JP5108271B2 - Biaxially stretched laminated polyester film - Google Patents

Description

  The present invention relates to a biaxially stretched laminated polyester film, and in particular, relates to a biaxially stretched laminated polyester film having high reflectivity and excellent heat resistance.

  Conventionally, a backlight system in which light is applied from the back of the display has been adopted in a liquid crystal display. However, in recent years, for example, a sidelight system as disclosed in Japanese Patent Application Laid-Open No. 63-62104 has a merit that it can be illuminated thinly and uniformly. Therefore, it has come to be widely used. In this sidelight system, a reflector is installed on the back surface. The reflector is required to have high light reflectivity and high diffusibility.

  As liquid crystal displays in recent years have become larger and higher brightness is strongly demanded, improvement in the reflectance of the reflector is expected. In order to meet this demand, the conventional technology has used a reflector that exhibits light reflection by stretching a film made of a composition of polyester and an incompatible resin. However, in the method of adding an incompatible resin to polyester, the film breaks when the addition amount increases, so there is a limit in film formation, and the addition amount has been limited to a limited range of at most 30% by weight. It was.

JP 63-62104 A JP-A-2-284929 Japanese Patent Laid-Open No. 3-76727 JP-A-4-239540 Japanese Patent Publication No. 8-16175 JP 2004-50479 A JP 2004-330727 A JP 2005-125700 A

  An object of the present invention is to solve the problems of the prior art, and can provide a stable film formation with sufficient reflection performance in the visible light region, and can be used for liquid crystal displays and internally illuminated signboards. An object is to provide a biaxially stretched laminated polyester film that can be suitably used as a reflector substrate.

  As a result of intensive studies, the inventors have used a copolymer polyester, and by adding polyolefin to the copolymer polyester at a higher concentration than before, it is possible to obtain a film having a higher reflectivity than before. It was found that the film can be formed stably.

That is, the present invention is composed of a reflective layer composed of 31 to 50% by weight of polyolefin and 69 to 50% by weight of a copolyester and a support layer made of polyester in contact with this layer . or reflective layer / supporting layer / reflective layer or the support layer / reflective layer / laminate a polyester film for the support layer three-layer structure der Ru biaxially oriented reflector substrate of,
The thickness of the laminated film after biaxial stretching is 25 to 350 μm, the thickness of the reflective layer is 50 to 80 with respect to the total thickness 100 of the laminated film,
The average reflectance at a wavelength of 400 to 700 nm on at least one surface is 95% or more,
The glossiness of the surface used on the side reflecting the light source is 65 or less,
It is a laminated polyester film for a reflector substrate.

  According to the present invention, it has sufficient reflective performance in the visible light region, can be stably formed, and can be suitably used as a reflector substrate for liquid crystal displays and internally illuminated signboards. A biaxially stretched laminated polyester film can be provided.

Hereinafter, the present invention will be described in detail.
The biaxially stretched laminated polyester film of the present invention comprises a layer comprising 50 to 69% by weight of a copolyester functioning as a reflective layer and 31 to 50% by weight of a polyolefin, and a support layer comprising a polyester that supports the reflective layer; Consists of.

[Reflective layer]
The reflective layer is composed of a copolyester and a polyolefin.

[Copolyester]
As the copolymerized polyester, a copolymerized polyethylene terephthalate containing a dicarboxylic acid component such as isophthalic acid or naphthalenedicarboxylic acid as a copolymerized component is preferable. The copolymerization ratio of the dicarboxylic acid as a copolymerization component is preferably 1 to 20 mol%, more preferably 3 to 18 mol%, and particularly preferably 5 to 15 mol%. If the copolymerization ratio is not within this range, the film is easily broken during film formation, resulting in a film having poor film forming property or lacking in thermal stability.

  As the copolymer polyester, from the viewpoint of obtaining a good film formation, 1 to 20 mol% of isophthalic acid and / or naphthalenedicarboxylic acid and 99 to 80 mol% of terephthalic acid are used as the dicarboxylic acid component, and ethylene glycol is used as the diol component. It is preferable to use a copolyester.

  It is preferable that the copolyester of the reflective layer contains substantially no antimony element. “Substantially not contained” means that the content is 20 ppm or less, preferably 15 ppm or less, more preferably 10 ppm or less. When the antimony element is substantially contained, in the case of a white film, it looks black and streaks, which may unfavorably deteriorate the film appearance.

