention in any way.

<Examples and Comparative Examples>
<Production of unstretched sheet>: Using a dicarboxylic acid component such as terephthalic acid and a diol component such as ethylene glycol or neopentyl glycol, chips of a copolymer polyester having an intrinsic viscosity of about 0.63 dl/g were produced. The chips were melted at 280°C in an extruder and extruded through a T-die, and then rapidly solidified on a cooling roller with a surface temperature of 20°C. At the same time, the chips were brought into close contact with the cooling roller using an electrostatic application method to obtain 10 test pieces of an amorphous unstretched sheet.

<Production of biaxially stretched film>: A series of processes for film production were carried out under the conditions listed in Table 1 below to obtain a biaxially stretched polyester film with a thickness of approximately 38 μm. The thickness of 38 μm is a thickness commonly used in fields related to the base film of polarizing plate release films used in polarizing plates. To reduce costs, there was some consideration of reducing the thickness to around 25 μm, but due to quality issues with curling during processing, a thickness of 38 μm is currently the standard.

<Evaluation of the film>
The evaluation items are as follows:

1. Presence or absence of unevenness (spots) on the polarizing mirror: Using a polarizing bending evaluation device (Shinto Scientific, Heidon-24W, Japan), the polarization axis of the lower stage was adjusted to be perpendicular (90°) to the polarization axis of the upper stage, and then a sample cut to A4 size was placed on the lower stage of the evaluation device, and the presence or absence of rainbow spots on the film was evaluated from above with the naked eye (see Figure 3). Specifically, if rainbow spots were visible, it was recorded as "present," and if rainbow spots were not visible, it was recorded as "absent."

2. Orientation angle (degree, °): Using a microwave molecular orientation instrument (Oji Scientific Instruments, MOA-7015, Japan), each film sample was attached to a dedicated sample holder and then inserted into the molecular orientation instrument to measure the orientation angle. Since the orientation angle measured by the instrument is a value based on MD, in order to record the orientation angle based on TD, the actual measured value was subtracted from 90° and the absolute value was recorded as the orientation angle.

3. In-plane retardation average (nm), in-plane retardation standard deviation (nm), and in-plane retardation deviation (%): The retardation value was measured at a measurement wavelength of 590 nm using a parallel Nicol rotation type retardation meter (Oji Scientific Instruments, KOBRA-WPR, Japan). Specifically, the in-plane retardation (Re) and its standard deviation (Re standard deviation) are calculated as follows:

[formula]
In-plane retardation (Re) = Re = (nx - ny) x d

where nx is the refractive index in the main alignment direction, ny is the refractive index perpendicular to the main alignment direction, and d is the thickness of the film.

[formula]

where is an individual value of the phase difference measurement, is the average of all the phase difference measurements, and n is the number of times the phase difference is measured.

In addition, a sample was prepared with a length of 20 cm and a width of 100 cm, and cut into 10 samples at 10 cm intervals in the width direction. The phase difference was measured using the phase difference measuring device described above, and the difference between the maximum and minimum phase difference values was divided by the average value and expressed as a percentage to obtain the deviation of the in-plane phase difference.

4. Thickness and thickness deviation R value (μm): The thickness of the produced film was measured at 5 cm intervals in the width direction using an electric micrometer (Mahr, Millimar-1240, Germany), and the thickness deviation R value was calculated from the measured thickness using the following formula.

(In the above formula, is the individual thickness measurement value, is the average thickness measurement value, and n is the number of thickness measurements)

6. Presence or absence of thermal wrinkles during release coating (post-processing replica): The presence or absence of thermal wrinkles during release coating was evaluated in the following order a) to d). The Taut thermal wrinkle evaluation device shown in Figure 4 was used.

a) A film was cut into pieces of 30 cm in MD and 80 cm in TD, and a 2,000 g weight was attached to the pieces.
b) After heating in a drying oven set at 130° C. for about 180 seconds,
c) Remove from the drying oven and allow to cool at room temperature for approximately 60 seconds.

d) After this, the treated samples are inspected for heat wrinkles with the naked eye, and the size and number of heat wrinkles are measured to determine the grade as follows:

-1 grade: no wrinkles -2 grade: 4 or less wrinkles with a width of 3 cm or more -3 grade: 5 or more wrinkles with a width of 3 cm or more -4 grade: 4 or less wrinkles with a width of 3 cm or less -5 grade: 5 or more wrinkles with a width of 3 cm or less

In Tables 1 and 2, comparing Examples 1-5, which satisfy the relational expressions, with Comparative Examples 1-5, which do not satisfy the relational expressions, it is confirmed that the Examples have less unevenness, a lower orientation angle, and a lower in-plane retardation and retardation (standard) deviation than the Comparative Examples. It is also confirmed that the Examples have a lower thickness deviation than the Comparative Examples, and are also superior in heat wrinkle evaluation.

