TECHNICAL FIELD

The present invention relates generally to thin metallized and non-metallized polymer films that incorpo¬ rate additional coatings and surface functionahzation, which impart application specific properties such as unproved thermal stability, abrasion resistance, moisture barrier and optical effects, that make these films useful in food pack¬ aging applications, electrical applications that include film capacitors and cables, metal evaporated magnetic tapes, printing, decorative wraps and optically variable films for security applications

BACKGROUND ART

Metallized and non-metallized films are commonly used in a variety of electrical, packaging and decorative applications Although the application field is quite broad, the desired properties of the different films are basically the same These desired properties include mechanical strength, thermal and chemical resistance, abrasion resis¬ tance, moisture and oxygen barrier, and surface functionality that aids wetting, adhesion, slippage, etc As a result, a multitude of hybrid films have been developed to service a wide range of applications

In general, hybrid metallized and polymer coated films utilize a variety of production methods For exam¬ ple metallized polymer films are usually corona-, flame-, or plasma-treated, to promote adhesion of the metal to the polymer surface (U S Patents 5,019,210 and 4,740,385), or ion beam-treated and subsequently electron beam- charged to promote adhesion and flattening of the film onto a substrate by electrostatic force (U S Patent 5,087,476) Polymer coatings that serve various functions such as printability, adhesion promotion, abrasion resis¬ tance, optical and electrical properties, have been produced using various techniques that include thermal cure, re active polymerization, plasma polymerization (U S Patent 5,322,737), and radiation-cuπng usmg ultra-violet and electron beam radiation (U S Patents 5,374,483, 5,445,871, 4,557,980, 5,225,272, 5,068,145, 4,670,340, 4,720,421, 4,781,965, 4,812,351, 4,67,083, and 5,085,911) In such applications, a monomer material is applied usmg conventional techniques of roll coating, castmg, spraying, etc , and the coating is subsequently polymerized under atmospheric pressure conditions

More recently, a new technique has been developed that allows the formation of radiation-curable coatings in the vacuum using a flash evaporation technique that leads to the formation of a vapor-deposited thm liquid

monomer which can be radiation-cured (U.S. Patents 4,842,893; 4,954,371; and 5,032,461 and European Patent Application 0 339 844). This technique overcomes the limitations of conventional techniques for applying the liquid monomers and requires relatively low doses of radiation for polymerization.

The vacuum polymer coating technique as descnbed in the above references was found to have some cnti- cal limitations on certain mechanical, thermal, chemical and morphological properties that can reduce their useful¬ ness in packaging films, capacitors, metal evaporated magnetic tapes and optically variable films. The invention disclosed herein overcomes these problems and extends the one-step polymer and metal vacuum coatmg technique to new functional products with a unique set of properties.

DISCLOSURE OF INVENTION

It is an object of the present invention to produce a hybrid polymer film that has superior nechanical, thermal, chemical and surface morphological properties. In conjunction with one or more metal coatings or a ce¬ ramic coating, the hybrid film can be used to produce improved packaging films, film capacitors, metal evaporated magnetic tapes and optically variable films.

It is also an object of this invention to produce hybrid films with controlled surface microroughness. This mcludes films that have a flatter surface than that of the base film, or a surface with controlled microroughness.

It is another object of the invention to provide an improved process for applying, polymerizing, and dis¬ charging, one or more layers of vacuum-deposited radiation-curable monomer films that are used to produce the hybrid polymer film in a one-step continuous process.

In accordance with the present invention, a hybrid polymer film comprises a first polymer film having a plasma-treated surface and a second polymer film having first and second surfaces, the first surface of the second polymer film being disposed along the first plasma-treated surface of the first polymer film.

The base, or first polymer, films used in the invention to produce the hybrid films are chosen from a group of thermoplastic films that include polypropylene, polyethylene terephthalate, high and low density polyethylene, polycarbonate, polyethylene-2,6-naphthalate, nylon, polyvinylidene difluonde, polyphenylene oxide, and polyphenylene sulfide, and thermoset films that include cellulose derivatives, polyimide, polyimide benzoxazole, and polybenzoaxozole. The second polymer films are radiation-polymerized monomer films that are multifunctional acrylate or acrylated monomers that contain double bonds capable of radical polymerization. Plasma treatment with gases from the group of N ₂ , Ar, Ne, 0 ₂ , C0 ₂ , and CF ₄ is used to functionalize the base film, to further improve the cross-linking of the acrylate film surface, and to remove surface charge, which improves winding and unwmdmg of the hybrid film. Inorganic layers may be used in combination with the polymer layers to produce different end use hybrid films; such inorganic layers include metals selected from the group consisting of aluminum, zinc, nickel, co¬ balt, iron, iron on aluminum, zinc on silver, zinc on copper, and zinc on aluminum, nickel-cobalt alloys, and nickel- cobalt-iron alloys, and ceramics selected from the group consisting of aluminum oxide, silicon oxides (SiO„ where x=l to 2), tantalum oxide, aluminum nitride, titanium nitride, silicon nitride, silicon oxy-nimde, zinc oxide, indium oxide, and indium t oxide.

The hybrid polymer film evidences both improved corrosion resistance and current carrying ability of met¬ allized capacitors compared to pπor art polymer films and overall reliability in demanding applications that require operations in extreme conditions of voltage current and temperature.

As incorporated in food packaging, the presence of the acrylate polymer on a thermoplastic polymer such as polypropylene improves the oxygen and moisture barrier of metallized and ceramic coated films, and it also im¬ proves the mechanical properties of the bamer layer to the extent that there is less damage of the bamer layer as a function of film elongation. By adjusting the chemistry of the acrylate coatings, the surface of the hybrid films can be made hydropho- bic/philic, oliophobic/philic and combinations thereof. This can accommodate different printing inks for packaging film applications in addition to the improvement of barrier properties. Such metallized printable film produced in a one-step process eliminates the lamination of an additional polymer film that is used to protect the metal layer and provide a printable surface. The hybrid films can have reduced surface microroughness, thus eliminating the need for costly flat films for magnetic tape applications. Increased and controlled surface microroughness on a hybrid film can result in lesser abrasion damage and the formation of unique interference effects cause color shifts with changing viewing angle.

As incorporated in electrical flexible cables, fluorinated acrylate polymers deposited on such thermoset polymer films as polyimide, polyimide benzoxazole (PIBO), and polybenzoaxozole (PBO), prevent electrical tracking, and only carbonize in die presence of electrical arcing.

Color shifting effects useful in decorative and security applications can be produced in a one-step low cost process by proper choice of the thickness of the metal and polymer layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of apparatus useful for carrying out the process of the invention;

FIG. 2, on coordinates of electrical resistance (in ohms) and time (in hours), is a plot showing changes in electrical resistance due to the corrosion of aluminum deposited on control polypropylene (PP) film and on PP/plasma/acrylate hybrid film, in which die aluminum metallized films were exposed to temperature and humidity ambient conditions of 70°C and 85% relative humidity, respectively;

FIGS. 3A-E depict various combinations of films that may be deposited on a film substrate, including an acrylate film on the substrate (FIG. 3A), a metal film on an acrylate film on the substrate (FIG. 3B), an acrylate film on a metal film on an acrylate film on the substrate (FIG. 3C), an acrylate film on a metal film on the substrate (FIG. 3D), and both sides of the substrate coated widi an acrylate film (FIG. 3E); FIG. 4, on coordinates of increase in electrical resistance (in percent) and elongation (in percent), plots the electrical resistance increase as a function of elongation, which is an indication of resistance to corrosion, for an un¬ treated metallized polymer film and for a metallized polymer film that is plasma-treated;

FIG. 5, on coordinates of increase in electrical resistance (in percent) and elongation (in percent), plots the electrical resistance increase as a function of elongation for an untreated metallized polymer film and for a metal- lized polymer film that is plasma-treated and coated;

FIG. 6, on coordinates of number of samples and oil/water index, plots the extent of wetting by oil- and aqueous-based liquids on polymeric samples;

FIG. 7A is a cross-sectional view of an acrylate-coated polypropylene film in which the surface of the acrylate coating is micro-roughened; FIG. 7B is a cross-sectional view of die coated polymer film of FIG. 7A, following deposition of a thm metal layer on the micro-roughened surface of the acrylate coating; and

FIG 8 is a cross-secπonal view of the hybπd polymer film of the invention configured for optical filter ap¬ plications

BEST MODES FOR CARRYING OUT THE INVENTION

The vacuum polymer coating technique as descπbed m U.S Patents 4,842,893, 4,954,371 , and 5,032,461 and European Patent Application 0 339 844 was found to have some critical limitations that mclude the following (a) when an acrylate monomer matenal is deposited on a polymer film substrate, adsorbed oxygen on the surface of the film scavenges radiation induced free radicals and inhibits the polymerization process in the interface area, (b) C contammants and low molecular weight species on the surface of most polymer mateπals can inhibit the wetting of the vapor deposited liquid monomer, resulting in thm polymer films with poor uniformity, (c) when this process is used to produce coatings on a polymer web tiiat is moving at high speed, the polymer coating does not always reach 100% polymerization; and (d) when electron radiation is used to cure the acrylate monomers, electrons trapped on the surface of the film cause electrostatic charging. The combination of partial cure and electrostatic charge (trapped 5 electrons) on the film surface, can cause the film to block or stick to itself when it is wound mto a roll.

The discussion below is directed in the mam to a hybπd film compnsing polypropylene (PP) coated with a vacuum-deposited, radianon-curable acrylate monomer film tiiat is polymerized upon curing However, it will be understood that this discussion is exemplary only, and is not intended to be limiting to the composition of the coated polymer or to the presence or absence of a metal coating on the hybπd film. 0 The hybnd film compnses PP film coated with a high temperature, cross-linked, acrylate polymer, depos¬ ited by a high speed vacuum process. The basic aspects of the process are disclosed in U.S Patent;. 4,842,893, 4,954,371; and 5,032,461. However, that process is modified for the purposes of the present invention.

