Method of manufacturing a biaxially stretched polyester film

Abstract

The present application relates to a biaxially-stretched polyester film and a method of manufacturing the same. According to the present application, a polyester film is provided which is improved in film uniformity and mechanical properties while suppressing a bending phenomenon and reducing a retardation deviation.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0105996, filed with the Korean Intellectual Property Office on August 11, 2021, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] This application relates to a method for manufacturing biaxially oriented polyester film. Background Technology

[0004] Optical film is a film used as an optical material in displays, and also serves as an optical material (component) for surface protection and process carriers in various optical displays such as BLUs (backlight units) or LCD touch panels. Polyester film can be used as such an optical film. For example, when polyester film is used in the manufacturing process of polarizing plates, during the inspection of polarizing plate defects, the long axis of the polarizing plate to which the release film is attached is arranged perpendicular to the crystal axis of the inspection machine. Therefore, only when the orientation angle of the polyester film, which serves as the substrate of the release film, is low can color distortion during inspection be prevented and the sensitivity of the inspection be improved.

[0005] Meanwhile, simultaneous biaxial stretching or sequential biaxial stretching processes are used in the manufacture of polyester films such as PET. For example, when manufacturing polyester films according to a sequential biaxial stretching process, after stretching the unstretched film in the longitudinal (MD) direction, sequential stretching in the transverse (TD) direction is performed, and heat treatment can be carried out in equipment called a tenter frame. However, through the heat treatment described above, a phenomenon known as bending occurs. Bending refers to the phenomenon in which a straight line stretched in the width direction of the film before stretching deforms into an arc-shaped (bent) shape after stretching and heat treatment. This phenomenon is related to retardation deviation within the film. Due to retardation deviation, uneven film color, spots, and interference colors appear; therefore, films with large retardation deviations become unsuitable for use as optical films.

[0006] In existing technologies, to suppress bending, methods are considered to reduce the degree of orientation in the longitudinal direction (MD) and minimize the shrinkage deviation in the transverse direction (TD) during heat treatment in a tenter frame. For example, a film stretched in the MD direction can be heat-treated in a tenter frame while its edges are secured with clamps. When the degree of MD orientation is reduced in this process (i.e., if the MD stretch ratio is reduced), the polymer chains oriented in the longitudinal direction (MD direction) exhibit reduced shrinkage behavior in the longitudinal direction (MD) due to the heat within the tenter frame between the edge of the film secured with clamps and the center of the film (not secured with clamps). As a result, a method for suppressing bending is employed.

[0007] However, when the orientation degree in the longitudinal direction (MD) is reduced, i.e., when the draw ratio in the longitudinal direction is reduced, there is a problem that the orientation in the longitudinal direction becomes lower and it is difficult to ensure the uniformity of stretching. Considering the characteristics of polyester film, in which the thickness becomes more uniform and the mechanical rigidity is higher with increasing stretching, existing techniques for reducing the orientation degree in the longitudinal direction need to be improved. In addition, when the orientation degree in the longitudinal direction is reduced, non-uniform polarization (polarization spots) occurs and thermal wrinkling defects occur due to insufficient in-plane orientation, so it needs to be improved. Summary of the Invention

[0008] Technical issues

[0009] One objective of this application is to address the problems arising from the prior art regarding the stretching and heat treatment processes performed during the manufacture of biaxially oriented polyester films.

[0010] Another object of this application is to provide a polyester film in which bending is suppressed and a method for manufacturing the polyester film.

[0011] Another object of this application is to provide a polyester film with reduced delay deviation and a method for manufacturing the polyester film.

[0012] Another object of this application is to provide a polyester film suitable for use as an optical or display component and a method for manufacturing said polyester film.

[0013] The above and other objectives of this application can be fully achieved by the following detailed description of this application.

[0014] Technical solution

[0015] According to one embodiment of this application, a method for manufacturing a biaxially oriented polyester film and a film manufactured therefrom are provided.

[0016] Specifically, the inventors of this application experimentally demonstrated the relationship between stretching speed and stretching temperature when stretching at a low stretch ratio (e.g., MD stretching) to suppress bending, a relationship that does not sacrifice film uniformity due to the low stretch ratio, thus completing the present invention. According to the present invention described below, a film can be provided that not only suppresses bending but also ensures film uniformity even when stretched at a low stretch ratio, thereby reducing retardation (or phase difference) deviation in the width direction.

[0017] The present invention will now be described in more detail.

[0018] In one embodiment related to this application, this application relates to a method of manufacturing a polyester film.

