JP2019056109A - Polyester film and method for producing the same - Google Patents

Abstract

To provide a polyester film in which L values on any surface of a polyester film are 12 or less, and a method for producing the same.SOLUTION: A polyester film contains a black pigment, where a thickness of the polyester film is 150 μm or more and 500 μm or less, a breaking strength of at least one of a longitudinal direction (MD direction) and a width direction (TD direction) of the film is 150 MPa or more and 250 MPa or less, and L values on any surface of the polyester film are 12 or less.SELECTED DRAWING: None

Description

  The present invention relates to a polyester film having an L value of 12 or less on the surface of the polyester film and a method for producing the same.

  Polyester films have excellent mechanical properties and are used in many fields. Among them, films having a concealing property such as a film containing a black pigment are used as a diaphragm for a digital camera, an inner member of an electronic component such as a laser beam printer, and a light shielding member. A metal member exists in the same application, but since the film member is lighter than the metal member, the share of the film member is increasing.

  Typical examples of black films are those made by kneading black pigments such as carbon black into inexpensive polyester resins such as polyester, which have been proposed as films with excellent concealability and durability. (Patent Documents 1 and 2). Moreover, as thickness of a black film, there exists a request | requirement of 10 micrometers-500 micrometers according to a use.

JP, 2014-192180, A JP 2014-145046 A

  Films with concealment used for electronic parts such as cameras and printers are required to have a certain thickness because durability is also required. However, as the thickness increases, the surface characteristics of both sides are equal. It becomes difficult to produce a film controlled to a high level.

  If a difference occurs in the surface characteristics and color tone (color tone) of the front and back sides of the polyester film, it becomes inconvenient because it is necessary to manage the front and back sides of the film when processing the film as a part. In addition, from the viewpoint of product design, it is required to provide a polyester film that is homogeneous on both sides.

  In the above-mentioned Patent Document 1 and Patent Document 2, a film having a thickness of 10 μm to 50 μm has been proposed as a film having excellent surface characteristics on both sides, but the surface characteristics on both sides when thickened have not been studied. It was.

  The problem to be solved by the present invention is to provide a polyester film containing a black pigment, sufficiently black on both sides, having a high-quality thickness of 150 μm or more and 500 μm or less, and a method for producing the same.

In order to solve the above problems, the present invention has the following configuration. That is,
[1] A polyester film containing a black pigment, wherein the thickness of the polyester film is 150 μm or more and 500 μm or less, and the breaking strength of at least one of the longitudinal direction (MD direction) and the width direction (TD direction) of the film is 150 MPa or more and 250 MPa. A polyester film having an L value on the surface of the polyester film of 12 or less on both sides.
[2] The polyester film according to [1], wherein the surface roughness (SRa) of the polyester film is 46 nm or less on both sides.
[3] The polyester film according to [1] or [2], wherein the content of the black pigment contained in the polyester film is 0.01% by mass or more and 1.5% by mass or less with respect to the entire polyester film.
[4] A polyester resin containing 0.01% by mass to 1.5% by mass of a black pigment is melt-extruded on a rotating cooling drum, and cooled to solidify to a temperature lower than the temperature lowering crystallization temperature Tmc (° C.) of the polyester resin. Is a method for producing a polyester film having a step of obtaining an unstretched polyester film of 1300 μm or more and 4500 μm or less, wherein the surface temperature of the polyester film opposite to the cooling drum of the polyester film melt-extruded on the cooling drum is Tmc The time taken to cool from (° C.) to (Tmc-70) (° C.) was cooled to 3.0 seconds or more and 9.0 seconds or less, and then peeled from the cooling drum to obtain an unstretched film. Then, the manufacturing method of the polyester film stretched at least uniaxially.
[5] In the step of obtaining the unstretched polyester film, air of 5 ° C. or more and 25 ° C. or less is blown at a speed of 10 m / second or more and 40 m / second or less on the surface of the polyester film opposite to the cooling drum on the cooling drum. The method for producing a polyester film according to [4].
[6] The method for producing a polyester film according to [4] or [5], wherein a surface temperature of the cooling drum is 10 ° C. or higher and 40 ° C. or lower.

  According to the present invention, even a thick film having a thickness of 150 μm or more and 500 μm or less can provide a polyester film in which the color tone (L value) on both sides is suppressed and a method for producing the same.

  Examples of the polyester resin constituting the polyester film of the present invention include an aliphatic dicarboxylic acid such as aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, and naphthalenedicarboxylic acid, adipic acid, sebacic acid, dodecadioic acid, and azelaic acid. Examples include acids and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid, and alcohol components such as aliphatic glycols such as ethylene glycol, diethylene glycol and polyethylene glycol, and aromatic glycols such as cyclohexanedimethanol. These polyester resins may be used alone or may be copolymerized with other components. Specifically, polyethylene terephthalate or polyethylene-2,6-naphthalate is preferable from the viewpoint of cost and characteristics.

