Description

[Title of the Invention]

Method for producing white film

Technical field

[0001]
The present invention relates to a white film production method. More specifically, it relates to a production method for a white film that is suitable as material for reflection members (reflecting plates and reflectors) of surface light sources and that is excellent in reflection characteristics and easy to produce.

Background art

[0002]
In recent years, liquid crystal displays have come in wide use as displaying components for personal computers, TVs, and mobile phones. Since such a liquid crystal display panel itself does not emit light, a surface light source, or a backlight, is provided to irradiate it from behind; To meet the requirement of irradiating the entire screen uniformly rather than simply illuminating it, the backlight is also in the form of a surface light source that has an edge type or a direct type structure. An edge type backlight is commonly used in the thin-type liquid crystal display in notebook computers and other apparatuses that are required to be thin and small. An edge type backlight illuminates the screen from its edges.

[0003]
An edge type backlight uses an array of cold cathode fluorescent lamp as an illumination light source to emit light from the edges of the light guide plate, which serves for unifonn propagation and diffusion of light to achieve uniform illumination of the entire liquid crystal display. For an increased illumination efficiency of this method, a reflector is provided around the array of cold cathode fluorescent lamp. In addition, a reflecting plate is provided under the light guide plates so that the light diffused by the light guide plate is reflected efficiently toward the liquid crystal screen. This serves to reduce the loss of light from the cold cathode fluorescent lamp, allowing the liquid crystal screen to be lit more strongly.

[0004]
On the other hand, a large screen such as built in a liquid crystal TV uses a direct type backlight. In a backlight of this type, cold cathode fluorescent lamp are provided below and parallel to the liquid crystal screen, that is, the cold cathode fluorescent lamp are provided above and parallel to the reflecting plate. The reflecting plate may be planar or in the form of a semicircle that is concave along the cold cathode fluorescent lamp.

[0005]
The reflectors, reflecting plates, and the like to be used in surface light sources for liquid crystal screens (hereinafter referred generically to surface light source reflection members) are required to be both excellent in reflection characteristics and small in thickness. For use in surface light source reflection members, films that contain fine voids with a refractive index largely different from that of the matrix resin, such as those produced by forming fine voids in a film so that light is reflected at the gas/solid interface, have been disclosed (Patent document 1).

[0006]
A method for fine void formation within a film has disclosed that comprises the step of introducing inert gas in a polyester resin sheet under compression and the step of heating the inert-gas-containing polyester resin sheet under atmospheric pressure to foam the film (Patent document 2).

[0007]
There is another disclosed method for white film formation in which immiscible polymer resin particles are introduced in a film followed by stretching it to produce voids around the resin particles (Patent document 3).

[0008]
In other studies, methods to produce a monolayer film containing both immiscible polymer resin particles and a light resistant component have been disclosed (Patent documents 4 to 6).

[0009]
For formation of a white film containing voids, a production method that uses an infrared heater to heat the film while stretching it has been disclosed (Patent document 7).

Prior art documents
Patent documents
[0010]
Patent document 1: JP 2002-40214 A Patent document 2: JP 2006-249158 A Patent document 3: JP 2009-98660 A Patent document 4: JP H08-48792 A Patent document 5: JP 4306294 B Patent document 6: JP2009-516049 A Patent document 7: JP 2006-241471 A

Summary of the invention

Problems to be solved by the invention

[0011]
The techniques disclosed in Patent documents 2 to 7, however, have problems as described below.
[0012]
The technique disclosed in Patent document 2 has difficulty in producing a thin film, and it is not suitable as a method to produce a film to be used in surface light source reflection member.

[0013]
To ensure increased light resistance and stable productivity, the technique disclosed in Patent document 3 requires formation of a layered structure consisting of an inner layer containing an immiscible polymer component at a high concentration for viod formation and a considerably void-free outer layer containing a titanium oxide component that makes the layer light resistant. The titanium oxide on the outer layer, however, slightly absorbs light to impede the improvement in reflectance. In addition. large-scale equipment is necessary to produce a layered structure, leading to a disadvantage in cost.

[0014]
In the techniques disclosed in Patent documents 4 to 6, undesirable voids are formed on the film surface while the film is stretched to cause viod formation in it, causing immiscible polymer resin particles to come off to contaminate the production line. Furthermore, although formation of a large number of voids is effective to improve the reflectance, it reduces the specific gravity and makes the film damageable, leading to a poor film production stability.

[0015]
In the technique disclosed in Patent document 7, the infrared heater used has a low output and is only able to assist the rolls in heating the film and voids are formed on the film surface, causing immiscible polymer resin particles to come off to contaminate the production line.

[0016]
The present invention solves these problems to provide a white film production method that is free from contamination of the production line and able to produce films stably without suffering from significant damage.

Means of solving the problems

[0017]
The present invention provides a production method for a white film containing voids in its interior and having a specific gravity of 0.55 or more and 1.30 or less, comprising the step of causing a film having a layer containing a main resin component and another component that is immiscible with said resin component to be stretched by rolls with different circumferential speeds, 3.0 times or more and 4.5 times or less in the film's length direction, while heating at least one of its surfaces by applying heat of 8.5 W/cm or more and 40 W/cm or less per surface, and the subsequent step of stretching it 3 times or more and 5 times or less in the film's width direction.

Effect of the invention

[0018]
The white film production method according to the present invention can produce a white film stably without causing contamination of the production line or breakage of the film.

Brief description of the drawings

[0019]
[Fig. 1] Fig. 1 is a schematic view seen from the film's width direction to illustrate the method to measure the heat quantity Q.

Description of embodiments

[0020]
(1) White film (1.1) Constitution of white film The white film produced according to the present invention is in the forni of a white film containing voids inside and having a specific gravity of 0.55 or more and 1.30 or less.

[0021]
The white film is required to contain voids in its interior. It is preferable that the white film is in the form of a monolayer film containing voids in its interior or has a layer containing voids in its interior at least as one of the outermost layer. The existence of a layer containing voids in its interior as an outermost layer serves to allow the white film to have good reflection characteristics. Such a layered white film may consist of a combination of a viod-containing layer and a void-free layer or a combination of two or more layers with different void contents.

[0022]
For the present invention, the viods contained in the film may be in the form of independent viods or groups of two or more voids in contact with each other. There are no specific limitations on the shape of the voids, but it is preferable that the cross sections of the voids have a circular shape or an ellipsoidal shape stretched in the film's plane direction so that many interfaces are formed in the film's thickness direction.

[0023]
As a preferable method for air viod formation, a mixture of a resin to constitute the main rein component (a) of the layer containing voids in its interior and a component (b) immiscible with this resin component (a) is melt-extruded and then stretched at least in one direction to produce voids in the interior. Here, the main resin component (a) refers to the component that accounts for more than 50% by mass of the entire layer containing voids in its interior. This method can form fine flattened voids and serves for efficient production of a white film with high reflection performance.

[0024]
In this method, flattened voids are fonned as a result of separation between the main resin component (a) and the immiscible component (b) that takes place during the stretching step. Accordingly, biaxial stretching is preferable to uniaxial stretching because the occupied volume of the voids and the number of their interfaces per unit film thickness can be increased to ensure improved reflection performance.
[0025]
The existence or absence of voids within a film can be checked by the following method. Specifically, a section in the direction parallel to the film's TD direction (film's width direction) is cut out with a microtome. Platinum-palladium is deposited on the section and its surface is observed by scanning electron microscope (hereinafter referred to as SEM) at an appropriate magnification (500x to 10,000x). The existence of voids observation can be confinned by inspecting the photograph obtained.

[0026]
It is preferable that the white film has a thickness of 30 pm or more and 500 pm or less. The lower limit of the thickness is more preferably 50 pm or more. The upper limit of the thickness is more preferably 300 pm or less. If the thickness is less than 30 pm, it will sometimes be impossible to achieve adequate reflection characteristics. If the thickness is more than 500 pm, the film is too thick for use in a liquid crystal display that has to be thin. For a white film with a layered structure, this refers to its entire thickness.

