FR3140790A1 - Glazing with floating antistatic layer - Google Patents

Abstract

Laminated glazing comprising a first outer sheet of glass (2) with a thickness of between 0.5 and 5 mm and at least one sheet of structural glass (3, 3') bonded two by two by an adhesive interlayer (4, 6) ), the laminated glazing being fixed by pinching to a mounting structure by a retainer (1) bolted to it and electrically secured to the mass thereof, the free surface of the first sheet of glass (2) supporting a floating antistatic layer (8) of electrical conductivity less than 10 MOhm per square, and located at most 10 mm from the retainer (1), or more than 10 mm from the retainer (1) and the main surface of the first sheet of glass (2) internal to the structure of the laminated glazing supporting an electrode (9) electrically secured to the ground of the mounting structure; use as aircraft glazing. Figure for abstract: Figure 2

Description

Glazing with floating antistatic layer

Aeronautical glazing is generally made of electrically insulating materials (glass, PMMA, PC, etc.) which tend to become electrically charged in certain environments through triboelectric effects. These environments are for example clouds of dust from volcanic eruptions, snow, ice crystals. Typical currents that may be encountered are a few hundred µA/m² of glazing. For such currents, the flows in the thickness and on the surface of the glazing can be neglected.

A surface potential of the glazing is then created which is different from that of the aircraft structure which is generally equipped with passive dissipators.

In addition, aeronautical glazing is generally equipped with an anti-frost system covering all or part of the laminated surface of the glass (organic or mineral) exposed to load phenomena on its other side (external glass sheet, or external glass).

Under loading conditions this results:
-an intense electric field in the thickness of the external glass (between its heating side and its charged side) which can induce a dielectric breakdown in its thickness and its ruin;
-significant electrical energy stored in the capacitor formed by the thickness of the external glass (dielectric) and the two electrodes which are its two surfaces. This energy can supply tree surface discharges between the structure of the aircraft at the periphery of the glazing and the surface of the glazing. These discharges cause:
-electromagnetic radiation that could disrupt the electronics of the aircraft;
-overvoltages or overcurrents in particular on the temperature probe circuits facing the heating layer and making it possible to regulate the temperature of the glazing to prevent icing;
-dazzling of pilots by the light radiation from the discharges.

Establishing a surface discharge requires a sufficiently energetic plasma at the discharge tip. In practice, for a glass 3 mm thick and under an atmospheric pressure of 1 bar, a surface potential of at least 25 kV is required for surface discharges to be established.

For all these reasons we are sometimes required to provide an electrostatic discharge system which can:
-be made up of a surface treatment of the glass aimed at giving it dissipative (electrical) properties and connected to the mass of the aircraft;
-equip the external surface of the glazing with metal points connected to the aircraft ground allowing early corona discharge. This second solution is not applicable to mineral glass because in practice it is necessary to pierce the external glass to route a wire to its surface. It is therefore more often used on pierceable plastic glazing. The first solution can be applied to mineral glasses by applying a durable coating of the indium tin oxide type (in English “Indium Tin Oxide” – ITO-) or diamond type carbon (in English “Carbon Like Diamond” - DLC) or substoichiometric oxide. On organic glasses, conductive layers can also be applied but they are generally sensitive to scratches. For all of these solutions, the electrical charges are drained to aircraft ground via electrical wires.

Managing the routing of electrical wires to aircraft ground induces complexities linked to issues of bulk, and/or visibility and/or interface and/or durability of the wire - dissipative layer connection. In particular, the dissipative layers being on the surface of the glazing they are subject to eroding environments. An electrical connection on this surface induces extra thickness increasing exposure to erosion and requiring a flush protection strategy (aerodynamic disturbance, aerodynamic noise, erosion and associated maintenance). Finally, the attachment of lightning to these drains connected to the aircraft ground in its internal part can induce:
-structural degradation of the glazing;
-electromagnetic radiation inside the aircraft structure;
-heating and projections of molten metals inside the aircraft which could cause fires.