  Examples of the polyester resin include polyethylene terephthalate, polyethylene-2,6-naphthalate, polypropylene terephthalate, polybutylene terephthalate, polylactic acid and other homopolymers, and copolymers thereof. The polyester resin constituting the film of the present invention is the above-mentioned polyester resin. One of these homopolymers and copolymers may be selected and used, or homopolymers or a mixture of homopolymers and copolymers may be used.

  Here, the homopolymer of the polyester resin is preferably polyethylene terephthalate, polyethylene-2, 6-naphthalate, polybutylene terephthalate, or polylactic acid from the viewpoint of film forming property. Among them, polyethylene terephthalate and polyethylene are preferable because of easy processability. -2,6-naphthalate is more preferable, and polyethylene terephthalate is particularly preferable because it is superior in film forming property.

  The polyester resin copolymer is a copolymer composed of either or both of different dicarboxylic acid components and diol components in less than 50 mol% of the entire polyester resin, and blended with a homopolymer. Is assumed, it is preferable to use a copolymer in which the same molecular structure as that of the target homopolymer constitutes 50 mol% or more of the whole.

  Here, as a copolymer of polyester resin, alicyclic dicarboxylic acid, isophthalic acid, naphthalenedicarboxylic acid is a diol component as a dicarboxylic acid component from the viewpoint of excellent polymerization suitability, thermal stability, and compatibility with a homopolymer. As these, those containing butanediol, ethylene glycol, spiroglycol, and cyclohexanedimethanol as co-components are preferably used, and these may be used alone or in combination as necessary. Among them, those containing isophthalic acid as a copolymer component as a dicarboxylic acid component and those containing cyclohexanedimethanol as a copolymer component as a diol component are more preferably used from the viewpoint of improving film forming properties.

  The polyester film of the present invention needs to contain a black pigment. By doing so, the polyester film can be concealed. In addition, a polyester film containing a black pigment is preferably used from the viewpoint of the design of electronic equipment that is processed as part of the product appearance, such as a camera shutter.

  The concealability of the film can be evaluated using the optical density as an index. If the optical density is 3.0 or more, it is considered that there is concealing property, which is preferable.

  The polyester film of the present invention needs to have a thickness of 150 μm or more and 500 μm or less. When the thickness is less than 150 μm, the rigidity of the film is insufficient, so that wrinkles are likely to occur when processed as an electronic component. In addition, when the thickness of the polyester film is more than 500 μm, since the film is too thick, the rigidity becomes too strong, which makes it difficult for the customer to carry and process it. More preferably, they are 180 micrometers or more and 320 micrometers or more.

  The polyester film of the present invention needs to have a breaking strength of 150 MPa or more and 250 MPa or less in at least one of the longitudinal direction (MD) and the width direction (TD) of the film. When the breaking strength is less than 150 MPa, the strength of the film is insufficient, and therefore, when used as an electronic component, there is a risk of causing problems such as breaking. When the breaking strength exceeds 250 MPa, the rigidity of the film is too strong, resulting in insufficient elongation and difficult product processing. It is more preferable that the breaking strength in the longitudinal direction (MD) and the width direction (TD) of the film is in the above range.

  In the polyester film of the present invention, the color tone (L value) on the surface of the polyester film needs to be 12 or less, and preferably 11 or less. It is preferable for the color tone (L value) to be in the above-mentioned range since the blackness of the film becomes strong, the design appearance is excellent, and the jet blackishness can be obtained. When the color tone (L value) on the surface of the polyester film exceeds 12, the film looks white and looks almost gray, and the product that can be used is limited from the viewpoint of design. Moreover, in this invention, when the color tone (L value) of the film of this invention shall be 12 or less, when the difference of the color tone (L value) of the front and back of a film shall be 3.0 or less, external appearance evaluation (whitening condition) and front and back This is preferable because both of the difficulty of identification are good. The difficulty in identifying the front and back is an index by which the difference in color tone can be identified when the front and back are visually confirmed. In the present invention, the measurement method described later is used. If there is a difference in appearance between the front and back of the film, it will be necessary to process the film while paying attention to the front and back of the film, making it difficult to handle. The difference in color tone (L value) between one side and the other side is 3.0 or less. Preferably, it is 2.0 or less, more preferably 1.0 or less. If there is a difference in appearance between the front and back of the film, the direction of unwinding of the film roll must be selected and installed in the machine equipment at the time of processing, and the film surface can be used as an opportunity equipment by rewinding the film roll before processing. It takes time and labor to take into account the film surface according to the equipment of the processing destination. Therefore, from the viewpoint of difficulty in identifying the front and back, the difference in color tone (L value) between the front and back of the film is preferably 0.0.

  The evaluation of the difficulty in identifying the front and back is as described later, but is an evaluation of how far the front and back cannot be identified.

