JP2015222763A - Black polyester film for solar batteries - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a black polyester film for solar batteries, which is superior not only in delusterability and light concealability, but also in hydrolysis resistance, which is arranged so that the degradation of a mechanical property can be suppressed even after having used under a harsh natural environment, e.g. at a high-temperature and high-humidity environment for a long time, and which is suitable for e.g. a backside protection film arranged so as to retain a satisfactory solar battery protection function even after having been used for a long time.SOLUTION: A black polyester film for solar batteries comprises a laminate including at least two layers composed of a pigment layer as a surface layer, and a delustering layer as a core layer. (1)The pigment layer includes a black pigment in a range of 0.5-2.4 wt% to the weight of the layer. (2) The delustering layer includes: particles of which the average particle diameter is 2.5-5.5 μm and the maximum particle diameter is 16 μm or less in a range of 1.5-4.5 wt% to the weight of the layer; and a black pigment in a range of 0.01-2.4 wt%. The degree of brilliance (G) the surface of the pigment layer is 20-50. The polyester film has an L* value of 13.5 or less in L*a*b* color system.

Description

  The present invention relates to a black polyester film for solar cells. More specifically, the present invention relates to a black polyester film for a solar cell that is excellent in matte property, light hiding property, and hydrolysis resistance.

  In recent years, a solar power generation system using a solar cell module has been spread as one of power generation means using clean energy. The back surface protective film of the solar cell is mainly required to have a protective function of the solar cell module, to have excellent mechanical properties, and to have various properties such as light resistance, heat resistance and hydrolysis resistance. Yes. In addition, when a solar cell is installed on a roof or the like, for example, the back sheet of the solar cell is also colored in black for the purpose of imparting designability, decoration, etc. suitable for the surrounding environment, and light concealment is required. There is. Furthermore, with regard to this type of black solar cell backsheet, the fine wrinkles and fingerprints attached to the surface that are generated when pasting with EVA (ethylene vinyl acetate) and other solar cell module fillers are inconspicuous. In recent years, it has been desired that the matting property is also good.

  In order to impart light hiding properties to a polyester film, it is known to use a pigment, for example. For example, as described in Patent Document 1, a polyester film having a good light concealing property containing carbon black is disclosed. However, in Patent Document 1, since recycled resin is mainly used, the hydrolysis resistance required as a solar cell back surface protective film is not fully satisfactory.

  In addition, it is known to use inorganic particles in order to impart matte properties to the polyester film. However, for example, as described in Patent Document 2, generally, when a large amount of inorganic particles are added to a resin, the inorganic particles have a slight amount of adsorbed water. It is necessary to knead with high shear in order to easily disperse a large amount of inorganic particles in the polyester film, and the molecular weight of the polyester resin is lowered due to shear heat generation during kneading. Since it becomes easy, it might be accompanied by a decrease in hydrolysis resistance.

Patent Document 3 discloses a biaxially stretched coextruded matte polyester film having good matting and transparency, and 1 to 10% by weight of particles having a particle size of 2 to 5 μm on one side of the laminated film. Although it is disclosed to be added, the film gloss (G ₆₀ ) specifically exemplified is about 50 to 70, and higher matteness is required.
As described above, although a black polyester film for solar cells to which carbon black has been added and a film having good matting properties to which inorganic particles have been added have been studied, hydrolysis resistance, light hiding properties, and matting properties have been studied. The present condition is that the black polyester film for solar cells excellent in any of these is not yet obtained.

JP 2002-302595 A JP 2010-64250 A JP 2002-200723 A

  The present invention has been made paying attention to the above-mentioned problems, and not only is it excellent in matting properties and light hiding properties, but also has excellent hydrolysis resistance at the same time under severe natural environments such as high temperature and high humidity. To provide a black polyester film for solar cells suitable for use as a back surface protective film that maintains a good solar cell protective function even when used for a long period of time. is there.

  As a result of intensive studies to solve the problems of the prior art, the present inventors have not been able to obtain sufficient black concealment when a matting agent and a black pigment are blended in a single layer or a surface layer. Or, in view of the fact that degradation of hydrolysis resistance due to a matting agent is likely to occur, when using particles as a matting agent for a black polyester film, a black pigment layer is arranged as a surface layer, and particles of a specific size are used as a core layer. By arranging the matte layer to be included, the matte effect can be achieved without using particles on the surface side due to the push-up effect by the particles in the core layer. For solar cell modules such as EVA (ethylene vinyl acetate) Fine wrinkles and fingerprints attached to the surface are not noticeable when pasting with a filler, etc., and the light hiding property is very good. It found that it is possible to obtain a black polyester film suitable for the protective film of a solar cell having excellent Lum appearance and hiding properties, and have completed the present invention.

