BE1020040A3 - HEATED GLAZING. - Google Patents

Abstract

The invention relates to a glazing comprising a network of conductive wires applied by screen printing on one side of the glazing, these wires being obtained by firing an enamel paste comprising metal particles consisting mainly of silver, a French fries glass and organic binders, characterized in that in the dough, the glass frit is in the form of grains whose average maximum dimensions are less than 0.8u, preferably less than 0.6u, and particularly preferably less than 0 , 5u.

Description

The invention relates to a heated glazing unit comprising a series of conductive wires spaced on the surface of the glazing, or a part of the surface of this glazing. Glazing of this type concern in particular, and not exclusively, automotive glazing. The heating of the latter is intended to prevent condensation or frost formation.

For the rear automobile glasses, the most usual technique for constituting the wires, because it integrates conveniently in the production process, and consequently is advantageous from the economic point of view, consists in the application of an enameled conductive composition. Once applied, the yarns are dried and fixed by cooking. The compositions used for this purpose are usually based on silver.

The application technique on the glazing is traditionally a screen printing technique. The composition for forming the conductive yarns, suitably conditioned, is applied to the glass sheet in the form of a paste comprising conductive particles in a suitable medium. The applied composition dries naturally in a very short time and is then subjected to a heat treatment which permanently fixes its constituents on the glass sheet.

The compositions of the pastes used, apart from the silver-conducting particles, contain a fuse of relatively low-temperature fusible glass, also in the form of particles, and a binder which is removed during the heat treatment.

The glass frit allows a strong attachment to the glass sheet on which the wires are applied. The binder and optionally a solvent serve only to give the dough the pasty consistency, in other words the rheological qualities, which allow the application through the sieve and the adhesion of the wires deposited to the glass for the preceding period. cooking enamel.

To meet the demand of car manufacturers, the trend is to increase the conductivity of formed wires. This increase allows a more intense heating of the glazing. It also makes it possible, if necessary, to modify the geometric characteristics of the wires applied. This is particularly sought after, to ensure that these son are as discrete as possible on the glazing, including reducing their width.

To improve this type of performance, different approaches have been proposed. The former consisted in increasing the content of conductive particles of the pastes applied by screen printing. The highest levels of silver particles of pasta marketed at present are thus of the order of 88% by weight. Of course the increase in the silver content to the detriment of the other constituents of these pasta can not exceed a certain limit. The subsequent reduction of the fried content, beyond a certain limit, would lead to insufficient fixing of the son on the glass sheet.

Another approach which is the subject of the application WO 2009/053469, aims at the use of screen printing screens with characteristics allowing the application of the dough in a better controlled manner.

Previous proposals have improved the characteristics of heating wires. Improvements are nevertheless still desirable in this area. In response to this request, the inventors have examined the structure of the wires applied in order to determine what could be the means leading notably to bringing the resistivity of the wires in question closer to that which corresponds to "theoretical" wires made of the metal of the particles used. . In other words the inventors were interested in how to bring the structure of these son of an ideal structure.

Micrographic son analysis and X-ray emission spectra have shown characteristics that may be responsible for the lower conductivity than might be expected. In particular the micrography of the sections of the wires applied under previous conditions, has shown the presence of inhomogeneities whose presence can reasonably be compared with this limited conductivity.

The inventors accordingly propose the implementation of the conditions which are the subject of claim 1 to lead to improved heating glazings.

A first observed element, the inhomogeneity of the distribution of the fried particles, and even more the presence of relatively large particles of fried, appeared as a factor that can limit the conductivity significantly. To minimize the negative impact of fries in these yarns, the inventors have shown that it is useful to reduce the size of the fried particles.

Apart from the benefit of improving the structure of the yarns produced, controlling the size of the fried particles in a direction reducing these dimensions, still allows the dough to retain adequate rheological properties while still increasing the silver content. In proportion, the fried content can also be slightly reduced without the enamelled son are more mechanically fragile, or their attachment to the glazing is less strong.

The heat treatment corresponding to the firing of the son applied by screen printing inevitably leads to a modification of the distribution of the constituents of the initial paste. During this cooking solvents and organic binders are removed leaving only the mineral constituents that are the fries on the one hand and money on the other hand. The manner in which the distribution of the chips is carried out in the thickness is the subject of hypotheses which have not yet been verified. One hypothesis is that the metal matrix has a tendency to repel the foreign grains constituted by the fried particles and that the latter, on the contrary, tend to cluster together by merging in contact with the constituents of the glass sheet.

Another phenomenon appeared to affect the conductivity. The analysis of the earlier products shows the presence of relatively large clusters of fries included in the metal. These clusters are of course detrimental to the conductivity of the metal matrix.

A first measure proposed by the inventors for improving the structure of the son is to use silver-based conductive pastes in which the fried particles before cooking have reduced dimensions. The inventors thus propose that the fried particles do not exceed on average 0.8μ, and preferably, are not greater than 0.6μ, and particularly preferably less than 0.5μ.

