U.S. patent application number 10/944909 was filed with the patent office on 2005-02-17 for soda-lime silica glass compositions and applications.
This patent application is currently assigned to SAINT-GOBAIN GLASS FRANCE. Invention is credited to El Khiati, Nathalie, Gy, Rene, Le Bourhis, Eric.
Application Number | 20050037912 10/944909 |
Document ID | / |
Family ID | 27217212 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050037912 |
Kind Code |
A1 |
El Khiati, Nathalie ; et
al. |
February 17, 2005 |
Soda-lime silica glass compositions and applications
Abstract
The subject of the invention is a glass composition of the
silica-soda-lime type, intended for the manufacture of substrates
or sheets, the said glass composition having a .phi. coefficient of
between 0.50 and 0.85 N/(mm.sup.2..degree. C.) and a working point
of less than 1200.degree. C.
Inventors: |
El Khiati, Nathalie; (Deuil
Ia Barre, FR) ; Gy, Rene; (Bondy, FR) ; Le
Bourhis, Eric; (Sarcelles Village, FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
SAINT-GOBAIN GLASS FRANCE
Courbevoie
FR
|
Family ID: |
27217212 |
Appl. No.: |
10/944909 |
Filed: |
September 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10944909 |
Sep 21, 2004 |
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09242803 |
Feb 24, 1999 |
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09242803 |
Feb 24, 1999 |
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PCT/FR98/00508 |
Mar 12, 1998 |
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Current U.S.
Class: |
501/72 ;
501/70 |
Current CPC
Class: |
C03C 23/007 20130101;
C03C 3/083 20130101; C03C 4/00 20130101; C03C 3/087 20130101; C03B
27/00 20130101 |
Class at
Publication: |
501/072 ;
501/070 |
International
Class: |
C03C 003/078; C03C
003/087 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 1997 |
DE |
197 10 289.1 |
Apr 30, 1997 |
FR |
97/05364 |
Jun 17, 1997 |
FR |
97/07521 |
Claims
1-18. (Canceled).
19. A silica-soda-lime glass composition having a .phi. coefficient
of between 0.5 and 0.85 N/(mm.sup.2..degree. C.) and a working
point of less than 1200.degree. C.
Description
[0001] The invention relates to silica-soda-lime glass compositions
suitable for being converted into a ribbon of glass from which may
be cut sheets which, in particular after treatment, exhibit heat
resistance.
[0002] Such sheets may more particularly be used for producing
fireproof glazing panels or for serving as substrates for the
manufacture of plasma screens, electroluminescent screens and
cold-cathode screens (field-emission display).
[0003] With regard more particularly to glazing panels which are
fire resistant according to the G fire resistance classes, these
consist of a thermally toughened sheet of glass and have properties
of a safety glass.
[0004] Glazing panels fire resistant in accordance with the G fire
resistance classes, together with their frames and their fittings,
must offer resistance, in a fire withstand test according to the
standard DIN 4102 or to the standard ISO/DIS 834-1, for a certain
time, to the passage of the fire and smoke. During this time, the
glazing panels must neither break, under the effect of the stresses
which occur as a result of the temperature gradients between the
surface of the glazing panel in contact with the heat and the
embedded edge, nor exceed their softening point, since they would
lose their stability and would thus expose the opening. They are
ranked in the fire resistance classes G 30, G 60, G 90 or G 120
depending on the time in minutes for which they withstand fire.
[0005] In general, fire-resistant glazing panels are held in frames
which protect, to a greater or lesser extent, the edge of the said
glazing panels from the effect of the heat. The temperature
gradient which thus occurs between the middle of the glazing panel
and the edge generates considerable tensile stresses in the
marginal region and results in the destruction of the glazing
panels if special measures are not taken to compensate for these
tensile stresses. These measures consist of thermally toughening
the glazing panels, this toughening making it possible to induce
large initial compressive stresses in the marginal region. The
thermal toughening gives the glazing panel additional properties of
a safety glass when the toughening is carried out in such a way
that, should the glazing panel break, it would do so by fragmenting
into tiny pieces.
