Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR DIRECT METAL-PLATING OF A PLASTIC SUBSTRATE
Field Of The Invention
The present invention relates to a process for direct metal-plating of a
plastic
substrate.
Description Of The Prior Art
Metal-plating of substrate materials is known. For example, chrome-plating of
automobile trim components is periodically popular.
Many years ago such chrome-plating was achieved by plating a metal substrate
(e.g., a bumper).
Over the years, chrome-plating of metal substrates for use in automobile trim
parts has been reduced for a variety of reasons, including: the need to reduce
the
weight of the vehicle, corrosion problems with the metal substrate, the
advance of other
systems for energy absorption if the substrate was being used as a bumper and
the like.
Thus, the state of the art has advanced over the years in the general subject
matter area
of metal-plating of substrates (both metal and non-metal).
United States patent 5,468,518 [Lein et al. (Lein)] teaches a combined
primer/basecoat island coating system for manufacturing a metallized part.
Generally,
the process relates to metallizing a substrate material selected from the
group
comprising thermoplastic urethane (TPU), TPU alloys, polyester alloys, nylon,
thermoplastic olefins (TPO) and aluminum. In the process, a protective layer
(primer/basecoat) is spray deposited, flashed and cured over the substrate.
The
protective layer comprises clear urethane resin, black pigment paste, a
solvent blend
and a catalyst solution. Thereafter, a layer of corrosion prone metal is
vacuum
deposited to form a discontinuous film covering the combined primer/basecoat
layer.
Finally, a layer of clear resinous protective dielectric topcoat is spray
deposited and
cured to completely cover the layer of corrosion prone metal material.
Apparently, the
process provides metallized parts which have a metallic rather than satin
appearance
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and which may be produced relatively rapidly and cost-effectively. Thus, Lein
teaches
a general plating technique for metal and non-metal substrates.
United States patent 5,591,488 [Schafer et al. (Schafer)] teaches a process
for
treatment of polymer-containing workpiece surfaces and to an aqueous non-
ageing
solution. The focus of Schafer is in a preconditioning process and materials
for use
therein to prepare a substrate material for application of a metal plating
layer. In
Schafer, the substrate is a polymeric material - e.g., polycarbonate,
optionally in
admixture with acrylonitrile-butadiene-styrene copolymer. Schafer is not
particularly
concerned with direct coating of a specific type of polymer substrate with a
metal
plating layer.
United States patent 5,693,209 [Bressel et al. (Bressel)] teaches a process
for
direct metallization of a circuit board having nonconductive surfaces. The
process
comprises reacting the nonconductive surface with an alkaline permanganate
solution
to form manganese dioxide chemically adsorbed on the nonconductive surface.
Thereafter, an aqueous solution of a weak acid and of a pyrrole or a pyrrole
derivative
and soluble oligomers is formed and contacted with the nonconductive surface
to
deposit an adherent, electrically conducting, insoluble polymer product on the
nonconductive surface. Thereafter, a metal is directly electrodeposited onto
the
nonconductive surface. The process is principally directed to the process
useful in 'the
production of circuit boards.
United States patent 5,882,736 [Stein et al. (Stein)] teaches a process for
deposition of a palladium layer on a metal surface. Specifically, the process
is for the
deposition of highly adhesive, permanently glossy palladium layers having very
few
pores on the metal surface. This is achieved by immersing the metal surface in
a
formaldehyde-free chemical bath, with or without pre-treatment.
United States patent 5,985,418 [Lein et al. (Lein)] teaches a process for
manufacturing a metallized substrate using a so-called island coating method.
Particularly adapted fox application of a metal layer to a substrate used in
the apparel
industry. The process comprises depositing a first coating layer containing a
radiation
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curable non-volatile film former. The coated substrate is then vacuum
metallized to
form the metal islands of the invention and thereafter, a layer of clear
resinous
protective dielectric top coat containing a radiation curable non-volatile
film former is
deposited to completely cover the layer of "metal islands" . The substrate can
be
polymeric or metallic.
