Transparent protective sheet for solar cell modules

Abstract

To provide a transparent protective sheet for a solar cell module that has a polyester resin as a base material resin, the transparent protective sheet achieving both transparency and weatherability at a higher level than before, the protective sheet having, as for the transparency, specifically a light transmittance of 90.0% or more in a wavelength of 400 nm or more and 1200 nm or less.SOLUTION: A transparent protective sheet 6 for a solar cell module includes a base material layer 60 containing a polyester resin as a base resin, and a weather resistant layer 63 laminated on one face of the base material layer 60 with a weather resistant adhesive layer 66 therebetween. In the transparent protective sheet 6, the refractive index of the weather resistant layer 63 is smaller than the refractive index of the base material layer; the difference between the refractive index of the weather resistant adhesive layer 66 and the refractive index of the weather resistant layer 63, and the difference between the refractive index of the weather resistant adhesive layer 66 and the refractive index of the base material layer 60 are both 0.20 or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a transparent protective sheet for a solar cell module, and more particularly to a transparent protective sheet that can be disposed as any of the outermost layers in a solar cell module. [Background technology]

[0002] In recent years, solar cells have been attracting attention as a clean energy source due to the growing awareness of environmental issues. In general, a solar cell module constituting a solar cell is configured by laminating a transparent front substrate, a front sealing material, a solar cell element, a back sealing material, and a protective sheet in this order from the light receiving surface side, and has a function of generating electricity when sunlight is incident on the solar cell element. The protective sheet is not only used as a back surface protective sheet, but may also be used as a front surface protective sheet in place of a transparent front substrate such as glass.

[0003] Solar cell modules are used outdoors for long periods of time, and therefore each of the components constituting the solar cell module is required to have durability capable of withstanding the harsh outdoor environment for a long period of time. In particular, the protective sheet is required to have high weather resistance.

[0004] Conventionally, fluorine-based films have been widely used as protective sheets for solar cell modules, which require high weather resistance. However, in response to the demand for reducing manufacturing costs and weight, a coating-type protective sheet with enhanced weather resistance has been proposed, which uses a polyester resin such as polyethylene terephthalate (PET) as a base sheet (see Patent Document 1).

[0005] Meanwhile, in recent years, there has been a demand for protective sheets that not only maintain the above-mentioned weather resistance but also have transparency as protective sheets for use in so-called see-through type or double-sided light-receiving type solar cell modules (see Patent Document 2).

[0006] Transparent protective sheets for solar cell modules are required to have high levels of weather resistance and transparency. To meet this requirement, various transparent protective sheets have been developed that have a multilayer structure in which a weather-resistant layer for imparting high weather resistance is further laminated onto a base layer formed of a transparent thermoplastic resin or the like (see Patent Document 3).

[0007] However, when the transparent protective sheet has a multi-layer structure, the light reflection at the interface caused by the difference in the refractive index between each layer prevents the improvement of transparency, so that the current situation is that there is a certain structural limit to the improvement of transparency in the transparent protective sheet having a multi-layer structure to ensure weather resistance. The refractive index of polyester resin is about 1.65 for general polyethylene terephthalate (PET) and about 1.77 for general polyethylene naphthalate (PEN). The light transmittance of the laminate of these resins at wavelengths of 400 nm to 1200 nm was limited to about 80 to 85% on the upper side.

[0008] In this specification, "transparent" means that "light in the visible light region and the near infrared region can be transmitted", and more specifically, means that the light transmittance at wavelengths of 400 nm to 1200 nm is at least 80%. Conventional transparent protective sheets can also be said to be transparent at this level, but the transparent protective sheet of the present invention has a high degree of transparency (light transmittance of 90.0% or more) that far exceeds this level. In addition, in this specification, "light transmittance" means the light transmittance at wavelengths of 400 nm to 1200 nm measured in accordance with JIS-K-7105 or JIS-K-7136, unless otherwise specified. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Special Publication No. 2010-519742 [Patent Document 2] JP 2012-040842 A [Patent Document 3] JP 2012-040842 A Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a transparent protective sheet for solar cell modules that uses a polyester-based resin as a base resin and combines a higher level of transparency and weather resistance than conventional methods, specifically, a transparent protective sheet having a light transmittance of 90.0% or more in the wavelength range of 400 nm or more and 1200 nm or less. [Means for solving the problem]

[0011] As a result of intensive research into solving the above problems, the present inventors discovered that the above problems can be solved by optimizing the refractive indexes of all layers, including the adhesive layers interposed between the layers, based on the refractive index of the base layer of the transparent protective sheet, and further by disposing a resin layer with a relatively small refractive index as the weather-resistant layer that comes into contact with the outside air, and thus completed the present invention. More specifically, the present invention provides the following.

