DE102014001792A1 - Lower Deck Plate - Google Patents

Abstract

The present invention is directed to a bottom cover plate wherein one or both surfaces of the bottom cover plate have hydrophobic properties.

Description

The invention relates to a lower cover plate for use as a substructure in roof structures. Such panels are generally placed on the rafter construction and fastened there before the actual roofing is applied. In addition to the actual task of representing a walkable and waterproof at least during the construction phase roof, such plates are also assigned to noise and heat insulating properties.

Generic insulation boards are well known from the prior art.

DE 4322745 describes a roof insulation board with a mountable on the rafters of a roof vapor-permeable insulation board, which is provided with a waterproof, vapor-permeable film to ensure waterproofness, for example, in driving rain and consists of compressed polyester fibers. Due to the waterproof, vapor-permeable film, however, this solution entails dangers such as, for example, the delamination of the layers or even the formation of holes in the film during the laying phase. When processing such panels, special care must be taken to avoid damage to the roof insulation panels.

In DE 29723930 an insulating element is disclosed with a walk-in cover plate, is fixed under a continuous insulating plate, are formed in the longitudinal and transverse ventilation channels.

The insulating plate consists for example of PU foam, stone and mineral wool or wood fiber material, while for the cover plate wood-based panels, gypsum and gypsum fiber boards are used.

The disadvantage, however, is that always two plate-shaped fabric must be joined together here. In addition, the introduction of ventilation ducts in the insulating plate means additional work in production, whereby such kind of insulation elements are correspondingly high priced.

Furthermore, there are lower cover plates made of compressed, chemically bonded wood pulp whose disadvantage in addition to the high weight of a plate and the low resistance to permanent moisture. For longer-lasting moisture, such plates tend to swell up to the formation of mold at the moist sites.

The object of the present invention is to avoid the mentioned disadvantages of the prior art with the features mentioned in claim 1 and in the subclaims.

The lower deck according to the invention is able to significantly increase not only the basic requirement for accessibility and permanent waterproofness, but also the noise insulation and the thermal insulation despite the small thickness of the panel in the roof space.

This is achieved by making this bottom cover plate of a carded, cross-laid nonwoven fabric of synthetic staple fibers which is mechanically consolidated and compacted, as well as thermally consolidated.

To ensure the driving rain safety at least one side of the lower cover plate is provided with hydrophobic properties. Furthermore, both the top and the bottom or, in preferred embodiments, the complete bottom cover plate, d. H. also possess the thickness, hydrophobic properties.

In order to reduce the carbon dioxide emissions caused by the heating of buildings, thermal insulation measures are carried out in the construction of new buildings and refurbishments. When renovating old buildings, the rafter height often places limits on the height of the insulation layer between the rafters. Even with new buildings, the rafter height is often limited by the financial framework of the client. For the client, both the external appearance and the cost issue are an important aspect to be considered. The legislation has narrow limits here, which must be complied with in order to achieve a permanent insulation from an economic point of view.

• • Wasserdichtheit
• • Begehbarkeit
• • Wärmedämmung
• • Geräuschisolierung
• • Gewichtseinsparung
• • Resistenz gegen Mikroben

in einem Bauteil zu vereinigen.Based on this basis, the invention is based on the idea of essential properties of a roof substructure such as:

• • Water tightness
• • accessibility
• • Thermal insulation
• • Noise isolation
• • Weight saving
• • Resistance to microbes

in one component.

An inventively designed lower deck plate makes it possible to move a portion of the thermal insulation in the outer area of the roof structure, so that it is not necessary to work with large thicknesses especially in the renovation of old buildings in the interior.

The undersheet according to the invention comprises a mechanically formed nonwoven made from a mixture of thermoplastic and / or duroplastic staple fibers with melt fibers, which is first mechanically consolidated and compacted. An adjoining thermal treatment activates the melt fibers, so that after cooling of the material results in the inventive rigid bottom cover plate. The raw materials used should also provide permanent mold resistance without relying on chemical treatment.

The underlying manufacturing processes can be found in the book "Nonwovens", Fuchs / Albrecht (eds.), Published in Viley VCH, 2012.

According to the invention, one or both sides of the lower cover plate is hydrophobic. This can be achieved by using a hydrophobic coating, or else according to the invention, by using at least one hydrophobic fiber type. Melt fibers are then used in the fiber mixture. In further preferred embodiments, other fibers with large fiber diameters are used.

