JP2015124226A - Easily adhesive polyester film for solar cell back protection sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easily adhesive polyester film for a solar cell back protection sheet, having excellent mechanical properties, heat resistance and moisture resistance, and exhibiting excellent adhesion to EVA even with short-time crosslinking.SOLUTION: Provided is a polyester film having a coating film of 20 to 100 nm thickness disposed on at least one surface of a polyethylene terephthalate film, the coating film being formed of a coating liquid containing, based on mass of the coating liquid component (excluding the solvent), 50 to 90 mass% of an aqueous polyurethane resin and 3 to 20 mass% of an epoxy-based crosslinking agent represented by the following formula.

Description

  The present invention relates to an easily-adhesive polyester film for a solar cell back surface protective film, and more specifically, an ethylene-vinyl acetate copolymer that is provided with an easily-adhesive coating film on at least one surface of a polyester film and is a sealing resin for solar cells ( Hereinafter, it may be abbreviated as EVA) and relates to an easily-adhesive polyester film for solar cell back surface protective film exhibiting excellent adhesion and a solar cell back surface protective film using the same.

  In recent years, a solar power generation system has been spreading as one of power generation means using clean energy. In general, a solar cell module has a structure in which a plurality of plate-like solar cell elements are sandwiched between a light-receiving side glass substrate and a back surface protective film, and an internal gap is filled with a sealing resin. As the sealing resin, EVA resin is used because of its high transparency and excellent moisture resistance.

As a protective film on the back side of the solar cell, for example, Japanese Patent Application Laid-Open No. 11-261085 and Japanese Patent Application Laid-Open No. 11-186575 describe that a polyethylene resin, a polyester resin sheet, or a fluororesin film is used. Yes. However, these protective films do not necessarily have sufficient adhesion to EVA, and there remains concern about long-term durability.
By the way, a polyester film, in particular, a biaxially stretched film of polyethylene terephthalate or polyethylene naphthalate has excellent mechanical properties, heat resistance, and moisture resistance, and has excellent performance as a solar cell back surface protective film. However, a polyester film, particularly a polyester film that is biaxially stretched and highly oriented and crystallized, has an inactive surface and poor adhesion to EVA.

  Recently, the production of solar cells has increased explosively, and its productivity is emphasized. In particular, EVA that can be crosslinked for a short time called fast cure and enables mass production of solar cells has come to be widely used, and it is required to produce more solar cells in a short time. However, the biaxially stretched polyester film has particularly poor adhesion to the fast cure type EVA, and cannot be used without surface modification.

  In order to improve the adhesion between the polyester film and EVA, Japanese Patent Application Laid-Open No. 2006-152013 proposes coating a resin film containing a crosslinking agent on the surface of the polyester film. Japanese Patent Laid-Open No. 2006-175664 proposes to provide a coating film using a water-based polyurethane resin and a crosslinking agent on the surface of a polyester film. However, the adhesion when the polyester film and Fast Cure EVA which are proposed in these are laminated by short-time cross-linking does not last for long-term outdoor use of solar cells, and in particular, film production using aqueous resin The thin coating film sometimes provided has poor moisture resistance and has a problem in long-term adhesion with EVA.

JP-A-11-261085 JP-A-11-186575 JP 2006-152013 A JP 2006-175664 A

  The present invention solves such problems of the prior art, and has excellent mechanical properties, heat resistance, and moisture resistance, and has excellent adhesion to EVA even for short-time crosslinking, and is an easily adhesive polyester for solar cell back surface protective film. The object is to provide a film.

  As a result of intensive studies to solve the above problems, the present inventors have used a urethane resin as a resin and a specific epoxy compound as a cross-linking agent even in the case of a thin coating film provided at the time of film production using an aqueous resin. As a result, it was found that the adhesion to the fast cure type EVA and its long-term durability were good, and the present invention was completed.

  That is, an object of the present invention is a solar cell back surface comprising a biaxially stretched polyester film having a thickness of 38 to 300 μm made of polyethylene terephthalate substantially free of a copolymer component and a coating film having a thickness of 20 to 100 nm provided on at least one side. A film for protective film, wherein the coating film is represented by 50 to 90% by mass of an aqueous polyurethane resin and 3 to 20% by mass of the following formula (Chemical Formula 1) based on the mass of the coating liquid component (excluding the solvent). It is achieved by an easy-adhesive polyester film for solar cell back surface protective film formed from a coating liquid containing the epoxy-based crosslinking agent A.

