JP2001288291A - Void-containing polyester based film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a void-containing polyester based film which is a polyester based film having :a light weight and high strength and, simultaneously, excellent processability and has a good dispersion state of voids and excellent reflectivity to visible rays and is suitably used as the material for various reflective plates. SOLUTION: The polyester based film is mainly composed mainly of a polyester based resin and contains voids in the inside with a void build-up number density of >=0.20/μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a void-containing polyester film, particularly to a void-containing polyester film having high reflectivity to visible light and suitable for use in various reflectors.

[0002]

2. Description of the Related Art In recent years, as information processing devices such as computers, word processors, and mobile phones have become higher in performance and smaller in size and lighter in weight, liquid crystal displays have been increasingly used as display devices to replace cathode ray tubes and LED panels that have been widely used. Spreading is progressing. In these information processing devices, there is a demand for weight reduction of each component of each part in order to reduce the size and improve the portability, and studies are also being continued to realize smaller and lighter liquid crystal displays. ing. In this trend, polyester-based films are used as reflectors and diffusers for backlights of liquid crystal panels, and are attracting attention as materials that are lightweight, durable, and have high processability that are not found in other materials such as glass and metal. I have.

[0003] The role of a reflector in a liquid crystal backlight is to improve the brightness of a display, and plays this role by reflecting light guided from the side through a light guide plate forward. For this reason, the reflector is required to have a high reflectance with respect to visible light, and is required to provide uniform reflected light at all wavelengths, and various techniques have been studied.

[0004] In general, as a means for making a polyester film opaque and imparting reflectivity to visible light, a white pigment is added and dispersed in the film. Many studies have been made on this method as a technique for using a polyester film as a printing / display material, and attempts have been made to add a wide variety of inorganic and organic white pigments such as calcium carbonate, titanium dioxide, and barium sulfate. ing. At present, one having a certain degree of reflectivity has already been obtained by this technique and is used in the actual production of liquid crystal panels, but at present some problems remain as unsolved problems.

[0005] One of the unsolved problems is the problem of film density. Originally, the purpose of using a polyester-based film as a reflector is to reduce the size and weight, and it is expected that a thin and light film as thin as possible will obtain high reflectivity. However, white pigments such as those described above are substances that are significantly more dense than the resin that forms the film in many cases, and adding a large amount of this to increase reflectivity results in increasing the density of the film itself. . This is to offset the lightness obtained by using the polyester film for the reflector, and is an important issue to be improved. Another unsolved problem is productivity and cost. Generally, these pigments are more expensive than the resin that forms the film, and adding these pigments makes the film more expensive. In addition, in the production of a film to which a white pigment is added, breakage during film formation or process contamination due to the pigment often occurs.
Generally, productivity tends to decrease. This is also a factor that increases the film price, and is another problem of the method by adding a pigment.

On the other hand, as a means for imparting reflectivity to a film without adding a white pigment, a method of including fine voids in the film has been studied.

[0007] As one of the typical methods for containing fine cavities in a film, a method is known in which a thermoplastic resin incompatible with a polyester constituting the base of the film is added. As for this method, many studies have already been made as a technique for using a polyester film as a printing / display material, and a resin such as polystyrene, polypropylene, or polymethylpentene has been used alone or in combination. (Eg, Japanese Patent Application Laid-Open No. 49-134755, Japanese Patent Publication No. 54-2955)
No. 0, Japanese Unexamined Patent Publication No. 8-143692). Above all, in Japanese Patent Application Laid-Open No. 8-143,692,
A method has been reported in which two types of polyolefin-based resin or polystyrene-based resin are contained in a polyester-based resin for forming a film to finely disperse the cavities, thereby preventing thermal wrinkles and thermal curls. Is greatly improved.

[0008]

However, the above-mentioned conventional void-containing polyester film has insufficient fine dispersion of cavities for use as a material for a reflector, and thus has a lamination of cavities necessary for improving the reflectivity to visible light. There was a problem that the number density was not achieved.