Thermoplastic Olefin (TPO) is commonly molded into exterior trim components
for automobiles. In particular, vehicles are commonly fitted with a front and
rear
fascia which cover or conceal a front and rear bumper beam assembly. Molded
TPO
rocker panels and fenders are also commonly utilized on vehicles. The trim
panels are
molded and then painted to match the color of the remaining portion of the
vehicle.
Prior to the use of molded fascia, vehicles were fitted with steel bumpers
that
had a chromed outer surface. Slowly, the design of vehicles shifted to the use
of color
matched molded fascia concealing the bumper beam minimizing the use of chrome.
However, there has been a "retro" trend in automotive design and designers are
again
requiring chromed exteriors .
However heretofore, conventional TPO does not accept metal plating despite
the advances in the art. There is a need for a relatively simple process which
would
allow for direct metal-plating on a polymer surface, particularly a
thermoplastic olefin
(TPO) surface of an article adapted to be used in an automotive trim
application.
Summary of the Invention
The disadvantages of the prior art may be overcome by providing a novel
process for direct metal-plating of a plastic substrate.
Accordingly, in one of its aspects, the present invention provides a process
for
direct metal-plating of a plastic substrate comprising the steps of:
(i) activating a surface of a TPO substrate to produce an active surface
thereof, the active surface having at least about 7 % of carbon atoms being in
the form
of carbonyl;
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(ii) electrochemically depositing metal layer on the activated surface.
The functionality of carbon atoms at the surface of an activate thermoplastic
olefin (TPO) substrate (i.e., the active surface of the substrate) to be metal-
plated plays
an important role in facilitating direct metal-plating. Specifically, it is
important that
the active surface of the substrate comprise carbon atoms of which at least
about 7
are in the form of carbonyl in order to produce a durable, good quality metal
plate
coating on the substrate. If the active surface of the substrate comprises
carbon atoms
of which is less than about 7 % in the form of carbonyl, the metal-plate
coating will be
inferior and subject to detachment from the thermoplastic olefin substrate. As
used
throughout this specification, the term " active surface" is intended in
denote a surface
layer of the TPO substrate having a thickness of from about 3 to about 5 nm.
As used throughout this specification, the term "modified TPO" is intended to
mean polypropylene-based materials such as polyolefin modified with a first
elastomeric material such ethylene propylene dime monomer (EPDM) and further
modified with a second elastomer including a dime and triene type polymer.
Description of the Drawings
In drawings which illustrate embodiments of the present invention,
Figure 1 is a perspective view of an automotive trim piece of the present
invention; and
Figure 2 is a cross sectional view of the automotive trim price of Figure 1.
Detailed Description of the Preferred Embodiment
The process of the present invention is useful to directly metallize a TPO
substrate. In the first preferred embodiment, the TPO substrate is a modified
TPO
comprising a polyolefin material modified with a first elastomer, such as EPDM
and
then modified with a second elastomer, including diene and triene type
polymers, such
as acrylonitrile and butadiene. The elastomers are added in effective amounts
providing a basis for an active surface as discussed below.
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The TPO is molded in a conventional manner into a substrate 10. Preferably,
the substrate is in the form of an automotive component such as a grill.
However,
other components such as fascia, trim panels, rocker panels, fenders, trim
strips are
also contemplated in the present invention. A surface of the TPO substrate 14
is
activated by one of the methods described below to provide an active surface
16.
Preferably, the active surface 16 is a presentation or exterior-facing surface
of the
molded part on which a metal layer 18 is deposited.
The active surface 16 of the TPO substrate 14 comprises carbon atoms of which
at least about 7 % are in the form of carbonyl. Preferably from about 7 % to
about 25
of carbon atoms at the active surface are in the form of carbonyl. More
preferably,
from about 7 % to about 20 % of carbon atoms at the active surface are in the
form of
carbonyl. Most preferably, about 7 % to about 15 % of carbon atoms at the
active
surface are in the form of carbonyl.