[0012] (1) A transparent protective sheet for a solar cell module, comprising: a substrate layer whose base resin is a polyester-based resin having a refractive index of 1.50 or more and 1.60 or less; and a weather-resistant layer laminated on one side of the substrate layer via a weather-resistant adhesive layer, wherein the refractive index of the weather-resistant layer is 1.35 or more and 1.45 or less, and the difference between the refractive index of the weather-resistant adhesive layer and the refractive index of the weather-resistant layer, and the difference between the refractive index of the weather-resistant adhesive layer and the substrate layer bonded to the weather-resistant adhesive layer are all 0.20 or less.

[0013] (2) The transparent protective sheet according to (1), wherein the substrate layer is a multi-layer laminate in which a plurality of substrate resin sheets made of the polyester-based resin are laminated via substrate adhesive layers, and the difference between the refractive index of the substrate adhesive layer and the refractive index of the substrate resin sheet facing the substrate adhesive layer is 0.01 or less.

[0014] (3) The transparent protective sheet according to (2), wherein each of the base resin sheets is made of a hydrolysis-resistant polyethylene terephthalate resin.

[0015] (4) The transparent protective sheet according to any one of (1) to (3), wherein an easy-adhesion layer is formed on the other surface of the base layer, and the difference in refractive index between the easy-adhesion layer and the base layer is 0.05 or less.

[0016] (5) The transparent protective sheet according to any one of (1) to (4), which has a light transmittance of 90.5% or more at a wavelength of 400 nm or more and 1200 nm or less.

[0017] (6) A solar cell module comprising a solar cell element and an adhesive, the transparent protective sheet according to any one of (1) to (5) being disposed as the outermost layer.

[0018] (7) A solar cell module comprising a solar cell element and an adhesive, in which the transparent protective sheet according to (4) is disposed as an outermost layer, and the adhesive is laminated to the easy-adhesion layer of the transparent protective sheet.

[0019] (8) The solar cell module according to (6) or (7), wherein the adhesive is a polyethylene resin or an ethylene-vinyl acetate copolymer resin. Effect of the Invention

[0020] According to the present invention, it is possible to provide a transparent protective sheet for a solar cell module that uses a polyester-based resin as a base resin and combines a higher level of transparency and weather resistance than conventional ones, specifically, a transparent protective sheet having a light transmittance of 90.0% or more in the wavelength range of 400 nm or more and 1200 nm or less. [Brief description of the drawings]

[0021] [Figure 1] FIG. 2 is a cross-sectional view showing an example of the layer structure of a solar cell module using the transparent protective sheet for a solar cell module of the present invention. [Diagram 2] FIG. 2 is a cross-sectional view showing an example of the layer structure of the transparent protective sheet for a solar cell module of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The transparent protective sheet for a solar cell module of the present invention will be described in detail below. The present invention is not limited to the embodiments described below.

[0023] <Basic structure of solar cell module> First, the basic structure of a solar cell module in which the transparent protective sheet of the present invention is used will be described. Fig. 1 is a cross-sectional schematic diagram showing an example of the layer structure of a solar cell module 1 according to one embodiment of the present invention. As shown in Fig. 1, the solar cell module 1 is configured by sequentially stacking a transparent front substrate 2, a front sealing material 3, a solar cell element 4, a rear sealing material 5, and a transparent protective sheet 6 from the light-receiving surface side. The transparent protective sheet in the present invention is not necessarily limited to being disposed on the non-light-receiving surface side, and as described above, it can also be used as a front-side transparent protective sheet disposed on the light-receiving surface side instead of a transparent front substrate made of glass.

[0024] <Transparent protective sheet> The transparent protective sheet 6 of the present invention will be described with reference to Fig. 2. The transparent protective sheet 6 is a protective sheet for a solar cell module, and is assumed to be disposed on any one of the outermost layers of the solar cell module 1. Therefore, it is required to have high weather resistance in addition to transparency.

[0025] The transparent protective sheet 6 is a resin sheet having a multi-layer structure, and is configured to include at least a base layer 60 and a weather-resistant layer 63 laminated on one surface thereof via a weather-resistant adhesive layer 66. In addition, it is preferable that an easy-adhesion layer 64 is formed on the other surface of the base layer 60 to improve adhesion with an adhesive for the solar cell module, such as a back sealing material 5.

[0026] The base material layer 60 may be a single-layer base material resin sheet, but is more preferably a laminate having a multi-layer structure in which a first base material layer 61 and a second base material layer 62 are laminated via a base material adhesive layer 65 as shown in FIG. 2. When the base material layer 60 has such a multi-layer structure, for example, a more preferable layer structure may be one in which the first base material layer 61 arranged on the weather-resistant layer 63 side is a layer having higher weather resistance than the second base material layer 62. Specifically, an example may be one in which the first base material layer 61 and the second base material layer 62 are made of the same type of polyester resin, while only the first base material layer 61 is made of a resin that has been subjected to an annealing treatment.

