Patent Application
20230166138
The invention relates to a seal for an aircraft incorporating a fire-resistant structure, and an aircraft incorporating at least one such seal between structural elements of the aircraft connected to each other at a zone of the latter which may be a fire zone according to the standard ISO 2685:1998 or the standard AC 20-135 of the FAA (Federal Aviation Administration). In particular, the invention applies to generally tubular seals for sealing and protecting from fire zones of an airplane or a helicopter selected from among those of the main reactors and of the auxiliary engines, such as an engine, a nacelle, a pylon or an auxiliary power unit (APU), while bearing in mind that any aerial or space vehicle likely to have at least one fire zone is concerned by the invention.
In an aircraft, many zones referred to as “fire zones” in accordance with the standard ISO 2685:1998 or AC 20-135 require using seals designed so as to ensure two main functions, the first one being a sealing function in normal flight conditions and the second being a fire-resistance (“fire-proof”) function in the event of a fire breakout or propagation in the or each considered zone of the aircraft.
In a known manner, these two functions are antagonist, given the fact that the sealing function is promoted by an increased flexibility of the seal which enables it to conform to the profile of the structural elements in contact with which it is mounted in the stressed state (pressure-barrier effect) and to simplify the installation and the tight closure of the connection by the elastic stiffness of the seal, whereas the fire-resistance function is on the contrary promoted by an increased stiffness of the seal by limiting the risk of perforation thereof by the flame (fire-barrier effect) and by limiting the transmission of heat to the other side opposite to the origin of the fire.
The sealing and fire-resistant seals designed to this end are usually made of an elastomer-fabric(s) composite by which an often unsatisfactory trade-off is achieved between these two functions, in particular for fire-resistance, and also with a significant impact resulting therefrom for the parts surrounding the sealed structural elements of the aircraft (which are usually made of a composite material or of titanium). Indeed, these parts should, on the one hand, support the stiffness of the seal constrained thereby without being deformed and, on the other hand, they often should incorporate thermal protections to guarantee fire resistance of the entirety of the considered zone including the seal and its adjacent elements.
U.S. Pat. No. 3,566,541 A discloses in
A major drawback of this barrier lies in the unsuitability of the polychloroprene seal to the high temperatures in the immediate proximity of aircraft engines, which, in normal operation (i.e. fire-free environment), usually vary between −55 and 250° C. Indeed, the intumescent mass according to this document would be the site of a premature expansion and the seal incorporating it would disintegrate in normal operation of an aircraft.
EP 2 412 409 A1 discloses a monolithic seal for a fire damper, made of an intumescent material completely covered with a profile made of a silicone elastomer, which is co-extruded in contact with this intumescent and crosslinked material without any heat supply by an UV radiation. For example, this intumescent material is formed by expandable graphite or by a silicate, and its expansion can start at a temperature of 100-110° C.
A major drawback of this monolithic seal also lies in its unsuitability to the already hot fire-free environment of aircraft engines in particular because of the premature expansion of the intumescent material, which further contributes, in this document, to sealing in the absence of fire as it is clasped by the seal.
The present invention aims to provide a seal, comprising:
To this end, a seal according to the invention is such that said at least one intumescent mass is made of a rubber composition having an intumescence trigger temperature equal to or higher than 270° C., measured by a plane-plane rotary rheometer with a temperature scan from 23 to 380° C. according to a temperature ramp of 10° C./min, with 1% of deformation and with an evolution, starting from 23° C., of the normal force Fn starting from 0.07 N and of the air gap h between the planes starting from 2 mm. This seal according to the invention is further configured to connect two structural elements to each other at a zone of an aircraft which is selected from among the main reactor and auxiliary engine zones, whose temperature in the absence of fire can vary from −55° C. to 250° C. and which is referred to as fire zone according to the standard ISO 2685:1998 or AC 20-135.
It should be noted that this seal of the invention thus integrates the sealing function in normal operation of the aircraft (i.e. at a temperature lower than 270° C. which may vary for example from −55 to 250° C.), and the two sealing and fire-barrier functions in the event of a fire in a zone of the aircraft in the immediate proximity of the seal, thanks to the expansion of said at least one intumescent mass starting from 270° C. or from a higher temperature. Indeed, the or each intumescent mass synergistically cooperates in the expanded state with the seal body that it completely fills, thereby providing an additional stiffness to the wall of the cavity of the seal body against which the or each mass bears and further effectively protecting this wall throughout the duration of the fire.