FIG. 1 depicts an example of apparatus 10 suitably employed in the practice of the present invention A vacuum chamber 12 is connected to a vacuum pump 14, which evacuates the chamber to the appropnate pressure 5 The essential components of the apparatus 10 within the vacuum chamber 12 mclude a rotating drum 16, a source spool 18 of polymer film 20, a take-up spool 22 for winding the coated polymer film 20', suitable guide rollers 24, 26, a monomer evaporator 28 for depositing a thm film of an acrylate monomer or mixture containing an acrylate monomer on the polymer film, and radiation curing means 30, such as an electron beam gun, for cross-linking the acrylate monomer to form a radiation-cured acrylate polymer. 0 Optionally, an evaporation system 32 for depositing an inorganic film on the acrylate film may be em¬ ployed. Also optionally, a second monomer evaporator 128 and radiation curing means 130 may be situated after the resistive evaporation system 32. These optional aspects are discussed in greater detail below

The vacuum employed in the practice of the invention is less tiian about 0001 atmosphere, or less uian about 1 millibar Typically, the vacuum is on the order of 2 10 ^~ * to 2x10 ⁵ Ton 35 In operation, the polymer film 20 is fed from die source spool 18 onto die rotating drum 16, which rotates in the direcnon shown by arrow "A", via guide roller 24 The polymer film passes through several stations, is picked off from the surface of die rotating drum 16 by guide roller 26, and is taken up by take-up spool 22 as coated film 20' As the polymer film 20 is taken off die source spool 18, it passes tilrough a first plasma treatment unit 36, where d e surface of the film to be coated is exposed to a plasma to remove adsorbed oxygen, moisture and any low J C molecular weight species from the surface of die film pπor to forming die acrylate coating tiiereon Just before the coated polymer film 20' is wound on the take-up spool 22, it passes through a second plasma treatment unit 38,

where the coated surface of the film is exposed to a plasma to finish cunng die acrylate coating and to remove any accumulated electronic charge

The conditions of die plasma treatment are not cntical, and the plasma source may be low frequency RF, high frequency RF, DC, or AC ; The rotating drum 16 is a water-cooled drum dnven by a motor (not shown) The drum 16 is cooled to a temperature specific to the particular monomer being used and generally in the range of -20° to 50°C to facilitate condensation of the monomer (m vapor form) The drum 16 is rotated at a surface speed within the range of 1 to 1000 cm/second

The polymer film 20 may compπse any of die polymers tiiat have the requisite properties to be treated as 0 descπbed below Examples of such polymers mclude die tiiermoplastic polymers such as polypropylene (PP), poly- ediylene terephthalate (PET), polycarbonate, polyethylene-2,6-naphfhalate, polyvinylidene difluoπde, polyphen¬ ylene oxide, and polyphenylene sulfide, and die thermoset polymers such as polyimide, polyimide benzoxazole (PIBO), polybenzoaxozole (PBO), and cellulose derivatives

The acrylate monomer is deposited on the polymer film 20 by the monomer evaporator 28, which is sup- 5 plied with liquid monomer from a reservoir 40 uirough an ultrasonic atomizer 42, where, witii the aid of heaters (not shown), die monomer liquid is instantiy vaporized, i.e., flash vaporized, so as to minimize the opportunity for po¬ lymerization pnor to being deposited on the polymer film 20. The specific aspects of tins part of the process are de- scnbed in greater detail in the above-mentioned U.S. Patents 4,842,893, 4,954,371, and 5,032,461 and do not form a part of the present invenoon. 0 The flash- vaporized monomer condenses on die surface of d e polymer film 20 tiiat is supported on die cooled rotating drum 16, where it forms a dun monomer film

The condensed liquid monomer is next radiation-cured by die radiation curing means 30. The radiation cunng means may compπse any of die common metiiods for opening die double bonds of die acrylate monomer, examples of suitable means mclude apparatus which emit electron beam or ultra-violet radiation Preferably, the ra- 5 diation-cuπng means 30 comprises a thermionic or gas discharge electron beam gun. The electron beam gun directs a flow of electrons onto die monomer, diereby cunng die matenal to a polymerized, cross-linked film. Cunng is controlled by matching die electron beam voltage to the acrylate monomer thickness on the polymer film 20 For example, an electron voltage in die range of about 8 to 12 KeV will cure about 1 μm tiiick of deposited monomer As with die specifics regarding the deposition of the acrylate monomer, die specific aspects of this part of the proc- 0 ess are descnbed m die above-mentioned U.S. Patents 4,842,893; 4,954,371; and 5,032,461, and do not form a part of die present invention

FIG 3 A depicts the coated film 20' at tins stage, compπsing an acrylate polymer film 122 on a polymer film web or substrate 120 Such a coated film 20' has a vaπety of uses, mcludmg high temperature electπcal cables and foil capacitors m which die film is wound with a metal foil. 5 The cured acrylate monomer, or cross-linked polymer, then passes to the optional resistive evaporation system 32, where an morganic matenal, such as aluminum or zinc, can, if desired, be deposited on die cured mono¬ mer layer Two such coated films 20' may be wound to form metallized capacitors FIG 3B depicts a coated film 20' widi a metallized layer 124 on the acrylate polymer film 122

The same matenal may be used for food packaging films, preferably with an additional acrylate coatmg

4 C over the aluminum metal layer to protect the thm metal layer and tiius improve die bamer properties of the film

FIGS 3C and 3D depict two alternate configurations useful in food packaging In FIG 3C, the additional acrylate

coating 122' is formed on top of die metal film 124, while in FIG. 3D, die metal film is first deposited on the poly¬ mer film substrate 120, followed by deposition of die acrylate film 122 thereon. The process for forming either of these configurations is discussed in greater detail below.

The resistive evaporation system 32 is commonly employed in die art for metallizing films. Alternatively, otiier metal deposition systems, such as a conventional electron beam vaporization device, and magnetron sputtering may be employed. The resistive evaporation system 32 is continually provided witii a source of metal from the wire feed 44.

The deposition of die metal film may be avoided, diereby providing simply a hybnd polymer film, which may have a vaπety of uses, as descπbed above, with reference to FIG. 3A. Following die optional metallization stage, a second, optional acrylate monomer deposition may be per¬ formed, using monomer evaporator 128 and radiation-curing means 130. This second deposition is used to form die coated films 20' shown in FIGS. 3C and 3D, discussed above. The composition of die second acrylate film 122' may the same or different as mat of die first acrylate film 122.

The apparatus shown in FIG. 1 is primarily directed toward die formation of an acrylate film 122, 122' on top of another polymer film 120, witii or widiout a layer of metal 124. Further, the uncoated side of die polymer film 20 could also be coated with an acrylate film 222 of die same or different composition, for example, by pro¬ viding a second rotating drum witiiin die vacuum chamber 12 and providing die same sequence of devices (monomer evaporator 28 and radiation curing means 30, with optional metallization device 32). Such a coated film 20' is depicted in FIG. 3E. The acrylate polymer film 122, 222 is formed to a thickness within the range of about 0.01 to 12 μm, de¬ pending on die particular application. While ti icker films than tiiis may be fabncated, no benefit seems to be de- πved using such thicker films.

The thickness of die polymer film substrate 20, 120 is typically witiiin die range of about 1 io 100 μm, again, depending on die particular application. When used witii conventional polymer films employed in metallized capacitors, such as PP, PET, or poly¬ carbonate, the duckness of the acrylate film 122, 222 is typically on die order of 0.1 to 1 μm. In such cases, the un¬ derlying base film (PP or polycarbonate) 20, 120 is much thicker; commercially avadable films of PP are m die range of 4 to 25 μm. Metallized thin film PP capacitors are used in low loss, AC applications, and the presence of the acrylate film provides a number of advantages, including greater reliability and, unexpectedly, improved corro- sion resistance. The dielectric constant of the acrylate film for use in such applications preferably is in the range of about 2.5 to 4.0.

On the otiier hand, diere are applications requiring energy storage in which higher dielectπc constants in die range of about 10 to 15 are desired. In such cases, acrylate polymers having such a high dielectπc constant are deposited on thinner films, and tiius die acrylate film compnses a substantial fraction of the hybnd film thickness. In packaging applications, the acrylate coatings 122, 222 are typically about 0.5 to 2.0 μm thick, deposited on PP or PET films 20, 120 tiiat are typically 12 to 35 μm diick.

Examples of acrylate monomers tiiat may be used in die practice of the present mvenhon ine ude ethoxy¬ lated bis-phenol diacrylate, hexadiol diacrylate, phenoxyethyl acrylate, acrylonitrile, 2,2,2-tπfluoromethyl acrylate, methylene glycol diacrylate, iso-decyl acrylate, alkoxylated diacrylate, tnpropylene glycol diacrylate, ethoxy ethyl acrylate, polyethylene glycol diacrylate, diethylene glycol diacrylate, tπmetiiylol propane macrylate, tetraethylene glycol diacrylate. cyano-ediyl (mono)-acrylate, octodecyl acrylate, dinitrile acrylate, pentafluorophenyl acrylate,

nitrophenyl acrylate, lso-bornyl acrylate, ms(2-hydroxyethyl)-ιso-cyanurate macrylate, tetrahydrofurfuryl acrylate, neo-pentyl glycol diacrylate, propoxylated neo-pentyl glycol diacrylate, and mixtures thereof.

The metal film 124 that is deposited in die case of metallized capacitors may compπse aluminum or zinc or a composite film of nickel on aluminum, iron on aluminum, zinc on silver, zmc on copper, or z c on aluminum The thickness of the metal film 124 for capacitor use is on the order of 100 to 300 A, and may be uniform across the widtii of die hybπd film 20' or may be thicker at die edges than m the central portion

The metal film 124 that is used m packagmg typically compnses aluminum The thickness of die metal film is witiiin the range of about 100 to 400 A

EXAMPLES

I. CAPACITORS

Capacitors employ low temperature thermoplastic dielectπc thin film polymers, such as polypropylene (PP), polyediylene terephdialate (PET), polycarbonate, polyethylene-2,6-naphthalate, polyvinyhdene difluoπde (PVDF), polyphenylene oxide, and polyphenylene sulfide, erther metallized or maintained between metal foil elec¬ trodes Metallized film capacitors are used extensively in a broad range of electncal and electromc equipment mat mclude motor run and motor start circuits for air conditioners, fluorescent and high intensity light ballasts, power supplies, telecommumcation equipment, instrumentation, and medical electromcs In many of these applications, the metallized capacitors are used to conserve energy by correcting die power factor of a circuit and m others they are used to perform specific functions, such as timmg, filtenng, and decouplmg. The advantages of metallized film over film foil capacitors mclude lower volume, weight, cost, and higher reliability due to die self-healing properties of die metallized films The low temperature metallized film dielectncs find use in such a multitude of high voltage and high frequency capacitor applications due to their low dielectric loss.