The present invention solves the problems of the above-mentioned conventional void-containing polyester film, and is a lightweight, high-strength and excellent workability polyester film, which has a good dispersion state of voids, An object of the present invention is to provide a void-containing polyester film having excellent reflectivity to visible light and suitable for use as a material for various reflectors.

[0010]

In order to achieve the above object, a void-containing polyester film according to the present invention is a polyester film mainly comprising a polyester resin and having voids therein, wherein the void-laminated polyester film has The number density is 0.20 / μm or more.

Here, the cavity lamination number density means a value (number / μm) obtained by dividing the number of cavities (the number of laminations) in the thickness direction in the cross section in the stretching direction of the film by the film thickness (μm).

The void-containing polyester film of the present invention having the above-mentioned structure is a lightweight, high-strength, and excellent workability polyester film, which has a good dispersion state of the voids and a high reflectivity to visible light. And is suitable for use as a material for various reflection plates.

In this case, the void-containing polyester film has a spectral reflectance of 9 to an electromagnetic wave having a wavelength of 450 nm.
It is preferably at least 8%.

In this case, the void-containing polyester film preferably contains a thermoplastic resin incompatible with the polyester resin.

In this case, it is preferable that the thermoplastic resin incompatible with the polyester resin is a polystyrene resin.

In this case, it is preferable that the thermoplastic resin incompatible with the polyester resin is a polystyrene resin or a polyolefin resin.

Preferably, at least a part of the polyolefin resin is a polymethylpentene resin.

In this case, the melt viscosity η of the polystyrene resin contained in the void-containing polyester film is
It is preferable that the melt viscosity η ₀ of _S and the polyolefin resin satisfy the following relational expression (1). η ₀ / η _S ≦ 0.80 (1)

In this case, the void-containing polyester film has an apparent density of 0.70 to 1.25 g / cm.
It is preferably ³ .

Further, in this case, the void-containing polyester film preferably has a content of white pigment particles of 5% or less.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the void-containing polyester film of the present invention will be described in detail.

In the void-containing polyester film of the present invention, the void number density of the voids must be at least 0.20 / μm, preferably at least 0.25 / μm, more preferably at least 0.30 / μm. / Μm or more. It is not preferable that the cavity stacking density is less than the requirement of the present invention because high reflectivity to visible light, which is the object of the present invention, is not exhibited.

Further, in the void-containing polyester film of the present invention, the spectral reflectance to an electromagnetic wave having a wavelength of 450 nm (that is, blue visible light) is preferably 98% or more, more preferably 100% or more. More preferably, it is most preferably at least 102%.
If the reflectance is less than 98%, high reflectivity to visible light, which is the object of the present invention, is not exhibited, which is not preferable.

The void-containing polyester film of the present invention has an apparent density of 0.70 to 1.2.
5 g / cm ³ , preferably 0.80 to 1.2
0 g / cm ³ , more preferably 0.85
More preferably, it is 1.15 g / cm ³ . If the apparent density is less than 0.70 g / cm ³ , the strength of the film tends to decrease, and there is also a problem in that the film frequently breaks during the stretching step during production and the productivity decreases. On the other hand, if the apparent density exceeds 1.25 g / cm ³ , the content of voids in the film is not sufficient, and the object of the present invention is not to provide sufficient reflectivity to visible light, which is not preferable.

The void-containing polyester film of the present invention contains voids in the film by dispersing and stretching a thermoplastic resin incompatible with the polyester resin.

The polyester resins used herein include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid and naphthalenedicarboxylic acid or esters thereof and ethylene glycol, diethylene glycol, 1,3-propanediol, 1,4-butanediol, It is a polyester produced by polycondensing a glycol such as neopentyl glycol. These polyesters may be prepared by directly reacting an aromatic dicarboxylic acid and a glycol, or by subjecting an alkyl ester of the aromatic dicarboxylic acid to a transesterification reaction with a glycol followed by polycondensation, or a diglycol ester of an aromatic dicarboxylic acid. Can be produced by a method such as polycondensation. Representative examples of such polyesters include polyethylene terephthalate, polytrimethylene terephthalate, polyethylene butylene terephthalate and polyethylene terephthalate.
2,6-naphthalate and the like. This polyester may be a homopolymer or a copolymer of the third component. In any case, in the present invention, the ethylene terephthalate unit, the propylene terephthalate unit, the butylene terephthalate unit or the ethylene-2,6-naphthalate unit is 70 mol% or more,
It is preferable to use a polyester having preferably 80 mol% or more, more preferably 90 mol% or more.

