Polyester film with UV-stability and high light transmittance

#This application is the national stage filing of PCT application number IB2011/000415, filed 4 Feb. 2011, and claims priority of British patent application number 1001947.9, filed 5 Feb. 2010, the entireties of which applications are incorporated herein by reference.

#The present invention is concerned with a polyester film which exhibits high UV-stability and high light transmittance, and which is of particular use in the manufacture of photovoltaic (PV) cells and other electronic devices, particularly flexible devices.

#The mechanical properties, dimensional stability, flexibility, weight, impact resistance and optical properties of polyester films offer advantages for their use in the manufacture of electronic or opto-electronic devices, such as electroluminescent (EL) display devices (particularly organic light emitting display (OLED) devices), electrophoretic displays (e-paper), photovoltaic (PV) cells and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). The use of flexible polyester film as layer(s) in electronic devices allows the manufacture of such devices in a reel-to-reel process, thereby reducing cost.

#According to the present invention, there is provided a composite film comprising: (i) a heat-stabilised oriented polyester substrate comprising a UV-absorber in an amount of from about 0.1 to about 10% based on the total weight of the polyester substrate, and (ii) on one or both surfaces of the substrate a polymeric coating layer, which has a thickness in the range of from about 10 nm to about 200 nm, and which comprises an ethylene acrylic acid (EAA) copolymer, wherein the composite film exhibits a shrinkage at 150° C. for 30 minutes of less than 0.1% in both the longitudinal and transverse dimensions of the film.

#The polyester substrate is a self-supporting film or sheet by which is meant a film or sheet capable of independent existence in the absence of a supporting base. The substrate is uniaxially or biaxially oriented, preferably biaxially oriented. The polyester(s) which constitute the substrate is/are typically synthetic linear polyester(s). Suitable polyesters are obtainable by condensing one or more dicarboxylic acid(s) or their lower alkyl (up to 6 carbon atoms) diesters with one or more diols. The dicarboxylic acid component typically contains at least one aromatic dicarboxylic acid, which is preferably terephthalic acid, isophthalic acid, phthalic acid, 1,4-, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, and is preferably terephthalic acid or 2,6-naphthalenedicarboxylic acid, and preferably terephthalic acid. The polyester may also contain one or more residues derived from other dicarboxylic acids such as 4,4′-diphenyldicarboxylic acid, hexahydro-terephthalic acid, 1,10-decanedicarboxylic acid, and in particular aliphatic dicarboxylic acids including those of the general formula CnH2n(COOH)2 wherein n is 2 to 8, such as succinic acid, glutaric acid sebacic acid, adipic acid, azelaic acid, suberic acid or pimelic acid, preferably sebacic acid, adipic acid and azelaic acid, and more preferably azelaic acid. The diol(s) is/are preferably selected from aliphatic and cycloaliphatic glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexanedimethanol, preferably from aliphatic glycols. Preferably the polyester contains only one glycol, preferably ethylene glycol. Preferably the synthetic linear polyester contains one aromatic dicarboxylic acid and one glycol. Polyethylene terephthalate (PET) or polyethylene 2,6-naphthalate (PEN), particularly PET, is the preferred polyester. The film-forming polyester resin is the major component of the substrate, and makes up at least 50% by weight of the total weight of the substrate layer, and in one embodiment at least 65%, typically at least 80%, more typically at least 90%, more typically at least 95%, and in one embodiment at least 99% by weight of the total weight of the substrate layer.

#Formation of the polyester is conveniently effected in a known manner by condensation or ester interchange, generally at temperatures up to about 295° C. Optionally, solid state polymerisation may be used to increase the intrinsic viscosity to a desired value, using conventional techniques well-known in the art, for instance using a fluidised bed such as a nitrogen fluidised bed or a vacuum fluidised bed using a rotary vacuum drier.