EP0513252A1

EP0513252A1 - High purity aromatic polyester, a film and other products and devices containing such polyester as well as a process for preparing such film

Info

Publication number: EP0513252A1
Application number: EP91915765A
Authority: EP
Inventors: Peter Fialla; Edward F. Hampl, Jr.; Mohammed M. Iqbal
Original assignee: Isonova Technische Innovationen Ges mbH
Current assignee: Isonova Technische Innovationen Ges mbH
Priority date: 1990-08-28
Filing date: 1991-08-28
Publication date: 1992-11-19
Also published as: WO1992003493A1; CA2071477A1

Abstract

Polyester aromatique extrêmement pur constitué principalement d'ensembles à répétition interpolymérisés, dérivés de 1,1-bis(4hydroxyphényle)-1-phényléthane et 9,9-bis(4-hydroxyphényle)fluorène ainsi que de chlorure d'acide isophthalique et/ou de chlorure d'acide téréphthalique, ledit polyester contenant moins de 4 % en poids d'espèces à poids moléculaire faible dont le poids moléculaire est inférieur à 8000 et formant des pellicules dont le point de rupture à l'élongation est d'au moins 50. On prépare ces polyesters au moyen d'un nouveau procédé de polymérisation interfaciale, de ce fait la proportion d'espèces à poids moléculaire faible est encore diminuée pendant la précipitation du polymère en utilisant, par exemple, MEK ou de l'acétone comme agent de précipitation. Les polyesters possèdent une excellente résistance à la traction et à l'élongation, ainsi que d'excellentes capacités de résistance chimique, à l'humidité et à la température, ainsi que des propriétés électriques telles qu'une résistance volumique et superficielle élevée, une résistance aux courants de fuite et à la formation d'arc, une constante et une résistance diélectriques élevées et un facteur de dissipation bas. On peut utiliser avantageusement le polyester comme pellicule ou couche déposée sur un substrat. On peut, par exemple l'utiliser comme isolant électrique, comme moyen diélectrique, particulièrement, dans un condensateur à enroulements en pellicules et comme moyen optique pour des fibres optiques.Extremely pure aromatic polyester consisting mainly of interpolymerized repeating assemblies, derived from 1,1-bis (4hydroxyphenyl) -1-phenylethane and 9,9-bis (4-hydroxyphenyl) fluorene as well as isophthalic acid chloride and / or of terephthalic acid chloride, said polyester containing less than 4% by weight of low molecular weight species having a molecular weight less than 8000 and forming films having an elongation breaking point of at least 50 These polyesters are prepared by means of a novel interfacial polymerization process, therefore the proportion of low molecular weight species is further reduced during precipitation of the polymer using, for example, MEK or acetone as an agent. of precipitation. Polyesters have excellent tensile strength and elongation, as well as excellent chemical, moisture and temperature resistance capabilities, as well as electrical properties such as high volume and surface resistance, resistance to leakage currents and arcing, high dielectric constant and resistance, and low dissipation factor. Polyester can advantageously be used as a film or layer deposited on a substrate. It can, for example, be used as an electrical insulator, as a dielectric medium, particularly in a capacitor with film windings and as an optical medium for optical fibers.

Description

HIGH PURITY AROMATIC POLYESTER, A FILM AND OTHER PRODUCTS AND DEVICES CONTAINING SUCH POLYESTER AS WELL AS A PROCESS FOR PREPARING SUCH FILM.

Technical Domain

The invention relates to a polyester consisting essentially o repeating interpolymerized units derived from 1,l-bis(4-hydro xyphenyl)-l-phenylethane or 9,9-bis(4-hydroxyphenyl)fluoren and isophthalic acid chloride and/or terephthalic acid chlori de. The invention relates further to a film and other product and devices and containing such polyester as well as a proces for preparing such film.

State of the Art Such polyesters are known per se from the EP-0064971 (derive from 1,l-bis(4-hydroxyphenyl)-l-phenylethane) and from the EP 00064972 (derived from 9,9-bis(4-hydroxyphenyl)fluorene. I these patents is reported that at least some of the polyeste have a "binodal" molecular weight distribution wherein 10 - 20 of the polyester weight is comprised of low molecular species.

