Title: Optical filters
Abstract: A hybrid film, comprising a first polymer film having a plasma-treated surface and a second polymer film having first and second surfaces, with the first surface of the second polymer film being disposed along the first plasma-treated surface of the first polymer film, has superior thermal and mechanical properties that improve performance in a number of applications, including food packaging, thin film metallized and foil capacitors, metal evaporated magnetic tapes, flexible electrical cables, and decorative and optically variable films. One or more metal layers may be deposited on either the plasma-treated surface of the substrate and/or the radiation-cured acrylate polymer. A ceramic layer may be deposited on the radiation-cured acrylate polymer to provide an oxygen and moisture barrier film. The hybrid film is produced using a high speed, vacuum polymer deposition process that is capable of forming thin, uniform, high temperature, cross-linked acrylate polymers on specific thermoplastic or thermoset films. Radiation curing is employed to cross-link the acrylate monomer. The hybrid film can be produced in-line with the metallization or ceramic coating process, in the same vacuum chamber and with minimal additional cost.
Patent Number: 6,838,183 Issued on 01/04/2005 to Yializis
| Inventors:
|
Yializis; Angelo (Tucson, AZ)
|
| Assignee:
|
Sigma Laboratories of Arizona, Inc. (Tucson, AZ)
|
| Appl. No.:
|
792197 |
| Filed:
|
February 21, 2001 |
| Current U.S. Class: |
428/457; 359/582; 359/585; 359/586; 428/458; 428/461; 428/463 |
| Intern'l Class: |
B32B 015/08 |
| Field of Search: |
359/586,585,588,582
428/463,457,458,461
|
References Cited [Referenced By]
U.S. Patent Documents
| 3858977 | Jan., 1975 | Baird et al. | 359/582.
|
| 4557980 | Dec., 1985 | Hodnett, III | 428/336.
|
| 4705356 | Nov., 1987 | Berning et al. | 359/590.
|
| 4740385 | Apr., 1988 | Feuerstein et al. | 427/38.
|
| 4812351 | Mar., 1989 | Okita et al. | 428/141.
|
| 4842893 | Jun., 1989 | Yializis et al. | 427/44.
|
| 4954371 | Sep., 1990 | Yializis | 427/44.
|
| 5019210 | May., 1991 | Chou et al. | 156/643.
|
| 5032461 | Jul., 1991 | Shaw et al. | 428/461.
|
| 5087476 | Feb., 1992 | Tohma et al. | 427/35.
|
| 5225272 | Jul., 1993 | Poole et al. | 428/323.
|
| 5270137 | Dec., 1993 | Kubota | 429/249.
|
| 5322737 | Jun., 1994 | Morra et al. | 428/412.
|
| 5374483 | Dec., 1994 | Wright | 428/412.
|
| 5440446 | Aug., 1995 | Shaw et al. | 361/301.
|
| 5445871 | Aug., 1995 | Murase et al. | 428/215.
|
| 5877895 | Mar., 1999 | Shaw et al. | 359/588.
|
| 6231939 | May., 2001 | Shaw et al. | 428/35.
|
| Foreign Patent Documents |
| 0339844 | Nov., 1989 | EP.
| |
| 375215 | Jun., 1990 | EP.
| |
| 62132940 | Jun., 1987 | JP.
| |
| 404152553 | May., 1992 | JP.
| |
Primary Examiner: Jackson; Monique R.
Attorney, Agent or Firm: Oremland, P.C.; Lawrence R.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The present application is a divisional of application Ser. No. 09/169,175,
filed Oct. 8, 1998 now U.S. Pat. No. 6,214,422, which is a divisional of
application Ser. No. 08/628,570, filed Apr. 4, 1996, now U.S. Pat. No.
6,083,628, issued Jul. 4, 2000, which in turn is a continuation-in-part
application of Ser. No. 08/334,739, filed Nov. 4, 1994 now abandoned.
Claims
What is claimed is:
1. An optical filter, comprising
a polymer substrate having a first vacuum formed plasma treated surface,
a vacuum deposited, first radiation polymerized acrylate monomer film
having first and second surfaces, the first surface of the first monomer
film being deposited on the first plasma treated surface of the polymer
substrate,
a metal film having first and second surfaces, the first surface of the
metal film deposited on the second surface of the first polymerized film,
wherein the second surface of the first polymerized film has a
microroughness produced by (a) radiation curing a portion of the first
polymerized film that includes the second surface, to partially cure the
portion of the first polymerized film, and (b) subsequent curing of the
entire first polymerized film.