[0006] The initial stress state is usually determined by means of
the flexural/tensile strength obtained by the toughening operation,
in accordance with the standard DIN 52303 or to the standard EN
12150. Experiments have in this case shown the need to guarantee a
flexural/tensile strength of at least 120 N/mm.sup.2 so that the
glazing panel can withstand the tensile stresses generated by the
temperature gradients at the edge. Given that untoughened glazing
panels have a basic flexural/tensile strength of approximately 50
N/mm.sup.2, this means that it is necessary to increase this
strength, by toughening, by at least 70 N/mm.sup.2. The value of
this increase in the flexural/tensile strength corresponds directly
to the value of the initial compressive surface stresses.
[0007] It is also possible to increase the fire resistance time by
increasing the depth of insertion of the glazing panel in the
frame. In the case of a flexural/tensile strength of the glazing
panel of 120 N/mm.sup.2 and an insertion depth of 10 mm, the
glazing panel conforms, for example, to the fire resistance class G
30, while an insertion depth of 20 mm allows it to achieve the fire
resistance class G 90.
[0008] Glazing panels made of the usual float glass
(soda-lime-based silica glass) may be suitably toughened by means
of conventional toughening plants, given that these glass
compositions have relatively high thermal expansion coefficients,
greater than 85.times.10.sup.-7 K.sup.-1. The usual float glass
allows flexural/tensile strengths possibly ranging up to 200
N/mm.sup.2 to be achieved. Under the effect of the tensile stresses
generated by the temperature gradients, the glazing panels
consequently do not break if the insertion depth is approximately
10 mm, but they lose their stability because of their relatively
low softening temperature of approximately 730.degree. C. Toughened
glazing panels made of float glass therefore conform, under
standard installation conditions, at the very most to the fire
resistance class G 30.
[0009] However, monolithic glazing panels of fire resistance class
G 60 and higher classes are also known. These glazing panels
consist of glass compositions having a high softening point of
greater than 815.degree. C. and consequently have a long resistance
time in a fire withstand test. In this case, borosilicate- and
aluminosilicate-based heat-resistant glasses prove to be
particularly suitable. However, these types of glass must also be
toughened thermally in order to be able to withstand the high
tensile stresses which occur in the marginal region in a fire
withstand test.
[0010] The use of thermal toughening for fireproof glazing panels
whose glass compositions are based on heat-resistant borosilicate
or on heat-resistant aluminosilicate is known from the documents DE
2,313,442 B2 and U.S. Pat. No. 3,984,252. According to these
documents, only suitable for toughening are glasses for which the
product of the thermal expansion a and the modulus of elasticity E
reaches 1 to 5 kp.cm.sup.-2..degree. C..sup.-1, i.e. borosilicate-
or aluminosilicate-based glasses having a thermal expansion of
.alpha..sub.20-300=30 to 65.times.10.sup.-7.degree. C..sup.-1.
However, the necessary toughening at the edge of these glazing
panels cannot be carried out by means of conventional
air-toughening plants but requires a special process in which the
glazing panels are placed, during the heating, between slightly
smaller ceramic tiles in such a way that the edge of the glazing
panel extends beyond the ceramic tiles and is therefore cooled more
rapidly, while the middle of the glazing panel cools more slowly
due to the effect of the ceramic tiles. The necessary toughening at
the edge may, to be sure, be achieved in this way, but the glazing
panels thus manufactured do not have any safety-glass
properties.
[0011] It is known from the document EP-A-638,526 to use, for the
manufacture of monolithic fireproof glazing panels, glass
compositions which have a thermal expansion coefficient .alpha. of
between 30 and 60.times.10.sup.-7 K.sup.-1, a .phi. coefficient of
between 0.3 and 0.5 N/(mm.sup.2.K), a softening point (=temperature
for a viscosity of 10.sup.7.6 poise) of greater than 830.degree. C.
and a working point (=temperature for a viscosity of 10.sup.4
poise) of between 1190.degree. and 1260.degree. C. The .phi.
coefficient or specific thermal stress is the specific parameter of
the glass calculated from the thermal expansion coefficient
.alpha., the modulus of elasticity E and Poisson's ratio .mu.
according to the formula .phi.=.alpha..E/(1-.mu.). Glazing panels
having these physical properties may acquire, in a conventional
air-toughening plant, both the initial compressive stresses
necessary at the edge and the toughening stresses exerted over the
entire surface and necessary for obtaining fragmentation into tiny
pieces, so that no particular measurement is necessary in respect
of the toughening operation and so that the manufacturing process
is thereby considerably simplified. However, glazing panels having
these physical properties necessarily contain B.sub.2O.sub.3,
Al.sub.2O.sub.3 and ZrO.sub.2 in quantities which complicate the
melting process and the conversion process. These glazing panels
thus cannot be manufactured using the floating process which has
proved to be exceptionally economical, given that their conversion
point is too high and that the melting furthermore requires special
measures.
