BIAXIALLY ORIENTED COPOLYESTER/POLYOLEFIN BI- OR TRILAYER FILM AND METHOD FOR MAKING THE SAME

Abstract: Disclosed herein is a bi- or trilayer film comprising one or two biaxially oriented copolyester film layers and a polyolefin layer bonded to one surface of said biaxially oriented copolyester film layer(s), wherein the copolyester is PENBB. The laminated film may also include a metal layer bonded to the copolyester surface opposite the surface bonded to the polyolefin. The metal layer preferably has a thickness of 0.03 to 0.3 mu m. The polyolefin preferably has a thickness from about 5 to about 30 mu m and a birefringence of 0 to about 0.02. PENBB as mentioned herein is a copolyester containing units of formula (I). Such films are useful in capacitor applications.

Description: BIAXIALLY ORIENTED COPOLYESTER/POLYOLEFIN Bl- OR TRILAYER FILM AND METHOD FOR MAKING THE SAME Background Of The Invention 1 ) Field Of The Invention The present invention relates to an improved biaxially oriented PENBB copolyester film carried on a substrate film for a metallized capacitor dielectric. The film has improved stiffness (tensile modulus), strength and shrinkage, thereby permitting thinner film and capacitors with a higher capacitance per unit volume. The bi- or trilayer film includes one or two copolyester layers and a polyolefin layer. The PENBB copolyester layer has at least 25 mole-% of the diacid derived units in the form of 4,4'-bibenzoate. 2) Prior Art With the recent reductions in the size and weight of electric appliances, the size of capacitors used therein has been gradually reduced, and thus, a reduction in the thickness of the films ' used for the capacitors has become necessary. The thickness of the thinnest polyethylene terephthalate film now available is 0.9 μm, but this fiim is very difficult to handle because of the extreme thinness. If a film having a further reduced thickness is developed, this film will be very effective for increasing the capacitance or reducing the size of a capacitor. However, if the film thickness is further reduced, the stiffness of the film is lowered proportionally and such operations as vacuum deposition on the film, slitting, element winding, coating, etc. become very difficult. Additionally, there is particular demand for miniature capacitors (chip type) with sufficient thermal resistance to withstand the solder bath conditions experienced in surface mounting circuit assembly. With poly¬ ethylene terephthalate as a dielectric film layer, as is known in the art, the high solder bath temperature (260 °C) melts the film, causing the capacitor to fail. A way to improve the poor processability of an ultra thin film is known in which the ultra thin film is coextruded with a different film to form a thick bi- or trilayer film and, after processing, the ultra thin film is peeled from the different film. For example, Japanese Unexamined Patent Publication No. 58- 5226 discloses a method in which a laminated sheet is prepared by co- extrusion of a polyolefin and polyethylene terephthalate, the laminated sheet is drawn, and the polyethylene terephthalate is then separated from the laminated sheet to obtain an ultra thin film. This publication also suggests that a metal may be vacuum deposited on this ultra thin film. European Patent Application No. 153,081 to Yoshii et al discloses a laminated film having a propylene copolymer film bonded to a polyethelene terephthalate (PET) film with an adhesive force of 0.1 to 2.0 g/cm. A metal layer can be applied to the polyester film layer. There are many methods known in which different polymers are laminated or coextruded, the laminate or coextrudate is drawn, and one polymer film is separated from the laminate to form an ultra thin film. These known methods are disclosed in, for example, Japanese Unexamined Patent Publications No. 58-132520, No. 58-136417, No. 57-176125, and No. 52- 37982. A defect of these known methods is that it is very difficult to obtain a long, unbroken ultra thin film. by continuously separating the ultra thin film from a substrate film.- In other words, the ultra thin film usually splits or tears during the separation. All of this prior art is based on the use of polyethylene terephthalate (PET) as the ultra-thin layer(s) to be separated from the polyolefin carrier. The lowest thickness achievable with PET is limited by the tensile modulus of PET, in part due to the tensions for separating and winding. On the other hand, the capacitance of a given film capacitor is inversely proportional to the square of the thickness of the film. For example, a 10 % increase in the tensile modulus allows a 10 % reduction in dielectric film thickness, which results in an increase in the capacitance of the capacitor manufactured from the same area this film is over 20 %, while reducing size. Accordingly, there is a need to provide a dielectric film with increased modulus, as this enables the production of smaller capacitors. U.S . Patent No. 3,008,934 discloses copolyesters containing as acid derived units 4,4'-bibenzoate and a host of other dicarboxylates including 2,6- naphthalic dicarboxylate. It also discloses oriented fibers and films prepared from these copolyesters, however, biaxially oriented PENBB films are not disclosed or envisioned. In particular, those films with improved stiffness (tensile modulus) and tensile strength in both MD and TD as well as thermostability, UV stability, hydrophobicity, dimensional stability and impermeability toward gases in comparison to PET film are not disclosed in U.S. Patent No. 3,008,934. It is a primary object of the present invention to provide a bi- or trilayer film with ultra-thin layers for capacitors, wherein the tensile modulus of the ultra-thin layer is increased versus PET and the thermal stability is sufficient to withstand surface mounting soldering conditions. Another object of the present invention is to provide a metal deposited bi- or trilayer biaxially oriented copolyester film for a capacitor, in which the foregoing defects are eliminated; that is, operational processability is good and an ultra thin film can be separated very easily. Still another object of the present invention is to provide a method for making a capacitor element by using the metal deposited laminated biaxially oriented copolyester film. Summary Of The Invention In one aspect of the present invention, there is provided a metal deposited bilayer film for a capacitor, which comprises a biaxially oriented PENBB copolyester film having a thickness of 0.1 to 2.0 μm, on one surface of which a metal is vacuum deposited, and on the other surface a polyolefin substrate film is bonded to the copolyester. In another aspect, there are provided two metal deposited biaxially oriented PENBB copolyester layers with a thickness of 0. 1 μm to 2μm are provided such that one layer is bonded to each side of the polyolefin substrate film. In yet another aspect of the present invention, there is provided a method for making capacitor elements, which comprises cutting the above- mentioned metal deposited biaxially oriented bi- or trilayer PENBB copolyester film to a predetermined width, and forming a capacitor element by separating the metal deposited biaxially oriented copolyester film from the cut laminated film and, successively, forming the separated metal deposited biaxially oriented copolyester film into a coil. In the broader sense, the present invention pertains to biaxially oriented PENBB copolyester film(s) having at least 25 mole percent of the dicarboxylic acid, or the ester equivalents, of 4,4'-bibenzoic acid having a thickness of 0.1 to 2 μm, bonded on one or both sides of a polyolefin, preferably a polypropy- lene copolymer, with an adhesive force of 0.1 to 2.0 g/cm. Description Of The Preferred Embodiments Generally the PENBB copolyesters of the present invention are made by reacting at least two dicarboxylic acids, or their ester equivalents with suitable diols, as is well known in the art. The copolyester of the present invention is derived primarily from dicarboxylic acids or their ester equivalents with at least 25 mole-% of the dicarboxylic acid being 4-4 ' -bibenzoic acid (or the ester equivalent) having the following repeat unit: (bibenzoate, BB) The remainder of the copolyester may be formed from other dicarbox¬ ylic acids or their ester ec - valents, such as terephthalic acid, isophthalic acid, phthalic acid, naphthalene-2, 6-dicarboxylicacid, 1 ,4-cyclohexanedicarboxylic acid, di-(4-phenyl)-acetyiene dicarboxylic acid; 1 ,2 di-(4-phenyl)ethylene- dicarboxylic acids, sebacic acid, malonic acid, adipic acid, azelaic acid, glutaric acid, suberic acid, succmic acid, and the like, or mixtures of these can be employed in the present invention. Naphthalene-2,6-dicarboxylic acid is the preferred remainder diacid . Suitable diols employed in the present invention include ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1 , 5-pentanediol, 1 ,6- hexanediol, neopentyl glycol, 1 , 10-decanediol, cyclohexane dimethanol, and the like. Ethylene glycol is the preferred diol. Suitable copolyesters of the present invention can comprise, for example, polyethylene terephthalate/4,4 '-bibenzoate, polybutylene terepht- halate/4,4 ' -bibenzoate, polypropylene terephthalate/4,4 ' -bibenzoate, polyethylene naphthalate/4,4 ' -bibenzoate, polyethylene terephthalate/isopht- halate/4,4 '-bibenzoate, polyethylene terephthalate/adipate/4,4 '-bibenzoate, polyethylene terepthalate/sulphoisophthalate/4,4 ' -bibenzoate, and the like. In order to achieve the desired mechanical properties in the biaxially oriented