PRINTED GLAZINGS

The present invention relates to compositions of inks used for printing onto glass, in particular, ink used to print electrical circuits onto glass used in automotive glazings.

Automotive glazings are often provided with heating circuits to enable the glazing to be demisted in humid, wet or cold weather conditions. In laminated glazings, the circuits may be provided by heating wires embedded in the interlayer material, or by printing on one of the inner surfaces of the plies of glass forming the laminated structure. In the case of single-ply glazings, such as sidelights and backlights, heating circuits are printed onto the side of the glazing which will face into the vehicle when fitted.

Heating circuits are printed using a high-silver content ink. Although elemental silver is virtually unreactive, the printed lines making up the heating circuit may be corroded by acid or alkaline cleaning agents, or by voltage dependent diffusion. However, the largest cause of failure of heating circuits is mechanical damage of the lines in the circuit. This can occur by scratching with sharp or blunt objects, or by abrasion. For example, in a hatchback vehicle, if the parcel shelf is removed, the boot may be overfilled with objects such as shopping, suitcases, building materials, garden waste. Alternatively, items such as keys and coats may be placed on the parcel shelf when it is in position. In both situations, objects come into contact with the printed heating circuit, and may damage the heating lines by scratching and abrasion. Aside from corrosion by cleaning materials, cleaning the inside of the glazing may also cause damage by scratching, for example, contact with cufflinks, watches and other jewellery, or if grit or other dirt is present on the cloth being used to clean.

In each of these situations, the scratch or abrasion made to the surface of the heating line may be complete, causing a break in the heating line, or partial, causing surface damage or partial breakage. If there is a break in the heating line, no current will flow, even at maximum voltage, due to the effectively infinite resistance of the line. If the break is partial, such that it is possible to obtain a resistance value, it often results in the silver being "blown out" or removed at a low initial voltage, with no further response, even at

maximum voltage. As current flow along a conductor is constant, any small break in the heating line causes a restriction, such that localised heating takes place, damaging the printed line.

Figures Ia, Ib and Ic are photographs that illustrate the situation where an AC voltage is applied to the heating line. Figure Ia shows an incomplete break in the heating line. Figure Ib shows the sudden failure when the voltage is applied, causing the ink to melt and vaporise. Figure Ic shows the heating line after failure, where a large portion of the ink has been removed.

Similar failure occurs when a DC voltage is applied to the heating line. However, a second form a failure may occur, which rather than resulting in a sudden catastrophic failure of the heating line produces a gradual failure and removal of the silver ink. This is illustrated in Figures 2a - 2f, photographs of a partial break in a heating wire under DC voltage.

Figures 2a and 2b show the initial break before and just after application of the DC voltage. In Figure 2c, a wave front of molten silver can be seen moving away from the break. As the voltage applied increases, this wave front progresses further and further away from the break in Figures 2d, 2e and 2f. The wave front travels towards the positive terminal, and leaves a gap in the heating line at the position of the original break.

Should either of the types of failure occur in the service life of the glazing, then the heating circuit will no longer work. It is therefore desirable to find a way to improve the durability of the printed heating circuit lines, such that everyday wear and tear when fitted into a vehicle does not lead to failure of the heating circuit, and the lifetime of the glazing is prolonged.

The present invention aims to address these problems by providing an autmotive glazing having an electrical circuit printed thereon, wherein the ink used to print the electrical circuit contains an alloy of silver and at least one other metal.

Preferably, the alloy has an annealed hardness on the Rockwell 15T of at least 50.

Preferably, the alloy has an electrical conductivity of at least 80% IACS.

Preferably, the alloy comprises silver and copper. More preferably the alloy comprises an amount of copper in the range 0.01%wt to 28.0wt%. Even more preferably, the alloy comprises 92.5% wt% silver and 7.5 wt% copper.

The invention will now be described by way of example only, and with reference to the accompanying drawings in which:

Figure Ia (referred to above) is a photograph showing a partial break in a heating line prior to testing with AC voltages;

Figure Ib (referred to above) is a photograph showing a partial break in a heating line after testing with AC voltages;

Figure Ic (referred to above) is a photograph showing a partial break in a heating line after testing with AC voltages;

Figure 2a (referred to above) is a photograph showing a partial break in a heating line prior to testing with DC voltages;

Figure 2b (referred to above) is a photograph showing a partial break in a heating line during testing with DC voltages;

Figure 2c (referred to above) is a photograph showing a partial break in a heating line during testing with DC voltages;

Figure 2d (referred to above) is a photograph showing a partial break in a heating line during testing with DC voltages;

Figure 2e (referred to above) is a photograph showing a partial break in a heating line during testing with DC voltages;

Figure 2f (referred to above) is a photograph showing a partial break in a heating line during testing with DC voltages; and

Figure 3 is a schematic diagram showing the printed glass samples used for testing purposes.