[0027] The transparent protective sheet 6 having the above basic structure has a light transmittance of 90.0% or more, preferably 90.5% or more, at a wavelength of 400 nm or more and 1200 nm or less. The above light transmittance of each layer constituting the transparent protective sheet 6 does not necessarily have to be 90.0% or more in all layers. As will be described in detail below, the transparent protective sheet 6 is characterized in that the reflectance at the interface between the layers is suppressed by optimizing the overall design so that the difference in refractive index between the resin layers that are in close contact with each other at the interface between the layers of the multilayer structure is within a predetermined range, thereby realizing a high level of transparency as a whole of the transparent protective sheet, that is, the above light transmittance, in terms of the layer structure. A specific description of the above embodiment in which the difference in refractive index between the resin layers is within a predetermined range will be given below together with the details of each layer.

[0028] When the transparent protective sheet 6 is integrated into the solar cell module 1, the weather-resistant layer 63 is exposed on the outermost layer side of the solar cell module 1. When the easy-adhesion layer 64 is formed, the easy-adhesion layer 64 is used so as to be an adhesive surface with other resin substrates such as the back sealing material 5.

[0029] [Base material layer] In order to provide the transparent protective sheet 6 with the above-mentioned high degree of transparency, the base layer 60, which is its main constituent part, is naturally required to have high transparency. Specifically, it is preferable that the light transmittance in the wavelength range of 400 nm or more and 1200 nm or less is 88% or more.

[0030] The substrate layer 60 is required to have not only the above-mentioned transparency but also a refractive index within a specific range specified in the present application. Specifically, a resin having a refractive index in the range of 1.50 to 1.60 can be used as the resin constituting the substrate layer 60 of the transparent protective sheet 6. The "refractive index" refers to the value obtained by dividing the light speed c in a vacuum by the light speed v in the resin substrate (medium) for light with a wavelength of 589.3 nm (D line of sodium). As a transparent resin that satisfies the requirements for the refractive index and is also excellent in weather resistance, hydrolysis-resistant polyethylene terephthalate (HR-PET) can be preferably used as the resin constituting the substrate layer 60 in the transparent protective sheet 6.

[0031] Here, in a protective sheet for a solar cell module, a resin material that is generally transparent and also has the weather resistance required for a protective sheet, taking into consideration economic efficiency, processability, and the like, can be used to form a base layer, and examples of such resins include polyester-based resins such as polyethylene naphthalate (PEN) and polyethylene terephthalate (PET). However, most general polyester-based resins usually have a refractive index of more than 1.60. The refractive index of general PET is about 1.65, and the refractive index of general PEN is about 1.77. Therefore, these general-purpose polyester-based resins are not suitable as resins for forming the base layer 60 of the transparent protective sheet 6. The transparent protective sheet 6 is a unique concept that focuses on the refractive index of each layer, which has been overlooked in conventional protective sheets for solar cell modules, and specifies the polyester resin constituting the base layer 60 as a polyester resin having a specific refractive index (refractive index of 1.50 or more and 1.60 or less), and as a preferred specific example, hydrolysis-resistant polyethylene terephthalate (HR-PET) having a refractive index of substantially about 1.58, which is a notable difference from the resin material selection in conventional general protective sheets. It goes without saying that a polyester transparent resin having the above refractive index conditions and used in place of HR-PET is also within the scope of the present invention.

[0032] In addition, as a resin material having the weather resistance required for a protective sheet for a solar cell module, a fluororesin may be selected. However, since fluororesins are soft and stretchy, they are prone to problems such as stretching during drying after application of an adhesive or the like. In this respect, polyester resins are more advantageous as resins for forming the base layer of a protective sheet for a solar cell module.

[0033] When the substrate layer 60 has a multilayer structure in which a first substrate layer 61 and a second substrate layer 62 are laminated via a substrate adhesive layer 65 as shown in FIG. 2, a transparent polyester-based resin having a refractive index in the range of 1.50 or more and 1.60 or less, such as HR-PET, can be used for both the first substrate layer 61 and the second substrate layer 62, as described above.

[0034] The base resin sheet constituting the base layer 60 can be obtained by forming a film from the above-mentioned HR-PET or the like using an extrusion method, a cast molding method, a T-die method, a cutting method, an inflation method, or other film forming methods. The thickness of the base layer 60 is not particularly limited, but the total thickness is preferably 200 μm or more and 300 μm or less. If the total thickness of the base layer 60 is less than 200 μm, it is not preferable in terms of reduced insulation. Also, if this thickness exceeds 300 μm, it is not preferable in terms of processability.

