Note: Descriptions are shown in the official language in which they were submitted.
2 ~
Go311/7033
WRM:PCL:sjd:1500O
PROCESS FOR ENHANCING ADHESION
BETr~EEN A METAL AND A POLYMERIC SUBS~RATE
,
Field of the Invention
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The present invention pertains to an improved
process for enhancing adhesion between a metal and a
polymeric substrate.
Description of the Prior Art
Plastics have replaced more traditional materials,
such as glass, in many applications for a variety of
reasons including better strength, lighter weight, and
lower cost. One such application is in metallization
processes, wherein plastics are preferred to glass as a
substrate material. Metalized plastics are commonly
used as reflectors, primarily in automotive headlamps~
A variety of polymeric substrates, including
polycarbonate and nylon, are commonly used in the
production of lighting reflectors. Although several
metals have been applied to these plastics, aluminum is
.~ currently used in all automotive headlamp reflectors.
Aluminum is commonly applied to a plasma-treated
polymer substrate in a vacuum evaporation process. In
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the firs~ step of such a process, the plastic substrates
are pu~ in-to a vacuum chamber and exposed to a glow
discharge, having an energy sufficient to yield a plasma
at the substrate surface. This discharge oxidizes
material on the substrate surface and promotes adhesion
through a cornbination of eliminating dirt and other
small molecules from the surface~ cross-linkirlg the
surface, and providing oxygen-containing functional
groups with which the subsequently applied aluminum will
react. Typically the pressure is then lowered, and the
aluminum is evaporated from heated tungsten filaments
onto the surface of the substrate to a thickness of
approximately 500-1000 angstrorns.
Freshly prepared aluminum surfaces have a
reflectivity of about 92~ over the visible wavelength
range. Utilizing a metal having a greater reflectivity
would increase the efficiency of a lighting system. For
example, silver, one of the best-known reflectors of
visible light, which has a reflectivity approaching 98~,
could theoretically increase the efficiency of a
lighting system by approximately 6% without any changes
to the lamp itself.
Additionally, silver is less reflective than
aluminum in the ultraviolet. For example, at 320
nanometers silver reflects less than 10~ while aluminum
reflects 92~ This lack of reflectivity in the
ultraviolet may be potentially useful in preventing
degradation of polymeric lenses by ultraviolet radiation.
Silver, however, has several drawbacks, relative to
aluminum, for use in lighting systems. These include
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higher cost, and a lack of environmental resistance of
the silver surface. In addition, if silver is
substituted ~or aluminum in the existing process of
vacuum evaporation, the adhesion of the silver to the
S surface of the substrate is not adequate. In many cases
the initial adhesion is insufficient, and after
exposure, as in several specific environmental tests,
adhesion decreases.
In automotive applications these tests are of three
types. The first test is immersion in water at an
elevated temperature, for example, 96 hours in deionized
water at 90 F; the second test is exposure to a salt
spray at elevated temperature, for example, 48 hours in
5% NaCl at 100F; the third test is exposure to humid
air, for example, 120 hours in 90% relative humidity at
100F.
It is therefore an object of this invention to
provide a process for enhancing adhesion between metals
and a polymeric substrate. A further object of the
invention is to provide a process for enhancing adhesion
between silver and a polymeric substrate.
SummarY of the Invention
According to the present invention, a process to
enhance adhesion between a metal and a polymeric
substrate utilizes a heating process.
In the process of the invention, a polymeric
substrate is coated with a metal, deposited by a vacuum
evaporation process. The metal coated substrate is then
heated for a predetermined time and temperature to
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enhance adhesion between the metal and the substrate,
without deforming the substrate.
For e~ample, in a preferred embodiment, a
silver-coated polycarbonate substrate is heated at
temperatures between about 100C to about 150C for
about 30 to 60 minutes. Temperatures higher than 150C
are impractical due to the softening of the polymer,
while temperatures lower than 100C are less effective
at promoting adhesion. After the heating pro~ess is
completed, greater than 99% of the silver adheres to the
polycarbonate substrate during a tape-peel test,
performed after exposure to environmental conditions.
Other objects and features of the present invention
will become apparent from the followin~ detailed
description.
Detailed Descr;Ption of the Invention
The present invention provides a process for
promoting adhesion between a metal and a polymeric
substrate.
In the process of the invention, a polymeric
substrate is placed into a reaction chamber wherein the
pressure is lowered, and a metal is vaporized~ The
polymeric substrate is then exposed to the metal vapor,
and the metal is deposited on the substrate. The metal
coated substrate is then heated for a predetermined time
and temperature to promote adhesion between the metal
and the substrate, without deforming the substrate.
