CN115697679A

CN115697679A - Polyester film and method for producing same

Info

Publication number: CN115697679A
Application number: CN202180036734.3A
Authority: CN
Inventors: 赵显国; 朴翰铢; 金起汉
Original assignee: Kolon Industries Inc
Current assignee: Kolon Industries Inc
Priority date: 2020-08-28
Filing date: 2021-08-25
Publication date: 2023-02-03
Also published as: WO2022045764A1; TW202214415A; JP2023527466A

Abstract

The invention provides a polyester film and a preparation method thereof. By using the embossing roller having the corrugation pattern formed on the outer circumferential surface thereof, the polyester film has corrugations on the surface thereof corresponding to the corrugation pattern and has low surface roughness while having excellent running properties and winding properties. In the production of a multilayer ceramic capacitor (MLCC), a polarizing plate, an optically clear adhesive, and the like, a polyester film may be suitably used as a base film for peeling.

Description

Polyester film and method for producing same

Technical Field

The present disclosure relates to a polyester film and a method for preparing the polyester film.

Background

The polyester film has excellent physical stability in a wide temperature range from low temperature to high temperature, and has excellent chemical resistance compared to other polymer resins. In addition, mechanical strength, surface characteristics, and thickness uniformity are good, and thus can be applied to various uses. Therefore, polyester films are applied to capacitors, photo films, labels, pressure-sensitive adhesive tapes, decorative laminates, transfer tapes, polarizing plates, release ceramic green sheets (green sheets for release), and the like, and the demand is gradually increasing.

Among them, in the case of electronic material films, the optical market has recently been stopped and the competition has been increased, and thus a higher level of physical properties and a reduction in cost are required.

In accordance with the trend toward miniaturization of electronic devices, electronic components such as capacitors and inductors are also being miniaturized, and ceramic green sheets themselves are also being thinned. Recently, it has become a major problem to laminate more ceramic layers in the same volume by thinning the ceramic green sheet.

The ceramic green sheet is a thin ceramic sheet prepared by coating a ceramic slurry on a silicone release layer formed on the surface of a polyester film. In the process of manufacturing a ceramic capacitor or the like, the polyester film is removed.

Generally, particles for controlling surface roughness are added to a polyester film. However, when the surface roughness of the polyester film is high, the film itself may satisfy drivability and windability, but there is a problem in that the projecting shape of the particles protruding from the surface of the polyester film is transferred to the silicone release layer. The transfer problem causes a decrease in workability, for example, causes an imbalance in coating of ceramic layers laminated on a polyester film and generates pinholes.

In contrast, in the case of reducing the surface roughness of a polyester film, when a ceramic slurry is applied to the polyester film, the coating stability and drivability of the film are deteriorated, and a roll formed during winding on a roll may be dropped. In addition, in the process of applying the silicone release layer to the surface of the polyester film, defects such as scratches may occur on the surface of the film.

Accordingly, there is an increasing demand for polyester films having excellent physical properties while being capable of solving the above-mentioned problems.

Disclosure of Invention

Technical problem

In the present disclosure, a polyester film having excellent drivability and winding properties and low surface roughness is provided, which is a base film for a polyester release film.

In addition, a method for preparing the polyester film is also provided.

Technical scheme

According to an embodiment of the present disclosure, there is provided a method of preparing a polyester film, including the steps of:

a melt of a resin composition containing a polyester resin is supplied onto an embossing roll having a concave-convex pattern formed on an outer circumferential surface thereof to obtain an unstretched film having a concave-convex surface corresponding to the concave-convex pattern.

In one embodiment, the melt supplied on the embossing roll may have a temperature of 200 ℃ to 300 ℃.

In one embodiment, the embossing roll may have an outer surface temperature of 25 ℃ to 130 ℃.

In one embodiment, the melt supplied on the embossing roller can be embossed by the embossing roller at the same time as the casting.

In one embodiment, the melt supplied on the embossing roller may pass through a gap between the embossing roller and a nip roller adjacent to the embossing roller.

In one embodiment, the nip roll may have an outer surface temperature of 25 ℃ to 130 ℃.

In one embodiment, the depth of the concave portions of the concavo-convex pattern formed on the outer circumferential surface of the embossing roller may be 5 μm to 100 μm and the period of the concave portions may be 10 μm to 100 μm.

In one embodiment, the concavo-convex pattern formed on the outer circumferential surface of the embossing roller may have an arc-shaped cross-sectional shape, or may have a cross-sectional shape having one or more internal angles.

In one embodiment, the method may further comprise the step of uniaxially or biaxially stretching the unstretched film having the concavo-convex surface.

