Plasma-polymerized vinyl trimethoxy silane films were deposited on transparent polycarbonate substrates. The adherent, clear films protected the substrates from abrasion and also served as antireflection coatings. Post-treatment of the vinyl trimethoxy silane films in an oxygen glow discharge further improved their abrasion resistance. The coatings were characterized by elemental analysis of the bulk, ESCA analysis of the surface, transmission, thickness, abrasion resistance, haze, and adhesion. This patented process is currently used by the world's largest manufacturers of non-prescription sunglasses to protect the plastic glasses from scratching and thereby to increase their useful lifetime

Wydeven, Theodore

English

NASA Technical Reports Server

N91-23043
PLASMA-POLYMERIZED COATING FOR POLYCARBONATE: SINGLE-LAYER,
ABRASION RESISTANT, AND ANTIREFLECTION
Wydeven
NASA Ames Research Center
MoffeU Field, California 94035
ABSTRACT
Plasma-polymerized vinyltrimethoxy silane films were deposited on mmsparent polycarbonate
subslrates. The adherent, clear films protected the substrates from abrasion and also served as antireflection
coatings. Post-treatment of the vinyltrimethoxy silane films in an oxygen glow discharge further improved
their abrasion resistance. The coatings were characterized by elemental analysis of the bulk, ESCA analysis
of the surface, transmission, thickness, abrasion resistance, haze, and adhesion. This patented process is
currently used by the world's largest manufacturer of non-prescription sunglasses to protect the plastic
glasses from scratching and thereby increase their useful lifetime.
INTRODUCTION
Traditionally,ground and polishedtemperedglasswas thematerialofchoiceformanufacturing
sunglass lenses. However, in recent years the many virtues of polarized and other plastic lenses have come
into sharp focus. The advantages of plastic lenses include excellent optics, lightweight and therefore more
comfortable than glass, easy to shape, reasonable to make, vastly better absorption of harmful ultraviolet
radiation, and resistance to shattering. The major disadvantage of most plastic lenses is susceptibility to
scratching. Uncoated plastic lenses develop a haze after a relatively short number of wearings which greatly
reduces visibility.
There are many other commercial applications for transparent plastics. Two large industries,
automobile and cons0"uction, have numerous needs for Iransparent plastics. Automobile lenses and
dashboard covers and glazing applications in construction are just some of the needs of these industries for
transparent plastics. As in the optics industry, resistance of the plastic to scratching and abrasion is
desirable for most automobile and conslruction applications.
Space helmet visors are also made of plastic and it is this application that led NASA to develop a
transparent scratch resistant coating for plastics. Polycarbonate, an impact resistant and transparent plastic
is generally used for space helmets. However, polycarbonate also suffers from the disadvantage of most
plastics for optics applications, i.e., it is highly susceptible to scratching. NASA therefore embarked on a
program to develop a scratch resistant coating for polycarbonate space helmet visors. The coating process
chosen for this development effort was plasma polymerization. This process was chosen for several
reasons;
a) deposition occurs near ambient temperature, making the process useful for coating temperature
sensitive plastics.
b) plasma polymerized coatings are conformal and uniform in thickness.
c) practically any organic vapor can be used as starting material for depositing coatings by plasma
polymerization; this feature allows one to deposit coatings covering a broad range of
composition and properties.
d) it can easily be scaled for coating large samples.
e) multiple treatments and processes can becarriedoutin the same plasma reactor.
The starting material chosen for preparing a scratch resistant coating was based primarily on its
chemical composition and structure. The composition was chosen so that the deposited ftlm would mimic
in some respects the composition of glass (a material rich in silicon and oxygen) which is known to be
much more scratch resistant than plastic. The structure was chosen so that reasonable deposition rates
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would be achieved thus minimizing the time required to deposit a coating of sufficient thickness to protect
the underlying plastic from being scratched. Enhanced deposition rates can generally be achieved by plasma
polymerization if the starting material contains a vinyl group or carbon-carbon double bond. With these
requirements in mind the starting material chosen was vinyltrimethoxy silane [(H3CO)3-Si-CHCH2]. Since
coatings are prepared from gases or vapors by plasma polymerization, the vapor pressure of the starting
material was also considered in the selection process. Vinyltrimethoxy silane is a liquid with sufficient
vapor pressure at room temperature to be a useful compound for plasma polymerization.
This paper describes the preparation and properties of a transparent scratch resistant and
antireflection coating for polycarbonate. A two-step process was used for preparing the coating. The first
step involved deposition of a plasma polymerized coating of vinyltrimethoxy silane on the polycarbonate
