Note: Descriptions are shown in the official language in which they were submitted.
CA 02063858 2001-02-02
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Preventing Silver Ion Migration In Enamels
This invention relates to glass frits which, screen-
printed on their own, in an overlayer or in an enamel, are
capable of stopping the heat-activated migration of Ag+ ions
between a support of float glass and a silver-based resistive
layer.
Enamel composition:a for glass are well known among
experts:
They are generally formed after firing by a vitreous
phase supplied by the frit(s) used and a coloring and
opacifying crystalline pigment phase to which other compounds
may be added to obtain various effects. They are suitable
for various application_~ and may be used inter alia for
forming borders at the periphery of sheets of flat float
glass which are thermoformed and then tempered for use as
glazing in motor vehicles.
Originally used with the sole object of protecting the
adhesive used to fix they sheets of glass (windscreens, rear
windows, side windows) t:o bodywork against the ultraviolet
radiation in sunlight, these strips of black or grey enamel
were subsequently the sL~bject of aesthetic research, attempts
having been made in particular to use these enamels to
conceal the lateral silver collectors of heated rear windows
and the welding spots of the connecting terminals of the
current supply wires. hfow, it has been found that, after the
heat treatment required for curving and tempering, none of
the enamels of conventional composition are capable of
completely concealing the silver collectors applied in
superposed form and heated with the enamel or the above-
mentioned weld spots.
2~~~~ ~~
2
Various studies have demonstrated the existence of heat-
activated transport of the Ag+ cations from the resistive
layer through the enamel to the enamel/glass interface, thus
revealing the presence of collectors behind the enamel by an
inter_se colorimetric contrast.
It is known from US-PS 4,837,383 that it is desirable to
prevent the migration of silver when using an enamel
consisting of a glass flux based on a glass frit, pigments,
elemental aluminium and/or lithium oxide and a vehicle for
paste formation. Published European patent application EP-A
0 377 062 claims improvements in the silver-based opacity of
enamels by the addition before paste formation of silicon,
boron, carbon, lead and/or silver in their elemental forms.
For obvious reasons of cost (silver), special operating
conditions necessitated by the introduction of highly
reducing elements, such as silicon or boron, stability to
acids (lithium) or toxicity (lead), additions of the type
mentioned have not proved entirely satisfactory. The enamels
developed with additions such as these in conjunction with
the present state-of-the-art in this field show that, despite
the aesthetic interest there is in completely concealing the
silver collectors by enamel, such solutions limit the range
of colours currently useable by the automotive industry to a
few blacks and to a few greys.
In addition, the quality of the masking effect obtained
is largely influenced by the firing cycle as a whole.
Accordingly, the present invention provides an enamel
barrier layer for stopping the migration of silver cations
from the collectors to the glass during the firing of a
combination of flat glass with an enamel barrier layer and,
CA 02063858 2001-02-02
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above this enamel layer,, a resistive layer of silver which
would have the disadvani:ages of the systems mentioned
above.
Thus, by eliminating the need to form a paste of the
additives mentioned above, the present invention offers a
general solution to the problems discussed in the
foregoing.
More particularly, the invention provides a glass
frit, comprising 0.05 to 15.0 mol% of sulphur, a sulfide or
a mixture thereof.
More specifically, the present invention provides a
glass frit, essentially free of lead, for forming an enamel
capable of preventing Ac~+ ion migration, the glass frit
comprising 15 to 70% by weight zinc oxide, 15 to 40% by
weight silicon dioxide and 5 to 25% by weight boron
trioxide, and further cc>mprising, dissolved in the glass,
0.05 to 15 mol% of at l.e~ast one member of the group
consisting of sulfur and sulfides.
The present invention also provides a glass surface
having deposited thereon an enamel barrier-layer capable of
preventing the migration o:f Ag+ ions, the layer being
formed from a glass frit, and optionally one or more
colorants and/or opacifiers, the glass frit being
essentially free of leaf. and comprising 15 to 70% by weight
CA 02063858 2001-02-02
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zinc oxide, 15 to 40% by weight silicon dioxide and 5 to
25~ by weight boron trioxide, and further comprising,
dissolved in the glass, 0.05 to 15 molo of at least one
member of the group con_~isting of sulfur and sulfides.