In other words, the seal of the invention with a seal body and separate intumescent mass(es) allows significantly improving the sealing and fire-barrier performances while decorrelating these two functions in normal operation of the aircraft, thanks to the non-participation of said at least one intumescent mass in the sealing function in normal operation which is promoted by being ensured only by the flexibility and elasticity of the seal body, while in the event of fire the expansion of the or each intumescent mass ensures the sealing and fire-barrier dual function by conferring an increased rigidity on the seal body throughout the duration of the fire, while limiting the heat-up of the wall of the seal body against which it bears.
It should also be noted that in addition to integrating these two antagonist functions in a joint and decorrelated manner without affecting either one neither in normal operation nor in the event of a fire, the seal of the invention allows simplifying technical constraints on the surrounding parts. Indeed, the metallic or composite structural elements connected to each other at a fire zone of the aircraft (as defined by § 2.1 of the standard ISO 2685:1998 or by the standard AC 20-135), can advantageously be designed thanks to the seal of the invention with a reduced sizing in comparison with fire-resistant structural elements of the prior art, because this seal remains more flexible in normal operation of the aircraft. And since in the event of a fire, the expansion of the intumescent mass contributes in limiting the heat-up of the wall in contact with the seal body and further improving the strength of the seal when the flame passes to the other side (as this will be discussed hereinafter), this results in that this seal of the invention allows suppressing all or part of the need for thermal protection devices on the surrounding structural elements on each side of the seal.
Advantageously, said intumescence trigger temperature may be comprised between 280 and 400° C., preferably between 320 and 360° C. and for example between 330 and 350° C.
It should be noted that these temperatures are substantially higher than the maximum temperature of about 250° C. which usually characterises the engines of aircrafts such as airplanes, in particular, which improves even more the decorrelation of the aforementioned two functions and therefore the sealing performance in normal operation, since it is thus ensured that any premature and therefore undesirable start of expansion of said at least one intumescent mass is avoided.
According to another feature of the invention, the rubber composition may have, in the expanded state, a volumetric expansion ratio equal to or higher than 800%, preferably comprised between 820 and 950% and for example between 850 and 900%, measured for 15 min, at 600° C.±10° C. in a Nabertherm® N17/HR muffle furnace with a useful volume of 17 dm3 on a test sample with a circular section with a 25 mm diameter made of the rubber composition. This expansion ratio (%) is calculated by the formula ((Ef-Ei)/Ei).100 with Ei referring to the initial thickness of the test sample equal to 2 mm and Ef the final thickness of the test sample.
It should be noted that this very high expansion ratio advantageously allows filling the corresponding cavity of the seal body while causing the application of a multi-directional force by the or each expanded mass on the entire wall of this cavity, which allows conferring the aforementioned additional stiffness on this wall contributing to the fire-barrier function.
It should also be noted that the or each mass thus expanded by intumescence (commonly called “carbon residue” in the prior art to refer to the thermal degradation cellular residue) advantageously has for this volumetric expansion ratio a value that is high enough (of at least 800%) to obtain filling and the aforementioned stiffness of the wall, but not excessive (preferably lower than 950%) so that this expansion does not affect the mechanical properties of the carbon residue. Thus, the fire-resistant structure forming this carbon residue by intumescence contributes in making the seal resistant enough in the event of a fire, by preventing destruction thereof by the combustion and the generated vibrations, as this will be discussed hereinbelow.
According to another feature of the invention, the rubber composition may be based on (i.e. made at more than 50% by weight, preferably at more than 75% by weight and more preferably made exclusively of) at least one silicone rubber, preferably a phenyl-vinyl-methyl silicone (phenylmethyl-, vinylmethyl- and dimethylsiloxane terpolymer, abbreviated PVMQ).
Indeed, the Applicant has revealed that the use of at least one silicone rubber specifically a PVMQ type one to form most or exclusively all of the weight of the elastomer matrix of the composition of the or each intumescent mass, unexpectedly allows significantly increasing the thermal stability of the or each mass (by reducing its mass loss when temperature increases beyond 600° C., for example), in comparison with other silicone rubbers such as vinylmethyl silicones (VMQ).