PENBB film it is recommended that the IV value (inherent viscosity, as measured in a 1 : 1 weight-ratio mixture of pentafluorophenol and hexafluoroisopropanol at a concentration of 0.2 g/dl and a temperature of 25 ° C) of the PENBB polymer after extrusion be > 0.5 dl/g and preferably > 0.55 dl/g. In a preferred embodiment, a polyethylene naphthalate/bibenzoate copolyester comprises roughly equimolar portions of the ethylene esters of 2,6-naphthalate and 4,4 ' -bibenzoate, or the acids of naphthalene-2,6- dicarboxyiic acid and 4,4 '-bibenzoic acid. The term "roughly equimolar" here indicates molar ratios from 6:4 to 4:6. The copolyester is obtained by polycondensation of the diacids or diesters with a diol. It may be ad¬ vantageous, however, to employ an excessive amount of the diol in order to influence the reaction kinetics. After the reaction of the diacids or diesters with the diol, a polycondensation reaction is carried out according to known processes. For the preferred embodiment, a mixture of two or more dicarboxylic acids/esters are mixed with at least 100 mole percent of the corresponding diol. The diacids or their ester equivalents and the diols are mixed uniformly and heated to approximately 200 ° C in the presence of a transesterification catalyst, as is well known in the art. The reaction yields oligomeric or low molecular weight polyester which is subsequently subjected to the polycondensation reaction in the presence of polycondensation catalyst. Additionally, stabilizers, antioxidants, delustrants, pigments, fillers, antistatic agents, etc., may be uniformly mixed with the copolyester. Suitable catalysts are antimony, manganese, cobalt, magnesium, zinc, calcium, etc., as are well known in the art. The preferred transesterification catalyst, where employed, would be manganese and/or cobalt compounds. The preferred polycondensation catalyst would be antimony compounds. Such catalysts are well known and conventional in the prior art. The biaxially oriented PENBB copolyester film of the present invention is obtained by biaxially drawing the bi- or trilayer film to impart a molecular orientation to the copolyester layer, which has a bibenzoate unit conent of at least 25 mole-%. The melting point of the copolyester is desirably at least 255 ° C, preferably at least 260 ° C. In the bi- or trilayer film of the present invention, the thickness of the biaxially oriented copolyester film layer(s) is 0.1 to 2.0 μm, preferably 0.1 to 1 .0 μm. If the thickness of the biaxially oriented copolyester film is smaller than 0.1 μm, the film will easily tear during the separating or peeling operation and thus have little practical utility. If the thickness of the biaxially oriented copolyester film layer is larger than 2.0 μm, the biaxially oriented copolyester film alone can be handled safely, and the formation of a bi- or trilayer film as claimed in the present invention is not necessary. The kind of metal to be vacuum deposited on one surface of the biaxially oriented copolyester film is not particularly critical, but aluminum, zinc, copper, silver, or mixtures thereof are preferred . The metal layer vacuum deposited on one surface of this biaxially oriented copolyester film facilitates the separation of the biaxially oriented co¬ polyester film from the polyolefin film. Preferably, the thickness of the vacuum deposited metal layer is 30 to 300 nm, more preferably 50 to 200 nm. If the thickness of the vacuum deposited metal layer is too small and outside the above range, it is difficult to maintain a sufficiently high quality of capacitor elements. Also, separation of the biaxially oriented copolyester film layers is not greatly facilitated by the metal layer. If the vacuum deposited metal layer is too thick and outside the above range, when a capacitor is fabricated by using the separated biaxially oriented copolyester film, the self- healing function of the capacitor may be impaired. The polyolefin film is preferably a propylene copolymer film composed of about 80 to 97 mole-% of propylene and 3 to 20 mole-% of at least one olefin other than propylene having 2 to 8 carbon atoms. It is preferred that the melting point of the polyolefin film be in the range of from 90 ° C to 140 ° C. As specific examples of the polyolefin (hereinafter referred to as "PO"), there can be mentioned a propylene/ethylene copolymer, a propylene/- butene copolymer, and a propylene/ethylene/butene terpolymer. A random structure is preferred for the copolymer, but a block copolymer may be used if the degree of biockiness is not so high as to induce overt crystallinity. Preferably, the polyolefin film bonded to the surface of the ultra-thin biaxially oriented copolyester film layer opposite to the metal vacuum deposited surface will have a thickness of 5 to 30 μm and a