That is, the object of the present invention is a polyester film in which at least two layers having a pigment layer as a surface layer and a matte layer as a core layer are laminated,
(1) The pigment layer contains a black pigment in a range of 0.5 wt% to 2.4 wt% based on the weight of the layer,
(2) The matte layer has a mean particle size of 2.5 μm to 5.5 μm and a maximum particle size of 16 μm or less based on the weight of the layer in a range of 1.5% to 4.5% by weight. And a black pigment in a range of 0.01 wt% to 2.4 wt%,
Achieved by a black polyester film for a solar cell in which the glossiness (G ₆₀ ) of the pigment layer surface is 20 or more and 50 or less, and the L * value in the L * a * b * color system of the polyester film is 13.5 or less. Is done.

  Moreover, the black polyester film for solar cells of this invention WHEREIN: As a preferable aspect, the weight change at 300 degrees C by the TG-DTA method of the said particle | grains contained in a mat layer is 0% or more and 3.0% or less, The particles contained in the extinguishing layer are synthetic zeolite, the half elongation time of the polyester film is 72 hours or more in an environment of a temperature of 121 ° C. and a humidity of 100% RH, for protecting the back surface of a solar cell. Also included are those having at least one of the above.

  According to the present invention, even when used for a long time in a harsh natural environment such as high temperature and high humidity, the deterioration of mechanical properties is suppressed, the hydrolysis resistance is excellent, and at the same time, matte and concealment properties are achieved. It is possible to provide a black polyester film for a solar cell suitable for a back surface protective film of a solar cell that has a good appearance and protection function even when used for a long time.

Hereinafter, the present invention will be described in detail.
[polyester]
Examples of the polyester constituting the polyester film of the present invention include polyethylene terephthalate and polyethylene naphthalate, and polyethylene terephthalate is particularly preferably used. The polyethylene terephthalate in the present invention is a polyester polymer whose main repeating unit is ethylene terephthalate, that is, terephthalic acid or a derivative thereof as a dicarboxylic acid component and ethylene glycol as a diol component. Here, the main repeating unit means that 90 mol% or more of all repeating units constituting the polyester is an ethylene terephthalate unit, more preferably 95 mol% or more, and particularly preferably 97 mol% or more.

The polyester in the present invention may be copolymerized with other components as long as the effects of the present invention are not impaired, and the copolymer component may be an acid component or an alcohol component. As acid components to be copolymerized, components other than the main components of aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid and naphthalenedicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid And alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid. Examples of the copolymerized alcohol component include aliphatic diols such as butanediol and hexanediol, and alicyclic diols such as cyclohexanedimethanol. These may be used alone or in combination of two or more.
When the copolymerization amount of the copolymer component exceeds 10 mol%, the heat resistance and hydrolysis resistance may be lowered.

  The intrinsic viscosity of the polyester in the present invention is preferably 0.62 to 0.90 dl / g, more preferably 0.67 to 0.85 dl / g. When the intrinsic viscosity is less than 0.62 dl / g, it is difficult to adjust the weight average molecular weight of the polyester in the film to the range of 44,000 to 61,000, which may be accompanied by a decrease in the hydrolysis resistance of the film. On the other hand, if the intrinsic viscosity of the polyester is higher than 0.90 dl / g, the melt extrudability may be lowered due to the high melt viscosity.

The polyester in the present invention is a method in which an aromatic dicarboxylic acid ester typified by dimethyl terephthalate and ethylene glycol are reacted by a transesterification reaction, followed by a polycondensation reaction, an aromatic dicarboxylic acid typified by terephthalic acid and ethylene glycol Any method of esterifying and may be used. In the case of using the transesterification method, the transesterification reaction was carried out while removing the generated alcohol in this production process, and then the phosphoric acid compound was added to substantially complete the transesterification reaction. At least one of an antimony compound and a titanium compound is added to the reaction product, and a polycondensation reaction is performed.
In order to obtain a higher hydrolysis-resistant polyester film, it is important to use a polyester material having a high intrinsic viscosity or to reduce the terminal carboxyl group concentration, and it is preferable to add solid-phase polymerization as one of the steps. .