The presence of these smaller particles leads to reduce the presence of clusters, assimilated to undesirable inclusions. This finding has led to another improvement consisting in reducing the overall fried content of the pasta used, while maintaining the level of adhesion which was that obtained previously with pasta of the prior art. The reduction of the fried content is to the essential benefit of a corresponding increase in the silver mass, and therefore an improvement of the conductivity.

The inventors recommend using pasta in which the content of fried particles of limited size, as indicated above, is not greater than 12% by weight of the dough, and preferably not more than 10%, and particularly preferably not more than 8%.

The reduction of the fried content in the dough results in a corresponding increase in the silver content present in the yarns after they have been cooked. Thus the implementation according to the invention leads to son whose silver content is not less than 90% by weight, and preferably not less than 92%.

The inventors have further observed that the crystallographic structure of the silver in the yarns cooked according to the invention has differences with those of the yarns obtained under the conditions of the prior art.

The products obtained under the conditions of the invention have dimensions of silver crystallites which are substantially larger than those of the prior art.

The determination of the size of the crystallites is carried out relatively. The determination is made by what is referred to as the "correlation length". This correlation length is made from the width of the X-ray diffraction intensity peaks. Qualitatively, the higher the peaks, the greater the correlation length. The relationship between correlation length and peak width is Debye Scherrer's relationship:

where: B is the width at half height (FWHM),

The is the correlation length, k is the form factor (0.89), D is the angular position of the peak and Q is the wavelength (Cu KO).

The measurement of the relative dimensions of the crystallites can only be obtained in the direction orthogonal to the glass sheets. The measurement of the correlation length is obtained by measuring the width at half height of the <111> diffraction peak characteristic of the crystal structure of silver. This preferred orientation is most represented in the direction orthogonal to the glass sheets. Nevertheless, the growth of these crystallites is not related to the direction envisaged, so that the measurement constitutes an adequate indication of what the dimensions are in directions parallel to the sheets. In the case of the invention, the latter are superior to those of the products of the prior art. This finding is apparently related to the increase in conductivity for a constant silver content. The conductivity is even higher as the interfaces between the crystallites are less numerous for the same length of wire. Crystallites of larger dimensions necessarily lead to reducing the number of these interfaces.

According to the methods of the invention, it appears possible to increase the size of the crystallites by a rate greater than 10% and advantageously greater than 20%, and under the best conditions by over 27%, compared to that of the wires obtained. with pasta of the prior art. The crystallites obtained according to the invention have average dimensions which are at least 124 nm, and advantageously at least 132 nm, and under the best conditions of at least 140 nm.

Under the conditions of the invention, the son after baking have a resistivity that is not greater than 2.8. 10-6Ω.αη, advantageously not greater than 2.5. 106μ.cm and particularly advantageously not greater than 2.2.10-6 Ω.αη.

The improvement of the conductivity of the yarns obtained according to the invention makes it possible to minimize their section while maintaining a resistance comparable to that of the yarns of the prior art. Overall this has the consequence, at equal strength, the possibility of having son that have less impact on the optical qualities of glazing on which these son are applied.

Yarns of reduced cross section lead to less important optical distortions. According to the invention this cross section can be reduced as much as previously indicated, the inclusions of frit irregularities, or even the porosities, are in smaller proportions compared to previous products. To experience the conductivity of the son is all the better than in the formation of their cross section the thickness to width ratio is higher. This ratio is partly at least adjustable depending on the conditions of application by screen printing, and in particular the structure of screen printing screens.

As previously indicated, particularly preferred screens for the implementation of the invention are of the type described in the aforementioned application. These screens have particular particularity, a more regular coating of the fabric. Controlling the thickness of this coating makes it possible, among other parameters, to adjust the thickness of the paste applied, and consequently that of the yarns.

In the preferred conditions according to the invention, for the constitution of heating glazing and in particular of rear glasses, the son advantageously have, after cooking, a cross-section which is not greater than 15 × 10 -3 mm 2, and preferably not greater than 13 × 10 -3 mm 2 .

Conductors are not strictly the same section over their entire length regardless of the precautions taken. In practice the cross-sectional variations do not exceed 8% and advantageously not 5%.

Reducing the section of the yarns by maintaining a width-to-thickness ratio as indicated above further permits a limitation of the width of the yarns on the glazing and contributes to the more discreet appearance of its yarns. For the resistors necessary to develop the power required by the manufacturers for the rear glasses in particular, the son in question are advantageously less than 0.2mm wide and preferably less than 0.15mm.

The invention is described below in detail with reference to the accompanying plates in which: - Figure 1 is a microphotographic section of a conductive wire obtained under the conditions of the prior art; - Figure 2 is a view similar to the previous for a wire obtained under the conditions of the invention; - Figure 3 is an X-ray diffractogram obtained on the son applied before cooking; FIG 4 comprises a diffractogram similar to that of Figure 3 corresponding to the son after cooking, and separately, three magnification of the peaks of this diffractogram.