In one embodiment, the Machine Direction (MD) of the unstretched film may be stretched 2 to 6 times, or the Machine Direction (MD) and the Transverse Direction (TD) of the unstretched film may be stretched 2 to 6 times, respectively.

According to another embodiment of the present disclosure, there is provided a polyester film having a concavo-convex surface on one surface and having a center line average roughness (Ra) of 7.0nm or more according to JIS B-0601.

In one embodiment, the height of the convex portion of the concavo-convex surface may be 0.1 μm to 50 μm, and the period of the convex portion may be 50 μm to 400 μm.

In one embodiment, the concave-convex surface may have an arcuate cross-sectional shape.

In one embodiment, the polyester film may have a static coefficient of friction (μ S) of 0.40 or less and a dynamic coefficient of friction (μ D) of 0.40 or less according to the standard test method of ASTM-D-1894.

Hereinafter, a polyester film according to an embodiment of the present invention and a method of manufacturing the same will be described in more detail.

Unless defined otherwise in this disclosure, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the relevant art to which this invention belongs. Therefore, the terminology used in the detailed description is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

The singular expressions of the present disclosure may include the plural expressions unless they are expressed differently in context.

The terms "comprises," "comprising," "including," and the like, in the present disclosure are used to specify the presence of certain features, regions, integers, steps, operations, elements, and/or components, and these do not preclude the presence or addition of other certain features, regions, integers, steps, operations, elements, components, and/or groups thereof.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. However, it is not intended to limit the invention to the particular forms disclosed, and it should be understood that the invention includes all modifications, equivalents, and alternatives falling within the spirit and technical scope of the invention.

In the present disclosure, when terms such as "upper", "above", "below", and "next" are used to describe a positional relationship between two parts, one or more other parts may be located between the two parts unless these terms are used together with the terms "immediately" or "directly".

In the case of describing temporal relationships, for example, when the temporal sequential relationships are described using terms such as "after 8230; \8230, after", "next", and "before 8230; \8230, before", the case may include a case where the temporal sequential relationships are not consecutive unless these terms are used with the terms "immediately" or "directly".

In the present disclosure, the term "at least one" includes all possible combinations of one or more related items.

I. Method for producing polyester film

a melt of a resin composition containing a polyester resin is supplied onto an embossing roll having a concave-convex pattern formed on the outer circumferential surface to obtain an unstretched film having a concave-convex surface corresponding to the concave-convex pattern.

Generally, particles for controlling surface roughness are added to the polyester film. The particles protrude on the polyester surface to increase the surface roughness. The polyester film having the surface roughness increased by adding the particles may exhibit excellent drivability and winding property. However, the projecting shape of the particles protruding from the surface of the polyester film is transferred to the release layer, and this transfer problem causes a decrease in workability, for example, causes coating imbalance of other layers laminated on the polyester film and generates pinholes.

However, as a result of the studies of the present inventors, it was confirmed that a polyester film having excellent drivability and winding properties and low surface roughness can be provided by supplying a melt of a resin composition comprising a polyester resin onto a pressing roll having a concavo-convex pattern formed on the outer circumferential surface to obtain an unstretched film and then stretching.

The production method according to the present embodiment provides a polyester film having a concave-convex surface formed by an embossing roll. That is, in the method of manufacturing a polyester film according to the present embodiment, unlike the conventional method of controlling surface roughness by adding particles, appropriate surface roughness may be provided without including particles by forming a concave-convex surface on one surface of a film using an embossing roller. Thus, the above-described problems caused by the conventional protruded particles can be solved by the production method of the present embodiment. In addition, the production method according to the present embodiment provides a polyester film that can exhibit excellent drivability and winding properties by forming a concavo-convex surface on one surface.

In manufacturing a highly smooth multilayer ceramic capacitor (MLCC), a polarizing plate, and an optically clear adhesive, a polyester film having these characteristics can be suitably used as a base film for peeling.

In one embodiment of the present disclosure, the resin composition comprises a polyester resin.

The resin composition may include two or more polyester resins having different compositions.

Optionally, additives commonly used in the art to which the present invention pertains, for example, a fixing agent (fixing agent), an antistatic agent, an ultraviolet stabilizer, a water repellent, a slip agent, a heat stabilizer, etc., may be added to the resin composition.

Further, the resin composition may further include: inorganic particles, for example, calcium carbonate, titanium dioxide, silica, kaolin, and barium sulfate; organic particles such as silicone resins, crosslinked divinylbenzene polymethacrylates, crosslinked polystyrene resins, benzoguanamine-formaldehyde resins, benzoguanamine-melamine-formaldehyde resins, and melamine-formaldehyde resins; or mixtures thereof.