The importance of the molecular weight of the 9,9-bis(hydroxy phenyl)-fluorene or 1,l-bis(4-hydroxγphenyl)-l-phenylethane isophthalic and terephtalic acid polyester (hereinafter, fo brevity, occasionally referred to as the FPE and CPE copolymer respectively) in achieving certain desirable physical proper ties has been discussed in the art. As noted above, the cite European Patent Specifications views the presence of 10 - 20 of oligomeric species as producing an advantageous FPE or CP coplymer property. Applicant was the first to note that eve presence of small amounts (i.e., between 2 and 10% of tota polyester weight) of oligomer species in the FPE copolymer ca result in inferior properties. This is true even where th copolymer has a high average molecular weight or high inheren viscosity. The applicant has determined that small amounts of oligomeric species in the FPE or CPE copolymer results in increased sol¬ vent sensitivity, lower elongation, poorer tensile strength, poorer chemical resistance, dimensional instabilitiy and variation in electrical properties when the material is exposed to high temperatures. Thus e.g., the presence of oligomer renders the FPE copolymer unsuitable for many applications such as those in which critical electrical properties must be main- tained under high temperature conditions. Dimensional instabi¬ lity and chemical instability renders the material unsuitable for application wherein the material is thermally printed. Applicant has further determined that the presence of even small amounts of oligomer will make the FPE copolymer sensitive to ultraviolet radiation and unstable under vacuum conditions. Films containing small amounts of oligomer will "yellow" o degrade upon limited exposure to ultraviolet radiation. This greatly limits the use of the film for applications in whic the optical properties of the film are important, such as packaging material. Vacuum instability results in weight los from the copolymer under vacuum conditions. This renders th material unsuitable for applications in which the material wil be subjected to high vacuum conditions (e.g., 10^~6 torr), suc as applications where vapor deposition or sputtering processe are used.

Description of the Invention

The invention has the primary objective to disclose polyester of the kind described above which contain very low level o oligomeric material.

The polyester according to the invention is characterized i that it contains less than 4% by weight of low molecula species of a molecular weight less than 8000 and forms film having an elongation at break of at least 50%. For polyesters derived from 1,l-bis(4-hydroxyphenyl)-l-phenylethane th elongation at break is advantageously at least 60%. For mos uses, the weight-average molecular weight is at least 500,000 and for other uses it is preferably at least 700,000 or 900,00 or higher.

The invention relates further to a film comprising a polyeste according to the invention the thickness of which is at least microns. Such film can advantageously have a vacuum-deposite coating which can comprise a material suitable for a magneti recording layer.

The invention further relates to a tape comprising th polyester film according to the invention coated with a adhesive.

The invention is further concerned with an electrical devic comprising an electrical conductor insulated with the polyeste according to the invention, which preferably has less than on percent oligomeric species. This conductor advantageously ca be a wire or a metallic foil or it can be present as a layer o electrically conductive material. Advantageously the polyeste is present in the form of film wrapped around the conductor According to another embodiment the electrical conductor i present as a layer of electrically conductive material, whereb the device incorporates a plurality of said layers assembled i overlying relationship, and said polyester is present as film or layers interleaved between adjacent layers of sai electrically conductive material. Thereby such layers of elec trically conductive material is vapor-deposited onto sai polyester layer. Such device can advantageously by a film woun capacitor.

According to the invention the polyester can be used as conductor of light, and thereby may be present in the form of an optical fiber or it may be present in a coating cladded onto an optical fiber.

The invention further relates to a fibrous web comprising fibers of the polyester according to the invention.

Finally, the invention is concerned with a process of preparing the polyester film comprising the steps of: a) dissolving the polyester in an appropriate organic solvent, b) casting a film of the desired thickness of the solution prepared in step (a) on a substrate, and c) evaporating the solvent and optionally removing the dried film from the substrate.