The present invention also provides a method of
preventing silver migration into glass, the method
comprising the steps of placing an enamel barrier-layer
formed from a glass fr.it, and optionally one or more
colorants and/or opaci~iers, between sheet glass and a
resistive-layer containing silver, wherein the enamel
barrier-layer is capable of preventing Ag+ ion migration,
and firing at a temperature of from 450°C to 800°C, wherein
the glass frit is essentially free of lead and comprises 15
to 70°s by weight zinc oxide, 15 to 40~ by weight silicon
dioxide and 5 to 25o by weight boron trioxide, and further
comprises, dissolved in the glass, 0.05 to 15 molo of at
least one member of the group consisting of sulfur and
sulfides.
The present invention also provides a composite glass
product comprising sheet glass, an enamel barrier-layer,
and a silver resistive-layer, the composite glass product.
made by a method comprising the steps of placing an enamel
barrier-layer formed from a glass frit, and optionally one
or more colorants and/or opacifiers, between sheet glass
and a resistive-layer containing silver, wherein the enamel
barrier-layer is capable of preventing Ag+ ion migration,
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and firing at: a temperature of from 450°C to 800°C, wherein
the glass fri.t is essent=ially free of lead and comprises 15
to 70% by weight zinc o:~ide, 15 to 40% by weight silicon
dioxide and 5 to 25% by weight boron trioxide, and further
comprises, dissolved in the glass, 0.05 to 15 mol% of at:
least one member of the group consisting of sulfur and
sulfides.
The present invention also provides a composite glass
article comprising a glass support-layer, an enamel
barrier-layer deposited on the glass support layer, and a
silver resistive-layer deposited on the barrier-layer, the
enamel barrier-layer being capable of preventing Ag+ ion
migration, and being formed from a glass frit, and
optionally one or more colorants and/or opacifiers, wherein
the glass frit is essentially free of lead and comprises 15
to 70% by weight zinc o~:ide, 15 to 40% by weight silicon
dioxide and 5 to 25% by weight boron trioxide, and further
comprises, dissolved i:n the glass, 0.05 to 15 mol% of at
least one member of the group consisting of sulfur and
sulfides.
The present invention also provides a process for the
production of a glass fri.t, essentially free of lead, for
forming an enamel capable of preventing Ag+ migration, the
glass frit comprising :15 to 70% by weight zinc oxide, 15 to
40% by weight silicon dioxide and 5 to 25% by weight boron
trioxide, and further comprising, dissolved in the glass,
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0.05 to 15 mol% of at .Least one member of the group
consisting of sulfur and. sulfides, the process comprising
the steps of melting a mixture comprising (a) glass-forming
oxide compounds, or non-oxide compounds which form glass-
forming oxides, corres):~onding to 15 to 70% by weight zinc
oxide, 15 to 40% by weight silicon dioxide and 5 to 25% by
weight boron trioxide, and (b) 0.05 to 15 mol% of at least
one member of the group consisting of sulfur and sulfides,
at a temperature of 1,000° to 1,300°C, to dissolve the
sulfur or the sulfides in the glass, cooling suddenly to
form granules, and grinding the granules to form a frit.
The present invention also provides a glass frit,
essentially free of lead, for forming an enamel capable of
preventing Age- migration,. the glass frit comprising 15 to
70% by weight zinc oxide, 7.5 to 40% by weight silicon
dioxide and 5 to 25% by 'weight boron trioxide, and further
comprising, dissolved i.n the glass, 0.05 to 15 mol% of at
least one member of the group consisting of sulfur and
sulfides, the glass frit being produced by a process
comprising the steps of: melting a mixture comprising (a)
glass-forming oxide compounds, or non-oxide compounds which
form glass-forming oxides, corresponding to 15 to 70% by
weight zinc oxide, 15 t:o 40% by weight silicon dioxide and
5 to 25% by weight boron trioxide, and (b) 0.05 to 15 mo:1%
of at least one member of the group consisting of sulfur
and sulfides, at a temperature of 1,000° to 1,300°C, to
dissolve the =sulfur or the sulfides in the glass, cooling
CA 02063858 2001-02-02
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suddenly to form granules; and grinding the granules to
form a frit.