[0035] For the above reasons, when the total thickness of the base material layer 60 is set to 200 μm or more, it can be formed of a single layer base material resin sheet having a thickness of 200 μm or more, but it is more preferable to form it by laminating multiple base material resin sheets having a thickness of 100 μm or more. Generally, when a base material resin sheet having PET as a base resin is formed into a sheet having a large thickness of about 150 μm or more, the stretching ratio decreases during film formation, thereby decreasing the molecular orientation, and increasing the risk of not fully exhibiting hydrolysis resistance. In addition, in this case, curling may occur during production due to the difference in orientation between the surface layer and the center of the base material resin sheet. Such a risk can be avoided by forming the base material layer 60 into a multilayer sheet in which base material resin sheets having a thickness of 100 μm or more and 150 μm or less are laminated.

[0036] The base layer 60 may contain components other than the above resins within a range that does not impair the effects of the present invention. In addition, for example, various plastic compounding agents, additives, other resins, etc. can be added for the purpose of improving or modifying processability, heat resistance, light resistance, weather resistance, mechanical properties, dimensional stability, antioxidant properties, slipperiness, mold releasability, flame retardancy, antifungal properties, electrical properties, etc. The amount of these additives to be added is not particularly limited, and can be added arbitrarily depending on the purpose. Examples of general additives include lubricants, crosslinking agents, antioxidants, ultraviolet absorbers, light stabilizers, fillers, lubricants, reinforcing fibers, reinforcing agents, antistatic agents, flame retardants, flame retardants, foaming agents, antifungal agents, modifying resins, etc.

[0037] (Base adhesive layer) When the base layer 60 has a multi-layer structure as shown in FIG. 2, the base adhesive layer 65 that bonds the first base layer 61 and the second base layer 62 is also required to be transparent. In addition, in the transparent protective sheet 6, the base adhesive layer 65 is not only transparent, but also its refractive index is adjusted to a specific range in relation to each resin layer that constitutes the base layer 60. As a result, when the base layer 60 has a multi-layer structure, the transparent protective sheet 6 reduces the light reflectance at each interface between the base adhesive layer 65 and the first base layer 61 and the second base layer 62, thereby maintaining the high transparency of the transparent protective sheet 6. Specifically, the refractive index of the base adhesive layer 65 is set so that the difference in refractive index between the base adhesive layer 65 and the base resin sheets that constitute the first base layer 61 and the second base layer 62 facing the base adhesive layer 65 is 0.01 or less.

[0038] The refractive index of each layer, including the adhesive layers constituting the transparent protective sheet 6, such as the substrate adhesive layer 65 and the weather-resistant adhesive layer 66 described below, can be measured in accordance with JIS K 7142 using an Abbe refractometer DR-M2 manufactured by Atago Co., Ltd. More specifically, the refractive index of the adhesive layer and primer layer is measured by coating the layer on a release film with a Mayer bar, drying the layer, and then coating and drying the layer again repeatedly to prepare a measurement sample with a thickness of about 200 μm, which can then be measured in accordance with JIS K 7142 using an Abbe refractometer DR-M2 manufactured by Atago Co., Ltd.

[0039] As an adhesive for forming the substrate adhesive layer 65 that is transparent and has a refractive index within the above range, for example, a "two-component type laminate adhesive consisting of a base agent and a curing agent, which includes a mixture of a specific polyurethane diol and an aliphatic polycarbonate diol" disclosed in JP 2012-79868 A can be used.

[0040] The main component of the adhesive is characterized by including a mixture of polyurethane diol and aliphatic polycarbonate diol. The polyurethane diol and aliphatic polycarbonate diol constituting the main component are both polyols having hydroxyl groups, and can form the substrate adhesive layer 65 by reacting with a curing agent having an isocyanate group.

[0041] The polyurethane diol, which is the main component, is a polyurethane having a urethane structure as a repeating unit and having hydroxyl groups at both ends. The hydroxyl value of the polyurethane diol is preferably in the range of 10 to 50 mgKOH / g. The polyurethane diol is obtained by reacting an aliphatic polycarbonate diol, 1,6-hexanediol, and isophorone diisocyanate as the main component of the adhesive to improve its adhesiveness and weather resistance.

[0042] The aliphatic polycarbonate diol may be a commercially available product. In order to obtain an adhesive having excellent durability, weather resistance, heat resistance, and hydrolysis resistance, for example, an aliphatic polycarbonate diol having a number average molecular weight of 1000 (manufactured by Asahi Kasei Chemicals Corporation, product name "Duranol T5651") or an aliphatic polycarbonate diol having a number average molecular weight of 2000 (manufactured by Asahi Kasei Chemicals Corporation, product name "Duranol T5662") may be suitably used.

[0043] In addition, the solvent that can be used when reacting an aliphatic polycarbonate diol, 1,6-hexanediol, and isophorone diisocyanate is not particularly limited as long as it can dissolve these compounds and does not react with the solvent. From the viewpoints of compatibility and processability during lamination, examples of the solvent that can be used include carboxylic acid ester-based solvents such as ethyl acetate.