birefringence of 0 to 0.02, more preferably, 0 to 0.01 . If the birefringence is too large and outside the above range, the vacuum deposited biaxially oriented copolyester film or the polyolefin film may be easily torn or split upon separation. The birefringeance is adjusted through variation in the draw ratio (MD/TD). Birefringeance as mentioned herein is the absolute value of the difference between the maximum and minimum refractive indices in the plane of the film, as measured on common instruments such as Abbe refractometer, optical bench or compensators. It is an indispensable feature of the present invention that the adhesive force between the biaxially oriented copolyester film and the polyolefin film be 0.1 to 2.0 g/cm, and it is preferably 0.2 to 0.8 g/cm. If the adhesive force is too large and outside the above range, the film will be easily torn or split at the separation stage. If the adhesive force is too small and outside the above range, smooth separation is impeded, causing formation of wrinkles in the film. As a means for maintaining the adhesive force between the biaxially oriented copolyester film and the polyolefin film within the above-mentioned range, a non-granular lubricant is incorporated into the biaxially oriented copolyester film and/or the polyolefin film by the addition of 0.001 to 1 % by weight, more preferably, 0.005 to 0.5 % by weight of the lubricant. The non-granular lubricant is a substance having a melting point or softening temperature lower than 200 ° C and imparting a lubricating property to the film, even if liquid or solid at room temperature. Where two or more of these substances are incorporated in the film, it is sufficient if the total amount is within the above-mentioned weight range. Specific examples of the non-granular lubricant are as follows. A. Aliphatic hydrocarbons such as liquid paraffin, microcrystalline wax, natural paraffin, synthetic paraffin, polyethylene wax, and polypropylene wax. B. Higher fatty acids and metal salts thereof such as stearic acid, calcium stearate, hydroxystearic acid, hardened oil, and sodium montanate. C. Fatty acid amides such as stearic acid amide, oleic acid amide, erucic acid amide, ricinoleic acid amide, behenamide, and methylene-bis- stearamide. D. Fatty acid esters such a n-butyl stearate, methyl hydroxystearate, myricyl cerotinate, a polyhydric alcohol fatty acid ester, and an ester type wax. E. Fatty acid ketones such as a ketone type wax. F. Aliphatic alcohols such as lauryl alcohol, stearyl alcohol, myristyl alcohol, and cetyl alcohol. G. Fatty acid-polyhydric alcohol partial esters such as a glycerol fatty acid ester, hydroxystearic triglyceride, and a sorbitan fatty acid ester. H. Non-ionic surface active agents such as a polyoxyethyiene alkyl ether, a polyoxyethyiene phenyl ether, a polyoxyethyiene alkylamide, and a polyoxyethyiene fatty acid ester. I. Silicone oils such as a linear methylsilicone oil, a methylphen- ylsilicone oil, and a polyoxyalkylene glycol-modified silicone oil. J. Fluorine-containingsurface active agents such as a fluoroalkylcar- boxylic acid, a perfluoroaklylcarboxylic acid, a monoperfluoroalkyl ethyl phosphate, arid a perfluoroalkylsulfonic acid salt. If an inorganic fine particle having an average particle size of 0.001 to 1 μm, such as silica, zeolite, calcium carbonate, calcium phosphate, kaolin, kaolinite, clay, talc, titanium oxide, alumina, zirconium oxide, or aluminum hydroxide, is incorporated in an amount of 0.01 to 0.5 percent by weight in the PET film and/or the polyolefin film in combination with the above- mentioned non-granular lubricant, the effect of the non-granular lubricant is synergistically enhanced in many cases. The layer structure of the present invention is not limited to the (metal deposited) copolyester/polyolefin bilayer film structure. For example, there may be a (metal deposited) copolyester/polyolefin/copolyester(with deposited metal) trilayer structure. This trilayer film structure is advantageous in that two ultra thin metal deposited films can be obtained and a capacitor element can be fabricated by winding these two films. The term "layer" as used herein refers to the polymeric material only. An embodiment of the process for the preparation of the metal deposited laminated film of the present invention will now be described, though the process is not limited to this embodiment. Copolyester pellets containing optional lubricant and a fine particulate filler and pellets of the polyolefin containing lubricant and a fine particulate filler are melted in separate extruders. The melt streams are combined laminarly to form a copolyester/polyolefin/copolyester layer structure either in a feed block or a multimanifold die and coextruded through a T-type slot die onto a chill roll, where the laminate