The invention was carried out by wondering whether the above problems cannot be eradicated by leaving the antistatic layer floating, that is to say not electrically connected to the ground of the mounting structure (in particular cabin of plane). The system must then be designed to promote discharges that do not degrade the performance of the aircraft. These questions could be answered favorably by the invention which, consequently, relates to laminated glazing comprising a first sheet of glass and at least one sheet of structural glass bonded two by two by an interlayer adhesive layer, the surface main free part of the first sheet of glass being intended to be in contact with the external atmosphere, the first sheet of glass being set back relative to said at least one sheet of structural glass, and having a thickness of between 0.5 and 5 mm, the edges of the first sheet of glass and the first adhesive interlayer, and the free part of the main surface of the structural glass sheet, overhanging the first sheet of glass, forming a reception cavity 'a retainer for fixing the laminated glazing by pinching to a mass mounting structure to which the retainer is electrically secured, said free surface of the first sheet of glass supporting an antistatic layer not electrically secured to the mass of the mounting structure , and electrical conductivity less than 10 MOhm per square, characterized in that at least one point of the antistatic layer is located at most 10 mm from the retainer, or the antistatic layer is entirely located more than 10 mm from the retainer and the main surface of the first sheet of glass internal to the structure of the laminated glazing supports an electrode electrically secured to the ground of the mounting structure.

When two flat electrodes are facing each other, from a threshold electric field (breakdown field), a discharge can begin and almost instantly generates an arc. The discharge phase corresponds to the establishment of an electric field sufficient for a first ionization (ionizing radiation for example) to generate an avalanche of ionizations. The electrons are in fact accelerated by the effects of the electric field and can then, by electronic bombardment of the gases, generate other ionizations. Other mechanisms are involved (excitation of molecular species, UV radiation, photo-ionization, electronic emission under ion and electron impact, electronic attachment, etc.).

The arc results from the creation of a conductive path in the air by heating the air by the passage of current. In plane/tip or tip/tip configurations, the electric field is not homogeneous and is more intense near the tips. Only a volume close to the tips is then capable of causing ionization avalanches. The discharge remains located near the peaks. We then speak of crown discharge or partial discharge. Despite its partial nature, this discharge is capable of discharging a current of the order of mA/cm for linear geometries, that is to say a high current compared to the charging currents on aeronautical glazing, of the order of a few hundred µA/m² in heavily loaded conditions.

The electric field generated by charged elements can generate atmospheric plasmas. The ionized species (ions and electrons) are then transported by electric fields and in particular charges can be transported towards insulating surfaces such as glass. In the event of initiation of partial discharges, the electric fields are modified by space charges in the air (separation of differently charged species and different mobility of species) and on insulating surfaces. The mobility of surface charges being much lower than that of charges in the atmospheric phase, it is the latter which have the greatest influence on the potentials. Thus for low intensity discharges, the potentials of the insulators will be established so that no net current is collected.

For practical reasons it is extremely complicated to ensure continuity between the antistatic layer and the retainer. There is therefore a dielectric space on the surface of the glazing between the edge of the antistatic layer and the metal retainer. In a charging environment, different potentials are established between the retainer, the surface of the antistatic layer and the insulating dielectrics. This results in intense electric fields, particularly through the peak effect at the edge of the metal structure and at the edge of the (very thin) antistatic layer. This results in the initiation of partial discharges generating a plasma charging the surface of the dielectrics.

In equilibrium, the surface of the dielectrics is found at intermediate potentials between metallic structure and antistatic layer. The peak effects are then screened by the space charges on the dielectrics. This results in an extinction of partial discharges. It is then possible to achieve in the air gap between the retainer and the antistatic layer potential lines relatively parallel to each other and perpendicular to the surface of the glazing, characteristic of plane/plane discharges and therefore the discharge criteria are governed by Paschen's law. These discharges have an immediate arc transition. The arc currents being very high, the charges of the antistatic layer cannot power the arc by conduction in the latter but only by movement of the arc head on the surface of the antistatic layer. This is the surface discharge mechanism already described. To avoid these discharges, it is necessary to ensure that the potential of the glass is lower than the potential necessary for the establishment of surface discharges (typically 25 kV for 3 mm glass). The breakdown fields described by Paschen's law are 30 kV/cm at atmospheric pressure (worst case). To allow partial discharges, the edge of the layer must then be located less than 8 mm from the retainer at at least one point.

Thus, according to the first alternative of the invention, the floating antistatic layer of electrical conductivity less than 10 MOhm per square positioned on a glass of 0.5 to 5 mm is located at at least one point less than 10 mm from a element electrically attached to the aircraft mass.

