JP2010031172A - Polyester film for solar cell back side-protecting film and solar cell back side-protecting film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyester film for a solar cell back side-protecting film, which is excellent in hydrolysis resistance and can maintain a partial discharge-starting voltage at a high voltage over a long period, and to provide the solar cell back side-protecting film using the same. <P>SOLUTION: Provided is the polyester film for the solar cell back side-protecting film, comprising a polyester composition which comprises 100 pts.wt. of a polyester (A), 0.5 to 30 pts.wt. of a styrenic polymer (B) having a syndiotactic structure, and 0.1 to 5 pts.wt. of a vinylic polymer (C) having oxazoline groups on side chains. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polyester film for a solar cell back surface protective film and a solar cell back surface protective film using the same. More specifically, the present invention relates to a polyester film for a solar cell back surface protective film excellent in durability over time of a partial discharge start voltage and a solar cell back surface protective film using the same.

  In recent years, the photovoltaic power generation system has been spreading as one of power generation means using clean energy. The structure of the solar cell module is generally such that a plurality of plate-like solar cell elements are sandwiched between a light receiving side glass substrate and a back side protective film, as described in, for example, Japanese Utility Model Laid-Open No. 6-38264. It takes a structure in which the internal gap is filled with sealing resin.

  When using a film as a member of a solar cell, for example, it is known to use a sheet of polyethylene resin or polyester resin, or to use a fluororesin film (Patent Documents 1 and 2). It is also known to use a multilayer polyester film instead of a fluororesin film (Patent Document 3).

JP-A-11-261085 JP-A-11-186575 JP 2005-11923 A

  A high voltage may be applied to the solar cell back surface protective film. However, in the conventional polyester film for solar cell back surface protective film, it was difficult to maintain the partial discharge start voltage high over a long period of time.

  The present invention provides a polyester film for a solar cell back surface protective film that can maintain a high partial discharge start voltage over a long period of time and is excellent in hydrolysis resistance, and a solar cell back surface protective film using the same. Let it be an issue.

  As a result of intensive studies to achieve the above object, the present inventor has found that the above object can be achieved by blending a vinyl polymer having a specific reactive group with a polyester and a styrene polymer. Reached.

  That is, the present invention relates to 100 parts by weight of polyester (A), 0.5 to 30 parts by weight of styrene polymer (B) having a syndiotactic structure, and vinyl polymer (C) 0 having an oxazoline group in the side chain. It is a polyester film for solar cell back surface protective films comprised from the polyester composition which consists of 0.1-5 weight part.

  This invention is also a solar cell back surface protective film using this polyester film for solar cell back surface protective films.

  According to the present invention, there is provided a polyester film for a solar cell back surface protective film that is excellent in hydrolysis resistance and can maintain a high partial discharge start voltage over a long period of time, and a solar cell back surface protective film using the same. Can do.

Hereinafter, the present invention will be described in detail.
[Polyester (A)]
As the polyester (A) of the polyester composition constituting the polyester film for solar cell back surface protective film of the present invention, a polyester comprising an aromatic dicarboxylic acid and a glycol component can be used. For example, aromatic dicarboxylic acid components such as terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, and ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedi An aromatic polyester composed of glycol components such as methanol and 1,6-hexanediol can be used, preferably polyethylene terephthalate, polyethylene-2,6-naphthalenedicarboxylate, particularly preferably polyethylene terephthalate. is there. These polyesters may be copolymer polyesters, but the copolymer component is preferably 20 mol% or less. In addition, other components may be blended as long as there is no problem in terms of mechanical properties and productivity.

[Styrene polymer (B)]
In the present invention, the polyester composition constituting the polyester film for solar cell back surface protective film contains a styrene polymer (B) having a syndiotactic structure. The syndiotactic structure of the styrenic polymer (B) has a stereochemical structure mainly of a syndiotactic structure, that is, a phenyl group or a substituted phenyl as a side chain with respect to a main chain formed from a carbon-carbon bond. It is a three-dimensional structure in which groups are alternately located in opposite directions.

