Etching of vacuum metallized indium

A plastic object is manufactured by a process including vacuum metallizing with a corrosion prone metal, namely indium, a dielectric substrate to form "islands" of the indium top coated with a clear resinous layer which encapsulates and insulates the islands, one from another. The indium islands are less than one thousand angstroms thick and have an average diameter of less than three thousand angstroms. The island structure is etched following the growth of the metal as it is deposited between the nucleation stage and the stage of channelization of formation of an electrically conductive film. The etchant is selected to clear channels between island structures to improve adhesion of a dielectric resinous top coat to the dielectric substrate by order of magnitude to adhesion to the islands. A preferred application of this invention is the manufacture of exterior automobile trim components the base structure of which is a flexible elastomer such as a thermoplastic urethane.

TECHNICAL FIELD 
This invention pertains to bright trim articles and more particularly to a 
method for manufacturing bright trim articles by vapor deposition of 
amphoteric materials. 
BACKGROUND ART 
Vacuum metallizing of plastic and similar dielectric substrates is 
disclosed in various forms including U.S. Pat. Nos. 
2,992,125; Fustier 
2,993,806; Fisher 
3,118,781; Downing 
3,914,472; Nakanishi 
4,101,698; Dunning 
4,131,530; Blum 
4,211,822; Kaufman 
4,215,170; Oliva 
My prior U.S. Pat. No. 4,431,711 issued Feb. 14, 1984, relates to metal 
film island structure and spacing to the appearance and performance of a 
commercial product, to the conductivity of the metal layer, to the 
corrosion resistance of the metal layer and/or to the adhesion of the top 
coat. It further relates to nucleation and film growth to a desired island 
structure and spacing that achieves these ends. 
With regard to the last statement, two excellent reference books are: 
Thin Film Phenomena, Kasturi L. Chopra, Robert E. Kreiger Publishing 
Company, Huntington, N.Y., 1979. See especially pp. 163 et seq. 
Handbook of Thin Film Technology, Leon I. Maissel and Reinhard Glang, 
McGraw-Hill Book Company, New York, N.Y., 1970. See especially pp. 8-32 et 
seq. 
These texts discuss and illustrate the stages of metal film growth by 
vacuum deposition from metal nucleation and nuclei growth, to liquid 
coalescence, to electrically discrete islands, channelization with 
incipient film conductivity, and finally, full continuous film formation. 
Film formation of vacuum deposited metals on plastic substrates for 
commercial products, especially on elastomeric plastic substrates, is not 
discussed. Nor is the interdependence of the natures of the metal film and 
the top coating correlated with product performance. 
My U.S. Pat. No. 4,431,711 shows the significant difference in performance 
to be obtained with a vacuum metallized flexible plastic product, top 
coated, where the metal particles are coalesced only to the island state 
instead of being allowed to coalesce to beyond the channelization stage 
where film conductivity is established. 
In the '711 patent, the separate islands are coalesced from separate 
nucleation points and are globular or rounded and fused appearing and are 
part of the nucleation and growth process. 
In general, the coalesced islands forming the indium films of the '711 
patent are smaller and there is a much greater spacing between them that 
can be filled with the resin of the top coating, in effect encapsulating 
the islands and binding them to the substrate surface. The rounded islands 
are better protected by the resin and the film over all is far more 
corrosion resistant, surprisingly so. The metal film is much more securely 
adhered to the substrate--a very significant advantage. The appearance of 
the globular island product is better--it is more specular, more 
reflective. 
The construction of the indium island structure in U.S. Pat. No. 4,431,711 
includes islands that are separated by channels which receive the top coat 
to bond the resinous film of the top coat to the substrate for the indium 
island structures. While the island structures are suitable for their 
intended purpose, it has been observed that the channels formed between 
the individual islands also contain many clusters and smaller islands of 
residual material. It is believed that this material reduces the total 
effective area of substrate material to which the top coat can be bonded. 