In accordance with the second alternative of the invention, an electric field singularity is generated in the vicinity of the edge of the floating antistatic layer by means of an electrode integral with the aircraft mass present in the laminate, ideally on the internal surface of the external glass. In this second alternative or configuration, the electric field is more influenced by the potential of the laminate electrode than by the retainer.

Here again the initiation of discharges generates a deposition of charges on the surface of the glass in such a way that the electric field on the surface of the glass is parallel to it in the active zone of the discharge. In this case the influence of the ground electrode makes it possible to create a peak effect at the edge of the layer.

If the edge of the antistatic layer is sufficiently far from the edge of the retainer, we can then generate a corona discharge which does not attach to the glazing retainer, thus avoiding any phenomenon of degradation of the latter.

Such a configuration is known as a dielectric barrier discharge and can typically initiate partial discharges for 3 mm of glass from a potential difference between the antistatic layer and the ground electrode of 6 kV for a glass of 3mm thick. It is therefore a gentler discharge mode than that of the first alternative of the invention.

According to the invention, a sheet of glass designates both a sheet of mineral glass of the soda-lime, aluminosilicate, borosilicate, float type, optionally thermally tempered or chemically reinforced, as well as a sheet of transparent polymer material such as poly(methyl methacrylate) (PMMA), polycarbonate (PC), poly(ethylene terephthalate) (PET), polyurethane (PU)… The first glass sheet is a relatively thin surfacing sheet, unlike the structural sheet(s). s), designating sheets capable of guaranteeing the mechanical properties required for laminated glazing, in particular in the case where it must delimit a pressurized volume such as an airplane. When the structural block only includes one sheet of glass, it is considered structural provided that its elastic modulus is at least equal to 1500 MPa for example. The structural glass sheet(s) is(are) for example made of mineral glass with a thickness of between 1.6 and 10 mm, or of a PMMA type polymer material with a thickness of between 3 and 30, preferably at most 20 mm.

An adhesive interlayer is made of a thermoplastic polymer, mainly polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), ethylene – vinyl acetate copolymer (EVA), casting resin, ionomer resin. The thickness of the first adhesive interlayer bonding the first sheet of glass to the first structural sheet is between 3 and 10, preferably 4 and 8 mm, while the thickness of the adhesive layer(s) ) next interlayer(s) gluing structural sheets two by two is between 0.5 and 4 mm, preferably at most equal to 2 mm.

Preferably, the antistatic layer is made of doped oxide such as indium-tin oxide (ITO) or aluminum-zinc oxide (AZO), of non-stoichiometric oxide such as SnO ₂ , or of diamond-type carbon (in English Diamond Like Carbon – DLC -), with a thickness of between 5 nm and 1 µm, preferably between 10 and 50 nm. The antistatic layer can be deposited on the glass by PVD (physical vapor deposition) means, for example by magnetic field-assisted cathodic sputtering – magnetron under reduced pressure, or by liquid means, for example sol – gel. .

Preferably, the antistatic layer has an electrical conductivity at least equal to 100 Ohm per square, preferably 10 kOhm per square, and at most equal to 100 kOhm per square.

Preferably, the electrode is closer to the retainer than the antistatic layer, by a distance preferably at least equal to the thickness of the first sheet of glass, the edge of the electrode distal to the retainer is closer of the latter than the proximal edge of the antistatic layer from a distance at most equal to the thickness of the first sheet of glass, further away from a distance at most equal to the characteristic dimensions of the main surfaces of the first sheet of glass. glass, and the length of the electrode in the direction normal to the retainer is at least equal to the thickness of the first sheet of glass.

Preferably, the proximal edge of the antistatic layer relative to the retainer is straight and parallel to the latter, or has tip effects to promote the initiation of electrical discharges.

In the latter case, the proximal edge of the antistatic layer relative to the retainer comprises points preferably in the form of triangles, rectangles longer than wide, rectangles with rounded corners or rectangles finished with triangles, the sides of the rectangles or triangles being rectilinear or curved so as to form concave or convex edges, and the points being fully facing the electrode if it exists. The tips can be obtained by selective ablation of antistatic layer, for example by means of a laser, by a mechanical or chemical method. They can be obtained by masking at the deposition stage.