  In order to obtain a copolyester substantially free of an antimony element, the polyester may be polymerized using a catalyst other than the antimony compound. As a catalyst used for polymerization of polyester, it is preferable to use any one of a manganese (Mn) compound, a titanium (Ti) compound, and a germanium (Ge) compound.

As the titanium compound, for example, titanium tetrabutoxide and titanium acetate can be used.
Examples of germanium compounds include amorphous germanium oxide, fine crystalline germanium oxide, a solution in which germanium oxide is dissolved in glycol in the presence of an alkali metal or alkaline earth metal or a compound thereof, and germanium oxide is dissolved in water. A solution can be used.

[Polyolefin]
The polyolefin used together with the copolymerized polyester in the reflective layer is peeled off at the interface with the copolymerized polyester when the film is stretched. As a result, many fine voids are formed in the film.

  As the polyolefin, for example, polyethylene, polypropylene, polybutene, and polymethylpentene can be used. Among them, polypropylene and polymethylpentene have high transparency, so that light absorption is suppressed, and as a result, high reflectance can be obtained and they are optimally used.

  The polyolefin is contained in an amount of 31 to 50% by weight, preferably 35 to 45% by weight, per 100% by weight of the total weight of the composition constituting the reflective layer. If the content is less than 31% by weight, the desired reflectance cannot be obtained, while if it exceeds 50% by weight, the stability of the film formation may be impaired.

  When polypropylene is used as the polyolefin, those having a melt flow rate of 1 to 30 g / 10 min (measured in JIS K7210: 99) are preferable from the viewpoint of obtaining good dispersibility and kneadability.

  When polymethylpentene is used as the polyolefin, those having a melt flow rate of 10 to 250 g / 10 min (measured by ASTM D1238) are preferable from the viewpoint of obtaining good dispersibility and kneadability.

[Support layer]
As the polyester of the support layer in the present invention, a thermoplastic polyester can be used, and preferably a copolyester is used. As the preferred copolyester, the same copolyester as mentioned in the reflective layer can be used. Further, from the viewpoint of obtaining good film forming properties, the polyester used in the support layer is preferably the same as the copolyester used in the reflective layer.

[Glossiness]
Since the biaxially stretched laminated film of the present invention obtains good diffuse reflection of light and suppresses luminance unevenness of the light source, the glossiness of at least one surface of the film is preferably 65 or less, more preferably 55 or less, particularly preferably 50 or less. In particular, it is preferable that the surface used on the side reflecting the light source has the above glossiness.

[Additive]
A lubricant may be blended in order to slide the film surface and improve the handleability. As the lubricant, either an organic substance or an inorganic substance may be used, and examples of the inorganic lubricant include titanium oxide, barium sulfate, calcium carbonate, silicon dioxide, and alumina particles. These particles may be either plate-like or spherical. From the viewpoint of dispersibility and slipperiness, these particles preferably have an average particle size of 0.1 to 5.0 μm, more preferably 0.2 to 4.0 μm. The lubricant is preferably blended in a layer that forms at least one surface of the laminated film of the present invention.

  You may mix | blend a fluorescent whitening agent with the biaxially stretched laminated polyester film of this invention. When the fluorescent whitening agent is blended in the reflective layer, the concentration per 100% by weight of the polyester composition constituting the reflective layer is, for example, 0.005 to 0.2% by weight, preferably 0.01 to 0.1% by weight. Blend. If it is less than 0.005% by weight, the reflectance in the wavelength region near 350 nm is not sufficient, so the meaning of adding it is not preferable. On the other hand, if it exceeds 0.2% by weight, the unique color of the fluorescent whitening agent appears. This is not preferable. As the fluorescent brightening agent, for example, OB-1 (manufactured by Eastman), Uvitex-MD (manufactured by Ciba Geigy), or JP-Conc (manufactured by Nippon Chemical Industry Co., Ltd.) can be used.

  To the biaxially stretched laminated polyester film of the present invention, an antioxidant, an ultraviolet absorber, and a fluorescent brightening agent may be added as necessary to further improve the performance. Moreover, you may apply | coat the coating agent containing these agents to a laminated | multilayer film.

  The thickness of the reflective layer is preferably 30 to 95, more preferably 35 to 90, with respect to the total thickness 100 of the laminated film. If it is less than 30, the reflectance may be inferior, which is not preferable. If it exceeds 95, it is not preferable from the viewpoint of stretchability.