Description
TECHNICAL FIELD
The present invention relates generally to thin metallized and
non-metallized polymer film that incorporate additional coatings and
surface functionalization, which impart application specific properties
such as improved thermal stability, abrasion resistance, moisture barrier
and optical effects, that make these films useful in food packaging
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 resistance, 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 example metallized polymer films are usually
corona-, flame-, or plasma-treated, to promote adhesion of the metal to
the polymer surface (U.S. Pat. Nos. 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.
Pat. No. 5,087,476). Polymer coatings that serve various functions such as
printability, adhesion promotion, abrasion resistance, optical and
electrical properties, have been produced using various techniques that
include thermal cure, reactive polymerization, plasma polymerization (U.S.
Pat. No. 5,322,737), and radiation-curing using ultra-violet and electron
beam radiation (U.S. Pat. Nos. 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 using
conventional techniques of roll coating, casting, 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 thin liquid
monomer which can be radiation-cured (U.S. Pat. Nos. 4,542,893; 4,954,371;
and 5,032,461 and European Patent Application 0 339 544). 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 described in the above references
was found to have some critical limitations on certain mechanical,
thermal, chemical and morphological properties that can reduce their
usefulness 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 coating
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 mechanical, thermal, chemical and surface morphological
properties. In conjunction with one or more metal coatings or a ceramic
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 includes 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 discharging, 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 difluoride, polyphenylene oxide, and polyphenylene sulfide,
and thermoset fins that include cellulose derivatives, polyimide,
polyimide benzoxazole, and polybenzoaxozole. The second polymer films are
radiation-polymerized monomer films that are multifunctional acrylate or
acrylate monomers that contain double bonds capable of radical
polymerization. Plasma treatment with gases from the group of N.sub.2, Ar,
Ne, O.sub.2, CO.sub.2, and CF.sub.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 unwinding 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, cobalt, 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.sub.x, where x=1 to 2), tantalum oxide, aluminum nitride,
titanium nitride, silicon nitride, silicon oxynitride, zinc oxide, indium
oxide, and indium tin oxide.
The hybrid polymer film evidences both improved corrosion resistance and
current carrying ability of metallized capacitors compared to prior 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
improves the mechanical properties of the barrier layer to the extent that
there is less damage of the barrier layer as a function of film
elongation.
By adjusting the chemistry of the acrylate coatings, the surface of the
hybrid films can be made hydrophobic/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 the 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 the aluminum metallized films
were exposed to temperature and humidity ambient conditions of 70.degree.
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 with 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 untreated 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
metallized 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 the coated polymer film of FIG. 7A,
following deposition of a thin metal layer on the micro-roughened surface
of the acrylate coating; and
FIG. 8 is a cross-sectional view of the hybrid polymer film of the
invention configured for optical filter applications.
BEST MODES FOR CARRYING OUT THE INVENTION
The vacuum polymer coating technique as described in U.S. Pat. Nos.
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 include the
following: (a) when an acrylate monomer material 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) contaminants and low molecular weight species on
the surface of most polymer materials can inhibit the wetting of the vapor
deposited liquid monomer, resulting in thin polymer films with poor
uniformity; (c) when this process is used to produce coatings on a polymer
web that 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 electrons) on the film surface, can cause the film to
block or stick to itself when it is wound into a roll.
The discussion below is directed in the main to a hybrid film comprising
polypropylene (PP) coated with a vacuum-deposited, radiation-curable
acrylate monomer film that 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 hybrid film
The hybrid film comprises PP film coated with a high temperature,
cross-linked, acrylate polymer, deposited by a high speed vacuum process.
The basic aspects of the process are disclosed in U.S. Pat. Nos.
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 appropriate pressure. The
essential components of the apparatus 10 within the vacuum chamber 12
include 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 thin 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.
Optionally, an evaporation system 32 for depositing an inorganic film on
the acrylate film may be employed. 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 than about
0.001 atmosphere, or less than about 1 millibar. Typically, the vacuum is
on the order of 2.times.10.sup.-4 to 2.times.10.sup.-5 Torr.
In operation, the polymer film 20 is fed from the source spool 18 onto the
rotating drum 16, which rotates in the direction shown by arrow "A", via
guide roller 24. The polymer film passes through several stations, is
picked off from the surface of the 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 the source spool 18, it passes through a first plasma
treatment unit 36, where the surface of the film to be coated is exposed
to a plasma to remove adsorbed oxygen, moisture and any low molecular
weight species from the surface of the film prior to forming the acrylate
coating thereon 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
curing the acrylate coating and to remove any accumulated electronic
charge.
The conditions of the plasma treatment are not critical, and the plasma
source may be low frequency RF, high frequency RF, DC, or AC.
