JP5028803B2 - White polyester film for light reflection - Google Patents

Description

  The present invention relates to a light-reflecting white polyester film containing a large number of fine voids therein. More specifically, the present invention relates to a light-reflecting white polyester film that can constitute a base material for a reflector that has excellent crease resistance.

  Polyesters represented by polyethylene terephthalate have excellent physical and chemical properties, and are widely used as fibers, films, and other molded articles. In film applications, white films are used as substrates for cards, labels, display boards, white boards, photographic paper, image paper, etc., and light transmission and light diffusibility are used as boards for electrical decoration boards, drafting, labels, etc. A milky white film is used.

  Conventionally, when illuminating a liquid crystal display, a backlight system in which light is applied from the back of the display or a sidelight system as disclosed in Patent Document 1 has been widely used because of its thinness and uniform illumination. In particular, the sidelight method is a method in which illumination such as a cold-cathode tube is applied from an edge of an acrylic plate or the like, so that an illumination light is dispersed throughout and a screen having uniform brightness is easily obtained. Furthermore, since the lighting is installed not on the back of the screen but on the edge portion, it has a feature that can be made thinner than the backlight method. In order to prevent the illumination light from escaping to the back of the screen, it is necessary to install a reflector on the back of the screen. This reflector is required to be thin and highly reflective. A white film in which fine bubbles are contained and light is scattered by the bubbles is generally used.

  Conventionally, it is well known to add an appropriate amount of white inorganic particles to a polyester in order to obtain a white film or a milky white film. Typical examples of the white inorganic particles include titanium oxide, calcium carbonate, barium sulfate, calcium sulfate, talc, and silicon oxide. For example, a biaxially stretched polyester film (see Patent Document 2) in which barium sulfate fine particles are dispersed and contained, an example in which titanium oxide is added (see Patent Document 3), an example in which calcium carbonate is added (see Patent Document 4), and the like. It can be seen.

  However, a polyester film added with a large amount of white inorganic particles having a large specific gravity, such as titanium oxide, has a high density and is difficult to handle. Therefore, a heterogeneous polymer having a low density has also been proposed. For example, polyolefin polymers such as polyethylene, polypropylene and polymethylpentene can be mentioned. An oriented film obtained by uniaxially or biaxially stretching an unstretched sheet obtained by blending these dissimilar polymers with polyester and melt-extruding has been proposed. Yes.

  In Patent Document 5, it has been proposed to contain 0.1 to 10% by weight of polymethylpentene as a different polymer and a thermoplastic polyester elastomer having a melting point of 160 ° C to 230 ° C. The technique of this patent has been confirmed to be effective for so-called delamination, which causes cohesive failure when a pressure-sensitive adhesive tape such as a cellophane tape is stretched and peeled off on the surface of the film and peels in layers. In addition, since the film produced by this technology is incompatible with polyester, polymethylpentene is dispersed in polyester, and when it is stretched to form a film, fine voids are formed with this as the core. . Thereby, the film becomes a white polyester film excellent in light reflectivity, and is also suitable as a light reflecting film used as a reflective base material of a backlight of a liquid crystal display.

However, since the film has many voids and low specific gravity, there is a problem that wrinkles tend to occur when folded. If wrinkles occur during the processing process when applied to a reflective substrate, the flatness deteriorates and the reflection characteristics around the wrinkled part become non-uniform, resulting in uneven brightness, and as a reflector There was a risk of reducing the required performance. In recent years, with improvements in functionality of large-sized liquid crystal displays, improvement in reflection characteristics is demanded in this application, while improvement in handling in processing is strongly desired.
JP 63-62104 A Japanese Patent Publication No. 60-30930 JP 59-8882 A Japanese Examined Patent Publication No. 43-122013 Japanese Patent Laid-Open No. 04-264141

  An object of the present invention is to provide a light-reflecting white polyester film that, when used as a light-reflecting film, obtains high reflection characteristics and luminance characteristics and is excellent in crease resistance.