The term melt fiber is used in the present specification synonymously for fibers which are usable for the thermal consolidation of nonwovens. Typically, such fibers contain thermoplastic regions which have a lower melting point than the remaining non-woven fabric forming fibers. These fibers may consist entirely of a low melting point polymer, but may also be so-called bicomponent side / side or core / sheath type fibers. As polymers all common variants of polyesters, polyamides or polyolefins are suitable. Also fusible fractions with higher crystalline proportions can be used. The proportion of melt fibers according to the invention is at least 40 weight percent. The fibers have a commercial crimp. The fiber diameter of the used melt fibers is between 10 and 60 microns.

To ensure watertightness, either a hydrophobic coating is applied to one or both sides of the bottom cover plate, or, which is preferred according to the invention, some of the fibers used have hydrophobic properties. The hydrophobic fibers have a smaller compared to the other fibers in the fiber mixture fiber diameter. The fiber diameter used are at a maximum of 20 microns, preferably less than 18 microns. In the following, these fibers are called fine fibers. Such fibers may consist of thermoplastic polymers, but thermoset polymers are also conceivable. The fine fibers have a crimp, which can be either two-dimensional but also three-dimensional nature. By crimping the fibers have a more or less strong restoring force against mechanical loads. The greater the crimping, expressed in terms of number of crimps / cm, the greater the restoring force. Fine fibers which have a number of at least 4 crimps per cm are suitable according to the invention; fibers having greater than 6 sheets / cm are preferred.

This restoring force can be further influenced by the polymer forming the fibers. Suitable are polyolefins, polyamides, polyacrylics and polyesters. Due to the good recovery properties, preference is given to fibers of linear macromolecules whose chain consists of at least 85 percent by weight of the ester of a diol with terephthalic acid. They are referred to in the context of the present invention as polyester fine fibers.

The hydrophobic properties of these fine fibers are based either on the fiber properties themselves, for example, when using polyolefins such as polypropylene or by so-called Finishes that impart a hydrophobic character to the fine fibers before processing, or to finishing systems that only develop their hydrophobic properties through crosslinking reactions during the thermal treatment.

The polymers from which the fine fibers are formed must be selected so that they have a melting point which is higher than the melting temperature of the melt fibers or higher than the temperature necessary for the aforementioned crosslinking reaction.

The proportion of hydrophobic fine fibers according to the invention is at least 30 weight percent.

In a preferred embodiment, fibers with a larger fiber diameter are also used. These fibers then have a diameter of at least 30 microns. Fiber diameters of greater than 40 μm are preferred. Such fibers impart to the lower cover plate according to the invention an increased stability especially under pressure loads, for example when walking on the plate. Such fibers are referred to as coarse fibers. The proportion of these coarse fibers, if they are used, at a maximum of 30 weight percent. The coarse fibers have commercial ripples. As polymers for the production of coarse fibers, polyolefins, polyamides, polyacrylics and polyesters are suitable.

Particularly preferred are polyester fibers of linear macromolecules whose chain consists of at least 85 percent by weight of the ester of a diol with terephthalic acid. They are referred to in the context of the present invention as polyester coarse fibers.

If the fiber types used are all based on the same polymer, then at least in the case of thermoplastic polymers, recycling during renovation / reconstruction of a roof is ensured. According to the invention and produced with preferred raw sub-top plates can easily be recycled.

In contrast to the prior art is used with polymers that can not be microbially metabolized. This ensures that even with permanent moisture in the environment, a lower cover plate according to the invention does not provide a breeding ground for the growth of mold or bacteria. This property proves to be particularly beneficial in new buildings, which are still in the phase of drying masonry or plaster in the interiors. During this dry phase, most of the moisture in the building diffuses outward through the roof area. Depending on the temperature conditions, it may lead to the formation of condensation on the Unterdeckpatten. An inventively designed lower cover plate behaves inert, since the moisture neither leads to swelling nor to other negative effects (mold growth).

Of crucial importance to the present invention is the step of mechanical consolidation and densification in conjunction with the use of hydrophobic fine fibers having the above fiber diameters.