[0019] Specifically, the method includes a first stretching step of stretching an unstretched polyester sheet in a first direction, wherein the stretching in the first direction is performed under the condition that the following relationship is satisfied:

[0020] [Relational Expression]

[0021] 0.095 < y < 0.110

[0022] In the relational expression, y = (a / b) x 100, where a is the tensile temperature (°C) in the first direction, b is the tensile speed (% / min) in the first direction, and y is a dimensionless constant.

[0023] At this point, the stretching speed in the first direction can be calculated using Equation 1 below.

[0024] [Equation 1]

[0025] Tensile speed in the first direction (% / min) = ∑S n / n

[0026] In equation 1 above, n is an integer greater than 1, and S n This represents the stretching speed (% / min) of each section in an n-stage stretching process including n+1 stretching rolls, and the stretching speed of each section can be calculated using Equation 2 below.

[0027] [Equation 2]

[0028] S n =E n / [L n / {(R n+1 -R n ) / 2}]

[0029] In equation 2 above, E n R represents the stretching amount (%) in each segment during the n-stage stretching process from segment 1 to segment n. n and R n+1 L is the rotational speed (m / min) of the (n+1)th and nth stretching rollers forming the nth section. n It refers to the distance (m) between the (n+1)th stretching roller and the nth stretching roller that form the nth segment.

[0030] At this point, the stretch amount can be used interchangeably with the stretch ratio used in the field of polyester film stretching technology. For example, "the stretch amount (%) in the first direction is 300%" means that the stretch ratio in the first direction is 3.00 times. Usually, the stretch ratio refers to the ratio of the length after stretching to the length before stretching (length after stretching / length before stretching), but in the use of roller-to-roll stretching, sometimes the ratio of the roller speed after stretching to the roller speed before stretching is used as the stretch ratio.

[0031] In one specific embodiment of this application, when the first direction is stretched by a two-stage stretching process, Equations 1 and 2 can be expressed as follows.

[0032] [Equation 1-1]

[0033] First direction stretching speed = (S 第一 +S 第二 ) / 2

[0034] In equation 1-1, S 第一 and S 第二 In a two-stage stretching process including the first to third stretching rolls, the stretching speed (% / min) of each segment of the first and second sections, represented by Equations 2-1 and 2-2, is...

[0035] [Equation 2-1]

[0036] S 第一 =E1 / [L1 / {(R2-R1) / 2}]

[0037] [Equation 2-2]

[0038] S 第二 =E2 / [L2 / {(R3-R2) / 2}],

[0039] In Equations 2-1 and 2-2, E1 and E2 are the stretching ratios (%) in each section of the two-stage stretching process, R1, R2 and R3 are the rotational speeds (m / min) of each individual stretching roll (roller, R / L), L1 is the distance (m) between the first stretching roll and the second stretching roll, and L2 is the distance (m) between the second stretching roll and the third stretching roll.

[0040] In one exemplary embodiment, the lower limit of the numerical value of the relational expression can be 0.096 or higher, 0.097 or higher, 0.098 or higher, 0.099 or higher, 0.100 or higher, 0.101 or higher, 0.102 or higher, 0.103 or higher, 0.104 or higher, or 0.105 or higher. Furthermore, the upper limit of the numerical value of the relational expression can be, for example, less than 0.109, less than 0.108, less than 0.107, less than 0.106, less than 0.105, less than 0.104, less than 0.103, less than 0.102, less than 0.101, or less than 0.100.

[0041] According to one specific embodiment of this application, sequential biaxial orientation can be performed, wherein a first stretching is first performed in a first direction, as described below. As demonstrated by the experimental examples below, it has been confirmed that when the process conditions for stretching in the first direction satisfy the range according to the above relational expression, the problems of the prior art can be solved. This is believed to be because the main cause of problems such as bending in sequential biaxial stretching originates from the degree of orientation in the first direction (e.g., the MD direction). The inventors have experimentally confirmed that, compared with the stretching conditions in the first direction satisfying the above relational expression, controlling the stretching conditions (e.g., the stretch ratio) in the second direction (e.g., the TD direction) does not provide meaningful results in improving the problems of the prior art.

[0042] There are no particular restrictions on the tensile temperature (°C) in the first direction, as long as the above relationship expression is satisfied.

[0043] For example, the first stretching can 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 embodiment of this application, stretching in the first direction can be performed, for example, at temperatures above 80°C, 85°C, 90°C, 95°C, or 100°C. Furthermore, there is no particular upper limit, but it can be, for example, below 120°C, below 115°C, below 110°C, below 105°C, below 100°C, or below 95°C.