The invention further provides a novel interfacial polymerization process for the preparation of the polyeste according to the invention.

Detailed Description of the Invention

Some of the polyesters of this invention consist of repeatin groups represented by the formula

-(-O-X-O-Y-)n- (I)

where X is a divalent organic radical derived from eithe terephthalic or isophtalic acid and each X is independently selected, Y is derived from 9,9-bis-(4-hγdroxyphenyl)fluoren or l,l-bis(4-hydroxyphenyl)-l-phenylethane, n is a whole numbe greater than or equal to 1. Applicants have been able to obtai very narrow molecular weight distributions. The higher th molecular weight, the better for many uses. For most uses, th weight-average molecular weight is at least 500,000, and for other uses it is preferably at least 700,000 or 900,000 o higher. The polyesters of this invention can be prepared by reactin bisphenols, 9,9-bis-(4-hydrxyphenyl)fluorene or l,l-bis(4 hydroxyphenyl)-l-phenylethane or functional derivatives thereo (e.g., an alkali metal diphenolate such as sodium, potassium or lithium diphenolate) with at least one acid selected fro the group consisting of terephthalic and isophthalic acid, o functional derivatives thereof (e.g., acid halides o anhydrides), in the presence of a phase transfer catalyst using an interfacial polymerization method.

While interfacial polymerization methods have been used t produce the FPE or CPE copolymer, the use of the nove interfacial polymerization method described herein permit production of the oligomer-free FPE or CPE copolymer.

The compounds, 9,9-bis(4-hydroxyphenyl)fluorene and 1,1-bis(4 hydroxyphenyl)-l-phenylethane, are known in the art and can b prepared according to a procedure similar to that described i U.S.Pat.No.4,503,266 (Szabolcs), which is herein incorporate by reference. Briefly, the process of preparation comprise reacting phenol and, e.g., fluorenone in the presence o gaseous hydrogen halide and catalytic amounts of at least on bivalent, trivalent or tetravalent metal halide. The 9,9-bis (4-hydroxyphenyl)fluorene used in this invention must b extremely pure, i.e., at least 99,8% pure and preferably 99,9 to 99,95% pure as measured by high pressure liquid chromato graphy. 9,9-bis(4-hydroxyphenyl)fluorene which is 99,95% pur typically has a melting point of 225.5 - 227°C as measured b differential scanning calorimetry (DSC) while the l,l-bis(4- hydroxyphenyl)-l-phte.nylethane of the same purity has a meltin point of 188 - 190°C. The higher the level or ortho-para-isome or dimer of unreacted phenol, the larger the fraction of oligo¬ meric species that will be present in the organic phase fro which the final FPE copolymer is precipitated. Generally, the higher the level of oligomeric species in the organic phase prior to precipitation, the higher the fraction of oligomeri species which will result in the FPE copolymer.

Isophthalic and terephthalic acid, acid halides thereof, an anhydrides thereof are known in the art. For purposes of thi invention, these compounds must be anhydrous, i.e, they mus contain no water of hydration or free water. Before using thes materials, they should be purified, e.g., by vacum distillin and using immediately. These compounds must also be at leas 99,90% pure as measured by high pressure liquid chromatography, and preferably they are at least 99,95% pure.

In reaction with the bisphenols in question, the acid chloride or their derivatives may be used individually or as mixtures, and are preferably used in a ratio of 70 to 30% of terephthali acid to 30 to 70% by weight of isophthalic acid. Most prefe rably, an equimolar mixture of terephthalic acid and isoph thalic acid are employed in this invention.

The phase transfer catalysts suitable for this inventio include tertiary amines, tertiary sulfonium salts an quaternary ammonium, phosphonium, and arsenium salts. Thes compounds are known in the art and are either commerciall available or can be synthesized using known methods. The phas transfer catalysts may be used individually or as mixtures, bu preferably they are used individually. The phase transfe catalysts need not be as pure as the bisphenols or dicarbox cylic acids, analytical reagent quality is sufficient pure fo use in this invention.