  Moreover, you may further laminate | stack another layer in order to provide another function to the laminated | multilayer film of this invention. Examples of other layers herein include a transparent polyester resin layer, a metal thin film, a hard coat layer, and an ink receiving layer. By providing these layers, it is possible to print on the reflective film or to provide functions other than the reflective function.

[Production method]
Hereinafter, a method for producing the biaxially stretched laminated polyester film of the present invention will be described. Here, an example of a method for producing a biaxially stretched laminated polyester film having a configuration of A layer / B layer will be described. The stretching may be a sequential biaxial stretching method or a simultaneous biaxial stretching method. Here, a production method by the sequential biaxial stretching method will be described.

  First, an unstretched laminated sheet is produced by simultaneous multilayer extrusion using a feed block of polyester melted from a die. That is, a melt of the copolymer polyester constituting the reflective layer (A layer) containing polyolefin and a melt of the polyester constituting the support layer (B layer) are converted into an A layer / B layer using a feed block. And then extruding by spreading on a die. At this time, the polymer laminated by the feed block maintains the laminated form.

  In the melting step, it is preferable to filter the molten polymer using a non-woven filter having an average opening of 10 to 100 μm, preferably an average opening of 20 to 50 μm, made of a fine stainless steel wire having a wire diameter of 15 μm or less. By performing this filtration, it is possible to obtain a film with few coarse foreign matters by suppressing the aggregation of particles that are generally likely to aggregate into coarse aggregate particles and foreign foreign matters.

  The unstretched laminated sheet extruded from the die is cooled and solidified by a casting drum to form an unstretched laminated film. This unstretched laminated film is heated by means such as roll heating or infrared heating, and stretched in the longitudinal direction to obtain a longitudinally stretched laminated film. This stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. The stretching temperature is preferably a temperature equal to or higher than the glass transition point (Tg) of the copolyester, and more preferably a temperature of Tg to (Tg + 70) ° C. The draw ratio is preferably 2.2 to 4.0 times, more preferably 2.3 to 3.9 times in the machine direction, although it depends on the required characteristics of the application. When the ratio is less than 2.2 times, the thickness unevenness of the film is deteriorated and a good film cannot be obtained. When the ratio exceeds 4.0 times, breakage is likely to occur during film formation.

  The laminated film after the longitudinal stretching is subsequently subjected to the treatment of transverse stretching, heat setting, and thermal relaxation to form a biaxially stretched laminated film, and these treatments are performed while the laminated film is running. The transverse stretching process starts from a temperature higher than the glass transition point (Tg) of the copolyester. In general, the temperature is raised to a temperature of (Tg + 5) to (Tg + 70) ° C. Although the temperature rise in the transverse stretching process may be continuous or stepwise (sequential), the temperature is usually raised sequentially. For example, the transverse stretching zone of the tenter is divided into a plurality along the film running direction, and the temperature is raised by flowing a heating medium having a predetermined temperature for each zone. The transverse stretching ratio is preferably 2.5 to 4.5 times, more preferably 2.8 to 3.9 times, although it depends on the required characteristics of this application. If the thickness is less than 2.5 times, the thickness unevenness of the laminated film is deteriorated, and a favorable laminated film cannot be obtained. On the other hand, if it exceeds 4.5 times, breakage is likely to occur during film formation. Absent.

  The laminated film after transverse stretching is heat-treated at a constant width or a width reduction of 10% or less from (Tm-20) to (Tm-100) ° C. of the polyester of the support layer while holding both ends, thereby reducing the heat shrinkage rate. It is good to let them. When the heat treatment is performed at a temperature higher than this, the flatness of the film is deteriorated, and the thickness unevenness is increased, which is not preferable. On the other hand, if the heat treatment temperature is lower than this, the heat shrinkage rate may increase, which is not preferable. In order to adjust the heat shrinkage in the region of (Tm-20) to (Tm-100) ° C. in the process of returning the film temperature to room temperature after heat setting, both ends of the laminated film being gripped are cut off, The take-up speed in the vertical direction can be adjusted and relaxed in the vertical direction. As a means for relaxing, the speed of the roll group on the tenter exit side is adjusted. As the rate of relaxation, the speed of the roll group is reduced with respect to the film line speed of the tenter, preferably 0.1 to 1.5%, more preferably 0.2 to 1.2%, particularly preferably 0.3. The film is relaxed by performing a speed reduction of ˜1.0% (this value is referred to as “relaxation rate”), and the longitudinal heat shrinkage rate is adjusted by controlling the relaxation rate. Further, the width of the film in the horizontal direction can be reduced in the process until both ends are cut off, so that a desired heat shrinkage rate can be obtained.