The rotating drum 16 is a water-cooled drum driven 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.degree. to 50.degree. C. to
facilitate condensation of the monomer (in 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 comprise any of the polymers that have the
requisite properties to be treated as described below. Examples of such
polymers include the thermoplastic polymers such as polypropylene (PP),
polyethylene terephthalate (PET), polycarbonate,
polyethylene-2,6-naphthalate, polyvinylidene difluoride, polyphenylene
oxide, and polyphenylene sulfide, and the 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 supplied with liquid monomer from a reservoir 40
through an ultrasonic atomizer 42, where, with the aid of heaters (not
shown), the monomer liquid is instantly vaporized, i.e., flash vaporized,
so as to minimize the opportunity for polymerization prior to being
deposited on the polymer film 20. The specific aspects of this part of the
process are described in greater detail in the above-mentioned U.S. Pat.
Nos. 4,842,893; 4,954,371; and 5,032,461 and do not form a part of the
present invention.
The flash-vaporized monomer condenses on the surface of the polymer film 20
that is supported on the cooled rotating drum 16, where it forms a thin
monomer film.
The condensed liquid monomer is next radiation-cured by the radiation
curing means 30. The radiation curing means may comprise any of the common
methods for opening the double bonds of the acrylate monomer, examples of
suitable means include apparatus which emit electron beam or ultra-violet
radiation. Preferably, the radiation-curing means 30 comprises a
thermionic or gas discharge electron beam gun. The electron beam gun
directs a flow of electrons onto the monomer, thereby curing the material
to a polymerized, cross-linked film. Curing is controlled by matching the
electron beam voltage to the acrylate monomer thickness on the polymer
film 20. For example, an electron voltage in the range of about 8 to 12
KeV will cure about 1 .mu.m thick of deposited monomer. As with the
specifics regarding the deposition of the acrylate monomer, the specific
aspects of this part of the process are described in the above-mentioned
U.S. Pat. Nos. 4,842,893; 4,954,371; and 5,032,461, and do not form a part
of the present invention.
FIG. 3A depicts the coated film 20' at this stage, comprising an acrylate
polymer film 122 on a polymer film web or substrate 120. Such a coated
film 20' has a variety of uses, including high temperature electrical
cables and foil capacitors in which the film is wound with a metal foil.
The cured acrylate monomer, or crosslinked polymer, then passes to the
optional resistive evaporation system 32, where an inorganic material,
such as aluminum or zinc, can, if desired, be deposited on the cured
monomer layer. Two such coated films 20' may be wound to form metallized
capacitors. FIG. 3B depicts a coated film 20' with a metallized layer 124
on the acrylate polymer film 122.
The same material may be used for food packaging films, preferably with an
additional acrylate coating over the aluminum metal layer to protect the
thin metal layer and thus improve the barrier 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 the
metal film 124, while in FIG. 3D, the metal film is first deposited on the
polymer film substrate 120, followed by deposition of the 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 the art for
metallizing films. Alternatively, other 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 with a source of metal from the wire feed 44.
The deposition of the metal film may be avoided, thereby providing simply a
hybrid polymer film, which may have a variety of uses, as described above,
with reference to FIG. 3A
Following the optional metallization stage, a second, optional acrylate
monomer deposition may be performed, using monomer evaporator 128 and
radiation-curing means 130. This second deposition is used to form the
coated films 20' shown in FIGS. 3C and 3D, discussed above. The
composition of the second acrylate film 122' may the same or different as
that of the first acrylate film 122.
The apparatus shown in FIG. 1 is primarily directed toward the formation of
an acrylate film 122, 122' on top of another polymer film 120, with or
without a layer of metal 124. Further, the uncoated side of the polymer
film 20 could also be coated with an acrylate film 222 of the same or
different composition, for example, by providing a second rotating drum
within the vacuum chamber 12 and providing the 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 .mu.m, depending on the particular application.
While thicker films than this may be fabricated, no benefit seems to be
derived using such thicker films.
The thickness of the polymer film substrate 20, 120 is typically within the
range of about 1 to 100 .mu.m, again, depending on the particular
application.
When used with conventional polymer film employed in metallized capacitors,
such as PP, PET, or polycarbonate, the thickness of the acrylate film 122,
222 is typically on the order of 0.1 to 1 .mu.m. In such cases, the
underlying base film (PP or polycarbonate) 20, 120 is much thicker,
commercially available films of PP are in the range of 4 to 25 .mu.m.
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 corrosion
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 other hand, there are applications requiring energy storage in which
higher dielectric constants in the range of about 10 to 15 are desired. In
such cases, acrylate polymers having such a high dielectric constant are
deposited on thinner films, and thus the acrylate film comprises a
substantial fraction of the hybrid film thickness.