Moreover, the easy-adhesive polyester film for solar cell back surface protective film of the present invention has, as a preferred embodiment, a water-based polyurethane resin is a polyester-based polyurethane resin having an aromatic polyester skeleton, and an aromatic component constituting the aromatic polyester skeleton. Is an aromatic dicarboxylic acid component, the proportion of which is 70 to 90 mol% based on the total of the aromatic dicarboxylic acid component and the isocyanate component constituting the polyurethane resin, and the coating film is the mass of the coating liquid component ( And a coating solution containing 50 to 90% by mass of an aqueous polyurethane resin, 3 to 20% by mass of the epoxy-based crosslinking agent A and 0.5 to 4% by mass of glycerin polyglycidyl ether. Rutile-type titanium oxide particles in the biaxially stretched polyester film 2% by mass, the COOH end group concentration of the film is 3 to 30 equivalent / ton, the weight average molecular weight is 38000 to 58000, the melting subpeak (Tsm) obtained by differential scanning calorimetry is 215 to 230 ° C., biaxial The stretched polyester film is a laminated polyester film composed of two or three layers of a surface layer (A) and a base material layer (B), and the surface layer (A) contains 10 to 20% by mass of rutile-type titanium oxide particles. The base material layer (B) contains 0.3 to 5% by mass of rutile-type titanium oxide particles, the base material layer (B) has a thickness of 75 to 94% of the total film thickness, The COOH end group concentration is 3 to 30 equivalents / ton, the weight average molecular weight is 38000 to 58000, and the melting subpeak (Tsm) obtained by differential scanning calorimetry is 215 to 230 ° C. Embodiments comprising at least any one of also encompassed.
Moreover, the solar cell back surface protective film comprised only from the easily adhesive polyester film for solar cell back surface protective films of this invention is also included by this invention.

  The easily-adhesive polyester film for solar cell back surface protective film of the present invention has excellent mechanical properties, heat resistance and moisture resistance, and is excellent in long-term adhesion with EVA even if it is a thin coating film. Even when laminated with EVA by time cross-linking, it has good long-term adhesion.

Hereinafter, the present invention will be described in detail.
<Biaxially stretched polyester film>
[polyethylene terephthalate]
The polyester constituting the biaxially stretched polyester film of the present invention (hereinafter sometimes abbreviated as a polyester film) is made of polyethylene terephthalate containing substantially no copolymer component. Here, the phrase “substantially free of copolymerization component” means that 97 mol% or more of all repeating units constituting polyethylene terephthalate are composed of ethylene terephthalate units. You may do it. When the ratio of the ethylene terephthalate unit is less than 97 mol%, it is not preferable because a decrease in heat resistance due to a decrease in the melting point of polyester and a decrease in hydrolysis resistance due to a decrease in crystallinity occur.

  The component copolymerized with polyethylene terephthalate may be a dicarboxylic acid component or a diol component. Examples of the dicarboxylic acid component of the copolymer component include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid and naphthalenedicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid and sebacic acid, and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid. An acid can be illustrated. Examples of the diol component of the copolymer component include aliphatic diols such as butanediol, hexanediol, and diethylene glycol, and alicyclic diols such as cyclohexanedimethanol.

The polyester of the present invention may contain fine particles as a lubricant, if necessary, in order to improve the rollability of the film at the time of film formation, the transportability of the film in the back surface protective film processing step for solar cells, and the like. . As such fine particles, either inorganic fine particles or organic fine particles may be used. Examples of inorganic fine particles include calcium carbonate, aluminum oxide, kaolin, silicon oxide, and zinc oxide. Examples of the organic fine particles include crosslinked acrylic resin particles, crosslinked polystyrene resin particles, urea resin particles, melamine resin particles, and crosslinked silicone resin particles.
Furthermore, a colorant, an antistatic agent, an antioxidant, a hydrolysis inhibitor such as an end-capping agent, a catalyst, and the like may be optionally added as long as the film characteristics are not impaired.

[Lamination structure of film]
The polyester film of the present invention may be a single layer film or a multilayer film using a plurality of extruders. In the case of a multilayer film, it is preferably a film having a two-layer structure of A / B or a three-layer structure of A / B / A.

