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Publication numberUS2921869 A
Publication typeGrant
Publication dateJan 19, 1960
Filing dateMay 22, 1956
Priority dateMay 22, 1956
Publication numberUS 2921869 A, US 2921869A, US-A-2921869, US2921869 A, US2921869A
InventorsThomas Mcbride Richard
Original AssigneeDu Pont
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Coated polymeric thermoplastic dielectric film
US 2921869 A
Abstract  available in
Images(4)
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Claims  available in
Description  (OCR text may contain errors)

COATED POLYIVIERIC THERMOPLASTIC DIELECTRIC FILM N Drawing. Application May 22, 1956 Serial No. 586,359

17 Claims. (Cl. 117138.8)

This invention relates to a process of improving the dielectric life of polymeric thermoplastic materials useful as electrical insulation and to the product of such process and, more particularly, to a coated polyethylene terephthalate film having an improved dielectric life.

Polyethylene terephthalate film, particularly oriented film (film which has been stretched and/ or rolled in two directions and heat-set at elevated temperatures within the range from 150-250 C.) has been found to possess a unique combination of electrical, physical and chemical properties which make it outstanding for use as a dielectric. That is particularly true of polyethylene terephthalate film which has been stretched and/ or rolled to substantially the same degree in both directions, e.g., 3X, where X equals the original dimension of the film, to obtain a substantially balanced film, which means that the physical properties of the film, as measured in both directions, are substantially the same. The oriented, heat-set, polyethylene terephthalate film is outstanding as a dielectric because it retains a considerable percentage of its electrical properties, strength, and durability at elevated operating temperatures. Consequently, the film is particularly useful as a dielectric in capacitors, motors, generators, transformers, etc., at not only moderate operating temperatures, but also at ambient temperatures approaching 150-l75 C.

In extending the usefulness of polyethylene terephthalate and like thermoplastic polymer film dielectric to a wider variety of electrical end uses, it became apparent that the dielectric life of the film required improvement, particularly for use in equipment subjected to high voltage stresses. As employed herein, the term dielectric life of the polymeric thermoplastic film applies to the actual time that a particular film may be subjected to a particular voltage stress under conditions of corona discharge before actual physical breakdown, i.e., rupture, of the film dielectric.

It is the action upon the polymeric film dielectric of corona discharge (in air or other gaseous medium) which causes actual physical breakdown of the film under conditions of a particular voltage stress (a potential difference large enough to produce a visible discharge but not large enough to produce instantaneous breakdown). Obviously, the lower the voltage stress, the longer the dielectric life of the film. Corona discharge? is defined as the discharge of electricity which appears upon the surface of a conductor when the potential gradient exceeds a certain value, For example, when a continuous potential applied to a pair of narrow wires is slowly incre'asedfthe voltage will'be reached at which a hissing noise is heard, and a pale violet light is visible (in the dark) around the'wir'e'sl" This'voltage is defined as the critical visual corona point. Corona is due to ionization of the air or surrounding gases (it does not occur in a vacuum). That is, the air in the ionized region is conducting, and this results in increasing the effective diameter of the conductor. For example, even though United States Patent a film dielectnc is wound tightly around a wire, air is not entirely excluded from the space between the conductor and the film dielectric; and it is the air or other gaseous medium which is ionized. The corona envelopes a conductor as a concentric cylinder, and the outside diameter becomes such that the gradient at that point decreases to the rupturing point of the air. Beyond this the film eventually ruptures; hence, this causes short cir-' cuits owing to the physical failure.

An object of the present invention is to provide a polymeric thermoplastic dielectric film having increased dielectric life. A further object is to provide a polyethylene terephthalate film dielectric having increased dielectric life. A further object is to provide a process of improving the dielectric life of polymeric thermoplastic dielectric compositions, particularly polyethylene terephthalate film. A still further object is to provide a novel coating composition useful for improving the dielectric life of polymeric thermoplastic dielectric film. Other objects will be apparent from the following description of the invention.

