Laminated glazing

Abstract

A laminated glazing comprising a first ply of glazing material and a second ply of glazing material joined by at least one ply of adhesive interlayer material is disclosed. The first ply of glazing material comprises a sheet of glass having a first composition and the second ply of glazing material comprises a sheet of glass having a second composition different to the first composition. The laminated glazing has (i) a peripheral region extending around the periphery of the laminated glazing, the laminated glazing having a surface compression stress in the peripheral region and (ii) an edge compression, wherein the magnitude of edge compression is greater than the magnitude of the surface compression stress in the peripheral region. A method of making such a laminated is provided. A glass sheet suitable for being incorporated in such a laminated glazing is also disclosed.

Claims

1. A laminated glazing comprising a first ply of glazing material and a second ply of glazing material joined by at least one ply of adhesive interlayer material, the first ply of glazing material comprising a sheet of glass having a first composition and the second ply of glazing material comprising a sheet of glass having a second composition different to the first composition, wherein the laminated glazing has (i) a peripheral region extending around the periphery of the laminated glazing, the laminated glazing having a surface compression stress in the peripheral region and (ii) an edge compression, wherein the magnitude of the edge compression is greater than the magnitude of the surface compression stress in the peripheral region, further wherein the second ply of glazing material is a sheet of glass that has been chemically strengthened to have a depth of layer between 10 m and 60 m.

2. A laminated glazing according to claim 1, wherein the second composition is an alkali aluminosilicate glass composition.

3. A laminated glazing according to claim 1, wherein the second composition includes at least about 6 wt % aluminium oxide.

4. A laminated glazing according to claim 1, wherein the second composition is (i) 68 mol % SiO.sub.2, 2.5 mol % Al.sub.2O.sub.3, 11 mol % MgO, 3.7 mol % CaO, 14.2 mol % Na.sub.2O and 0.6 mol % K.sub.2O, or (ii) wherein the second composition comprises 67-72 mol. % SiO.sub.2, 1-4 mol. % Al.sub.2O.sub.3, 8-15 mol. % MgO, 1-8 mol. % CaO and 12-16 mol. % Na.sub.2O, or (iii) wherein the second composition comprises (by weight) 58% to 70% SiO.sub.2, 5% to 15% Al.sub.2O.sub.3, 12% to 18% Na.sub.2O, 0.1% to 5% K.sub.2O, 4% to 10% MgO and 0% to 1% CaO with the provisos that the sum of the Al.sub.2O.sub.3 and MgO exceeds 13%, that the sum of the amounts of Al.sub.2O.sub.3 plus MgO divided by the amount of K.sub.2O exceeds 3 and that the sum of the Na.sub.2O plus K.sub.2O plus MgO exceeds 22%.

5. A laminated glazing according to claim 1, wherein the peripheral region extends in a band around the entire periphery of the laminated glazing.

6. A laminated glazing according to claim 5, wherein the band extends a distance from the periphery of the laminated glazing wherein the distance is between 200 mm and 400 mm.

7. A laminated glazing according to claim 1, wherein the surface compression stress in the peripheral region is between 5 MPa and 25 MPa.

8. A laminated glazing according to claim 1, wherein the first ply of the laminated glazing has an edge region having a residual edge stress with a net tension and wherein the residual edge stress of the first ply in the edge region has a net tension less than 15 MPa.

9. A laminated glazing according to claim 1, wherein the edge compression of the first ply is greater than 24 MPa.

10. A laminated glazing according to claim 1, wherein the first ply of glazing material has a thickness between 1.5 mm and 2.5 mm.

11. A laminated glazing according to claim 1, wherein the second ply of glazing material is a sheet of glass that has been chemically strengthened to have a surface compressive stress greater than 400 MPa.

12. A laminated glazing according to claim 1, wherein the second ply of glazing material is a sheet of glass that has been chemically strengthened to have a surface compressive stress between 450 MPa and 675 MPa.