  As the styrene polymer (B), polystyrene, poly (alkylstyrene), poly (halogenated styrene), poly (alkoxystyrene), poly (vinyl benzoate) can be used. These may be used alone or in combination. When using two or more, it is good also as a mixture and good also as a copolymer.

Examples of poly (alkyl styrene) include poly (methyl styrene), poly (ethyl styrene), poly (isopropyl styrene), and poly (tertiary butyl styrene).
Examples of poly (halogenated styrene) include poly (chlorostyrene), poly (bromostyrene), and poly (fluorostyrene).
Examples of poly (alkoxystyrene) include poly (methoxystyrene) and poly (ethoxystyrene).

  Among these, as the styrenic polymer (B), polystyrene, poly (p-methylstyrene), poly (m-methylstyrene), poly (p-tertiarybutylstyrene), poly (p-chlorostyrene), poly (M-chlorostyrene) and poly (p-fluorostyrene) are preferable, and a copolymer of styrene and p-methylstyrene is preferable.

  The weight average molecular weight of the styrenic polymer (B) is preferably 10,000 to 500,000, more preferably 50,000 to 400,000. When the weight average molecular weight is less than 10,000, the heat resistance is not sufficient because it is not preferable. On the other hand, when it exceeds 500,000, dispersibility is likely to be poor.

[Vinyl polymer (C)]
In the present invention, the polyester composition constituting the polyester film for solar cell back surface protective film contains a vinyl polymer (C) having an oxazoline group in the side chain. This vinyl polymer (C) preferably further has an aromatic group. That is, a vinyl polymer having a side chain having an oxazoline group and an aromatic group is preferred as the vinyl polymer.

  Such vinyl polymer (C) includes an addition-polymerizable oxazoline compound represented by the following formula (I) and an addition-polymerizable vinyl compound having an aromatic group exemplified by styrene, α-methylstyrene, and p-methylstyrene. And an aliphatic addition-polymerizable vinyl compound exemplified by acrylonitrile, methyl methacrylate, methyl acrylate, and butyl acrylate as necessary.

  If the proportion of the oxazoline group in the vinyl polymer (C) is too small, the effect of maintaining the partial discharge starting voltage high over a long period of time will be small. On the other hand, if the proportion is too large, the mechanical properties will be lowered, which is economically disadvantageous. Therefore, the proportion of the component polymerizable oxazoline compound constituting the vinyl polymer (C) in the vinyl polymer (C) is preferably 0.5 to 50% by weight, particularly preferably 0.8. ~ 30% by weight.

  If the proportion of the aromatic group in the vinyl polymer (C) is too small, the thermal stability is lowered and the strength retention in a moist heat environment is lowered. On the other hand, if it is too much, the vinyl polymer in the polyester (A). Since the dispersibility of (C) is likely to decrease, the proportion of the addition polymerizable vinyl compound having an aromatic group as a component constituting the vinyl polymer (C) in the vinyl polymer is preferably 50 to 95. % By weight, particularly preferably 60 to 90% by weight.

  Preferred as the vinyl polymer (C) are 2-vinyl-2-oxazoline / styrene copolymer, 5-methyl-2-vinyl-2-oxazoline / styrene copolymer, 4,4-dimethyl-2- Vinyl-2-oxazoline / acrylonitrile / α-methylstyrene copolymer, 5-methyl-2-vinyl-2-oxazoline / methyl methacrylate / butyl acrylate / styrene copolymer.

  The number average molecular weight of the vinyl polymer (C) is preferably 1000 to 40000, more preferably 1500 to 30000 in the measurement by GPC (gel permeation chromatography). By using the vinyl polymer (C) having a number average molecular weight within this range, the vinyl polymer (C) in the polyester (A) is excellent while maintaining the partial discharge start voltage high over a long period of time. Dispersibility can be obtained.

[Intrinsic viscosity, terminal carboxyl group concentration]
This polyester composition has an intrinsic viscosity of the polyester measured in the state of the composition (measured at a temperature of 35 ° C. using an orthochlorophenol solvent), preferably 0.6 to 1.0 dl / g, particularly preferably 0.8. 7-0.9 dl / g. When the intrinsic viscosity is within this range, a film having excellent mechanical properties and hydrolysis resistance can be produced with high productivity without increasing the load during melt extrusion.