[0044] There are no particular restrictions on the stretching speed (% / min) in the first direction, as long as the above relationship expression is satisfied.

[0045] For example, according to one embodiment of this application, the tensile speed in the first direction can 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. Furthermore, the upper limit can be, for example, below 95,000% / min, below 94,500% / min, below 94,000% / min, below 93,500% / min, below 93,000% / min, below 92,500% / min, below 92,000% / min, below 91,500% / min, below 91,000% / min, below 90,500% / min, or below 90,000% / min.

[0046] As experimentally verified, when the first stretching is performed within the above temperature and stretching speed range under the premise of satisfying the above relationship expression, the uniformity of the film will not deteriorate.

[0047] According to one embodiment of this application, the stretching amount (%) in the first direction can typically be kept below the stretching amount to ensure film uniformity. For example, in the latest techniques related to sequential biaxial stretching, stretching in the first direction (e.g., MD) is performed at a level of about 350% or more, such as about 330% to 400%, but in the manufacturing method of this application, a lower stretching amount in the first direction can be used.

[0048] For example, stretching in the first direction can be performed with a stretching amount of 320% or less, 310% or less, 300% or less, 290% or less, 280% or less, or 270% or less. As mentioned above, reducing the stretching amount in the prior art is used as a means to suppress bending phenomena, and low stretching amounts (stretch ratios) have the problem of deteriorating the uniformity of the polyester film. However, in this application, since the relationship between stretching speed and stretching temperature is controlled as in the above-described relational expression, the uniformity of the film caused by stretching at a low stretch ratio is not sacrificed. There is no particular limitation on the lower limit of the stretching amount in the first direction, but when the stretching amount is too low, it is difficult to increase the strength of the film. Therefore, taking this into consideration, the lower limit of the stretching amount in the first direction can be 250% or more, 260% or more, 270% or more, 280% or more, 290% or more, 300% or more, or 310% or more.

[0049] In one exemplary embodiment, the first direction of stretching can be the machine direction (MD) (or longitudinal direction). However, the first direction can be the transverse direction (TD) (or transverse direction) perpendicular to the machine direction, but is not limited thereto.

[0050] In one exemplary embodiment, the method may be a method of performing sequential biaxial stretching. Specifically, the method may be a method that, after a first stretching step, further includes a second stretching step of stretching the membrane stretched in the first direction in a second direction intersecting the first direction. There is no particular limitation on the angle at which the first and second directions intersect, but it may be a right angle.

[0051] According to an embodiment of the method of this application for performing sequential biaxial stretching, the first direction can be longitudinal (MD) and the second direction can be transverse (TD).

[0052] In one exemplary embodiment, the method may further include preheating the stretched film in a first direction after the first stretching step and before the second stretching step. There is no particular limitation on the temperature at which the preheating is performed, but for example, preheating can be carried out 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 can vary depending on its intrinsic viscosity (IV), depending on factors such as the degree of polymerization. The glass transition temperature is typically in the range of 65°C to 85°C. With this in mind, preheating can, for example, be carried out at 65°C to 95°C, but the preheating temperature is not specified within the aforementioned temperature range.

[0053] There is no particular limitation on the temperature at which the stretching is performed in the second direction. In one embodiment of this application, the temperature at which the stretching is performed in the second direction can be above 80°C, above 85°C, above 90°C, above 95°C, above 100°C, above 105°C, or above 110°C, and its upper limit can be, for example, below 140°C, below 135°C, below 130°C, below 125°C, below 120°C, below 115°C, below 110°C, or below 105°C.

[0054] In one exemplary embodiment, the stretching in the second direction can be performed at a temperature higher than that in the first direction.

[0055] When stretching in the second direction is performed under the temperature conditions described above, a stable process can be carried out without damaging the film.

[0056] In one embodiment of this application, the stretching in the second direction can be performed in a tenter frame, wherein the stretching speed (% / min) in the second direction can be in the range of 4,500% / min to 5,500% / min. In this case, the stretching speed in the second direction can be calculated using Equation 3 below.

[0057] [Equation 3]

[0058] Second-direction stretching speed (%) / min = E / (L / LSP)

[0059] In Equation 3, E is the stretching amount (%) in multiple stretching zones during the stretching process of the tenter frame, L is the total length (m) of multiple stretching zones, and LSP is the linear speed (m / min) during the stretching process of the tenter frame.

[0060] Regarding the stretching in the second direction as described above, multiple stretching zones can be, for example, two or more, such as two-stage stretching including the first to third stretching rollers, without particular limitation.

