Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1998-03-06
2001-02-13
Codd, Bernard (Department: 1753)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S441000, C136S251000, C156S099000
Reexamination Certificate
active
06187448
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to encapsulant materials for various applications. More particularly, the invention relates to an encapsulant material for solar cell module and laminated glass applications.
BACKGROUND
Transparent encapsulant materials are used in numerous applications, including solar cell module and laminated glass applications. In solar cell applications, transparent encapsulants protect and seal the underlying solar cells without adversely affecting the optical properties of such underlying materials. In laminated glass applications, transparent encapsulants minimize any possible hazards from broken glass. In these applications, the encapsulant is exposed to the ultraviolet (UV) rays of the sun and this exposure can result in the yellowing and physical degradation of the polymer. To prevent this, UV stabilizers are added to the encapsulant.
In the manufacture of crystalline silicon solar cell modules, a transparent encapsulant material is used to protect the brittle silicon solar cells from breakage and to help seal these cells into the overall module structure. The encapsulant material is usually a thermoplastic. The thermoplastic is melted, then flows to fill in any open spaces in the module and bonds to all adjacent surfaces. The most widely used encapsulant material for solar cell modules is a copolymer of vinyl acetate and ethylene, known as ethylene vinyl acetate (EVA). EVA is used to encapsulate and seal both thin film and crystalline silicon solar cell modules.
There are several disadvantages associated with using EVA as an encapsulant material that adversely affect the quality and manufacturing cost of the solar cell modules. First, an organic peroxide is added to EVA in order to cross-link it using the heat which accompanies the lamination process. The cross-linking is necessary to increase the creep resistance of the encapsulated structure. However, the peroxide is not completely consumed during the cross-linking process, and the remaining peroxide can promote subsequent oxidation and degradation of EVA. In addition, the EVA must be laminated in a vacuum when making a module because of the presence of peroxide in the EVA. The reason for this is that oxygen lowers the degree of cross-linking, producing an unsatisfactory encapsulant. Second, the preferred EVA usually contains 33% (by weight) of vinyl acetate, and thus is a very soft and tacky substance that tends to stick to itself. This tackiness makes handling of the EVA material in a manufacturing environment much more troublesome and also makes it more expensive to manufacture the base resin. As such, the EVA material requires a release paper or liner material to use the material after it has been made into sheet. Third, peroxide cured EVA has been known to turn yellow and brown under extensive exposure to sunlight for several years. Yellowing and browning causes reduction in solar module power output. Fourth, EVA can produce acetic acid under processing conditions which can then foster metal contact corrosion. Fifth, EVA is known to be fairly permeable to water and is, therefore, far from ideal as a sealant.
Although virtually any transparent polymer eventually shows some degradation and yellowing after exposure to sunlight, an encapsulant material that can withstand degradation and yellowing for a longer period of time than EVA is desirable. Ideally, a solar cell module should last for thirty years without showing much sign of degradation. EVA is unlikely to satisfy this thirty year duration requirement. In addition to finding a suitable replacement for EVA (or PVB, which is described below), it is also necessary to develop a suitable UV light stabilization package for the encapsulant.
In laminated glass applications, the laminated glass is made by forming a sandwich of two pieces of glass with a sheet of a transparent polymer disposed between the two pieces. This transparent polymer sheet serves to prevent the glass in the laminated structure from shattering into dangerous shards when the glass is broken. Windshields on automobiles and architectural glass are manufactured in this manner. Poly vinyl butyral (PVB) is a widely used material in such polymer sheets in the foregoing laminated glass applications. PVB, however, has several drawbacks. First, PVB is extremely hydroscopic (i.e. it absorbs moisture readily). Therefore, it must be kept refrigerated and maintained under special atmospheric conditions before it can be successfully laminated. Second, PVB is also extremely soft and tacky and, therefore, must be used with a release or liner layers.
SUMMARY OF THE INVENTION
This invention features an encapsulant material that may be used in solar cell modules, laminated glass and a variety of other applications. The encapsulant material is a three layer structure. A middle layer is formed of metallocene polyethylene and disposed between two outer layers of an acid copolymer of polyethylene. The layer of metallocene polyethylene can comprise copolymers of ethylene with butene, hexene, or octene.
The acid copolymer layers can be derived from any direct or grafted ethylene copolymer of an alpha olefin having the formula R—CH═CH
2
, where R is a radical selected from the class consisting of hydrogen and alkyl radicals having from 1 to 8 carbon atoms and alpha, beta-ethylenically unsaturated carboxylic acid having from 3 to 8 carbon atoms. The acid moieties are randomly or non randomly distributed in the polymer chain. The alpha olefin content of the copolymer can range from 50-92% while the unsaturated carboxylic acid content of the copolymer can range from about 2 to 25 mole percent, based on the alpha olefin-acid copolymer.
The layers of metallocene polyethylene and acid copolymer are exceptionally transparent. In one detailed embodiment, the metallocene polyethylene layer is formed from a copolymer of ethylene and octene, and the acid copolymer layers are based on an ethylene methacrylic acid copolymer.
An encapsulant material which is a combination of these two materials allows for the exploitation of the best properties of each material while overcoming the limitations of either EVA or of each material if used alone. The outer acid copolymer layers allow the encapsulant material to bond strongly to all the adjacent surfaces (e.g. a backskin formed of any suitable material). The inner metallocene polyethylene layer which forms the bulk of the encapsulant material is a highly transparent, low cost thermoplastic material. The two acid copolymer layers are thin (i.e., the order of 0.001-0.004″ thick), and may be based on either an ethylene methacrylic acid copolymer or ethylene acrylic acid copolymer (such copolymers containing 7-15% by weight of the carboxylic acid). This level of acid content promotes strong adhesive bonds with the glass superstrate, the silicon cells, and the rear backing surface of the encapsulated system as well as exhibiting high light transmission. The metallocene polyethylene has excellent optical clarity and superior physical properties compared to the acid copolymers. These superior properties are derived from the fact that the metallocene catalyst used results in a polymer with narrow molecular weight distribution.
There are several advantages associated with an encapsulant material which is a combination of the metallocene polyethylene and the acid copolymer materials. One of these advantages involves the bonding strength of the encapsulant with all adjacent surfaces. The bonding strength is qualitatively described by the method in which the bond failure occurs under test conditions. Adhesive bond failure describes the case in which the bond at the interface fails. Cohesive failure describes a much stronger bond, whereby the polymer material itself fails before the interface bond. In this case, the interface bond is stronger than the internal bonding of the polymer (i.e., the encapsulant) itself. Because of their acid functionality, the outer acid copolymer layers will result in cohesive bond failure. The metallocene polyethylene, on the other
Hanoka Jack I.
Klemchuk Peter P.
Codd Bernard
Evergreen Solar Inc.
Testa Hurwitz & Thibeault LLP
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