[0044] The thickness of the adhesive layer may be appropriately changed depending on the transparency and adhesive strength required for the transparent protective sheet 6, and may be, for example, from 1.0 μm to 10 μm.

[0045] [Weatherproof layer] The weather-resistant layer 63 is a layer laminated on the outermost layer of the transparent protective sheet 6 to protect the base layer 60. The weather-resistant layer 63 is formed by laminating a weather-resistant resin sheet (weather-resistant resin sheet) on one surface of the base layer 60 via a weather-resistant adhesive layer 66. In order to maintain the transparency of the transparent protective sheet 6, the weather-resistant layer 63 is required to have the same transparency as the base layer 60. Furthermore, the resin constituting the weather-resistant layer 63 of the transparent protective sheet 6 is a resin whose refractive index is in the range of 1.30 or more and 1.50 or less, preferably 1.35 or more and 1.45 or less.

[0046] Generally, a fluororesin that combines transparency and excellent weather resistance is used as a resin for forming the weather-resistant layer of a transparent protective sheet. Specific examples of such fluororesins include ETFE (tetrafluoroethylene-ethylene copolymer), PCTFE (polychlorotrifluoroethylene), PFA (tetrafluoroethylene-perfluoroalkylvinyl-ester copolymer), PTFE (polytetrafluoroethylene), PVDF (polyvinylidene fluoride), etc., and in the transparent protective sheet 6, ETFE, which has a refractive index of about 1.4, can be preferably used as a weather-resistant resin sheet for forming the weather-resistant layer 63.

[0047] (Weather-resistant adhesive layer) The weather-resistant adhesive layer 66 that bonds the weather-resistant layer 63 to the surface of the base material layer 60 is also required to have the same transparency as the above-mentioned base material adhesive layer 65. In addition, in the transparent protective sheet 6, the weather-resistant adhesive layer 66 is used not only to have transparency but also to have a refractive index within a specific range relative to the refractive index of the weather-resistant layer 63. As a result, the transparent protective sheet 6 reduces the light reflectance at each interface between the weather-resistant adhesive layer 66 and the base material layer 60 and the weather-resistant layer 63, and maintains the high transparency of the transparent protective sheet 6. Specifically, the refractive index of the weather-resistant adhesive layer 66 is set so that the difference in refractive index between the weather-resistant layer 63 facing the base material adhesive layer 65 and the substrate layer 60 bonded to the substrate adhesive layer 65 is 0.20 or less. In addition, when the substrate layer 60 is composed of a first substrate layer 61 and a second substrate layer 62 as shown in Figure 2, the above ``difference in refractive index'' means the difference in refractive index between the weather-resistant adhesive layer 66 and the first substrate layer 61 that is in contact with the weather-resistant adhesive layer 66.

[0048] The adhesive for forming the weather-resistant adhesive layer 66, which is transparent and has a refractive index within the above range, may be the same as the adhesive constituting the above-mentioned substrate adhesive layer, or may be a two-component laminate adhesive made of a base material and a curing agent, including a mixture of a specific polyurethane diol and an aliphatic polycarbonate diol, to which an ultraviolet absorber has been added, and which has excellent weather resistance.

[0049] [Easy adhesive layer] The easy-adhesion layer 64 is a so-called primer layer, and is formed on the other surface of the substrate layer 60 of the transparent protective sheet 6 opposite to the surface on which the weather-resistant layer 63 is laminated, as shown in FIG. 2. The easy-adhesion layer 64 is also required to have the same transparency as the substrate layer 60 in order to maintain the transparency of the transparent protective sheet 6. The easy-adhesion layer 64 of the transparent protective sheet 6 is formed so that the difference in refractive index between the easy-adhesion layer and the substrate layer 60 is 0.05 or less. As a primer composition constituting the easy-adhesion layer 64 having such a refractive index range, a primer composition containing an olefin resin and having an aqueous medium as the main solvent can be particularly preferably used in the transparent protective sheet 6. In addition, when the substrate layer 60 is composed of a first substrate layer 61 and a second substrate layer 62 as shown in FIG. 2, the above-mentioned "difference from the refractive index" means the difference in refractive index between the easy-adhesion layer 64 and the second substrate layer 62 in contact with it.

[0050] The olefin resin contained in the primer composition (hereinafter, also simply referred to as "primer composition") used to form the easy-adhesion layer 64 is preferably an acid-modified polyolefin resin containing an olefin component and an unsaturated carboxylic acid component. The acid-modified polyolefin resin preferably has an MFR of 0.01 g / 10 min or more and less than 100 g / 10 min at 190° C. and a load of 2.16 kg, measured in accordance with JIS K7210. This allows the easy-adhesion layer 64 to be formed, which maintains good dispersibility in an aqueous medium at the primer composition stage and has excellent adhesion to the olefin resin that forms the back sealing material 5.