In the polyester composition of the present invention, the terminal carboxyl group concentration of the polyester measured in the composition state is preferably 10 to 30 eq / T, more preferably 12 to 28 eq / T. When the terminal carboxyl group concentration is within this range, a film excellent in hydrolysis resistance can be produced with high productivity.
The polyester composition in the present invention may contain conventionally known additives as long as the object of the present invention is not impaired. Examples of the additive include organic or inorganic lubricant particles, colorants, antistatic agents, antioxidants, ultraviolet absorbers, and lubricants.

[Partial discharge start voltage]
The polyester film for a solar cell back surface protective film of the present invention preferably has a partial discharge start voltage of 1000 eV or more after 50 hours in an environment of 121 ° C. and 100% RH. By providing a partial discharge start voltage of 1000 eV or more, the partial discharge start voltage can be maintained high for a long period of time when used as a solar cell back surface protective film.

[Hydrolysis resistance]
The polyester film for solar cell back surface protective film of the present invention has a breaking elongation retention after 75 hours in an environment of a temperature of 121 ° C. and a humidity of 100% RH, preferably 75% or more. By providing a break elongation retention of 75% or more, good hydrolysis resistance can be maintained over a long period of time.

[Polyester composition]
The polyester composition constituting the polyester film for a solar cell back surface protective film of the present invention contains the polyester (A), the styrene polymer (B) and the vinyl polymer (C).

  The content of the styrenic polymer (B) in the polyester composition is 0.5 to 30 parts by weight, preferably 1.0 to 20 parts by weight, more preferably 3.10 parts by weight with respect to 100 parts by weight of the polyester (A). 0 to 10 parts by weight. When the styrenic polymer (B) is less than 0.5 parts by weight, the heat-and-moisture resistance is inferior and unfavorable. On the other hand, when it exceeds 30 parts by weight, the mechanical properties of the film become inferior and the productivity is lowered. Absent.

  The content of the vinyl polymer (C) in this polyester composition is 0.1 to 5 parts by weight, preferably 0.5 to 4 parts by weight, particularly preferably 1 to 100 parts by weight of the polyester (A). 3 parts by weight. If the content is less than 0.1 part by weight, the dispersibility of the styrene polymer becomes poor, and the effect of improving hydrolysis resistance becomes insufficient. On the other hand, when the amount exceeds 5 parts by weight, the surface properties of the resulting film tend to be rough, which is not preferable.

  In the polyester composition of the present invention, the polyester (A), the styrenic polymer (B), and the vinyl polymer (C) may not be reacted with each other, but may be in a mixture state. May be in a state in which a part of the mixture has reacted, that is, a mixture and a reactant coexist, or may exist as a reaction product in which each component is completely reacted.

[Production method]
In the present invention, the vinyl polymer (B) having an oxazoline group in the side chain is prepared by subjecting an addition-polymerizable oxazoline compound, a vinyl monomer, and, if necessary, another monomer to a conventionally known polymerization method. It can be obtained by polymerizing with. When the vinyl polymer (B) having an oxazoline group and an aromatic group in the side chain is used, the vinyl polymer (B) includes an addition polymerizable oxazoline compound and a vinyl monomer having an aromatic group, and If necessary, it can be obtained by polymerizing with other monomers by a conventionally known polymerization method. Examples of the polymerization method include a solution polymerization method, a suspension polymerization method, an emulsion polymerization method, and a bulk polymerization method.

  The styrenic polymer (B) having a syndiotactic structure in the present invention uses, for example, a condensation product of a titanium compound, water and a trialkylaluminum as a catalyst in an inert hydrocarbon solvent or in the absence of a solvent. It can be produced by polymerizing a styrene monomer (a monomer corresponding to a styrene polymer). Polymerization methods are known and are described, for example, in JP-A-62-187708. A method for producing poly (halogenated alkylstyrene) is also known, and can be obtained by the method described in JP-A-1-46912, for example.