  The lower limit of the color tone (L value) need not be limited, but is substantially 8.0 or more. The method of setting the color tone (L value) on both sides of the film in the above range is not particularly limited, but the difference in the black pigment content of the layer having the surface layer (A if it is a two-layer laminated film of A layer / B layer) The difference between the black pigment content of the layer and the black pigment content of the B layer, the difference between the black pigment content of the A layer and the black pigment content of the C layer if it is a three-layer laminated film of A layer / B layer / C layer ) Is reduced to 0.1% by mass or less (in the case of a single layer film, the difference is 0), and a method of obtaining a polyester film by using a production method described later is exemplified.

  The color tone is an index for evaluating the color of the film, and the L value thereof evaluates the brightness of the color. The L value has a range from 0 to 100. The larger the L value is, the more white the color is, and the smaller the value is, the closer it is to black.

In the polyester film of the present invention, the content of carbon black in the polyester composition constituting the polyester film is preferably 0.01% by mass or more and 1.5% by mass or less based on the entire polyester composition. If the carbon black content is less than 0.01% by mass, the concealability is not sufficient, and if it exceeds 1.5% by mass, the crystallization speed of the polyester composition increases, so the color tone (L value) of the film surface is The surface roughness SRa, which will be described later, is also easily roughened. More preferably, it is 0.1 mass% or more and 1.5 mass% or less, More preferably, it is 0.3 mass% or more and 1.2 mass% or less.

  In the polyester film of the present invention, the surface roughness SRa on both sides of the film is preferably 46 nm or less, and more preferably 37 nm or less. When the surface roughness SRa of the film exceeds 46 nm, the smoothness of the film surface may deteriorate, the glossiness may deteriorate, and the appearance of the design may be deteriorated. In addition, transfer marks may be generated at the time of film roll creation due to coarse protrusions. As a result, there is a limit to the use of the film. From the viewpoint of the smoothness of the present invention, the surface roughness SRa of the film is preferably a lower value, but it is wound up as a film roll from the viewpoint of runnability during the film production process or film processing. From the viewpoint of blocking between the films, the SRa on the front and back sides of the film is preferably 10 nm or more, more preferably 15 nm or more. From the viewpoint of running performance, the surface roughness of the film having a certain degree of gloss is selected, but the front and back are selected, and the surface having a higher surface roughness SRa is often used on the surface side in contact with the process equipment line. It is necessary to perform processing while paying attention to the front and back sides, and the smaller the difference in surface roughness SRa between the front and back sides, the better the handleability. In the film of the present invention, the difference in surface roughness SRa between one surface and the other surface is preferably 30 nm or less, and more preferably 20 nm or less. The method for setting the SRa on both sides of the film in the above range is not particularly limited, but the difference in the sum of the content of the particles of the surface layer and the black pigment (if it is a two-layer laminated film of A layer / B layer) The difference between the sum of the contents of the A layer particles and the black pigment and the sum of the contents of the B layer particles and the black pigment, and if the A layer / B layer / C layer three-layer laminated film, the A layer particles and the black pigment The difference between the sum of the pigment contents and the sum of the contents of the C layer particles and the black pigment) is reduced to 0.1% by mass or less (in the case of a single layer film, the difference is 0), And the method of obtaining a polyester film using the manufacturing method mentioned later is mentioned.

  Examples of black pigments include carbon black (furnace black, channel black, acetylene black, thermal black, lamp black, etc.), graphite, copper oxide, manganese dioxide, aniline black, perylene black, titanium black, cyanine black, activated carbon, ferrite, Black pigments such as magnetite, chromium oxide, iron oxide, molybdenum disulfide, chromium complexes, complex oxide black pigments, anthraquinone organic black pigments can be used alone or in combination of two or more. It is preferable to use carbon black that is excellent in light shielding properties and dispersibility in the resin.

  The polyester film of the present invention may contain various additives excluding the black pigment as necessary. The content of the additive is preferably 1.5% by mass or less, more preferably 1.2% by mass or less, together with the black pigment, with respect to the entire film. As the additive, various inorganic and organic particles can be used, and those having various shapes such as true spherical particles, aggregated particles, flake shaped particles, and beaded particles can be used. As the organic particles, those made of a thermoplastic resin such as cross-linked polystyrene, cross-linked acrylic resin, fluororesin, and imide resin can be used. When the particles are contained, the film can be easily slipped and processed easily. If the various additives exceed 1.5% by mass, the crystallization speed of the polyester composition increases, so the color tone (L value) of the film surface tends to increase, and the surface roughness SRa described later also tends to be rough.

  The polyester film of the present invention is at least a uniaxially oriented film and exhibits the main characteristics necessary for the function of the film, such as improved mechanical strength, improved thermal stability, improved chemical resistance, and improved electrical properties. Therefore, it is preferable. The term “uniaxial orientation” as used herein refers to a uniaxial orientation pattern obtained by wide-angle X-ray diffraction. In general, the uniaxially oriented film can be obtained by stretching an unstretched sheet in the sheet longitudinal direction or the width direction by about 2.5 to 5.0 times, and then performing heat treatment to complete the crystal orientation. In addition, in the case of a biaxially oriented film, the stretching in the longitudinal direction and the width direction may be a so-called sequential biaxial stretching method that is sequentially performed individually or a simultaneous biaxial stretching method that is simultaneously performed.

