JP4902464B2 - Solar cell back surface protective film - Google Patents

Description

  It is related with the film for solar cell back surface protective films.

  In recent years, a solar power generation system has been spreading as one of power generation means using clean energy. The structure of a solar cell module is generally a plurality of plate-like solar cells between a glass substrate on the light receiving side and a protective film on the back side, as described in, for example, Japanese Utility Model Publication No. 6-38264. It has a structure in which an element is sandwiched and a sealing resin is filled in an internal gap. The protective film on the back side is generally formed by laminating 2 to 3 resin films via an adhesive. For example, JP-A-11-261085 and JP-A-11-186575 describe that a polyethylene-based resin, a polyester-based resin sheet, or a fluororesin film is used as the protective film on the back surface. However, these protective films have portions where the adhesion of each layer is not always sufficient and the durability is inferior.

  Polyester films, especially biaxially stretched films of polyethylene terephthalate and polyethylene naphthalate, have excellent mechanical properties, heat resistance, and moisture resistance, so they are used for magnetic tapes, ferromagnetic thin film tapes, photographic films, packaging films, and electronic parts. It is widely used as a film, an electrical insulating film, a metal plate laminating film, and a display member-required film, and has excellent performance as a solar cell back surface protective film. However, since a molecular chain is oriented in the stretching direction, that is, the in-plane direction of the film, the polyester film, particularly a biaxially stretched polyester film, tends to weaken the cohesive force in the thickness direction of the film and easily break into layers. For this reason, there exists a problem that delamination of a film generate | occur | produces and durability deteriorates.

Japanese Utility Model Publication No. 6-38264 JP-A-11-261085 JP-A-11-186575

  An object of the present invention is to solve the above-described problems of the prior art, and to provide a back surface protective film for a solar cell that is suppressed in hydrolysis, has high durability, and has excellent processability.

  That is, the present invention consists of polyethylene-2,6-naphthalenedicarboxylate, and has a thickness of 1 to 38 μm, a plane orientation coefficient of 0.240 to 0.255, and a breaking strength of 250 to 280 Mpa, It is the film for back surface protection films of a solar cell.

  ADVANTAGE OF THE INVENTION According to this invention, a back surface protective film of the solar cell which is suppressed hydrolysis, has high durability, and has the outstanding workability can be provided.

Hereinafter, the present invention will be described in detail.
[Polyethylene-2,6-naphthalenedicarboxylate]
The film for solar cell back surface protective film of the present invention comprises polyethylene-2,6-naphthalenedicarboxylate. This polyethylene-2,6-naphthalene dicarboxylate is a polyester comprising 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component and ethylene glycol as a diol component.
Polyethylene-2,6-naphthalenedicarboxylate is preferably a homopolymer in order to obtain high hydrolysis resistance and strength, but the copolymerization component is copolymerized up to 3 mol% for each of dicarboxylic acid and diol. May be.

[Intrinsic viscosity]
The intrinsic viscosity of polyethylene-2,6-naphthalenedicarboxylate is preferably 0.40 dl / g or more, more preferably 0.40 as the intrinsic viscosity calculated from measurement at 35 ° C. in o-chlorophenol. ~ 0.90 dl / g. If the intrinsic viscosity is less than 0.40 dl / g, process cutting may occur frequently, which is not preferable. There is not preferable.

[Thickness]
The film for a solar cell back surface protective film of the present invention preferably has a film thickness of 1 to 38 μm, more preferably 6 in order to obtain strength required when used as a solar cell back surface protective film and flexibility to some extent. It is -38 micrometers, Most preferably, it is 12-25 micrometers. If the thickness is less than 1 μm, the strength as a solar cell back surface protective film is insufficient, and if it exceeds 38 μm, the total thickness when the insulating film is laminated becomes too thick, so that the flexibility is not preferable.

[Surface orientation coefficient]
The film for solar cell back surface protective film of the present invention has a plane orientation coefficient of 0.240 to 0.255. If the plane orientation coefficient is less than 0.240, the orientation is too low, resulting in a film with poor thickness unevenness and poor flatness, and is easily hydrolyzed and lacks strength. On the other hand, if it exceeds 0.255, the orientation is too high and delamination tends to occur. In addition, the plane orientation coefficient of the polyethylene-2,6-naphthalene dicarboxylate film is usually about 0.260, the orientation is too high, and delamination is likely to occur.

[Breaking strength]
The film for solar cell back surface protective film of the present invention has a breaking strength of 250 to 280 Mpa. If the breaking strength is less than 250 MPa, long-term durability cannot be obtained. On the other hand, if it exceeds 300 MPa, the orientation is too high and delamination occurs.
The film for protecting a back surface of a solar cell of the present invention has a breaking strength of the film after an acceleration test in which the film is allowed to stand for 300 hours in an environment of 121 ° C., 100 RH%, 2 atm from the viewpoint of maintaining durability to hydrolysis over a long period of time. Is preferably 100 Mpa or more.