  In order to solve the above-mentioned problems, the white polyester film for light reflection of the present invention is achieved by satisfying the following conditions.

That is,
(1) A polyester film stretched in at least a uniaxial direction,
The film contains 5 to 25% by weight of a resin incompatible with polyester,
Furthermore, the white polyester film for light reflections which uses the polyester film which contains 12-30 weight% of thermoplastic polyester elastomers in the range of 160 ° C to 230 ° C.

(2) The white polyester film for light reflection according to (1), wherein the resin incompatible with the polyester is polymethylpentene,
(3) A white polyester film for light reflection constituted by laminating a layer substantially free of voids on at least one side of the polyester film according to (1) or (2),
(4) The white polyester film for light reflection according to any one of (1) to (3), which is used as a reflector of a liquid crystal display,
It is.

  The white polyester film for light reflection of the present invention is excellent in reflection characteristics, luminance characteristics, etc., and exhibits high characteristics as a reflector in a surface light source for illuminating a liquid crystal screen, and also has excellent crease resistance. Value is high.

  The polyester constituting the present invention is a polymer obtained by condensation polymerization from a diol and a dicarboxylic acid. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid, sebacic acid, and the like. The diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol and the like. Specific examples include polymethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylenedimethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate.

  Of course, these polyesters may be homopolyesters or copolyesters. Examples of copolymer components include diol components such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, adipic acid, sebacic acid, and phthalic acid. And dicarboxylic acid components such as isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 5-sodiumsulfoisophthalic acid.

  Further, in this polyester, a trifunctional or higher functional compound or a monofunctional compound may be bound within a range considered to be substantially linear. Various known additives such as antioxidants and antistatic agents may be added.

  As the polyester used in the present invention, polyethylene terephthalate and polyethylene naphthalate which are excellent in water resistance, durability, chemical resistance and the like are particularly preferable.

  On the other hand, the incompatible resin used in the present invention is a thermoplastic resin other than polyester, and is a thermoplastic resin that is incompatible with the polyester. In the polyester, a resin dispersed in a particulate form is used. To tell. A resin having a high effect of forming voids in the film by stretching is preferred.

  The melting point of such a resin is preferably lower than the melting point of the polyester and higher than the temperature (heat treatment temperature) when the film is heat-fixed and oriented during film formation. From this point, among the incompatible resins, polyolefin resins such as polyethylene, polypropylene, polybutene, and polymethylpentene, polystyrene resins, polyacrylate resins, polycarbonate resins, polyacrylonitrile resins, polyphenylene sulfide resins, fluorine resins, and the like. Preferably used.

  Among them, polyolefin resins such as polypropylene and polymethylpentene having a small critical surface tension are preferable, and polymethylpentene is most preferable. Since the polymethylpentene has a relatively large difference in surface tension from polyester and a high melting point, it has a feature that the effect of forming voids per added amount is large, and is particularly preferable as an incompatible resin. These incompatible resins may be homopolymers or copolymers, and two or more incompatible resins may be used in combination.

  The content of the incompatible resin in the polyester film needs to be 5% by weight or more and 25% by weight or less with respect to the entire polyester film. More preferably, it is 10 to 20% by weight. If it is less than this content, the effect of whitening will be diminished and it will be difficult to obtain a high reflectivity. On the other hand, if it is too high, the mechanical properties such as the strength of the film itself will be low and it may be easily broken.