[Pigment layer]
The black polyester film for solar cells of the present invention has a configuration of a polyester film in which at least two layers having a pigment layer as a surface layer and a matte layer as a core layer are laminated. Here, the surface layer in the case where the polyester film has a two-layer structure is a layer disposed on the side opposite to the side to be bonded to the module when the polyester film is disposed as a member of the solar cell module. The core layer in this case refers to a layer disposed on the side to be bonded to the module.

The pigment layer in the present invention contains a black pigment, and the content thereof is 0.5% by weight or more and 2.4% by weight or less based on the weight of the layer. By using the pigment layer as a surface layer, a sufficient black appearance and light shielding property can be obtained.
The pigment layer is composed of the polyester described above, and the content of the black pigment is preferably 1.0% by weight to 2.4% by weight, more preferably 1.0% by weight to 2.0% by weight. is there. When the content of the black pigment in the pigment layer exceeds the upper limit, the light hiding property is increased, but the hydrolysis resistance and matting properties required for the black solar cell protective film are reduced, and the film forming property is also reduced. Decreases and tears easily occur. On the other hand, when the content of the black pigment in the pigment layer is less than the lower limit, sufficient light hiding properties and black appearance are not exhibited.

  The black pigment used in the pigment layer may be either inorganic or organic. As the inorganic black pigment, carbon black (channel, furnace, acetylene, thermal, etc.), carbon nanotube (single layer, multilayer), aniline black, Examples thereof include black iron oxide. Examples of organic black pigments include condensed azo, phthalocyanine, cynaridon, oxazine, xanthene, isoindolinone, quinophthalone, and anthraquinone. Among these, inorganic pigments are superior to organic pigments in heat resistance during melt molding of polyester and weather resistance when used outdoors. Further, among these, carbon black is more preferable in consideration of the similarity in color tone with the solar battery cell, the coloring power and economy of the color pigment, and the fact that there is almost no influence such as promoting the decomposition of the polyester.

  Moreover, it is preferable from a hydrolysis resistant viewpoint that the pigment layer of this invention does not contain the particle | grains (matting agent) which express a matting effect. Here, the particles exhibiting the matting effect represent particles having a particle size used in the matting layer described later, and the absence of such particles means 0.5% by weight or less based on the weight of the layer. Means within range.

[Matte layer]
The black polyester film for solar cells of the present invention has a configuration in which a pigment layer is used on the surface layer side and a matte layer is used on the core layer side. Since the polyester film of the present invention has a matte layer on the core layer side, the matte effect is exhibited on the pigment layer surface by the particle push-up effect by the core layer while maintaining the light concealment by the pigment layer. Due to the matte effect of the pigment layer surface, the fine wrinkles and fingerprints attached to the surface are not noticeable when bonded to a filler for solar cell modules such as EVA (ethylene vinyl acetate), and the light is concealed. Can be obtained, and a black polyester film suitable for a protective film of a solar cell, which is excellent in hydrolysis resistance, film appearance and concealment, can be obtained.

The matte layer in the present invention is a layer containing particles having a specific size and a black pigment, which will be described later, and includes particles of a specific size exhibiting a matte effect, and is therefore referred to as a matte layer in the present invention.
The matte layer is composed of the above-mentioned polyester, and in order to obtain the matteness (glossiness) of the present invention, the matte layer has an average particle size of 2.5 μm or more and 5.5 μm or less, In addition, it is necessary to contain particles having a maximum particle size of 16 μm or less in a range of 1.5 wt% to 4.5 wt% based on the weight of the layer.

  When the content of such particles is less than the lower limit, a sufficient matte appearance cannot be imparted to the pigment layer surface. On the other hand, when the content of the particles exceeds the upper limit, hydrolysis resistance, which is a characteristic required for a polyester film used in a solar cell, is remarkably reduced, and film forming property is lowered, and tearing is likely to occur. Film formation itself becomes difficult.