The polyester composition in this invention can be manufactured by melt-kneading polyester (A), a styrene-type polymer (B), and a vinyl-type polymer (C), for example using a biaxial kneading extruder. Further, when the polycondensation reaction of the polyester (A) is completed, a predetermined amount of the vinyl polymer (C) is added to the polyester (A), and further, the styrene polymer (B) is added, and melt mixed. It can also be obtained by extrusion.
For this melt-kneading and extrusion, for example, a normal injection molding machine, a so-called injection compression molding machine, a twin screw extruder, a single screw extruder, a twin screw extruder with a vent, or a single screw extruder with a vent is used. be able to.

  The polyester film for solar cell back surface protective film of the present invention can be produced, for example, as follows. The glass transition temperature is referred to as Tg. First, a polyester composition obtained by melt-kneading a polyester (A), a styrene polymer (B) and a vinyl polymer (C) to which a lubricant has been added in advance is melt-extruded into a film and cooled and solidified with a casting drum The unstretched film is stretched once or twice or more in the longitudinal direction at Tg to (Tg + 60) ° C. so that the total magnification becomes 3 to 6 times, and then Tg to (Tg + 60) ° C. The film can be stretched in the width direction so that the magnification is 3 to 5 times, and further subjected to heat treatment at 180 ° C. to 255 ° C. for 1 to 60 seconds as necessary.

  Stretching in the longitudinal direction and the transverse direction may be carried out by sequential biaxial stretching method or simultaneous biaxial stretching method. In order to increase the dimensional stability during heating, for example, a method of shrinking in the vertical direction in a heat treatment step shown in, for example, JP-A-57-57628, or a film shown in, for example, JP-A-1-275031, is used. A relaxation heat treatment method in a suspended state may be used. The thickness of the film after stretching is preferably 25 to 300 μm, more preferably 38 to 250 μm.

  In the polyester film for a solar cell back surface protective film of the present invention, an easily adhesive coating layer may be provided on at least one surface thereof. The coating layer can be formed by applying a coating solution containing a crosslinking agent and an acrylic resin and / or a polyester resin to a stretchable polyester film, drying, stretching, and heat-treating. The thickness of the coating film is preferably 0.01 to 1 μm.

[Solar cell back surface protective film]
The polyester film for solar cell back surface protective film of the present invention can be used as a solar cell back surface protective film, either alone or in combination of two or more.
When used as a solar cell back surface protective film, a water vapor barrier layer may be laminated for the purpose of imparting water vapor barrier properties. The solar cell back surface protective film having this configuration preferably has a water vapor transmission rate of 5 g / (m ² · 24 h) or less as measured according to JIS Z0208-73.

  As such a water vapor barrier layer, a film or foil having a water vapor barrier property can be used. Examples of the film include a polyvinylidene chloride film, a polyvinylidene chloride coated film, a polyvinylidene fluoride coated film, a silicon oxide deposited film, an aluminum oxide deposited film, and an aluminum deposited film. Examples of the foil include an aluminum foil and a copper foil. Can be illustrated.

  Using these films or foils, from the side on which the EVA (ethylene vinyl acetate) layer for sealing the solar cell element is placed, the white film / water vapor barrier layer / the film for protecting the back surface of the solar cell of the present invention, or the white film It can be used as a solar cell back surface protective film in the form of a laminate structure such as a film for solar cell back surface protective film / water vapor barrier layer of the present invention.

  Hereinafter, the present invention will be described more specifically with reference to examples. The characteristics in the present invention were measured and evaluated by the following methods.

(1) Intrinsic Viscosity The viscosity of a solution using an orthochlorophenol solvent was measured and determined at 35 ° C.

(2) Number average molecular weight The number average molecular weight was measured by gel permeation chromatography (GPC: manufactured by Nippon Waters) using tetrahydrofuran as a solvent.

(3) Carboxyl group concentration It dissolved in benzyl alcohol under nitrogen atmosphere, and measured by the titration method.