[0012] Borosilicate-based glass compositions are known, from the
document FR-2,389,582, which are provided, to be sure, for use in
fireproof glazing panels which, because of their relatively low
conversion point, may melt during the floating process and also be
toughened by means of conventional toughening plants. However,
these glasses contain from 11.5 to 14.5% of B.sub.2O.sub.3 and also
have physical properties similar to those of the glasses known from
the document EP-A-638,526. Even in the case of these glasses, the
initial compressive stresses and the flexural or the tensile
strength which may be achieved by air toughening are limited to
relatively low values and these glasses also have the known
difficulties and drawbacks when melting borosilicate-based
glasses.
[0013] With regard to the manufacture of emissive screens of the
plasma-screen type, the substrate is subjected to several heat
treatments for the purpose of stabilizing the dimensions of the
said substrate and of fixing a series of layers of various
compounds, such as enamels, deposited on its surface. Fixing these
relatively thick layers requires the substrate to be heated to
temperatures greater than 550.degree. C. If the expansion
coefficient of the silica-soda-lime glass used is of the same order
of magnitude as that of the compounds deposited on its surface, its
temperature withstand is insufficient and it is necessary to place
it on a ground slab during the heat treatments in order to avoid
any deformation.
[0014] Novel families of glass compositions have been developed and
described in the patent WO-96/11887 so as to mitigate these
drawbacks, especially so as to be able to manufacture sheets or
substrates undergoing virtually zero deformation during heat
treatments of the order of 550 to 600.degree. C. and capable of
generating, by thermal toughening, stress levels comparable to
those obtained with standard silica-soda-lime glass.
[0015] However, it appears that these glasses may undergo breaks
during the deposition of certain layers, including when the methods
of depositing these layers result in local temperatures of the
glass which do not exceed about a hundred degrees Celsius.
[0016] The inventors have thus sought to remedy these breaks,
which, albeit infrequent, disrupt the manufacturing plants.
[0017] The subject of the invention is novel glass compositions
allowing the manufacture of substrates whose deformation remains
virtually zero when they are subjected to temperatures of about
600.degree. C. and which do not deteriorate when depositing layers
on their surface, i.e. which do not break immediately and which do
not have flaws which may lead to an eventual break.
[0018] The subject of the invention is also novel glass
compositions for the manufacture of glazing panels which are fire
resistant according to the G fire resistance classes which, on the
one hand, may be thermally toughened by means of conventional
plants and which, on the other hand, can be melted without any
economic and/or technological problems and which can be converted
into flat glass using the float process.
[0019] The subject of the invention is also glass compositions
which allow the manufacture of glazing panels whose appearance and
optical properties are comparable to those of known float
glass.
[0020] These objects are achieved according to the invention by a
glass composition intended for the manufacture of thermally stable
substrates, the said glass composition having a thermal stress
factor or .phi. coefficient of between 0.5 and 0.85
N/(mm.sup.2..degree. C.) and a working point or conversion point
(viscosity=10.sup.4 dpa.s) of less than 1200.degree. C.
[0021] As mentioned previously, the .phi. coefficient is defined
according to the relationship:
.phi.=.alpha..E/(1-.mu.)
[0022] where
[0023] .alpha.: expansion coefficient
[0024] E: modulus of elasticity
[0025] .mu.: Poisson's ratio.
[0026] The modulus of elasticity and Poisson's ratio are determined
by the following test: a glass test piece having the dimensions
100.times.10 mm.sup.2 and a thickness of less than 6 mm is
subjected to 4-point bending in which the outer bearing points are
separated by 90 mm and the inner bearing points 30 mm. A strain
gauge is bonded to the centre of the glass plate. The main strains
(in the length of the plate and in its width) are deduced
therefrom. The stress applied is calculated from the force applied.
The equations between the principal stress and strains allow the
modulus of elasticity and Poisson's ratio to be determined.
[0027] According to a preferred variant of the invention, the glass
compositions according to the invention have a softening point
(viscosity=10.sup.7.6 poise) of greater than 750.degree. C. Also
preferably, the working point of the glass compositions according
to the invention is less than 1190.degree. C.