The thermoplastic resin incompatible with the polyester resin used herein is not particularly limited, and may be a homopolymer or a polymer having a copolymer component. It is preferable to use a mixture of a polyolefin resin in addition to the resin or the polystyrene resin. The polystyrene resin used is not necessarily limited to a homopolymer, and may be a copolymer obtained by copolymerizing various components. However, when a copolymer is used, the copolymer component hinders the effects of the present invention. It is necessary that there is no. Examples of the polyolefin resin to be used include polyethylene, polypropylene, polybutene, and polymethylpentene. Polymethylpentene resin is preferred because it hardly softens even at a high temperature and exhibits excellent cavitation. When polymethylpentene resin is used as the main component of the polyolefin resin, it is not always necessary to use it alone, and another polyolefin resin may be added as an auxiliary component. Examples of the resin used as the auxiliary component include polyethylene, polypropylene and those obtained by copolymerizing various components thereof, but are not particularly limited. The viscosity of the polyolefin-based resin added as an auxiliary component is not particularly limited, but it is necessary that the addition amount does not exceed the addition amount of the resin added as a main component.

When a polyolefin resin and a polystyrene resin are used together as the thermoplastic resin, the ratio (η) of the melt viscosity η ₀ (poise) of the polyolefin resin to the melt viscosity η _S (poise) of the polystyrene resin is used. ₀ / η _S ) is preferably 0.80 or less,
It is more preferably 0.60 or less, and most preferably 0.50 or less. The above melt viscosity ratio is 0.8
If it exceeds 0, the distribution of the polystyrene-based resin phase in the dispersed particles will be uniform and the phase structure of the dispersed particles will be unstable.
Therefore, it is not preferable because the dispersion state of the dispersed particles is deteriorated, and it becomes difficult to satisfy the stacking density of the cavities defined in the present invention.

In the void-containing polyester film of the present invention, the content of white pigment particles is preferably 5% by weight or less, more preferably 2% by weight or less, and more preferably 1% by weight or less. Is most preferred. The white pigments here are titanium dioxide, calcium carbonate,
It is a fine powder of barium sulfate and zinc sulfide. If the amount of the white pigment particles exceeds the amount specified here, the reflection of light by the cavities is blocked by the pigment particles, and the reflectance is undesirably reduced. In addition, it is not preferable because the cost of the raw material increases and breakage occurs in the stretching step.

In the void-containing polyester film of the present invention, the thickness of the layer is not particularly limited, but is preferably from 20 to 500 μm, and more preferably from 40 to 250 μm depending on the use. .

In the void-containing polyester film of the present invention, a layer made of a polyester resin or a resin having an adhesive property to the polyester resin on one or both sides thereof for improving the slipperiness of the film or for other purposes. May be laminated by co-extrusion. Further, the resin to be laminated may contain the same type or different types of cavity-forming agents as the film of the present invention. However, when a layer containing no voids is laminated, if the thickness ratio of this layer to the total thickness of the film is significantly increased, the reflectivity naturally decreases, which is not preferable.

The void-containing polyester film of the present invention may have a coating layer on one or both surfaces. Here, by providing the coating layer, the adhesiveness and the antistatic property can be improved.
As the compound constituting the coating layer, a polyester-based resin is preferable. In addition, a polyurethane resin, a polyester-urethane resin, an acrylic-based resin, and the like are disclosed as means for improving the adhesiveness / antistatic property of a normal polyester film. And the like are applicable.

As a method of providing a coating layer, a commonly used method such as a gravure coating method, a kiss coating method, a dip method, a spray coating method, a curtain coating method, an air knife coating method, a blade coating method, and a reverse roll coating method is applied. it can. As the step of applying, any method such as a method of applying before stretching the film, a method of applying after longitudinal stretching, and a method of applying to the surface of the film after the orientation treatment is possible.