surface and the second step involved hardening of the coating with an oxygen plasma. The second step of
the process has been granted US Patent 4,137,365.
EXPERIMENTAL
Deposition Chamber and Materials
The deposition chamber shown in Figure 1 was used to deposit and harden the transparent scratch
resistant coating. Copper sheet electrodes were wrapped around the outside of the reactor and were attached
to a 13.56 MHz radiofrequency power supply through a manually controlled impedance matching network.
The transparent plastic substrates used for the coating experiments were cut from 0.64 cm thick
polycarbonate sheet stock manufactured by Rohm & Haas and sold under the tradename Zelux-C. The
square substrates had an exposed surface area of 6.45 cm 2 during deposition and were held in a plastic frame
supported on a glass plate located inside the reactor as shown in the figure. The frame ensured uniformity
in thickness of the deposited coatings from the center to the edge of the samples. The substrates were
cleaned before being coated by degreasing them in vapors of trichlorolrifluoroethane. The plasma
polymerized coatings were prepared from vinyltrimethoxy silane vapor that was delivered to the deposition
chamber from a degassed thermostatted reservoir of the liquid. The coatings were further hardened in the
glow of an oxygen plasma. The oxygen for these experiments was obtained from a cylinder of oxygen
having a reported purity of 99.5%.
+
NLET
TO VACUUM
Figure 1. Chamber for depositing and hardening coatings using a low temperature plasma.
Deposition and Film Hardening Procedure
The reactor, with substrate in position, was initially evacuated to a background pressure of 0.7
N/m 2 (0.005 torr) or less before initiating flow of vinyltrimethoxy silane vapor. The flow rate of the
monomer was adjusted to 3.2x10"gm 3 (STP)/sec (0.19 cm3(STP)/min), which yielded a pressure of 2.8
N/m 2 (0.021 ton), before striking a discharge. A pulsed plasma mode, instead of a continuous wave (CW)
mode, was used for film deposition. The pulsed mode reduces substrate heating during deposition. The
peak-to-peak voltage during the on-time of the pulse was 400 V and the peak-to-peak current to ground was
0.6 A. The pulse time was 1.5 msec on and 1.5 msec off. At the end of a pre-determined deposition time,
185
monomer flow to the deposi6on chamber was shutoff and residual monomer was pumped from the chamber.
The hardeningofthedepositedcoalingwas thenbegunwithoutremovingthecoatedsubstratefromthe
reactor. The following conditions were used for hardening the siloxane coating with an oxygen plasma:
Mode of plasma operation: CW
Peak-to-peak voltage: 390 V
Peak-to-peak current: 0.58 A
Oxygen flow rate: 2.9x10 "s m3(STP)/sec (1.7 cm3(STP)/min)
Initial oxygen pressure (discharge off): 13 N/m2 (0.098 tort)
Post-treatment duration: 600 sec
Following the hardening step, subsumes were removed from the reactor for evaluation of the coatings.
AnalyticalProceduresforEvaluatingtheCoatings
The test equipment and procedmes followed for determining the elemental analysis, x-ray
photoelectron spectra (ESCA), transmission, abrasion resistance, haze, adhesion, and thickness of the
siloxane coatings on polycarbonate can be found elsewhere (1,2).
RESULTS AND DISCUSSION
Samples for elemental analysis of plasma polymerized vinyltrimethoxy silane films were obtained
by scraping the coating from a glass plate which was coated with the polymer. The average weight percent
of carbon, hydrogen and silicon dioxide in plasma polymerized vinyltrimethoxy silane films (without
oxygen post-treatment) derived from elemental analysis by a combustion method of three samples was:
Element Weight Percent
C 35.84
H 6.64
SiO2 48.56
Si 22.69
Since this analysis was done by a combustion method, the amount of oxygen in the samples could not be
measured. The amount of silicon in the samples was calculated from the amount of silicon dioxide. The
stoichiomelric composition of vinyltrimethoxy silane is 40.52, 8.16, and 18.95 weight percent carbon,
hydrogen, and silicon, respectively. In comparing the elemental analysis of the bulk plasma polymer with
the starting monomer composition it is apparent that plasma polymerized vinyltrimethoxy silane films are
deficient in carbon and hydrogen and rich in silicon relative to the starting monomer A deficiency in
carbon and hydrogen could have arisen if a carbon/hydrogen entity, such as a methyl group, was stripped
from the starting monomer during plasma deposition, leaving a film with an enhanced amount of silicon
but deficient in carbon and hydrogen.
X-ray photoelectron spectral analysis was chosen to determine the elemental composition of the
surfaces of untreated and oxygen plasma post-treated vinyllrimethoxy silane plasma polymerized films. The
remits derived from these analyses were:
Element Untreated film, 0 2 plasma treated,
atom % atom %
C 21.7 13.7
0 12.1 24.2
Si 13.3 9.21
Since ESCA cannot be used to analyze for hydrogen, it was assumed that the hydrogen concentration on
the surface was the same (52.87 atom %) as in the bulk plasma polymer before calculating surface
composition from the ESCA data. The atom % of the elements carbon, oxygen, and silicon in
stoichiometric vinyltrimethoxy silane monomer are 23.81, 14.28, and 4.76, respectively. It is clearly
186
evident from the ESCA results that oxygen post-treatment of the plasma polymerized vinyltrimethoxy
silane coating enriched the surface in oxygen at the expense of carbon, possibly by volatilizing the carbon
