Base film for membrane switch and membrane switch

#The present invention relates to a membrane switch base film composed of a biaxial oriented polyester film comprising polyethylene-2,6-naphthalenedicarboxylate as the major component. More specifically, it relates to a membrane switch base film with excellent handling properties and durability, and especially to a membrane switch base film which is suitable for use in automobile interior devices which are potentially exposed to high temperature.

#A membrane switch comprises two base films lying on either side of a spacer, with contact points (electrodes) corresponding to each of the opposing surfaces. Switching action between conduction and insulation can be easily accomplished by pressing the base films, i.e. varying the spacing between the base films. Membrane switches have recently come into wide use for the keypads of cellular phones or portable personal computers or the various control panel switches of household electrical appliances such as VTRs and microwave ovens.

#Because the switching action of a membrane switch involves repeated pressing, the base films used therefor must be flexible and resistant to deformation. Base films used for conventional membrane switches have generally been polyethylene terephthalate (hereinafter also abbreviated as “PET”) films because of their deformation resistance, adhesion with electrodes, bonding with printing pastes, and other properties.

#<Polyethylene-2,6-naphthalenedicarboxylate>

#The polymer composing the biaxially oriented polyester film of the invention comprises polyethylene-2,6-naphthalenedicarboxylate (hereinafter also abbreviated as “PEN”) as the major component, and it may be also be a copolymer or blend. The term “major” as used in the present invention means that ethylene-2,6-naphthalenedicarboxylate constitutes at least 80 mole percent, preferably at least 90 mole percent and more preferably at least 95 mole percent of the total repeating units of the polymer. It is only necessary to guarantee permanent deformation resistance with use under high temperature, without extreme loss of the original properties of the biaxially oriented polyester film of the invention.

#In the case of a copolymer, compounds having two ester-forming functional groups in the molecule may be used as the copolymerizing components of the copolymer other than polyethylene-2,6-naphthalenedicarboxylate as the major component. As such compounds there may be used, preferably, dicarboxylic acids such as oxalic acid, adipic acid, phthalic acid, sebacic acid, dodecanedicarboxylic acid, isophthalic acid, terephthalic acid, 1,4-cyclohexanedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, phenylindanedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, tetralindicarboxylic acid, decalindicarboxylic acid and diphenyletherdicarboxylic acid. There may also be suitably used oxycarboxylic acids such as p-oxybenzoic acid and p-oxyethoxybenzoic acid. In addition, there may be suitably used dihydric alcohols such as propylene glycol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, cyclohexanemethylene glycol, neopentyl glycol, bisphenolsulfone ethylene oxide addition product, bisphenol A ethylene oxide addition product, diethylene glycol and polyethylene oxide glycol.