(Film production method)
About the manufacturing method of the polyester film of this invention, manufacturing by the process shown below, for example is preferable.
First, the manufacturing method of resin which comprises a polyester film can be manufactured with the following method.

  The polyester resin can be obtained by subjecting a dicarboxylic acid or an ester derivative thereof and a diol to an ester exchange reaction or an esterification reaction by a known method. Examples of conventionally known reaction catalysts include alkali metal compounds, alkaline earth metal compounds, zinc compounds, lead compounds, manganese compounds, cobalt compounds, aluminum compounds, antimony compounds, titanium compounds, and phosphorus compounds. Preferably, an antimony compound, a germanium compound, or a titanium compound is added as a polymerization catalyst at an arbitrary stage before the production method of the polyester resin is completed. As such a method, for example, when a germanium compound is taken as an example, it is preferable to add the germanium compound powder as it is.

  Examples of the method of incorporating the black pigment into the polyester film of the present invention include melt extrusion of a polyester resin containing 0.01% by mass to 1.5% by mass of the black pigment. There is a method (masterbatch method) in which a raw material in which a black pigment is masterbatched at a high concentration is prepared and diluted to a black pigment concentration in the above range with a polyester resin that does not contain a black pigment when charged into an extruder. Specifically, after 1 to 30% by mass of carbon black is kneaded in a polyester resin to obtain a carbon black-containing polyester chip, a polyester resin not containing carbon black and a desired black pigment are contained. Can be blended to make an amount.

  The polyester resin raw material containing the black pigment is put into an extrusion hopper, the resin is melted at 270 to 320 ° C. in the extruder, extruded into a slit-shaped die, wound around a cooling drum having a surface temperature of 10 to 40 ° C., and cooled. Air is blown onto the polyester film on the side opposite to the drum to cool and solidify it to make an unstretched film. Subsequently, the unstretched film is stretched 2.5 to 3.5 times in the longitudinal direction at 60 to 120 ° C. to obtain a uniaxially oriented film. When producing a biaxially oriented film, the uniaxially oriented film is then introduced into the tenter, preheated at 90 to 140 ° C., and then stretched 2.0 to 4.0 times in the width direction to obtain a biaxially oriented film. . Thereafter, the uniaxially or biaxially oriented film may be heat treated at 210-240 ° C. It is possible to increase the strength by further stretching in the vertical or horizontal direction, or both in the vertical and horizontal directions before the heat treatment. When the draw ratio in the film width direction is 4.0 or more, the thermal shrinkage rate in the film width direction becomes too large, and when the product is produced, there may be a problem in a processing process with precise dimensional regulation.

  In the manufacturing method of the polyester film of this invention, when the thickness of the polyester film after extending | stretching is 150 micrometers-500 micrometers, it is preferable that the thickness of the unstretched film extruded to the cooling drum and solidified by cooling is 1300 micrometers or more and 4500 micrometers or less. When the thickness of the unstretched film is 1300 μm or less, the film thickness after stretching is insufficient, and the strength of the polyester film is insufficient. When the thickness of the unstretched film exceeds 4500 μm, the cooling of the unstretched film by the cooling drum and air becomes insufficient, and it becomes difficult to suppress the color tone (L value) and surface roughness SRa of the film surface opposite to the cooling drum. . The thickness of the unstretched film after cooling and solidification is more preferably 1600 μm or more and 3000 μm or less.

  In the method for producing a polyester film of the present invention, in the step of melt-extruding a polyester resin on a rotating cooling drum and cooling and solidifying to obtain an unstretched polyester film, what is the cooling drum of the polyester film melt-extruded on the cooling drum? The time required for the surface temperature of the opposite polyester film to cool from Tmc (° C.) to (Tmc-70) (° C.) is preferably 3.0 seconds or more and 9.0 seconds or less. However, Tmc is the temperature-falling crystallization temperature of a polyester resin. When the time taken for the surface temperature of the film opposite to the cooling drum of the polyester film to cool from Tmc (° C.) to (Tmc-70) (° C.) exceeds 9.0 seconds, the cooling drum of the polyester film Since the cooling of the opposite surface is insufficient, crystallization proceeds and the color tone (L value) and surface roughness SRa cannot be suppressed. In order to cool an unstretched film of 1300 μm or more, the time required for the surface temperature of the film opposite to the film cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.) is It is substantially difficult to make the time less than 3.0 seconds. More preferably, it is 4.0 seconds or more and 7.0 seconds or less. The method of controlling the time taken for the surface temperature of the polyester film opposite to the cooling drum of the polyester film melt-extruded on the cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.) is particularly Although not limited, the method of adjusting the temperature and wind speed of air blown to the polyester film surface opposite to the cooling drum on the cooling drum, which will be described later, and adjusting the rotation speed of the cast drum, The method of adjusting the time which the film is contacting is mentioned.

