FR2762838A1 - New soda-lime glass compositions - Google Patents

Abstract

A soda-lime glass composition for substrate or plate manufacture, having a coefficient phi (thermal stress coefficient, phi = alpha \*E/(1- mu )) of 0.50-0.85 N/mm<2>. deg C and a working point of less than 1200 deg C, is new. Preferably, the composition contains (by wt.) (a) 55-75% SiO2, 0-7% Al2O3, 0-8% ZrO2, 5-10% Na2O, 0-8% K2O and 8-12% CaO; or (b) 55-75% SiO2, 0-7% Al2O3, 0-8% ZrO2, 2-8% Na2O, 2-8% K2O, 4-11% CaO and 0-4% MgO. A preferred glass composition has a coefficient phi of less than 0.84, a strain point of above 507 deg C and a resistivity such that log rho (250?oC) is greater than 6.6 and contains 55-75% SiO2, 0-5% Al2O3, 3-8% ZrO2, 4.5-8% Na2O, 3.5-7.5% K2O and 7-11% CaO.

Description

1-

SILICO-SODO-CALCIUM GLASS COMPOSITIONS

AND THEIR APPLICATIONS

The present invention relates to glass compositions capable of being transformed into a ribbon of glass from which plates can be cut, which are very resistant to heat. These plates can more particularly be used to produce fire-resistant glazing or serve as substrates for the manufacture of plasma screens, electroluminescent screens and

of cold cathode screens (Field-Emission-Displays).

The glass initially used to make such substrates is a glass belonging to the family of silico-soda-lime glasses, commonly used to manufacture glazing intended for buildings or motor vehicles. While this type of glass is generally satisfactory as regards its chemical resistance and flatness, its temperature resistance sometimes leaves much to be desired. During the manufacture of emissive screens of the plasma screen type, the substrate is subjected to several heat treatments which aim to stabilize the dimensions of said substrate and to fix a series of layers of different compounds, such as enamels, deposited on its surface. . The fixing of these layers of greater or lesser thicknesses requires that the substrate be brought to temperatures above 550 C. If the coefficient of expansion of the silico-soda-lime glass used is of the same order of magnitude as that of the deposited compounds on its surface, its resistance to -2- temperature is insufficient and it is necessary to place it on a slab

rectified during heat treatments to avoid any deformation.

The glasses used for the manufacture of fire-resistant glazing generally belong to the family of borosilicate glasses. These glasses, which have very good resistance to heat and thermal shock, are generally characterized by a low coefficient of expansion. This last characteristic does not allow the development in these glasses of strong stresses by thermal toughening, and the increase in their resistance

mechanical by this means is limited.

New families of glass compositions have been developed and described in patent WO-96/11887, in order to overcome these drawbacks, in particular in order to be able to manufacture plates or substrates with almost zero deformation during heat treatments of the glass. around 550 to 600 C and able to present, by thermal quenching, stress levels

comparable to those obtained with standard silico-soda-lime glass.

However, it appears that these glasses can exhibit breakage during the deposition of certain layers, including when the methods of deposition of these layers lead to local temperatures of the glass not exceeding one.

hundred degrees Celsius.

The inventors thus set themselves the task of remedying these

breakages which, although infrequent, disrupt manufacturing facilities.

The object of the invention is new glass compositions making it possible to manufacture substrates whose deformation remains almost zero when they are subjected to temperatures of the order of 600 ° C. and which do not deteriorate during the deposition of layers. their surface, that is to say which do not break immediately and which do not present any defect that could lead to

subsequent breakage.

This object is achieved according to the invention by a glass composition intended for the manufacture of thermally stable substrates or plates, said glass composition having a coefficient (p less than 0.84 N / (mm2.0C), its temperature below annealing (strain point) being greater than 507 C and its electrical resistivity being such that log P (2500c is greater than 6.6. The coefficient (p is defined according to the relation: <p = xE / 1-p with: -3 - X: coefficient of expansion E: modulus of elasticity

PL: Poisson's ratio.

It is commonly accepted that glass no longer has any viscous behavior below a characteristic temperature called the lower annealing temperature (strain point), which corresponds to a viscosity of the order of 1014.5 poises. Therefore, this temperature is a benchmark

interesting for evaluating the temperature resistance of a glass.