One disadvantage, however, is lower current-carrying capacity due to die thm metallized electrodes that are deposited on low temperature thermoplastic dielectncs, such as PP, PET, and polycarbonate In particular, reliabihty issues arise when metallized capacitors are forced to carry high levels of pulse or AC current. The present mvenuon addresses die production of a hybnd film tiiat has improved mechanical and tiiermal properties that improve ca¬ pacitor performance and reliability.

The following discussion is directed to low loss, AC applications, mvolvmg die deposition of an acrylate film on a polypropylene film, followed by metallization to form a dun film metallized capacitor. It will be readily apparent to die person skilled in tius art, however, that tiiese teachings may be extended to other base polymer films for other applications

Over 100 acrylate monomer mateπals have been polymerized and tested for dielectnc constant and dissi¬ pation factor as a funcnon of temperature and frequency, and oil and water surface wetting Some of these matenals were sourced from vanous vendors, others were formulated usmg propπetary mixtures of commercially available monomers, and some were molecularly syntiiesized. The acrylate polymer mateπals tested mclude the following 2,2,2-tnfluoroethylacrylate, 75% 2,2,2-mfluoroethyl-acrylate/25% C„ diol diacrylate, 50% acrylonιtnle/50% C _:o diol diacrylate, dimerol diacrylate; 50% acrylomtπle/50% C„ diol diacrylate, Erpol 1010 diacrylate, 50% hexane diol dιacrylate/50% C _;o mol diacrylate, 50% 2,2,2-tnchloroacrylate/50% C ₁₉ diol diacrylate, 75% 2,2,2- tnfluoroethylacrylate/25% C„ diol diacrylate, 50% octane diol diacry late/50% 2-cyanoethyl acrylate, 67% 2,2,2- tnfluoroetiιylacrylate/33% C _;o mol macrylate, lauryl acrylate, tnmediylolpropane macrylate, ethoxyetiioxy ethyl

acrylate, pentaerythπtol tetraacrylate, neo-pentyl glycol diacrylate; octyldecyl acrylate; tetraethylene glycol diacry¬ late, tnpropylene glycol diacrylate; octane diol diacrylate; 1,8-alkoxylated ahphanc acrylate; decanediol diacrylate, ethylene glycol diacrylate; iso-bomyl acrylate; butanediol diacrylate; 93% hexane diol dιacrylate/7% KENSTAT qlOO, 95% hexane diol dιacrylate/5% chlorinated polyester diacrylate, trimethylolpropane ethoxylate tπacrylate, tnmediylolpropane propoxylate macrylate; neo-pentyl glycol propoxylate diacrylate; bisphenol A etiioxylate dia¬ crylate, alkoxylated aliphatic diacrylate ester; 50% 2-cyanoethyl acrylate/50% C„ diol diacrylate, 92 % tnmethyiol- propane tπacrylate/8% acrylonimle; 50% iso-bomyl acrylate/50% pentaerydiπtol macrylate, 83% tnme _' hylolpro- pane tπacrylate/17% FLUORAD FX189; 50% acrylonιmle/50% trimethylolpropane macrylate, 70% mmethylol- propane tnacryiate/30% acrylo mle, 40% FLUORORAD FX 189/60% trimethylolpropane macrylate, 70% hexane diol dιacrylate/30% iso-bomyl acrylate; 50% hexane diol dιacrylate/50% iso-bomyl acrylate, 12.5% aliphatic ure¬ thane tnacrylate/87.5% hexane diol diacrylate; 31% KENSTAT qlOO/69% C _l4 diol diacrylate; 69% C, ₄ diol diacry- late/31% acrylommle, 80% C ₁₄ diol dιacrylate/20% KENSTAT qlOO; 94% trimethylolpropane propoxylate tnacry- late/6% KENSTAT qlOO, 50% trimethylolpropane tnacrylate/50% acetommle; 70% tnmediylolproparie tnacry- late/30% acetommle, 88% phenol ethoxylate mono-acrylate/12% acetommle; 80% C _u diol dιacrylate/20% acetoni- le, 80% C, ₄ diol dιacrylate/20% KENSTAT qlOO; 12% trimethylolpropane macrylate/88% iso-bomyl acrylate, 69% C, ₄ diol dιacrylate 23% KENSTAT qlOO/8% trimethylolpropane macrylate; 33% acetommle/33% polyaimne aery late/34% iso-bomyl acrylate; 75% aliphatic amine acrylate/ 17% KENSTAT q 100/8% trimethylolpropane ma¬ crylate, 80% C, ₄ diol dιacrylate/20% FLUORAD FX189; 80% phenol etiioxylate monoacrylate/-20% pentaerythπ¬ tol macrylate; 80% hexane diol dιacrylate/20% acrylommle; 70% hexane diol diacrylate/ 15% acrylonimle/ 15% tnmediylolpropane macrylate; propoxylated glycerine macrylate; ethoxylated trimethylolpropane acrylate; ca- prolactone acrylate; 90% alkoxylated tnfunctional acrylate/10% beta-carboxyethyl acrylate; 90% polyethylene gly¬ col 200 diacrylate/ 10% pentaenthntol di tn tetraacrylate; 75% hexane diol dιacrylate/25% KenReact LICA 44; 50% pentaerythrytol tetraacrylate/50% hexane diol diacrylate; pentaerythntol polyoxyethylene petraacylate; tetrahy- drofurfuryl acrylate; 25% KENSTAT ql00/75% hexane diol diacrylate; 50% tettahydrofurfuryl acrylate/50% poly- ethylene glycol 200 diacrylate; 88% tetrahydrofurfuryl acrylate/12% trimethylolpropane macrylate; 88% caprolac- tone acrylate/12% trimethylolpropane macrylate; EBECRYL 170; 80% EBECRYL 584/20% beta-carboxyetiiyl acrylate; 88% tetrahy&o-urfuryl acrylate/12% beta-carboxyethyl acrylate; 88% EBECRYL 170/12% beta- carboxyethyl acrylate; aliphatic polyesther hexaacrylate oligomer; aliphatic urethane diacrylate; tnpropylene glycol diacrylate, hexane diol diacrylate; 88% iso-bomyl acrylate/12% beta-carboxyethyl acrylate; 90% polyetitylene gly- col 200 dιacrylate/5% iso-bomyl acrylate/5% pentaerythritol macrylate; 82% polyethylene glycol 200 diacry¬ late/ 12% hexane diol dιacrylate/6% tnmediylolpropane macrylate; 75% tetiahydrofurfuryl acrylate/ 15% hexane diol dιacrylate/5% trimethylolpropane tnacrylate/5% oligomer; 44% polyethylene glycol 200 diacrylate '44% acry¬ lommle/ 12% hexane diol diacrylate; 70% C _M diol dιacrylate/30% aliphatic urethane acrylate oligomer trimethyl¬ olpropane ethoxylate macrylate, and 50% PHOTOMER 6173/50% hexane diol diacrylate 50% Notes KENSTAT qlOO and KenReact LICA 44 are trade names of Kennch Corp , FLUORAD FX189 is a trade name of 3M Industnal Chemical Corp., EBECRYL 170 and EBECRYL 184 are trade names of UCB Radcure Corp , and PHOTOMER 6173 is a tradename of Henkel Corp

Several acrylate polymers were identified as candidate matenals for AC voltage capacitor designs These are low dissipation factor (DF) polymers with DF <001, l e , <1%, and dielectnc constant (K) m the range 2 5 < K < 4 0

PP-acrylate hybnd films were produced usmg a production-size vacuum metallizing chamber that was ret¬ rofitted to allow deposition of the acrylate coatings in line with the metallization process. PP films of 6 μm, 8 μm, 12 μm, and 19 μm were first treated witii a gas plasma and tiien coated witii acrylate polymer films with dncknesses of about 0.2 to 1.0 μm. Dielectnc characterizanon of small area stamp capacitors, with PP (control) and hybnd Ξ films, showed that the PP-acrylate films had supenor current carrying capability, higher resistance to degradation from partial discharges (corona), and breakdown voltage equal to or higher than PP films of equal thickness.

An additional benefit that was unexpected was unproved corrosion resistance of die metallized aluminum electrodes when deposited on die acrylate coating rather than on the PP film. This is very significant because it al¬ lows die use of thinner aluminum layers, which mcreases the self-healing properties of the hybπd film capacitors. o Several full-size capacitor designs were produced and tested. Capacitors with ratings of 8 μF/330 VAC, 0.1 μF/1200 VDC, and 0.1 μF/2000 VDC were built usmg acrylate-coated 6 μm, 12 μm, and 19 μm PP films, respec¬ tively. Short-term current, humidity, and breakdown voltage tests showed that the acrylate-PP hybnd capacitors followed die performance of the small area stamp capacitors. That is, the hybrid film capacitors had significantly higher performance than that of die PP control capacitors. In a cπtical high dV/dt test that applied 5000 V pulses 5 with a πse time of 1000 V/50 ns, die 0.1 μF/2000 V capacitors out-performed by far commercial capacitors tiiat used double metallized PET film electrodes (for higher current carrying capacity) and equal tiuckness dielectπc. The commercial capacitors degraded or failed after 2400 pulses, while there was no degradation in the 19 μm hy¬ brid film capacitors. In fact, die 12 μm hybrid film capacitors (0.1 μF/1200 V) mat were less than half the volume of the 19 μm capacitors were also superior to the commercial capacitors when tested under die same conditions. A 0 comparison of 12 μm PP lighting ballast capacitors with 12 μm acrylate-PP film capacitors using 3000 V pulses showed no degradation in the hybrid film capacitors, while die performance of conventional capacitors from two different manufacturers varied from significant degradation to complete failure.

Large quantities of acrylate hybrid films were coated, metallized, and wound into capacitors usmg conven¬ tional winding equipment. The acrylate-PP film handled as well as any capacitor film through the vaπous process 5 steps. Since the acrylate polymer can be deposited in-lme witii die metallization, and in die same vacuum chamber, added labor cost is minimal. The base acrylate monomers are available at a cost of $3/lb to $4/lb and at a thickness of about 1 μm, added matenal cost will be minimal.