Representative phase transfer catalysts suitable for use i this invention include benzyltriethylammonium chloride an tetrabutylammonium iodided and blends thereof. Analytica reagent grade phase transfer catalyst is sufficiently pure o use in this invention. The alkaline hydroxides include materials which can generate in admixture with water, a pH of at least ten. The preferre alkaline hydroxides comprise the alkali metal hydroxides, e.g. sodium, lithium, potassium hydroxides. Sodium hydroxide i highly preferred and is employed in a mole ratio of about 2. to 3.0 moles of hydroxide per mole of the bisphenol, and prefe rably at a mole ratio of 2.3 moles of hydroxide per mole of th bisphenol. Levels of hydroxide above 3.0 moles are discourage since it merely increased the level of ionic impurity in th organic phase prior to precipitation of the copolymer whic must be removed by washing steps. Very high purity of th alkaline hydroxides is not required. Commercially availabl analytical reagent (AR) quality materials are suitable for us in this invention.

The preferred method of producing the oligomer-free FPE or CP copolymer comprises preparation of a preemulsion mixture com prising the bisphenol and phase transfer catalyst. The preemul sion is prepared by making a mixture of the bisphenol (at leas 99,5% pure), sodium hydroxide, distilled water, and an organi cosolvent such as dioxane or acetone. A 2.3 to 1 molar ratio o sodium hydroxide to bisphenol is employed. The organic cosol vent need not be anhydrous, however if dioxane is used, it mus be stripped of peroxide compounds, e.g., passing it throug deactivated basic aluminium oxide. The resulting mixture i heated to its boiling point, cooled to room temperature, an charged into an appropriate reaction vessel equipped with high speed stirrer and thermometer. Preferably the reactio vessel is equipped with a cooling jacket. To this stirre mixture, a room temperature solution of the phase transfe catalyst, preferably benzyltriethylammonium chloride, dis solved in distilled water is added, followed by a chilled, organic solvent such as 1,2-dichlorethane (DCE) or methylen chloride forming the preemulsion. It is adavantageous to chill the organic solvent. To the stirred preemulsion, a mixture con- taining an equimolar amount of isophthaloyl chloride an terephthaloyl chloride in an anhydrous, organic solvent such a DCE is added. Stirring is continued while the reaction pro ceeds. The pH of the reaction mixture should then be adjuste such that it is acidic; preferably to a pH of about 2 to 3. Th resulting solution is then allowed to stand until an aqueou phase separates from an organic phase. The organic phase con tains the copolymer. Additional DCE may be added if necessar to reduce the solution viscosity to allow stirring. The resul ting organic phase is then mixed with approximately an equa volume of deionized water to extract water soluble impurities such as ionic impurities, from the organic phase. The water an dissvolved impurities are allowed to separate from the organi phase and are decanted. This procedure is repated until th conductivity of the decanted water is 5 μS (micro-Siemens) o less. After the final washing step, the oligomer-free copolyme is precipitated from the organic phase by stirring into th organic phase an excess (approximately two times the organi phase volume) of an organic solvent with selective solubility that is an organic solvent in which the oligomeric species ar soluble but the polymer is not soluble. The preferred organi solvents for precitipation are the lower ketones containing to 10 carbon atoms, more preferred are those containing 3 to carbon atoms such as methyl ethyl ketone. The most preferre organic solvent for precipitation is acetone. Optionally, th precipitated polymer may be washed with distilled water. Th polymers of this invention can be cast into films or coating using known methods such as dissolving the polymer is a appropriate organic solvent, casting or forming a film of th solution on the desired substrate, evaporating the solvent, an if desired, removing the film from the substrate. The polymers of this invention can be formed into self-supporting films a least as thick as 5 microns. The dissolved polymer can also b coated on to metal foils, wire or other substrates in thick nesses at least as thick as 0.5 micron. The FPE and CPE copolymers of this invention have excellen tensile strength and elongation, chemical resistance, moistur resistance, temperature resistance (e.g.,FPE is nonflammabl and dimensional stability to 300°C), electrical propertie (e.g., high volume resistivity, high surface resistivity, hig tracking and arc resistance, high dielectric constant, hig dielectric strength, and low dissipation factor), ultraviole resistance, vacuum stability, and exhibit good adhesion t metals. As a result, films or coatings of the FPE material ca be used for many applications in which polyimides, such a Kapton available from E.I. Dupont deNemours & Company, ar currently used. Electrical conductors may be insulated b wrapping the conductor with a f lm, or with an adhesive-coate film or tape, or by applying a solution of the polymer, (e.g., dissolved in n-methylpyrrolidone, cyclohexanone, or tetrahydro- furan) to the conductor and evaporating the solvent. Th coating or film has a high volume resistivity over a wide range of temperatures up to at least 300°C. Film-wound capacitors using film of the invention as the dielectric are especiall useful because of their high-temperature capability (to 200°C or higher) , low dissipation factor over the entire temperature range, high dielectric strength and high dielectric constant. Such capacitors can be made by conventional techniques, e.g., winding alternating layers of film of the invention and metal¬ lic conductive foils or vapor-depositing metallic conductors onto film of the invention and wrapping such metal-coated films in a spiral. Film of the invention is also especially useful as a substrate for vacuum-deposited coatings such as aluminium, copper, or magnetic recording layers, such as chrome or cobalt.