  In the present invention, the content of the thermoplastic polyester elastomer (abbreviated as TPEE) in the film needs to be 12 to 30% by weight. More preferably, it is 15 to 25% by weight. Conventionally, it has been known that the dispersibility of an incompatible resin can be improved by adding a small amount of TPEE or the like as a dispersant for the incompatible resin. Demonstrate. However, it has been found that when the content of TPEE is 12% by weight or more, it is effective for the crease resistance of the film. The reason for this is not clear, but, for example, by increasing the TPEE content, the adhesion at the interface between the incompatible resin, which is the core of the void (void), and PET is improved, and as a result, the external force that bends and bends into the film is increased. When working, it is presumed that peeling of the incompatible resin and PET is suppressed, and the film is not easily folded and wrinkled. However, if the content exceeds 30% by weight, the strength of the whole film is lowered and the film tends to be broken. In addition, as TPEE in this invention, the resin incompatible with the said polyester is not included.

  TPEE in the present invention is a polyether ester block copolymer obtained by copolymerizing thermoplastic polyester as a hard segment and poly (alkylene oxide) glycol as a soft segment.

  For example, terephthalic acid, isophthalic acid, phthalic acid, naphthalene-2,6-dicarboxylic acid, naphthalene-2,7-dicarboxylic acid, diphenyl-4,4′-dicarboxylic acid, diphenoxyethanedicarboxylic acid, sodium 3-sulfoisophthalate Aromatic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, alicyclic dicarboxylic acids such as succinic acid, oxalic acid, adipic acid, sebacic acid, dodecanedioic acid, dimer acid, or esters thereof Aliphatic diols such as at least one dicarboxylic acid component selected from formable derivatives, ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, neopentyl glycol, decamethylene glycol, etc. At least one low molecular diol component selected from alicyclic diols such as 1,1-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, tricyclodecane dimethanol, or ester-forming derivatives thereof, and an average molecular weight About 400-5000 poly (ethylene oxide) glycol, poly (1,2- and 1,3-propylene oxide) glycol, poly (tetramethylene oxide) glycol, copolymers of ethylene oxide and propylene oxide, ethylene oxide and tetrahydrofuran A block copolymer consisting of at least one of three poly (alkylene oxide) glycols such as a copolymer can be exemplified.

  In the present invention, a block copolymer polyether ester having polytetramethylene terephthalate polyester as a hard segment and poly (tetramethylene oxide) glycol as a soft segment is particularly preferable. The polyester portion constituting the hard segment is mainly composed of polytetramethylene terephthalate composed of terephthalic acid or an acid component obtained by combining this with isophthalic acid and a tetramethylene glycol component, but a part of this acid component (usually 30 A polyester in which a part of the glycol component (usually 30 mol% or less) is replaced with a low molecular glycol component other than the tetramethylene glycol component. Also good.

  Further, a part of the polyether portion constituting the soft segment may be a polyether replaced with a dioxy component other than the tetramethylene glycol component.

  Such TPEE may contain a stabilizer, an ultraviolet absorber, a thickening branching agent, a matting agent, a colorant, and other various improving agents as required.

  The melting point of such TPEE is preferably in the range of 160 to 230 ° C, more preferably in the range of 180 to 220 ° C. When the melting point of TPEE is less than 160 ° C., there is a problem that uniform dispersion, which is the object of the present invention, is difficult to obtain, and when it is higher than 230 ° C., a dispersion effect is hardly recognized, which is not preferable.

  In the film of the present invention, voids are formed around the incompatible resin dispersed in a granular or spherical shape in the polyester, and the apparent density is required to be low. Therefore, the polyester containing the incompatible resin and TPEE is used. It is necessary to stretch the melt-extruded sheet at least in a uniaxial direction, preferably in a biaxial direction. The film of the present invention has excellent crease resistance and is characterized by containing uniform and extremely fine fine voids.

