U.S. patent application number 09/724467 was filed with the patent office on 2001-04-19 for silver film incorporating protective insoluble metallic salt precipitate.
This patent application is currently assigned to Lilly Industries (USA), Inc.. Invention is credited to Soltys, Joseph.
Application Number | 20010000332 09/724467 |
Document ID | / |
Family ID | 22505144 |
Filed Date | 2001-04-19 |
United States Patent
Application |
20010000332 |
Kind Code |
A1 |
Soltys, Joseph |
April 19, 2001 |
Silver film incorporating protective insoluble metallic salt
precipitate
Abstract
A method is provided for making mirrors having enhanced
reflective layer resistance to corrosion. The reflective layer of
the mirror, typically silver, is contacted, preferably
simultaneously, with a first solution containing a specific cation
and a second solution containing a specific anion, or alkaline
material which forms hydroxyl ions, the specific cation and
specific anion or hydroxyl ion reacting to form a water insoluble
precipitate on the silver surface. The mirror may then be painted
to provide additional corrosion resistance to the mirror. The
method eliminates the need for a copper layer on the silver surface
and the method may be incorporated into existing mirror production
lines as a replacement for the copper layering step. Also provided
are an apparatus for making the mirrors and the mirrors made using
the method and apparatus of the invention. A preferred cation
containing solution contains tin (e.g., SnCl.sub.2) and a preferred
anion containing solution contains hydroxyl ions (e.g., NaOH).
Inventors: |
Soltys, Joseph; (London,
CA) |
Correspondence
Address: |
DELIO & PETERSON
121 WHITNEY AVENUE
NEW HAVEN
CT
06510
|
Assignee: |
Lilly Industries (USA),
Inc.
|
Family ID: |
22505144 |
Appl. No.: |
09/724467 |
Filed: |
November 29, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09724467 |
Nov 29, 2000 |
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09420100 |
Oct 18, 1999 |
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09420100 |
Oct 18, 1999 |
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09143685 |
Aug 28, 1998 |
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6017580 |
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Current U.S.
Class: |
428/434 ;
359/838; 427/162; 427/165; 427/168 |
Current CPC
Class: |
C03C 17/3605 20130101;
C03C 17/38 20130101; Y10S 423/08 20130101; C03C 17/36 20130101;
C03C 17/3663 20130101; C03C 17/3644 20130101 |
Class at
Publication: |
428/434 ;
359/838; 427/162; 427/165; 427/168 |
International
Class: |
G02B 005/08; B32B
015/04; B32B 017/06; B05D 005/06; B05D 005/00 |
Claims
Thus, having described the invention, what is claimed is:
1. A method for making mirrors wherein the mirror comprises a glass
substrate and a reflective metallic film layer on the glass
substrate comprising: supplying a glass substrate; sensitizing the
glass substrate; coating a layer of silver on the glass substrate;
contacting the silver coated glass substrate with a first solution
containing a specified cation and a second solution containing a
specified anion, or alkaline material which forms hydroxyl ions,
preferably simultaneously by spraying or otherwise directing
streams of the solutions so that the two solutions meet at the
silver surface, the specified cation and specified anion or
hydroxyl ion being reactive to form a water insoluble precipitate
on the silver surface, which precipitate enhances the corrosion
resistance of the silver layer.
2. The method of claim 1 wherein the specified cation of the first
solution is selected from the group consisting of Sn.sup.2+,
Sn.sup.4+, Bi.sup.3+, Ag.sup.+, Zn.sup.2+, Pb.sup.2+, Ce.sup.3+,
Al.sup.3+, Fe.sup.2+, In.sup.3+, Ti.sup.3+and La.sup.3+.
3. The method of claim 2 wherein the specified anion of the second
solution is selected from the group consisting of OH.sup.-,
CO.sup.2-.sub.3, HPO.sup.2-.sub.3, SiO.sup.2-.sub.3
B.sub.4O.sub.7.sup.2-and BO.sub.2.sup.- alkaline materials which
form hydroxyl ions.
4. The method of claim 3 wherein the first solution is SnCl.sub.2
or SnF.sub.2 alone or acidified with HCl or H.sub.2SO.sub.4.