These objects are realized in accordance with the present invention which, briefly stated, comprises providing a dielectric base film of thermoplastic polymeric material, e.g., polyethylene terephthalate, with a relatively thin, adherent, continuous coating consisting essentially of a homogeneous mixture of from 95% to 40% by Weight of (A) a cured resinous compound selected from the group consisting of (l) phenol formaldehyde resins, (2) alkyd resins resulting from the inter-reaction of a polybasic carboxylic acid and a polyhydric alcohol, (3)

long chain fatty acid-modified alkyd resins, and (4) the condensation product of epichlorohydrin and bis p,p'- hydroxyphenyldimethylmethane, and from 5% to 40% by Weight of (B) finely divided particles from the group consisting of amorphous silica and estersils, said particles having an ultimate average particle size between 5 millimicrons and 5 microns.

The present invention will be further described specifically with respect to improving the dielectric life of polyethylene terephthalate film but it should be understood that the invention is applicable as Well to enhancing the dielectric life of other types of unplasticized polymeric thermoplastic films useful as dielectrics, such as fihns of polyethylene, polytetrafiuoroethylene, polystyrene, polyamides, etc.

Any phenol-formaldehyde resin is operable in this invention; It has been found that both heat-reactive and non-heat-reactive resins (i.e., thermosetting resins and non-thermo-setting resins) and both oil-reactive and nonoil-re'active resins are'operative. The term phenol includes beside phenol itself, phenol derivatives such as cresol, paraphenylphenol, xylenol, ethyl phenols, salicyclic acid, and resorcinol and chlorinated phenols. The oilreactive phenols may be modified by the addition of up to by weight of a drying or se '-drying oil. Preferably, the oil is added to a solution of the resin in a volatile organic solvent, e.g., toluene, xylene, mineral spirits, etc.

The alkyd resins utilized in the coating compositions characterizing the present invention may be prepared in known manner by inter-reacting a polycarboxylic acid or the anhydride thereof, having from 2 to 3 carboxyl of from 15% toj8 0% of a higher fatty acid, i.e., a 1

fatty acid having at least 8 carbon atoms. The fatty.

acids are preferably selected from the group comprising unsaturated dry oil type fatty acids, e.g., coconut oil acid,

and saturated non-drying monocarboxylic fatty acids, e.g., Z-ethyl hexoic acid.

The epoxide resins employed in this invention are formed by the condensation of epichlorohydrin and bisp,p'-hydroxy-phenyldimethylmethane in the presence of sodium hydroxide. During the condensation HCl or NaOl is split out to form polyethers which have epoxy groups on the end of the chain. The degree of polymerization is expressed in terms of epoxy equivalents. The term epoxy'equivalent is defined as the weight of the resin in grams which contains 1 gram equivalent Weight of the epoxy groups. The resins of the above type are available commercially. One brand'of'such resins is sold by the Shell Chemical Corporation under the name Epon.1 7

Types of finely divided particles which are to be blended with the blend of alkyd or phenolic resins are selected from the following compounds: (1) amorphous silica, and (2) estersils as defined in U.S. Patent 2,657,149. The essential requirement is that whenever any of the above compounds are blended with the alkyd, phenolic, or epoxy resins characteristic of the coating compositions of the present invention, the compound must be in finely divided form such that the ultimate particles, either in the form of super-colloidal aggregates or. individual particles, have an average greatest dimension ranging between 5 millimicrons and 5 microns, and preferably below 50 millimicrons. Some types of the above compounds may not be commercially available in finely divided form wherein the average particle size is 5 microns or less. In such cases, it is necessary to process the particles by ball-milling or other techniques to produce the size of the average particle.

The commercially available types of amorphous silicas may be satisfactorily employed in the present invention.