Consequently, the resultant bright trim article may be subject to 
undesirable delamination between the top coat and the substrate material. 
The prior art does not set forth a proven process for forming a clear 
channel configuration by use of etchants so as to improve adhesion of a 
top coat. 
STATEMENT OF INVENTION AND ADVANTAGES 
The present invention includes a process of manufacturing a corrosion 
resistant vacuum metallized article of bright metallic material in which a 
dielectric substrate surface has a vacuum deposited layer of metal 
selected from a group consisting of indium and alloys thereof which alloys 
are predominantly of indium and wherein the vacuum deposition is continued 
only until there is a formation of discrete islands which visually appear 
as a continuous film, but which have channels formed between the discrete 
islands of a dimension that will maintain the islands electrically 
non-conductive over the surface area of the substrate, wherein the process 
improvement includes etching the vacuum deposited discrete islands with a 
solvent which slowly dissolves residual amounts of indium from the 
channels between the distinct islands so as to clear the channels to 
expose additional bonding surfaces on the substrate for increasing the 
surface area of adhesion between the substrate and a protective dielectric 
top coat. 
The deposited islands are formed by indium which is amphoteric and thus has 
some solubility in both acids and bases. 
As deposited, the indium metal layer is composed of tiny islands ranging 
from tiny clusters of 25 angstroms or less in diameter. The tiny clusters 
are barely resolvable in the transmission electronic microscope. The 
islands can increase in diameter to sizes as large as 2,000 angstroms in 
diameter. Each of the islands is separate by channels which can be several 
hundred angstroms wide. However, in the deposition process to form the 
aforedescribed indium island structure, it is observed that many clusters 
and small islands of residual indium material may exist in the channels 
which produce the desired electrically non-conductive characteristics 
across the surface of the substrate. 
In accordance with the present invention, the process includes etching the 
previously deposited indium material with a solution that slowly dissolves 
the small clusters and islands to clean the channels and thereby define an 
additional surface area against which the top coat can adhere to the base 
coat so as to improve its adhesion to the base coat. 
The typical adhesion strength of a top coat material to a base coat 
material is in the order of 2 orders of magnitude stronger than the 
adhesion strength of the top coat to the metal making up the individual 
island structures separated by the channels. 
The treatment steps for vacuum deposited islands just before top coating 
consists of rinsing the part in a 10% NaOH solution for 60 to 90 seconds 
in a temperature range of 150.degree.-160.degree. F. followed by two water 
rinses and a second rinse with deionized water. This etch treatment step 
greatly improves the adhesion of top coat material of the type set forth 
in U.S. Pat. No. 4,431,711. While the flexible substrate described in U.S. 
Pat. No. 4,431,711 has sufficient adhesion to pass most automotive 
specification tests, it is desirable to improve the adhesion in such 
article so that it will consistently pass an X-scribed type taped adhesion 
test after either Florida exposures or accelerated weathering tests 
including (QUV, weatherometer, xenon, dual carbon arc weatherometer). With 
increasing emphasis on quality in American made cars, such tests are now 
beginning to show up in automotive specifications (see, for example, 
Fisher Body FBMS 1-51 specification). While etching the island containing 
metal layers of the type described in U.S. Pat. No., 4,431,711, an 
improved adhesion between top coat and base coat materials results so that 
such X-scribed standards can be met. 
PRESENT INVENTION 
The present invention includes use of such an etchant step to improve an 
article of manufacture comprising an organic dielectric base or substrate 
having a smooth surface such as a molded plastic, a macroscopically 
continuous-appearing very thin layer thereon of a vacuum deposited 
corrosion prone metal, specifically indium and alloys thereof consisting 
predominantly of indium and acting in much the same manner as pure indium. 
Preferably, the alloys each have a melting point in the range of 
125.degree. to 250.degree. C. The resultant metal is in the form of minute 
specular electrically discrete rounded metal islands with channels formed 
therebetween. The part is etched subsequent to island formation and prior 
to application of a protective top coat, so as to clear residual deposits 
of metal from the channels thereby to define a high adhesion force bonding 
surface between the top coat and the article of manufacture. Then a top 
coating is applied over the metal film encapsulating and protecting the 
metal particles and binding them firmly to the substrate. 