solidifies. Alternatively, it is also possible to produce a bilayer copolyester/polyolefin structure in a similar fashion. The laminate film is then drawn in MD and TD, each at draw ratios of 2 to 5, at suitable temperatures between the glass transition temperature and 30 ° C above the cold crystallization temperature of the particular copolyester in use. These drawing processes can be carried out simultaneously, first in machine direction (MD), then in transverse direction (TD), or in TD then MD. It is also possible to carry out the total stretching in MD or TD in several smaller steps such as: MD, - MD 2 - TD 3 MD, - TD 2 - MD 3 MD, - TD 2 - MD 3 - TD 4 or MD, - TD 2 - simultaneous 3 The biaxially drawn PENBB-polyolefin film is then heat set to crystallize the film by exposing it to elevated temperatures, ranging from about the cold crystallization temperature (T cc ) to about the melt temperature (T m ) of the co¬ polyester, for a period from about 1 second to about 1 minute, while maintaining the film under tension in MD and TD. The cold crystallization temperature as well as the melt temperature can be determined on the amorphous sheet by DSC; T cc being identified by an exothermic peak and T m by an endothermic peak. The heat treated film is then gradually cooled to room temperature to obtain a laminated film comprising a center polyolefin layer having a thickness of 1 5 μm and copolyester layers each having a thickness of 0.5 μm, which are bonded closely to both sides of the polyolefin layer, respectively. The thus obtained film is placed in a vacuum deposition vessel and a metal is vacuum deposited on the surfaces of the biaxially oriented co¬ polyester films according to customary procedures. Known vacuum deposition methods include the resistance heating method, an electron beam heating method, a sputtering method, and an ion plating method. A margin (a non-metallized portion) necessary for a capacitor may be formed during the vacuum deposition stage. For example, a masking tape is placed between a metal source and the surface of the film in such a manner that the metal is not deposited on the portion of the film surface covered by the tape. If a strip shaped mask is placed in parallel to the continuous film, a continuous margin in the longitudinal direction of the film can be formed, and if the masking tape is placed in a position intersecting the running direction of the film, a margin extending in the lateral direction of the film or in an oblique direction can be formed. According to the above-mentioned procedures, there can be obtained a bi- or trilayer film comprising a polyolefin film and a metal deposited, biaxially oriented copolyester film bonded closely to the polyolefin film. The thus-prepared metal deposited layered film is slit in a predetermined width, and the metal deposited biaxially oriented copolyester film is separated and successively formed into a capacitor element, whereby the production of capacitor elements can be greatly facilitated. When a laminated capacitor is made according to an embodiment of the present invention, the following operations are performed . An organic solvent solution of a soluble resin ts ch as polycarbonate or polyphenylene oxide) as an adhesive is coated on tne metal deposited surface of a laminated film comprising a propylene copoiymer film and a metal deposited biaxially oriented copolyester film, and the coated surface is dried by hot air to form a coating layer having a thickness of 0.05 to 2.0 μm. Then, the coated metal deposited biaxially oriented copolyester film is separated from the propylene copolymer film and is wound on a large diameter roll (so individual regions have low curvature) to obtain an intermediate wound roll. The roll is heated and compressed by means of a press to obtain a laminated sheet in which several hundreds to several thousands of metal-deposited film layers are laminated together. The laminated sheet is cut into a laminated capacitor chip having desired size (for example, 3 mm x 5 mm), and a metal is applied to both end faces of the chip to form the contact to the electrodes. Thus, a laminated capacitor is obtained. With this method of separating, the metal deposited biaxially oriented copolyester film is separated from the laminated film from the polyolefin substrate, directly while forming the intermediate wound roll, the rate of breakage of the ultra thin dielectric layer (copolyester) is reduced, which increases the overall production yield of the thin layer film chip capacitors. The physical properties of films used in the specification are determined as follows. ( 1 ) Thickness of Film With respect to films having a thickness larger than 1 μm, the thickness is measured by a mechanical gauge. Where a film has a thickness of 1 μm or less, the film is p'aced on a