5. The method of claim 4 wherein the second solution is NaOH or
NH.sub.4OH.
6. The method of claim 1 wherein the first solution and second
solution simultaneously contact the silver surface.
7. The method of claim 1 wherein either the first solution or
second solution first contacts the silver surface followed by
contacting with the other solution.
8. A mirror made by the method of claim 1.
9. A mirror made by the method of claim 3.
10. A mirror made by the method of claim 5.
11. A mirror made by the method of claim 6.
12. A mirror made by the method of claim 7.
13. A method for enhancing the corrosion resistance of a silver
surface comprising: contacting the silver surface with a first
solution containing a specified cation and a second solution
containing a specified anion, or alkaline material which forms
hydroxyl ions, the specified cation and specified anion or hydroxyl
ion reacting to form a water insoluble precipitate on the silver
surface.
14. The method of claim 13 wherein the silver surface is the
reflective layer of a mirror which has a glass substrate.
15. The method of claim 14 wherein the first solution comprises tin
ions.
16. The method of claim 15 wherein the second solution comprises
OH.sup.- ions.
17. A mirror made by the method of claim 13.
18. A mirror made by the method of claim 16.
19. An apparatus for making mirrors comprising: means for moving a
sheet of glass preferably along a horizontal path; means for
cleaning the sheet of glass to remove oils, grease, powder,
interleaving material and the like; means for sensitizing the
cleaned sheet of glass to promote silver deposition thereon; means
for applying a layer of silver on the sensitized glass surface;
means for contacting a first solution containing a specified cation
and a second solution containing a specified anion, or alkaline
material which forms hydroxyl ions at the silver surface,
preferably simultaneously, the specified cation ion, and specified
anion or hydroxyl ion reacting to form a water insoluble reaction
product precipitate on the silver surface; and means for optionally
painting or applying another protective layer on the precipitate to
form the mirror product.
20. The apparatus of claim 19 wherein either the first solution or
second solution first contacts the silver surface followed by
contacting with the other solution.
Description
BACKGROUND OF THE INVENTION
1. 1. Field of the Invention
2. This invention relates to mirrors, and to a method for making
mirrors comprising applying a reflective layer such as silver to a
glass surface and, in particular, to treating the silver surface to
provide enhanced corrosion resistance of the silver surface and
mirror without the need for a corrosion inhibiting copper layer on
the silver surface.
3. 2. Description of Related Art
4. For convenience, the following description will be directed to
mirrors and improving the corrosion resistance of the reflective
layer used to make the mirror but it will be understood to those
skilled in the art that other metal containing substrates and metal
particles may be treated using the method and apparatus of the
invention to enhance the corrosion resistance of the metal.
5. Typical mirrors are made of a glass sheet and a thin layer of a
reflective metallic film applied to the back of the sheet. The
metallic film layer applied directly to the glass is usually a film
of silver, although other metallic films may also be used, such as
copper. When silver is used as the primary reflective layer, it is
commonly protected by a second metallic film layer of copper to
inhibit corrosion of the silver layer. A layer of paint is also
typically used over the silver or copper layer for enhanced
corrosion and abrasion resistance. Each step in the mirror making
process is usually performed in an automated process with the glass
sheet positioned horizontally as the sheet moves continuously
through the various steps of the process. The time and need for
each step is therefore very important from a commercial standpoint
and the elimination of a step or substitution of a more efficient
and environmentally acceptable step is a continuing goal of
industry.
6. Mirrors are generally made continuously by a sequence of steps
on a mirror conveyor. The first step lightly polishes and cleans
the glass surface and after rinsing, the next step sensitizes the
surface with an aqueous stannous chloride solution. The silver film
layer is then deposited on the sensitized glass surface by one of
many methods such as described in U.S. Pat. No. 4,737,188 to Bahls.
Typically, an ammoniacal silver nitrate solution and a reducing
agent solution containing a strong base are sprayed on and combined
at the sensitized glass surface to deposit the silver film.