Typical inks for printing electric circuits on the surfaces of automotive glazings comprise between 50 and 83wt% of elemental silver in powdered form (as either particles or flakes), 3-6% glass frit, 1-12% other additives (colour improvers), and small amounts of oils, polymers and flow agents. Although silver has high electrical conductivity (104% IACS - International Annealed Copper Standard), it is relatively soft, having a hardness, when annealed, of 30 on the Rockwell 15T scale. As the remaining ink components have little effect on the ability of the printed, annealed heating line to withstand damage, it has been appreciated in the present invention that by replacing the elemental silver powder within the ink used to print the heating lines with a silver-based alloy powder having a high electrical conductivity and increased hardness, the durability of the printed lines on the surface of the glazing can be improved.

Alloying silver with other metals, such that the electrical conductivity is preserved as much as possible, increases the hardness of the alloy, compared with elemental silver, when annealed. Suitable metals include copper, cadmium, palladium, gold and nickel. Particularly suitable are silver-copper alloys, having up to 28wt% copper. Table 1 below shows the electrical conductivity (on the IACS scale) and annealed hardness (on the Rockwell 15T scale) of three silver-copper alloys:

Table 1 : conductivity and hardness of silver-copper alloys

Alloying silver with even a small amount of copper leads to an increase in hardness. Although the electrical conductivity of the alloy decreases with increasing copper content, even at the eutectic point (28% added copper) the electrical conductivity of the alloy is still 84% IACS. Therefore, by substituting silver-copper alloy powder in the above ink composition, after printing and firing, the resulting lines have a higher resistance to wear, scratching and abrasion than lines printed with elemental silver powder in the ink

composition. Whether an ink is resistant to scratching or not can be indicated in its performance as a printed conductive line on glass, as this is the basic function of the ink in use. If the ink resists scratching sufficiently to maintain its original resistance with little or no change, then it may be regarded as scratch resistant.

In order to examine the effect of using a silver alloy in preference to elemental silver in inks used for printing automotive glazings, test samples were prepared using two ink compositions in order to compare the scratch resistance behaviour:

Composition 1: SP1773, available from Ferro Corporation, 1000 Lakeside Avenue, Cleveland, Ohio 44114-1147, USA

Composition 2: SP1773 containing sterling silver in place of elemental silver, composition: 92.5% wt% silver and 7.5% wt% copper.

Each sample consisted of a series of lines printed onto the surface of a ply of 4mm thick float glass, in the configuration illustrated in Figure 3. Figure 3 is a schematic diagram showing the printed glass samples used for testing purposes. Two broad lines 1, 2, both 89mm in length and 12mm in width were printed along opposite edges of the upper surface of the glass 3, with eight evenly spaced lines 4, 60mm long and ending in a 4mm diameter circle 5 were printed between the broad lines 1, 2. The samples were fired at 650°C. The physical properties of the printed lines are summarised in Table 2 below:

Table 2: physical properties of printed lines for initial testing.

Two tests were used to determine the scratch resistance of each ink composition. Initially, the resistance of each line was measured. Once this was known, the line was either scratched across its width using a razor blade, or was scratched using an Erichsen pen. The change in resistance after testing was taken to be indicative of the scratch

resistance of the ink composition: the smaller the change in resistance, the higher the scratch resistance of the ink.

Table 3 shows the results of both of these tests for each ink composition given above (all figures other than percentages are in Ohms, percentages are given to the nearest whole number):

Table 3: Initial resistance and resistance after 2ON, 30N Erichsen Pen or razor blade tests in ω.

Following the scratch tests described above, further tests were carried out using the razor blade on printed lines having various thicknesses and initial resistances. The samples were printed as described above and shown in Figure 3, with line properties as given in Table 4 below. Table 4 also shows initial, pre-test resistances and final post-test resistances. The following abbreviations are used: WFT: wet film thickness (ink thickness before firing); R _a : average roughness; R _z : ten point height. Percentages are given to the nearest whole number.

Table 4: Initial resistance and resistances after razor blade tests for ink compositions 1 and 2.

From the above results it is seen that including a silver alloy in the ink composition, rather than elemental silver, reduces the change in resistance in the printed line if the line is scratched. Consequently, ink compositions containing such alloys exhibit a higher resistance to scratching than other ink compositions containing elemental silver.