[0051] Furthermore, it is clear from the description in, for example, JP 2013-74172 A that the easy-adhesion layer 64 of the present invention can significantly increase the adhesion between, for example, a base material layer 60 made of a polyester-based resin, and a rear sealing material 5 made of an olefin-based resin such as EVA or low-density polyethylene.

[0052] The olefin component, which is the main component of the acid-modified polyolefin resin contained in the primer composition, is not particularly limited. Alkenes having 2 to 6 carbon atoms, such as ethylene, propylene, isobutylene, 2-butene, 1-butene, 1-pentene, and 1-hexene, are preferred, and mixtures thereof can also be used. The acid-modified polyolefin resin contained in the primer composition is a polyolefin resin that has been acid-modified with an unsaturated carboxylic acid component. Examples of the unsaturated carboxylic acid component include acrylic acid and methacrylic acid.

[0053] The content of the unsaturated carboxylic acid component in the acid-modified polyolefin resin is 0.1% by mass or more and 10% by mass or less, preferably 0.2% by mass or more and 8% by mass or less. If the content of the unsaturated carboxylic acid component is less than 0.1% by mass, the dispersibility in the aqueous dispersion tends to be insufficient. On the other hand, if the content exceeds 10% by mass, the adhesion to the olefin resin forming the back sealing material 5 and the water resistance tend to decrease.

[0054] The number average particle size of the acid-modified polyolefin resin in the aqueous dispersion containing the acid-modified polyolefin resin is preferably 0.5 μm or less, more preferably 0.01 to 0.4 μm, and particularly preferably 0.02 to 0.3 μm, from the viewpoint of adhesion and dispersion stability. The pH of the aqueous dispersion containing the acid-modified polyolefin resin is preferably in the range of pH 7 to 12, more preferably pH 8 to 11, from the viewpoint of dispersion stability.

[0055] Specific examples of acid-modified polyolefin resins include ethylene-(meth)acrylic acid ester-(anhydride)maleic acid copolymers such as ethylene-ethyl acrylate-(anhydride)maleic acid copolymer and ethylene-butyl acrylate-(anhydride)maleic acid copolymer, ethylene-propylene-(meth)acrylic acid ester-(anhydride)maleic acid copolymer, ethylene-butene-(meth)acrylic acid ester-(anhydride)maleic acid copolymer, propylene-butene-(meth)acrylic acid ester-(anhydride)maleic acid copolymer, ethylene-propylene-butene-(meth)acrylic acid ester-(anhydride)maleic acid copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-maleic anhydride copolymer, ethylene-propylene-(anhydride)maleic acid copolymer, ethylene-butene-(anhydride)maleic acid copolymer, propylene-butene-(anhydride)maleic acid copolymer, and ethylene-propylene-butene-(anhydride)maleic acid copolymer.

[0056] In the primer composition, it is necessary to use the acid-modified polyolefin resin as an aqueous dispersion. The acid-modified polyolefin resin can be processed into an aqueous dispersion by dispersing it in an aqueous medium. As a dispersion method, a known dispersion method such as a self-emulsification method or a forced emulsification method may be adopted. As an aqueous dispersion containing an acid-modified polyolefin resin, it is preferable from the viewpoint of adhesion to use an anionic aqueous dispersion obtained by neutralizing the unsaturated carboxylic acid component of the acid-modified polyolefin resin in an aqueous medium with a basic compound.

[0057] In the primer composition, the aqueous medium used as the main solvent when dispersing the acid-modified polyolefin resin in water is water or a medium consisting of a liquid containing water. Specifically, the solvent of the primer composition uses an aqueous medium as the main solvent, and the content of the aqueous medium relative to 100 parts by mass of the solvent is more than 50 parts by mass and 100 parts by mass or less, preferably 70 parts by mass or more and 100 parts by mass or less.

[0058] In addition, the primer composition may further contain an organic solvent having a specific range of SP value as an auxiliary solvent in the aqueous dispersion containing the acid-modified polyolefin resin, within a range that does not inhibit the development of the above-mentioned effects of the present invention. Specifically, when the solvent of the primer composition contains an organic solvent as an auxiliary solvent, the content of the organic solvent relative to 100 parts by mass of the solvent is 0 parts by mass or more and less than 50 parts by mass, preferably 0 parts by mass or more and less than 30 parts by mass. In addition, it is preferable to appropriately select this organic solvent from the viewpoint that it has an SP value that reduces the solubility of the resin according to the SP value of the main resin constituting the infrared-transmitting dark layer. For example, if polycarbonate polyurethane, which generally has an SP value of about 10, is used as the main resin of the infrared-transmitting dark layer, an organic solvent having an SP value of about 12 can be used as an auxiliary solvent as necessary. Representative examples of such organic solvents include isopropyl alcohol (SP value: 11.9), 1-propanol (same: 11.9), ethanol (same: 12.7), etc.