[0027]
The specific gravity of the white film is 0.55 or more and 1.30 or less. It is more preferably 0.55 or more and 0.99 or less, and still more preferably 0.55 or more and 0.90 or less. The specific gravity referred to herein is that of the entire white film. If the specific gravity is less than 0.55, it is not preferable because the film is low in strength and can be easily broken, leading to a poor productivity. It is not preferable also because the film can suffer from creases during fabrication of a liquid crystal display. If the specific gravity is more than 1.30, it is not preferable because the reflection characteristics led by voids gets insufficient.

[0028]
The methods to produce a white film with a specific gravity of 0.55 or more and 1.30 or less include: 1) increasing the content of the immiscible component (b), 2) using of resin particles as the immiscible component (b), 3) using a immiscible component (b) with a small volume average particle diameter Dv, and 4) stretching the film to a high draw ratio.

[0029]
The density of craters on the white film surface is preferably one or less per 100 | According to a study of the inventors, contamination of the production line is attributable to particles (immiscible component) coming off from craters and adhering to the production equipment. Thus, contamination of the production line can be prevented if the density of craters on the white film surface is maintained at one or less per 100 pm^. The lower limit of the density is 0 or more per 100 pm^. To achieve the intended effect, the density of craters is preferably one or less per 100 pm^ on both surfaces of the white film. A crater as referred to for the present invention is a depressed portion in the film surface having a maximum width of 1 pm or more as observed in a 2,500x SEM photograph.

[0030]
The methods to achieve a crater density of one or less per 100 pm^ include, as described below, applying a specific quantity of heat to the film (that is, heating it) with a high-power infrared heater while stretching it.

[0031]
(1.2) main resin component (a)
The main resin component (a) constitutes the matrix resin component of the layer that contains voids. The main resin component (a) is preferably a polyester resin (a1). A polyester resin is a polymer produced through condensation polymerization of a diol component and a dicarboxylic acid component. Major examples of said dicarboxylic acid component include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalene dicarboxylic acid, adipic acid, and sebacic acid. Major examples of said diol component include ethylene glycol, trimethylene glycol, tetramethylene glycol, and cyclohexane dimethanol. Specific examples of said polyester resin include polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate (polyethylene naphthalate), polypropylene terephthalate, and polybutylene terephthalate.

[0032]
Needless to say, these polyesters may be homopolyesters or copolyesters. Examples of copolymerization components include, for instance, diol components such as diethylene glycol, neopentyl glycol, and polyalkyiene glycol; and dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, 2,6-naphthaiene dicarboxylic acid, and 5-sodium sulfoisophthalic acid.

[0033]
The use of one of the above resins as polyester resin (a1) serves to produce a film with high mechanical strength while persistently maintaining a colorless state. It is more preferably polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) from the viewpoint of their low price and high heat resistance.

[0034]
(1.3) immiscible component (b)
There are no specific limitations on the immiscible component (b) as long as it is immiscible with the main resin component (a), that is, the matrix resin component, and it may be a thermoplastic resin (b1) or an inorganic particle material (b2) that are immiscible with the matrix resin. These components may be used singly or as a combination of two or more thereof. The combined use of a thermoplastic resin (b1) and an inorganic particle material (b2) as the immiscible component (b) is one of the preferable embodiments.

[0035]
It is preferable that the white film has a layer comprising a polyester resin (a1) and a component (b) immiscible with the polyester resin (a1) and that such a layer constitutes at least one of the outermost layers of the white film. Such a constitution serves for efficient formation of voids in the film, making it possible to produce a white film with good reflection characteristics. It is more preferable that the white film is constituted only of a layer comprising a polyester resin (a1) and an immiscible component (b). The use of a void-free layer at the surface of a white film is not preferable because it can allow the polymer to be yellowed as a result of degradation caused by ultraviolet ray.

[0036]
(1.3.1) thermoplastic resin (b1) If a thermoplastic resin (b1) is used as the immiscible component (b), the resin may be crystalline or amorphous and either is preferable. Specifically, preferable examples include linear, branched or cyclic polyolefin resins such as polyethylene, polypropylene, polybutene, polymethylpentene, and cyclopentadiene; acrylic resins such as poly(meth) acrylate; and others such as polystyrene and fluorine resins. These immiscible polymer resins may be either homopolymers or copolymers, and two or more immiscible polymer resins may be used in combination. Of these, polyolefin is preferable because it is high in transparency and heat resistance. Specifically, preferable crystalline resins include polypropylene and polymethylpentene while amorphous ones include cycloolefin copolymers.

[0037]
If a polyester resin (a1) is used as the main resin component (a) while using a thermoplastic resin (b1) as the component immiscible with the polyester resin (a1), the crystalline resin is more preferably a polymethylpentene, as a specific example. from the viewpoint of transparency and heat resistance. Said polymethylpentene preferably contains units derived from 4-methylpentene-1 in the backbone of its molecule up to a content of 80 mol% or more, more preferably 85 mol% or more, and particularly preferably 90 mol% or more. Other usable derived units include ethylene unit, propylene unit, butene-1 unit, 3-methylbutene-1, and hydrocarbons with a carbon number of 6 to 12 except 4-methyl pentene-1. The polymethylpentene to be used may be a homopolymer or a copolymer. Two or more polymethylpentene of different compositions and melt viscosities may be used together, and they may be used in combination with other olefin resins or other types of resins.

[0038]
If an amorphous resin is to be used as the thermoplastic resin (b1), the use of a cyclic olefin copolymer resin is particularly preferable. A cyclic olefin copolymer is a copolymer consisting of at least one cyclic olefin selected from the group of cycloalkene, bicycloalkene, tricycloalkene, and tetracycloalkene, and a linear olefin such as ethylene and propylene.

[0039]
Major cyclic olefins used for said cyclic olefin copolymer resin include bicyclo[2,2,1]hept-2-ene, 6-methyl-bicyclo[2,2,1]hept-2-ene, 5,6-dimethyl-bicyclo[2,2,1]hept-2-ene, 1-methyl-bicyclo[2,2,1]hept-2-ene, 6-ethyl-bicyclo[2,2,1 ]hept-2-ene, 6-n-butyl-bicyclo[2,2,1 ]hept-2-ene, 6-i-butyl bicycio[2,2,1]hept-2-ene, 7-methyl-bicyclo[2,2,1]hept-2-ene, tricyclo[4,3,0,1 ^ ^]-3-decene, 2-methyl-tricyclo[4,3,0,1 ^ ^]-3-decene, 5-methyl-tricyclo[4,3,0,1^^]-3-decene, tricyclo[4,4,0,1^^]-3-decene, and 10-methyl-tricyclo[4,4,0,1 ^ ^]-3-decene.

[0040]
Major linear olefins for said cyclic olefin copolymer resin include ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-octadecene.

[0041]
Of those resins cited as the thermoplastic resin (b1), an amorphous cyclic olefin 10 copolymer resin is particularly preferable. A cyclic olefin copolymer resin can be further dispersed as a result of interaction with the alicyclic diol and the alicyclic dicarboxylic acid contained the matrix described later, leading to further improved reflection characteristics.

[0042]
The thermoplastic resin (b1) preferably has a glass transition temperature Tg of 170°C or more. It is more preferably 180°C or more. If it is 170°C or more, the resin can be dispersed more finely in the matrix resin during kneading to serve for the formation of voids during the stretching step and for more efficient prevention of loss of voids during the heat treatment step. Its upper limit is preferably 250°C. If it is more than 250°C, a higher extrusion temperature will be required for film production, possibly leading to inferior processability.

[0043]
In particular, if a cyclic olefin copolymer resin is used as the thermoplastic resin (b1) and its glass transition temperature Tg is less than 170°C, the cyclic olefin copolymer resin, which acts as nucleating agent, can suffer defonnation when the film is heat-treated to improve dimensional stability. As a result, voids formed on it acting as nucleus may be broken partly or entirely, possibly leading to deterioration in reflection characteristics. If the heat treatment temperature is lowered in an attempt to maintain reflection characteristics, the film can suffer from deterioration in dimensional stability.