[Film thickness]
The total thickness of the polyester film of the present invention is suitably in the range of 38 to 300 μm for the solar cell back surface protective film, the lower limit is preferably 50 μm, particularly preferably 75 μm, and the upper limit is preferably 250 μm, particularly preferably 200 μm. is there. When the total thickness is less than the lower limit, the long-term reliability is deteriorated. On the other hand, when the total thickness exceeds the upper limit, the economical efficiency is deteriorated.

[Titanium oxide]
The polyester film of the present invention more preferably has a function of reflecting the light that has entered the solar cell back surface protective film and returning it to the solar cell element to improve the conversion efficiency of the solar cell. It is desirable to be reflective. In order to achieve this reflection characteristic, the polyester film is preferably colored white. As the pigment for coloring the polyester film white, titanium oxide, barium sulfate, calcium carbonate, and the like are conceivable. Among these, titanium oxide is preferable. Rutile type titanium oxide is more preferable because it can suppress discoloration of the polyester film and deterioration of mechanical properties due to ultraviolet rays.

When the polyester film has a single layer configuration, the preferable range of the content of titanium oxide is 0 to 18% by mass, preferably 5 to 15% by mass. If it exceeds 18% by mass, the film becomes brittle and it is difficult to obtain practical mechanical strength.
On the other hand, in the case of a multilayer structure, it is composed of a surface layer (A) containing 10 to 20% by mass of rutile-type titanium oxide and a base material layer (B) containing 0.3 to 5% by mass, and (A) / (B ) Or a three-layer structure of (A) / (B) / (A). From the viewpoint of achieving both hydrolysis resistance and reflection characteristics of the polyester film, such a multilayer structure is preferable to a single layer structure. If the titanium oxide content of the surface layer (A) is less than 10% by mass, it is difficult to obtain a highly reflective film, and if it exceeds 20% by mass, the mechanical strength of the layer decreases. Moreover, even if the titanium oxide content of the base material layer (B) is less than 0.3% by mass, the reflection characteristics of the film are lowered, and when it exceeds 5% by mass, the hydrolysis resistance of the film is liable to be lowered. Furthermore, the preferable thickness range of the base material layer (B) is 75 to 94% of the total film thickness. If it is less than 75%, it becomes difficult to ensure the hydrolysis resistance of the film. On the other hand, if it exceeds 94%, a highly reflective film cannot be obtained.

[Concentration of terminal carboxyl group of film]
In the polyester film of the present invention, the terminal carboxyl group concentration of polyethylene terephthalate in the film is preferably 3 to 30 equivalent / ton, more preferably 3 to 25 equivalent / ton, and particularly preferably 3 to 20 equivalent / ton. When the terminal carboxyl group concentration exceeds 30 equivalents / ton, the hydrolysis resistance of the film is poor, and the mechanical properties of the film tend to deteriorate when used for a long time under high temperature and high humidity conditions. On the other hand, a film of less than 3 equivalents / ton is difficult to manufacture even if an end-capping agent is used, and is not practical. In addition, when the polyester film of this invention is comprised with a some polyester layer, it is preferable that the terminal carboxyl group density | concentration measured as the whole polyester film becomes the above-mentioned range, and also has the terminal carboxyl group density | concentration also about each layer. More preferably.

[Weight average molecular weight of film]
In the polyester film of the present invention, the polyethylene terephthalate in the film preferably has a weight average molecular weight of 38,000 to 58,000. When the weight average molecular weight is less than 38,000, the hydrolysis resistance of the film becomes insufficient. On the other hand, in order to produce a polyester film exceeding 58,000, a polyester raw material with a higher degree of polymerization is required, the polymerization time becomes longer, and the melt viscosity of the raw material is high in the film production process, so that the production efficiency is high. However, it is uneconomical. In addition, when the polyester film of the present invention is composed of a plurality of polyester layers, the weight average molecular weight measured as the whole polyester film is preferably in the above-mentioned range, and each layer also has such a weight average molecular weight. Further preferred.
In order to obtain a polyester film having such a molecular weight range, it is preferable to use a raw material polyethylene terephthalate having an intrinsic viscosity of 0.60 to 0.85 dl / g. The intrinsic viscosity of polyethylene terephthalate is a value obtained from a measured value at 35 ° C. after being dissolved in a phenol: tetrachloroethane mixed solvent having a mass ratio of 6: 4.