  In the method for producing a polyester film of the present invention, in the step of obtaining an unstretched polyester film, it is preferable to blow air of 5 ° C. or more and 25 ° C. or less on the surface of the polyester film opposite to the cooling drum on the cooling drum. When the temperature of the air exceeds 25 ° C., the unstretched polyester film is not sufficiently cooled, and the crystallization of the polyester film proceeds. Therefore, the color tone (L value) and surface roughness of the film surface opposite to the cooling drum are increased. SRa may not be suppressed. When the temperature of the air is less than 5 ° C., condensation occurs in the cooling drum or the air blowing device, causing the polyester film to be hydrolyzed, resulting in uneven thickness.

  In the method for producing a polyester film of the present invention, in the step of obtaining an unstretched polyester film, air is blown at a speed of 10 m / second to 40 m / second on the surface of the polyester film opposite to the cooling drum on the cooling drum. Is preferred. When the air speed is less than 10 m / sec, cooling of the unstretched polyester film is insufficient, and crystallization of the polyester film proceeds. Therefore, the color tone (L value) of the film surface opposite to the cooling drum or It becomes impossible to suppress the surface roughness SRa. When the air speed exceeds 40 m / sec, the color tone (L value) tends to be 12 or less, but the unstretched film may flutter and may cause uneven thickness, resulting in a low product value and realistic. is not. More preferably, it is 12 m / sec or more and 35 m / sec or less.

  The present inventors presume that a difference occurs in the color tone (L value) and surface roughness SRa between one film surface and the other film surface depending on the cooling condition of the polyester film having a large layer thickness. doing. When the polyester film is insufficiently cooled, crystallization of the polyester proceeds with the particles as nuclei, forming a whiteish gray polyester film and forming surface protrusions. In a general polyester film manufacturing method, after melt extrusion, the polyester film is cooled by bringing one side of the polyester film into contact with the cooling drum, and in a thick polyester film, the polyester surface in contact with the cooling drum; Since the cooling conditions on the polyester surface not in contact with the cooling drum are greatly different, the above-mentioned phenomenon occurs remarkably. Since carbon black, which is a typical black pigment, can also be a crystal nucleus material, if the content of carbon black particles is brought close to the lower limit of the present invention, the surface roughness SRa can be easily reduced. Depending on the cooling conditions, crystallization of the polyester proceeds and it is difficult to reduce the color tone (L value).

  In the method for producing a polyester film of the present invention, in the step of obtaining an unstretched polyester film, the surface temperature of the cooling drum is preferably 10 ° C. or higher and 40 ° C. or lower. When the surface temperature of the cooling drum exceeds 40 ° C., the cooling of the unstretched polyester film is insufficient, and the crystallization of the polyester film proceeds. Therefore, the L value and surface roughness of the film surface opposite to the cooling drum are increased. In some cases, SRa cannot be suppressed. If the surface temperature of the cooling drum is less than 10 ° C., condensation occurs on the cooling drum, causing hydrolysis of the polyester film, resulting in uneven thickness. More preferably, it is 11 degreeC or more and 24 degreeC.

  Regarding the cooling of the polyester film on the cooling drum, the surface roughness SRa of the film surface on the cooling drum side is in close contact with the cooling drum and easily satisfies the scope of the present invention, but the color tone (L value) of the film surface on the cooling drum side Tends to exceed the scope of the present invention when cooling is slow.

  The polyester film of the present invention has excellent light-shielding properties, and there is no difference in appearance on the front and back of the film, so there is no need to process it while paying attention to the front and back of the film. It can be suitably used by being incorporated in an electronic device or apparatus such as a camera, a video camera, a laser beam printer, or a lithium ion battery. Since the film of the present invention has high-quality surface properties on both sides, it can be easily processed into a substrate, and can be a lightweight, high-strength substrate with excellent design.

(Measurement method of physical properties)
(1) Thickness (μm)
The slice piece of the film cut in cross section using the electric microtome ST-201 manufactured by Japan Microtome Research Laboratory was observed with a transmission light microscope, and the thickness (μm) was measured.

(2) Breaking strength (MPa)
An A4 size film was cut and used as a sample. Using a constant-speed tension type tensile testing machine, pulling in the sample length direction at an original length (distance between chucks) of 100 mm and a pulling speed of 200 mm / min. The tensile load value when the film is broken is the cross-sectional area of the film before the test of the sample. The value (MPa) divided by was calculated. The measurement was performed 5 times for each of the longitudinal direction (MD direction) and the width direction (TD direction) of the film, and the average value was defined as the breaking strength. In addition, when the longitudinal direction cannot be specified, the ultrasonic conduction velocity in the film plane is measured by SONIC SHEEET TESTER SST-4000 (manufactured by Nomura Shoji Co., Ltd.). Is regarded as the width direction.

(3) Color tone (L value)
Using a spectroscopic color difference meter (Nippon Denshoku Industries Co., Ltd. ZE-2000), the color tone (L value) of any film was measured by a reflection method. For standard matching, an attached standard white plate (X value, Y value, and Z value in the above-described measurement method was 93.11, 95.11, 112.05) was used.