[0044]
In cases where a cyclic olefin copolymer resin is used as the thermoplastic resin (b1), its glass transition temperature Tg can be controlled in the range of 170°C or more and 250°C or less by, for instance, increasing the content of the cyclic olefin component in the cyclic olefin copolymer while decreasing the content of the linear olefin component such as ethylene. Specifically, it is preferable that that the cyclic olefin component accounts for 60 mol% or more of the cyclic olefin copolymer while the linear olefin component such as ethylene accounts for less than 40 mol%. It is more preferable that the cyclic olefin component accounts for 70 mol% or more while the linear olefin component such as ethylene accounts for less than 30 mol%, and still11 more preferably, the cyclic olefin component accounts for 80 mol% or more while the linear olefin component such as ethylene accounts for less than 20 mol%. It is particularly preferable that the cyclic olefin component accounts for 90 mol% or more while the linear olefin component such as ethylene accounts for less than 10 mol%. By maintaining the contents in these ranges, the glass transition temperature Tg of a cyclic olefin copolymer can be increased to 170°C.

[0045]
In cases where a cyclic olefin copolymer resin is used as the thermoplastic resin (b1), there are no specific limitations on the linear olefin component, but it is preferably an ethylene component from the viewpoint of its reactivity.

[0046]
There are no specific limitations either on the cyclic olefin component, but is preferably bicyclo[2,2,1]hept-2-ene (norbornene) or its derivative from the viewpoint of its productivity, transparency, and possibility for increasing its Tg.

[0047]
Thus, it is preferable that the white film has a layer comprising a polyester resin (a1) and an immiscible component (b) and that the immiscible component (b) is a thermoplastic resin (b1) with a glass transition temperature of 170°C or more and 250°C or less. The thermoplastic resin (b1) is more preferably an amorphous resin. The thennoplastic resin (b1) is particularly preferably an amorphous cyclic olefin copolymer resin.

[0048]
The content of the thermoplastic resin (b1) is preferably 3 mass% or more and 25 mass% or less of the entire layer that contains voids. The lower limit of the content is more preferably 5 mass% or more. The upper limit of the content is more preferably 10 mass% or less. If the content of the thermoplastic resin (b1) is less than 3 mass%, an adequate number of voids will not be formed within the film, possibly leading to inferior whiteness and light reflection characteristics. If the content of the thermoplasfic resin (b1) is more than 25 mass%, the film can deteriorate in strength and suffer from breakage during stretching. If the content is maintained in the range of 12 3 mass% or more and 25 mass% or less, the film can have adequate whiteness, reflection characteristics and lightweight.

[0049]
(1.3.2) inorganic particle material (b2) If an inorganic particle material (b2) is to be used as the immiscible component (b), it may be glass, silica, barium sulfate, titanium oxide, magnesium sulfate, magnesium carbonate, calcium carbonate, or talc.

[0050]
In cases where a polyester resin (a1) is used as the main resin component (a), it is preferable to use a particle material (b2) which is at least one or more selected from the group of titanium oxide, calcium carbonate and barium sulfate, among other inorganic particle materials listed above, from the viewpoint of overall effect for air viod formation, whiteness, and optical density. Titanium oxide is particularly preferable.

[0051]
The content of the inorganic particle material (b2) is preferably 5 mass% or more and 60 mass% or less of the entire layer that contains voids. The lower limit of the content is more preferably 10 mass% or more. The upper limit of the content is more preferably 20 mass% or less. If the content of the inorganic particle material (b2) is less than 5 mass%, an adequate number of voids will not be formed within the film, possibly leading to inferior whiteness and light reflection characteristics. If the content of the inorganic particle material (b2) is more than 60 mass%, the film can deteriorate in strength and suffer from breakage during stretching. If the content is maintained in the range of 5 mass% or more and 60 mass% or less, the film can have adequate whiteness, reflection characteristics and lightweight.

[0052]
The combined use of a themnoplastic resin (b1) and an inorganic particle material (b2) as the immiscible component (b) is one of the preferable embodiments. In particular, it is preferable that the white film has a layer comprising a polyester resin (a1) and an immiscible component (b) and that a thennoplastic resin (b1) with a glass transition temperature of 170°C or more and 250°C or less and an inorganic particle material 13 (b2) which is at least one or more selected from the group of titanium oxide, calcium carbonate, and barium sulfate are used as the immiscible component (b). Furthermore, it is preferable that the thermoplastic resin (b1) accounts for 3 mass% or more and 25 mass% or less of the entire layer that contains voids while the inorganic particle material (b2) accounts for 5 mass% or more 60 mass% or less of the entire layer that contains voids.

[0053]
(1.4) other additives
For use as the matrix resin component of the layer that contains voids, the mixture of a polyester resin (a1) may be mixed with a copolymerized polyester resin (c) containing a copolymerization component. There are no specific limitations on the quantity of the copolymerization component, its content is preferably 1 mol% or more and 70 mol% or less, more preferably 10 mol% or more and 40 mol% or less, for both the dicarboxylic acid component and the diol component from the viewpoint of transparency and moldability as well as from the viewpoint of making the copolymer amorphous, which will be addressed later.

[0054]
As the copolymerization resin (c), furthemnore, it is preferable to use a polyester material made amorphous by copolymerization. Its examples include copolymerized polyester resin in which alicyclic glycol accounts for a major part of the diol component, and a copolymerized polyester resin in which alicyclic dicarboxylic acid constitutes the acid component. In particular, an amorphous polyester produced through copolymerization with cyclohexane dimethanol, which is a alicyclic glycol, as the diol component is preferred from the viewpoint of transparency, moldability, and the effect of finely dispersing the immiscible resin, which will be addressed later. In this case, the cyclohexane dimethanol used as the diol component of the copolymerized polyester resin (c) should preferably account for 30 mol% or more from the viewpoint of maintaining an amorphous state.

[0055]
In the case where a cyclic olefin copolymer resin, i.e. a thermoplastic resin (b1), is 14 used as the immiscible component (b), the introduction of a copolymerized polyester resin (c) into the matrix resin of the layer that contains voids allows the cyclic olefin copolymer resin to be finely dispersed in the matrix resin as a result of interaction between the cyclic aliphatic hydrocarbon portion of the copolymerized polyester resin (c) and the cyclic olefin portion of the cyclic olefin copolymer resin, leading to good reflection characteristics, high whiteness, and lightweight. Furthemnore, addition of a copolymerized polyester resin (c) serves to improve the stretchability and film-forming properties.

[0056]
The content of the copolymerized polyester (c) is preferably 1 mass% or more and less than 50 mass% per 100 mass% of the total quantity of the resins constituting the matrix of the layer that contains voids. The lower limit of the content is more preferably 1.5 mass% or more. The upper limit of the content is more preferably 40 mass% or less, and particularly preferably 35 mass% or less. If the content of the copolymerized polyester (c) is less than 1 mass%, it will sometimes be difficult to allow the thermoplastic resin (b1) to be finely dispersed in the matrix. If the content of the copolymerized polyester (c) is 50 mass% or more, the heat resistance will decrease, and heat treatment of the film performed to improve its dimensional stability can cause the matrix resin to soften, possibly leading to breakage or loss of voids and deterioration in reflection characteristics. If the heat treatment temperature is lowered in an attempt to maintain reflection characteristics, the film can suffer from deterioration in dimensional stability. Controlling the content of the copolymerized polyester (c) in the range of 1 mass% or more and less than 50 mass% serves to maintain required film-forming properties and mechanical characteristics while achieving adequate effect on dispersing said immiscible component. As a result, both high reflectance and dimensional stability are maintained at the same time.