[0059] In addition, as a non-volatile dispersion aid for promoting the aqueous dispersion of the acid-modified polyolefin resin, for example, an emulsifier, a compound having a protective colloid action, a modified wax, an acid-modified compound with a high acid value, a water-soluble polymer, etc. can be appropriately added. In addition to the non-volatile dispersion aid, various additives that can be added to the primer composition to further improve its physical properties and other added resins, more specifically, the non-volatile dispersion aids disclosed in the above patent document (JP Patent Publication No. 2014-240174) and various other additives can be appropriately and preferably used.

[0060] [Refractive index of each layer] As described above, in the transparent protective sheet 6, the difference in refractive index between each resin layer is set to be within a predetermined range. Specifically, based on the refractive index of HR-PET used as the base layer being about 1.58, a resin having a refractive index of 1.45 or less is placed in the weather-resistant layer 63, which is the interface with the outside air when integrated into the solar cell module 1, to reduce the theoretical reflectance calculated by the following formula 1 at the outermost surface, and further minimize the difference in refractive index at other interfaces to minimize the reflectance at the interfaces. As an example, the reflectance (%) at the interface between air (refractive index: 1.00) and the weather-resistant layer 63 (refractive index: 1.45) is {(1.00-1.45) / (1.00+1.45)} 2 × 100 = 3.4 (%). By setting the difference in refractive index between each adhesive layer and each resin substrate facing it within the above-mentioned range, the reflectance at each interface can be set to less than 0.1%.

[0061] Reflectance (R)=((n1-n2) / (n1+n2) )) 2 (n1 and n2 are the refractive indices of the resin substrates that are in contact with each other at the interface) (Equation 1)

[0062] <Solar cell module manufacturing method> The solar cell module 1 can be manufactured, for example, by sequentially laminating the members consisting of the transparent front substrate 2, the front sealing material 3, the solar cell element 4, the back sealing material 5, and the transparent protective sheet 6, integrating them by vacuum suction or the like, and then thermocompression molding the members into an integrally molded body by a molding method such as lamination. For example, when using vacuum thermal lamination, the lamination temperature is preferably within a range of 130°C to 190°C. The lamination time is preferably within a range of 5 to 60 minutes, and particularly preferably within a range of 8 to 40 minutes. In this manner, the solar cell module 1 can be manufactured by thermocompression molding the layers into an integrally molded body. EXAMPLES

[0063] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0064] <Manufacturing of transparent protective sheets> Transparent protective sheets of Examples and Comparative Examples were produced using the following resin sheets, material compositions, adhesives, etc. The layer structure of each transparent protective sheet was as shown in Table 1. First, each resin sheet was bonded to a laminate using the following adhesive, and these were used as the transparent protective sheets of Example 1 and Comparative Example. However, for Example 2, an easy-adhesion layer was further formed on the above laminate by the following method to produce the transparent protective sheet.

[0065] (base material layer) As the base resin sheet constituting the base layer, a hydrolysis-resistant polyethylene terephthalate (HR-PET) film (AP (manufactured by Teijin DuPont Films)) having a thickness of 125 μm or 250 μm was used. In Table 1, the film having a thickness of 125 μm is indicated as "PET125" and the film having a thickness of 250 μm is indicated as "PET250". Note that only the base resin sheet constituting the base layer of Comparative Example 3 used a polyethylene naphthalate (PEN) film having a thickness of 250 μm (Q51 (manufactured by Teijin DuPont Films)), which is indicated as "PEN" in Table 1.

[0066] (weather resistant layer) A 25 μm thick ETFE (tetrafluoroethylene-ethylene copolymer) film (Afflex 25ND (manufactured by Asahi Glass Co., Ltd.)) was used as the weather-resistant resin sheet constituting the weather-resistant layer. This is shown in Table 1 as "ETFE25."

[0067] (Easy adhesion layer) The following primer composition was used as a material for forming an easy-adhesion layer on the substrate layer. The primer composition was applied in an amount of 1.0 g / m 2 The resulting mixture was gravure coated on the surface of the base layer, and a coating film was formed to form an easy-adhesion layer having a thickness of 1.0 μm. This is shown in Table 1 as “Pr1”. Primer composition: A primer composition consisting of 59 parts by mass of water, 18 parts by mass of acid-modified polyolefin resin, less than 1 part by mass of acrylic resin, 18 parts by mass of IPA, less than 1 part by mass of triethylamine, and 3 parts by mass of wax.

