Method of manufacturing a tank

Abstract

The invention relates to a method of manufacturing at least one hollow element of a fuel system (3), especially a casing of a fuel tank. It comprises the following steps: (a) introducing the element or elements to be treated into a treatment chamber (2) equipped with at least one microwave generator, (b) injecting a reaction gas into the chamber while enabling the gas to enter inside the said elements, (c) subjecting this gas to microwave radiation delivered by the microwave generator (4), at a frequency and intensity chosen so as to create a plasma in the chamber and to bring about the coating of a material, arising from the decomposition of the reaction gas, on the said element or elements, preferably on both the inner and outer faces of the said element or elements.

Description

[0001] The present invention relates to a method for depositing a coating on an element of a motor vehicle fuel system, especially a tank casing.

[0002] It is known to treat motor vehicle fuel tanks made of plastic so as to make them impermeable to hydrocarbons.

[0003] Such a treatment may consist in superimposing a layer forming a hydrocarbon barrier onto the plastic constituting the casing of the tank.

[0004] U.S. Pat. No. 5,677,010 describes a method for producing such a layer on the inner surface of a fuel tank, in which a reaction gas is introduced inside the tank by means of a probe and this gas is activated using microwave radiation in order to generate a plasma therein. A layer forming a barrier, arising from the decomposition of the reaction gas, is thus formed on the inner surface of the tank. Several tanks can be treated simultaneously by this method by introducing an individual probe into each of them. It is thus necessary to provide as many individual probes as there are tanks to be treated simultaneously, which makes the method relatively complex to implement.

[0005] German Patent Application DE 44 37 050 describes a device for treating the inner surface of a fuel tank by means of a plasma. The tank to be treated is placed in a cavity defined by two sheet-metal elements, this cavity being shaped so as more or less to match the shape of the tank.

[0006] French Patent Application FR 2 776 540 describes a method for producing a layer forming a barrier on the inner or outer surface of a bottle, a plasma being generated, depending on the case, in the bottle itself or in the reactor, outside the bottle, the latter then being stoppered.

[0007] There is a need to improve the performance of motor vehicle fuel tanks, in particular their impermeability to constituent components of the fuel, in particular the hydrocarbons.

[0008] There is also a need to be able to treat several tanks simultaneously in a relatively simple way.

[0009] The invention meets these needs at least partially using a new method of manufacturing at least one hollow element of a fuel system, especially a casing of a fuel tank, characterized in that it comprises the following steps:

[0010] (a) introducing the element or elements to be treated into a treatment chamber equipped with at least one microwave generator,

[0011] (b) injecting a reaction gas into the chamber while enabling the gas to enter inside the said elements,

[0012] (c) subjecting this gas to microwave radiation delivered by the microwave generator, at a frequency and intensity chosen so as to create a plasma in the chamber and to bring about the coating of a material, arising from the decomposition of the reaction gas, on the said element or elements, preferably on both the inner and outer faces of the said element or elements.

[0013] By virtue of the invention, it is possible to produce a tank, the casing of which is made of plastic and has, on its inner and outer surfaces, a coating layer of a material different from the casing, arising from the decomposition of the reaction gas, chosen to improve the properties of the tank, for example with respect to hydrocarbons, ultraviolet radiation, abrasion or heat.

[0014] By depositing the material arising from the decomposition of the reaction gas both on the inner and outer faces of the element or elements treated, a double barrier is obtained, which is more effective than a single barrier which would be present only on one of the faces of the element or elements.

[0015] Furthermore, where the element or elements placed in the chamber comprise(s) one or more attached elements such as a valve, a gauge mounting or an electrical connector in addition to the tank casing, the inner and outer coating layers are deposited both on the casing and on the attached elements and are not interrupted at the joins between the attached elements and the casing.

[0016] It is thus possible to obtain good impermeability of the assembly in particular to hydrocarbons, and to comply with the most stringent standards concerning emissions.

[0017] Preferably, several elements are treated simultaneously in the chamber.

[0018] In this case, advantageously only a single plasma is generated for the entire chamber, which avoids having to introduce a multitude of individual probes into each of the elements to be treated.

[0019] Advantageously, between the abovementioned steps a) and b), the chamber is evacuated such that the absolute pressure therein is less than 0.05 mbar (5 Pa), preferably less than 0.01 mbar (1 Pa).

[0020] It is possible to eliminate the traces of gas which are harmful to the plasma treatment, in particular oxygen, in a particularly effective way by creating a fluorine-based plasma in the chamber after having evacuated it, the oxygen atoms that combine with the fluorine atoms then being pumped out of the chamber.

[0021] The reaction gas may comprise a hydrocarbon compound, preferably with at least 5 carbon atoms.

[0022] In particular, it is possible to use an alkane or an alkene or a mixture thereof.

[0023] Preferably, a mixture of methane and propane is used, such a mixture being inexpensive and enabling a layer of carbon with a polymer tendency to be deposited onto the tank, constituting an effective hydrocarbon barrier.