This resultant product is particularly useful in the automotive 
applications as an automobile exterior trim component to replace heavier 
and more expensive conventional chrome plated metal parts. 
The present process retains the thin vacuum metallized layer as an indium 
layer deposited or coalesced into electrically discrete islands which are 
maintained electrically non-conductive. However, it improves over the 
prior art by improving adhesion of the topcoat to protect the indium 
against corrosion even though it is a metal that is especially corrosion 
prone. 
The invention will now be described by way of the following examples and 
with reference to the accompanying drawing, with it being understood that 
other advantages and a more complete understanding of the invention will 
be apparent to those skilled in the art from the succeeding detailed 
description of the invention and the accompanying drawing hereto.

In both FIGS. 1 and 2, vacuum deposited indium film was microtomed to give 
slices that were 20 to 50 microns thick. These slices were encapsulated in 
an epoxy and were then microtomed or shaved to a tiny tip which contained 
the sample. The tip was then microtomed into approximately 1,000.degree. A 
thick specimens which were floated onto tiny copper grids. A diamond 
microtome was used in the specimen preparation. 
Photographs were taken of the indium layer at the 100,000 magnification. 
As can be seen from FIG. 1, prior to etching the primary indium islands are 
widely separated. However, the channels include the presence of clusters 
and small islands of indium material that effectively prevent the full 
surface are of the bottom of the separating channels between the larger 
island structures to be bonded to a top coat material. 
As seen in FIG. 2, following the etching step to be described, the indium 
islands are still separated by channels of a wide spacing as set forth in 
FIG. 1. However, the cleaning out of the channels by removing the residual 
clusters and small islands from the channel is clearly shown. 
Measurements of the surface energy of the metal layer both prior and 
following the etching step shows that there is no significant change in 
the surface energy of the metal layer due to etching. 
The etched islands as shown in FIG. 2 are slightly smaller than the 
unetched islands, and it is apparent that there are a greater number of 
middle sized island structures. 
Such increase in polydispersity can result in a lower reflectance and an 
increased haze level after etching. Accordingly, it is important to 
control the degree of etch to optimize both adhesion and resultant 
appearance of the bright trim article. 
From experiments with different acids and bases, it has been found that 
several acids or bases can etch the previously deposited indium metal 
island structures to produce the desired results. 
Table 2 shows the result of etching with a number of acids and bases with a 
flexible bright trimmed island deposition of indium material over TPU 
(thermoplastic polyurethane). 
All acids and bases evaluated as etchants were found to improve adhesion. 
All gave better adhesion and lower reflectance with increasing etch 
concentration, etch time and etch temperature. The concentrations, etch 
times and temperatures and pH to give an optimum etch with each acid or 
base varied widely. 
The optimum reflectance to give acceptable adhesion results (very little or 
no loss in a customer specified X-scribed adhesion test range) is in the 
mid to upper 50s regardless of the type of etchant used. Such reflectance 
is measured by a diffuse illumination of the part surface by an 18 inch 
diameter sphere by a standard broad band light source according to a 
manufacturer's instructions. A barium sulfide surface has a diffuse 
reflectance of 100% on such a scale. FIG. 3 is an example of the 
invention's reflectance from a 220.degree. A thick coating of indium with 
a clear top coat after the etchant step. It can be seen that the diffuse 
reflectance of the indium coating accomplishes the objective of a diffuse 
reflectance in the range of 50 throughout the wave length band of energy 
imposed thereon. 