It should be noted that the silicone rubber preferably used in the rubber composition allows significantly improving the fire resistance of the or each intumescent mass at very high temperatures which may reach 1,100° C., typically, while complying with the operating temperature range of the seal and of the structural elements that it connects in the considered fire zone of the aircraft.
Nevertheless, it should be noted that the vinyl-methyl silicone (VMQ) and fluorosilicone (FVMQ) rubbers can also be used in the composition of said at least one intumescent mass, even though PVMQs are preferred.
According to a preferred embodiment of the invention, the rubber composition further comprises:
Advantageously, the rubber composition forming said at least one intumescent mass is not totally crosslinked, which rubber composition may be non-crosslinked or else only partially crosslinked. Preferably, the rubber composition that incorporates this crosslinking system according to a reduced amount of peroxide is crosslinked only slightly (by heating at a temperature for example comprised between 160 and 220° C.). Indeed, the Applicant has noticed a significant improvement of the mechanical properties of the carbon residue resulting from the expansion of a composition according to the invention that is not totally crosslinked inside the cavity of the seal, whereas a totally crosslinked composition has not been capable of expanding enough therein.
It should be noted that this non-totally crosslinked rubber composition is supposed to be flexible enough so that the or each intumescent mass may be inserted inside the cavity of the seal while resisting the possible deformations of the seal throughout the different phases of its service life, including in particular storage thereof and mounting thereof on the aircraft.
According to an example of this preferred embodiment of the invention, the rubber composition comprises for 100 pce of said at least one silicone rubber (pce: parts by weight for 100 parts of elastomer(s)):
It should be noted that the quartz, the metal oxide, the dimethyl siloxane with a dimethylvinyl terminal group and the platinum mentioned hereinabove are mentioned without limitation, bearing in mind that other fireproof agents selected from among cerium hydroxide, dimethyl siloxane with a hydroxyl terminal group, dimethyl, methylvinyl siloxane with a dimethylvinyl terminal group, carbon black and mixtures of at least two of these other agents, can also be used in said flame-retardant system by being dispersed beforehand in a silicone oil.
Alternatively, it is possible to use in said flame-retardant system an ammonium polyphosphate solution, for example mixed with pentaerythritol and with melamine, in particular.
It should also be noted that the aforementioned amount of expandable graphite of 10-20 pce (preferably of 13-17 pce) unexpectedly allows achieving a trade-off between the volumetric expansion ratio of the rubber composition in the expanded state and the mechanical properties obtained for the carbon residue generated by this expanded composition, bearing in mind that an increase in the used amount of expandable graphite results not only in raising this expansion ratio, but also in reducing the mechanical properties of the carbon residue.
It should further be noted that the rubber composition according to the invention may be devoid of any reinforcing charge, and that the possible use of the aforementioned mineral fibres in the composition can allow mechanically reinforcing the carbon residue formed upon expansion of this composition which contains the peroxide (for example dicumyl peroxide) according to the reduced amount of 0.05-0.5 pce, preferably of 0.1-0.3 pce, to limit the subsequent hot creeping of the rubber composition without affecting expansion thereof by intumescence.
Advantageously, the rubber composition may have, in the non-crosslinked state, a Mooney viscosity ML(1+4) at 40° C., measured according to the standard ASTM D-1646, which is comprised between 15 and 25 and for example between 18 and 22.
It should be noted that these Mooney viscosity ranges for the non-crosslinked rubber composition demonstrate its ability to be implemented (i.e. shaped according to a determined shape) following mixing thereof, possibly before the partial crosslinking thereof under heat.
According to another aspect of the invention, the rubber composition of the or each intumescent mass, in the expanded state, advantageously withstands the vertical flame test according to the standard FAR25.853, Appendix F part I (a) (1) (i), the rubber composition then having no residual flame after removal of a methane flame at 843° C. for a flammability time of 60 seconds.
It should be noted that this absence of residual flame upon completion of this vertical flame test (in which the tested samples are supported vertically, as specifically described in this standard FAR25.853, Appendix F part I (b) (1) to (4)), which may be reflected by the fact that the expanded and ignited rubber composition is extinguished directly after removal of the methane flame generated by a burner, demonstrates the reduced flammability of the or each intumescent mass according to the invention.