[0068] (Adhesive layer) An adhesive for forming a substrate adhesive layer and a weather-resistant adhesive layer was manufactured by the following steps (a) to (c). The adhesive was dissolved in ethyl acetate, gravure coated so that the film thickness after curing would be 5.0 μm, and cured by aging treatment at 30 to 50°C for 70 to 200 hours. This is indicated as "AD" in Table 1. (a) Manufacturing of the main component In a nitrogen atmosphere, 100 parts by mass of an aliphatic polycarbonate diol having a number average molecular weight of 1000 (manufactured by Asahi Kasei Chemicals Corporation, product name "Duranol T5651"), 1,6-hexanediol (5 parts by mass), isophorone diisocyanate (27.5 parts by mass), and ethyl acetate (132.5 parts by mass) were added to a flask equipped with a stirrer. -1 The mixture was heated under reflux until the absorption of isocyanate disappeared, to obtain a 50% solution of polyurethane diol. As the aliphatic polycarbonate diol, which is the main component, an aliphatic polycarbonate diol (manufactured by Asahi Kasei Chemicals Corporation, product name "DURANOL T5651") was prepared. The main component was prepared by using the polyurethane diol and aliphatic polycarbonate diol produced above, which are the main component components. The main component was prepared by mixing 15 parts by mass of the aliphatic polycarbonate diol with 100 parts by mass of the polyurethane diol. (b) Manufacturing of hardeners The curing agent materials used were a nurate form of isophorone diisocyanate and a hexamethylene diisocyanate-based bifunctional polyurethane diisocyanate (Duranate D101, manufactured by Asahi Kasei Chemicals Corporation). The blending ratio (mass) of the nurate form of isophorone diisocyanate: the hexamethylene diisocyanate-based bifunctional polyurethane diisocyanate was 40:60. The blending ratio (mass) is the solid mass ratio not including the solvent, but was adjusted to 50% solids during production. (c) Mixing the base resin and hardener The adhesive was manufactured using the base agent and hardener manufactured above. The base agent and hardener were mixed by dissolving them in a solvent to make each 50% by mass (ethyl acetate solution). The hardener was added with 3-glycidoxypropyltrimethoxysilane as a silane coupling agent at a ratio of 1.2% relative to the total adhesive.

[0069] <Evaluation of transparent protective sheets> To evaluate the transparency of each of the transparent protective sheets of the Examples and Comparative Examples, a JASCO V670 was used to measure light transmittance at wavelengths of 400 nm or more and 1200 nm or less in accordance with JIS K7361-1: 1997. The results are shown in Table 1.

[0070] [Table 1]

[0071] <Refractive index of each layer of the transparent protective sheet and reflectance at the interface between each layer> As a reference test to confirm the correlation between the configuration and the action and effect of the present invention, the refractive index of each layer of the transparent protective sheet of Example 3 was measured by the measurement method described in <paragraph 0038>, and the theoretical value of the reflectance at each interlayer interface was calculated by formula 1 described in <paragraph 0059>. The measurement and calculation results are shown in Table 2. It was assumed that the weather-resistant layer was in contact with the outside air (air), and the easy-adhesion layer was in contact with an encapsulant made of EVA resin to form an interface. The refractive index of other resin sheets was also measured, and for PEN, it was 1.77.

[0072] [Table 2]

[0073] From Tables 1 and 2, it can be seen that the transparent protective sheet of the present invention has extremely high transparency, even though it is a multi-layer protective sheet in which functional layers are laminated to enhance weather resistance and preferably adhesion to substrates. [Explanation of symbols]

[0074] 1. Solar cell module 2 Transparent front board 3 Front sealing material 4. Solar cell elements 5 Rear sealing material 6 Transparent protective sheet 60 Base material layer 61 1st base layer 62 2nd base layer 63 Weatherproof layer 64 Easy adhesive layer 65 Base adhesive layer 66 Weatherproof adhesive layer

Claims

1. A transparent protective sheet for a solar cell module, A base layer having a polyester resin as a base resin; A weather-resistant adhesive layer; a weather-resistant layer which is a resin layer having a refractive index smaller than that of the base layer, the weather-resistant layer is an outermost layer of the transparent protective sheet, The difference between the refractive index of the base layer and the refractive index of the weather-resistant adhesive layer is 0.01 or less, The light transmittance at a wavelength of 400 nm or more and 1200 nm or less is 90.0% or more. Transparent protective sheet.

2. The total light reflectance (R) at the interface between the weather-resistant adhesive layer and each of the weather-resistant layers calculated by the following formula (1) is less than 0.1%; The transparent protective sheet according to claim 1 . (Formula 1) Reflectance (R) = ((n1-n2) / (n1+n2)) 2 (n1 and n2 are the refractive indices of the resin substrates that are in close contact with each other at the interface)

3. The refractive index of the weather-resistant layer is 1.35 or more and 1.45 or less. The transparent protective sheet according to claim 1 or 2.

4. The refractive index of the base layer is 1.50 or more and 1.60 or less. The transparent protective sheet according to claim 1 .

5. A solar cell module comprising a solar cell element and a sealing material, the transparent protective sheet according to claim 1 being disposed as an outermost layer.

6. The sealing material is a polyethylene resin or an ethylene-vinyl acetate copolymer resin. The solar cell module according to claim 5 .

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