glass support in the unwrinkled state. The stylus of a surface prof;lometer is placed on the film and traversed over the edge of the film on to the glass substrate. The difference in height is designated as the film thickness. (2) Thickness Of Vacuum Deposited Metal Layer With respect to each metal, the value of the electric resistance is measured and converted to the thickness by utilizing the relationship between the thickness of tne metal deposited layer and the electric resistance thereof. (3) Melting Point The temperature is increased at a rate of 20 ° C/min by using DSC (Differential Scanning Colorimeter), and the temperature corresponding to the apex of the endothermic peak by melting is designated as the melting point (the sample quantity is 10 g). Where at least two melting point peaks are observed, the temperature corresponding to the apex of the higher peak is designated as the melting point. (4) Birefringence Refractive indexes of the film in the longitudinal and lateral directions are measured by an Abbe refractometer, and the absolute value of the difference between the two measured values is designated as the birefringence. (5) Adhesive Force The metal deposited biaxially oriented copolyester film layer is continuously separated from the polyolefin layer having a width of W (cm) at a peeling angle of 180° and a separating speed of 2 m/min, and the tension imposed on the metal deposited biaxially oriented copolyester film at this separation step is measured by a Tensiometer. If the measured tension is T (g), the adhesive force is determined by T/W (g/cm). The present invention will now be illustrated by the following examples. Example 1 289 parts by weight of dimethyl 2,6-naphthalenedicarboxyiate, 322 parts by weight of dimethyl 4,4 ' -bibenzoate, 368 parts by weight of ethylene glycol and 0.7 parts of manganese acetate tetrahydrate are initially introduced into a conventional polycondensation reactor provided with a blanketing gas line (N 2 ), pressure equalization, a thermometer, a condenser, a vacuum connection and a stirred. The mixture is heated at 220 °C for 2.5 hours, during which time methanol is distilled off. 0.675 parts by weight of triphenyl phosphate and 0.2259 parts of antimony trioxide are then added as polycon¬ densation catalysts along with 0.15 % by weight of dry process silica with an average particle size of 0.05 μm dispersed as a slurry in ethylene glycol. The mixture is heated to 270 °C, with stirring. Vacuum is applied and the temperature is raised to 285 °C and maintained for 2.5 hours. The residual melt is granulated. The granules are white, opaque and crystalline. An IV value of 0.56 dl/g is determined for the granules (measured at a concentration of 0.1 g/ml in pentafluorophenol/hexafluoroisopropanol (weight ration 1 : 1 ) at 25 °C) . The granules are further condensed for 20 hours at 240 °C under vacuum in the solid phase. After this treatment the IV value is 1 .1 dl/g. As expected, no T g or T cc is discernable in the DSC recording for the crystalline granules condensed in the solid phase; the melting point (T m ) is 281 °C. EXAMPLE 2 Copolyester: The copolyester of Example 1 is mixed with 0.08 % by weight of calcium montanate. Polyolefin: Propylene/ethylene random copolymer having an ethylene content of 5.5 percent by weight is used. The polyolefin copolymer has a melting point of 1 25 ° C and contains 0.5 percent by weight of stearic acid amide and 0.4 percent by weight of kaolinite having an average particulate size of 0.7 μm. Both the starting materials are supplied to different extruders and melt extruded at 31 5 ° C, and the melts are joined in a dual-manifold die to form a film having a copolyester/polyolefin bi-layer structure. The film is wound on a cooling drum maintained at 25 ° C and is cooled and solidified to obtain a bi¬ layer laminated film. The copolyester layer had a thickness of 5 μm and the substrate polyolefin layer had a thickness of 1 50 μm. The film is heated at 1 25 ° C, drawn at a draw ratio of 3.1 in the longitudinal direction, and then drawn at a draw ratio of 3.2 in the lateral direction. The film is heat treated at 260 ° C for 10 seconds, under tension, and gradually cooled to room temperature, and then wound . The thickness of the copolyester layers on both sides is 0.5 μm, the thickness of the central polyolefin layer is 1 5 μm, and the birefringence of the polyolefin layer is 0.01 . When continuous separation of the surface copolyester layer is attempted in this state, the films are sometimes torn and continuous separation cannot be performed smoothly. This laminated film is placed in a vacuum deposition vessel, and aluminum is vacuum deposited in a thickness of 0.08 μm on the surfaces of the copolyester layers on both sides at a vacuum degree of 10 '5 mmHg and film running