[Heat shrinkage]
It is preferable that the film for solar cell back surface protective films of this invention is 0.5 to 1.5% of the heat shrinkage rate of a longitudinal direction and the heat shrinkage rate of the width direction. If it is less than 0.5%, it is not preferable because sufficient strength cannot be obtained, and if it exceeds 1.5%, warp and wrinkles are generated due to heat in the lamination process with the insulating film.

[Easily adhesive layer]
In the solar cell, a solar cell element sealing material is provided on the back surface protective film of the solar cell. As the sealing material, an ethylene-vinyl acetate copolymer resin (hereinafter abbreviated as EVA) is generally used. In order to improve the adhesion between the film and the sealing material, it is preferable that an easy adhesion layer is provided on the film.

  The thickness of the easy adhesion layer is preferably 10 to 200 nm, more preferably 20 to 150 nm. If the thickness of the easy-adhesion layer is less than 10 nm, the effect of improving adhesion is poor, which is not preferable. If it exceeds 200 nm, cohesive failure of the easy-adhesion layer tends to occur and the adhesion may be lowered, which is not preferable.

  The easy-adhesion layer is preferably a constituent material that exhibits excellent adhesion to both polyester film and EVA. Specifically, polyester resin, acrylic resin, urethane acrylic resin, silicon acrylic resin, melamine resin, polysiloxane Resins can be exemplified. These resins may be used alone, or two or more kinds thereof may be used as a mixture, for example.

  The easy adhesion layer may be provided after the film is formed, or may be provided by coating in the production process of the polyester film. In particular, when stretching, heat setting, etc. are carried out in the production of a polyester film, it is preferably applied before these steps are completed. Here, the plastic film before crystal orientation is completed is an unstretched film, a uniaxially oriented film in which the unstretched film is oriented in either the longitudinal direction or the transverse direction, and further in two directions, the longitudinal direction and the transverse direction. And the like that have been oriented at a low magnification (biaxially stretched film before being finally re-stretched in the machine direction or the transverse direction to complete orientation crystallization).

[Production method]
The polyethylene-2,6-naphthalenedicarboxylate in the present invention can be produced by a conventionally known method. For example, it can be produced by a method of directly obtaining a low-polymerization degree polyester by reaction of dicarboxylic acid and glycol. Further, for example, it can be obtained by a method in which a lower alkyl ester of dicarboxylic acid and glycol are reacted using a conventionally known transesterification catalyst and then a polymerization reaction is carried out in the presence of a polymerization catalyst. Polyethylene-2,6-naphthalenedicarboxylate may be converted into chips after melt polymerization, and further subjected to solid phase polymerization under heating under reduced pressure or in an inert gas stream such as nitrogen.

In the film of the present invention, polyethylene-2,6-naphthalene dicarboxylate as a raw material is melt-extruded into a sheet shape, cooled and solidified with a casting drum to form an unstretched film, and this unstretched film is longitudinally stretched at Tg to (Tg + 60) ° C. Stretched biaxially at a magnification of 3.6 to 4.0 times in the direction and a magnification of 3.9 to 4.3 times in the transverse direction, heat-set at a temperature of 240 to 250 ° C. for 1 to 100 seconds, and after heat setting, the width It can be obtained by 0-2% relaxation (toe-in) in the direction.
The stretching method can be performed by a generally used method such as a method using a roll or a method using a stenter. The stretching method may be performed simultaneously in the longitudinal direction and the transverse direction, or may be sequentially performed in the longitudinal direction and the transverse direction.

In the case of sequential stretching, the easy-adhesion layer can be applied by applying an aqueous coating solution to a uniaxially oriented film stretched in one direction, stretching in the other direction as it is, and heat fixing.
As a 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. The coating amount is, for example, 0.5 to 20 g, preferably 1 to 10 g per 1 m ² of the running film. The aqueous coating liquid is preferably used as an aqueous dispersion or emulsion.

Hereinafter, the present invention will be described in detail with reference to examples.
The physical property values and characteristics were measured as follows.

(1) Film thickness Film thickness was measured at 30 g of needle pressure using an electronic micrometer (trade name “K-312A type” manufactured by Anritsu Corporation).

(2) Breaking strength When the film was cut into a sample width of 10 mm and a length of 15 cm, the chuck feeling was 100 mm, the tensile speed was 100 mm / min, and the chart speed was 100 mm / min. The strength was measured.