The void-containing polyester film of the present invention may have a thin metal layer on one or both surfaces. Here, by providing the thin metal layer, the reflectance can be further improved. As a means for providing the thin metal layer, a commonly used method such as vapor deposition or spatting coat can be applied.

The method for producing the void-containing polyester film of the present invention is arbitrary and not particularly limited. For example, a mixture having the above-described composition is melted and extruded into a film to form an unstretched film. Thereafter, a general method of stretching the unstretched film can be used.

In the present invention, a thermoplastic resin incompatible with the polyester resin is dispersed in the process of melting and extruding the raw material. In the present invention, the polyester resin and the resin to be mixed with the polyester resin are those supplied in the form of pellets, but the present invention is not limited thereto. Raw materials to be fed into an extruder for melt molding into a film were prepared by mixing these resins in pellets according to the desired composition. However, the polyester resin and the polyolefin resin used in the present invention generally differ greatly in specific gravity, and it is preferable to add a device that does not re-separate the pellets once mixed in the process of being supplied to the extruder.
As a preferable example of the measure for this, there is a method in which a part or all of the raw material resins are combined in advance, kneaded and pelletized, and prepared as master batch pellets. Although this method was used in the present invention, it is not particularly limited as long as the effects of the present invention are not hindered.

Further, in the extrusion of a mixed system of these incompatible thermoplastic resins, even after mixing and finely dispersing in a molten state, the resin is re-agglomerated due to the function of reducing the interfacial energy of the resin. There is. This is a phenomenon that, when the unstretched film is extruded, the cavity developing agent is coarsely dispersed, which hinders the development of the desired physical properties. In order to prevent this, when forming the film of the present invention, it is preferable to use a twin-screw extruder having a higher kneading effect to finely disperse the cavity-forming agent in advance. When this is difficult, as an auxiliary means, it is also preferable to finely disperse the cavity developing agent by passing the resin discharged from the extruder through a static mixer, and then supply the resin to a feed block or a die. .
As the static mixer used here, a static mixer, an orifice, or the like can be used. However, when these methods are employed, care must be taken since the thermally degraded resin may stay in the melt line. Since reagglomeration of the incompatible resin in the molten state is considered to proceed with time in a low shear state, reducing the time that the resin stays in the melt line from the extruder to the die is a fundamental solution. In the present invention, this time is 30
Minutes or less, and more preferably 15 minutes or less.

The conditions for stretching and orienting the unstretched film obtained as described above are closely related to the physical properties of the film. In the following, the stretching and orientation conditions will be described by taking as an example a sequential biaxial stretching method most preferably used, particularly a method of stretching an unstretched film in the longitudinal direction and then in the width direction. In the longitudinal stretching step, stretching is performed between two or many rolls having different peripheral speeds. As a heating means at this time, a method using a heating roll, a method using a non-contact heating method may be used, or both may be used. Next, the uniaxially stretched film is introduced into a tenter, and Tg or more Tm-10 ° C. in the width direction.
The film is stretched 2.5 to 5 times at the following temperature (where Tg is the glass transition point of the polyester and Tm is the melting point of the polyester).

The above-mentioned biaxially stretched film is subjected to a heat treatment as required. The heat treatment is preferably performed in a tenter, and is preferably performed in the range of Tm-60 ° C to Tm.

The void-containing polyester film thus obtained exhibits high reflectivity to visible light due to the fine voids dispersed in the polyester resin, and is suitable for use as a material for various reflectors. .

[0041]

Next, examples of the present invention and comparative examples will be described. First, the measurement and evaluation methods used in the present invention will be described below.