A. Hvbπd Film Production Process Development 0 Acrylate-PP hybrid films were produced usmg 6 μm, 8 μm, 12 μm, and 19 μm PP films tiiat were readily available. Rolls of film 32 cm wide were used that were at least a few tiiousand feet long (typical size for small met¬ allizing runs). The apparatus employed in the coating process is shown schematically FIG. 1. The liquid mono¬ mer was pumped from die reservoir 40 that was located outside die vacuum chamber 12 mto die flash evaporator 28. The liquid monomer was atomized into microdroplets witii the use of the ultrasonic atomizer 42 that was posi-

35 tioned on top of die evaporator 28. The evaporator 28 was held at a temperature which was above the boiling pomt of the liquid, but below its decomposition point. This caused die monomer to flash evaporate before it cured The molecular vapor exited at supersonic speeds and condensed on the film 20 that was m intimate contact with the chilled rotating drum 16. The condensed thin film deposit then moved in front of die electron beam gun 30 where it was polymerized.

Examples of suitable acrylates useful in the practice of the invention mclude iso-bomyl acrylate _, hexane diol diacrylate, and tπpropylene glycol diacrylate tiiat were formulated for fast cure, proper viscosity, and good ad _¬ hesion to the PP film. These acrylates all have a dissipation factor of less than 0.01.

Several large rolls of 6 μm and 8 μm PP film were coated under various conditions until a set of parame- ters tiiat produced a well-cured, uniform coating was obtained. The major process parameters are as follow ₅ :

1. Drum Temperature: Good films may be made within the temperature range from room temperature to -20°C. The lower temperature appeared to increase somewhat the condensation rate with some monomers, but not with others. It is estimated tiiat the residence time of the film on die drum was not sufficient to transfer much of the drum temperature to the top surface of the film. As a result, die difference m drum temperature did not have a ma _j or affect on the monomer condensation rate.

2. Drum Speed: Good films were produced with drum surface speeds anywhere in die range of 50 to 1000 feet/min (25 to 500 cm/sec).

3. Radiation Dose: The accelerating potential was 12 KeV and die current delivered to die monomer was about 2 mA for cunng monomer films 1 μm thick. 4. Final Monomer Mixture: Most of the monomer mixtures that were processed produced good quality coatings when considering parameters such as uniformity, degTee of cure, and adhesion to the PP film. One example of a suitable monomer mixture comprised a mixture of 70% hexanediol diacrylate, 20% iso-bornyl acrylate, and 10% tnpropylene glycol diacrylate.

5. Plasma Treatment Prior to Acrylate Deposition: The PP film was plasma-treated pnor to the deposition of the monomer vapor for the following reasons: a. The PP film surface has absorbed oxygen and moisture that interferes with the polymerization of die acrylate monomer. Oxygen is a free radical scavenger that neutralizes free radicals created by die electron beam, tiius inhibiting the polymerization process. b. The plasma treatment etches and cleans the film surface from low molecular weight residue created by the corona treatment process. This improves die wetting of die monomer to the film, resulting in a more uniform coating.

6. Plasma Treatment After Deposition of the Acrylate Polymer: This is a post-treatment that was used to reduce die static charge on the coated film and also complete the polymerization process on the acrylate surface. This process step reduced die "stickiness" on the acrylate-PP wound roll,

B. Evaluation of die Hybrid Films Using Small Area Films

The hybrid films were first evaluated using small area stamp capacitors, and dien full-size- wound capaci¬ tors were fabπcated for further evaluation. Stamp capacitors were used to evaluate breakdown strength, current car¬ rying capacity, partial discharge (corona) degradation of die polymer films, and corrosion resistance of the metal- lized electrodes.

1. Dielecmc Constant and Dissipation Factor

The acrylate coating, due to its low thickness, had minor influence on both the and DF of die hybrid films. Dependmg on die particular hybrid, the dielecmc constant of the acrylate-PP films was up to 7% highe- than that of plam PP film

2 DC Breakdown Strength

This measurement was made to ensure tiiat die hybnd film had a breakdown strength at least as high as that of the onginal PP film, plus an additional amount due to the acrylate coating The breakdown measurements on the films were done usmg a dry double-metallized and non-contact measurement technique that was performed at high vacuum (<10"" Torr), to eliminate partial discharges and surface flashovers The breakdown system was built in a turbomolecularly pumped stamless-steel chamber Metallization masks were designed and fabricated that allowed metallization of small area (about 1 square inch) stamp capacitors for die breakdown measurements The electrode contact was made to metallized pads diat were outside die active area to prevent film damage The voltage was ramped at about 500 V/sec For every breakdown measurement tiiat is reported below, at least 18 stamp capacitors were tested to ensure tiiat the data is statistically significant.

The results of the DC breakdown tests of 6 μm and 8 μm films coated witii about 0 5 μm of acrylate poly¬ mer are shown m Table I The acrylate polymer is produced by electron beam cunng of a 70% hexanediol diacry¬ late, 20% iso-bomyl acrylate, and 10% tnpropylene glycol diacrylate monomer film deposited m die vacuum usmg die experimental apparatus descnbed m FIG. 1 The PP film was first plasma-treated usmg an argon gas plasma and die acrylate monomer was flash evaporated on the treated surface.

TABLE I

FILM THICKNESS BREAKDOWN (μm) VOLTAGE (KV)

Control PP 6 3.87

PP/plasma acrylate 6.5 4.25

Control PP 8.0 4 84

PP/plasma/acrylate 8.5 5.34

Control PP 12.0 5 30

PP/plasma/acrylate 12.5 6.26

The breakdown voltage of die control and acrylate hybnd films was measured by metallizing one square inch electrodes on opposing sides of die films. Each measurement represents an average of at least eighteen individ- ual breakdown measurements The data in Table I show tiiat the acrylate-coated PP films have a breakdown strength that is higher than the control PP film by about 10% Given that the coating thickness is about 0 5 μm, the break¬ down strength of the acrylate-PP films is equal to or higher tiian PP films. Similar measurements on smgle acrylate layers have shown that the breakdown strength of 1 μm thick films is 20 to 24 KV/mil, which is equal to or higher than that of PP film 3 Current-Carrying gapapity

It is well-known that due to the low melting pomt of PP film, metallized PP capacitors become quite unre¬ liable at high current applications due to thermal damage of the termination The current generates I R losses (R = Equivalent Seπes Resistance, ESR) at the termination, which raises the temperature, which in turn damages the ter¬ mination and increases the ESR This process eventually causes a catastrophic failure Life test data showed that many conventional PP capacitor designs have marginal current carrying performance For this reason, in some high current designs, double metallized paper or double metallized PET film tiiat has a higher melting pomt than PP is used to carry the current These PET PP designs are inefficient and result m expensive capacitor products

To simulate the current flow from the sprayed termination to the metallized film, and die ability of the film to carry high currents without thermal damage, a simple test was developed to measure die maximum power that a metallized film can dissipate pnor to a tiiermal failure. Current was forced dirough a section of metallized film and the power (IxV) was increased until the dissipated heat forced the film to fail. The fixture had 1/2 mch w de contact electrodes which were placed 5 mches apart. AC voltage was applied to die electrodes and die cuπent ttirough the circuit was recorded. The voltage was raised until the power loss thermally degraded die metallized film _t o the poin _t of failure. In die films that were tested, die failure was an open circuit caused by melting of the PP film at some pomt close to die middle of die 5 inch strip. Table II shows the average power to failure (eight samples were tested for each condition), for both coated and uncoated 8 μm duck polypropylene. The acrylate polymer was produced by electron beam cunng of a 70% hexanediol diacrylate, 20% iso-bomyl acrylate, and 10% tripropylene glycol diacry¬ late monomer film deposited in die vacuum usmg the experimental apparatus described in FIG. 1. The PP film was first plasma-treated usmg an argon gas plasma and die acrylate monomer was flash-evaporated on the neated sur¬ face.

TABLE II.

FILM TYPE POWER IMPROVEMENT OVER

(Watts) CONTROL PP (%)

Control PP 13.5 --

PP/plasma/0.2 μm 14.0 3.7 acrylate

PP/plasma/0.4 μm 14.8 9.6 acrylate

PP/plasma/0.6 μm 15.6 15.6 acrylate

PP/plasma/0.8 μm 19.4 43.7 acrylate

PP/plasma/l.O μm 22.0 63.0 acrylate

The results show that die coated films have a higher thermal capacity tiiat varies from 4% to 63%. The vanation is due mostly to the thickness of the acrylate coating.

It is interesting to note that although die coated films failed at significantly higher power levels, they did not deform and shrink as much as die plain films. The higher thermal capacity of the hybrid films was a key objec¬ tive of this work and it is clear that tiiin coatings of die high temperature acrylate films can have a significant impact in the thermal properties of PP.

4. Resistance to Degradation from Partial Discharges (Coronal

One of the common mechanisms of failure in high voltage (V >300 VDC) film capacitors is damage to the polymer dielecmc from partial discharge activity (corona) in the capacitor windings. The partial discharges or co¬ rona pulses are generated m inter-electrode areas tiiat have large enough air gaps to sustain a certam level of loniza- tion. The high temperature corona pulses, although tiiey physically move around, can degrade the polymer dielecmc and cause a breakdown. Dry metallized capacitors are particularly susceptible to corona damage, especially at the

outer and innermost turns which are looser than the rest of the roll Metallized capacitors that have electrodes with reasonably high resistance (3 to 8 ohm/sq) have good self-healing properties, and die corona-induced cleanngs will result in some capacitance loss witii no further damage to die capacitor Capacitors widi electrodes of 2 to 3 ohm/sq have poorer clearing properties The resulting high levels of corona can lead to major capacitance loss, mcreased dissipation factor, higher leakage current, and often catastrophic failures

Resistance to die damaging diermal effects of die corona pulses mcreases as the thermal stability of the polymer film mcreases Control polyvinylidene difluonde (PVDF) and polypropylene (PP) films were tested along with PVDF/plasma/acrylate and PP/plasma/acrylate hybπd films for resistance to corona degradation The acrylate polymer was produced by electron beam cunng of a 70% hexanediol diacrylate, 20% iso-bomyl acrylate, and 10% mpropylene glycol diacrylate monomer film deposited in the vacuum usmg the experimental apparatus descπbed in FIG 1 The PP film was first plasma-treated usmg an argon gas plasma and die acrylate monomer was flash- evaporated on tile treated surface