(4) Black pigment content (% by mass)
A film piece of 5 mg was placed in a differential thermal analyzer (TA Instruments (model number Q5000IR) and heated in air from room temperature to 1000 ° C. at a rate of 2 ° C./min. From 550 ° C. at that time The weight loss (mass%) between 900 ° C. was analyzed as the weight loss of the black pigment, and the mass weight loss rate of 0.3% or less is within the apparatus detection error range.

(5) Surface roughness SRa (nm)
An arbitrary surface of the film was measured by a stylus method under the following conditions using a three-dimensional surface roughness meter ET4000AK (manufactured by Kosaka Laboratory). The surface roughness SRa (nm) is the difference between the height of the roughness curved surface and the height of the center surface of the roughness curved surface, and represents the average value of the absolute values.
Needle diameter 2μmR
Needle pressure 10mg
Measurement length 500μm
Vertical magnification 20000 times CUT OFF 250μm
Measuring speed 100μm / s
Measurement interval 5μm
Number of records 80 lines Hysteresis width ± 6.25nm
Reference area 0.1 mm ² .

(6) Temperature drop crystallization temperature (Tmc) (° C)
The film piece was melted at a temperature of 300 ° C. for 5 minutes using a differential scanning calorimeter TAC7DX and then cooled at a cooling rate of 20 ° C./min, and the temperature of the exothermic peak accompanying crystallization was read as Tmc (° C.).

(7) Time (seconds) required for the surface temperature of the unstretched polyester film opposite to the cooling drum surface on the cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.)
Using a Keyence non-contact thermometer IT2-80, the temperature of the unstretched polyester film on the cooling drum was measured at an emissivity of 0.95, the position (point A) at which the film temperature was Tmc (° C.), and the film temperature Was searched for (Tmc-70) (° C.). Thereafter, the side surface of the cooling drum was marked, and the time (seconds) during which the cooling drum rotated and passed between the points A and B was measured.

(8) Air temperature (° C) and speed (m / s)
The temperature (° C.) and speed (m / s) of the tip of the supply nozzle of the air blowing device were measured using an Anemo master speedometer Model 6112 manufactured by Canomax. Five points in the width direction of the nozzle were measured and averaged.

(9) Cooling drum surface temperature (° C)
The temperature (° C.) on the cooling drum was measured with an emissivity of 0.95 using a Keyence non-contact thermometer IT2-80.

(10) Appearance evaluation (A) Film surface whitening condition evaluation The evaluation criteria for the film surface whitening condition are as follows according to the L value of the film surface of the surface having the higher color tone (L value) of the front and back of the film evaluated in (3). And ○ and △ were regarded as acceptable.
Any film surface has an L value of 11 or less:
Any film surface has an L value of more than 11 and 12 or less:
Any film surface has an L value greater than 12: x
(B) Evaluation of film surface smoothness The evaluation criteria for the surface roughness (SRa) of the film surface having the higher surface roughness (SRa) of the front and back surfaces of the film measured in (5) are defined as follows. , ○ and △ were regarded as acceptable.
Any film surface has SRa of 37 nm or less:
Any film surface has SRa of more than 37 nm and 46 nm or less: Δ
Any film surface has an SRa greater than 46 nm: x
(C) Evaluation of difficulty in discriminating between front and back of film The evaluation criteria are defined as follows to determine whether the film surface is difficult to discriminate due to the difference in color tone (L value) between the front and back of the film evaluated in (3). , ○ and △ were regarded as acceptable.
L value difference is more than 0.0 and 1.0 or less:
L value difference exceeds 1.0 and 2.0 or less: ○ L value difference exceeds 2.0 and 3.0 or less: Δ
Difference in L value exceeds 3.0: x.

[Production of film]
(Production method of resin)
1. Polyethylene terephthalate (PET)
100 parts by mass of dimethyl terephthalate, 57.5 parts by mass of ethylene glycol, 0.03 parts by mass of magnesium acetate dihydrate, 0. 03 parts by mass were melted at 150 ° C. in a nitrogen atmosphere. While stirring this melt, the temperature was raised to 230 ° C. over 3 hours to distill methanol, and the transesterification reaction was completed. After completion of the transesterification reaction, an ethylene glycol solution (pH 5.0) in which 0.005 parts by mass of phosphoric acid was dissolved in 0.5 parts by mass of ethylene glycol was added. The intrinsic viscosity of the polyester composition at this time was less than 0.2. Thereafter, the polymerization reaction was carried out at a final temperature of 285 ° C. and a degree of vacuum of 0.1 Torr to obtain polyethylene terephthalate having an intrinsic viscosity of 0.65 and a terminal carboxyl group amount of 34 equivalents / ton.
2. CB master batch (CB-MB)
Above 1. 95% by mass of polyethylene terephthalate obtained in accordance with the above item and 5% by mass of carbon black (CB-1) produced by a furnace method having a primary particle diameter of 18 nm are melt-kneaded in a vented 280 ° C. extruder, and a CB master batch. 1 was produced.
3. Silica master batch (Si-MB)
Above 1. The mixture was melt-kneaded in an extruder at 280 ° C. in which 98.0% by mass of the polyethylene terephthalate obtained in the above section and 2.0% by mass of silica particles having a primary particle size of 2.6 μm were vented to prepare a SiO ₂ master batch.