[0061] In one exemplary embodiment, the stretching in the second direction can be performed at a stretching amount (%) of less than 500%. For example, the stretching amount in the second direction can be less than 490%, less than 480%, less than 470%, less than 460%, less than 450%, less than 440%, less than 430%, less than 420%, less than 410%, or less than 400%. Furthermore, the lower limit can be, for example, more than 200%, more than 250%, more than 300%, more than 350%, or more than 400%.

[0062] The amount of stretching in the second direction can also be used interchangeably with the stretching ratio. As mentioned above, the stretching ratio refers to the ratio of the length after stretching to the length before stretching (length after stretching / length before stretching). However, in the case of tenter frame stretching, the stretching ratio is sometimes defined by the ratio of the width W2 (m) on the exit side of the tenter frame to the width W1 (m) on the inlet side.

[0063] When the stretching speed and stretching amount in the second direction are met, it is beneficial to manufacture a membrane whose physical properties do not deteriorate or break.

[0064] In one exemplary embodiment, the method may further include a heat treatment (or heat setting) step performed after the second stretching step. The heat treatment involves exposing the membrane to a warm or heated atmosphere, and this heat treatment can enhance the mechanical properties of the membrane (e.g., stiffness, etc.).

[0065] There are no particular restrictions on heat treatment, but it can be carried out at a higher temperature than the temperature at which the first and second stretching are performed.

[0066] In one exemplary embodiment, the heat treatment can be performed at a temperature ranging from 120°C to 250°C. Specifically, the lower limit of the heat treatment temperature can be, for example, above 130°C, above 140°C, above 150°C, above 160°C, above 170°C, above 180°C, above 190°C, or above 200°C, and the upper limit can be, for example, below 240°C, below 230°C, below 220°C, below 210°C, or below 200°C.

[0067] According to one specific embodiment of this application, heat treatment can be performed stepwise at different temperatures. Specifically, the heat treatment steps may include a first heat treatment step performed at a relatively low temperature and a second heat treatment step performed at a relatively high temperature.

[0068] Considering the mechanical properties of the film (e.g., rigidity), the temperatures of the first and second heat treatments can be appropriately adjusted, and for example, the first heat treatment can be carried out at 120°C to 180°C. Furthermore, the second heat treatment can be carried out, for example, at a temperature above 160°C, specifically at a temperature between 180°C and 250°C. As experimentally verified, controlling the temperature of the second heat treatment within the aforementioned range is more beneficial in ensuring the rigidity of the polyester film.

[0069] There are no particular restrictions on the order of the first and second heat treatments, but it is possible, for example, to allow the biaxially oriented film to be sequentially processed by zones in a tenter frame set to different temperatures.

[0070] In one exemplary embodiment, the method may further include a step of relaxing the heat-treated film after the heat treatment step. When a film that has been stretched and heat-treated as described above is post-processed as is, there is a problem of dimensional changes due to heat. The relaxation process can prevent this dimensional change problem.

[0071] There are no particular limitations on this relaxation process, but it can be carried out by known methods, for example, by reducing the track of the clamps that hold the film in the tenter frame in the width direction, or by reducing the distance between the clamps in the longitudinal direction (MD).

[0072] Since the relaxation process requires a certain amount of heat, it can be performed together with the heat treatment described above. However, the method for performing the relaxation process is not limited to this method, and if necessary, the relaxation process can be performed in a cooling section (cooling zone) separate from the heat treatment section.

[0073] In one exemplary implementation, relaxation can be performed in a first direction and / or a second direction.

[0074] In one exemplary embodiment, a relaxation process can be performed to achieve a relaxation rate in the range of 2.0% to 5.0%. In this case, the relaxation rate can be used interchangeably with the total relaxation rate used in the field of polyester film stretching techniques.

[0075] Processes following 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 performed using equipment known as a tenter frame, without particular limitation. In this case, the space of the tenter frame can contain or be divided into separate areas where the various processes or treatments can be performed, such as including a preheating zone, a second-direction stretching zone, a heat treatment zone, other cooling zones, etc. The film after relaxation is discharged from the tenter frame.

[0076] In one exemplary embodiment, the method may further include a step of winding the relaxed membrane after the relaxation step. There are no particular limitations on the method or apparatus used for winding, and known methods may be appropriately used.

[0077] There are no particular limitations on the method of manufacturing unstretched sheets that have undergone steps such as stretching in the first direction. For example, they can be manufactured by (melt) extrusion of polyester resin or a composition or fragment containing polyester resin. For extrusion, melt extrusion using a T-die, which is widely used in the prior art, can be carried out, and the extruded unstretched sheet can be cooled.

