# Patent

## Title
Laminated glazing for use as a head-up display screen

## Abstract

A laminated glazing used as a head-up display screen includes two sheets of glass and a viscoelastic plastic interlayer arranged between the sheets of glass. The interlayer includes at least one layer made of viscoelastic plastic with vibro-acoustic damping properties. The interlayer has a cross section that decreases in a wedge shape from the top to the bottom of the glazing. A resonant frequency f2 of the second resonance mode of a laminated glazing bar with a surface area of 25 mm×300 mm composed of two glass sheets each 2.1 mm thick, between which is incorporated the interlayer, determined by measuring the mechanical impedance (MIM) at 20° C. according to standard ISO 16940, is between 760 Hz and 1000 Hz and the loss factor η2 of the second resonance mode of the same bar, determined by MIM under the same conditions, is greater than or equal to 0.25.

## Background
BACKGROUND
The invention relates to a laminated glazing for use as a head-up display screen. Head-up displays are useful in all types of vehicles, in particular motor vehicles.
Head-up displays display information projected onto a laminated glazing, which is reflected towards the driver or the observer. These systems can especially give information to the vehicle driver without the driver having to divert his gaze from the front field of vision of the vehicle, so as to ensure safety during driving. The driver then sees a virtual image located a certain distance behind the windscreen.
If standard laminated glazing is used for such a system, the driver sees a double image: a first image reflected by the surface of the windscreen oriented into the cockpit, and a second image reflected by the outer surface of the windscreen, these two images being slightly shifted relative to each other. This shift may disrupt the viewing of the information. To overcome this problem, use may be made of a laminated windscreen formed from two sheets of glass and an interlayer made of polyvinyl butyral (PVB), the two outer faces of which are not parallel but wedge-shaped, such that the image projected by a display source and reflected by the face of the windscreen oriented into the cockpit is virtually superimposed on the same image originating from the same source reflected by the face of the windscreen oriented outwards. To produce this wedge-shaped laminated glazing, use is made of an interlayer sheet whose thickness decreases from the upper edge of the glazing to the lower edge.
Moreover, among all the qualities contributing to comfort in modern transportation means such as trains and motor vehicles, silence has become a determining factor.
Acoustic comfort has been improved over several years now, by treating noise, such as noise from the engine, rolling or suspension noise, either at their source or during their propagation through the air or through solids, by means, for example, of absorbent coatings or elastomeric connecting pieces.
The shapes of vehicles have also been modified to improve the penetration in air and to reduce the turbulence, which are themselves sources of noise.
In recent years, attention has been focused on the role that glazing might play in improving acoustic comfort, in particular laminated glazing comprising plastic interlayer films. Laminated glazing also has other advantages, such as elimination of the risk of projection of fragments in the event of sudden breakage, constituting an effraction retarder.
It has been demonstrated that the use of standard plastic films in laminated glazing is unsuitable for improving acoustic comfort. Specific plastic films were then developed, which had damping properties that allow an improvement in the acoustic comfort.
Moreover, in the existing windscreens, the thickness of the glass sheet intended to be turned facing the exterior of the vehicle is generally 2.1 mm and the thickness of the glass sheet intended to be turned facing the interior of the vehicle is generally 1.6 mm. However, the tendency is towards lightening motor vehicles in order to reduce their consumption and the induced discharge of CO2. One means is to propose lighter motor vehicle glazing. One solution for reducing the weight of glazing is to reduce the thickness of the glass sheets. However, this reduction of thickness results in a degradation of the acoustic properties of the laminated glazing.
There is thus a need for a laminated glazing intended to be used as a head-up display screen, the laminated glazing also having vibro-acoustic damping properties and allowing a reduction in the thickness of the glass sheets without degradation of the vibro-acoustic damping properties.
BRIEF SUMMARY
To do this, the invention proposes a laminated glazing intended to be used as a head-up display screen, comprising:

    
    
        two sheets of glass,
        a viscoelastic plastic interlayer arranged between the two sheets of glass, the interlayer comprising at least one layer made of a viscoelastic plastic material with vibro-acoustic damping properties and the interlayer having a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing,
    