  The thus obtained biaxially stretched polyester film of the present invention has a heat shrinkage rate of 85 ° C. of 0.5% or less, more preferably 0.4% or less, most preferably 0 in both orthogonal directions. .3% or less.

  The thickness of the laminated film after biaxial stretching is preferably 25 to 350 μm, more preferably 40 to 320 μm, and particularly preferably 50 to 300 μm. If the thickness is less than 25 μm, the reflectance is lowered, which is not preferable. On the other hand, if the thickness exceeds 350 μm, the reflectance cannot be increased even if the thickness is further increased.

  The biaxially stretched laminated polyester film of the present invention thus obtained has a reflectance of at least one surface of 95% or more, more preferably 96% or more, more preferably an average reflectance of a wavelength of 400 to 700 nm. Preferably, the reflectance is 97% or more. If it is less than 95%, it is not preferable because sufficient screen brightness cannot be obtained.

Hereinafter, the present invention will be described in detail by way of examples. Each characteristic value was measured by the following method.
(1) Film thickness A film sample was measured for 10-point thickness with an electric micrometer (K-402B manufactured by Anritsu), and the average value was taken as the thickness of the film.

(2) Thickness of each layer A sample was cut into a triangle, fixed in an embedded capsule, and then embedded in an epoxy resin. Then, after embedding the sample with a microtome (ULTRACUT-S) into a thin film section having a thickness of 50 nm in parallel with the microtome, the specimen was observed and photographed with a transmission electron microscope at an acceleration voltage of 100 kv. The thickness of each layer was measured and the average thickness was determined.

(3) Reflectivity A spectrophotometer (UV-3101PC manufactured by Shimadzu Corporation) is attached with an integrating sphere, and the reflectance when the BaSO ₄ white plate is 100% is measured over a wavelength range of 400 to 700 nm, and 2 nm intervals from the obtained chart. I read the reflectance. When the configuration of the film was a reflective layer (A layer) on one side and a support layer (B layer) on the other side, the measurement was performed from the A layer side. An average value was determined within the above range.

(4) Stretchability The film was stretched 2.5 to 3.4 times in the longitudinal direction and 3.5 to 3.7 times in the transverse direction to form a film, and it was observed whether it could be stably formed. Evaluation was made according to the following criteria.
○: Stable film formation over 1 hour ×: Cutting occurs within 1 hour and stable film formation is not possible

(5) Thermal shrinkage rate The film was held in an oven set at 85 ° C. for 30 minutes in an unstrained state, the distance between the gauge points before and after the heat treatment was measured, and the thermal shrinkage rate (85 ° C. thermal shrinkage rate) according to the following formula: ) Was calculated.
Thermal shrinkage% = ((L ₀ −L) / L ₀ ) × 100
L ₀ : Distance between gauge points before heat treatment L: Distance between gauge points after heat treatment

(6) Glass transition point (Tg), melting point (Tm)
Using a differential scanning calorimeter (TA Instruments 2100 DSC), the measurement was performed at a heating rate of 20 m / min.

(7) Deformation due to heat (evaluation of deflection)
A film sample was cut into A4 plate, and after processing for 30 minutes in an oven heated to 80 ° C. with the four sides of the film fixed with a metal frame, the deformation (the film deflection state) was visually observed and the following criteria were used. evaluated.
○: A bent state is not observed.
Δ: Slight deflection is observed in part.
X: There is a bent portion, and unevenness of the deflection is observed as a bulge of 5 mm or more.

(8) Glossiness 60 ° gloss was measured using a measuring instrument of GM-268 manufactured by Minolta.