In the case of mechanical consolidation, in addition to the partial reorientation of the staple fibers from the horizontal to the vertical dimension, the density of the material, in terms of density per unit area, in short RG, with the unit kg / m ³ , is increased.

The inventive use of hydrophobic fine fibers, a high number of individual fibers per unit of space is available. The compaction during the mechanical consolidation reduces the distances between the individual fibers in such a way that, especially in the area of the outer surfaces, a pore-poor, almost closed surface of fibers results. This surface is for drop-shaped impinging water, for example, rain virtually dense, but still permeable to water vapor. Conventional mechanical needling techniques are used; needle machines which are preferred are a method sequence according to the EP 1644565 enable. By means of this preferred needling technique, the nonwoven fabric thus obtained obtains high internal consolidation, which thereby ensures the penetration resistance due to high shear strength in the undercover panel according to the invention.

The weight per unit area of the fibrous web, which is supplied to the mechanical consolidation, according to the invention is greater than 1000 g / m ² , preferably greater than 2000 g / m ² . As a result of the step of mechanical solidification, the fibrous web is solidified into a flexible nonwoven and compressed in such a way that, after completion of the thermal solidification stage for the finished lower cover plate, room weights of less than 200 kg / m ³ , preferably less than 160 kg / m ³ , are particularly preferred of less than 130 kg / m ³ .

According to the invention, this mechanical solidification is followed by a thermal hardening stage. This can for example consist of a heat treatment by means of flowing through hot air. For example, conventional belt dryers are used. The hot air melts the fusible portions of the melt fibers, the melt migrates to points of intersection of unmelted fibers and preferably deposits there. At the end of the thermal solidification step, the still low-melt mat is subjected to a cooling treatment, for example by means of cold air, so that the molten areas cool and the nonwoven fabric is solidified to form the intrinsically rigid lower cover plate according to the invention.

Depending on the design of the fine fibers, the thermal solidification stage may also result in a crosslinking reaction of softening components on the fiber surface. If, during the production of the fine fibers, the fiber surface is provided, for example, with starting materials for a hydrophobic coating of fluorocarbon, the crosslinking reaction can also be activated within the scope of the thermal solidification stage. As a result, a bottom cover plate produced in accordance with the invention thus produced has a water-tightness which is superior to the state of the art and has mechanical resistance to stress on the viewing surface.

The volumetric weight can be further increased during the thermal consolidation. For example, can connect to the heating stage in the dryer a Kalibrierband or a sizing roll, which further compresses the hot melt web and this state is fixed by the subsequent cooling. Thus, the fibers are further pressed together in the surface area, a further improvement of the watertightness is the result.

Thus prepared, the lower deck panels according to the invention carried out have densities of less than 200 kg / m ^3, preferably less than 160 kg / m ³ and particularly preferably less than 130 kg / m ^3. The thicknesses are in a range of 10 to 60 mm, with a thickness in the range 15 to 30 mm being preferred.

Characteristic of the present invention is a surface which consists of highly compressed, hydrophobic fine fibers and is formed at least on one side of the lower cover plate. The water-repellent properties of this surface remain - in contrast to the prior art - even after punctual loads or overload, such as the impact of a nail for attachment of the lower deck plate to the substructure or when walking, obtained.

Conventional materials as described in the prior art require a sealed layer to achieve watertightness. This is, since the demand for water vapor permeability exists, generally formed from a waterproof, but water vapor permeable film. The disadvantage is that these films have only very low mechanical strengths due to their porosity. Partial overstretching or hole formation is not compensated.

If such a film layer corresponding to the prior art is fastened to the substructure by means of nails or staples, a hole is created around the penetrating point, which can further increase during elongation, for example during walking. But hole formation can also be done by partial overstretching, z. B. a stone stuck in the shoe profile of a roofer. The stone presses into the surface and perforates the film, the roof has defects. This is also the case with chemically bonded wood fiber boards. Once there is a partial overload, such a bottom plate is disturbed in its integrity and vulnerable to impinging water.

This is different with a lower cover plate designed according to the invention. The hydrophobic fine fibers are strongly pressed together by the compression. If a nail penetrates into the lower cover plate, then the fine fibers are forced out of the nail to the side, d. h further compressed. By virtue of the restoring force inherent in the fine fibers, both the penetration point and the contact surface of the nail to the fine fibers are now sealed by the hydrophobic fine fibers via the thickness of the lower cover plate. This especially prevents water penetration in the rafter area. Drop-shaped water can not penetrate either on the tread or in the area of the nail points. Prior art materials require just around the nail points around on the rafters still Abdichthilfen.