The additives and particles may be added during polymerization of the polyester resin, during preparation of a masterbatch sheet comprising the polyester resin, or during preparation of the resin composition.

In one embodiment of the present disclosure, there is no particular limitation on the type of the polyester resin.

The polyester resin can be obtained by polycondensing an acid component containing a dicarboxylic acid as a main component and a diol component containing an alkylene diol as a main component.

As dicarboxylic acid, terephthalic acid or its alkyl or phenyl esters can be used. In addition, difunctional carboxylic acids, for example, isophthalic acid, ethyl p-hydroxybenzoate, adipic acid, sebacic acid, and sodium 5-sulfoisophthalate, or ester-forming derivatives thereof, may be used as the acid component.

As the diol component, ethylene glycol may be mainly used, and propylene glycol, neopentyl glycol, 1, 3-propylene glycol, 1, 4-cyclohexanediol, 1, 4-cyclohexanedimethanol, 1, 4-dioxyethoxybenzene (1, 4-bisoxobenzene), bisphenol, polyoxyethylene glycol, and the like may be used together.

As a non-limiting example, the polyester resin may be obtained by polycondensing 50mol% of a glycol component comprising diethylene glycol and ethylene glycol in a molar ratio of 5.

Preferably, polyethylene terephthalate, polyethylene naphthalate, or the like can be used as the polyester resin.

It is advantageous that the polyester resin has a weight average molecular weight of 40,000g/mol to 70,000g/mol, so that the polyester film has appropriate solvent resistance and mechanical properties. Preferably, the weight average molecular weight of the polyester resin can be 40,000g/mol to 70,000g/mol, 45,000g/mol to 65,000g/mol, or 50,000g/mol to 60,000g/mol.

In the present disclosure, the weight average molecular weight refers to a weight average molecular weight in terms of polystyrene measured by GPC. In the measurement of the polystyrene-reduced weight average molecular weight by GPC measurement, a conventionally known analysis apparatus, such as a differential refractometer and a column for analysis, may be used, and general temperature conditions, solvents and flow rates may be employed.

As a specific example of the measurement conditions, the polymer resin was dissolved in Tetrahydrofuran (THF) at a concentration of 1.0 (w/w)% in THF (solid content about 0.5 (w/w)%), then filtered using a 0.45 μm-pore-size syringe filter, and then 20 μ l was injected into GPC. The mobile phase of GPC was Tetrahydrofuran (THF), and flowed at a flow rate of 1.0 mL/min. The measurement was carried out using a chromatography column with 1 Agilent PLgel 5 μm Guard column (7.5 x50mm) and 2 Agilent PLgel 5 μm Mix D columns (7.5 x300mm) connected in series, using an Agilent 1260Infinity II system, RI detector at 40 ℃. Polystyrene standard samples (STD a, B, C, D) obtained by dissolving polystyrene having various molecular weights in tetrahydrofuran at a concentration of 0.1 (w/w)% were filtered through a 0.45 μm-pore-size syringe filter and then injected into GPC, and the weight average molecular weight (Mw) of the polymer was obtained using the calibration curve thus formed.

STD A(Mp)：791,000/27,810/945

STD B(Mp)：282,000/10,700/580

STD C(Mp)：126,000/4430/370

STD D(Mp)：51,200/1920/162

Heating a resin composition comprising a polyester resin in a melt extruder to form a melt having a temperature of 200 ℃ to 300 ℃.

Referring to fig. 1, a melt of a resin composition comprising a polyester resin is continuously extruded through a T-die 10 provided at one end of a melt extruder.

The melt extruded from the T-die 10 is supplied to an embossing roll 20 formed with a concavo-convex pattern on the outer circumferential surface thereof.

The melt supplied on the impression roller 20 is cooled and processed into a film. That is, the melt supplied on the embossing roller 20 is embossed by the embossing roller 20 while being poured.

The embossing roller 20 has a concavo-convex pattern including repeated and continuous concave and convex portions formed on the outer circumferential surface of the embossing roller 20. Thus, the unstretched film 100 having the concavo-convex surface corresponding to the concavo-convex pattern is obtained by film processing. The film processing of the melt and the formation of the concavo-convex surface are carried out simultaneously and continuously.

The concave-convex pattern formed on the outer circumferential surface of the embossing roller 20 may have an arc-shaped cross-sectional shape. That is, the concavo-convex pattern may have a circular shape such as an arc or circular arc shape having no interior angle in cross section.