speed of 30 m/min. When continuous separation of the metal deposited copolyester layers from the metal deposited laminated biaxially oriented copolyester film is carried out, the formation of wrinkles and tearing the film is substantially reduced, and an excellent quality, long, ultra thin, metal deposited biaxially oriented copolyester film is obtained. The adhesive force between the copolyester layer and the polyolefin layer is 0.4 g/cm. The metal deposited laminated film is micro-slit in a width of 1 cm, and the metal deposited biaxially oriented copolyester film is continuously separated from both sides and the two separated films are piled and wound to form a capacitor element. The capacitor element is heat pressed, and a metal is flame sprayed at both ends to form electrodes, whereby a capacitor having a capacitance of 0.1 μF and excellent electric characteristics was obtained. EXAMPLE 3 Copolyester: A copolyester of about 40 mole percent 2,6-dimethyl naphthalate and 60 mole percent 4,4'-bibenzoate with 100 mole percent of ethylene glycol is prepared as in Example 1 . To this copolyester 0.1 percent by weight of myricyl cerotinate (carnauba wax) and 0.2 percent by weight of dry process silica having an average particle size of 0.05 μm is added. Polyolefin: Propylene/ethylene/butene random terpolymer having an ethylene content of 4 percent by weight, and a butene content of 1 .5 percent by weight is used. The polyolefin has a melting point of 122 ° C and contains 0.3 percent by weight of calcium stearate, 0.2 percent by weight of behenamide, and 0.4 percent by weight of calcium carbonate having an average particle size of 0.9 μm. The two starting materials are supplied to different extruders and melt extruded at 320 ° C, and the melts are extruded from a 3-slot die to form a copolyester/polyolefin/copoiyester three-layer structure. The film is extruded on a cooling drum maintained at 30 β C and is cooled and solidified . The thickness of each of the copolyester layers on both sides is 4 μm, and the thickness of the central polyolefin layer is 140 μm. The film is heated at 140 ° C and simultaneously drawn in both the longitudinal and lateral directions at a draw ratio of 3.2 in each direction, heat treated at 280 ° C for 5 seconds while keeping the film under tension, and is gradually cooled to room temperature. In the final film, each of the copolyester layers on both sides has a thickness of 0.4 μm, and the central polyolefin layer has a thickness of 14 μm and a birefringence of 0.005. When continuous separation of the surface copolyester layers is tried in this state, the films are sometimes torn and continuous separation can not be performed smoothly. The laminated film is placed in a vacuum deposition vessel and aluminum is vacuum deposited in a thickness of 0.07 μm on the surfaces of the copolyester layers according to customary procedures. An organic solvent solution of polycarbonate is coated on the metal deposited surfaces of the metal deposited copolyester laminated film, and the coated surfaces are dried so that the thickness of the coating layer was 0.4 μm. The three-layer film having the metal/copolyester layer is continuously separated from the polyolefin substrate layer. The adhesive force between the copolyester layer and the polyolefin layer is 0.3 g/cm. Two separated metallized biaxially oriented copolyester films are piled on a flat plate. Then, the flat plate is heat pressed to form a laminate comprising 1000 film layers. A chip having a size of 6 mm x 8 mm is cut from the laminate, and electrodes are attached to the chip to obtain a laminated capacitor. The electric characteristics of this laminated capacitor are very good. EXAMPLE 4 The shrinkage of the separated biaxially oriented copolyester film prepared according to Example 2 is compared to that of typical commercial polyethylene terephthalate-PET capacitor dielectric grade film. The shrinkage is measured as the dimensional change after heat treatment of the film for 1 5 minutes at 150 ° C in a forced air oven. Shrinkage Percent MD TD Present Invention 0.3 0.3 Typical PET > 1.0 >0.5 EXAMPLE 5 The mechanical properties and solder bath resistance of the separated biaxially oriented copolyester film prepared according to Example 1 is compared to that of typical commercial PET capacitor dielectric grade film. The F-5 valve is the tensile strength at 5 percent elongation. Thus it is apparent that there has been provided, in accordance with the invention, a metal laminated copolyester having at least 25 mole percent of bibenzoate and a resulting capacitor from the metal laminated film that fully satisfies the objects, aims, and aspects set forth above. While the invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the sphere and broad scope of the present invention.