[0057]
To achieve finer dispersion of the thermoplastic resin (b1) in the matrix resin of the layer that contains voids, it is preferable that the matrix resin contains a dispersing agent (d) in addition to said polyester resin (a1) and copolymerized polyester resin (c). 15 Inclusion of a dispersing agent (d) serves to further reduce the dispersion diameter of the thermoplastic resin (b1). Accordingly, the flattened voids formed by stretching can be made finer, allowing the film to improve in whiteness, reflection characteristics, and lightweight properties.

[0058]
There are no specific limitations on the type of the dispersing agent (d), and usable examples include olefin-based polymers and copolymers having polar groups such as carboxyl group and epoxy groups or functional groups reactive with polyester; and others such as diethylene glycol, polyalkylene glycol, surface active agent and heat-bonding type resin. Needless to say, these may be used singly or as a combination of two or more thereof. In particular, a polyester-polyalkylene glycol copolymer (d1) composed of a polyester component and a polyalkylene glycol component is preferable.

[0059]
In this case, said polyester component is preferably one composed of an aliphatic diol portion with a carbon number of 2 or more and 6 or less and a terephthalic acid and/or isophthalic acid portion. Preferable examples of said polyalkylene glycol component, on the other hand, include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol.

[0060]
For particularly preferable combinations, that polyethylene terephthalate or polybutylene terephthalate should be used as said polyester component in combination with polyethylene glycol or polytetramethylene glycol used as said polyalkylene glycol component. Among others, the combination of polybutylene terephthalate as said polyester component and polytetramethylene glycol as said polyalkylene glycol component and the combination of polyethylene terephthalate as said polyester component and polyethylene glycol as said polyalkylene glycol component are particularly preferable.

[0061]
The content of the dispersing agent (d) is preferably 0.1 mass% or more and 30 16 mass% or less per 100 mass% of the total resins that constitute the matrix. The lower limit of the content is more preferably 1 mass% or more, and particularly preferably 1.5 mass% or more. The upper limit of the content is more preferably 25 mass% or less, and particularly preferably 20 mass% or less. If the content is less than 0.1 wt%, the effect of forming fine voids can be decreased. If the content is more than 30 mass%, the heat resistance will decrease, and heat treatment of the film performed to improve its dimensional stability can cause the matrix to soften, possibly leading to breakage or loss of voids and deterioration in reflection characteristics. If the heat treatment temperature is lowered in an attempt to maintain reflection characteristics, the film can suffer from deterioration in dimensional stability. In addition, there can arise problems such as a decrease in production stability or an increase in cost. Controlling the content of the dispersing agent (d) in the range of 0.1 mass% or more and 30 mass% or less serves to maintain required film-forming properties and mechanical characteristics while achieving adequate effect on dispersing said immiscible component, consequently allowing both high reflectance and dimensional stability to be maintained at the same time.

[0062]
The white film may contain, as needed, appropriate additives including, for instance, thennal stabilizer, oxidation-resistant stabilizer, ultraviolet absorber, ultraviolet stabilizer, organic lubricant, organic fine particles, filler, nucleating agent, dye, dispersing agent, and coupling agent, as long as they do not impair the effect of the present invention.

[0063]
(2) film characteristics
The total light transmittance of the white film is preferably 1.5% or less. It is more preferably 1.2% or less, and more preferably 1.0% or less. The total light transmittance referred to herein is determined in accordance with JIS-K7361-1 (1997 edition). If the total light transmittance is maintained at 1.5% or less, light leakage from the reverse side can be prevented. As a result, it is possible to produce a white film with high whiteness and reflection characteristics. If used in liquid crystal displaying equipment, in particular, the film will serve effectively to achieve a highly enhanced brightness.

[0064]
The relative reflectance of the white film is preferably 100% or more. It is more preferably 100.5% or more, and still more preferably 101% or more. There are no specific limitations on the upper limit, but practically, it is 120% or less. If the relative reflectance is 100% or more, it will be possible to produce a white film with high whiteness and reflection characteristics. If used in liquid crystal displaying equipment, in particular, the film will serve effectively to achieve a highly enhanced brightness.
[0065]
The total light transmittance and relative reflectance of the white film can be maintained in said ranges by, for instance, 1) controlling the dispersion diameter and density of the resin particles within the film in said ranges or 2) increasing the film thickness.

[0066]
(3) production method The white film production method according to the present invention is illustrated in detail below, but the invention should not be construed as being limited thereto except for the description on stretching methods.

[0067]
A mixture of a polyester resin (a1) and an immiscible component (b) is adequately vacuum-dried as needed and supplied to the heated extruder (main extruder) of an extruder-equipped film production apparatus. Addition of the immiscible component (b) may be carried out by supplying master chips prepared in advance by melt-kneading for uniform blending, or by supplying the material directly to the kneading extruder. The use of master chips prepared in advance by melt-kneading a mixture of a polyester resin (a1) and an immiscible component (b) is more preferable because the dispersion of the immiscible component (b) is promoted more strongly.

[0068]
In the melt-extruding step, it is preferable that the material is filtrated first through a 18 filter with a mesh of 40 pm or less and introduced to the T-die orifice, followed by extrusion molding to produce a molten sheet. This molten sheet is brought electrostatically into contact with a drum cooled to a surface temperature of 10°C or more and 60°C or less so that the sheet is cooled and solidified into an unstretched film.

[0069]
This unstretched film is introduced to a group of rolls heated at temperatures of 40°C or more and 120°C or less so that the film is stretched between two sets of rolls having different circumferential speeds in the film's travelling direction (film's length direction). Thus, the difference in circumferential speed between the rolls is made use of for the stretching. During this stretching step, at least one of the surfaces of the film is heated with a heat quantity Q of 8.5W/cm or more and 40W/cm or less per surface.

The lower limit of the heat quantity Q is preferably 10W/cm or more. The upper limit of the heat quantity Q is preferably 25W/cm or less. The heat quantity Q is defined as the quantity of heat applied to the film's surface per cm in the film's width direction.

[0070]
If the heat quantity Q is less than 8.5W/cm, the temperature of the film surface will not be increased sufficiently and craters will be formed on the surface, leading to contamination of the process equipment with generated powder or the like. If the heat quantity Q is more than 40W/cm, the film can be softened during the longitudinal stretching step to prevent stable film production.

[0071]
The useful heat sources for the heating include infrared ray heater and hot air. The use of an infrared ray heater is preferable from the viewpoint of energy efficiency.

[0072]
There are no specific limitations on the type of the infrared ray heater, and tools such as near-infrared ray heater and carbon heater may be used. The use of a carbon heater is more preferable from the viewpoint of the balance between heating performance and operating life. The rear surface of said infrared ray heater is preferably provided with a gold reflective coat. A light-gathering apparatus may be 19 used additionally. Examples of such a heater include a Twin Tube transparent quartz glass carbon heater supplied by Heraeus K.K.

[0073]
Said infrared ray heater installed so that the length direction of the heater is in parallel with the film's width direction. To ensure that the film is heated uniformly in the film's width direction, it is preferable that the length of the infrared ray heater is larger than the film's width. Heating may be performed by using either only one infrared ray heater or two or more arranged in the film's length direction. Only one heater will work sufficiently in cases where the film production speed is low, but use of a line of two or more heaters is preferable in cases where the film production speed is high. There are no specific limitations on the upper limit of the number, but practically, the upper limit may be 4 from the viewpoint of the gap between the rolls.

[0074]
These infrared ray heaters may be provided either on one side or on both sides. They should be provided at least on the side toward the layer containing voids.

[0075]
The output (heat quantity) S emitted from the infrared ray heaters toward the film is preferably 35W/cm or more and 150W/cm or less per surface. In cases where two or more infrared ray heaters are provided on one side of the film, said output is equal to the output of one infrared ray heater emitted toward the film multiplied by the number of heaters provided on that side. The total output of an infrared ray heater is not entirely emitted toward the film, but it includes loss that does not reach the film surface.

The output emitted toward the film can be calculated by multiplying the rated output (W/cm) of the infrared ray heater by a heat exposure efficiency rate unique to the infrared ray heater. The output of infrared ray heaters emitted toward the film referred to herein is calculated by the following equation.