Preferably, the electrode constitutes the anti-frost heating layer or is an additional electrode such as indium tin oxide (ITO) or equivalent. If the laminated electrode is the heating layer itself, we will then ensure that in the non-heating phases, the latter remains at the potential of the mounting structure.

The invention also relates to the use of laminated glazing described above as glazing for aircraft cockpits, in particular windshields, in particular for medium and long-haul commercial aircraft, business and tourism aircraft.

The invention will be better understood in the light of the description which follows of the appended drawings in which
is a schematic sectional view of a glazing of the state of the art;
is a schematic sectional view of glazing according to the first alternative of the invention;
is a schematic sectional view of glazing according to the second alternative of the invention;
is a partial schematic representation of glazing according to the second alternative of the invention;
And [ ] are two schematic front views of the exterior of glazing conforming to the invention.

In reference to the , a commercial aircraft windshield representative of the state of the art consists of a thin outer sheet 2 of glass bonded to a relatively thick sheet of structural glass 3 via a relatively thick layer 4 of TPU. The glass sheet 3 constitutes the structural block of the laminate with the relatively thick structural glass sheet 3', to which it is bonded by a fairly thin layer 6 of PVB. The laminated glazing rests on an interior retainer 1', which can be the aircraft structure, with the interposition of a wedge 7. The glazing is fixed to the mounting structure by the retainer 1, an exterior retainer consisting of a press glass which, by being bolted to the aircraft structure, exerts pressure on the edge of the structural block 3, 6, 3' via the silicone seal 5.

An electrical connection is established between the antistatic layer 8 and the aircraft ground not shown. This connection can be obtained by bonding using a conductive glue 11, a first conductive connection element 10 and a routing wire 10' towards a connection to the aircraft ground. Depending on the case, 10 and 10' can be a single cable or preferably a thin metallic foil type element for 10 and a 10' cable for example brazed to each other. Such an assembly requires protection 12 from the eroding environment and moisture penetration. The internal face of the laminated structure of the first glass sheet 2 or outer glass sheet supports an anti-frost heating layer 9 in ITO, connected to the ground of the aircraft structure via electrical sources. This configuration has the disadvantages described above.

There describes the first alternative of the invention. Here, the antistatic layer 8 is floating, or in other words, is not electrically connected to the ground of the aircraft structure. The antistatic layer 8 is spaced at most 10 mm from the retainer 1 (or ice press). A corona discharge 20 is shown at the edge of the antistatic layer 8, a corona discharge 21 at the edge of the retainer 1, as well as an electric arc 22 between the antistatic layer 8 and the retainer 1. The transition from the corona discharges 20 and 21 to the arc electrical 22 can exceptionally occur but in an unstable manner and without causing surface discharge.

There describes the second alternative of the invention, according to which the antistatic layer 8 is located entirely more than 10 mm from the retainer 1 and the heating layer or electrode 9 is electrically secured to the mass of the mounting structure, so that the discharges Electric charges are preferably made at the edge of the antistatic layer thanks to the amplification of the electric field generated by the proximity of the electrode 9. The electric charges are then put into the air and can be all or in part recollected by the plane but without discharge phenomenon. The crown discharge 21 and the electric arc 22 of the no longer occur, unlike the crown discharge 20 which is maintained.

In reference to the , we represent the relative positions of the antistatic layer 8, of the laminated electrode 9 and of the retainer 1 or ice press, according to the second alternative of the invention. These relative positions can be defined by the fact that
u is positive, preferably greater than a thickness e of external glass 2;
v between –e and any positive value less than the characteristic size of the exterior glass 2 (length L and width l of its main surfaces);
u+v >e; And
D > 10mm.

The edge of the antistatic layer 8 can be generally straight or textured so as to form points or fingers oriented towards the retainer 1 (first alternative of the invention), and/or entirely facing the laminated electrode ( second alternative of the invention). On the And are represented the retainer 1 and two configurations of antistatic layer 8 having a pointed edge, configurations representative of both the first alternative of the invention (D ≤ 10 mm), and the second (D > 10 mm). Typically we will choose e' less than or equal to the thickness of the exterior glass 2, d greater than or equal to the thickness of the exterior glass 2 and L greater than the thickness of the exterior glass 2. The discharge zone consists of a layer of the same nature as the antistatic layer 8 and whose edges are formed by
masking when depositing,
ablation after deposition for example by laser, or
additional deposition of conductive elements by bonding or screen printing for example.

