FR2735064A1 - Prodn. of composite material microporous sandwich panel skins - Google Patents

Abstract

Composite material, microporous skin, partic. for making sandwich construction parcels, is produced by: (a) laying a skin (16) of fine mesh fabric coated with resin over a mould release agent covered metal tooling (15); (b) putting in place necessary surrounding consumable prods.; (c) putting the skin, tooling and consumables into a flexible bag (18) connected to a vacuum source; (d) applying vacuum to flatten the skin against the tooling; and (e) placing the bag and its contents into an oven, curing the resin then demoulding the microporous skin.

Description

PROCESS FOR PRODUCING MICROPOROUS SKIN
IN COMPOSITE MATERIAL
The present invention relates to a method of manufacturing a microporous skin of composite material, in particular a method of manufacturing at least one of the skins of a sandwich panel made of composite material as well as the microporous skin obtained.

 In the case of an aircraft fuselage in particular, it is sought to reduce the noise for better comfort of passengers and flight personnel who are subjected very regularly and for long periods to an environment with high acoustic level, due to the engine, vibrations and air flows.

 The frequencies are more particularly generated in ranges higher than 500 Hz, more particularly comprised between 1200 and 2000 Hz and these frequencies must be treated in priority because the body of the passengers is very sensitive to it and especially the intelligibility of the speech s' feels it.

 Acoustic problems posed by aircraft cells must also be resolved by suitable panels, but taking into account additional constraints such as mass, thickness, price, but also aesthetic constraints, because such panels must be able to be decorated on the inside, cabin side.

 In aircraft, at present, provision is made for such panels with a high sound absorption coefficient, comprising at least one layer of open-cell foam to prevent direct transmission from the source to the interior of the cabin, but they are not entirely satisfactory because they do not respond well to the additional constraints set out above.

 The decorative and aesthetic constraints are not the least because the side facing the interior of the cabin must be coated to give a feeling of comfort and a certain aesthetic appearance.

 This interior face of the panel, in visual and physical contact with the passengers, must also be able to be easily cleaned during the interior maintenance of the cabin.

 In addition, the panel must be rigid enough so that a person can lean on it, even in the event of an impact, without breaking it, which implies good mechanical resistance.

 The solution adopted for the invention consists, for example, in producing cladding panels with high acoustic attenuation the aim of which is' to dampen the noises in the cabin, generated by the various acoustic connections or bridges, knowing that an insulated shell , without leakage, is not practical in practice for an aircraft cabin. It is therefore necessary a panel which makes it possible to fight against reverberations, all the more numerous since the cabin is small, which is light, which is simple to manufacture so that the cost price remains within lower limits than those of the panels. existing.

 Fire resistance is another constraint to be observed for the materials used in the construction of aircraft cabin walls. It is therefore necessary that the compounds as used during the manufacture of said wall comply with the standard in force, FAR 25 853 (Fire / SmokeToxicity criteria) applicable to commercial aircraft.

 In different applications, there are documents of the prior art which propose rigid panels made of composite material intended to attenuate the noises.

We know from patent application FR-A-2,710,874, in the name of
Applicant, a rigid and self-supporting sheet material for acoustic attenuation and a panel made of such a material.

This material includes
- a core with an alveolar structure, the alveoli being open, more
particularly a honeycomb,
- an acoustically absorbent material, filling the cells,
- bands of resistant fibers forming openings between them,
on each side, and
- a treiilis covering the whole, with large meshes.

 This type of panel thus produced is used as transverse partition walls, in particular in aircraft cabins. Such a panel allows excellent absorption of noise in the cabin, in particular for aircraft of the turboprop type, but such a panel is too heavy, too thick and above all less effective in covering the partition, said partition generally already comprising other acoustically absorbent materials. .

US-A-3,439,774 describes a panel forming a casing for an aircraft reactor, intended to absorb the noise generated by the operation of this reactor. For this purpose, it is provided in superposition, from the outside to the inside:
- a full sheet forming an envelope,
- a first core with a honeycomb structure, of the cross type,
- a first sheet of microperforated metallic material with
holes from 50 to 500 lim, the process for producing microperforations
not being indicated,
- a second core with honeycomb structure, of the cross type, and
- a second sheet of microperforated metallic material, with a
superior permeability.

 Thus, the high frequency noises propagating from the inside to the outside are absorbed preferentially by the second sheet and the low frequencies are absorbed by the first sheet after treatment by passage through the second sheet.