. S=S'xExN S: the output (W/cm) of the infrared ray heaters emitted toward the film S': the rated output (W/cm) of one infrared ray heater E: the heat exposure efficiency rate of the infrared ray heaters 20 N: the number of heaters provided on one side of the film The lower limit of the output emitted toward the film is more preferably 40W/cm or more and particularly preferably 50W/cm or more. The upper limit of the output emitted toward the film is more preferably 100W/cm or less and particularly preferably 80W/cm or less. If the output of the infrared ray heaters emitted toward the film during the longitudinal stretching step is more than 150W/cm, the film can suffer from softening during the longitudinal stretching step. This can make stable film production impossible. If the output of the infrared ray heaters emitted toward the film during the longitudinal stretching step is less than 35W/cm, it can be impossible to sufficiently increase the temperature of the film surface. As a result, craters can be formed on the surface, leading to contamination of the process equipment with generated powder or the like.

[0076]
The distance from the infrared ray heaters to the film surface is preferably 5 mm or more and 100 mm or less. The lower limit of the distance is more preferably 10 mm or more. The upper limit of the distance is more preferably 50 mm or less, and particularly preferably 20 mm or less. If the distance from the infrared ray heaters to the film surface is more than 100 mm, the output of the infrared ray heaters in the above range can decay largely before reaching the film, failing to heat the film surface up to a required temperature. If the distance from the infrared ray heaters to the film surface is less than 5 mm, the output of the infrared ray heaters in the above range can cause softening across the entire thickness of the film. This can make stable film production impossible. The distance from an infrared ray heater referred to herein is defined as the distance from the central axis of the heating tube of the infrared ray heater to the surface of the film.

[0077]
The time it takes for the film to pass through the heat exposure zone is preferably 0.2 second or more and less than 2 seconds. The lower limit of the passage time is more preferably 0.4 second or more. The upper limit of the passage time is more preferably 1 second or less. The heat application zone of a heater extends over a distance of 40 21 mm centering the heating tube in the film's length direction (20 mm on the upstream side and 20 mm on the downstream side from the position of the heating tube). If there are two or more infrared ray heaters, it is the sum for all heat application zones of the heaters minus the lengths of the overlaps. If the passage time is less than 0.2 second, the film may not be heated sufficiently. If the passage time is 2 seconds or more, the film's internal temperature can become so high that sufficiently large voids cannot form. As a result, the reflectance can become too small.

[0078]
If the film surface is heated during stretching, the tension caused by the stretching at the film surface is decreased, impeding the fomiation of voids. At the same time, the heat conductivity within the film decreases as voids begin to form, and accordingly the interior of the film is heated less efficiently than the surface. As a result, an adequate tension is caused by the stretching in the interior of the film to promote the fomnation of voids. Thus, this leads to the formation of a white film with a smaller number of voids at the surface and a larger number of voids in the interior.

[0079]
Assuming that the main resin component (a) has a glass transition temperature of Tg (°C), the temperature of the unstretched film is preferably (Tg - 20°C) or more and Tg or less. The temperature of the unstretched film is defined as the temperature of the film before it is fed to a set of heated rolls to receive heat of a quantity Q for surface heating. The temperature of the unstretched film can be determined by setting the longitudinal stretching ratio during the film production process to 1.0 and using a radiation thermometer to measure the temperature of the film passing through the stretching zone where the film travels without undergoing surface heating with a heat quantity Q. The lower limit of the temperature of the unstretched film is more preferably (Tg - 15°C) or more. The upper limit of the temperature of the unstretched film is more preferably (Tg - 5°C) or less. If the main resin component (a) is PET, in particular, the temperature of the unstretched film is preferably 60°C or more and 80°C or less. If it is maintained in the range of (Tg - 20°C) or more and Tg or less, large voids are formed within the film, leading to enhanced reflection performance. If it 22 is less than (Tg - 20°C), the film will not be stretched largely enough, possibly leading to breakage of the film. If it is more than Tg, the film will not undergo a sufficiently large tension in the stretching step and separation between the main resin component (a) and the immiscible component (b) will not take place smoothly at their interface, impeding the formation of voids. This may cause the reflecting plate to fail in having a sufficient reflection performance. With respect to the method for control of the temperature of the unstretched film, the temperatures of the heated rolls in the roll group may be adjusted in accordance with the heat transfer coefficient that depends on the traveling speed of the film, type of the roll material and type of the film material.

[0080]
While being heated as described above, the film is stretched 3.0 times or more and 4.5 times or less in the film's length direction and subsequently cooled between rolls maintained at temperatures of 20°C or more and 50°C or less. The stretch ratio in the film's length direction is preferably 3.4 times or more and 4.5 times or less. If the stretch ratio is less than 3.0 times, adequately large voids will not be formed, failing to develop a sufficient reflectance. If the stretch ratio is more than 4.5 times, it is not preferable because the film will be easily broken during the subsequent transverse stretching (stretching in the film's width direction) step, leading to a poor productivity.

[0081]
Then, with its both ends held by clips, the film is introduced in a tenter, and stretched 3 times or more and 5 times or below in the direction perpendicular to the film's length (i.e., film's width direction) in an atmosphere heated at a temperature in the range of QOX or more and 150°C or less. If the stretch ratio is less than 3 times, the voids formed will be small, failing to develop a sufficient reflectance. If the stretch ratio is more than 5 times, it is not preferable because the film will be easily broken, leading to a poor productivity. The reflection performance can be further enhanced by increasing the product of the stretch ratio in the film's length direction and that in the film's width direction.

[0082] To ensure the completion of oriented crystallization in the resulting biaxially stretched 23 film to allow the film to have planarity and dimensional stability, it is further heat-treated in the tenter at a temperature of 150°C or more and 240°C or less for 1 second or more and 30 seconds or less, followed by uniform cooling down to room temperature. Subsequently, appropriate treatment such as corona discharge is carried out as needed to further enhance its contact with other materials, followed by winding it up. In the heat treatment step, the film may be subjected to relaxation treatment, as needed, in the range of 3% or more and 12% or less in the film's width direction or film's length direction. In some backlight lamps, the air temperature inside the lamps can rise up to about 100°C, and accordingly, it is desired that white films have a certain level of dimensional stability at high temperatures. In general, films heat-treated at higher temperatures are higher in dimensional stability at high temperatures, and therefore, it is preferable that heat treatment is performed at a high temperature of 190°C or more.

[0083]
(4) measuring method A. heat quantity Q
The quantity Q of heat that reaches the film surface is determined as described below. The film is placed at the same distance from the heat source as on the film production conditions. A thennocouple is attached to the surfaces of the film, and the average of the temperatures at the two surfaces is taken as the temperature of the film. The film is maintained in a stationary state and exposed to the heat source for heating, followed by measuring the heating rate a (°C/sec). The heat quantity Q is calculated by the following equation. Fig. 1 is a schematic view of the measuring equipment. • Q=axDxMxC Q: quantity of heat reaching the film (for each surface of the film) (W/cm) a: heating rate a (°C/sec). D: length in the film's length direction, of the portion of the film surface exposed to heat (cm) M: weight of the film per cm^ of the film's surface (g/cm^) C: specific heat of the film (J/(g • °C)) 24 In cases where an infrared ray heater is used, the length D (cm) is the same as the length of the heat exposure zone. The heat exposure zone of an infrared ray heater extends over a distance of 40 mm centering the heating tube in the film's length direction (20 mm on the upstream side and 20 mm on the downstream side from the position of the heating tube). If there are two or more infrared ray heaters, it is the sum for all heat exposure zones of the heaters minus the lengths of the overlaps. The specific heat C (J/(g'°C)) can be determined according to JIS K7123 (1987 edition). C=1.25 (J/(g-°C)) for PET films.

