Glazing for motor vehicles

Abstract

The invention relates to a laminated glass panel for an automobile, having a curved shape resulting from the assembly of a first glass sheet, which is curved before said assembly, with an intermediate thermoplastic sheet and a second glass sheet, the thickness of which does not exceed one third of that of the first sheet, the second glass sheet not being curved, or having a curvature that is substantially smaller than that of the first sheet before the assembly thereof with the latter and the intermediate thermoplastic sheet.

Claims

1. A laminated curved automotive glazing assembly, comprising: a first glass sheet initially curved in this assembly, a thermoplastic interlayer sheet, and a second glass sheet with a thickness that is not greater than a third of that of the first sheet, wherein the second glass sheet does not have a curvature, or has a curvature that is less than that of the first sheet, before its assembly with the first glass sheet and the thermoplastic interlayer sheet, wherein in the assembly the second glass sheet has a surface stress that is not more than 50 MPa, and wherein a total thickness of the first glass sheet, thermoplastic interlayer, and second glass sheet is not more than 2.8 mm.

2. The glazing according to claim 1, wherein the first glass sheet has a thickness that is not more than 2.1 mm.

3. The glazing according to claim 1, wherein the thickness of the second glass sheet is not more than 0.8 mm.

4. The glazing according to claim 1, wherein the thickness of the second sheet is not less than 0.2 mm.

5. The glazing according to claim 1, wherein the curvature of the first glass sheet is cylindrical and a radius of curvature is not less than 1 m.

6. The glazing according to claim 1, wherein the second glass sheet is not curved before assembly.

7. The glazing according to claim 1, wherein the interlayer sheet has a thickness that is not more than 0.8 mm.

8. The glazing according to claim 1, wherein at least the second glass sheet is chemically toughened.

9. The glazing according to claim 1, wherein the second glass sheet bears a system of functional layers and the system of layers comprises a layer system for filtering infrared rays on a face in contact with the thermoplastic interlayer.

10. The glazing according to claim 1, forming a side window of a motor vehicle.

11. The glazing according to claim 1, wherein the thickness of the second glass sheet is not greater than a fourth of that of the first glass sheet.

12. The glazing according to claim 1, wherein in the assembly the surface stress of the second glass sheet is not more than 30 MPa.

13. The glazing according to claim 1, wherein in the assembly the surface stress of the second glass sheet is not more than 20 MPa.

14. The glazing according to claim 1, wherein in the assembly the surface stress of the second glass sheet is not more than 10 MPa.

15. The glazing according to claim 1, wherein the curvature of the first glass sheet is cylindrical and a radius of curvature is not less than 1.5 m.

Description

(1) The invention is described in detail below with reference to the sets of figures, wherein:

(2) FIG. 1 is a diagram showing the principle of the implementation of the invention;

(3) FIG. 2 is a diagram in which the dimensional elements of the glazing units according to the invention are illustrated;

(4) FIG. 3 is a graph of the first step of a cycle of the assembly of the glazing units according to the invention;

(5) FIG. 4 is a graph of a second step of a cycle of the assembly of the glazing units according to the invention;

(6) FIG. 5 gives the example of the ratio of radius of curvature to thickness for three levels of corresponding stresses.

(7) FIG. 1a shows the principle of the combination of two glass sheets according to the invention. A glass sheet 1 is initially bent in a usual technique for bending a single sheet. This bending can be achieved by pressing, by gravity or a combination of pressing and gravity.

(8) A glass sheet 2 has a substantially gentler curvature, or better is plane.

(9) The two sheets have substantially identical dimensions so that once they are assembled, their edges are contiguous. This arrangement, which is not represented precisely, means that when the sheets are superposed the plane, or almost plane, sheet 2 projects very slightly in relation to the initially bent sheet 1. The diagram exaggerates the differences in shape for the purposes of understanding. In practice, since the curvatures are relatively gentle, the initial overlap remains very small, which allows an adequate final assembly edge to edge.

(10) As indicated, to enable sheet 2 to be consistent with sheet 1, their respective thicknesses are very different, wherein sheet 2 has to undergo a mechanical deformation operation utilising its flexibility, and has a small thickness compared to that of sheet 1, which is necessarily more rigid. The deformation of sheet 2 is achieved during assembly by pressure exerted onto the outside faces of the two sheets.

(11) In FIG. 1b the final assembly comprises, in addition to the two glass sheets, a thermoplastic interlayer sheet 3 of the type traditionally used in laminated glazing units.

(12) The method of composition of the glazing units according to the invention, which is based on differences in rigidity of the sheets necessarily leads to a curvature that progresses in an essentially “cylindrical” manner, i.e. in a single direction, as opposed to a “spherical” curvature that progresses in two directions. Any spherical curvature involves an “extension” in the plane of the glass sheet, which extension only being possible during the softening at elevated temperature. When cold, glass sheets do not have the elasticity that would allow a significant spherical bending. For this reason, the figures show glass sheets curved in a single direction.