These silicas may be arbitrarily divided into four 'classifications, namely: aerogel, precipitated, vapor phase, and diatomaceous types. Santocel CS, manufactured by the Monsanto Chemical Company is an example of the aerogel type. This is slightly acidic and contains a small amount of alcohol, water and sodium sulfate as the chief impurities. The particlediameter is approximately 30 millimicrons, with a surface area of approximately 100-15O mi /gm. (squme meters per gram). The precipitated silicas may be represented by Hi-Sil X-303 or Hi-Sil XW, manufactured by the Columbia-Southern Chemical Corporation' This material has an average 7 particle diameter of 20-25 millitnicrons and a surface area within the range of 140-160 m. /g'm. A typical representative of the vapor phase type of silica is Aero sil, manufactured by Godfrey L. Cabot, Inc. This material has an average particle diameter approximately 14-20 millimicrons and a surface area of 175-200 mF/gm. The diatomaceous types of silicas may be further divided into two groups which are important in compounding the present coating compositions. These are the heat-calcined, and flux-calcined materials. Both are relatively free from organic contaminants. Particle sizes are'mnch larger than those of the silicas mentioned above, ranging fiom 1-6 microns. The surfaces areas are larger than would be calculated from particle diameter owing to high internal area in this form of silica. In the heat-calcined group are such products as Celite 270, manufactured by Johns-Manville Corporation and Dicalite PS, manufactured by Great Lakes Carbon Corporation. In the flux-calcined group are Dicalite White and Celite Superfloss.

The estersils useful for purposes of this invention are 4 described in U.S. Patent 2,657,149 to Ralph K. Iler. In brief, the estersils are esterified super-colloidal substrates, that is, a substrate in the form of particles coated with OR groups, the substrates having a surface of silica and having a specific surface area of at least 1 m. /gm., the coating of -OR groups being chemically bound to said silica, and R being a hydrocarbon radical of at least 2 carbon atoms wherein the carbon atom attached to oxygen is also attached to hydrogen. In general, the estersils have a specific area of at least 1 m. /gm., usually within the range of l-900 m.'-/gm.

The coating compositions .may conveniently be prepared by first dissolving the resin component ina suitable volatile organic solvent such as toluene, xylene or mineral spirits, and ball-milling the resulting solution admixed with the finely divided particles of silica or estersil to form a homogeneous blend. The composition should contain from 10 to 60%, and preferably from 20 to 40% by weight of total solids.

The coating composition may be applied to one or both surfaces, preferably both surfaces, of the base film by any desired expedient; and the coated film may thereafter be dried at room temperature or moderately elevated temperatures to remove solvent. Curing of the resin/particle blend to harden the resin should be carried out at elevated temperatures, e.g., l00-l75 C., for durations from 10 minutes to 60 minutes. In some instances, air-drying at room temperature, is sufiicient to effect hardening of the coating.

It may be necessary, depending upon the end use for the coated dielectric films of the present invention, to apply a subcoating to the base dielectric film, e.g., oriented, heat-set, polyethylene terephthalate film, to improve adhesion between the base film and the resin/ particle coating. When applying the subject coating compositions to polyethylene terephthalate film, for example, it is preferred to apply a subcoating which is chemically similar to the base film. Any suitable subcoatings may be employed provided that they improve the adhesion betweenthe base film and the present coating and do not adversely affect the original combination of physical, electrical and chemical properties of the base film. Preferred subcoatings for pclyethyleneterephthalate film include copolyesters derived by reacting glycol, terephthalic acid, or low alkyl ester thereof with a second acid 'or alkyl ester thereof from the group consisting of sebacic acid, isophthalic acid and hexahydroterephthalic acid. Normally, in preparing suitable subcoatings, it is preferred that the subcoating composition EXAMPLE 1 The following formulation:

Parts Super-Beckacite 1003 (Thermosetting varnish oil reactive-soluble, phenol formaldehyde resin- Reichold Chemical Company) -L. 75 Estersil (as described in U.S. Patent 2,657,149) 20 Toluene V 173 was mixed (35.5% solids) and placed in a ball-mill apparatus. The contents were agitated for 116 hours. Polyethylene terephthalate (0.5 mil in thickness) was coated with this composition and air-dried at room temperature.