[0084]
In cases where an infrared ray heater is used, the quantity Q of heat that reaches the film surface can also be determined by the following equation. In cases where two or more infrared ray heaters are used, the equation can work only when the distance to the film surface is the same for all infrared ray heaters. • Q = S X (0.4 - 0.055 X |n(L)) S = S' X E X N Q: the quantity of heat reaching the film (for each surface of the film) (W/cm) S: the output of the infrared ray heaters emitted toward the film (for each film surface) (W/cm) L: the distance from the infrared ray heaters to the film surface (mm) S': the rated output per infrared ray heater (W/cm) E: the heat exposure efficiency N: the number of heaters provided on one side of the film

[0085]
B. density of craters on the film surface After deposition of platinum and palladium on the film surface, a magnified photograph was taken by using a field emission scanning electron microscope at a magnification of 2,500x. In the magnified photograph, the number of concave craters with a maximum width of 1 pm or more contained in square portions with 10 pm sides was counted. The number was determined for 10 different square portions, and their average was taken as the density of craters. The measurement was carried out for 25 both surfaces of the film, and the larger value was adopted. A JSM-6700F field emission scanning electron microscope supplied by JEOL Ltd. was used.

[0086]
C. relative reflectance
A spectroscopy photometer equipped with a 60-diameter integrating sphere and a 10° inclined spacer was used to measure the light reflectance at 560 nm. The light reflectance was measured for both surfaces of the white film, and the larger value was taken as the reflectance of the white film. A U-3410 spectrophotometer supplied by Hitachi, Ltd., a Model 130-0632 60-diameter integrating sphere (with barium sulfate inner wall) supplied by Hitachi, Ltd., and a Model 210-0740 standard white plate (aluminum oxide) supplied by Hitachi Instruments Service Co., Ltd were used. The relative reflectance was rated as follows. A specimen was assumed to be acceptable if rated as S, A, or B. Specimens rated as S or A are preferable.

• S: The relative reflectance is 101% or more and less than 120%
• A: The relative reflectance is 100% or more and less than 101%
• B: The relative reflectance is 99% or more and less than 100%
• C: The relative reflectance is less than 99%

[0087]
D. specific gravity
A specimen with a size of 5 cm x 5 cm was cut out of a white film, and measurements were made using an electronic gravimeter according to JIS K7112 (1980 edition). Five specimens were prepared from one white film and subjected to measurement separately, and their average was taken as the specific gravity of the white film. A SD-120L electronic gravimeter supplied by Mirage Trading Co., Ltd. was used. The specific gravity was rated as follows. A specimen was assumed to be acceptable if rated as S, A, or B.

• S: The specific gravity is 0.55 or more and 0.9 or less
• A; The specific gravity is more than 0.9 and 1.0 or less
• B: The specific gravity is more than 1.0 and 1.3 or less
• C: The specific gravity is more than 1.3

[0088]
E. film-forming properties

Film-forming properties were evaluated based on the frequency of film breakage during film production. Film-forming properties of rank S, A or B are required for mass production. A larger cost reduction can be achieved if the film is ranked S or A.

• S: Film breakage takes place once or less per week.
• A: Film breakage takes place twice or more and 5 times or less per week.
• B: Film breakage takes place 6 times or 7 times per week.
• C: Film breakage takes place 8 times or more per week.

[0089]
F. evaluation in film production line contamination

In the longitudinal stretching step for film production, film production line contamination was evaluated according to the length of the film that has passed the cooling rolls before contamination was detected over the entire face or in a portion of the face of any of the rolls on which the film was traveling. In cases where contamination occurs, cleaning will be necessary and production has to be suspended during cleaning operations, and accordingly, the film should be ranked S or A to be accepted to achieve a required productivity. Rank S is more preferable.

• S: Contamination not found after passage of 50,000 m
• A: Contamination not found after passage of 10,000 m, but found after passage of 50,000 m
• B: Contamination not found after passage of 2,000 m, but found after passage of 10,000 m
• C: Contamination found after passage of 2,000 m

[0090]
G. glass transition temperature of immiscible component (b) In cases where the immiscible component (b) was obtained independently, 5 mg of the immiscible component (b) was melted and quenched to prepare a sample, and it was then heated from 25°C at a heating rate of 20°C/min in a differential scanning calorimeter to determine the midpoint glass transition temperature according to J IS K7121 (1987 edition), which was taken as the glass transition temperature of the component. A DSC-2 differential scanning calorimeter supplied by PerkinElmer was used.

[0091]
In cases where the immiscible component (b) is not obtained independently, the immiscible component (b) is separated from the white film and its glass transition temperature is determined by using a differential scanning calorimeter. In the case of a white film consisting of a polyester resin (a1), a cyclic olefin copolymer as immiscible thermoplastic resin (b1), and an inorganic particle material (b2), for instance, white film is dissolved in a 1:1 (by volume) mixed solution of methanol and chloroform and filtrated to collect the undissolved material. The undissolved material is dissolved in chloroform, and the resulting undissolved material is removed and dissolved again in a 1:1 (by volume) mixed solution of hexafluoroisopropanol and chloroform. The solution is subjected to centrifugal separation in a centrifugal separator, and the cyclic olefin copolymer component can be obtained by collecting the suspended matter. Then, 5 mg of the cyclic olefin copolymer thus obtained was melted and quenched to prepare a sample, and it is heated from 25°C at a heating rate of 20°C/min in a differential scanning calorimeter to determine the midpoint glass transition temperature according to JIS K7121 (1987 edition), which can be taken as the glass transition temperature of the copolymer. For instance, a DSC-2 differential scanning calorimeter supplied by PerkinElmer may be used.

[0092]
H. temperature of unstretched film Film production conditions are set so as to include a longitudinal stretching ratio of 1.0 and exclude the heating with infrared ray heaters. The temperature of a film passing through the stretching zone for longitudinal stretching was measured 5 times with a radiation thermometer, and the average was taken as the temperature of the unstretched film. In advance, emissivity correction for the film had been carried out. A IT2-80 radiation thermometer supplied by Keyence Corporation was used.
Examples

[0093]
The present invention will be illustrated below in greater detail with reference to Examples, but it should be understood that the invention is not construed as being limited thereto.

[0094]
(Material) • polyester resin (a1-1) Terephthalic acid and ethylene glycol were used as the acid component and the glycol component, respectively, and antimony trioxide (polymerization catalyst) was added so that the antimony atom would account for 300 ppm relative to the resulting polyester pellets, and condensation polymerization reaction was carried out to provide pellets of polyethylene terephthalate (PET) having a limiting viscosity of 0.63 dl/g and containing 40 equivalents/ton of the carboxyl end group. The crystalline heat effusion was not less than 1 cal/g when measured with a differential thermal analyzer, showing that the material was a crystalline polyester resin. The melting point Tm of this resin was measured and found to be 250°C.

[0095]
• cyclic olefin copolymer resin (b1-1) A Topas cyclic olefin resin (supplied by Polyplastics Co., Ltd.) with a glass transition temperature of 178°C and a MVR (260°C / 2.16 kg) of 4.5 ml /10 min was used. The crystalline heat of fusion was less than 1 cal/g when measured with a differential thermal analyzer, showing that the material was an amorphous resin.

[0096]
• cyclic olefin copolymer resin (b1-2) A Topas cyclic olefin resin (supplied by Polyplastics Co.,Ltd.) with a glass transition temperature of 158°C and a MVR (260°C / 2.16 kg) of 4.5 ml /10 min was used. The crystalline heat of fusion was less than 1 cal/g when measured with a differential thermal analyzer, showing that the material was an amorphous resin.

[0097]
29 • olefin resin (b1-3) ATPX olefin resin PMP (polymethylpentene) (supplied by Mitsui Chemicals, Inc.) with a glass transition temperature of 25°C, a melting point of 235°C, and a MFR (260°C / 5 kg) of 8 g /10 min was used.

[0098]
• copolymerized polyester resin (c-1) PET copolymerized with CHDM (cyclohexane dimethanol) was used. This resin is PET consisting of a copolymerizable glycol component copolymerized with 30 mol% cyclohexane dimethanol. The crystalline heat of fusion was less than 1 cal/g when measured with a differential thermal analyzer, showing that the material was an amorphous resin.

