METHOD FOR ATTACHING A LAYER OF ELASTOMERIC MATERIAL TO AN ALUMINUM SUBSTRATE

Description

[0001] The present invention relates to a method of fixing a layer of elastomeric material onto an aluminum substrate.

[0002] It finds application particularly in the aeronautical field, for example in the manufacture of fuel tanks or de-icers. STATE OF THE ART

[0003] In many industrial sectors, manufactured products are made from assemblies of materials including elastomers. To ensure their mechanical strength, the substrates onto which the elastomers are deposited are coated with bonding agents.

[0004] The need for high mechanical performance has led to more complex deposition processes for these agents. Indeed, the deposition procedure is often manual, involves the repetitive application of one or more adhesive layers, and depends on the complex geometries of the parts to be assembled.

[0005] Thus, in the aeronautical field, it is known to manufacture fuel tanks and de-icers from elastomeric products for which bonding to metallic, thermoplastic and vulcanized rubber substrates is necessary.

[0006] In the case of tanks, these incorporate accessories of varying chemical compositions that are bonded to the rest of the product wall. These bonds must therefore provide a perfect seal and significant mechanical resistance. For example, to pass a crash test, these tanks must maintain their seal when dropped from a standardized height of 15 meters.

[0007] Adhesion between the accessories and the tank wall is therefore a key parameter for these assemblies.

[0008] Regarding de-icers, we are dealing with less mechanical stress. On the other hand, these assemblies must withstand significant temperature variations during operation (de-icing cycles), which can range from -55°C at altitude (in the absence of icing conditions) to temperatures of around 100°C at the level of the metallic thermal resistance assembled by bonding.

[0009] The stresses are induced by the differential expansion of the constituents (metal / rubber). These products, due to their use in an environment external to the aircraft, are subjected to aggressive environmental conditions (temperature variation, humidity, presence of ozone, abrasion, etc.).

[0010] Such assemblies are carried out until now in the following manner: surface preparation of the metallic substrate (such as sandblasting, grinding, sanding), cleaning, application of an adhesion primer, application of a layer of glue and finally bonding with the elastomer layer.

[0011] These assemblies are integrated into the manufacture of the product (reservoir or defroster) which is then vacuum-sealed and vulcanized in an autoclave.

[0012] Primer and adhesive layers are generally liquid phases (water-based or organic). As a result, operators are exposed to chemicals that can potentially be hazardous to their health. Furthermore, guidelines regarding chemical risks are becoming increasingly stringent, and some adhesive products are already facing obsolescence.

[0013] Thus, some adhesion primers use ethyl alcohol and ethyl acetate, both of which are highly flammable, with ethyl acetate also being an irritant.

[0014] Prior art of interest consists of document US2017 / 154866 and the article "Characterization of functionalized coatings prepared from pulsed plasma polymerization" (JI Marisol et al. / 2021).

[0015] The present invention aims to overcome the problem described above and to propose for this purpose a metal / elastomer assembly technique, in this case aluminium / elastomer, which is carried out via the preparation of the substrate by dry means, that is to say without the use of adhesion primers or glue at the metal / rubber interface. PRESENTATION OF THE INVENTION

[0016] Thus, the present invention relates to a method for fixing a layer of elastomeric material onto an aluminum substrate, characterized in that it comprises the implementation of the following steps: a) Treatment of the surface of said aluminium substrate so as to make it rough; b) Treatment of said surface using an argon plasma; c) Exposure of said surface to a plasma, in the presence of a gaseous chemical precursor, said precursor being selected from epoxides, acrylics, alkenes, alkynes and imides, until a deposit of chemical species with a thickness of between 5 and 110 nanometres is obtained; d) Contacting said layer of elastomeric material (CE) with the surface (S) of said aluminium substrate; e) Vulcanization of said layer of elastomeric material, said step c) being carried out at low pressure, i.e. under a pressure of between 10⁻² and 10⁻⁵ mbar.

[0017] Thanks to the invention, the use of liquid adhesion chemicals used until now is eliminated.

[0018] Moreover, unlike the prior art, which exhibited heterogeneities in the preparation of the metal surface before bonding due to a manual process, the process according to the invention is simpler to implement, particularly due to the reduction in the number of steps and a saving of labor in the preparation of the metal surfaces.