    


in which the interlayer is such that the resonant frequency f2 of the second resonance mode of a laminated glazing bar with a surface area of 25 mm×300 mm composed of two glass sheets each 2.1 mm thick, between which is incorporated the interlayer, determined by measuring the mechanical impedance (MIM) at 20° C. according to standard ISO 16940, is between 760 Hz and 1000 Hz and the loss factor η2 of the second resonance mode of the same bar, determined by MIM under the same conditions, is greater than or equal to 0.25 a viscoelastic plastic interlayer intended to be incorporated between two glass sheets to form a laminated glazing with vibro-acoustic damping properties, the interlayer comprising at least one layer made of viscoelastic plastic with vibro-acoustic damping properties, the interlayer being such that the resonant frequency f2 of the second resonance mode of a laminated glazing bar with a surface area of 25 mm×300 mm composed of two glass sheets each 2.1 mm thick, between which is incorporated the interlayer, determined by measuring the mechanical impedance (MIM) at 20° C. according to standard ISO 16940, is between 760 Hz and 1000 Hz and the loss factor η2 of the second resonance mode of the same bar, determined by MIM under the same conditions, is greater than or equal to 0.25.
According to another particular feature, the layer is based on polyvinyl butyral and plasticizer.
According to another particular feature, the sheets of glass are curved.
According to another particular feature, the resonant frequency f2 is between 800 and 900 Hz and preferably between 800 Hz and 850 Hz.
According to another particular feature, the loss factor η2 is greater than 0.30.
According to another particular feature, the interlayer also comprises two outer layers made of standard PVB, the layer being between the two outer layers.
According to another particular feature, only one of the two outer sheets has a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing, the layer of viscoelastic plastic material with vibro-acoustic damping properties and the other outer layer having a constant cross section from the top to the bottom of the laminated glazing.
According to another particular feature, the two outer layers have a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing, the layer of viscoelastic plastic material with vibro-acoustic damping properties having a constant cross section from the top to the bottom of the laminated glazing.
According to another particular feature, the measurement of the mechanical impedance is performed at least one month after assembling the laminated glazing bar, the laminated glazing bar having itself been assembled at least one month after the manufacture of the interlayer.
The invention also relates to glazing comprising:

    
    
        one of the glass sheets is between 0.5 mm and 2.6 mm thick,
        one of the glass sheets is between 0.5 mm and 1.6 mm thick, the total thickness of the glass sheets being strictly less than 3.7 mm.
    
    


The invention also relates to a motor vehicle comprising glazing described above, the glass sheet between 0.5 mm and 2.6 mm thick being turned facing the exterior of the vehicle and the glass sheet between 0.5 mm and 1.6 mm thick being turned facing the interior of the vehicle.
The invention also relates to the use of the glazing described above as a motor vehicle windscreen.



BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention will now be described with regard to the drawings, in which:
FIG. 1 shows a curve of the acoustic insulation as a function of the frequency, measured on three windscreens;
FIG. 2 shows a view in cross section of glazing according to the invention.