The particles contained in the matte layer preferably have a change in weight at 300 ° C. by the TG-DTA method of 0% to 3.0%, more preferably 0.5% to 2.5%. Preferably, it is preferably 1.0% or more and 2.0% or less.
Specifically, the high-temperature weight change of the particles is obtained by measuring a change in weight at 300 ° C. when the temperature is increased from 30 ° C. to 500 ° C. at a temperature increase rate of 10 ° C./min with a TG-DTA apparatus.

  The particles contained in the matte layer of the present invention may be inorganic particles or organic particles as long as they have such a particle size, and may be amorphous silica (colloidal silica), silica, talc, calcium carbonate, carbonic acid. Examples include magnesium, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, alumina, carbon black, titanium dioxide, kaolin, synthetic zeolite, crosslinked polystyrene particles, and crosslinked acrylate particles. Among these particles, synthetic zeolite is particularly preferable from the viewpoint of high temperature heat resistance stability.

  Further, in order to improve the light hiding property as a film, the matte layer also contains a black pigment in a range of 0.01 wt% to 2.4 wt% based on the weight of the layer, preferably 0.1 wt%. To 2.0 wt%, more preferably 0.5 to 1.8 wt%, and still more preferably 0.8 to 1.5 wt%.

[Glossiness]
The polyester film of the present invention has a glossiness (G ₆₀ ) of 20 to 50 on the pigment layer surface. The glossiness in the present invention is defined by values measured at 60 ° for both the incident angle and the light receiving angle in accordance with JIS standard Z8741. The polyester film of the present invention is characterized in that a matte effect is exhibited on the pigment layer surface by the particle push-up effect by the matte layer disposed on the core layer side, and the light shielding property and hydrolysis resistance are extremely good. At the same time, fine wrinkles generated when pasting with EVA (ethylene vinyl acetate) and other fillers for solar cell modules, fingerprints attached to the surface, etc. are inconspicuous, and the solar cell has excellent film appearance and concealment A black polyester film suitable for the back surface protective film can be obtained.

The lower limit of the glossiness of the pigment layer surface is preferably 25, more preferably 30. The upper limit of the glossiness is preferably 45, and more preferably 42. If the glossiness of the pigment layer surface is to be lowered from the lower limit value, the matte layer contains excessive particles, so that the film-forming property and the hydrolysis resistance required for the polyester film for solar cells are not sufficient. On the other hand, when the glossiness exceeds the upper limit value, fine wrinkles generated when pasted with a filler for solar cell modules such as EVA, fingerprints attached to the surface, etc. are easily noticeable on the pigment layer surface.
The polyester film of the present invention preferably has a matte surface glossiness (G ₆₀ ) of 20 or more and 50 or less.

[Concealment]
The black polyester film for solar cells of the present invention has an L * value in the L * a * b * color system of 13.5 or less, preferably 13.0 or less, more preferably 12.6 or less. When the L * value in the L * a * b * color system exceeds the upper limit value, the light hiding property required for the black film of the solar cell is not sufficiently exhibited. This L * characteristic is achieved by the fact that the pigment layer used as the surface layer contains a certain amount of black pigment and does not contain a matting agent, and further includes a certain amount of black pigment on the core layer side containing the matting agent. Is done.

[Hydrolysis resistance]
The black polyester film for solar cells of the present invention has a retention rate of breaking elongation before and after aging in an environment of a temperature of 121 ° C. and a humidity of 100% RH (hereinafter sometimes referred to as a breaking elongation retention rate) of 50%. Is preferably 72 hours or more, and the retention of the breaking elongation before and after aging in the same environment for 72 hours is preferably 52% or more, particularly It is preferably 58% or more.
The black polyester film for solar cells of the present invention has such a breaking elongation retention characteristic, so that the film of the present invention has excellent hydrolysis resistance even when used outdoors for a long time without causing deterioration over a long period of time. Good mechanical strength can be maintained.

[Film thickness]
The black polyester film for solar cells of the present invention preferably has a film thickness of 10 to 75 μm, more preferably 20 to 50 μm.

[Layer structure]
The black polyester film for a solar cell of the present invention has a layer structure in which at least two layers having a pigment layer as a surface layer and a mat layer as a core layer are laminated, the A layer as a pigment layer, and the B layer as a mat. In the case of a layer, not only the two-layer structure of A layer / B layer but also the three-layer structure of A layer / B layer / A layer and A layer / B layer are used as repeating units within the range not impairing the characteristics of the film. A multilayer structure may be adopted.
Moreover, regarding the thickness of the pigment layer in the present invention, the proportion of the A layer in the entire film is 5 to 50% in order to achieve both the matte properties, the light hiding properties and the hydrolysis resistance of the present invention. Preferably, it is 10 to 30%, more preferably 15 to 25%.