Example 1
The raw material polymers were mixed at the blending ratio shown below, and a film was obtained by the following procedure.
PET: 85.5% by mass
・ CB-MB: 13% by mass
・ Si-MB: 1.5% by mass
The raw material chips were dried at 160 ° C. for 2 hours under a reduced pressure of 3.0 kPa or less using a rotary vacuum dryer, then supplied to the extruder and melt-extruded at 280 ° C. After the stainless steel fiber is filtered through a sintered and compressed filter, it is extruded into a sheet form from a T-shaped die, wound around a mirror surface cooling drum having a surface temperature of 20 ° C. using an electrostatic application casting method, and an air nozzle is placed on the cooling drum. On the other hand, with an air blowing device having 13 in parallel, air at 12.4 ° C. and a speed of 19.4 m / sec was blown onto the film on the cooling drum to cool and solidify. The time required for the surface temperature of the unstretched polyester film opposite to the cooling drum surface on the cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.) was 5.7 seconds. This unstretched film is preheated to 80 ° C. with a preheating roll, and then stretched 2.8 times in the longitudinal direction using the difference in peripheral speed between the rolls while heating to 90 ° C. using a radiation heater, and subsequently used as a cooling roll. And cooled to 40 ° C. to obtain a uniaxially oriented film. The both ends of the uniaxially oriented film in the width direction were held with clips and preheated in an oven at an ambient temperature of 130 ° C., and continuously stretched 3.3 times in the width direction. The obtained biaxially oriented film was subsequently heat-treated in a heating zone at 230 ° C., and a relaxation treatment of 2.3% was performed by reducing the interval between clips facing in the width direction while cooling to 150 ° C. Then, after further cooling, the film holding the both ends in the film width direction is separated and the film is taken out from the oven, the both ends in the width direction are cut and removed, wound up after being transported by a transport roll, and the film thickness is 250 μm. A biaxially oriented polyester film was obtained.

(Examples 2 to 19)
A polyester film was obtained in the same manner as in Example 1 except that it was described in the table.

(Comparative Examples 1-8)
A polyester film was obtained in the same manner as in Example 1 except that it was described in the table.

(Reference Example 1)
A polyester film was obtained in the same manner as in Example 1 except that it was described in the table.

(Summary of observation)
When the polyester films obtained in Examples 1 to 7 and 9 to 13 were observed, they were high-quality polyester films without any whitening or poor smoothness on either side.

  In Example 8, since the air speed blown to the cooling drum is slower than that in Example 1, the color tone (L value) of the film surface opposite to the cooling drum surface and the surface roughness SRa are slightly higher, but whitening, A polyester film in which almost no smoothness was found was obtained.

  In Example 14, since the air temperature blown to the cooling drum is higher than that in Example 1, the surface roughness SRa of the film surface opposite to the cooling drum surface is slightly high, but there is no whitening and poor smoothness. A polyester film with almost no spots was obtained.

  In Example 15, the air speed blown onto the cooling drum is slower than that in Example 1 and the air temperature is high, so the color tone (L value) and surface roughness SRa of the film surface opposite to the cooling drum surface are slightly higher. Although it was high, a polyester film in which whitening and poor smoothness were hardly seen was obtained.

  In Example 16, the time taken for the unstretched polyester film surface temperature on the opposite side of the cooling drum surface on the cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.) was compared with Example 1. Because of its long length, a polyester film having a slightly high surface roughness SRa on the side opposite to the cooling drum surface but no whitening and almost no poor smoothness was obtained.

  In Example 17, since the carbon black concentration in the polyester film is higher than that in Example 1, the color tone (L value) of the film surface opposite to the cooling drum surface is slightly higher, but there are no places with poor smoothness. A polyester film with almost no whitening was obtained.

  In Example 18, the time taken for the surface temperature of the unstretched polyester film on the opposite side of the cooling drum surface on the cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.) is compared with Example 1. Since the film was long, the color of the film surface opposite to the cooling drum surface (L value) and the surface roughness SRa were slightly high, but a polyester film with almost no whitening or poor smoothness was obtained.

  In Example 19, since the speed of air blown onto the cooling drum was faster than that in Example 1, local thickness unevenness was observed in the film, but a polyester film with no whitening or poor smoothness was obtained. It was.

  In Comparative Example 1, the time taken for the surface temperature of the unstretched polyester film opposite to the cooling drum surface on the cooling drum to cool from Tmc (° C.) to (Tmc-70) (° C.) exceeded 9 seconds. Therefore, the crystallization of the polyester film proceeds on the surface opposite to the cooling drum surface, the color tone (L value) of the surface opposite to the cooling drum exceeds 12, and the surface roughness SRa exceeds 46 nm. As a result, whitening of the surface opposite to the cooling drum surface and deterioration of smoothness appeared clearly, and a low-quality polyester film was obtained.