[Example 1]
132 parts by weight of dimethyl terephthalate, 23 parts by weight of dimethyl 2,6-naphthalenedicarboxylate (12 mol% based on the acid component of the polyester), 96 parts by weight of ethylene glycol, 3.0 parts by weight of diethylene glycol, 0.05 weight of manganese acetate And 0.012 parts by weight of lithium acetate were charged into a rectification column and a flask equipped with a distillation condenser, and heated to 150 to 235 ° C. with stirring to distill methanol to conduct a transesterification reaction. After the methanol was distilled off, 0.03 part by weight of trimethyl phosphate and 0.04 part by weight of germanium dioxide were added, and the reaction product was transferred to the reactor. Subsequently, while stirring, the pressure in the reactor was gradually reduced to 0.5 mmHg and the temperature was raised to 290 ° C. to carry out a polycondensation reaction. The obtained copolymer polyester had a diethylene glycol component amount of 2.5% by weight, a germanium element amount of 50 ppm, and a lithium element amount of 5 ppm. This copolymerized polyester is used for the reflective layer (A layer) and the support layer (B layer), the polyolefin resin shown in Table 1 is used for the reflective layer (A layer), and the average particle diameter of 1. is used for the support layer (B layer). 0.5 μ% of 0 μm calcium carbonate particles were added. It is supplied to two extruders each heated to 285 ° C., and the A layer polymer and the B layer polymer are merged using a two layer feed block apparatus in which the A layer and the B layer are A / B. While maintaining the laminated state, it was formed into a sheet from a die. Further, an unstretched film obtained by cooling and solidifying the sheet with a cooling drum having a surface temperature of 25 ° C. was heated at the described temperature, stretched in the longitudinal direction (longitudinal direction), and cooled by a roll group at 25 ° C. Subsequently, the film was stretched in a direction perpendicular to the longitudinal direction (lateral direction) in an atmosphere heated to 120 ° C. while being guided to a tenter while holding both ends of the longitudinally stretched film with clips. Thereafter, heat setting was performed in the tenter at the temperature shown in Table 2, and longitudinal relaxation and lateral width insertion were performed under the conditions shown in Table 2, and the film was cooled to room temperature to obtain a biaxially stretched laminated film. Table 2 shows the physical properties of the obtained laminated film as a reflector.

[Examples 2 to 5]
The addition amount shown in Table 1, the polyolefin resin, and the acid component of the polyester were adjusted and added. Under the film forming conditions shown in Table 2, a biaxially stretched laminated film was produced and evaluated.

[Example 6]
Polymer of isophthalic acid copolymer by changing 23 parts by weight of dimethyl 2,6-naphthalenedicarboxylate of Example 1 to 18 parts by weight of dimethyl isophthalate (12 mol% with respect to the acid component of the polyester) in the step of preparing the polymer Was used as a reflective layer (A layer), and a film was prepared and evaluated under the conditions shown in Tables 1 and 2.

[Examples 7 to 12]
A laminated film which was added and biaxially stretched as shown in Tables 1 and 2 was prepared and evaluated.
In some cases, polyester was polymerized using dimethyl 2,6-naphthalenedicarboxylate and dimethyl isophthalate and adjusted to a certain ratio before use.

[Comparative Examples 1 to 9]
Films were prepared by addition as shown in Tables 1 and 2 and evaluated.
In some cases, dimethyl isophthalate, dimethyl terephthalate, or dimethyl 2,6-naphthalenedicarboxylate was used as the acid component of the polyester. The results are as shown in Table 2, but in part, the stretchability was poor and the film was not formed.

  The biaxially stretched laminated polyester film of the present invention has a high light reflectivity, and should be suitably used as a reflector for various reflectors, especially a reflector for liquid crystal displays and a slightly illuminated interior signboard. Can do. Moreover, it can use optimally for the back seat | sheet of a solar cell. Other uses include paper substitution, ie cards, labels, stickers, home delivery slips, video printer paper, inkjets, barcode printer paper, posters, maps, dust-free paper, display boards, white boards, thermal transfer, offset It can also be used as a substrate for receiving sheets used for various printing records such as printing, telephone cards and IC cards.

Claims (3)

It is composed of a reflective layer composed of 31 to 50% by weight of polyolefin and 69 to 50% by weight of copolyester and a support layer composed of polyester in contact with this layer, and is composed of a two-layer structure of reflective layer / support layer, or reflective layer / supporting layer / reflective layer or the support layer / reflective layer / laminate a polyester film for the support layer three-layer structure der Ru biaxially oriented reflector substrate of,
The thickness of the laminated film after biaxial stretching is 25 to 350 μm, the thickness of the reflective layer is 50 to 80 with respect to the total thickness 100 of the laminated film,
The average reflectance at a wavelength of 400 to 700 nm on at least one surface is 95% or more,
The glossiness of the surface used on the side reflecting the light source is 65 or less,
Laminated polyester film for reflector substrate.   The lamination polyester film according to claim 1 whose glossiness of the surface used for the side which reflects a light source is 20 or less. The copolyester is a copolyester having 1 to 20 mol% of isophthalic acid and / or naphthalenedicarboxylic acid, 99 to 80 mol% of terephthalic acid as a dicarboxylic acid component, and ethylene glycol as a diol component. Or the laminated polyester film of 2.