  Moreover, this white polyester film for light reflection is a laminated film in which a layer substantially free of voids is formed on at least one side of a polyester film containing the incompatible resin and TPEE in the range of the specific amount. Is more preferable. Specifically, it is a laminated film having a configuration such as A / B, A / B / A, or A / B / C (B layer is a polyester film layer containing specific amounts of incompatible resin and TPEE). Such a configuration has an effect of further improving the effect of folding resistance and wrinkle resistance, and is also preferable from the viewpoint of compatibility between film forming properties and high reflectance. In addition, the A layer and / or C layer corresponding to the surface of the white polyester film for light reflection is substantially free of voids for the purpose of improving the concealing property and adjusting the glossiness, whiteness, etc. of the surface. About a certain amount of inorganic particles or fluorescent brightening agent may be added. In addition, the thickness of these substantially void-free layers is desirably about 30% or less of the entire light-reflecting white polyester film after lamination so as not to impair the high reflectance. Note that substantially having no voids means that the average void content of the corresponding layer is about 15% by volume or less.

  Next, the film production method of the present invention will be specifically described. The present invention is not limited to the method described below.

  The raw material which mix | blended the polyester chip | tip, the PMP chip | tip, and the TPEE chip | tip is supplied to the extruder A heated at the temperature of 260-320 degreeC. If necessary, polyethylene terephthalate containing an inorganic additive is supplied to Extruder B by a conventional method, and A / B / A is used so that the polymer on Extruder B side is on both surfaces of the T-die 3 layer die. You may laminate in the structure which becomes.

  It melts in this way, is extruded into a sheet through a die, and is rapidly cooled to about 70 ° C. or less to obtain an unstretched sheet.

The melted sheet is tightly cooled and solidified by electrostatic force on a drum cooled to a drum surface temperature of 10 to 60 ° C., and the unstretched film is led to a group of rolls heated to 80 to 120 ° C. in the longitudinal direction. The film is stretched 2.0 to 5.0 times longitudinally and cooled with a roll group of 20 to 50 ° C. Subsequently, the film is stretched in the direction perpendicular to the longitudinal direction in an atmosphere heated to 90 to 140 ° C. while being guided to a tenter while holding both ends of the longitudinally stretched film with clips. The stretching ratio is 2.5 to 4.5 times in the longitudinal and lateral directions, and the area ratio (longitudinal stretching ratio x lateral stretching ratio) is preferably 9 to 16 times. If the area magnification is less than 9 times, the whiteness of the film obtained becomes poor. Conversely, if it exceeds 16 times, the film tends to be broken during stretching and the film-forming property tends to be poor. In order to give the flatness and dimensional stability of the biaxially stretched film in this manner, heat setting at 150 to 230 ° C. in a tenter, uniform cooling, and cooling to room temperature, winding up the film of the present invention. obtain.
The white polyester film of the present invention thus obtained can exhibit crease resistance while maintaining excellent reflection characteristics. Accordingly, the white polyester film of the present invention can be suitably used as a film for reflecting light such as a reflector in a surface light source for illuminating a liquid crystal screen.

[Measurement of physical properties and measurement of effects]
The physical property value measuring method and the effect evaluating method in the present invention are as follows.

(1) Apparent density The film is cut into 100 × 100 mm square, the thickness of at least 10 points is measured with a dial gauge attached, and the average thickness d (μm) is calculated. Moreover, this film is weighed with a direct balance, and the weight w (g) is read to a unit of 10 ⁻⁴ g. At this time, apparent specific gravity = w / d × 100.

(2) Folding resistance: A white polyester film is cut into a length of 200 mm and a width of 15 mm to produce a sheet for evaluation of folding wrinkles. One end of the sheet was fixed, and the other end was pulled at 200 mm / second while turning 180 degrees around a 5 mm diameter iron circle with a 200 g weight connected to both sides with a wire. The state of occurrence was observed with a stereomicroscope (magnification 10 times) and judged as follows:
Class A: 0 wrinkles / 10 mm ²
Class B: generation of wrinkles 1 to 3/10 mm ²
Class C: generation of wrinkles is 4 to 9/10 mm ²
Class D: Generation of wrinkles is 10 or more / 10 mm ² .

