GB2449174A

GB2449174A - Printed automotive glazing

Info

Publication number: GB2449174A
Application number: GB0808421A
Authority: GB
Inventors: Michael Lyon; Julie Houghton; Lee Morris
Original assignee: Pilkington Group Ltd
Current assignee: Pilkington Group Ltd
Priority date: 2007-05-09
Filing date: 2008-05-09
Publication date: 2008-11-12
Also published as: GB0808421D0; GB0708913D0

Abstract

An automotive glazing having an electric circuit printed thereon is disclosed. The electric circuit is printed using a silver-containing ink, such that the roughness <B>R</B>z of the printed circuit is less than or equal to 81.m, and the force required to remove the printed circuit from the surface of the glass using a Rockwell C diamond tip tester in a progressive load scratch test is equal to or greater than 10N. A method of printing the circuit is also disclosed.

Description

PRiNTED AUTOMOTIVE GLAZING 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 arc 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 or outer 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 oblects, 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, lb 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 lb shows the sudden failure when the voltage is applied, causing the ink to melt and vaporise. Figure 1 c 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 of 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 automotive glazing having an electric circuit printed thereon, the electric circuit being printed using a silver-containing ink, wherein the roughness R of the printed circuit is less than or equal to 8lm, and the force required to remove the printed circuit from the surface of the glass using a Rockwell C diamond tip tester in a progressive load scratch test is equal to or greater than ION.

By minimising the roughness of the printed circuit, its scratch resistance can be increased.

Preferably, the roughness R of the printed circuit is less than or equal to 5.5tm.

The invention also provides a method of printing an automotive glazing with an electric circuit, the electric circuit being printed using a silver-containing ink, comprising printing the circuit using a screen having a mesh of greater than or equal to 100 threads per cm, wherein the roughness R of the printed circuit is less than or equal to 8tm, and the force required to remove the printed circuit from the surface of the glass using a Rockwell C diamond tip tester in a progressive load scratch test is equal to or greater than 1ON.

The invention will now be described by way of example only, and with reference to the accompanying drawings in which: Figure la (referred to above) is a photograph showing a partial break in a heating line from AC voltages; Figure lb (referred to above) is a photograph showing a partial break in a heating line from AC voltages; Figure ic (referred to above) is a photograph showing a partial break in a heating line from AC voltages; Figure 2a (referred to above) is a photograph showing a partial break in a heating line from DC voltages; Figure 2b (referred to above) is a photograph showing a partial break in a heating line from DC voltages; Figure 2c (referred to above) is a photograph showing a partial break in a heating line from DC voltages; Figure 2d (referred to above) is a photograph showing a partial break in a heating line from DC voltages; Figure 2e (referred to above) is a photograph showing a partial break in a heating line from DC voltages; Figure 2f (referred to above) is a photograph showing a partial break in a heating line from DC voltages; Figure 3 is a schematic diagram showing the printed glass samples used for testing purposes; and Figure 4 is a graph showing the relationship between the roughness of an electrically conductive ink and the load required to scratch the surface of the il1k.

In the present invention, it has been appreciated that the durability of the electrically conductive inks used in printing circuits onto automotive glazings can be improved by minimising the roughness of the printed ink on the surface of the glass, and making the printed ink as smooth as possible. Typically, the ink is applied as a paste, comprising silver (for electrical conductivity), glass frit (for adhesion and strength), a lacquer (for printing, comprising a resin and a solvent), which have been mixed together in a 3 roll mill with a viscosity modifier to obtain suitable viscosity for screen printing. The ink is then screen printed onto a glazing, either directly onto the glass, or onto a printed ceramic region known as an obscuration band.

In order to understand the manner in which the durability of an electrically conductive ink varies with ink content, glass samples having bus bars printed using ten different inks were prepared and tested using the method described below. Each glass sample comprised a single ply of 4mm thick thermally toughened glass, 100mm x 100mm in size, having a pattern of lines and dots screen printed on one surface using the ink being tested. 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, acting as busbars 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 I, 2. The average line width for each sample was approximately 0.3mm. The average line thickness varied between 3.9 and 22.2pm.

The behaviour of individual electrically conductive inks varies due to several factors, including firing temperaturc and time. For comparison purposes, each ink sample shown in Table I below was fired until it reached 650 C.

Each glass sample was tested using a ST300I Teer machine, designed for adhesion and wear resistance testing of thin coatings. The machine was used to carry out scratch testing of the busbars 1, 2 on the printed samples using a Rockwell C diamond tip to determine the load at which the printed line failed. Two modes of testing arc available: PLST (progressive load scratch testing), which determines the load at which a coating undergoes complete adhesive failure, and CLST (constant load scratch testing), which is used to confirm the results of PLST and to identify inconsistencies in coating properties.

Each of the glass samples was tested using PLST.

The following parameters were used in the PLST test: Load Range: ON-40N Path Length: up to 80mm Load Rate: up to 20N/min Linear Velocity: up to 80mm/mm.

A summary of the electrically conductive inks used, and their properties, is given in Table Ink Silver (wt%) Frit (wt%) Roughness Load (N) (Ri, pm) 1 78 2 3.45 12.33 2 80 4 3.45 10.40 3 76 6 5.43 8.53 4 65 3. 5 4.75 8.27 81 3 7.65 14.13 6 80 2 6.59 8.93 7 55 3 7.93 3.07 8 60 3 11.26 2.93 9 78 4 5.25 10.27 50 4 5.92 1.89 Table 1: Summary of electrically conductive inks used in tests Figure 4 is a graph showing the relationship between the roughness of the printed electrically conductive ink and the load required to scratch the surface of the ink. The general trend is that the load required to scratch the surface of the ink increases with decreasing roughness. Preferably, the roughness R of the printed circuit is less than or equal to 8.tm, more preferably less than or equal to 5.5lim, such that the force required to remove the printed circuit from the surface of the glass using a Rockwell C diamond tip tester in a progressive load scratch test is equal to or greater than I ON. From the data in table 1, a best fit using the least squares method reveals a regression coefficient of -0.9, indicating a good match to a trend of increasing load with decreasing R7.

Various factors may influence the roughness of the printed ink surface. All of the above inks were printed using the same mesh print screen. However, preferably, the mesh size used on the screen is greater than or equal to 100 threads per cm. In addition, both the medium used in the ink (which affects the flowability of the ink though the screen) and the burn out of the ink on firing are preferably adjusted to determine the optimum printing conditions for each ink.

Claims

An automotive glazing having an electric circuit printed thereon, the electric circuit being printed using a silver-containing ink, wherein the roughness R of the printed circuit is less than or equal to 8tm, and the force required to remove the printed circuit from the surface of the glass using a Rockwell C diamond tip tester in a progressive load scratch test is equal to or greater than iON.
2. The automotive glaing of claim 1, wherein the roughness R7 of the printed circuit is less than or equal to 5.5pm.
3. A method of printing an automotive glazing with an electric circuit, the electric circuit being printed using a silver-containing ink, comprising printing the circuit using a screen having a mesh of greater than or equal to 100 threads per cm, wherein the roughness R of the printed circuit is less than or equal to 8.tm, and the force required to remove the printed circuit from the surface of the glass using a Rockwell C diamond tip tester in a progressive load scratch test is equal to or greater than I ON.