Film of the invention is also useful as a substrate for thermal printing processes. Polyesters of the invention are also useful in optical uses because of the desirable higher index of re- fraction, low birefringence coefficient, and ability to with¬ stand high temperatures. For example, the polyesters can be used as optical fibers or as cladding for optical fibers. It also can be used in optical recording discs, where its high dimensional stability is of advantage.

Polyesters of the invention are also useful as fibers, e.g., as solution-blown fibers and microfibers or as spun-drawn fibers.

The following non-limiting examples are provided to further illustrate the invention.

To start, a general procedure is described, wherein the general term "bisphenol" is used instead of 9,9-bis(4-hγdroxyphenol)- fluorene or l,l-bis(4-hydroxyphenyl)-l-phenylethane.

For the preparation of the solution of the bisphenolate 9 moles of the bisphenol, 20.7 moles of the alkali hydroxide and 0.9 moles of benzyltriethylammoniumchloride are dissolved in a mixture of 27 liter deionized water and 9 liter of the freshly distilled solvent defined later on in the specific examples in a 200 liter vessel that is equiped with a high speed stirrer and thermometer.

For the preparation of the acid dichloride solution 9 moles of the freshly distilled acid chlorides, i.e. 4.5 moles of isoph- thaloyl chloride and 4.5 moles of terephthaloyl chloride ar dissolved in 6 liters of thoroughly dried chlorinated hydro carbon also defined in the specific examples.

Next, a preemulsion of the so-called aqueous phase containin the bisphenolate with the said chlorinated hydrocarbon is prepared by mixing into the aqueous phase 45 liter of chille chlorinated hydrocarbon-with the high speed stirrer for abou 2-2.5 minutes and thereafter stirred for additional five mi nutes to get said premulsion while its temperature is kep between 15 and 20°C. Thereafter, while still stirring, the acid-dichloride solution is added to the preemulsion durin about 2 minutes and stirred for a further five minutes. Durin this time the viscosity of the reaction mixture increases an the temperature rises from 15 to 20°C to about 30°C. Then th mixture is neutralized with 270 ml of aqueous HC1 (37%).

After stopping the stirring, the mixture is allowed to stan for 15 minutes until the aqueous phase separates as an uppe layer from the organic phase. After decanting the aqueou phase, the remaining organic phase, which contains the synthe sized polyester in solution is washed four times with 25 lite or deionized water each time. After the final washing step th conductivity of the aqueous phase after washing is 5 μS (micro Siemens) or less.

After the final washing 50 liter of acetone is slowly adde with stirring to the remaining aqueous phase to precipitate th copolyester. The precipitate is collected in a centrifuge an washed with acetone and then with water. The resulting copoly mer is then oven dried at 120°C for 18 hours.