DETAILED DESCRIPTION
The invention relates to a laminated glazing intended to be used as a head-up display screen, comprising two sheets of glass and a viscoelastic plastic interlayer arranged between the two sheets of glass, the interlayer comprising at least one layer made of viscoelastic plastic material with vibro-acoustic damping properties and the interlayer having a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing.
The interlayer is such that the resonant frequency f2 of the second resonance mode of a laminated glazing bar with a surface area of 25 mm×300 mm composed of two glass sheets each 2.1 mm thick, between which is incorporated the interlayer, determined by measuring the mechanical impedance (MIM) at 20° C. according to standard ISO 16940 (with only one difference concerning the thickness of the glass sheets of the bar, which is 2.1 mm instead of 4 mm), is between 760 Hz and 1000 Hz and the loss factor η2 of the second resonance mode of the same bar, determined by MIM under the same conditions, is greater than or equal to 0.25.
The inventors have demonstrated, as will be seen later, that an interlayer comprising these characteristics makes it possible to obtain thinned laminated glazing whose acoustic performance qualities are equivalent or even superior to those of laminated glazing with standard thicknesses of glass incorporating a known interlayer with improved acoustic properties.
FIG. 2 shows a view in cross section of glazing according to the invention.
The glazing comprises two glass sheets, 1, 2 between which is inserted the interlayer according to the invention. The solid connection of the interlayer to the glass sheets is made by known means, for example by stacking the glass sheets and the interlayer and by passing the assembly into an autoclave.
The glass sheet 1 of the glazing is intended to be turned facing the exterior of the vehicle, whereas the glass sheet 2 is intended to be turned facing the interior of the vehicle. The glass sheet 1 is preferably thicker than the glass sheet 2 such that the glazing affords better protection against external attack (inclement weather, projection of gravel, etc.). Specifically, the thicker the glass, the greater its mechanical strength. However, the thicker the glass is, the heavier it is. A compromise must thus be found between the mechanical strength and the weight of the glazing. Thus, the thickness of the glass sheet 1 is, for example, between 0.5 mm and 2.6 mm and preferably between 1.4 mm and 2.0 mm, and the thickness of the glass sheet 2 is, for example, between 0.5 mm and 1.6 mm and preferably between 1.1 mm and 1.5 mm.
In the existing glazing, the thickness of the glass sheet 1 is generally 2.1 mm and the thickness of the glass sheet 2 is generally 1.6 mm, i.e. a total glass thickness of 3.7 mm.
The glazing according to the present invention comprises a total glass thickness strictly less than 3.7 mm, preferably less than or equal to 3.2 mm.
Preferably, according to the invention, the thickness of the glass sheet 1 is 1.8 mm and the thickness of the glass sheet 2 is 1.4 mm so as to limit the weight of the glazing, which makes it possible to reduce the consumption of fuel of a vehicle equipped with such glazing. This also makes it possible to manipulate the glazing more easily and to save on material.
The glazing according to the invention may also have a glass sheet 1 1.6 mm thick and a glass sheet 2 1.2 mm thick, or a glass sheet 1 1.4 mm thick and a glass sheet 2 1.1 mm thick.
The interlayer is constituted by at least one layer 3 made of viscoelastic plastic with vibro-acoustic damping properties. It is preferably based on polyvinyl butyral and plasticizer. The content and nature of the plasticizer and the degree of acetalization of the polyvinyl butyral make it possible to modify in a known manner the rigidity of a component based on polyvinyl butyral and plasticizer.
In the example of FIG. 2, the interlayer also comprises two layers 4, 5, referred to as outer layers, between which is inserted the layer 3.
The outer layers 4, 5 are preferably made of standard PVB. The layer 3 is less rigid than the outer layers 4, 5 so as to vibrate correctly to ensure the desired acoustic damping.
As a variant, the interlayer may comprise at least two layers made of viscoelastic plastic with vibro-acoustic damping properties, optionally surrounded by layers of standard PVB.
The acoustic characteristics of the interlayer are determined by measuring the mechanical impedance (MIM) at 20° C. according to standard ISO 16940 of a laminated glazing bar with a surface area of 25 mm×300 mm composed of two glass sheets each 2.1 mm thick (and not 4 mm thick as recommended in standard ISO 16940), between which is incorporated an interlayer according to the invention, i.e. an interlayer comprising at least one layer of viscoelastic plastic with vibro-acoustic damping properties.
MIM makes it possible to determine the resonant frequencies and the loss factors of the various resonance modes of the laminated glazing bar.
The interlayer is in accordance with the invention if the resonant frequency f2 of the second resonance mode of the laminated glazing bar determined by MIM is between 760 Hz and 1000 Hz and the loss factor η2 of the second resonance mode of the laminated glazing bar determined by MIM is greater than or equal to 0.25.
Preferably, the resonant frequency f2 is between 800 Hz and 900 Hz, which makes it possible to have improved acoustic performance qualities while degrading less the level of weakening of the laminated glazing before the critical frequency. More preferably, the resonant frequency f2 is between 800 Hz and 850 Hz, which makes it possible to have more improved acoustic performance qualities while degrading even less the level of weakening of the laminated glazing before the critical frequency.
Preferably, the loss factor η2 is greater than 0.30, which makes it possible to have improved acoustic performance qualities while improving the acoustic damping.
The measurement of the mechanical impedance (MIM) is performed at least 1 month after assembling the laminated glazing bar, the laminated glazing bar itself having been assembled at least 1 month after the manufacture of the interlayer. This makes it possible to be sure that the interlayer and the laminated glazing have achieved stable states and thus to determine reliable values.
FIG. 1 shows a curve of the acoustic insulation as a function of the frequency, measured on three windscreens. The acoustic insulation of a glazing takes into account the acoustic performance qualities that may be observed on a vehicle equipped with the said glazing.
Thus, a first windscreen (21-16 known) comprises:

    
    
        two glass sheets 2.1 mm and 1.6 mm thick, respectively, and
        an interlayer comprising two outer layers made of standard PVB and a central layer made of viscoelastic plastic with vibro-acoustic damping properties, the interlayer having a resonant frequency f2 of 675 Hz (±15 Hz) and a loss factor η2 equal to 0.35 (±0.03).
    
    


The first windscreen corresponds to a standard windscreen with a known interlayer with acoustic damping properties.
The curve of acoustic insulation (shown as diamonds) of the first windscreen shows a dip at about 6500 Hz.
A second windscreen (18-14 known) comprises:

    
    
        two glass sheets 1.8 mm and 1.4 mm thick, respectively, and
        an interlayer comprising two outer layers made of standard PVB and a central layer made of viscoelastic plastic with vibro-acoustic damping properties, the interlayer having a resonant frequency f2 of 675 Hz (±15 Hz) and a loss factor η2 equal to 0.35 (±0.03).
    