Further, the concavo-convex pattern formed on the outer circumferential surface of the embossing roller 20 may have a cross-sectional shape having one or more internal angles. Here, the internal angle means an angle formed on a cross-section of the concavo-convex pattern by connecting two or more straight lines. For example, when there is one inner angle, it represents a triangular cross-section, and when there are two inner angles, it represents a rectangular cross-section. Preferably, the concavo-convex pattern may have a cross-sectional shape having 1 to 5 interior angles.

As one non-limiting example, the concave-convex pattern formed on the outer circumferential surface of the embossing roller may have concave portions in the form of a cone, a triangular pyramid, or a quadrangular pyramid, wherein the concave portions of the concave-convex pattern have a triangular cross section.

In one embodiment of the present disclosure, the depth of the concave-convex pattern of the concave portions formed on the outer circumferential surface of the embossing roller 20 may be 5 μm to 100 μm, and the period of the concave portions may be 10 μm to 100 μm.

The depth of the recess refers to the vertical distance from the outer circumferential surface of the impression roller 20 to the lowest point of the recess. The period of the concave portion refers to the length of an arc connecting any point of one arbitrary concave portion and a corresponding point of another concave portion adjacent thereto.

Preferably, the depth of the concave portions of the concavo-convex pattern formed on the outer circumferential surface of the embossing roller 20 is 5 μm to 50 μm, or 5 μm to 20 μm; and the period of the recesses is 10 to 70 μm, or 30 to 60 μm.

Preferably, the ratio (T: D) of the period (T) of the recess to the depth (D) of the recess portion is 1.

In the concavo-convex pattern, when the depth (D) of the concave portions is increased as compared with the period (T) of the concave portions, the melt of the resin composition is difficult to penetrate into the concavo-convex pattern, so that a uniform concavo-convex surface cannot be obtained. Therefore, the ratio (T: D) is preferably 1.

However, when the depth (D) of the concave portion is too small compared to the period (T), the height of the concave-convex surface formed on the unstretched film is low, and the height of the concave-convex surface is further reduced by stretching the unstretched film, so that it is difficult to have appropriate surface roughness. Therefore, the ratio (T: D) is preferably 1.

Meanwhile, the melt supplied on the embossing roller 20 preferably has a temperature of 200 ℃ to 300 ℃.

That is, in order to maintain sufficient processability of the resin composition in a molten state, the melt preferably has a temperature of 200 ℃ or higher. However, when the resin composition is heated to a high temperature, the components may be denatured by deterioration, and the cooling efficiency of the melt in the platen roller 20 may be reduced. Therefore, the melt preferably has a temperature of 300 ℃ or less.

In particular, the embossing roll 20 preferably has an outer surface temperature of 25 ℃ to 130 ℃. In particular, the embossing roller 20 may have an outer surface temperature of 25 ℃ to 130 ℃, 25 ℃ to 100 ℃, 30 ℃ to 100 ℃, or 30 ℃ to 80 ℃.

When a polyester resin (particularly polyethylene terephthalate) is heated, crystallinity increases. However, when the crystallinity is excessively increased, the breakage rate may be increased during the stretching process. Therefore, the outer surface temperature of the platen roller 20 is preferably 130 ℃ or less, 100 ℃ or less, or 80 ℃ or less.

However, when the melt is processed at a low temperature, the concavo-convex pattern may not be properly formed on the melt. Therefore, the temperature of the outer surface of the platen roller 20 is preferably 25 ℃ or more or 30 ℃ or more.

Referring to fig. 2, the melt supplied on the embossing roll 20 may pass through a gap (also referred to as a nip) between the embossing roll 20 and a nip roll 30 adjacent thereto. Thereby, an unstretched film having a uniform thickness and a uniform concavo-convex pattern can be obtained.

The size of the gap may be determined in consideration of the thickness of the polyester film. For example, the gap may have a size of 200 μm to 2000 μm.

In order to more efficiently perform the thin film process using the embossing roller 20 and the nip roller 30, the material of the nip roller 30 is preferably metal or polymer.

Here, the pressure exerted by the nip roller 30 on the impression roller 20 is important. When a constant pressure is applied by the nip roller 30, the melt penetrates into the concave-convex pattern formed on the embossing roller 20 and is cooled, thereby forming an unstretched film having a concave-convex surface corresponding to the concave-convex pattern.