  In Comparative Example 2, since the air speed blown to the cooling drum is lower than 10 m / sec, whitening of the surface opposite to the cooling drum surface and deterioration of smoothness clearly appear, and a low-quality polyester film is produced. Obtained.

  In Comparative Example 3, since air was not blown to the surface opposite to the cooling drum surface, the crystallization of the polyester film progressed on the surface opposite to the cooling drum surface, and the surface opposite to the cooling drum. The color tone (L value) exceeds 12 and the surface roughness SRa exceeds 46 nm. As a result, whitening of the surface opposite to the cooling drum surface and deterioration of smoothness clearly appear, and low-quality polyester. A film was obtained.

  In Comparative Example 4, since the carbon black content in the polyester film exceeded 1.5% by mass and no air was blown onto the surface opposite to the cooling drum surface, the polyester film crystals Progressing on the surface opposite to the cooling drum surface, the color tone (L value) on the surface opposite to the cooling drum exceeded 12, and the surface roughness SRa exceeded 46 nm. Whitening of the surface opposite to the surface and deterioration of smoothness appeared clearly, and a low-quality polyester film was obtained.

  In Comparative Example 5, in addition to the fact that the sum of the carbon black content and the Si particle content in the polyester film exceeds 1.5% by mass, air is not blown onto the surface opposite to the cooling drum surface. Therefore, the crystallization of the polyester film proceeds on the surface opposite to the cooling drum surface, the color tone (L value) of the surface opposite to the cooling drum exceeds 12, and the surface roughness SRa exceeds 46 nm. As a result, whitening of the surface opposite to the cooling drum surface and deterioration of smoothness appeared clearly, and a low-quality polyester film was obtained.

  In Comparative Example 6, since the carbon black content in the polyester film exceeds 1.5% by mass, crystallization proceeds on both sides of the polyester film, and the color tone (L value) on both sides of the film exceeds 12. In addition to this, since the speed of air blown to the cooling drum exceeds 40 m / s, uneven thickness occurs due to film fluttering, resulting in a low-quality polyester film in which both sides are whitened and uneven thickness is clearly seen. It was.

  In Comparative Example 7, since the carbon black content in the polyester film exceeds 1.5% by mass, the color tone (L value) of the film surface opposite to the cooling drum exceeds 12. Since the temperature of the air blown onto the cooling drum is below 10 ° C, condensation has formed on the air blowing device and the cooling drum, resulting in uneven thickness of the film. A low-quality polyester film as shown in FIG.

  In Reference Example 1, the unstretched film thickness on the cooling drum was 660 μm, and both sides of the film were sufficiently cooled without air blowing, and the crystallization of the polyester was suppressed. A high-quality polyester film having no poor smoothness was obtained.

  In Comparative Example 8, since the carbon black content in the polyester film exceeds 1.5% by mass, crystallization proceeds on both sides of the polyester film, and the color tone (L value) of the film ND surface exceeds 12. A low-quality polyester film having a whitened ND surface was obtained.

Claims (7)

A polyester film containing a black pigment, wherein the thickness of the polyester film is 150 μm or more and 500 μm or less, and the breaking strength of at least one of the longitudinal direction (MD direction) and the width direction (TD direction) of the film is 150 MPa or more and 250 MPa or less. A polyester film having an L value on the surface of the polyester film of 12 or less on both sides. The polyester film according to claim 1, wherein the surface roughness (SRa) of the polyester film is 46 nm or less on both sides. The polyester film according to claim 1 or 2, wherein a content of the black pigment contained in the polyester film is 0.01% by mass or more and 1.5% by mass or less with respect to the entire polyester film. A polyester resin containing 0.01% by mass or more and 1.5% by mass or less of a black pigment is melt-extruded on a rotating cooling drum, cooled and solidified to a temperature lower than the temperature lowering crystallization temperature Tmc (° C.) of the polyester resin, and a thickness of 1300 μm or more A method for producing a polyester film comprising a step of obtaining an unstretched polyester film of 4500 μm or less, wherein the polyester film has a surface temperature Tmc (° C.) opposite to the cooling drum of the polyester film melt-extruded on the cooling drum. To (Tmc-70) (° C.) after cooling to 3.0 seconds or more and 9.0 seconds or less, after peeling from the cooling drum to obtain an unstretched film, at least A method for producing a uniaxially stretched polyester film. In the step of obtaining the unstretched polyester film, air of 5 ° C or more and 25 ° C or less is blown at a speed of 10 m / second or more and 40 m / second or less on the surface of the polyester film opposite to the cooling drum on the cooling drum. 4. A method for producing a polyester film according to 4. The method for producing a polyester film according to claim 4 or 5, wherein a surface temperature of the cooling drum is 10 ° C or higher and 40 ° C or lower. The method for producing a polyester film according to claim 4, wherein the stretched polyester film has a thickness of 150 μm or more and 500 μm or less.