13. A laminated glazing according to claim 1, wherein the first ply of glazing material has a concave surface and an opposing convex surface, wherein the concave surface faces the ply of adhesive interlayer material.

14. A laminated glazing according to claim 1, wherein the adhesive interlayer material comprises polyvinyl butyral (PVB), EVA, PVC, EMA, polyurethane, acoustic modified PVB or Uvekol (a liquid curable resin) and/or wherein the adhesive interlayer material has a thickness between 0.3 mm and 2.3 mm.

15. A laminated glazing according to claim 1, wherein the first composition is a soda-lime-silicate glass composition having a composition by weight of SiO.sub.2 69-74%; Al.sub.2O.sub.30-3%; Na.sub.2O 10-16%; K.sub.2O 0-5%; MgO 0-6%; CaO 5-14%; SO3 0-2% and Fe.sub.2O.sub.3 0.005-2%.

16. A vehicle glazing comprising a laminated glazing according to claim 1, wherein the vehicle glazing is a windscreen, backlight, side window, or sunroof.

17. A laminated glazing comprising a first ply of glazing material and a second ply of glazing material, the first ply of glazing material having a thickness between 1.5 mm and 2.5 mm and being annealed soda-lime-silica glass, the second ply having a thickness between 0.4 mm and 1.2 mm and being chemically toughened glass, the first ply of glazing material being joined to the second ply of glazing material by at least one ply of adhesive interlayer material, the first ply of glazing material prior to being incorporated in the laminated glazing having a peripheral region having a first residual surface compression stress, wherein when incorporated in the laminated glazing the first ply of glazing material has a second residual surface compression stress in the peripheral region, further wherein the second ply of glazing material is a sheet of glass that has been chemically strengthened to have a depth of layer between 10 m and 60 m.

18. A laminated glazing according to claim 17, wherein the first ply of glazing material after being incorporated in the laminated glazing has a residual surface compression stress in the peripheral region between 10 MPa and 20 MPa.

19. A laminated glazing according to claim 17, wherein the second ply of glazing material is a sheet of glass that has been chemically strengthened to have a surface compressive stress greater than 400 MPa.

20. A laminated glazing according to claim 17, wherein the second ply of glazing material has a composition comprising (i) 67-72 mol. % SiO.sub.2, 1-4 mol. % Al.sub.2O.sub.3, 8-15 mol. % MgO, 1-8 mol. % CaO, and 12-16 mol. % Na.sub.2O; or (ii) wherein the second ply of glazing material has a composition comprising (by weight) 58% to 70% SiO2, 5% to 15% Al.sub.2O.sub.3, 12% to 18% Na.sub.2O, 0.1% to 5% K.sub.2O, 4% to 10% MgO and 0% to 1% CaO with the provisos that the sum of the A1.sub.2O.sub.3 and MgO exceeds 13%, that the sum of the amounts of Al.sub.2O.sub.3 plus MgO divided by the amount of K.sub.2O exceeds 3 and that the sum of the Na.sub.2O plus K.sub.2O plus MgO exceeds 22%; or (iii) wherein the second ply of glazing material has the composition 68 mol % SiO.sub.2, 2.5 mol % Al.sub.2O.sub.3, 11 mol % MgO, 3.7 mol % CaO, 14.2 mol % Na.sub.2O and 0.6 mol % K.sub.2O; or (iv) wherein the second ply of glazing material is an alkali aluminosilicate glass composition.