[0078] The polyester resin contained in the composition or fragment can be obtained by the reaction between a known component, such as an acid component (e.g., a dicarboxylic acid component) and a diol component.

[0079] There are no particular restrictions on the specific types of dicarboxylic acids that can be used, but for example, terephthalic acid or its alkyl or phenyl esters can be used. In some cases, those substituted with difunctional carboxylic acids or their ester-forming derivatives can be used, such as isophthalic acid, oxyethoxybenzoic acid, adipic acid, sebacic acid, and sodium isophthalate-5-sulfonate.

[0080] Furthermore, there are no particular restrictions on the types of glycol components that can be used, but for example, one or more components selected from propylene glycol, neopentyl glycol, trimethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanediol, 1,4-dioxyethoxybenzene, bisphenol and polyethylene glycol can be used as ethylene glycol and other mixed components.

[0081] In one exemplary embodiment, the polyester resin may be polyethylene terephthalate (PET). Specifically, the polyester resin may be polyethylene terephthalate prepared by using terephthalic acid as the dicarboxylic acid component and ethylene glycol as the diol component.

[0082] In one exemplary embodiment, polyester resins having an intrinsic viscosity of 0.6 dl / g to 0.7 dl / g can be used. Polyester resins that meet the above intrinsic viscosity requirements are beneficial for imparting heat resistance to the film and ensuring interfacial stability, stable lamination, and operational stability between the layers forming the film.

[0083] In one exemplary embodiment, the unstretched polyester sheet may further comprise other components in addition to the polyester resin. That is, the composition or fragments used to manufacture the unstretched polyester sheet may also contain other components besides the polyester resin. These other components may be, for example, various known additives or particles.

[0084] There are no particular restrictions on the types of additives that can be included in the film, and examples can be one or more selected from fixatives, antistatic agents, UV stabilizers, waterproofing agents, slip agents, and heat stabilizers. In the case of light stabilizers, for example, benzophenone compounds, benzotriazole compounds, benzoxazinone compounds, benzoate esters, phenyl salicylate compounds, hindered amine compounds, etc., can be used.

[0085] The particles contained in the membrane can be organic or inorganic particles. These particles can be, for example, organic particles, inorganic particles, or mixed particles, which act as an anti-blocking agent for membrane manufacturing processes. Inorganic particles can be, for example, one or more selected from calcium carbonate, silica, titanium dioxide, kaolin, barium sulfate, aluminum silicate, and calcium carbonate, but are not limited thereto. Organic particles can be, for example, one or more selected from silicone resins, cross-linked divinylbenzene polymethyl methacrylate, cross-linked polymethyl methacrylate, cross-linked polystyrene resin, benzoguanamine-formaldehyde resin, benzoguanamine-melamine-formaldehyde resin, and melamine-formaldehyde resin, but are not limited thereto. Mixed particles can refer to, for example, particles in which two different components respectively form a core and a shell (coating component). When particles are used, for example, based on the total membrane, the content (by weight) can range from 200 ppm to 2,000 ppm or from 400 ppm to 1,000 ppm. In addition, the particles used may have a length with the longest dimension in the range of, for example, 0.01 μm to 5 μm or 0.1 μm to 3 μm, or may have an average particle size in the range of that range.

[0086] In one exemplary embodiment, the unstretched sheet can be a single-layer or multi-layer film. A single-layer film may contain the aforementioned polyester resin component, and in a multi-layer film, at least one of the two layers forming the multilayer film may contain the aforementioned polyester resin component.

[0087] In one specific embodiment of this application, when the unstretched sheet is a multilayer film, the multilayer film can be manufactured by co-extruding two or more layers formed from two or more fragments each having a different composition.

[0088] When the unstretched sheet is a multilayer film, the film may include: a core layer; and a surface layer wherein at least one layer is laminated on each of the two surfaces of the core layer. They may be co-extruded to form the unstretched sheet.

[0089] The surface layer may contain particles with anti-adhesion properties without particular limitation, and the core layer may not contain particles with the aforementioned properties. In this case, the multilayer membrane may include a core layer comprising 70% to 90% of the total membrane and a surface layer comprising 10% to 30% of the total membrane.

[0090] In one exemplary embodiment, the method may further include the step of preparing a multilayer unstretched sheet by melting and then co-extruding the core layer fragments and the surface layer fragments, respectively. In this case, there is no particular limitation on the co-extrusion temperature, but considering processing performance, it can be between 260°C and 300°C. In some cases, the co-extruded film may be further subjected to a cooling process. The cooling temperature may be, for example, below 40°C or below 30°C.