In this way, the present application ensures film uniformity (phase difference deviation and thickness) while using a low stretch amount of 320% or less in the first direction (low stretch ratio of 3.2 or less), and also ensures mechanical rigidity such as no heat wrinkles. This is an effect that cannot be achieved with conventional technology, which employs a low stretch ratio in the MD direction to suppress the bowing phenomenon, thereby sacrificing uniformity and rigidity, as in the comparative example.

For reference, the bowing phenomenon occurs when a film stretched in the MD and TD directions is held by clips in a tenter and then heat-treated. When the stretch amount in the MD direction is 340-350%, the orientation angle caused by the bowing phenomenon is usually about 30-40°. Therefore, in the examples of this application in which the orientation angle is reduced to 12° or less, it is believed that the bowing phenomenon was suppressed.

Claims (15)

A method for producing a polyester film, comprising a first stretching step of stretching an unstretched polyester sheet in a first direction,
The stretching in the first direction is performed under conditions that satisfy the following relationship:
[Relational expression]
0.095<y<0.110
(In the above formula, y=(a/b)×100, a is the stretching temperature in the first direction (° C.), b is the stretching speed in the first direction (%/min), and y is a dimensionless constant.
Here, the stretching speed in the first direction is calculated according to the following Equation 1.
[Formula 1]
Stretching speed in first direction (%/min)=ΣS _n /n
In the above formula 1, n is an integer of 1 or more, and _Sn means a stretching speed (%/min) for each section in an n-stage stretching process having n+1 stretching rolls, and the stretching speed for each section is calculated by the following formula 2.
[Formula 2]
S _n = E _n / [L _n / {(R _n+1 - _{R n} ) / 2} ]
In the above formula 2, E _n is the amount of stretching (%) in each section in the process in which n-stage stretching is performed from the first section to the nth section, R _n and R _n+1 are the rotation speeds (m/min) of the n+1th stretching roll and the nth stretching roll forming the nth section, and L _n is the distance (m) between the n+1th stretching roll and the nth stretching roll forming the nth section. The method for producing a polyester film according to claim 1, wherein the stretching in the first direction is carried out at a temperature in the range of 80 to 120°C. The method for producing a polyester film according to claim 1, wherein the stretching in the first direction is performed at a stretching speed in the range of 80,000 to 95,000%/min. The method for producing a polyester film according to claim 1, wherein the stretching in the first direction is performed by an amount of stretching of 320% or less. a second stretching step of stretching the film stretched in the first direction in a second direction intersecting the first direction after the first stretching step;
The method for producing a polyester film according to claim 1 , further comprising: The method for producing a polyester film according to claim 5, wherein the first direction is the machine direction (MD) and the second direction is the transverse direction (TD). The method for producing a polyester film according to claim 5 or 6, wherein the stretching in the second direction is performed at a temperature higher than the stretching in the first direction. The method for producing a polyester film according to claim 5 or 6, wherein the stretching in the second direction is carried out at a temperature in the range of 80 to 140°C. The method for producing a polyester film according to claim 5, wherein the film stretched in the first and second directions has a thickness within the range of 20 to 60 μm. A heat treatment step is performed after the second stretching step;
The method for producing a polyester film according to claim 5 , wherein the heat treatment is performed at a temperature higher than the stretching temperature in the first direction and the stretching temperature in the second direction. The method for producing a polyester film according to claim 10, wherein the heat treatment step is carried out at a temperature in the range of 120 to 250°C. The heat treatment step includes a first heat treatment step performed at a relatively low temperature and a second heat treatment step performed at a relatively high temperature,
The method of claim 10 or 11, wherein the first heat treatment is performed at 120 to 180° C. The method for producing a polyester film according to claim 1, wherein the unstretched sheet is produced by extruding chips containing a polyester resin. The method for producing a polyester film according to claim 13, wherein the chips further contain one or more selected from additives and particles. The method for producing a polyester film according to claim 13, wherein the unstretched sheet is a multilayer film, and the multilayer film is produced by co-extruding two or more layers from two or more chips having different compositions.