[0024] Such a layer of carbon with a polymer tendency has strong chemical bonds with the substrate and a certain deformability, which enables it to follow the deformations of the casing to some extent, without a significant risk of failure or of detachment.

[0025] The reaction gas is a siloxane, preferably hexamethyldisiloxane in vapour form.

[0026] The reaction gas can be injected at a nominal pressure of between 0.05 mbar (5 Pa) and 0.5 mbar (50 Pa), for example, preferably equal to about 0.1 mbar (10 Pa).

[0027] The microwaves may be produced at a frequency of between 2 GHz and 3 GHz for example, preferably about 2.45 GHz.

[0028] Advantageously, the microwaves are generated at a power which varies with time, so as to reduce the heating of the casings of the tanks and enable the latter to keep their shape throughout the treatment.

[0029] Advantageously, the aforementioned steps b) and c) are repeated at least once with a reaction gas which is different from the previous gas(es) in order to deposit several materials onto the element or elements to be treated.

[0030] It is thus possible to produce a tank having at least one layer forming a hydrocarbon barrier but also a layer offering better ultraviolet or abrasion resistance, for example, depending on the materials deposited on the casing.

[0031] Advantageously, pressure variations are created in the chamber, especially pressure oscillations, so as to make the casing(s) breathe and to improve the diffusion of the plasma thereinside.

[0032] The relative amplitude of these pressure oscillations is, for example, greater than a factor of five and may be equal to a factor of about ten.

[0033] A favoured manner of varying the pressure in the chamber consists, on the one hand, in applying continuous suction to the chamber and, on the other hand, in injecting the reaction gas with a flow rate which varies over time, this injection preferably being carried out by intermittently opening a valve for controlling the supply of the reaction gas.

[0034] The oscillation period may be of the order of a few seconds to several tens of seconds.

[0035] The plasma is generated in one particular embodiment of the invention at ambient temperature, which minimizes the heating and the deformation of the tank casing.

[0036] Another subject of the invention is an element of a fuel system, especially a tank casing, comprising a substrate layer having an inner surface and an outer surface, characterized in that each of these surfaces is coated with at least one coating material which is different from that of the substrate, arising from the decomposition of a chemical compound under the effect of plasma-generating microwave radiation.

[0037] Such a coating material may be chosen so as to form a hydrocarbon barriers or to protect from ultraviolet radiation, to increase the abrasion resistance of the tank or its resistance to heat.

[0038] Advantageously, the coating material comprises carbon with a polymer tendency and/or silica.

[0039] The substrate layer may receive a layer of the same material or several layers of different materials on each face, the thickness of each layer preferably being between 0.3 and 1 &mgr;m.

[0040] The element or elements treated are advantageously made of a thermoplastic, especially HDPE.

[0041] As indicated above, the coating material deposited on the substrate layer is preferably bonded thereto by strong chemical bonds.

[0042] In the case of carbon, these strong chemical bonds are obtained by the surface carbon atoms of the substrate layer combining with the atoms of the coating material.

[0043] As indicated above, the fuel system element treated according to the invention may comprise, other than a casing, attached elements such as a valve or an electrical connector, assembled before the plasma treatment.

[0044] The subject of the invention is also a fuel tank comprising, on at least one of its inner or outer faces, a coating of carbon or of silica with a polymer tendency.

[0045] Other characteristics and advantages of the present invention will emerge on reading the following detailed description of a non-limiting exemplary embodiment of the invention, and on examining the single FIGURE showing very schematically a plant for treating fuel tanks.

[0046] A treatment plant 1 according to the invention is shown in the drawing, comprising a treatment chamber 2 in which the tanks 3 to be treated are placed.

[0047] These tanks 3 have been produced, in the example in question, by blow moulding high-density polyethylene.

[0048] Of course, the invention is not limited to the treatment of tanks made by blow moulding and is in general applicable to the treatment of tanks obtained by thermoforming, rotomoulding or injection moulding, for example.

[0049] Each tank 3 is especially equipped with a valve 15 fastened thereon before its introduction into the treatment chamber 2.

[0050] Each tank may of course be equipped with other attached elements, such as nozzles, gauge mountings or electrical connectors, in a conventional manner.

[0051] The chamber 2 is equipped with a magnetron 4, known per se, enabling a plasma, to be created thereinside, as will be explained below.

[0052] The nominal power of the magnetron 4 is pulsed at a low frequency, it being possible for the duration of the pulses to vary from 0.5 to 30 s, so as to avoid excessive heating of the tanks 3, which could cause their deformation.

[0053] The tanks 3 are held in the chamber 2 by support means which have not been shown for the sake of clarity in the drawing.

[0054] The plant 1 comprises a two-stage suction system 5 and 6 which operates continuously and enables a relatively high vacuum to be created in the chamber 2, the absolute pressure therein being between 0.01 and 0.1 mbar in the example described.

[0055] The chamber 2 is supplied with a reaction gas contained in a storage tank 7 via a solenoid valve 8 connected to a control device 9.

[0056] The chamber 2 may be big enough so that a large number of tanks can be treated simultaneously in batches, it being possible for this number to be greater than 100, for example.