[0091] In one embodiment of this application, the biaxially oriented film may have a thickness ranging from 20 μm 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, less than 55 μm, less than 50 μm, less than 45 μm, less than 40 μm, or less than 35 μm.

[0092] Films manufactured by the method can have low orientation angles, small in-plane retardation, small in-plane retardation deviations, and thickness deviations.

[0093] In one embodiment of this application, the biaxially oriented film may have an orientation angle of less than 12°, less than 11°, or less than 10°. In this case, the orientation angle is the dominant orientation angle of the molecules in the width direction (TD), referring to the direction in which the refractive index of the film is at its maximum, and can be measured or calculated according to the method described in the experiment below. When polyester films exceeding the above range are used as the base film for optical components, such as polarizer release films, the inspection sensitivity in foreign matter and defect inspection of crossed polarizers decreases.

[0094] In one embodiment of this application, the biaxially oriented film may have an in-plane retardation standard deviation of less than 100 nm. The in-plane retardation standard deviation can be measured or calculated according to the method described in the experiments below. Specifically, the in-plane retardation standard deviation may be, for example, less than 90 nm, less than 80 nm, less than 70 nm, less than 60 nm, or less than 50 nm. When it exceeds the above range, the uniformity of light transmission is insufficient, and therefore, it is unsuitable for use as an advanced optical component.

[0095] In one specific embodiment of this application, the in-plane retardation deviation of the biaxially oriented film can be within 5%. The in-plane retardation deviation can be measured or calculated according to the method described in the experiment below. Specifically, the in-plane retardation deviation can be, for example, below 4.9%, below 4.8%, below 4.7%, below 4.6%, below 4.5%, below 4.4%, below 4.3%, below 4.2%, below 4.1%, below 4.0%, below 3.9%, below 3.8%, below 3.7%, below 3.6%, or below 3.5%. Meeting the above ranges is beneficial for suppressing polarization inhomogeneity.

[0096] Although there are no particular limitations, provided that the standard deviation (nm) and retardation deviation (%) of the in-plane retardation are met, the average in-plane retardation of the biaxially oriented film of this application can be in the range of 0 nm to 1700 nm. The average in-plane retardation can be measured or calculated according to the method described in the experiment below. Specifically, the average in-plane retardation can be, for example, below 1650 nm, below 1500 nm, below 1450 nm, below 1400 nm, below 1350 nm, below 1300 nm, below 1250 nm, or below 1200 nm.

[0097] In one embodiment of this application, the thickness deviation of the biaxially oriented film can be within 1.0 μm. The thickness deviation can be measured or calculated according to the method described in the experiment below. Specifically, the film thickness deviation can be, for example, less than 0.9 μm, less than 0.8 μm, less than 0.7 μm, less than 0.6 μm, or less than 0.5 μm. If the thickness deviation is small, it can contribute to the improvement of the film's performance when used as a polarizing plate component as described above.

[0098] There are no particular restrictions on the use of biaxially oriented films. For example, they can be used for surface protection of optical displays, as carriers in manufacturing processes, or as release films for optical films such as polarizing plates.

[0099] Beneficial effects

[0100] According to this application, the problems of the prior art regarding the stretching and heat treatment processes performed during the manufacture of biaxially oriented polyester films can be solved. According to embodiments of this application, a polyester film is provided in which bending phenomena are suppressed, delay deviations are reduced, and simultaneously, film uniformity and mechanical properties (e.g., rigidity) are excellent. Furthermore, this application has the inventive effect of providing a polyester film suitable for use as an optical component or display component. Attached Figure Description

[0101] Figure 1 This is a schematic diagram illustrating the stretching speed in the first direction, and shows the lengths (L1, L2) of two sections using three rollers R1, R2, and R3. At this time, the three rollers can each have a V... R1 V R2 and V R3 The rotational speed.

[0102] Figure 2 This is a schematic diagram illustrating the stretching speed in the second direction, and it shows the stretching zone in the stretching device of a tenter frame. At this time, the widths of the inlet and outlet sides in the TD direction stretching of the tenter frame are W1 and W2, respectively, and the total length of the stretching zone is L.

[0103] Figure 3 These are images used to illustrate methods related to the evaluation of the presence of rainbow spots on a membrane.

[0104] Figure 4 These are images used to illustrate and evaluate methods related to the presence of thermal wrinkles during the release coating process. Detailed Implementation

[0105] In the following description, the effects and functions of the invention will be described in more detail with reference to specific embodiments thereof. However, these embodiments are presented for illustrative purposes only, and the scope of the invention is not limited thereto in any way.