The microphotographic section of FIG. 1 is that of a wire applied to a glass sheet 4, obtained with a paste of the prior art. The screen printing screen used is of the type described in the previous application WO 2009/053469. The paste comprises 88% by weight of silver particles and 10% by weight of fries, the particles of which have average dimensions of the order of 1.5μ, the remainder being binder and solvent.

The cut is made on a wire 0.3mm wide and about 10 / x thick.

The section reveals a number of inhomogeneities. Among these are frit particles 1 corresponding approximately to the size of the initial particles of the order of 1.4μ. These irregularities in the structure of the wire are the cause of a conductivity much lower than that which would correspond to the section of a wire of the same dimensions but consisting exclusively of homogeneous silver.

The fried particles are roughly distributed evenly throughout the thickness of the wire. In other words, the entire thickness is affected by the presence of inhomogeneities of the wire.

The micrograph of FIG. 2 is obtained on a wire prepared under the conditions of the invention. The same screen printing screen as before is used. In the paste the silver content is 92% by weight.

The appearance of the cup whose magnification is the same as in Figure 1 is much more homogeneous. Frit particles 1, which appear as dark spots, are small. The average dimensions are of the order of 0.42 / x.

Figure 3 shows a crystallographic analysis. The analyzes are done on a device "Bruker D8 Advance in téta / téta scan". The beam is CuKQ. The scanned domain is 5 to 70 °.

The diffractogram of silver varies a little according to the origin of the analyzed pulp, but all the diffractograms show peaks for the orientations noted <111>, <200> and <220>. The most intense peak is always the peak <1>.

In FIG. 3, the diffractogram corresponds to a paste according to the invention before firing. Figure 4 corresponds to the diagram obtained for the same paste after baking at 690 ° C. The diffractogram shows a certain evolution which appears more clearly on the enlargements made of each of the peaks concerned. Each peak appears split, showing the appearance in money of two distinct modes of crystallization (see Figure 4).

The determination of the average correlation lengths from this type of diffractogram is based on the calculation of the correlation lengths of each peak (<111> a and <111> b) and takes into account their intensity ratio. This method makes it possible to evaluate the dimensions of the silver crystallites present in the conductive wires. For son formed in the conditions of the prior art the crystallites have at best dimensions of about 11Onm. For son made with pasta meeting the characteristics of the invention, and under identical conditions of application and heat treatment, the crystallites have dimensions of just over 132 nm. Consequently, the crystallites according to the invention being larger in size, the interfaces are less numerous and / or less extensive, which leads to improved conductivity.

On son after firing whose sections are respectively 19.10-4mm2 and 12.10-4mm2 for the product according to the prior art and according to the invention, the respective resistivities measured are 3.04.10'6Ω.αη, and 2.4.10 '6Ω.αη is a decrease of the order of 20%. It is all the more remarkable that the resistivity is lower according to the invention, while the son concerned have a reduced section of about one-third, and that the effects of eg surface irregularities should relatively play disadvantageously.

The son applied according to the provisions of the invention are also the object of mechanical strength tests. These tests are intended to determine the good adhesion of the son to the glass sheet on which they are applied.

The test is carried out after aging of the heating glazing (aging made according to DIN 50018 and ISO 6988). The procedure is as follows: - the aged glazing is cleaned; - application of the adhesive (TESA 4657) on the glazing comprising the son; - rest for at least 5 minutes; - rapid tearing of the adhesive strip at one time at an angle of 60 °; - verification of the wires.

No degradation of the wire network is observed after 5 cycles.

Claims (8)

1. Glazing comprising a network of son son applied by screen printing on one side of the glazing, these son being obtained by baking an enamel paste comprising metal particles consisting mainly of silver, a glass frit and organic binders, characterized in that in the dough, the glass frit is in the form of grains whose average maximum dimensions are less than 0.8μ, preferably less than 0.6μ, and particularly preferably less than 0.5μ . 2. Glazing according to the preceding claim wherein the weight content of the fried in the dough is not greater than 12%, preferably not more than 10% and particularly preferably not greater than 8%. 3. Glazing according to one of the preceding claims wherein the silver-based metal content of the conductors after firing is not less than 88% by weight, preferably not less than 90% and so particularly preferred is not less than 92%. 4. Glazing according to one of the preceding claims wherein the conductive son have a resistivity not greater than 2.8.10'6ü.cm, preferably not greater than 2.5. 10'6i2.cm and particularly preferably not greater than 2.2. 10'6fl.cm. 5. Glazing according to one of the preceding claims wherein the conductive son have a section which is not greater than 15.10 '3mm2, and preferably less than 13.10'3mm2. 6. Glazing according to one of the preceding claims wherein the maximum width of the conductive son and less than 0.2mm and preferably less than 0.15mm. 7. Glazing according to one of the preceding claims wherein the average dimension of the silver crystallites in the son, is not less than 124nm and preferably not less than 132nm. 8. Glazing according to one of the preceding claims wherein the son in their length have a cross section whose variations are not greater than 8% and preferably not greater than 5%.