[0028] In an advantageous variant of the invention, the thermal
expansion coefficient .alpha..sub.20-300 of the glass compositions
is between 60 and 88.times.10.sup.-7.degree. C..sup.-1 and
preferably less than 85.times.10.sup.-7.degree. C..sup.-1.
[0029] Also preferably and more particularly in the case of the
production of fireproof glazing panels, the glass composition
according to the invention satisfies the relationship:
.phi..sup.2.c/a<2 MPa.sup.2/.degree. C..sup.2.
[0030] The "c/a" value is defined by the brittleness test described
below: the glass is firstly annealed so as to remove the residual
stresses. The glass is heated at its annealing point for 1 hour and
then cooled at 2.degree. C./min to ambient temperature. The glass
test piece to be tested is indented with a 200 g load for 30
seconds at ambient temperature. The diagonals of the Vickers
impression and the size of the radial cracks (Lawn and Marshall, J.
Am. Cer. Soc. 62, 347-350 (1979); Sehgal et al., J. Mat. Sci. Let.
14, 167-169 (1995)) are measured 72 hours after indentation. The
c/a ratio, i.e. length of the radial cracks/semi-diagonal, is
measured on 10 indentations so as to obtain sufficient
statistics.
[0031] Preferably, the glass composition according to the invention
satisfies the relationship:
.phi..sup.2.c/a>0.70 MPa.sup.2/.degree. C..sup.2.
[0032] Also preferably, the product .phi..sup.2.c/a is greater than
1 and preferably less than 1.8.
[0033] In one embodiment of the invention and more particularly in
the case of the production of substrates for plasma screens, the
composition has a strain point of greater than 570.degree. C. and
preferably greater than 600.degree. C. More particularly also for
applications of the plasma-screen type, the .phi. coefficient is
between 0.75 and 0.85 and preferably less than 0.8.
[0034] For fireproof glazing panel applications, the .phi.
coefficient is advantageously less than 0.8 and preferably greater
than 0.7.
[0035] The inventors have been able to demonstrate that glasses
having the properties in accordance with the invention may not only
melt relatively well but, in addition, are particularly suitable
for the manufacture of monolithic fireproof glazing panels insofar
as, even in the case of conventional air toughening, they have a
flexural/tensile strength markedly greater than that of the known
borosilicate- and aluminosilicate-based glasses for the manufacture
of fireproof glazing panels. By virtue of their higher thermal
expansion coefficient and their higher .phi. coefficient, it is
possible in fact to obtain, by means of standard toughening plants,
flexural/tensile strengths markedly greater, i.e. markedly greater
initial compressive stresses, so as to increase substantially the
resistance to the temperature difference which may exist between
the embedded cold edge and the hot centre of the glazing panel.
Furthermore, it was apparent that the resistance of these glasses
was entirely sufficient for meeting the fire resistance class G 30
even in the case of a depth of insertion in the frame of 10 mm.
However, the glasses used in accordance with the invention also
make it possible to achieve superior fire resistance classes of G
60, G 90 or even G 120 when, as required, thicker glazing panels
are used and a frame is used in which they are embedded more
deeply, i.e. a frame which covers the edge of the glazing panel to
a greater extent, for example up to 25 mm.
[0036] According to a preferred embodiment of the invention, the
glass composition contains the constituents below in the following
proportions by weight:
1 SiO.sub.2 55-75% Al.sub.2O.sub.3 0-7% ZrO.sub.2 0-8% Na.sub.2O
5-10% K.sub.2O 0-8% CaO 8-12%.
[0037] According to another embodiment of the invention, the glass
composition contains the constituents below in the following
proportions by weight:
2 SiO.sub.2 55-75% Al.sub.2O.sub.3 0-7% ZrO.sub.2 0-8% Na.sub.2O
2-8% K.sub.2O 2-8% CaO 4-11% MgO 0-4%.
[0038] According to another variant and more particularly in the
case of the manufacture of substrates for emissive screens, the
glass composition according to the invention has a .phi.
coefficient of less than 0.84 N/(mm.sup.2..degree. C.), its
strain-point temperature being greater than 507.degree. C. and its
electrical resistivity being such that log .rho..sub.(25.degree.
C.) is greater than 6.6.
[0039] It is commonly accepted that glass no longer behaves in a
viscous manner below a characteristic temperature called the
strain-point temperature which corresponds to a viscosity of the
order of 10.sup.14.5 poise. This temperature is therefore a useful
reference point for evaluating the temperature withstand of a
glass.