Likewise, it is in the above example with a stone in the shoe profile. The lower cover plate is briefly compressed by the load but not perforated. Once the load has been removed, the fine fibers return to their original position so that water can not penetrate the point of loading.

The lower cover plate according to the invention according to the embodiment 1 consists of the following fiber mixture:

Melting fiber:

• Share 50% by weight, bicomponent melt fiber with core / shell structure of co-polyester / polyester, wherein the shell has a melting point of about 110 ° C and the core has a melting point of about 250 ° C. The fiber diameter is about 20.1 microns, the fiber length 51 mm

Coarse Fiber:

• Share 20% by weight of a polyester coarse fiber with a melting point of about 250 ° C The fiber diameter is about 50.8 microns, the fiber length 76 mm.

Fine fiber:

• Proportion 30% by weight of a polyester fine fiber with a melting point of about 250 ° C The fiber diameter is about 12.5 microns, the fiber length 38 mm. The fibers have a crimp of on average 7 sheets / cm. The Avivage system consists of a non-crosslinked fluorocarbon-based overlay, which only forms the hydrophobic properties after thermal treatment.

The aforementioned fiber types are mixed homogeneously and fed to a carding machine. An unstiffened batt with a weight per unit area of about 2400 g / m ^{2 is} produced by means of a stacker.

This batt is fed to a needle loom, as in EP 1644565 is described. The batt is run at a speed through the needle system, resulting in a number of 140 stitches per cm ^{2 of} nonwoven surface. It is worked with so-called 1-Barb needles.

The fibrous web thus produced is fed to a belt dryer, which melts the fusible shell portions of the melt fibers contained in the batt. The dryer temperature is around 175 ° C.

Finally, the heated batt is cooled so that the invention, inherently rigid plate with a thickness of 24.5 mm at a basis weight of 2309 g / m ² , corresponding to a density of 94 kg / m ³ results.

The plate thus produced has hydrophobic properties both on both surfaces and over the entire thickness of the plate. As a result, the cut edges of the plate are water repellent.

In further embodiments of the invention, the lower cover plate can be treated on at least one side of a surface treatment by means of pressure and / or heat so that the plate surface is smoothed. Smoothing in the case of the present invention means a change in the fiber surfaces, so that they do not have a round cross-section in the region of the plate surface but a cross-section which is flattened on one side.

This is achieved by the application of pressure to the surface to be smoothed and, if necessary, heat support. Suitable for this purpose are, for example, belt presses, in which the surfaces of the underlay panels are smoothed under pressure and heat due to the smooth band structure. A treatment with calender brings similar effects.

Such a smoothing treatment results in an improved closure of the thus treated surface of the lower cover plate, so that the driving rain density is increased.

Embodiment 2 shows an inventively designed, smoothed on both sides lower cover plate, which was first prepared according to Embodiment 1. Subsequently, the lower cover plate was subjected to a smoothing treatment by means of a flat bed laminating machine at a temperature of 180 ° C. and a belt gap of 21 mm. The press had a smooth Teflon coating on the upper and lower belt. The speed was 2.1 m / min.

The behavior of materials according to the invention in comparison with the prior art can be seen from Table 1.

The following methods were used as test methods:
Basis weight: according to EDANA WSP 130.1. with a pattern size of 25 * 25 cm. The measurement result is given in g / m ² .
Thickness: according to EDANA WSP 120.6., Measuring instrument according to paragraph 4.1 and test method according to paragraph 7.2 option B. The measurement result is given in mm
Air permeability: according to EDANA WSP 70.1, determined at a differential pressure of 100 Pa. The result is given in l / m ² / s.
Water absorption: first, the dry weight T1 is determined in g of a specimen of size 100 * 100 mm. Thereafter, the test specimen is immersed in a water bath filled with ion exchange water having a conductivity of less than 0.5 .mu.S / cm within 5 seconds so far that results in a water level of 150 mm above the specimen. After immersion for 60 seconds, the specimen is removed from the water bath within 5 seconds. After another 60 seconds dripping time, the wet weight N1, also determined in g.