Preferably, the pressure applied to the embossing roller 20 by the nip roller 30 may be 1kgf/cm ² To 100kgf/cm ² Or 1kgf/cm ² To 50kgf/cm ² . The pressure applied to the embossing roller 20 by the nip roller 30 is preferably 1kgf/cm for good penetration of the melt into the concave-convex pattern of the embossing roller 20 ² The above. However, when the pressure is too great, it may be difficult to control the thickness of the melt passing through the gap. Therefore, the pressure applied to the embossing roller 20 by the nip roller 30 is preferably 100kgf/cm ² Below or 50kgf/cm ² The following.

To achieve improved processability, it may be preferred that the nip roller 30 have an outer surface temperature comparable to the embossing roller 20. For example, the nip roll 30 may have an outer surface temperature of 25 ℃ to 130 ℃, 25 ℃ to 100 ℃, 30 ℃ to 100 ℃, or 30 ℃ to 80 ℃.

Meanwhile, according to another embodiment of the present disclosure, as shown in fig. 3, a step of obtaining an unstretched film having a concave-convex surface corresponding to a concave-convex pattern using an embossing nip roll 35 (imprint nip roll) having a concave-convex pattern on an outer circumferential surface may be performed.

In particular, it is possible to carry out: a step of supplying a melt of a resin composition containing a polyester resin on a casting roll 25 (casting roll), and a step of obtaining an unstretched film having a concave-convex surface corresponding to a concave-convex pattern by passing the melt through a gap between the casting roll 25 and an embossing nip roll 35 adjacent to the casting roll 25 and having the concave-convex pattern formed on an outer circumferential surface.

Here, the description of the concave-convex pattern formed on the outer circumferential surface of the embossing nip roller 35 may refer to the description of the concave-convex pattern formed on the outer circumferential surface of the above-described embossing roller 20.

Meanwhile, an unstretched film (or called "cast sheet") having a concave-convex surface corresponding to the concave-convex pattern is obtained by the above-described method.

In one embodiment of the present disclosure, the unstretched film obtained by the above-described method may have a concave-convex surface corresponding to a concave-convex pattern on one surface thereof.

In general, how much melt of the resin composition is filled in the concavo-convex pattern formed on the outer circumferential surface of the embossing roller may vary depending on the viscosity of the melt of the resin composition, the density of the concavo-convex pattern, the processing speed, and the like. Accordingly, the height of the convex portions of the concavo-convex surface of the unstretched film may be the same as or less than the depth of the concave portions of the concavo-convex pattern.

Meanwhile, the method for preparing the polyester film may further include the step of uniaxially or biaxially stretching the unstretched film having the concavo-convex surface.

The uniaxial stretching may be performed in the machine direction (referred to as MD, or length direction) or the transverse direction (referred to as TD, or width direction) of the unstretched film.

The biaxial stretching may be performed by stretching in the Machine Direction (MD) and then in the Transverse Direction (TD) of an unstretched film successively, or by stretching in both the Machine Direction (MD) and the Transverse Direction (TD) simultaneously.

The non-stretched film may be stretched 2 to 6 times in the Machine Direction (MD), or in both the Machine Direction (MD) and the Transverse Direction (TD), for example, the Machine Direction (MD), or the Transverse Direction (TD) may be uniaxially stretched 2 to 6 times. The Machine Direction (MD) and the Transverse Direction (TD) may be biaxially stretched 2 to 6 times, respectively.

The stretched film obtained by the stretching step has a reduced thickness as compared with the unstretched film, and the shape of the concave-convex surface formed on one surface thereof is changed.

Referring to fig. 4, the change of the concavo-convex surface and the cross-sectional shape thereof may be confirmed according to the continuous biaxial stretching of the unstretched film. Fig. 4 (a) shows an unstretched film having a concavo-convex surface corresponding to a concavo-convex pattern. As shown in fig. 4 (b), when the unstretched film is longitudinally stretched, the height of the convex portions of the concavo-convex surface is reduced, the period of the convex portions is increased, and the thickness of the film is reduced. As shown in fig. 4 (c), when the longitudinally stretched film is stretched in the transverse direction, the height of the convex portions of the concave-convex surface is further decreased, the period of the convex portions is further increased, and the thickness of the film is further decreased.

Fig. 5 is a cross-sectional view of a polyester film according to the present disclosure, in which (a) shows an unstretched film 100 and (b) shows a stretched film 105.

In one embodiment of the present disclosure, in the case of an unstretched film having a concavo-convex surface on one surface, by stretching, the period (T) of the convex portions of the concavo-convex surface is increased and the height (H) of the convex portions is decreased.

Preferably, the stretched film obtained by stretching may have a concavo-convex surface on one surface thereof, the depth of the convex portion is 0.1 μm to 50 μm, 0.2 μm to 20 μm, or 0.4 μm to 10 μm, and the period of the convex portion is 50 μm to 400 μm, 50 μm to 300 μm, 100 μm to 300 μm, or 150 μm to 250 μm.