It turned out during the tests that in particular the combination of these lower annealing temperature values and the coefficient (p allow the production of a thermally stable substrate or plate and not undergoing any deterioration or breakage during the treatment phases for the deposition of The electrical resistivity values in particular limit the diffusion into the glass, for example of silver ions contained in the layers deposited at

1 5 the surface of the substrate.

According to a preferred embodiment of the invention, the coefficient of expansion of the glass composition, measured in a known manner by expansion differences at temperatures of 25 C and 300 C, is between 65 and 88.10-7 C '. Such values are interesting in particular for their compatibility with those of the glass frits usually used for

for example, making barriers for plasma screens.

More preferably, the coefficient of expansion is between 80 and

8 -7OC-1.

85.10'7 o-C.

A glass composition which is more particularly advantageous according to the invention, in particular in terms of resistance to thermal breakage and of cost, has a coefficient (p less than 0.8 N / (mm2.0C) and preferably

still greater than 0.7 N / (mm2.0 C).

Also to reduce the cost of the glass composition, the latter advantageously has a lower annealing temperature (strain

point) less than 590 C and preferably less than 580 C.

Again advantageously and in particular to reduce the compaction of the substrate during treatment at relatively high temperatures, the glass composition has a lower annealing temperature (strain - 4 - point) greater than 530 ° C. and preferably greater than 550 C. lower annealing temperature values allow good control and good precision of the deposits that can be made at temperatures

of the order of 600 C.

More preferably, the electrical resistivity of the glass composition according to the invention is such that log P (2500c) is greater than 8; this makes it possible to further prevent the diffusion into the glass of ions originating from the deposited layers. According to a preferred embodiment of the invention, the glass composition comprises the constituents below in the following weight proportions SiO2 55-75%

A1203 0 -5%

ZrO2 3 -8% Na20 4.5 - 8%

K20 3.5-7.5%

CaO 7 - 1% The glass compositions according to the invention have in particular the advantage of being able to be melted and transformed into a glass ribbon at

temperatures close to those adopted for the manufacture of silicate glass

i 5 classic soda-lime.

In this respect SiO2 plays an essential role. In the context of the invention, the SiO2 content must not exceed approximately 75%; beyond that, the melting of the vitrifiable mixture and the refining of the glass require high temperatures which cause accelerated wear of the refractories of the furnaces. Below

55% by weight of SiO2, the stability of the glasses according to the invention is insufficient.

Alumina acts as a stabilizer. This oxide increases to some extent the chemical resistance of the glass and promotes the increase in the lower annealing temperature. The percentage of A1203 should not exceed 5%, and preferably 3%, in particular so as not to increase in

the viscosity of glass at elevated temperatures is unacceptable.

ZrO2 also plays a stabilizing role. This oxide increases to some extent the chemical resistance of the glass and promotes the increase of the lower annealing temperature. The percentage of ZrO2 must not exceed 8%, otherwise the fusion will be too difficult. If 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 proportions as SiO2 and A1203. The oxide B203 can also be present with a content of at most 3%, and preferably less than 2%. This oxide makes it possible to fluidify

the glass without lowering the strain point.

Overall, the melting of the glasses according to the invention remains within acceptable temperature limits, provided that the sum of the contents of the oxides SiO2, A1203 and ZrO2 remains equal to or less than 75%. By acceptable limits, it should be understood that the temperature of the glass corresponding to

log 1 = 2 does not exceed about 1550 C and preferably 1510 C.

Moreover, it appeared that these glasses lead to low corrosion of the refractories, of the AZS (alumina-zirconia-silica) type, usually used in this type of furnace. These glasses thus guarantee an optimization of the

duration of use of the oven.

On the other hand, the glass compositions according to the invention have a sufficient difference between the glass forming temperature and its liquidus temperature; in fact, in the float glass technique in particular, it is important that the liquidus temperature of the glass remains equal to or less than the temperature corresponding to logq = 3.5, which is the case for glasses

according to the invention. This difference is advantageously at least 10 C to 30 C.