Description

(1) The present invention will now be described with reference to the following figures in which:

(2) FIG. 1 is a cross section view of a laminated glazing according to the present invention;

(3) FIGS. 2-4 are plan views of laminated glazings in accordance with the present invention;

(4) FIG. 5 shows a schematic perspective representation of a glass ply useful for making a vehicle windscreen;

(5) FIG. 6 shows a cross-sectional view of a glass ply to show the stresses therein;

(6) FIG. 7 shows a graphical representation of the average stresses through the thickness of a glass ply as a function of distance from the edge of the glass ply;

(7) FIG. 8 is a schematic flowchart of the lamination process used to produce a laminated glazing in accordance with the present invention;

(8) FIG. 9 is a plan view of an outer ply of glass prior to being incorporated into a laminated glazing; and

(9) FIG. 10 is a plan view of the outer ply of FIG. 9 when incorporated in a laminated glazing of the type shown in FIG. 1.

(10) In the following description of the present invention, the surface compression stress measurements are made using a Strainoptics Laser GASP-CS (http://www.strainoptics.com/files/Laser%20GASP-CS%20Quick-Start%20(English).pdf). Such equipment is available from Strainoptics, Inc., 108 W. Montgomery Avenue, North Wales, Pa. 19454 USA.

(11) The edge stress measurements are made using a Sharples S-69 Stress Meter in reflection, such equipment being available from Sharples Stress Engineers Ltd, Unit 29 Old Mill Industrial Estate, School Lane, Bamber Bridge, Preston, Lancashire, PR5 6SY UK (http://www.sharplesstress.com/edgestress.htm). Edge stress measurements may also be made in transmission if no obscuration band (or the like) is on one or more of the glass surfaces being measured.

(12) FIG. 1 shows a cross section of a curved laminated glazing in accordance with the present invention.

(13) The laminated glazing 1 has a first ply 3 of soda-lime-silicate glass having a composition such as clear float glass, typically with the addition of iron oxide as a tinting agent to provide the laminated glazing with some form of solar control. The first ply 3 has a thickness of 2.3 mm although the thickness may be in the range 1.4 mm to 2.5 mm or in the range 1.6 mm to 2.3 mm.

(14) A typical soda-lime-silicate glass composition is (by weight), SiO.sub.2 69-74%; Al.sub.2O.sub.3 0-3%; Na.sub.2O 10-16%; K.sub.2O 0-5%; MgO 0-6%; CaO 5-14%; SO3 0-2%; Fe.sub.2O.sub.3 0.005-2%. The glass composition may also contain other additives, for example, refining aids, which would normally be present in an amount of up to 2%. The soda-lime-silica glass composition may contain other colouring agents such as Co.sub.3O.sub.4, NiO and Se to impart to the glass a desired colour when viewed in transmitted light. The transmitted glass colour may be measured in terms of a recognised standard such as BS EN410.

(15) The laminated glazing 1 also has a second ply 7 of chemically strengthened alkali aluminosilicate glass such as Gorilla glass available from Corning Incorporated. The second ply 7 has a thickness of 1 mm although the thickness of the second ply may be in the range 0.4 mm to 1.2 mm or in the range 0.5 mm to 1 mm.

(16) A specific composition for the second ply 7 is 68 mol % SiO.sub.2, 2.5 mol % Al.sub.2O.sub.3, 11 mol % MgO, 3.7 mol % CaO, 14.2 mol % Na.sub.2O, 0.6 mol % K.sub.2O. For this composition MgO+CaO is 14.7 mol % and Na.sub.2O+K.sub.2O is 14.8 mol %. This is composition number 13 in table 2 on page 20 of WO2014/148020A1 as published. The iron oxide (Fe.sub.2O.sub.3) content of the second ply may be low, being less than 0.1 percent by weight i.e. about 0.012 percent by weight.

(17) The first ply 3 is joined to the second ply 7 by an adhesive interlayer 5. The adhesive interlayer 5 is a 0.76 mm thick ply of PVB. The adhesive interlayer 5 may have a thickness between 0.3 mm and 1.8 mm.

(18) Other suitable adhesive interlayers include PVC, EVA, EMA and polyurethane.

(19) The laminated glazing 1 is curved in one or more directions. The radius of curvature in one of the one or more directions is between 1000 mm and 8000 mm.