[0019] Furthermore, there is a homogeneity in the deposition of chemical species, both in composition and thickness, regardless of the geometry of the part.

[0020] Furthermore, we gain in thickness with thinner deposits (5 to 100 nm) compared to 100 µm for the primaries of the prior art.

[0021] It should also be noted that steps b) and c) can be carried out in mask time, since they do not require manpower during plasma treatment.

[0022] Finally, this process contributes to reducing the environmental footprint of this technique, in particular by decreasing the use of chemicals that are contrary to current or future environmental standards and regulations.

[0023] According to other advantageous and non-limiting features of the invention: In step a), the surface is treated until a roughness Ra of between 2 and 20 micrometers is obtained; in step a), the surface treatment is carried out by laser ablation or by mechanical treatment such as shot blasting, sanding or scraping; step c) is carried out with an argon plasma; step c) is carried out with the following operating conditions: Pulsed plasma; Power: between 5 and 600W, and preferably between 5 and 100W; Frequency: between 5 and 50KHz, and preferably between 5 and 30 KHz; Argon gas flow rate: between 5 and 50 cm³ / minute and preferably between 10 and 40 cm³ / minute; Total duration: between 11 and 60 minutes; Duty cycle: between 4 and 30% for pulsed mode, and preferably between 8 and 11%. DESCRIPTION OF THE FIGURES

[0024] Other features and advantages of the invention will become apparent from the description which will now be given, with reference to the attached drawing, which represents, by way of example but not limitation, one possible embodiment.

[0025] In these drawings: there figure 1 is a very simplified diagram of an assembly obtained in accordance with the process according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] As mentioned above, the method according to the invention comprises the implementation of the following steps: a) Treating the surface of the aluminum substrate to make it rough; b) Treating the surface of the substrate with an argon plasma; c) Exposing the surface of the substrate to a plasma, in particular argon, in the presence of a chemical precursor, said precursor being selected from epoxides, acrylics, alkenes, alkynes and imides, until a deposit of chemical species with a thickness of between 5 and 110 nanometers is obtained; d) Contacting said layer of elastomeric material with the surface of the aluminum substrate; e) Vulcanizing the layer of elastomeric material.

[0027] Each of these steps is described below, along with variations and / or preferred methods of carrying them out. Treating the surface of the aluminum substrate to make it rough.

[0028] The purpose of this treatment is to prepare the free surface S of the metallic substrate SM (see figure 1) in order to promote the subsequent anchoring of the layer of chemical species generated during the later plasma treatment steps.

[0029] Its purpose is also to remove the usual anti-corrosion protective layer from the surface of the metallic substrate, such as the sulfuric anodic oxidation protective layer (abbreviated "OAS").

[0030] Preferably, the surface is treated until a roughness Ra of between 2 and 20 micrometers is obtained.

[0031] To achieve this operation, one can use laser ablation or resort to a mechanical treatment aimed at creating a rough surface such as shot blasting, sanding or scraping. Surface treatment of the substrate using an argon plasma

[0032] The main purpose of this treatment is to remove any oxides that may have formed on the surface S of the substrate SM, as well as any organic pollutants. Indeed, between the previous step and this one, a "re-oxidation" may have occurred.

[0033] Advantageously, this treatment can be implemented while respecting the following parameters: Continuous plasma mode; Power: 200W; Argon gas flow rate: 20 cm³ / minute; Duration: 20 minutes. Exposure of said surface to a plasma, for example argon, in the presence of a chemical precursor

[0034] The precursor allows the creation, by plasma deposition, of a polymer layer which will allow the metallic surface to be chemically compatible with the elastomer.

[0035] The thickness of this layer must be sufficient to allow inter-diffusion of the polymer.

[0036] According to the invention, said precursor is selected from epoxy compounds, acrylics, alkenes, alkynes and imides, until a deposit of chemical species of thickness (referenced DP in the figure 1 ) between 5 and 110 nanometers.

[0037] This includes, for example, acetylene gas.

[0038] It should be noted that a thickness less than that indicated above results in lower mechanical strength than required. This could also lead to poor metal coverage.

[0039] Beyond 110 nanometers, no significant improvement in adhesion is detected and a layer that is too thick could cause embrittlement by internal stress.