In packaging applications, the acrylate coatings 122, 222 are typically
about 0.5 to 2.0 .mu.m thick, deposited on PP or PET films 20, 120 that
are typically 12 to 35 .mu.m thick.
Examples of acrylate monomers that may be used in the practice of the
present invention include ethoxylated bis-phenol diacrylate, hexadiol
diacrylate, phenoxyethyl acrylate, acrylonitrile, 2,2,2-trifluoromethyl
acrylate, triethylene glycol diacrylate, isodecyl acrylate, alkoxylated
diacrylate, tripropylene glycol diacrylate, ethoxy ethyl acrylate,
polyethylene glycol diacrylate, diethylene glycol diacrylate, trimethylol
propane triacrylate, tetraethylene glycol diacrylate, cyano-ethyl
(mono)-acrylate, octodecyl acrylate, dinitrile acrylate, pentafluorophenyl
acrylate, nitrophenyl acrylate, iso-bornyl acrylate,
tris(2-hydroxyethyl)-iso-cyanurate triacrylate, tetrahydrofurfuryl
acrylate, neo-pentyl glycol diacrylate, propoxylated neo-pentyl glycol
diacrylate, and mixtures thereof.
The metal film 124 that is deposited in the case of metallized capacitors
may comprise aluminum or zinc or a composite film of nickel on aluminum,
iron on aluminum, zinc on silver, zinc on copper, or zinc on aluminum. The
thickness of the metal film 124 for capacitor use is on the order of 100
to 300 .ANG., and may be uniform across the width of the hybrid film 20'
or may be thicker at the edges than in the central portion.
The metal film 124 that is used in packaging typically comprises aluminum.
The thickness of the metal film is within the range of about 100 to 400
.ANG..
EXAMPLES
I. Capacitors
Capacitors employ low temperature thermoplastic dielectric thin film
polymers, such as polypropylene (PP), polyethylene terephthalate (PET),
polycarbonate, polyethylene-2,6-naphthalate, polyvinylidene difluoride
(PVDF), polyphenylene oxide, and polyphenylene sulfide, either metallized
or maintained between metal foil electrodes. Metallized film capacitors
are used extensively in a broad range of electrical and electronic
equipment that include motor run and motor start circuits for air
conditioners, fluorescent and high intensity light ballasts, power
supplies, telecommunication equipment, instrumentation, and medical
electronics. In many of these applications, the metallized capacitors are
used to conserve energy by correcting the power factor of a circuit and in
others they are used to perform specific functions, such as timing,
filtering, and decoupling. The advantages of metallized film over film
foil capacitors include lower volume, weight, cost, and higher reliability
due to the self-healing properties of the metallized films. The low
temperature metallized film dielectrics 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 the
thin metallized electrodes that are deposited on low temperature
thermoplastic dielectrics, such as PP, PET, and polycarbonate. In
particular, reliability issues arise when metallized capacitors are forced
to carry high levels of pulse or AC current The present invention
addresses the production of a hybrid film that has improved mechanical and
thermal properties that improve capacitor performance and reliability.
The following discussion is directed to low loss, AC applications,
involving the deposition of an acrylate film on a polypropylene film,
followed by metallization to form a thin film metallized capacitor. It
will be readily apparent to the person skilled in this art, however, that
these teachings may be extended to other base polymer films for other
applications.
