Metal oxide films such as SiO2 are known to provide an effective barrier to the transport of moisture as well as gaseous species through polymeric films. Such thin film coatings have a tendency to crack upon flexure of the polymeric substrate. Sputter co-deposition of SiO2 with 4 to 15 percent fluoropolymers was demonstrated to produce thin films with glass-like barrier properties that have significant increases in strain to failure over pure glass films which improves their tolerance to flexure on polymeric substrates. Deposition techniques capable of producing these films on polymeric substrates are suitable for durable food packaging and oxidation/corrosion protection applications

Banks, Bruce A.

Mirtich, Michael J.

Nahra, Henry

Rutledge, Sharon K.

Sovey, James S.

English

NASA Technical Reports Server

N91"24062
FLEXIBLE FLUOROPOLYMER FILLED PROTECTIVE COATINGS
Bruce A. Banks, Michael J. Mirtich, James S. Sovey,
Hem-y Nahra, and Sharon K. Rutledge
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
Metal oxide films such as SiO2 are known to provide an effective barrier to the transport of moisture
as well as gaseous species through polymeric films. Such thin film coatings have a tendency to crack upon
flexure of the polymeric substrate. Sputter co-deposition of SiOz with 4%-15% fluoropolymers has been
demonstrated to produce thin films with glass-like barrier properties that have significant increases in strain-
to-failure over pure glass films, thus improving their tolerance to flexure on polymeric substrates. Deposition
techniques capable of producing these films on polymeric substrates are suitable for durable food packaging
and oxidation/corrosion protection applications.
_ntroduction
Polymeric materials used on spacecraft in the low-earth-orbital environment have been shown to
degrade through oxidation because of reaction with the environmental atomic oxygen (ref. 1). Atomic
oxygen in the low-earth-orbital environment is formed by photodissociation of the diatomic oxygen by solar
photons whose wave length is less than 2430A. Atomic oxygen is a predominant species between the
altitudes of 180 and 650 km (ref. 2). Polymers have not yet been developed which are durable to atomic
oxygen interaction. As a result, current approaches to prevent atomic oxygen degradation have utilized
protective coatings over the oxidizable substrates. These coatings have consisted of metal oxides such as SiO2
and AIzO3, as well as mixed metal oxide/fluoropolymer films (refs. 1,3and 4). Because metal oxides are
typically in their highest oxidation state, they act as ideal atomic oxygen protective barriers to prevent
oxidation of the underlying polymeric materials. However, pure metal oxide films such as SiO z have a very
limited strain-to-failure, thus decreasing their utility on substrates which are subjected to flexure. The
addition of a small fluoropolymer content to the predominantly SiO2 films greatly increases the strain-to-
failure of the films while still maintaining their protective properties. Numerous aerospace spinoff
applications exist such as: hermetic coatings, protective transparent coatings, and non-stick coatings, this
paper will present deposition techniques for the application of flexible thin film fluoropolymer filled SiO2
protective coatings, as well as discuss film properties and potential applications.
Deposition Technique
Molecularly mixed metal oxide/fluoropolymer films can be produced by simultaneous sputter etching
of sputter targets consisting of silicon dioxide and polytetrafluoroethylene (PTFE-Teflon) (ReL 3). Figure 1
shows the ion beam sputter deposition configuration for co-deposition of SiO2 or AI203 with a fluoropolymer.
this apparatus consists of an electron bombardment ion source operating on 500-1000eV argon ions to
sputter-etch targets of metal oxide simultaneous with the polytetrafluoroethylene (PTFEoTeflon). As a result
of the simultaneous bombardment of both types of materials, a molecular mixture of metal oxide and
fluoropolymer scission fragment species deposits on surfaces exposed to the sputter ejecta from these targets.
Because the ion beam current density distribution is typically normally distributed, the ratio of deposition
species can be readily controlled by utilizing a pie-shaped polytetrafluorocthylene target segment on top of a
circular SiO2 sputter target (tel 5). Figure 2 shows a typical plot of the volume fraction of fluoropolymer in
the deposited SiO2/fluoropolymer-mixed film as a function of the polytetrafluoroethylene target angle. The