[0040] It has proved to be the case in tests that, in particular,
the combination of these values of strain-point temperature and of
the .phi. coefficient allow the production of a substrate or sheet
which is thermally stable and undergoes no deterioration or break
during the layer-deposition treatment phases. The electrical
resistivity values limit, in particular, the diffusion into the
glass of, for example, silver ions contained in the layers
deposited on the surface of the substrate.
[0041] According to a preferred embodiment of the invention, the
expansion coefficient of the glass composition is between 65 and
88.times.10.sup.-7.degree. C..sup.-1. Such values are particularly
advantageous for their compatibility with those of the glass frits
normally used for producing, for example, barriers for plasma
screens.
[0042] Also preferably, the expansion coefficient is between 80 and
85.times.10.sup.-7.degree. C..sup.-1.
[0043] A more particularly advantageous glass composition according
to the invention, in particular in terms of thermal-break
resistance and cost, has a .phi. coefficient of less than 0.8
N/(mm.sup.2. C) and preferably greater than 0.7
N/(mm.sup.2..degree. C.).
[0044] Also to decrease the cost of the glass composition, the
latter advantageously has a strain-point temperature of less than
590.degree. C. and preferably less than 580.degree. C.
[0045] Also advantageously, and in particular for decreasing the
compaction of the substrate during treatment at relatively high
temperatures, the glass composition has a strain-point temperature
of greater than 530.degree. C. and preferably greater than
550.degree. C. Such strain-point temperature values allow good
control and high precision in the deposition operations which may
be carried out at temperatures of about 600.degree. C.
[0046] Also preferably, the electrical resistivity of the glass
composition according to the invention is such that log
.rho..sub.(250.degree. C.) is greater than 8; this makes it even
more possible to prevent diffusion into the glass of ions
originating from the deposited layers.
[0047] According to a preferred embodiment of the invention, the
glass composition contains the constituents below in the following
proportions by weight:
3 SiO.sub.2 55-75% Al.sub.2O.sub.3 0-5% ZrO.sub.2 3-8% Na.sub.2O
4.5-8% K.sub.2O 3.5-7.5% CaO 7-11%.
[0048] The various families of glass compositions according to the
invention have, in particular, the advantage of being able to be
melted and converted into glass ribbon form using the float process
at temperatures close to those adopted for the manufacture of
conventional silica-soda-lime glass.
[0049] In this regard, SiO.sub.2 plays an essential role. In the
context of the invention, the SiO.sub.2 content must not exceed
approximately 75%; above this, the melting of the batch and the
refining of the glass require high temperatures which cause
accelerated wear of the furnace refractories. Below 55% by weight
of SiO.sub.2, the glasses according to the invention are
insufficiently stable.
[0050] Alumina acts as a stabilizer. This oxide to some extent
increases the chemical resistance of the glass and increases the
strain-point temperature. The percentage of Al.sub.2O.sub.3
advantageously does not exceed 5% and more preferably does not
exceed 3%, in particular so as not to increase unacceptably the
viscosity of the glass at high temperatures.
[0051] ZrO.sub.2 also acts as a stabilizer. This oxide to a certain
extent increases the chemical resistance of the glass and increases
the strain-point temperature. The percentage of ZrO.sub.2 must not
exceed 8% for fear of making the melting operation too difficult.
Although this oxide is difficult to melt, it has the advantage of
not increasing the viscosity of the glasses according to the
invention at high temperatures, in the same way as SiO.sub.2 and
Al.sub.2O.sub.3. The oxide B.sub.2O.sub.3 may also be present with
a content of at most 3%, and preferably less than 2%. This oxide
makes it possible to increase the fluidity of the glass without
lowering the strain point.
[0052] Overall, the melting of the glasses according to the
invention remains within acceptable temperature limits, as long as
the sum of the SiO.sub.2, Al.sub.2O.sub.3 and ZrO.sub.2 oxide
contents remains less than or equal to 75%. The expression
"acceptable limits" should be understood to mean that the
temperature of the glass corresponding to log .eta.=2 does not
exceed approximately 1550.degree. C. and preferably 1510.degree.
C.
[0053] Moreover, it seems that these glasses lead to little
corrosion of the refractories of the AZS (alumina-zirconia-silica)
type normally used in this type of furnace. These glasses thus
guarantee that the operating time of the furnace is optimized.