The acrylate-PVDF hybnd was mcluded because PVDF film is die highest energy density capacitor di¬ elecmc tiiat is commercially available, and an acrylate-PVDF hybπd may present some unique product opportuni- ties Furthermore, die PVDF film has about d e same melting pomt as PP The PVDF stamp capacitors were liquid impregnated and tested at 1200 VAC The PP capacitors were dry and tested at 350 VAC This test was an acceler¬ ated corona test, where the level of partial discharges reflected eitiier poor impregnation (impregnated cap), or loose outer turns in a dry capacitor To date, several stamp capacitors have been tested, and die results are shown in Table III TABLE III

CAPACITOR FILM TYPE TIME TO BREAK¬ NUMBER DOWN (mm )

1 Control PVDF, 12 μm 3 0

2 Control PVDF, 12 μm 3.8

3 Control PVDF, 12 μm 5 1

4 Control PVDF, 12 μm 3 8

5 PVDF/plasma/1 μm 47 8 acrylate)

6 PVDF/plasma/1 μm 29 0 acrylate)

7 PVDF/plasma/1 μm 32 0 acrylate)

8 PVDF/plasma 1 μm 33 8 acrylate)

9 Control PP, 8 μm 19 0

10 PP/plasma/1 μm acrylate 133 0

As seen m Table III, the hybnd films had about one order of magnitude longer time to failure. This can raise the reliabϋity level of the capacitors significantly. In the case of a pulse-type capacitor, it may represent a large number of additional pulses pnor to failure from corona degradation. 5. Conosion Resistance Capacitance loss due to electrode corrosion is die most common failure mechanism m metallized capaci¬ tors The corrosion resistance of aluminum electrodes deposited on control PP and PP/plasma/acrylate films was tested. The corrosion stability of aluminum metallized PP and PP/plasma/acrylate films was tested by measuring the change in electncal resistance of a 5 inch by 1 mch metallized stnp, after exposure in a temperature humidity envi¬ ronment of 70 C/85% RH. The 5 mch strips were cut out of large bobbins of matenal which were also used to make full size capacitors. In order to assure that a good electrode contact was made at each measurement (especially as the electrode starts to conode), a tiύcker aluminum pad was deposited at botii ends of die 5 mch stnp.

The control PP and hybrid films were produced from die same roll of PP film by plasma treating and acry¬ late coating half die roll (32 cm wide and about 10,000 feet long) and then metallizing die entire roll in a smgle metallizing run. FIG. 2 shows the change in electrical resistance as a function of time for 6 μm control PP film (Curve 52) metallized with aluminum side by side widi PP/plasma/1 μm acrylate film (Curve 50). The acrylate polymer was produced by electron beam curing of a 70% hexanediol diacrylate, 20% iso-bomyl acrylate, and 10% mpropylene glycol diacrylate monomer film deposited in the vacuum using the experimental apparatus descnbed FIG. 1. The PP film was first plasma-treated using an argon gas plasma and die acrylate monomer was flash- evaporated on die treated surface. The data shows that the metallized aluminum electrodes on die hybnd films have superior corrosion resistance.

The surface properties of die polymer film play a major role in the structural and chemical stability of die metallized electrodes. Several studies have been conducted in the past that show tiiat surface treatment of the poly¬ mer films prior to the metallization improves die adhesion and corrosion resistance of die aluminum electrodes. Film cross-linking, polar chemical functionalities and lack of low molecular weight matenal are parameters that fa- vor the acrylate surface. In addition, PP film, as a mle, is always corona or flame treated pnor to metallization. This is done in-line with the film manufacturing process. Subsequent exposure to ambient conditions leads to moisture adsorption by the carboxyl, carbonyl, and hydroxy groups formed by die treatment process. The adsorbed moisture will react with the aluminum deposit and degrade its chemical stability.

The above tests indicate that the acrylate hybrid films have unique surface properties surface that enhance die chemical resistance of evaporated aluminum metal coatings.

C. Evaluation of Hybrid Films Using Full Size Capacitors

Full size capacitors based on an 330 VAC design were fabricated using control 8 μm PP films and

PP/plasma/0.7 μm acrylate hybrid film. The acrylate polymer was produced by electron beam cunng of a 70% hex- anediol diacrylate, 20% iso-bomyl acrylate, and 10% tripropylene glycol diacrylate monomer film deposited in the vacuum usmg the experimental apparatus described m FIG. 1. The PP film was first plasma-treated usmg an argon gas plasma and die acrylate monomer was flash-evaporated on the treated surface. The capacitors were wound in solid hex-cores and they had a cylmdncal shape.

Full size capacitors were built and life-tested for electrode corrosion resistance under the same conditions C as the small area films. Instead of measuring die capacitance change, due to the short test peπod, die optical density of the electrodes was measured. In tins manner, the aluminum oxidation could be detected even when die electrode

was conducting, which would not result m a capacitance loss. The optical density was mitially measured on the bobbms that were used to make die capacitors. After die test, die capacitors were unwound and the optical density of the metallized film was measured at the outer part of the roll, at some fixed distance away from the end of the winding The results of tins test are shown m Table IV Four sets of capacitors were made usmg control PP and PP/plasma/0.7 μm acrylate hybnd film metallized witii dun aluminum (optical density 1.25 and 1 51, denoted as ( 1 ) in Table IV) and tiucker aluminum (optical density 1.68 and 1.76, denoted as (2) in Table IV).

TABLE IV

CHANGES IN OPΗCAL DENSITY OF ALUMINUM METALLIZED CAPACITOR FILMS OF PP AND

PP/PLASMA/0.7 μm ACRYLATE FILMS, DUE TO CORROSION IN A 70°C AND 85% RH ENVIRONMENT

OPTICAL OPTICAL PERCENTAGE DENSITY, DENSITY, CHANGE OVER 214

MATERIAL 0 HOURS 214 HOURS HOURS

Hybnd (1) 1.25 0.98 -21.60

Control PP (1) 1.51 0.55 -63.58

Hybnd (2) 1.68 1.38 -17.86

Control PP (2) 1 76 1.41 -19.89

The results in Table IV indicate that d e corrosion performance of control PP and acrylate-PP hybπd films, when wound mto capacitors rolls, is basically die same as that of die small area films (Hybnd (1) is die same film matenal tested in small area samples, see FIG. 2). That is, metallized capacitors wound with acrylate hybrid films suffer less electrode corrosion than similar parts produced witii metallized control PP films.

This is a significant advantage for die hybrid films. The improved corrosion resistance of the metallized electrodes will allow use of tiiis product m applications where the capacitors are exposed to higher humidity and temperature.

A high frequency, high current low voltage test was developed to evaluate the current carrying capacity of the capacitor termination. The current through die capacitors was altered by varying die voltage amplitude of die high frequency signal. After testmg several capacitors to detemune a set of conditions that would degrade the ca¬ pacitor termination by some measurable amount, both plain PP and hybrid film capacitors were tested. The test data as shown in Table V demonstrates tiiat capacitors built with die acrylate-PP hybnd film dielectncs have supeπor parametnc stability than sister units produced with control PP film.

TABLE V

DISSIPATION FACTOR AND ESR OF CAPACITORS BUILT WITH

PP AND ACRYLATE-PP HYBRID FILM. THEY ARE TESTED AT 40 KHz BEFORE AND AFTER THE HIGH

CURRENT TEST.

INITIAL VALUES

UNΓT # CAP (μf) DF (%) ESR (m ohms)

Control PP 8.92 1.13 5.0

Hybπd (2) 8.86 0.65 2.9 C

AFTER 10 MIN. 25 KHz 50 A PEAK AT 60 V PEAK

UNIT # CAP (μf) DF (%) ESR (m ohms)

Control PP 8.76 1 15 5.20

Hybnd (2) 8 82 0.69 3.10

AFTER 5 ADD'L MIN. 25 KHz 75 A PEAK AT 85 V PEAK

UNIT # CAP (μf) DF (%) ESR (m ohms)

Control PP 7.40 11.00 59.20

Hybπd (2) 8.59 0.71 3.30

The current-carrying performance of die full size capacitors is in line witii die current or power dissipation performance of die small area metallized films (Table IV). Furthermore, it is interesting to note that die full size hy¬ bnd film capacitors were fabncated witii Hybnd (2) film (see Table IV), which did not have die highest power dis- sipation performance. This suggests that capacitors built with optimized hybπd films could have far supenor per¬ formance to the capacitors that were tested.

Capacitors with 0.1 μF capacitance were fabncated, using 19 μm PP/ρlasma/1.0 μm acrylate (referred to as Hybπd A in Table VI) and 12 μm PP/plasma/1.0 μm acrylate (referred to as Hybπd B in Table VI). These capaci¬ tors were arbitraπly rated 0.1 μF/2000 VDC and 0.1 μF/1200 VDC, respectively. The acrylate polymer was pro- duced by electron beam cunng of a 70% hexanediol diacrylate, 20% iso-bomyl acrylate, and 10% mpropylene gly¬ col diacrylate monomer film deposited in the vacuum using the experimental apparatus described FIG. 1. The PP film was first plasma-treated using an argon gas plasma and die acrylate monomer was flash-evaporated on the treated surface.

The capacitors were compared with state-of-the-art high current commercial capacitors, which although tiiey had a rating of 0.1 μF/1200 V, diey also utilized 19 μm PP dielecmc and double metallized polyester (PET) film electrodes, to carry die high current. These capacitors were about the same size as the Hybπd (A) film 0.1 μF metallized capacitors and more man double d e volume of the Hybrid (B) 0.1 μF capacitors.