While the process of the present invention may be
used to produce various products, of particular interest
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is the production of reflectors and, more specifically,
automotive headlamp reflectors. Typically, various
polymers are used in this application, including
polycarbonate and nylon, Most preferably, polycarbonate
is used as the polymeric substrate due to its light
weight, high impact strength, moldability and rigidity.
It should be noted that vari.ous other polymers, in
addition to those used for automotive headlamp
reflectors, may be utilized in the process o the
present invention to produce a variety of metalized
products, including, for example polypropylene,
polymethyl methacrylate, high density polyethylene,
polyethylene terephthalate, acrylic, and phenolic resins,
Once molded into the desired shape, the polymeric
substrate may be cleaned with soap and water and/or
plasma treated. Typically, a substrate to be plasma
treated is placed into a reaction chamber and exposed to
a glow discharge having an energy sufficient to yield a
plasma. At reduced pressures, glow discharges may be
produced by the use of a high frequency, such as radio
or microwave frequency, alternating current passed
through a coil surrounding the chambsr, or between two
external electrodes attached to the chamber. This
discharge excites molecules within the chamber which
oxidize the substrate's surface. Typically, any gas,
such as oxygen, nitrogen, or air, may be used within the
chamber; in the present embodiment, the air present in
the chamber is used to oxidize material on the substrate
surface. The excited molecules on the substrate's
surface promote adhesion by cleaning the surface of
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dirt, and perhaps other small molec:ules, cross--linking
the surface molecules, and providing oxygen-containing
functional groups.
The plasma may alternatively be formed from a corona
discharge. A corona discharge may occur at any pressure
and in all types of yases. This discharge is physically
similar to a glow discharge in a highly non-uniform
electric field. The electric energy in a corona
discharge is converted chiefly into heat in the gas
within the chamber.
After the polymeric substrate is cleaned and/or
exposed to the plasma treatment, the pressure in the
reaction chamoer is lowered from about 1 X 10 2 mbar
to about 1 X 10 4 mbar. In the low pressure
atmosphere the metal is evaporated, and is subsequently
deposited on the surface of the substrate.
Many different types of metals may be used in the
; process of the present invention. However, for
automotive headlamp reflectors, an important feature to
be considered when selecting metals is reflectivity.
Possible metals include copper, aluminum, gold and
silver. Of these, silver, when freshly deposited, is
the best reflector of visible light known, and is
therefore the most preferred metal coating.
The metal coatings are formed by condensation of
metal vapor. Typically, a metal holder is positioned on
a filament, which is connected to electrodes. A current
is passed through the filament to vaporize the metal
which is placed into the metal holder. Typically, the
metal holcler is constructed of a material which has a
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higher melti~ point than the meta:L to be vaporized, and
which ~ill not react with the se:Lected metal during the
process. Preferably, the holder mateeial is tungsten,
molybdenum, tantalum or a ceramic material. Most
preferably, a tungsten filament is used for heating. In
the chamber, the filament, and therefore the vaporizing
metal, is typically within three feet of the polymeric
substrate material. Most preferably, the vaporizing
metal is within two feet of the substrate; a relatively
short distance is preferred due to the nature of the
vaporized metal atoms which are deposited upon
everything in their path while being transerred from
the filament to the substrate.
A quartz crystal monitor, positioned at the same
distance from the vaporizing metal as the substrate, is
used to determine the transfer rate of the vaporized
metal. Typical thin metal film deposition rates are
between about 1 to 20 angstroms per second. A
relatively fast metal deposition rate, of about 10
angstroms per second, is preferred to prevent impurities
from being deposited with the metal onto the substrate.
By adjusting the voltage applied to the filament, any
desired deposi~ion rate may be achieved. Generally,
these rates do not vary with the substrate temperature.
; 25 The ~uartz crystal monitor is also used to determine
the total amount, or thickness, of the metal deposited
on the substrate. To provide adequate reflectivity a
thickness of at least 500 angstroms is required, as
metal thicknesses less then 500 angstroms are often
. 30 transparent. Further, although a vacuum evaporation
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process may be used to deposit metal coatings of a
thickness of up to one micron, at that thickness the
stress may be too high, and cracks in the coating
surface may occur. A preferred thickness of the metal
s coating upon the substrate is between about 500
angstroms and about lOoo angstroms. When the quartz
crystal monitor determines that a desired metal
thickness is achieved, the process may be stopped, by
blocking the path of the metal vapor to the substrate
mechanically, or by shutting off the voltage applied to
the filaments, or by burning out (evaporating) all of
the metal.