 Such a material is too thick to be used in the cabin wall, as a coating. The material thus disclosed is also too heavy and this type of panel remains limited to the particular application envisaged, namely the reduction in the propagation of noise from an aircraft engine.

 Finally, mention may also be made of patent application DE-A-3,913,255 which describes a panel comprising a succession of a plate comprising holes, a sheet of non-woven fabric, a honeycomb core, a plate provided smaller diameter holes and a thick layer of rock wool.

 Such a panel aims at acoustic attenuation but it is too thick, too heavy, too complex for an application to interior lining panels of an aircraft cabin.

 Furthermore, the industry shows a high demand for microporous skins, of great mechanical strength, of great dimensional stability, which can be conformed in a complex manner, without these criteria being able to be considered as exhaustive.

 The object of the invention is therefore to be able to produce a skin with a single fold with microperforations having in particular a controlled geometry and dimensions.

 The invention also relates to the production of a plywood.

 Such skins can then be used in particular for the production of acoustic damping panels but not exclusively since applications to filtration for example can also be envisaged.

 On the other hand, it is known that the production of a skin of composite material has micropores of more or less large dimensions. In addition, for the cost price to be affordable, mechanical machining, laser or any other similar method must be avoided.

 It therefore appears advantageous to be able to produce a microporous skin, either independently so as to be subsequently included in a multilayer panel or in other structures. either by co-cooking, simultaneously with the production of a sandwich panel.

To this end, the method of manufacturing a microporous skin according to the invention is characterized in that it comprises the following steps
- Draping of a skin of fine mesh woven material, coated with
resin, on a metal tool, to the shape of the skin to be obtained,
coated with a release agent,
- Implementation of environmental products,
- Introduction of the skin and the tools in a flexible bladder connected
vacuum means,
- Depression of the bladder so as to press the skin on
the tools,
- Introduction of the bladder with its contents and tools in a
oven,
- Cooking cycle, and
- Demoulding of the microporous skin thus produced.

 According to another characteristic of the invention, the resin used is a phenolic resin and the woven material is a glass fabric.

The invention also has for application a method of manufacturing a sandwich panel comprising at least one microporous skin and is characterized in that it comprises the following steps
- Draping of the first skin in glass fabric with large mesh and
possible reinforcements on a metal tool in the shape of the
panel to be obtained, coated with a release agent,
- Placement of the honeycomb core on this first skin,
-Deposition on the honeycomb core 14 of the second skin 16 in
fine mesh fabric,
- Implementation of environmental products, including
necessary consumables, of known type, in particular a
layer of perforated non-stick material, for example in
fluoroethylene, a layer of draining material such as
wadding which allows in addition to a good diffusion of depression, a
absorption of excess resin,
- Installation of a flexible bladder, generally made of nylon,
surrounding the entire sandwich and consumables,
The sealing of the bladder with the tool being obtained by
cords of a suitable material, of the putty type,
- Depression of the interior of the bladder to a vacuum level
between 5.104 and 9.104 Pa,
- Introduction of the bladder and the whole sandwich in a
oven,
- Cooking cycle, and
- Demoulding of the part thus produced.

According to another characteristic of the invention, the method of manufacturing a microporous skin and a sandwich panel is characterized in that the cooking cycle comprises the following stages
- Rise to polymerization temperature,
- Maintaining at this polymerization temperature up to the degree of
desired crosslinking,
- Heating stops and free cooling,
- Vacuum break and return to atmospheric pressure when the
temperature is preferably lower than TG (temperature of
glass transition).

The method is described below, according to a particular, nonlimiting embodiment, given by way of example, this description referring to the various appended drawings in which
- Figure 1 shows a perspective view of a real mode
skin co-cooking simultaneously with manufacturing
an acoustic attenuation covering panel,
- Figure 2 shows a cross-sectional view of a panel
as shown in figure 1, set up in a cabin
aircraft,
- Figure 3 shows a view of a panel with skin
microporous in a bladder, and
- Figure 4 shows a microscope view of a skin
microporous obtained by implementing the method according to
the invention.

 According to the embodiment of FIG. 1, the panel shown comprises from the inside I towards the outside E, a layer 10 of decoration proper, a first skin 12 made of wide mesh fabric, a core 14 and a second skin 16 with micropores, skin which directly concerns the process according to the invention.

The resins of the different skins are phenolic because they have many advantages
- very good fire resistance,
- very low heat output, and
- low emission of toxic products in the event of inflammation.

 The decoration layer 10 comprises a decoration fabric better known in the trade under the name of AERIA T1 from the company TEXAA.