All capacitors were tested usmg 5,000 V pulses with a dV/dt of 1000 V/ns. The capacitance and ESR were measured usmg a conventional electronic capacitance bndge. The initial capacitance and ESR are compared to die after test capacitance and ESR m Table VI below

TABLE VI. VOLTAGE AND CURRENT CARRYING COMPARISON OF STATE OF THE . IT COMMERCIAL PP CAPACITORS AND CAPACITORS BUILT WITH ACRYLATE HYBRID FILMS OF THE

PRESENT INVENTION

Commercial Capacitor

Initial Capacitance Initial ESR Final Capacitance Final ESR @ 100 KHz (nf) @ 100 KHz @ 100 KHz (nf) @ 100 KHz (milliohms) (milliohms)

98.53 5 63.72 2115

98.34 6 33.91 8294

98.36 5 98.11 5

101.72 7 33.97 7768

97.69 5 62.94 2050

97.30 6 51.81 3249

98.35 7 51.30 3112

98.11 3 98.00 6

Average Change: -37.4% +3319.5 mΩ

Present Invention, 19 μm Hybrid (0.1 μF/2000 V)

Initial Capacitance Initial ESR Final Capacitance Final ESR @ 100 KHz (nf) @ 100 KHz @ 1O0 KHz (nf) @ 100 KHz (milliohms) (milliohms)

102.13 8 101.90 9

103.43 9 103.37 9

102.83 6 102.83 9

103.30 4 103.28 4

103.77 4 103.76 4

95.08 4 95.08 4

103.61 9 103.59 9

103.43 8 103.43 9

Average Change: -0.05% +0.62 mΩ

Present Invention, 12 μm Hybrid (0.1 μF/1200 V)

Initial Capacitance Initial ESR Final Capacitance Final ESR @ 100 KHz (nf) @ 100 KHz @ 100 KHz (nf) @ 100 KHz (milliohms) (milliohms)

100.06 5 99.79 10

99.82 11 81.89 395

101.30 9 82.70 302

101.70 11 94.80 229

100.30 10 86.10 901

102.60 14 80.00 411

100.80 10 87.10 439

101.90 10 99.90 45

Average Change: -11.9% β31.5 mΩ

It can be seen that the 0 1 μF/2000 VDC (Hybπd (A)) design, outperforms the equal size commercial de¬ sign by a very large margin In fact, the data shows tiiat all but two of the commercial capacitors faded, *hιle none of die capacitors prepared by die process of die present invention had any significant degradation Witii regard to the 0 1 μF/1200 V (Hybnd (B)) capacitors prepared accordance with the present mvention (that have less than half the size of the commercial capacitors), the data show that although these capacitors degraded, on average, they performed better than die state of die art commercial capacitors

U. FOOD PACKAGING FILMS In packaging applications, bamer films are of particular mtcrest for separating products such as food and electromc components from environments that reduce product life Such bamer films may be achieved eidier usmg films in which the bamer is built into die film or by applying a coating to a film The bamer coating may be either transparent, compnsing oxide mateπals for example, or opaque or semi-transparent, compnsing metals for example

Extension of shelf life at a competitive cost has been a continuing challenge for film producers, metallizing houses and laminators Specifically, for food applications, the metallized film acts as (a) an oxygen and moisture bamer, to help keep food fresh and cnsp, (b) a light bamer, to reduce rancidity m fatty foods, and (c) ar. aroma bar¬ rier, to keep the onginal flavor intact Additional requirements mclude processability of die metallized film, abra¬ sion resistance, mechanical robustness, reliability and cost

A Opaque Metallized Bamer Films

The bamer properties of d e metallized aluminum films depend almost entirely on die lntegπty of the met¬ allized aluminum layer The quality of die metallized layer depends on defects on die surface of d e polymer film Gross defects are manifested mainly the form of pinholes in the metallization that are visible with ths optical mi¬ croscope Pinhole defects are usually traced to particles on d e film surface and abrasion of the aluminum on film fibnls and other surface protrusions caused by antioxidants and slip agents These features dot the surface landscape of the films and are charactenstic of a particular film vendor and manufacturmg process Although the number and size of pinholes can dominate the transmission of oxygen through the film, there are additional micrc structure de¬ fects that detemune die ultimate bamer possible for a given film Microstructure defects can have several forms that mclude micropinholes, microcracks, and microareas of very dun aluminum These defects can be traced to proper- ties of die substrate film tiiat mvolve die basic film chemistry and manufacturmg process Parameters such as the molecular weight of the resin used to make the film additives to die resin, process conditions and surface treatment of die film Improved barner performance can be achieved by minimizing botii gross and microstructure defect den¬ sity on die film surface

Several matenals were investigated for use in the production of high barner metallized films One prereq- uisite is that die monomer and of course the polymer coatings must have a low amount of volatile components, to assure that the package matenal does not produce an odor In order to qualify polymer matenals, a standard "aroma test" was conducted, and many of the polymer matenals that were tested passed the test A deodorized version of mpropylene glycol diacrylate was arbitranly chosen to conduct oxygen barner tests

For opaque high oxygen bamer packages (potato chip bags), tiie industry standard is polypropylene film (PP) metallized wuh a dun layer of aluminum (Al) The bamer matenal development mcluded the following de¬ signs of metallized PP and acrylate hybπd films

1. PP Plasma/Al Acryl/Plasma

The acrylate polymer was produced by electron beam curing of a tripropylene glycol diacrylate monomer film deposited in the vacuum using die experimental apparatus described in FIG. 1. The tiiickness of die acrylate polymer was about 1.0 μm diick. The PP film was first plasma-treated using a gas plasma produced by an AτfN g ^as mixture with a 10%/90% ratio, and aluminum was applied on top of die plasma-treated surface. The acrylate poly¬ mer was applied on top of die thin aluminum metal to prevent pinhole formation from abrasion of die film surface on rollers in the vacuum chamber and in subsequent processes such as slitting and laminating. The structure is de¬ picted in FIG. 3D. The plasma treatment runctionalization of PP film resulted in higher nucleation rates of the aluminum and finer crystal structure. The formation of finer aluminum crystallites was confirmed using X-ray diffraction (XRD) analysis. Specifically, the metallized films were tested using a Scintag XRD system, which revealed tiiat the alumi¬ num 2.344 CPS peak for die plasma-treated film was at least three times less intense and about twice as broad as die non-treated metallized film. Also, transmission electron microscopy (TEM) and electron diffraction analysis con- ducted on a Hitachi TEM showed that the crystals in the aluminum deposited on the plasma-treated PP film were much smaller witii very broad diffraction rings (when compared to die aluminum deposited on control PP). The finer aluminum crystallites result in a more flexible metal film that resists microcracking during stretching. This was measured by monitoring die electrical resistance of a metallized film while die film was elongated at a fixed speed, using a computer-controlled system. In a metallized film, microcracking of the metal layer will result in an mcrease of its electrical resistance. FIG. 4 shows die change in resistance versus percent elongation for a control PP/aluminum film (Curve 54) and PP/plasma aluminum (Curve 56). As can be seen, the aluminum deposited on die functionaiized plasma-treated surface has significantly more microcracking resistance than the control. 1. PP/Plasma/Acrvl/AI The acrylate polymer was produced by electron beam curing of a tripropylene glycol diacrylate monomer film deposited in die vacuum using die experimental apparatus described in FIG. 1. The thickness of die acrylate polymer was about 1.0 μm thick. The PP film was first plasma-treated using a gas plasma produced by an Ar/N ₂ gas mixture witii a 10%/90% ratio, and die acrylate monomer was flash-evaporated on the treated surface. The acrylate polymer was applied below the aluminum layer to smooth out the rough surface of the PP film and to also provide a thermally and mechanically superior substrate. As shown in the capacitor section above, this results in higher corro- sion stability of the aluminum layer. The structure is depicted in FIG. 3B.

In addition, the surface of die PP film contains low molecular weight material composed of PP, degraded PP, and slip agents added to die film during manufacture. It was discovered that these lower molecular weight spe¬ cies reflow when exposed to die heat generated from die evaporation sources and die condensation of die aluminum deposit. The reflow prevents nucleation of aluminum atoms on the surface of these cites, thus creating pinholes that degrade die moisture and oxygen barrier properties. This effect is eliminated or minimized when die high tempera¬ ture acrylate polymer is deposited on die PP surface and tiien metallized.

3. PP/Plasma/Acrvl/Al/Acrvl/Plasma

The acrylate polymer was produced by electron beam curing of a tripropylene glycol diacrylate monomer film deposited in die vacuum using the experimental apparatus described in FIG. 1. The tiiickness of the acrylate polymer was about 1.0 μm thick. The PP film was first plasma-treated using a gas plasma produced by an Ar/N _: gas mixture with a 10%/90% ratio, and die acrylate monomer was flash-evaporated on die treated surface. This is a

combination of the two above systems. Other combinations with a second aluminum layer are also possibl , but less cost effective. The structure is depicted m FIG. 3C.

In order to make the hybrid film production economically more attractive, lower grades of PP films (lower cost) may be used tiiat when metallized have Oxygen Transmission Rates (OTR) m the range of 3 to .0, versus higher quality films that have OTRs m die range of 1 to 3. The data m Table VII below show that the OTR of con¬ trol films (PP/A1) can be reduced from 6.17 to as low as 0.07 usmg the hybrid film designs descnbed above. All OTR and MVTR banner measurements reported herem were performed by a dnrd party, usmg commercial equip¬ ment. Considermg that die hybπd films can be produced in the same metallization equipment that die control films are made and witii little additional cost, the OTR values of die hybπd films represents an unprecedented level of improvement, and are far superior to commercially available films.

TABLE VII. OXYGEN TRANSMISSION RATE (OTR)

OF CONTROL METALLIZED PP (PP/A1) AND

ACRYLATE-PP HYBRID FILMS

FILM TYPE OTR cm ³ / 100 in ² /day

PP/A1 (control) 6.17

PP/plasma/Al acryl/plasma 0.59

PP/plasma/acryl/Al 2.54

PP/plasma/acryl Al/acryl plasma 0.07

It should be noted that the polypropylene was plasma-treated in the case that the acrylate polymer was de¬ posited directly on die polypropylene (configurations 2 and 3 above). The plasma treatment was found to be advan¬ tageous for two reasons, (a) it removes adsorbed air and moisture frflm the surface of the PP film, which enhances die degree of cross-linking on die PP/acrylate interface, and (b) it functionalizes the PP film, which facilitates wet¬ ting of the acrylate monomer. Several gases were used for die plasma treatment operation that mcluded Ar, N ₂ , 0 ₂ , Ne, CO,, and mixtures thereof. Ar and N ₂ mixed in a 10%/90% ratio produced a very effective mixture as measured by wetting and uniformity of the polymer layer, and so did mixtures witii 5% 0 ₂ . Another effective gas mixture in¬ corporates 99.9%Ne and 0.1%Ar, and mixture of tiiat witii 10% N ₂ , or 5% of 0 ₂ , or 5% of C0 ₂ . The long lived me- tastable levels in the Ne atom enhance the ionization level of d e mixture and increase significantly the lomzation current and surface treatment. Another effective mixture is 96% CF ₄ and 4% 0 ₂ . It was also found tiiat at times, after die hybrid film was wound into a roll, a certam level of "blocking" or tackiness developed, because the back side of the polypropylene film had a soft copolymer layer deposπed on it tiiat is used to tiiermally seal the plastic bag. The tackiness was a result of die combined effect of partial cure on the sur¬ face of the acrylate coating and electrostatic charge (electrons) that can be trapped in deep and shallow potential wells m die acrylate coatmg. It was found tiiat the blocking could be eliminated witii the addition of a second cross-linking and dis¬ charge station, after the acrylate polymer is formed in tiie first station, The second cross linking station can be an¬ other lower energy electron beam, or a plasma treatment station. An electron beam with 600 to 1200 eV energy was used to add surface cure and discharge die surface of die acrylate polymer. At tins voltage level, most polymer ma¬ tenals have a secondary electron emission coefficient that is higher than 1.0, tiius leadmg to a surface discharge, be-

cause for every incoming electron more than one electrons are emitted from the surface. A plasma treatment station accomplishes the same result, altiiough the mechanism is different. The radiation in a plasma discharge mcludes ions and UV radiation. The UV radiation mcludes very energetic (15 eV) vacuum UV photons tiiat enhance surface cross-linking, while the ions help discharge die film surface. Ξ An alternate method of preventing blockmg of the film in die roll is to use monomer formulations that in¬ clude small quantities of additives such as mono-acrylate monomers that cannot fully cross-link, or that are partially conductive This leads to die formation of a "slippery" and/or antistatic acrylate surface that inhibits blockmg and static charge formation.