Thereafter, a copper film may be applied to and over the silver
film by any of a variety of prior art procedures such as a galvanic
process which utilizes an aqueous suspension of iron powder and an
aqueous solution of copper sulfate or by the disproportionation of
cuprous ions on the silver surface. The latter process is described
in U.S. Pat. No. 5,419,926 to Soltys. The copper layer is normally
painted to produce the finished mirror, or another protective
coating such as a hardened organic resin incorporating a corrosion
inhibitor may likewise be applied as shown in U.S. Pat. No.
5,156,917 to Sanford. The above patents are hereby incorporated by
reference. A standard mirror making process thus comprises a series
of steps which steps are performed in sequence on a conveyor as
part of a continuous mirror making process.
7. A serious problem of the mirror manufacturing industry is the
need for a copper layer on the silver layer to inhibit corrosion of
the silver. The application of copper to the silver surface
necessarily produces copper containing waste streams which must be
environmentally treated or processed for recycling. Typically, the
copper streams are treated to remove copper before discharge to the
effluent and this procedure is complex and costly. The copper film
on the mirror is also a weak link in the life of a conventional
mirror. The copper film is easily corroded when the mirror is
subjected to ammonia or alkaline glass cleaners because these cause
the edges of mirrors to corrode and turn black in color thereby
shortening the life of the mirror.
8. A number of patents have been issued for improving treatment of
the silver surface in the mirror making process to eliminate the
copper layering step. In U.S. Pat. No. 5,374,451 to Servais et al.,
a mirror is shown having a reflective layer of silver which has
been treated with a solution containing ions of at least one of the
group consisting of: Cr (II); V (II or III); Ti (III or II); Fe
(II); In (I or II); Cu (I); and Al (III). The solution may also
contain Sn(II) ions. It is preferred in the patent that a
protective layer of paint still be used to protect the treated
silver layer. A similar treatment is disclosed in U.S. Pat. No.
5,240,776 which uses stannous ions to contact the silver layer
followed by a silane treatment.
9. All the above patents are incorporated herein by reference.
10. Unfortunately, current methods of treating the silver surfaces
of mirrors to inhibit corrosion thereof are unreliable, and
specially developed corrosion inhibiting coatings are required, and
the processes of U.S. Pat. Nos. 5,240,776 and 5,374,451, supra,
only treat the silver surface with a simple metal solution to
augment the metal atoms on the silver surface as stated in the
5,240,776 patent but which processes may not be entirely effective
for the myriad of mirror products made in industry.
11. Bearing in mind the problems and deficiencies of the prior art,
it is an object of the present invention to provide a method for
making mirrors wherein the copper layering process is replaced with
an environmentally friendly process step which protects the silver
layer against corrosion and which may be used in existing
commercial mirror making conveyor systems without having to add
extra sections to the system.
12. It is another object of the present invention to provide a
method for enhancing the corrosion resistance of a metal surface
such as the silver surface of a mirror and, additionally, to treat
the silver surface of the mirror to enhance the corrosion
resistance of the silver.
13. It is a further object of the invention to provide an apparatus
for making mirrors without the need for the copper layering
step.
14. Another object of the invention is to provide an improved
corrosion inhibiting mirror and other metal substrate and metal
articles of manufacture.
15. Still other objects and advantages of the invention will in
part be obvious and will in part be apparent from the
specification.
SUMMARY OF THE INVENTION
16. The above and other objects and advantages, which will be
apparent to those skilled in the art, are achieved in the present
invention which is directed to, in a first aspect, a method for
making mirrors having enhanced reflective metal, e.g., silver,
corrosion resistance and to eliminate the need for a copper
protective layer on the reflective layer, the method
comprising:
17. supplying a glass substrate;
18. sensitizing the glass substrate by one of several prior art
methods;
19. coating a layer of silver on the glass substrate;
20. contacting the silver coated glass substrate with a first
solution containing a specified cation and a second solution
containing a specified anion, or alkaline material which forms
hydroxyl ions, preferably simultaneously by spraying or otherwise
directing streams of the solutions so that the two solutions meet
at the silver surface, the specified cation and specified anion or
hydroxyl ion being reactive to form a water insoluble precipitate
on the silver surface, which precipitate enhances the corrosion
resistance of the silver layer.