The water absorption is according to the equation Water absorption [%] N1 - T1 / T1 = × 100% determined and expressed in%.

Penetration resistance: The test setup is as in 1 shown. The test simulates a load such as occurs when a roof is walked on by a roofer. To determine the penetration resistance is a test specimen 3 made of size 50 * 300mm. The examinee 3 is based on two parallel support bearings spaced 230 mm apart 1 placed so that a roof-like construction results, in which the two support bearings 1 the rafters and the test specimen lying thereon 3 represent the bottom cover plate. A 55 mm wide pressure fin 2 will then be in the middle of the distance of the two support bearings 1 , at 115 mm, so on the specimen 3 set that the contact line of the pressure fin 2 parallel to the support lines of the test object 3 on the support bearings 1 runs and the pressure fin 2 on each side of the specimen 3 Protrudes 2.5 mm. The pressure fin 2 presses after test start on the examinee 3 and deformed it at a speed of 100 mm / min until a distance of 30% of the support bearing distance was covered. Finally, the Finn 2 moved out at the same speed in the opposite direction, the test specimen 3 relieved. The test is passed if the examinee does not break, ie a person can not break through the resulting defect.

Permanent deformation: The test procedure corresponds to that under the description penetration resistance. He is only possible with materials that do not break. Once the penetration test has been completed, the difference becomes after 60 seconds in which the DUT is completely relieved 4 in mm as height difference of the position of the surface of the test object 3 before testing the position of the surface of the test piece 3a after examination, determined.

The "permanent deformation" is expressed as a percentage of the difference 4 calculated on the support bearing distance according to the following formula: Permanent deformation [%] = difference 4 [mm] / support bearing distance [mm] × 100%

The aim is to achieve values of less than 8% permanent set, preferably values which are less than 5%.

Looking at Table 1, it can be seen that, when determining the penetration resistance, the material designed according to the invention does not crack or break under pressure, for example when a panel is walked on. There is only a deformation that disappears immediately after the loss of stress again. A prior art lower cover plate made of compressed wood fibers, listed in Table 1 as a comparative example, breaks even at a deformation by 3% of the support bearing distance, in the present case already at a deformation of only 6.9 mm. Plates made according to the invention can easily be deformed 10 times, up to 30% without breaking.

It is the same with the parameter of permanent deformation. As mentioned, a plate of the prior art already breaks at low deformation. The inventively designed material goes back almost immediately after a load of 30% back to the original position. It therefore shows no permanent deformation, the measured value is 0 mm. This property is necessary especially for panels that are not covered but laid by tongue and groove. A permanent deformation would lead to tongue and groove no longer interlocking, it would lead to a defect in the cover. Furthermore, this property minimizes the risk of edge damage during transport / handling of panels on a construction site. If the edge of a lower cover plate designed according to the invention is subjected to a mechanical load, it deviates from the load, but returns to the starting position after the load has been eliminated. In a prior art plate, the edge would be irreparably damaged.

Furthermore, one recognizes the enormous weight advantage over the prior art. A commercially available wood fiber board has the same thickness 2.1 times the weight of a inventively designed lower deck plate. The handling on a construction site when covering a roof is correspondingly simpler.

A lower cover plate according to the prior art has almost no air permeability according to the above test method. If you consider the air permeability of an inventively designed lower cover plate, so you can see air permeability, this is in the embodiments in the range of 120 l / m ² / s. Depending on the application, the air permeability may vary. This has a very positive effect on the climate of the roof construction. It can not come to moisture accumulation, since moisture can easily diffuse through an inventively designed lower cover plate. Likewise, if due to adverse weather circumstances, water has penetrated into the matrix of the lower cover plate, this moisture can easily be released again, the lower cover plate dries off easily.

The inventively designed Unterdeckplatten have Wärmeleitfähigkeiten less than 0.04 W / mK, in preferred embodiments, less than 0.035 W / mK. Hardboard from the embodiment are here by 0.05 W / mK significantly higher. Due to these properties, a bottom cover plate designed according to the invention can also be included for the calculation of the U value.

Due to the fact that already a part of the external structure of the roof takes over heat-insulating function. The thickness of a Zwischensparrendämmung can be significantly reduced, which leads, especially in Altbausanierungen by the different rafter heights to reduce costs.