Preferably, the f:ri_t:s contain zinc sulfide.
In another embodiment of the invention, the frits
contain zinc oxide, preferably in a quantity of 30 to 65%
by weight.
In another embodiment of the invention, the frits
contain coloring metal oxides.
In another embodiment, the invention relates to a
glass frit obtained by fusion of a mixture of oxides and/or
non-oxide compounds (f.lu.orides, carbonates) at temperatures
of 1,000 to 1,300°C, followed by sudden cooling,
characterized in that .it contains from 0.05 to 15 molo
sulfur, zinc sulfide o:r other sulfides.
In another embodiment, the invention relates to the
use of the glass frits to form enamel barrier layers for
stopping the migration of silver from a resistive silver
layer to the
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4
glass during the firing of a combination of sheet glass 4rith
one or more enamel layers, including the enamel barrier
layer, and - above this layer - a resistive silver layer.
It has been found that the addition of 0.05 to 15.0 mal
- ~ sulfur or sulfides (preferably zinc sulfide) to a mixture
of oxides and/or non-oxide compounds for the production of a
glass frit provides the frit with the property of stopping
the heat-activated diffusion within the frit of the Ag+
cations from the collectors of the resistive network based on
silver.
In addition to sulfur and/or sulfides, the frits consist
of a suitable association of oxides in a vitrifiable
composition. The expert knows many different associations of
oxides which, after fusion, give glass frits, see for example
US 4,892,847, EP-A-0 377 062 and EP-A-0 294 502.
The choice of the oxides determines the properties of
the glass frits, for example their softening point, their
thermal expansion coefficient and their stability to bases
and acids. In general, the colorless glass frits contain
oxides selected from:
Si02, A1203, Ti02, ZnO, MgO, Li20,
B203 Bi203 Sn02 Pb0 Ca0 Na20
La203 Zr02 Mn0 Ba0 K20
V205 Mo03
although other oxides, for example P205, Cr203, Fe203, CuO,
NiO, CoO, W03, Ce02, etc. may also be present. According to
2~~~~~~
the invention, frits based on borosilicates, particularll
zinc borosilicates, are preferred. It is for this reason
that they contain silicon dioxide in a quantity of 15 to 40%
5 by weight and boron trioxide in a quantity of 5 to 25% by
weight and preferably 15 to 70% by weight and, above all, 30
to 65% by weight zinc oxide.
The introduction of sulfur during dry-mixing of the
composition before melting may be carried out either in the
1o form of flowers of sulfur or in the form of sulfides,
although dissolution and retention in the structure of the
glass vary according to the method of introduction selected.
The advantage of ZnS over elemental sulfur and the other
sulfides is that it remains stable up to the fusion of the
eutectics of the oxides of the mixture and it is only within
this molten mixture that ZnS is progressively dissolved.
Accordingly, the elimination of S02 and the loss of elemental
sulfur are avoided with ZnS. In addition, it has been found
that zinc is an element which promotes stabilization of the
sulfur in the vitreous atomic structure.
In principle, other sulfides than ZnS, for example
sulfides of alkali metals, alkaline earth metals or of Mn,
Fe, Co, Ni, Cu, Cd, Sn, Pb, Sb, Bi, may be introduced into
the mixture to be melted. However, it should be pointed out
that some of these sulfides, such as for example MnS, FeS,
Cu2S, CdS, PbS, color the vitreous mass. In addition, the
introduction of these sulfides in large quantities can cause
liquid-phase separation within the molten mass. It is
pointed out that the presence of elemental sulfur in the
frits according to the invention is necessary for the
interaction with the Ag+ rations and that, introduced in the
form of sulfides, these sulfides are at least partly
dissociated (ZnS begins to dissociate at around 900°C) during
6
melting of the fusible mixture so that, thereafter, it is
only the sulfur which reacts with the Ag+ rations in an
enamel barrier layer containing the vitreous structure of
such frits.