[0057] The plant 1 operates as follows.

[0058] The tanks 3 are introduced into the chamber 2, then the vacuum is created.

[0059] Next, a fluorine-based plasma is generated and the residual oxygen atoms combined with the fluorine atoms are eliminated by the two-stage suction system 5 and 6.

[0060] Then, a reaction gas contained in the storage tank 7 is introduced into the chamber by opening the solenoid valve 8.

[0061] In the example in question, this reaction gas consists of a mixture of methane and propane and its nominal pressure of injection into the chamber 2 is 0.1 mbar.

[0062] The reaction gas introduced into the chamber 2 is ionized by the microwaves generated by the magnetron 4 while the material constituting the casing of the tanks 3 is activated by the same microwaves.

[0063] The use of a reaction gas consisting of a propane and methane mixture makes it possible to deposit amorphous carbon with a polymer tendency, bonded by very strong chemical bonds to the material constituting the casing of the tanks, onto the inner and outer surfaces of the tanks 3.

[0064] The carbon layer thus deposited is therefore intimately bonded to the casing of each tank.

[0065] Furthermore, the aforementioned chemical bonds enable the carbon layer to follow any deformation of the casing while preserving its continuity.

[0066] The plasma may be generated at ambient temperature, which minimizes the heating and the deformation of the tank casings during the treatment.

[0067] Once the treatment by the reaction gas is completed, a second reaction gas different from the first can be introduced into the chamber 2 in order to deposit a layer of a material giving additional properties to the tanks, onto the tanks 3.

[0068] It is thus possible for several materials respectively impermeable to the various compounds constituting petrol, to be deposited in succession.

[0069] By way of example, a second reaction gas consisting of hexamethyldisiloxane may be introduced into the chamber 2, which gas enables a silica coating to be obtained on the tanks 3 under the effect of the microwaves.

[0070] The solenoid valve 8 is advantageously intermittently opened and closed by the control device 9, so as to create pressure oscillations in the chamber 2, the stages 5 and 6 operating continuously.

[0071] Variations of a factor 10 of the pressure inside the chamber 2 can thus be produced.

[0072] These pressure variations enhance the penetration of the reaction gas inside the tanks.

[0073] Of course, the invention is not limited to the exemplary embodiments which have just been described.

[0074] In particular, it is possible to introduce more than two reaction gases into the treatment chamber 2 in succession, so as to deposit various materials chosen according to the properties that it is desired to give the tanks.

[0075] In particular, it is possible to deposit various materials aiming to solve problems of conductivity, ultraviolet resistance, abrasion resistance, resistance to given chemicals or sensitivity to heat or flame.

[0076] The tanks 3 may be treated while they are without an attached element.

[0077] The invention is not limited to treating fuel tanks for motor vehicles, but is applicable more generally to any type of tank.

Claims

1. Method of manufacturing at least one hollow element of a fuel system (3), especially a casing of a fuel tank made of plastic, characterized in that it comprises the following steps:

(a) introducing the element or elements to be treated into a treatment chamber (2) equipped with at least one microwave generator,

(b) injecting a reaction gas into the chamber while enabling the gas to enter inside the said elements,

(c) subjecting this gas to microwave radiation delivered by the microwave generator (4), at a frequency and intensity chosen so as to create a plasma in the chamber and to bring about the coating of a material, arising from the decomposition of the reaction gas, on the said element or elements, preferably on both the inner and outer faces of the said element or elements.

2. Method according to the preceding claim, characterized in that, between the steps a) and b), the chamber is evacuated such that the absolute pressure therein is less than 0.05 mbar (5 Pa).

3. Method according to any one of the preceding claims, characterized in that the reaction gas is a hydrocarbon compound, preferably with at least 5 carbon atoms, in particular an alkane or an alkene or a mixture thereof, preferably a mixture of methane and propane.

4. Method according to either of claims 1 and 2, characterized in that the reaction gas is a siloxane, preferably hexamethyldisiloxane in vapour form.

5. Method according to any one of the preceding claims, characterized in that the reaction gas is injected at a nominal pressure of between 0.05 mbar (5 Pa) and 0.5 mbar (50 Pa).

6. Method according to any one of the preceding claims, characterized in that the element or elements placed in the chamber comprise at least one element attached thereto, such as a valve (15) or an electrical connector, in addition to the tank casing.

7. Element of a fuel system, especially a tank casing, comprising a substrate layer having an inner surface and an outer surface, characterized in that each of these surfaces is coated with at least one material which is different from that of the substrate, arising from the decomposition of a chemical compound under the effect of plasma-generating microwave radiation.

8. Element according to the preceding claim, characterized in that the substrate layer comprises, on each face, a layer of the same material or several layers of different materials, the thickness of each layer being between 0.3 and 1 &mgr;m.

9. Element according to either of claims 7 and 8, characterized in that the coating material comprises carbon with a polymer tendency and/or silica.

10. Fuel tank made of high-density polyethylene comprising, on at least one of its inner or outer faces, a coating of carbon and/or of silica with a polymer tendency.