21. The reaction of the first solution and the second solution may
generally be shown by the following equation:
AB+CD.fwdarw.AD.dwnarw.+BC
22. wherein A is the specified cation, D is the specified anion or
alkaline material, AB and CD are water soluble compounds and BC is
a water soluble reaction product and AD is a water insoluble
reaction product precipitate of the specified cation A and
specified anion or hydroxyl ion D. The .dwnarw. indicates a
precipitated compound. When the first solution AB and second
solution CD are mixed, a supersaturated solution of product AD is
formed and an opalescent mixture is obtained showing the presence
of a precipitate.
23. The treated silver surface may be optionally painted or
otherwise coated to provide the mirror product. Any organic-based
mirror backing paint may be used, leaded or lead-free, and
water-based mirror backing paint being a typical and alternate
paint. Lilly Industries mirror backing paints are preferred.
24. In another aspect of the invention a method for enhancing the
corrosion resistance of a metal coating such as the silver surface
of a mirror comprises contacting the metal coating with a first
solution containing a specified cation and a second solution
containing a specified anion, or alkaline material which forms
hydroxyl ions, preferably simultaneously by spraying or otherwise
directing a stream of each solution so that the solutions meet at
the metal coating surface, the specified cation and specified anion
or hydroxyl ion being reactive to form a water insoluble reaction
product precipitate on the metal surface to enhance the corrosion
resistance of the metal surface.
25. In another aspect of the invention, either the first solution
containing a specified cation or the second solution containing a
specified anion, or alkaline material which forms hydroxyl ions,
may be applied separately and then the other solution applied to
form the reaction product precipitate. For this technique it is
preferred that the solution first applied to the surface be
maintained in a liquid form on the metal surface so that the
cations and anions or hydroxyl ions of the solution are in the
ionized form and suitable for reaction with the solution applied
thereto to form the reaction product precipitate.
26. In a further aspect of the invention, an apparatus is provided
for making mirrors comprising:
27. means for moving a sheet of glass preferably along a horizontal
path;
28. means for cleaning the sheet of glass to remove oils, grease,
powder, interleaving material, etc.;
29. means for sensitizing the cleaned sheet of glass by one of
several known methods to promote silver deposition thereon;
30. means for applying a layer of silver on the sensitized glass
surface;
31. means for contacting a first solution containing a specified
cation and a second solution containing a specified anion, or
alkaline material which forms hydroxyl ions, at the silver surface,
preferably simultaneously, the specified cation ion and specified
anion or alkaline material reacting to form a water insoluble
reaction product precipitate on the silver surface;
32. means for optionally applying a silane treatment over the
precipitate by techniques known to those familiar with the art;
and
33. means for optionally painting or applying another protective
layer on the precipitate to form the mirror product.
34. In another aspect, an improved mirror and other metal substrate
and metal articles of manufacture are provided made by the method
and/or apparatus of the invention.
35. In another aspect of the invention, the precipitated protective
layer can comprise more than one substance such as a hydroxide of a
specified cation along with the reaction product AD.
36. The term "water insoluble reaction product precipitate" is
intended to mean that the precipitate is substantially insoluble in
water as will be understood by those skilled in the art. The
solubility product constant in water at 25.degree. C. should
generally be less than Ksp=10.sup.-6, and preferably less than
Ksp=10.sup.-8.
DETAILED DESCRIPTION OF THE INVENTION
37. The glass substrate on which the mirror is made may be any of
the conventional glasses used in mirror making manufacture. Such
glasses include soda lime glass and other conventional glass
products. The conventional method of preparing the glass substrate
to make a mirror is to clean the glass to remove grease, oil, etc.
using cerium oxide and/or a detergent. Other substrate materials
such as plastics, including Lexan and polycarbonate, may also be
used as a metal covered or coated substrate. Metal particles such
as silver flakes and powders, metal (silver) coated mica, metal
(silver) coated metal particles, e.g., Ni or Cu, and the like may
also be treated using the method and apparatus of the invention to
increase the corrosion resistance of the metal particles.