deposition can be performed by either ion beam or RF magnetron deposition with the substrate at room
temperature. The deposition rates produced by ion beam sputter-deposited films are typically lower than
those which may be produced by RF magnetron deposition processes.
179
https://ntrs.nasa.gov/search.jsp?R=19910014749 2020-03-19T17:44:27+00:00Z
Thin Film Properties
Although silicon dioxide and aluminum oxide are known to be durable to atomic oxygen attack, pure
fluoropolymer deposited films are oxidizable by atomic oxygen attack. However, small quantities of
fluoropolymers can be molecularly mixed with metal oxide films and still provide atomic oxygen protection if
the thin film thickness is slightly increased over that of the pure metal oxide film. Figure 3 shows the
minimum thickness fluoropolymer-filled SiO 2 film necessary for permanent atomic oxygen protection as a
function of fiuoropolymer content fraction in the SiO 2 film (reL 3). Thinner protective coatings do not
provide durable protection thus allowing atomic oxygen reaction with the underlying polymer. For atomic
oxygen protective coatings in space, the film thicknesses typically between 650-1500_are used.
Fluoropolymer filled SiO 2 protective coatings appear colorless and quite transparent on polymeric
substrates. Figure 4 illustrates the differences in reflectance, absorptance, and transmittance of SiO, films
containing up to 49_ fluoropolymer sputter deposited on 127#m thick polyimide Kapton.
The value of adding the small addition of fluoropolymer to the metal oxide films is to increase the
strain to which the silicon dioxide can be subjected witfiout-brittle fractures developing. Figure 5 illustrates
the increase of strain-to-failure by small additions of fluoropoT-ymer to the film as well as the minimum
radius curvature which film can survive when deposited on a 127#m thick polyimide Kapton substrate without
brittle fracture. As can be seen from Figure 5, a_luoropolymer fill fraction of 15% willinerease the strain-
to-failure by a factor of three. In addition, thin polymeric materials with a 15% fluoropolymer fillfraction
can be bent around a radius of curvature of approximately lmm without brittle fracture. However as can be
seen from Figure 3, such films must be at least 600/kto provide atomic oxygen oxidation protection. Oxygen
diffusion through thin films appears to be limited to not more than 300/i. based on isotope labeled diffusion
studies conducted in reference 6. However, it appears that 15% fluoropolymer-filled $iO 2 films whose
thicknesses exceed 600/k are quite _and atomic oxygen durable. Sputter deposition of these co-
deposited films has been shown to produce an intrinsic stress which can tend to deform the substrate
polymers. Figure 7 is a plot of the intrinsic stress of co-deposited thin films on silicon suhstrates for various
composition films and film thicknesses (reL 7).
Atomic oxygen durability of pure SiO 2 and 4% fluoropolymer-filled SiO 2 films were evaluated in
space and found to reduce oxidation by a factor of 3,7500over that of unprotected polyimide Kapton (ref. 7).
Potential Application
One of the primary space applications considered for such films is for the protection of flexible
polyimide Kapton solar array blankets that would be used in low earth orbit, the fluoropolymer content of
such films may enable inflatable structures to be protected from oxidation as well. Polymeric materials
which must be subjected to flexure or small radius of curvature would be ideally suitable for use with
fluoropolymer filled films because of their tolerance to high strain in comparison to that of pure metal oxide
films.
Because the fluoropolymer-filled SiO 2 films are clear, flexible, corrosion-resistant, chemically
inactive, and an effective moisture harrier, there may be numerous non-aer0space applications. One Of the
most obvious applications is the packaging of food products where it is desirable to prevent diffusion of water
vapor through the thin film polymeric container. Generally acceptable long-term hermetic protection is
achieved by metallization of plastic film packages. However, such-c_ffta_ners do not allow visual inspection
of the content of the containers. The flexible fluoropolymer-filled SiO_ films would be both visibly
transparent and offer a hermetic protection in excess of that of uncoated polymeric packaging. The
fluoropolymer content of the film may be useful in cases where a slight reduction in coefficient of friction is
also important. There may be obvious non-stick applications where the durability of the SiO 2 component can
