ELECTRICAL INSULATION FROM BIAXIALLY ORIENTED PENBB FILM

Abstract: Disclosed herein is a biaxially oriented copolyester film containing inert particles, wherein the copolyester is PENBB and wherein the inert particles are either generated in situ, added to the monomers before or during polycondensation or added to the PENBB before the film-forming process. Depending on the particular application, the film may have various thickness, may include pigment matter, and/or may be coated or surface treated. PENBB as mentioned herein is a copolyester containing units of formula (I).

Description: Electrical Insulation from biaxially oriented PENBB Film Background of Invention Flexible polymeric films have proved to be of great value as insulating films in electrical equipment and machinery. In particular biaxially oriented poly ethylene terephalate films are useful in transformer insulation, motor generato insulation, wire and cable insulation and as insulating supports for flexible circuits. In these applications, high stiffness and strengths including resistance to cut-through are important. Also, these applications require property retention after exposure to elevated temperatures. In the case of flexible circuiting, the manufacturing process requires resistance to soldering melt temperatures found in the wave soldering process. Motors, generators, transformers, and wire and cable may also be used where long term (years) of exposure to high humidities (and temperatures) may be a problem. Good hydrolytic stability is necessary. Design of equipment is limited by the present property combination of current materials. Scope of Invention The resent invention provides a biaxially oriented film of PENBB co- polyester with excellent stiffness, high temperature strength, dimensional stability, and hydrolysis resistance for use as electrical insulation. Description It has now been found that biaxial oriented, heat set films made from PENBB copolyester resin exhibit greater stiffness, strength and higher melting point than films made from polyethylene terephtalate. In addition, this film has improved hydrolytic stability. Such improvements are important to the use of film in electrical insulation applications where high temperatures or humidities may be experienced over long periods of time. Such uses might include transformer insulation, motor and generator insulation, flexible circuit substrate, and the ' .ke. PENBB as used herein refers to a copolyester containing acid-derived knit of which at least 25 mole % are of the formula: (bibeπzoate, BB] Preferably, the bibenzoate units comprise 40 - 60 mole % of the aci derived units of the copolyester, and at least 80 mole . ό of the diol-derived unit are -O-(CH 2 ) 2 -O- (ethylene glycol units) . The remaining acid-derived units of thi copolymer are preferably of the formula: (2,6-naphthalate, N) U.S. Patent No. 3,008,934 discloses copolyesters containing as aci derived units 4,4'-bibenzoate and a host of other dicarboxylates including 2, naphthalic dicarboxylate. It also discloses oriented fibers and films prepared fro these copolyesters, however, biaxially oriented PENBB films are not disclosed envisioned . In particular, those films with improved stiffness (tensile modulu and tensile strength in both MD and TD as well as thermostability, UV stabilit hydrophobicity, dimensional stability and impermeability toward gases i comparison to PET film are not disclosed in U.S. Patent No. 3,008,934. The copolyester is obtained by polycondensation of the correspondin diacid or lower dialkyl diester and the corresponding diol. Both component should preferably be employed in roughly equimolar ratios. It may however b advantageous to employ one of the components - especially the diol - in exces for instance in order to influence the reaction kinetics or to serve as a solven The polycondensation is carried out according to known processes used e g. in the production of polyethylene terephthalate (PET) Usually about 1 00 mole- % of the dicarboxylic acid or dialkyldicarboxylate mixture are mixed with > 1 0 mole-% of the corresponding dιol(s), in the presence of a transesteπficatio catalyst, and heated until sufficient lower alkyl alcohol or water has bee removed from the mixture via distillation This reaction yields an oligomer or low molecular weight polyester, which is subsequently subjected to polyconden sation, preferably in the presence of a stabilizer and/or catalyst. Useful stabilizers and catalysts can be polyphosphates, tπorganyi phosphates, antimony tπoxide or tetraalkoxy titanate (IV) or mixtures of tπphenylphosphate and antimony tπoxide A preferred process for the production of such copolyester is described in U S Patent Application Ser No. 07/735, 553 which is incorpora ted herein by reference A further increase in molecular weight can be achieved by solid phase polycondensation at a temperature just below the melting poin under vacuum, or a stream of dry air or inert gas. In order to achieve the desired mechanical properties in the biaxiall oriented PENBB film it is recommended that the IV value (inherent