38. After the glass surface has been cleaned and preferably rinsed,
it is then sensitized using, for example, conventional sensitizing
solutions. In general a conventional stannous ion solution is
preferably used. The sensitizing solution can be applied to the
glass surface by pouring, immersion, dipping, spraying or rinsing
the solution over the glass surface. An acidic stannous solution is
generally used to sensitize the glass surface, although the
stannous solution alone can be used if it is prepared shortly
before use. A stannous ion concentration 10-1000 mg/l and a pH of
2-5 is typically employed but these sensitizing solutions may vary
widely in composition and concentration.
39. The silver film is then applied to the sensitized glass surface
following conventional coating techniques such as described in U.S.
Pat. No. 4,737,188, supra. Basically, a silver solution and a
reducing solution are brought together before or at contact with
the substrate to be silvered by pouring or metering the solutions
such that they meet just before contact with the substrate.
Alternatively, the component solutions may be sprayed using an air
or airless system prior to or simultaneously with intermixing at
the surface of the substrate.
40. The corrosion resistance of the silver film is enhanced by
using the method and apparatus of the invention which broadly
comprises forming a water insoluble reaction product precipitate on
the silver surface. The specified cation containing solution and
specified anion, or alkaline material which forms hydroxyl ions
containing solution which react to form the water insoluble
reaction product precipitate may be employed at varying
concentrations, temperatures and silver contacting conditions. The
solution temperature may vary widely up to boiling, e.g., 5.degree.
C. to 95.degree. C. and it is preferred that a solution temperature
of about 20.degree. C. to about 45.degree. C., preferably
25.degree., be used. A contact time on the silver surface of the
two solutions is up to about 2 minutes, e.g., 5 seconds to 2
minutes, preferably 20-40 seconds, e.g., 30 seconds. A
concentration up to saturation may be employed with generally a
stoichiometric amount being used. Amounts of 0.01 mM to 0.1 M of
specific cation solution have been found useful.
41. Any suitable water soluble components may be employed to form
the reacting solutions with the proviso that the specified cation
of one of the solutions and specified anion or alkaline material
which forms hydroxyl ions of the other solution react to form a
water insoluble precipitate of the specified cation and specified
anion or hydroxyl ion on the metal surface. Exemplary specific
cations include Sn.sup.2+, Bi.sup.3+, Ag.sup.+, Zn.sup.2+,
Pb.sup.2+, Ce.sup.3+, Al.sup.3+, Fe.sup.2+, In.sup.3+and
La.sup.3+cations. Titanium cations, Ti.sup.3+, may be added as a
secondary cation in a minor amount, preferably with the stannous
cation in, for example, an 80%/20% ratio of Sn.sup.2+to
Ti.sup.3+.
42. Desirable stannous salts include stannous chloride
(SnCl.sub.2), stannous fluoride (SnF.sub.2), stannous sulfate
(SnSO.sub.4), stannous bromide, stannous fluorborate, and stannous
methane sulfonate, preferably stannous fluoride. Stannic tin,
Sn.sup.4+, in the form of SnCl.sub.4, SnF.sub.4 or SnBr.sub.4, for
example, may also be added in minor amounts to the stannous
solution, such as in an 90%/10% ratio of stannous to stannic, or
used alone as the cation.
43. Exemplary specific anions or hydroxyl ions include OH.sup.-,
CO.sub.3.sup.2-, HPO.sub.3.sup.2-, SiO.sub.3.sup.2-,
B.sub.4O.sub.7.sup.2-, and BO.sub.2.sup.-anions. The alkaline
materials which form hydroxyl ions may vary widely and are
generally compounds of Group I and Group II elements such as NaOH,
KOH, LiOH, RbOH, CsOH, Mg(OH).sub.2, Ca(OH).sub.2, Sr(OH).sub.2,
Ba(OH).sub.2, NaBO.sub.2 and Na.sub.2B.sub.4O.sub.7. Soluble
compounds of the Group I and Group II cations which form hydroxyl
ions are also included such as alkaline salts of weak acids such as
sodium carbonate, trisodium phosphate, sodium borate, sodium
silicate, sodium phosphite, and the like. Another source of
hydroxyl ions is aqueous ammonia (NH.sub.4OH) and other amines such
as hydrazine, free base hydroxyl amine, aliphatic amines, hydroxyl
amines such as ethanolamine and polyamines. Ammonia gas and other
gases may be used as one of the reactants to form the metallic salt
precipitate.