According to a particular embodiment of the invention, the fire-resistant structure further may comprise an envelope which is separated from the seal body and which encapsulates said at least one intumescent mass in particular to protect it from surrounding fluids, the envelope being for example based on a non-crosslinked silicone rubber and may form a “skin” in contact with the or each mass.
It should be noted that this protective encapsulation of the or each intumescent mass by such an envelope may further allow limiting the movement of this mass inside the corresponding cavity of the seal, in normal operation of the aircraft.
According to another feature of the invention, the fire-resistant structure may be disposed over an inner zone of the seal body independent of the tightness ensured by the seal without the fire-resistant structure being fastened to the seal body.
It should be noted that this separate arrangement (i.e. not fastened) of the fire-resistant structure on the seal body may be reflected by an absence of physical or chemical adhesion between the fire-resistant structure and the seal body, until intumescence of the or each mass.
According to another feature of the invention, the fire-resistant structure may have, in a non-expanded state before intumescence of the or each mass, any geometry for example a generally parallelepiped one (i.e. a prismatic type one), preferably defined in this case by a width along a transverse dimension of said at least one cavity, by a height perpendicular to and smaller than said width and by a length for example equal to that of said at least one cavity.
Alternatively, the fire-resistant structure may have any other elongate geometry extending over the length of the seal body, for example generally cylindrical with a circular section, or not.
According to another feature of the invention, the seal body may be entirely or partially made of an elastomeric material based on at least one silicone rubber, preferably a terpolymer derived from phenylmethyl-, vinylmethyl- and dimethylsiloxane (PVMQ). This elastomeric material of the seal body may be made at more than 50% by weight, preferably at more than 75% by weight and more preferably made exclusively of said at least one silicone rubber.
It should be noted that the silicone rubber used in the elastomeric material of the seal body allows significantly improving its fire resistance at very high temperatures up to 1,100° C., typically, in the considered fire zone of the aircraft.
It should also be noted that fluorosilicone rubbers (FVMQ) cannot be used in the elastomeric material of the seal body.
According to preferred embodiments of the invention, the seal body is for example selected from among tubular seals with an Ω-like cross-section, a P-like cross-section, and from among annular bellows for conduits.
It should be noted that other geometries can be used for the seal body, in particular among those commonly used for seals on the aeronautical industry.
According to a particular embodiment of the invention, the seal body may be made of said elastomeric material, being devoid of a reinforcing layer embedded in said elastomeric material.
It should be noted that this exclusively elastomeric structure of the seal body has the advantage of a simplified manufacture of the latter which may be implemented by a unique extrusion step, for example.
According to another embodiment of the invention, the seal body may be made of a composite comprising said elastomeric material and at least one ply of a fabric, said at least one ply being embedded in said elastomeric material and being selected from among glass fabrics, aromatic polyamide fabrics (for example of aramid) and combinations thereof.
It should be noted that this composite structure of the seal body, obtained by a common confection method, thus requires one or more plies (respectively consisting of identical or different fabrics), the or each fabric should have enough resistance at temperatures of about 1,100° C. in the event of a fire.
For example, it is possible to use a unique glass fabric ply to reinforce the seal body, bearing in mind that this reduced reinforcement of the seal body is advantageously compensated by the intumescence capability of the fire-resistant structure and by the mechanical properties of the carbon residue obtained after intumescence, upon exposure to fire.
According to another general aspect of the invention which may be combined with any one of the aforementioned features and examples of the seal (including the seal body and/or the fire-resistant structure), the seal withstands fire according to the standard ISO 2685:1998, being capable of resisting for 15 minutes:
A seal according to the invention also withstands fire according to the standard AC 20-135, by being capable of resisting for 15 minutes:
It should be noted that this satisfactory resistance to heat and vibrations during the 15 minutes of the fire test, as specifically described in the standard ISO 2685:1998 or AC 20-135, demonstrates the fact that the combination according to the invention of the seal body and of the fire-resistant structure forming the aforementioned carbon residue by intumescence allows making the seal resistant enough in the event of a fire, by preventing destruction or disintegration thereof by the combustion and the vibrations generated by the fire.