Over 100 acrylate monomer materials have been polymerized and tested for
dielectric constant and dissipation factor as a function of temperature
and frequency, and oil and water surface wetting Some of these materials
were sourced from various vendors, others were formulated using
proprietary mixtures of commercially available monomers, and some were
molecularly synthesized. The acrylate polymer materials tested include the
following: 2,2,2-trifluoroethylacrylate; 75%
2,2,2-trifuoroethyl-acrylate/25% C.sub.19 diol diacrylate; 50%
acrylonitrile/50% C.sub.20 diol diacrylate; dimerol diacrylate; 50%
acrylonitrile/(50% C.sub.19 diol diacrylate; Erpol 1010 diacrylate; 50%
hexane diol diacrylate/50% C.sub.20 triol diacrylate; 50%
2,2,2-trichloroacrylate/50% C.sub.19 diol diacrylate; 75%
2,2,2-trifluoroethylacrylate/25% C.sub.19 diol diacrylate; 50% octane diol
diacrylate/50% 2cyanoethyl acrylate; 67% 2,2,2-trifluoroethylacrylate/33%
C.sub.20 triol triacrylate; lauryl acrylate; trimethylolpropane
triacrylate; ethoxyethoxy ethyl acrylate; pentaerythritol tetraacrylate;
neo-pentyl glycol diacrylate; octyldecyl acrylate; tetraethylene glycol
diacrylate; tripropylene glycol diacrylate; octane diol diacrylate;
1,8-alkoxylated aliphatic acrylate; decanediol diacrylate; ethylene glycol
diacrylate; iso-bornyl acrylate; butanediol diacrylate; 93% hexane diol
diacrylate/7% KENSTAT q100; 95% hexane diol diacrylate/5% chlorinated
polyester diacrylate, trimethylolpropane ethoxylate triacrylate;
trimethylolpropane propoxylate triacrylate; neo-pentyl glycol propoxylate
diacrylate; bisphenol A ethoxylate diacrylate; alkoxylated aliphatic
diacrylate ester; 50% 2-cyanoethyl acrylate/50% C.sub.19 diol diacrylate;
92% trimethylolpropane triacrylate/8% acrylonitrile; 50% iso-bornyl
acrylate/50% pentaerythritol triacrylate; 83% trimethylolpropane
triacrylate/17% FLUORAD FX189; 50% acrylonitrile/50% trimethylolpropane
triacrylate; 70% trimethylolpropane triacrylate/30% acrylonitrile; 40%
FLUORORAD FX189/60% trimethylolpropane triacrylate; 70% hexane diol
diacrylate/30% iso-bornyl acrylate; 50% hexane diol diacrylate/50%
iso-bornyl acrylate; 12.5% aliphatic urethane triacrylate/87.5% hexane
diol diacrylate; 31% KENSTAT q100/69% C.sub.14 diol diacrylate; 69%
C.sub.14 diol diacrylate/31% acrylonitrile; 80% C.sub.14 diol
diacrylate/20% KENSTAT q100; 94% trimethylolpropane propoxylate
triacrylate/6% KENSTAT q100; 50% trimethylolpropane triacrylate/50%
acetonitrile; 70% trimethylolpropane triacrylate/30% acetonitrile; 88%
phenol ethoxylate monoacrylate/12% acetonitrile; 80% C.sub.14 diol
diacrylate/20% acetonitrile; 80% C.sub.14 diol diacrylate/20% KENSTAT
q100; 12% trimethylolpropane triacrylate/88% iso-bornyl acrylate; 69%
C.sub.14 diol diacrylate/23% KENSTAT q100/8% trimethylolpropane
triacrylate; 33% acetonitrile/33% polyamine acrylate/34% iso-bornyl
acrylate; 75% aliphatic amine acrylate/17% KENSTAT q100/8%
trimethylolpropane triacrylate; 80% C.sub.14 diol diacrylate/20% FLUORAD
FX189; 80% phenol ethoxylate monoacrylate/-20% pentaerythritol
triacrylate; 80% hexane diol diacrylate/20% acrylonitrile; 70% hexane diol
diacrylate/15% acrylonitrile/15% trimethylolpropane triacrylate;
propoxylated glycerine triacrylate; ethoxylated trimethylolpropane
triacrylate; caprolactone acrylate; 90% alkoxylated trifunctional
acrylate/10% beta-carboxyethyl acrylate; 90% polyethylene glycol 200
diacrylate/10% pentaerithritol di tri tetraacrylate; 75% hexane diol
diacrylate/25% KenReact LICA 44; 50% pentaerythrytol tetraacrylate/50%
hexane diol diacrylate; pentaerythrtol polyoxyethylene petraacylate;
tetrahydrofurfuryl acrylate; 25% KENSTAT q100/75% hexane diol diacrylate;
50% tetrahydrofurfuryl acrylate/50% polyethylene glycol 200 diacrylate;
88% tetrahydrofurfuryl acrylate/12% trimethylolpropane triacrylate; 88%
caprolactone acrylate/12% trimethylolpropane triacrylate; EBECRYL 170; 80%
EBECRYL 584/20% beta-carboxyethyl acrylate; 88% tetrahydrofurfuryl
acrylate/12% beta-carboxyethyl acrylate; 88% EBECRYL 170/12%
beta-carboxyethyl acrylate; aliphatic polyesther hexaacrylate oligomer;
aliphatic urethane diacrylate; tripropylene glycol diacrylate; hexane diol
diacrylate; 88% iso-bornyl acrylate/12% beta-carboxyethyl acrylate; 90%
polyethylene glycol 200 diacrylate/5% iso-bornyl acrylate/5%
pentaerythritol triacrylate; 82% polyethylene glycol 200 diacrylate/12%
hexane diol diacrylate/6% trimethylolpropane triacrylate; 75%
tetrahydrofurfuryl acrylate/15% hexane diol diacrylate/5%
trimethylolpropane triacrylate/5% oligomer, 44% polyethylene glycol 200
diacrylate/44% acrylonitrile/12% hexane diol diacrylate; 70% C.sub.14 diol
diacrylate/30% aliphatic urethane acrylate oligomer, trimethylolpropane
ethoxylate triacrylate; and 50% PHOTOMER 6173/150% hexane diol diacrylate
50%. Notes: KENSTAT q100 and KenReact LICA 44 are trade names of Kenrich
Corp.; FLUORAD FX189 is a trade name of 3M Industrial 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 materials for AC
voltage capacitor designs. These are low dissipation factor (DF) polymers
with DF <0.01, i.e., <1%, and dielectric constant (.kappa.) in the
range 2.5<.kappa.<4.0.