I C B. Transparent Ceramic-Coated Bamer Films

Environmental constraints are leading to die replacement of certam chloπne-containing high bamer poly¬ mer films, with polyester films coated with transparent bamer layers of inorganic matenals such as aluminum oxide and silicon oxides (SiO _x , where x=l to 2).

Some food manufacturers have been slow in accepting the higher cost coated transparent bamer films due 15 to the higher pnce of die polyester film that appears to be necessary for the production of transparent high bamer coatings. For this, several attempts have been made to produce lower cost transparent barrier usmg a polypropylene (PP) substrate films. This work has shown that the low melting point PP polymer films are in general a poor sub¬ strate for the deposition of the higher melting point inorganic coatings. Thus, ceramic-coated transparent barner films that are presently in die market place utilize polyester film as a substrate that has supenor tiiermomechanical 20 properties to PP.

The present invention addresses the low cost production of high barrier metallized films as well as high bamer transparent PP films, using a combination of vacuum polymer coating and surface modification by plasma treatment. PP film was plasma-treated using a 90% N ₂ -10% Ar gas mixture and a low aroma acrylate monomer was deposited and cross-linked witii electron beam radiation. The accelerating voltage is typically 10 to 12 KeV for 25 about 1.0 μm of acrylate coating. The current (or number of electrons) varies widi web speed. For a one foot wide web movmg at 100 to 200 ft/mm, 5 to 10 mA of current was used.

The transparent films employed in die practice of die present invention include aluminum oxide, silicon oxides (SiO„ where x=l to 2), tantalum oxide, aluminum nitride, titanium nitride, silicon nitnde, silicon oxy-nitnde, zmc oxide, indium oxide, and indium tm oxide. The thickness of the ceramic coating may range from about 5 to 100

30 nm for use as a barrier layer. Preferably, the best barrier properties are obtamed in die lower portion of die tiiickness range. The ceramic coating may be deposited by electron beam evaporation.

Two types of transparent barner films were produced; one using a thm inorganic film of SiO„ where x=l to 2, and A1,0 ₍ . The SiO, film was deposited usmg plasma deposition from a silane gas mixture and die A1 ₂ 0 ₃ film was deposited usmg electron beam evaporation. In this experiment, die objective was to see if die thermomechani- 35 cally supenor acrylate hybrid film had improved bamer properties, rather than optimizing the barner properties to a minimum. The results as shown m Table VIII indicate that the oxygen and moisture barner properties of die trans¬ parent PP/plasma/acrylate/ceramic film are supenor to die control PP/ceramic film.

TABLE VIII. OXYGEN (OTR) AND MOISTURE (MVTR) TRANSMISSION RATE OF CONTROL PP/CERAMIC AND PP/PLASMA ACRYLATE/CERAMIC HYBRID

FILMS

FILM TYPE MVTR OTR g/100 in ² /day cm ³ / 100 in ² /day

PP/SιO _x (control) 0.25 9.2

PP/A1 ₂ 0, (control) 21.4

PP/plasma acryl SiO 0.10 2.5

PP/plasma/acryl/AUOj 4.0

In packaging applications, it is important that the barrier properties of a film are maintained dirough die process of processing the film into a bag. This process exposes the film to some elongation and stretching as die film is pulled over a forming collar that forces the flat film to conform into a cylindrical opening that is sealed first on the side, then the bottom and on the top after food is automatically inserted in the bag. If conventional metallized PP film is elongated, then the aluminum layer can easily crack, leading to loss of barrier. Since barrier properties as a function of elongation is difficult to measure, die cracking of the aluminum as a function of elongation was meas¬ ured indirectly by measuring the resistivity of die aluminum. FIG. 5 shows that the aluminum deposited on PP (Curve 58) has far inferior mechanical properties tiiat of a PP/plasma/aluminu /acrylate/plasma hybrid film (Curve 60), using 90%N ₂ -10%Ar gas plasma and adhesion promoted deodorized tripropyleneglycol diacrylate monomer.

Further investigation on die stmcture of the aluminum using X-ray diffraction analysis, transmission elec- tron microscopy, and electron diffraction analysis show that die aluminum formed on the control PP surface has large crystals, while aluminum deposited in die hybrid film has much finer crystal stmcture. Such crystal stmcture in combination with superior aluminum adhesion to the nitrided PP surface and die protective acrylate coating, rninimize the formation of microcracks and preserve die original barrier properties of the film. Several packaging trials conducted by a leading manufacturer of snack foods have shown that bags produced witii a monoweb of PP/plasrna/aluminum/acrylate. The acrylate polymer was produced by electron beam curing of a tripropylene glycol diacrylate monomer film deposited in the vacuum using the experimental apparatus described in FIG. 1. The tiiick¬ ness of the acrylate polymer was about 1.0 μm thick. The PP film was first plasma-treated using a gas plasma pro¬ duced by an Ar/N ₂ gas mixture with a 10%/90% ratio, and aluminum was applied on top of the plasma-treated sur¬ face. The acrylate polymer was applied on top of die dun aluminum metal to prevent pinhole formation from abra- sion of the film surface on rollers in die vacuum chamber and in subsequent processes such as slitting and laminat¬

The hybrid film maintained low MVTR and OTR, while die control PP/A1 film can increase by about an order of magnitude. Table IX shows OTR and MVTR data following film production, after printing and following bag production.

TABLE IX. OXYGEN (OTR) AND MOISTURE (MVTR) TRANSMISSION RATE OF PP/PLASMA/ALUMINUM/ACRYLATE/PLASMA HYBRID FILMS AT

DIFFERENT PRODUCTION STAGES OF A SNACK FOOD BAG

FILM TYPE MVTR OTR PP/plasma/metal acry late/plasma g/100 in ² /day cπv lOO πvVday

Film Production 0.004 0.34

Printing 0.003 0.35

Bag Production 0.01 1.41

ID. FILMS FOR PRINTING APPLICATIONS

In the above description of packaging applications, die PP/plasma/metal/acrylate/plasma hybrid film can be printed directly after production without any special preparations. The printing industry employs a broad range of printing inks that include three main categories; water-based, solvent- (oil) based, and 100% solids. Although these inks are formulated to wet different kinds of surfaces, surfaces are often formulated to be wetted by a particular ink process and equipment that a food manufacturer has in place. The acrylate hybrid films can be designed to be wet¬ ted by various ink chemistries. The hydrophobic/philic and oliophobic/philic surface wetting properties were meas- ured using d e solutions shown in Table X. A drop of each of die solutions in Table X was deposited on die surface of an acrylate polymer, obtained by electron beam curing of the acrylate monomer. If the liquid drop of a particular composition (1-6) does not wet the surface within d e specified period (see foot notes in Table X), then die surface is considered phobic for that liquid. TABLE X. SOLUTION CHEMISTRY USED FOR WETTABILITY TESTS, NUMBER "1 " IS MOST PHILIC AND NUMBER "6" IS MOST PHOBIC

Oil Rating Number Composition

1 Kaydol (mineral oil)

2 65/35 Kaydol n-hexadecane

3 n-hexadecane

4 n-tetradecane

5 n-dodecane

6 n-decane

30 seconds wetting period

Water Rating Number % iso-Propanol % Water

1 2 98

2 5 95

3 10 90

4 20 80

5 30 70

6 40 60

10 seconds wetting penod

About 100 different acrylate formulations were produced and tested for basic wetting properties (see list under Capacitors, above) The results m FIG 6 show tiiat hybnd films can be formulated with specific surface wet¬ ting charactenstics to accommodate a broad range of ink cheπustnes Some examples of acrylate polymer formula¬ tions and their corresponding wetting properties are shown in Table XI

TABLE XI OIL AND WATER WETTING PROPERTIES OF ACRYLATE HYBRID FILMS (PP/plasma/Acrylate), SEE TABLE X FOR WETTING RATING NUMBERS

Oil Water Acrylate Polymer

1 6 neo-pentyl glycol diacrylate

5 6 octyldecyl acrylate

3 6 octane diol diacrylate 1,8

3 4 akoxylated aliphatic diacrylate ester

3 6 decanediol diacrylate

1 3 93% hexane diol dιacrylate/7% KENSTAT lOO

Note KENSTAT q 100 is a trade name of Kennch Petrochemicals, Inc.

In addition to printing applications, the surface wetting funcπonalization of the PP/acrylate films can be useful m other applications where surface wetting or lack of it is important. For example, a Teflon-like surface that repels water can have many industπal and commercial applications. An oliophihc surface could be used m film/foil capacitor applications where die wound rolls have to be thoroughly impregnated witii a dielecmc oil-based fluid

IV. MAGNETIC TAPES

The need for increasingly higher density storage media, the widespread use of narrow format video tapes, and die development of digital audio and video tape dnves are leading to shorter wavelengui recording, which al¬ lows higher data storage per umt length of tape. Such recordmg tapes utilize a dim metal magnetic coating that can be obtamed by metal evaporation, sputtering and plating techniques This approach eliminates the polymer bmder tiiat is utilized m conventional magnetic coatings resulting m ferromagnetic layers with higher saturation magneti¬ zation, which is suitable for higher storage densities.