Preferably, in the concavo-convex surface formed on one surface of the stretched film, the ratio (T: H) of the period (T) to the height (H) of the convex portions may be 1.

In order to prevent the film from being broken during the stretching process while ensuring the efficiency of the stretching process, it is preferable to perform the stretching at 80 to 120 ℃, 90 to 110 ℃, or 90 to 100 ℃.

Further, the unstretched film is preferably provided at a temperature equal to or higher than the glass transition temperature (Tg) of the polyester resin during stretching to ensure an appropriate level of crystallinity. Preferably, the temperature of the unstretched film provided during stretching may be 80 ℃ to 120 ℃, 90 ℃ to 110 ℃, or 90 ℃ to 100 ℃.

Meanwhile, after the stretching process, a heat treatment and a heat treatment step of relaxation-stretching the film may be further performed as necessary.

The heat treatment step may be performed at a temperature of 180 ℃ to 260 ℃, 190 ℃ to 250 ℃, or 200 ℃ to 240 ℃. The relaxation rate at the time of relaxation can be adjusted to 0.1% to 10% with respect to the longitudinal direction and the transverse direction.

When the temperature of the heat treatment step is too low, the crystallinity of the polyester may be reduced, and thus mechanical properties may be reduced or residual stress may not be sufficiently reduced, thereby increasing heat shrinkage. When the temperature of the heat treatment step is too high, mechanical properties may be deteriorated due to thermal decomposition of the polyester, or quality may be deteriorated due to thermal deformation of the film at high temperature.

II. Polyester film

According to another embodiment of the present disclosure, there is provided a polyester film having a concave-convex surface on one surface and having a center line average roughness (Ra) of 7.0nm or more according to JIS B-0601.

The polyester film can be obtained by the above "I, method for producing a polyester film".

Preferably, the polyester film may be a biaxially stretched film according to the above-described production method.

The concave-convex surface formed on one surface of the polyester film may have an arc-shaped cross-sectional shape. That is, the concave-convex surface may have a circular shape such as an arc or circular arc whose cross section does not have an inner angle.

In one embodiment of the present disclosure, the height of the protrusions of the concavo-convex surface formed on one surface of the polyester film may be 0.1 μm to 50 μm, and the period of the protrusions may be 50 μm to 400 μm.

Preferably, the height of the convex portion of the concavo-convex surface formed on one surface of the polyester film may be 0.1 μm to 50 μm, 0.2 μm to 20 μm, or 0.4 μm to 10 μm; and the period of the convex portions may be 50 μm to 400 μm, 50 μm to 300 μm, 100 μm to 300 μm, or 150 μm to 250 μm.

Preferably, in the concavo-convex surface formed on one surface of the polyester film, the ratio (T: H) of the period (T) to the height (H) of the convex portion may be 1.

When the height (H) of the convex portion in the concavo-convex surface is too small compared to the period (T), it may be difficult to have appropriate surface roughness due to the concavo-convex surface. Therefore, the ratio (T: H) is preferably 1.

However, when the height (H) of the convex portion in the concavo-convex surface is excessively large compared to the period (T), since the surface roughness of the concavo-convex surface becomes excessively large, workability is reduced, for example, coating of any layer laminated on the polyester film is unbalanced. Therefore, the ratio (T: D) is preferably 1.

The polyester film having a concavo-convex surface on one surface may exhibit excellent drivability and winding properties as well as low surface roughness.

In one embodiment of the present disclosure, the polyester film may have a center line average roughness (Ra) of 7.0nm or more according to JIS B-0601. Preferably, the polyester film may have a center line average roughness (Ra) of 7.0nm to 35.0nm, 7.5nm to 33.0nm, or 7.6nm to 33.0nm according to JIS B-0601.

In one embodiment of the present disclosure, the polyester film may have a static coefficient of friction (μ S) of 0.40 or less, or 0.35 to 0.40, and a dynamic coefficient of friction (μ D) of 0.40 or less, or 0.33 to 0.40, according to the standard test method of ASTM-D-1894.

The polyester film may have a thickness of 10 μm to 250 μm, 10 μm to 200 μm, 20 μm to 150 μm, or 30 μm to 100 μm. Here, the thickness refers to a measured thickness of the convex portion included in the concavo-convex surface formed on one surface of the polyester film.

Further, the haze of the polyester film may be 1.0% or less, 0.1% to 1.0%, or 0.3% to 0.6%; and the total light transmittance may be 90.0% or more, 90.0% to 92.0%, or 90.0% to 91.0%.