(1) Density of Cavity Lamination Number The value obtained by dividing the number of cavities (the number of laminations) in the thickness direction in the cross section in the stretching direction of the film by the thickness (μm) of the film was defined as the density of the number of laminations (cavities / μm). The measurement was performed by using a scanning electron microscope and observing a cross section parallel to the longitudinal stretching direction of the film and perpendicular to the film surface at five different portions of the sample. The microscope was inspected at an appropriate magnification of 300 to 3000 times, and a photograph was taken in which the dispersion state of the cavities in the entire thickness of the film could be confirmed. A straight line was drawn in a vertical direction on the film surface at an arbitrary position on the photographic image, and the number of cavities (the number of layers) intersecting the straight line was counted. Further, the total thickness (μm) of the film was measured along this straight line, and the number of laminated layers of the cavity was divided by the total thickness of the film to determine the density of the number of laminated layers (units /
μm). In addition, the measurement was performed at five places per one photograph, and the average value of a total of 25 places was obtained to obtain the density of the number of hollow layers of the sample.

(2) Number of cavities A value obtained by counting the number of cavities observed in the cross section of the film in the electron micrograph taken in the above (1). This was divided by the area of the observation part and normalized, then multiplied by 2500 to 50 μ
It was converted to the number of cavities contained in m squares. In addition, the measurement was performed using five photographs of five different portions of the sample, and five photographs were taken, and the average value of a total of 25 locations was obtained as the number of cavities of the sample.

(3) Spectral reflectance A value obtained by mounting an integrating sphere on a spectrophotometer (Hitachi Spectrophotometer U-3500 type) and measuring the spectral reflectance (%) with respect to light having a wavelength of 450 nm. Alumina white plate as standard reflection plate (manufactured by Hitachi Instrument Service Co., Ltd./product number 210-0740)
The spectral reflectance of the sample was measured with the reflectance being 100%. The measurement was performed by stacking 1 to 5 sample films, and the relationship between the thickness and the reflectance was determined. From this, the reflectivity at a thickness of 188 μm was determined by interpolation, and was taken as the reflectivity of the sample. The higher this value, the higher the reflectivity to visible light was evaluated.

(4) Melt Viscosity (η ₀ , η _S ) The melt viscosity at 285 ° C. was measured using a flow tester (CFT-500, manufactured by Shimadzu Corporation). The measurement of the melt viscosity at a shear rate of 100 sec ^-1
Since it is difficult to carry out the measurement at ⁰ sec ^-1 , the melt viscosity is measured at an arbitrary shear rate of less than 100 sec ^{-1 and at} an arbitrary shear rate higher than the speed by using an appropriate load. The melt viscosity was plotted on the graph, and the shear rate was plotted on the horizontal axis, and plotted on a graph. The above two points are connected by a straight line, and the melt viscosity at a shear rate of 100 sec ^-1 (η: pois
e) was determined.

(5) Film Thickness and Apparent Density Four pieces of the film were cut into 5.00 cm squares to obtain samples. The four sheets were stacked, and the thickness was measured at four points using a micrometer at four significant figures, and the average value of the stacked thickness was determined. The average value was divided by 4 to round it to three significant figures, and the average film thickness per sheet (t: μm) was obtained. In addition, the weight (w:
g) was measured using an automatic precision balance with four significant figures, and the apparent density was determined by the following equation. The apparent density was rounded to three significant figures. Apparent density (g / cm ³ ) = w × 10 ⁴ /(5.00×5.00×t×4)

(6) Intrinsic viscosity of polyester resin 60% by weight of phenol and 40% by weight of tetrachloroethane
Was dissolved in a mixed solvent of (1) and the measurement was carried out at 30 ° C.

Next, examples of the present invention will be described in order, and the main conditions relating to the film production method are shown in Table 1, and the evaluation results of the obtained film are shown in Table 2.

(Example 1) (Preparation of master pellet) Melt viscosity (η ₀ ) is 130
0 weight polymethylpentene resin (manufactured by Mitsui Chemicals, Inc./DX820) 60% by weight, melt viscosity (η _s ) 390
20% by weight of 0 poise polystyrene resin (manufactured by Nippon Polystyrene / G797N) and a melt viscosity of 2000p
A mixture of pellets of 20% by weight of polypropylene polypropylene resin (manufactured by Grand Polymer Co., Ltd./J104WC) was supplied to a vented twin-screw extruder whose temperature was controlled at 285 ° C., and was pre-mixed. This molten resin was continuously supplied to a vent-type single-screw kneader, kneaded and extruded, and the obtained strand was cooled and cut to prepare a cavity developing agent master pellet (MA).