44. The solutions, particularly the cation containing solutions,
may be prepared and used immediately as-is, or may be slightly
acidified for storage purposes to increase their shelf life.
Typically an acid such as HCl, H.sub.2SO.sub.4, HNO.sub.3, acetic,
lactic, glycolic, formic or other organic acid may be used to
provide a pH up to about 6, preferably 1-3. Also contemplated
herein are the use of solutions containing multiple specific
cations and/or multiple specific anions or alkaline materials
and/or the use of multiple solutions containing one or more
specific cations and one or more specific anions or alkaline
materials to form a mixed water insoluble reaction product
precipitate on the silver surface. Silanes are conventionally used
to provide adherence between the silver coating and paint as
described in U.S. Pat. No. 5,240,776, supra, and such techniques
can be used herein. It is understood that each solution contains
both cations and anions or hydroxyl ions for electron neutrality
with the condition that the specific cation of one solution and the
specific anion or hydroxyl ion of the other solution react to form
a water insoluble precipitate.
45. The reaction product water insoluble precipitate coated silver
film is then preferably rinsed and may be painted using
conventional paints and techniques or covered with a polymer
coating to provide a finish surface which will further protect the
mirror against abrasion and corrosion of the silver film.
46. It will be appreciated by those skilled in the art that since
the mirror making process comprises a series of steps, each step is
important to the mirror making process and that an improvement to
any of the steps will improve the mirror making process and the
resulting mirror. It is an important feature of the invention that
the precipitate producing step may be used on existing mirror
production lines since it replaces the environmentally undesirable
copper layering process step with the environmentally friendly
precipitate coating step. The mirrors made according to the
invention have enhanced mirror properties, particularly enhanced
resistance to corrosion and, consequently, a longer useful mirror
life.
47. The mechanism of how the precipitate coats the silver surface
is not known, but a substantial amount of precipitate coats the
silver surface in a sufficient quantity to be detected with
chemical spot checks of the surface (e.g., by a chemical spot test
described in Example 1) or by EDS (Energy Dispersive Spectroscopy)
equipment. For example, using a SnCl.sub.2 solution (62.3 mM) and
NaOH solution (140 mM) and spraying the solutions so they meet at a
silver surface, it has been determined that the thickness of the
formed precipitate is between 86 .ANG. and 114 .ANG.. This is
significantly thicker than a single molecular monolayer resulting
from contacting a silver surface with a single solution of a metal
salt. A single molecular monolayer would have a thickness generally
less than 2 .ANG..
48. Using SnCl.sub.2/NaOH EDS (Energy Dispersive Spectroscopy)
found 2 to 3 mg per sq. ft, which is ({fraction (3/70)}) 100%
Sn/Ag=4.2% Sn/Ag. 70mg/ft.sup.2 of silver is typical on a mirror.
This is about 100 times more tin on the surface than when the
silver surface is treated with only a SnCl.sub.2 solution as in
U.S. Pat. No. 5,240,776, supra. That the EDS method compared the
entire silver film to tin content is evident from the fact that Si,
Ca, Mg, Na were detected (i.e., glass components) which means that
the electron beam traveled all the way through the silver film
during the reading process.
49. The following non-limiting examples are provided to illustrate
the method of the invention.
50. The examples have been subjected to different accelerated aging
tests.
51. One indication of the resistance to aging of a mirror
incorporating a metallic film is given by subjecting it to a
copper-accelerated acetic acid salt spray test known as the CASS
Test in which the mirror is placed in a testing chamber at
50.degree. C. and is subjected to the action of a fog formed by
spraying an aqueous solution containing 52 g/l sodium chloride, 0.2
g/l anhydrous cuprous chloride with sufficient glacial acetic acid
to bring the pH of the sprayed solution to between 3.1 and 3.3.