Advantageous effects

In the present disclosure, a polyester film having excellent drivability and winding properties and low surface roughness and a method for preparing the same are provided. The polyester film may be suitably used as a base film for peeling in the manufacture of multilayer ceramic capacitors (MLCCs), polarizing plates, and optically transparent adhesives.

Drawings

Fig. 1 illustrates one embodiment of a method of preparing a polyester film according to the present disclosure.

Fig. 2 shows another embodiment of a method for producing a polyester film according to the present disclosure.

Fig. 3 illustrates yet another embodiment of a method of manufacturing a polyester film according to the present disclosure.

Fig. 4 illustrates the change of the concavo-convex surface and the cross-sectional shape by the continuous biaxial stretching of an unstretched film in the preparation of a polyester film according to the present disclosure.

Fig. 5 is a cross-sectional view of a polyester film according to the present disclosure, in which (a) shows the polyester film before stretching and (b) shows the polyester film after stretching.

< description of reference >

10: t-shaped die head

20: embossing roller

25: pouring roller

30: clamping roller

35: impression nip roller

100: unstretched film

105: stretched film

Detailed Description

Hereinafter, preferred examples are given to aid understanding of the present invention. However, the following examples are provided only for easier understanding of the present invention, and the present invention is not limited thereto.

Preparation example 1

The polyester resin was obtained by polycondensing 50mol% of a glycol component comprising diethylene glycol and ethylene glycol in a molar ratio of 5 and 50mol% of an acid component comprising terephthalic acid and sulfoterephthalic acid in a molar ratio of 8.5.

Example 1

The polyester resin obtained in production example 1 was put into a melt extruder and formed into a melt (intrinsic viscosity: 0.63) at 300 ℃. The melt is continuously extruded through a T-die 10.

The melt is supplied on an impression roller 20. As the platen roller 20, a platen roller having a concave-convex pattern and a concave portion on the outer circumferential surface thereof with a period (T) of 50 μm and a depth (D) of the concave portion of 19.2 μm was used. The concave portion has a conical shape with its apex directed toward the center of the platen roller.

The temperature of the outer surface of the embossing roll 20 was maintained at 80 ℃.

The melt passes through the gap between the embossing roll 20 and the nip roll 30 adjacent thereto. The temperature of the outer surface of the nip roller 30 was maintained at 80 ℃. The pressure applied to the embossing roller 20 by the nip roller 30 was maintained at 50kgf/cm ² 。

As the melt passes through the gap, an unstretched film having a concave-convex surface corresponding to the concave-convex pattern of the embossing roller 20 on one surface thereof is formed.

The unstretched film was transferred to a stretching process while the temperature was 90 ℃ higher than the glass transition temperature (Tg) of the polyester resin. The unstretched film was stretched 2 times in the Machine Direction (MD) and 3 times in the Transverse Direction (TD) at a temperature of 95 ℃.

After the stretching step, the stretched film was heat-treated at 230 ℃ for 10 seconds to obtain a polyester film.

Examples 2 to 6 and comparative examples 1 to 4

Polyester films of examples 2 to 6 and comparative examples 1 to 4 were obtained in the same manner as in example 1, except that the concavo-convex pattern, the outer surface temperature, and the stretching conditions of the embossing roller 20 were changed as shown in table 1.

[ Table 1]

Examples of the experiments

(1) Crystallinity of unstretched film: the crystallinity was calculated by the following formula using the measured density of the sample (measured at 25 ℃ using a gradient tube). The values are shown in table 1 above.

* Crystallinity = { [ (density of sample) - (density of 100% amorphous PET) ]/[ (theoretical density of 100% crystalline PET) - (density of 100% amorphous PET) ] } × 100

(density of 100% amorphous PET: 1.335g/cm ³ Density of theoretical 100% crystalline PET: 1.455g/cm ³ )

(2) Film thickness: the thickness (based on the convex portion of the concave-convex surface) at 5 points was measured at intervals of 1cm in the width direction of the film using an electrometer measuring device (manufactured by Mahr, millimar-1240, germany), and the average value thereof was calculated.

(3) Roughness (Ra) of thin film: the sample was prepared by selecting an arbitrary center point from the entire width of the prepared polyester film, and cutting the film into dimensions of 5cm in width and 5cm in length based on the point. Ra (center line average roughness) of the sample was measured using a two-dimensional contact surface roughness meter (manufactured by KOSAKA, SE-3300, japan) in accordance with JIS B-0601. The surface roughness meter was set to have a cut-off (. Lamda.c) value of 0.08mm and a measurement length of 1.50mm.