PP-acrylate hybrid films were produced using a production-size vacuum
metallizing chamber that was retrofitted to allow deposition of the
acrylate coatings in line with the metallization process. PP films of 6
.mu.m, 8 .mu.m, 12 .mu.m and 19 .mu.m were first treated with a gas plasma
and then coated with acrylate polymer films with thicknesses of about 0.2
to 1.0 .mu.m Dielectric characterization of small area stamp capacitors,
with PP (control) and hybrid films, showed that the PP-acrylate films had
superior 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 improved corrosion resistance
of the metallized aluminum electrodes when deposited on the acrylate
coating rather than on the PP film. This is very significant because it
allows the use of thinner aluminum layers, which increases the
self-healing properties of the hybrid film capacitors.
Several full-size capacitor designs were produced and tested. Capacitors
with ratings of 8 .mu.F/330 VAC, 0.1 .mu.F/1200 VDC, and 0.1 .mu.F/2000
VDC were built using acrylate-coated 6 .mu.m, 12 .mu.m, and 19 .mu.m PP
films, respectively. Short-term current, humidity, and breakdown voltage
tests showed that the acrylate-PP hybrid capacitors followed the
performance of the small area stamp capacitors. That is, the hybrid film
capacitors had significantly higher performance than that of the PP
control capacitors. In a critical high dV/dt test that applied 5000 V
pulses with a rise time of 1000 V/50 ns, the 0.1 .mu.F/2000 V capacitors
out-performed by far commercial capacitors that used double metallized PET
film electrodes (for higher current carrying capacity) and equal thickness
dielectric. The commercial capacitors degraded or failed after 2400
pulses, while there was no degradation in the 19 .mu.m hybrid film
capacitors. In fact, the 12 .mu.m hybrid film capacitors (0.1 .mu.F/1200
V) that were less than half the volume of the 19 .mu.m capacitors were
also superior to the commercial capacitors when tested under the same
conditions. A comparison of 12 .mu.m PP lighting ballast capacitors with
12 .mu.m acrylate-PP film capacitors using 3000 V pulses showed no
degradation in the hybrid film capacitors, while the 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 using conventional winding equipment. The
acrylate-PP film handled as well as any capacitor film through the various
process steps. Since the acrylate polymer can be deposited in-line with
the metallization, and in the 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 .mu.m, added material cost will be
minimal.
A. Hybrid Film Production Process Development
Acrylate-PP hybrid films were produced using 6 .mu.m, 8 .mu.m, 12 .mu.m,
and 19 .mu.m PP films that were readily available. Rolls of film 32 cm
wide were used that were at least a few thousand feet long (typical size
for small metallizing runs). The apparatus employed in the coating process
is shown schematically in FIG. 1. The liquid monomer was pumped from the
reservoir 40 that was located outside the vacuum chamber 12 into the flash
evaporator 28. The liquid monomer was atomized into microdroplets with the
use of the ultrasonic atomizer 42 that was positioned on top of the
evaporator 28. The evaporator 28 was held at a temperature which was above
the boiling point of the liquid, but below its decomposition point. This
caused the monomer to flash evaporate before it cured. The molecular vapor
exited at supersonic speeds and condensed on the film 20 that was in
intimate contact with the chilled rotating drum 16. The condensed thin
film deposit then moved in front of the electron beam gun 30 where it was
polymerized.
Examples of suitable acrylates useful in the practice of the invention
include iso-bornyl acrylate, hexane diol diacrylate, and tripropylene
glycol diacrylate that were formulated for fast cure, proper viscosity,
and good adhesion to the PP film. These acrylates all have a dissipation
factor of less than 0.01.
Several large rolls of 6 .mu.m and 8 .mu.m PP film were coated under
various conditions until a set of parameters that produced a well-cured,
uniform coating was obtained The major process parameters are as follows:
1. Drum Temperature: Good films may be made within the temperature range
from room temperature to -20.degree. C. The lower temperature appeared to
increase somewhat the condensation rate with some monomers, but not with
others. It is estimated that the residence time of the film on the drum
was not sufficient to transfer much of the drum temperature to the top
surface of the film. As a result, the difference in drum temperature did
not have a major affect on the monomer condensation rate.