When short wavelength recordmg is used, dropout or the loss of information is much higher for a given spacmg between the recordmg medium and the magnetic head The spacmg loss is represented by the formula 54 6 d/w, where d is the tape to head distance, and w is die recordmg wavelengtii Therefore, if the surface of the ag-

netic medium, or even the opposite side are rough, loss of information will occur as die tape is running m front of the head For this reason, films that are used for metallized magnetic tapes are quite costly because die surface roughness of both die front and die back surfaces have to be well-controlled to assure low degree of roughness Ii should be noted that if the roughness of the back side is too low, dien the tape can bmd and be damaged An additional problem with thm metal tapes is damage of the magnetic due to abrasion resistance and envi¬ ronmental corrosion Tape manufacturers have devised different ways of dealing witii tiiese limitations Radiation- curable mateπals are commonly used to coat bodi die front and back surface of the substrate tape and often inor¬ ganic additives for controlled roughness and antistatic properties are used on the back side coating (U S Patents 5,068, 145, 4,670,340, 4,720,421 4,781,965, 4,812,351 , 4,67,083, and 5,085,911) The existing methods for apply- mg the radiation-curable coatings use conventional applications methods (roll coating, casting, and solution based) under atmospheπc conditions, and the cure utilizes UV or electron beam, performed in air or in a nitrogen environ¬ ment

In the present invention, acrylate coatings are vapor-deposited in vacuum which allows for thinner, pinhole free and more uniform coatings, in-line with die deposition of die magnetic coating Acrylate polymer films depos- ited on one surface of polyester film can flatten surface irregulanties by one to two orders of magnitude, dependmg on die thickness of die acrylate layer For example, a 2 0 μm acrylate coating on a surface with 400 nm aspenties reduces die average surface roughness to less than 50 nm The acrylate polymer was produced by electron beam cunng of a hexane diol diacrylate monomer film deposited in die vacuum usmg the experimental apparatus de¬ scnbed m FIG 1 The substrate film was first plasma-treated usmg a gas plasma produced by an Ar/N ₂ gas mixture with a 10% 90% ratio If a flatter film is used wtth 50 nm aspenties, tiien a 1 0 μm or less of hexane diol diacrylate acrylate layer results m a virtually flat film surface with a surface roughness of die order of 1 to 2 nm

As mentioned above, a surface with a controlled roughness on die backside of die magnetic tape is often desirable to minimize friction over rollers A hybnd film witii a controlled microroughness was produced in the following manner After the deposition of a radiation-curable monomer, an electron beam is used to polymerize die dun monomer film For a film thickness of 1 0 μm, a beam with an accelerating voltage of about 8 to 12 KeV may be used If die beam voltage is reduced considerably, die penetrating depth of the electrons is reduced proportion¬ ally For example, an accelerating voltage of about 600 to 1000 electron volts will cure the surface of the acrylate film, thus creating a dun skm The heat produced in die exothermic polymerization reaction and die shrinkage of the acrylate film dunng polymerization causes the skm to wrinkle m a very controlled way A cross-section of the wrinkled surface 62a is shown in FIG 7 A The penodicity and deptii of die wrinkles can be controlled based on the thickness of the monomer film, accelerating voltage, current (number of electrons) and film shrinkage, that can be vaπed by changing the acrylate chemistry Acrylate mateπals with film shrinkage greater than 5% are desirable for this process, with shrinkage levels of about 15% being preferable Such monomers mclude mateπals such as pentaerythntol macrylate, hexane diol diacrylate, tnmediylolpropane macrylate, pen- taerythπtol tetraacrylate, di-tnmediylolpropane tetraacrylate, dipentaerythntol pentaacrylate, mpropylene glycol diacrylate and tetraetiiylene glycol diacrylate After die production of the wrinkled surface, a second higher voltage electron beam is used to polymerize die whole monomer film thickness

The reduced voltage may be obtamed by placmg a smaller electron beam gun 230, shown in phantom in FIG 1 , between the flash evaporator 28 and die radiation cunng means 30

The roughened surface 62a shown m FIG 7A can be used m the backside of a magnetic tape to minimize abrasion and facilitate free movement over rollers Such a surface may also be useful in film/foil capacitors to fa¬ cilitate impregnation of capacitor rolls with a dielecmc fluid

V. OPTICAL FILTERS

Polymer films with optical effects such as color shift and holographic images are used m vanous applica¬ tions that mclude wrapping films and tamper proof and anti-counterfeiting medallions Two such acrylate hybnd films were produced in a one-step low cost manufacturmg process

(A) The wrinkled film descnbed above, when metallized after the second electron beam cure (FIG 7B), 0 creates bnlhant color shifting reflections if the wrinkles size is small enough to interfere with the ambient light Al¬ though in tius work only a uniform interference pattem was produced across die film surface, color shifting images can be produced by patterning die wrinkle area 62b and by varying the wrinkle size within an image This can be accomplished by replacing the wide electron beam curtain witii a smgle pomt electron beam that can be computer controlled to move in the X-Y axis and also have vaπable current by modulation of the Z axis -.5 (B) A uniform color shifting film 64 was produced usmg a relatively flat substrate film 120, or a relatively rough film that was flattened with an acrylate coating The configuration of the color shifting film (shown in FIG 8), was as follows polyester/plasma metal 1/acrylate 1/metal 2/acrylate 2/plasma The acrylate polymer was pro¬ duced by electron beam cunng of a hexane diol diacrylate monomer film deposited m die vacuum usmg the expeπ- mental apparatus descnbed m FIG 1 The polyester substrate film was first plasma-treated usmg a gas plasma pro- 20 duced by an Ar/N ₂ gas mixture with a 10%/90% ratio Metal 1 was an alummum film 124 that is highl/ reflective with an with an optical density of about 3 to 4 Acrylate 1 was a dim acrylate coating 122 with a thickness that was '/ _< wavelength of visible tight (04 to 07 μm) Metal 2 was a thm semi-transparent layer 124' with optical density of about 0 5 to 1.5 Acrylate 2 is an optional protective coating 122' that can have any thickness (typically 0 5 to 3 μm) that provides adequate abrasion resistance Such hybnd film results m a uniform bπght color tiiat shifts to higher or 25 lower frequency color as die viewmg angle changes

Light beam 66 is mcident on die color shifting device 64, and a portion passes through the semi-transparent metal layer 124' and is reflected as beam 68a by die opaque metal layer 124 Another portion of mcident light beam 66 is reflected as beam 68b by me semi-transparent metal layer 124'

The above color shift hybnd film could be made to color shift on botii sides That is, it could also color 30 shift tiirough die transparent polyester substrate film, if die color shifting stack is symmemcal Such structure would be as follows Polyester/semi-transparent aluimnum/'Λλ acrylate/reflective aluminum/V-λ acrylate/semi-transparent alummum Given the above hybnd film design, it will also be relatively easy to someone skilled m die art to pro¬ duce a color shift pigment by releasing the color shifting stack from the polyester substrate This can be accom¬ plished by coating die polyester substrate with a layer of polystyrene The color shifting stack is tiien deposited on 35 the polystyrene, which can be dissolved m a solvent to produce symmetnc color shifting flakes, that can be reduced to a finer color shifting pigment

VI. FLEXIBLE CABLES

In electncal cables, such as tiiose used in aircraft to direct power to vanous parts of the airplane, arcing can

4 C occur that creates so-called "electrical tiackmg", which may result in fires The use of these films in military aircraft has been banned by the U S Navy, and later the U S Army and Au Force, due to poor electncal tracking resistance

that has resulted m several documented cases of fires, and even loss of military aircraft. One solution has been to replace the thermally and mechanically robust polyumde films that have poor trackmg properties with polytetrafluoroethylene- ("Teflon") coated wire, which has excellent arc trackmg properties However, die Teflon coating is rather soft and it can be easily damaged. A solution to tins problem is to clad Teflon on polyimide film (see Chemfab entry in Table XII, below). This hybπd film has trackmg properties similar to Teflon, with supenor thermal and mechanical properties.

In the present invention, reduction of electncal trackmg is accomplished witii die use of a hybπd film that utilizes a combination of a Teflon-like vacuum polymer coatmg and surface modification by plasma treatment An electncal arc trackmg test was developed to simulate the performance of cable insulation. A high voltage, high im¬ pedance AC voltage (5 to 15 KV), was applied between two flat metal plates tiiat were 5 mches long, 2 inches wide and 1/16 mch duck. The test samples (m die form of 1 to 6 mil duck films) were wrapped around die plates and die two plates were fixtured agamst each other along their length on one plane. The gap between the plates was adjusted to about 1 mm and die voltage was raised until an arc initiated at one of the two opposing sets of comers m the two plates that was intentionally exposed (not covered witii die test films) to allow die arc to strike. The polymer films start to bum as soon as the arc strikes. The rate of arc movement (film burning) accurately replicates the ability of the matenal to arc-track. The system was tested usmg known matenals such as Teflon and Kapton films diat have low and high arc trackmg rates, respectively.

TABLE XII. ARC TRACKING RESISTANCE OF SEVERAL POLYMER FILMS

LAYERS/ TRACKING DIS¬

FILM THICKNESS TANCE (mch)

PBO/Fluoroacrylate, 12 μm 4/1 mil no arc

PTFE 4/1 mil <l/32

Chemfab, Style 072992A 2/2 mil <l/32

Polyester (PET) 8/0.5 mil <l/32

Polypropylene (PP) 8/0.5 mil 4/32

Kapton/Fluoroacrylate, 12 μm 4/1 mil 4/32

ETFE (Tefzel film) 4/1 mil 12/32

PFCB 2/0.8 mil (best of 3) 30/32

PIBO 4/1 mil 32/32

Polycarbonate (PC) 8/0.5 mil 40/32

PBO 4/1 mil 60/32

Kapton 2/2 mil 60/32

A functionally similar hybnd film was produced by depositing a fluorine-containing acrylate film on one or botii sides of Kapton (DuPont film) and PBO (Dow Chemical film). Table XII shows a clear relationship between the tiiickness of a fluoro-acrylate coating on 25 μm of PBO and Kapton films. The fluoroacrylate coatmg was com¬ posed of a 25%.75% mixture of hexane diol diacrylate (HDODA):perfluoro-alkysulfonamide, and tiien cross-linked with an election beam, usmg die apparatus descnbed m FIG. 1. For die PBO-coated films, die improvement in arc trackmg paralleled the coatmg thickness until a critical thickness of about 12 μm was reached Beyond that pomt.