The proposed solution is to be compared to glazing not provided with an antistatic solution on the one hand and to glazing equipped with an antistatic layer discharged by a drain on the other hand. Compared to solutions without an antistatic layer, the solution of the invention makes it possible to eliminate dazzling and radiant discharges (electromagnetic, visual and sound disturbances).

The implementation of a conductive drain between the external surface of a glazing and the internal structure of the aircraft has the following limits:
complexity of implementing a reliable electrical connection between a non-metallic layer and a metal drain (bonding, differential expansions, sensitivity to humidity, etc.);
protrusion (aerodynamic impact, including noise);
risk of glass breakage or internal fire in the aircraft in the event of lightning attaching to the drain.

These limitations are resolved by the present invention.

Claims (8)

Laminated glazing comprising a first sheet of glass (2) and at least one sheet of structural glass (3, 3') bonded two by two by an adhesive interlayer (4, 6), the main free surface of the first sheet of glass (2) being intended to be in contact with the external atmosphere, the first sheet of glass (2) being set back relative to said at least one sheet of structural glass (3, 3'), and having a thickness between 0.5 and 5 mm, the edges of the first glass sheet (2) and the first interlayer adhesive layer (4), and the free part of the main surface of the structural glass sheet (3), overhanging the the first sheet of glass (2), forming a cavity for receiving a retainer (1) for fixing the laminated glazing by pinching to a mass mounting structure to which the retainer (1) is electrically secured, said surface free of the first sheet of glass (2) supporting an antistatic layer (8) not electrically secured to the mass of the mounting structure, and of electrical conductivity less than 10 MOhm per square, characterized in that one point at least the antistatic layer (8) is located at most 10 mm from the retainer (1), or the antistatic layer (8) is entirely located more than 10 mm from the retainer (1) and the main surface of the first glass sheet ( 2) internal to the structure of the laminated glazing supports an electrode (9) electrically secured to the ground of the mounting structure. Laminated glazing according to claim 1, characterized in that the antistatic layer (8) is made of doped oxide such as indium-tin oxide (ITO) or aluminum-zinc oxide (AZO), of non-stoichiometric oxide such as SnO ₂ , or diamond-like carbon (DLC), with a thickness of between 5 nm and 1 µm, preferably between 10 and 50 nm. Laminated glazing according to one of the preceding claims, characterized in that the antistatic layer (8) has an electrical conductivity at least equal to 100 Ohm per square, preferably 10 kOhm per square, and at most equal to 100 kOhm per square. Laminated glazing according to one of the preceding claims, characterized in that the electrode (9) is closer to the retainer (1) than the antistatic layer (8), by a distance preferably at least equal to the thickness of the first sheet of glass (2), in that the edge of the electrode (9) distal to the retainer (1) is closer to the latter (1) than the proximal edge of the antistatic layer (8) d 'a distance at most equal to the thickness of the first sheet of glass (2), further away from a distance at most equal to the characteristic dimensions of the main surfaces of the first sheet of glass (2), and in that the length of the electrode (9) in the direction normal to the retainer (1) is at least equal to the thickness of the first glass sheet (2). Laminated glazing according to one of the preceding claims, characterized in that the proximal edge of the antistatic layer (8) relative to the retainer (1) is straight and parallel to the latter (1), or has point effects to promote the initiation of electric discharges. Laminated glazing according to one of the preceding claims, characterized in that the proximal edge of the antistatic layer (8) relative to the retainer (1) comprises points in the form of triangles, rectangles longer than wide, rectangles at the corners rounded or rectangles ending with triangles, the sides of the rectangles or triangles being rectilinear or curved so as to form concave or convex edges, and the points being fully facing the electrode (9) if it exists. Laminated glazing according to one of the preceding claims, characterized in that the electrode (9) constitutes the anti-frost heating layer or is an additional electrode such as indium-tin oxide (ITO) or equivalent. Use of laminated glazing according to one of the preceding claims as aircraft cockpit glazing, in particular windshields, in particular medium and long-haul commercial aircraft, business aircraft, tourism aircraft.