[Film Melting Subpeak Tsm]
The polyester film of the present invention preferably has a melting subpeak temperature (Tsm) of 215 to 230 ° C. The melting subpeak temperature is a minute endothermic peak that appears before crystal melting by differential scanning calorimetry, and this melting subpeak (Tsm) is observed as a minute peak at a temperature corresponding to the heat fixing temperature of the film. This is because an incomplete portion (pseudocrystal) of the formed crystal structure is melted. When the melting sub-peak temperature is lower than 215 ° C., the mechanical strength in the thickness direction of the film is lowered and the film tends to be broken, or the film has a high heat shrinkage rate and may be difficult to process. On the other hand, when the temperature exceeds 230 ° C., the hydrolysis resistance of the film is inferior, and the mechanical properties of the film are likely to deteriorate when used for a long time under high temperature and high humidity conditions.

<Coating film>
The coating film provided on one side of the polyester film of the present invention is formed by applying a coating liquid containing an aqueous polyurethane resin and an epoxy-based crosslinking agent A described later.

[Waterborne polyurethane resin]
The aqueous polyurethane resin used in the present invention is not particularly limited as long as it can be dissolved, emulsified or dispersed in water, but a polyester-based polyurethane resin formed from a polyester polyol and a polyisocyanate is preferable, and an aromatic dicarboxylic acid is particularly preferable. A polyester-based polyurethane resin having an aromatic polyester skeleton formed from an aromatic polyester polyol using an acid component and a polyisocyanate is preferable. When such a polyester polyurethane resin, particularly a polyester polyurethane resin having an aromatic polyester skeleton, is used, the adhesion between the polyester film and the coating film is improved.
The aromatic dicarboxylic acid component used in the aromatic polyester polyol is not particularly limited, but terephthalic acid and isophthalic acid are preferable, and the alcohol component is not particularly limited, but ethylene glycol, diethylene glycol, and polyethylene glycol are preferable.

In the polyester polyol, preferably the aromatic polyester skeleton-containing polyol, the aromatic component constituting the aromatic polyester skeleton is an aromatic dicarboxylic acid component, and the proportion thereof constitutes the aromatic dicarboxylic acid component and the polyurethane resin. It is preferably used in the range of 70 to 90 mol%, more preferably 80 to 90 mol%, based on the total with the isocyanate component. By setting it as this range, it is possible to maintain high adhesion strength with the polyester of the base material without impairing the moisture resistance of the coating film.
The ratio of the aqueous polyurethane resin in the coating liquid is 50 to 90% by mass, preferably 50 to 80% by mass, based on the mass of the coating liquid component (excluding the solvent). When the ratio of the aqueous polyurethane resin is less than 50% by mass, the adhesion with EVA becomes insufficient, which is not preferable. On the other hand, if it exceeds 90% by mass, the long-term durability of the coating film becomes insufficient, such being undesirable.

[Epoxy crosslinking agent A]
The epoxy-based crosslinking agent A used in the present invention is N, N, N ′, N′-tetraglycidyl-metaxylylenediamine, N, N, N ′, N ′ represented by the above chemical formula (Formula 1). -It is important that it is tetraglycidyl-1,3-bis (aminomethyl) cyclohexane or N, N, N ', N'-tetraglycidyl-4,4'-diamino-diphenylmethane, and in the coating liquid It is important that the ratio is 3 to 20% by mass, preferably 5 to 16% by mass, based on the mass of the coating liquid component (excluding the solvent). By doing so, it is possible to combine both excellent moisture resistance that does not cause deterioration of the coating film and decrease in adhesion strength due to the influence of heat and moisture, and excellent adhesion to the polyester substrate. If the ratio of the epoxy-based crosslinking agent A is less than 3% by mass, sufficient adhesion strength cannot be obtained, and the effect of improving adhesion durability (moisture resistance durability) becomes insufficient, which is not preferable. On the other hand, when it exceeds 20% by mass, it is difficult to form a coating film. As a result, the adhesion strength with EVA may be lowered, and the adhesion with a polyester film is also lowered, which is not preferable.