The water absorption of a plate according to the invention is, in each case in comparison to the prior art, at a lower density, and increased or even existing air permeability at a comparable level. In particular, one recognizes the influence of surface smoothing on this parameter. A two-sided heat treatment reduces the water absorption by 2.4% to 4.5%.

In further embodiments, the side edges of ready-laid under deck panels may be made smooth, but may be configured so that the edges have a tongue and groove or beveled structure.

According to the invention, the tongue and groove structure can be achieved by stacking a plurality of cut bottom cover plates offset one above the other and resulting in the tongue and groove structure. A mechanical milling treatment for producing the grooves / springs is feasible.

By this design, the watertightness is also guaranteed at the transition points between two plates when laying on impact. Also, the stiffness is further increased.

It is also conceivable to use a hydrophilic coating or plate on the side of the lower cover plate facing the structure. Due to the hydrophilic properties, the formation of droplets of condensation is prevented, thereby creating intelligent, condensation-tolerant moisture management.

Surprisingly, it was found that undercover panels produced according to the invention, in addition to thermal insulation, also contribute to sound insulation. The sound absorption is favored by porous, open-cell structures. The sound energy is converted by friction into heat energy. The existence of open and deep pores is important here. An inventively executed material has just such pore structures. These structures affect the so-called specific flow resistance, which is calculated according to the formula: Specific flow resistance [Pa s / m] = pressure difference [Pa] / flow rate air [m / s]

Thus, the plate manufactured according to the embodiment was examined for its sound insulation. The air permeability was determined according to the method described above. Values of 770 Pa.s / m for Embodiment 1 and of 810 Pa.s / m for Embodiment 2 were found for the plates constructed according to the invention. Values between 700 and 3000 Pa.s / m are aimed for. Values greater than 3000 Pa · s / m are to be avoided, since then a sound reflection takes place.

Undercover panels so produced continue to have flammability properties that meet common market requirements such as DIN 4102, B2 correspond. If higher demands are made, these can be achieved by using flame retardant treated fibers such as Trevira CS or chemical equipment.

The areas of use of such undercover panels are not limited to the roof area. They can also be used in the area of exterior walls as a heat-insulating construction material.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 4322745 [0003]
• DE 29723930 [0004]
• EP 1644565 [0030, 0043]

Cited non-patent literature

• DIN 4102, B2 [0071]

Claims (14)

Lower cover plate, characterized in that • one or both surfaces of the lower cover plate have hydrophobic properties. Bottom cover plate according to claim 1, characterized in that • the bottom cover plate consists of a nonwoven fabric formed in the dry process Bottom cover plate according to one of the preceding claims, characterized in that • the nonwoven fabric is mechanically consolidated and compacted, and thermally bonded. Bottom cover plate according to one of the preceding claims, characterized in that • the nonwoven fabric is formed from synthetic staple fibers which have a crimp Bottom cover plate according to one of the preceding claims, characterized in that • at least 30% by weight of the fibers forming the nonwoven fabric consist of staple fibers which have hydrophobic properties before and / or after thermal treatment. and have a fiber diameter of less than 21 microns Bottom cover plate according to one of the preceding claims, characterized in that • at least 40% by weight of the fibers forming the nonwoven fabric consist of melt fibers. Bottom cover plate according to one of the preceding claims, characterized in that • a maximum of 30% by weight of the fibers forming the nonwoven fabric are coarse fibers. Bottom cover plate according to one of the preceding claims, characterized in that • the surface of the upper side and / or the surface of the underside is smoothed. Lower cover plate according to one of the preceding claims, characterized in that • the lower cover plate has a thickness between 10 mm to 60 mm. Lower cover plate according to one of the preceding claims, characterized in that • the lower cover plate has a density of less than 200 kg / m ³ . Lower cover plate according to one of the preceding claims, characterized in that • the lower cover plate has a coefficient of thermal conductivity of less than 0.04 W / mK Lower cover plate according to one of the preceding claims, characterized in that • the lower cover plate has an air permeability. Bottom cover plate according to one of the preceding claims, characterized in that the bottom cover plate has a permanent deformation of the test method specified in the description of less than 10%. Lower cover plate according to one of the preceding claims, characterized in that • the lower cover plate has a specific flow resistance greater than 700 Pa · s / m.

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