(3) Heat shrinkage rate Marks are applied to film samples at intervals of 30 cm, heat treatment is performed for a predetermined time in an oven at a predetermined temperature without applying a load, and the interval between the heat marks after the heat treatment is measured to form a film continuously. In the direction (MD direction) and the direction (TD direction) perpendicular to the film forming direction, the thermal shrinkage rate was calculated by the following formula.
Thermal shrinkage (%)
= (Distance between the pre-heat treatment marks -Distance between the heat treatment marks) / Distance between the pre-heat treatment marks x 100

(4) Planar orientation coefficient (NS)
Using an Abbe refractometer, the refractive index is measured using sodium D line (589 nm) as a light source to determine nMD, nT, D and nZ.
The plane orientation coefficient (NS) was determined by the following formula.
NS = (nMD + nTD) / 2-nZ
(In the formula, nMD represents the refractive index in the continuous film-forming direction of the film, nTD represents the refractive index in the width direction, and nZ represents the refractive index in the thickness direction.)

(5) Delamination resistance A 125 μm polyethylene terephthalate film is laminated on a film sample with a urethane-based adhesive {polyester-based main agent + (isophorone diisocyanate / xylylene diisocyanate) -based curing agent} by a dry laminating method, and then 180 °. Was peeled off, and the peeling interface formed at that time was observed and evaluated according to the following criteria.
○: Peeling occurs due to destruction of the adhesive layer ×: Peeling occurs due to cohesive failure of the film itself (Delami)

(6) 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 is set to 100 in an environmental test machine set to 121 ° C., 100% RH, 2 atm, wet saturation mode. Left for hours. Thereafter, the sample piece was taken out, its longitudinal breaking strength was measured 5 times, and the average value was obtained. Hydrolysis resistance was evaluated according to the following criteria.
○: Breaking strength after treatment 100 Mpa or more ×: Breaking strength after treatment <100 Mpa

[Example 1]
0.03 part of manganese acetate tetrahydrate is added to a mixture of 100 parts of dimethyl 2,6-naphthalenedicarboxylate and 60 parts of ethylene glycol, and transesterification is performed while gradually raising the temperature from 150 ° C to 240 ° C. It was. On the way, when the reaction temperature reached 170 ° C., 0.024 part of antimony trioxide was added, and further 0.13% by weight of spherical silica particles having an average particle size of 0.25 μm and a particle size ratio of 1.1 was added. When the reaction temperature reached 220 ° C., 0.042 part (corresponding to 2 mmol%) of 3,5-dicarboxybenzenesulfonic acid tetrabutylphosphonium salt was added. Subsequently, a transesterification reaction was carried out. After the transesterification reaction, 0.023 parts of trimethyl phosphate was added. Next, the reaction product was transferred to a polymerization reactor, heated to 290 ° C., subjected to a polycondensation reaction under a high vacuum of 0.2 mmHg or less, and the intrinsic viscosity measured with an o-chlorophenol solution at 25 ° C. was inherent. Polyethylene-2,6-naphthalenedicarboxylate having a viscosity of 0.66 dl / g was obtained.

  The polyethylene-2,6-naphthalenedicarboxylate pellets were dried at 170 ° C. for 6 hours, then fed to an extruder hopper, melted at a melting temperature of 305 ° C., and filtered through a stainless steel fine wire filter having an average opening of 17 μm. The film was extruded through a 2 mm slit die on a rotary cooling drum having a surface temperature of 60 ° C. and rapidly cooled to obtain an unstretched film. The unstretched film thus obtained was preheated at 120 ° C. and further heated by a 900 ° C. IR heater 15 mm above between low-speed and high-speed rolls and stretched in the longitudinal direction to 3.8. Subsequently, it was supplied to a tenter and laterally at 145 ° C. The film was stretched 4.1 times, further heat-set at 245 ° C. for 5 seconds and 1.0% relaxed (toe-in) in the width direction to obtain a film having a thickness of 12 μm. The properties of the obtained film are shown in Table 1.

[Examples 2-6, Comparative Examples 1-6]
The same procedure as in Example 1 was performed except that the film forming conditions of the film were changed as shown in Table 1. The properties of the obtained film are shown in Table 1.

  The film for solar cell back surface protective film of the present invention can be used as a solar cell back surface protective film.

Claims (4)

  A solar cell back surface protective film comprising polyethylene-2,6-naphthalenedicarboxylate, having a thickness of 1 to 38 μm, a plane orientation coefficient of 0.240 to 0.255, and a breaking strength of 250 to 280 Mpa. Film.   The film for solar cell back surface protective films of Claim 1 whose heat shrinkage rate of the longitudinal direction of a film and the heat shrinkage rate of the width direction are both 0.5 to 1.5%.   The film for protecting a back surface of a solar cell according to claim 1, wherein the film has a breaking strength of 100 Mpa or more after being left in an environment of 121 ° C, 100RH%, 2 atm for 300 hours.   The solar cell back surface protective film which consists of a film for solar cell back surface protective films of Claim 1.