Claims: THAT WHICH IS CLAIMED IS: 1 . A bi- ortrilayer film comprising one ortwo biaxially oriented copolyester film layers and a polyolefin layer bonded to one surface of said biaxially oriented copolyester film layer(s), wherein the copolyester is PENBB. 2. The film of Claim 1 , wherein the biaxially oriented copolyester film layer(s) has (have) a thickness of 0.05 to 2 μm. 3. The film of Claim 1 or 2, wherein the polyolefin layer is bonded to the surface(s) of the biaxially oriented copolyester film layer(s) with an adhesive force of 0.1 to 2.0 g/cm. 4. The film of Claim 1 , wherein said biaxially oriented copolyester film layer(s) includes a metal layer thereon having a thickness of 0.03 to 0.3 μm. 5. The film of Claim 1 , wherein said polyolefin film layer is a polypropylene copolymer film having a thickness of 5 to 30 μm and a birefringence of 0 to 0.02. 6. The film of Claim 5, wherein said polypropylene copolymer is composed of 80 to 97 mole percent of propylene derived units and 3 to 20 mole percent of units derived from at least one non-propylene olefin having 2 to 8 carbon atoms. 7. The film of any one or more of Claims 1 to 6 wherein PENBB is a co¬ polyester wherein at least 25 mole-% of the acid derived units are of the formula (bibenzoate, BB) 8. The film of Claim 7, wherein said PENBB is selected from the class consisting of polyethylene terephthalate/4,4 '-bibenzoate, polybutylene terephthalate/4,4 '-bibenzoate, polypropyleneterephthalate/4,4 '-bibenzoate, polyethylene naphthalate/4,4 '-bibenzoate, polyethylene terephthalate/isopht- halate/4,4 ' -bibenzoate, polyethylene terephthalate/adipate/4,4 ' -bibenzoate, > and polyethylene terephthalate/sulphoisophthalate/4,4 '-bibenzoate. 9. The film of any one or more of claims 1 to 8, wherein the birefringean¬ ce of the copolyester layer(s) is < 0.2 and the IV of the copolyester if > 0.5 dl/g. 10. The film of any one or more of Claims 1 to 9, wherein a metal is deposited on a surface of the biaxially oriented copolyester film layer(s). 1 1 . The film of Claim 10, wherein said metal is selected from the class consisting of aluminum, zinc, copper, silver and mixtures thereof. 12. A metallized biaxially oriented copolyester film obtainable by separating the metallized biaxially oriented copolyester film layer(s) from the polyolefin layer in a film according to Claims 9 or 10. 13. The metallized film of Claim 12 further characterized in that the metal which is deposited on a surface of the biaxially oriented copolyester film layer is coated. 14. A capacitor containing a metallized biaxially oriented copolyester film according to Claim 12 or 13.

Inventors: BENNETT CYNTHIA, CHOE E-WON, FLINT JOHN ANTHONY, KUHMANN BODO, BENNETT, CYNTHIA, CHOE, E-WON, FLINT, JOHN, ANTHONY, KUHMANN, BODO
Applicants: BENNETT CYNTHIA, CHOE E WON, FLINT JOHN ANTHONY, HOECHST AG, HOECHST CELANESE CORP, KUHMANN BODO, BENNETT, CYNTHIA, CHOE, E-WON, FLINT, JOHN, ANTHONY, HOECHST AKTIENGESELLSCHAFT, HOECHST CELANESE CORPORATION, KUHMANN, BODO
Application year: 1992
Main classification: B32B27
IPC codes: B32B15, B32B27, H01G4