For determining the inherent viscosity (IV in dl/g), viscosit measurements are made with a solution of 125 mg of polyester i 25 ml of a mixture of 60 parts (by weight) of phenol and 4 parts 1-,1-,2-,2-tetrachloroethane at 30°C.

Molecular weight distribution is measured with a high pressur liquid chromatograph Hewlett Packard 1090 with five ULTRASTYRA GEL gel permeation columns in series (10^A, 10^A, 10^A, 10-^ and 500A of Water Associates using the Water Associates poly¬ styrene standards). The solvent is 1,2-dichlorethane, distille over CaH2 and filtered through a 0.5 μm filter (type FH, Milli pore) : The detector is a filter photometric detector wit deuterium discharge lamp (190 nm to 600 nm wave length wit maximum emitted energy at 240 nm and a filter at 254 nm) . For preparing the samples, 0.1 mg of the polyester are dissolved i 5 ml of the solvent and filtered through a 0.5 μm Millex S filter unit (Millipore) . The flowrate of the pure solvent i 0.8 ml, the injected volume of a sample 100 μl.

By the general procedure described above copolyesters have bee prepared according to the specific examples 1 through 6 a following:

EXAMPLE 1

Bisphenol: 9,9-bis(4-hydroxyphenyl)fluorene chlorinated hydrocarbon: 1,2-Dichloroethane cosolvent: Dioxane alkali hydroxide; NaOH results: IV = 3,40 dl/g

MW = 1,053,000 low molecular weight content

(MW < 8000) : 1.6%

EXAMPLE 2

Bisphenol: 9,9-bis(4-hydroxyphenyl)fluorene chlorinated hydrocarbon: 1,2-Dichloroethane cosolvent: Dioxane alkali hydroxide: IV = 3.10 dl/g

MW = 964,000 low molecular weight content

(MW < 8000) : 1.8%

EXAMPLE 3 Bisphenol: 9,9-bis(4-hydroxyphenyl)fluorene chlorinated hydrocarbon: Dichloroethane cosolvent: Acetone alkali hydroxide: LiOH results: IV = 2.85 dl/g

MW = 724,000 low molecular weight content

(MW < 8000) : 2.5% EXAMPLE 4

Bisphenol: 9,9-bis(4-hydroxyphenyl)fluorene chlorinated hydrocarbon: Dichloromethane cosolvent: Acetone alkali hydroxide: KOH results: IV = 3.80 dl/g

MW = 1,133,000 low molecular weight content

(MW < 8000) : 2.4%

EXAMPLE 5

Bisphenol: 1,1-bis(4-hydroxyphenyl)-1-phenylethane chlorinated hydrocarbon: Dichloromethane cosolvent: Acetone alkali hydroxide: KOH results: IV = 3.24 dl/g

MW = 823,000 low molecular weight content

(MW < 8000) : 3.3%

EXAMPLE 6

Bisphenol: 1,1-bis(4-hydroxyphenyl)-1-phenylethane chlorinated hydrocarbon: Dichloroethane cosolvent: 2-Propanol alkali hydroxide: NaOH results: IV = 2.05 dl/g

MW = 299,100 low molecular weight content

(MW < 8000) : 4.0%

In the following examples comparative tests were made t demonstrate the influence of the precipitant on the oligomers content. A copolyester was synthesized according to the general procedure as described above with

EXAMPLE 7

Bisphenol: 9,9-bis(4-hydroxyphenyl)fluorene chlorinated hydrocarbon: 1,2-Dichloroethane cosolvent: Acetone alkali hydroxide: KOH results: IV = 2.37 dl/g

MW = 545,100 low molecular weight content (MW < 8000) : 3.42%

Instead of precipitation with acetone the copolyester was precipitated out of the organic phase of the same batch wit different precipitants according to the following table:

Example Precipitant MW IV Low Molecular Weight Conten (MW < 8000)