(20) When the laminated glazing is curved in two directions, suitably each direction of curvature is orthogonal to the other. Suitably the radius of curvature in one or both directions of curvature is between 1000 mm and 8000 mm.

(21) The first ply 3 has a concave surface and an opposing convex surface. The second ply 7 has a convex surface and an opposing concave surface. The concave surface of the first ply 3 is in contact with the adhesive interlayer 5 and the convex surface of the second ply 7 is in contact with the adhesive interlayer 5. Using conventional nomenclature, the convex surface of the first ply 3 is surface one (or S1) of the laminated glazing 1, the concave surface of first ply 3 is surface two (or S2) of the laminated glazing 1, the convex surface of second ply 7 is surface three (or S3) of the laminated glazing 1 and the concave surface of second ply 7 is surface four (or S4) of the laminated glazing 1. As can be seen from FIG. 1, the edge of the laminated glazing comprises the edge of the first ply of glazing material 3, the edge of the second ply of glazing material 7 and the edge of the adhesive interlayer 5.

(22) The laminated glazing 1 may be a vehicle glazing such as a vehicle windscreen, a vehicle sunroof, a vehicle sidelight or a vehicle backlight. The laminated glazing 1 may be a glazing for a building, in which case the laminated glazing would typically be flat.

(23) In use, surface one faces the exterior of the vehicle or building in which the laminated glazing 1 is installed, and surface four faces the interior of the vehicle of building in which the laminated glazing 1 is installed. In accordance with the present invention the laminated glazing has a surface residual compression stress in a 300 mm band extending around the periphery of the windscreen ranging from 10 MPa to 20 MPa. This is the surface compression stress for the convex surface of the first ply 3. The first ply 3 of the laminated glazing has an edge region having a residual edge stress with a net tension below 11 MPa and an edge compression above 25 MPa.

(24) When viewed in the direction of arrow 9, the plan view of the laminated glazing may have a major surface having a periphery as shown in FIGS. 2, 3 and 4.

(25) In FIG. 2, the periphery is typical of a vehicle sidelight. In FIG. 3 the periphery is typical of a vehicle windscreen. In FIG. 4 the periphery is typical of a vehicle sunroof. In all the FIGS. 2, 3 and 4 there is shown peripheral region in the form of a band 10 extending around the periphery of the respective laminated glazing. In these examples the band extends 300 mm from the edge of the glazing towards the centre of the major surface of the glazing, all the way around the periphery as shown.

(26) Each FIGS. 2, 3 and 4 shows the respective laminated glazing has a central region 12 inboard of the peripheral region 10, the surface being in compression in region 12.

(27) FIG. 5 shows a schematic perspective view of a glass ply 71 used to make a laminated glazing having a periphery as shown in FIG. 3 i.e. a glass ply for a vehicle windscreen.

(28) The glass ply 71 is a sheet of soda-lime silicate glass i.e. float glass and has a thickness of 2.1 mm. The glass sheet has an iron oxide content expressed as Fe.sub.2O.sub.3 in the range 0.1-2% by weight, in particular 0.6-1.2% by weight.

(29) When viewed in the direction of arrow 73 the glass ply 71 has a first major surface 72 and an opposing major surface 72 (not labelled in the figure). The direction specified by arrow 73 is in a direction normal to the first major surface 72. The edge of the glass ply 71 is the minor surface extending around and between the first and second major surfaces. In FIG. 5 the edge is made up of edge portions including edge portion 75 and edge portion 77. When viewed in the direction of the arrow 73 the glass ply has a periphery defined by the lines joining points ABCD. The other opposing major surface has corresponding point ABCD (only B, C and D are shown in the figure).

(30) The edge portion 75 is the minor surface bounded by lines CD, DD, CD and CC.

(31) In FIG. 5, edge portions 75, 77 are shown as flat but may be curved, for example because the initially cut edge has been edge worked.