[0040] Advantageously, this step is implemented with the following operating conditions: Pulsed plasma; Power: between 5 and 600W, and preferably between 5 and 100W; Frequency: between 5 and 50KHz, and preferably between 5 and 30 KHz; Argon gas flow rate: between 5 and 50 cm³ / minute and preferably between 10 and 40 cm³ / minute; Total duration: from 11 to 60 minutes; Duty cycle: between 4 and 30% for pulsed mode, and preferably between 8 and 11%.

[0041] These operating conditions promote the growth of the DP deposit, with inter-diffusion between this deposit and the elastomer layer in the following step. The elastomer material layer is brought into contact with the surface of the aluminum substrate.

[0042] The SM substrate is then used for assembly with an elastomer layer (referenced CE to the figure 1 ) such as a PVC-NBR (mixture of polyvinyl chloride and acrylonitrile butadiene).

[0043] If necessary, the contacting surface of the elastomer will have been previously and advantageously treated with a solvent to give it a "sticky" surface before assembly. In other words, this operation not only cleans the surface but also causes the elastomer to swell, promoting diffusion / migration to the deposit.

[0044] Elastomers such as polyurethane-based elastomers, EVA (ethylene-vinyl acetate), etc. can also be used. Vulcanization of the elastomer material layer.

[0045] The resulting assembly is, for example, placed under vacuum and vulcanized. During this step, chemical bonding occurs, achieved through the reactivity of the DP plasma deposition with the elastomer to be vulcanized.

[0046] An alternative method involves vulcanizing under pressure.

[0047] A layer of material, such as an AC anti-crash material, can then be fixed to the surface opposite the CE elastomer layer.

[0048] This crash-resistant material is typically a polyamide-based fabric with an elastomer coating. The coated side of this fabric is manually pressed into contact with the exposed layer of the CE-marked elastomer. After this contact, the assembly is compressed using a notched roller.

[0049] Alternatively, one or more layers of material reinforced by a textile or other compositions / substrates can be used depending on the intended applications.

[0050] On the figure 1 attached, the thicknesses shown do not reflect reality.

[0051] For guidance purposes only, these thicknesses are as follows: SM: less than 4 millimeters; DP: between 5 and 110 nanometers; CE: between 0.2 and 2 millimeters; AC: between 0.1 and 10 millimeters.

[0052] Adhesion test specimens were assembled and vulcanized in an autoclave according to the present process.

[0053] The adhesion values ​​obtained as is are higher than the requirements of the TSO-C80 standard (2 N / mm).

[0054] Regarding the stability of the metal / elastomer interface in contact with a fuel, an adhesion value greater than 1.08 N / mm after immersion in an isooctane / toluene mixture is observed.

Claims

1. A method for bonding a layer of elastomeric material (CE) to an aluminum substrate (SM), characterized in that it comprises carrying out the following steps: a) Treating the surface (S) of said aluminum substrate (SM) so as to roughen it; b) Treating said surface (S) by means of an argon plasma; c) Exposing said surface (S) to a plasma, in the presence of a gaseous chemical precursor, said precursor being chosen from the epoxides, acrylics, alkenes, alkynes and imides, until a deposit of chemical species with a thickness of between 5 and 110 nanometers is obtained; d) Contacting said layer of elastomeric material (CE) with the surface (S) of said aluminum substrate (SM); e) Vulcanizing said layer of elastomeric material, said step c) being carried out at low pressure, i.e. under a pressure of between 10-2 and 10-5 mbar.

2. The method according to claim 1, characterized in that, in step a), said surface (S) is treated until a roughness Ra of between 2 and 20 micrometers is obtained.

3. The method according to one of claims 1 and 2, characterized in that, in step a), said surface treatment (S) is carried out by laser ablation or by mechanical treatment such as shot blasting, sanding or scraping.

4. The method according to one of claims 1 to 3, characterized in that step c) is carried out with an argon plasma.

5. The method according to one of the preceding claims, characterized in that said step c) is implemented with the following operating conditions: - Plasma in pulsed mode; - Power: comprised between 5 and 600 W, and preferably between 5 and 100 W; - Frequency: comprised between 5 and 50 KHz, and preferably between 5 and 30 KHz; - Argon gas flow rate: comprised between 5 and 50 cm3 / minute and preferably between 10 and 40 cm3 / minute; - Total duration: comprised between 11 and 60 min; - Duty cycle: between 4 and 30% for pulsed mode, and preferably between 8 and 11%.