When the desired thickness of the metal coating is
deposited upon the polymeric substrate, and the
deposition process is terminated, the metal coated
polymeric substrate is removed from the chamber and is
placed into an oven. Typically, the heating process is
carried out in an air atmosphere; other gases however,
such as nitrogen, may be present. Depending on the
polymeric substrate chosen, the metal coated substrate
is heated for a predetermined time and temperature to
enhance the adhesion between the metal and the
substrate. Polycarbonate substrates, for example, are
heated at temperatures between about 100C to about
- 25 150C for about 30 to 60 minutes. Temperatures higher
than 150C are impractical due to the softening of the
polymer, while temperatures lower than 100C are less
effective. The most preferred heating temperature for
polycarbonate and silver is about 130C for about one
hour. Polyetherimide substrates generally require
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higher temperatures of about 200C for periods of about
30 minutes to promote adhesion between the metal and the
substrate.
It is believed that the heating process step
utilized in the present invention enhances the adhesion
of a metal to a polymeric substrate by either
reorganizing the chains of the substrate, which results
in better contact between t;he metal and the polymer, or
by inducing a physical or chemical reaction between the
metal and the polymer.
The present invention will be further illustrated by
the following examples which are intended to be
illustrative in nature and are not to be construed as
limiting the scope of the invention.
Example I
The adhesion of silver to a polycarbonate substrate
was compared with the adhesion of aluminum to a
polycarbonate substrate, utilizing the known vacuum
~: evaporation process, without the subsequent heating
; process. The experimental results are presented in
Table 1.
The first samples were not cleaned prior to testing;
the second samples were cleaned by scrubbing with soap
and water, and were then dried prior to testing; and the
third samples were similarly cleaned, and subjected to
an air plasma (which simulates the glow discharge used
in the above-described commercial process). The
adhesion was tested first, initially after the vacuum
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evaporation process, and second, after water immersion
for 18 hours at 25C. A "Pass" mea.rls that when adhesive
tape tPermanent Mending Tape 3M Corporation, St. Paul,
~N) is applied to the metal:ized substrate, which has
been scored with a razor bl,~de into a 1 centimeter
square grid, and then peeled up, less than 1~ of the
silver peeled off the surface with the tape. The
percentage indicated in failed samples is that portion
of the silver that peeled UI?
TABLE l
A. Silver Adhesion
After Water
Initial Immersion
Sam~le SubstrateAdhesion (18h, 25C)
: 1. PolycarbonateFail (80%)Fail (80~o)
2. PolycarbonateFail (70%)Fail ~90%)
(Cleaned).
3, PolycarbonateFail (90~Fail (100%)
(Cleaned then
Air Plasma)
B. Aluminum Adhesion
After Water
Initial Immersion
SamPle SubstrateAdhesion ~18h, 25C)
1. PolycarbonatePass Fail (100%)
2. PolycarbonatePass Fail (100~)
(Cleaned).
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3. Polycarbonate Pass Pass
(Cleaned then
Air Plasma)
This experiment shows that when silver is
substituted for aluminum in a standard vacuum
evaporation process, adhesion is not adequate initially
and is less adequate after water immersion. It is also
noted that aluminum adhesion is enhanced by utilizing
the air plasma prior to the vacuum evaporation process.
Example II
A silver coated polycarbonate substrate, prepared
with a cleaned, b~t not plasma treated, polycarbonate
sheet (such as Sample 2, Example I) in a vacuum
evaporation process, was heated to permit better
interaction between the metal and the substrate and to
enhance overall adhesion. The experimental results are
presented in Table 2.
The samples were subjected to various temperatures
for various periods to determine the optimum bake
conditions to improve adhesion of silver deposited by a
vacuum evaporation process onto a polycarbonate
substrate. After the heating process, the samples were
scored into a 1 centimeter square grid with a razor
blade and exposed to deionized water, at 90F for 96
hours, before being dried and subjected to a tape peel
test. A "pass" indicates that less than 1% of the
silver peeled off of the surface with the tape.
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TA:BI,E 2
Post-Environment
Sample 3ake Condi_ions Tape Peel Test
1. None Fail
2. 50C, lh Fail
3. 100C, lh Fail
4. 130C, lh Pass
5. 150C, 30 min Sample distorted
in oven
This data shows that the polycarbonate substrate
exhibits improved adhesion to silver, deposited by
vacuum evacuation, when heated for one hour a-t 130C
~Sample 4). Temperatures of 100C or less failed the
tape peel test, while temperatures of 150C or higher
softened the polymer.
Example III
The experiment run in Example II was repeated to
determine the optimum heating process timP necessary at
a given temperature. The experimental results are
presented in Table 3.