(Preparation of film raw material) 91.0% by weight of a polyethylene terephthalate resin having an intrinsic viscosity of 0.62 dL / g (hereinafter referred to as dry polyethylene terephthalate resin) subjected to vacuum drying at 140 ° C. for 8 hours and 90 ° C. for 4 hours The above-mentioned master pellet (MA) 9.0 which was vacuum-dried
By weight, pellets were mixed to obtain a film raw material (I).

(Preparation of Unstretched Film) The above-mentioned film raw material (I) was supplied to a twin-screw extruder controlled at 285 ° C. and kneaded. It was extruded from a T-die onto a cooling roll adjusted to 25 ° C. to produce an unstretched film having a thickness of 480 μm. At this time, the time during which the molten resin stays in the melt line is about 3 minutes, and the shear rate received from the T-die is about 100 seconds.
ec ^-1 .

(Preparation of Biaxially Stretched Film) The obtained unstretched film was uniformly heated to 65 ° C. using a heating roll, and longitudinally stretched 3.4 times between two pairs of nip rolls having different peripheral speeds. At this time, as an auxiliary heating device for the film,
An infrared heater (a rated output of 20 W / cm) having a gold reflective film in the middle of the nip roll was placed at a position 2 cm from the film surface facing both surfaces of the film and heated. The thus obtained uniaxially stretched film is guided to a tenter, heated to 150 ° C., stretched 3.7 times in a transverse direction, fixed in width, subjected to a heat treatment at 220 ° C. for 5 seconds, and further heated at 200 ° C. in the width direction. To give a void-containing polyester film having a thickness of about 47 μm.

Example 2 86.0% by weight of a dry polyethylene terephthalate resin and 14.0% by weight of the above-mentioned master pellet (MA), which had been vacuum-dried at 90 ° C. for 4 hours, were mixed as a pellet to obtain a film raw material (I). And This film raw material (I) was supplied to a twin-screw extruder whose temperature was controlled at 285 ° C. and kneaded. It was extruded from a T-die onto a cooling roll adjusted to 25 ° C. to produce an unstretched film having a thickness of 620 μm. Other conditions were the same as in Example 1, and the thickness was about 7
A 4 μm void-containing polyester film was obtained.

Example 3 The film raw material (I) used in Example 1 was fed to a single screw extruder whose temperature was controlled at 285 ° C., and the dried polyethylene terephthalate resin was used alone for a twin screw extruder whose temperature was controlled at 290 ° C. Each was supplied separately. The molten resin discharged from the single-screw extruder is guided through an orifice, and the resin discharged from the twin-screw extruder is guided to a feed block through a static mixer. A layer made of a terephthalate resin (layer A) was laminated in the order of layer A / layer B / layer A. This is 2
It was co-extruded from a T-die onto a cooling roll adjusted to 5 ° C.
The discharge rate of each extruder was adjusted so that the thickness ratio of each layer was 1: 8: 1, and an unstretched film having a thickness of 580 μm was produced. At this time, the time for the molten resin of the raw material I to stay in the melt line was about 12 minutes, and the shear rate received from the T-die was about 150 sec ^-1 . The other conditions were the same as in Example 1 to obtain a void-containing polyester film having a thickness of about 58 μm.

Comparative Example 1 Melt viscosity (η ₀ ) was 2000
Poise polypropylene resin (J104WC manufactured by Grand Polymer Co., Ltd.) was supplied to a vented twin-screw extruder whose temperature was controlled at 285 ° C., and was pre-mixed. The molten resin was continuously supplied to a vent-type single-screw kneader, kneaded and extruded, and the obtained strand was cooled and cut to prepare a cavity developing agent master pellet (MC). A mixture of 50% by weight of dry polyethylene terephthalate resin and 50% by weight of anatase type titanium dioxide particles having an average particle diameter of 0.3 μm was pre-kneaded in the same manner as the master pellet (MC).
It was extruded and cut to prepare a white pigment master pellet (MB). Next, 87% by weight of the dried polyethylene terephthalate resin and 4% by weight of the above-mentioned master pellet (MB) subjected to vacuum drying at 140 ° C. for 8 hours, and the above-mentioned master pellet (MC) 9 subjected to vacuum drying at 90 ° C. for 4 hours 9 By weight, pellets were mixed to obtain a film raw material (I). Further, 70% by weight of a dry polyethylene terephthalate resin and 30% by weight of a master pellet (MB) were pellet-mixed to obtain a film raw material (II).