 This layer 10 does not intervene at all in terms of acoustic damping properties since it is transparent to sounds, but its role is to mask the first skin 1 2 with large meshes from view without modifying the frequency spectrum of the sounds which spread in the cabin.

 The first skin 1 2 of large mesh fabric contributes to the mechanical reinforcement of the panel. This skin is obtained from a glass fabric impregnated with phenolic resin.

The characteristics of the glass fabric are as follows
- Area weight: 278 g / m2;
- thickness: 0.4 mm
- square mesh of the order of 1.8 mm between the axes of the wires.

 This first skin 1 2 is optionally supplemented by reinforcements 13 also produced from a glass fabric impregnated with phenolic resin, placed at the appropriate locations on the panel, determined by calculations of the resistance of materials within the reach of man. art.

The core 14 is made of honeycomb, in particular of aramid fiber paper such as that known commercially as "NOMEX".

 The second skin 1 6 is the essential element of the combination, by the micropores which it presents, as will be explained later.

This skin is obtained from a glass fabric coated with phenolic resin, the characteristics of which are as follows
2
- Area weight: 107 g / m2
- Thickness: 9/100 mm
- Armor: Satin of 4
- Number of warp threads per cm: 24
- Number of weft threads per cm: 23
As for the impregnation, it is carried out with a nominal mass rate of 40% in resin.

 We know that, in addition to their intrinsic qualities, phenolic resins polymerize by a polycondensation reaction which is accompanied by the evolution of water in the form of vapor at temperatures above 1000C, the firing temperature generally being 1500C.

 We can advance a first hypothesis of micropore formation according to which the gases, by escaping, make appear in the tissue of the micropores. Generally, manufacturers of composite parts fight against such micropores, in particular by placing the fabric which it is desired to obtain a seal against a mold or, more exactly, against the face of the tooling and / or by introducing degassing phases in the polymerization cycle.

 We can also put forward a second hypothesis according to which, during polymerization, the polymer firstly passes through a sufficiently liquid phase with a simultaneous shrinking between two fibers, which tends to cause a hole to appear in line with the thin layer. . As the fabric has mesh, the hole tends to appear in the center of the mesh. The polymerization then blocks the polymer in this situation, which generates the micropores sought.

 Reference may be made to FIG. 4 which shows a microscope view, magnification of the order of 60, of a microporous skin thus obtained.

Contrary to this technique and overcoming the prejudice which results from it, the method of manufacturing a panel with a microporous skin according to the invention comprises the succession of the following steps, according to FIG. 3
- Draping of the first skin 1 2 in glass fabric with large mesh
and any reinforcements 1 3 on a metal tool to the shape
of the panel to be obtained, coated with a release agent,
- Placement of the honeycomb friend 14 on this first
skin,
-Deposition on the honeycomb core 14 of the second skin 16 in
fine mesh fabric,
- Implementation of environmental products, including
necessary consumables, of known type, in particular a
layer 21 of perforated non-stick material, for example in
fluoroethylene, a layer 22 of draining material such as
wadding which allows in addition to a good diffusion of depression, a
absorption of excess resin,
- Installation of a flexible bladder 18, generally made of nylon,
surrounding the entire sandwich and consumables,
The sealing of the bladder 18 with the mold 15 being obtained by
cords 24 of a suitable material, of the mastic type,
- Depression of the interior of the bladder 18 at a level of
vacuum between 5.104 and 9.104 Pa,
- Introduction of the bladder 1 8 and the entire sandwich in
an oven,
- Cooking cycle
. Rise to polymerization temperature,
. Maintaining at this polymerization temperature up to the degree
desired crosslinking,
Heating stops and free cooling,
- Vacuum break and return to atmospheric pressure when the
temperature is preferably lower than TG (temperature of
glass transition), and
- Demoulding of the part thus produced.

 It is noted that during this polymerization, micropores were formed whose geometry, number and size depend in particular on the polymerization conditions. These parameters can be checked and reproduced.

Under the following cooking conditions
- temperature rise at a speed of 50C per minute
- maintaining at a temperature of 1500C for a period of
1 h 30
- breaking of the vacuum when the temperature is below 400C, micropores are obtained whose geometry is substantially rectangular, the sides having dimensions of between 0.1 and 0.3 mm (cf. FIG. 4).

It should be noted that the important parameters to be controlled are
- maximum temperature to reach for polymerization,
- vacuum level,
- mold release temperature,
- mass rate of resin,
- properties of the resin.

 - choice of armor.