Of all of the examples of etchants used, the preferred etchant is a 10% 
sodium hydroxide solution. The preferred etch conditions are a 60-90 
second etch period at a temperature of 150.degree.-160.degree. F. Such a 
solution and etch period and temperature produces consistently good 
performance. The higher solution concentration and greater length of etch 
time results in better control over the etching conditions. Most of the 
etch studies set forth herein have been done by dipping the metallized 
plaques or parts into the etch solution. Preliminary experiments indicate 
that spraying the etchant onto the parts will also perform suitably in the 
process. 
At least two rinses are necessary after the etching period. The water from 
the final rinse should be deionized water that is rapidly blown off the 
part with high velocity air to prevent streaking on the metal layer 
defined by the indium island structures. 
The improved weathering results of a bright trim indium island structure 
system over a TPU base with and without etching is shown in the following 
Table I. 
TABLE I 
______________________________________ 
WEATHERING RESULTS WITH DAVIDSON CC-2042 
BRIGHT TRIM SYSTEM OVER TPU 
% LOSS AFTER X-SCRIBE ADHESION TEST 
WITH 3M - 610 TAPE 
______________________________________ 
1000 Hours 1000 Hours 
Sample Etch Weatherometer Xenon 
______________________________________ 
776-152G 
Yes 0 0 
776-48C No 25 -- 
______________________________________ 
1000 Hours 
1000 Hours Dual Carbon Arc 
12 Months 
Sample QUV Weatherometer Florida 
______________________________________ 
776-152G 
0 0 0 
776-48C 25 -- 25 
______________________________________ 
Adhesion results with different acids and bases are set forth in the 
following Table II. 
TABLE II 
______________________________________ 
ADHESION RESULTS WITH 
DIFFERENT ACIDS AND BASES 
______________________________________ 
Treatment 
Reflec- Temp. of Time 
Sample tance Acid or Base Acid or Base 
(Sec.) 
______________________________________ 
834-29I 
65 None -- -- 
834-26B 
60 5% NaOH 150.degree. F. 
80 
834-26C 
59 10% NaOH 150.degree. F. 
40 
834-26D 
59 10% NaOH 150.degree. F. 
80 
834-26F 
56 15% NaOH 150.degree. F. 
80 
834-27B 
64 10% KOH 150.degree. F. 
80 
834-25B 
57 0.01 N Hcl Ambient 40 
834-25F 
54 0.01 N Hcl Ambient 40 
834-27F 
54 1.0 N Phosphoric 
130.degree. F. 
80 
834-28D 
51 0.1 N Nitric Ambient 40 
834-29A 
51 0.1 N Sulfuric 
Ambient 40 
834-29F 
49 0.1 N Acetic 130.degree. F. 
160 
______________________________________ 
1000 Hours Accelerated 
pH of Hydrolytic Tests 
Weathering Tests 
Acid Multiple Tape Multiple Tape 
or Crosshatch Test 
X-Scribe 
Crosshatch 
Sample Base (% Loss) (% Loss) 
(% Loss) 
______________________________________ 
834-29I 
-- 90 35 100 
834-26B 
13.0 0 20 100 
834-26C 
13.3 0 12 97 
834-26D 
13.3 0 0 83 
834-26F 
13.5 0 6 73 
834-27B 
13.9 0 23 100 
834-25B 
2.1 0 3 79 
834-25F 
0.55 0 1 60 
834-27F 
2.2 0 0 54 
834-28D 
2.0 0 14 99 
834-29A 
1.3 0 0 68 
834-29F 
3.6 0 3 50 
______________________________________ 
Table II shows that the adhesion test for a nonetched material will produce 
substantial percentage losses under hydrolytic tests of a moldable tape 
cross hatch test, while there is no loss under such a test where an 
etchant has been used to clear the channels for better bonding of the top 
coat to the base material. 
The X-scribed percent loss following etching is less than with no etching 
for all the etchant solutions. The etching step also improves a multiple 
tape cross hatch percentage of loss in all cases except for the use of 
potassium hydroxide in a 10% solution range. 
Representative embodiments of different etchant processes have been shown 
and discussed, those skilled in the art will recognize that various 
changes and modifications may be made within the scope and equivalency 
range of the present invention.