(4) Partial discharge start voltage Water vapor is obtained by providing a vapor-deposited thin film layer of silicon oxide with a thickness of 80 nm on one side of a biaxially stretched polyethylene terephthalate film with a thickness of 12 microns (trade name: Teijin Tetron NS manufactured by Teijin DuPont Films Ltd.) As a barrier film, a biaxially stretched polyethylene terephthalate film having a thickness of 50 microns (trade name: Teijin Tetron U2 manufactured by Teijin DuPont Film Co., Ltd.) is prepared as a white film. Takeda Pharmaceutical Co., Ltd. polyurethane adhesive (main agent Takelac A515 / curing agent Takenate A50 = 10/1 solution) in order of white film / water vapor barrier film / film for solar cell back surface protective film of the present invention. Use a dry laminating machine to bond Created a solar cell back surface protective film.

Using this solar cell back surface protective film, the partial discharge start voltage (Umax) before and after wet heat treatment (temperature of 121 ° C. and humidity of 100% RH for 50 hours) was used as a partial discharge manufactured by Kikusui Electronics Corporation. Using a tester (model KPD2050), the measurement was performed in accordance with IEC606664-1 partial discharge test. The measurement was performed on 10 samples and the average value was taken. The partial discharge start voltage was evaluated according to the following criteria.
○: 1000 eV or more ×: less than 1000 eV

  Originally, it is desirable to evaluate the partial discharge start voltage after about 20 years in the outdoor exposure state as a solar cell back surface protective film, but it is difficult to perform an outdoor exposure test over such a long time. Therefore, instead of this evaluation, an accelerated test is performed for 50 hours in an environment of a temperature of 121 ° C. and a humidity of 100% RH, and the partial discharge start voltage is evaluated.

(5) Hydrolysis resistance A strip-shaped sample piece cut to a length of 100 mm in the vertical direction and a width of 10 mm in the horizontal direction was left in an environmental test machine set at a temperature of 121 ° C. and a humidity of 100% RH for 50 hours. Thereafter, a sample piece was taken out and the elongation at break in the longitudinal direction was measured 5 times to obtain an average value. A value obtained by dividing the average value by the measured value of the breaking elongation before standing was defined as the breaking elongation retention rate (%), and the hydrolysis resistance was evaluated according to the following criteria.
Breaking elongation retention rate (%)
= (Breaking elongation after 50 hours) / (initial breaking elongation) × 100
○: Breaking elongation retention ratio of 75% or more ×: Breaking strength retention ratio of less than 75%

  Originally, it is desirable to evaluate the hydrolysis resistance after 20 years in the outdoor exposure state as a solar cell back surface protective film, but it is difficult to perform such an outdoor exposure test over a long period of time. Therefore, instead of this evaluation, an accelerated test is performed for 50 hours in an environment of a temperature of 121 ° C. and a humidity of 100% RH, and the elongation at break is evaluated.

[Reference Example 1]
To a reaction vessel with an internal volume of 2 liters, 1.0 liter of purified styrene and 1 mmol of triethylaluminum were added and heated to 80 ° C., followed by a premixed catalyst (pentamethylcyclopentadienyl titanium trimethoxide 90 μmol, dimethylaniline). 16.5 ml of nitrotetra (pentafluorophenyl) borate 90 μmol, toluene 29.1 mmol, triisobutylaluminum 1.8 mmol) was added, and a polymerization reaction was carried out at 80 ° C. for 4 hours. After completion of the reaction, the product was repeatedly washed with methanol and dried to obtain 380 g of a polymer.

This polymer had a weight average molecular weight of 240,000 as measured by gel permeation chromatography at 130 ° C. using 1,2,4-trichlorobenzene as a solvent. Moreover, it confirmed that this polymer was a polystyrene of a syndiotactic structure by < ¹³ > C-NMR measurement.

[Example 1]
As a polyester (A), homopolyethylene terephthalate pellets (intrinsic viscosity 0.85, melting point 257 ° C.), as a styrene polymer (B), in a Kobe Steel twin screw extruder KTX-46 equipped with three quantitative feeders Syndiotactic polystyrene (obtained in Reference Example 1), Nippon Shokubai Epochros RPS-1005 as vinyl polymer (C), polyester (A) 100 parts by weight, styrene polymer (B) 3 weights, respectively. Parts and vinyl polymer (C) at a ratio of 0.4 parts by weight, and these are melt-kneaded to contain 800 ppm of massive silicon oxide particles having an average particle diameter of 2.5 μm with respect to the total amount of the polymer. A polyester composition blended in was prepared. The twin screw extruder was 260 ° C., 250 rpm, 50.0 kg / h discharge. The polyester composition thus kneaded was discharged to a water-cooled bath through a die plate having two holes with a diameter of 6 mm, and pelletized with a small pelletizer (manufactured by Isuzu Chemical Industries, Ltd., HSCF200).