(32) When viewed in the direction of arrow 73, the band 70 corresponds to the band 10 shown in FIG. 3. Inboard of the band 70 is a central region 74.

(33) Although in FIG. 5 the glass ply 71 is shown as being flat, the glass ply 71 may be curved such that the major surface 72 is a convex surface and the opposing major surface 72 is a concave surface.

(34) FIG. 6 is a cross section of the glass ply 71 in the plane m-m. In FIG. 6 the edge portion 75 is shown as being rounded, but may be flat. The glass ply 71 can be seen to have a compression layer 81 and a tension layer 83. The surface compression stress is the compressive stress in surface 72 and is measured using a Strainoptics Laser GASP-CS. The edge compression is the compressive stress at the edge portion 75 and is measured using a Sharples S-69 Stress Meter in reflection or transmission.

(35) FIG. 7 shows how the average stresses vary with distance from the edge of the glass ply when measured through the glass thickness i.e. in the direction of arrow 80 in FIG. 6 (which is normal to the surface 72). The vertical axis 82 is used to represent the physical edge of the glass and the axis 85 denotes the distance therefrom. Axis portion 87 denotes average tension and axis portion 89 denotes average compression (both when measured through the glass thickness). The net stress when measured through the glass thickness i.e. in the direction of arrow 80, is given by line 90.

(36) At the edge region the glass has a compression region 86 with a maximum edge compression 88 at the physical edge of the glass. Moving inboard the edge there is a region of net tension 84 with a maximum net tension 89.

(37) Absent any other external stresses, for example when the glass ply is incorporated in a frame or vehicle body, the stresses in the glass ply are known as residual stresses.

(38) With reference to FIG. 8, a laminated vehicle windscreen according to the present invention is manufactured as follows.

(39) Initially a sheet of soda-lime silicate glass is provided at step 21. The soda-lime-silicate glass may be clear or tinted. In this example a sheet of clear float glass is provided at step 21. By clear float glass, it is meant a glass having a composition as defined in BS EN 572-1 and BS EN 572-2 (2004).

(40) The sheet of clear float glass is 2.3 mm thick and at step 23 is cut to the desired shape using conventional techniques. Once bent, the cut sheet of clear float glass will be the outer ply of the windscreen. The cut sheet of clear float glass before being bent is known as the outer blank.

(41) The sheet of clear float glass may have a thickness between 1.6 mm and 2.3 mm.

(42) At step 25 the edges of the outer blank are edge worked following which the outer blank is washed.

(43) After washing, one or both major surfaces of the outer blank may be printed with a layer of ink, which may be optically opaque and/or electrically conductive. Such printed inks may be used to form an obscuration band around the periphery of the outer ply, as is conventional in the art.

(44) At step 27 the outer blank is heated to glass softening temperature in a suitable furnace. The heat softened glass is press bent between a pair of complimentary shaping members to impart a desired curvature to the outer ply. Press bending allows a precise control of the shape of the outer ply.

(45) Example press bending stations are described in WO2005/033026A1 and EP0677486A2.

(46) To control the stresses in the outer ply, the upper and/or lower press bending member is heated to control the residual edge stress and/or the edge compression of the outer ply.

(47) By selection of the temperature of the upper and/or lower press bending members it is possible to produce an outer ply having an edge region having a residual edge stress with a net tension below 11 MPa and an edge compression above 25 MPa.

(48) The residual surface stress can be controlled by directing cooling air around the periphery of the bent outer ply shortly after the completion of the press bending operation and before the bent outer ply is cooled to room temperature.

(49) By blowing cooling air around the glass edges the residual surface stress is controlled to 15-20 MPa in a band extending around the periphery of the bent outer ply, the band extending 300 mm from peripheral edge of the bent outer ply. This is shown in FIG. 9.