(3) Film-forming property When the film is formed, the melted sheet is cooled and solidified, and the film is judged as follows according to the frequency of film breakage when the unstretched sheet is stretched in the longitudinal and transverse directions under predetermined conditions:
○: Breaking frequency 1 time / day Δ: Breaking frequency 2 to 3 times / day ×: Breaking frequency 4 times / day

(4) Average reflectance A spectrophotometer (U-3310) manufactured by Hitachi High-Technologies Co., Ltd. is attached with an integrating sphere, and the relative reflectance when the standard white plate (aluminum oxide) is 100% is in the wavelength range of 400 to 700 nm. To measure. In the obtained chart, the reflectance of each wavelength is read every 5 nm, and the average value is defined as the average reflectance.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to a following example, unless the meaning is exceeded.

[Comparative Example 1]
A polyethylene terephthalate chip having an intrinsic viscosity of 0.65 is vacuum-dried in hot air at 170 ° C. for 3 hours in advance, and 15 parts by weight of polymethylpentene chip and 10 parts of TPEE having a melting point of 170 ° C. are added to obtain 75 parts by weight. The parts by weight were blended, supplied to an extruder heated to 290 ° C., extruded through a die into a cooling drum cooled to 25 ° C., and cooled rapidly. Subsequently, the film was stretched 3 times at 90 ° C. in the longitudinal direction and 3.1 times at 130 ° C. in the transverse direction, and then heat-set at 230 ° C. in a tenter, uniformly cooled, and then cooled to room temperature. The film was wound up to obtain a film having a thickness of 100 μm.
The physical properties of the obtained film are as shown in Table 1, and the physical properties as a light reflecting film used for a liquid crystal display and the like were satisfactory, but the evaluation of the crease resistance was insufficient.

[Examples 1 to 6]
A polyethylene terephthalate chip having an intrinsic viscosity of 0.65 was previously vacuum-dried in hot air at 170 ° C. for 3 hours, and a polymethylpentene chip and TPEE having a melting point of 170 ° C. were blended in the proportions shown in Table 1, followed by comparison. Six kinds of films having a thickness of 100 μm were finally obtained in the same manner as in Example 1.
The physical properties of the obtained film are as shown in Table 1, and satisfactory physical properties as a light reflecting film and crease resistance were obtained.

[Example 7]
TPEE was changed to a chip having a melting point of 210 ° C., the raw material compounding ratio was set to the value shown in Table 1, and a film having a thickness of 100 μm was finally obtained in exactly the same manner as in Examples 1-6. Table 1 shows the evaluation results of the physical properties and folding resistance of the obtained film as a light reflection film.

[Comparative Example 2]
A film was formed in the same manner as in Examples 1 to 6 except that 20 parts by weight of PMP and 35 parts by weight of TPEE were used, and finally a film having a thickness of 100 μm was obtained.
When the film was stretched, the film was broken, and it was difficult to stably produce a long-winding product. The physical properties of the obtained film as a light reflecting film and the evaluation results of the crease resistance are as shown in Table 1. [Comparative Example 3]
Film formation was performed in the same manner as in Examples 1 to 6 except that 30 parts by weight of PMP and 15 parts by weight of TPEE were used to obtain a final 100 μm film.
However, the film was broken during stretching in the transverse direction, and a predetermined film could not be obtained.

  The results thus obtained are summarized in Table 1.

Claims (4)

A polyester film stretched in at least a uniaxial direction,
The film contains 5 to 25% by weight of a resin incompatible with polyester,
Furthermore, the white polyester film for light reflections which uses the polyester film which contains 12-30 weight% of thermoplastic polyester elastomers in the range of 160 ° C to 230 ° C. 2. The white polyester film for light reflection according to claim 1, wherein the resin incompatible with the polyester is polymethylpentene. The white polyester film for light reflection comprised by laminating | stacking the layer which does not have a space | gap substantially on the at least one side of the polyester film of Claim 1 thru | or 2. The white polyester film for light reflection according to any one of claims 1 to 3, which is used as a reflection plate of a liquid crystal display.