(4) Shape of concave-convex surface: after the film was cut using a Microtome apparatus (manufactured by LEICA, EM-UC7, germany), its cross section was placed on a sample holder of an optical microscope (manufactured by Olympus, BX51, japan). The line width of the concave-convex surface and the depth of the concave portion on the cut surface of the film were measured.

(5) Haze and total light transmittance: haze (%) and total light transmittance (T.t,%) of the film were measured according to the standard test method of ASTM D-1003. A haze meter (manufactured by NIPPON DENSHOKU, NDH-5000, japan) was prepared and the instrument was calibrated. The sample (5 cm in width and 5cm in length) was placed on the sample holder of the instrument, the sample was held by the holder, and then the measurement was performed by pressing the start button. 5 measurements were made and the average was derived.

(6) Coefficient of friction: the static friction coefficient (. Mu.S) and the dynamic friction coefficient (. Mu.D) of the film were measured according to the standard test method of ASTM-D-1894.

A friction coefficient tester (manufactured by Toyoseiki, TR-2, japan) was prepared. The sample (20 cm wide and 11cm long) was placed on the measuring table of the tester without wrinkles and then attached to the back of a slider (slid) (10.5 cm wide and 9.5cm long). The slider was connected to a force cell, the static friction coefficient (μ S) and the dynamic friction coefficient (μ D) were measured 5 times, respectively, and the average values thereof were derived.

(7) Drivability: drivability was evaluated using a dynamic friction coefficient according to the following criteria.

* Very good (excellent) -coefficient of dynamic friction: less than 0.35

* Good (∘) -coefficient of dynamic friction: 0.35 to 0.40

* Difference (X) -coefficient of dynamic friction: greater than 0.40

(8) Stretch processability: the stretch processability was evaluated based on whether or not a break occurred in the film stretching step.

* X: not broken

* O: fracture of

[ Table 2]

Referring to table 2, the films of examples had higher surface roughness and exhibited excellent drivability without breaking in the stretching process, compared to the films of comparative examples 1 to 3, which had no concavo-convex surface.

It was confirmed that the films of comparative examples 1 to 3, which had no concavo-convex surface, had lower surface roughness and thus had inferior drivability than the films of examples. It was confirmed that the film of comparative example 4 did not exhibit appropriate surface roughness due to the concavo-convex surface including the concave portions having too shallow a depth, and had poor drivability.

Claims

1. A method of preparing a polyester film comprising the steps of:

2. A method of producing a polyester film according to claim 1, wherein the melt supplied on the embossing roll has a temperature of 200 ℃ to 300 ℃.

3. A polyester film production method according to claim 1, wherein the embossing roll has an outer surface temperature of 25 ℃ to 130 ℃.

4. A method of producing a polyester film according to claim 1, wherein the melt supplied on the embossing roller is embossed by the embossing roller while being cast.

5. A method of producing a polyester film according to claim 1, wherein the melt supplied on the embossing roll passes through a gap between the embossing roll and a nip roll adjacent to the embossing roll.

6. A method of producing a polyester film according to claim 5 wherein the nip roll has an outer surface temperature of 25 ℃ to 130 ℃.

7. A polyester film production method according to claim 1, wherein the depth of the recesses of the concavo-convex pattern formed on the outer circumferential surface of the embossing roller is 5 μm to 100 μm and the period of the recesses is 10 μm to 100 μm.

8. A polyester film production method according to claim 7, wherein the concavo-convex pattern formed on the outer circumferential surface of the embossing roll has an arc-shaped cross-sectional shape or a cross-sectional shape having one or more internal angles.

9. A method of producing a polyester film according to claim 1, further comprising a step of uniaxially or biaxially stretching the unstretched film having the concavo-convex surface.

10. A polyester film production method according to claim 9, wherein the machine direction MD of the unstretched film is stretched 2 to 6 times, or the machine direction MD and the transverse direction TD of the unstretched film are each stretched 2 to 6 times.

11. A polyester film having a concavo-convex surface on one surface and having a center line average roughness Ra of 7.0nm or more according to JIS B-0601.

12. A polyester film according to claim 11, wherein the height of the protrusions of the concavo-convex surface is 0.1 μm to 50 μm, and the period of the protrusions is 50 μm to 400 μm.

13. A polyester film according to claim 12, wherein the concave-convex surface has an arcuate cross-sectional shape.

14. A polyester film according to claim 11 wherein the polyester film has a static coefficient of friction μ S of 0.40 or less and a dynamic coefficient of friction μ D of 0.40 or less according to the standard test method of ASTM-D-1894.