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TABLE 3
Post-Environment
SamE~ Bake Condit,ions TaPe Peel Test
1. None 30% Silver Loss
2. 130C, 1 mln 20% Silver Loss
3. 130C, 5 mi.n 5% Silver 1Oss
4. 130C, 30 min 5% Silver Loss
5. 130C, 60 min Pass (<1% Silver
Loss)
6. 130C, 135 min ~ass
The polycarbonate substrate appears to require
greater than 30 minutes at 130C to insure passing the
tape peel test. Note also that periods of up to 135
minutes at 130C resulted in less than 1% silver loss.
Example IV
Experiments for other polymeric substrates
demonstrate that the trend o Examples II and III is a
gsneral one. Polyetherimide silver coated substrates,
prepared similarly to the polycarbonate samples of
Sample 2, Example I in a vacuum evaporation process,
pass the tape peel test after water immersion (as
described in Example I) without being subjected to any
post deposition heating process. The samples, however,
do not pass the tape peel test after being subjected to
a 5% salt spray at 100F for 48 hours. The experimental
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results are presented in Table 4.
TA:BLE 4
Post-Environment
Sarnpl_ Bake CondltionsTape Peel Test
1. ~one 75~ Silver Loss
2. 200C, lh 1530 Silver Loss
3. 200C, 4h Silver Surface
Damage
These results show that exposure to heat (Sample 2.
200C for one hour) increases the a~hesion of silver to
the polyetherimide substrate, as the silver loss was
decreased from 75% to 15~ after exposure to the salt
spray.
Example V
Samples of polyetherimide typically require cleaning
before metal deposition to ob-tain optimum adhesion
results. However a post-deposition heating process can
effectuate similar results. Polyetherimide silver
coated substrates, prepared similarly to those in
Example IV, were subjected to a post deposition heating
process. The samples were then exposed to deionized
water at 90F for 96 hours before being dried and
subjected to a tape peel test. Experimental results are
presented in Table 5.
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TA3LE 5
Post-Environmen'c
: SamPle Bake Conditions Ta~e Peel Test
l. Cleaned None Pass ~<l~ silver
loss)
2. Cleaned 200C lh Pass
3. Not Cleaned None 40 - 100% Silver
Loss)
4. Not Cleaned 130C, lh 10~ Silver Loss
5. Not Cleaned 200C, 30 min Pass
The above results show that a cleaned polyetherimide
substrate does not require the post-deposition heating
to pass the tape peel test after being subjected to the
water immersion test described above. Similar results
are obtained when the clean sample is subjected to a
post-deposition heating process. However, when the
polyetherimide sample is not cleaned or subjected to a
post-deposition hearing process the sample fails the
tape peel test. Further, while the heating cycle of
130C for one hour provided sufficient adhesion between
the polycarbonate substrate and silver to pass the water
immersion test (see Example II, Table 2), it is not
sufficient for use with polyetherimide. An unclean
sample of a polyetherimide substrate requires
: 15 temperatures of about 200 for a period of about 30
minutes to pass the tape peel test. This test confirms
that the amount of heating reguired depends on the
nature of the polymeric substrate chosen.
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Example VI
The experiment run in Example II was repeated using
copper coated (loO0 angstroms thick) polycarbonate
substrates and gold coated (1000 angstroms thick)
polycarbonate substrates to observe whether heating
would enhance overall adhesion of polycarbonate to
metals other than silver. The experimental results are
presented in Table 6.
The samples were not subjected to any additional
environmental test conditions after the heating
process. After heating, the samples were scored into a
1 centimeter square grid with a razor blade and
subjected to a tape peel test. A "pass" indicates that
less than 1% of the metal coating peeled off the surface
with the tape. The percentage indicated in failed
samples is that portion of the metal which peeled up.
TABLE 6
Post-Environment
Sample Metal Bake Conditions TaPe Peel Test
. 1. ~old None Fail (100%)
2. Gold 130C, 1 h Fail (15~)
3, ~old 130C, 18 h Fail (1%)
4. Copper None Fail (100%)
5. Copper 130C, 1 h Pass
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6. Copper 130C, 5 min Pass
7. Coppee 130C, 1 min Fail (1%)
8. Copper 130C, 30 sec Fail (10%~
This data shows that the polycarbonate substrate
exhibits improved adhesion to gold and copper, deposited
by vacuum evaporation, when heated for a period of
time. Gold adhesion improved with time, and approached
passing at a period of 18 hours at 130C, Higher
temperatures are not practical (as shown in Example
II3. Copper adhesion also improved with time; at a
;temperature of 130C the copper coated polycarbonate
substrate passed the tape peel test when heated for at
least 5 minutes.
Although particular embodiments of the invention
have been described in detail for purposes of
illustration, various modifications may be made without
` departin~ from the spirit and scope of the present
invention. Accordingly, the invention is not to be
limited except at by the amended claims.
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