be utilized in concert with the non'stick fiuoi0poiymer component for such applications as cookware.
180
The chemical properties of the fluoropolymer ion beam sputter-deposited component of the films is
thought to be similar to that of polytetrafluoroethylene based on the deposition of pure fluoropolymer films
from polytetrafluoroethylene targets (ref. 8). The sputter-deposited fluoropolymer films are very similar to
that of the bulk polytetrafluoroethylene except the sputter-deposited fluoropolymer is slightly more cross-
lined, which produces a higher thermal decomposition temperature (tel 8). Applications where abrasion-
resistance, oxidation protectiveness, and transparency are requirements include durable acrylic coatings for
surface protection of floor coverings, sky light transparency protective coatings, and protection of
polycarbonate face sh/elds. These oxidation protective coatings, their deposition process, and associated
apparatus have been patented by NASA under U.S. Patent Numbers 4,664,980,4, 560,577and 4,604,181
respectively.
SUMMARY
Molecularly mixed fluoropolymer-silicon dioxide films cannot be fabricated by conventional bulk
melting or mixing processes. However, these mixed films can be deposited by ion sputter co-deposition, as
well as magnetron sputter deposition processes. Fluoropolymer contents up to 15_ by volume enable a
factor of three (3) gain in strain-to-failure of the silicon dioxide films, thus allowing a much greater flexibility
in the films to be applied on thin polymeric substrates. The co-deposited films are clear, atomic oxygen
durable, and pose potential for hermetic coatings for applications ranging from food stuff packaging to face
shields. The fluoropolymer content in these coatings may be advantageous to non-stick applications as well.
REFERENCES
. Banks, B. A., et ah Sputtered Coatings for Protection of Spacecraft Polymers. NASA TM-83706,
1984.
. Banks, B. A., et ah Atomic Oxygen Undercutting of Defects on SiO2 Protected Polyimide Solar
Array Blankets. Presented at the Materials Degradation in Low Earth Orbit Symposium of the
119th TMS Annual Meeting and Exhibit, Anaheim, CA, 1990.
. Banks, B. A., et al: Protection of Solar Array Blankets from Attack by Low Earth Orbital Atomic
Oxygen. Presented at the Eighteenth IEEE Photovoltaic Specialists Conference, Las Vegas, NV,
1985.
. Banks, B. A., et ah Performance and Durability of Space Solar Dynamic Power System Optical
Surfaces. Presented at the ASME International Solar Energy Conference, Miami, FL, 1990.
, Nahra, H.: Protective Coatings for Polymers Exposed to Atomic Oxygen in the Low Earth Orbits.
M.S. Thesis, Cleveland State University, Cleveland, OH, 1983.
6, Gulino, D. A., et al: Isotopic Study of Oxygen Diffusion in Silicon Dioxide Thin Films. Published in
the Journal of Thin Solid Films, 188 (1990) 237-246.
7, Banks, B. A., et al: Ion Beam Sputter-Deposited Thin Film Coatings for Protection of Spacecraft
Polymers in Low Earth Orbit. NASA TM-87051, 1985.
, Banks, B. A., et al: Ion Beam Sputter Etching and Deposition of Fluoropolymers. NASA TM-78888,
1978.
181
ION
SOURCE
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Kapton ® SUBSTRATE IN
SPUTTERCLEANING POSITIONJ
SiO2 OR I
AI203 TARGET_ _
BEAM / _\ xO/ L OPTIONALFLUOROPOLYMER TARGET
FORCO-DEPOSITION
/ _ Kapton ®
I
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/ IN DEPOSITION/
i POSITION
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Figure 1. Ion beam sputter, deposition syste.m for molecularly
mixed metal oxide fluoropolymer thin films.
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Figure 2. Fluoropolymer volume fraction of fluoropolymer filled
SiO2 sputter deposited films.
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200
182
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Figure 3. Minimum film thickness for atomic oxygen protection
as a function of fluoropolymer content in fluoropolymer film
Si02 sputter deposited films.
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WAVELENGTH,_, pm
Figure 4. Optical properties of 2 96% SiO: -< 4% PTFE coated Kapton
samples unexposed and exposed to low-earth-orbital environment
compared with uncoated arzd unexposed Kapton,
183
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co-deposited Si02-fluoropolymer film (IO0OA) can survive without
brittle failure.
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Figure 6. Intrinsic stress of co-deposited thin films on silicon substrates.
184


Flexible fluoropolymer filled protective coatings

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