[Production of film]
The black polyester film for solar cells of the present invention can be produced according to a conventionally known film forming method. An example is shown below.
First, the polyester raw material of each layer is dried as necessary, and then each is melt-mixed with a separate extruder, laminated using a feed block, spread on a slit die, melt-extruded into a film shape, and a casting drum And solidified by cooling to form an unstretched film, and the obtained unstretched sheet is stretched in at least a uniaxial direction, preferably in a biaxial direction. The stretching may be sequential biaxial stretching or simultaneous biaxial stretching.

  For example, a method by sequential biaxial stretching will be described. An unstretched film is heated by roll heating, infrared heating, or the like, and stretched in the longitudinal direction (hereinafter, sometimes referred to as a mechanical axis direction, continuous film forming direction, or MD direction). To obtain a longitudinally stretched film. This stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. The stretching temperature in the machine direction is preferably a temperature equal to or higher than the glass transition point (Tg) of the polyester, and more preferably a temperature higher by Tg to 70 ° C. The film after longitudinal stretching is then subjected to lateral stretching (hereinafter sometimes referred to as the width direction and TD direction), and further subjected to heat fixing and thermal relaxation treatments as necessary to obtain a biaxially oriented film. However, these processes are performed while the film is running. The transverse stretching step starts at a temperature higher than the glass transition point (Tg) of the polyester and is performed while raising the temperature to a temperature higher by 5 to 70 ° C. than Tg. 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 draw ratio is preferably 2.8 to 4.0 times, more preferably 3.0 to 3.8 times in both the longitudinal direction and the transverse direction. If the draw ratio is less than the lower limit, film thickness unevenness may occur, which tends to lead to a decrease in heat resistance and hydrolysis resistance. On the other hand, if the stretching ratio exceeds the upper limit in both the longitudinal direction and the transverse direction, breakage tends to occur during film formation.

  The film after transverse stretching is dimensionally stable when the thermal shrinkage is lowered by heat treatment at (Tm-55) to (Tm-20) ° C. under a constant width or a width reduction of 10% or less while holding both ends of the film. Is preferable. Furthermore, when performing relaxation treatment, a known relaxation treatment method can be used.For example, in the process of returning the film temperature to room temperature after heat setting, both ends of the gripped film are cut off, and the take-up speed in the film longitudinal direction is adjusted. A method of relaxing in the vertical direction can be mentioned, and the heat shrinkage rate in the vertical direction can be further reduced. Further, as a method for further improving the dimensional stability in the lateral direction, a desired heat shrinkage rate can be obtained by reducing the lateral width in the process until the film ends are cut off.

[Method for adding black pigment]
In the present invention, the method for incorporating the black pigment into the polyester film is not particularly limited, and a conventionally known method can be used. For example, it can be added at any stage where the polyester is polycondensed, and it may be added preferably after the esterification stage or after completion of the transesterification reaction to advance the polycondensation reaction. Alternatively, a black pigment slurry dispersed in ethylene glycol or water and a polyester raw material may be blended using a kneading extruder with a vent.
In particular, a so-called master batch chip containing a high concentration of black pigment is produced using a kneading extruder, and if necessary, this master batch chip is kneaded and extruded with a polyester raw material containing no or a small amount of black pigment. It is most preferable to produce a polyester film containing a predetermined amount of black pigment by mixing using a machine.

[Black polyester film for solar cells]
The black polyester film for solar cells of the present invention can be suitably used as a solar cell protective film of a solar cell module, and can be particularly suitably used as a back surface protective film of a solar cell. For example, EVA (ethylene vinyl acetate) is used. It is preferable to be used in combination with the first filler for solar cell modules.

  The solar cell back surface protective film composed of the polyester film for solar cells of the present invention is excellent in matte properties, so that fine wrinkles generated when pasting with EVA or the like, and fingerprints attached to the surface are less noticeable, In addition, since the light hiding property is very good, a good black appearance can be obtained. In addition, the polyester film for solar cell of the present invention has a half-break time of breaking elongation of 72 hours or more before and after aging in an environment of a temperature of 121 ° C. and a humidity of 100% RH, and has high durability. The protective function as a film can be maintained for a long time, and the power generation efficiency of the solar cell can be maintained over a long period of time.