These working deviations or levels, which could appear "narrow" for standard soda-lime-silicate glasses intended for manufacturing glazing, are here sufficient to ensure quality forming without adopting conditions that are too extreme for the operation of the furnace. These are in fact quite specific glasses, for high-tech applications with high added value, such as plasma screens where we can "afford" a very precise control and adequacy of the operation of the oven: we stay in "accessible" working levels without disruption or risk taking

in the oven.

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 on their properties, is as follows: the oxides Na2O and K20 make it possible to maintain the temperature of melting of the glasses according to the invention and their viscosities at high temperatures 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 silico-soda-lime glass, the simultaneous presence of these two oxides in the glasses according to the invention, sometimes in similar proportions, makes it possible to considerably increase their chemical resistance, more precisely their hydrolytic resistance, as well as their electrical resistivity. The increase in the electrical resistivity of the glasses makes it possible to reduce the diffusion of ions, for example silver, in the glass coming from the layers deposited on the surface of the substrates, in particular in the case of the production of plasma screens. The increase in the electrical resistivity of the glasses is also of interest in certain applications, more precisely when they serve as a substrate for screens with a cold i15 cathode. In these screens arise surface electric fields which cause a localized concentration of electrons. This concentration can cause an undesirable migration of alkalis as a reaction if the

resistivity of the glass is insufficient, as in the case of a soda-silicate glass

ordinary calcium.

If the two types of alkaline oxides Na2O and K20 are necessary, on the other hand, if we want to increase the overall content, it is preferable to promote an increase in K20, which has the advantage of making the glass more fluid without lower the strain point, without therefore being too penalizing on the hardness properties of the glass after forming. In addition, K20 promotes a reduction in the modulus of elasticity in the glass compositions according to the invention. There is thus advantageously and preferably provided a ratio of weight percentages K20 / Na2O of at least 1.2, and preferably of at least

minus 1.4.

It is also possible to provide for incorporating lithium oxide Li20 into the glass composition according to the invention, in particular as a melting agent, in contents which may reach 3% and preferably not

not exceeding 1%.

-7- The alkaline earth oxides introduced into the glasses according to the invention have the overall effect of raising the lower annealing temperature, this is the reason why the sum of their weight contents must be at least equal to 12% . Beyond approximately 20%, the ability of the glasses to devitrify may increase in proportions incompatible with the float process on a metal bath. In order to maintain the devitrification of the glasses within acceptable limits, their CaO and MgO weight contents must not exceed 11 and 5% respectively. The MgO content is preferably equal to or

less than 2%.

MgO, CaO and to a lesser extent SrO make it possible to raise the lower annealing temperature; BaO and SrO increase resistance

of the glasses according to the invention as well as their resistivity. Alkaline

earthy also have the effect of decreasing the melting temperature as well

than the viscosity of glasses at high temperatures.

However, BaO is preferably present with a content of less than 2%, these low contents make it possible to limit the formation of crystals of barium sulphate BaSO4, which proves to be detrimental in terms of optical quality. If a total absence of BaO is not excluded, a low content is preferred because of the properties of BaO previously stated. When BaO is present, it is still possible to slightly adapt the heat treatment conditions of the substrate to make them unfavorable to the formation of crystals.

of BaSO4.

The advantages presented by the glass compositions according to the invention will be better appreciated through the examples collected in the attached table.

in the appendix.

In this table, are grouped together, for each of these examples, the chemical formulations with the contents expressed in weight percentages, the coefficient values (p expressed in N / (mm2.0C), the temperature values below the annealing temperature of the glasses (known as strain point) Ti.r, the coefficients of thermal expansion aC (25-300oc) of the glasses in CO, the log of their resistivities in ohm.cm log p, their liquidus temperatures Tliq, their temperatures at viscosities in poises corresponding respectively to - 8 - logr1 = 2 and logq = 3.5, Tlogl = 2 and Tlogq = 3.5. All temperatures are

expressed in degrees Celsius.

The tests carried out and / or the data in the appendix, and more particularly the last three lines, when the measurements have been made, which indicate temperatures corresponding for the first, to the viscosity Tlogl = 2, which is the temperature in the molten bath, for the second at viscosity Tlog = 3.5 which is the chosen temperature of entry of the glass to the molten metal bath and finally for the third to liquidus, it is first of all checked that the glasses according to invention can be melted in a furnace

fusion and that their forming on a tin bath does not pose a problem.