%

8 Methanol 415,000 2.00 7.24 9 Isopropano1 (IPA) 436.500 2.09 9.89

10 Acetone/IPA = 1:1 487,000 2.08 5,97 11 Methalethalketone 614,300 2.31 2.50

Experience has shown that the lower content of lower molecula weight species in the FPE or CPE copolyesters is, the better o better reproduceable are the electrical properties of the film case from FPE or CPE copolyester, respectively. This may be result of the fact that lower content of low molecular weigh species leads to a reduced content of polyester end groups. Because a higher end group concentration may reduce electrica properties of the polyester; lower end group concentration are, therefore, advantageous.

Further the films cast from the polymers FPE and CPE having such low content of low molecular weight species have elonga¬ tions at break (measured according to ASTM D882-75) of more than 50% with a low mean variation. Higher content of low molecular weight species reduces the elongation at break. The films cast from the polyester according to this invention have for FPE films 50 - 70% and for CPE films 60 - 100% elongations at break in a good reproduceable manner.

Claims

CLAIMS :

1. A polyester consisting essentially of repeating inter polymerized units derived from 1,1-bis(4-hydroxyphenyl)-l phenylethane or 9,9-bis(4-hydroxyphenyl)fluorene and iso phthalic acid chloride and/or terephthalic acid chloride characterized in that the polyester contains less than 4% b weight of low molecular weight species of a molecular weigh less than 8000 and forms films having an elongation at break o at least 50%.

2. A polyester of claim 1, characterized in that the lo molecular weight species is less than 2% by weight, preferabl less than 1% by weight.

3. A polyester of claim 1, based on 1,1-bis(4-hydroxyphenyl) 1-phenylethane, characterized in that it has an elongation a break of at least 60%.

4. The polyester of claim 1, characterized in that it has weight-average molecular weight of at least 500,000.

5. The polyester of claim 1, characterized in that it has weight-average molecular of at least 700.000.

6. The polyester of claim 1, characterized in that it has weight-average molecular weight of at least 900,000.

7. The polyester of claim 1, characterized in that it i derived from equimolar amounts of isophthalic and terephthali acid.

8. A film, characterized in that it is comprising th polyester of claim 1.

9. The film of claim 8, characterized in that its thickness is at least 5 microns.

10. The film of claim 8, characterized in that it has a vacuum-deposited coating.

11. The film of claim 10, characterized in that said vacuum- deposited coating comprises a material suitable for a magnetic recording layer.

12. A tape, characterized in that it is comprising the film of claim 8 coated with an adhesive.

13. An electrical device, characterized in that it is comprising an electrical conductor insulated with the polyester of claim 1.

14. An electrical device, characterized in that sai electrical conductor is a wire.

15. The device of claim 13, characterized in that sai electrical conductor is a metallic foil.

16. The device of claim 14, characterized in that sai electric conductor is present as a layer of electricall conductive material, the device incorporates a plurality o said layers assembled in overlying relationship, and sai polyester is present as film interleaved between adjacen layers of said electrically conductive material.

17. The device of claim 16, characterized in that said laye of electrically conductive material is vapor-deposited ont said polyester layer.

18. The device of claim 16, characterized in that said devic is a film wound capacitor.

19. The device of claim 16, characterized in that sai polyester layer has less than one percent oligomeric species.

20. The device of claim 13, characterized int that sai polyester is present in the form of film wrapped around th conductor.

21. An optical device, characterized in that the polyester o claim 1 is used as a conductor of light.

22. The optical device, characterized in that the polyester i present in the form of an optical fiber.

23. An optical fiber, characterized in that it is cladded wit a coating comprising the polyester of claim 1.

24. Fibrous web, characterized in that it is comprising fiber of the polyester of claim 1.

25. A process of preparing the film of claim 8 comprising th steps of: a) dissolving the polyester in an appropriate organic solvent, b) casting a film of the desired thickness of the solutio prepared in step (a) on a substrate, and c) evaporating the solvent and optionally removing the dried film from the substrate.