The evaluation method is shown below.
(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) Glossiness (G ₆₀ )
Based on JIS standard (Z8741), it measured using the gloss meter "VGS-SENSOR" by Nippon Denshoku Industries Co., Ltd. With respect to the pigment layer surface, both the incident angle and the light receiving angle were measured at N = 5 at 60 °, and the average value was used. Further, the matte property was judged according to the following criteria.
◎: 20 or more and 35 or less ・ ・ ・ Excellent mattness ○: More than 35 and 50 or less ・ ・ ・ Good mattness ×: More than 50 ・ ・ ・ Poor mattness

(3) Average particle diameter, maximum particle diameter (d ₉₈ )
The particles were stirred with a mixer so as to have a concentration of 3% by weight in ethylene glycol, and measured using a laser scattering particle size distribution analyzer SALD-7000 manufactured by Shimadzu Corporation. The 50% volume particle size (D50) was determined from the particle size distribution measurement result, and this was used as the average particle size.
The maximum particle size was a median particle diameter d _98. Note the value of d ₉₈ is the particle diameter at 98% cumulative particle size distribution curve.

(4) TG-DTA evaluation Weight reduction of particles at 300 ° C. when the temperature was raised from 30 ° C. to 500 ° C. at a rate of temperature increase of 10 ° C./min using a TG-DTA apparatus (SII, SSC5200 TG / DTA220). It calculated | required from rate (%).

(5) Evaluation of light concealment property Measured by a white plate reflection method using an automatic color difference meter (SE-6000) manufactured by Nippon Denshoku Industries Co., Ltd., and L * value in the L * a * b * color system was obtained. The matte property was judged according to the following criteria.
○: 13.5 or less ・ ・ ・ Extremely good light concealment × More than 13.5 ・ ・ ・ Inadequate light concealment

(6) Evaluation of hydrolysis resistance Using strip-shaped sample pieces cut to a length of 100 mm in the vertical direction and a width of 10 mm in the horizontal direction, the film was aged for 72 hours in an environmental test machine set at a temperature of 121 ° C. and a humidity of 100% RH. I do. Five samples subjected to aging were prepared, and the breaking elongation in the longitudinal direction of the sample was measured by n = 5, and the average value of breaking elongation after aging for 72 hours was determined from the average value. The tensile test was performed using Toyo Baldvin (trade name “Tensilon”), and the initial chuck distance was 50 mm and the tensile speed was 50 mm / min. Similarly, the breaking elongation was measured at five points for the test piece before aging in the environmental testing machine, and the average value of the initial breaking elongation was obtained from the average value thereof. The value obtained by dividing the average value of the five points of rupture elongation after aging for 72 hours by the average value of the initial rupture elongation is the rupture elongation retention [%] after aging for 72 hours, and is resistant to hydrolysis according to the following criteria: Sex was evaluated.
○: Retention rate at break after treatment for 72 hours in an environment of 121 ° C. and 100% RH is 50% or more. Extremely good hydrolysis resistance ×: Treatment for 72 hours in an environment of 121 ° C. and 100% RH Breaking elongation retention rate is less than 50%.

[Example 1]
Polyethylene terephthalate (inherent viscosity 0.63 dl / g) is used as a polyester raw material, and as shown in Table 1, carbon black is added as a black pigment so as to be 1.2% by weight based on the layer weight of the pigment layer (A layer). In addition, carbon black as a black pigment and synthetic zeolite having an average particle size and a maximum particle size shown in Table 1 as particles are 1.2% by weight, respectively, based on the layer weight of the matte layer (B layer). Add to 0% by weight, feed from two extruders each heated to 280 ° C, and merge the A-layer polymer and B-layer polymer using a two-layer feedblock device that is A / B Then, while maintaining the laminated state, the sheet was melt-extruded from a die on a rotary cooling drum maintained at 20 ° C. to obtain an unstretched film. Stretched 3.6 times in the longitudinal direction at ℃, then stretched 3.5 times in the transverse direction at 125 ℃, and further heat-fixed at 220 ℃, the layer A thickness 10 μm, layer B thickness 40 μm, total layer thickness 50 μm A biaxially stretched polyester film was obtained. The evaluation results of the obtained film are shown in Table 2.