[0106] Examples and Comparative Examples

[0107] Manufacturing of unstretched sheets: Copolyester fragments with an intrinsic viscosity of approximately 0.63 dl / g are manufactured using dicarboxylic acid components such as terephthalic acid and glycol components such as ethylene glycol or neopentyl glycol. The fragments are melted in an extruder at 280°C and extruded through a T-die, then rapidly cooled and solidified on cooling rollers with a surface temperature of 20°C. Simultaneously, an electrostatic application process is used to ensure close adhesion to the cooling rollers, resulting in ten amorphous unstretched sheet samples.

[0108] Manufacturing of the biaxially oriented film: A series of processes related to film manufacturing are performed under the conditions described in Table 1 below to obtain a biaxially oriented polyester film with a thickness of approximately 38 μm. For reference, a thickness of 38 μm is the thickness typically used in the art relative to the base film of the polarizer release film used for polarizers. To reduce costs, a thickness reduction to approximately 25 μm has been considered; however, a thickness of 38 μm is currently the standard due to quality issues related to curling during process use.

[0109] [Table 1]

[0110]

[0111] Membrane evaluation

[0112] The evaluation items are as follows.

[0113] 1. Presence of polarizer inhomogeneity (spots): Using a polarization bending evaluation device (Heidon-24W, Shinto Scientific, Japan), align the lower and upper polarization axes to be orthogonal (90°), then place an A4-sized sample at the bottom of the evaluation device. Visually evaluate the presence of rainbow spots on the film from above (see...). Figure 3 Specifically, if a rainbow-colored patch is visually recognized, it is described as "yes"; if no rainbow-colored patch is visually recognized, it is described as "no".

[0114] 2. Orientation Angle (degrees, °): Each membrane sample was mounted onto a dedicated sample holder using a microwave molecular orientation meter (MOA-7015, Oji Scientific Instruments, Japan), and then inserted into the molecular orientation meter to measure the orientation angle. The orientation angle measured by the orientation meter is based on the MD value. The actual measured value was subtracted from 90° to record the orientation angle relative to TD, and the absolute value was recorded as the orientation angle.

[0115] 3. Mean in-plane delay (nm), standard deviation of in-plane delay (nm), and in-plane delay deviation (%): The phase difference at a measurement wavelength of 590 nm was measured using a parallel Nicol rotating phase difference measuring instrument (KOBRA-WPR, Oji Scientific Instruments, Japan). Specifically, the in-plane delay (Re) and its standard deviation (Re standard deviation) can be calculated as follows.

[0116] [Equation]

[0117] In-plane phase difference (Re) = Re = (nx - ny) × d

[0118] Where nx is the refractive index in the direction of the principal orientation axis, ny is the refractive index in the direction perpendicular to the principal orientation axis, and d is the thickness of the film.

[0119] [Equation]

[0120]

[0121] Where, x i It is a single numerical value of the delay measurement. is the average of all delay measurements, and n is the number of delay measurements.

[0122] Furthermore, the retardation values ​​were measured for 10 samples using the aforementioned retardation measuring device. Ten samples were prepared by cutting specimens at 10cm intervals in the width direction, each specimen having a dimension of 20cm in the longitudinal direction and a dimension of 100cm in the width direction. The difference between the maximum and minimum retardation values ​​was divided by the average value, and the value expressed as a percentage was taken as the in-plane retardation deviation.

[0123] 4. Thickness and thickness deviation R-value (μm): The thickness of the prepared film was measured at 5 cm intervals in the width direction using an electrical micrometer (Mahr, Millimar-1240, Germany), and the thickness deviation R-value was calculated by measuring the measured thickness using the following equation.

[0124]

[0125] (where yi It is a single value from the thickness measurement. (This represents the average thickness measurement, and n is the number of thickness measurements.)

[0126] 5. Whether thermal wrinkling exists during the release coating process (post-processing simulation): Evaluate the presence of thermal wrinkling during the release coating process according to steps a) through d) below. At this time, use... Figure 4 The Taut thermal wrinkle evaluation device shown is shown.

[0127] a) Install 2,000g of the film onto a membrane cut to 30cm on the MD and 80cm on the TD.

[0128] b) Heat in a drying oven set to 130°C for approximately 180 seconds.

[0129] c) Remove from the drying oven and cool to room temperature for about 60 seconds.

[0130] d) Then, visually inspect the thermal wrinkles of the processed sample, measure the size and number of thermal wrinkles, and classify them as follows.

[0131] - Grade 1: No wrinkles.