Full details of this test are set out in International Standard ISO
3770. Mirrors may be subjected to the action of the saline fog for
different lengths of time, whereafter the reflective properties of
the artificially aged mirror may be compared with the reflective
properties of the freshly formed mirror. As outlined in ISO 3770,
an exposure time of 120 hours gives a useful indication of the
resistance of a mirror to aging. The CASS Test was performed on 10
cm by 10 cm square (100 square cm) mirror coupons, and after
exposure to the copper-accelerated acetic acid salt spray for 120
hours, each coupon was subjected to microscopic examination. The
principal visible evidence of corrosion was both a darkening of the
silver layer and peeling of the paint around the margins of the
mirror. The extent of corrosion is noted along the entire portion
of all four edges of the coupon, and the mean of these measurements
is calculated.
52. A second indication of the resistance to aging of a mirror
incorporating a metallic film can be given by subjecting it to a
U.S. Federal Specification Salt Fog Test(DD-M-00411C) which
comprises subjecting the mirror in a chamber maintained at
35.degree. C. to a salt fog formed by spraying an aqueous solution
containing 230 g/l sodium chloride. An exposure time of 300 hours
to the Salt Fog Test gives a useful indication of the resistance of
a mirror to aging. The mirrors were subjected to microscopic
examination, and the corrosion present at the margin of the coupon
is measured to obtain a corrosion result in micrometers, in the
same way as in the CASS Test.
EXAMPLE 1
53. The formation of water insoluble precipitates on a silver
coated glass surface was performed by spraying two (2) solutions
together on a sheet about 12 inch.times.24 inch so the solutions
met at the surface and was used to simulate such a treatment step
in a commercial conveyor mirror making system. All samples were
coated with a low lead content mirror backing paint with a measured
dry film thickness of 1.0 to 1.2 mil (0.025 to 0.030 mm) after full
baking to about 130.degree. C. for 2 minutes at that temperature. 6
inch.times.6 inch square samples were then cut from the layer for
testing. The following specific cation containing solutions and
specific containing solutions as shown in Table 1 were used. All
solutions formed a precipitate when combined at the silver surface.
A silver coated glass surface without any treatment was used as one
control and another silver coated glass surface coated with a
conventional copper layer was used as the other control.
1TABLE 1 RUN CATION ANION EDGE FAILURE EDGE FAILURE No. SOLN CONC.
(mM) SOLN CONC. (mM) SALT (.mu.m) CASS (.mu.m) A 2420 700 B 4080
2310 1 SnCl.sub.2 0.062 NaOH 0.14 480 312 2 SnCl.sub.2 0.62 NaOH
1.40 360 216 3 SnCl.sub.2 6.23 NaOH 14.0 390 222 4 SnCl.sub.2 62.3
NaOH 140. 546 150 5 SnCl.sub.2 0.062 Na.sub.2CO.sub.3 0.14 450 126
6 SnCl.sub.2 0.62 Na.sub.2CO.sub.3 1.40 570 210 7 SnCl.sub.2 6.23
Na.sub.2CO.sub.3 14.0 360 180 8 SnCl.sub.2 62.3 Na.sub.2CO.sub.3
140 500 150 9 SnCl.sub.2 3.12 NH.sub.4OH 10 0 270 282 10 SnCl.sub.2
6.23 Na.sub.2HPO.sub.3 6.94 560 282 11 SnCl.sub.2 6.23
(NH.sub.4).sub.2CO.sub.3 10.4 342 282 12 SnF.sub.2 4.78
Na.sub.2SiO.sub.3 9.01 320 180 mM = millimolarity A = Control with
copper coating B = Control with silver only
54. Run No. 4 was measured for the amount of tin present on the
silver surface by SEM/EDS (Scanning Electron Microscopy/Energy
Dispersive Spectroscopy). The amount of tin metal was determined to
be between 2.40 mg/ft.sup.2 and 3.22 mg/ft.sup.2. The total weight
of the tin precipitate would of course be higher based on the
molecular weight of the insoluble tin precipitate components formed
on the silver surface.
55. Spraying of the panel with only a SnCl.sub.2 solution did not
leave any detectable tin on the silver surface using the spot test
or EDS method.