[0099]
• copolymerized polyester resin (c-2) PET copolymerized with CHDM (cyclohexane dimethanol) was used. This resin is PET consisting of a copolymerizable glycol component copolymerized with 60 mol% cyclohexane dimethanol. The crystalline heat of fusion was less than 1 cal/g when measured with a differential thermal analyzer, showing that the material was an amorphous resin.

[0100]
• copolymerized polyester resin (c-3) PET copolymerized with isophthalic acid was used. This resin is PET consisting of a copolymerizable dicarboxylic acid component copolymerized with 17.5 mol% isophthalic acid. The crystalline heat of fusion was less than 1 cal/g when measured with a differential thermal analyzer, showing that the material was an amorphous resin.

[0101]
• dispersing agent (d-1) APBT-PAG (polyalkylene glycol) copolymer was used. This resin is a block copolymer of PBT (polybutylene terephthalate) and PAG (mainly polytetramethylene glycol). The crystalline heat of fusion was not less than 1 cal/g when measured with a differential 30 thermal analyzer, showing that the material was a crystalline resin.

[0102]
(Example 1) A mixture of the materials listed in Table 1 was vacuum-dried at a temperature of 180°C for 3 hours and then supplied to an extruder. It was melt-extruded at a temperature of 280°C, filtrated through a 30 pm cut filter, and introduced to a T-die orifice.

[0103]
Then, it was extruded from the T-die orifice into a sheet-like form to provide a molten monolayer sheet. The molten monolayer sheet was then electrostatically brought into close contact with a drum with a surface temperature maintained at 25°C to achieve cooling and solidification to produce an unoriented monolayer film. In this step, the surface of the film that is in contact with the drum is referred as the reverse face, while that exposed to air is referred to as the top face. Subsequently, the unstretched monolayer film was preheated by a group of rolls heated at a temperature of 85°C, and while heat was applied to both faces of the film from infrared ray heaters set under the conditions given in Table 4, the film was stretched 3.6 times in its length direction by rolls with different circumferential speeds and cooled by a group of rolls maintained at a temperature of 25°C to provide a uniaxially stretched film. For the infrared ray heating, two CZB8000/1OOOG carbon heaters supplied by Heraeus K.K. were used to heat each face. Each heater had a length of 1 m and a rated output of 80W/cm. The electric power supplied to the heaters was adjusted so that the infrared ray heating conditions would be as described in Table 4. The figures included in the heating conditions described in the table are for one face of the film.

[0104]
The resulting uniaxially stretched film, with its ends held by clips, was supplied to the preheat zone maintained at a temperature of 95°C in a tenter, and continuously fed to a heating zone with a temperature of 105°C where it was stretched 3.6 times in the direction (width direction) perpendicular to the length direction. In succession to this, heat treatment was carried out at a temperature of 190°C for 20 seconds in the heat 31 treatment zone in the tenter, followed by relaxation treatment at a temperature of 180°C by 6% in the width direction and additional relaxation treatment at a temperature of 140°C by 1% in the width direction. Then, it was cooled slowly and uniformly, and wound up. Thus, a white monolayer film with a thickness of 188 pm was obtained. Observation of a cross section of this white film showed that the film contained many fine voids in its interior. The film did not suffer from significant crater formation on its surfaces, did not cause contamination of the rolls, and had good film-funning properties. Table 7 gives various characteristics of the film.

[0105]
(Examples 2 to 7) Except that the infrared ray heating conditions were as listed in Table 4, the same film production procedure as in Example 1 was carried out to provide a white monolayer film with a thickness of 188 pm. Observation of a cross section of each white film showed that the film contained many fine voids in its interior. The films did not suffer from significant crater formation on their surfaces, did not cause contamination of the rolls, and had good film-forming properties. Table 7 gives various characteristics of the films.

[0106] (Examples 8 to 15, 17, and 22 to 24) Except that the material compositions given in Tables 1 and 2 were adopted, the same film production procedure as in Example 1 was carried out to provide white monolayer films with a thickness of 188 pm. Observation of a cross section of each white film showed that the film contained many fine voids in its interior. The films did not suffer from significant crater formation on their surfaces, did not cause contamination of the rolls, and had good film-forming properties. Tables 7 and 8 give various characteristics of the films.

[0107]
(Example 16) A mixture of the materials listed in Table 2 was vacuum-dried at a temperature of 180°C for 3 hours and then supplied to an extruder (A). Elsewhere, a polyester resin 32 (a1-1) was dried at a temperature of 180°C for 3 hours and supplied to an extruder (B). The materials supplied to the extruder (A) and those supplied to the extruder (B) were melted at a temperature of 280°C and supplied to the respective feedblock. In the feedblock, the layer composed the materials supplied to the extruder (A), (layer A), and the layer composed the materials supplied to the extruder (B), (layer B), were stacked in their thickness direction to form a two-layered (layer A/layer B) structure and introduced to the T-die orifice.

[0108]
Then, it was extruded from the T-die orifice into a sheet-like form to provide a molten two-layered sheet consisting of layer A and layer B. The molten two-layered sheet was then electrostatically brought into close contact with a drum with a surface temperature maintained at 25°C to achieve cooling and solidification to produce an unstretched two-layered film. At this point, the surface of the layer B was in contact with the drum while the surface of the layer A is exposed to air. Thus, the surface of the layer B constitutes the reverse face while the surface of the layer A constitutes the top face.

[0109]
Subsequently, the unstretched two-layered film was preheated by a group of rolls heated at a temperature of SS^C, and while heat was applied to only the surface of the layer A (top face) from infrared ray heaters set under the conditions given in Table 5, the film was stretched 3.6 times in its length direction by rolls with different circumferential speeds and cooled by a group of rolls maintained at a temperature of 25°C to provide a uniaxially stretched film.

[0110]
The resulting uniaxially stretched film, with its ends held by clips, was supplied to the preheat zone maintained at a temperature of 95°C in a tenter, and continuously fed to a heating zone with a temperature of 105°C where it was stretched 3.6 times in the direction (width direction) perpendicular to the length direction of the film. In succession to this, heat treatment was carried out at a temperature of 190°C for 20 seconds in the heat treatment zone in the tenter, followed by relaxation treatment at a 33 temperature of 180°C by 6% in the width direction and additional relaxation treatment at a temperature of 140°C by 1 % in the width direction. Then, it was cooled slowly and uniformly, and wound up. Thus, a white layered film with a thickness of 188 pm was obtained. Observation of a cross section of this white film showed that the layer A contained many fine voids in its interior. The film did not suffer from significant crater formation on both top and reverse faces, did not cause contamination of the rolls, and had good film-forming properties. Table 8 gives various characteristics of the film.

[0111]
(Examples 18 to 21 and 27) Except that the films were stretched to the stretch ratios given in Table 5, the same film production procedure as in Example 1 was carried out to provide white monolayer films with a thickness of 188 |jm. Observation of cross sections of these white films showed that the films contained many fine voids in their interior. The films did not suffer from significant crater formation on their surfaces, did not cause contamination of the rolls, and had good film-forming properties. Table 8 gives various characteristics of the films.

[0112]
(Examples 25 and 26) Except that the preheating roll temperatures and the infrared ray heating conditions were as listed in Table 5, the same film production procedure as in Example 1 was carried out to provide white monolayer films with a thickness of 188 pm. Observation of cross sections of these white films showed that the films contained many fine voids in their interior. The films did not suffer from significant crater formation on their surfaces, did not cause contamination of the rolls, and had good film-forming properties. Table 8 gives various characteristics of the films.

[0113]
(Comparative examples 1, 2, and 4) Except that the infrared ray heating conditions were as listed in Table 6, the same film production procedure as in Example 1 was carried out to provide white monolayer films with a thickness of 188 pm. The film-forming properties were poor as compared 34 with Example 1. Observation of cross sections of these white films showed that the films contained fine voids in their interior. Because the heat quantity Q was less than 8.5 W/cm, the temperature of the film surface did not rise sufficiently, leading to a large number of craters on the film surfaces. Consequently, the rolls suffered from considerable contamination and required frequent cleaning. Table 9 gives various characteristics of the films.