    


The second windscreen corresponds to a thinned windscreen with an interlayer identical to that of the first windscreen.
The curve of acoustic insulation (shown by squares) of the second windscreen shows similar behaviour to that of the first windscreen up to about 5000 Hz, but a dip shifted towards the high frequencies, at about 8000 Hz. This shift in the dip is very troublesome since it implies that this windscreen allows aerial noise to pass at high frequencies that are irritating to the human ear.
A third windscreen (18-14 invention) comprises:

    
    
        two glass sheets 1.8 mm and 1.4 mm thick, respectively, and
        an interlayer comprising two outer layers made of standard PVB and a central layer made of viscoelastic plastic with vibro-acoustic damping properties, the interlayer having a resonant frequency f2 of 800 Hz (±15 Hz) and a loss factor η2 equal to 0.30 (±0.03).
    
    


The third windscreen corresponds to a thinned windscreen with an interlayer according to the invention.
The curve of acoustic insulation (shown by triangles) of the third windscreen shows behaviour similar to that of the first windscreen, with a dip centred at about 6500 Hz and acoustic insulation values similar to those of the first windscreen.
The windscreen with an interlayer according to the invention thus indeed makes it possible to compensate for the acoustic degradation associated with the thinning of the glazing.
The laminated glazing according to the invention may be used as a motor vehicle windscreen. In this case, it of course satisfies all the conditions of rule No. 43 of the United Nations (known as rule R43) of hard impact strength for ensuring its mechanical strength. To do this, in the case of an interlayer comprising two outer layers made of standard PVB, the thickness of these outer layers 4, 5 is, for example, adapted in a known manner, for example by patent application FR 09 52567.
In order for the laminated glazing according to the invention to be able to be used as a head-up display screen, the interlayer has a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing. Preferably, only one or the two outer layers has/have a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing, layer 3 made of viscoelastic plastic material with vibro-acoustic damping properties having a constant cross section from the top to the bottom of the laminated glazing. Furthermore, the two sheets of glass 1, 2 are curved.

## Claims

1. A laminated glazing to be used as a head-up display screen, comprising:
two sheets of glass;
a viscoelastic plastic interlayer arranged between the two sheets of glass, the interlayer comprising at least one layer made of viscoelastic plastic material with vibro-acoustic damping properties and the interlayer having a cross section that decreases in a wedge shape from a top to a bottom of the laminated glazing,
in which the interlayer being such that a resonant frequency f2 of a second resonance mode of a laminated glazing bar with a surface area of 25 mm×300 mm composed of two glass sheets each 2.1 mm thick, between which is incorporated the interlayer, determined by measuring a mechanical impedance (MIM) at 20° C. according to standard ISO 16940, is between 760 Hz and 1000 Hz and a loss factor η2 of the second resonance mode of the same bar, determined by MIM under the same conditions, is greater than or equal to 0.25.

2. The laminated glazing according to claim 1, in which the layer includes polyvinyl butyral and plasticizer.

3. The laminated glazing according to claim 1, in which the sheets of glass are curved.

4. The laminated glazing according to claim 1, in which the resonant frequency is between 800 and 900 Hz.

5. The laminated glazing according to claim 1, in which the loss factor η2 is greater than 0.30.

6. The laminated glazing according to claim 1, further comprising:
two outer layers made of standard PVB, the layer being between the two outer layers.

7. The laminated glazing according to claim 6, in which only one of the two outer layers has a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing, the layer made of viscoelastic plastic material with vibro-acoustic damping properties and the other outer layer having a constant cross section from the top to the bottom of the laminated glazing.

8. The laminated glazing according to claim 6, in which the two outer layers have a cross section that decreases in a wedge shape from the top to the bottom of the laminated glazing, the layer made of viscoelastic plastic material with vibro-acoustic damping properties having a constant cross section from the top to the bottom of the laminated glazing.

9. The laminated glazing according to claim 1, in which the measurement of the mechanical impedance is performed at least one month after assembly of the laminated glazing bar, the laminated glazing bar having been assembled at least one month after manufacture of the interlayer.

10. The laminated glazing according to claim 1, in which:
one of the glass sheets is between 0.5 mm and 2.6 mm thick,
one of the glass sheets is between 0.5 mm and 1.6 mm thick,
a total thickness of the glass sheets being less than 3.7 mm.

11. A motor vehicle, comprising:
the glazing according to claim 9, the glass sheet between 0.5 mm and 2.6 mm thick being turned facing an exterior of the vehicle and the glass sheet between 0.5 mm and 1.6 mm thick being turned facing an interior of the vehicle.

12. The glazing according to claim 1, wherein the glazing is a motor vehicle windscreen.

13. The glazing according to claim 1, in which the resonant frequency f2 is between 800 and 850 Hz.