(50) After the cooling air is directed onto the edges for the suitable length of time to produce the required residual stress in the cooled bent outer ply, the bent outer ply is controllably cooled at step 29 to room temperature in a suitable annealing furnace.

(51) Bending the outer plies may be part of a batch process, with a number of outer plies being bent one after another.

(52) The outer ply may be bent in one or more directions. The curvature in the one or more directions may have a radius of curvature between 1000 mm and 8000 mm.

(53) The inner ply of the laminated glazing is produced as follows.

(54) At step 31 a sheet of alkali aluminosilicate glass such as Gorilla glass available from Corning Incorporated is provided and will be used for the inner ply of the windscreen. Suitable alkali aluminosilicates compositions are described in U.S. Pat. No. 7,666,511 B2, WO2014/148020A1 and WO99/48824A1.

(55) A specific composition for the inner ply is 68 mol % SiO.sub.2, 2.5 mol % A1.sub.2O.sub.3, 11 mol % MgO, 3.7 mol % CaO, 14.2 mol % Na.sub.2O, 0.6 mol % K.sub.2O. For this composition MgO+CaO is 14.7 mol % and Na.sub.2O+K.sub.2O is 14.8 mol %. This is composition number 13 in table 2 on page 20 of WO2014/148020A1 as published.

(56) The sheet of alkali aluminosilicate glass is 1 mm thick and is cut at step 33 to have the same periphery as the unbent outer blank. Before being bent, the cut sheet of alkali aluminosilicate glass is known as the inner blank. The sheet of alkali aluminosilicate glass may have a thickness between 0.4 mm and 1.2 mm, or a thickness between 0.5 mm and 1 mm.

(57) At step 35, the inner blank is suitably edge worked and washed prior to being bent.

(58) At step 37 the inner blank is placed on a suitable ring mould to support the inner blank close to the periphery thereof. The inner blank is heated to sufficient temperature to cause the alkali aluminosilicates glass to soften and sag under the influence of gravity, conventionally referred to as sag bending. The glass sag bends to a shape close to that of the bent outer ply. However the curvature of the inner ply may not be the same as the outer ply.

(59) At step 39 the bent inner ply is then annealed using controlled cooling to reduce the temperature to room temperature.

(60) At step 40 the bent inner ply of alkali aluminosilicate glass is chemically strengthened using an ion exchange process. Typically sodium ions are chemically exchanged for potassium ions.

(61) For the specific composition mentioned above, it is possible to chemically strengthen the inner ply to have surface compressive stress greater than 400 MPa, typically between 450 MPa and 675 MPa. The depth of layer (DOL) of the chemically strengthened glass ply was between 10 m and 60 m.

(62) It is also envisaged that the bent inner ply of alkali aluminosilicate glass may be thermally toughened although it is difficult to thermally toughen plies of glass that have a thickness of 1 mm or less.

(63) At step 41 a bent outer ply (following steps 21-29) and a bent inner ply (following steps 31-40) are provided.

(64) The pair of bent inner and outer plies are washed at step 42 and at step 44 a ply of interlayer material having a thickness between 0.3 mm and 1.5 mm is positioned between the inner ply and the outer ply. In this particular example the interlayer material was a 0.76 mm thick ply of PVB, although other suitable adhesive interlayer material may be used, for example acoustic modified PVB.

(65) At step 46 the assembly of inner ply and outer ply with PVB ply therebetween are laminated using suitable lamination conditions to join the inner ply to the outer ply via the PVB ply.

(66) The laminated glazing so produced at step 48 is washed and inspected prior to being delivered to a customer.

(67) The resulting laminated windscreen has a surface residual stress where there is a surface compression band ranging from 10 MPa to 20 MPa extending around the periphery of the bent outer ply, the band extending 300 mm from peripheral edge of windscreen. The windscreen has an edge region having a residual edge stress with a net tension less than 11 MPa and an edge compression greater than 25 MPa. The outer ply in the laminated glazing may have a different surface compression stress in the peripheral region when compared to the bent outer ply prior to being incorporated in the laminated glazing because of the stress imparted to the outer ply from the lamination process.