In the present invention, in addition to the epoxy-based crosslinking agent A, other compounds having an epoxy group (sometimes referred to as epoxy-based crosslinking agent B) are 0 based on the mass of the coating liquid component (excluding the solvent). You may use together in the range of 0.5-4 mass%. The compound may be a tri- or higher functional polyepoxy compound, a diepoxy compound, or a monoepoxy compound, but glycerin polyglycidyl ethers such as glycerin diglycidyl ether and glycerin triglycidyl ether are particularly preferable.
In addition, when using together epoxy-type crosslinking agent A and B, it is preferable to adjust so that the sum total may be 3.5-24 mass% on the basis of the mass of a coating liquid component (except a solvent).

[Other coating film components]
In addition to the water-based polyurethane resin and the epoxy-based crosslinking agent, the coating film of the present invention contains fine particles having an average particle diameter of 20 to 200 nm for the purpose of preventing film blocking, and the mass of the coating liquid component (excluding the solvent). You may contain 3-20 mass% as a reference | standard. When the content of the fine particles is less than 3% by mass, the anti-blocking effect is reduced. On the other hand, when it exceeds 20 mass%, adhesiveness with EVA may fall. Moreover, if the particle size of the fine particles is less than 20 nm, the anti-blocking effect is poor, and if it exceeds 200 nm, the adhesion strength with EVA is lowered, or the particles are likely to fall off the coating film.

Moreover, from a viewpoint of improving the adhesiveness of a coating film and EVA, it may contain water-based polyolefin resin, and it is preferable that it is the range of 10-40 mass% on the basis of the mass of a coating liquid component (except a solvent). . By setting it as this range, it becomes easy to achieve both the long-term durability of the coating and the adhesion to EVA.
Furthermore, a wetting agent, an antistatic agent, a colorant, a surfactant, an ultraviolet absorber and the like may be contained.

[Thickness of coating film]
The thickness of the coating film comprising the above-mentioned components needs to be 20 to 100 nm, preferably 20 to 60 nm. When the thickness of the coating film is less than 20 nm, it is not preferable because sufficient adhesion strength with EVA cannot be obtained. On the other hand, even if it is thicker than 100 nm, the adhesion strength with EVA is not increased any more, and it is not only excessive quality, but also the adhesion strength may be lowered.

<Film manufacturing method>
The manufacturing method of the easily-adhesive polyester film for solar cell back surface protective films of this invention demonstrated above is not specifically limited, It can manufacture based on a conventionally well-known film forming method. An example is shown below.

First, the raw material polyester is melt-extruded into a film form from a slit die, cooled and solidified by a casting drum to form an unstretched film, and the resulting unstretched sheet is stretched in the biaxial direction. Stretching may be sequential biaxial stretching or simultaneous biaxial stretching. For example, in the case of sequential biaxial stretching, an unstretched film is heated by roll heating, infrared heating, or the like, and stretched in the longitudinal 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 film after longitudinal stretching is subsequently subjected to lateral stretching and heat setting to form a biaxially stretched film, which is performed while the film is running. In the case of a multilayer structure, after drying the raw materials of each layer as necessary, each is melt-mixed with a separate extruder, laminated using a feed block, and then developed on a slit die to obtain an unstretched film Implement by method.
As a method of adjusting the amount of heat shrinkage of the obtained film and making it easy to use in subsequent processing steps, heat treatment is performed while holding the both ends of the film after transverse stretching while keeping the width constant or 10% or less. It is also possible to use a method of cutting in the longitudinal direction by cutting off both ends of the film being gripped in the process of returning the film temperature to room temperature after heat setting, and adjusting the take-up speed in the longitudinal direction of the film.

The easily adhesive polyester film for solar cell back surface protective film of the present invention is provided with a coating film formed from a coating liquid containing the above component on at least one surface of the polyester film. In the above-described film forming method, this film is preferably applied by applying an aqueous liquid containing the above components at a stage where the polyester film can be stretched, and then drying, stretching, and heat-treating as necessary. . The coating liquid component concentration of this aqueous liquid is usually 30% by mass or less, and more preferably 10% by mass or less. Moreover, the lower limit of the coating liquid component concentration of the aqueous liquid is preferably 2% by mass, more preferably 3% by mass.
The stretchable polyester film is an unstretched polyester film, a uniaxially stretched polyester film, or a biaxially stretched polyester film. Of these, a longitudinally stretched polyester film uniaxially stretched in the film extrusion direction (longitudinal direction) is particularly preferred.