The principle of the process for producing the glass
frits is known to the expert. To make the glass frits
according to the invention, a mixture consisting of (a)
oxides and/or compounds which form oxides by melting into a
vitrifiable composition corresponding to the composition of
the frit and (b) one or more compounds selected from sulfur,
zinc sulfide and other sulfides in a quantity of 0.05 to 15
mol-% of the mixture is melted at temperatures of 1,000 to
1,300oC, after which the molten glass is suddenly cooled in
water.
Subsequent grinding of the granules obtained provides
the ready-to-use frit powder.
The mixture mentioned above is preferably composed of
oxides, although alkali metal and alkaline earth metal oxides
are often introduced in the form of carbonates. Melting is
carried out conventionally in a refractory ceramic crucible
or in a furnace having a suitable lining. The fragments of
the frits are ground after cracking (preferably 90% smaller
than 20 ' 10-6m) by known grinding techniques for glass
frits.
By virtue of their ability to stop the migration of
silver, the glass frits according to the invention may be
used to form enamel barrier layers. These enamels may be
colorless or colored, transparent or opaque. The enamel
barrier layers may contain other frits as the vitrifiable
compound although the essential constituent is always a frit
2~~~~~~
containing sulfur. The enamel barrier layers according to
the invention stop the migration of silver during firing of
the combinations of sheet glass with one or more enamel
layers, including the barrier layer, and - above the last
enamel layer - a resistive silver layer. The function which
the vitreous structures of the enamel barrier layer perform
in stopping the migration of silver is also effective at the
temperatures required for curving and tempering the sheets of
glass. The enamel barrier layer may be situated between the
glass, for example a sheet of flat glass, and a resistive
network based on silver and conceals the silver collectors if
the enamel is colored and preferably opaque. Now, the
barrier layer may be situated between an opaque colored
enamel applied to the glass and a resistive silver layer.
The enamel of the unfired barrier layer is formed by one
or more frits according to the invention, colorants and/or
opacifiers where a decorative effect is required and a
vehicle for paste formation, for example a screen-printing
oil based on solvents, natural resins and/or thermoplastics.
The vehicle for paste formation is selected according to the
final application envisaged. It must provide for effective
suspension of the particles and must be completely consumed
during firing and tempering. The enamel of the barrier layer
and/or a subjacent enamel of conventional composition have to
meet certain requirements and, in the case of decorative
enamels, must have above all a sufficient pigment content to
satisfy the color and opacity requirements.
The enamels of the barrier layers according to the
invention are applied by screen printing either directly or
by way of a transfer or another intermediate support. The
average thickness per layer after firing is between 5 ' 10-6m
and 50 ' 10-6m.
~0~3~~~
8
In cases where a barrier layer which is colorless or
colored in the mass is interposed between a crude and dry
enamel which itself is applied by screen printing and the
resistive silver layer, disorientation of the meshes of the
screen printing cloths used for one or other of the
applications will be favorable. In this way, firing of the
combination of glass (with all the enamel layers) and the
resistive silver layer (for example 60 to 80% by weight
silver) is carried out in a single operation.
The heat treatment, i.e. firing followed by tempering, of the
combinations of glass/enamel barrier layer/resistive silver
layer or glass/conventional enamel layer/enamel barrier
layer/resistive silver layer is carried out in various types
of furnace, according to the intended application of the
enamelled glass. The firing temperature is also determined
by the composition of the frit(s) used in the enamels. In
general, the firing temperature varies from 450~C to 8000C
and preferably from 550oC to 750oC.
In tests (see Example 10) it was found that, in the
enamels described in DE-OS 38 32 937 and DE-OS 38 38 196
which are formed by a conventional frit, a vehicle and, in
addition, a sulfide of copper, zinc, iron, tin or aluminium,
there was no favorable action in stopping the Ag+ cations,
i.e. there was no effective interaction between sulfide and
Ag+.