2. Drum Speed: Good films were produced with drum surface speeds anywhere
in the range of 50 to 1000 feet/min (25 to 500 cm/sec).
3. Radiation Dose: The accelerating potential was 12 KeV and the current
delivered to the monomer was about 2 mA for curing monomer films 1 .mu.m
thick.
4. Final Monomer Mixture: Most of the monomer mixtures that were processed
produced good quality coatings when considering parameters such as
uniformity, degree 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% tripropylene glycol diacrylate.
5. Plasma Treatment Prior to Acrylate Deposition: The PP film was
plasma-treated prior 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 the acrylate monomer. Oxygen is a free radical
scavenger that neutralizes free radicals created by the electron beam,
thus 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 the wetting of the 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 the static charge on the coated
film and also complete the polymerization process on the acrylate surface.
This process step reduced the "stickiness" on the acrylate-PP wound roll.
B. Evaluation of the Hybrid Films Using Small Area Films
The hybrid films were first evaluated using small area stamp capacitors,
and then full-size-wound capacitors were fabricated for further
evaluation. Stamp capacitors were used to evaluate breakdown strength,
current carrying capacity, partial discharge (corona) degradation of the
polymer films, and corrosion resistance of the metallized electrodes.
1. Dielectric Constant and Dissipation Factor
The acrylate coating, due to its low thickness, had minor influence on both
the and DF of the hybrid films. Depending on the particular hybrid, the
dielectric constant of the acrylate-PP films was up to 7% higher than that
of plain PP film.
2. DC Breakdown Strength
This measurement was made to ensure that the hybrid film had a breakdown
strength at least as high as that of the original PP film, plus an
additional amount due to the acrylate coating. The breakdown measurements
on the films were done using a dry double-metallized and non-contact
measurement technique that was performed at high vacuum (<10.sup.-4
Torr), to eliminate partial discharges and surface flashovers. The
breakdown system was built in a turbomolecularly pumped stainless-steel
chamber. Metallization masks were designed and fabricated that allowed
metallization of small area (about 1 square inch) stamp capacitors for the
breakdown measurements. The electrode contact was made to metallized pads
that were outside the active area to prevent film damage. The voltage was
ramped at about 500 V/sec. For every breakdown measurement that is
reported below, at least 18 stamp capacitors were tested to ensure that
the data is statistically significant.
The results of the DC breakdown tests of 6 .mu.m and 8 .mu.m films coated
with about 0.5 .mu.m of acrylate polymer are shown in Table I. The
acrylate polymer is produced by electron beam curing of a 70% hexanediol
diacrylate, 20% iso-bornyl acrylate, and 10% tripropylene glycol
diacrylate monomer film deposited in the vacuum using the experimental
apparatus described in FIG. 1. The PP film was first plasma-treated using
an argon gas plasma and the acrylate monomer was flash evaporated on the
treated surface.
TABLE I
THICKNESS BREAKDOWN
FILM (.mu.m) 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 2.5 6.26
The breakdown voltage of the control and acrylate hybrid films was measured
by metallizing one square inch electrodes on opposing sides of the films.
Each measurement represents an average of at least eighteen individual
breakdown measurements. The data in Table I show that 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 .mu.m, the
breakdown strength of the acrylate-PP films is equal to or higher than PP
films. Similar measurements on single acrylate layers have shown that the
breakdown strength of 1 .mu.m thick films is 20 to 24 KV/mil, which is
equal to or higher than that of PP film.
3. Current-Carrying Capacity
It is well-known that due to the low melting point of?P film, metallized PP
capacitors become quite unreliable at high current applications due to
thermal damage of the termination The current generates I.sup.2 R losses
(R=Equivalent Series Resistance, ESR) at the termination, which raises the
temperature, which in turn damages the termination 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 that has a higher
melting point than PP is used to carry the current. These PET/PP designs
are inefficient and result in expensive capacitor products.
To simulate the current flow from the sprayed termination to the metallized
film, and the ability of the film to carry high currents without thermal
damage, a simple test was developed to measure the maximum power that a
metallized film can dissipate prior to a thermal failure. Current was
forced through a section of metallic film and the power (I.times.V) was
increased until the dissipated heat forced the film to fail. The fixture
had 1/2 inch wide contact electrodes which were placed 5 inches apart. AC
voltage was applied to the electrodes and the current through the circuit
was recorded The voltage was raised until the power loss thermally
degraded the metallized film to the point of failure. In the films that
were tested, the failure was an open circuit caused by melting of the PP
film at some point close to the middle of the 5 inch strip. Table II shows
the average power to failure (eight samples were tested for each
condition), for both coated and uncoated 8 .mu.m thick polypropylene. The
acrylate polymer was produced by electron beam curing of a 70% hexanediol
diacrylate, 20% iso-bornyl acrylate, and 10% tripropylene glycol
diacrylate monomer film deposited in the vacuum using the experimental
apparatus described in FIG. 1. The PP film was first plasma-treated using
an argon gas plasma and the acrylate monomer was flash-evaporated on the
treated surface.