[Example 2]
Biaxial stretching was carried out in the same manner as in Example 1 except that a three-layer feed block device in which the layered structure of the pigment layer (A layer) and matte layer (B layer) was A / B / A was used. A biaxially stretched polyester film having a subsequent layer thickness of 5 μm / 40 μm / 5 μm (total layer thickness 50 μm) was obtained. The evaluation results of the obtained film are shown in Table 2.

[Example 3]
A film having a total thickness of 50 μm was obtained in the same manner as in Example 1 except that the content of carbon black in the pigment layer (A layer) was 2.4% by weight. The evaluation results of the obtained film are shown in Table 2.

[Comparative Examples 1-3]
The same operation as in Examples 1 to 3 was performed except that particles were added to the pigment layer (A layer) and no particles were used in the matte layer (B layer) to obtain a film having a total thickness of 50 μm. . Since Comparative Examples 1 to 3 contained carbon black, which is a black pigment, and particles in the same layer, the light hiding property and the hydrolysis resistance were insufficient, which was unsuitable as a black polyester film for solar cells.

[Comparative Examples 4 to 5]
The same operation as in Example 1 was performed except that particles of different types and particle sizes were added to the pigment layer (A layer) and no particles were used in the matte layer (B layer). A film was obtained. In Comparative Examples 4 to 5, since carbon black, which is a black pigment, and particles were contained in the same layer, the light hiding property was insufficient, and was unsuitable as a black polyester film for solar cells.

[Comparative Example 6]
A film having a thickness of 50 μm was obtained by performing the same operation as in Example 1 except that the single layer design having only the pigment layer was adopted. Since Comparative Example 6 did not contain particles of a specific size, the glossiness was insufficient, and was unsuitable as a black polyester film for solar cells.

[Comparative Examples 7-8]
A film having a thickness of 38 μm and 18 μm was obtained in the same manner as in Example 1 except that carbon black and particles were contained, and amorphous silica having a small average particle diameter was used as the particles. . In Comparative Examples 7 to 8, carbon black, which is a black pigment, and inorganic particles are contained in the same layer, so that the light shielding property and hydrolysis resistance are insufficient, and sufficient glossiness is not exhibited. It was unsuitable as a black polyester film for protection.

[Comparative Examples 9 to 10]
A film having a thickness of 50 μm was obtained in the same manner as in Example 1 except that the average particle diameter of the particles used for the matte layer (B layer) was changed. In Comparative Example 9, since the average particle size of the particles was small, the glossiness of the pigment layer (A layer) was insufficient. Further, Comparative Example 10 had insufficient hydrolysis resistance due to the large average particle size of the particles.

  According to the present invention, even when used for a long time in a harsh natural environment such as high temperature and high humidity, the deterioration of mechanical properties is suppressed, the hydrolysis resistance is excellent, and at the same time, matte and concealment properties are achieved. It is possible to provide a black polyester film for a solar cell suitable for a back surface protective film of a solar cell that has a good appearance and protection function even when used for a long time.

Claims (5)

It is a polyester film in which at least two layers having a pigment layer as a surface layer and a matte layer as a core layer are laminated,
(1) The pigment layer contains a black pigment in a range of 0.5 wt% to 2.4 wt% based on the weight of the layer,
(2) The matte layer has a mean particle size of 2.5 μm to 5.5 μm and a maximum particle size of 16 μm or less based on the weight of the layer in a range of 1.5% to 4.5% by weight. And a black pigment in a range of 0.01 wt% to 2.4 wt%,
The glossiness (G ₆₀ ) of the pigment layer surface is 20 or more and 50 or less, and the L * value in the L * a * b * color system of the polyester film is 13.5 or less. Black polyester film.   The black polyester film for solar cells according to claim 1, wherein a change in weight of the particles contained in the matte layer at 300 ° C by a TG-DTA method is 0% or more and 3.0% or less.   The black polyester film for solar cells according to claim 1 or 2, wherein the particles contained in the matte layer are synthetic zeolite.   The black polyester film for solar cells according to any one of claims 1 to 3, wherein the polyester film has an elongation at break half time of 72 hours or more in an environment of a temperature of 121 ° C and a humidity of 100% RH.   The black polyester film for solar cells according to any one of claims 1 to 4, which is used for protecting a back surface of a solar cell.