The film raw material (I) was separately supplied to a single screw extruder whose temperature was controlled at 285 ° C., and the film raw material (II) was separately supplied to a twin screw extruder whose temperature was controlled at 290 ° C. Each of the melted raw materials was led to a feed book, and a layer (layer B) composed of the film raw material (I) and a layer (layer A) composed of the film raw material (II) were laminated in the order of layer A / layer B / layer A. The other conditions were the same as in Example 3, and an unstretched film having a thickness of 600 μm was prepared and stretched to obtain a void-containing polyester film having a thickness of about 53 μm.

(Comparative Example 2) In Comparative Example 1, the white pigment particles used were changed to calcium carbonate particles having an average particle diameter of 0.7 μm, and the white pigment-containing master pellets (MD) were used in the same manner as the white pigment-containing master pellets (MB). ) Was adjusted. Other conditions were the same as in Comparative Example 1, and the thickness was 650.
A non-stretched film having a thickness of about 55 μm is prepared and stretched.
m was obtained.

The following can be considered from the above Examples and Comparative Examples. In Examples 1 to 3, the density of the cavity lamination number of the film was 0.2 due to the effect of the optimized cavity developing agent resin, the effect of the twin screw extruder, and the effect of the static mixer.
A void-containing polyester film having a high reflectivity and satisfying the requirements of the present invention of 6 to 0.37 / μm was obtained. On the other hand, in Comparative Examples 1 and 2, the cavity-containing polyester film which does not satisfy the requirement of the cavity lamination density defined in the present invention and has sufficient reflectivity to visible light, and is suitable for use as a material of various reflectors is Could not be obtained.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

According to the void-containing polyester film of the present invention, the polyester film is lightweight, has high strength, and is excellent in workability. It has excellent performance and is suitable for use as various reflection plates.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) C08L 67/02 C08L 67/02 (72) Inventor Koji Yamada 2-1-1 Katata, Otsu-shi, Shiga Pref. F-term in Reference Laboratory (reference) 4F074 AA16 AA26 AA32 AA66 AA66A CA01 CA03 CC02Y CC22X DA02 DA24 DA59 4J002 BB00Y BB17Y BC02X CF041 FD096 GP00

Claims (9)

[Claims] 1. A method mainly comprising a polyester resin,
A polyester film having voids therein, wherein the number density of laminated voids is 0.20 / μm or more. 2. The void-containing polyester film according to claim 1, wherein the spectral reflectance to an electromagnetic wave having a wavelength of 450 nm is 98% or more. 3. The void-containing polyester film according to claim 1, wherein the polyester resin contains a thermoplastic resin incompatible with the polyester resin. 4. The void-containing polyester film according to claim 3, wherein the thermoplastic resin incompatible with the polyester resin is a polystyrene resin. 5. The void-containing polyester film according to claim 3, wherein the thermoplastic resin incompatible with the polyester resin is a polystyrene resin or a polyolefin resin. 6. The void-containing polyester film according to claim 5, wherein at least a part of the polyolefin resin is a polymethylpentene resin. 7. The void-containing polyester film according to claim 5, wherein the melt viscosity η _S of the polystyrene resin and the melt viscosity η ₀ of the polyolefin resin satisfy the following relational expression (1). . η ₀ / η _S ≦ 0.80 (1) 8. An apparent density of 0.70 to 1.25 g / c
Claim 1, 2, 3, 4, characterized in that the m ^3,
8. The void-containing polyester film according to 5, 6, or 7. 9. The void-containing polyester film according to claim 1, wherein the content of the white pigment particles is 5% or less.