TABLE II
FILM POWER IMPROVEMENT OVER
TYPE (Watts) CONTROL PP (%)
Control PP 13.5 --
PP/plasma/0.2 .mu.m 14.0 3.7
acrylate
PP/plasma/0.4 .mu.m 14.8 9.6
acrylate
PP/plasma/0.6 .mu.m 15.6 15.6
acrylate
PP/plasma/0.8 .mu.m 19.4 43.7
acrylate
PP/plasma/1.0 .mu.m 22.0 63.0
acrylate
The results show that the coated films have a higher thermal capacity that
varies from 4% to 63%. The variation is due mostly to the thickness of the
acrylate coating.
It is interesting to note that although the coated films failed at
significantly higher power levels, they did not deform and shrink as much
as the plain films. The higher thermal capacity of the hybrid films was a
key objective of this work and it is clear that thin coatings of the high
temperature acrylate films can have a significant impact in the thermal
properties of PP.
4. Resistance to Degradation from Partial Discharges (Corona)
One of the common mechanisms of failure in high voltage (V>300 VDC) film
capacitors is damage to the polymer dielectric from partial discharge
activity (corona) in the capacitor windings. The partial discharges or
corona pulses are generated in inter-electrode areas that have large
enough air gaps to sustain a certain level of ionization. The high
temperature corona pulses, although they physically move around, can
degrade the polymer dielectric 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 the corona-induced
clearings will result in some capacitance loss with no further damage to
the capacitor. Capacitors with electrodes of 2 to 3 ohm/sq have poorer
clearing properties. The resulting high levels of corona can lead to major
capacitance loss, increased dissipation factor, higher leakage cue and
often catastrophic failures.
Resistance to the damaging thermal effects of the corona pulses increases
as the thermal stability of the polymer film increases. Control
polyvinylidene difluoride (PVDF) and polypropylene (PP) films were tested
along with PVDF/plasma/acrylate and PP/plasma/acrylate hybrid films for
resistance to corona degradation. The acrylate polymer was produced by
electron beam curing of a 70% hexanediol diacrylate, 20% iso-bornyl
acrylate, and 10% tripropylene glycol diacrylate monomer film deposited in
the vacuum using the experimental apparatus described in FIG. 1. The PP
film was first plasma-treated using an argon gas plasma and the acrylate
monomer was flash-evaporated on the treated surface.
The acrylate-PVDF hybrid was included because PVDF film is the highest
energy density capacitor dielectric that is commercially available, and an
acrylate-PVDF hybrid may present some unique product opportunities.
Furthermore, the PVDF film has about the same melting point 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 accelerated
corona test, where the level of partial discharges reflected either poor
impregnation (impregnated cap), or loose outer turns in a dry capacitor.
To date, several stamp capacitors have been tested, and the results are
shown in Table III.
TABLE III
CAPACITOR TIME TO BREAK-
NUMBER FILM TYPE DOWN (min.)
1 Control PVDF, 12 .mu.m 3.0
2 Control PVDF, 12 .mu.m 3.8
3 Control PVDF, 12 .mu.m 5.1
4 Control PVDF, 12 .mu.m 3.8
5 PVDF/plasma/1 .mu.m 47.8
acrylate)
6 PVDF/plasma/1 .mu.m 29.0
acrylate)
7 PVDF/plasma/1 .mu.m 32.0
acrylate)
8 PVDF/plasma/1 .mu.m 33.8
acrylate)
9 Control PP, 8 .mu.m 19.0
10 PP/plasma/1 .mu.m acrylate 133.0
As seen in Table III, the hybrid films had about one order of magnitude
longer time to failure. This can raise the reliability level of the
capacitors significantly. In the case of a pulse-type capacitor, it may
represent a large number of additional pulses prior to failure from corona
degradation.
5. Corrosion Resistance
Capacitance loss due to electrode corrosion is the most common failure
mechanism in metallized capacitors. The corrosion resistance of aluminum
electrodes deposit 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 electrical resistance of a 5
inch by 1 inch metallized strip, after exposure in a temperature/humidity
environment of 70 C/85% RH The 5 inch strips were cut out of large bobbins
of material 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 corro