The film was then cured in an oven at C. for 10 minutes.

was mixed (35.3% solids) and agitated in a ball-mill apparatus for 72 hours. Polyethylene terephthalate film (0.5 mil in thickness) was coated with this mixture and air-dried at room temperature. cured in an oven at 150 C. for 10 minutes.

EXAMPLE 3 The following formulation:

Parts Glyptal 2502 (60% solvent solution-glyceryl 'phthalate having high hydroxy content-Gen eral Electric Company) 110.6 Toluene 109 Cab-O-Sil (finely divided silica-Godfrey L.

Cabot, Inc.) 27

was mixed (37.7% solids) and agitated in a ball-mill apparatus for 168 hours. This mixture was applied to 0.5 mil thick polyethylene terephthalate film and the film was air-dried at room temperature. The coated film was then cured in an oven at 150 C. for 10 minutes.

EXAMPLE 4 The following formulation:

Parts Epon (epoxide resinAhell Chemical Company) 40 Estersil (as described in US. Patent 2,657,149) 10 Toluene 100 Methyl ethyl ketone 100 was mixed solids) and agitated in a ball-mill apparatus for 240 hours. Polyethylene terephthalate film (0.5 mil in thickness) was coated with this mixture and air-dried at room temperature. The coated film was cured in an oven at 150 C. for 10 minutes.

The coated film was" thermoplastic film applies to the actual below.

was mixed (37.7%) and agitated in a ball-mill apparatus for 168 hours. This mixture was applied to 1.5 mil thick polyethylene film and the film was lair-dried at room temperature. The coated film was then cured in an oven" at 75 C. 'for 20 minutes.

Table I, below, records the corona life of the coatings described in Examples l5, inclusive. The fifth failure in 10 samples was taken as representative of possible" corona life. of the coated Also listed are overall thickness (gauge) under Examples sides of the film. I, were subjected l-5, inclusive, were coated on both The coated films, as described in Table Dielectric life test on top of the film sample and normal to the brass plate (the film sample was sandwiched between the brass plate and an'end of the brass rod). The rod served as the ground electrode. The end of the rod touching the film sample-was rounded off at a radius of curvature of The pressure exerted on the film was due only to the gravitational forces on the rod. The entire test apparatus was set up in air. Sufiicient voltage was applied to the plate to give a voltage stress of 1,000'volts per mil across the sample. rapid increase in the flow of current between the brass plate and brass rod. At the moment an arc was struck between the electrodes, the are passing through the hole in the film caused by the failure. Theabrupt increase in the flow of current was used to trip a relay giving a record of the failure. Ten samples were treated simultaneously. The time to the failure of the" fifth sample was used as a measure of the dielectric life (under corona discharge conditions) of the material tested. As employed herein, dielectric life of a polymeric time that a par ticular film may be subjected to a particular voltage stress under conditions of corona discharge before actual physical breakdown, i.e., rupture, of the dielectric.

* TABLE 1 1 Dielectric life (corona resistance) of polyethylene terephwith resin/finely Dielectric Life- Wt. Ratio Overall Ball Mill at 1,000 Volts/ Example No. Coating of Gauge Time Mil (Time to Compo- (Thiclmess) ,(Hours) fifth failure 'of nents (Mils) lo-samples) (Hours) PART I0.5 MIL POLYLTHYLENE TEREPHTHALATE FILM None 0. 5 7. 5 "Snper-Beckacite 1 1003/Estersil 80/20 0. 84 '168 36 GlyptaY' 2502 lEstersil 2 80/20 1. 04 72 162 Glyptal" 2502 /C8b-OSil 70/30 1. 28 168 51 Epon 1009 EstersilL 80/20 1. 67 240 94 PART 1I1.5 MIL POLYETHYLEN E FILM 5 "GlyptaY 2502 =/"Oab-o-si1" 70/30 2. 14 168 155 Control N o Coating 1. 5 25 1 Super-Beckacite l003-phenol-formaldehyde resin-Reicholc. Chemical Co.