As for the decorative layer 10, it is then added by bonding to the first skin 12 using a suitable adhesive, for example sprayed vinyl glue (4301-8 from the French Seal) so as not to disturb fire resistance and acoustic transparency. One can use in particular the fabric sold under the name AERIA T1 of the Company
TEXAA.

 In Figure 2, there is shown a sectional view of a covering panel thus obtained and arranged in the immediate vicinity of an aircraft cabin partition comprising a layer 20 of damping material such as open-cell foam. This foam has a damping effect for direct noise propagating from outside sources towards the interior, through said partition, but also an absorption role for noise propagating in the cabin and treated by the micropore panel obtained by the process. above described.

 Indeed, the noises propagating in the cabin pass through the decoration layer 10, through the first skin 12 with large meshes then through the micropores of the second skin 16, to be absorbed by the layer of cell foam open 20.

 It is found that such a panel has a small thickness, good mechanical strength due to its sandwich structure as well as a certain ability to receive a decorative fabric.

 In the case where the microporous skin is placed in view, one can also provide, as a layer of decorative material, the deposition of a veil of paint so as to avoid clogging the micropores.

 It is also noted that the panel is produced in a single step which simultaneously allows the fixing of the two skins on the honeycomb core and the production of the micropores without any other intervention.

 It is noted that in the case of integrated manufacturing, the panel is obtained directly, in a single operation. The assembly of the skins with the honeycomb core, an operation necessary in all cases for the production of a sandwich structure, is used to simultaneously generate the micropores, without rework, without any mechanical machining, without intermediate blowing. .

 This process allows to easily realize, in a single step, an effective sandwich panel, which is simple in its structure, light, mechanically resistant, to fire standards because the materials used are well known and few, moreover they have already proven themselves for their qualities in the event of a fire.

 It is also noted that the microporous skin can be obtained alone for use in the production of multilayer panels in composite material or for any other application such as filtration.

 In this case, the manufacturing process is modified since only the fine mesh fabric coated with resin is placed on the mold, the shape of which corresponds to the profile of the skin to be obtained. It is then advisable to modify the mass content of resin and the firing conditions to obtain the desired micropores.

 The invention makes it possible to produce a multilayer microporous skin in the same way by playing on the polymerization parameters and the mass rates of resin in particular.

Claims (4)

 1. Method for manufacturing a microporous skin of composite material, in particular for producing a sandwich panel, characterized in that it comprises the following steps  - Draping of a skin (16) of woven material with fine mesh, coated with  resin, on a metal tool (15), in the shape of the skin  obtain, coated with a release agent,  - Implementation of environmental products (21,22),  - Introduction of the skin and the tools into a flexible bladder (18)  connected to vacuum means,  - Depression of the bladder so as to press the skin on  the tools,  - Introduction of the bladder with its contents and tools in a  oven,  - Cooking cycle, and  - Demoulding of the microporous skin thus produced.  2. A method of manufacturing a microporous skin according to claim 1, characterized in that the resin is a phenolic resin and the woven material is a glass fabric.  3. A method of manufacturing a sandwich panel comprising at least one microporous skin obtained by the method according to any one of claims 1 or 2, characterized in that it comprises the following steps  - Draping of the first skin (12) in large mesh glass fabric  and possible reinforcements (13) on a metal tool at the  shape of the panel to be obtained, coated with a release agent,  - Placement of the honeycomb friend (14) on this first  skin,  -Deposition on the honeycomb core (14) of the second skin (16)  in fine mesh fabric,  Implementation of environmental products, including  necessary consumables, of known type, in particular a  layer (21) of perforated non-stick material, for example in  fluoroethylene, a layer (22) of draining material such as  wadding which allows in addition to a good diffusion of depression, a  absorption of excess resin,  - Installation of a flexible bladder (18), generally made of nylon,  surrounding the entire sandwich and consumables,  sealing the bladder (18) with the tool (15) being obtained  by cords (24) of a suitable material, of the mastic type,  Depression of the interior of the bladder (18) to a level of  vacuum between 5.104 and 9.104 Pa,  - Introduction of the bladder (18) and the entire sandwich into  an oven,  - Cooking cycle, and:  - Demoulding of the part thus produced.  4. Manufacturing method according to claim 1 or 3, characterized in that the cooking cycle comprises the following steps  - Rise to polymerization temperature,  Maintaining at this polymerization temperature up to the degree of  desired crosslinking,  - Heating stops and free cooling,  Vacuum break and return to atmospheric pressure when the  temperature is preferably lower than TG (temperature of  glass transition).