When applying the aqueous coating liquid to the film, as a pretreatment for improving the coating properties, the film surface is subjected to physical treatment such as corona surface treatment, flame treatment, plasma treatment, etc. It is preferable to use a coating liquid containing a chemically inert surfactant.
Such a surfactant promotes the wetting of the aqueous coating liquid onto the polyester film. For example, polyoxyethylene alkylphenyl ether, polyoxyethylene-fatty acid ester, sorbitan fatty acid ester, glycerin fatty acid ester, fatty acid metal soap, Examples include anionic and nonionic surfactants such as alkyl sulfates, alkyl sulfonates, and alkyl sulfosuccinates. It is preferable that 1-10 mass% of surfactant is contained in the coating liquid component. If it is this range, the surface tension of a coating liquid can be 40 mN / m or less, and the repellency of a coating liquid can be prevented.

Any known coating method can be applied as the coating method. For example, a roll coating method, a gravure coating method, a roll brush method, a spray coating method, an air knife coating method, an impregnation method, and a curtain coating method can be used alone or in combination. The coating amount of the coating liquid is preferably 0.5 g to 20 g, more preferably 1 to 10 g per 1 m ² of the running film. The aqueous liquid is preferably used as an aqueous dispersion or emulsion.
The stretchable polyester film coated with the aqueous liquid is led to a drying and stretching treatment step. Preferred conditions for such treatment include, for example, drying at a temperature of 90 to 130 ° C. for 2 to 10 seconds, and the stretching temperature is 90 to 140 ° C., the stretching ratio is 3.0 to 5.0 times in total in both the longitudinal direction and the transverse direction, and the heat setting is 215 to 250 ° C., particularly 225 to 240 ° C. and 2 to 60 seconds.

<Manufacture of backside protective film>
The easily adhesive polyester film for solar cell back surface protective film of the present invention can be used alone as a solar cell back surface protective film, and a laminate laminated with other sheets is used as a solar cell back surface protective film. It is also possible. When the polyester film of the present invention is used alone as a solar cell back surface protective film, it is preferably a white film containing rutile-type titanium oxide as described above, and the content is more within the above range. preferable. As an example of laminating with other sheets to form a solar cell back surface protective film, a laminated body bonded with a film made of a highly weather resistant resin such as polyvinyl fluoride or a high molecular weight polyester film for the purpose of improving durability. For example, a laminate in which a water vapor barrier film is bonded for the purpose of imparting water vapor barrier properties can be exemplified. When these are laminated on the polyester film of the present invention, they are laminated on the opposite surface of the coating film.

  Hereinafter, the present invention will be described in more detail with reference to 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 coating film After fixing the film with an embedding resin, the section was cut with a microtome, stained with 2% osmic acid at 60 ° C. for 2 hours, and then using a transmission electron microscope (JEM 2010 manufactured by JEOL Ltd.) It was measured.

(3) Adhesion strength with EVA A 450 μm thick EVA (CIK FH type) is coated on both sides with a polyester film coating, and reduced pressure at 150 ° C. for 1 minute using a desktop vacuum laminator PVL0202S manufactured by Nisshinbo Mechatronics. After that, it was laminated by pressing at 150 ° C. for 12 minutes. When laminating, the part not to be laminated was left as a part for starting a peel test, and the polyester films on both sides were peeled from this part, and the peel strength was measured. The peeling test was performed under the conditions of a sample width of 15 mm, a peeling angle of 90 ° (T-shaped peeling), and a peeling speed of 200 mm / min.
A: Peel strength is 50 N / 15 mm or more B: Peel strength is 30 N / 15 mm or more and less than 50 N / 15 mm X: Peel strength is less than 30 N / 15 mm

(4) Durability for adhesion to EVA (moisture resistance of coating)
After the laminate sample prepared in (3) above was treated for 1000 hours in an atmosphere of 85 ° C. and 85% RH, the peel strength was measured in the same manner as in (3) above, and compared with the peel strength before treatment. evaluated. In addition, it was set as (triangle | delta) about the case where durability of film base material itself is scarce and peeling strength cannot be measured.
◎: Adhesion strength retention 75% or more ○: Adhesion strength retention 50% or more, less than 75% ×: Adhesion strength retention 50% or less

(5) Concentration of terminal carboxyl group of film 10 mg of a sample was dissolved in 0.5 ml of a mixed solvent of HFIP (hexafluoroisopropanol): deuterated chloroform = 1: 3, a few drops of isopropylamine were added, and ¹ H-NMR method (50 (C, 600 MHz).