[0054] Moreover, there is a sufficient difference in the glass
compositions according to the invention between the glass-forming
temperature and its liquidus temperature; this is because, in the
float-glass technique in particular, it is important that the
liquidus temperature of the glass remain equal to or less than the
temperature corresponding to log .eta.=3.5, which is the case with
the glasses according to the invention. This difference is
advantageously at least 10.degree. C. to 30.degree. C. These
differences or working ranges, which might seem "narrow" for
standard silica-soda-lime glasses intended for manufacturing
glazing panels, are sufficient here to ensure high-quality forming
without adopting excessively extreme conditions for operating the
furnace. This is because the glasses are quite special, for
applications of the high-tech, high value-added type, such as
plasma screens, in which one may "indulge" in very precise control
and suitability of the operation of the furnace: "accessible"
working ranges are maintained without upsetting the furnace or
exposing it to risk.
[0055] The influence of the other oxides on the ability of the
glasses according to the invention to be melted and floated on a
metal bath, as well as their properties, is as follows: the oxides
Na.sub.2O and K.sub.2O make it possible to maintain the melting
temperature of the glasses according to the invention and their
high-temperature viscosities within the limits indicated above. To
do this, the sum of the contents of these oxides remains greater
than 8% and preferably greater than 10%. Compared with an ordinary
silica-soda-lime glass, the simultaneous presence of these two
oxides in the glasses according to the invention, sometimes in
similar proportions, considerably increases their chemical
resistance, more specifically their hydrolytic resistance, as well
as their electrical resistivity. Increasing the electrical
resistivity of the glasses decreases the diffusion of ions, for
example silver ions, coming from the layers deposited on the
surface of the substrates, into the glass, especially in the case
of the production of plasma screens. Increasing the electrical
resistivity of the glasses is also advantageous in certain
applications, more specifically when they are used as a substrate
for cold-cathode screens. In these screens, surface electric fields
are created which cause a localized concentration of electrons.
This concentration may cause, in reaction, undesirable migration of
the alkali elements if the resistivity of the glass is not high
enough, as in the case of an ordinary silica-soda-lime glass.
[0056] However, although both types of alkali metal oxides
Na.sub.2O and K.sub.2O are necessary, it is preferable, if it is
desired to increase their overall content, to favour an increase in
the K.sub.2O content, which has the advantage of increasing the
fluidity of the glass without lowering the strain point, and hence
without excessively compromising the hardness properties of the
glass after forming. In addition, K.sub.2O is conducive to
decreasing the modulus of elasticity in the glass compositions
according to the invention. Preferably, a K.sub.2O/Na.sub.2O weight
percentage ratio of at least 1.2, and preferably at least 1.4, is
thus advantageously provided.
[0057] Provision may also be made to incorporate lithium oxide
Li.sub.2O in the glass composition according to the invention,
especially as a fluxing agent, with contents of possibly as much as
3% and preferably not exceeding 1%.
[0058] The alkaline-earth metal oxides introduced into the glasses
according to the invention have the overall effect of raising the
strain-point temperature, and it is for this reason that the sum of
their weight contents must be at least equal to 12%. Above
approximately 20%, the ability of the glasses to devitrify may
increase to an extent incompatible with the process of floating on
a metal bath. In order to keep the devitrification of the glasses
within acceptable limits, their CaO and MgO weight contents must
not exceed 12%, preferably 11%, and 5%. The MgO content is
preferably equal to or less than 2%.
[0059] MgO, CaO and, to a lesser degree, SrO make it possible to
increase the strain-point temperature; BaO and SrO make it possible
to increase the chemical resistance of the glasses according to the
invention as well as their resistivity. The alkaline-earth metals
also have the effect of decreasing the melting temperature and the
high-temperature viscosity of the glasses.
[0060] However, BaO is preferably present with a content of less
than 2%; these low contents make it possible to limit the formation
of barium sulphate BaSO.sub.4 crystals, which would impair the
optical quality. Although complete absence of BaO is not excluded,
a low content is preferred because of the above-mentioned
properties of BaO. When BaO is present, it is possible as well to
modify the substrate heat-treatment conditions slightly in order to
make them less conducive to the formation of BaSO.sub.4
crystals.
[0061] The advantages afforded by the glass compositions according
to the invention will be more fully appreciated from the examples
given below.
[0062] The first examples relate more particularly to compositions
intended for the manufacture of fireproof glazing.