[0132] - Level 2: 4 or fewer folds with a width of more than 3cm

[0133] - Level 3: 5 or more folds with a width of 3cm or more

[0134] - Level 4: 4 or fewer folds with a width of 3cm or less

[0135] - Level 5: 5 or more folds with a width of less than 3cm

[0136] [Table 2]

[0137]

[0138] As can be seen in Tables 1 and 2, when comparing Examples 1-5 that satisfy the relational expression with Comparative Examples 1-5 that do not satisfy the relational expression, the Examples have less inhomogeneity, lower orientation angle, and lower in-plane delay and delay (standard) deviation compared to the Comparative Examples.

[0139] Therefore, this application can ensure film uniformity (delay deviation and thickness) while using a low stretch amount of less than 320% (a low stretch ratio of less than 3.2) in the first direction, and also ensure mechanical rigidity and prevent thermal wrinkling. This is an effect that cannot be ensured in the prior art, such as in the comparative example, where a low stretch ratio in the MD direction is used to suppress bending, thereby sacrificing uniformity and rigidity.

[0140] For reference, bending occurs during heat treatment following the clamping of the film stretched in the MD and TD directions in a tenter frame. When the stretch in the MD direction is 340% to 350%, the orientation angle typically exhibits a bending phenomenon of approximately 30° to 40°. Therefore, it can be seen that in the embodiments of this application where the orientation angle is reduced to below 12°, bending is suppressed.

Claims

1. A method for manufacturing a polyester film, comprising: The first stretching step involves stretching the unstretched polyester sheet in a first direction. The stretching in the first direction is performed under the condition that the following relationship is satisfied: [Relational Expression] 0.095 < y < 0.110 In the relational expression, y = (a / b) x 100, where a is the tensile temperature (°C) in the first direction, b is the tensile speed (%) / min in the first direction, and y is a dimensionless constant. The tensile speed in the first direction is then calculated using Equation 1 below: [Equation 1] Tensile velocity % / min in the first direction = ∑S n / n In equation 1 above, n is an integer greater than 1, and S n This represents the stretching speed % / min for each section in an n-stage stretching process involving n+1 stretching rolls, and the stretching speed for each section is calculated using the following Equation 2: [Equation 2] S n = E n / [L n / {(R n+1 -R n ) / 2}] In equation 2 above, E n R is the stretching amount (%) in each segment of the n-stage stretching process from segment 1 to segment n. n and R n+1 L is the rotational speed (m / min) of the (n+1)th and nth stretching rollers forming the nth section. n It refers to the distance m between the (n+1)th stretching roller and the nth stretching roller that form the nth segment.

2. The method for manufacturing a polyester film according to claim 1, wherein: The stretching in the first direction is performed at a temperature in the range of 80°C to 120°C.

3. The method for manufacturing 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% / min to 95,000% / min.

4. The method for manufacturing a polyester film according to claim 1, wherein: The stretching in the first direction is performed with a stretching amount of less than 320%.

5. The method for manufacturing a polyester film according to claim 1, further comprising: Following the first stretching step, a second stretching step is performed, in which the membrane stretched in the first direction is stretched in a second direction intersecting the first direction.

6. The method for manufacturing a polyester film according to claim 5, wherein: The first direction is longitudinal (MD), and the second direction is transverse (TD).

7. The method for manufacturing a polyester film according to claim 5 or 6, wherein: The stretching in the second direction is performed at a temperature higher than that in the first direction.

8. The method for manufacturing a polyester film according to claim 5 or 6, wherein: The stretching in the second direction is performed at a temperature ranging from 80°C to 140°C.

9. The method for manufacturing a polyester film according to claim 5, wherein: The membrane stretched in the first and second directions has a thickness in the range of 20 μm to 60 μm.

10. The method for manufacturing a polyester film according to claim 5, further comprising: The heat treatment step performed after the second stretching step. 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.

11. The method for manufacturing a polyester film according to claim 10, wherein: The heat treatment step is carried out at a temperature ranging from 120°C to 250°C.

12. The method of manufacturing a polyester film according to claim 10 or 11, wherein: The heat treatment process 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 first heat treatment step is performed at a temperature of 120°C to 180°C. The second heat treatment step is carried out at 180°C to 250°C.

13. The method for manufacturing a polyester film according to claim 1, wherein, The fragments containing polyester resin are extruded to produce the unstretched polyester sheet.

14. The method for manufacturing a polyester film according to claim 13, wherein: The fragments further comprise at least one selected from additives and granules.

15. The method for manufacturing a polyester film according to claim 13, wherein: The unstretched polyester sheet is a multilayer film, and The multilayer film is manufactured by co-extruding two or more layers formed from two or more fragments each having a different composition.

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