  Subsequently, this pellet was melt-extruded on a rotary cooling drum maintained at 20 ° C. using an extruder heated to 285 ° C. to obtain an unstretched film. The film was stretched 3.6 times in the longitudinal direction at 100 ° C., then stretched 3.9 times in the transverse direction at 110 ° C., and heat-set while shrinking 3% in the width direction at 225 ° C. to a thickness of 50 μm. The polyester film for solar cell back surface protective films was obtained.

A deposited thin film layer of silicon oxide having a thickness of 80 nm is formed on one side of the obtained polyester film for solar cell back surface protective film and a 12 micron biaxially stretched polyethylene terephthalate film (trade name: Teijin Tetron NS, manufactured by Teijin DuPont Films Ltd.). The provided film is used as a water vapor barrier film, and a biaxially stretched polyethylene terephthalate film (trade name: Teijin Tetron U2 manufactured by Teijin DuPont Films Co., Ltd.) having a thickness of 50 microns is prepared as a white film. White film / water vapor barrier film / solar cell A solar battery back surface protective film was prepared by laminating with a dry laminating machine so as to be in the order of the polyester film for the back surface protective film.
Using this solar cell back surface protective film, the partial discharge start voltage before and after wet heat treatment (temperature of 121 ° C. and humidity of 100% RH in 50 hours) was determined. The evaluation results are shown in Table 1.

[Example 2]
In Example 1, the polyester (A), the styrene polymer (B) and the vinyl polymer (C) were changed so as to be supplied at a ratio of 100 parts by weight, 20 parts by weight and 3 parts by weight, respectively. The same operation as in Example 1 was performed. The evaluation results are shown in Table 1.

[Comparative Example 1]
It implemented like Example 1 except not adding a styrene-type polymer (B). The evaluation results are shown in Table 1.

[Comparative Example 2]
It implemented like Example 2 except not adding a vinyl-type polymer (C). The obtained film was significantly deteriorated in mechanical properties, frequently cut during production, and greatly inferior in stable film formation. The evaluation results are shown in Table 1.

[Comparative Example 3]
In Example 1, except that the polyester (A), the styrene polymer (B) and the vinyl polymer (C) were changed so as to be supplied at a ratio of 100 parts by weight, 40 parts by weight and 6 parts by weight, respectively. Was carried out in the same manner as in Example 1. The obtained film was significantly deteriorated in mechanical properties, frequently cut during production, and greatly inferior in stable film formation. The evaluation results are shown in Table 1.

  The polyester film for solar cell back surface protective film of the present invention can be suitably used as a solar cell back surface protective film. In addition, a solar cell back surface protective film is a structural member of a solar cell arrange | positioned at the back surface of a solar cell.

Claims (5)

  100 parts by weight of polyester (A), 0.5-30 parts by weight of styrene polymer (B) having a syndiotactic structure and 0.1-5 parts by weight of vinyl polymer (C) having an oxazoline group in the side chain The polyester film for solar cell back surface protective films comprised from the polyester composition which consists of.   The polyester film for a solar cell back surface protective film according to claim 1, wherein the vinyl polymer (C) is a vinyl polymer having an oxazoline group and an aromatic group in a side chain.   The polyester film for solar cell back surface protective films according to claim 1, wherein 80 mol% or more of all repeating units of the polyester (A) are ethylene terephthalate units.   The polyester film for a solar cell back surface protective film according to claim 1, wherein a partial discharge start voltage after elapse of 50 hours in an environment of 121 ° C. and 100% RH is 1000 eV or more.   The solar cell back surface protective film using the polyester film for solar cell back surface protective films in any one of Claims 1-4.