[0063] A glass composition is made which contains the constituents
below in the following proportions by weight, the first column
indicating the desired values and the second column the measured
values:
4 Desired Measured SiO.sub.2 69.60% 69.60% Al.sub.2O.sub.3 0.90%
0.90% ZrO.sub.2 2.60% 2.62% Na.sub.2O 7.10% 7.07% K.sub.2O 2.90%
2.91% CaO 10.50% 10.50% MgO 2.00% 1.98% SrO 3.90% 3.86%
Fe.sub.2O.sub.3 <0.15% 0.055% Other oxides <0.50%.
0.505%.
[0064] The glass composition has the following properties:
5 .phi. coefficient: 0.77 N/(mm.sup.2 .multidot. .degree. C.)
Expansion coefficient .alpha..sub.20-300: 76.6 .times. 10.sup.-7
.degree. C..sup.-1 Modulus of elasticity: 78.6 .times. 10.sup.3
N/mm.sup.2 Poisson's coefficient: 0.22 .phi..sup.2 .multidot. c/a
1.64 MPa.sup.2/.degree. C..sup.2 Softening point: 805.degree. C.
Liquidous temperature T.sub.liq: 1160.degree. C. Temperature
T.sub.log.eta.=2, 1500.degree. C. corresponding to a viscosity such
that log.eta. = 2: Temperature T.sub.log.eta.=3.5, 1176.degree. C.
corresponding to a viscosity such that log.eta.=3.5: Temperature
T.sub.log.eta.=4, 1100.degree. C. corresponding to a viscosity such
that log.eta. = 4: Relative density 2.59 TL 84.48% TE 81.46%
[0065] It is apparent first of all, from the liquidus temperature,
from T.sub.log.eta.=2, which is the temperature in the melting
bath, and from T.sub.log.eta.=3.5, which is the chosen entry
temperature of the glass on the bath of molten metal, that the
glass composition may be melted in a melting furnace and that the
forming process (float process) on a bath of tin poses no
problem.
[0066] Sheets of glass were thus produced with thicknesses of
between 5 and 10 mm. After having subjected their edges to a
polishing treatment, the sheets of glass were toughened, in a
horizontal position, in a conventional air toughening plant.
[0067] Next, the sheets of glass were fitted in frames with rabbet
depths varying from 10 mm to 25 mm.
[0068] It has proved to be the case that the glazing panels thus
produced according to the invention showed, in fire-withstand tests
in accordance with the standard DIN 4102 or the standard ISO/DIS
834-1, that they met the conditions of fire-resistance classes G 30
to G 120 depending on their thickness and on the depth of the
frame's rabbet.
[0069] The glass composition described below, which may also be
melted and obtained in the form of a ribbon using the float
technique, may also be used for producing fireproof glazing meeting
the conditions of the G fire-resistance classes:
6 SiO.sub.2 74.40% Al.sub.2O.sub.3 0.95% Na.sub.2O 9.05% K.sub.2O
0.45% CaO 9.10% MgO 5.65% Fe.sub.2O.sub.3 0.10% Other oxides
0.30%.
[0070] It has the following properties:
7 .phi. coefficient: 0.71 N/(mm.sup.2 .multidot. .degree. C.)
Expansion coefficient .alpha..sub.20-300: 75.6 .times. 10.sup.-7
.degree. C..sup.-1 Modulus of elasticity: 75.4 .times. 10.sup.3
N/mm.sup.2 Poisson's coefficient: 0.20 .phi..sup.2 .multidot. c/a
1.56 MPa.sup.2/.degree. C.sup.2.
[0071] The glass compositions described in the table below may also
be melted and obtained in the form of a ribbon of glass using the
float technique and may be used for producing fireproof glazing
panels meeting the conditions of the G fire-resistance classes. The
glass compositions given in this table have an even higher
(Littleton) softening point compared to the previous compositions,
thereby further improving the fire resistance.