The presence of the carbonyl groups at the active surface may be confirmed by
conventional techniques - e.g., FTIR (Fourier Transform Infrared
spectroscopy). The
concentration of carbonyl groups at the active surface may be confirmed by
conventional techniques - e.g., XPS (X-ray photoelectron spectroscopy).
In the first preferred method, a modified TPO is utilized and the desired
amount
of carbonyl content can be conferred to the surface of a modified TPO
substrate by
controlling preferred pretreatment steps in the process.
It is preferred to pretreat the modified TPO substrate using an etching
technique
- i.e., this is a preferred embodiment of Step (i) of the present process.
Preferably, the
etching technique comprises contacting the modified TPO substrate with an
etching
solution comprising chromic acid or a mixture of chromic acid and sulfuric
acid.
During this etching step the active surface is formed. It is believed that
chromic acid
is reduced on the surface of the modified TPO substrate to provide oxygen
containing
moieties (e.g., hydroxyl, ether and the like), including the desired carbonyl
groups.
The amount of desired carbonyl groups may be controlled by the time period
during
which the substrate is immersed in the etching solution. The chemicals) used
in the
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above-mentioned etching step may be obtained commercially from Atotech Canada
Ltd.
under the tradename FuturonTM
Next, it is preferred to subject the active surface of the modified TPO
substrate
to chromic acid reduction to reduce any residual chromic acid left in the
pores of the
substrate from Cr6+ to Cr3+ (the reason for this is that Crb~ is detrimental
if present
during later steps in the process).
Next, it is preferred to subject the active surface of the modified TPO
substrate
to a pre-dipping step in which the active surface of the modified TPO
substrate is
contacted with an acid. A non-limiting useful such acid is hydrochloric acid.
The chemicals used in the above-mentioned chromic acid reduction and pre-
dipping steps are commercially available under the tradename FuturonT"' from
Atotech
Canada Ltd.
The active surface of the modified TPO substrate is then ready for further
activation wherein a surface thereof is fully activated for electrolytic
metallization.
This involves contacting the active surface with an aqueous solution
comprising a
palladium salt and a tin salt in hydrochloric acid. Preferably, the solution
comprises
palladium chloride and stannous chloride.
Practically, it is preferred to immerse the substrate in a bath containing
stannic
palladium in a concentration of from about 5 to about 20,000 ppm (O.OOS to 20
g/L),
preferably 20 to about 300 ppm (0.02 to 0.3 g/L), more preferably from about
20 to
about 250 ppm (0.02 to 0.25 g/L) for a period of at least about one minute,
preferably
from about one minute to about ten minutes, more preferably from about two
minutes
to about four minutes.
This activation step serves to anchor the palladium/tin complex to the surface
of the TPO substrate. By controlling the oxygen content in the substrate, a
desirable
level of palladium/tin complex is dispersed over the surface of the substrate.
Preferably, the next step in the process is to exchange the tin in the
palladium/tin complex with copper. This is done in a conventional manner.
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The chemicals) used in the above-mentioned electrolytic metallization steps
may be obtained commercially from Atotech Canada Ltd. under the tradename
FuturonTm .
Optionally, a layer of nickel or copper can be deposited on the active surface
in a conventional manner, including electroless deposition whereby nickel or
copper
salt is reduced onto the active surface of the TPO substrate. Advantages of
the nickel
or copper layer include increased conductivity, reduced bath time and lower
palladium
concentrations ( < 100ppm or 0.1 g/L) in the palladium bath discussed above.
Step (ii) of the present process comprises electrochemically depositing a
metal
layer 1 ~ on the active surface 14 of the TPO substrate 14. This can be done
in a
conventional manner.
Embodiments of the present invention will be described with reference to the
following examples which are provided for illustrative purposes only and
should not
be used to construe or Iimit the scope of the invention.
Examples
In the Examples, a modified TPO obtained from Solvay Engineered Polymers
was activated, treated and electrolytically metallized in the following
manner.