as carbon monoxide or carbon dioxide. The oxygen plasma treated surface is also greatly enriched in oxygen
and silicon and deficient in carbon relative to the starting monomer.
The intent in treating plasma polymerized vinyltrimethoxy silane coatings with an oxygen plasma
was to change their composition into more of a glass-like material, rich in silicon and oxygen, while
maintaining I]_msparency and improving resistance to scratching and abrasion. It is clear from analysis of
the treated surface that the composition has been transformed by the oxygen plasma into a more oxygen
rich material having less surface carbon.
An image viewed through a lense covered with an antireflection coating appears sharper than the
image viewed through an uncoated lense. Hardened plasma polymerized vinyltrimethoxy silane was found
to be an antireflection coating for polycarbonate. Figure 2 shows transmission spectra for an uncoated
polycarbonate substrate and substrates coated with oxygen post-treated plasma polymerized vinyltrimethoxy
silane films. It is apparent from these spectra that the coated samples of polycarbonate transmit more light
in the visible region than the uncoated sample. For example, at 5500 A, the midpoint of the visible region
of the spectrum, the transmission of the coated samples is greater by about 5%. In the eases of the coated
samples, two different coating thicknesses were deposited, 939 and 2818 A. These physical thicknesses
correspond to optical thicknesses of 1375 and 4125 A, respectively. The optical thickness, nd, where n is
the refractive index and d is the physical thickness of the coati.ng, was calculated using 1.464 for the
refractive index of the coating. The refractive index at 5893 A (the Na o line) was measured with an Abbe
refractometer.
100
_ 001"-
u
Z
,<
Z
<
,," 80
I--
4OO
; t I I l
o
s
500 600 700
WAVELENGTH. nm
Figure 2. Transmission spectra of uncoated and coated (both sides) polycarbonate. D uncoated, - - -
coated with 939 A of plasma polymerized vinyltrimethoxy silane, and - - - coated with 2818 A of
vinyltrimethoxy silane.
The abrasion resistance of samples of uncoated and coated polycarbonate were determined by
measuring haze before and after abrasion with an eraser. The force applied to the eraser when abrading the
samples using the apparatus and procedure described previously was 1.4x10 j kg/m z (1). The following
table shows haze values for the various samples studied:
Sample description % Haze Notes
Uncoated PC
*Coated PC
*Coated & 0 2 treated PC
Uncoated & unabraded PC
*Coated & unabraded PC
5.2+0.8 Ave. for 10 samples
4.1+0.5 Ave. for 3 samples
2.5+0.2 Ave for 6 samples
2.3+0.1 Ave. for 10 samples
2.2_+0.1 Ave. for 3 samples
PC = polycarbonate
*Coating thickness = 2818A
187
From this data it is evident that the least amount of damage from abrL_on occurred in those coated samples
which were post-treated in an oxygen plasma. The haze for oxygen post-treated samples was only 0.2%
greater than uncoated and unabraded polycaflxmate. It is also evident from this data that coated and
tmabraded samples had the same haze as uncoated and unal0raded polycarbonalc within experimental error.
This f'mding shows that neither the plasma polymerized silane coating nor oxygen post-treatment increased
the haze ofpolycarbonate.Anotherimportantconclusionthatcan be derivedfrom the haze data is that
oxygen post-lreatment improved the abrasion resistance as evidenced by a reduction in haze of 1.6% when
compared with coated polycarhonate without oxygen treatmenL It should also he noted that untreated
plasma polymerizedvinyilrimethoxy silane coatings also increased the abrasionresistanceof polycarbonate
as indicated by a reduction in haze of 1.1%. The improved abrasion resistance that the coatings developed
here provide for a pol_ surface may be due to a greater hardness of the coating compared with
uncoated polycarhonate, as well as less friction between the coating and the abrading material, in this case
the eraser.
Adhesion of the plasma coating to polycaxbonate was measured using the cellophane tape test as
described in Military Specification MIL-C-675A. The coatings consistently passed this test without any
special treatment of the polycarbonate surface other than the cleaning Woc_ure deacrihed in the
experimental section of this paper.
CONCLUSIONS
A two step low temperatme plasma ixocess has been developed for depositing a transparent and
antireflection scratch resistant coating on polycarbonate. The first step of the process involves the
delX_ition of a plasma polymerized coaling of a siloxane on a clean polycarbona_ surface. The second step
is a hardening step and involves the treatment of the siloxane coating with an oxygen plasma. Both steps
are carried out in the same reaction chamber.
REFERENCES
1. Wydeven, T.: Plasma Polymerized Coating for Polycarbonate: Single layer, Abrasion Resistant, and
Antireflection. Applied Optics vol 16, no. 3, 1977, pp. 717-721. See also Applied Optics vol 16, no. 5,
1977, p. 1138.
2. Wydeven, T.; and Hollahan, J.: United States Patent 4,137,365: Oxygen Post-treatment of Plastic
Surfaces Coated with Plasma Polymerized Silicon Containing Monomers, Jan. 3, 1979.
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Plasma-polymerized coating for polycarbonate: Single-layer, abrasion resistant, and antireflection

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