(13) The assembled sheets have very different thicknesses. Assemblies that respond satisfactorily in terms of optical quality are obtained, for example, by assembling glass of 0.9 mm and 3.8 mm, but this type of assembly is primarily of interest in forming laminated structures of very small thickness. By way of example, glazing units formed with a sheet of 0.4 mm and a sheet of 1.6 mm are of particular interest. In association with an interlayer sheet of 0.76 mm, they have a total thickness of 2.76 mm and are lighter than the traditional monolithic glazing units of a greater thickness.

(14) Compared to these monolithic glazing units, the glazing units according to the invention additionally provide advantageous mechanical characteristics. In this type of assembly the anti-intrusion characteristics are added by the presence of the thermoplastic sheet. Moreover, as known, impact tests referred to as the “gravel test” show a better resistance by virtue of the assembly of two sheets of different thicknesses.

(15) FIG. 2 schematically shows the structural parameters associated with the laminated glazing units according to the invention besides those relating to the thicknesses h.sub.1 and h.sub.2 shown in FIG. 1a. The radii of curvature R.sub.1 and R.sub.2 are shown with the greatest distance d existing between the sheets before their assembly. In the most usual case according to the invention sheet 2 is plane, the radius R.sub.2 is infinite.

(16) The assembly of the sheets comprises two steps, the conditions of implementation of which are shown in FIGS. 3 and 4.

(17) The first step that takes the sheets to an assembly of the structural elements of glass sheets and thermoplastic interlayer comprises placement under pressure and the elevation of temperature.

(18) FIG. 3 indicates the development in time expressed in minutes of the pressure exerted in bar and the temperature. The pressure is expressed negatively to illustrate the fact that a relative vacuum is formed, which allows not only the pressure to be exerted on the sheets by means of the envelopes in which they are placed, but at the same time enables elimination of the air present between these sheets.

(19) Low pressure is exerted throughout the operation and is only relaxed after the temperature is brought back to ambient temperature, wherein the thermoplastic interlayer is then perfectly stabilised after the necessary softening time for adhesion of the glass sheets. This adhesion occurs in the phase corresponding to the temperature level in the order of 110° C. for a polyvinyl butyral interlayer.

(20) During this first step the low pressure presses the sheets against one another, the less rigid sheet 2 moulding to the shape of sheet 2. The interlayer does not pose any resistance to this shaping operation which is much easier when sheet 2 is less thick and therefore more flexible.

(21) At the end of the adhesion step, the sheets are well secured. The laminated glazing unit also requires an oven stage under pressure which provides it with its final transparency. In fact, after adhesion the glazing units also has a haze due to the adhesion not being perfectly uniform between the surfaces of the glass sheets and those of the interlayer. The oven at a higher temperature, e.g. 140° C., and at a pressure of 12 bar for a PVB interlayer enables the light transmission to be increased. During the oven stage the assembly must be held under pressure to prevent delamination.

(22) FIG. 4 shows the desired oven temperature (straight lines) and the temperature of the glazing (curved line). The pressure is represented by broken lines.

(23) FIG. 5 shows the relation existing between the thickness of the second sheet and the radius of curvature imposed on it during assembly. The graph shows this relationship when a surface stress is set that is not to be exceeded. As an indication, three stress values are shown: 10 MPa, 20 MPa and 50 MPa.

(24) This graph shows very clearly that the radius of curvature for the same thickness is greatly dependent on the acceptable stress level. As an indication, for a sheet 0.4 mm thick, if a stress of 50 MPa is accepted, the radius of curvature can be as small as 300 mm. If the stress must not exceed 10 MPa, the radius of curvature must not be less than 1500 mm.

(25) If the thickness of the sheet increases, the acceptable radii of curvature are also more significant. In other words, to be able to provide the sheets with a relatively small radius of curvature, it is necessary to choose sheets with as small a thickness as possible.

(26) The advantage of using a very thin second sheet is also found in the noted modifications to the final glazing in relation to the initial curvature of the previously bent sheet. Measurements of maximum deformation existing between the initial shape and that corresponding to the assembly?

(27) As an indication, for sheets extending over 50 cm in the direction of curvature and a largest initial spacing (d FIG. 2) between the two sheets and depending on the respective sheet thicknesses, a variation is noted as follows: sheet 1 1.6 mm; d 8.5 mm; sheet 2 0.4 mm; modification after assembly about 1 mm; sheet 2 3.8 mm, d 12.3 mm; sheet 2 0.4 mm; modification after assembly about 0.4 mm.

(28) As predictable, the modification is less perceptible when sheet 1 is thicker. These values correspond to relatively significant curvatures. Naturally, the choice of sheets and the initial curvature of sheet 1 must take into account these modifications in the production of glazing units with the dimensions required by the manufacturers.

(29) Tests have also been conducted to verify impact resistance. In practice, the “gravel test” consists of dropping a ball onto the glazing from increasing height. The height, at which the sheet onto which the ball falls is broken, is established. According to the requirements of the manufacturers and for the side windows the impact under consideration will be on the sheet directed towards the passenger compartment. In these conditions the impact is therefore on the thin sheet under stress. In sheet 1/sheet 2 assemblies of 1.6/0.4 mm, since the thin sheet is toughened, the determined heights are equivalent to those corresponding to those observed for monolithic glazing units.