All samples were TPO panels which were treated using chemicals commercially
available from Atotech Canada Ltd. in the FuturonTM pre-plating process. As is
known
in the art, the FuturonTM pre-plating process comprises the following general
steps: (i)
cleaner (optional); (ii) etching (typically with chromic sulfuric acid); (iii)
reduction
(typically with Cr (VI), (iv) pre-dip solution; (v) Pd/Sn activation and (vi)
Cu-link. In
the Examples, the modified TPO substrate was treated according to the
particulars set
out in Tables 1 and 2.
Etch time was varied as set out in Table 3 below.
Four panels per Example were processed through to copper plating.
Pre-plate tank conditions (dwell times, concentrations, temperatures) are
reported below. All panels were plated using a conventional acid copper-
electroplating
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bath, followed by conventional electroplating nickel and chrome baths. Panels
were
plated for 60 minutes at 30 amps/frz to achieve 40+/- 5 microns of copper
thickness for
peel testing as defined in ASTM B533.
The test panels were stripped of palladium and copper using a solution of Aqua
Regia (1:1). Solutions were analysed for metal content using Atomic Adsorption
spectrophotometry. Samples were tested for peel strength using the 90°
tensile test, on
an InstronT"" instrument in accordance with ASTM B533. Each sample panel was
cut
into three strips and each strip tested. The overall average of the strips for
each
Example is reported as the peel strength of the panel.
The peel test results illustrate that significantly improved adhesion of the
metal
plating layer is achieved in Examples 2 and 3 (carbonyl content of at least 7
% )
compared with that achieved in Example 1 (carbonyl content less than 7 % ) .
While the present invention has been described with reference to preferred and
specifically illustrated embodiments, it will of course be understood by those
skilled in
the art that various modifications to these preferred embodiments and
illustrated
embodiments may be made without departing from the scope of the invention as
defined
in the appended claims.
While the carbonyl content described in the Examples was achieved with
chemical etching of a modified TPO, it is of course possible to confer the
carbonyl
functionality to the surface of TPO using other conventional techniques such
as
flaming, vacuum plasma and electrical discharge (e.g., Corona surface
treatment).
In the case of vacuum plasma and Corona surface treatment, it is possible to
achieve the requisite active surface 16 of a conventional TPO substrate 14
without the
need for modifying the TPO with the second elastomer.
Still further, while the carbonyl content described in the Examples was varied
by varying the period chemical etching, it is of course possible to control
the amount
of carbonyl functionality on the substrate surface using other means such as
temperature
of the etching bath and/or the concentration the chemicals used in the etching
bath.
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Other modifications which do not depart from the scope of the present
invention will
be apparent to those of skill in the art.
All publications, patents and patent applications referred to herein are
incorporated by reference in their entirety to the same extent as if each
individual
publication, patent or patent application was specifically and individually
indicated to
be incorporated by reference in its entirety.
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TABLE 1
Tank Immersion Time (minutes)Temperature (C)
Chromo/Sulphuric Varied (2-20) (60-70)
Etch
Chrome Reducer 0.5 25
Pre-Dip 1 25
Pd/Sn Activator 4 44
Cu-Link 3 55
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TABLE 2
Re-
Etch ducer Pre-DipActivator Cu-Link
Cr(VI) HzS04 Cr(III)CrVR HCI Pd HCl Sn+2 Part Part
A B
(g/L) (g/L) (g/L) (g/L) ( % (ppm) (mL/L) (g/L) (mL/L) (mL/L)
)
390 400 25 11 30 250 30 11 100 400
TABLE 3
Example Etch time Carbonyl Pd Cu Average
(min.) content Adsorption2Adsorption2Peel
(%)1 Strength
(lbf/in)
1 2 4.1 16.5 4.5 4.1
2 11 17 127.5 69 5.6
3 15 18 246.5 117 5.4
IReported as a percentage of total carbon.
ZReported as mg/m2.
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