The S - Ag+ interaction and the resulting arrest of the
Ag+ cations is thus a quality unique to enamels of which the
frits contain elemental sulfur and/or a sulfide dissolved in
the vitreous mass. Frits which do not contain dissolved
sulfur and which are applied on their own or in association
9
cations during their heat-activated migration towards the
interface with the glass. In addition, the direct
introduction of sulfides into the enamel, as in the documents
cited above, i.e. as an associated pigment charge, causes a
significant colorimetric drift whereas the use of a frit
according to the invention (containing sulfur) causes no such
drift.
The invention is illustrated by the following Examples
in conjunction with the accompanying drawing, in which:
Figure 1 shows the X ray diffraction spectrum obtained
with the frit of Example 3.
Examples 1 to 4
Frits having the following composition were prepared for
colorless barrier layers:
Composition 1 2 3 4
of the frit
-o
mol
___________ ________________________________________
ZnS 1.98 3.99 4.82 5.00
Si02 21.86 34.99 19.76 29.00
B203 12.64 20.08 11.42 18.00
Zn0 63.52 30.09 64.00 35.45
Na20 6.30 6.00
Ba0 4.55 4.65
Zr02 1.90
~0~3~58
Dry mixing of the oxides except for Na20 and BaO, which were
added with Na2C03 and BaCOg, melting at 1,150 to 1,200oC,
quenching in water, grinding, stowing. To obtain the enamel
5 barrier layer, the frits were made into a paste with a
vehicle and the enamel was applied by screen printing.
Examples 5 and 6
Introduction of the sulfur in the form of flowers of
sulfur or cadmium sulfide.
10 Composition of the 5 6
mixture for melting
mol°%
S 15.00 -
CdS - 6.04
S102 18.95 20.95
B203 10.97 12.11
Zn0 55.08 60.89
___________________________________________________
The frits were produced in the same manner as in
Examples 1 to 4.
Example 7
The frit may be pigmented as such without affecting its
barrier properties. In Example 7, the dissolution of a black
colored inorganic pigment of the copper chromite spinal type
enables a brown frit to be obtained with its barrier
properties intact. Dry mixing of the frit of Example 2
(85.36% by weight) with a black spinal pigment (14.64% by
11
weight), melting, quenching, grinding and stowing to obtain
an intermediate frit.
The brown frit consists of 70% by weight of the frit of
Example 1 and 30% by weight of the intermediate frit
mentioned above. To obtain the enamel for a barrier layer:
disintegration of the two frits in alcohol to form a
homogeneous mixture, stowing, paste formation.
Examples 8 and 9
Black and grey opaque enamels masking the silver collectors.
Composition of the enamel 8 9
by weight
Frit of. Example 4 75 77.5
Black pigment, copper chromite
spinel type 25 12.5
Titanium dioxide 10.0
Color of the enamel Black Grey
____________________________________________________
Disintegration of the compounds,in alcohol, stowing and paste
formation for application as a barrier layer.
lz
Example 10
(for comparison with DE-OSS 38 32 nd 38 38 196)
937 a
Tests for introduction of sulfides the enamel
into
(composition in parts by weight):
a b b d a f
Vitreous enamel
powder VA 641
(DPC LIMOGES) 7.2 7.2 7.2 7.2 7.2 7.2
ZnS (ALDRICH) 1.6
ZnS (PROLABO) 1.6
CuZS 1. 6
FeS 1.6
CdS 1.6
MnS 1.6
VEHICLE MX 54 2.8 2.8 2.8 2.8 2.8 2.8
(DPC LIMOGES)
COLOUR OF BLUISH BLUISH DEEP BROWN- GREEN GREEN
ENAMEL GREY GREY GREY- ISH GREY GREY
RED GREY
COLORIMETRIC
DIFFERENCES
D E
IN RELATION 13.40 10.11 6.15 14.80 5.30 11.40
TO THE ENAMEL
VA 641 AS
REFERENCE
Application of the enamels to glass mm); superposition
(4 of
the enamelled glass with the silver
paste SP 1230 (Degussa
AG): vertical firing and tempering 3.5 minutes at 680oC.
for
None of the enamels of Example 10
is effective in stopping
the migration of the silver.
Ba0 4.55 4.65