An aircraft according to the invention, in particular an airplane or a helicopter, comprises:
It should be noted that said at least one fire zone of the aircraft, such as an aerial or space vehicle, may be defined in a way other than according to the standard ISO 2685:1998 or AC 20-135, and that it may thus concern zones other than those identified hereinabove.
Other features, advantages and details of the present invention will appear upon reading the following description of several embodiments of the invention, provided for illustrative and non-limiting purposes with reference to the appended drawings, among which:
The seal 10 according to the invention visible in
Only for indication, the fire-resistant structure 12 has, for example, a rectangular section (i.e. a generally parallelepiped geometry over the length of the cavity 10A), and this fraction occupied by the fire-resistant structure 12 is, for example, comprised between only 5 and 20% of said height H (which is defined from the inner face of the base 11B to that of the top 11A of the cavity 10A).
As explained hereinabove, the seal body 11 is, for example, entirely or partially made of an elastomeric material based on a silicone rubber, in which material is optionally embedded at least one reinforcing fabric ply. As regards the fire-resistant structure 12, it is, for example, made of an intumescent mass formed by a rubber composition based on a silicone rubber and further comprising:
After intumescence of the fire-resistant structure 12 in the event of a fire on-board the aircraft, one could see in
The Ω-like seal 10 according to the invention visible in
The seal 10 according to the invention visible in
The seal 10 according to the invention visible in
The Ω-like seal body illustrated in
As illustrated in
As illustrated in
The intumescent mass of the fire-resistant structure 12 of this seal 10 according to the invention has been prepared as described hereinafter.
The formulation of this intumescent mass is indicated in Table 1 hereinafter, representative of an example of said first composition according to the invention.
More specifically, amongst these ingredients:
The composition of the flame-retardant system is indicated in Table 2 hereinafter.
The following properties of the rubber composition obtained by thermomechanical mixing of the aforementioned ingredients have been measured, whose values are reported in Table 3 hereinafter. For this mixing, a Haake® tangential mixer with a useful volume of 250 cm3 with a fill coefficient equal to 1 has been used, this mixing having been implemented at a temperature of 30° C. for a mixing duration of 2 min. 30 s.
These properties of the obtained rubber composition in the non-crosslinked (Mooney viscosity and creeping) and partially crosslinked (expansion ratio, mechanical properties and flammability) were well suited for the obtainment of a fire barrier of the seal 10 according to the invention incorporating a fire-resistant structure 12 made of this composition.
This TGA analysis of
As illustrated in
The analysis illustrated by
For the fire tests concerning the seal 10 according to the invention of
a) a fuel (i.e. kerosene) burner,
b) a K-type thermocouple for measuring temperature during step 1 of calibrating the flame generated by the burner, these K thermocouples being remote by 100 mm from the burner,
c) a calorimetric device, comprising a copper tube and PT 100 type thermocouple forming a calorimeter used for calculating the heat flux in order to calibrate the flame during step 2 of calibrating the heat flux of the flame, and a flowmeter for measuring the water flow rate in this tube during step 2,
d) a digital camera and three video cameras (arranged at the front and at the rear of the test device), which cameras are visible in
e) a test device (visible in
f) a vibrating table for imparting to each tested sample the vibrations required by the standard ISO 2685:1998.
The monitored conditions for these fire tests being those prescribed in the standard ISO 2685:1998, they will not be detailed hereinafter, while pointing out that each fire test, implemented for 15 minutes, has subjected the tested seal sample to:
(i) the heat generated by a kerosene calibrated flame at 1,100° C.±80° C. with a heat flux density absorbed by the standardised apparatus described in B.4.2 of the standard ISO 2685:1998 which is 116±10 kW/m2, and to
(ii) vibrations of 50 Hz and 0.8 mm peak-to-peak as described in this same standard.
A thermal camera has been disposed at the rear of each assembly to measure the temperatures at the rear of each tested “control” sample and according to the invention (i.e. on the side opposite to the flame, cf. the left side of
These measurements of temperature at the rear of each seal sample have revealed:
To conclude, and as this is confirmed by the photographs of
Indeed, one could see in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2112682 | Nov 2021 | FR | national |