viscosity, a measured in a 1 A weight-ratio mixture of pentafluorophenol and hexafluoroi sopropanol 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 order to provide good handling and winding properties the film composition must be such that the surface is rough. This can be achieved b incorporating fine inert insoluble particles into the film One method of doing this is producing inert particulates in situ by reactin the metal salt transesteπfication catalyst with phosphoric acid derivatives in th course of the polycondensation step, which causes fine particulate matter t precipitate in the polymer Another method is the addition of fine particles to th monomers before or during polycondensation or to the polymer before extrusion Such inert fine particles can be kaolin, talc, silica, carbonates of magnesium calcium or barium, sulphates of calcium or barium phosphates of lithium, calciu or magnesium, titanium oxide, lithium fluoride, carbon black and the organic aci salts of calcium, barium, zinc and manganese. It is also possible to use fin particles made of cross-linked polymers. The particles may be of one type o mixtures of several types. The shape of the particles can be irregular, flaky spherical or elongated . The hardness, density and the color of the particles i immaterial. The average size of the particles should be less than 10 μ preferably less than 3 μm. The amount incorporated in the film should in th range of 0.01 % to 2.00 % (by weight), preferably between 0.05 % and 0.8 (by weight) . In order to render the film more durable in a variety of applications common additives such as flame retardaπts, anti-oxidants etc. may be incorpor ted in the composition. To produce the film, the polymer melt is extruded through a die onto a chi roll where it solidifies, is then biaxially oriented, heat set, optionally post treate and wound on a roll. For a multi layer film known methods for coextrusion, in-lin or off-line coating can be used . The solidified film as extruded on the chill ro should be obtained in an essentially amorphous state. To achieve this, the mel film must be pinned to the chill roll by a known method such as electrostati pinning or vacuum, air knife or the like. The biaxial orientation of the film is achieved by stretching the film a elevated temperature in the machine (MD) and traverse direction (TD) . Thi stretching can be either simultaneous or sequential. In the case of sequenti stretching the first stretching step can be in either MD or TD, followed b stretching in the perpendicular direction. The orientation in MD can also b achieved in several steps, either one after another prior to stretching in TD, o before and after the TD stretching. Preferred temperatures for stretching li between the glass transition temperature and about 30°C above the col crystallization temperature of the PENBB copolymer composition in use (bot temperatures can easily be measured on amorphous films by DSC) . The tot stretch ratios (/.) in MD and TD lie between 1 : 2 and 1 : 1 0, preferably betwee 1 : 2.5 and 1 : 5. The product of the total stretch ratios should be between 1 to 30 preferably between 5 to 20. In order to optimize properties, relaxation steps can be included in the orientation and heat setting processes. Biaxial drawing is performed such that the birefringeance is < 0.2, preferably < 0.1 to ensure adequately isotropic properties. 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. The heat setting takes place at a temperature between the cold crystalliza- tion temperature and the melt temperature of the copolymer composition. In one embodiment, the copolyester film for electrical insulation is preferably coated with a primer or surface treated via known methods such as corona, plasma, or flame treatment to improve adhesion to other sheet-like materials such as fiber sheet it might be laminated to or to impregnation resins in insulation applications. In another embodiment, useful for slot liners, the film contains up to 5 % inert, opaque, white or colored fillers that render the film opaque, which makes the completed insertion of the slot liner in motor or generator manufacture visually apparent. Films according to the present invention used for electrical insulation application typically are about 75-350 μm thick. In flexible printed circuit applications, a film thickness of about 25-1 50 μm is typically suitable. A thickness of about 1 5-75 μm maybe preferable for cable and/or transformer applications. For membrane switches or touch panels, film 20-300 μm thick may be needed. A particular application will be readily ascertainable by one of ordinary skill in the art in each case. The following Examples illustrate certain aspects of the invention. However, the invention is not limited to the embodiments illustrated or described . EXAMPLE 1 Production of Polymer Polymer with a T B of 123°C and a T m of 281°C 289 parts by weight of dimethyl 2, 6-naphthalene dicarboxylate, 322 part 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 conventional polycondensation reactor provided with a blanketing gas line (N 2 ) pressure equalization, a thermometer, a condenser, a vacuum line and stirred fo 2.5 hours, during which time methanol is distilled off. 0.675 parts by weight o triphenyl phosphate, 0.2259 parts of antimony trioxide and 23 parts of a slurr prepared by dispersing 5.8 parts by weight of BaSO 4 with an average particl diameter of 1 μm in 1 7.2 parts by weight of ethylene glycol are then added a polycondensation catalysts and the mixture is heated to 270 °C, with stirring Vaccum is applied and the temperature is raised to 285 ° C and maintained fo 2.5 hours. The 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 concen tration of 0.1 g/ml in pentafluorophenol/Heyaf luoroisopropanol [weight ratio 1 : 1 at 25 °C) . The granules are further condensed for 20 hours at 240 ° C under vacuu in the solid phase. After this treatment the IV value is 1 .1 dl/g. The meltin point (TJ is 281 °C. Film The PENBB granules are melted in a single screw extruder at temperature of 280 - 320 °C and extruded through a sheet dye onto a cooling roll temperatu re controlled at 30 °C. An amorphous film is obtained which is clear and transparent. This PENB prefilm is then sequentially biaxially oriented (first transversely, then longitudinal ly: 3.5x3.5) at 1 30 °C on a film stretching device . A strong clear film i obtained . The film is finally heat treated at 260 ° C for 1 0 minutes. The annealed biaxially oriented PENBB film produced as above has th following mechanical properties This film has a thermal shrinkage of 0.3% in the MD and TD directio when heated for 1 5 minutes at 1 50°C. A test of hydrolytic stability is made on this film, and on polyethylen terephthalate film of like thickness for comparison. The samples are immerse in water in an autoclave which is heated to 120°C for 48 hours. Tensil properties measurements show that under these conditions the PENB copolyester film retaines 84% of initial strength, while the PET film retaines onl 52% . EXAMPLE 2 The dielectric properties of the biaxially oriented film in Example 1 ar measured in comparison to standard commercial PET at 1 kHz, and 50% r.h. : As can clearly be seen the biaxially oriented PENBB films according to th invention are superior over common biaxially oriented PET films. laims

Claims: THAT WHICH IS CLAIMED IS: 1 . Biaxially oriented copolyester film containing inert particles, characterise in that the copolyester is PENBB and in that the inert particles are generated i situ in the course of the polycondensation of the PENBB. 2. Biaxially oriented copolyester film containing inert particles, characterise in that the copolyester is PENBB and in that the inert particles are added to th monomers which form the PENBB before or during the polycondensation of th PENBB. 3. Biaxially oriented copolyester film containing inert particles, characterise in that the copolyester is PENBB and in that the inert particles are added to th PENBB before or during the film-forming process. 4. Biaxially oriented copolyester film according to any one of the preceedin claims wherein the inert particles are selected from the group consisting of: kaolin, talc, silica, MgC0 3 , CaC0 3 , BaCO 3 , CaSO 4 , BaSO 4 , Li 3 PO 4 , Ca 3 (PO 4 ) 3 Mg 3 (P0 4 ) 3 , TiO 2 , LiF, C, organic acid salts of Ca, Ba, Zn, Mn, cross-linke polymers and mixtures thereof. 5. Biaxially oriented copolyester film according to any one of the preceedin claims wherein the shape of the inert particles is irregular, flaky, spherical o elongated . 6. Biaxially oriented copolyester film according to any one of the preceedin claims wherein the average diameter of the inert particles is less than 1 0 μm. 7. Biaxially oriented copolyester film according to any one of the preceeding claims wherein the inert particles are present in an amount of 0.01 to 2.00 weιght-%, based on the total weight of the copolyester film. 8. Biaxially oriented copolyester film according to any one of the preceeding claims wherein the film has a thickness of 1 5 - 350 μm. 9. Biaxially oriented copolyester film according to any one of the preceeding claims wherein the film contains up to 5 % colored pigment material. 10. Biaxially oriented copolyester film according to any one of the preceeding claims which has been coated with an adhesion promoting primer or subjected to surface treatment by corona, plasma or flame. 1 1 . Biaxially oriented copolyester film according to any one of the preceeding claims wherein the birefringeance of the PENBB film is < 0.2 and wherein the IV-value of the PENBB is > 0.5 dl/g. 1 2. Use of a film according to any one of claims 1 to 1 1 as a substrate for flexible printed circuits. 1 3. Use of a film according to any one of claims 1 to 1 1 as cable and transformer insulation. 14 Use of a film according to any one of claims 1 to 1 1 for the production of membrane switches or touch panels. 1 5. Use of a film according to any one of ciaims 1 to 1 1 for motor and generator insulation

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: B32B33
IPC codes: B29C55, B29D7, B29K67, B29L7, B32B15, B32B27, C08G63, C08J5, C08K3, C08K7, C08L67, D06N7, H05K1