56. The results clearly show the enhanced corrosion resistance of a
reaction product precipitate coated silver mirror treated according
to the invention.
57. Several additional spot tests as described below were performed
on the tin precipitated silver surface (without paint) of Run 4.
The first test uses two solutions: concentrated sulfuric acid
diluted 1:75, and a silver nitrate solution, 2 mg/mL. Two drops of
the silver solution were placed on a sample with the tin ppt. and a
silver only panel. Then two drops of acid were added to the silver
solution treated surface. Almost instantly, the tin precipitate
sample formed an intricately divided ring-like pattern with a
brown-black tint on the film surface. The silver only sample formed
a very slight milky-white haze on the film surface. This test
showed that the presence of the tin precipitate can be confirmed.
The spot test results in the dissolution of the protective
precipitate which reacts with the silver nitrate to form
black/brown colloidal silver, i.e.,
Sn.sup.2++2 Ag.sup.+.fwdarw.2 Ag.sup.0+Sn.sup.4+
58. The second qualitative test was similar in nature. The same
silver solution was used, but nitric acid was used instead of the
sulfuric. The nitric acid was again diluted 1:75. Two drops of
silver solution were placed both on the tin precipitate panel and
on the silver only panel. Then two drops of nitric acid were
applied to the silver on each panel. The solutions were allowed to
react for 20-30 seconds. The solutions were rinsed from the panels
with deionized water. The panel with the tin precipitate remained
intact while the silver on the untreated panel delaminated from the
glass. This test showed that the tin reaction product compound
present on the silver film protected the integrity of the silver
film in the presence of materials known to delaminate silver from
glass.
59. The third test is described in the book "Mirrors" by Bruno
Schweig, published by Pelham Books, London in 1973. A crystal of
diatomic iodine is placed on a surface, such as a benchtop.
Spacers, such as matchsticks are used to hold silver panels 1-2 mm
above the crystals, without touching them. The silver layer of the
panels face downward toward the crystals. After a few minutes, the
iodine vapors convert the silver metal to silver iodide, leaving a
transparent hole in the silver film. On the silvered panel with the
tin precipitate, this hole did not form as readily, and the pattern
of rings left on the silver surface was very deformed and obscured.
There were small areas of the silver that the tin precipitate
protected completely from the reactive vapor. The tin precipitate
appeared to inhibit the reaction between the silver and the iodine
vapor.
EXAMPLE 2
60. Example 1 was repeated using the cation and anion solutions
shown in the following Table 2.
2TABLE 2 RUN CATION ANION EDGE FAILURE EDGE FAILURE No. SOLN. CONC.
(mM) SOLN. CONC. (mM) SALT (.mu.m) CASS (.mu.m) 13
Ag(NH.sub.3).sub.2NO.sub.3 1.47 (NH.sub.4).sub.2CO.sub.3 10 4 675
96
61. These results are comparable to the controls of Example 1 and
show the increased corrosion resistance of the silver coated glass
treated according to the invention.
EXAMPLE 3
62. Example 1 was repeated using the cation and anion solutions
shown in the following Table 3.
3TABLE 3 EDGE EDGE FAILURE FAILURE RUN CATION CONC. ANION CONC.
SALT CASS No. SOLN. (mM) SLN. (mM) (.mu.m) (.mu.m) 14
Ce.sub.2(SO.sub.4).sub.3 3.01 Na.sub.2CO.sub.3 14.1 510 1450 15
Ce(NO.sub.3).sub.3 1.15 Na.sub.2HPO.sub.3 4.62 420 72 16
Ce(NO.sub.3).sub.3 1.15 Na.sub.2HPO.sub.3 2 31 222 90
63. These results are comparable to the controls of Example 1 and
show the increased corrosion resistance of the silver coated glass
treated according to the invention.
64. While the present invention has been particularly described, in
conjunction with a specific preferred embodiment, it is evident
that many alternatives, modifications and variations will be
apparent to those skilled in the art in light of the foregoing
description. It is therefore contemplated that the appended claims
will embrace any such alternatives, modifications and variations as
falling within the true scope and spirit of the present
invention.
* * * * *