Glasses according to the invention have thus been obtained using the float technique in the form of a strip of 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 having the particular object of stabilizing the dimensions of said sheets. On these sheets were then placed

layers such as those leading to the production of plasma screens.

First of all, the substrates showed quite satisfactory thermal stability. Moreover, during the coating deposition treatments, it

no breakage of said substrates appeared.

The glass compositions thus presented according to the invention therefore meet the requirements set, that is to say they make it possible to produce thermally stable substrates or plates exhibiting resistance to

increased thermal breaks compared to already known glasses.

-9- co cri oe APPENDIX ["Composition 1 2 3 4 5 6 7 8 9 10 1 1 12 SiO2 68 65 64.5 65 67.5 64.5 66 65 69 67.5 69.5 70

A1203 O O 1 1 1 1 O 1 0 0 1 0.5

ZrO2 4 7.5 7 6 3 6.5 6.5 6.5 4 4.5 3 3 Na2O 5 5 5 5.5 5 7.5 7 9 6 5 6.5 6.5

K20 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 1 9.5 11 1 1 11 11 MgO 0.5 0 0 0 1.5 2 2 2 2 2 2 1.5 BaO OO 0 0 1.5 OO 0 0 OOO SrO 4 4 4 3.5 4 3 3 3 4 3.5 3, 5 4

0.75 0.79 0.79 0.79 0.75 0.79 0.8 0.8 0.74 0.76 0.75 0.73

Tr. 580 583 581 582 573 567 570 558 - - - -

Iogp (250 C) - - - - - 7.9 - - - - - -

a 82 81 81 81 81 79.8 80.1 83.2 78.1 77.4 73.1 75.5 Tlogil = 2 1496 1491 1497 1491 1498 1500 1490 1480 1490 1485 1500 1498 Tlogil- 3.5 1171 1186 1189 1186 1171 1185 1175 1170 1170 1173 1180 1169

Tliq - - - - 1120 1140 1090 - - - -

-: value not measured

- 10 -

Claims (10)

1. Glass composition intended for the manufacture of thermally stable substrate or plate, characterized in that it has a coefficient (p less than 0.84, in that its lower annealing temperature (strain point) is greater than 507 C, and in that its electrical resistivity is such that log p <250oc) is greater than 6.6. 2. Glass composition according to claim 1, characterized in that its coefficient of expansion is between 65 and 88.10-7 C-1 and preferably between 80 and 85.10-70C-1. 3. Glass composition according to one of claims 1 or 2, characterized in that the coefficient (p is less than 0.8 and preferably greater than 0.7. 4. Glass composition according to one of the preceding claims, characterized in that its lower annealing temperature (strain point) is between 530 and 590 C and preferably between 550 and 580 C. 5. Glass composition according to one of the preceding claims, characterized in that its electrical resistivity is such that log p (250 C) is greater than 8. 6. Glass composition according to one of the preceding claims, characterized in that it comprises the constituents below in the following weight proportions SiO2 55 - 75% A1203 0 - 5% ZrO2 3 - 8% Na2O 4.5 - 8% K20 3.5-7.5% CaO 7 -11% 7. Glass composition according to claim 6, characterized in that it satisfies the following weight percentage: K20 / Na20> 1.2 and preferably K20 / Na2O> 1.4. 8. Glass composition according to one of claims 6 or 7, characterized in that it verifies, in weight proportions: - 11 - 12% _ MgO + CaO + SrO + BaO 20% 9. Glass composition according to one of claims 6 to 8, characterized in that the BaO content by weight is less than 2%. 10. Glass composition according to one of the preceding claims, characterized in that it is capable of being transformed into a ribbon of glass according to the float casting process on a bath of molten tin. 1 1. Use of glass compositions as defined by one any of the preceding claims for the manufacture of substrates for emissive screens of the plasma screen, electroluminescent screen or cold cathode screen type, in particular from a sheet of glass cut from a ribbon of glass obtained by floating the glass on a bath of molten metal.