[0114]
(Comparative example 3) Except that the infrared ray heating conditions were as listed in Table 6, the same procedure as in Example 1 was carried out to produce a film. Because the heat quantity Q was more than 40 W/cm, the film softened when stretched in the length direction (longitudinal direction) and the film sagged due to heat, making it impossible to complete the intended film production.

[0115]
(Comparative example 5) Except that the infrared ray heating conditions were as listed in Table 6, the same film production procedure as in Example 16 was carried out to provide a white layered film with a thickness of 188 pm. Breakage took place frequently, and the film-forming properties were poor as compared with Example 16. Observation of a cross section of this white film showed that the layer produced from the material mixture given in Table 3 contained fine voids in its interior. Because the heat quantity Q was less than 8.5 W/cm, the temperature of the film surface did not rise sufficiently, leading to a large number of craters on the film surfaces. Consequently, the rolls suffered from considerable contamination and required frequent cleaning. Table 9 gives various characteristics of the film.

[0116]
(Comparative example 6) Except that the material composition given in Table 3 was adopted, the same film production procedure as in Example 1 was carried out to provide a transparent monolayer film with a thickness of 188 pm. The edge portions sagged due to heat 35 during the longitudinal stretching step and the longitudinally stretched film suffered from a width variation, resulting in the formation of a film with uneven thickness. Contamination of the rolls did not take place because no immiscible components were contained. However, because the film contained no voids, its reflectance was small, making the film unsuitable for use as reflector film. Table 9 gives various characteristics of the film.

[0117]
(Comparative examples 7 and 12) Except that the material compositions given in Table 3 were adopted, the same film production procedure as in Example 1 was carried out to provide white monolayer films with a thickness of 188 pm. Observation of a cross section of the white film prepared in Comparative example 7 showed that the film contained large voids connected to each other. The film production process was unstable and breakage took place frequently.

Observation of a cross section of the white film prepared in Comparative example 12 showed that the film contained fine voids in its interior. Because the heat quantity Q was less than 8.5 W/cm, the temperature of the film surface did not rise sufficiently, leading to a large number of craters on the film surfaces. Consequently, the rolls suffered from considerable contamination and required frequent cleaning. Table 9 gives various characteristics of the film.

[0118]
(Comparative examples 9 to 11 and 13) Except that the films were stretched to the stretch rations given in Table 6, the same film production procedure as in Example 1 was carried out to provide white monolayer films with a thickness of 188 pm. Observation of cross sections of these white films showed that the films contained many fine voids in their interior.

The stretch ratio in the film's length direction was more than 4.5 in Comparative example 9 and the stretch ratio in the film's width direction was more than 5 in Comparative example 11, leading to an increased number of craters on the film surfaces. Consequently, the rolls suffered from considerable contamination and 36 required frequent cleaning.

The stretch ratio in the film's width direction was less than 3 in Comparative example 10 and the stretch ratio in the film's width direction was less than 2.9 in Comparative example 13, leading to a small number of craters on the film surfaces and nonsignificant contamination of the rolls. However, the film was low in reflectance and was not suitable for use as reflector film. In Comparative example 13, breakage easily took place in the transverse stretching step. Table 9 gives various characteristics of the films.

[0119] [Table 1]

[Table 1] -2

[0120] [Table 2]

[0121] [Table 3]


[0122] [Table 4]
production method longitudinal stretching transverse stretching preheating roll temperature (°C) unstretched film temperature (X) heat quantity Q (W/cm) infrared ray heater ratio stretched temperature (°C) ratio

[0123] [Table 5]
production method longitudinal stretching transverse stretching preheating roll temperature (°C) unstretched film temperature (°C) heat quantity Q (W/cm) infrared ray heater ratio stretching temperature (°C) ratio

[0124] [Table 6]
production method longitudinal stretching transverse stretching preheating roll temperature (°C)unstretched film temperature CC) heat quantity Q (W/cm) infrared ray heater ratiostretching temperature (X) ratio output (W/cm) distance (mm) time (sec)

[0125] [Table 7]

Film thickness (Mm) relative Reflectance (%) specific gravity number of craters (top/reverse) (per 100 pm^) roll contamination film-forming properties

[0126] [ Table 8]

[0127] [Table 9]

[0128]
The item "heat quantity Q (W/cm)" in Tables 4 to 6 refers to the quantity of heat reaching either surface of the film.

The item "infrared ray heater/output (W/cm)" in Tables 4 to 6 refers to the output of the infrared ray heaters emitted toward either surface of the film.

The item "infrared ray heater/distance (mm)" in Tables 4 to 6 refers to the distance from the infrared ray heaters to the film surface.

The item "infrared ray heater/time (sec)" in Tables 4 to 6 refers to the time required for the film to pass through the heat exposure zone.
Industrial applicability

[0129]
The white film production method according to the present invention can provide a white film with good film-forming properties, high whiteness, and good reflection characteristics. The use of this white film serves to produce a surface light source with good luminance characteristics.

Explanation of numerals

[0130]
1:film
2: thermocouple
3: heat source (infrared ray heater)

D: length in the film's length direction, of the portion of the film surface exposed to heat (length of heat exposure zone in the film's length direction) L: distance from the heat source (infrared ray heaters) to the film surface (mm)

Claims

[Claim 1]

A production method for a white film containing voids in its interior and having a specific gravity of 0.55 or more and 1.30 or less, comprising the step of causing a film having a layer containing a main resin component and another component that is immiscible with the resin component to be stretched by rolls with different circumferential speeds 3.0 times or more and 4.5 times or less in the film's length direction, while heating at least one of its surfaces by applying heat of 8.5 W/cm or more and 40 W/cm or less per surface, and the subsequent step of stretching it 3 times or more and 5 times or less in the film's width direction.

[Claim 2]
A white film production method as claimed in claim 1 wherein the stretch ratio in the film's length direction is 3.4 or more and 4.5 or less.

[Claim 3]
A white film production method as claimed in either claim 1 or 2 wherein the film is preheated so that the temperature of the film before being stretched in the film's length direction is Tg - 20 (°C) or more and Tg (°C) or less where Tg (°C) denotes the glass transition temperature of the main resin component.

[Claim 4]
A white film production method as claimed in any of claims 1 to 3 wherein the surfaces of the film are heated by heat of 8.5 W/cm or more and 40 W/cm or less per surface by: installing an infrared ray heater at least on one side of the film, adjusting the distance from the film surface to the infrared ray heater to 5 mm or more and 100 mm or less, and adjusting the output of the infrared ray heater emitted toward each surface of the film to 35 W/cm or more and 150 W/cm or less.

[Claim 5]
A white film production method as claimed in any of claims 1 to 4 wherein the white 49 film has a layer comprising a polyester resin and a component immiscible with the polyester resin, with such a layer constituting at least one of the outermost layers of the white film, and the immiscible component is composed of a thermoplastic resin with a glass transition temperature of 170°C or more and 250°C or less and/or an inorganic particle material which is at least one or more selected from the group of titanium oxide, calcium carbonate and barium sulfate.

[Claim 6]
A white film production method as claimed in claim 5 wherein the immiscible component is composed of a thermoplastic resin with a glass transition temperature of 170°C or more and 250°C or less and an inorganic particle material which is at least one or more selected from the group of titanium oxide, calcium carbonate and barium sulfate.

[Claim 7]
A white film production method as claimed in either claim 5 or 6 wherein the inorganic particles account for 5 mass% or more and 60 mass% or less of the layer containing the polyester resin and the immiscible component.

[Claim 8]
A white film production method as claimed in any claims 1 to 7 wherein the density of craters on the white film surface is one or less per 100 pm^.

[Claim 9]
A white film production method as claimed in any claims 1 to 8 wherein the white film has a relative reflectance of 100% or more and 120% or less.