(6) Weight average molecular weight of the film 0.5 ml of HFIP: chloroform (1: 1) was added to 1 mg of the film sample and dissolved (overnight), 9.5 ml of chloroform was added immediately before the measurement, and the membrane filter was 0.1 μm. And GPC analysis was conducted. Measuring equipment and conditions are as follows.
GPC: HLC-8020 manufactured by Tosoh Detector: UV-8010 manufactured by Tosoh Column: TSK-gel GMHHR · M × 2 manufactured by Tosoh Mobile phase: chloroform for HPLC Flow rate: 1.0 ml / min
Column temperature: 40 ° C
Detector: UV (254 nm)
Injection volume: 200 μl
Sample for calibration curve: Polystyrene (EasyCal “PS-1” manufactured by Polymer Laboratories)

(7) Melting sub-peak Tsm of film
A DSC curve was drawn with a film sample of 20 mg and a heating rate of 20 ° C./min using Q100 manufactured by TA Instruments, and the endothermic peak at a lower temperature side than the clear endothermic peak due to crystal melting was defined as the melting subpeak temperature. .

(8) Layer constitution ratio of film A sample was cut into a triangle, fixed in an embedding capsule in a vertical direction, and then embedded in an epoxy resin. And after making the embedded sample into a 50 nm-thick thin-film slice with a microtome (ULTRACUT-S), it was observed and photographed using a scanning electron microscope (trade name: S-2400, manufactured by Hitachi, Ltd.), and photograph The thickness of each layer was measured.

[Examples 1-5 and Comparative Examples 1-2]
A polyethylene terephthalate chip having an intrinsic viscosity of 0.78 was obtained by an ordinary melt polycondensation method followed by a solid phase polycondensation method (PET-a). Subsequently, 50 parts by mass of the obtained polymer (PET-a) and 50 parts by mass of rutile titanium oxide particles TCR-52 (average particle size 0.2 μm) manufactured by Sakai Chemical Co., Ltd. were melt-kneaded to obtain a master of titanium oxide. A pellet was obtained (PET-b).
PET-a and PET-b were mixed at 70:30, the chip was dried to reduce the water content to 50 ppm or less, melted at 285 ° C., melt-extruded on a cooled drum, and made into an unstretched film. Next, the film was stretched 3.2 times in the longitudinal direction at 100 ° C. to obtain a longitudinally uniaxially stretched film. The aqueous coating liquid shown in Table 1 was uniformly applied to one side of the film with a coating liquid thickness of 2.0 μm using a roll coater. Subsequently, this film was stretched 3.4 times in the transverse direction in an atmosphere of 120 ° C. Thereafter, heat setting was performed in a tenter at 230 ° C. for 15 seconds, and then a width of 4.0% was inserted in the transverse direction, and then both ends were cut off and relaxed at a relaxation rate of 2.0% in the longitudinal direction. After cooling to room temperature, a white polyester film having a thickness of 188 μm was obtained. In addition, the coating film thickness of the obtained film was 30 nm. The characteristics of the obtained film are shown in Tables 1 and 2. In Table 2, those with at least one x were determined to be comprehensive evaluation x.

In addition, the coating liquid components in Table 1 are as follows.
Aqueous polyurethane: polyester system in which aromatic carboxylic acid and isocyanate component are 43 mol% terephthalic acid / 43 mol% isophthalic acid / 14 mol% isophorone diisocyanate, and glycol component is 40 mol% ethylene glycol / 60 mol% polyethylene glycol Polyurethane. Solid content 25%.
Aqueous polyolefin: 75 mol% polypropylene / 1-butene 25 mol%
Crosslinking agent A-1: N, N, N ′, N′-tetraglycidyl-metaxylylenediamine crosslinking agent A-2: N, N, N ′, N′-tetraglycidyl-1,3-bis (aminomethyl) ) Cyclohexane crosslinking agent B-1: Glycerin diglycidyl ether Surfactant: Polyoxyethylene (n = 7) lauryl ether

[Comparative Example 3]
A white polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the coating thickness of the aqueous coating liquid was changed to 1.0 μm. The characteristics of the obtained film are shown in Tables 1 and 2. As shown in Table 2, the adhesive strength with EVA was weak and unsuitable as an easy-adhesive film for a solar cell back surface protective film.