8 SiO.sub.2 70 66.1 65.6 Al.sub.2O.sub.3 0 0.5 0.5 ZrO.sub.2 3 6.5
6.5 MgO 2 1 2 CaO 6 7 5 SrO 8.5 7.5 9 BaO 0 0 0 Na.sub.2O 5 5 5
K.sub.2O 5.4 5.9 6.4 Softening point, .degree. C. 811 825 821
Strain point, .degree. C. 577 581 574 Expansion coefficient
.alpha..sub.20-300, .degree. C..sup.-1 77.8 78 80 Modulus of
elasticity, 10.sup.3 N/mm.sup.2 75 76.7 76 .phi. coefficient,
N/(mm.sup.2 .multidot. .degree. C.) 0.75 0.77 0.78
T.sub.log.eta.=3.5, .degree. C. 1182 1197 1191 T.sub.log.eta.=2,
.degree. C. 1528 1522 1515
[0072] The second series of examples relates more particularly to
glass compositions intended for manufacturing substrates for plasma
screens. These examples are combined in the table attached as an
annex.
[0073] This table gives, for each of these examples, the chemical
formulations with the contents expressed in percentages by weight,
the values of the .phi. coefficient expressed in
N/(mm.sup.2..degree. C.), the strain-point temperature values of
the glasses T.sub.sp, the thermal expansion coefficients
.alpha..sub.(25-300.degree. C.) of the glasses in .degree.
C..sup.-1, the log of their resistivities log .rho. in ohm.cm,
their liquidus temperatures T.sub.liq, their temperatures at
viscosities, in poise, corresponding respectively to log.eta.=2 and
log.eta.=3.5, T.sub.log.eta.=2 and T.sub.log.eta.=3.5. All
temperatures are expressed in degrees Celsius.
[0074] From the tests carried out and/or given in the annex, and
more particularly from the last three lines, when the measurements
were made, which indicate temperatures corresponding, in respect of
the first, to the viscosity T.sub.log.eta.=2, which is the
temperature in the melting bath, in respect of the second, to the
viscosity T.sub.log.eta.=3.5, which is the chosen entry temperature
of the glass on the bath of molten metal, and finally, in respect
of the third, to the liquidity, it is firstly verified that the
glasses according to the invention may be melted in a melting
furnace and that their forming on a bath of tin poses no
problem.
[0075] It was thus possible to obtain glasses according to the
invention, using the float technique in the form of a ribbon having
a controlled thickness, which may vary from 0.5 to 10 mm. Sheets of
glass were then cut to the desired format and subjected to a heat
treatment whose purpose was to stabilize the dimensions of the said
sheets. Next, layers were deposited on these sheets, such as those
leading to the production of plasma screens.
[0076] First of all, the substrates exhibited quite satisfactory
thermal stability. Moreover, during the layer-deposition
treatments, no break of the said substrates occurred.
[0077] The glass compositions thus presented according to the
invention therefore meet the stipulated requirements, that is to
say that they make it possible to produce substrates or plates
which are thermally stable and have an increased thermal-break
resistance over the glasses already known.
9 ANNEX Composition 1 2 3 4 5 6 7 8 9 10 11 12 SiO.sub.2 68 65 64.5
65 67.5 64.5 66 65 69 67.5 69.5 70 Al.sub.2O.sub.3 0 0 1 1 1 1 0 1
0 0 1 0.5 ZrO.sub.2 4 7.5 7 6 3 6.5 6.5 6.5 4 4.5 3 3 Na.sub.2O 5 5
5 5.5 5 7.5 7 9 6 5 6.5 6.5 K.sub.2O 7.5 7.5 7.5 7.5 7 5.5 4.5 4 4
6 3.5 3.5 CaO 11 11 11 10.5 10.5 10 11 9.5 11 11 11 11 MgO 0.5 0 0
0 1.5 2 2 2 2 2 2 1.5 BaO 0 0 0 0 1.5 0 0 0 0 0 0 0 SrO 4 4 4 3.5 4
3 3 3 4 3.5 3.5 4 .phi. 0.75 0.79 0.79 0.79 0.75 0.79 0.8 0.8 0.74
0.76 0.75 0.73 T.sub.s.p. 580 583 581 582 573 567 570 558 -- -- --
-- log.rho..sub.(250.degree. C.) -- -- -- -- -- 7.9 -- -- -- -- --
-- .alpha. 82 81 81 81 81 79.8 80.1 83.2 78.1 77.4 73.1 75.5
T.sub.log.eta.=2 1496 1491 1497 1491 1498 1500 1490 1480 1490 1485
1500 1498 T.sub.log.eta.=3.5 1171 1186 1189 1186 1171 1185 1175
1170 1170 1173 1180 1169 T.sub.liq -- -- -- -- -- 1120 1140 1090 --
-- -- -- "--": value not measured
* * * * *