(68) With reference to FIGS. 9 and 10, there is shown in FIG. 9 a plan view of a bent outer ply 57 produced at step 29 i.e. a bent ply of soda-lime-silicate glass. The outer ply 57 has four corners A, B, C and D. The line connecting corner A to corner C is substantially straight. The line connecting corner B to corner D is substantially straight. The line connecting corner A to corner B is curved and the line connecting corner C to corner D is curved. The periphery of the outer ply 57 is defined by the lines AB+BD+DC+CA.

(69) FIG. 9 illustrates the band that extends around the periphery. Within the periphery of the outer ply 57 is a band i.e. band is inboard of the periphery. The band has a width 52, 54, 56 and 58. Typically the distances 52, 54, 56 and 58 are the same. In this example the distances 52, 54, 56 and 58 are 300 mm. The band can be thought as defining a surface compression stress region 60, beyond which (i.e. inboard of) is a central region 62 where the surface in the central region 62 is in compression. The central region is defined by the points E, F, G and H. The line connecting points E and G is substantially straight and parallel to line AC. The line connecting points F and H is substantially straight and parallel to line BD. The line connecting points E and F is curved and parallel to line AB. The line connecting points G and H is curved and parallel to line CD.

(70) The surface compression stress region 60 is equivalent to the region 10 shown in FIG. 3 i.e. the surface compression stress region 60 is a peripheral region. In this example the surface compression stress in the surface compression stress region 60 is between 15 MPa and 20 MPa.

(71) With reference to FIG. 8 and FIG. 10, FIG. 10 shows a plan view of laminated glazing produced at step 48 (as viewed in the direction of arrow 9 of FIG. 1).

(72) The periphery of the laminated glazing 51 is defined in terms of the points A, B, C and D. These points correspond to the points A, B, C and D of FIG. 9.

(73) The surface compression stress region 60 of the laminated glazing 51 is defined in the same way as the surface compression stress region 60 i.e. the surface compression stress region 60 is a peripheral region. The surface compression stress in the surface compression region 60 is between 10 MPa and 20 MPa.

(74) Bounded by the peripheral stress region 60 is a central region 62 where the surface in central region 62 is in compression. As for the outer ply shown in FIG. 9, the distances 52, 54, 56 and 58 are 300 mm.

(75) The present invention provides a laminated glazing, in particular a vehicle windscreen, sunroof, backlight or sidelight, comprising an annealed outer glass with a thickness ranging from 1.6 mm to 2.3 mm laminated with an adhesive interlayer to a chemically toughened inner ply with a thickness ranging from 0.5 to 1 mm. The outer ply is produced by press bending to fully meet customer surface design with a residual surface compression stress on a 300 mm band around the windscreen contour ranging from 15 MPa to 20 MPa and a residual edge stress with a net tension below 11 MPa and an edge compression above 25 MPa. The inner ply is bent using a gravity bending process to an intermediate shape able to give a surface residual stress after lamination on the 300 mm band around the windscreen ranging from 10 MPa to 20 MPa, and with a residual edge stress of the outer ply with a net tension below 11 MPa and an edge compression above 25 MPa.

(76) In accordance with the present invention it has been found that due to the strength of the chemically strengthened inner ply it is not necessary to bend the inner ply to the same degree of precision as the outer ply. The shape of the windscreen is essentially fixed by the outer ply and the inner ply is able to flex slightly to be able to conform to the shape of the outer ply when joined thereto via an adhesive interlayer. The stress in the resulting laminated windscreen can be modified by the stress profile in the surface of the outer ply and the stress characteristics of the chemically strengthened inner ply. It has been found that by having a windscreen made in accordance with the present invention it is possible to have a lightweight windscreen that has superior stone impact performance compared to conventional windscreens.