[Comparative Example 4]
A white polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the coating thickness of the aqueous coating solution was 8.5 μm. The characteristics of the obtained film are shown in Tables 1 and 2. As shown in Table 2, the adhesive strength with EVA was weak and unsuitable as an easy-adhesive film for a solar cell back surface protective film.

[Example 6]
Two extruders are used, extruder A is supplied with PET-a and PET-b mixed and dried at a ratio of 64:36, and extruder B is 95 with PET-a and PET-b. : After supplying the mixture mixed and dried at a ratio of 5, each was melt-extruded at 285 ° C., and the resin was merged into a three-layer structure of A / B / A and extruded onto a sheet from a slit die to form an unstretched film Was carried out in the same manner as in Example 1 to obtain a white bilayer film of 188 μm. The thickness of the base material layer (B) on the extruder B side was adjusted to be 80% of the film layer thickness. The characteristics of the obtained film are shown in Tables 1 and 2.

[Example 7]
A white three-layer film of 188 μm was obtained in the same manner as in Example 6 except that heat setting was performed at 220 ° C. for 15 seconds. The characteristics of the obtained film are shown in Tables 1 and 2.

  Since the easy-adhesive polyester film of the present invention has excellent mechanical properties, heat resistance and moisture resistance, it has good long-term adhesion even when laminated with EVA by short-time crosslinking, It can be suitably used for a solar cell back surface protective film.

Claims (7)

A film for a solar cell back surface protective film comprising a biaxially stretched polyester film having a thickness of 38 to 300 μm and comprising a coating film having a thickness of 20 to 100 nm provided on at least one side, comprising a polyethylene terephthalate substantially free of a copolymer component, The coating film comprises 50 to 90% by mass of an aqueous polyurethane resin and 3 to 20% by mass of an epoxy crosslinking agent A represented by the following formula (Chemical Formula 1) based on the mass of the coating liquid component (excluding the solvent). An easily-adhesive polyester film for a solar cell back surface protective film, which is formed from a coating solution containing the solar cell.

  The easily adhesive polyester film for solar cell back surface protective films according to claim 1, wherein the aqueous polyurethane resin is a polyester-based polyurethane resin having an aromatic polyester skeleton.   The aromatic component constituting the aromatic polyester skeleton is an aromatic dicarboxylic acid component, and the proportion thereof is 70 to 90 mol% based on the total of the aromatic dicarboxylic acid component and the isocyanate component constituting the polyurethane resin. The easily adhesive polyester film for solar cell back surface protective films of Claim 2.   The coating film is 50 to 90% by mass of an aqueous polyurethane resin, 3 to 20% by mass of the epoxy-based crosslinking agent A and 0.5 to 4% by mass of glycerin poly based on the mass of the coating liquid component (excluding the solvent). The easily-adhesive polyester film for solar cell back surface protective films in any one of Claims 1-3 formed from the coating liquid containing a glycidyl ether.   The biaxially stretched polyester film contains 0 to 18% by mass of rutile-type titanium oxide particles, the film has a COOH end group concentration of 3 to 30 equivalent / ton, a weight average molecular weight of 38000 to 58000, and is obtained by differential scanning calorimetry. The easily adhesive polyester film for solar cell back surface protective film according to any one of claims 1 to 4, wherein the melting subpeak (Tsm) is 215 to 230 ° C.   The biaxially stretched polyester film is a laminated polyester film composed of two or three layers of a surface layer (A) and a base material layer (B), and in the surface layer (A), rutile type titanium oxide particles are contained in 10-20. % By mass, containing 0.3 to 5% by mass of rutile titanium oxide particles in the base material layer (B), and the base material layer (B) has a thickness of 75 to 94% of the total film thickness, The COOH end group concentration of the film is 3 to 30 equivalent / ton, the weight average molecular weight is 38000 to 58000, and the melting subpeak (Tsm) obtained by differential scanning calorimetry is 215 to 230 ° C. The easily-adhesive polyester film for solar cell back surface protective films as described in 2.   The solar cell back surface protective film comprised only from the easily adhesive polyester film for solar cell back surface protective films of Claim 5 or 6.