2 Estersil-desoribed in 17.8. Patent 2,657,149.

3 Glyptal 2502-alkyd resin- 4 Cab-O-Sil"finely divided General Electric Company. silicaG. L. Cabot, Inc.

' Epon"epoxide resin-Shell Chemical Company.

EXAMPLE 5 The following formulation:

Parts Glyptal 2502 solvent solution-glyceryl phthalate having high hydroxy content-General Electric Company) 110.6 Toluene 109 Cab-O-Sil (finely divided silica-Godfrey L.

Cabot, Inc.)

27 jected to high voltage stresses.

film and the temperature at which the corona life of the film was tested; All of the films listed to the dielectric life test as described Failure of the sample was indicated by a' current flowed,

a 40% by weight of a cured ultimate average particle size I claim: 1. An electrical insulating material comprising a polymeric thermoplastic dielectric base film having an adherent continuous coating consisting essentially of from 95 %v to 40% by weight of (A) a cured resinous compound selected from the group consisting of (1) phenol-formaldehyde resins, (2), alkyd resins resulting from the interreaction of a polybasic carboxylic acid and a polyhydric alcohol (3) long chain fatty acid-modified alkyd resins, and (4) the condensation product of epichlorohydrin and his p,p'-hydroxyphenyldimethylmethane and from 5% to 60% by weight of (B) finely divided particles from the group consisting of amorphous silica and estersils, said particles having an ultimate average particle size between 5 millimicrons and 5 microns.

2. The product of claim 1 wherein the base film is polyethylene terephthalate film;

3. The product of claim '1 wherein the base film is balanced, heat-set, polyethylene terephthalate film.

4. An electrical insulating material comprising polyethylene terephthalate base film having an adherent, continuous coating consisting essentially of from 95% to 40% by weight of a cured, phenol-formaldehyde resin and from 5% to 60% by weight of finely divided particles from the. group consisting of amorphous silica and estersils, said particles having an ultimate average particle size between '5, millimicrons and 5 microns.

5. The product of claim 4 wherein the base film is balanced, heat-set, polyethylene terephthalate film.

6. An electrical insulating ethylene terephthalate base film having an adherent, continuous coating consisting essentially of from 95 to alkyd resin resulting from the inter-reaction of a polybasic carboxylic acid and a polyhydric alcohol and from 5% to 60% by weight of finely divided particles from the group consisting of amorphous silica and estersils, said particles having an ultimate average particle size between 5 millimicrons and 5 microns.

7. The product of claim 6 wherein the base film is balanced, heat-set, polyethylene terephthalate film.

8. An electrical insulating material comprising polyethylene terephthalate base film having an adherent, continuous coating consisting essentially of from 95% to 40% by weight of a cured higher fatty acid-modified alkyd resin of a polybasic carboxylic acid and a polyhydric alcohol, andfrom 5% to 60% by weight of finely divided particles from the group consisting of amorphous silica and estersils, said particles having an between 5 rnillimicrons and 5 microns.-

9. The product of claim 8 wherein the base film is balanced, heat-set, polyethylene terephthalate film.

material comprising poly-,

10. An electrical insulating material comprising polyethylene terephthalate base film having an adherent, continuous coating consisting essentially of from 95% to 40% by weight of the cured condensation product of epichlorohydrin and bis p,p'-hydroxyphenyldimethylrnethane, and from 5% to by weight of finely divided particles from the group consisting of amorphous silica and estersils, said particles havingan ultimate average particle size between 5 millimicrons and5 microns.

11. The product of claim 10 wherein the base film is balanced, heat-set, polyethylene terephthalate film. 12. A process for coating non-fibrous polyethylene terephthalate film which comprises applying to said film a dispersion in a solvent of 10-60% solids, said solids consisting essentialy of from to 40% by weight of (A) a cured resinous compound selected from the group consisting of (1) phenol-formaldehyde resins, (2) alkyl resins resulting from the interreaction of a polybasic carboxylic acid andia polyhydric alcohol, (3) long chain fatty acid-modified alkyd resins, and (4) the condensation product of epichlorohydrin and bis p,p'-hydroxy phenyldimethylmethane and from 5% to 60% by weight of (B) finely divided particles from the group consisting of amorphous silica and estersils, said particle size between S millimicrons and 5 microns; driving otf said solvent to dry the coated film; and curing the coated film.

13. A process as in claim 12 wherein the base film is balanced, heat-set, polyethylene terephthalate film.

14. A process as in claim 12 wherein said cured resinous compound is a phenol-formaldehyde resin.

15. A process as in claim 12 wherein said cured resinous compound is an alkyd resin resulting from the interreaction of a polybasic carboxylic acid and a polyhydric alcohol.

16. A process as in claim 12 wherein said cured resinous compound is a long chain fatty acid-modified alkyd I'CSll].

17. A process as in claim 12 wherein said cured resinous compound is a condensation product of epichlorohydrin and bis p,p-hydroxyphenyldimethylmethane.

References Cited in the file of this patent UNITED STATES PATENTS 2,653,891 Gentle et a1. Sept. 29, 1953 2,657,149 Iler Oct. 27, 1953 2,733,523 Beacham Jan. 31, 1956 2,741,568 Hayek Apr. 10, 1956 2,751,316 Philips June 19, 1956 2,753,316 Campbell July 3, 1956 2,760,941 Iler Aug. 28, 1956

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2653891 *Jun 27, 1950Sep 29, 1953British CelaneseProcess of treating fibrous materials to improve their bonds with rubber and the products thereof
US2657149 *Oct 21, 1952Oct 27, 1953Du PontMethod of esterifying the surface of a silica substrate having a reactive silanol surface and product thereof
US2733523 *Oct 30, 1952Feb 7, 1956 murray
US2741568 *Dec 5, 1951Apr 10, 1956Du PontWater insoluble polymeric quaternary ammonium carboxylate salts and the treatment of textiles therewith
US2751316 *Mar 30, 1955Jun 19, 1956Hercules Powder Co LtdCoated film
US2753316 *May 26, 1952Jul 3, 1956Port Oils Inc XProcess of forming a condensation product of an alkyl ether of phenol and an aldehyde and the resulting composition in combination with colloidal silica
US2760941 *Sep 9, 1953Aug 28, 1956Du PontElastomer foam containing colloidal silica
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3154432 *Jun 15, 1961Oct 27, 1964Gen ElectricCoated polycarbonate resin recording sheet
US3373143 *May 24, 1963Mar 12, 1968Ici LtdManufacture of polyurethane solutions
US4085245 *Apr 15, 1976Apr 18, 1978Xerox CorporationTransparencies for color xerographic copies
US4189514 *Mar 17, 1978Feb 19, 1980Graham Magnetics, Inc.Process of making high-temperature magnetic tape
US4931330 *Mar 29, 1988Jun 5, 1990Stier Sam SPrefabricated, slip-resistant surface coating
WO1987000019A1 *Jul 2, 1986Jan 15, 1987Sam S StierPrefabricated, slip-resistant surface coating
Classifications
U.S. Classification428/331, 427/375
International ClassificationC08J7/04, H01B3/30, C08J7/00, H01B3/42
Cooperative ClassificationC08J7/047, H01B3/42, H01B3/30
European ClassificationH01B3/30, C08J7/04L, H01B3/42